EP3393652A1

EP3393652A1 - A method for the synthesis of a bivalent tin oxy-hydroxide adsorbent for the removal of hexavalent chromium from water, particularly drinking water, the adsorbent and its use

Info

Publication number: EP3393652A1
Application number: EP16834237.6A
Authority: EP
Inventors: Manassis Mitrakas; Efthymia KAPRARA; Konstantinos Symeonidis
Original assignee: Aristotle University of Thessaloniki
Current assignee: Aristotle University of Thessaloniki
Priority date: 2015-12-21
Filing date: 2016-12-21
Publication date: 2018-10-31
Also published as: GR1008962B; WO2017109521A1

Abstract

The present invention refers to a method for the synthesis of an adsorbent which is realized by the production of a bivalent tin oxy-hydroxide [Sn_xO_y(OH)_z], with 1<x<6, 0<y<4 and 0<z<4, into a two-stage continuous flow reactor and a pH range 2-12. The material can be applied for the removal of hexavalent chromium, bromate, chlorate and perchlorate ions in drinking water treatment units for house, municipal and industrial use. The invention also relates to its adsorbent, resp. the use thereof.

Description

A METHOD FOR THE SYNTHESIS OF A BIVALENT TIN OXY-HYDROXIDE ADSORBENT FOR THE REMOVAL OF HEXAVALENT CHROMIUM FROM WATER, PARTICULARLY

DRINKING WATER, THE ADSORBENT AND ITS USE

Field of the invention

The invention belongs to the field of chemical engineering and specifically to the water purification technology using solid adsorbents. Solid adsorbents are applied by the current state of the art as filling media in column beds for the removal from water and wastewater of oxidative ions like bromates, chlorates, perchlorates, as well as chromates (hexavalent chromium) from drinking water or wastewater by adsorption.

Background of the invention

Chromium exists in nature in the rocks/subsoil, as well as in the ground water sources. Despite the variety of its possible oxidation states, chromium is usually found in the environment as trivalent [Cr(lll)] and hexavalent [Cr(VI)]. Among these forms, Cr(lll) is considered as an essential trace element for human nutrition, while Cr(VI) is recognized as a toxic and carcinogenic agent.

The presence of chromium in water may be attributed to anthropogenic (tannery, plating, cooling towers, etc.) or natural origin with the latter explained by its release during water contact with alluvial precipitates formed by the erosion and weathering of ultramafic rocks. Since Cr(lll) solubility in the common pH range of drinking water (6.5-8.5) is estimated lower than 5 μg L, higher concentrations of dissolved chromium in natural water should be related to the presence of the more soluble Cr(VI) form.

Due to its high toxicity and the confirmation of its natural formation in groundwater, Cr(VI) is considered as a priority pollutant by the World Health Organization. The maximum contaminant level (MCL) of 100 μg/L for total chromium has been set by U.S. Environmental Protection Agency, while the corresponding value for E.U. is 50 \ig/L. Such legislation for total chromium instead of its specification for Cr(VI) arises by the initial consideration that the presence of Cr(VI) in the environment is solely caused by anthropogenic activities. Nevertheless, recent studies indicate natural procedures as the main source of water pollution by chromium and that the Cr(VI) is the dominant form (N. Kazakis et al., Sci. Total Environ. 514 (2015) 224, E. Kaprara et al., J. Hazard. Mater, 281 (2015) 2, S. Saputro et al., Chem. Geol. 364 (2014) 33). Under this prism, there is an increasing pressure to the authorities in world level to establish a new drinking water MCL specified for Cr(VI). A first step in this direction came from the California State which, by July 2014, applied the new MCL of 10 pg/L for Cr(VI) in drinking water (Memorandum, California Department of Public Health, State Adoption of a Hexavalent Chromium MLC, 2014). This is not only expected to affect the regulation worldwide but also force the re-evaluation of existing processes for the removal of Cr(VI) from drinking water.

In the last years, various processes have been studied for the removal of Cr(VI) from wastewaters and aqueous solutions, including chemical precipitation, adsorption, ion exchange, reverse-osmosis or nanofiltration membranes, electrocoagulation, phytoremediation, floatation and extraction. However, there is a number of requirements that should be fulfilled by each method before its application in drinking water treatment including

achievement of low residual Cr(VI) concentrations, if possible even lower than 1 pg Cr(VI)/L;

non alteration of physical and chemical characteristics of water;

^ absence of chemical reagents addition that could cause deterioration of water's quality;

short process time;

possibility for application at large-scale under continuous flow operation;

very low fixed and operational cost.

