Remediation of mine-impacted water using crabshell chitin as an electron donor and alkalinity source

A major environmental problem faced by the mining industry is the production of mine impacted water (MIW). These acidic, metal contaminated streams have to be properly managed and remediated to prevent adverse effects on humans and wildlife. Three basic requirements need to be satisfied to achieve a successful remediation of MIW: pH neutralization, reduction of sulfate concentrations, and removal of dissolved metals. The aim of this study was to evaluate the sustainable waste material, raw crabshell chitin, as a substrate for MIW remediation in both batch microcosm and continuous-flow column tests. For microcosm tests, samples of MIW and associated benthic sediments were collected from three different sites within Centre County, Pennsylvania. For each site, a series of duplicate sacrificial microcosms containing 120 mL of MIW, 1.0 g of sediment (bacterial source), and 0.5 g of SC-20 crabshell chitin were prepared and incubated in the dark under anoxic conditions. Continuous-flow tests were conducted in duplicate for one site. Clear PVC columns were packed with a mixture of 25 g of chitin, 50 g of sediment, and ∼500 g of silica sand within the first 1-ft segment of the column and the remainder was packed with silica sand. A singlet control column without chitin was also evaluated. Influent MIW was pumped upward through the columns at a continuous rate of 0.25 ml/min. In microcosm tests, the dissolution of carbonate minerals associated with the crabshell chitin caused rapid changes in the systems. In only two days, near neutral conditions were reached from an initial pH 3.5. Alkalinity was steadily generated while acidity was removed at rates of 37.9 and -27.5 mg CaCO₃ /L-d, respectively. No significant differences were observed in the results from the three evaluated sites. The activity of sulfate reducing bacteria was evident after 8 days of incubation, with reduction rates of -11.9 - -16.5 mg SO₄ ^2-/L-d. Results from the column tests indicate that a retention time of 11.2 h is enough to raise the pH from 3.5 to ∼7.5. Alkalinity was generated at a rate of 50 ± 20 mg Ca-CO ₃/d, without showing signs of exhaustion by the end of the test (273 pore volumes). Metals were removed at 100% efficiency for the first 80 days (175 pore volumes). Metal removal was likely due to the precipitation of insoluble phases: aluminum hydroxides, manganese carbonate (rhodochrosite), and iron hydroxides and/or sulfides. Manganese and iron breakthroughs were observed after 80 and 109 days, respectively, whereas aluminum was completely removed throughout the duration of the test. The obtained results show crabshell chitin as a promising electron-donor and alkalinity source for microbial activity and neutralization of acidic streams. In particular, the removal of manganese at relatively low pH, compared to the high pH commonly required in conventional Mn-removal processes, may represent a major advantage of this material over alternative substrates.