Ultrathin and Metal Ions-Free Polyionic Nanogel Coating Membrane for Efficient and Fast Oil-in-Water Emulsions Separation and Cr(VI) Removal: Adsorption Properties, Kinetics, and Mechanism

Abstract

The problem of oil fouling and the complex composition of oily wastewater are bottlenecks that urgently need to be addressed for the application of membrane separation technology in the field of oil−water separation. Therefore, ultrathin TA/SA/PEI polyionic nanogel coatings were constructed on hydrophobic PVDF membrane surfaces through surface grafting and dopamine-like self-polymerization reactions. The functional coated membranes (TSP-3) retain superhydrophilicity and underwater superoleophobicity, even in harsh environments such as strong acids, strong alkalis, various salt solutions, ultrasound, and abrasion. The TSP-3 membrane exhibited excellent separation efficiency (>99%) for a wide range of oil-in-water emulsions stabilized by surfactants, with virtually no deterioration in flux (>8014.70 ± 298.86 L•m −2 •h −1 •bar −1 ) over five dead-end cycle separations. The formation process of the oil film filter cake was also analyzed in crossflow mode. It was assumed that the accumulation of oil droplets on the membrane surface to form a film under certain conditions was beneficial to the membrane's antioil fouling performance. Meanwhile, the TSP-3 membrane possessed high removal efficiency (96.13%) and recoverability for Cr(VI), and the phenolic hydroxyl groups could also reduce Cr(VI) to the lower toxicity Cr(III). Various adsorption kinetic and isotherm models were used to analyze the adsorption process, and the results fit well with the pseudo-second-order kinetic model (R 2 = 0.99820) and the Langmuir model (R 2 = 0.99986). This indicated that the adsorption of Cr(VI) by TSP-3 was monomolecular layer chemical adsorption on heterogeneous surfaces, and the adsorption was favorable (0 < R L < 1). Electrostatic attraction and hydrogen bonding were considered to play a key role in the adsorption of Cr(VI) by XPS characterization analysis and DFT theoretical calculations. This strategy provided a feasible approach for constructing ultrathin polyanionic nanogel membranes to achieve an antioil barrier, which could effectively and rapidly separate a wide range of oil-in-water emulsions stabilized by surfactants, while removing and reducing Cr(VI).