According to such an evaluation, adsorption is the optimum method for Cr(VI) removal since it does not only comply with these criteria but it also provides flexibility in the design and operation of the process. Recently, a large variety of adsorbents, like biological material, inorganic oxides, activated carbon and others are attracting the interest of researchers. Activated carbons are the most widely used adsorbents for water and wastewater treatment. Particularly, they are promoted due to their high specific surface area and their wide availability in market. In most of relevant studies, the main mechanism of Cr(VI) removal is considered to be the surface reduction followed by the adsorption as Cr(lll) (D. Mohan, C.U. Pittman Jr., J. Hazard. Mat, B137 (2006) 762). However, their efficiency is relatively limited and drastically decreased in the pH range 6-8 (L Khezamietal., J. Hazard. Mater., 123 (2005) 223), whereas there is no evidence for any possibility to obtain a residual Cr(VI) concentration below the expected regulation limit of 10 pg/L. Another category of adsorbents studied for Cr(VI) removal are the low-cost materials received as byproducts of agricultural and industrial processes or recycling, like those described in documents CN104190372(A), CN10394921 1 (A) and CN 103910437(A). Among them, fruit shells, leaves, roughage and spent car tires have a zero cost but they are lacking of acceptable efficiency, while the significant downgrade of water quality is another important disadvantage for their implementation in drinking water treatment. Similarly, drawbacks also stand for bio-adsorbents (bacteria, algae, fungi), which additionally demand large contact times to maximize their activity. Overall, the application of bio-adsorbents cause deterioration in the quality of drinking water as a result of its enrichment by organic compounds and microorganisms.

Some of the adsorbents consisting of inorganic phases, which have already been successfully tested in water treatment for the removal of other pollutants, were also tested for the uptake of Cr(VI). As described in the publications of M. Lehmannetal., Chemosphere, 39 (1999) 881, E.A. Deliyanni et al., Environ. Sci. Pollut. Res. Int., 11 (2004) 18, N.K. Lazaridis et al, Chemosphere, 58 (2005) 65 and H. Tel et al., J. Hazard. Mater, 112 (2004) 225, activated alumina, bauxite, iron oxides (y-Fe₂03) and oxy-hydroxides (FeOOH), ΤΊΟ2, zeolites, lignite and industrial byproducts (flying ash, rust) are some of them. Although such materials are operating well in wastewaters with high Cr(VI) concentrations, they fail to achieve a sufficient efficiency at lower concentrations due to the absence of any reducing agent and therefore, cannot decrease residual concentration at levels below 10 pg/L In addition, Cr(VI) is weakly physisorbed by the formation of outer-sphere complexes initiating an environmental issue concerning the safe disposal of adsorbents after their use.

Metals and metal oxides that can act as electron donors appear advantageous due to their ability to reduce Cr(VI) to insoluble Cr(lll) hydroxides, which are then retained onto their surface. In that way operate zero-valent iron, magnetite (FeaC ), copper, zinc, magnesium and alloys (e.g. zinc-copper). However, in this case there are also issues related to the surface passivation during treatment (e.g. Fe°), water quality modification due to metal ions dissolution (e.g. Fe²⁺, Cu²⁺, Zn²⁺, Mg²⁺) at levels above MCLs, as well as to the efficiency decrease at pH > 7 (e.g. Fe²⁺, Cu²⁺, Fe₃0₄).

Prior Art

Concerning document XP055308620 of F. Pinakidou et al., about "Metal (hydr)oxides for removal of Cr(VI) from drinking water: a XAFS study", XXXI Panhellenic Conference on Solid State Physics and Materials Science, Book of Abstracts, 20-23 September 2015, 20 September 2015 (20-09-20), pages 32-33, it discloses information which is not sufficiently clear to reveal the required development aimed at herewith, since it does not provide details of the environmental conditions and the pH values of synthesis reaction. This document is focused on the mechanisms of octahedral Cr(lll) ions uptake on the Sn02, which result after the reaction of Cr0₄ ²- with Sn60₄(OH)₄ (chemical reaction 1 ), rather than the technological part of the current invention, which is the uptake efficiency of hexavalent chromium and how this is defined by the synthesis conditions.

Sn₆0₄(0H)₄ + 4CrO ^" + 8H₂0→ 6Sn0₂ + 4Cr(OH)₃ + 80H^~ [1]

It is further clarified that the tin precursor reagent e.g. SnS0₄ / SnC / Sn(N03)2, as well as the concentration and the kind of alkaline reagent do not suggest any active contribute to the aimed invention in that it lies on the reaction conditions of the Sn²⁺ precursors with the alkaline reagents for the synthesis of Sn60₄(OH) , so as to achieve a maximum positive surface charge density and therefore, maximize the removal efficiency for hexavalent chromium and keep a very low fixed and operational water treatment cost. Importantly, the treatment process of the produced dispersion, i.e. its thickening in a tank for 24 h under mild stirring, the mechanical dewatering, the drying and extrusion -in granules or powder- of the solid is a typical procedure applied for the recovery of dispersion and therefore, does not reveal the required development aimed at.

