Calcite-Induced Epitaxial Crystallization of Ca Arsenite Enhances As(III) Stability

Calcite plays a significant role in regulating As speciation in natural and engineering systems. However, known processes, including surface adsorption and probable structural incorporation, likely underestimate the effect of calcite-induced surface reactions on the As fate. This study reveals the epitaxial crystallization of calcium arsenite (CaHAsO₃) on calcite surfaces, which is responsible for the enhanced As(III) stability caused by calcite during lime interaction with As(III). SEM images showed that Ca arsenite grew in a "standing" geometry on the calcite (104) face and appeared in order as micrometer-sized, well-crystallized platy crystals, rather than poorly crystallized nanosheets formed in the absence of calcite. The higher crystallinity/order and larger size of the epitaxial Ca arsenite led to its lower As(III) solubility. By electron diffraction analyses, lattice matching between calcite and Ca arsenite was evidenced. Despite the lack of Ca arsenite's crystallographic data, the epitaxial relationship was determined as (hkl)_d=3.05Å// (104)_calcite; [uvw]_{Ca-As-Ca PBC}// [-441]_calcite, where PBC stands for periodic bond chain. In contrast, no epitaxial crystallization of Ca arsenate minerals was observed on calcite surfaces; thus, the presence of calcite had little effect on As(V) stability. Our findings improve the understanding of calcite's role in affecting As speciation and broaden the application of calcite-containing materials in environmental remediation.