WO2019050471A1

WO2019050471A1 - A method of producing bismuth vanadate and related products

Info

Publication number: WO2019050471A1
Application number: PCT/SG2017/050453
Authority: WO
Inventors: Guowu ZHAN; Chi-Hwa Wang; Wei Cheng NG
Original assignee: National University Of Singapore; Sembcorp Industries Ltd
Priority date: 2017-09-08
Filing date: 2017-09-08
Publication date: 2019-03-14

Abstract

There is provided a method of producing bismuth vanadate, the method comprising preparing a mixture having a pH of no more than 3.0 by adding a leaching solution containing vanadate ions to a bismuth salt solution, the leaching solution being derived from a vanadium source, and precipitating bismuth vanadate from the mixture. In particular, said vanadium source is an oil refinery waste or carbon black waste. Also provided are bismuth vanadates obtained therefrom.

Description

A METHOD OF PRODUCING BISMUTH VANADATE AND RELATED

PRODUCTS TECHNICAL FIELD

Various embodiments disclosed herein relate broadly to a method of producing bismuth vanadate from a vanadium source and related products. Vanadium source includes but is not limited to vanadium-based waste such as fossil fuel waste.

BACKGROUND

The amount of waste generated has dramatically increased with the rising consumption and resource use by a rapidly growing world population. An increasing amount of research interest has therefore been focused on building opportunities from waste, including food waste, biomass waste, oil fly ash, and the like. Such waste may sometimes contain useful components that still possess good market value, but the components are simply disposed of instead of recovered due to technological or economical limitations. An economically feasible recovery of the useful components may also contribute to a sustainable society. For instance, waste could be used as a valuable resource for the preparation of fuels and high-value chemicals/materials.

Vanadium is an excellent but toxic heavy metal that occurs in crude oil, coal, oil shale and tar sands. In particular, vanadium is the most abundant metal in crude oil, reaching concentration of up to 1 580 ppm of total crude, which is largely derived from chlorophyll of dead organisms. As crude oil is processed and refined into more useful fuels and intermediate products by oil refinery plants, vanadium will finally exist in carbon black waste (a major oil refinery waste). Carbon black waste is therefore a potential but untapped source of vanadium. Further, disposal of carbon black waste containing vanadium poses a pollution risk as vanadium is hazardous to human health and was found to impair the antioxidant enzymatic activities of human cell lines.

Typically, for metal recovery from carbon black waste, complete dissolution of valuable metals from carbon black waste is achieved by advanced leaching processes. Various technologies have been developed for the subsequent separation and recovery of metal in the leaching solutions, such as chemical precipitation, reactive crystallization, adsorption, ion exchange, electrochemical removal, biotechnological processes, and membrane separations. However, separation and recovery of metal, particularly vanadium, in the leaching solutions remains a challenge. Furthermore, the studies on transformation of waste metal ions, particularly vanadium ions, into value-added products are still insufficient in both academia and industry. In view of the above, there is thus a need to address or at least ameliorate one of the above problems.

SUMMARY In one aspect, there is provided a method of producing bismuth vanadate, the method comprising:

preparing a mixture having a pH of no more than 3.0 by adding a leaching solution containing vanadate ions to a bismuth salt solution, the leaching solution being derived from a vanadium source,

precipitating bismuth vanadate from the mixture.

In one embodiment, the mixture has a pH of no more than 2.0.

In one embodiment, said adding the leaching solution to the bismuth salt solution comprises adding the leaching solution dropwise into the bismuth salt solution. In various embodiments, the bismuth salt solution is selected from the group consisting of bismuth nitrate, bismuth acetate, bismuth halide and combinations thereof. In various embodiments, the bismuth halide is selected from the group consisting of bismuth fluoride, bismuth chloride, bismuth bromide, bismuth iodide and combinations thereof.

In one embodiment, the pH of the bismuth salt solution is no more than 0.5.

In one embodiment, the pH of the bismuth salt solution is no more than 0.

In one embodiment, the mole ratio of bismuth to vanadium in the mixture is from 0.8:1 .0 to 1 .2:1 .0.

In one embodiment, the method further comprises adding at least one of an acid or a base to the vanadium source to form the leaching solution containing vanadate ions. In one embodiment, the method further comprises, prior to the step of preparing the mixture:

(i) adding at least one of an acid or a base to the vanadium source to obtain a first leachate containing vanadate ions;

(ii) adding the leachate obtained from the preceding step to the vanadium source to obtain a second leachate containing a higher concentration of vanadate ions;

(iii) optionally repeating step (ii) for one or more times to obtain a third leachate or subsequent leachate;

wherein the leachate obtained after completion of the above steps forms the leaching solution containing vanadate ions for use in the step of preparing the mixture.

In one embodiment, the leaching solution has a pH greater than 7. In one embodiment, the method further comprises subjecting the precipitated bismuth vanadate to heat treatment at a temperature greater than 120°C.

In one embodiment, the heat treatment is selected from the group consisting of: hydrothermal treatment, calcination and combinations thereof.

In one embodiment, the addition of at least one of an acid or a base to the vanadium source selectively leaches vanadium as compared to other metals from the vanadium source to form the leaching solution containing vanadate ions, wherein said other metals are one or more metals selected from the group consisting of nickel, iron, aluminium, zinc, chromium, cadmium, cobalt and copper. In one embodiment, the bismuth vanadate is substantially free of impurities.

In one embodiment, the bismuth vanadate comprises monoclinic bismuth vanadate. In one embodiment, the vanadium source is an oil refinery waste.

In one embodiment, the vanadium source is a carbon black waste.

In one aspect, there is provided bismuth vanadate derived from a mixture of a leaching solution containing vanadate ions and a bismuth salt solution, wherein the mixture has a pH of no more than 3.0, wherein the bismuth vanadate has a surface area of at least 4 m²/g.

In one embodiment, the bismuth vanadate comprises monoclinic bismuth vanadate.

In one embodiment, the bismuth vanadate has been heat pre-treated at a temperature greater than 1 20°C. In various embodiments, the bismuth vanadate has one or more of the following properties selected from the group consisting of:

a) an average particle size of at least 1 pm;

b) an X-ray diffraction pattern corresponding to that of FIG. 5A or FIG.

5B; and

c) an optical band gap of between 2.30 eV and 2.45 eV.

In one aspect, there is provided a method of catalysing a reaction, the method comprising:

adding the bismuth vanadate according to any one of the above aspects or embodiments to the reactants, and

irradiating the bismuth vanadate with visible light to catalyse the reaction. DEFINITIONS

The term "vanadium source" as used herein broadly refers to materials that contain detectable amounts of elemental vanadium having a ground oxidation state and/or species containing vanadium that exists in a variety of oxidation states. Examples of "vanadium source" include but are not limited to vanadium- based waste which in turn includes but is not limited to fossil fuel waste.

The term "waste" as used herein broadly refers to unwanted materials that are left over from a process and are intended to be disposed of. The term encompasses but is not limited to waste generated from industrial process plants such as oil refinery or petroleum refinery, power plant, chemical plant and water and wastewater treatment plant. The term "vanadium-based waste" as used herein broadly refers to waste materials that contain detectable amounts of elemental vanadium having a ground oxidation state and/or species containing vanadium that exists in a variety of oxidation states. Examples of "vanadium- based waste" include but are not limited to fossil fuel waste, oil refinery waste, petroleum coke and carbon black waste. "Carbon black waste" may be understood to be a carbon-rich solid residue generated from incomplete combustion of hydrocarbon or cracking of oil under high temperatures in an oil refinery.

The term "leaching" as used herein refers to a process of extracting metal species from a material containing the metal species with a leaching agent. The term "leaching solution" as used herein refers to a solution resulting from the addition of the leaching agent to the material containing the metal species and the dissolution of the metal species. The leaching process may comprise a step of removing insoluble solids from the leaching solution subsequent to the dissolution of the metal species to form the leaching solution. Accordingly, the leaching solution may be substantially free of insoluble solids. The leaching solution may include both an intermediate and a final leaching solution/leachate. In some embodiments, as an intermediate leaching solution, it may be cycled repeatedly with the materials containing the metal species to eventually obtain a final leaching solution with an increased metal concentration. In some embodiments, the leaching process may comprise adjusting the pH of the leaching solution with an acid or a base.

The term "vanadium" as used herein broadly refers to elemental vanadium having a ground oxidation state and species containing vanadium that exists in a variety of oxidation states such as +2, +3, +4 and +5. Examples of such species encompass but are not limited to vanadium(ll), vanadium(lll), vanadium(IV), vanadyl, vanadium(V), vanadate ions, salts, compounds or complexes.

The term "vanadate ions" as used herein broadly refers to species containing an oxoanion of vanadium. Examples include but are not limited to VO3^" , V04^3", V₂07⁴-, V₃09^3", ν₄Οΐ2^4", VsOu³- , protonated vanadate ions such as HVO4²- , H2VO4^" and polyvanadate ions. Therefore, the term "vanadate ions" may also be understood to include oxoanions of vanadium where vanadium has an oxidation state of +5.

