MOBILIZATION OF ADSORBED CADMIUM AND LEAD IN AQUIFER MATERIAL BY BACTERIAL EXTRACELLULAR POLYMERS

The mobility of cationic trace metals, such as ph and Cd, in porous media can be severely limited by their adsorption at the solid/solution interface. The transport of metals can be enhanced by complexation with a ligand of ''carrier'' that (i) is soluble in water and does not strongly sorb to surfaces, (ii) has a high metal binding affinity and (iii) is not readily altered in soil by chemical or biological reactions. Extracellular polymers of bacterial origin are plausible carriers for metals in soil or aquifer systems. Bacterial extracellular polymers occur naturally in groundwaters and some have well established metal binding properties. In this study, extracellular polymers from 13 bacterial strains, including five subsurface isolates, were screened for their ability to mobilize Pb and Cd adsorbed to an aquifer sand. Batch adsorption isotherms were employed to screen polymers for their effect on metal phase distribution. Air of the extracellular polymers tested reduced the linear distribution coeffients of Cd and Pb. Reductions in metal adsorption by over 90% were achieved at an extracellular polymer concentration of 10.6 mg l(-1) The sorption isotherm of a selected extracellular polymer indicated that it had a low affinity for the sand sorbent and suggested that the polymer would be mobile in the porous sand medium. The distribution coefficient of the polymer for the sand was not effected by the presence Cd at low concentrations. Independently determined distribution constants for Cd and extracellular polymer with the sand and the binding constant for Cd to polymer yielded reasonable estimates of the observed distribution of Cd in the presence of the extracellular polymer. Column experiments performed with Cd in the presence and absence of the selected extracellular polymer confirmed that application of polymer solutions can enhance metal mobility in porous media.