No abstract

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 5 times higher than 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 shortcircuited, 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 timedependent 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 of 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.key-words: water desalination; porous electrode theory; capacitive (non-faradaic) electrochemical cells; electrostatic double layer theory.2

No abstract

e 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.

No abstract

No abstract

A laboratory study was performed to investigate the effects of desorption from soil aggregates on the biomineralization kinetics of a-hexachlorocyclohexane (a-HCH). Desorption and biodegradation of a-HCH in mixed soil suspensions at 20 "C were shown to be controlled by intraparticle mass-transfer processes. Two models were applied to the desorption and biodegradation kinetic data, a first-order model (FOM) and a sorption-retarded radial diffusion model (RDM). Only the RDM could explain the effect of aggregate size on HCH desorption and bioconversion rate. It also yielded the best fit to the desorption kinetic data. Estimated effective diffusivities were in the order of 5 X lo-'' m2/s. Biodegradation kinetics of a-HCH could only be described by the RDM by assuming that microorganisms could penetrate the inner parts of the aggregates.

No abstract