Effects of geochemical conditions, surface modification, and arsenic (As) loadings on As release from As-loaded nano zero-valent iron in simulated groundwater

Arsenic (As) released from As-loaded nano zero-valent iron (nZVI) after As(v) remediation could become an environmental hazard, thus deserving in-depth investigations. This study investigated As release from As-loaded nZVI under different geochemical conditions (i.e., pH, Ca2+, and humic acid (HA) concentrations), as well as the effects of nZVI surface modification (chitosan and polyaniline) and As loadings (low and high) on As release. Alkaline condition (pH = 9) and the presence of humic acid were found to enhance As release from all the types of nZVI samples. Ca2+ had no effect on As release for all the types of nZVI at both As loadings, except for chitosan- and polyaniline-modified nZVI at high As loading when Ca2+ concentrations increased up to 16 mg L-1. At low As loading, surface modification had a limited impact on As release from As-loaded nZVI. When As loading was high, a higher percentage of As was released from pristine nZVI than those from both the types of modified nZVI. This is because pristine nZVI either formed only monodentate complexes or had a higher ratio of As(iii)/As(v). Under the same geochemical conditions, As released from all the types of As-loaded nZVI at high As loadings was lower than that at low As loadings. This is due to the formation of bidentate complexes with stronger bindings and lower ratios of As(iii)/As(v) for high As loadings. These findings provide a fundamental understanding regarding the environmental behaviors of As-loaded nZVI under natural conditions and the risk associated with the release of As from As-loaded nZVI.