Impact of cell wall structure on the behavior of bacterial cells in the binding of copper and cadmium

In this study, Fourier transform infrared spectroscopy, potentiometric titration, along with sorption experiments using chemically modified bacteria were conducted to compare the behavior of Gram-positive Bacillus thuringiensis and Gram-negative Escherichia coli as sorbents for Cu2+ and Cd2+ ions. The IR spectra showed that there were obvious changes connected with the–COOH groups for Cu(II)- and Cd(II)-loaded B. thuringiensis. A three site non-electrostatic model provides an excellent fit to the titration curves of both E. coli and B. thuringiensis with the first corresponding pKa values of 4.16 ± 0.18 and 3.30 ± 0.24, respectively, implying that B. thuringiensis contains more carboxyl groups than E. coli. Chemical modification and metal sorption experiments further confirmed that carboxyl groups may play a more important role in the binding Cd(II) and Cu(II) for B. thuringiensis than for E. coli, which could be attributed to the higher concentration of carboxyl sites of B. thuringiensis than E. coli. These results show that the main chemical functional group responsible for the binding of metal ions on B. thuringiensis and E. coli is obviously different because of the different composition and structure of bacterial cell walls. The concentrations of the chemical functional groups on bacterial cell walls are regarded to govern their role in the binding of metal ions and affect the affinity between the bacterial cells and metal ions.