Concerning document XP028697741 of Liang Ying et al. entitled "Synthesis and characterization of SnO with controlled flowerlike microstructures" Materials Letters, 2013, 108, 235-238, and according to the description of the experimental part in this document, a synthesis is performed in an alkaline environment, so as to form tin oxides. Although the authors do not provide data for the pH value control during the addition of SnCl2-2H20, the reaction of SnO formation seems to take place in a strongly alkaline environment -excess of NaOH-, since 0.005 mol of acidic salt (SnCl2-2H20) are not adequate to neutralize 0.01 mol (double quantity) of a strong base (NaOH). In other words, SnCl2-2H20 is hydrolyzed by 1 N NaOH (0.4 g are dissolved in 10 mL of distilled water), that is, at a strongly alkaline environment. However, it is observed in the present introduction that synthesis under high pH values leads to the formation of oxides. In order to avoid oxidation of Sn²⁺ and favor bivalent tin oxide (SnO), the reaction takes place under heating, which diminishes the concentration of dissolved oxygen and in this way limits oxidation to Sn02. The formation of Sn604(OH)₄ as an intermediate step before SnO production is something that is merely assumed in said document but not proved therein though , wherein it is stated that «With the increasing of SnCh-2H20 amount, the solution turned to ivory-white suspension, and then to light yellow suspension indicating the formation of Sne04(OH)4.

Conclusively, said document discloses SnO synthesis in conditions of batch mode, high temperatures and alkaline environment, and for this reason the produced material is not expected to show any efficiency in the removal of hexavalent chromium.

Concerning document XP055309210 of «TJ Xiao, Q L Wu, P Liu, Y Liang, H B Li, M M Wu and G W Yang» for «Highly stable sub-5 nm Sn60₄(OH)₄ nanocrystals with ultrahigh activity as advanced photocatalytic materials for photodegradation of methyl orange", Nanotechnology 25 (2014) 135702: doi: 10.1088/0957- 4484/25/13/135702. In this document, metal tin is used as a precursor reagent, which is oxidized by a laser beam in a distilled water environment in order to prepare tin oxy-hydroxides. The correlation of this document with the present development is based on the final product Sn60₄(OH)4 which is only identical in its crystal structure, but it is prepared in a completely different way. Therefore, it is not featured by the novel characteristics of the material presented in the present development. In particular, the layer ablation liquid phase method (LAL) is a production method of high energy demands, taking place in batch mode, in a quartz cell where the laser beam is directed to and it requires tin of high purity (99.99 %). It is a quite good method to produce materials of high quality and accurate morphology, but only for quantities of few milligrams up to some grams, which are considered very small for environmental applications with demands of thousands of tons.

In conclusion, this document discloses a batch mode synthesis method for bivalent tin oxy-hydroxides, with low production rate and high energy demands, implying a huge production cost and a non-acceptable water treatment cost. Furthermore, it is obvious that the synthesis conditions cannot cover in a large-scale the requirements of the use mentioned in the latter document (photocatalytic oxidation of methyl red) independently from the treatment cost. Conclusively, the synthesis conditions are not focused in the optimization of the surface charge so as to achieve an at least acceptable removal efficiency for hexavalent chromium.

Concerning document EP 2 578 535 A2 of DOW GLOBAL TECHNOLOGIES LLC 10/04/2013, it describes the synthesis of SnO (Table 1 , page 6) aiming at a use in electronic devices. Its synthesis takes place in a weakly alkaline (pH 8.5-12) -but not acidic- environment, aiming to hydrolysis under heating (> 70 °C, § 0016) of the intermediate tin oxy-hydroxides Sn302(OH)2 (§ 0015) to SnO. As explained in the introduction, oxidation of Sn²⁺ to Sn⁴⁺ in alkaline environment is very rapid (AE°cei = +1.3 V). Under such conditions, to avoid the transformation of tin to Sn02 the process is performed in an environment free of oxygen. The SnO is oxidized rapidly to Sn02 and for this reason its drying takes place at inert conditions without oxygen (§ 0018). In addition, SnO, as all metal oxides, has a low surface charge especially when synthesized at alkaline conditions. Therefore, it is not expected to show any significant removal efficiency for hexavalent chromium, as presented for the case of Sn02 in Figure 8.

