Mechanisms of arsenic retention on titanosilicate sitinakite: A spectro-computational study

Arsenic mobility in natural waters is strongly governed by mineral–water interfacial processes; however, the role of redox-inert titanosilicates remains poorly constrained compared with that of iron and manganese oxides and hydroxides. Here, batch adsorption experiments conducted in deionized, tap, and lake waters, combined with synchrotron-based spectroscopy and density functional theory (DFT) calculations, were employed to elucidate the retention of arsenic by sitinakite (Na 2 Ti 2 SiO 7 ·2H 2 O). Rapid arsenic uptake was observed, reaching equilibrium within 120 min, with a maximum capacity of 49.46 mg·g −1 derived from the Langmuir model. At an environmentally relevant concentration of 1.0 mg·L −1 , the adsorption capacities were 0.39, 0.36, and 0.32 mg·g −1 in deionized, tap, and lake waters, respectively, while trace-level experiments (10–100 μg·L −1 ) yielded distribution coefficients (K d ) of 0.333–0.479 L·g −1 . X-ray absorption spectroscopy (XAS) confirmed that adsorbed arsenic remains pentavalent, and EXAFS fitting indicated tetrahedral As–O coordination without a resolvable, well-ordered As–Ti/Si second shell. DFT calculations supported hydrated outer-sphere configurations stabilized by electrostatic interactions and hydrogen bonding, whereas the Ti-associated inner-sphere configuration was less favorable within the adopted protonated-channel model. Together, these results reveal a titanosilicate-mediated pathway for arsenic immobilization with implications for its cycling and treatment design.