Abstract
Acid mine drainage (AMD) generated at active and abandoned mines shows a low pH (<3.0) and sometimes contains high amounts of iron (Fe) (~1000 mg/L) and arsenic (As) (~50 mg/L). Although neutralization using chemical reagents is a common method for removing these elements, microorganisms such as Fe-oxidizing bacteria (FeOB) are also employed as a more efficient treatment option. One of the major challenges in the biological treatment of AMD is maintaining microbial activity regardless of seasonal variations in temperature and chemical composition (especially pH). In this study, we investigated the kinetic effect of FeOB on Fe removal from AMD using a biogenic Fe-sludge collected from an aerating reactor installed at the Date mine in Hokkaido, Japan. Arsenic removal experiments were also conducted to examine the potential secondary use of the Fe-sludge, which contains schwertmannite as an adsorption material. For the Fe removal experiment, the Fe-sludge was reacted with natural AMD (Fe: 148 mg/L; As: 0.24 mg/L; SO₄²⁻: 587 mg/L) at different solid/liquid ratios (1–10 v/v%) and pH conditions (3.0 and 3.5) for 120 min, and Fe2+ and Fe3+ concentrations were analyzed to determine the oxidation rate of Fe2+ using PHREEQC calculations. At pH 3.0, the Fe²⁺ oxidation rate was 4.5 × 10⁴ times higher with 5 v/v% Fe-sludge than without sludge addition, and a positive correlation was observed between the amount of Fe-sludge added (1–10 v/v%) and the Fe²⁺ oxidation rate constants. Little Fe²⁺ oxidation was observed when FeOB in the Fe-sludge was inactivated by freezing at −80 °C, indicating that Fe²⁺ oxidation is derived from the oxidation by FeOB in the Fe-sludge rather than by abiotic processes such as adsorption on schwertmannite. A higher Fe²⁺ oxidation rate was observed at pH 3.5 by the addition of Fe-sludge and most of Fe³⁺ was precipitated from solution, although Fe3+ remained in the solution at pH 3.0. Our results, therefore, indicate that accumulation of Fe-sludge and pH control in the reactor are required to maintain FeOB activity for effective Fe removal from AMD. For the As adsorption experiments, the Fe-sludge was reacted with simulated AMD containing As (8 mg/L) of different oxidation states (As(III) or As(V)) at pH 3–6 for 60 min. Experimental result shows that the biogenic Fe-sludge has a lower adsorption capacity than synthetic schwertmannite for both As(III) and As(V). Zeta potential measurements indicated that general schwertmannite has a positive surface charge at pH 2–3, whereas the Fe-sludge used in this study showed a negative surface charge across the entire experimental pH condition. The predominant species are H₃AsO₃0 neutral molecule for As(III) and H₂AsO₄⁻ anion for As(V) at the acidic condition solution, respectively. Therefore, the negatively charged surface of Fe-sludge could inhibit As adsorption, which would be caused by the incorporation of SO₄²⁻ and/or the accumulation of microbial cells in the Fe-sludge.