The distribution, solid-phase speciation, and desorption/dissolution of As in waste iron-based drinking water treatment residuals

Abstract

Arsenic concentrations and solid-phase speciation were assessed as a function of depth through Fe-media beds for two commercially available products (Granular Ferric Hydroxide ®—GFH and Bayoxide E33 ®—E33) from pilot-scale water treatment field tests. These results were compared with data from solution (de-ionized water—DI-H 2O) concentrations of As equilibrated with Fe-media in an anoxic environment at 4 °C. The materials had a high capacity for As (GFH media 9620 mg kg −1 As, E33 Media 5246 mg kg −1). Arsenic concentrations decreased with bed depth. For E33, X-ray absorption near-edge spectroscopy results showed that As(V) was the dominant solid-phase species. For GFH, As(III) was detected and the proportion (relative to As(V)) of As(III) increased with bed depth. Arsenic concentrations in DI-H 2O equilibrated with the media were low (⩽35 μg l −1) over a period of 50 d. Arsenic concentrations in the equilibrated solutions also decreased with depth. Results from tests on soluble As speciation show that As in solution is in the form of As(V). Kinetic desorption experiments carried out at different pH values (3, 5, 7, 8, and 9) show that the media exhibit some acid/base neutralization capacity and tend to bind As sufficiently. Concentrations of As in the pH desorption experiments were in the same order of magnitude as the toxicity characteristic leaching procedure extractions (tens of μg l −1) except at low pH values. For the GFH media tested at a pH of three, As increases in solution and is mainly associated with colloidal (operationally defined as between 0.1 and 1.0 μm) iron.