FellowInna Melnyk
Project NameBifunctional silica and magnetite spherical particles with tailored porosity and surface chemistry for complex water treatment
Host organisationInstitute of Geotechnics
Duration of the project01.03.2016 - 31.12.2018

The purpose of the present project is to develop bi(or multi)functional silica and magnetically removable adsorbents for water purification and remediation securing removal of chemical pollutants both organic and inorganic nature. SASPRO will unite the efforts of the scientists and their expertise in two adjacent fields - synthesis and functionalization of mesoporous silica and silica-based adsorbents from Ukraine and advanced experience in physicochemical characterization of surface binding with valuable experience of their implementation in water purification from the Slovak side. Our specific aims: 1) Creation of silica microsheres with well defined composition for selective uptake of metal cations. 2) Сombination the properties of magnetite (easy removal from the reaction medium, recovery) with the assets of functionalized polysiloxane (specific groups, large specific surface area) and contribute to the selective removal of contaminating cations and organic molecules simultaneously. 3) Synthesis of nanocomposite multi-functional microparticles, testing them on wastewater samples modeling complex water systems. The goals will be achieved through coordinated research uniting the efforts of specialists in materials synthesis, characterization and application. In addition these materials are promising for applications in chromatography, drug delivery, creation of chemo- and biosensors, etc.

Project Summary with Interim Results

The work was carried out in 3 directions:

1. Synthesis and study of bifunctional silica nanoparticles with amino and amino/methyl (phenyl) groups

Silica particles with bifunctional surface layers and high content of available aminopropyl groups were successfully produced by a modified one-step Stöber approach. Combining hydrophobic ligands with the aminopropyl groups in the surface layers of hybrid silica particles appears to offer generally smaller particles. Their surface is apparently porous when methyl groups are introduced as co-ligand. The content of functional groups and thermal stability of the surface layers were analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and 13C and 29Si solid-state NMR spectroscopy revealing their composition and organization. The fine chemical structure of the surface in the produced hybrid adsorbent particles and the ligand distribution were further investigated by electron paramagnetic resonance (EPR) and electron spectroscopy of diffuse reflectance (ESDR) spectroscopy using Cu2+ ion coordination as a probe. The composition and structure of the emerging surface complexes were determined and used to provide an insight into the molecular structure of the surfaces. It was demonstrated that the introduction of short hydrophobic (methyl) groups improves the kinetic characteristics of the samples during the sorption of copper(II) ions and promotes fixation of aminopropyl groups on the surface of silica microspheres. The methyl groups dilute the aminopropyl groups uniformly, opening for uptake of higher amounts of Cu2+ ions used as probes in relation to the amount of grafted amino functions. This should even improve potentially the reactivity used for further functionalization of amino modified silica. As a consequence the amino functions become apparently partly inaccessible for the complexation with Cu2+ as testified by low Cu2+/Lig ratios (0.35 and lower), while the simultaneous EPR and ESDR measurements reveal the formation of the trans Cu(Lig)22+ complexes. The size and aggregation/morphology of obtained particles was optimized controlling the synthesis conditions, such as concentrations of reactants, basicity of the medium, and the process temperature. Changes in the surface geometry in the bifunctional layers even result in unusual patterns of adsorption of small organic molecules, making the particles capable to use both hydrophobic and hydrophilic functions simultaneously.

A one-stage method of synthesis of new silica nanoparticles containing 3-aminopropyl and phenyl groups was developed. Spherical silica particles with bifunctional (≡Si(CH2)3NH2/≡SiC6H5) surface layers were synthesized by the Stöber method using ternary alkoxysilanes systems. According to SEM data, upon heating the reaction solution (from 2 to 45°C) the diameter of particles and their aggregation could be regulated from 50 to 846 nm. Elemental analysis and Zeta potential measurement point to the same content of functional groups, but different surface charges depending on the synthesis temperature. Silica nanoparticles with bifunctional surface layer contain ~2.2 mmol/g of amino groups and ~1.1 mmol/g of phenyl groups. The variation of electric charges of N-functional groups was disclosed in obtained nanospheres and attributed to different surface location of these groups and their surrounding with other groups. The sorption of Cu(II) ions by functionalized silicas depends on the concentration of amino-groups, which correlates with the isoelectric point values (determined to vary from 8.26 to 9.21). Bifunctional nanoparticles adsorb 99.0 mg/g of methylene blue, compared with 48.0 mg/g by silica sample with amino groups, which may indicate a partially non-specific interaction of methylene blue molecules with the surface phenyl groups. Silica nanoparticle sample with amino-/phenyl- functional surface layer demonstrate up to 65.7% of antibacterial activity against S. aureus during 60 min., whereas for the same sample with adsorbed copper(II) ions, the antibacterial activity is much higher, and reaches 97.8%.

2. Development of methods for obtaining bridged particles with 3-mercaptopropyl groups

We considered the possibility to obtain mercapto-containing spherical particles based on the 1.2-bis(triethoxysilyl)ethane (BTESE) and 3-mercaptopropyltrimethoxysilane (MPTMS) using the modified Stöber method. Hydrolytic polycondensation reaction of alkoxysilanes was carried out in the presence of a fluoride-ion catalyst in an ethanol solution. The influence of various factors (ratio of reacting components, the amount of solvent and water) on the morphology, particle size, porosity, and content of SH-groups was studied.

The synthesized materials were investigated by scanning electron microscopy, photon cross-correlation spectroscopy, Fourier-transform infrared spectroscopy, N2 adsorption–desorption isotherms, elemental analysis. The effects of the contact time and the initial concentrations of Ag(I), Pb(II) and Cd(II) ions were studied by conventional static mode adsorption test. The obtained materials remove metal ions from the wastewater.

3. Magnetic nanoparticles with thiol-containing groups: synthesis and application

Functionalization of magnetite nanoparticles with mercaptopropyl groups and bifunctional mercaptopropyl-and-alkyl groups bearing silica shells were carried out by one-step synthesis. It was shown that content of 3-mercaptopropyl groups was increasing in the preparation of materials with a bifunctional surface layer due to different rates of the silanes’ hydrolysis. The presence of alkyl groups was demonstrated to prevent the formation of disulfide bridges on the particle surface helping to maintain the functionality. It was shown that the gradual introduction of the sol and the presence of catalyst contribute to better coverage with polysiloxane layer characterized by well-developed surface, but also increase the number of functional groups introduced in their surface. The samples obtained after functionalization retain magnetic properties, and can be easily removed from an aqueous medium using a magnet. The influence of synthetic conditions was revealed leading to increased amounts of active functional groups on the surface and improved capacity in uptake of Ag(I), Hg(II), Cd(II) and Pb(II) cations. The obtained materials were characterized by TEM, SEM, XRD, FTIR, Raman, TGA and chemical microanalysis shedding light on the efficiency and mechanisms of the synthetic procedures. The composition, stability and kinetics of the surface complexes with heavy main group metal cations were investigated opening for large scale production of improved adsorbents. Bifunctional samples show better removal efficiency of metal ions from industrial water, namely the values of efficiency can reach 90% for cadmium (II), 98.3% for lead (II) and 89% for zinc(II). Absorption properties of bifunctional sample with 3-mercaptopropyl and propyl groups almost restored after regeneration by nitric acid.

Web page of the project was created - http://ugt.saske.sk/en/saspro/