Molecular size distribution characteristics of the metal–DOM complexes in stream waters by high-performance size-exclusion chromatography (HPSEC) and high-resolution inductively coupled plasma mass spectrometry (ICP-MS)

Abstract

In this work, the complexation of the metals Fe, V, Ce, Th, U, Mo, Cu, Ni, Co, Cr, Zn, Pb and Cd with different molecular size (MS) fractions of dissolved organic matter (DOM) in natural waters was investigated. In order to assess the MS distribution of the metal–DOM complexes, DOM samples from stream waters were concentrated by reverse osmosis, and were then analyzed by high-performance size-exclusion chromatography (HPSEC) coupled with on-line UV-Vis absorbance and high-resolution inductively coupled plasma mass spectrometry (ICP-MS). The MS distribution of overall DOM and its metal-bound complexes was evaluated. The results indicate the following order of decreasing number-averaged molecular weight: Cu > Ni > (Co, Zn, Cr) > Pb > Cd for the DOM-bound complexes of transitional metals, which is consistent with Irving-Williams series, and (Fe, V, Ce) > Th > U > Mo for the DOM-bound complexes of the other metals. The results suggest that the metal distribution among the different MS fractions was closely related to metal binding strength; metals with high binding strength were distributed more in the larger MS fractions, and metals with low strength were distributed more in the smaller MS fractions. Possible mechanisms for these observations were discussed. This study is significant to the understanding of metal complexation with DOM in natural waters.