Solidification processes produce monolithic blocks of waste with high structural integrity. The contaminants do not necessarily interact chemically with the solidification reagents (typically cement/ash) but are mechanically locked within the solidified matrix. Stabilization methods usually involve the addition of chemical binders, such as cement, silicates, or pozzolans, which limit the solubility or mobility of waste constituents even though the physical handling characteristics of the waste may not be changed or improved. Methods involving S/S techniques are often proposed in RODs and RI/FSs for lead battery recycling sites. Solidification/stabilization of contaminated soil can be conducted either in situ or ex situ. In situ S/S techniques are now considered innovative and are discussed in Section 4.
Excavation and off-site disposal of contaminated soil (with or without solidification/stabilization pretreatment) to a landfill have been performed extensively at many sites. Landfilling of hazardous materials, especially hazardous wastes, is becoming increasingly difficult and expensive as a result of growing regulatory control, and may be cost-prohibitive for sites with large volumes, greater depths, or complex hydrogeologic environments. In addition, disposal capacity for radioactive and mixed waste is extremely limited. Determining the feasibility of off-site disposal requires knowledge of land disposal restrictions and other regulations developed by state governments.
Acid extraction uses an acid, such as hydrochloric acid, to extract heavy metal contaminants from soils. In this process, soils are first screened to remove coarse solids. Hydrochloric acid is then introduced into the soil in the extraction unit. The residence time in the unit varies depending on the soil type, contaminants, and contaminant concentrations, but generally ranges between 10 and 40 minutes. The soil-extractant mixture is continuously pumped out of the mixing tank, and the soil and extractant are separated using hydrocyclones.