Precipitation and processing of solids from high pH, high Si groundwater

Abstract

This research is concerned with the treatment of highly contaminated groundwater from the former Occidental Chemical Corporation manufacturing site at Tacoma, WA. During various stages of production that took place at this site from 1927 till mid-1980’s, highly caustic brines and chlorinated solvents were released into the ground resulting in widespread contamination of groundwater, notably the elevation of its pH to > 12 and the presence of volatile organic compounds (VOCs) across the site. Elevated pH of the groundwater has resulted in the dissolution of significant amounts of Si (concentrations as high as 48000 ppm). This silica could precipitate on granular activated carbon (GAC) used for VOC removal. This necessitates Si removal from high pH, high Si groundwater before VOC treatment, when this groundwater is extracted and treated ex-situ. On the other hand, groundwater can also be acidified in-situ to precipitate Si containing solids in the aquifer and form a hydraulic barrier to prevent contaminant plume propagation. In this study, groundwater samples from the site were analyzed for Si and various other elements. Visual Minteq modelling was used to predict and interpret observed concentration vs pH trends for Si, Al, Ca and Mg. Experiments were conducted to study the precipitation of dissolved Si by acidification and CaCl2 addition. A treatment approach was explored that reduced the volume of generated solids (that would require handling and disposal) to 16% of the groundwater volume. The generated solids were analyzed to be 82% SiO2, indicating that they might find application in the cement industry. Experiments to study the formation and stability of a hydraulic barrier in a laboratory scale set-up indicated that in-situ precipitation can be induced, but the effects on hydraulic conductivity of the soil are difficult to predict.