WO2022260623A1

WO2022260623A1 - A shungite based hydrogel to remove rare earth elements from an aqueous media and it's preparation method

Info

Publication number: WO2022260623A1
Application number: PCT/TR2022/050371
Authority: WO
Inventors: Seda DEMIREL TOPEL
Original assignee: Antalya Bilim Universitesi Rektorlugu
Priority date: 2022-04-26
Filing date: 2022-04-26
Publication date: 2022-12-15

Abstract

Invention is about a sustainable adsorber hydrogel comprised of natural shungite dust with 35-80 or 20-35 mass percentages of carbon content for sorption of critical rare earth elements (REEs) such as Dy³⁺, Er³⁺, Nd³⁺, Y³⁺ and La³⁺ ions from aqueous solution or industrial wastewater. The produced shungite-based hydrogel has lower energy consumption, high selectivity towards REEs and easy of scalability.

Description

A SHUNGITE BASED HYDROGEL TO REMOVE RARE EARTH ELEMENTS FROM AN AQUEOUS MEDIA AND IT’S PREPARATION METHOD

TECHNICAL FIELD

BACKGROUND

Over the last thirty years, there has been an explosion in the applications of REEs and their alloys in several technology devices such as car catalysts (Ce), hybrid vehicles (Dy, La, Nd) or modern green energy technologies such as wind turbines (Pr, Nd, Sm, Dy), batteries (La) or fluorescent and luminescent phosphor lamps (La, Gd, Tb, Eu, Yb), 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 noted by several government agencies, such as the European Union Commission and the US Department of Energy, have raised awareness regarding our current and future needs in REEs. Currently, China produces 75% of lanthanides and more than 90% of REEs worldwide via mining. However, ore mining requires large amount of energy and generates large quantities of waste. For example, production of 1 tonne of rare earth oxide (REO) in China generate 60,000 m³ of waste gases, 200 m³ of acidified water and 1 .4 tonnes of radioactive waste since of most REEs deposits contain uranium or thorium (Amato et al., 2019).

Since 2006, China, one of the most abundant REEs reserves in the world, constrained the mining of RE sources for protecting the environment and started to develop innovative technologies which are more sustainable and environmentally friendly. Particularly, increasing strict export controls and many countries’ lacks minable repositories within their boundaries are causing them to seek alternative strategies which require less energy consumption, less harsh chemical treatments for acquiring REEs supply for high technological industries. Henceforth, numerous advanced methods were reported worldwide (Binnemas et al., 2013). However, the most promising alternative supply of REEs are based on the recycling processes which is a key factor in circular economy.

Various wastes contain REEs at various concentrations. In case of industrial wastewaters with relatively high REE concentration, they can be recovered by ion- exchange, biosorption, extraction, and adsorption methods with traditional adsorbents such as active carbon, clay, and zeolite (Xie et al. , 2014). Among these available methods, adsorption has gained a significant attention due to its simplicity, high efficiency, and low cost. However, wastewater includes very low concentrations of REEs, hence utilization 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). Besides their high adsorption efficiency, nanomaterials enable a simple removal process of target RE³⁺ ions from the wastewater area (Callura et al., 2018).

One of the most promising nanosorbent manufactured from ore called “Shungite” include carbon nanostructures and turbostratic carbon with different types of minerals such as quartz, pyrite, chlorite and sericite. The types of the shungite carbon are comprised of amorphous carbon, ordered graphite, fullerene, graphene layers and glassy carbon. Literature survey shows that shungite has excellent adsorption properties towards various organic compounds, and heavy metals such as Zn, Cd, Pb, Mn, Cr, Cu and Ni in aqueous solution. There is myriad of patents about developing water treatment device that contains shungite in granular form [US 2021/0094844 A1 ; RU 2448 676 C1 ; WO 2021/123498 A1] and production of sorbent materials composed of shungite for drinking water treatment [WO 2020/039299 A1 ; RU 2191 748 C2; RU 2696165 C1 ]. However, it is interesting to note that, there is no study on the recovery of REEs from industrial wastewater.

