Aggregation of humic substances by metal ions measured by ultracentrifugation

UV scanning analytical ultracentrifugation has been used to measure the aggregation of dissolved humic substances by metal ions. An adaptation of the Archibald approach to equilibrium technique has been used to monitor the change in average molecular weight, both with time, and also with mass of sample. The results have been analysed to give molecular weight distributions in the aggregated systems. For each metal studied, it was found that there was a threshold concentration below which no aggregation was observed, the magnitude of that concentration depending upon the metal. The range of concentrations from the onset of aggregation to the precipitation of the humic also depends upon the metal. It was found that the metal ions promoted aggregation in the order: La3+>Cu2+>Cd2+>Ag+. Tl4+, however, were found not to induce aggregation. In addition, the effects of ageing time, solution pH and ionic strength have been studied. It was also found that many of the aggregated humic populations were stable in solution for periods of weeks, and that the aggregation threshold concentration was independent of solution ionic strength. It was found that the aggregation was not the result of simple double layer compression as predicted by the Schulze-Hardy rule. Rather, the metal ions were reducing the intermolecular repulsion by reducing the humic charge.