WO2014094130A1

WO2014094130A1 - Graphene oxide for use in removing heavy metal from water

Info

Publication number: WO2014094130A1
Application number: PCT/CA2013/001061
Authority: WO
Inventors: You FU; Jingyi Wang; Qingxia Liu; Hongbo Zeng
Original assignee: The Governors Of The University Of Alberta
Priority date: 2012-12-19
Filing date: 2013-12-19
Publication date: 2014-06-26

Abstract

A product is provided for binding heavy metals in. Specifically, the substance is a derivative of graphite, namely graphene oxide nano-platelets (GO). In another embodiment of the present invention, a product comprising GO grafted with magnetic particles (MGO particles) is provided. The process for removing heavy metal from an aqueous solution is also provided. The process includes mixing GO or MGO particles with water containing a heavy metal, whereby the heavy metal is absorbed by the GO, forming a slurry. Where MGO particles are used, the process can also include using a magnetic field to separate the heavy metal loaded MGO from the remaining aqueous solution. Once separated, the heavy metal can be stripped from the heavy metal loaded MGO particle so that it can be reused.

Description

GRAPHENE OXIDE FOR USE IN REMOVING HEAVY METAL FROM

WATER

Inventors: You Fu, Jingyi Wang, Qingxia Liu, and Hongbo Zeng

Field of Invention

This invention relates to the removal of metal from water using an absorbent.

Background

Selenium (Se) is an essential nutrient element for life at trace concentrations, but can be toxic in higher concentrations. It is one of a small group of anionic elements that potentially pose a particular hazard to organism health and to the environment and thus can have a significant impact on the quality of drinking water. During mining and mineral processing, selenium can be released into surrounding water and soil. It is then taken up by plants and animal organisms, and thereby makes its way into the ecosystem.

The average Se content is 0.2 parts per billion (ppb) for river water and 0.1 ppb for seawater. The regulations provided by the Environmental Protection Agency state that the maximum contaminant level of Se is 50 ppb for primary drinking water. The normal human dietary intake of Se is about 200 g per day, while Se toxicity may manifest at dietary levels of 600 μg to 6,340 μg per day. Ingestion of too much Se from drinking water could significantly impact human health with undesirable physical manifestations. Therefore, a product and process that provides for efficient separation of trace amount of Se is important to protecting human health, as well as the environment.

Selenium can exist in four oxidation states: selenide (Se ^"), elemental selenium (Se ), selenite (Se0₃ ), and selenate (Se0₄ ^"). Among all these forms, selenate (Se ) and selenite (Se⁴⁺) are more mobile and toxic, and they are commonly found in arid regions. Selenate can occur in oxidized soils and alkaline surface waters, and is more mobile due to its high water solubility and poor soil adsorption characteristics. Selenate can be reduced to selenite, which is hard to dissolve in water and easily adsorbed by soil colloids including Iron (Fe) and Aluminum (Al) oxides. As a result, selenate is the major species of Se in water.

Different methods have been proposed to remove Se from drinking water such as ion exchange, insoluble complex technique, air flotation, chemical precipitation, emulsion liquid membranes, nanofiltration, reductive cementation, biological reduction, reverse osmosis, solvent extraction, precipitation, reduction processes, lime softening, and adsorption. The adsorption method has typically been preferred for its relatively fast removal speed and minimum sample pretreatment.

However, commonly used adsorbents, such as silica gel, clay, activated carbon, molecular sieve, ferrihydrite, ferric oxyhydroxide/peat/resins, and activated aluminium oxide, are limited by their low adsorption, high price, non-renewability and narrow application range in large-scale application.

Moreover, most adsorbents show poor performances in selenate removal. This is because selenate forms an outer-sphere adsorption on adsorbents such as ferrihydrite, while selenite can be absorbed by the adsorbents in an inner-sphere adsorption manner, which is much stronger than the outer-sphere adsorption. As a result, some adsorption methods require a complicated pre-reduction of selenate.

Therefore, an effective selenium adsorbent product and process is needed. Summary

In one aspect of the present invention a product for use in binding heavy metals is provided. Specifically, the substance is graphene oxide nano-platelets (GO).

In another embodiment of the present invention, a product comprising GO grafted with magnetic particles (MGO particles) is provided.

In a further aspect of the present invention a process for removing heavy metal from an aqueous solution is provided. The process comprises mixing GO with water containing a heavy metal, whereby the heavy metal is absorbed by the GO, creating a slurry. The GO may have been grafted with magnetic particles prior to mixing.

