Calcium–phosphate treatment of contaminated soil for arsenic immobilization

► As concentration in soil was 278ppm after >5 decades of As-herbicide application. ► Phosphate treatments were tested for in situ fixation of arsenic in soil. ► Phosphate only treatment was not able to immobilize arsenic in soil. ► Phosphate–calcium treatment helped immobilize arsenic in soil at pH>7. ► Arsenic immobilization occurred by precipitation of As-phosphate phases. 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=278mg/kg) collected from an industrial site located in the southeastern USA was subjected to inorganic phosphate (Pi) treatments. Although Pi treatments have been previously used for flushing As from contaminated soils, in this study, contaminated soil was amended with Pi to study the possible immobilization of As through a co-precipitation mechanism. Specifically, the Pi 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 Pi treatment experiments were performed at different pH conditions, with and without the addition of Ca. Sorption of Pi 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 HNO3-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 Pi sorption experiments. This increased mobility of As after the addition of Pi resulted from the competitive desorption of As from the soil. Although Pi sorption at high pH (>8) was largely controlled by precipitation, As did not co-precipitate with Pi. Aqueous geochemical modeling indicated that the lack of As co-precipitation during Pi-only treatment primarily resulted from the deficiency of Ca in the system. When additional Ca (16.9mmol) was supplied along with Pi (3.38mmol), 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–Pi 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–Pi treatment for remedi