APPLICATION OF ION EXCHANGE RESINS TO RECOVER URANIUM FROM ACID MINE DRAINAGE.

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1 Proceedings of the 13 th International Conference of Environmental Science and Technology Athens, Greece, 5-7 September 2013 APPLICATION OF ION EXCHANGE RESINS TO RECOVER URANIUM FROM ACID MINE DRAINAGE. A.C.Q. LADEIRA 1 and L.C. SICUPIRA 1. 1 Centre for Development of Nuclear Energy, Department of Mineral Processing, Av. Antônio Carlos 6627 CEP Belo Horizonte MG - Brazil ana.ladeira@cdtn.br ABSTRACT Acid mine drainage (AMD) can be considered one of the greatest and most technically challenging issues of the mining industry environment. It is caused by the oxidation of sulfites that produce acidity which, consequently, dissolves metals from the soils. Once favorable conditions to AMD have been created, significant impacts may persist for hundreds of years and may be very expensive to stop. The main impacts can be contamination of waters, sediments and soils. In Brazil, AMD has been prominent in one uranium mine whose activities have ceased for over twenty years. The acid waters have ph around 3 and contain uranium (8 to 11mg/L), besides other elements such as sulfate (1350 to 2000mg/L). Uranium is mainly in the form of UO 2(SO 4) 3-4. The presence of uranium in acid mine drainage is of special concern if environment and health issues are to be considered. The aim of this study is to recover the uranium present in two acid mine waters (effluent A and B) from this mining using three different strong base resins (Amberlite IRA910U, and ) through column experiments. Elution was also investigated by using different solutions, i.e. NaCl and H 2SO 4, to determine the influence of competitive anions such as iron. The loading capacity of the resins was determined and presented the highest capacity for uranium, i.e., mg/g, for effluent A and mg/g for effluent B. Elution, carried out in two steps, extracted up to 99.7% of uranium from for effluent A; 6% corresponds to uranium eluted in the initial stage for iron removal. Keywords: uranium, ion exchange resins, column experiments, elution 1. INTRODUCTION Acid mine drainage (AMD) is present in most sectors of the mining industry, including coal, precious metals, base metals, uranium and industrial minerals. It can be considered one of the most detrimental effects of mining on the environment and also one of the most technically challenging. Significant impacts may persist for hundreds of years and may be very expensive to stop. The main impacts can be contamination of waters, sediments and soils. In the past, mining activities that damaged ecosystems and impacted heavily on communities were largely condoned. Today, poor practice cannot be tolerated if mining is to be sustainable. In the southeast of Brazil, AMD has been prominent in the Plateau of Poços de Caldas, more specifically, in one uranium mine whose activities have ceased for over twenty years. Two main waste rock piles, totaling 33 million tons of sulfidic material, promote the acidification of seepage waters. The waters are collected in two distinct dams (figure 1).

2 A B Figure 1: The dam in the forefront and the waste rock #4 in the background (A). The dam in the forefront and the waste rock #8 in the background (B). Those acid waters contain, besides other elements, uranium in concentrations around 10mg/L. Among the many proven technologies for removing different types of dissolved substances, the ion exchange process is one of the most appropriate for the removal of uranium present in low levels (Ladeira and Gonçalves, 2007, 2008). In recent years, much attention has been given to the development of processes for the treatment of AMD. Due to stricter legislation, more effective treatments based on adsorption and ion exchange techniques have been used in order to meet the low concentration levels required by law. According to Katsoyiannis (2007), only anionic exchangers and titanium dioxide are capable of removing uranium to levels below1 μgl -1. Several studies such as Ladeira and Morais, 2005a and b, Braun et al.,2008, Katsoyiannis, 2007 and Mohan and Pittman, Jr., 2006, refer to adsorption of metals, where a variety of adsorbents with different prices have been assessed according to its capacity of removing metal ions, including uranium. The aim of this study is to recover uranium from two acid mine waters using three different strong base resins. Column experiments were carried out to determine the uranium loading capacity of the resins. Elution was also investigated to determine the influence of competitive anions such as iron in the final recovery. The samples used were acid mine waters. The recovery of uranium from the acid waters has the advantage of generating a product of significant commercial value and reducing the concentration of this metal in the final effluent to the limit set for discharging, thus preventing human contamination due to uranium chemical toxicity.

