Abstract
Chromium is found in the environment mainly as hexavalent chromium, Cr(VI), and the trivalent form, Cr(III). Cr(VI) is carcinogenic and mutagenic to living organisms including humans whereas trivalent chromium is 1,000 times less toxic than Cr(VI). Most of the anthropogenic Cr discharged to the environment is in the hexavalent state since most human activities oxidise chromium ore (Cr(III)) to chromates and dichromates (Cr(VI)). The detoxification of Cr therefore involves reduction of Cr(VI) to Cr(III). Unfortunately, for in situ bioremediation systems, this causes accumulation of the Cr(III) precipitate, Cr(OH)3(s). The accumulated precipitate, if the water is underground, reduces the porosity and permeability of aquifer medium. In this study, a cleanup method for an in situ underground remediation barrier system is evaluated. The process involved the remobilisation of the Cr(OH)3 precipitate in a horizontal flow aquifer microcosm reactor using a dilute acid (0.1 % HCl) followed by recover of Cr(III) species on the negatively charged electrode (anode) of an electrokinetic field located downstream of the remediation zone. The reactor was operated for 28 d under soil washing with 0.1 % HCl and electrokinetics remediation with a DC voltage of 50-150 V. An increase in total chromium (73 %) was observed suggesting that the trapped chromium species in the barrier was effectively remobilized. A gradient of yellow and green precipitate was observed around the anode confirming the migration of Cr(III) species toward the anode. A non-competitive inhibition model for Cr(VI) reduction successfully predicted effluent conditions at different loading conditions and during barrier regeneration by acid washing and electrokinetic remobilization. Optimum parameters that resulted in the best fit to experimental datacomprised of maximum Cr(VI) reduction rate coefficient km = 0.221 h-1, half velocity concentration Kc =11.6 mg.L-1, non-competitive inhibition coefficient K = 145 mg.L-1, and the cells’ Cr(VI) reduction capacity Rc = 0.964 g.g-1. The results demonstrated for the first time, the potential for sustainable long-term operation of biological permeable reactive barriers for treatment of Cr(VI).
