It is currently accepted –and repeated in the introduction of many articles and books– that humic substances are largely responsible for the complexation of trace elements by natural organic matter (NOM) in waters and soils. For this reason, the study of trace element complexation by humic substances has been the focus of considerable research and an impressive amount of literature exists on the topic, with the earliest studies dating back to the late 1950s. However, in spite of the amount of work done and the fact that the use of speciation codes embedding humic binding models that imply a high degree of fitting is increasingly observed in some fields (e.g. bioavailability studies), trace element complexation by humics has been, and essentially remains, a controversial issue.
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