Evaluating Of Holey Graphene Oxide Modified Ultrafiltration Membrane And The Performance For Natural Organic Matter Removal

Membrane technology has been widely used in the drinking water and wastewater treatment and reuse for its high separation efficiency,low energy consumption,small footprint,etc.However,it suffered from membrane fouling.During the filtration process,the pollutants deposited on the membrane surface and adsorbed in the pore walls of the membrane,which caused reduction of flux,higher trans-membrane pressure and frequent backwashing.Membrane fouling can be alleviated by pre-treatment,adjustment of operating conditions and membrane modification.Among them,membrane modification is aimed to improve the membrane properties to endow the membrane with anti-fouling ability.In recent yeas,many functional nano-materials have been applied to membrane performance.Graphene oxide(GO)and its functionalized derivatives are considered as ideal modification materials because of their monoatomic thickness and ultra-high hydrophilicity.GO was stacked to form channels between nanosheets by pressure filtration or vacuum filtration,so that organic pollutants could be rejected by the channels.However,GO coated membrane were sufferd from serious flux decline,in this work,GO as hydrothermally etched to synthetize porous graphene oxide(HGO)as a modified material for ultrafiltration membranes.The composite membrane was prepared by a twostep modification method based on dopamine self-polymerization.A variety of characterization methods were applied to characterize the morphology,physical properties,chemical properties and antifouling properties.First,the adhesion ability of polyethersulfone(PES)commercial membrane was improved by polydopamine(PDA).Then the HGO solution was filtrated on the PES membrane by vacuum filtration.The differences in morphology,surface properties,flux,and antifouling properties between the PES and the HGO composite membrane were investigated.The results showed that the surface of the HGO membrane was dense and uniform when coated with less HGO,and more wrinkles were observed when coated more HGO.Due to the porous and hydrophilic properties of HGO,the hydrophilicity of the membrane was significantly enhanced,and the transfer resistance of water molecules was reduced.The flux loss of HGO membrane was less than GO membrane.Moreover,the rejection rate of HGO coated membrane on natural organic matter(NOM)such as humic acid(HA),bovine serum albumin(BSA),and sodium alginate(SA)was also increased from 56%,51%,30% to 88%,78%,65%,respectively.Second,the modification process was optimized.The effects of HGO loading rate on the flux,hydrophilicity and selectivity of the membrane was investigated to obtain the optimal loading rate for membrane modification.It was confirmed that due to the hydroxyl groups and carboxyl groups in HGO,the hydrophilicity of the modified membrane was significantly improved.At the optimal loading of 0.04g/m~2,the contact angle was reduced from 71° to 35°,and the removal rate of HA,BSA and SA were up to 92%,62%,60%,respectively.When the loading rate was higher than 0.04 g/m~2,the removal rate of HA and BSA was decreased from 92%,83% to 80%,75%,respectively.The pure water flux reached 52L/m~2h·bar,which was several times higher than that of typial GO composite membranes.It indicated that the porous structure improved the membrane flux and reduce the resistance caused by the stacking of traditional GO nanosheets.Finally,the membrane fouling resistance distribution of each HGO membranes was analyzed given HA as model pollutant,and the effects of HGO on mitigating the organic fouling of Polyethersulfone(PES)membranes were discussed.The HA fouling was alleviated by HGO coating.The total membrane fouling resistance decreased from 3.45 to 1.73 with HGO coating,and the irreversible fouling also decreased by 62.86%-95.83%.The finest flux recovery rate(96.04%)was achieved at 0.02 g/m~2 HGO.The mechanism of fouling mitigation was concluded as follows: the hydroxyl and carboxyl groups of HGO endowed the PES membrane with hydrophilicity,which was beneficial for reducing the hydrophobic-hydrophobic interaction between membrane surface and HA molecules,so that contaminants adhered on HGO layer could be effectively removed by backwashing;HGO coating kept the HA fouling in HGO layers,as a result,the standard blocking was dominated during filtration.It was also found that increasing the loading of HGO could delay the conversion of pore blocking to the cake layer.