Effective removal of sulfate and heavy metals from wastewater by ettringite precipitation using aluminum-based sludge

The increasing generation of aluminum-based sludge (Al sludge) from water treatment poses significant environmental and economic challenges. Despite growing attention to resource recovery, most Al sludge is still landfilled, and its reuse (primarily in agriculture) is limited due to concerns about leachable contaminants such as heavy metals. This study introduces a sustainable strategy to valorize raw Al sludge as a reactive agent for treating sulfate-rich wastewater via ettringite (AFt) precipitation. Batch experiments demonstrated rapid sulfate removal from 1627 mg/L to <10 mg/L within 10 min under optimized conditions (Ca/S = 5, Al/S = 1.5), with pH identified as a critical control factor. Characterization (SEM-EDS, XRD) confirmed AFt as the dominant product, accompanied by minor monosulfate and calcite. The treated effluent contained only trace Al (∼0.7 mg/L) and co-occurring metals (e.g., Zn, As, Cr). Mechanistic analysis revealed that contaminant removal occurred through precipitation, surface adsorption, and structural incorporation. Density functional theory (DFT) calculations showed that the substitution of representative oxyanions (As(III/V), Se(IV/VI), Cr(VI)) into the lattice is thermodynamically favorable, and the substitution energies follow the same trend as the removal efficiencies, suggesting that incorporation likely plays a significant role. A techno-economic analysis estimated the reagent cost at $0.40 / m³, substantially lower than conventional treatments using commercial Al-salt ($1.88 / m³) or barium precipitation ($14.52 / m³), primarily due to reduced reagent and sludge disposal expenses. Overall, this approach enables the direct reuse of untreated Al sludge for efficient, simultaneous removal of sulfate and heavy metals from complex wastewater matrices.