Natural brown coal as an adsorbent for manganese removal from groundwater: A mechanistic and operational evaluation

This study investigates the potential of natural Brown Coal (BC) as a sustainable, cost-effective adsorbent for the removal of manganese (Mn²⁺) from contaminated groundwater. A series of batch adsorption experiments was conducted to assess the influence of key operational parameters—such as solution pH, initial Mn²⁺ concentration, BC dosage, temperature, and the presence of competing ions—on Mn²⁺ removal efficiency. The environmental compatibility and regeneration potential of BC were also evaluated to determine its practical viability for repeated use. To better understand the adsorption behaviour, equilibrium and kinetic data were analysed using established isotherm and kinetic models, while thermodynamic parameters were computed to assess the spontaneity and thermal characteristics of the adsorption process. Furthermore, geochemical modelling and comprehensive BC characterization—including surface morphology, mineralogical and elemental composition, and functional group analysis—were performed to elucidate Mn²⁺ speciation under varying environmental conditions and to uncover the underlying adsorption mechanisms. Results showed that Mn²⁺ removal efficiency increased with higher pH, temperature, and BC dosage, but declined at elevated initial Mn²⁺ concentrations due to active site saturation. The process was spontaneous and endothermic, with the Langmuir isotherm model (R² = 0.994) and pseudo-second-order kinetic model (R² = 0.996) providing the best fit to experimental data. Mechanistic analysis indicated that chemisorption, primarily through ion exchange and inner-sphere complexation, was the dominant mode of Mn²⁺ uptake. The presence of competing cations, especially Fe³⁺ and Cu²⁺, significantly hindered Mn²⁺ removal due to preferential binding. Importantly, BC exhibited strong reusability, maintaining over 80% removal efficiency across four adsorption–desorption cycles without evidence of secondary pollutants. These findings demonstrate the potential of natural BC as an efficient, reusable, and environmentally benign material for treating manganese-contaminated groundwater.