THORIUM REMOVAL FROM RARE EARTH INDUSTRIAL RESIDUE USING NATURAL AND MODIFIED ZEOLITES

Abstract

The rare-earth industries produce a significant quantity of radioactive residue that contains elevated thorium concentrations. Therefore, thorium removal from radioactive waste is regarded as a crucial issue in the treatment of such waste due to its radiotoxicity, chemical toxicity, and long half-life, which could potentially have adverse effects on the environment and human health. Furthermore, the removal of thorium from radioactive residues using appropriate treatments can reduce the amount of waste that needs to be disposed of. This study investigates the removal of thorium ions from synthetic thorium solutions and industrial rare earth residue known as Water Leach Purification (WLP) using natural and modified zeolites. The natural zeolite (clinoptilolite) was modified with sulphate and phosphate anions. The physical and chemical characterization of natural zeolite (NZ), phosphate-modified zeolite (PZ), and sulfate-modified zeolite (SZ) was conducted using various characterization techniques. Experiments were conducted in the laboratory to assess the suitability of the examined adsorbent materials for the removal of thorium. Adsorption of thorium from the aqueous solutions was investigated via a batch method. Furthermore, isotherm models, kinetic models, and other experimental parameters that affect the adsorption process were also investigated. The successful modification of NZ with sulfate and phosphate anions was confirmed through shifts observed in the Fourier-transformed infrared (FTIR). Furthermore, the energy dispersive X-Ray (EDX) analysis results unequivocally verified the presence of sulfate and phosphate elements in the structures of SZ and PZ, respectively. Findings proves that the modification to the surface of NZ has been successfully carried out. The modification of NZ reduced its specific surface area but increased its thorium adsorption capacity. The obtained results from the adsorption of thorium ions on NZ, PZ and SZ demonstrated that the investigated adsorbents were capable of removing thorium ions from aqueous solutions. The results indicated that the experimental maximum adsorption capacities of NZ, PZ, and SZ for thorium are found to be 12.7, 17.4, and 13.8 mg/g, respectively. The analysis of isotherms indicated that the Langmuir isotherm model provided the best fit to the experimental data. The modification of NZ with sulfate anions did not lead to a notable enhancement in its thorium ion adsorption capacity. In contrast, the adsorption capacity was significantly improved when it was modified with phosphate anions. The effect of experimental variables, including contact time, mass of the adsorbent, initial thorium concentration, and pH level of the solution, was scrutinized utilizing the batch mode method to identify the optimal adsorption conditions. The findings revealed that the best parameters for thorium adsorption included a 24-hour contact time, 0.03 grams of PZ, a pH of 3, and a temperature of 25°C. Studies on the kinetics of adsorption showed that the thorium adsorption onto PZ was a good fit for the pseudo-second-order model. The application of PZ to treat actual industrial rare earth residue (WLP) under optimized conditions demonstrated that almost complete thorium removal (> 99%) was achieved from the leached WLP solution. The results obtained indicated that PZ proved to be an exceptionally effective adsorbent material for thorium removal from real industrial rare earth residue using the adsorption method, leading to a decrease in waste volume for final disposal.