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

Abstract

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.

Electroreductive Defluorination of Unsaturated PFAS by a Quaternary Ammonium Surfactant-Modified Cathode via Direct Cathodic Reduction

“…The rate constant for the cathode modified by OTAB was 0.2158 h –1 , which was 9.38, 22.48, and 38.54 times higher than CTAB (0.0230 h –1 ), TTAB (0.0096 h –1 ), and DTAB (0.0056 h –1 ), respectively. The positive correlation between the removal efficiency and the alkyl chain lengths of the surfactants was associated with the more hydrophobic nature of longer-chain surfactants. − Through stronger hydrophobic interactions, more PFMeUPA molecules were adsorbed onto the cathode surface and degraded by the electrons. Without surfactant modification, the defluorination efficiency was less than 20% (Figure e).…”

Engineering Nano-Au-Based Sensor Arrays for Identification of Multiple Ni(II) Complexes in Water Samples

“…Besides, we recently reported that heavy metal complexes mixed with surfactants could form selfassemblies via electrostatic and hydrophobic interactions. 34 The self-assembly process is highly dependent on the charge of the terminal group and the length of the alkyl chain of surfactants. Inspiringly, in this study, 3-mercapto-1-propanesulfonic acid sodium salt (MPS) with a negative terminal group (−SO 3 − ) and a propane hydrophobic chain (C3) 35 and tributylhexadecylphosphonium bromide (THPB) with a positive terminal group (−P + (C 4 H 9 ) 3 ) and a cetyl hydrophobic chain (C16) 36 are selected as the surface modifiers of Au nanosensors, aiming to provide different hydrophobic and electrostatic interactions toward Ni(II) species.…”

“…As reported, N -acetyl- l -cysteine (NAC) and adenosine monophosphate (AMP) have been employed to chelate Ni­(II) via the amino and phosphate groups, exhibiting extremely high selectivity toward Ni­(II) over other metal ions in alkaline conditions. Besides, we recently reported that heavy metal complexes mixed with surfactants could form self-assemblies via electrostatic and hydrophobic interactions . The self-assembly process is highly dependent on the charge of the terminal group and the length of the alkyl chain of surfactants.…”