Effects Of Carbon Nanotubes And Cellulose Nanpcrystals On Membrane Properties Improvement

Ultrafiltraion technology can guarantee the biological safety of drinking water.Pressure-driven membrane process can be highly automated,require a small plant footprint,and are readily adaptable to modular configurations.Membrane technology is regarded as an eco-friendly process with no chemical reaction or second-pollution.Membrane technology has been widely applied for water treatment.However,membrane material related issures including membrane fouling,hydrophobicity and weak strength restrick the development of membrane technology.To address problems above,enhancing properties of the membranes,membrane modification meothds were conducted in this work.Surface coating and blending methods were used to modify polyethersulfone(PES)membrane,with carbon nanotubes(CNTs)and cellulose nanocrystals(CNCs)as functional additives.Effects of CNTs surface coating on membrane fouling control and fouling resistances distribution were systematically investigated.The morphologies,hydrophilicity and mechanical properties of blended membranes incorporated with CNTs and CNCs were studied and compared,and there mechanisms were discussed.The effects of operation conditions on membrane fouling control were investigated using raw water.Contaminants analysis showed that insoluble particles,dissolved organic carbon and natural organic matters were accumulated in retentate solution during ultrafiltration process.To alleviate membrane fouling due to contaminants accumulation,operation conditions including filtration mode,hydraulic backwashing and sludge discharge were studied and optimized.In addition,effects of sludge discharge intervals(SDI)on membrane fouling and net water production rate were further investigated.Results showed that concentration polymerization and cake layer fouling can be controlled by optimized operation conditions,leading to the decrease of reversible fouling.The transition from loose cake layer to compressed cake layer can be controlled by short SDI.Weak adsorption fouling was reduced,leading to the decrease of part of the irreversible fouling.Net water production was decreased as the decrease of SDI.The water production rate was 96.31 %,92.61 %,85.23 % at an SDI of 24 h,12 h,and 6 h,respectively.Optimizing operation conditions showed positive effect on reversible fouling control.However,their effect on irreversible fouling was limited.Hence,it is insufficient to solve membrane fouling issue,especially for irreversible fouling just from the aspect of optimizing operation conditions.To improve antifouling ability,a PES ultrafiltration membrane was coated with multi-walled carbon nanotubes(MWCNTs).MWCNTs were chemically treated and named according to functional groups,i.e.Raw-MWCNTs,COOH-MWCNTs,PEG-MWCNTs,respectively.Characterization of MWCNTs results showed that abundant hydrophilic groups were existed in functionalized MWCNTs.With the same pH condition,negative charge of the investigated MWCNTs decrease in the order: COOH-MWCNTs > PEG-MWCNTs > Raw-MWCNTs.The mean diameter in aqueous solution decrease in the order: Raw-MWCNTs > COOH-MWCNTs >PEG-MWCNTs.Subsequently,the membranes were coated with the three MWCNTs,and named as Control-M,Raw-M,COOH-M,PEG-M according to the type of the coating material.Compared to the Control-M,pure flux was slightly dropped for the modified membranes.Surface roughness of the investigated membranes decrease in the order: Raw-M > COOH-M > PEG-M > Control-M.Natural organic matter(NOM)models were used to investigate the mechanisms of surface coating on membrane fouling control.HA filtration experiments showed that irreversible fouling was decreased by 68.8%,75.0% and 93.8% for Raw-M,COOH-M and PEG-M,respectively.Negative charge and hydrophilic surface are the key foctors.BSA filtration experiments showed that irreversible fouling was decreased by 81.0%,90.5% and 66.7% for Raw-M,COOH-M and PEG-M,respectively.Negative charge and surface roughness are the key factors.SA filtration experiments showed that irreversible fouling was decreased by 69.4%,93.1% and 98.3% for Raw-M,COOH-M and PEG-M,respectively.Negative charge and hydrophilic surface are the key foctors.For the three types of NOM model contaminants,surface coating with CNTs exhibited excellent performance in controlling irreversible fouling.Stability evaluation results showed that,for both ultrapure water and permeat backwashing,dislodged contents of MWCNTs for the modified membrabes were in the order of: PEG-M>COOH-M>Raw-M.To enhance hydrophilicity and mechanical properties of the membranes,CNTs and CNCs were used as additives to prepare blending nanocomposite membranes.Materials characterization results showed that hydroxyl groups were existed in both CNTs and CNCs,indicating hydrophilicity of CNTs and CNCs.The resultant membranes were named according to their weight concentration of CNCs or CNTs.A control PES membrane(named Control-M)with no CNCs or CNTs was also fabricated via the same procedures.Effects of CNTs and CNCs on pore geometry,structure and porosity of the nanocomposite membranes were investigated.Results showed that the Control-M polymers were densely packed in the top layer,and few voids were observed.However,the polymers were loosely packed in the top layer of both CNTs-M and CNCs-M.Compared to the Control-M,the overall porosity,surface porosity and mean pore radius of the CNTs-M and CNTs-M were greatly increased,and surface roughness were slightly increased.The surface of the Control-M was hydrophobic while the surface was hydrophilic of both CNTs-M and CNCs-M.With the same addition content,the hydrophilicity of the CNCs-M was greater than the CNTs-M.Pure water flux of nanocomposite membranes was improved as the increasing of addition content for both CNTs and CNCs.Enhanced hydrophilicity and increased porosity are responsible for the phenomenon.With the same addition content,pure water flux of the CNCs-M was greater than the CNTs-M.Mechanical properties investigation results showed that both CNTs and CNCs present positive effects on enhancing strength properties of the nanocomposite membranes.This is because nanocomposite membranes followed be mechanical behavior of their building blocks.With the same addition content,Young's modulus and tensile strength of the CNTs-M were greater than that of the CNCs-M.The most promising finding was that CNC-M could attain same level in Young's modulus and tensile strength as the CNT-M by doubling the CNCs content.Considering the advantages of CNCs compared to CNTs of environmental friendly and cheap cost,CNCs were a possible alternative to CNTs in enhancing hydrophilicity and mechanical properties of the nanocomposite membranes.