The effect of groundwater velocities on sulfidation of arsenic-bearing ferrihydrite: insight from column experiments

Abstract The sulfidation of Fe oxides/hydroxides has an important effect on the migration and transformation of As in groundwater. We examine the sulfidation of As (III)-bearing ferrihydrite and subsequent As repartitioning induced by sulfide in column experiments. Groundwater velocities has an obvious influence on sulfidation and the subsequent formation/transformation of secondary minerals. At a slower flow 1 pore volume·d-1), adsorbed As blocks the ferrihydrite surface, resulting in a slower sulfidation and thus slower generation of S0 and Fe2+. Mackinawite is subsequently generated by reaction of Fe2+ with excess sulfide and then reacts with S0 to form greigite. At a faster flow (5 pore volumes·d-1), stronger water elution results in desorption of much of the adsorbed As. Faster sulfidation produces considerable amounts of S0 and Fe2+ and restricts the formation of mackinawite. Fe2+ then adsorbs and catalyzes the conversion of residual ferrihydrite to hematite. The mackinawite also reacts with S0 to form pyrrhotite rather than greigite. Overall, heavy sulfidation does not effectively prevent the release of As caused by the reduction of ferrihydrite at either groundwater velocities. Sufficient As retention may be achieved by adjusting the hydrodynamic intensity of the system to determine the product of the sulfidation.