Preparation Of Loose Nanofiltration Membrane By Pyrogallol And Its Separation Performance For Dye/salt

"Promoting the recycling of industrial wastewater"is a research hotspot in the field of water treatment and printing and dyeing industry in recent years.Printing and dyeing wastewater has the characteristics of large discharge,complex composition,large environmental harm,etc.,so it is necessary to treat printing and dyeing wastewater to recover valuable substances and environmental benefits.In the printing and dyeing wastewater treatment method,nanofiltration technology is widely used.Nanofiltration membranes（NFs）can effectively trap dyes and allow water and small molecules of low-valent salts to pass through,achieving the purpose of dye and salt separation.However,NFs have high retention for high-valent salts,cannot effectively separate divalent salts.Therefore,in order to achieve green sustainability goals,the development of effective resources and the development of high-performance films are very important for the treatment of printing and dyeing wastewater.Loose nanofiltration membranes（LNMs）have a larger pore size and higher molecular weight than NFs,which can enable high-valent salt ions to pass through the membrane to achieve effective dye/salt separation.The biomimetic coating method in the preparation method of LNMs is used because of its universal support layer,easy operation and green process.Polyphenolic compounds have been widely studied because of their special functional groups and general adhesion properties,among which dopamine（DA）and tannin acid（TA）,which are widely used,have the problems of high price and unstable ester structure under alkaline conditions,respectively.Therefore,in this study,the PG/PEI binary biomimetic system LNMs were prepared by using pyrogallol（PG）with similar polyphenol structure and relatively stable properties.The results show that although the PG/PEI system can effectively separate dye/salt,it has problems such as long reaction time and thick selection layer.In order to solve the above problems,trivalent iron ions(Fe³⁺)were added to the PG/PEI system to form a PG/PEI/Fe³⁺ternary biomimetic system,and highly permeable LNMs with ultra-thin selection layers were prepared.The specific results of the study are as follows:（1）The PG/PEI binary biomimetic system composed of PG and PEI was prepared for LNMs,and their physicochemical properties and dye/salt separation property was investigated.The obtained LNMs surfaces were hydrophilic and negatively charged.The higher the content of PEI in the binary biomimetic system,the thicker the coating and the rougher the membrane surface,which obviously affects the morphology and hydrophilicity of the film,and then affects the permeability.The PG/PEI-1:0.1 exhibited a desirable water permeability of 46.2 LHM·bar^-1,high rejection of Congo Red（99.9%）as well as high transmission of salts（98.7%for Na Cl,97.2%for Na₂SO₄,98.6%for Mg Cl₂,and 96.3%for Mg SO₄,respectively）.Even at 40 g/L salt content,the high retention rate of CR and the high permeability of salt can still be maintained.In addition,PG/PEI-1:0.1 membranes have excellent stability and resistance to organic matter pollution.（2）The characterization results of the PG/PEI/Fe³⁺ternary system show that the addition of Fe³⁺limits the formation of covalent bonds（CB）between PG and PEI through Michael addition and Schiff base reactions,as it can form coordination bonds（COB）with the-OH of PG and-NH groups of PEI,thereby further reducing the selective layer thickness and improving the permeability of the membrane.The content of PEI and Fe³⁺in the ternary biomimetic system increases,the polymers and oligomers on the membrane surface also increase,and the thicker the membrane surface coating,which in turn affects the permeability.More importantly,PG/PEI/Fe³⁺-1:0.1:2 membrane had an ultrahigh water flux of 141.5 LHM·bar^-1,a high rejection of Congo red（CR,99.5%）and Alcian blue 8GX（ALBL,99.7%）,and a high transmission rate（98.8%for Na Cl,97.3%for Na₂SO₄,98.2%for Mg Cl₂,96.1%for Mg SO₄）.High CR retention and high salt pass rates are maintained even at 60 g/L salt content.In addition,PG/PEI/Fe³⁺-1:0.1:2 also has excellent stability and anti-dye contamination ability.In summary,the prepared LNMs by biomimetic coating method were used in this study,and the prepared membrane had high dye retention rate and high salt transmittance,which could effectively separate the dye/salt and have excellent stability performance.Compared with PG/PEI,PG/PEI/Fe³⁺is thinner than that of LNMs prepared by PG/PEI,the water flux is greatly improved,and the coating time is effectively shortened.Therefore,the pyrogallol can be used in practical applications as a promising method to achieve efficient dye/salt separation and thus sustainable remediation of textile wastewater.