Chitin complex for the remediation of mine impacted water: Geochemistry of metal removal and comparison with other common substrates

Abstract

Abstract Remediation of mine impacted water (MIW) generally requires decreasing the acidity and concentrations of dissolved and/or particulate contaminants ( SO 4 2 - , metals and metalloids). By fulfilling these requirements in both laboratory and field trials, the sustainable composite waste material, crab-shell chitin complex (CC) has proven to be a promising substrate for MIW remediation, but has not yet been directly compared with other substrates under controlled conditions. In this study, remediation rates and metal removal mechanisms promoted by CC were evaluated and compared to the more commonly used lactate and spent mushroom compost (SMC) using sacrificial batch microcosms and geochemical modeling. Under comparable conditions with equivalent mass of substrate to water ratios, increases in pH were much faster in the microcosms containing CC than with the other substrates: CC increased the pH from pH 3.0 to near neutral in 3 d. In microcosms containing CC, steady alkalinity generation and acidity removal were observed at average rates of 26.5 and −25.2 mg CaCO3/L-d, respectively. The activity of SO 4 2 - -reducing bacteria was evident after 9 d of incubation, with average reduction rates of −17.8 mg SO 4 2 - /L-d. Similar changes in alkalinity, acidity, and SO 4 2 - were also observed in lactate-containing microcosms, but only after a 27 d lag period. No alkalinity generation or SO 4 2 - reduction activity was observed in bottles containing SMC. Aluminum removal (100%) was eventually observed with all substrates, but occurred much faster with CC. Results from thermodynamic geochemical modeling indicate that Al removal was consistent with the precipitation of hydroxides and/or alunite. Iron removal was consistent with precipitation of Fe(III) oxides and Fe(II) sulfides, as well as sorption onto CC and SMC. The addition of Na lactate interfered with such mechanisms due to complexation effects. Chitin complex was the only substrate able to partially remove Mn (>73%), likely due to the formation of rhodochrosite. The results of this study indicate that CC is an attractive substrate for treating metal-laden waste streams, especially those which are high in Mn.