Research On Membrane Technology For Removal Of Heavy Metal Ions And Trace Detection Of Cr³⁺

Nanofiltration membrane,as a novel and side-effect-free separation technology for the removal of heavy metal ions from water,has attracted more and more attention in the fields of wastewater treatment and drinking water purification.Polyamide（PA）nanofiltration membrane is a conventional nanofiltration membrane used in water treatment and has been widely used in actual water treatment process.However,because of the negative charged groups（such as carboxyl group）on the surface of polyamide nanofiltration,the removal efficiency of the positive charged pollutants such as heavy metal ions in water is far lower than the removal efficiency of the negative charged organic pollutants in water.Therefore,it is more and more necessary to improve the performance of PA nanofiltration membrane to remove heavy metal ions.At the same time,the detection of heavy metal ions is also an indispensable part of the water treatment field.Recently,a novel detection technique has been developed to realize trace detection of the target by using the super sensitivity of the transmembrane current of the nuclear pore membrane containing nanochannels to the target by constructing a single or multiple nanochannels inside the nuclear pore membrane.In terms of the removal of heavy metal ions in water by PA nanofiltration membrane,the surface of PA nanofiltration membrane was chemically modified by positively charged PEI（polyethylenimine）and Zr⁴⁺in this paper,and the PA nanofiltration membrane with PEI positively charged modification and with PEI/Zr⁴⁺double positive functionalization were obtained respectively.The removal efficiency of two kinds of positively charged PA nanofiltration membranes on Cu²⁺and Cr³⁺,as well as their sustainable performance and anti-pollution performance were investigated.In the aspect of trace detection of heavy metals by nuclear pore membrane,conical nanochannels were constructed in the PET（Polyethylene terephthalate）nuclear pore membrane by asymmetic chemical ething method,and the inner wall of the nanochannels was modified with the functional molecule ANBA（5-amino-2-nitro-benzoic acid）,which has a spectific recognition of Cr³⁺.Different concentrations of Cr³⁺combined with ANBA caused changes in the transmembrane current of the PET nuclear pore membrane,and on this basis,the standard curve of Cr³⁺was further analyzed and calculated to achieve the purpose of trace detection of Cr³⁺.In chapter 3,based on the amide reaction of dehydration condensation of amine and carboxylic groups,the carboxylic activators of EDC（1-（3-dimethylaminopropyl）-3-ethylcarbodiimide hydrochloride）and NHS（N-hydroxy succinimide）were used to modify positively charged PEI on the surface of PA nanofiltration membrane.After PEI 70000 modification,the zeta potential on the surface of PA nanofiltration membrane increased from-6.97 m V to+35.5 m V,and the negative charge on the surface of PA nanofiltration membrane was successfully transformed into positive charge.Although the decrease degree of water flux of PA nanofiltration membrane modified by PEI increased with the increase of molecular weight of PEI,its rejection of Cu²⁺increased obviously.The Cu²⁺rejection increased with the increase of PEI molecular weight and PEI 70000 concentration,and the maximum Cu²⁺rejection was more than 92%.The results of detailed analysis showed that the rejection ability of PEI modified PA nanofiltration membrane for Cu²⁺mainly was from the electrostatic repulsion of surface positive charge on Cu²⁺and the interception of water channel aperture,but not from the adsorption of Cu²⁺by the membrane surface.Subsequent experimental results also showed that the PEI-modified PA nanofiltration membrane had a good sustainable ability for application,and its Cu²⁺rejection performance was not affected by the negative charged organic matter（humic acid,HA）in water,and the pollution degree of the PEI-modified PA nanofiltration membrane received HA would not be further enhanced with the increased filtration number of HA.In chapter 4,the surface of PA nanofiltration membrane modified with PEI 70000was further loaded with Zr⁴⁺,and it was found that the water flux of PA nanofiltration membrane with PEI/Zr⁴⁺double positive functionalization was improved.Compared with modification by only PEI,the water flux increased by 12%～15%.At the same time,the rejection performance of Cr³⁺increased slightly after loading Zr⁴⁺.Both the treatment of low and high concentration of Cr³⁺feed solutions,the PA nanofiltration membrane with PEI/Zr⁴⁺double positive functionalization had a good sustainable ability for application,and all the Cr³⁺rejection reached a high level.More importantly,not only did the negatively charged substance（bovine albumin,BSA）in water have no significant effect on the Cr³⁺rejection performance of PA nanofiltration membrane with PEI/Zr⁴⁺double positive functionalization,but also the loading of Zr⁴⁺could reduce the HA contamination of the positive charged PA nanofiltration membrane and enhance the anti-fouling ability of the positive charged PA nanofiltration membrane.In chapter 5,the nanochannels were constructed inside the PET nuclear pore membrane,and ANBA was modified on the inside wall of the nanochannels.The pore sizes of the tip side and base side were about 40 nm and 180 nm,respectively.The results of XPS,water contact angles and zeta potentials on solid surface clearly demonstrated the ability of ANBA to“capture”Cr³⁺.The appropriate p H value and response time of ANBA-modified nanochannels to Cr³⁺recognition were 6.06 and 60min,respectively.The selectivity experiment showed that the nanochannels modified by ANBA had a good selectivity to Cr³⁺.The results of five cycles also showed that the nanochannels modified by ANBA had a reliable stability and repeatability for the recognition of Cr³⁺.The final analytical parameters showed that the detection limit of Cr³⁺was as low as 4.10 n M.In addition,this method and ICP-MS were used to detect Cr³⁺in real water at the same time,and the recovery rates of both methods for Cr³⁺in real water samples were within a reasonable range of 80%～120%,indicating that this detection method basically reached the detection level of ICP-MS.