Sequential acid–base treatment enables aluminum extraction, speciation transformation, and risk reduction for polyaluminum chloride sludge land disposal

To address critical knowledge gaps concerning the behavior of residual aluminum (Al) in Al-recovered polyaluminum chloride (PAC) sludge for land disposal, this study systematically investigated the release kinetics, speciation transformation, and ecological risks of sludge-bound Al during sequential acid–base treatment and subsequent application to alkaline (pH 8.59) and acidic (pH 5.54) soils. The results demonstrated that H₂SO₄-mediated acid extraction (pH 2.0) recovered 66.54 ± 3.16 % of Al, preferentially through the leaching of exchangeable Al (Al_ex). Subsequent NaOH neutralization further enhanced Al immobilization through the formation of stable Al(OH)₃ precipitates and colloidal Al species in the residual sludge. Column leaching tests with irrigation water (pH 8.21) and simulated acid rain (pH 5.40) revealed that the release of Al from soil amended with Al-extracted sludge was significantly lower ( p < 0.05) than that from raw sludge-amended soil. At the 1.6 % sludge dosage, the peak Al concentrations decreased by 78.26 % in alkaline soil and 86.80 % in acidic soil, while the cumulative release over 10 leaching cycles decreased by 85.15 % and 84.03 %, respectively. Thirty-day incubation experiments confirmed the complete elimination of Al_ex following the application of Al-extracted sludge. Consequently, both the geoaccumulation index and the phytotoxicity assessments verified the environmental safety of the Al-extracted sludge for land disposal.