Size distribution of copper complexing ligands in tropical freshwaters

The size distribution of soluble copper complexing ligands (<0.45 mum) present in water samples collected from a tropical river system was characterised using a combination of equilibrium dialysis and differential pulse anodic stripping voltammetry. The proportion of copper complexing capacity associated with small molecular weight (<1,000 Dalton) ligands increased progressively from 40% in the upper catchment to 79% at a lowland floodplain/wetland location. This increase indicated the dominant role of the floodplain as a source of low molecular weight ligands to the river system. A significant proportion of copper complexation was also associated with the >14,000 Dalton fraction. As the majority of soluble iron was also associated with this fraction, it is likely that this complexing pool contains organic ligands adsorbed to the surface of inorganic colloids as well as large molecular weight ligands. Destruction of dissolved organic matter by sample pre-treatment with UV light resulted in a complete loss in copper complexation capacity detectable by anodic stripping voltammetry. This confirmed that the observed complexation of copper was associated with organic ligands and not inorganic components of the sample. A statistically significant linear correlation was observed between copper complexation capacity and fluorescence (340 nm excitation/440 nm emission). This indicated that the ligands were most likely to be associated with humic and fulvic compounds having conjugated molecular structures. Overall, the study emphasises the role of low molecular weight organic ligands in controlling copper speciation in tropical freshwater systems.