A SHUNGITE-BASED HYDROGEL FOR THE REMOVING OF RARE EARTH ELEMENTS FROM AN AQUEOUS MEDIUM AND ITS PRODUCTION METHOD

Abstract

A shungite-based hydrogel for the removal of rare earth elements (REE 3+ ) from an aqueous medium, where the rare earths are an element of the group of dysprosium (Dy 3+ ), neodymium (Nd 3+ ), Erbium (Er 3+ ) and Lathan (La 3+ ) comprising a hydrogel further comprising shungite dust, a cross-linking agent and a polymer, characterized in that the polymer is formed from at least one of: 2-acrylamide-2-methylpropanesulfonic acid (AMPS), methacrylamide (MAM), 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl methacrylate (HPMA), N-isopropylacrylamide (NIPAm); or PVA, polyacrylamide (PAM), guar gum (GG), polyacrylic acid (PAA), hydroxyapatite (HA), methoxy polyethylene glycol monoacrylate (mPEGMA), alginate, chitosan, carboxymethyl cellulose (CMC); the crosslinking agent is at least one of the following: borax, NN'-methylenebisacrylamide (MBA), ethylene glycol diacrylate (EGDA), PEG diacrylate (PEGDA); and the dust has a weight ratio of shungite to polymer between 50:50% and 60:40%.

Description

TECHNICAL FIELD

[0001] The invention relates to a sustainable adsorbent hydrogel consisting of natural shungite dust with a carbon content of 35-80 or 20-35 mass percent and used for the sorption of critical rare earth elements (REEs) such as Dy 3+ , He 3+ , Nd 3+ , Y 3+ and La 3+ Ions from aqueous solutions or industrial wastewater. The resulting shungite-based hydrogel is characterized by low energy consumption, high selectivity towards REEs, and easy scalability. BACKGROUND

[0002] Over the past thirty years, there has been an explosive growth in the applications of rare earth elements (REEs) and their alloys in various technological devices, such as automotive catalysts (Ce), hybrid vehicles (Dy, La, Nd), and modern green energy technologies such as wind turbines (Pr, Nd, Sm, Dy), batteries (La), fluorescent and luminescent phosphor lamps (La, Gd, Tb, Eu, Yb), and magnetic resonance imaging (MRI) agents (Gd) (Langkau et al. 2021; Morimoto et al. 2021). These applications, combined with the monopolistic nature of the REE market, recently highlighted by several government agencies such as the European Commission and the US Department of Energy, have raised awareness of our current and future needs for REEs. Currently, China produces 75% of the lanthanides and more than 90% of the world's mined REEs. However, mining ores requires large amounts of energy and produces large amounts of waste.For example, the production of 1 ton of rare earth oxide (REO) in China generates 60,000 m. 3 Exhaust fumes, 200 m 3 acidic water and 1.4 tons of radioactive waste, since most REE deposits contain uranium or thorium (Amato et al. 2019).

[0003] Since 2006, China, one of the world's largest rare earth depositors, has restricted the mining of RE sources to protect the environment and developed innovative technologies that are more sustainable and environmentally friendly. In particular, increasingly strict export controls and the lack of mineable deposits in many countries are prompting them to develop alternative strategies that require less energy consumption and less aggressive chemical treatments to meet the demand for REEs for high-tech industries. Therefore, numerous advanced methods have been reported worldwide (Binnemas et al. 2013). However, the most promising alternative source of REEs is based on recycling processes, which are a key factor in the circular economy. Different wastes contain REEs in different concentrations.In the case of industrial wastewater with relatively high concentrations of REEs, these can be recovered through ion exchange, biosorption, extraction, and adsorption with conventional adsorbents such as activated carbon, clay, and zeolite (Xie et al. 2014). Among these available methods, adsorption has gained considerable attention due to its simplicity, high efficiency, and low cost. However, industrial wastewater contains very low concentrations of REEs, so the use of nanomaterials as adsorbents offers a promising tool due to their high potential adsorption efficiency (Xiaoqian et al. 2021; Legaria et al. 2017; Anastopoulus et al. 2016). In addition to their high adsorption efficiency, nanomaterials enable a simple removal process for the target REEs. 3+ -ions from wastewater (Callura et al. 2018).

