Efficient removal of chromium from soil in a modified electrokinetic system using microbial consortium

Severe chromium contamination is a worldwide ubiquitous environmental issue [۱]. Remediation of Cr contaminated soils are amidst the highest priorities of various developed countries [۲]. Therefore, it is urgent to propose an economical, sustainable, and effective technique to remediate Cr from soil and reduce the risk of toxicity [۳]. Electrokinetic (EK) remediation process coupled with other remediation technologies was found to enhance the remediation more effectively by extracting/eliminating Cr out of soil matrix is more desirable and controllable, rather than immobilization/passivation of Cr in soil [۴].This technology involves the application of a direct current across the electrodes to remove contaminants from the soil based on the three fundamental mechanisms of electro-migration, electro-osmosis, and electrophoresis [۵]. In addition, a better choice of inexpensive and eco- friendly enhancing agent is the microorganisms capable of reducing Cr (VI) to Cr (III). The chromium-reducing bacteria were found not only to reduce, but also to favor the precipitation and adsorption of chromium [۶]. Different reports have shown that the novel combination of EK and bio-electrokinetic (BEK) methods realized the accelerated transportation of contaminated and enhanced EK performance [۷, ۸]. Therefore, the present study is attempted to demonstrate BEK and EK towards Cr removal from soil using chromium-reducing consortium microorganisms.The mixture of Cr-reducing microorganisms was isolated from tannery effluent in the biotechnology laboratory by enrichment culture technique. Sandy soil free of chromium, was collected from a non- agricultural area. The EK cell was divided into three compartments. The middle one was loaded with K۲Cr۲O۷-spiked soil (۵۰۰ mg/kg Cr (VI)). In the BEK experiment, ۱۰% (v/w) of the bacterial cultures was inoculated into the soil and the final humidity of soil was ۲۰%. The side compartment was designed to hold distilled water as electrolytes. Two graphite electrodes were used as anode and cathode. Constant voltage of ۱ v/cm was applied for ۸ days. Electrolyte of both the compartmentswas refreshed daily. Two sets of experiments were performed under the title of BioEK (bioelectrokinetic) and EK (electrokinetic). At the end of ۸ days, the Cr (VI) in the soil samples was extracted by alkaline digestion (US EPA ۳۰۶۰A method) and was quantified via the Diphenyl Carbazide colorimetric method. To separate out total Cr, each part of the soil was subjected to acid digestion and analyzed by inductively coupled plasma–atomic emission spectroscopy (ICP-AES).As it is seen in Fig. ۱, there was a significant difference in the residual concentration of chromium after ۸ days of treatment in electrokinetic and bio-electrokinetic experiments. The average residual concentration of Cr (VI) in the reaction chamber of EK was ۹۱.۲ mg/kg, which was significantly higher than ۲۱ mg/kg of BioEK. This result demonstrated that bacterial consortium was satisfactory enhancement techniques for EK remediation of Cr (VI)-contaminated soil.Various reports on active participation of biological organisms in combination with EK remediation have been reported by many researchers. A maximum Cr(VI) removal of ۹۸.۳۳% was reported using bio-EK method [۹]. The potentiality of acidophilic bacterium towards zinc removal using an integrated approach of bioleaching and EK remediation was reported by Selvi and Rajasekar [۱۰].