Fixed-bed column study and reactive transport modeling of manganese removal from acidic wastewater using activated carbon-functionalized fly ash-based geopolymer

Manganese contamination in mining wastewater presents serious environmental and health risks. Developing efficient removal methods is essential to the mitigation of these impacts and water resources protection. This study investigated a fixed-bed column combined with reactive transport modeling to evaluate the removal of manganese from acidic wastewater using an activated carbon-functionalized fly ash-based geopolymer. Given the environmental risks associated with manganese in mining effluents, the effects of key parameters, including influent manganese concentration (300–700 mg/L), solution pH (1–3), flow rate (0.5–2 mL/min), column bed depth (4–12 cm), and column diameter (2.5–7.6 cm), were systematically evaluated in terms of adsorption performance. The results demonstrated that higher influent manganese concentrations accelerated adsorption kinetics but led to earlier column saturation, reduced breakthrough and exhaustion times, and an expanded mass transfer zone (MTZ). Increasing the solution pH enhanced the manganese removal efficiency, reaching 67.26% at a pH value of 3. However, a pH value of 2 was selected as a representative condition for typical mining wastewater. Lower flow rates improved the residence time and adsorption efficiency, while greater column bed depths extended column operation, yielding breakthrough and exhaustion times up to 3420 min and 4230 min, respectively. Column diameter exhibited the most pronounced influence on performance. A diameter of 7.6 cm achieved a maximum manganese removal efficiency of 99.26%, an adsorption capacity of 16.16 mg/g, and a treated effluent volume of 21.25 L, while simultaneously minimizing the MTZ. Breakthrough behavior was accurately described by the Thomas, Adams–Bohart, Yoon–Nelson, and bed depth service time models, all of which showed strong correlations. Furthermore, the multicomponent reactive transport modeling in variably saturated porous media (MIN3P) reactive transport code effectively simulated the interactions between manganese and the geopolymer. This study highlights the potential of the modified geopolymer as a sustainable adsorbent for manganese removal and environmental remediation in mining-impacted areas.