TREATMENT OF ARSENIC USING PERMEABLE REACTIVE BARRIERS

Arsenic is a common and widespread groundwater contaminant, which is derived from natural or anthropogenic sources. Arsenic commonly occurs in groundwater as an oxyanion, and is mobile under the conditions that prevail in most aquifers. Several permeable reactive barrier (PRB) systems have been installed for treatment of As-contaminated groundwater. The PRB sites include mine tailings impoundments, fertilizer manufacturing sites, thermal-electric power generation and transmission facilities and chemical production plants. A variety of reactive materials have been used in these installations. Some of the PRBs use reactive materials which include zero valent iron (ZVI) filings either with or without an organic carbon (OC) source such as wood chips or compost. Other installations contain basic oxygen furnace (BOF) slag, a byproduct of the steel refining process. Removal of As to below drinking water guideline concentrations has been observed in all of these PRBs. The PRBs range from a few meters in length to 600 m long. The mechanisms responsible for As removal in the PRBs containing ZVI and OC include microbially mediated sulfate reduction followed by metal (As) sulfide precipitation and secondarily by adsorption and coprecipitation on the ZVI corrosion surfaces. In the BOF slag barriers As is removed by adsorption and secondary mineral precipitation. Studies of the reactive material using selective chemical extractions, mineralogical examination and surface analytical techniques (e.g. SEM and synchrotron radiation based methods) are being conducted to determine the specific forms of the reaction products.