Kinetics of Cu(II) Reduction
by Natural Organic Matter
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Abstract
The kinetics of Cu(II) reduction by Suwannee River fulvic
acid
(SRFA) at concentrations from 0.25 to 8 mg L<sup>–1</sup> have
been investigated in 2 mM NaHCO<sub>3</sub> 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<sup>–1</sup> [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<sup>–1</sup>. 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<sup>–4</sup> equiv of e<sup>–</sup> (g of SRFA)<sup>−1</sup>) that reduced Cu(II) relatively
rapidly and a less labile donor (with a concentration of 3.1 ×
10<sup>–4</sup> equiv of e<sup>–</sup> (g of SRFA)<sup>−1</sup>) that reduced Cu(II) more slowly