Application of a new method of analysis of ultracentrifugation data to the aggregation of a humic acid by copper(II) ions

UV scanning analytical ultracentrifugation has been used to measure the aggregation of dissolved humic substances by copper(II) ions. An adaptation of the Archibald approach to equilibrium technique has been used which monitors the charge in average molecular weight, both with time, and also with mass of sample. Also, a novel adaptation of the method was developed which, for the first time, allows molecular weight distributions to be determined for highly polydisperse populations. The method has been applied to the aggregation by copper (II) ions, of an aquatic humic acid isolated from surface water on Whitray Fell in the Forest of Bowland, UK. It is found that there is a threshold ion concentration of approximately 5×105M at which the copper(II) ions initiate aggregation of the humic substance. The aggregated systems contain aggregates with molecular masses up to 500,000, consisting of agglomerates of at least 10 species present in the metal-free samples.