This process may further include using a magnetic field to separate the heavy metal loaded MGO particles from the remaining aqueous solution. Once separated, the heavy metal can be stripped from the heavy metal loaded MGO particles so that the MGO particles can be reused. Description of the Figures

Figure 1 shows a flow diagram for the synthesis of magnetic graphene oxide nano- platelets (MGO particles);

Figure 2 shows the process of MGO particle synthesis, water purification and recycling of MGO particles;

Figure 3 is a graph showing the removal percentage for Se (IV) using MGO particles, GO or Fe₃0₄ as an adsorbent;

Figure 4 is a graph showing Q-C and q-C curves with different initial Se⁴⁺/Se⁶⁺ concentrations; Figure 5A shows a slurry before separation of the heavy metal loaded MGO particles; Figure 5B shows a slurry after separation of the heavy metal loaded MGO particles using a hand magnet in lab; and

Figure 5C is a diagram of magnetic separation of heavy metal loaded MGO using drum magnetic separator.

Description of the Preferred Embodiment

Compared with the hydrophobic natural graphite, graphene oxide is a hydrophilic monolayer substance. Its hydrophilic characteristics are due to the large quantity of surface hydroxyl and carboxyl groups. Graphene oxide also has a high specific surface area, large interlayer distance, abundant hydroxyl content, and is stable under normal conditions.

Figure 1 outlines one example of one embodiment of a method of making graphene oxide nano-platelets (GO), as well as one example of one embodiment of the additional steps can be used to transform GO into magnetic graphene oxide particles (MGO particles).

In general, the process of making GO starts with the oxidation of graphite flakes to make graphite oxide. Graphite oxide then undergoes ultrasonic treatment and is dispersed as GO with only one or few layers.

More specifically, one possible way to make GO starts with preparing graphite oxide from graphite flakes using a modified Hummers method. For example, 2 g of natural graphite flake (7-10 μηι) and 1.5 g of sodium nitrate (NaN0₃) is mixed in a three necked bottle. Then 150 ml of 98 wt% sulfuric acid (H₂S0₄) is added to the mixture in an ice-bath while mechanical agitation is maintained. Next, 9 g of potassium permanganate (ΚΜη0₄) is slowly added, while stirring and kept in an ice-bath for 2 hours. The mixture is removed from the ice-bath, but the agitation continues for another 5 days at room temperature. A 6 ml portion of H₂0₂ is added to the mixture to neutralize unreacted KMn0₄. The generated 0₂ assists with the exfoliation of GO layers.

After another 2 hours of agitation, the resulting bright yellow suspension is diluted and washed with a 250 ml liquid mixture of 7.5 ml 98% H₂S0₄, 4.17 ml 30 wt% H₂0₂ and Milli-Q water. Then sediment is washed with Milli-Q water until neutral. The sediment is dialyzed for 5 days to clean out remnant salt. After an ultrasonic dispersion, fluffy golden flocci of GO were attained using freeze-dry technology known in the art.

Figure 3 shows that GO on its own is capable of absorbing selenite with approximately the same efficacy as Fe₃0₄, however it does not remove selenate (see Table 1). In addition, GO is highly hydrophilic and therefore it is difficult to remove from water after selenium adsorption.

In a second embodiment of the present invention magnetic particles are grafted on the GO to form MGO particles. In one example of one embodiment, this may be done using a modified Shen's method. By way of example, the method may include taking GO (100 mg) and dispersing it in I-methyl-2pyrrolidone (NMP) (30ml) by ultrasonication at room temperature. The mixture is then heated to 190°C under a nitrogen atmosphere. Fe(acac)₃ (1.413 g, 4 mmol) is dissolved in 20ml of NMP and added dropwise for about 1 hour to the GO/NMP solution under vigorous stirring. In contrast to the traditional Shen's method, the stirring is continued for another 4 hours after the all of the Fe(acac)₃ is added. This modified procedure may increase the magnetism of the composite. After cooling down to room temperature, the mixture is washed several times with alternating acetone and water. The precipitate is collected by magnetic separation and is then dispersed in water using sonication. The resulting black powder is collected using freeze-dry technology which is well known in the art.

Alternatively, other magnetic particles may be used, for example, Ag⁺, as it has a similar oxidixability to Fe³⁺.

Once grafted, the magnetic particles are firmly entrapped and immobilized onto GO sheets via strong covalent bonding. The surface of MGO is very coarse and highly covered with magnetic nanoparticles. The high specific surface area facilitates introduction of magnetic nanoparticles from both sides of the nanosheets, which in turn enlarges the distance between layers and prevent GO from stacking back to the original graphitic structure. Moreover, the GO sheets protect magnetic nanoparticles from surface oxidation, providing a support matrix for application in flow water.

The large surface areas of both GO and MGO particles (approximately 1 13.06

2 2

m /g and 131.73 m /g, respectively) as well as the inter-layer distance of the matrix, which enable the accessibility of binding sites, and the strength of the chemical adsorption ability of surface hydrolysis of magnetic Fe₃0₄ particles (in the case of MGO), which is guaranteed by the abundant surface functional groups, make them useful products to use for absorption.

