Surface charge-mediated heteroaggregation of chloropyromorphite with natural inorganic colloids: Mechanisms and remediation implications

Phosphorus-induced immobilization of labile lead (Pb) into chloropyromorphite (CPY, Pb 5 (PO 4 ) 3 Cl), is a key strategy for remediating Pb-contaminated soils, but the long-term environmental risk of CPY depends critically on its colloidal stability. This study investigated the heteroaggregation behavior of CPY with two ubiquitous natural colloids—negatively charged montmorillonite (Mont) and positively charged goethite (Goe)—under varied hydrochemical conditions (pH, ionic strength (IS), cation type). Results showed that Mont inhibited CPY aggregation by enhancing electrostatic repulsion (e.g., decreasing CPY zeta potential from -20.1 mV to -25.4 mV at pH 7, 10 mM NaCl), thereby increasing CPY stability. In contrast, Goe promoted rapid aggregation via charge neutralization, even reversing CPY surface charge to positive values (e.g., +5.2 mV). The critical coagulation concentration of CPY increased in NaCl with Mont, but decreased with Goe. Divalent Ca 2+ accelerated aggregation more than Na + due to charge screening and bridging effects. Sedimentation experiments revealed that after 120 min, the normalized concentration (C/C 0 ) of Goe-CPY aggregates dropped below 20% in 10 mM NaCl at pH 7, compared to ~80% for pure CPY and Mont-CPY aggregates. Interaction energy calculations based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory corroborated these trends. Thus, CPY fate in aquatic systems is controlled by mineral surface charge and water chemistry. Goe-rich or high-hardness (high Ca 2+ ) environments favor its immobilization, while Mont or acidic/low IS conditions promote dispersion and increase migration risk, providing key insights for optimizing P-based remediation and assessing the long-term stability of immobilized Pb.