Testing the component additivity approach to surface complexation modeling using a novel cadmium-specific fluorescent probe technique

Abstract

Over the past 40 years, laboratory experiments involving single metal-single sorbent systems have been conducted in order to determine thermodynamic stability constants for metal-bacteria and metal-mineral surface complexes. The component additivity (CA) approach to surface complexation modeling (SCM) represents one method for using these experimentally-derived stability constants to predict the extent of metal adsorption in complex, multi-sorbent systems. However, quantitative tests of the CA approach are rare due to difficulties in determining the distribution of metals in complex multi-sorbent systems. In this study, we use a novel technique that couples the use of a cadmium(Cd)-specific fluorescent probe with confocal scanning laser microscopy to quantify Cd adsorption to bacteria in fully hydrated multi-sorbent samples that contain different ratios of Bacillus subtilis bacterial cells, the clay mineral kaolinite, and the aqueous chelating ligand EDTA. In this approach, we directly determine the distribution of Cd by measuring the total concentration of adsorbed Cd and the concentration of Cd that is adsorbed to bacterial cells, and by difference we calculate the concentration of Cd that is adsorbed to kaolinite. We compare these experimental measurements to the extent of Cd adsorption that is calculated using a CA approach to predict the distribution of Cd under our experimental conditions. In general, the CA predictions of the distribution of Cd between the aqueous phase and the two sorbents agree within uncertainties with the measured concentrations of Cd in each reservoir in both the EDTA-free and the EDTA-bearing experimental systems. This study demonstrates that the Cd-fluorescent probe technique is a suitable, and relatively simple, option for quantitatively testing CA surface complexation models. Our results suggest that although the CA approach can yield reasonable predictions of the distribution of Cd in mixed sorbent systems, the accuracy of the predictions depends directly on the accuracy of the measurements of stability constants for both the aqueous and surface metal-ligand complexes that occur in a system of interest.