[0004] One of the most promising nanoadsorbents, made from an ore called "shungite," contains carbon nanostructures and turbostratic carbon with various minerals such as quartz, pyrite, chlorite, and sericite. Shungite carbon types include amorphous carbon, ordered graphite, fullerene layers, graphene layers, and glassy carbon. Literature shows that shungite exhibits excellent adsorption properties toward various organic compounds and heavy metals such as Zn, Cd, Pb, Mn, Cr, Cu, and Ni in aqueous solution. Numerous patents exist for the development of water treatment systems containing shungite in granular form [US 2021 / 0094844 A1; RU 2448 676 C1; WO 2021 / 123498 A1] and for the production of adsorption materials containing shungite for drinking water treatment [WO 2020 / 039299 A1; RU 2191 748 C2; RU 2696165 C1].However, it is interesting to note that there are no studies on the recovery of REEs from industrial wastewater.

[0005] To recover REEs from solid waste, strong acids are used to extract them from their solid source. The acid leachate is further processed to recover all valuable products. Unfortunately, these valuable elements exist in low concentrations (3-7 ppm) in the strong acid leachate before being diluted to even lower concentrations (0.5-0.6 ppm). In the case of REEs recovery from industrial wastewater, many techniques exist, such as ion exchange, electrodialysis, membrane filtration, reverse osmosis, chemical precipitation and adsorption, etc. However, most of these techniques are insufficient for large-scale processes and rapid separations. Among these techniques, adsorption is one of the most practical, cost-effective, and environmentally friendly. Conventional adsorbents are typically made of carbon-, silicon-, and polymer-based materials.Nevertheless, there is a need to explore new and efficient adsorption materials for the removal of REEs. In recent years, hydrogels have gained considerable attention as adsorption materials due to their superior properties such as high adsorption capacity, high surface-to-volume ratio, and mechanical strength. In this invention, the introduction of shungite into the hydrogel system not only increased the adsorption capacity but also dramatically improved the mechanical strength compared to pure PVA / borax hydrogel. The advantages of the prepared shungite-based hydrogel are that it is cost-effective, nature-based, easily scalable, environmentally friendly, suitable for industrial production, and highly adsorptive toward critical REEs.

[0006] AUBAKIROVA, Roza et al.: “Sorption extraction of heavy metal ions from wastewater by natural and synthetic sorbents,” in: Chemical Engineering Transactions, Vol. 81, 2020, pp. 343-348. - ISSN 1974-9791 concerns a polymer-protected hydrogel for the removal of copper and zinc from wastewater. BRIEF DESCRIPTION OF THE INVENTION

[0007] The invention, as defined in the claims, provides a shungite-based hydrogel for removing rare earth elements y 3+ , He 3+ , Nd 3+ and La 3+ from an aqueous medium which has a very good adsorption affinity, as well as a process for its preparation,