To recover the REEs from solid waste, strong acids are used to leach them from their solid source. The acid leachate is further processed to recover any high-value products. Unfortunately, these valuable elements exist in low concentrations (3-7 ppm) in the strong acid leachate before dilution to even lower concentrations (0.5-0.6 ppm). In case of the recovery of REEs in industrial wastewaters, there are plenty of techniques such as ion exchange, electrodialysis, membrane filtration, reverse osmosis, chemical precipitation, and adsorption etc. However, most of these techniques are not sufficient for bulky processes and fast separation. Among these techniques, adsorption is one of the most practical, low-cost, and environmentally friendly technique. Conventional adsorbents are generally comprised of carbon, silica and polymer-based materials. Nevertheless, there is a demand to explore new and efficient adsorbent materials for the removal of REEs. In recent years, hydrogels as adsorbent material has gained considerable attention due to their superior properties such as high adsorption capacity, high surface to volume ratio and mechanical strengths etc. In this invention, the introduction of shungite to the hydrogel system not only increase its adsorption capacity, but also resulted to dramatically increased its mechanical strength compared to pure PVA/borax hydrogel. The advantages of the produced shungite based hydrogel are that it is low-cost, natural based, easy to scalability, environmentally friendly, suitable for industrial scale production and has high adsorption capacity towards to critical REEs.

BRIEF DESCRIPTION OF THE INVENTION

Invention is about a sustainable adsorber hydrogel comprised of natural shungite dust with 35-80 or 20-35 mass percentages of carbon content for sorption of critical REEs such as Dy³⁺, Er³⁺, Nd³⁺ and La³⁺ from aqueous solution or industrial wastewater. The produced shungite-based hydrogel has lower energy consumption, high selectivity towards REEs and easy of scalability.

The hydrogel was generated using a polymer crosslinked with a crosslinking agent via freezing-thawing method with 3 cycles. 40-60 mass % of dust shungite with 6 pm diameter in size, may involve in the 60-40 mass % of polymer hydrogel composition. As adsorption kinetics models (shungite-PVA/borax hydrogels are taken as an example), pseudo first-order and the pseudo second-order were applied to the data in order to clarify the adsorption mechanism of REEs onto the shungite- PVA/borax hydrogels. The best fit was found to be pseudo second-order model (Fig.1 and Table 1 ). For the adsorption isotherm, Langmuir and Freundlich, models were applied to the obtained data and Langmuir model was found to represent the experimental data rather than the other model (Fig. 2 and Table 2). By increasing the mass percentage of borax in shungite-PVA hydrogel system, the reusability may increase for three cycles.

Table 1. Adsorpsion kinetic models using shungite PVA/borax hydrogel at T=298 K

Table 2. Adsorption isotherm models shungite PVA/borax hydrogel at T = 298K.

LIST OF FIGURES

Figure 1. Adsorption kinetics of the Shungite hydrogel towards to Dy³⁺, Er³⁺, La³⁺ and Nd³⁺ ions.

Figure 2. Uptake of the Dy³⁺, Er³⁺, La³⁺ and Nd³⁺ ions using Shungite PVA/borax hydrogel

DETAILED DESCRIPTION The present invention is to provide a method for the recovery of rare earth elements (REEs) from industrial wastewater using a shungite based polyvinyl alcohol (PVA)/borax adsorbent hydrogels. The shungite based hydrogel system exhibit high affinity towards to the critical REEs such as Dy³⁺, Nd³⁺, Er³⁺ and La³⁺ ions in aqueous solution. The uptake of the RE³⁺ ions were reached to an equilibrium between 4-6 hours and adsorption kinetic model was fit best to the pseudo-second order equation (Table 1 ) (Anastopoulos et al., 2016). On the other hand, adsorption isotherms were fitted to Langmuir model with the maximum adsorption capacity followed by 160.2 mg La³7g, 127.0 mg Nd³7g, 86.9 mg Er³7g and 85.7 mg Dy³7g. In the desorption studies, Nd³⁺ adsorbed shungite hydrogel exhibited the best release of Nd³⁺ from the hydrogel treating with 20 mL of 0.5 M HCI for 3 hours with 80% Nd³⁺ ion release. Under the same conditions, Dy³⁺, La³⁺, Er³⁺ were also successfully released with 65%, 33% and 22% (w/w), respectively. As a result, Nd³⁺, La³⁺ and Dy³⁺ have a good adsorption and desorption behaviour towards to shungite PVA/borax hydrogel system. The shungite hydrogel was stable for sorption and desorption process (using 0.2-0.5 M HCI) for three cycles.

Preparation of Shungite based Polymer/Crosslinking agent hydrogels

A polymer solution was prepared by dissolving in a proper solvent such as organic or aqueous solution. The mentioned polymer can be obtained from a monomer or a macromer. The monomer can be at least one of the 2-acrylamide-2-methyl- propanesulfonic acid (AMPS), methacrylamide (MAM), 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl methacrylate (HPMA), /V-isopropylacrylamide (NIPAm) and the macromer can be at least one of the PVA, polyacrylamide (PAM), guar gam (GG), polyacrylic acid (PAA), hydroxyapatite (HA), methoxyl poly(ethylene glycol) (PEG) monoacrylate (mPEGMA), alginate, chitosan, carboxymethyl cellulse (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 35-80 or 20-35 mass percentages of carbon content was added to the prepared polymer solution.

