Design And Preparation Of Polyvinylidene Fluoride Functional Separation Membrane And Its Application For Removal Of Heavy Metals

With the rapid development of membrane technology,the membrane separation technology is more and more popular in the field of wastewater treatment.At present,the common membrane separation technologies applicable for the removal of heavy metals are electrodialysis,nanofiltration and reverse osmosis,as well as polymer adsorption and micelle enhanced microfiltration/ultrafiltration.Among these approaches,electrodialysis,nanofiltration and reverse osmosis have the disadvantages of high treatment cost and strict pretreatment requirements;polymer adsorption and micelle enhanced ultrafiltration/microfiltration are at risk of secondary contamination.Therefore,the development of a new modified microfiltration/ultrafiltration functional separation membrane,i.e.,endowing the excellent adsorption performance for heavy metals and achieving the removal and recycling of heavy metal pollutants,will significantly expand the application of micro/ultrafiltration membrane in wastewater treatment.Bearing this viewpoint in mind,the author attempted to design and fabricate three kinds of polyvinylidene fluoride-type(PVDF)functional separation membranes,carried out a series of experimental studies,and evaluated their performance of adsorption of heavy metal ions,thereby provide a valuable guidance in the fabrication of functional membranes.The contents and major conclusions are reported as follows:(1)In this research,three polyvinylidene fluoride-type functional separation membranes bearing/grafting the polyamino carboxylic acid and the polyamino phosphonic acid functional groups with strong chelation and coordination in PVDF matrix were fabricated by physical blending or chemical grafting(only bearing polyamino phosphonic acid denoted as EDTMPA-TBOT/PVDF,only bearing polyamino carboxylic acid denoted as DTPA-DETA-VBC-PVDF,bearing polyamino carboxylic acid and polyamino phosphonic acid denoted as EDTMPA-TBOT-APTMS-DTPA/PVDF).(2)The structures and morphologies of the three fabricated PVDF functional separation membranes were characterized using the field emission scanning electron microscopy.The techniques of Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,energy dispersive X-ray spectrometry and nuclear magnetic resonance spectroscopy were employed to characterize the chemical components and functional groups of fabricated membranes.The results confirm that the chelating functional groups have been successfully incorporated into the PVDF matrix,and the three fabricated PVDF functional membranes are competent for the capture of heavy metal pollutants(such as Pb²⁺and Ni²⁺)from the solution.(3)The antifouling performances of the three fabricated PVDF functional separation membranes were assessed.The results show that the antifouling performances of three fabricated membranes are considerably improved compared with that of the pristine PVDF membrane.Especially,the PVDF functional separation membranes prepared by the chemical grafting method exhibits the best hydrophilic characteristic and the strongest antifouling performance.(4)The effects of conventional adsorption parameters and coexisting substances on the Pb²⁺adsorption of fabricated PVDF functional membranes were investigated by static adsorption experiments;the adsorption kinetics and adsorption thermodynamics were analyzed to reveal the inherent adsorption feature of Pb²⁺captured onto the surfaces of membranes.The results indicate that the Pb²⁺adsorption on these membranes are seriously dependent on the change of p H,and the amount capture of Pb²⁺can be ignored at a very low p H value(such as p H=2).All coexisting cations(Ca²⁺,Mg²⁺,Cu²⁺,Ni²⁺,Cd²⁺,Zn²⁺,Fe³⁺and Cr³⁺)show a negative effect on Pb²⁺adsorption of the three PVDF functional separation membranes,but the disturbing orders of them on the Pb²⁺adsorption for the three PVDF functional membranes are not coincident.The interferences of three coexisting complexing reagents(citrate,ethylene diaminetetraacetic acid(EDTA)and nitrilotriacetic acid(NTA))on Pb²⁺adsorption of the three membranes follow the same sequence:citrate<NTA<EDTA.The pseudo-second-order kinetic model and Langmuir isothermal adsorption model can be excellently competent for the descriptions in adsorption kinetics and adsorption isotherms of the three membranes toward Pb²⁺.The adsorption of the three membranes toward Pb²⁺is a spontaneous and exothermic chemisorption process.(5)The effects of influent flow rate,influent concentration and membrane stack thickness on the Pb²⁺adsorption of the three PVDF functional membranes were studied by the dynamic adsorption experiments,and the experimental data of breakthrough curves were fitted and analyzed by Thomas and BDST models.The results illustrate that Pb²⁺adsorption capacities of the three membranes are greater when the influent flow rate is lower,and the influent concentration is higher.The breakthrough time of the three membranes can be prolonged at the conditions of decreases in influent flow rate and influent concentration and the increase in the thickness of the membrane stack.The Thomas model shows an excellent fit to the obtained breakthrough curves,and the calculated maximum adsorption amount is close to the equilibrium adsorption value obtained from the experimental test.In addition,the three PVDF functional separation membranes show a promising regeneration performance,thereby exhibiting the potential engineering application for the recovery of heavy metal ions from the aqueous solution.(6)The series of calculations of density functional theory(DFT)were implanted.Results of DFT simulation are in accordance with the experiment data,indicating that DFT molecular simulation can be advantageous to investigate the adsorption mechanism of the PVDF functional separation membranes toward heavy metal ions from a molecular level.