Biocontainment of PCBs on flat concrete surfaces and coprecipitation of PCBs and metals in boiler chemical cleaning wastewater by microbial carbonate precipitation.

摘要

Some industrial equipment such as electric transformers and capacitors are located in vaults and substations on flat concrete surfaces. Due to accidents, normal routine maintenance or replacements, these equipments may spill oil which may contain PCBs on these surfaces. These spills must be cleaned up in accordance with United States Environmental Protection Agency's (USEPA) guidelines. Current cleanup operations are abiotic in nature, encompassing both physical and chemical methods. These methods generate enormous amounts of impacted wastewater and solid wastes which have to be disposed of carefully at a cost. In addition, some chemicals used are harmful to both humans and other organisms. Epoxy coatings have been used to encapsulate PCBs on concrete surfaces. However, these coating systems can be ineffective because the adhesion with concrete is easily damaged by elevated temperatures causing failure of the coating system.;The use of biosealant obtained from microbial carbonate precipitation (MCP) on PCB-contaminated concrete surfaces was studied as a possible alternative to epoxy coatings. Sporosarcina pasteurii strain ATCC 11859 was used for this purpose. This bacterium metabolizes urea, and in a calcium-rich environment precipitates calcite which deposits on the surface.;This research therefore exploits this unique microbial activity to determine optimum conditions for MCP, and uses these conditions to lay a biosealant on the Polychlorinated Biphenyls (PCBs) contaminated concrete surface. An investigation into the possibility of solid-phase capture (by co-precipitation) of PCBs and metals in boiler chemical cleaning wastewater (BCCW) was also explored. The results indicate that the presence of bacteria and bacterial cell concentration have a significant influence in MCP and the rate of urea hydrolysis. At 25 mM/L Ca2+ concentration, increasing bacteria cell concentration from 106 to 108 cells/mL increases the CaCO3 precipitated and CO2 sequestrated by over 30%. However, when Ca2+ concentration is increased 10-fold to 250 mM/L Ca2+, the CaCO3 precipitated and CO2 sequestrated increased by more than 100% irrespective of urea concentration. This result shows that the amount of CaCO3 precipitated is determined by the concentration of the Ca2+ ions present. Reduction in the coefficient of water permeability by 1-5 orders of magnitude and high resistance to carbonation was also exhibited by the biosealed surfaces indicating a greater potential for obtaining a stronger coherent impermeable durable surface by MCP.;Coprecipitation results have shown that the PCBs and BCCW had little effect on the activity of the urease enzyme. Consequently, the hexane-based PCBs and the metals in the BCCW were effectively coprecipitated. However, no coprecipitation occurred in the oil-based PCBs. Chemical and Energy Dispersive X-ray (EDX) analyses on the coprecipitated solids confirmed the precipitation of metals in the BCCW. However, the ASTM Method D3987 test to determine the leaching potential of the coprecipitated metals was inconclusive, and more research needs to be done in this area.