Microbial remediation of heavy metal ion polluted soil and water

Sammanfattning: Contamination of soil and water by heavy metal ions (HMs) is a problem all over the world.The use of biological methods alone or in combination with other techniques for treatment of such polluted sites constitutes a major challenge. The present thesis summarizes work based on four basic technologies: 1) use of Sulphate Reducing Bacteria (SRBs) for production of hydrogen sulphide which is used to treat heavy metal contaminated water; 2) mobilization of HMs present in soil or biomass and transfer to a water phase; 3) use of chelating agents such as ethylenediaminetetraacetic acid (EDTA) to form complexes with the HMs; 4) degradation of the chelating agent (EDTA) since it is recalcitrant and thus the spreading of it in to the environment must be controlled. Thus, a few different strategies were used to fulfil the demands to remove HMs from the environment. SRBs where grown on carriers as a biofilm allowing production of a high and stable concentration of H2S from the bioreactor. Precipitation of HMs was done from a water phase in a separate reaction chamber, thus allowing recovery of the precipitated metal for either reuse or safe disposal. To facilitate long term operation of the reactors, a poorly soluble carbon source was used. Newsprint was a suitable alternative carbon source that was made available for the SRBs by enzymatic hydrolysis. To that end, a culture of SRBs capable of hydrolyzing cellulose was isolated. Precipitation of HMs as metal sulphides is shown to have a good potential for the three metals (Cu2+, Pb2+, Hg2+) studied also when they were present in chelate complexes prior to precipitation. The removal of metal ions from the complex with EDTA thus allows reuse of EDTA in soil washing processes. When the remediation is finished EDTA is degraded in a specially designed bioreactor process. Biodegradation of EDTA was performed in an aerobic biofilm system with suspended carriers allowing treatment of solutions with up to 1000 mg EDTA/litre. The experiences from electroreclamation of Hg2+-polluted soil using iodine as chelating substance was similar to that from EDTA. Removal of HMs present in biomass was studied during production of biogas by circulating the leachate water trough a macroporous polyacrylamide adsorbent containing the chelating agent iminodiacetate (IDA). The trapped metal ions were then released during column regeneration and could subsequently be precipitated in a separate step. This treatment resulted in a potential biofertiliser (the residue from the biogas production process)with reduced content of heavy metals.