3 2. METHODOLOGY Liquid Samples: Two samples, identified as A and B, were supplied by Indústrias Nucleares do Brasil S.A and were collected at two different water dams located at the old uranium mine on the plateau of "Poços de Caldas" as seen in figure 1. Chemical Analyses: Uranium in concentrations > 10mg.L -1 was determined by X-ray fluorescence KEVEX-RAY (model SIGMAX-9050) and for concentrations < 10 mg.l -1 two techniques were employed: inductively coupled plasma mass spectrometry (ICP- MS)/SERTA and neutron activation technique by using a Triga Marki IPR-R1 reactor. Total iron was determined by atomic absorption spectrometry VARIAN, model AA240FS. Sulfate was analyzed by energy dispersive X-ray technique using Shimadzu model EDX Resins: The resins were strongly basic and were used in the form of chloride and sulfates. Table 1: Anion-exchange resins and their main properties. Commercial Name Matrix Functional Group Density (g/ml) Amberlite Polystyrene RA910U reticular Dowex MSA-2 Dowex TM RPU Copolymer styrene -divinyl benzene Copolymer styrene -divinyl benzene Benzyldimethyl ethanolamine Dimethyl ethanolamine Quaternary Amine Grain size (mm) ,70 to < 95% < <85% <0.84 Column Experiments: Adsorption and elution experiments were carried out in glass columns with diameter of 16mm and 75mm high (ratio high/diameter equal to 4.7). The columns were filled with 15mL (1 bed volume) of wet resins which correspond to g of Amberlite IRA910U, g of and g of Dowex TM RPU. The operation was performed by downstream flow using a peristaltic pump (masterflex, model ) at a constant flow rate of 2.5 ml/min (10BV/h) which implies in a residence time of 6min. For the adsorption tests the effluents A and B were used, whose uranium content is 8mg/L and 11 mg/l, respectively, temperature 25 o C and ph around 3. Elution experiments for the resins loaded with effluent A were carried out in two steps. The first one with H 2SO 4 0.1mol.L -1, aiming at removing the adsorbed iron. The second one with a solution of NaCl 1.5mol.L -1 and H 2SO mol.L -1. For the resins loaded with effluent B, only the NaCl 1.5mol.L -1 solution was used in the elution. 3. RESULTS AND DISCUSSION 3.1. Chemical Characterization of the liquid samples The two acid water samples were chemically characterized for the most important contaminants and the results are presented in table 2. Besides uranium, sulfate appears as the predominant ion. In general, uranium in acidic medium is found in a hexavalent state as UO However, it is well known that it forms complexes with a variety of ligands such as CO 3 2-, PO 4 3 and SO 42. In these acidic waters, where sulfate is present in concentration higher than 2.0 gl -1, uranium forms complexes with sulfate the main species being [UO 2(SO 4) 3] 4-. Uranium content, in an order of milligrams per liter (8 to 11mg/L), can be recovered by ion exchange and the final concentrate can reach grams per liter (1 g/l) i.e., a hundred times more concentrated than the feed.

4 Table 2: Chemical analyses for effluent A and B. Sample U (mg/l) Fe (mg/l) 2- SO 4 ph (mg/l) Effluent A Effluent B Effluent permissible limit ND 15 ND 6.0 to 9.0 Water permissible limit to 9.0 ND = not determined by Brazilian legislation, Conama n 0 35, mar According to table 2 which shows the permissible limit for effluent discharging, the ph values are the only parameters which do not comply with the legislation. However, taking into account the permissible limit for surface waters, the sulfate and the uranium are well above what is determined by the legislation, thus making these effluents potentially harmful to the environment. In the case of the ion exchange process, it is important to observe the presence of competitor ions that have a deleterious effect. Sulfate, for example, in high concentrations, as the ones found in the acidic waters (1.3 to 2.0g/L), compete with the uranium by occupying the specific sites for uranium hindering its adsorption. Similarly, the soluble iron in the form of ([Fe(SO4)n]) (3-2n), despite being in low concentration, can be adsorbed by the resins which could cause the contamination of the uranium product. In addition, the presence of iron can cause technical problems such as the precipitation of iron hydroxide (Fe(OH) 3) in the column 3.2. Column Experiments Column experiments were carried out in order to assess the main parameters of the ion exchange process. Figures 2 and 3 show the adsorption profiles for the effluents A and B where the uranium concentration in the outflow is plotted against cumulative bed volume (BV). Bed volume refers to the volume of solution equivalent to the resin volume in place. The loading capacities were calculated by integrating the area above the curves in Figs. 2 and 3, and represent the maximum amount of solute the column can store. 9 Uranium in the outflow (mg.l -1 ) Amberlite IRA910U Figure 2: Adsorption profiles for three different resins for effluent A. Flow rate = 2.5mL.min -1, temperature 25 o C. Resins activated with sodium chloride.

