| Oily wastewater,generated from diverse sources,contains complex pollutants.Traditional treatment process,oil separation-air floating-biochemical treatment,is applicable for floating oil and dispersed oil in oily wastewater.However,the emulsified oil has stable physical and chemical properties due to its small droplet size,and the dissolved organic pollutants have poor biodegradability.Thus,the above traditional treatment process cannot remove these two kinds of pollutants efficiently.Membrane separation has the merits of high separation efficiency and stable effluent quality,showing promise in oily wastewater treatment.Whereas,oil is a kind of viscous and deformable liquids,which can lead to serious membrane fouling and have the risk of oil penetration.Moreover,the dissolved organic pollutants with molecular scale in oily wastewater,such as surfactants,corrosion and scale inhibitors,cannot be rejected by traditional oil separation membranes.Therefore,the fouling resistance and retention of membranes for oily wastewater treatment need to be improved.To solve the above issues,membranes with hydrophilcity and oleophobicity were fabricated based on carbon nanotubes(CNT)and MXene to alleviate membrane fouling by reducing oil adhesion according to the different wettability of oil-water on material surface.Then,electro-assisted membrane separation process was designed to improve the retention and fouling resistance of emulsified oil droplets and dissolved organic pollutants by enhancing the electrostatic repulsion between the membrane surface and the above charged pollutants.The main research content and results of the paper are as follows:(1)Carbon nanotube-polyvinyl alcohol(CNT-PVA)membrane with super-hydrophilicity,underwater super-oleophobicity and three dimensional interconnected pores was prepared by vacuum filtration and chemical crosslinking.The pore size of CNT-PVA membrane was 120 nm and the pure water flux reached 4200 L m-2 h-1 bar-1,which was 1.5 times of commercial polyvinylidene fluoride(PVDF)membrane with a pore size of 110 nm.Besides,the oil contact angle under water of CNT-PVA membrane was 150°,higher than PVDF(90°)and polyethersulfone(PES)membrane(110°).The experimental results for the treatment of simulated emulsified oil wastewater(640 mg L-1)showed that the oil retention of CNT-PVA membrane exceeded 95%,which was comparable with PVDF and PES membranes.While,the permeation flux of CNT-PVA membrane reached 376 L m-2 h-1 bar-1,which was 2.0 times of PVDF membrane and 11.4 times of PES membrane.Therefore,CNT-PVA membrane showed superior performance in emulsified oil wastewater treatment.(2)In order to further improve the membrane separation performance for emulsified oil,electro-assisted membrane separation process was designed by utilizing the conductivity of CNT-PVA membrane.When the CNT-PVA membrane was used as working electrode and a potential of-1.5 V(vs Ag/AgCl)was applied,the oil retention increased from 96.8%to 99.4%.Additionally,the permeation flux under electro-assistance was 559 L m-2h-1 bar-1,which was 1.5 times of the single membrane process.In addition,the calculation results of Hermia fouling model showed that the pollution indexes were decreased by 51.0?79.3%by the electroassistance.According to the electrochemical experiment results,the above improvement in retention and anti-fouling performance was due to the enhancement of the electrostatic repulsion effect between the negatively charged oil droplets and the membrane cathode surface.(3)In order to simultaneously remove emulsified oil and dissolved organic pollutants(usually with molecular weight of 300-2000 Da)in oily wastewater,a hydrophilic and oleophobic MXene/CNT membrane with pore size in nanoscale was designed and constructed.The membrane had a pore size of?1.5 nm and a pure water flux of 30.4 L m-2 h-1 bar-1.Besides,the water contact angle of the membrane was 62° and the underwater oil contact angle was 135°.The retention of MXene/CNT membrane for surfactant Tween 20,corrosion and scale inhibitor ethylenediamine tetramethylene phosphonate,polysaccharide sodium alginate and bovine serum albumin was above 87.3%,demonstrating promising removal performance for a variety of common dissolved organic pollutants in oily wastewater.For treatment of Boost cutting fluid wastewater with oil concentration of 100 mg L-1 and COD of 773 mg L-1,no oil was detected in the effluent,and the effluent COD decreased to-45 mg L-1,realizing the efficient removal of emulsified oil and dissolved organic pollutants at the same time.(4)For further improvement of the membrane performance in treating dissolved organic pollutants in oily wastewater,an electro-assisted membrane separation process was designed by utilizing the conductivity of MXene/CNT membrane.In a two-electrode system,when the membrane was used as cathode and a voltage of 3 V was applied,the retention of sodium dodecylbenzene sulfonate and ethylene-diamine-tetra-acetic acid disodium increased from 57.7%to 89.0%and 76.3%to 96.8%,respectively.Besides,the permeation flux increased by 67.6%and 42.3%respectively,indicating the fouling resistance of MXene/CNT membrane was also improved by the electro-assistance.The theory calculation results showed that the electroassistance enhanced electrostatic repulsive force between the pollutant and the membrane surface by 9.2 times,thereby improving the retention and anti-fouling performance of the membrane.For treatment of oil refining wastewater(COD was 109 mg L-1),the electroassistance reduced the effluent COD from-60 mg L-1 to?40 mg L-1,and increased the permeation flux from 8.4 L m-2 h-1 bar-1 to 14.1 L m-2 h-1 bar-1.In summary,the construction of membranes with hydrophilicity and oleophobicity,and the enhancement of electrostatic repulsion by electro-assistance were effective to improve the membrane performance in oily wastewater treatment,providing new insight and theoretical basis for efficient and stable oily wastewater treatment by membrane separation technology. |
PDF Full Text Request