Adsorption Mechanisms of Manganese (II) Ions onto Acid-treated Activated Carbon

In this work, modification of Granular Activated Carbon (GAC) using nitric acid as an oxidizing agent was investigated to develop a novel adsorbent that can effectively remove manganese from aquatic environments. The results revealed that the acid modification method reduced BET surface area, micropore volume, and the point of zero charge (pH(PZC)) but produced more carboxylic acid groups on the M-GAC surface. Batch adsorption experiment results indicated that pH, adsorbent dosage, initial Mn(II) concentration, and solution temperature dramatically influenced the Mn(II) ions adsorptive behavior. The M-GAC showed maximum adsorption capacity for Mn(II) ions (9.25 mg/g) significantly higher compared to that of R-GAC (1.29 mg/g). Equilibrium data were evaluated by Langmuir, Freundlich, Sips, and Tempkin isotherm models, and the results suggested that the Sips model is the most suitable to expound the adsorption behavior of Mn(II) ions on both M-GAC and R-GAC. The pseudo second order kinetic model exhibited a better fit with the adsorption kinetic data of both M-GAC and R-GAC. The three-stage kinetic model revealed that the enhancement of Mn(II) adsorption on M-GAC was exclusively due to an increase in adsorption on the external and internal surfaces of M-GAC. Also, external mass transfer and intraparticle diffusion were both involved in the rate controlling step of the Mn(II) adsorption onto M-GAC. In addition, the resistance of mass transfer during the adsorption was verified primarily dependent on the external mass transfer via the mass transfer factor model. Overall, the results obtained from this study provides an insight into the adsorption mechanisms of Mn(II) ions onto the acid treated GAC.