Size fractionation of trace metal species in freshwaters: implications for understanding their behaviour and fate

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Data on both the colloid size distribution present in freshwaters and the extent to which trace metal ions associate with the different size fractions within the so-called ‘dissolved’ phase in these systems are examined. Several key features are identified: (i) division of total metal into ‘particulate’ and ‘dissolved’ fractions via a single ca. 450 nm filtration procedure provides no useful information, (ii) within the ‘dissolved’ fraction, a continuum of particle sizes exists in the colloidal size range in freshwaters, (iii) for the trace metals Cu, Cd, Zn, Cr, Co, and Ni, the greatest proportion of dissolved trace metals are in the ‘low molecular mass’ fraction, together with the largest proportion of DOC, (iv) in all rivers, and in lakes with a low DOC content, the predominant determiner of the size distribution of trace metals is the lower molecular size, more strongly complexing, pedogenic organic matter fraction.

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