Conclusively, this document describes the synthesis of a different compound (SnO) under different conditions from those to be developed in the present application. Aim of the invention

In summary, it is aimed at in the present invention to the synthesis of a novel adsorbent for hexavalent chromium, consisting of a positively-charged bivalent tin oxy-hydroxide with a structure Sn604(OH)4, under conditions which introduce a number of novel characteristics compared to other similar materials disclosed in the latter cited documents of the current state of the art. In particular, synthesis in continuous-flow operation and environmental temperature implies the possibility for illimitable quantities with low production cost. In combination with the high positive charge density, leading to high removal efficiency for hexavalent chromium, and the simplicity of use in water treatment facilities, an extremely low fixed and operational cost is ensured when compared to other adsorbents of the aforementioned current state of the art.

The analysis of the main differences between the latter earlier documents, on the one hand, and the aimed invention on the other hand, reveals distinctive features of the subject method and the involvement of a non-obvious development respective the current state of the art, and especially by the specific documents.

Summary of the invention

In the present invention, there is proposed a synthesis method for an adsorbent consisting of bivalent tin oxy-hydroxide of the type Sn_xO_y(OH)z, with 1<x<6, 0<y<4 and 0<z<4, taking place in the pH range 2-12 and followed by a thickening and drying process. The aim of this invention is the preparation of an adsorbent which combines reducing and adsorbing properties achieving a high Cr(VI) removal efficiency from water. Such properties are accomplished by the hydrolysis of Sn(ll) salts in a two-stage continuous flow reactor where pH is adjusted to the targeted point by the addition of an alkaline solution.

The present invention faces the problem of Cr(VI) removal from water in the following way, thereby providing a suitable solution to it.

The adsorbent successively reduces Cr(VI) to Cr(lll), since it shows reducing potential without passivation problems, and then captures Cr(lll) in the structural unit of the oxide/oxy-hydroxide. More specifically, during contact with polluted water, Cr(VI) is initially adsorbed on the surface of Sn60₄(OH)₄ and then, the reduction of Cr(VI) takes place following reaction [2]:

Sn₆0₄(OH)₄ +4Cr04²- + 8H2O→ 6Sn0₂ + 4Cr(OH)₃ + 80H^" [2] A strong uptake of Cr(lll) by the resulted Sn02 is achieved by the formation of inner- sphere complexes (Figure 1), as X-ray absorption fine structure spectroscopy studies indicate. The strong adsorption of Cr(lll) together with the extremely low solubility of tin, explain the high efficiency of Cr(VI) removal succeeding residual concentrations below 1 pg/L and the insignificant leaching of tin, the concentration of which was always around the detection limit of graphite-furnace atomic absorption spectroscopy (10-20 pg/L). Therefore, the non-reversible capture of Cr(lll) ensures an environmentally friendly behavior of the adsorbent after disposal. It should be mentioned that until now, there is no regulation limit for the concentration of tin in water. As described in the Background document for development of WHO Guidelines for Drinking-water Quality "Inorganic Tin in Drinking-water", WHO/SDE/WSH/03.04/115, tin toxicity is so low that a suggestive drinking water limit would be 3 orders of magnitude higher than the average presence of tin in water (1-2 pg/L). This is actually the reason for the application of the 50 % of tin's world production in plating for food packaging and preparation facilities (JECFA, 1989).

The invention provides a method for the synthesis of bivalent tin oxy-hydroxide with chemical formula Sn60₄(OH)₄ under acidic environment (pH<4) in a continuous-flow reactor. The main feature of the method is the stabilization of the crystal structure Sn60₄(OH)₄ against oxidation or dehydroxylation combined with the development of a dense positive charge on its surface. According to this, the synthesis of positively- charged Sn60₄(OH)₄, as well as the conditions defining the production of this material, differentiates the method according to the invention from any other method producing the same phase, but with much different physical properties and technological features. The combination in the specific phase of the highly reducing Sn²⁺ with the high density of positive surface charge, makes this material ideal for hexavalent chromium anions (Cr0₄ ^2") removal from water through its reduction to the trivalent form, which is chemisorbed on the material's surface. To prove this, there is provided the variation of positive surface charge density of Sn60₄(OH)₄ synthesized by the chemical precipitation in various pH values as shown in Figure 7, which represents an effect of synthesis conditions (pH) of Sn604(OH)4 in the positive surface charge density.

