|Abstract:||Heavy metal ions, such as Zn2+, Cd2+, Pb2+, Ni2+, Cu2+, Mn2+, Hg2+, Co2+, Al3+, Fe3+, Cr6+,… are mainly discharged into the environment as industrial wastewaters, causing serious soil and water pollution and consequently effect the health of living organisms due to ecotoxical properties. Their removal is achievable with membrane technologies, by using appropriate nanofiltration membrane. Since NF membrane’s efficiency varies regarding the material used and preparation process of the manufacturer, surface modification with alkoxysilanes via sol-gel process seems promising for a more successful selective removal of heavy metal ions.
Surface modification of membranes by alkoxysilane precursors via sol-gel process in terms of additional surface coating can further stabilize the membrane, and particularly improve the selective removal of specific heavy metals due to steric effects, electrosteric properties and functional groups with a tendency to heavy metal ions on the surface membrane.
The research was focused on the basic characterization of the selected thin-film composite NF membrane (NF2, Inc. Sepro.). Sols were prepared in accordance with the sol-gel process pure and combined components of alkoxysilane precursors, i.e. tetraethoxysilane (TEOS), 3-(trimethoxysilylpropyl)diethylenetriamine (DETA), 3-(mercaptopropyl)trimethoxysilane (MPTMS) for particle. Sol formation was based on water-to-precursor ratio: (r525, r264 and r175) and combinations of precursors in the proportions 1:1, 1:2, 1:4, 2:1 and 4:1, in alkaline and acidic pH medium (~4.5 and ~10), from 1 to 14 hours aging time and temperatures from 25 °C to 50 °C. Prepared sols and modified membranes were characterized via several different characterization techniques. The purpose of basic research was to examine the parameters effect of preparation and sol-gel coating (type and concentration of sol-gel precursor, catalyst, molar ratio water to precursor, sol aging, and preparation time of sol) on the final microstructure of membrane surface modification. Based on the analyzed properties of sol-gel alkoxide materials and surface layer of the modified membranes, information of the basic parameters that influence the effect of retention of heavy metals ions (Zn2+, Cd2+, Hg2+, Pb2+) from wastewater was obtained. Surface-modified membranes with the best sol-gel coatings were tested for efficiency of heavy metal ions removal using model water at various filtration conditions.
The characterization results of non-modified, commercially available thin-film composite nanofiltration membrane showed an unstable structure and varying retention rates of ions and molecules of different sizes and weights as its drawback.
Selected alkoxysilane precursors (TEOS, DETA and MPTMS) forming coatings in sol-gel process with optimized sol composition under optimized sol-gel procedure enable to form the functional particles with affinity to membranes and to heavy metal ions and to apply them as a functional and stable monolayer onto the selected membrane.
The results of the filtration show that the surface modification of NF membranes can be effective in terms of selective retention of heavy metal ions. Particularly the selective retention of Hg2+ and Pb2+ ions is improved by using a combination of precursor TEOS:MPTMS and TEOS:DETA 1:1 ratio, at temperature 50 °C, and TEOS with a r525 at room temperature.
The filtration experiments using a laboratory scale filtration unit demonstrated that by modifying the membrane in terms of application of additional functional layers, formed of particles on the basis of selected alkoxysilane precursors by the sol-gel method, the selectivity and stability of selected TFC nanofiltration membrane can be significantly improved (more than 100% for Hg2+ and up to 10% the Pb2+).
Extensive experimental work, the results of which are monitored and proven by selected analytical methods (SEM/TEM/EDXS microscopy, ATR-FTIR spectroscopy, measurements of zeta potential of colloids, atomic absorption spectroscopy), lead to great opportunities for furthe|