Sorption model for dissolved aluminium in freshwaters

A series of aluminium speciation measurements were made in the freshwaters of the Yare and Great Ouse rivers (England). Samples were analysed for dissolved and particulate Al, suspended particulate matter (SPM), and other allied chemical species in order to assess the role of sorption processes on the suspended phases in controlling dissolved Al levels. Partitioning of Al occurs between solid and solution phases with a distribution coefficient (K-d) which varies over about one and a half orders of magnitude for suspended particle concentrations of comparable magnitude. A sorption model is proposed for dissolved Al concentrations in these freshwaters with most of the data fitting the model defined by a zone with K-d values of 0.316x10(6) and 10(7). However, a few data points lie outside this zone, suggesting that other processes may also influence dissolved Al distributions. Nevertheless, the model may serve as a starting point for predicting concentrations of dissolved Al in rivers where SPM levels are moderate to high (>1 mg l(-1) but <75 mg l(-1)), and indeed, this model works reasonably well for the Conway system (Wales). Further, the empirical distribution coefficient, K-d, decreases with increasing suspended particle concentration, which may be due in part to colloidal phenomena.