5 Uranium in the outflow (mg.l -1 ) 12 Amberlite IRA910U Figure 3: Adsorption profiles for three different resins for effluent B. Flow rate = 2.5mL.min -1, temperature 25 o C. Resins activated with sodium chloride. The breakthrough points for the experiments with effluent A (figure 2) for the resins Amberlite IRA-910U, and are 2000 BV, 2750 BV and 1900 BV, respectively. This point represents the BV in which the uranium is first detected in the outflow solution. In the present study the detection limit of the uranium chemical analyses was 1mg.L -1. The saturation point, defined as the BV in which the uranium concentration in the outflow is equal to the initial uranium concentration in the feed (inflow solution), i.e., 8 mg.l -1 was reached at 3240 BV, 4320 BV and 3000 BV for Amberlite IRA-910U, Dowex MSA-2 and, respectively. The maximum amount of uranium stored by each column is: , and mg, respectively. Figure 2 shows that the resin presents a better adsorption profile while the profiles for and IRA 910U are very similar. The maximum loading capacities for effluent A, for the resins Amberlite IRA-910U, and determined as the area above the curves, are: 59.00, and 43.91mg.g -1, respectively. For the experiments with the effluent B, as seen in figure 3, the breakthrough point for the resins Amberlite IRA-910U, and was 800 BV, 1200 BV and 1200 BV, respectively. The saturation point was 2400 BV and 3600 BV for the Amberlite IRA910U and. However, the saturation point for the resin was not achieved once the column was blocked, maybe because of the precipitation of iron hydroxide and, consequently, the feed of column had to be discontinued. The maximum amount of uranium stored was and mg for Amberlite IRA-910U and Dowex MSA-2, respectively, which corresponds to the maximum loading capacity of and mg.g -1.Over again, the performance of the resin was superior. It is interesting to observe that the results for the effluent A and effluent B are rather different (figure 2 and 3). The maximum loading capacity of the resins when using the effluent B was 60 % higher (approx. 40mg.g -1 ), that is explained in part because of the lower uranium concentration in solution A (8mg.L -1 ) than in solution B (11mg.L -1 ). Therefore, the difference of the uranium concentration in the liquid phase and the solid phase for effluent B is higher. The increase of the driving force intensifies the mass transfer as for the same mass of resins there is a greater amount of uranium available for the ion exchange. In addition, the concentration of sulfate in solution B (1350 mg/l), considered the major interfering ion on the process, is lower than the one presented by solution A (2070 mg/l).

6 After the saturation of the columns, the resins were submitted to elution with a solution of sulfuric acid 0.1mol.L -1 and another solution of sulfuric acid 0.05mol.L -1 and sodium chloride 1.5mol.L -1. The former was used in the first step to elute the adsorbed iron and the latter to elute the uranium. The elevate concentration of Cl -1 in the solution used for elution is necessary as to compensate the lower affinity of the resins with this kind of ion compared to the uranium ions. In order to assess the uptake of iron by the resins, the elution was carried out with a sulfuric acid (0.1mol.L -1 ) and the spent eluent was analyzed for total iron. The iron competed with the uranium and decreased the loading capacities of the resins as it occupies the sites of the resins available for the uranium. Figure 4 shows the iron elution for the experiments with effluent A, which presents an iron content slightly more elevated than effluent B, i.e., 2mg/L. Fe total in the spent eluent (mg.l -1 ) Amberlite IRA910U (A) Uranium in the spent eluent (g/l) 3.0 (B) Amberlite IRA910U Figure 4: First step elution profiles for 3 different resins for experiments with effluent A. (A) Iron profile, (B) Uranium profile. Flow rate = 2.5mL.min -1, temperature = 25 o C, sulfuric acid 0.1mol.L -1. According to figure 4, the uptake of iron is evident. However, it is observed that the behavior of the resins is not similar. presented the greatest amount of eluted iron (Figure 4(A). Although the aim of the first step was the elution of iron, it is observed from figure 4 (B) that a significant portion of uranium was also removed in this step for the resin IRA 910U. The concomitant elution of iron and uranium presented by this resin is impeditive because iron is a contaminant in the product. The other two resins presented a better performance and very little uranium was eluted in the first step. Up to now, the best resin is, which presents low uranium and low iron elution in the first step. The subsequent step was the elution of uranium as shown in Figure 5.

