Enhanced degradation of roxarsone and removal of total arsenic by zero-valent iron with ligand under aerobic condition

The slow rate of oxidative destruction of organoarsenics and the inadequate immobilization of produced inorganic arsenic are the bottlenecks that must be overcome for full remediation of organoarsenics from environmental matrix. Herein, roxarsone was treated by zero-valent iron (ZVI) with ligands under aerobic condition. We found that two typical ligands ethylenediamine tetraacetate (EDTA) and diethylenetriamine pentaacetate (DTPA) could simultaneously enhance the degradation of roxarsone and the removal of total arsenic by ZVI under aerobic condition. After 180 min reaction, the removal ratio of roxarsone reached 99.5 % and 98.9 % in the ZVI/EDTA/Air and ZVI/DTPA/Air respectively. The roxarsone removal rate constant in the ZVI/EDTA/Air system and ZVI/DTPA/Air system were calculated to be 0.030 min−1 and 0.025 min−1 respectively, which was about 10 times higher than that in the ZVI/Air system (0.0032 min−1). At the same time, negligible organic intermediates and inorganic arsenic (As(III) and As(V)) were retained in water after the treatment. With the pH increased from 3 to 7, the removal ratio of roxarsone decreased from 99.5 % to 72.3 % and 98.9–64.5 % in ZVI/EDTA/Air and ZVI/DTPA/Air respectively. Quenching experiments confirmed that both the ZVI reduction and •OH oxidation contributed to the degradation of roxarsone. X-ray photoelectron spectroscopy (XPS) characterization confirmed the generated inorganic arsenic was finally co-precipitated with iron oxyhydroxides. This study suggests ligands enhanced molecular oxygen activation with ZVI is a promising approach for efficient abatement of organoarsenic caused water pollution in view of its high efficiency, low cost and simple operation.