When the polymer solution is prepared with PVA, the ratio of shungite: PVA is 60:40% (w/w). The mixture of shungite-PVA was stirred at 45 °C for 2-5 mins. Shungite dust should be added at this stage, because a homogeneous mixture couldn’t be obtained if it is added in the later stages. Higher ratios for shungite:polymer, such as more than 70:30% (w/w) of shungite:polymer was resulted to mechanically weak composites, and easily leaching in acidic solutions (HCI, HNCb etc.). The optimum ratio was found to be 60:40% (w/w) shungite: polymer. In the case of using other polimers synthesized from monomers and macromers mentioned above, the ratio of the shungite/polymer could be 50-60/50-40 (w/w) percentages. If the percentage of shungite in the hydrogel drops below to 50%, the adsorption capacity of REE³⁺ could be decreased dramatically. When mixing shungite dust with the polymer solution, a homogenous black viscous solution was obtained. In this step, 7-10% (w/w) of a crosslinking agent was introduced to the black mixture and continued to be stirring at 70°C for 30 mins. When PVA was used as a polymer, borax should be used as the crosslinking agent. Because, between PVA and borax, two different complex structures occur as follows (a) a monodiol complex generated by the reaction of the tetrahydroxy borate anion with the diol (-OH) groups in PVA (b) bidiol complex formed by the complexation between two hydroxyl groups in borate anion with another adjacent diol in PVA. Otherwise, at least one of these reagents such as N,N'- methylenebis(acrylamide) (MBA), ethylene glycol diacrylate (EGDA) or PEG diacrylate (PEGDA) was used as a crosslinking agent via copolymerization. Increasing the mass percentage of the crosslinking agent in polymer solution was caused to mechanical strain hardening in the composite. The highly viscous gel was then transferred to a glass petri dish, and the excess of water was evaporated in the oven at 120°C for 30 mins. Freezing and thawing method was applied to the shungite based hydrogel for 3 cycles by freezing at -4°C and thawing at 120°C in the oven. Their swelling capacity, contact time with REEs (Dy³⁺, Nd³⁺, Er³⁺ and La³⁺) and adsorption kinetics and adsorption isotherms were investigated.

Swelling Studies of the Shungite based PVA/borax hydrogels

Dynamic swelling studies were made by gravimetric measurements. The hydrogel samples were placed in falcon tube and suspended in 20 mL of dd. H2O at 25 °C. The hydrogel samples were removed at different time intervals (1 h, 2h, 3h, 4h, 6h, 16h and 24h) and weighed on an analytical balance after excess solution was blotted free of the surface water using a filter paper. In this experiment, the mean values of three duplicate measurements were presented. The results were calculated in the following Equation-1 :

where, Q is the swelling ratio in percentage (%), mi is the swollen mass, and m2 is the dried mass. The swelling capacity of the shungite PVA/borax hydrogel was found to be 61 %.

Study of adsorpsion kinetics

In a 50 mL of falcon tube, 25 mL of 400 ppm concentration of lanthanide (Dy³⁺, Nd³⁺, Er³⁺ or La³⁺) solution and 50 mg of shungite based hydrogel adsorbent was added. The falcon tube was replaced to an open-air shaker with a constant speed (200 rpm) at room temperature. The lanthanide concentrations in the solution at specific time intervals (0.5, 1 , 2, 3, 4, 6, 16, 24 hours) were determined with titration using tethylene diamine tetraacetic acid (EDTA). EDTA was standardized with magnesium sulphate (MgSC ) before starting the titrations with lanthanides.

Determination of Lanthanide Concentrations by EDTA Titrations

To a 5 mL of lanthanide (Dy³⁺, Nd³⁺, Er³ or La³⁺) solution (400 ppm), 20 mL of deionized water, 1 mL of acetate buffer (pH=4.40-4.75) and 1 drop of xylenol orange indicator was added. After the indicator was introduced to the mixture, the solution colour changed to yellow. The solution was titrated with 9.3x10^_4M of EDTA until the yellow colour turns to purple at the equivalence point.

Study of adsorpsion isoterms

50± 0.1 mg of shungite-based hydrogels were placed into the 50 mL of falcon tubes. 20 mL of 400, 600, 800, 1000, 1200, 1400, 1600, 1800 and 2000 ppm concentration of REEs (Dy³⁺, Nd³⁺, Er³⁺ and La³⁺) were introduced to the falcon tube, and shaked at 200 rpm in an air-open shaker for 6 hours. After adsorption of REEs, the shungite-based hydrogels were removed from the tube, and the residue solution were titrated with EDTA to determine the mass of the adsorbed REEs per gram of shungite-based hydrogel.

