Efficient metal removal and neutralization of acid mine drainage by crab-shell chitin under batch and continuous-flow conditions

Crab-shell chitin was evaluated as a multifunctional substrate for treating acid mine drainage (AMD) in both batch-microcosms and continuous-flow column tests. In microcosms, crab-shell chitin was able to treat AMD from three different sites with similar results: pH increased from 3.5 to ∼7.5 within 2 days; alkalinity increased at a rate of 37.9 ± 2.2 mg CaCO3/L day; and sulfate was reduced at a rate of -13.6±2.6mgSO42-/Lday. In columns, a hydraulic retention time of 11.2 h was enough to raise the pH from 3.5 to ∼7.5. Alkalinity increased at a rate of 50 ± 20 mg CaCO3/day, and lasted throughout the duration of the test (125 days, 268 pore volumes (PV)) without showing signs of exhaustion. Metals (Al, Fe, and Mn) were completely removed for 171 PV, and geochemical modeling indicates that they likely precipitated as insoluble hydr(oxides), sulfides, and carbonates. Manganese and iron breakthroughs occurred after 174 and 234 PV, respectively, whereas aluminum breakthrough was never observed. These results demonstrate for the first time that crab-shell chitin can completely remove metals and neutralize the pH of AMD under continuous-flow conditions.