EVALUATION OF CHITINOUS MATERIALS AS A MULTIFUNCTIONAL SUBSTRATE FOR THE REMEDIATION OF MINE IMPACTED WATER
Open Access
- Author:
- Robinson Lora, Mary Ann
- Graduate Program:
- Environmental Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 18, 2009
- Committee Members:
- Rachel Alice Brennan, Dissertation Advisor/Co-Advisor
Rachel Alice Brennan, Committee Chair/Co-Chair
Brian Dempsey, Committee Member
Bruce Ernest Logan, Committee Member
Susan Louise Brantley, Committee Member
Peggy Ann Johnson, Committee Member - Keywords:
- precipitation
metal biosorption
manganese removal
passive treatment
acid mine drainage
sulfate reduction - Abstract:
- The generation of mine impacted waters is one of the most serious environmental problems originated by the mining industry. For the remediation of these metal-laden and often acidic streams, several passive treatment technologies have been applied. The success of these technologies greatly depends on the selection of the substrate. This study evaluates the ability of crab-shell chitin (SC-20) as a multifunctional substrate for MIW treatment. Both biologically active and abiotic tests were conducted to 1) assess the contributions of each of the components of SC-20 (chitin, proteins, and minerals) to the observed changes, and 2) compare the performance of crab-shell chitin to other commonly used materials. In biologically active microcosm tests, SC-20 demonstrated a superior performance in comparison to spent mushroom compost and sodium lactate. The addition of SC-20 increased the pH from 3.0 – 3.5 to near neutral in 2 – 3 days, steadily generated alkalinity at a rate of 26.5 – 40.3 mg CaCO3 /L-d, and supported the activity of sulfate reducing bacteria (SRB) with a lag period of 7 – 9 d and sulfate reduction rates of 11.9 – 17.8 mg SO42-/L-d. While no major changes were observed using spent mushroom compost as a sole substrate, alkalinity generation and sulfate reduction rates promoted by sodium lactate (30.3 mg CaCO3 /L-d and 24.8 mg SO42-/L-d) were comparable to those obtained with SC-20, but such changes only occurred after 27 days of incubation. Al and Fe removal was observed with all three materials, but it was much faster with SC-20. The latter was the only substrate able to partially remove manganese (>73%). In biologically active columns using SC-20, 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 or 268 pore volumes (PV)) without showing signs of exhaustion. Metals (Al, Fe, and Mn) were completely removed for 171 PV. Manganese and iron breakthroughs occurred after 174 and 234 PV, respectively, whereas aluminum breakthrough was never observed. The steady generation of alkalinity in SC-20-treated systems was attributed to the dissolution of chitin-associated carbonates (mainly calcite), while the prompt onset of the SRB activity was supported by the fermentation of chitin and its associated proteins. Results from thermodynamic geochemical modeling using PHREEQC indicate that Al removal was likely due to the precipitation of hydroxides and/or alunite (KAl3(SO4)2(OH)6). Iron removal appeared to be driven by precipitation of ferric oxides at the beginning of the test, and by iron sulfide precipitation once the SRB became active. The partial removal of manganese could be explained by the formation of rhodochrosite (MnCO3), although other mechanisms like sorption or precipitation of other minerals cannot be discarded. Abiotic and anoxic tests were conducted to isolate the chemical and physical treatment mechanisms from those driven by biological activity, using SC-20 with different grades of purity. In particular, the generation of alkalinity and the removal of manganese due to mineral dissolution and precipitation were evaluated and compared to those obtained using limestone in closed-system and kinetic tests. In closed systems with a contact time of 72 h, manganese removal ≥95% (initial concentration = 10 mg/L) was obtained using only 5 g/L of SC-20 (raw or deproteinized); the pH was increased from 3 to 9.2-10.2; and 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%). Results from kinetic tests indicated that removal of ≥95% of the initial Mn load by SC-20 was achieved after 48 h. Geochemical calculations (PHREEQC) indicate that limestone-treated systems were close to equilibrium with calcite, while octacalcium phosphate (Ca4H(PO4)3) appears to be the controlling phase in systems treated with SC-20. The removal of manganese could be attributed to the precipitation of rhodochrosite (MnCO3) and/or MnHPO4. The faster changes observed with the two grades of SC-20 compared to limestone could be attributed to their larger surface area and their distinct composition, including phosphates and soluble organic compounds. The sorption of manganese onto the organic components of SC-20 (chitin and proteins) was evaluated using two purities of SC-20 (demineralized (chitin + proteins) and demineralized/deproteinized (“pure” chitin)) under different pH conditions by means of kinetic tests and sorption isotherms. The kinetics of manganese adsorption onto both types of solids was well described by the pseudo-second order model, with faster changes occurring under alkaline conditions and with “pure” chitin. The equilibrium of adsorption was well described by the Langmuir model. The maximum sorption capacity (qm) was found to depend greatly on the pH of the solution, with minimal or no sorption observed at pH <5. At higher pH regimes, qm values ranged from 0.165 (at pH 5.4) to 0.981 (at pH 8.7) for “pure” chitin, and increased from 0.878 (at pH 5.2) to 5.437 (at pH 8.6) when both chitin and protein were present. Results clearly suggest that the chitin-associated proteins offer additional sorption sites for manganese.