The term "oxidation state" as used herein refers to the degree of oxidation of an atom which is represented by zero, or by a positive or negative number. An increase in oxidation state is referred to as an oxidation process, while a decrease in oxidation state is referred to as a reduction process.

The term "nano" as used herein is to be interpreted broadly to include dimensions between 1 nm - 100 nm. Accordingly, a term carrying "nano" as prefix as used herein may include particles of sizes that are between about 1 nm to about 100 nm. The sizes of the particles may be no more than about 100 nm, no more than about 90 nm, no more than about 80 nm, no more than about 70 nm, no more than about 60 nm, no more than about 50 nm, no more than about 40 nm, no more than about 30 nm, no more than about 20 nm, or no more than about 10 nm.

The term "micro" as used herein is to be interpreted broadly to include dimensions no more than about 1000 pm. Accordingly, the term "micropartides" as used herein may include particles of sizes that are no more than about 1000 pm, no more than about 900 pm, no more than about 800 pm, no more than about 700 pm, no more than about 600 pm, no more than about 500 pm, no more than about 400 pm, no more than about 300 pm, no more than about 200 pm, or no more than about 100 pm. The term encompasses but is not limited to micropartides that are aggregation of smaller constitutent particles such as for example nanoparticles.

The term "size" when used to refer to a "nanostructure", "nanorod" "nanoparticle", "nanomaterial", "microstructure", "microparticle" or the like may broadly refer to the largest dimension of the nanostructure, nanorod, nanoparticle, nanomaterial, microstructure, microparticle or the like. For example, when the structure is substantially spherical, the term "size" can refer to the diameter of the structure; or when the structure is substantially non-spherical, the term "size" can refer to the largest length of the structure although not necessarily limited to that.

The term "precipitate" as used herein broadly refers to a solid formed from a solution and the terms "precipitating" and "precipitated" shall be construed accordingly. The term "and/or", e.g., "X and/or Y" is understood to mean either "X and Y" or "X or Y" and should be taken to provide explicit support for both meanings or for either meaning. The terms "coupled" or "connected" when used in this description are intended to cover both directly connected or connected through one or more intermediate means, unless otherwise stated.

The term "associated with", used herein when referring to two elements refers to a broad relationship between the two elements. The relationship includes, but is not limited to a physical, a chemical or a biological relationship. For example, when element A is associated with element B, elements A and B may be directly or indirectly attached to each other or element A may contain element B or vice versa. Further, in the description herein, the word "substantially" whenever used is understood to include, but not restricted to, "entirely" or "completely" and the like. In addition, terms such as "comprising", "comprise", and the like whenever used, are intended to be non-restricting descriptive language in that they broadly include elements/components recited after such terms, in addition to other components not explicitly recited. Further, terms such as "about", "approximately" and the like whenever used, typically means a reasonable variation, for example a variation of +/- 5% of the disclosed value, or a variance of 4% of the disclosed value, or a variance of 3% of the disclosed value, a variance of 2% of the disclosed value or a variance of 1 % of the disclosed value.

Furthermore, in the description herein, certain values may be disclosed in a range. The values showing the end points of a range are intended to illustrate a preferred range. Whenever a range has been described, it is intended that the range covers and teaches all possible sub-ranges as well as individual numerical values within that range. That is, the end points of a range should not be interpreted as inflexible limitations. For example, a description of a range of 1 % to 5% is intended to have specifically disclosed sub-ranges 1 % to 2%, 1 % to 3%, 1 % to 4%, 2% to 3% etc., as well as individually, values within that range such as 1 %, 2%, 3%, 4% and 5%. The intention of the above specific disclosure is applicable to any depth/breadth of a range.

Additionally, when describing some embodiments, the disclosure may have disclosed a method and/or process as a particular sequence of steps. However, unless otherwise required, it will be appreciated the method or process should not be limited to the particular sequence of steps disclosed. Other sequences of steps may be possible. The particular order of the steps disclosed herein should not be construed as undue limitations. Unless otherwise required, a method and/or process disclosed herein should not be limited to the steps being carried out in the order written. The sequence of steps may be varied and still remain within the scope of the disclosure.

DESCRIPTION OF EMBODIMENTS Exemplary, non-limiting embodiments of a method of producing bismuth vanadate and related bismuth vanadate products are disclosed hereinafter.

In various embodiments, there is provided a method of producing bismuth vanadate from a vanadium source, the method comprising preparing a mixture by adding a leaching solution containing vanadate ions to a bismuth salt solution under conditions suitable for forming bismuth vanadate, the leaching solution being derived from the vanadium source, and precipitating bismuth vanadate from the mixture.

In various embodiments, the method does not comprise adding a vanadate source selected from the group consisting of V2O5, NaVO3 and NH₄VO3. The method may not comprise adding a commercially available vanadate source such as a substantially purified vanadate compound.

Advantageously, embodiments of the method allow for the sustainable transformation of a vanadium source, for example a vanadium-based waste, into value-added products such as bismuth vanadate. Embodiments of the method efficiently recover vanadium from a vanadium source, for example a vanadium-based waste, and reduce the production cost for bismuth vanadate, particularly the cost of obtaining commercial vanadate sources such as V2O5, NaVO3 and NH₄VO3 as raw materials for production. Embodiments of the method also produce high quality bismuth vanadate suitable for industrial application, such as for use as pigments or photocatalysts.

In various embodiments, the precipitating step of the method comprises stirring the mixture. In various embodiments, said stirring comprises vigorous stirring. In various embodiments, said stirring is performed for up to about 5 hours, up to about 6 hours, up to about 7 hours, up to about or about 8 hours. In various embodiments, said stirring is performed at room temperature. It may be appreciated that stirring the mixture may promote the reaction between the vanadate ions and the bismuth ions from the bismuth salt solution to form bismuth vanadate. In various embodiments, the bismuth salt solution comprises a bismuth salt (III) solution. In various embodiments, the bismuth salt solution is selected from the group consisting of bismuth nitrate, bismuth acetate, bismuth halide and combinations thereof. In various embodiments, the bismuth halide is selected from the group consisting of bismuth fluoride, bismuth chloride, bismuth bromide, bismuth iodide and combinations thereof.

As may be appreciated by a person skilled in the art, undesirable bismuth hydroxide may be formed from the mixture of the bismuth salt solution and the leaching solution. Accordingly, in various embodiments, the conditions suitable for forming bismuth vanadate comprise conditions suitable for preferentially forming bismuth vanadate as compared to bismuth hydroxide. In an example embodiment, the bismuth salt solution is acidic, i.e. has a pH less than 7.

In further embodiments, the pH of the bismuth salt solution is no more than about 1 .0, no more than about 0.9, no more than about 0.8, no more than about 0.7, no more than about 0.6, no more than about 0.5, no more than about 0.4, no more than about 0.3, no more than about 0.2, no more than about 0.1 or no more than about or about 0. In an example embodiment, the pH of the bismuth salt solution is no more than about 0.5. In another example embodiment, the pH of the bismuth salt solution is no more than about 0. In various embodiments, the method further comprises a step of optionally adjusting the pH of the bismuth salt solution to be in accordance with the pH values disclosed herein.

The pH of the bismuth salt solution may affect product quality and yield. Generally, low pH level was found to favourably affect product quality and yield. Advantageously, when the pH of the bismuth salt solution is in accordance with the pH values disclosed herein, embodiments of the method produce bismuth vanadate preferentially to bismuth hydroxide, and show efficient recovery of vanadium from the leaching solution. Particularly, the vanadium recovery efficiency is sensitive to the pH of the bismuth salt solution; a change in the pH of the bismuth salt solution was found to affect the vanadium recovery efficiency. In various embodiments, the conditions suitable for forming bismuth vanadate relate to the mole ratio of bismuth to vanadium in the mixture. In various embodiments, the mole ratio of bismuth to vanadium in the mixture is from about 0.8:1 .0 to about 1 .5:1 .0. In various embodiments, the mole ratio of bismuth to vanadium in the mixture is from about 0.8:1 .0 to about 1 .2:1 .0. In one embodiment, the mole ratio of bismuth to vanadium in the mixture is 1 :1 . Advantageously, when the mole ratio of bismuth to vanadium in the mixture is in accordance with the ratios disclosed herein, embodiments of the method show efficient recovery of vanadium from the leaching solution. It was observed that a higher molar amount of bismuth in the mixture, such that the mole ratio of bismuth to vanadium in the mixture is not in accordance with the ratios disclosed herein, did not increase vanadium recovery, and even reduced vanadium recovery.

In various embodiments, the method has a vanadium recovery efficiency of at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, at least about 99.1 %, at least about 99.2% or at least about 99.3% from the leaching solution. Advantageously, embodiments of the method show a high vanadium recovery efficiency from the leaching solution.

