Negatively-charged Enhanced Nanofiltration Membrane And Its Application

As a new type of separation technology with strong operability and high adaptability,nanofiltration（NF）is more and more widely used in all aspects of hour daily life,such as sewage treatment,drinking water purification,product purification,etc.Most of commercial NF membranes used now are prepared by interfacial polymerization reaction of amine monomers and acid chloride monomers.And the performance of the NF membrane is relatively stable,which can meet the requirements of most wastewater treatment.However,a disadvantage of this type of NF membrane is that the charged groups on the membrane surface would be protonated with the p H value of the solution decrease,and the negative charge density of the NF membrane is gradually weakened.This paper would introduce strongly acidic groups（-SO₃H）into the separation layer of NF membrane to improve the negative charge stability of the NF membrane under low p H environment,which would further expand the application of NF membrane.Firstly,2,5-diaminobenzenesulfonic acid（DABSA）were mixed with piperazine（PIP）,and reacted with TMC on the surface of UF membrane supportion to prepare a typeⅠnegatively enhanced nanofiltration membrane（NF-PD）.At the same time,piperazine-polyamide nanofiltration membrane（NF-P）with same permeat flux as NF-PD were prepared.Field emission scanning electron microscopy（FE-SEM）,X-ray photoelectron spectroscopy（XPS）,zeta potential were used to characterize the surface morphology,chemical composition and charging performance of the NF membrane.The pore size of selective layer,permeability and salt separation performance of the NF membrane were further characteried.Finally,the separation performances of the NF membrane for Na₂SO₄ and Cr（VI）under different conditions were explored systematically.The results indicated that compared to NF-P membranes,NF-PD showed obviously better retention rates for Na₂SO₄ and Cr（VI）at low p H values.What’s more,the advantages become more and more obvious as the p H value decreases,indicating that the negative charge strengthened NF membrane（NF-PD）is more suitable for the separation process under low p H conditions.The influence of environmental conditions on the salt separation performance of negatively charged enhanced NF membranes were explored in further.The retention rate changes of NF-PD and NF-P for different concentrations of Na₂SO₄,Mg SO₄ and Mg Cl₂ under different p H values were analyzed.The results showed that retention rate of both NF membranes for Na₂SO₄ decreased with the decrease of p H value.However,the retention rate change degree of NF-PD is lower than that of NF-P.For Mg Cl₂,the rejection situation was just the opposite.For Mg SO₄,p H value change has little effect on the change of its rejection rate due to they are mainly rejected by the pore size sieving effect of NF membrane.4,4’-diaminostilbene-2,2’-disulfonic acid（DSDA）were used as the aqueous phase monomer in order to further enhance the negative charge density on NF membrane surface.TypeⅡnegatively-charged enhanced nanofiltration membrane（NF-D）were preapred by interfacial polymerization on UF membrane supportion.The performance of the NF-D was optimized by changing the TMC concentration,DSDA concentration,SDS concentration,p H value of the aqueous phase and reaction time.Elemental analysis,FE-SEM,zeta potential were used to characterize the surface morphology,chemical composition,and chargeability of the NF membrane.At the same time,the separation performance of the NF-D was investigated by measuring the permeate flux and salt separation performance.The retention rate changes of NF-D and NF-P for different concentration phosphate and Cr（VI）under different p H conditions were systematically investigated.The zeta potential characterization results indicated that NF-D showed the strongest negative charge compared with NF-P and NF-PD.NF-D presented relative higher retention rate for phosphate under low p H value environments,and its chromium removal performance is obviously better than that of NF-P.The result showed that NF-D can still show excellent Donan repulsion effect in strong acidity environment,which will broaden the application area of NF membrane furtherly.