Cationic Surfactant-Mediated Coagulation for Enhanced Removal of Toxic Metal–Organic Complexes: Performance, Mechanism, and Validation

Toxic metal ions tend to complex with coexisting organic ligands in contaminated waters, challenging their efficient removal via traditional processes such as adsorption, coagulation, or precipitation. In this study, we demonstrate a proof-of-concept strategy for the removal of metal–organic complexes using cationic surfactants as the coagulant. Using cetyltrimethylammonium bromide (CTAB) as the model one, such a simple strategy is applicable for efficient water decontamination from various metals [Cr(III), Ni(II), Cu(II), Zn(II), and Cd(II)] complexed with different ligands (citrate, malate, tartrate, and oxalate), outperforming direct alkaline precipitation and Al(III) coagulation remarkably. In the case of the Cr(III)-citrate complex, the CTAB coagulation could result in the Cr(III) reduction from 10.4 to only 0.2 mg/L. A negligible effect of nine ubiquitous cations or anions was observed on the process, while the carbon chain of surfactants larger than C16 is required to achieve a satisfactory removal of the target complexes. The strong electrostatic interaction between the negatively charged Cr(III)-citrate species and the positively charged CTA+ results in the dehydration of the head group of CTAB and the formation of unstable aggregates that precipitate from the solution. Furthermore, the CTAB coagulation is demonstrated for effective removal of Cr(III) complexes in real tannery wastewater, resulting in the residual Cr(III) below the discharge standard of China. This study may present a new option for water decontamination from metal–organic complexes.