Crystallinity and dissolution-recrystallization mechanism controlled As(V) retention by calcium phosphate

Retention of toxic metals/metalloids like arsenic via mineral-water interaction plays a crucial role in the environmental behavior of pollutants. However, the influence of mineral crystallinity on the retention of toxic elements, the evolution of liquid composition, and the interaction mechanism are poorly understood. This study investigated the interaction between As(V) and calcium phosphate (CaP) under oxic conditions with varying crystallinities, particularly amorphous CaP (ACP), across varying As(V) concentrations and pH conditions. Results revealed that the amorphous phase substantially influenced As(V) fate, with the As(V) retention potential of ACP and poorly crystalline hydroxylapatite (HAP) being 13.65 and 12.61 times higher than highly crystalline HAP, respectively. As(V) retention involves the dissolution of ACP and the recrystallization of As(V)-substituted HAP, correlated with three distinct ACP transformation stages during recrystallization. The lower pH (7.5) facilitated ACP dissolution, and the elevated Ca²⁺ concentration enhanced the volume of CaP recrystallization. Conversely, higher pH levels (8.0, 8.5, and 9.0) promoted a higher degree of recrystallization, evidenced by reduced residual Ca²⁺ levels after 48 hrs (post-crystallization stage). Meanwhile, As-bearing CaP forms with greater competition between PO₄^3- and AsO₄^3- at higher initial As(V) concentrations than lower ones. Additionally, lattice distortion, increases in species of surface bond groups, and reduced crystallinity were observed in the As(V)-bearing CaP product. Overall, this study underscores the pivotal role of ACP and its poorly crystalline counterparts in arsenic retention through the dissolution-recrystallization mechanism.