English

Abstract

Crab-shell chitin has proven to be an efficient multifunctional substrate for the biological treatment of mine impacted waters (MIW). Beyond its capacity as an electron donor source, this material has shown high efficiency in the neutralization of acidic water and in the removal of metals, especially Mn. In this study, the performance of crab-shell chitin as a substrate for abiotic and anoxic MIW treatment was assessed to isolate its chemical and physical treatment mechanisms. Alkalinity generation and metal (Mn, Fe, Al) removal with crab-shell chitin were evaluated and compared to those obtained using limestone in closed-system and kinetic tests. Raw (R-SC20) and deproteinized (DP-SC20) crab-shell chitin were tested and compared to evaluate the effect of chitin-associated proteins. Anoxic, synthetic MIW (SMIW), with individual metal concentrations of 10 mg/L, was used in all tests. Systems for all tests were prepared and operated inside an anaerobic chamber by mixing crab-shell chitin or limestone with SMIW at predetermined ratios. In closed systems, 5 g/L of R- or DP-SC20 completely removed (≥95%) both Mn and Fe from single-metal SMIW. After 72 h, pH increased from 3 to 9.2-10.2, while 83-187 mg CaCO3/L of alkalinity was generated. In contrast, 5-125 g- limestone/L only raised the pH to 7.8-8.3, leading to lower alkalinity levels (56- 63 mg CaCO3/L) and poor metal removal efficiencies (≤85%). In kinetic tests with 5 g-DP-SC20/L, removal of ≥95% of the initial metal load was achieved after 0.5, 6, and 48 h for Al, Fe, and Mn, respectively. Geochemical calculations (PHREEQC) indicate that precipitation of Al-hydroxides and rhodochrosite (MnCO3) and/or MnHPO4 are the probable mechanisms for Al and Mn removal. In the case of iron, geochemical calculations point to hydroxides precipitation; however, visual observations suggest the formation of green rust, a precursor of other more stable phases like goethite or lepidocrocite. The faster changes observed with DP-SC20 compared to limestone could be attributed to its significantly larger surface area. These results are the first to verify and quantify the capacity of crab-shell chitin to treat MIW abiotically. Additional keywords: Manganese removal, passive treatment