In various embodiments, the conditions suitable for forming bismuth vanadate relate to the pH of the mixture resulting from the addition of the leaching solution to the bismuth salt solution. In various embodiments, the pH of the mixture is no more than about 7, no more than about 6, no more than about 5, no more than about 4, no more than about 3, no more than about 2.9, no more than about 2.8, no more than about 2.7, no more than about 2.6, no more than about 2.5, no more than about 2.4, no more than about 2.3, no more than about 2.2, no more than about 2.1 or no more than about 2, no more than about 1 or no more than about 0. In various embodiments, the pH of the mixture is from about 0 to about 7, from about 0 to about 5, from about 0 to about 3 or from about 1 to about 3. In various embodiments, the method further comprises a step of optionally adjusting the pH of the mixture to be in accordance with the pH values disclosed herein. It may be appreciated that the pH of the mixture may affect the bismuth vanadate obtained such as in terms of its product quality including the particle size, crystal form, morphology and chemical purity. Advantageously, when the pH of the mixture is in accordance with the pH values disclosed herein, embodiments of the method produce bismuth vanadate preferentially to bismuth hydroxide. Further advantageously, when the pH of the mixture is in accordance with the pH values disclosed herein, embodiments of the method produce desirable bismuth vanadate of monoclinic scheelite structure preferentially to bismuth vanadate of tetragonal scheelite structure and zircon-type structure. In one embodiment, the method is devoid of a pH adjustment step and/or a pH detection step after precipitating bismuth vanadate from the mixture.

In various embodiments, there is provided a method of producing bismuth vanadate, the method comprising preparing a mixture having a pH of no more than 3.0 by adding a leaching solution containing vanadate ions to a bismuth salt solution, the leaching solution being derived from a vanadium source, and precipitating bismuth vanadate from the mixture. In some embodiments, the mixture has a pH of no more than 2.0. In one embodiment, the method produces monoclinic bismuth vanadate. As may be appreciated by a person skilled in the art, among the three polymorphs of bismuth vanadate with different crystal structures, namely, tetragonal scheelite structure, monoclinic scheelite structure and zircon-type structure, bismuth vanadate having monoclinic scheelite structure is the most excellent photocatalyst due to its enhanced photon absorption and low band gap energy.

In various embodiments, one or more conditions relating to the leaching solution may also be controlled. In various embodiments, the leaching solution has a pH greater than about 7, greater than about 8, greater than about 9, greater than about 10, greater than about 1 1 , greater than about 12, greater than about 13, greater than about 13.1 , greater than about 13.2, greater than about 13.3, greater than about 13.4, greater than about 13.5, greater than about 13.6, greater than about 13.7, greater than about 13.8, greater than about 13.9 or greater than or about 14.0. In one embodiment, the leaching solution is alkaline. In various embodiments, the method further comprises a step of optionally adjusting the pH of the leaching solution to be in accordance with the pH values disclosed herein. Without being bound by any theory, it is believed that vanadium may exist primarily in the form of VO4^3" and undesirable formation of vanadate polyanions, which may reduce the availability of vanadate ions to react with the bismuth ions to produce bismuth vanadate, may be suppressed when the pH of the leaching solution is in accordance with the pH values disclosed herein.

In various embodiments, said adding the leaching solution to the bismuth salt solution comprises adding the leaching solution dropwise into the bismuth salt solution. In various embodiments, said adding the leaching solution dropwise into the bismuth salt solution comprises adding the leaching solution dropwise at a rate of no greater than about 30 mL/min, no greater than about 25 mL/min, no greater than about 20 mL/min, no greater than about 15 mL/min, no greater than about or about 10 mL/min. Advantageously, when the addition of the leaching solution is in accordance with embodiments of the method disclosed herein, the formation of the undesirable by-product bismuth hydroxide is minimized or prevented. Without being bound by any theory, when the addition of the leaching solution is in accordance with embodiments of the method disclosed herein, it is believed that VO4^3" ions in the leaching solution would react with bismuth cations in the bismuth salt solution to form bismuth vanadate, while the hydroxide ions in the leaching solution would react with the protons in the bismuth salt solution to form water and would not react with the bismuth cations to form the undesirable by-product bismuth hydroxide.

In various embodiments, the method further comprises adding at least one of an acid or a base to the vanadium source to form the leaching solution containing vanadate ions. Advantageously, it was found that vanadium can be leached from a vanadium source by using acid or base as leaching agents. Particularly, vanadium was found to be selectively leached by using alkaline solutions such as sodium hydroxide as leaching agents with high efficiency, whereas other metals such as nickel and iron which were contained in the vanadium source were hardly leached. Accordingly, in various embodiments, the addition of at least one of an acid or a base to the vanadium source selectively leaches vanadium as compared to other metals from the vanadium source to form the leaching solution containing vanadate ions. In various embodiments, said other metals comprise one or more metals selected from the group consisting of nickel, iron, aluminium, zinc, chromium, cadmium, cobalt and copper. In various embodiments, the leaching solution is substantially free from or contains no more than about 3 ppm, no more than about 2.5 ppm, no more than 2 ppm or no more than about 1 .9 ppm of one or more metals selected from the group consisting of nickel, iron, aluminium, zinc, chromium, cadmium, cobalt and copper. Advantageously, embodiments of the method, by enabling the selective leaching of vanadium, produce bismuth vanadate with substantially high purity.

In one embodiment, adding at least one of an acid or a base to the vanadium source to form the leaching solution comprises adding an acid to the vanadium source to leach vanadium followed by adding a base to increase the pH of the leaching solution. In another embodiment, adding at least one of an acid or a base to the vanadium source to form the leaching solution comprises adding a base to the vanadium source to leach vanadium followed by adding an acid to reduce the pH of the leaching solution. In various embodiments, the method further comprises, prior to the step of preparing the mixture, (i) adding at least one of an acid or a base to the vanadium source to obtain a first leachate containing vanadate ions, (ii) adding the leachate obtained from the preceding step (i.e. the first leachate) to the vanadium source to obtain a second leachate containing a higher concentration of vanadate ions, and optionally repeating step (ii) for one or more times to obtain a third leachate or subsequent leachate, wherein the leachate obtained after completion of the above steps forms the leaching solution containing vanadate ions for use in the step of preparing the mixture. In some embodiments, the method further comprises a step of removing insoluble solids from the leachate before proceeding to the next step. Advantageously, embodiments of the method which relate to, for instance, recycling of a leachate to leach a same vanadium source repeatedly, maximises the leaching of a metal species, for example vanadium, from the vanadium source with minimal amount of an acid or a base.

In various embodiments, the leaching solution may be an intermediate leaching solution or a final leaching solution. As an intermediate leaching solution, it may be further used to leach more vanadium ions from the vanadium source. For example, it may be cycled repeatedly with the vanadium source to obtain a leaching solution with an increased vanadium concentration or a final leaching solution having a vanadium concentration that is suitable for use in subsequent processing steps to eventually produce the bismuth vanadate. It may be appreciated that if the concentration of vanadium present in the leaching solution/leachate is insufficient for subsequent steps of producing the bismuth vanadate, cycling leaching can be performed to increase the concentration of vanadium in the leaching solution/leachate. In various embodiments, the cycling leaching comprises subjecting a leaching solution/leachate having a low concentration of vanadium to one or more additional leaching process(es), without introducing any additional leaching agents. In other words, the leaching solution/leachate having a low concentration of vanadium may act as the leaching agent during the cycling leaching process. In various embodiments, the concentration of vanadium in the leaching solution/leachate increases with the number of times the leaching solution/leachate is cycled with the vanadium source. For example, if the concentration of vanadium in the first leaching solution/leachate is low, a vanadium source may be added to the first leaching solution/leachate to perform a second leaching process. The second leaching process may comprise the steps of dissolving vanadium and removing insoluble solids to form the second leaching solution/leachate, which has a higher vanadium concentration than the first leaching solution/leachate. In each leaching cycle, the leaching process may be repeated at least once, at least twice, or at least thrice to obtain a second leaching solution/leachate, third leaching solution/leachate or subsequent leaching solution/leachate, wherein the final leaching solution/leachate forms the final leaching solution containing vanadate ions.

In some embodiments, the concentration of vanadium in the first, second, third subsequent or final leaching solution/leachate is at least but not limited to about 100 ppm, about 200 ppm, about 300 ppm, about 400 ppm, about 500 ppm, about 600 ppm, about 700 ppm or about 800 ppm. In some embodiments, the concentration of vanadium in the second, third, subsequent or final leaching solution/leachate is at least but not limited to about 1000 ppm.

