A NOVEL IN-SITU CHEMICAL REMEDIATION TECHNIQUE FOR ARSENIC-CONTAMINATED SOILS: PRELIMINARY RESULTS

Toxicity of arsenic (As) in soils is determined by its solubility and bioavailability. In-situ immobilization of metals using inexpensive amendments such as minerals (apatite, zeolite, or clay minerals) or waste by-products (steel shot, beringite, iron-rich biosolids) to reduce bioavailability is an inexpensive alternative to the more expensive ex-situ remediation methods. Water Treatment facilities typically use Fe salts, alum or lime to remove colloids, color, and chemicals from water. Water treatment residuals (WTRs) are by-products of drinking water production and generally consist of sediments, metal oxides, activated C, and polymers. The WTRs are currently disposed off in landfills (at great expense to municipalities), stored in onsite lagoons, or discharged to sanitary sewer systems. Recent studies on land application of WTRs (thereby minimizing disposal costs) revealed their unusually high oxyanion retention capacity (e.g. phosphate). Since As is chemically similar to phosphorus, the oxyanions arsenate (As-V) and arsenite (As-III) may have the potential of being retained by the WTRs. Our working hypothesis is that WTRs have the capability of irreversibly retaining As, thereby reducing its bioavailability. Two soils were chosen for the study based on their potential differences with respect to As reactivity: an acid sand and an organic (muck) soil. The soils were spiked with 450 mg/kg of sodium arsenate, and equilibrated for a period of 1 year. The equilibrated soils were then amended using two WTRs (Al-WTR and Ca-WTR) at three rates (0, 5 and 10 mg/kg). A laboratory incubation study (3 mo) is in progress to assess As immobilization by the WTRs, and their effects on As bioavailbility. A sequential extraction scheme is being used to identify the various geochemical forms of As. Concentrations of these operationally-defined soil As forms will be correlated with the “in-vitro” bioavailable fractions of As to identify the As species that are least bioavailable. Our expectation, based on the successful studies previously conducted on phosphorus is that the cationic components of WTR would be able to significantly increase the adsorption of As oxyanions, thereby converting labile As species to irreversibly bound soil forms, hence, decreasing As bioavailability, and consequently, human health risk.