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The electrokinetic remediation of an historically contaminated soil is described. The soil was contaminated with a range of metals including lead, zinc, manganese, copper and arsenic, polycyclic aromatic hydrocarbons (PAHs) and benzene, toluene, ethylbenzene and xylene (BTEX). A small-scale experiment (973.2 g dry weight soil), utilising a planar electrode con®guration, investigated the potential for moving metals and organics. After 23 days treatment at a current density of 3.72 A /m À2 , 44% of calcium and 29% of manganese were removed from the soil at the cathode. Of the other contaminating metals, zinc and lead moved towards the cathode, but with no signi®cant removal from the soil. Movement of PAHs was also observed, with a 94% reduction in concentration in the third of the soil closest to the anode after 23 days. A larger scale experiment (46.7 kg dry weight soil) utilised a hexagonal array of tubular anodes surrounding a central tubular cathode. Treatment for 112 days led to acidi®cation of the soil to pH 2.59 closest to the anode in a direct line between the anode and cathode. Soil not directly in line between the electrodes was not acidi®ed signi®cantly. Movement of metal ions was observed, in line with the electrodes, with concentrations of lead and arsenic increasing to 162% and 171% of starting concentrations closest to the anode, respectively, and those of zinc, copper and manganese decreasing to 42%, 68% and 57%, respectively. At positions not directly in line with the electrodes, no signi®cant metal movements were observed. Overall, there was no signi®cant removal of contaminating metals from the soil. PAHs and BTEX compounds were moved by electroosmosis towards the cathode, with soil concentrations of PAHs reduced from 720 mgkg À1 to 4.7 mgkg À1 after 22 days. PAHS (28 mg) and benzene (9660 mg) were recovered in granular activated carbon (GAC) columns.

The combination of bioleaching and electrokinetics for the remediation of metal contaminated land has been investigated. In bioleaching, bacteria convert reduced sulfur compounds to sulfuric acid, acidifying soil and mobilizing metal ions. In electrokinetics, DC current acidifies soil, and mobilized metals are transported to the cathode by electromigration. When bioleaching was applied to silt soil artificially contaminated with seven metals and amended with sulfur, bacterial activity was partially inhibited and limited acidification occurred. Electrokinetic treatment of silt soil contaminated solely with 1000 mg/kg copper nitrate showed 89% removal of copper from the soil within 15 days. To combine bioleaching and electrokinetics sequentially, preliminary partial acidification was performed by amending copper-contaminated soil with sulfur (to 5% w/w) and incubating at constant moisture (30% w/w) and temperature (20 °C) for 90 days. Indigenous sulfur-oxidizing bacteria partially acidified the soil from pH 8.1 to 5.4. This soil was then treated by electrokinetics yielding 86% copper removal in 16 days. In the combined process, electrokinetics stimulated sulfur oxidation, by removing inhibitory factors, yielding a 5.1-fold increase in soil sulfate concentration. Preacidification by sulfur-oxidizing bacteria increased the cost-effectiveness of the electrokinetic treatment by reducing the power requirement by 66%.

The coupling of electrokinetic movement of an organic contaminant, 2,4-dichlorophenoxyacetic acid (2,4-D), through soil and its biodegradation in situ has been demonstrated. In a first experiment, the direction and rate of movement of 2,4-D were determined using homogeneously contaminated soil (864 mg 2,4-D/kg dry weight soil) compacted into six individual compartments, 6 cm long, 3 cm wide, and 4 cm deep. Each compartment was bordered by a carbon felt anode and a stainless steel cathode. The application of a current density of 3.72 A/m(2) led to migration of 2,4-D towards the anode at a rate of approximately 4 cm/day. In a second experiment, electrokinetic movement and biodegradation were combined in situ. Sterilized silt soil contaminated with ring-labeled 14C-2,4-D (811 mg 2,4-D/kg dry weight soil) was compacted into a single soil compartment, 22 cm long, 7 cm wide, and 4 cm deep, in a 4.5 cm region adjacent to the cathode. The remainder of the compartment was filled with sterilized soil (to a total weight of 1,015 g). Burkholderia spp. RASC c2 (1.88 x 10(11) cells), a tetracycline-resistant bacterium with chromosomally encoded degradative genes for 2,4-D, was inoculated into the soil at a position 14-16 cm from the cathode. The reactor was placed within a sealed perspex box, with a constant air flow connected to sodium hydroxide traps. Under an applied current density of 0.89 A/m(2), the pollutant moved towards the bacteria. As it reached the inoculated region, its concentration decreased in the soil and 14CO2 was recovered in the traps. At the end of the experiment, 87.1% of radiolabel had been removed from the soil, 5.8% of which was recovered as 14CO2. A third, control, experiment showed a significant contrast in the absence of an electric current, where a slow rate of diffusion controlled the movement of both 2,4-D and bacteria in the soil and biodegradation occurred at the interface between the diffusing fronts.

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