Dispersed alkaline substrate passive treatment technology for highly contaminated acid mine drainage: 20 years of...

Over the past two decades, Dispersed Alkaline Substrate technology (DAS) has emerged as a highly effective passive approach to treating acid mine drainage with extreme acidity and metal loading. By mixing alkaline materials - such as limestone, magnesia, barium carbonate or industrial by-products - into an inert wood-chip matrix, DAS systems maintain high porosity and neutralization capacity without rapidly clogging. This treatment technology has evolved from laboratory trials to pilot and full-scale field testing, using multi-step systems integrated with Natural Fe-Oxidizing Lagoons (NFOL) for pre-oxidation, which have demonstrated their long-term effectiveness for acidity and metal removal. The sustainability and applicability of the process has been improved by the search for new reagents (e.g. MgO for divalent metals removal, BaCO₃ for sulfate removal, wood ash or calcite-rich waste for cost reduction). Field trials in diverse regions - from the Iberian pyrite belt to South Africa, Canada, South America, Asia, Europe and Oceania - report net acid removal often exceeding 95 % and near complete retention of metal(loid)s such as Al, Cu, Zn, Pb and As. Mineralogical analyses indicate that contaminants precipitate primarily as hydroxides, oxyhydroxides, and sulfates (e.g., schwertmannite, basaluminite, barite), allowing targeted valorization of metal-rich sludges. Remaining challenges include assessing long-term reagent life under variable hydrological conditions, extending full-scale use to phosphogypsum and other industrial leachates, and developing strategies for residue stabilization and resource recovery. The aim of this review is to synthesise these developments, assess current performance and identify future research needs for the advancement of passive DAS treatment technology.