METAL ADSORPTION ONTO ORGANIC COMPONENTS IN GEOLOGIC SYSTEMS: BUILDING GENERALIZED PREDICTIVE MODELS

Our ability to construct models capable of predicting the distribution and speciation of metals in near-surface geologic systems is limited by our inability to quantify adsorption reactions with organic components such as dissolved humic substances and bacteria. Existing models are capable of describing adsorption reactions that occur in relatively simple laboratory systems; however, real systems with dozens of bacterial species, various organic macromolecules, and many competing metals pose substantial obstacles to modeling. In this study, we continue experimental work on bacterial consortia from a variety of natural environments by conducting Pb, Ni, Sr, and Cd adsorption experiments at ionic strengths of 0.1 M and 0.001 M in an effort to develop ‘universal’ thermodynamic modeling parameters. Additionally, we conduct adsorption experiments in a Cd-bacteria-humic/fulvic acid system to test the accuracy and utility of our recently proposed model that accounts for competition of metals between dissolved humic substances and bacterial surfaces (Borrok & Fein, GCA 68, 3043-52). Results suggest that at the same ionic strength, consortia of bacteria from natural environments adsorb roughly similar extents of Pb, Ni, Sr, and Cd, and that adsorption reactions can be described (for each metal separately) by a single set of ‘universal’ equilibrium constants. However, for some of the metals substantial differences in metal adsorption were observed at different ionic strengths, suggesting that an electrostatic correction is necessary. The joint humic substance/bacteria adsorption model yields reasonably accurate predictions of Cd adsorption in our multicomponent experimental system. However, the model fit is greatly improved by incorporation of ternary humic-bacteria-Cd complexes, which were discovered to be abundant under the experimental conditions. These results significantly improve our ability to quantify the effects of organic-metal adsorption reactions on metal speciation and mobility in realistic environments.