Effect of pH and chloride on the kinetics and mechanism of copper transformation and associated production of oxidative products in aquatic systems

Abstract

This thesis explores the effects of pH and chloride on Cu transformations in aqueous solutions typical of both natural waters and physiological conditions: i) The effect of pH on the interaction between Cu and H2O2 in 0.7 M NaCl solution was investigated. A speciation-kinetic model was developed to explore the mechanism. While the rate of Cu(I) oxidation by H2O2 did not change with pH, the rate of reduction of Cu(II) by H2O2 was highly pH dependent due to changes in Cu and H2O2 speciation. ii) The production of oxidizing intermediates increased with increasing pH, with their reactivity assessed by phthalhydrazide hydroxylation and formate degradation experiments, both in presence and absence of additional probe compounds. By comparing observed reactivity with that expected for HO• it was shown that a higher oxidation state of copper, Cu(III), not HO•, is the oxidant formed between pH 6 and 8. Model simulations under experimentally inaccessible environmental conditions showed that Cu(III) production rates did not vary strongly with pH, despite large changes in cycling of Cu between its +I and +II oxidation state. iii) The impact of Cu on Menadiol (MNH2Q) oxidation was examined between pH 6.0 and 7.5. The oxidation of MNH2Q occurred in the open air and the autoxidation rate by O2 increased with pH, with Cu significantly accelerating the oxidation. The results were well-described by a kinetic model where mono-deprotonated menadiol caused the pH dependence, Cu(II) initiated the MNH2Q oxidation, and semiquinone and superoxide played important roles. The high [Cu(I)]ss attained in the system and the production of H2O2 also lead to formation of Cu(III). iv) The impact of chloride and the well-characterized Suwannee River Fulvic Acid (SRFA) on Cu-transformations was investigated. High chloride concentrations lead to highly-reactive Cu(II) complexes but more stable Cu(I) complexes with respect to reaction with all reductants/oxidants in the system. In the absence of O2, SRFA acted like a hydroquinone in the reduction of Cu(II); in the presence of O2 additional reactions mediated by superoxide that consumed the SRFA reductant needed to be considered, as well as complexation of Cu(I) by an oxidized SRFA moiety. A kinetic model that incorporated these key features described the key observations.