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 1.0x10^-4.0 and 3.2x10^-5 M with adsorbent mixtures proportioned on a 1:1 mass/volume basis.  The log of the apparent surface complex formation constants (log K^S_M) 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 (r² > 0.9), where Cd²⁺ exhibited somewhat lower sorption affinities (log K^S_M = -3 for ferrihydrite, -1.7 for B. subtilis – ferrihydrite, and –1.1 for E. coli – ferrihydrite) than Pb²⁺ (log K^S_M = -0.9 for ferrihydrite, -0.2 for B. 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 (Cd²⁺_{B. subtilis – ferrihydrite}: 0.29 mmole/g (observed) < 0.57 mmole/g (calculated); Cd²⁺_{E. coli – ferrihydrite}: 0.15 mmole/g (observed) < 0.44 mmole/g (calculated); Pb²⁺_{B. subtilis – ferrihydrite}: 0.5 mmole/g (observed) < 0.805 mmole/g (calculated); Pb²⁺_{E. coli – ferrihydrite}: 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.