It can be concluded that high positive surface charge density is favoured at low pH values, a fact which is not straight obvious in se, while this property is proved to be critical for the removal efficiency against hexavalent chromium anions, as derived by the breakthrough curves from an adsorption column for the materials synthesized at various pH values as shown in Figure 8 showing breakthrough curves for Cr(VI) in rapid small scale tests for Sn60₄(OH)4/Sn02 samples synthesized under various pH values and a corresponding Sn0₂ sample. Initial concentration 100 Cr(VI)/L, water pH 7.1+0.1 , empty bed contact time 4 min.

Furthermore, the method according to the invention has an additional important advantage: the high stability of the structure Sn60₄(OH)₄ against oxidation effects or transformation to oxides (Sn0₂) with very limited removal efficiency for hexavalent chromium. The low efficiency of Sn02 is observed in the corresponding breakthrough curve of Figure 8. The specific advantage is also attributed to the acidic conditions of synthesis reaction as analysed below. The oxidation of bivalent tin is a spontaneous process due to the presence of dissolved oxygen in water. Under acidic environment, the oxidation reaction of Sn²⁺ with the simultaneous reduction of oxygen is described by the following electrochemical equations:

2x Sn²⁺→Sn ⁺ + 2e -E° = -0.15 V

1 x 0₂ + 2H₂0 + 4e→ 40H^~ E° = +0.401 V

2Sn²⁺ + 0₂ + 2H₂0→ 2Sn⁴⁺ + 40H^" AE°ceii = +0.247 V

Similarly, under alkaline environment the oxidation of hydroxylated form of bivalent tin, Sn(OH)3^", proceeds as follows:

2x Sn(OH)₃ ^" + 30H-→ Sn(OH)₆ ^2~ + 2e -E° = -0.900 V

1 x 0₂ + 2H₂0 + 4e→ 40H^" E° = +0.401 V

2Sn(OH)₃ ^~ + 0₂ + 2H₂0 + 20H^~→ 2Sn(OH)₆ ^2~ ΔΕ°α>ιι = +1.301 V Therefore, by the cell potential AE°_Ceii, it comes out that the oxidation rate of Sn²⁺in alkaline environment is almost five times higher than the one in acidic conditions. This fact implies that in the method according to the invention, the precipitation reaction of Sn²⁺ for the formation of Sn604(OH)₄ is favoured at low pH values, where the tendency of Sn²⁺ oxidation, which would cause the formation of Sn⁴⁺ phases, is restricted. As an example, the transformation of Sn(OH)6^2" initially formed at high pH values to the thermodynamically stable phase Sn0₂ (chemical reaction 2) is mentioned

Sn(OH)₆ ^2~→ Sn0₂ + 2H₂0 + 20H^~ [2]

The effect of synthesis conditions variation in the stability of the final product and the surface charge density are clarified by the thermodynamic equilibrium phase diagram of tin in water represented in Figure 9 as a function of the pH value and the redox potential.

In particular, the selection of pH values in the range 2-4 and the proportional adjustment of redox potential in the range from 0 to -0.3 V, favours the formation of thermodynamically stable Sn²⁺ hydroxides and the preservation of positive charge excess in the surface of the formed solid phase. On the opposite, at higher pH values, the formation of Sn02 is favoured in any case, which is equivalent to (i) the oxidation of Sn²⁺ to Sn⁴⁺ with loss of the reducing potential and (ii) the transformation of oxy-hydroxide structure to an oxide after dehydroxylation.

The remarkable distinctive contribution of the method according to the invention is based on the fact that the synthesis reaction of the material takes place under a constant pH value between 2 and 4 and the control of redox potential at a constant value in the range from 0 to 0.3 V, respectively, following a continuous-flow two- stage process. In this way, the already known Sn60₄(OH)₄ phase is formed, but with characteristics that are completely different from the material prepared by the known chemical precipitation methods of the current state of the art. The bivalent tin oxy- hydroxide Sn60₄(OH)₄ of the method according to the invention should be considered as a distinctive chemical compound, although it is synthesized by a modification of a common reaction, due to the high chemical stability and the excess of positive surface charge preserved. Actually, these are the two main characteristics desired by a solid adsorbent that efficiently captures anions of CrC ^2- , which is the common form of hexavalent chromium in natural water. Proportionally, the reaction leading to the production of the specific material and the conditions under which it takes place should also be considered as distinguishing over the cited prior art. In addition, the method has important differences, which are not obvious with regard to other methods for the synthesis of Sn60₄(OH)4.

Brief description of the drawings

The present invention can be fully understood by the following detailed description of the synthesis method, the corresponding figures and the application examples. Figure 1 is a diagrammatic representation of the chemical reaction involved in the method according to the invention.

