Cadmium Adsorption by Mixed-Culture Biofilms under Metabolizing and Non-Metabolizing Conditions

Heavy metals are a common cause of pollution. They include several elements essential for growth, reproduction and/or survival of living things, some with no known biological function and many with economic, industrial and military uses. Metals are non-degradable and tend to accumulate in the environment. Their discharge into the environment by a number of industries constitutes one of the major causes of land and water pollution and results in high concentrations of the metals relative to normal background levels. Conventional methods of treatment for heavy metals are becoming increasingly expensive. In recent years there has been an increasing interest in the use of microorganisms, in particular in immobilized systems, to treat heavy metal-polluted wastes. They can accumulate trace levels of heavy metal ions, many toxic, from aqueous solutions and play a major role in the modification, activation and detoxification of heavy metals. The current studies investigated the extent of cadmium adsorption onto a mixed-culture bacterial system under two conditions. First we looked at cadmium adsorption with biofilms that were metabolically active. We then compared this to cadmium adsorption with biofilms that were metabolically inactive. Biofilms were grown using a continuous flow packed bed reactor. The reactor was inoculated with activated sludge and the feed solution consisted of a 16mM phosphate buffer with 1.664g of potassium acetate. Cells were inactivated by exposition to 35,000 rads/hr of cesium irradiation. Adsorption experiments were carried out using a solution that had no ammonium, 2 ppm Cd, and 150 ppm acetate where the bacteria was then added. Acetate adsorbed was determined through ion-chromatography and cadmium in bulk liquid by ICP-OES. We observed that bacterial metabolism significantly affected the extent of cadmium adsorption from solution. Metabolizing bacteria adsorbed approximately 27% of the cadmium from solution. Non-metabolizing bacteria adsorbed approximately 49% of the cadmium in the samples. It was also observed that samples with varying amounts of acetate adsorbed around the same percent of cadmium under both conditions. These results are important because in both natural and engineering systems, bacteria tend to grow in biofilms so our results likely demonstrate the tendency for biomass adsorption of cadmium better than experiments conducted with suspended cells. Our results suggest that bacterial metabolism can play an important role in controlling heavy metal adsorption efficiency, and therefore metabolic effects must be accounted for when modeling the speciation and fate of heavy metals in both natural systems and bioreactors.