A “Three Birds with One Stone” Strategy for the Pretreatment of Swine Wastewater Using Modified Attapulgite: Purification Mechanisms of Crystallization–Coagulation–Adsorption

This study developed a novel attapulgite-based coagulant (ATPC) to address the high turbidity, high organic content, and high NH 4 + concentration of swine wastewater. ATPC was synthesized from natural attapulgite (palygorskite) (ATP) through a synergistic modification strategy that combined high-temperature calcination, sulfuric acid leaching, and alkali-induced polymerization. The optimal preparation conditions were determined as follows: calcination temperature of 600°C, sulfuric acid concentration of 25%, solid–liquid ratio of 1:5, and polymerization pH of 3. Under these conditions, ATPC achieved a turbidity removal efficiency of 98.8% for swine wastewater. Compared with conventional polyaluminum chloride (PAC), ATPC formed denser flocs that settled considerably faster. Coupling ATPC with magnesium salts induced struvite crystallization and increased the NH 4 + removal efficiency from less than 10% to approximately 50%. At the optimal aging time of 12 h, the functional component accounted for 48.8% of [Fe(III) + Al(III)] T . The zeta potential trend indicates that charge neutralization is the primary mechanism of ATPC, in which high-valent cations are essential for destabilizing colloids, while the insoluble particles strengthen the floc structure and promote sedimentation through bridging effects. The as-prepared ATPC is an eco-friendly natural mineral coagulant with promising prospects for the advanced treatment of swine wastewater.