Sulfate Removal from Water and Wastewater: A Systematic Review of Chemical, Membrane, Sorptive, Biological and Electrochemical Technologies with Unit-Cost Benchmarking

Sulfate is a soluble and chemically stable anion that cannot be destroyed, only separated, concentrated or transformed, and its removal from mine water and industrial effluents is increasingly driven by site-specific discharge limits, commonly of 250–1000 mg L−1. This systematic review critically evaluates chemical precipitation, membrane, sorptive, biological and electrochemical technologies in terms of removal mechanism, achievable residual sulfate and unit treatment cost. Web of Science Core Collection and Scopus were searched on 20 June 2026 (publications 1999 to May 2026) and supplemented by citation searching, publisher platforms and regulatory sources. Eligible sources reported sulfate as an explicit target species with a removal mechanism, performance, operating or cost figure, whereas sources in which sulfate was only a background electrolyte, or which studied sulfide or sulfite without a sulfate figure, were excluded. Evidence was appraised by scale, water matrix and cost basis, without a formal risk of bias tool, and synthesized narratively. Search reproducibility, identification routes, screening and a random 20% sample of the data extraction were cross-checked by a second reviewer. A broadened sensitivity search indicated that relevant studies on matrices other than mine water and wastewater were under-retrieved. Reported and stoichiometrically derived costs were normalized per cubic metre and per kilogram of sulfate removed. Ninety-three sources were included (59 from the database searches and 34 from other sources), mostly journal articles, with evidence ranging from laboratory to full scale. Under representative mine water conditions, gypsum solubility limits inexpensive lime and limestone treatment to residuals of approximately 1200–1800 mg L−1. Unit costs span nearly three orders of magnitude, from about 0.02 USD m−3 for constructed wetlands to about 18 USD m−3 for single-pass barium chloride precipitation and the heterogeneous evidence indicates an apparent tradeoff between cost and achievable residual. No technology combines low cost with low residual. Crossing the gypsum ceiling raises cost roughly three- to tenfold for mainstream polishing routes. In four common reference scenarios, treating concentrated water to 250 mg L−1 costs about two to five times as much as partial treatment to about 1500 mg L−1 for mainstream treatment trains at the range midpoints, with widely overlapping ranges. The evidence is limited by heterogeneous matrices, scarce standardized cost reporting and, for several technologies, laboratory-scale data. Hybrid trains pairing low-cost bulk removal with selective polishing and resource recovery emerge as the most defensible strategy. This review was not registered. Funding: European Union—NextGenerationEU.