Arsenic(III) Oxidation by Iron(VI)\n(Ferrate) and Subsequent Removal\nof Arsenic(V) by Iron(III) Coagulation

Abstract

We investigated the stoichiometry, kinetics, and mechanism\nof arsenite [As(III)] oxidation by ferrate [Fe(VI)] and\nperformed arsenic removal tests using Fe(VI) as both an\noxidant and a coagulant. As(III) was oxidized to As(V)\n(arsenate) by Fe(VI), with a stoichiometry of 3:2\n[As(III):Fe(VI)]. Kinetic studies showed that the reaction of\nAs(III) with Fe(VI) was first-order with respect to both\nreactants, and its observed second-order rate constant at\n25 °C decreased nonlinearly from (3.54 ± 0.24) × 10⁵ to\n(1.23 ± 0.01) × 10³ M^-1 s^-1 with an increase of pH from 8.4\nto 12.9. A reaction mechanism by oxygen transfer has\nbeen proposed for the oxidation of As(III) by Fe(VI). Arsenic\nremoval tests with river water showed that, with minimum\n2.0 mg L^-1 Fe(VI), the arsenic concentration can be\nlowered from an initial 517 to below 50 μg L^-1, which is\nthe regulation level for As in Bangladesh. From this result,\nFe(VI) was demonstrated to be very effective in the\nremoval of arsenic species from water at a relatively low\ndose level (2.0 mg L^-1). In addition, the combined use\nof a small amount of Fe(VI) (below 0.5 mg L^-1) and Fe(III)\nas a major coagulant was found to be a practical and\neffective method for arsenic removal.