Calcium–phosphate treatment of contaminated soil for arsenic immobilization

Abstract The application of As-based herbicides at several industrial sites has resulted in numerous localized areas of As-contaminated soil. In this study, an As-contaminated soil (As = 278 mg/kg) collected from an industrial site located in the southeastern USA was subjected to inorganic phosphate (P i ) treatments. Although P i treatments have been previously used for flushing As from contaminated soils, in this study, contaminated soil was amended with P i to study the possible immobilization of As through a co-precipitation mechanism. Specifically, the P i amendment was aimed at simultaneous flushing of As from the soil with orthophosphoric acid and co-precipitating it as Ca–phosphate–arsenate phases. Bench-scale P i treatment experiments were performed at different pH conditions, with and without the addition of Ca. Sorption of P i on BH soil in the presence or absence of additional Ca was determined, along with the associated mobilization of As from the soil. A significant amount of the HNO 3 -digestible As (up to 55% at pH 4, 10–15% at pH 8, and ∼30% at pH 11) was released from the contaminated soil during the P i sorption experiments. This increased mobility of As after the addition of P i resulted from the competitive desorption of As from the soil. Although P i sorption at high pH (>8) was largely controlled by precipitation, As did not co-precipitate with P i . Aqueous geochemical modeling indicated that the lack of As co-precipitation during P i -only treatment primarily resulted from the deficiency of Ca in the system. When additional Ca (16.9 mmol) was supplied along with P i (3.38 mmol), the mobility of As decreased significantly at circum-neutral to high solution pH. Geochemical modeling suggested that the leachable As in the soil was potentially precipitated as As-bearing Ca–P i phases. X-ray diffraction analysis of precipitates separated from the treated soil and from the synthetic leachate confirmed that the formation of a poorly crystalline carbonate apatite phase occurred as a consequence of the treatment. The results of this study support the potential application of Ca–P i treatment for remediation of As-contaminated soil at environmentally relevant pH conditions.