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Purpose The aim of this study was to evaluate polychlorinated biphenyl (PCB) removal in relation to the associated bacterial community composition in Ohio River sediments (USA) using field and laboratory approaches. Materials and methods PCB removal was studied in different sediment layers applying dialysis equilibrators in the field for 4 months. In parallel, dissolved chemical constituents and bacterial community composition using the 16S rDNA cloning-sequencing approach were determined within the appropriate sediment layers. The effects of several bioremediation treatments on PCB removal and bacterial community composition were evaluated in laboratory experiments. Sediments spiked with 245-2'4'5' hexachlorobiphenyl were incubated under different conditions for up to 250 days, including alternating anaerobic-aerobic conditions, amendments with electron donors, FeSO 4 , and incubations at different temperatures (10°C, 25°C, 40°C). Results and discussion PCB levels remained stable in the sediment layers under field conditions. Surface sediments (0-26 cm) had elevated levels of nitrate+nitrite and sulfate, while subsurface sediments (27-35 cm) were characterized by high concentrations of ammonium, methane, and dissolved organic carbon. Oxic/anoxic conditions were also reflected in the bacterial community, with aerobic bacteria in the Bulkholderiales order enriched in the surface sediments, and anaerobic bacteria in the Clostridiales, Syntrophobacteriales, and Desulfobacterales orders more prevalent in the subsurface sediments. Although the chemical status was favorable and the potential degraders were present, PCB transformation did not take place under the prevailing environmental conditions. Under laboratory conditions, PCB transformation was positively influenced by lowered redox conditions with or without amendments with FeSO 4 , electron donors, or alternating anaerobic-aerobic conditions. The main PCB transformation process in all anaerobic treatments at 25°C was reductive dechlorination of 245-2'4'5' hexachlorobiphenyl to 24-2'4'5' pentachlorobiphenyl and 24-2'4' tetrachlorobiphenyl, which occurred at rates between 60-85% in 177 days. Exposing sediments to air for an additional period of 100 days in the sequential anaerobic-aerobic treatment did not result in additional mineralization of PCBs. Temperature, electron donors, and redox conditions significantly affected the abundance of bacteria in the Bacteroidales, Hydrogenophilales, Pseudomonales, Myxococcales, Syntrophobacterales, Acidobacterales, Caldilineales, and Clostridiales orders. The PCB dechlorinator Dehalococcoides and other dechlorinating bacteria were detected in anaerobic sediments, including Desulfitobacterium, Desulfuromonas, Anaeromyxobacter, Geobacter, and Desulfomonile. The data suggested that shifts in the bacterial communities due to abiotic conditions affected PCB transformation, either directly (degraders) or indirectly (bacterial syntrophic associations with degraders). Conclusions The data indicated that elevated redox status and low...

Hydrolysis between 1.5 < pH < 4 of five and six membered cyclic phosphoramides has been followed by UV and 3'PNMR spectroscopy. The observed rates fit the equation: k(obs) = k(H2O) [H+]/([H+] + Ka) + k'(H2O), where k(H2O) and k'(H2O) are the pseudo first-order rate constants of water attack on the protonated phosphoramide and its unprotonated form, respectively, and Ka is the phosphoramide acidity equilibrium constant. Although, faster hydrolysis rates on the five membered ring are expected due to the energy released in going from a strained cyclic to a "strained free" trigonal-bipyramidal-pentacoordinated intermediate, with one of the cyclic nitrogens occupying the apical position. these compounds react slightly faster (k(H2O) values) but slower regarding the k'(H2O) values than the six membered analogs. The balance in reactivity is attributed to the additional stability obtained in the six membered cyclic compounds by a syn orientation of the two lone pairs of the cyclic nitrogen to the water attack. This stabilization does not exist in the five membered phospholidines since the water attack is perpendicular to the electron pairs of the cyclic nitrogen. In agreement with the incoming water orientation, the product ratios from the hydrolysis show that in the five membered rings the main product is the one produced by endocyclic cleavage; meanwhile, in the six membered cyclic phospholines the kinetic product is the one produced by exocyclic cleavage. The syn orientation of two electron pairs on nitrogen stabilizes the transition state of water approach to the phosphoramides by ca. 3 kcal mol(-1) when compared to the orthogonal attack.