ARSENIC REDUCTION BY SULFIDE WITH ACTIVATED CARBON, FE(II), AND CITRATE

The rate and mechanism of reaction between arsenate and sulfide were measured as a function of pH, Fe(II), citrate, granular activated carbon (GAC) or graphite, and after various procedures for removal of chemi-sorbed O2 and oxidizing sites from GAC and graphite. The study was motivated by our observations that Fe-tailored GAC was significantly more effective than other Fe(III) (hydr)oxides, for removal of As(V) from water. The removal of As increased significantly after the tailored GAC was allowed to “rest”. It was hypothesized that sulfur and other functional groups in GAC were competent reducing agents for As(V), resulting in precipitation of As(III) sulfides such as realgar or orpiment. Reactions proceeded rapidly at pH 4 in the absence of the additives; arsenate initially complexed with sulfide to form thioarsenate species, consistent with literature reports, prior to reduction to As(III). Addition of Fe(II) resulted in a longer lag prior to initial reduction to As(III). Addition of GAC or graphite inhibited reduction to As(III) even at pH 4. Sequential degassing did not result in increased rate of reduction. A mechanism for reduction of As(III) was proposed that involved inhibition of thioarsenate formation by oxygen. We are now producing high-S GAC and also pre-treating GAC with reducing agents, using an O2 trap inside an anaerobic chamber in order to avoid traces of O2. It was also hypothesized that reducing conditions within GAC could lead to formation of nano-particle magnetite. Column and batch experiments both demonstrated that magnetite could be formed due to reducing conditions in GAC; adsorption of arsenate was much greater than for other iron (hydr)oxides.