In one embodiment, the concentration of vanadium in a first leaching solution/leachate is in a range of about 500 ppm to about 800 pm. The first leaching solution/leachate may act as a leaching agent in a second leaching process. After undergoing the second leaching process, the concentration of vanadium in the second leaching solution/leachate may increase to about 1 000 ppm. In some embodiments, if the concentration of vanadium in the second leaching solution/leachate is determined to be low, the second leaching solution/leachate obtained may be further cycled through the cycling leaching process in order to increase the concentration of vanadium in the final leaching solution/leachate. In some other embodiments, if the concentration of vanadium in the second leaching solution/leachate is determined to be sufficient, the second leaching solution/leachate may form the final leaching solution/leachate containing vanadate ions. In various embodiments, the minimum concentration of vanadium in a final leaching solution/leachate containing vanadate ions may be, but not limited to, 100 ppm. In various embodiments, the acid added to vanadium source may be any suitable acid that effectively and/or preferentially leaches vanadium from the vanadium source. In some embodiments, the acid comprises a strong acid. In some embodiments, the acid may be selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, acetic acid and phosphoric acid. In some embodiments, the concentration of the acid is from about 0.01 mol/L (M) to about 10 M, from about 0.1 M to about 5 M, from about 0.5 M to about 3 M or from about 0.8 M to about 1 .5 M. In one embodiment, the concentration of the acid is about 1 M. In another embodiment, the concentration of the acid is about 0.5 M . In some embodiments, the alkali/base added to vanadium source may be any suitable alkali/base that effectively and/or preferentially leaches vanadium from the vanadium source. In some embodiments, the alkali/base comprises a strong alkali/base. In some embodiments, the alkali/base may be selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, ammonium hydroxide, and mixtures thereof. In one embodiment, the alkali/base comprises sodium hydroxide. In some embodiments, the concentration of the alkali/base is from about 0.01 M to about 10 M, from about 0.1 M to about 5 M, from about 0.5 M to about 3 M or from about 0.8 M to about 1 .5 M. In one embodiment, the concentration of the alkali/base is about 1 M. In another embodiment, the concentration of the alkali/base is about 0.5 M.

In various embodiments, the vanadium source comprises a solid containing moisture. In various embodiments, the method further comprises drying the vanadium source to obtain a dry solid prior to adding at least one of an acid or a base to the vanadium source to form the leaching solution. In various embodiments, said drying the vanadium source comprises drying the vanadium source in an oven at a temperature greater than about 80 °C, greater than about 85 °C, greater than about 90 °C, greater than about 95 °C, greater than about 100 °C or greater than about or about 105 °C for a duration of at least about 5 hours, at least about 6 hours, at least about 7 hours, at least about 8 hours, at least about 9 hours, at least about 10 hours, at least about 1 1 hours, at least about or about 12 hours.

In various embodiments, the concentration of the vanadium source, for example a vanadium-based waste, in the leaching solution is from about from about 0.01 weight/volume percentage (w/v%) to about 90 w/v%, from about 1 w/v% to about 50 w/v%, from about 3 w/v% to about 30 w/v% or from about 5 w/v% to about 20 w/v%. As may be appreciated by a person skilled in the art, a 5 w/v% of vanadium source in leaching solution may be obtained, for instance, by mixing 2.5 g of vanadium source, for example 2.5 g of vanadium-based waste, with 50 imL of leaching solution.

In various embodiments, the step of adding at least one of an acid or a base to the vanadium source to form the leaching solution, comprises dispersing the vanadium source in the at least one of an acid or a base to extract/leach vanadium from the vanadium source. In various embodiments, said dispersing may be achieved by using techniques such as sonication, agitation, mixing and the like. In various embodiments, the vanadium source and/or the leaching solution comprises vanadium with an oxidation state of +5. The vanadium source and/or the leaching solution may be substantially free from vanadium having oxidation states other than +5 (e.g. +3 or +4 oxidation states). In various embodiments, the vanadium source and/or the leaching solution may be substantially free from VO3^" ions and/or V2O5. In various embodiments, the vanadium in the leaching solution exists as VO4^3" ions.

In various embodiments, the vanadium source comprises a vanadium-based waste. In various embodiments, the vanadium-based waste comprises fossil fuel waste. In various embodiments, the vanadium-based waste comprises an industrial waste. In various embodiments, the vanadium-based waste comprises an oil refinery waste. In various embodiments, the vanadium-based waste comprises a carbon black waste. As may be appreciated by a person skilled in the art, crude oil and by- products of crude oil refinery process, including carbon black waste, contain a substantial amount of vanadium. It may be further appreciated that excessive level of vanadium is considered toxic to organisms and harmful to the environment. Advantageously, embodiments of the method mitigate the risk of toxicity and pollution associated with the disposal of vanadium-based waste such as oil refinery waste and carbon black waste by reducing the vanadium content in such waste. In this regard, in various embodiments, the vanadium source disclosed herein is distinct and different from a purified vanadium source which is commercially available and which is substantially free from other metals and/or chemical compounds.

In various embodiments, the method further comprises subjecting the precipitated bismuth vanadate to heat treatment at a temperature greater than about 100°C, greater than about 1 10°C, greater than about 120°C, greater than about 130°C, greater than about 140°C, greater than about 150°C, greater than about 160°C, greater than about 170°C, greater than about 180°C, greater than about 190°C, greater than about or about 200°C. In further embodiments, the method further comprises subjecting the precipitated bismuth vanadate to heat treatment at a temperature greater than about 500°C, greater than about 510°C, greater than about 520°C, greater than about 530°C, greater than about 540°C, greater than about 550°C, greater than about 560°C, greater than about 570°C, greater than about 580°C, greater than about 590°C, greater than about or about 600°C.

In some embodiments, the heat treatment comprises a wet-heating treatment. In some embodiments, the heat treatment comprises a dry-heating treatment. In one embodiment, the wet-heating treatment comprises hydrothermal treatment. As may be appreciated by a person skilled in the art, hydrothermal treatment may be performed, for instance, by subjecting precipitated bismuth vanadate in wet suspension to heating in a sealed Teflon-lined reactor. Calcination may be performed, for instance, by subjecting dry precipitated bismuth vanadate to heating in a crucible in a furnace. Accordingly, in some embodiments, the method further comprises drying the precipitated bismuth vanadate prior to heat treatment. Accordingly, in various embodiments, the heat treatment is selected from the group consisting of hydrothermal treatment, calcination and combinations thereof.

In various embodiments, the heat treatment promotes the formation of a stable, monoclinic crystal structure of bismuth vanadate. In various embodiments, the heat treatment promotes the formation of bismuth vanadate having an X-ray diffraction pattern corresponding to that of FIG. 5A or FIG 5B. In various embodiments, the heat treatment facilitates crystallization of particles, leading to the transformation of an unstable precursor into a stable crystal product of bismuth vanadate. In one embodiment, the heat treatment facilitates aggregation of nanoparticles to form crystalline microparticles.

In various embodiments, the duration of the heat treatment is no more than about 50 hours, no more than about 45 hours, no more than about 40 hours, no more than about 35 hours, no more than about 30 hours, no more than about 25 hours, no more than about 20 hours, no more than about 15 hours or no more than about 10 hours. In various embodiments, the duration of the heat treatment is no less than about 5 hours or no less than about 10 hours. In one embodiment, the duration of the heat treatment is about 15 hours. In another embodiment, the duration of the heat treatment is about 10 hours.

Advantageously, embodiments of the method comprising a heat treatment step in accordance with one or more conditions and/or parameters, two or more conditions and/or parameters or three or more conditions and/or parameters disclosed herein are capable of producing a stable, monoclinic bismuth vanadate that is suitable for industrial application. It may be appreciated that heat treatment conditions and/or parameters such as temperature and duration may affect the bismuth vanadate product in terms of its crystal structure, stability, size, surface area, morphology and/or colour hue.

In various embodiments, the method further comprises a step of recovering the precipitated bismuth vanadate by at least one of centrifugation, filtration or a combination thereof. In various embodiments, the method further comprises washing the precipitated bismuth vanadate to substantially remove impurities. In various embodiments, the method further comprises filtering the leaching solution to remove impurities and/or sediments prior to adding the bismuth salt solution to the leaching solution. In various embodiments, said filtering comprises using a membrane filter. In various embodiments, the membrane filter has a pore size of no greater than about 0.80 pm, no greater than about 0.75 pm, no greater than about 0.70 pm, no greater than about 0.65 pm, no greater than about 0.60 pm, no greater than about 0.55 pm, no greater than about 0.50 pm, no greater than about or about 0.45 pm. In various embodiments, said filtering is conducted under vacuum.

Advantageously, embodiments of the method enable easy recovery and obtaining of a bismuth vanadate product that is substantially free of impurities.

In various embodiments, the method, from the step of preparing a mixture to the step of precipitating bismuth vanadate from the mixture, requires no more than about 25 hours, no more than about 24 hours, no more than about 23 hours, no more than about 22 hours, no more than about 21 hours, no more than about 20 hours, no more than about 19 hours, no more than about 18 hours, no more than about 10 hours, no more than about 9 hours or no more than about 8 hours to complete. In various embodiments, the method, from the step of drying the vanadium source to the step of precipitating bismuth vanadate from the mixture, requires no more than about 40 hours, no more than about 39 hours, no more than about 38 hours, no more than about 37 hours, no more than about 36 hours, no more than about 35 hours, no more than about 34 hours, no more than about 33 hours, no more than about 25 hours, no more than about 24 hours or no more than about 23 hours to complete. In one embodiment, the method comprises an industrial method.

Advantageously, embodiments of the method are feasible on a large scale. In some embodiments, at least about 1000 kg/day, at least about 1 100 kg/day, at least about 1200 kg/day, at least about 1300 kg/day, at least about 1400 kg/day, or at least about 1450 kg/day of bismuth vanadate is obtainable by the industrial method. In some embodiments, the industrial method is capable of recovering vanadium from at least about 10 tons/day, at least about 15 tons/day, at least about 20 tons/day, at least about 25 tons/day, or at least about 30 tons/day of wet carbon black waste.

