Kinetics of Cu(II) Reduction by Natural Organic Matter

The kinetics of Cu­(II) reduction by Suwannee River fulvic acid (SRFA) at concentrations from 0.25 to 8 mg L^–1 have been investigated in 2 mM NaHCO₃ and 0.7 M NaCl at pH 8.0. In the absence of oxygen, SRFA reduced Cu­(II) to Cu­(I) in a biphasic manner, with initial rapid formation of Cu­(I) followed by a much slower increase in Cu­(I) concentration over time. When present, oxygen only had a noticeable effect on Cu­(I) concentrations in the second phase of the reduction process and at high [SRFA]. In both the absence and presence of oxygen, the rate of Cu­(I) generation increased with increasing [SRFA]. At 8 mg L^–1 [SRFA], nearly 75% of the 0.4 μM Cu­(II) initially present was reduced to Cu­(I) after 20 min, although the yield of Cu­(I) relative to [SRFA] decreased at [SRFA] > 1 mg L^–1. Two plausible kinetic modeling approaches were found to satisfactorily describe the experimental data over a range of [SRFA]. Despite some uncertainty as to which approach is correct, common features of both approaches were complexation of Cu­(II) by SRFA and reduction of Cu­(II) by two different electron donor groups within SRFA: a relatively labile electron donor (with a concentration of 1.1 × 10^–4 equiv of e^– (g of SRFA)⁻¹) that reduced Cu­(II) relatively rapidly and a less labile donor (with a concentration of 3.1 × 10^–4 equiv of e^– (g of SRFA)⁻¹) that reduced Cu­(II) more slowly