DOES METAL ADSORPTION ONTO BACTERIAL SURFACES INHIBIT OR ENHANCE AQUEOUS METAL TRANSPORT? COLUMN AND BATCH REACTOR EXPERIMENTS ON CD-BACILLUS SUBTILIS-QUARTZ SYSTEMS

Abstract

In this study, we investigate the effects of bacteria on metal transport through mineral-filled columns, and how the effect varies with pH and mineralogy. We measured Bacillus subtilis and aqueous Cd transport through quartz and Fe-coated quartz columns as a function of pH, using a separation technique to determine the absolute concentrations of aqueous and bacterially bound Cd in the effluent. Experimental results indicate that under certain conditions, bacteria enhance Cd transport through the columns. In these cases, the migration of Cd through the column is facilitated by Cd adsorption onto bacterial surfaces, and by transport of the bacteria. However, under other conditions, the transport of bacteria is inhibited, causing a retardation in Cd mobility. Under these conditions, the bacteria are immobile due to bacterial adsorption onto mineral surfaces and/or straining by the sand matrix.In separate experiments, we test whether a surface complexation modeling approach can be used to account for metal adsorption in complex systems such as these that contain both bacterial and mineral surfaces. We performed batch adsorption experiments with aqueous Cd, B. subtilis, and quartz, quantifying metal and bacterial adsorption as a function of pH. We use these experiments as a rigorous test and extension of the surface complexation approach to more realistic geologic systems than have been studied previously. The experimental results show that thermodynamic stability constants, determined from binary systems, can be used to successfully quantify the distribution of Cd between the aqueous phase and the bacterial and mineral surfaces, and can be used to estimate the distribution of mass in systems not directly studied in the laboratory. The column results indicate that modeling of contaminant transport in bacteria-bearing systems requires not only accurate flow models, but also chemical speciation models that quantify the role of bacterial adsorption under a range of subsurface conditions. Surface complexation modeling offers a means to account for the adsorption chemistry in these complex systems.