The process for removing Se from an aqueous solution involves first mixing GO with wastewater containing selenium such that GO absorb the Se. Alternatively, MGO particles can be used for mixing and absorbing the Se. Figure 2 outlines the process that can be employed in the event that MGO particles are used. After GO (10) and the magnetic particles (12) are combined (16) to form the MGO (14), the MGO is mixed with wastewater in a container (18). The hydrolyzed MGO (20) absorbs the heavy metal (22) in the wastewater to form a heavy metal slurry (24). The heavy metal loaded MGO particles (32) can be separated from the remaining solution by applying a magnetic field (28) to the slurry, for example a magnet (26) as shown in Figure 5B. Other alternatives for separation using a magnetic field would be obvious to a skilled person.

In the presence of a magnetic field, the heavy metal loaded MGO particles move towards the source of the field. The heavy metal loaded MGO particles can then be removed (30) from the container and separated from the remaining aqueous solution.

One method of separating the heavy metal loaded MGO particles from the remaining solution is shown in Figure 5C. Specifically, the slurry is passed under a rotating steel drum (34) having a magnet unit (36) inside. The heavy metal loaded MGO particles are attracted to the surface of the drum (42) and remain attached past the clean water outlet (38). A drum scrapper (40) is used to remove the separated heavy metal loaded MGO particles from the drum surface.

After separation, the heavy metal can be stripped (44) from the MGO particles, for example by washing it with diluted HC1 (0.001N). This allows the MGO particles to be reused (46) to treat a new sample of contaminated water.

In experiments, after Se separation, only negligible trace amounts of MGO particles remained in the water after magnetic separation. Additionally, there was no evidence of pollution another metal ion (for example iron). Therefore, it is unlikely that the MGO particles will cause secondary pollution.

In order to test the absorption ability of GO and MGO particles, 20 mg of adsorbent (the dosage of absorbents was 0.1 wt. %) was added to 20 ml of Se⁴⁺ and Se⁶⁺ standard liquid (prepared from diluting 1000 ppm Na₂Se0₃ and Na₂Se0₄ stock water solutions), mixing uniformly. While constant temperature was remained, samples were placed on a standard shaker and oscillated for given hours at 300 rpm. NaOH and HCl was used to adjust systematic pH. Selenium ion concentration was analyzed in solutions by ICP-MS. After washing with diluted HCl (0.001 N), MGO particles were reused to repeat the exact test with the same initial selenium concentration and oscillating time.

The results shown in Table 1 demonstrate the efficacy of GO and MGO particles to remove Se ions from wastewater.

Table 1. Comparison between different adsorbents for selenium removal.

While the examples described herein refer to the removal of selenium from wastewater, MGO particles are also able to adsorb other heavy metal ions such as mercury, chromium, lead, cadmium, copper, zinc and arsenic. Therefore, it is contemplated that the MGO particles can be used to remove other heavy metals from aqueous solutions.

Figure 4 shows the adsorption capacity (Q-C curve) and removal ratio (q-C curve) of MGO particles with different initial Se⁴⁺/Se⁶⁺ concentrations. Similar trends are seen for Q-Co and q-C₀ in both Se⁴⁺ and Se⁶⁺ curves. They indicate that the higher the initial concentration of Se ions, the stronger the final adsorption capacity is. This suggests that the adsorption process is a monolayer adsorption using strong chemical bonding. In particular, Se ions are adsorbed on special adsorbing sites, which are hydroxyl functional groups.

Compared with selenite, selenate is a weaker binding anion, which could only be adsorbed through electrostatic attraction to the MGO particle surface, forming an outer- sphere complex with one water molecular between the MGO particle surface site and the selenate ligand. The outer-sphere is less stable, which makes it harder to form. This is why most adsorbents are ineffective in removing selenate. However, since the MGO particle is highly water-soluble, the interaction between the MGO particle and H₂0 is very tight and stable. Moreover, the high surface area and nanolayer structure make it easier to retain water and selenate molecules. Therefore, the outer-sphere is relative stable and has a high adsorption rate. Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention as defined by the claims.

Claims

1. A product for use in binding heavy metals in wastewater comprising, graphene oxide nano-platelets.

2. A product for use in binding heavy metals in wastewater comprising, magnetic particles grafted to the graphene oxide nano-platelets.

3. The product of claim 2, wherein the magnetic particles are iron oxide.

4. A process for removing a heavy metal from an aqueous solution, comprising, mixing graphene oxide nano-platelets with wastewater containing heavy metal, whereby the graphene oxide nano-platelets absorb the heavy metal.

5. The process of claim 4, wherein the graphene-oxide nano-platelets have magnetic particles grafted thereto.

6. The process of claim 5, further comprising, using a magnetic field separate the heavy metal slurry from the aqueous solution.

7. The process of claim 6, further comprising the removal of the heavy metal slurry from the aqueous solution.

8. The process of claim 7, further comprising stripping the heavy metal from the graphene-oxide nano-platelets and separating the heavy metal form the graphene oxide nano-platelets.

9. The process of claim 8, further comprising mixing the stripped graphene oxide nano-platelets with wastewater containing a heavy metal.

10. The process of any one of claims 4 to 9, wherein the heavy metal is selenium.

1 1. The process of any one of claims 5 to 10, wherein the magnetic particles are iron oxide.