Reactivity of ferric oxides toward H2S at low pH.

Goethite (gt), 2-line (21fh), 6-line ferrihydrite (61fh), and schwertmannite (shm) were reacted with H2S under steady-state conditions at pH 2.5-5 using a novel setup that consisted of an electrochemical sulfide generator transporting H2S into a suspension of oxides in a fluidized bed-reactor. The reactants were stoichiometrically converted into Fe2+ and So. Sulfate significantly inhibited the rates. Surface area normalized rates increased with pH. They depended on the mineral type following the order gt > 21fh > 61fh, contrasting previous observations at circumneutral pH with an inverse order. The rate of shm dissolution was highest probably due to direct reduction of dissolved ferric iron by H2S. A linear relationship between the observed rate and the surface species of H2S was derived from a surface complexation approach that allowed for the estimation of intrinsic rate constants kintr for the various oxides (38 min(-1), 1.4 min(-1), 0.29 min(-1) for gt, 21fh, and 61fh, respectively). kint decreased with increasing deltaGf for the reactions and seems to depend on the reduction potential of the solution. From this observation we derived the hypothesis that kint is determined by the flat-band potential of the mineral/solute interface at low pH while being affected by surface interactions at higher pH.