Abstract
This work has investigated the use of charcoal in the remediation of land contaminated with mixtures of metals and hydrocarbons using two strategies. Firstly, the removal of metal toxicity using highly metal adsorbent charcoals to stimulate the degradation of hydrocarbons and secondly, the use of charcoal as a microbial carrier to provide a population of specific contaminant degraders to enhance their degradation. In a soil contaminated with a mixture of heavy metals, arsenic and phenanthrene, addition of highly metal adsorbing charcoal (nettle charcoal) and iron filings, alone and in combination, were effective at reducing copper leaching, restoring microbial function and enhancing phenanthrene degradation. In addition, iron filings were effective at reducing leachable arsenic levels, but had a negative effect on the soil structure. Wood charcoals were evaluated as microbial carriers for use in bioaugmentation. Survival of P. fluorescens colonising wood charcoals was found at copper concentrations thirty times higher than that toxic to free cells. This effect could be attributed to the formation of a biofilm within the porous charcoal structure. Microcosm trials using charcoals inoculated with populations of hydrocarbon degraders were carried out to determine the necessity of bioaugmentation in the bioremediation of soils contaminated with both metals and hydrocarbons. The efficacy of microbial communities enriched on different hydrocarbon substrates to enhance the degradation of polyaromatic hydrocarbons and diesel range organics was assessed. In the soil examined, the use of specifically selected degrading populations were found to be unnecessary for enhancing the biodegradation of diesel range organics and phenanthrene once metal toxicity was removed by the addition of nettle charcoal. Removal of pyrene was low, despite the provision of pyrene degraders on inoculated charcoal, probably due to its low bioavailability to microorganisms. Removal of metal toxicity by charcoal amendment is highly effective at enhancing the degradation of organic compounds but low bioavailability remains a key limiting factor in bioremediation.