Preparation Of Composite Membrane With Superamphiphobic Properties For Coking Wastewater Treatment By Membrane Distillation

After the biochemical treatment of coking wastewater,the pollutant emission is still difficult to satisfy the pollutant discharge standard,so it is necessary to carry out advanced treatment of coking wastewater to achieve the“zero discharge”requirement.“Double-membranes process”is the best choice for advanced treatment of coking wastewater at present.However,the membrane is easy to be polluted and many pretreatments need to be protected.The whole process is complex and complicated.Therefore,there is an urgent need to seek a membrane treatment process with a relatively simple process to make the demonstrations for achieving“zero emission”of coking wastewater.Membrane distillation?MD?combines the advantages of distillation and membrane separation technology.In principle,100%separation of water vapor and other non-volatile components can be achieved,achieving“one-step”water purification.Due to the utilization of waste heat from coking plants,MD has a certain application potential for the advanced treatment of coking wastewater.The membrane fouling problems during the operation of MD is also unavoidable with other membrane separation processes,and it is urgent to prepare a membrane distillation anti-fouling dedicated membrane.People are inspired by the anti-fouling and self-cleaning of“Lotus Effect”,numerous superhydrophobic membranes have been developed which mimicked biological systems.Whereas,the superhdrophobic membrane was susceptible for fouling and wetting by some organic foulants such as humic acid via an adsorption–desorption mechanism,which limited the application of superhydrophobic membranes.Recent studies have found that the superamphiphobic surfaces were resistant to oil and other hydrophobic substances.In this study,traditional hydrophobic membranes were subjected by superamphiphobic modification.The superamphiphobic composite membranes were used for direct contact membrane distillation?DCMD?treatment of biologically treated coking wastewater?BTCW?and test the performance of anti-fouling and anti-wetting.?1?SiO₂ nano-particles and perfluorodecyltriethoxy-silane?17-FAS?were used to modified the PVDF membrane,prepared membrane was characterized by contact angle instrument,scanning electron microscopy,X-ray energy spectrum and Fourier transform infrared spectroscopy,and used for advanced treatment of coking wastewater by membrane distillation.The results showed that,the contact angles of water and ethylene glycol of the modified membrane were 154.8°and 137.0°,respectively,and the membrane had good superhydrophobicity and oleophobicity.The membrane surface had multi-scale roughness and great alteration of chemical functional groups which proved SiO₂/17-FAS/PVDF composite membrane was successfully prepared.The composite membrane demonstrated a stable membrane flux,which obviously improved the attenuation of raw membrane flux?decline for about 41%?.The mass concentration of TOC in the effluent of the composite membrane was reduced from 10.0 mg/L?unmodification?to 2.0 mg/L,and other parameters such as color,turbidity,NH₄⁺-N content and UV₂₅₄ were also improved notably.The modified PVDF membrane had a good anti-fouling performance and could be used in membrane distillation of coking wastewater.?2?Response surface methodology was used to further optimize the conditions for membrane modification.A?NaOH concentration?,B?Mass percentage of SiO₂?and C?17-FAS content?were selected as independent variables to carry out experimental design.The variance analysis of the regression equation showed that the factors affecting the water contact angle on the membrane surface were:B>A>C;the interaction effects were:BC>AC>AB.The main factor effects the oil contact angle is C,which interaction effects with BC>AB,and the effect on ethylene glycol is the most significant.Through the evaluation and verification of the model,the prediction model was accurate and reliable.Finally,the optimal modification conditions of the modified membrane are as follows:NaOH 7.52 mol/L,SiO₂ 0.05 wt%,and 17-FAS 1.90%?V/V hexane?.The contact angles of the composite membrane to water,diiodomethane and ethylene glycol were 161.18°,151.50°and 153.11°,respectively.?3?By the optimal membrane modification conditions,the superamphiphobic composite membrane-1 and superamphiphobic composite membrane-2 were prepared by electrostatic attraction and covalent grafting,respectively.It was found that there were intensive multi-scale micro-nanostructures of re-entrant texture on composite membrane-2by SEM.It was demonstrated that the composite membrane-2 was long-lasting by a flexible rinsing experiment.The superamphiphobic composite membrane was used for the DCMD treatment of BTCW.After 120 hours,the original membrane flux was reduced by65%,and the membrane surface was covered with a large amount of inorganic salts and humic acid,heterocyclic compounds?such as pyridine,furan,quinoline?and the like.The attenuation of flux for composite membrane-1 and composite membrane-2 were 15%and6%,respectively,and no contaminants were detected on the membrane surface.This is mainly due to the presence of a superamphiphobic coating on the surface of the composite membrane,which has a strong repulsive effect on the coking wastewater,and it is difficult for the organic pollutants to contact the membrane surface,and the membrane is less contaminated and the flux decay is reduced.By studying the law of water conductivity and TOC change of membrane distillation,it can be known that the effluent quality of the superamphiphobic composite membrane is stable and better than that of the original membrane.The 3D-EEM results further showed that the presence of humic acid contaminants was detected in the effluent of the original membrane,indicating that the original membrane was partially wetted,and the superamphiphobic membrane had good anti-wetting effect.The above results show that the superamphiphobic composite membrane has better anti-fouling and anti-wetting,while the composite membrane-2 has the best stability and is beneficial to the long-term operation of the MD.Regulating the surface wettability of superamphiphobic provides a feasible way for the anti-fouling modification of MD membrane.