Size distribution of copper complexing ligands in tropical freshwaters

The size distribution of soluble copper complexing ligands (<0.45 mum) pres ent in water samples collected from a tropical river system was characteris ed using a combination of equilibrium dialysis and differential pulse anodi c stripping voltammetry. The proportion of copper complexing capacity assoc iated with small molecular weight (<1,000 Dalton) ligands increased progres sively from 40% in the upper catchment to 79% at a lowland floodplain/wetla nd location. This increase indicated the dominant role of the floodplain as a source of low molecular weight ligands to the river system. A significan t proportion of copper complexation was also associated with the >14,000 Da lton fraction. As the majority of soluble iron was also associated with thi s fraction, it is likely that this complexing pool contains organic ligands adsorbed to the surface of inorganic colloids as well as large molecular w eight ligands. Destruction of dissolved organic matter by sample pre-treatm ent with UV light resulted in a complete loss in copper complexation capaci ty detectable by anodic stripping voltammetry. This confirmed that the obse rved complexation of copper was associated with organic ligands and not ino rganic components of the sample. A statistically significant linear correla tion was observed between copper complexation capacity and fluorescence (34 0 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 molecul ar weight organic ligands in controlling copper speciation in tropical fres hwater systems.