Figure 2 shows a simplified flow diagram of the synthesis procedure for the adsorbent according to the method of the invention.

Figure 3 shows an X-ray diffraction diagram of the material prepared according to the procedure given in Example 1 of method's application, verifying the existence of one bivalent tin oxy-hydroxide phase with the structure Sn604(OH)4.

Figure 4 similarly shows an X-ray diffraction diagram of the material prepared according to the procedure given in Example 2 of the method's application, verifying the existence of one bivalent tin oxide phase with the structure SnO.

Figure 5 further shows an X-ray diffraction diagram of the material prepared according to the procedure given in Example 4 of the method's application, verifying the existence of a major bivalent tin oxy-hydroxide phase with the structure Sn604(OH)4 and a small contribution by a bivalent tin oxide phase with the structure SnO.

Figure 6 shows breakthrough curves of laboratory rapid small scale column tests for Cr(VI) adsorption test from Sn604(OH)4 adsorbent synthesized at pH 9, which illustrates the low influence of adsorption pH values in the range 7 - 8 commonly encountered in drinking water. Initial concentration 100 \sg Cr(VI)/L, empty bed contact time 2 min, grains sized 0.25-0.5 mm and temperature 20±1°C.

Figure 7 is a representation of a curve showing the effect of synthesis conditions (pH) of Sn60₄(OH)4 in the positive surface charge density. Figure 8 shows a set of breakthrough curves for Cr(VI) in rapid small scale tests for Sn604(OH)4/Sn02 samples synthesized under various pH values and a corresponding Sn02 sample. Initial concentration 100 g Cr(VI)/L, water pH 7.1 ±0.1 , empty bed contact time 4 min, grains sized 0.25-0.5 mm and temperature 20±1°C. Figure 9 shows a thermodynamic equilibrium phase diagram of tin in water as a function of the pH value and the redox potential.

Description More in detail, a synthesis method of the adsorbent [Sn_xOy(OH)_z], with 1 <x<6, 0<y<4 and 0≤z<4, is performed as follows: a continuous flow and stirred reactor is used consisting of two stages (1) and (2) in each of which the retention time is at least 30 min as shown in Figure 2. An aqueous solution of SnSCU or SnC or Sn(N03)2 with concentration 1-100 g/L is continuously fed in the reactor 1 with a flowrate Q. The hydrolysis/precipitation reaction of bivalent tin takes place mainly in reactor 1 under intense stirring. Reactor 1 is connected to reactor 2 where the reaction is finalized under mild stirring conditions. The quantity of the product is defined by the flowrate and concentration of Sn(ll) salt solution. In both reactor 1 and reactor 2, pH value is adjusted throughout the reaction duration at a constant point in the range 2-12 the addition of one or a combination of more than one of alkaline reagents NaOH, NaHCC-3, Na2C03, KOH, KHCO3, K2CO3, Ca(OH)₂. The outflow from reactor 2 is collected in a thickening tank 3 under mild stirring for a period of 1-48 h to stabilize the nano-crystalline geometry of the material and a size in the range of 20-50 nm, e.g. nanocrystal size is around 45 nm for the material prepared according to Example 1 of method's application and around 20 nm for the material prepared according to Example 2 of method's application. The precipitate after thickening is mechanically dewatered 4, extruded in grains 5 sized 100-2000 pirn 5 and dried 6 as represented in Figure 2. Following the described procedure, a bivalent tin oxy-hydroxide of the type SnxOy(OH)z, with 1<x<6, 0<y<4 and 0<z<4, may be formed in the pH range 2-12. However, its synthesis in acidic environment with the presence of excess of H⁺ ensures the high positive charge density in its surface, therefore, favoring the approaching of chromate anions (Cr0₄ ^2_) and providing higher uptake of chromium.

The efficiency of the material to completely remove Cr(VI) from natural water under continuous flow conditions is evaluated by the diagram of Figures 6 and 8 , where the breakthrough curves of rapid small scale column tests are shown. For these experiments, Sn604(OH)₄ was introduced as filling material for the adsorption of Cr(VI) by NSF challenge water solution containing 100 pg Cr(VI)/L, using a contact time of 2 min, grains sized 0.25-0.5 mm and temperature 20±1°C, at various pH values. The test water characteristics follow the NSF standard for the composition of a typical natural water so as to include all possible parameters that could interfere chromium adsorption. Particularly, it consists of 89 mg/L Na⁺, 40 mg/L Ca²⁺, 12.7 mg/L Mg²⁺, 183 mg/L HCOr, 50 mg/L S0₄ ²-, 71 mg/L CI^", 2 mg/L N-NO3-, 1 mg/L P , 0,04 mg/L P-PO4³- and 20 mg/L S1O2.

