Immobilization of low-concentration uranium by mineralization with phosphates: dissolution stability of meta-autunite products in the presence of coexisting anions.

The process and mechanism of low-concentration U(VI) immobilization by mineralization with phosphates were investigated. Under optimized conditions (initial U concentration of 50 μg/L, seed crystal dosage of 0.5 g/L, Ca:U:P molar ratio of 50:1:10, pH of 6.0, temperature of 25°C, and reaction time of 30 min), immobilization efficiency reached 92%, with residual U of 4 μg/L (below WHO's 30 μg/L). Multi-scale characterizations (XRD, SEM, TEM, SAED, and HRTEM) substantiated a seed-induced heterogeneous nucleation mechanism, leading to the formation of meta-autunite (Ca(UO2)2(PO4)2·3H2O) through Ca-P-U synergistic complexation. Systematic investigations on the effects of environmental anions (Cl-, SO42-, HCO3- introduced as ammonium salts) and cations (NH4+) on product stability demonstrated that Cl- induced congruent dissolution without altering the crystalline structure. SO42- exhibited a concentration-dependent effect: it initially facilitated dissolution, followed by re-precipitation accompanied by partial amorphization. Notably, HCO3- elevated the pH and accelerated the dissolution of meta-autunite, and the released U(VI) subsequently formed stable carbonate complexes, namely [UO2(CO3)3]4- and [UO2(CO3)2]2-. In the presence of NH4+, [UO2(CO3)3]4- further precipitated as ammonium uranyl carbonate ((NH4)4UO2(CO3)3) on the mineral surface. This dissolution-complexation-remobilization pathway reveals a key mechanism for secondary mineralization in high-carbonate environments, advancing both uranium wastewater treatment and understanding of U geochemical behavior.