Modelling the interactions of Hg(II) and methylmercury with humic substances using WHAM/Model VI

Abstract

WHAM, incorporating Humic Ion Binding Model VI, was used to analyse published data describing the binding of Hg(II) and methylmercury (CH 3Hg) by isolated humic substances. For Hg(II), the data covered wide ranges of pH and levels of metal binding, whereas for CH 3Hg the range of metal binding was relatively narrow. Data were fitted by adjustment of a single model parameter, log K MA, the intrinsic equilibrium constant characterising, in the standard version of the model, the binding of metal ions and their first hydrolysis products to humic carboxylic acid groups. Other model parameters, including those characterising the tendency of metal ions to interact with “softer” ligand atoms (N and S), were held at their default values. The importance of the first hydrolysis products in binding was considered, and also the possible influence of competition by residual Fe(III), bound to the humic matter. Of the 11 data sets for Hg(II), eight gave results reasonably consistent with one another, and with the previously-estimated default values of log K MA. There was no consistent indication that assuming the presence or absence of competing Fe(III) gave superior fits; neither did the inclusion or exclusion of HgOH + binding provide consistently better results. The experimental data and the model show that apparent binding strength towards Hg(II) is highly dependent upon the metal loading, reflecting the high degree of heterogeneity in binding sites for the metal. Of the 24 metals to which WHAM/Model VI has now been applied, Hg(II) shows the strongest binding to humic substances, and the greatest range in binding affinities. The relatively few data characterising the interactions of CH 3Hg with humic substances can be approximately fitted with the model. The results show that CH 3Hg binding is appreciably weaker than that of Hg(II). New default values of log K MA are 3.6 for Hg(II)–HA binding, 3.1 for Hg(II)–FA and 0.3 for CH 3Hg–HA and CH 3–FA.