The role of precipitation and biosorption in the abiotic removal of manganese with crab-shell chitin

This study evaluated manganese removal using crab-shells (SC-20) under abiotic, anoxic conditions. Tests were conducted to assess the contributions of each of the components of SC-20 (chitin, proteins, and minerals) to the observed changes. Manganese removal due to precipitation was evaluated in closed-system tests using raw or deproteinized SC-20. Manganese removal due to adsorption onto the organic components of SC-20 (chitin and protein) was evaluated under different pH conditions using sorption isotherms. In closed-system precipitation tests with a contact time of 72 h and an initial manganese concentration (Mn_o) of 10 - 250 mg/L, removals of ~60 to >95% were obtained. Manganese removal decreased with increasing Mno and could be attributed to the precipitation of rhodochrosite (MnCO₃) and/or MnHPO₄. Significant increases in pH (from pH 3 to pH 7.7 - 9.9) and alkalinity (from 0 to 55 - 525 mg CaCO₃/L) were also observed as a result of the dissolution of chitinassociated minerals. These changes appear to be promoted by the relatively large surface area and distinct composition of SC-20, including phosphates and soluble organic compounds. The equilibrium of adsorption was well described by the Langmuir model. The maximum sorption capacity (q_m) depended greatly on the system's pH, with negligible sorption at pH < 5. At higher pH regimes, q_m ranged from 0.165 to 0.981 for chitin. When both chitin and protein were present, q_m increased 5 - 7 times, suggesting that chitin-associated proteins offer additional sorption sites for manganese. Results demonstrate that Mn removal with SC-20 is achieved by a combination of the reactivity of chitin-associated minerals and the sorption capacity of chitin and its associated proteins. These characteristics could be easily exploited in polishing systems for the removal of the historically difficult Mn in mine impacted waters.