In various embodiments, there is provided a method of preparing bismuth vanadate pigments and/or photocatalyts containing the following steps: chemically leaching an industrial waste, for example carbon black waste, making a precursor solution of bismuth ions, adjusting the pH of the leaching solution resulting from the chemical leaching of the industrial waste and the pH of the precursor solution, mixing the leaching solution with the precursor solution by vigorous stirring, collecting the precipitated bismuth vanadate precursor solid by filtration or centrifugation methods and subjecting the bismuth vanadate precursor solid to post-treatments such as hydrothermal treatment or calcination.

In various embodiments, there is provided bismuth vanadate derived from a vanadium-based waste. Embodiments of the bismuth vanadate, being partly derived from a vanadium-based waste, are an environmentally friendly and sustainable product. It will be appreciated that the starting vanadium source may have an effect on the final bismuth vanadate product, such that bismuth vanadate produced from different vanadium sources may possess different characteristics in terms of, for instance, the homogeneity of the bismuth vanadate particles and their morphology.

In some embodiments, the bismuth vanadate comprises nanoparticles. In some embodiments, the bismuth vanadate comprises spherical nanoparticles. In various embodiments, the bismuth vanadate comprising nanoparticles has an average particle size of no more than about 30 nm, no more than about 29 nm, no more than about 28 nm, no more than about 27 nm, no more than about 26 nm, no more than about 25 nm, no more than about 24 nm, no more than about or about 23 nm. In one embodiment, the bismuth vanadate comprising nanoparticles is an orange/yellow powder in appearance. In various embodiments, the bismuth vanadate comprising nanoparticles has not been heat-treated. Notably, embodiments of the bismuth vanadate having one or more of the characteristics disclosed herein may serve as a precursor to the synthesis of micro-sized bismuth vanadate particles.

Accordingly, in some embodiments, the bismuth vanadate comprises microparticles. In some embodiments, the bismuth vanadate has an average particle size of at least about 1 pm, at least about 1.5 pm, at least about 2 pm, at least about 2.5 pm, at least about 3 pm, at least about 3.5 pm, at least about 4 pm, at least about 4.5 pm, at least about 5 pm, at least about 5.5 pm, at least about 6 pm, at least about 6.5 pm, at least about 7 pm, at least about 7.5 pm or at least about 8 pm. In some embodiments, the bismuth vanadate has a broad size distribution of from about 1 pm to about 8 pm or from about 2 pm to about 7 pm. In some embodiments, the bismuth vanadate has a substantially uniform average particle size of about 5 pm, about 5.5 pm, about 6 pm, about 6.5 pm, about 7 pm, about 7.5 pm or about 8 pm. As may be appreciated by a person skilled in the art, the particle size and particle size distribution of bismuth vanadate may influence its optical performance as a pigment. For instance, the particle size and particle size distribution of a pigment may have an effect on the hue, tint strength, opacity, gloss, flocculation, viscosity, stability and weather resistance of the pigment. Advantageously, embodiments of the bismuth vanadate possess a size distribution and/or an average particle size that makes them suitable for use as pigments. At the same time, the size distribution and/or an average particle size of embodiments of the bismuth vanadate are also sufficiently small to render them suitable for use in photocatalytic applications.

In various embodiments, the bismuth vanadate has a surface area of at least about 1 m²/g, at least about 2 m²/g, at least about 3 m²/g, at least about 4 m²/g, at least about 5 m²/g, at least about 6 m²/g, at least about 7 m²/g, at least about 8 m²/g, at least about 9 m²/g or at least about 10 m²/g when measured by BET nitrogen adsorption. Advantageously, embodiments of the bismuth vanadate possess favourable characteristics, such as a high surface area, making them particularly desirable for use in photocatalytic applications.

In some embodiments, there is provided bismuth vanadate derived from a mixture of a leaching solution containing vanadate ions and a bismuth salt solution, wherein the mixture has a pH of no more than 3.0, wherein the bismuth vanadate has a surface area of at least 4 m²/g.

In various embodiments, the bismuth vanadate comprises monoclinic bismuth vanadate. It will be appreciated that monoclinic bismuth vanadate is considered the most excellent photocatalyst among the three polymorphs of B1VO4 with different crystal forms {i.e., tetragonal scheelite, monoclinic scheelite, and zircon-type structures). In various embodiments, the bismuth vanadate is substantially free of impurities. Advantageously, embodiments of the bismuth vanadate are suitable for industrial and further applications.

In various embodiments, the bismuth vanadate has been heat pre-treated at a temperature greater than about 100°C, greater than about 1 10°C, greater than about 120°C, greater than about 130°C, greater than about 140°C, greater than about 150°C, greater than about 160°C, greater than about 170°C, greater than about 180°C, greater than about 190°C, greater than about or about 200°C. In further embodiments, the bismuth vanadate has been heat pre-treated at a temperature greater than about 500°C, greater than about 510°C, greater than about 520°C, greater than about 530°C, greater than about 540°C, greater than about 550°C, greater than about 560°C, greater than about 570°C, greater than about 580°C, greater than about 590°C, greater than about or about 600°C. In various embodiments, the bismuth vanadate is substantially free from moisture. In various embodiments, the bismuth vanadate has an X-ray diffraction pattern corresponding to that of FIG. 5A or FIG. 5B. In various embodiments, the bismuth vanadate has an X-ray diffraction pattern that is substantially similar before and after storing for about 48 hours and above. Advantageously, embodiments of the bismuth vanadate, having been heat- treated, have a stable, monoclinic crystal structure which does not collapse after about 48 hours or more of storage, about 72 hours and above, about 96 hours and above or about 120 hours and above. Embodiments of the heat pre-treated bismuth vanadate also possess desirable properties in terms of its size, surface area, morphology and/or colour hue.

Embodiments of the bismuth vanadate have an optical band gap of from about 2.30 eV to about 2.45 eV, from about 2.30 eV to about 2.40 eV or from about 2.33 eV to about 2.40 eV. In one embodiment, bismuth vanadate has an optical band gap of about 2.35 eV. In another embodiment, bismuth vanadate has an optical band gap of about 2.39 eV. As may be appreciated by a person skilled in the art, embodiments of the bismuth vanadate possess an optical band gap that is close to, or corresponds with the reported optical band gap of bismuth vanadate compound.

In various embodiments, the bismuth vanadate is capable of catalysing a reaction under visible light irradiation. For instance, embodiments of the bismuth vanadate are capable of catalysing the degradation of organic compounds such as organic dye methylene blue under visible light irradiation. Advantageously, embodiments of the bismuth vanadate are good performing photocatalyst due to their favourable properties such as a substantially large surface area and small size.

In various embodiments, there is provided a method of catalysing a reaction, the method comprising adding the bismuth vanadate to the reactants, and irradiating the bismuth vanadate with visible light to catalyse the reaction. The step of irradiating the bismuth vanadate with visible light comprises irradiating with light having a wavelength of about 400 nm or more. Advantageously, embodiments of the method show good performance in catalytic reactions attributable to favourable properties of the bismuth vanadate such as a substantially large surface area and small size.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a schematic flowchart for illustrating a method of producing bismuth vanadate in example embodiments with hydrothermal treatment or calcination as post- treatment. FIG. 2 shows a ⁵¹V nuclear magnetic resonance (NMR) spectrum of a leaching solution of carbon black waste.

FIG. 3 shows the X-ray diffraction (XRD) patterns of illustrative bismuth vanadate samples obtained in accordance with various embodiments disclosed herein with the starting bismuth nitrate (A) at different pH conditions in solution and (B) at different amounts.

FIG. 4 shows the XRD patterns of an illustrative bismuth vanadate sample that has not been heat-treated (B1VO4-P) after storing for 48 hours.

FIG. 5 shows the XRD patterns of illustrative bismuth vanadate samples obtained by (A) calcination (B1VO4-C) and (B) hydrothermal treatment (B1VO4-H) in accordance with various embodiments disclosed herein.

FIG. 6 shows the scanning electron microscopy (SEM) images of illustrative bismuth vanadate samples (A) B1VO4-P, (B,C) B1VO4-H and (D,E,F) B1VO4-C, (G) transmission electron microscopy (TEM) image of a nanoplate of an illustrative bismuth vanadate sample B1VO4-H and its selected area electron diffraction (SAED) pattern at the inset, (H) energy-dispersive X-ray spectroscopy (EDX) elemental maps of the B1VO4 sample shown in the TEM image, and (I) EDX spectrum of illustrative bismuth vanadate samples B1VO4-H and B1VO4-C as-obtained.

FIG. 7 shows plots of (A) (ahv)² versus E (or hv) based on UV-Vis diffuse reflectance spectrum, where a is the absorption coefficient based on Kubelka-Munk function, h is the Planck constant, and D Dis the photon's frequency, (B) Fourier transform infrared spectroscopy (FTIR) spectra, (C) thermogravimetric analysis (TGA) curves, (D) X-ray photoelectron spectroscopy (XPS) spectra in 4f peaks of Bi, (E) XPS spectra in 2p peaks of V, and (F) XPS spectra in 1 s peaks of O of illustrative bismuth vanadate samples in accordance with various embodiments disclosed herein.

