BIOAVAILABILITY OF ARSENIC AND PHOSPHORUS IN A FLORIDA SOIL AMENDED WITH WATER TREATMENT RESIDUALS: EFFECT OF EQUILIBRATION TIME

The USEPA has classified Arsenic (As) as a Group A human carcinogen. Indiscriminate anthropogenic use of arsenical compounds have resulted in the contamination of a wide variety of environmental media, including mine sites, cattle dipping vat sites, wood treatment sites, pesticide treated agricultural fields, fruit orchards, etc. Ex-situ methods for remediation of As-contaminated sites are highly expensive as well as destructive to the ecosystem. Hence, in-situ methods of remediation, which are less disruptive and less expensive are becoming increasingly popular for cleanup of As-contaminated sites. In this study, we explored the feasibility of land application of water treatment residuals (WTRs) to remediate As-contaminated soils. Metal salts, such as ferric chloride and aluminum sulfate are commonly used in the municipal water treatment processes to destabilize colloids for subsequent flocculation and water clarification. WTRs are waste materials from these treatment plants, typically composed of Fe/Al-oxides, activated C, and high molecular weight, long-chain polymers. Several studies have demonstrated the tremendous capacity of the WTRs to adsorb phosphorus (P), thereby decreasing its bioavailability. Since P and As have similar geochemical properties, we assumed that the WTRs can similarly retain As in contaminated soils. In this laboratory incubation study, Immokalee soil (a sandy Spodosol from Florida with minimal As-retention capacity) was spiked with 90 mg/kg of As in the form of sodium arsenate and 115 mg/kg of P in the form of Triple Super Phosphate; the P-fertilized, As-contaminated soil was then amended with 3 WTRs (Al-, Fe- and Ca-WTR) at 5 rates, ranging between 0 to 5% wt/wt. Bioavailability of As and P were determined at 0-time (immediately after WTR-amendment), after 6-months and 12-months of equilibration, using an in-vitro gastrointestinal model for As and the Mehlich-III extraction method for P. Results from the three time periods clearly demonstrate the ability of Fe- and Al-WTRs (but not Ca-WTRs) to decrease As bioavailability, even in the presence of high concentrations of P. Results also indicate a significant effect of equilibration time on As and P bioavailability; as the soil-pesticide-fertilizer-WTR mix aged, the bioavailable concentrations of As decreased.