It is found that Sn60₄(OH)₄ is able to achieve residual concentrations lower than 1 pg/L, while its efficiency slightly increases when the pH increases in the range 7-8, which is the value for most drinking water containing Cr(VI). In particular, the adsorption capacity of Sn604(OH)₄ synthesized at pH 9 for residual Cr(VI) concentration 10 pg/L and adsorption pH 7.2±0.1 is estimated at 4.8 mg Cr(VI)/g, while for adsorption pH 7.7±0.1 the corresponding value reaches 6 mg Cr(VI)/g. Moreover, the adsorption capacity of Sn60₄(OH)₄ synthesized at pH 2 for residual Cr(VI) concentration 10 pg/L and adsorption pH 7.1+0.1 is estimated at 19 mg Cr(VI)/g. These results set the use of Sn60₄(OH)₄ for the removal of Cr(VI) from drinking water as rather an attractive material, since no adjustment of pH is required. It should be mentioned that the pH of groundwaters with elevated Cr(VI) concentrations ranges between 7.5 and 8 at a percentage higher that 95 % due to their contact with ultramafic rocks as described by the publication of E. Kaprara et al., J. Hazard. Mater., 281 (2015) 2.

Example 1 of method's application A solution of 25 g/L SnCI₂ is pumped by a flowrate of Q=1 m³/h in the 1 m³ continuous stirred reactor (CSTR) (1). The reaction pH is adjusted to 2±0.1 by the addition of a 30 % w/w NaOH solution. The pH is similarly adjusted to the 1 m³ CSTR (2). The product flowing out from the CSTR (2) is directed to the thickening tank to remain under mild stirring for 24 h, then mechanically dewatered by either a centrifuge or by a filter-press, extruded in grains sized 100-2000 μιη and dried at 80-100°C. The produced material consists of a bivalent tin oxy-hydroxide with the structure Sn60₄(OH)4 as represented in Figure 3.

Example 2 of method's application

A solution of 20 g/L SnCb is pumped by a flowrate of Q=1 m³/h in the 1 m³ CSTR (1). The reaction pH is adjusted to 4±0.1 by the addition of a 30 % w/w NaOH solution. The pH is similarly adjusted to the 1 m³ CSTR (2). The product flowing out from the reactor (2) is directed to the thickening tank to remain under mild stirring for 24 h, then mechanically dewatered by either a centrifuge or a filter-press, extruded in grains sized 100-2000 μιη and dried at 40 °C. The produced material mainly consists of a bivalent tin oxide with the structure SnO referred to in Figure 4.

Example 3 of method's application

A solution of 20 g/L SnS0₄ is pumped by a flowrate of Q=20 m³/h in the 10 m³ CSTR (1). The reaction pH is adjusted to 3±0.1 by the addition of a 30 % w/w NaOH solution. The pH is similarly adjusted to the 10 m³ CSTR (2). The product flowing out from the reactor 2 is directed to the thickening tank to remain under mild stirring for 24 h, then mechanically dewatered by either a centrifuge or a filter-press, extruded in grains sized 100-2000 pm and dried at 80-100 °C. The produced material consists of a bivalent tin oxy-hydroxide with the structure Sn60₄(OH)4.

Example 4 of method's use A solution of 20 g/L SnSC is pumped by a flowrate of Q=20 m³/h in the 10 m³ CSTR (1). The reaction pH is adjusted to 10±0.1 by the addition of a 30 % w/w NaOH solution. The pH is similarly adjusted to the 10 m³ CSTR (2). The product flowing out from the reactor 2 is directed to the thickening tank to remain under mild stirring for 24 h, then mechanically dewatered by either a centrifuge or a filter-press, extruded in grains sized 100-2000 μηη and dried at 40-100 °C. The produced material consists of a bivalent tin oxy-hydroxide (Sn60₄(OH)4), as the major structural phase, with a small percentage of bivalent tin oxide (SnO) referred to in Figure 5.

The synthesis method for bivalent tin oxy-hydroxide allows its production in granular units, so as to assist its use as a filling material in adsorption column beds, which is the only way of adsorbents application in large-scale. The production in a continuous flow reactor allows the accurate control of synthesis parameters, and as a result the optimization of adsorbents efficiency, while mild synthesis conditions favor high production rate at low cost implying to a minimum water treatment cost. In addition, accurate reaction control inhibits the production and release of toxic wastes to the environment. The adsorbent can be used for the removal of Cr(VI) from water supplied for drinking purposes since it is completely harmless for public health and does not involve byproducts formation which could potentially cause a reduction to the water quality. Its use is addressed to drinking water treatment units for house, municipal and industrial demands.

