Bioremediation of diesel-contaminated soil by microorganisms immobilised in polyvinyl alcohol

It is argued that bioaugmentation is best reserved for particularly recalcitrant contaminants where an autochthonous population may be missing, and that selection has produced the microbial diversity required for clean-up of less recalcitrant contaminants. However, biodegradable contaminants may persist as a result of sub-optimal environmental conditions such as temperature, pH, electron acceptor availability and biotic factors such as predation by protozoa. A role for immobilised-cell bioaugmentation may therefore still be envisaged for the bioremediation of, say, persistent fuel spills under such conditions, or in extreme environments. In this laboratory-scale study we examined the potential of immobilised hydrocarbon-degrading microorganisms for the clean up of diesel-contaminated soil. We used polyvinyl alcohol (PVA) cryogelation as an entrapment technique and microorganisms indigenous to the site. We constructed laboratory biopiles to compare immobilised bioaugmentation with liquid culture bioaugmentation and biostimulation. In terms of percentage removal of diesel after 32 days, the immobilised systems were found to be the most successful, with greatest removal in a co-immobilisation system containing PVA-entrapped microorganisms and a synthetic oil absorbent. Least success was achieved with a commercial liquid bioaugmentation agent containing surfactants and having a low pH, which also produced significant phytotoxicity. Other advantages of PVA cryogelation are discussed.