Alkaline Soil Extraction and Subsequent Mineralization of 2,6-Dichlorophenol in a Fixed-Bed Bioreactor

Soil contaminated with moderate concentrations (0.1 g to a few grams) of several chlorophenol (CP) congeners can be remediated by a combination of alkaline extraction and mineralization of the extracted CP in a bioreactor. This method could substitute energy-demanding thermal treatment or space-requiring composting of moderately CP-contaminated soils. 2,6-dichlorophenol (2,6-DCP) served as a model compound to study the alkaline extraction of a loamy sand soil, followed by a biological treatment of the extract. Alkaline extraction is shown to be applicable to different types of soil and a wide range of chlorophenol concentrations. Soil washing was optimal with 10 mM NaOH (pH 12). The procedure yielded 2,6-DCP comparable to amounts obtained by Soxhlet- or ethanol-extraction. With the model soil used in this study, three subsequent extraction steps led to 97% removal of the initially spiked 6.17 mmol 2,6-DCP × kg^-1 soil (=1 g/kg), thus reaching the remediation goal of ≤ 0.2 mmol/kg remaining contaminant concentration. The resulting aqueous extract contained up to 6.8 mM 2,6-DCP and was treated in an aerobic fixed-bed bioreactor. The extraction medium was fed into a recirculation loop in order to dilute the pollutant to concentrations tolerated by the mixed bacterial culture in the reactor. 2,6-DCP was degraded to below the quantification limit (1.8 μiM), and significant detoxification was reached at volumetric loading rates up to 2.1 g/L-d.