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The magnetite nanoparticles were functionalized with silica shells bearing mercaptopropyl (monofunctional) and mercaptopropyl-and-alkyl groups (bifunctional) by single-step sol-gel technique. The influence of synthetic conditions leading to increased amounts of active functional groups on the surface and improved capacity in the uptake of Ag(I), Cd(II), Hg(II), and Pb(II) cations was revealed. The physicochemical properties of obtained magnetic nanocomposites were investigated by FTIR, Raman, XRD, TEM, SEM, low-temperature nitrogen ad-/desorption measurements, TGA, and chemical microanalysis highlighting the efficiency of functionalization and mechanisms of the preparation procedures. The removal of the main group of heavy metal cations was studied in dependence from the pH, contact time and equilibrium concentration to analyze the complexes composition for the large scale production of improved adsorbents. It was demonstrated that introduction of the alkyl groups into the surface layer prevents the formation of the disulfide bonds between adjacent thiol groups. The obtained adsorbents were employed to treat real wastewater from Ruskov, Slovakia with concentration of Fe 319 ng/cm3, Cu 23.7 ng/cm3, Zn 36 ng/cm3, Mn 503 ng/cm3, Al 21 ng/cm3, As 34 ng/cm3, Pb 5.8 ng/cm3, Ni 35 ng/cm3, Co 4.2 ng/cm3, Cr 9.4 ng/cm3, Sb 6 ng/cm3, Cd 5 ng/cm3. These materials proved to be highly effective in the removal of 50% of all metal ions, espeсially Zn, Cd, and Pb ions from it and turned recyclable, opening for their sustainable use in water purification.

Iron-mediated synthesis of alkali metal iodides was quite unexpectedly demonstrated to be able to serve as a cost-efficient and reliable source of spherical single crystalline near-stoichiometric magnetite (Fe3O4) nanoparticles as revealed by TEM and XRD studies and also by XANES spectroscopic quantification of the Fe2+-content. Using the particles as nuclei for the Stoeber synthesis of silica nanoparticles, core-shell magnetic material has been produced. The nature of the magnetic component was probed by XANES spectroscopy. The size of the particles is dependent on the synthesis conditions and Si : Fe ratio but can be kept below 100 nm. It is the Si : Fe ratio that determines the stability of the particles in acidic medium. The latter was investigated spectrophotometrically as leaching of Fe3+-cations. Considerable stability was observed at Si : Fe > 10, while at Si : Fe >= 20 no measurable leaching could be observed in over 10 days. Magnetic nanoparticles with improved stability in acidic medium provide an attractive basis for creation of adsorbent materials for applications in harsh media.

The surface of magnetite nanoparticles was coated with functional polysiloxane layers using reaction of hydrolytic copolycondensation of tetraethoxysilane and 3-aminopropyltriethoxysilane (or N-[3-trimethoxysilylpropyl]ethylendiamine), and also that of tetraethoxysilane, 3-aminopropyltriethoxysilane and methyltriethoxysilane (or n-propyltriethoxysilane). It was shown that these functionalized magnetically controllable particles (about 60–150 nm in size as aggregates), as opposed to magnetite, adsorb urease well from aqueous solutions (up to 1 g/g), and that the level of residual activity of adsorbed layers is up to 84 % in the case of a bifunctional sample. It was established that the activity of immobilized urease is normally gradually reduced during storage of the samples, but in the case of ethylenediamine functional group is not decreased for 45 days. The synthesized samples are promising for use as magnetically directed biocatalysts.

Magnetic nanoparticles modified with siloxane layers bearing amino and thiol functions have been used for immobilization of urease either by adsorption or via surface grafting. The activity of the immobilized enzyme in the hydrolysis of urea extended to the levels typical of the native enzyme, while its long-term stability in combination with magnetic retraction opened for its repeated use in both analysis and detoxification of bio-fluids. The immobilized urease revealed strongly enhanced stability and 65% activity in the presence of 0.1mmol/l of Hg(2+) or 0.3mmol/l of Cu(2+) while the native urease did not retain any activity at all. The enzyme grafting was shown to be a potentially perspective tool in alleviation of heavy metal poisoning and to be providing an opportunity for use of the developed adsorbents as both biosensors and bio-reactants for removal of urea from biofluids.

Sol-gel technology is a versatile tool for preparation of complex silica-based materials with targeting functions for use as adsorbents in water purification. Most efficient removal of organic pollutants is achieved by using enzymatic reagents grafted on nano-carriers. However, enzymes are easily deactivated in the presence of heavy metal cations. In this work, we avoided inactivation of immobilized urease by Cu (II) and Cd (II) ions using magnetic nanoparticles provided with additional complexonate (diethylene triamine pentaacetic acid or DTPA) functions. Obtained nanomaterials were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). According to TGA, the obtained Fe3O4/SiO2-NH2-DTPA nanoadsorbents contained up to 0.401 mmol/g of DTPA groups. In the concentration range Ceq = 0–50 mmol/L, maximum adsorption capacities towards Cu (II) and Cd (II) ions were 1.1 mmol/g and 1.7 mmol/g, respectively. Langmuir adsorption model fits experimental data in concentration range Ceq = 0–10 mmol/L. The adsorption mechanisms have been evaluated for both of cations. Crosslinking of 5 wt % of immobilized urease with glutaraldehyde prevented the loss of the enzyme in repeated use of the adsorbent and improved the stability of the enzymatic function leading to unchanged activity in at least 18 cycles. Crosslinking of 10 wt % urease on the surface of the particles allowed a decrease in urea concentration in 20 mmol/L model solutions to 2 mmol/L in up to 10 consequent decomposition cycles. Due to the presence of DTPA groups, Cu2+ ions in concentration 1 µmol/L did not significantly affect the urease activity. Obtained magnetic Fe3O4/SiO2-NH2-DTPA-Urease nanocomposite sorbents revealed a high potential for urease decomposition, even in presence of heavy metal ions.