7 Uranium in the spent eluent (g/l) Amberlite IRA910U (A) Uranium in the spent eluent (g/l) Dowex MSA Amberlite IRA 910 (B) Figure 5: Uranium elution for 3 different resins. (A) Experiments with effluent A; (B) Experiments with effluent B. Flow rate = 2.5mL.min -1, temperature = 25 o C. Elution with solution of sodium chloride 1.5mol.L -1 and sulfuric acid 0.05mol.L -1. Ion exchange is a reversible process, so uranium was expected to be easily stripped from the resins when submitted to favorable conditions. The anion exchanger was eluted with chloride; however, given the high selectivity for the tetravalent uranium [UO 2(SO 4) 3-4 ], an excess of chloride several times the stoichiometry quantity is required. The results depicted in Figure 5 show that the exchangers were converted from the uranium form to the chloride form. The percentage of uranium stripped from the resins Amberlite IRA- 910U, and for the experiments with effluent A was 89.70, and 99.7%, respectively. For assays with effluent B desorption was 86.6, 94.1 and 92.7%, respectively. Ladeira and Morais, 2005a reported desorption percentage of uranium from 98 to 99% using ammonium carbonate as eluent, from the resin IRA 910U and effluent A. The results are similar, since in the present study 99.7% and 94% of overall recovery was achieved. The resins IRA-910U and MSA are effective and similar in the uptake, however the uranium elution from MSA for experiments with effluent A was considerably lower (70.15%). Moreover, the content of sulfate in effluent A is two times higher than what is found in effluent B and it probably affects the performance of the resins since the results for effluent B for all resins are much better. This fact should be assessed in depth. Table 4 summarizes the data for loading and elution for the three resins. Table 4: Results of loading capacity and % of elution for all resins Resins IRA 910 U Dowex MSA Loading Capacities (mg/g) Effluent A / Effluent B / / / -- % of elution 89.7/ / / CONCLUSIONS The use of ion exchange resins for the recovery of uranium from acid mine drainage is promising. By continuous column experiments with solution A, the anionic resin Dowex MAS-2 showed the highest maximum loading for uranium, 63.6 mg g -1. Also, for the experiments with solution B, showed the best result, i.e., 109mg.g -1. The difference between the experiments with solution A and B occurs because the latter has a

8 higher concentration of uranium, which favors the mass transfer from the liquid phase to a solid one. In addition, effluent A presents a higher concentration in sulfate which inhibits the process by competing with the uranium. The presence of iron in the effluent A requires a pre-elution of this species with a slightly acidic solution before the stripping of the uranium. The percentage of uranium eluted was no higher than 99.7% for the resin in experiments with effluent A. For effluent B, elution was at most 94.1% when using. According to the results presented here, it can be stated that it is possible to recover the uranium present in the acid mine drainage with ion exchangers, which makes this method recommended for the acid mine drainage from the Poços de Caldas Plateau. Acknowledgements: The authors would like to thanks Fapemig (PPM and INCT-Acqua) and CNPq (INCT- Acqua) for financial support and INB for supplying the samples. 5. REFERENCES 1. Braun, L., Märten, H., Raschke, R., Richter, A., Sommer, K., Zimmermann, U., Flood Water Treatment at the Former Uranium Mine Site Königstein a Field Report., CONAMA, Conselho Nacional do Meio Ambiente, Resolucão n 357, 17 março Brasília: Ministério do Meio Ambiente, MMA. Available: 3. Katsoyiannis, I. A., Carbonate effects and ph-dependence of uranium sorption onto bacteriogenic iron oxides: kinetics and equilibrium studies, J. Hazard. Mater, Ladeira A.C.Q., Gonçalves C.R., Influence of anionic species on uranium separation from acid mine water using strong base resins. J. Hazard. Mater. 148, , Ladeira, A. C. Q., Gonçalves, C. R., Uranium recovery and manganese removal from acid mine drainage. WIT Transactions on Ecology and the Environment. v. 111, p.wp080451, Ladeira, A. C. Q., Morais, C. A., Uranium recovery from industrial effluent by ion exchange column experiments, Miner. Eng. 18, Ladeira, A. C. Q., Morais, C. A., Effect of ammonium, carbonate and fluoride concentration on the uranium recovery by resins. Radiochim. Acta 93, , Mohan, D., Pittman Jr, C. U., Activated carbons and low cost adsorbents for remediation of triand hexavalent chromium from water. Journal of Hazardous Materials B137, , 2006.