Supramolecular Assembly-Enabled Removal of Stable Ni(II)-Carboxylate Complexes from Wastewater by CTAC-Mediated Coagulation

The stable complexation of nickel [Ni(II)] with organic ligands in industrial wastewater presents a major environmental challenge by increasing metal mobility and limiting the effectiveness of conventional treatment methods. Here, we propose a supramolecular assembly strategy using cetyltrimethylammonium chloride (CTAC) as a coagulant for the efficient removal of Ni(II)-carboxylate complexes, including citrate, oxalate, tartrate, and malate, to ultralow levels. Using citrate-complexed Ni(II) as a model pollutant, CTAC-mediated coagulation achieved a removal efficiency of 98.2 ± 0.5%, decreasing residual Ni(II) from 11.7 to 0.2 mg/L, far surpassing alkaline precipitation (<10%) and PAC/PAM coagulation (∼30%). Notably, negligible CTAC residues were detected in the treated wastewater, suggesting little risk of surfactant-induced secondary pollution. Mechanistic studies showed that cationic surfactants with alkyl chains of C16 or longer promote the hydrophobic collapse of ternary complexes through synergistic electrostatic interactions, accounting for the highly effective removal of Ni(II)-carboxylate complexes. The process was largely unaffected by common inorganic anions and low-molecular-weight organic species, while humic acid exhibited inhibition only at high concentrations. Full-scale application further demonstrated 99.0% Ni(II) removal from real electroplating wastewater containing highly stable organic complexes. These findings establish a noncovalent interaction-driven paradigm for metal speciation control and provide new insights into supramolecular assembly strategies for persistent metal-complex remediation.