Preparation And Desalination Performance Of Highly Stable Zr-based Metal-organic Framework Membrane

In recent years,the world has been struggled to address water scarcity and pollution caused by rapid development of economy and industrial activities.Membrane-based desalination has been considered as an effective way to obtain freshwater from saline water resources.Pervaporation technology（PV）,a thermal-membrane hybrid method,has been receiving increasing attentions for hypersaline water treatment.However,current membranes suffer from challenging issues such as low flux and insufficient stability.Robust crystalline molecular sieve membranes such as zeolite and metal-organic frameworks（MOFs）have received increasing attention in desalination application,due to their unique merits such as well-defined pore structure,high porosity and diverse building blocks.In particular,highly stable Zr-MOF UiO-66 membranes can be considered as promising candidate for desalination.However,they largely focus on low-concertation saline waters,and still suffers from insufficient permeability and salt rejection.In this thesis,an in situ nano-seeding followed by secondary growth strategy was proposed to fabricate high-quality stable MOF thin membrane（UiO-66）for high performance pervaporation desalination of hypersaline waters.To address the issue of membrane quality,a TiO₂ nano-interlayer was introduced on coarse mullite substrates for better growth of UiO-66 nano-seed layer,on which a well-intergrown UiO-66selective membrane layer was finally produced via subsequent secondary growth.Separation performance for hypersaline waters was systematically investigated at different salt concentrations,feed temperatures and long-term operation in different chemical environments via a PV process.Furthermore,fouling or scaling behavior was also studied with presence of foulant models in hypersaline water.Main results and conclusions are shown as follows:（1）Fabrication of mullite substrate and TiO2 nano-interlayer.Using inorganic solid waste fly ash and natural mineral bauxite as raw materials,a low-cost mullite hollow fiber ceramic substrate was prepared via the wet-spinning followed by sintering process.However,mullite substrate exhibited high roughness,macroporous structure and low concentration nucleation sites.To improve substrate quality,a TiO₂ nano-interlayer was introduced onto coarse mullite substrate via dip-coating technique.This nano-interlayer exhibits significantly lowered surface roughness,decreased pore size and enhanced hydroxyl concentration,which was beneficial to not only the in situ growth of uniform Ly distributed UiO-66 nano-seed layer,but preventing their growth inside microporous substrate.（2）Fabrication and characterization of defect-free UiO-66 thin membrane.Via heterogeneous nucleation from precursor solution,UiO-66 nano-seeds（～180 nm）were in situ grown and uniform Ly distributed onto mullite-TiO₂ substrate.After secondary growth on nano-seed layer,a high quality UiO-66 thin membrane was formed,featuring well-intergrown defect-free morphology,sufficient hydrophilicity and lowered membrane thickness（ca.1μm）.The UiO-66 membrane was characterized by XRD,FTIR etc.,which showed a pure phase and typic groups.Single-gas permeation results once again indicate a defect-free characteristic of UiO-66 thin membrane.（3）Desalination performance and long-term operation stability for UiO-66membrane.The desalination performance of UiO-66 membrane was investigated in PV process.The permeate flux increased with feed temperature but decreased with feed salt concentration while salt rejection remained a high level（over 99.9%）.Besides almost complete rejection（99.9%）,UiO-66 membrane exhibited high flux(37.4 L·m^-2·h^-1)for hypersaline waters（7 wt.%）,outperforming current existing zeolite and MOF membranes for treatment of lower concentration saline waters（3.5 wt.%）.The UiO-66 membrane was also demonstrated a superior long-term operational stability even at harsh environments（such as hot and acidic hypersaline solutions）.（4）Fouling behavior of UiO-66 membrane during long-term operation.The adsorption was investigated for Zr-MOF on common organic pollutants.Humic acid,sodium alginate and mineral ions were respectively used as models to probe fouling behavior during long-term desalination operation of UiO-66 membrane.The chemisorption between Zr-MOF and tetracycline is one of the key reasons for membrane fouling.For the organic fouling caused by humic acid and sodium alginate,the fouling process can be divided into two stages（i.e.,membrane-foulant interaction stage and foulant layer-foulant interaction stage）.According to the quantitative calculation by XDLVO theory,the interactions exhibited similar trend at all stages.The negative energy profiles at separation distance demonstrated certain attraction interactions at the interface.When close to membrane surface（low separation distance）,the short-range AB interaction dominated membrane fouling.At large separation distance,a long-range LW interaction dominated over other interactions.Severe scaling caused by mineral ions(such as Ca²⁺,Mg²⁺)blocked the water transport channels of UiO-66membrane,consequently decreasing water flux.The proposed preparation protocol in this study is not only applicable for design of high-quality UiO-66 membrane enabling hypersaline water treatment,but can be potentially extended to other MOF membranes for more separation applications.In addition,some effective strategies need to be considered to improve the anti-fouling ability of UiO-66membrane when treating real brine.