Decrease in zinc adsorption onto soil in the presence of EPS-rich and EPS-poor Pseudomonas aureofaciens

Abstract

The adsorption of Zn onto the humic and illuvial horizons of the podzol soil in the presence of soil bacteria was studied using a batch-reactor technique as a function of the pH (from 2 to 9) and the Zn concentration in solution (from 0.076 mM to 0.760 mM). Exopolysaccharides-forming aerobic heterotrophs Pseudomonas aureofaciens were added at 0.1 and 1.0 gwet L−1 concentrations to two different soil horizons, and Zn adsorption was monitored as a function of the pH and the dissolved-Zn concentration. The pH-dependent adsorption edge demonstrated more efficient Zn adsorption by the humic horizon than the mineral horizon at otherwise similar soil concentrations. The Zn adsorption onto the EPS-poor strain was on slightly lower than that onto EPS-rich bacteria. Similar differences in the adsorption capacities between the soil and bacteria were also detected by “langmuirian” constant-pH experiments conducted in soil-Zn and bacteria-Zn binary systems. The addition of 0.1 gwet L−1 P. aureofaciens to a soil–bacteria system (4 gdry L−1 soil) resulted in statistically significant decrease in the adsorption yield, which was detectable from both the pH-dependent adsorption edge and the constant-pH isotherm experiments. Increasing the amount of added bacteria to 1 gwet L−1 further decreased the overall adsorption in the full range of the pH. This decrease was maximal for the EPS-rich bacteria and minimal for the EPS-poor bacteria (a factor of 2.8 and 2.2 at pH = 6.9, respectively). These observations in binary and ternary systems were further rationalized by linear-programming modeling of surface equilibria that revealed the systematic differences in the number of binding sites and the surface-adsorption constant of zinc onto the two soil horizons with and without bacteria. The main finding of this work is that the adsorption of Zn onto the humic soil–bacteria system is lower than that in pure, bacteria-free soil systems. This difference is statistically significant (p < 0.05). As such, EPS-rich bacteria are capable of efficiently shielding the soil particles from heavy-metal adsorption. The removal efficiency of heavy metals in an abiotic organic-rich soil system should therefore be significantly higher than that in the presence of bacteria. This effect can be explained by the shielding of strongly bound metal sites on the organic-rich soil particles by inert bacterial exopolysaccharides.