[0008] The invention accordingly relates to a sustainable adsorbent hydrogel consisting of natural shungite dust with a carbon content of 35-80 or 20-35 mass percent and used for the sorption of critical REEs such as Dy 3+ , He 3+ , Nd 3+ and La3+ from aqueous solutions or industrial wastewater. The prepared shungite-based hydrogel is characterized by low energy consumption, high selectivity towards REEs, and easy scalability. The hydrogel was prepared using a polymer crosslinked with a crosslinking agent through a 3-cycle freeze-thaw process. 40–60 mass percent of shungite dust with a particle size of 6 µm can be incorporated into the 60–40 mass percent of the polymer hydrogel composition. For the adsorption kinetics models (shungite-PVA / borax hydrogels as an example), the pseudo-first-order and pseudo-second-order models were applied to the data to clarify the adsorption mechanism of REEs onto the shungite-PVA / borax hydrogels. The best agreement was found with the pseudo-second-order model ( Fig.and Table 1). For the adsorption isotherms, the Langmuir and Freundlich models were applied to the obtained data, and it was found that the Langmuir model represents the experimental data better than the other model ( Fig. and Table 2). By increasing the mass fraction of borax in the shungite-PVA hydrogel system, reusability can be increased for three cycles. Table 1. Adsorption kinetic models using the shungite-PVA / borax hydrogel at T=298 K Samples Pseudo-First Order Pseudo-second order Adsorbed REE 3+ qe,exp(mg / g) qe,cal(mg / g) K1(1 / min) R 2 qe,cal(mg / g) K2(g / mg.min) R 2 The 3+ 121.85 31.2 0.2693 0.8749 125 0.0066 0.9988 Er 3+ 68.28 8.9 0.1040 0.7698 68.027 0.0057 0.9986 Those 3+ 71.4 8.4 0.1212 0.7708 71.942 0.005 0.9998 Nd 3+ 73.64 14.8 0.1483 0.9354 75.758 0.0102 0.999 Table 2. Adsorption isotherm models of the shungite-PVA / borax hydrogel at T = 298 K Samples Langmuir Friendly Adsorbed REE 3+ q e (mg / g) q max (mg / g) K L (L / mg) R 2 1 / n K F (L / mg)1 / n R 2 The 3+ 160.20 178.571 0.0048 0.9966 0.1994 36.5932 0.9158 Er 3+ 84.23 94.340 0.0066 0.9999 0.1447 29.8195 0.9807 Those 3+ 85.00 91.743 0.0072 0.9997 0.1318 31.9963 0.9804 Nd 3+ 127.12 17.241 5.0539 0.9943 0.4274 5.4626 0.9681 BRIEF DESCRIPTION OF THE CHARACTERS Fig. 1. Adsorption kinetics of shungite hydrogel towards Dy 3+ , He 3+ , La 3+ and Nd 3+ ions. Fig. 2. Recording of Dy 3+ , He 3+ , La 3+ and Nd 3+ions using the shungite-PVA / borax hydrogel. DETAILED DESCRIPTION

[0009] The present invention relates to a process for the recovery of rare earth elements (REEs) from industrial wastewater using a shungite-based polyvinyl alcohol (PVA) / borax adsorbent hydrogel. The shungite-based hydrogel system exhibits a high affinity towards critical REEs such as Dy 3+ , Nd 3+ , He 3+ and La 3+ ions in aqueous solutions. The absorption of REE 3+ The ions reached equilibrium after 4–6 hours, and the adsorption kinetics model best fit the pseudo-second-order equation (Table 1) (Anastopoulos et al. 2016). On the other hand, the adsorption isotherms best fit the Langmuir model with a maximum adsorption capacity of 160.2 mg La. 3+ / g, 127.0 mg Nd 3+ / g, 86.9 mg He 3+ / g and 85.7 mg Dy 3+ / g. In the desorption studies, the Nd 3+ -adsorbed shungite hydrogel the best release of Nd 3+ from the hydrogel after treatment with 20 ml of 0.5 M HCl for 3 hours, with a release of 80% Nd 3+ -ions. Under the same conditions, Dy 3+ , La 3+ and He 3+ also successfully released with 65%, 33% and 22% (w / w) respectively. As a result, Nd 3+ , La 3+ and Dy 3+ Good adsorption and desorption behavior compared to the shungite-PVA / borax hydrogel system. The shungite hydrogel was stable for three cycles of adsorption and desorption (using 0.2-0.5 M HCl). Preparation of shungite-based polymer crosslinking agent hydrogel

[0010] A polymer solution was prepared by dissolving it in a suitable solvent, such as an organic or aqueous solution. Said polymer can be derived from a monomer or a macromer. The monomer can be at least one of the following: 2-acrylamide-2-methylpropanesulfonic acid (AMPS), methacrylamide (MAM), 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl methacrylate (HPMA), N-isopropylacrylamide (NIPAm), and the macromer can be at least one of the following: PVA, polyacrylamide (PAM), guar gum (GG), polyacrylic acid (PAA), hydroxyapatite (HA), methoxypolyethylene glycol monoacrylate (mPEGMA), alginate, chitosan, carboxymethylcellulose (CMC). When the polymer to be used is PVA, the polymer solution has a ratio of 10% (w / w) (MW: 89,000). Shungite dust with a carbon content of 35-80 or 20-35 mass percent was added to the prepared polymer solution.

