Competition for oxygen by iron and 2,4,6-trichlorophenol oxidizing bacteria in boreal groundwater

Abstract

Kinetics of simultaneous iron and 2,4,6-trichlorophenol (TCP) oxidation by groundwater enriched cultures were studied in order to reveal the competition for oxygen in aerobic in situ bioremediation of boreal groundwater. Chemical iron oxidation at near neutral pH in synthetic groundwater depended by the first order on the concentrations of ferrous iron and dissolved oxygen and by the second order on pH. The chemical iron oxidation rate constant was on average 2.2×10 13 mol −2 L 2 atm −1 min −1 . Chemical iron oxidation was insignificantly affected by natural organic matter, 2,4,6-tri-, 2,3,4,6-tetra- or pentachlorophenol in groundwater. Biological oxidation of iron followed zero-order kinetics. At pH of 6.3 and dissolved oxygen (DO) concentration of 11.5 mg L −1 , the rate of biological iron oxidation was 3.8×10 −4 mmol L −1 min −1 and up to one order of magnitude higher than the chemical oxidation rate, 5.2×10 −6 mmol L −1 min −1 . Biological oxidation of iron was completely inhibited by pentachlorophenol at 23 μmol −1 . With a groundwater enriched culture, oxygen was consumed at higher rates by 2,4,6-TCP oxidizers (2.5−7.6×10 −5 mmol DO L −1 min −1) than the iron oxidizing bacteria (0.8−3.1×10 −5 mmol DO L −1 min −1) at both low and saturated DO-concentrations. The results indicate that in situ iron oxidation is predominantly biogenic in the studied boreal aquifer. 2,4,6-TCP degrading bacteria consumed DO at higher rates than the iron oxidizing bacteria and thereby, favour bioremediation of the polychlorophenol contaminated groundwater.