Waste-derived alkali-activated adsorbents for multimetal acid mine drainage: synthetic and field AMD evaluation

Acid mine drainage (AMD) is a significant environmental problem associated with coal and metal mining, characterized by extreme acidity and high levels of dissolved heavy metals. This work investigates the efficacy of alkali-activated fly ash (AFA) and water treatment residue (AWTR) adsorbents for the removal of multimetal pollutants from synthetic and field-collected AMD samples. Batch studies were performed under varying contact times, adsorbent dosages, and initial metal concentrations, targeting Fe, Al, Mn, Cu, Zn, and Ni. Adsorption kinetics were best described by pseudo-second-order and Elovich models, indicating chemisorption, surface complexation, and co-precipitation as dominant mechanisms. The Langmuir isotherm model revealed maximum adsorption capacities of 61.4 mg g−1 (Al, AFA) and 64.8 mg g−1 (Fe, AWTR), reflecting the high adsorbent affinity. Thermodynamic analysis demonstrated spontaneous and favorable removal for selected metals, with negetive ΔG° values confirming adsorption-driven processes enhanced by metal hydroxide precipitation as pH increased. Characterization confirmed the formation of Fe/Al hydroxides and the interaction of metal ions with the silicate–aluminate framework. Field AMD treatment using 30 g L−1 adsorbent at pH 2.6 and 180 min contact time significantly reduced metal concentrations. This study highlights the potential of industrial by-product-derived alkali activated adsorbents as sustainable, low-cost materials for multimetal AMD remediation.