REDUCTION OF CR(VI) IN THE PRESENCE OF EXCESS SOIL FULVIC-ACID

The rate of hexavalent chromium reduction by a soil fulvic acid (SFA) was measured in aqueous solutions where concentrations of Cr(VI), H+, and SFA were independently varied. Rates of reduction increase strongly with decreasing pH. Typical Cr(VI)-SFA reactions display a nonlinear reduction of Cr(VI) with time that cannot adequately be modeled by either first-order or second-order rate equations. An-empirical rate equation that treats the SFA as a continuum of reactive groups which reduce Cr(VI) at varying rates adequately describes the effects of solution parameters on the rates of Cr(VI) reduction. The rate equation is R = k(r)[HCrO4-][SFA][Cr(VI)](p)(O)[H+](q) where [Cr(VI)](O) is the initial Cr(VI) concentration. The rate coefficient (k(r)) varies with the fraction (X(e)) of SFA reducing equivalents oxidized and can be written as kX(e)(n), where k is (4.27 +/- 0.68) x 10(-7) L mol(-1) s(-1) and n is -0.94 +/- 0.04. The parameters p and q are 0.47 +/- 0.11 and 0.45 +/- 0.03, respectively. The rate equation is based on experiments where up to 5% of the SFA was oxidized. Experiments conducted at higher [Cr(VI)](0) show excellent agreement between experimental and modeled results even when 21% of the SFA is oxidized.