Influence of water chemistry and dissolved organic matter (DOM) molecular size on copper and mercury binding determined by multiresponse fluorescence quenching

The effects of water chemistry (i.e. pH and Ca2+ concentration) dissolved organic carbon (DOC) concentration) and DOM quality (i.e. composition and molecular weight) on metal complexation were successfully investigated by a combination of tangential flow filtration, excitation-emission matrix fluorescence, parallel factor analysis (PARAFAC), and fluorescence quenching on four freshwater samples and one extracted Suwannee River fulvic acid (SRFA). Two terrestrial and one microbial humic-like components were found in this study. Despite strong correlation between the Ryan-Weber model and the multiresponse model, the latter is more appropriate for the calculation of binding parameters in multiple-ligand DOM system. Decreasing pH from 6 to 4 significantly reduced log K-Cu2+ from 5.22 +/- 0.24 to 4.60 +/- 0.30 at pH 6 and 4, respectively (p < 0.001), while the impacts of Ca2+ and DOC were not discernible at concentrations <100 ppm and <2.06 ppm, respectively. For natural freshwater DOM binding, the three humic-like components had similar log K values for both metals. High molecular weight (>1 kDa) DOM generally had higher log K and binding fluorophore abundance than bulk (unfractionated) and low molecular weight (<1 kDa) DOM for both metals. This trend however was not always true for Hg2+ where the binding parameters were quite variable. Overall the combined results provide evidence that binding parameters are not only affected by water chemistry, but also depend on DOM molecular weight. (c) 2013 Elsevier Ltd. All rights reserved.