FIG. 8 shows (A) the UV-vis absorption spectra of methylene blue (MB) as a function of time by using an illustrative bismuth vanadate sample B1VO4-C as catalysts and (B) the plots of the MB concentration vs light illumination time with Co being the initial concentration, Ct being the measured value at a specific time, and Ct Co being expressed in percentage (%) at the y-axis. FIG. 9 is a block flow diagram for illustrating an industrial process to produce bismuth vanadate in accordance with various embodiments disclosed herein.

FIG. 10 is a general scheme of a method of producing bismuth vanadate from a vanadium source in accordance with various embodiments disclosed herein.

DETAILED DESCRIPTION OF FIGURES

FIG. 1 is a schematic flowchart for illustrating a method of producing bismuth vanadate by using waste vanadium ions originating from carbon black waste in an example embodiment. Overall, the process can be divided into three well-controlled steps: leaching, recovery and post-treatments. Colour code in the crystal structure of monoclinic B1VO4: dark grey, O; light grey, Bi; and black, V.

FIG. 2 shows a ⁵¹ V NMR spectrum of a leaching solution of carbon black waste. Based on the figure, it can be seen that vanadium mainly existed as monomeric VO4^3" anions because of the single resonance at chemical shift of -537 ppm.

FIG. 3 shows the XRD patterns of illustrative bismuth vanadate samples obtained in accordance with various embodiments disclosed herein when the starting bismuth nitrate was (A) at different pH levels in solution and (B) at different amounts. In (A), at the inset, the photograph on the left shows that the synthetic solution prepared from a pH of 0 was orange in colour and the photograph on the right shows that the synthetic solution prepared from a pH of 7 was white in colour. Without the addition of HCI to the bismuth nitration solution (i.e., pH of 7), the obtained product could be attributed as bismuth oxide hydrate (JCPDS PDF#54-0344). In (B), pH of the bismuth nitrate solution was controlled as 0 while using different amounts of bismuth nitrate. At low bismuth amounts (17 mg), the XRD pattern of the product obtained seems to be amorphous without any intensive peaks. There was no distinct difference in the XRD patterns of the products obtained when the bismuth nitrate amount was 34 mg as compared to when the bismuth nitrate amount was 68 mg.

FIG. 4 shows the XRD patterns of an illustrative bismuth vanadate sample that has not been heat-treated (B1VO4-P) after storing for 48 hours. The crystal structure collapsed after storing for 48 hours as evident from the XRD characterization.

FIG. 5 shows the XRD patterns of illustrative bismuth vanadate samples obtained by (A) calcination (B1VO4-C) and (B) hydrothermal treatment (B1VO4-H) in accordance with various embodiments disclosed herein. The peaks were indexed based on monoclinic B1VO4 (JCPDS PDF#14-0688). As depicted, both XRD diffraction patterns of illustrative bismuth vanadate samples B1VO4-C and B1VO4-H matched well with the standard monoclinic BiVO4 with scheelite structures (JCPDS PDF#14-0688). FIG. 6 shows the SEM images of illustrative bismuth vanadate samples (A)

B1VO4-P, (B,C) B1VO4-H and (D,E,F) B1VO4-C, (G) TEM image of a nanoplate of an illustrative bismuth vanadate samples B1VO4-H and its SAED pattern at the inset, (H) EDX elemental maps of the B1VO4 sample shown in the TEM image, and (I) EDX spectrum of illustrative bismuth vanadate samples B1VO4-H and B1VO4-C as-obtained. As depicted, B1VO4-H and B1VO4-C were larger in size than B1VO4-P, and seemed to be composed of aggregates of smaller plate-like particles. The SAED pattern suggests the single crystal nature of the solid sample and the EDX results indicate that the nanoplate contained homogenously distributed elements of Bi, V and O, and that the sample was substantially free of impurities.

FIG. 7 shows plots of (A) (ahv)² versus E (or hv) based on UV-Vis diffuse reflectance spectrum, where a is the absorption coefficient based on Kubelka-Munk function, h is the Planck constant, and□ Dis the photon's frequency, (B) FTIR spectra, (C) TGA curves, (D) XPS spectra in 4f peaks of Bi, (E) XPS spectra in 2p peaks of V, and (F) XPS spectra in 1 s peaks of O of illustrative bismuth vanadate samples in accordance with various embodiments disclosed herein. The E_g values of the samples B1VO4-H and B1VO4-C were close to the reported E_g value of bismuth vanadate. The Eg value of the sample B1VO4-P was 2.57eV. The FITR spectra and the TGA results suggest that moisture was present in sample B1VO4-P but not in samples B1VO4-H and B1VO4-C. XPS analysis shows that the elemental oxidation states of the samples B1VO4-H and B1VO4-C were Bi³⁺, V⁵⁺, and that in the O 1 s spectra, the main peak could be assigned to surface lattice oxygen while the shoulder peak was caused by surface adsorbed oxygen species.

FIG. 8 shows (A) the UV-vis absorption spectra of methylene blue (MB) as a function of time by using an illustrative bismuth vanadate sample B1VO4-C as catalysts and (B) the plots of the MB concentration vs light illumination time with Co being the initial concentration, Ct being the measured value at a specific time and Ct C₀ being expressed in percentage (%) at the y-axis. As shown in the figure, MB was successfully degraded by the sample.

FIG. 9 is a block flow diagram for illustrating an example of an industrial process to produce bismuth vanadate in accordance with various embodiments disclosed herein.

As shown in the diagram, in accordance with various embodiments disclosed herein, a vanadium source raw material, such as carbon black waste, is first processed in a crusher CR-1 to break up the solid components. Next, the crushed carbon black waste is fed into a dryer, for instance a batch type tray dryer RD-1 , to obtain dried carbon black waste. In this example, drying by an indirect method of heating is shown. Atmospheric air is directed to a heat exchanger HE-1 by an axial flow fan BL-1 , and then heated by use of low pressure steam through the heat exchanger HE-1 , before the air, now heated, is passed through or circulated in the batch type tray dryer RD-1 to remove moisture from the crushed carbon black waste. The moist air then exits from the batch type tray dryer RD-1 . Separately, condensate exits from the heat exchanger HE-1 . The dried carbon black waste obtained is stored in storage TK-1 , where it may be immediately or subsequently transported, for instance by a conveyor CV-1 , to reactor RE-1 where leaching of vanadium from the dried carbon black waste may take place. Next, water is fed into reactor RE-1 via pump RG-2, and a leaching agent, for instance 50% NaOH, is fed into reactor RE-1 from storage TK-2 via pump RG-1 . Leaching of vanadium from the carbon black waste is then allowed to take place, where vanadium from carbon black waste is envisaged to preferentially dissolve in the leaching agent, in this case, NaOH, to form, in this case, VO4^3" ions. The leachate containing the vanadate ions then exits from reactor RE-1 via pump RG-3, and is subsequently passed through a filter FP-1 remove the insoluble solids to obtain an alkaline leachate that is substantially free from insoluble solids for further processing. The insoluble solid remnants which form the carbon black waste filter cake is removed.

Following the above, the alkaline leachate containing vanadate ions is stored in storage TK-3, where it may be immediately or subsequently fed, dropwise or otherwise, via pump RG-5 to reactors RE-2A and RE-2B where the reaction with bismuth ions to form bismuth vanadate is to take place. Here, a bismuth salt, such as bismuth nitrate, is fed into reactors RE-2A and RE-2B from a storage TK-5. Water is fed into reactors RE-2A and RE-2B via pump RG-6, and an acid, for instance 36% HCI, is fed into reactors RE-2A and RE-2B from storage TK-4 via pump RG-4 to adjust the pH of the bismuth solution or reacting mixtures in the reactors. It is therefore envisaged that certain conditions, such as an acidic pH, of the bismuth solution or reacting mixtures may be controlled, maintained and/or monitored at this stage. Reaction between the bismuth ions and the vanadate ions in the reacting mixtures to form bismuth vanadate precipitate is allowed to take place, and may be facilitated by stirring.

Subsequently, the reacting mixtures containing bismuth vanadate precipitate are allowed to exit via pump RG-7, and are then passed through a filter FP-2 to recover the precipitated bismuth vanadate from the remnants of the reacting mixture which form the acidic wastewater filtrate. The precipitated bismuth vanadate is then subjected to further heat treatment such as calcination in a heating chamber HE-2 to obtain a calcinated bismuth vanadate product. Various aspects of the industrial process described herein, including the batch leaching reactor (i.e. reactor RE-1 ) and the batch crystallizers (i.e. reactors RE-2A and RE-2B) are considered to mimic the experimental conditions that are in accordance with various embodiments as described herein above and also employed in the examples that follow. The design capacity of the industrial plant described herein was set to recover vanadium from 30 tons/day of wet carbon black waste. Based on the design capacity, an overall production rate of bismuth vanadate at 1460 kg/day is expected to be attained at the end of the process.

FIG. 10 is a general scheme of a method of producing bismuth vanadate from a vanadium source in accordance with various embodiments disclosed herein. As shown in the figure, a vanadium source such as a vanadium based waste may undergo a waste utilization process in accordance with various embodiments disclosed herein to eventually obtain bismuth vanadate.