To summarize, table 1 provides a synopsis showing the differences between the method according to the invention and the cited documents which indicates the appearance of a number of distinguishing characteristics of the current invention. Table 1 below highlights these differences between the present invention and said reference documents. Current EP2 578 535

Parameter XP055308620 XP028697741 XP055309210

invention A2

Highly

Positively- Nanoparticles

Composition Sn₆0₄(OH)4 SnO Oxides of Sn²⁺ charged Sn₆0₄(OH)

Preparation Sn²⁺ salts SnCI₂ SnCI₂ Laser ablation Sn²⁺ salts method precipitation precipitation precipitation of Sn precipitation

Unidentified,

Strongly

Reaction pH <4 NaOH Unidentified 8.5-12 alkaline

addition

Reaction

20 °C 20 °C 30-90 °C High >70 °C temperature

Characteristic High reducing Reducing Optical Photocatalytic Semiconducting property activity activity properties activity properties

Continuous Continuous

Preparation Batch mode Batch mode Batch mode flow flow

mode production production production production production

Production Extremely

Low Low Intermediate Intermediate cost high

Table 1

In summary, the method of the invention refers to the synthesis of a remarkable adsorbent for hexavalent chromium, consisting of a positively-charged bivalent tin oxy-hydroxide with a structure Sn604(OH)₄, under conditions which introduce a number of distinctive characteristics compared to other similar materials disclosed in the current state of the art. In particular, synthesis in continuous-flow operation and environmental temperature implies the possibility for illimitable quantities with low production cost. In combination with the high positive charge density, leading to high removal efficiency for hexavalent chromium, and the simplicity of application in water treatment facilities, an extremely low fixed and operational cost is ensured when compared to other adsorbents of current state of the art.

Claims

1. A method characterized by the synthesis of a bivalent tin oxy-hydroxide of the type Sn_xOy(OH)z], with 1 <x<6, 0<y<4 and 0<z<4, performed in a two-stage continuous flow reactor realized by the following steps:

i) in a first reactor (1), SnS0₄ or SnC or Sn(N03)2 is added as a Sn(ll) source with a concentration 1-100 g/L, wherein the reaction is performed at a constant pH value in the range 2-12 by the addition of one or a combination of more than one of the alkaline reagents NaOH, NaHCOs, NaaCOs, KOH, KHCOs, K2CO3, Ca(OH)₂;

ii) in a next step, the reaction is then finalized in a second reactor (2), which is connected to said first reactor (1 ), wherein the pH value in said second reactor (2) is similarly adjusted in the range 2-12 by the addition of one or a combination of more than one of the alkaline reagents NaOH, NaHC03, Na2C03, KOH,

wherein the retention-reaction time is at least 30 min in each of the said two reactors (1 ) and (2);

iii) further wherein, the product is then outflowed from said second reactor (2), it enters a thickening tank (3) wherein it is collected under mild stirring for a period of 1-45 h and wherein said product is thickened, the material has a nanocrystalline geometry which is stabilized, a mechanical dewatering of the product follows (4), an extrusion thereof in grains (5) takes place and it is dried at a temperature comprised in the range of 40-100°C (6).

2. Method according to claim 1 , characterized in that an aqueous solution of SnS0₄ or SnC or Sn(N03)2 is fed in said first reactor (1), with a flowrate Q and a concentration of 1-100 g/L, wherein the hydrolysis/precipitation reaction of bivalent tin is accomplished mainly in said first reactor (1) under intense stirring, whereas the outflow from said second reactor (2) is collected under mild stirring, wherein said second reactor (2) is connected stream-downwardly with respect to said first reactor (1).

3. Method according to claim 1 or 2, characterized in that with said thickening, a precipitate is generated which is dewatered mechanically.

4. Method according to claim 3, characterized in that said precipitate is then extruded in grains (5) sized 100-2000 μιη.

5. Method for the synthesis of a bivalent tin oxy-hydroxide according to claim 1 , characterized by its optimum application when the reaction pH is 3±1.

6. An adsorbent which is produced according to the method as defined in one of the preceding claims 1 to 5.

7. Use of an adsorbent as produced in the preceding claim, in an adsorption column bed or a powder dispersion for the removal from water of hexavalent chromium essentially.

8. Use of the adsorbent as produced in the preceding claim 6, in an adsorption column bed or a powder dispersion for the removal from water of hexavalent chromium, as well as other oxidative ions like bromate, chlorate and perchlorate ions.