Environ. Sci. Technol. 2000, 34, 1081-1087
An Integrated Method Incorporating Sulfur-Oxidizing Bacteria and Electrokinetics To Enhance Removal of Copper from Contaminated Soil GIACOMO MAINI,§ AJAY K. SHARMAN,§ GARRY SUNDERLAND,‡ CHRISTOPHER. J. KNOWLES,† AND S I M O N A . J A C K M A N * ,† IBS-Viridian Ltd., 114-116 John Wilson Business Park, Thanet Way, Whitstable, Kent CT5 3QT, U.K., EA Technology Ltd., Capenhurst, Chester CH1 6ES, U.K., Oxford Centre for Environmental Biotechnology, Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, and NERC Institute of Virology & Environmental Microbiology, Mansfield Road, Oxford OX1 3SR, U.K.
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 sulfuroxidizing 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%.
Introduction In the U.K. alone, 200 hundred years of industrialization has resulted in an estimated 200 000 contaminated sites (1). A significant number of these contain cocktails of toxic chemicals that provide a hazard to both human health and the environment. The Confederation of British Industry has recently estimated the cost of remediating this contaminated land at £ 20bn (1). The implementation of the U.K. 1995 * Corresponding author phone: +44 1865 281630; fax: +44 1865 281696; e-mail:
[email protected]. § IBS-Viridian Ltd. ‡ EA Technology Ltd. † University of Oxford and NERC Institute of Virology & Environmental Microbiology. 10.1021/es990551t CCC: $19.00 Published on Web 02/17/2000
2000 American Chemical Society
Environment Act, increases in landfill tax. and a commitment by the U.K. government to build 60% of new houses on previously developed sites (2) have increased the pressure to develop effective remediation technologies. Over 60% of sites are co-contaminated with metals and organics. The metals are frequently found as complex mixtures. and many former industrial sites contain highly toxic metals such as cadmium, nickel. and arsenic. Therefore, in the U.K., as in other countries, the potential for developing these sites together with the possibility that metals may leach into groundwater or enter the food chain through plant material means that remediation of metal-contaminated land is a priority. There are currently few comprehensive remediation technologies available for such sites. Within the mining industries, technologies have been developed for the recovery of valuable metals from ores and rock/soil materials. Indeed the use of bacteria in bioleaching has become a prominent method of recovery (3). Naturally occurring iron- and sulfur-oxidizing bacteria from mining wastes have been isolated and strains selected for their ability to solubilize metals from different substrates (4). In this respect, the bacteria involved in bioleaching processes are able to convert metal sulfides to their respective sulfates, thereby transforming them from insoluble to soluble salts. The metals can be recovered by washing and further processing. This established technology is now being extended to investigate the leaching of contaminating metals from soils as a bioremediation technique (5). The application of mixed cultures of bacteria has proved successful in mobilizing metals within different soils. Difficulties, however, may be presented by inhibition of microbial activity due to the mobilized metals, with the potential for the entire bioleaching process to come to a halt (6). The presence of anionic species is also known to inhibit sulfur oxidation as pH values are reduced (7). The potential for metals to leach from soils means that to avoid groundwater contamination the process has either to be conducted ex situ, or, if in situ, with a metal removal system. Electrokinetics is an emerging engineering technique for the remediation of contaminated land (8-10). The application of a direct current to soil, by insertion of electrodes, leads to the generation of hydrogen ions at the anode and hydroxyl ions at the cathode. These migrate into the soil and the hydrogen ions can displace adsorbed metal ions into the pore fluid of the soil. By manipulation of the conditions surrounding the cathode, an acidic pH can be maintained throughout the soil. Metal ions, once solubilized, can be transported by electromigration through the soil and recovered at the cathode. The process has been effective in both model and real systems and has been applied to sites in the United States (11) and in Europe (12). The combination of bioleaching and electrokinetics has the potential to overcome several of the limitations of the individual techniques with the possibility that some of the combined attributes may prove to be synergistic. In particular, electrokinetics mobilizes metal ions provided their speciation is appropriate. Metals as hydroxides or oxides may be solubilized by the electrokinetic acidification, but those present as insoluble sulfides, a common speciation in former gasworks sites and mining wastes, will not be extracted by this method. However, the bacteria involved in bioleaching processes can convert metal sulfides to sulfates, thereby enabling their solubilization and subsequent transport by electromigration. In addition, the directional transport of metal ions by electrokinetics is a useful complement to bioleaching as solubilized metals can be removed at the VOL. 34, NO. 6, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Characteristics of Standardized Silt Soil parameter pH soil density (kg/dm3) extractable phosphorus (mg/L) particle size, diameter (mm) cationic exchange capacity (me/100 g) soil buffering capacity (g/kg) (calcd as the amount of [H+] from sulfuric acid required to acidify the soil to pH 2) permeability, k (m/s) copper concn (mg/kg) calcium carbonate (%) total sulfur (%) total nitrogen (%) organic matter (%) organic carbon (%) C:N ratio
value 8.1 1.350 24