[0011] When the polymer solution is prepared with PVA, the shungite ratio is 60:40% (w / w). The shungite-PVA mixture was stirred at 45°C for 2-5 minutes. Shungite dust should be added at this stage, as a homogeneous mixture cannot be achieved if added at later stages. Higher shungite ratios, such as more than 70:30% (w / w) of shungite, resulted in mechanically weak composites and easy leaching in acidic solutions (HCl, HNO3, etc.). The optimal ratio was found to be 60:40% (w / w) shungite. When using other polymers synthesized from the above-mentioned monomers and macromers, the shungite ratio could be 50:60 / 50:40 (w / w). If the shungite content in the hydrogel falls below 50%, the adsorption capacity of REE could be reduced. 3+drastically decrease. When shungite dust was mixed with the polymer solution, a homogeneous black viscous solution was obtained. In this step, 7-10% (w / w) of a cross-linking agent was introduced into the black mixture, and stirring was continued at 70 °C for 30 minutes. When PVA was used as the polymer, borax should be used as the cross-linking agent. Two different complex structures form between PVA and borax: (a) a monodiol complex, formed by the reaction of the tetrahydroxyborate anion with the diol (-OH) groups in PVA, and (b) a bidiol complex, formed by the complexation between two hydroxyl groups in the borate anion and another adjacent diol in PVA. Otherwise, at least one of these reagents, such as NN'-methylenebisacrylamide (MBA), ethylene glycol diacrylate (EGDA), or PEG diacrylate (PEGDA), should be used as the cross-linking agent via copolymerization.Increasing the mass fraction of the crosslinker in the polymer solution led to mechanical strengthening of the composite. The highly viscous gel was then transferred to a glass dish, and the excess water was evaporated in an oven at 120 °C for 30 minutes. The freeze-thaw process was applied to the shungite-based hydrogel for three cycles by freezing it at -4 °C and thawing it at 120 °C in an oven. Their swelling ability, contact time with REEs (Dy. 3+ , Nd 3+ , He 3+ and La 3+ ) as well as adsorption kinetics and adsorption isotherms were investigated. Swelling studies of the shungite-based PVA / borax hydrogel

[0012] Dynamic swelling studies were conducted using gravimetric measurements. The hydrogel samples were placed in a Falcon tube and suspended in 20 mL of distilled water at 25 °C. The hydrogel samples were removed at various time intervals (1 h, 2 h, 3 h, 4 h, 6 h, 16 h, and 24 h) and weighed on an analytical balance after excess water was removed from the surface with filter paper. In this experiment, the average values ​​of three duplicate measurements were reported. The results were calculated using the following equation: Q(%)=m1−m2m2×100 where Q is the swelling ratio in percent (%), m1 is the swollen mass, and m2 is the dry mass. The swelling capacity of the shungite-PVA / borax hydrogel was determined to be 61%. Adsorption kinetics study

[0013] In a 50 mL Falcon tube, 25 mL of a 400 ppm solution of lanthanide (Dy 3+ , Nd 3+ , He 3+ or La 3+) and 50 mg of the shungite-based hydrogel adsorbent. The Falcontube was placed in an open shaker at constant speed (200 rpm) at room temperature. The lanthanide concentrations in the solution were determined at specific time intervals (0.5 h, 1 h, 2 h, 3 h, 4 h, 6 h, 16 h, 24 h) by titration with ethylenediaminetetraacetic acid (EDTA). EDTA was standardized with magnesium sulfate (MgSO4) before the start of the lanthanide titrations. Determination of lanthanide concentrations by EDTA titrations

[0014] To 5 mL of a lanthanide solution (Dy 3+ , Nd 3+ , He 3+ or La 3+ ) (400 ppm), 20 mL of deionized water, 1 mL of acetate buffer (pH=4.40-4.75), and 1 drop of xylenol orange indicator were added. After adding the indicator, the color of the solution changed to yellow. The solution was diluted with 9.3×10 -4 M EDTA until the transition point at which the yellow color changes to purple. Study on adsorption isotherms

