Solubility of lanthanide arsenates in aqueous, sulfate and chloride solutions – an alternative for arsenate disposal

This is an investigation into the solubility of synthetic lanthanide arsenates (La, Ce, and Nd) under various hydrometallurgical conditions, to evaluate the potential for lanthanides to be used for arsenic immobilization. Fresh lanthanide arsenate and calcined (800 °C in air) precipitates were synthesized and tested in aqueous, sulfate ([SO 4 ] T = 1 mol L -1 ), and chloride ([Cl] T = 1 mol L -1 ) solutions at 25°C, across pH ranges of 1.0-5.5 for fresh precipitates and 1.0-4.5 for calcined samples. The results demonstrate that lanthanide arsenates exhibit low solubility (logK s = -23.10, -23.33, and -22.44, for LnAsO 4 (Ln = La, Ce, and Nd, respectively) across all tested conditions. In aqueous solutions, arsenic activity plateaus were observed due to surface adsorption effects, particularly pronounced in fresh precipitates with high specific surface area (~100 m 2 g -1 ) compared to calcined samples (~0.1 m 2 g -1 ). Chemical thermodynamic calculations correlated well with experimental data, with calculated Gibbs free energy of formation value variation within ±2% as a function of pH. Surface charge analysis showed positive zeta potential values with isoelectric points at pH 4.3-4.7, favouring arsenic adsorption. Sulfate solutions increased arsenate solubility by 1.5-3.0 times compared to aqueous solution, with formation of minor hydroxy-sulfate arsenate phases at low pH. Most significantly, chloride solutions demonstrated the greatest stabilizing effect, reducing solubility by 5.0-15.0 times compared to aqueous and sulfate systems, respectively. The study provides chemical thermodynamic data previously unavailable for hydrated lanthanide arsenate phases, with ∆G f ° = -1686.83 kJ mol -1 , -1676.67 kJ mol -1 , and -1667.58 kJ mol -1 , for LnAsO 4 ·nH 2 O (s) (Ln = La, n = 1.0, Ln = Ce, n = 1.1, and Ln = Nd, n = 1.2, respectively) at temperature of 25 ± 2 °C.