|Mikutta, R; Baumgärtner, A; Schippers, A; Haumaier, L; Guggenberger, G: Extracellular polymeric substances from Bacillus subtilis associated with minerals modify the extent and rate of heavy metal sorption., Environmental Science & Technology, 46, 3866-3873 (2012), doi:dx.doi.org/10.1021/es204471x|
Extracellular polymeric substances (EPS) are an important source of organic matter in soil. Once released by microorganisms, a portion may be sorbed to mineral surfaces, thereby altering the mineral̀s ability to immobilize heavy metals. EPS from Bacillus subtilis were reacted with Ca-saturated bentonite and ferrihydrite in 0.01 M KCl at pH 5.0 to follow the preferential uptake of EPS-C, -N, and -P. The sorption kinetics of Pb2+, Cu2+, and Zn2+ to the resulting EPS-mineral composites was studied in single and binary metal batch experiments ([metal]total = 50 μM, pH 5.0). Bentonite sorbed much more EPS-C (18.5 mg g−1) than ferrihydrite (7.9 mg g−1). During sorption, EPS were chemically and size fractionated with bentonite favoring the uptake of low-molecular weight components and EPS-N, and ferrihydrite selectively retaining high-molecular weight and P-rich components. Surface area and pore size measurements by N2 gas adsorption at 77 K indicated that EPS altered the structure of mineral-EPS associations by inducing partial disaggregation of bentonite and aggregation of ferrihydrite. Whereas mineral-bound EPS increased the extent and rate of Pb2+, Cu2+, and Zn2+ sorption for bentonite, either no effect or a decrease in metal uptake was observed for ferrihydrite. The extent of sorption always followed the order Pb2+ > Cu2+ > Zn2+, which also prevailed in binary Pb2+/Cu2+ systems. In consequence, sorption of EPS to different minerals may have contrasting consequences for the immobilization of heavy metals in natural environments by inducing mineral-specific alterations of the pore size distribution and, thus, of available sorption sites.