EXAMPLES

Example embodiments of the disclosure will be better understood and readily apparent to one of ordinary skill in the art from the following examples, tables and if applicable, in conjunction with the figures.

Materials

Carbon black waste was collected from an oil refinery factory in Singapore. The following chemicals were used as received without further purification: bismuth nitrate pentahydrate (>98%, Sigma-Aldrich), sodium hydroxide (99%, VWR Chemicals), hydrochloric acid (VWR Chemical, 32%), and methylene blue (Merck). Deionized water was used for all experiments. Leaching Process

The as-received wet carbon black waste sample was dried in an oven at 105°C for 12 h. Then, 2.5 g of the dry solid was dispersed in 50 imL of aqueous NaOH solution (1 M) in a sonicator (150 W) for 1 h. Thereafter, the leaching solution was collected by a membrane filter (Nylon, pore size: 0.45 m) under vacuum filtration condition. The pH value of the leaching solution was around 13.8. The concentrations of different metals in this leaching solution are shown in Table 1 below. The NMR indicates that the vanadium existed as VO4^3" anions (See FIG. 2), so the chemical form of the vanadium in the leaching solution should have been present as Na3VO4.

Table 1 Elemental concentrations in leaching solutions of carbon black waste³.

^a sample was tested by inductively coupled plasma-atomic emission spectrometry (ICP-OES, Optima 7300DV, Perkin Elmer). Since the vanadium ions in leaching solution were mainly in +5 oxidation state as monomeric anion VO4^3", the cationic ions in the leaching solutions were likely Na⁺, derived from the leaching agent.

Synthesis of bismuth vanadate For the synthesis of B1VO4-C, firstly, 34 mg of bismuth nitrate pentahydrate was mixed with 5 mL of aqueous HCI solution (1 M) under vigorous stirring. The pH of the bismuth solution was around -0.04. Then, 5 mL of the prepared alkaline leaching solution was added dropwise (duration 30 s). The stoichiometry of the precursor solution was Bi:V = 1 :1 . An orange/yellow precipitated powder designated B1VO4-P was formed after the addition of the leaching solution. After stirring at room temperature for 8 h, the solid was collected by centrifugation and washed with water/or ethanol for two times. Finally, the wet solid was dried in an electric oven at 60°C for 6 h, followed by calcination at a box furnace at 600°C for 10 hours with a heating ramping rate of 5°C/min. In the case of B1VO4-H, the synthetic solution after mixing at room temperature for 10 min was subsequently hydrothermally treated at 200°C for 15 hours before the centrifugation and washing treatments. The samples that were synthesized are summarized in the Table 2 below.

Table 2 Summary of samples that were synthesized

Characterization methods

Scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscopy (TEM, JEM-2010) were used to characterize the morphology of the samples. Selected-area electron diffraction (SAED, JEM-21 OOF) and X-ray diffraction (XRD, Bruker D8 Advance) equipped with a Cu Ka radiation source were adopted to identify the crystallographic structures. The optical band gaps of the samples were tested in the solid state at room temperature by diffuse reflectance spectroscopy, which were collected using a Shimadzu 3600 UV-VIS-NIR spectrophotometer equipped with an integrating sphere in the 190-2000 nm range (BaSO4 was used as a white standard). The elemental mapping was carried out by energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments, Model 7426). BET specific surface areas of samples were calculated using N2 physisorption isotherms at 77 K (Quantachrome NOVA-3000 system). The chemical compositions of the samples were analyzed by X- ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical). Thermogravimetric analysis (TGA) studies were carried out on a thermobalance (TGA- 2050, TA Instruments) with flowing air atmosphere (flowing rate: 50 imL/min) at a temperature ramping rate of 10°C/min. Metal concentrations in leaching solutions were measured by inductively coupled plasma-atomic emission spectrometry (ICP- OES, Optima 7300DV, Perkin Elmer). The chemical structures of the samples were also characterized by Fourier transform infrared spectroscopy (FTIR, Bio-Rad). And ⁵¹V nuclear magnetic resonance (NMR) solution spectra were recorded on a Bruker AV500 (500 MHz) spectrometer. Photocatalytic degradation of organic dye

The reactions were carried out in a closed box, where the visible light source was provided by a high-pressure mercury lamp (Philips HPR 125W) equipped with an ultraviolet cutoff filter (λ > 400 nm, Edmund Optics). Firstly, 80 mg of B1VO4 sample was dispersed into 49 mL of water via sonication for 5 min. Then, 1 mL of MB aqueous solution (500 mg/L) was added. The suspensions were magnetically stirred in dark for 30 min to ensure adsorption equilibrium, before the light was turned on. The samples (1 mL) were withdrawn regularly from the reactor. The catalysts were separated from the sample solution (after diluting 4 times by water) by filtration via syringe filters (PES, 0.22 pm) and the MB concentration in the liquid was quantitatively determined by measuring its absorption at 664 nm with a UV-Vis spectrophotometer (Shimadzu UV- 2450). Example 1

This example shows the effects of pH of the precursor solution and the amount of bismuth nitrate on the product quality. It was found that pH value of the bismuth nitrate solution is an important factor. As the leaching solution was added dropwise to bismuth nitrate solution, the bismuth cations would either react with vanadate anions to form precipitate or hydrolyze to form bismuth hydroxide. When the pH of bismuth nitrate solution was 7, only white colour bismuth hydroxide was obtained, as confirmed from the XRD pattern (FIG. 3A). When the pH of the bismuth nitrate solution was decreased to 0 or 0.3 {via adding HCI solution), both conditions resulted in yellow precipitates. As shown in FIG 3B, the yellow colour solution obtained was not crystalline B1VO4, and the precipitate was labelled as precursor of B1VO4 (B1VO4-P) since it showed four intensive XRD diffraction peaks at 12°, 26°, 32.7°, and 33.6°, respectively. The formation of the yellow colour precipitate was very fast, with precipitation observed after 1 hour of the mixing of the leaching solution and the bismuth nitrate solution. Regarding the vanadium recovery efficiency, there was still 77.7% of vanadium ions existing in the supernatant liquid when the initial pH of bismuth nitrate was 0.3. When the pH of bismuth nitrate solution was further reduced to 0, a high vanadium precipitation of 99.3% was achieved. It should be noted that since the carbon black waste leaching solution was prepared in alkaline condition (pH value around 13.8), the final pH of the suspension was about 2.0 when controlling the pH of bismuth nitrate solution as 0. The amount of bismuth nitrate is another factor affecting the precipitation process. When the amount of bismuth nitrate was 34 mg, the mole ratio of bismuth and vanadium was 1 :1 . Interestingly, higher amount of bismuth nitrate did not result in higher vanadium recovery. The vanadium recovery efficiencies were 48.9%, 99.3% and 28.3%, respectively, as the amount of bismuth nitrate was increased from 17 mg to 34 mg and 68 mg. At low bismuth nitrate amount (17 mg), the XRD pattern of the product seemed to be amorphous without any intensive peaks (FIG. 3B). Meanwhile, there was no distinct difference in the product XRD patterns when the bismuth nitrate amount was adjusted from 34 mg to 68 mg. Example 2

In addition, the intermediate solid (B1VO4-P) was not stable, and the crystal structure collapsed after storing for 48 hours as evident from the XRD characterization (FIG. 4), indicating that B1VO4-P was just an intermediate to B1VO4 product. To convert the bismuth vanadate precursor (B1VO4-P) to the crystal phase of B1VO4, two different post-treatment processes were developed to induce the phase formation, namely, hydrothermal reaction at 200°C for 15 hours and high temperature calcination at 600°C for 10 h. The resulting BiVO4 were labelled as B1VO4-H and B1VO4-C, respectively. As depicted in FIG. 5, both XRD diffraction patterns of B1VO4-C and B1VO4-H matched well with the standard monoclinic B1VO4 with scheelite structures (JCPDS PDF#14- 0688). In the case of B1VO4-C, no peaks for impurities were detected. However, it should be noted that several small peaks located around 2Θ of 24.4°, 25.9° and 27.4° were found in B1VO4-H sample, due to the presence of a very small amount of other impurity phases. In addition, calcination temperature was found to affect the formation of monoclinic crystals. Example 3

This example shows the morphology of the bismuth vanadate. The panoramic morphology and microstructure of the as-prepare B1VO4 products and precursor were revealed by SEM images, as illustrated in FIG. 6A-F. As shown in FIG. 6A, B1VO4-P comprised spherical nanoparticles with an average size of 23 nm. However, both B1VO4-H and B1VO4-C comprised spherical microparticles with a much larger size. B1VO4-H has a broad size distribution of 2 to 7 μιη. In comparison, B1VO4-C has a more uniform size of ca. 6.5 μιη. A closer observation indicates that these microparticles were composed of many smaller plate-like particles serving as building blocks which were gradually aggregated into microparticles. TEM image of an individual plate-like particle on B1VO4-H is displayed in FIG. 6G, and the EDX elemental maps in FIG. 6H further confirmed that the nanoplate contained homogenously distributed elements of Bi, V, and O. Moreover, the SAED pattern (inset in FIG. 6G) of the B1VO4 with plate-like form suggests the single crystal nature of the solid. In addition, the elemental composition of the obtained B1VO4 is indicated in the EDX spectra (FIG. 61). As shown, in both samples, only bismuth, vanadium, and oxygen signals were observed, and the molar ratio of Bi:V was around 1 :1 , consistent with their formula of B1VO4. The result also indicates that the sodium, chloride and nitrate ions were totally removed via washing treatments. The surface area of the B1VO4 samples were determined by N2 physisorptions. The specific BET surface area of B1VO4-H and B1VO4-C were 4.4 m²/g and 1 0.5 m²/g, respectively, both of which are slightly higher than the reported samples. Example 4

