The effect of metal loading on bacterial Hg adsorption

Abstract

The mechanism of metal adsorption onto bacteria can change as a function of metal loading of the binding sites, and therefore the mechanisms of adsorption and the thermodynamic stability constants for the important bacterial surface complexes must be determined in order to accurately model metal behavior in near-surface geologic systems. In this study, batch metal adsorption experiments were conducted as a function of pH from 4 to 8, and Hg loading from 10 to 200 μmol Hg/g (wet weight bacteria). The adsorption data are in poor agreement with predictions of the extent of Hg adsorption that used previously published Hg-bacteria stability constants generated from high metal loading experiments. The extent that the thermodynamic model underpredicts the measured Hg adsorption increases with decreasing Hg loading and pH, suggesting that the mechanism of adsorption changes as a function of Hg loading and that an additional Hg-bacterial surface complex becomes important under low, <200 μmol/g, Hg loading and low, <6.5, pH conditions. The new Hg-bacterial surface complex involves a high affinity, low abundance site, which is likely a sulfhydryl moiety based on x-ray absorption spectroscopy, and is important only under low Hg loading conditions. We model the enhanced adsorption onto this high affinity site using an additional Hg-bacterial surface complex to those considered in the model derived from the high Hg loading data, and we use our data to constrain values of the stability constants for this new complex. Our modeling results provide reasonable fits to the data, and the proposed set of stability constants should improve the accuracy of quantitative models of Hg complexation with bacterial surfaces as a function of Hg loading and pH.