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Biosorption of Nonpolar Hydrophobic Organic Compounds to Escherichia Coli Facilitated by Metal and Proton Surface Binding

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journal contribution
posted on 15.04.2007, 00:00 authored by Lin Xiao, Xiaolei Qu, Dongqiang Zhu
We observed that the presence of transition metal ion, Ag+, Cu2+, or Fe3+, at a concentration of 3 mg L-1 increases sorption of two nonpolar hydrophobic organic compounds (HOCs), phenanthrene (PHEN), and 1,2,4,5-tetrachlorobenzene (TeCB) by 1.5−4 times to Gram-negative bacteria Escherichia coli. Complexation of transition metals with the deprotonated functional groups (mainly carboxyl) of bacterial cell walls neutralizes the negative charge, making the bacterial surface less hydrophilic and enhancing hydrophobic partition of HOCs. This is evidenced by the fact that the zeta potential (ζ) value of bacteria becomes less negative when a transition metal is present. Furthermore, the observed higher sorption of PHEN than TeCB at low pH (3.8) cannot be fully explained by the pH-dependent hydrophobic effects. The results led us to propose two specific sorption mechanisms for π-donor compounds:  cation−π interactions with protonated amines and π H-bonding with protonated carboxyls. The biosorption of PHEN was best described as π-donor compared to the biosorption of TeCB considered non-π-donor. Results of the present study highlight that the presence of coexisting transition metals and changes on pH have a major effect on the biosorption of nonpolar HOCs.