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

UV scanning analytical ultracentrifugation has been used to measure the agg regation of dissolved humic substances by copper(II) ions. An adaptation of the Archibald approach to equilibrium technique has been used which monito rs the charge in average molecular weight, bath with time, and also with ma ss of sample. Also, a novel adaptation of the method was developed which, f or the first time, allows molecular weight distributions to be determined f or highly polydisperse populations. The method has been applied to the aggr egation 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 x 10(-5) M at which the copper(II) ions initiate aggregation of the humic substance. The aggregate d systems contain aggregates with molecular masses up to 500,000, consistin g of agglomerates of at least 10 species present in the metal-free samples. (C) 2001 Elsevier Science B.V. All rights reserved.