Sorption of cadmium and lead by bacteria-ferrihydrite composites

The sorptive behavior of bacteria-iron oxide composites was investigated in batch metal sorption assays using ferrihydrite in isolation (0.13 and 0.14 g/L ferrihydrite in cadmium and lead systems, respectively) as well as in combination with Bacillus subtilis (0.25 g/L adsorbent mixture) and Escherichia coli (0.27 g/L adsorbent mixture). A pH range from 3.0 to 6.5 was studied using total metal concentrations of 1.0 x 10(-4.0) and 3.2 x 10(-5) M with adsorbent mixtures proportioned on a 1:1 mass/volume basis. The log of the apparent surface complex formation constants ( log K-M(S)) and sorption capacity (S-max) values were determined by fitting the experimental data to one-site Langmuir sorption isotherms. The one-site model effectively described the sorption data (r(2) > 0.9), where Cd2+ exhibited somewhat lower sorption affinities (log K-M(S) = -3 for ferrihydrite, -1.7 for B. subtilis-ferrihydrite, and -1.1 for E. coli-ferrihydrite) than Pb2+ (log K-M(S) = -0.9 for ferrihydrite, -0.2 forB. subtilis-ferrihydrite, and -0.1 for E. coli ferrihydrite). The corresponding S-max values for Cd2+ and Pb2+ on ferrihydrite were 0.78 mmole/g and 1.34 mmole/g, respectively. For the B. subtilis-ferrihydrite composites, Cd2+ and Pb2+ S-max values were lower at 0.29 mmole/g and 0.5 mmole/g, respectively. Similar values were determined for the E. coli-ferrihydrite composites (0.15 mmole/g and 0.68 mmole/g for Cd2+ and Pb2+, respectively). The sorption of Cd2+ and Pb2+ by each of the sorbent systems exhibited a strong dependence on pH with sorption edges in the range of pH 4.0 to 7.3. The observed S-max of the composites were lower than values predicted upon available site additivity (Cd-B. subtilis-ferrihydrite(2+) : 0.29 mmole/g (observed) < 0.57 mmole/g (calculated); Cd-E. coli-ferrihydrite(2+) : 0.15 mmole/g (observed) < 0.44 mmole/g (calculated); Pb-B. subtilis-ferrihydrite(2+) : 0.5 mmole/g (observed) < 0.805 mmole/g ( calculated); Pb-E. coli-ferrihydrite(2+) : 0.68 mmole/g (observed) < 0.775 mmole/g (calculated)), implying that a masking of reactive surface sites by attachment had occurred between the bacteria and ferrihydrite. Electrophoretic mobility analysis indicated that the ferrihydrite surface properties dominate the net surface charge for each composite system with lesser contributions from the bacteria.