Functionalized UiO-66 Mixed Matrix Membranes for the Removal of Selected Heavy Metals from Industrial Effluents

Abstract

Heavy metals are solicitous environmental pollutants due to persistent nature, toxic effect and tendency to bioaccumulate. Environmental Protection Agency (EPA) compels regulations on various industries to decrease the level of heavy metal contamination to safe limit. In modern era of technology, usage of nanofiltration (NF) membranes is introduced in different industries such as petrochemical, desalination, biotech and chemical etc. NF technology not only overcomes the operational and economic problems of conventional separation techniques but also cope over with efficiency and environmental sustainability. Mostly, NF membranes are thin-film nanocomposite (TFNs) membranes comprised of synthetic polymers (PSf, PES, SPEEK etc.) integrated with inorganic fillers (silica, zeolite, alumina etc.) which enhances the separation of charged metal ions from aqueous medium. Moreover, separation of both inorganic and organic particles through TFNs takes place in various methods; Donnan effect (diffusion of solution as well as sieving), electro-migration and dielectric exclusion. The aim of current research study was to fabricate promising mixed matrix membranes i.e. TFNs integrated with functionalized UiO-66 metal organic frameworks (MOFs) with heavy metal ions (Ca, Cd, Cu, Hg and Pb) selectivity and permeability. Recent work focuses firstly, on synthesis of highly stable zirconium based UiO-66 MOF, its functionalization with sulfonic group (UiO-66-SO3H) via solvothermal method and investigates the effects of synthesis conditions and chemical additives on MOFs preparation. Secondly, fabrication of thermally stable mixed matrix membranes i.e. TFNs incorporated with different loadings of MOFs (UiO-66 and UiO-66-SO3H) by interfacial polymerization (IP) technique followed by a reaction between piperazine (PIP)-aqueous and trimesoyl chloride (TMC)-organic solvent (hexane) at the surface of commercially available polyether sulfone (PES) substrate. PIP helped in the formation of polyamide (PA) thin layer as compared with the PA dense layer by m-phenylenediamine (MPD), commonly used in TFN membranes preparation. Thirdly, synthesized MOFs and fabricated TFNs were characterized by Fourier Transform Infrared (FTIR) spectroscopy, X-Ray Diffraction (XRD), Brunauer–Emmett–Teller (BET), Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive X-ray Spectroscopy (EDX), Atomic Force Microscopy (AFM), and Spectroscopic Ellipsometry (SE). Lastly, environmental remediation of heavy metal ions was examined via both MOFs and TFNs from water using batch adsorption and filtration experiments, respectively. Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) was used to identify metal ions concentrations in feed and permeate solutions. The results highlighted by current study revealed that functionalized MOFs (UiO-66-SO3H) achieved effective removal of metal ions such as 98% for calcium and 97.5% for cadmium at low concentration of 0.2 gm of adsorbent in 1000 ppm solution at pH 6 and 25°C temperature after 3hrs. It was further revealed that TFNs (incorporated with 0.2 wt % of UiO-66-SO3H), exhibited the best performance amongst all other TFC and TFN membranes. This TFN membrane displayed an excellent permeability of 9.57 LMH/ bar and higher rejections of 90%, 87.8%, 85%, 84.7% and 79.5% against CaSO4, CdCl2, Cu(CH3COO)2, HgCl2 and PbSO4, respectively. Good performance of TFN membranes with UiO-66-SO3H nanoparticles as compared to TFN membranes with UiO-66 nanoparticles and TFCs was due to enhanced hydrophilicity, exceptional aperture and presence of sulfur group (-SO3H). It was also found that loading of MOFs (wt %) is an essential factor to fabricate TFN membranes for removal of heavy metal ions. The present study will provide a favorable platform for the fabrication of TFN membranes with other functionalized MOFs of UiO-66 for heavy metal removal from aqueous medium. It is summarized from results of the present study, that sulfo-functionalized MOFs possess excellent adsorption properties. It is projected by this work that UiO-66 functionalized MOFs will play a vital role to reduce the heavy metal pollutant load in aqueous environment. Moreover, there is a possibility to commercialize these fabricated TFN membranes for the application of waste water treatment technologies based on green synthesis and environmentally sustainable.