Chemical heterogeneity of humic substances and its impact on metal complexation in natural waters

The objective of this research was to achieve a better understanding of the effects of chemical heterogeneity of humic substances (HS) on their complexation of metals in natural waters. The apparent stability and lability of metal-HS complexes were found to be dependent on the metal loading and heterogeneity of the humic substance. Heterogeneity was found to play a crucial role in metal-DOC interactions and to control trace metals competitions with target metals in natural waters. Anodic Stripping Voltammetry with Hanging Mercury Drop Electrode was used to investigate the Differential Equilibrium Function (DEF) and the heterogeneity parameters of metal-HS complexes in natural waters. The heterogeneity of metal-HS complexes was also investigated by kinetic speciation of the metals. Dissociation rate coefficients of metal-HS complexes were studied in order to determine the effects of heterogeneity of HS in metal-contaminated natural waters collected from Sudbury, Ontario. The kinetic speciation approach also revealed that the stability of metal-DOC complexes increased with the increase in heterogeneity of humic substances and with low metal loading (low [M]/[DOC] mole ratio). It was found that HS had different heterogeneity for different metals because of the availability of different complexing sites to different metals. This study also included investigation of the nature of metal-DOC bonding, covalence and electrovalence and their strengths (stability constants) in model solutions and in natural waters. Competing Ligand Exchange Method (CLEM) along with Adsorptive Stripping Voltammetry (AdSV) or Graphite Furnace Atomic Absorption Spectrometry (GFAAS) were used to investigate the reaction kinetics. Diffusive Gradients in Thin films (DGT) technique was also used to investigate the physical heterogeneity of HS.