This example is intended to show other characterization results of the prepared bismuth vanadate. The optical band gaps of B1VO4 were tested by diffuse reflectance spectroscopy. The diffuse reflectance spectra were translated into the absorption spectra by the Kubelka-Munk method (a = (1 - R)²/2R, R is the diffuse reactance). Then, the optical band gaps E_g was evaluated from the following theoretical equations x E = K x (E - E_g)ⁿ, where E is the photon energy (E = hv, where h is the Planck constant, and□□ is the photon's frequency), K is a constant, and n is a constant (n = 1 /2 for directly allowed transitions and n = 2 for indirectly allowed transitions). Since B1VO4 was found to be a direct band gap semiconductor, E_g was obtained from the plots of the (ahv)² versus photon energy (ahv = 0, calculated by extrapolating the straight line shown in FIG. 7A). As shown, the E_g values for B1VO4-C and B1VO4-H were 2.35 eV and 2.39 eV, respectively, which were very close to the reported data (~2.4 eV). The E_g value for B1VO4-P was 2.57 eV, which exhibited a yellow colour due to absorption of light above 2.6 eV.

FTIR spectra of B1VO4 and the precursor (viz., B1VO4-P) are shown in FIG. 7B. As shown, the two bands at 3436 cm^-1 and 1624 cm^-1 are associated to the lattice water, which was only present in the B1VO4-P but not in both the B1VO4 samples. The bands around 700 cm^-1 are due to the different stretching modes of V-O-V. As revealed from the TGA results in FIG. 7C, there was no weight loss for B1VO4 samples in the temperature range of 30 to 1000°C. But, it was found that the weight loss was 9% in B1VO4-P sample due to the presence of moisture and weight loss during phase transformation. The elemental oxidation states of the as-prepared products were analyzed by XPS. FIG. 7D-F give the XPS spectra of the two B1VO4 samples in Bi 4f, V 2p, and O 1 s, respectively. The binding energies of Bi 4fz/2 and 4fs/2 were found to be 158.5 eV and 163.8 eV, respectively, in accordance with Bi³⁺. The 2p32 and 2pi/2 levels of vanadium were 516.1 eV and 523.9 eV, corresponding to V⁵⁺. And in the O 1 s spectra, the main peak at 529.2 eV could be assigned to the surface lattice oxygen, and the shoulder peak at 531 .6 eV was caused by the surface adsorbed oxygen species. But, there is no appreciable difference in peaks positions for the two different bismuth vanadate products.

Example 5

This example demonstrates the photocatalytic ability of the prepared bismuth vanadate. As mentioned above, bismuth vanadate is an n-type semiconductor with bandgap values of 2.35 eV and 2.39 eV for B1VO4-C and B1VO4-H, respectively. Therefore, the produced B1VO4 would be an active photocatalyst under visible-light irradiation (λ > 400 nm). The visible light absorption observed in monoclinic B1VO4 may be attributed to the transition from Bi6s (or a hybrid orbital of Bi6s and O2_P) to V3d. Decolorization of organic dye effluents is an effective way to evaluate the performance of photocatalysts. Herein, the photocatalytic performances of the two prepared B1VO4 were tested on the photodegradation of organic dyes {e.g., MB, 10 ppm). As shown in FIG. 8A, the characteristic absorption peak of MB at about 664 nm was chosen for the photo-degradation analysis, which gradually decreased with increasing illumination time. Prior to illumination, the suspensions were vigorously stirred in dark for 30 min to eliminate the adsorption effect. As shown in FIG. 8B, both B1VO4 samples exhibited good performance for the degradation of dye. For instance, the concentrations of MB were decreased to 3.8 ppm and 4.1 ppm for using B1VO4-C and B1VO4-H as catalysts, respectively, after irradiation for 7 h. As a control, no appreciable decrease in MB concentration was found when no bismuth vanadate was added.

Example 6 This example gives a demonstration on the plant scheme to produce bismuth vanadate. Based on the conditions used in the experiment, a demonstration plant scheme was designed to evaluate the feasibility of the recovery process on a large scale. Both batch leaching reactor and batch crystallizers were considered to mimic the experimental conditions employed. The design capacity of the plant was set to recover vanadium from 30 tons/day of wet carbon black waste. Based on the design capacity, an overall production rate of bismuth vanadate at 1460 kg/day was attainable at the end of the process. A basic block flow diagram of the designed recovery scheme for carbon black waste is shown in FIG. 9.

APPLICATIONS

Embodiments of the present disclosure are capable of removing toxic vanadium from carbon black waste through the formation of bismuth vanadate which has various useful applications.

Various embodiments of the present disclosure provide bismuth vanadate that are non-toxic, chemically stable and useful in diverse applications such as eco-friendly yellow pigment (as an alternative to hazardous yellow formulations containing lead, chromium or cadmium pigment), ferroelastic material, photoelectrode, semiconductor, photocatalyst, etc.

Particularly, embodiments of the presently disclosed bismuth vanadate are envisaged to meet the expanding demand for bismuth vanadate pigments. Embodiments of the presently disclosed bismuth vanadate are also envisaged to serve as important visible-light-driven photocatalysts.

Claims

1 . A method of producing bismuth vanadate, the method comprising:

precipitating bismuth vanadate from the mixture.

2. The method of claim 1 , wherein the mixture has a pH of no more than 2.0.

3. The method of claim 1 , wherein said adding the leaching solution to the bismuth salt solution comprises adding the leaching solution dropwise into the bismuth salt solution.

4. The method of claim 1 , wherein the bismuth salt solution is selected from the group consisting of bismuth nitrate, bismuth acetate, bismuth halide and combinations thereof.

5. The method of claim 4, wherein the bismuth halide is selected from the group consisting of bismuth fluoride, bismuth chloride, bismuth bromide, bismuth iodide and combinations thereof.

6. The method of claim 1 , wherein the pH of the bismuth salt solution is no more than 0.5.

7. The method of claim 6, wherein the pH of the bismuth salt solution is no more than 0.

8. The method of claim 1 , wherein the mole ratio of bismuth to vanadium in the mixture is from 0.8:1 .0 to 1 .2:1 .0.

9. The method of claim 1 , further comprising adding at least one of an acid or a base to the vanadium source to form the leaching solution containing vanadate ions.

10. The method of claim 1 , further comprising, prior to the step of preparing the mixture:

(iii) optionally repeating step (ii) for one or more times to obtain a third leachate or subsequent leachate; wherein the leachate obtained after completion of the above steps forms the leaching solution containing vanadate ions for use in the step of preparing the mixture.

1 1 . The method of claim 1 , wherein the leaching solution has a pH greater than 7.

12. The method of claim 1 , further comprising subjecting the precipitated bismuth vanadate to heat treatment at a temperature greater than 120°C.

13. The method of claim 12, wherein the heat treatment is selected from the group consisting of: hydrothermal treatment, calcination and combinations thereof.

14. The method of claim 9, wherein the addition of at least one of an acid or a base to the vanadium source selectively leaches vanadium as compared to other metals from the vanadium source to form the leaching solution containing vanadate ions, wherein said other metals are one or more metals selected from the group consisting of nickel, iron, aluminium, zinc, chromium, cadmium, cobalt and copper.

15. The method of claim 14, wherein the bismuth vanadate is substantially free of impurities.

16. The method of claim 1 , wherein the bismuth vanadate comprises monoclinic bismuth vanadate.

17. The method of claim 1 , wherein the vanadium source is an oil refinery waste.

18. The method of claim 1 , wherein the vanadium source is a carbon black waste.

19. Bismuth vanadate derived from a mixture of a leaching solution containing vanadate ions and a bismuth salt solution, wherein the mixture has a pH of no more than 3.0, wherein the bismuth vanadate has a surface area of at least 4 m²/g.

20. Bismuth vanadate of claim 19, wherein the bismuth vanadate comprises monoclinic bismuth vanadate.

21 . Bismuth vanadate of claim 19, wherein the bismuth vanadate has been heat pre-treated at a temperature greater than 120°C.

22. The bismuth vanadate of claim 19, wherein the bismuth vanadate has one or more of the following properties selected from the group consisting of:

a) an average particle size of at least 1 pm;

b) an X-ray diffraction pattern corresponding to that of FIG. 5A or FIG. 5B ; and

c) an optical band gap of between 2.30 eV and 2.45 eV.

23. A method of catalysing a reaction, the method comprising:

adding the bismuth vanadate of claim 19 to the reactants, and irradiating the bismuth vanadate with visible light to catalyse the reaction.