Some Studies on Contaminated Soil Stabilization
The rapid industrialization has brought about a great amount of industrial waste, which requires special attention in terms of its safe and proper disposal. Depending upon the severity of the contaminants and toxins present in the waste, a proper retention strategy should be adopted. If the retention strategy has its limitations, either in terms of its cost effectiveness or handicaps associated with its execution, a suitable remediation strategy must be employed. Remediation of the hazardous waste sites, around the world, is posing a very challenging task to the present day engineering professionals in terms of the financial burden to be borne by the industry, the government and the taxpayers. As such, the development of more effective, less costly remediation technologies must be worked out. One of the major contaminants found in soils, at such hazardous waste sites, are heavy metals coming out of various, small as well as large, industries viz., battery recycling plants and industrial effluent treatment facilities. The most frequently found heavy metal contaminants, at such industrial sites and facilities, include lead (Pb), mercury (Hg), arsenic (As), chromium (Cr), cadmium (Cd), and copper (Cu), zinc (Zn), cobalt (Co), tin (Sn) and nickel (Ni). Normally, two basic strategies being adopted, world wide, for the remediation of heavy metal contaminated soils are; minimization of migration potential of the contaminant metals (e.g., solidification, stabilization, vitrification) and extraction/separation of the contaminant metals (e.g., soil washing, soil flushing). Stabilization has been found to be particularly well suited for the sites where the hazard involves large quantities of soils contaminated at low levels. In many instances it may not be environmentally sound nor cost-effective to excavate, transport, and landfill or incinerate soils contaminated with low levels of pollutants. This may be either due to the additional air pollution caused by excavation equipment, trucks, and the exposure of buried contaminated soils to the air, which enhances the volatilization of organics or the increased risks as a result of traffic accidents. In situ stabilization is an alternative which reduces some of the hazards normally associated with the stabilization of contaminated soil, at the site. In situ stabilization of contaminated soil with the aid of additives such as fly ash helps in reducing the hazard of both the contaminants present in both the soil as well as the fly ash. By using this approach we are able to effectively dispose of both the waste as well as the fly ash. It is necessary to determine the exact methodology for the process for successful stabilization of the waste. The efficacy of the process can undergo significant variations with respect to various parameters such as the type of the additive (fly ash), the proportion of waste:additive ratio, the pH of the mixture, and the pressure applied during the stabilization process. Generally, the main aim of these processes is to meet the toxicity characteristic (TC) values specified by the USEPA or other such regulatory agencies. However, an important parameter to evaluate the efficiency of this process would be to determine the long-term leachability of the contaminants from such a stabilized mass. There has been much concern regarding the long-term stability of immobilized hazardous wastes. Certain leaching procedures claim to be equivalent to tens of thousands of years of natural leaching action in the environment. However, no actual long-term data are available because the technology has only been practised for about 25 years at present. Nevertheless, there is no established alternative to S/S technology for management of hazardous metals. Therefore, models are needed to make intelligent estimates of long-term effects.
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