The oxidation,fate and effects of iron during on-site bioremediation of groundwater contaminated by a mixture of polychlorophenols

Kinetics of simultaneous iron and polychlorophenol (CP) oxidation by groundwater enriched cultures were studied in laboratory and during actual remediation in orderto reveal the fate and effects of iron on aerobic on-site bioremediation of boreal groundwater. 2,4,6-tri- (TCP), 2,3,4,6-tetra- (TeCP) and pentachlorophenol (PCP)were degraded in fluidized-bed bioreactor (FBR) by over 99%, over 99%, and over96%, respectively. The oxygen consumption rate for CP-biodegradation was 1.31mol DO L^-1 min^-1 and 0.29 mol DO L^-1 min^-1 for iron oxidation, i.e. approximately 12% of the oxygen was consumed by iron oxidation during normal FBR operation. Mineralization of CPs was confirmed by DOC removal and chloride release of 158% and 78%, respectively. Excess DOC removal was due to partial degradation of the natural organic matter (NOM) (1.1 mg L^-1 or 24% DOC removal) in the groundwater. Removal of NOM consumed 0.91 mol DO L^-1min^-1. Iron oxidation in the FBR was over 94% of which chemical Fe(II) oxidation accounted for up to 10%. Fe(III) partially accumulated (58 to 69%) in the system. The TCP- and CP-biodegradation consumed DO at two times higher rates than the Fe(II)-oxidation in both, laboratory and full-scale, respectively. The batch assays atvarious TCP and Fe(II) ratios and DO concentrations showed simultaneous oxygenconsumption by TCP and Fe-oxidizers and that increased Fe concentrations do notoutcompete the bioremediation of CP's for available oxygen. Abbreviations: DAPI: 4',6'-Diamidino-phenylindole; DO:Dissolved oxygen; DOC: Dissolved organic carbon; FBR: Fluidized-bed bioreactor;HRT: Hydraulic retention time; NOM: Natural organic matter; PCP: Pentachlorophenol; SEM: Scanning-electron microscopy, TeCP: 2,3,4,6-Tetrachlorophenol; TCP: 2,4,6-Trichlorophenol