Microbial community composition and diversity at a diesel-contaminated railway site were investigated by pyrosequencing of bacterial and archaeal 16S rRNA gene fragments to understand the interrelationships among microbial community composition, pollution level, and soil geochemical and physical properties. To this end, 26 soil samples from four matrix types with various geochemical characteristics and contaminant concentrations were investigated. The presence of diesel contamination significantly impacted microbial community composition and diversity, regardless of the soil matrix type. Clean samples showed higher diversity than contaminated samples ( P < 0.001). Bacterial phyla with high relative abundances in all samples included Proteobacteria , Firmicutes , Actinobacteria , Acidobacteria , and Chloroflexi . High relative abundances of Archaea , specifically of the phylum Euryarchaeota , were observed in contaminated samples. Redundancy analysis indicated that increased relative abundances of the phyla Chloroflexi , Firmicutes , and Euryarchaeota correlated with the presence of contamination. Shifts in the chemical composition of diesel constituents across the site and the abundance of specific operational taxonomic units (OTUs; defined using a 97% sequence identity threshold) in contaminated samples together suggest that natural attenuation of contamination has occurred. OTUs with sequence similarity to strictly anaerobic Anaerolineae within the Chloroflexi , as well as to Methanosaeta of the phylum Euryarchaeota , were detected. Anaerolineae and Methanosaeta are known to be associated with anaerobic degradation of oil-related compounds; therefore, their presence suggests that natural attenuation has occurred under anoxic conditions. This research underscores the usefulness of next-generation sequencing techniques both to understand the ecological impact of contamination and to identify potential molecular proxies for detection of natural attenuation.

In a combined experimental and theoretical study, we show that capacitive charging of porous electrodes in multicomponent electrolytes may lead to the phenomenon of time-dependent ion selectivity of the electrical double layers (EDLs) in the electrodes. This effect is found in experiments on capacitive deionization of water containing NaCl/CaCl(2) mixtures, when the concentration of Na(+) ions in the water is five times the Ca(2+)-ion concentration. In this experiment, after applying a voltage difference between two porous carbon electrodes, first the majority monovalent Na(+) cations are preferentially adsorbed in the EDLs, and later, they are gradually replaced by the minority, divalent Ca(2+) cations. In a process where this ion adsorption step is followed by washing the electrode with freshwater under open-circuit conditions, and subsequent release of the ions while the cell is short-circuited, a product stream is obtained which is significantly enriched in divalent ions. Repeating this process three times by taking the product concentrations of one run as the feed concentrations for the next, a final increase in the Ca(2+)/Na(+)-ratio of a factor of 300 is achieved. The phenomenon of time-dependent ion selectivity of EDLs cannot be explained by linear response theory. Therefore, a nonlinear time-dependent analysis of capacitive charging is performed for both porous and flat electrodes. Both models attribute time-dependent ion selectivity to the interplay between the transport resistance for the ions in the aqueous solution outside the EDL, and the voltage-dependent ion adsorption capacity of the EDLs. Exact analytical expressions are presented for the excess ion adsorption in planar EDLs (Gouy-Chapman theory) for mixtures containing both monovalent and divalent cations.

The reversibility and mechanisms of adhesion of various pseudomonads and coryneform bacteria having different hydrophobicities and negative cell surface charges on negatively charged Teflon and glass were studied. Adhesion at an ionic strength of 0.1 M was irreversible and corresponded to activation Gibbs energies for detachment higher than 5 kT for 19 out of 20 combinations of bacterial strains and surfaces. The data further demonstrate the importance of two groups of interactions: (i) the electrostatic and van der Waals interactions as described by the DLVO model, and (ii) the interactions between the outer cell surface macromolecules and the solids (steric interactions). At an ionic strength of 0.1 M, steric interactions control adhesion for all but two bacterium/substratum combinations tested. These interactions are attractive for seven moderately to strongly hydrophobic strains on Teflon and prevented detachment upon decreasing the ionic strength to less than 0.0001 M and also after applying shear forces. In contrast, steric interactions inhibited adhesion for more hydrophilic bacterium/substratum combinations for which detachment occurred upon reducing the ionic strength to less than 0.0001 M and/or after applying shear. The importance of the interactions included by the DLVO model is demonstrated by the following. (i) Two hydrophobic strains adhere irreversibly on glass by strong van der Waals attraction in a secondary DLVO minimum at an ionic strength of 0.1 M and detach when the ionic strength is reduced to less than 0.0001 M. (ii) Electrostatic repulsion inhibits deposition at lower ionic strength. The practical implications of these findings are discussed.