Modification of waste coal gangue and its application in the removal of Mn²⁺ from aqueous solution

We developed a new calcination method to convert coal gangue (CG), a common waste generated from coal production process, into a modified form, which could be used as an adsorbent to remove Mn²⁺ from aqueous solution. Sodium tetraborate (Na₂B₄O₇·10H₂O) was added into the CG calcination process as an additive, and the concentrations of Na₂B₄O₇·10H₂O were optimized along with the calcination temperature to obtain the best adsorbent capacity of modified coal gangue (MCG). We applied multiple analytical methods such as scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and Brunauer–Emmett–Teller analysis to characterize the MCG. The results showed it had a smaller particle size and a larger specific surface area and pore volume after modification. It also indicated that the phase of CG transformed from kaolinite to metakaolinite after calcination. Moreover, a new substance was generated with two new peaks at 1,632 cm⁻¹ and 799 cm⁻¹. The Mn²⁺ absorption capacity of MCG was evaluated using a series of experiments with different adsorbent doses, pH values and initial Mn²⁺ concentrations during the adsorption process. We found that Mn²⁺ adsorbent capacity of MCG increased by more than seven-fold compared to that of CG. The Langmuir isotherm model and the pseudo-second-order kinetic model provided the best fit to the adsorption processes.