High-performance polymeric aluminum ferric sulfate from Baihe pyrite slag: Enhanced coagulation in soil leaching alkaline wastewater and mechanistic insights.

Efficient removal of inorganic suspended matter is crucial for the recovery of soil leaching alkaline wastewater. This study aimed to develop a high-alumina inorganic polymer coagulant, polymerized aluminum ferric sulfate (PAFS), specifically targeting inorganic suspended matter in alkaline soil leaching wastewater. PAFS was prepared from aluminum ferric sulfate (Al0.862Fe0.138(SO4)3·nH2O) synthesized from Baihe pyrite slag, enabling resource utilization of this industrial waste. The physicochemical properties of the slag, intermediate sulfate, and final PAFS were characterized by XRD, SEM, and FT-IR. The optimal synthesis conditions were pH 3.0, aging temperature 80 °C, aging time 75 min, and combined AlFe content 11 %. The coagulation mechanism was systematically investigated, revealing a three-stage pH-dependent synergy of "charge neutralization - adsorption bridging - precipitation": (1) At pH 1.0-2.0, low-polymerization PAFS destabilizes colloids via charge neutralization; (2) At the optimal pH 3.0, enhanced hydrolysis produces multi-nuclear hydroxo-complexes ([Fe2(OH)2]4+ and [Al13O4(OH)24]7+) that efficiently bridge particles into dense, settleable flocs through hydroxo bridging and sweep flocculation; (3) The sulfate-linked FeAl copolymer structure imparts superior alkali resistance, while over-hydrolysis at pH > 3.0 causes precipitation and efficiency loss. In simulated wastewater treatment, PAFS achieved a 99.9 % turbidity removal rate under alkaline conditions (pH 8.0-11.0). This work provides an efficient, low-cost solution for alkaline soil leaching wastewater by converting pyrite slag into a high-performance coagulant, offering notable economic and environmental benefits.