Iron and copper competitions during redox reactions in natural waters

Abstract

Laboratory and field work have demonstrated the relevance of trace metal interaction in the redox chem- istry of essential metals like copper and iron. However, the competition of both metals has not been studied in solution. The oxidation-reduction of both, copper and iron was studied under air-saturated conditions bubbling ultra-pure air thorough the solution for 1 h previous and during the experiments. The redox kinetics of copper was studied at constant pH and temperature and as a function of Fe concen- tration (0-200 nM) and H2O2 (0-300 nM). In the case of iron redox reactions, the competition between Fe(II) and copper species has been studied in seawater at different initial Cu(II) and Cu(I) concentration (0-200nM). In addition, the effect of pH (6.2-8.5), bicarbonate concentration (2-9 mM) and H2O2 concentration (0-500 nM) on the Fe(II) rate constant were also studied. The oxidation rate of Cu(I) was retarded in the presence of Fe(II) and accelerated when Fe(III) was also in solution. The log k’ (min−1) as a function of Fe(II) and Fe(III) concentrations revealed slopes of -2.7± 0.2x10−4 and 8.0±0.5x10−5, respectively. On the other hand, the reduction of Cu(II) underwent significant changes. The Cu(II) back-reaction formed 20 nM of Cu(I) over 40 min. The reduction achieved 40-60% in the first 2 min of reaction when Fe(II) was present in solution and the Cu(I) regenerated was oxidized. The Cu(I) oxidation rate was lower than that for the experiments where Cu(I) was initially added. In addition, the half-life time was independent of the initial Fe(II) levels in solution. Moreover, Fe(III) did not present any effect on the Cu(II) reduction.

The Fe(II) oxidation is accelerated in the presence of copper, and a first-order pH dependence was always observed. The rate constant of Fe(II), without copper, was a second order pH dependent function at pH over 7.5 and a first order function for lower pH values (6.2-7.5). Thus, in the presence of copper, the speciation of Fe(II) is affected. FeOH+, FeCO3 and probably FeHCO+ are involved in the process. In addition, Fe(II) oxidation rate was increased only when both carbonate and Cu(II) concentrations was over 6 mM and 100 nM, respectively. The effect of H2O2 concentrations was also function of the initial Cu(II) additions, but the oxidation rate was equally affected by H2O2, thus the observed effect must be only due to the presence of copper species in solution.