The groundwater chemistry of an industrial site near the centre of a historical town in Flanders is characterized by chromium(VI) contamination. Since the potential for natural attenuation was not sufficient to control the groundwater plume as observed from different analyses like pH, Eh, soil oxidation and reduction capacity, the feasibility of site remediation based on in-situ bioprecipitation of Cr was examined in lab microcosm studies. By creating reducing conditions by addition of a carbon source in the groundwater, Cr(VI) can be biologically reduced to Cr(III), which can form an insoluble hydroxide precipitate. Between pH 6 and pH 12, the precipitated phase Cr(OH)_3 predominates (Loyauw-Lawniczak et al., 2001). Microcosms contained contaminated groundwater and aquifer samples that were collected and manipulated under anaerobic conditions. A rapid conversion of Cr(VI) to Cr(III) was observed for all conditions tested (three different carbon sources). However, extensive removal of Cr from the groundwater by precipitation was only achieved for a limited number of conditions. In general, lactate was found to be the most effective carbon source, slightly better than the lowest concentration of sugar molasses that was tested, which itself was more effective than molasses based on potato. Higher concentrations of molasses, although causing a steeper decline in redox potential, resulted in less effective Cr removal. Typically, the fermentation of molasses results in acidification and this drop in pH may hinder the formation of insoluble Cr(III)-hydroxides. It may also be possible that certain components of the molasses form soluble complexes with Cr(III). For lactate, no significant drop in pH was observed. Since the outcome of these tests was positive, a pilot scale test was implemented in the field in April 2005 until June 2006. Two different substrates, the cheap waste product sugar molasses and lactate were injected creating two different impact zones and typical parameters were monitored. Frequent re-injections were important to maintain reduced conditions and are carried out three-weekly up to now. To study the stability of precipitates formed, mesocosm socks containing aquifer material were installed in the monitoring wells and harvested at regular intervals. Results of this pilot test showed efficient chromate removal from the groundwater for the lactate injection zone within 200 days, while for the molasses zone efficient removal was observed only after injecting additional substrate at a depth between 8-12 m-bg after +/- 400 days. Based on the success of this field test, a full scale process will be designed and implemented as remediation strategy.
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