Bioremediation of organic and metal co-contaminated environments: Effects of metal toxicity, speciation, and bioavailability on biodegradation

Forty percent of the hazardous waste sites on the U. S. Environmental Protection Agencys National Priority List (NPL) are co-contaminated with metal and organic pollutants (Sandrin et al. 2000). Metals most frequently found at Superfund sites include arsenic, barium, cadmium, chromium, lead, mercury, nickel and zinc. Common organic cocontaminants include petroleum, chlorinated solvents, pesticides and herbicides. Conventional approaches to removing the organic pollutants at these sites, such as pump and treat, are costly and often ineffective (NRC 1994). Bioremediation is a viable alternative to conventional technologies, but metal toxicity at co-contaminated sites may limit its utility. Many studies report that metals inhibit general microbial activity (e.g., litter decomposition, methanogenesis, acidogenesis, nitrogen transformation), but a few have specifically investigated the impact of metals on organic pollutant biodegradation. The fact, that metals affect a myriad of microbial activities suggests that metals have the potential to affect the biodegradation of organics in co-contaminated environments. In some studies, metals have no impact or have a stimulatory effect on microbial activity. Thus, the effect of metals on organic pollutant biodegradation remains poorly characterized. This review discusses: 1) the toxicity of metals to microorganisms, 2) the roles metal speciation and bioavailability play in governing the extent to which metals affect organic pollutant biodegradation, 3) reported effects of metals on aerobic and anaerobic biodegradation, 4) patterns in which metals affect biodegradation, and 5) approaches to increasing organic biodegradation in co-contaminated systems.