[0015] 50 ± 0.1 mg of the shungite-based hydrogel was placed in a 50 mL Falcon tube. 20 mL of a solution of REEs (Dy 3+ , Nd 3+ , He 3+ and La 3+) at concentrations of 400, 600, 800, 1000, 1200, 1400, 1600, 1800, and 2000 ppm were added to the Falcontube and shaken at 200 rpm in an open shaker for 6 hours. After adsorption of the REEs, the shungite-based hydrogels were removed from the tube, and the residual solution was titrated with EDTA to determine the mass of adsorbed REEs per gram of shungite-based hydrogel. Our invention could be applied in the recycling industry, but in general, REEs can be used in various technological devices such as computer memory, DVDs, rechargeable batteries, catalysts, super magnets, mobile phones, LED lighting, superconductors, fluorescent materials, phosphate binders, solar panels, and magnetic resonance imaging (MRI) devices.

Claims

[1] A shungite-based hydrogel for the removal of rare earth elements (REE 3+ ) from an aqueous medium, where the rare earths are an element of the group of dysprosium (Dy 3+ ), neodymium (Nd 3+ ), Erbium (Er 3+ ) and Lathan (La 3+ ) comprising a hydrogel further comprising shungite dust, a cross-linking agent and a polymer, characterized by , that the polymer is formed from at least one of: 2-acrylamide-2-methylpropanesulfonic acid (AMPS), methacrylamide (MAM), 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl methacrylate (HPMA), N-isopropylacrylamide (NIPAm); or PVA, polyacrylamide (PAM), guar gum (GG), polyacrylic acid (PAA), hydroxyapatite (HA), methoxy polyethylene glycol monoacrylate (mPEGMA), alginate, chitosan, carboxymethyl cellulose (CMC); the crosslinking agent is at least one of the following: borax, NN'-methylenebisacrylamide (MBA), ethylene glycol diacrylate (EGDA), PEG diacrylate (PEGDA); and the dust has a weight ratio of shungite to polymer between 50:50% and 60:40%. [2] The shungite-based hydrogel according to claim 1, characterized in that the shungite dust has a carbon content of 35-80 or 20-35 mass percent. [3] The shungite-based hydrogel according to claim 1, characterized by a cross-linking agent with a weight ratio of 7-10% when using a shungite solution. [4] The shungite-based hydrogel according to claim 3, characterized in that the crosslinking agent is borax. [5] A method for preparing a shungite-based hydrogel for the removal of rare earth elements (REE 3+ ) from an aqueous medium, where the rare earths are an element of the group of dysprosium (Dy 3+), neodymium (Nd 3+ ), Erbium (Er 3+ ) and Lathan (La 3+ ) are, characterized by the following steps: - Preparation of a solution in an organic or aqueous solution - Addition of shungite dust to the prepared solution - Stir the mixture at 45 °C for 2-5 minutes - After obtaining a homogeneous black viscous solution, a crosslinking agent is added and the mixture is stirred at 70 °C for 30 minutes - Transfer the resulting viscous gel into a glass dish and evaporate the excess water in an oven at 120 °C for 30 minutes - Application of the freeze-thaw method to the shungite-based hydrogel for 3 cycles by freezing at -4 °C and thawing at 120 °C in the oven, wherein the prepared solution is a polymer solution, wherein the polymer is formed from at least one of: 2-acrylamide-2-methylpropanesulfonic acid (AMPS), methacrylamide (MAM), 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl methacrylate (HPMA), N-isopropylacrylamide (NIPAm); or PVA, polyacrylamide (PAM), guar gum (GG), polyacrylic acid (PAA), hydroxyapatite (HA), methoxy polyethylene glycol monoacrylate (mPEGMA), alginate, chitosan, carboxymethyl cellulose (CMC). wherein the crosslinking agent is at least one of the following: borax, NN'-methylenebisacrylamide (MBA), ethylene glycol diacrylate (EGDA), PEG diacrylate (PEGDA), and the dust has a weight ratio of shungite to polymer between 50:50% and 60:40%. [6] The method according to claim 5, characterized by a ratio of 10% (w / w) if the polymer is PVA. [7] The method according to claim 5, characterized in that the shungite dust has a carbon content of 35-80 or 20-35 mass percent. [8] The method according to any one of claims 5 to 7, characterized byBorax as a crosslinking agent with a weight ratio of 7-10% when PVA is used as a polymer.

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