The chemical and physical treatment mechanisms by which crab shell removes metals from mine impacted water (MIW) were evaluated under anaerobic and biologically limited conditions in closed systems and kinetic tests. Raw (R-SC20) and deproteinized (DP-SC20) crab shell were tested and compared to limestone to quantify the contribution of chitin-associated minerals and proteins to alkalinity generation and metal precipitation. Single-metal closed systems (initial Mn and Fe=0.18mM and Al=0.34mM) containing 5g/L of either R- or DP-SC20, yielded an increase in pH from 3 to 9.2-10.2, generation of 0.83-1.87mM of alkalinity, and resulted in ≥95% removal of metals within 72h. In contrast, 5-125glimestone/L only raised the pH to 7.8-8.3, produced lower alkalinity (0.56-0.63mM), and resulted in less metal removal (≤85%). In kinetic tests with 5g-DP-SC20/L, removal of ≥95% of the initial metal load was achieved after 0.5, 6, and 48h for Al, Fe, and Mn, respectively. Geochemical calculations (PHREEQC) indicate that limestone-treated systems were close to equilibrium with calcite (CaCO3), whereas octacalcium phosphate (Ca4H(PO4)3) appears to be a controlling phase in systems treated with R- and DP-SC20. The probable mechanisms for Mn removal are the precipitation of rhodochrosite (MnCO3) and/or sorption. In the case of Al and Fe, geochemical calculations point to the precipitation of hydroxides; however, visual observations in Fe systems suggest the formation of green rust, a precursor of other, more stable phases like goethite or lepidocrocite. Several factors may account for the faster changes observed with R- and DP-SC20 compared to limestone: increased dissolution and degree of supersaturation, the presence of phosphates, the release of organic compounds, and a significantly larger surface area. These results are the first to verify and quantify the capacity of crab shell-associated minerals to treat MIW under biologically limited conditions.
All Science Journal Classification (ASJC) codes
- Environmental Chemistry
- Geochemistry and Petrology