Our invention could be used in recycle industry, but in general aspects, REEs can be used in several technology devices such as computer memory, DVDs, rechargeable batteries, autocatalytic converters, super magnets, mobile phones, LED lighting, superconductors, fluorescent materials, phosphate binding agents, solar panels, and magnetic resonance imaging (MRI) agents.

Claims

1. A shungite based hydrogel to remove rare earth elements (REE³⁺) from an aqueous media characterized by; comprising a hydrogel further comprising a shungite dust, a crosslinking agent and a polymer;

2. The polymer of Claim 1 characterized by being formed with at least one monomer.

3. The polymer of Claim 1 characterized by being formed with at least one macromer;

4. The monomer of Claim 2 characterized by being at least one of the 2-acrylamide- 2-methyl-propanesulfonic acid (AMPS), methacrylamide (MAM), 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl methacrylate (HPMA), N- isopropylacrylamide (NIPAm).

5. The macromer of Claim 3 characterized by being at least one of the PVA, polyacrylamide (PAM), guar gam (GG), polyacrylic acid (PAA), hydroxyapatite (HA), methoxyl polyethylene glycol) (PEG) monoacrylate (mPEGMA), alginate, chitosan, carboxymethyl cellulse (CMC).

6. The crosslinking agent of Claim 1 characterized by being at least one of the borax, A/,/V'-methylenebis(acrylamide) (MBA), ethylene glycol diacrylate (EGDA) or PEG diacrylate (PEGDA).

7. The shungite based hydrogel to remove rare earth elements from an aqueous media of Claim 1 characterized by comprising the shungite dust with a shungite:polymer ratio of between 50:50% (w/w) and 60:40% (w/w).

8. The shungite dust of Claim 7 characterized by having a carbon content with 35- 80 or 20-35 mass percentages.

9. The shungite based hydrogel to remove rare earth elements (REE³⁺) from an aqueous media of Claim 1 characterized by comprising the crosslinking agent with a ratio of 7-10% (w/w) when a shungite: PVA solution is used.

10. The crosslinking agent of Claim 9 characterized by being borax.

11 . A method for preparing a shungite based hydrogel to remove rare earth elements (REE³⁺) from an aqueous media characterized by comprising the steps below;

- Preparing a solution in an organic or aqueous solution,

- Adding a shungite dust to the prepared solution, - Stirring the mixture at 45 °C for 2-5 mins,

- After a homogenous black viscous solution was obtained, introducing a crosslinking agent and stirring at 70°C for 30 mins,

- Transferring the obtained viscous gel to a glass petri dish and evaporating the excess of water in an oven at 120°C for 30 mins,

- Applying freezing and thawing method to the shungite based hydrogel for 3 cycles by freezing at -4°C and thawing at 120°C in the oven.

12. The prepared solution mentioned in Claim 11 characterized by being a polymer solution.

13. The polymer mentioned in Claim 12 characterized by being formed with at least a monomer or a macromer.

14. The monomer mentioned in Claim 13 characterized by being at least one of the 2-acrylamide-2-methyl-propanesulfonic acid (AMPS), methacrylamide (MAM), 2-hydroxyethyl methacrylate (HEMA), 2-hydroxypropyl methacrylate (HPMA), /V-isopropylacrylamide (NIPAm).

15. The macromer mentioned in Claim 13 characterized by being at least one of the PVA, polyacrylamide (PAM), guar gam (GG), polyacrylic acid (PAA), hydroxyapatite (HA), methoxyl poly(ethylene glycol) (PEG) monoacrylate (mPEGMA), alginate, chitosan, carboxymethyl cellulse (CMC).

16. The prepared solution mentioned in Claims between 12-15 characterized by having a ratio of 10% (w/w) when the polymer is PVA.

17. The shungite dust mentioned in Claim 11 characterized by having a 35-80 or 20-35 mass percentages of carbon content.

18. The crosslinking agent in Claim 11 characterized by being at least one of the borax, A/,/V'-methylenebis(acrylamide) (MBA), ethylene glycol diacrylate (EGDA) or PEG diacrylate (PEGDA).

19. The method for preparing a shungite based hydrogel to remove rare earth elements (REE³⁺) from an aqueous media mentioned in any above Claims characterized by having borax as a crosslinking agent with a ratio of 7-10% (w/w) when the PVA is used as a polymer.