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

Data on both the colloid size distributionpresent in freshwaters and the extent to whichtrace metal ions associate with the differentsize fractions within the so-called dissolved phasein these systems are examined. Several keyfeatures are identified: (i) division of totalmetal into particulate and dissolved fractions via a single ca. 450 nm filtrationprocedure provides no useful information, (ii)within the dissolved fraction, a continuum ofparticle sizes exists in the colloidal sizerange in freshwaters, (iii) for the tracemetals Cu, Cd, Zn, Cr, Co, and Ni, the greatestproportion of dissolved trace metals are in the low molecular mass fraction, together withthe largest proportion of DOC, (iv) in allrivers, and in lakes with a low DOC content,the predominant determiner of the sizedistribution of trace metals is the lowermolecular size, more strongly complexing,pedogenic organic matter fraction.