Re-use of passive treatment Fe-sludge for remediation of As-contaminated waters

Abstract

With a number of legacy metal mine sites in Japan, it is becoming increasingly important to accelerate the technological shift towards more sustainable and economical acid mine drainage (AMD) treatment. As one of the few passive treatment sites in the country to date, this study monitored the Fe-removal performance of a large-scale microbiological passive treatment plant and its required Fe-sludge waste disposal. The plant has been successfully operating for the past 4 years, with average Fe2+ oxidation and Fe removal efficiencies of 92% and 82%, respectively, to meet the effluent standard. The resulting Fe-sludge waste had to be removed twice to date and its reusability was investigated. The Fe-sludge, consisting mainly of amorphous goethite, was found to be an effective adsorbent, especially for As(V). The As(V) adsorption capacity of acidic as-received Fe-sludge was 49.2 mg/g, and the As(III) adsorption capacity of alkali pre-washed Fe-sludge was 25.1 mg/g. Based on a series of continuous column tests, for As solutions of pH 7–5, the removal capacity of Fe-sludge was almost identical for As(V) and As(III) (∼19 mg/g at HRT 6 h for the period of >99% As removal). For acidic As solutions of pH 5–2, the removal capacity of As(V) became greater (∼40 mg/g) than that of As(III) (∼13 mg/g) at HRT 6 h for the period of >99% As removal. A significant amount of As(III) can be rapidly desorbed from the Fe-sludge surface, especially at acidic pH, so the usability of the Fe-sludge was found to be more favorable for As(V). The application of a post-treatment at pHinitial 1.0, 70 °C was effective in inducing a phase transformation of As(V) ions to scorodite-like minerals. This effect was significantly accelerated by the addition of scorodite seed crystals to the post-treatment reaction, resulting in a > 100-fold improvement in TCLP As leachability from 45.1 (mg/L) to 0.37 (mg/L), well below the US disposal standard of 5.0 mg/L.