Research On Straw Cellulose And Carbon Quantum Dots Modified Forward Osmosis Membrane And Its Properties

As a new problem in environmental field,the shortage of fresh water has attracted more and more attention in the world.At present,forward osmosis(FO)as a new technology provides a new way to solve the problem of water scarcity.FO can obtain fresh water from the feed solution(low osmotic pressure)to the drive solution(high osmotic pressure)through the osmotic driving force,there is no need to apply external pressure in separation process so that the separation process running with a lower fouling tendency and consumption than reverse osmosis(RO).Despite the performance of FO membrane is excellent,the tradeoff among flux,desalination,chlorination and surface scaling is still a difficult problem to the application of the thin film composite in forward osmosis(TFC-FO).In order to solve the above problems,the research on TFC-FO membrane in recent years mainly focuses on the controlling interface polymerization(IP)process,modification of surface substitution by monomer and the addition of hydrophilic additives or functional nanoparticles.In view of the main problems TFC-FO membrane faced,polyethersulfone membrane was selected as the substrate,and IP method is used to modify the membrane to improve the performance and chlorine resistance of forward osmosis membrane.The main research results of this paper are as follows:The study selects straw as raw material to obtain cellulose nanocrystals(CNCs)by acid-base method.The typical TFC-FO membrane modified by CNC_S are prepared to the structure of double layers and three layers on PES substrate,and the parameters are measured to determine whether the modification is successful.The results show that PTFC_CNC has the best performance of FO compared with the unmodified origin membrane(TFC)and TFC_CNC.On this basis,0.001 wt.%of CNCs is selected as the best modification concentration,and the effects of CNCs on the surface composition,surface morphology,surface roughness,pore size and FO performance of TFC-FO membrane are studied.The results showes that the water flux of the modified PTFC_CNC membrane is as high as 41.6�5.4 LMH in the mode of"active layer towards draw solution"(PRO),which is about four times that of the original membrane,and the ratio of water flux to reverse salt flux(J_s/J_w)is greatly reduced(0.15).The highly hydrophilic cellulose layer between the polyethylsulfone support layer and the polyamide(PA)skin layer provides a water transport channel for the forward osmosis membrane and increases the roughness and compactness of the PA layer,hence,the effective area of osmotic pressure increases with the low reverse salt flux.In addition,this study tests the osmosis performance and stability of the modified membrane in simulation experiment of seawater desalination with the draw solution of ammonium bicarbonate.The results show that the flux and salt rejection of TFC_CNC and PTFC_CNC are good,and the flux(32LMH)and salt cutting rate(>97%)are matain a high level within 120min.The performance of PTFC_CNC is better than that of TFC_CNC and TFC,which provides a simple and effective method for the preparation of TFC-FO films with high separation performance and high practicability.The low permeability and chlorine resistance of thin film composite membranes limit their practical applications in many fields.In this study,carbon quantum dots(CQDs)derived from corn stalk are added into the selective polyamide(PA)layer to construct the polydopamine(PDA)sublayer,and a high-performance chlorine-resistant composite forward osmosis membrane is prepared.The results show that TFC_CQD(without PDA pretreatment)and PTFC_CQD films had thinner film thickness(less than68nm)with the addition of CQD_S.After pretreatment with CQDs and PDA,the surface roughness of PTFC_CQD membrane decreased significantly,and the microspheres with dense porous structure coexisted with the flocs.With the change of concentration and type of draw solution,the surface hydrophilicity of PTFC_CQD membrane is enhanced,the permeability path is shortened,and the value of permeability coefficient J_s/J_w is low(0.1?0.25).Under the chlorination of 16,000 ppm�h,the reverse salt flux of PTFC_CQDmembrane(8.4 g m^-2 h^-1)was much lower than that of TFC_CQD and TFC membrane.The super salt repellent ability is due to the presence of a large number of N-H bonds(CQDs),which are preferentially chlorinated by free chlorine to reduce corrosion of the PA layer.Similarly,this part tests the desalination performance and stability of modified membranes TFC_CQD and PTFC_CQD in simulation experiment of seawater desalination with draw solution of ammonium bicarbonate.The results showed that the flux and salt rejection of TFC_CQD and PTFC_CQD are maintained at a high level,and the flux(31LMH)and salt rejection(>99.1%)remained high within 120min.This study provides more possible application for the biomass-derived nanomaterials modified membranes.