Sol-gel approach based on hydrolytic copolycondensation of tetraethoxysilane (TEOS) with ethylthiocarbamidepropyl triethoxysilane (ETUS) was used to deposit functional layers with thiourea functions on the surface of macroporous ceramic alumina membrane supports. According to SEM images, such layers are composed of nanoparticles of about 60-70 nm in diameter, while IR spectroscopy data confirmed the presence of functional groups introduced during the synthesis. Such functionalization technique allows combining sorption and membrane technology and could be successfully used to remove trace quantities of copper, cadmium, and silver metal ions from aqueous solutions.
Methods of membrane separation used in water purification technology are conventionally divided into dialysis, electrodialysis, microfiltration, ultrafiltration, and reverse osmosis. In the process of ultrafiltration, the macromolecular substances exceeding the membrane pore sizes are retained, while the solvent freely permeates through the membrane. From ordinary filtration, ultrafiltration is distinguished by the smaller size of particles undergoing separation, while the pore size of the membrane should not exceed the size of the particles. In addition, this process results in the concentration of initial solution or suspension, not the precipitate formation as in conventional filtration. The mechanism of ultrafiltration is close to ordinary filtering [
Membranes including thiourea groupings on the basis of different supports were synthesized and studied by many scientists due to their selectivity [
Various adsorbents with thiourea groups were synthesized using the sol-gel technique and analyzed in terms of their adsorption properties. Porous materials with thiourea groups in the surface layer are proved to be perspective for the sorption of heavy metal ions from aqueous solutions. Earlier, the scientists from our research team developed a one-step technique for the synthesis of the xerogels [
Therefore, in the current research, we considered the possibility to deposit active layers containing thiourea functions [≡Si(CH2)3NHC(S)NНC2H5] on the surface of planar and tubular ceramic membrane supports.
Planar ceramic Al2O3 membrane supports are purchased from Anodisc, Whatman, with a diameter of 25 mm and an average pore size of 0.2
To prepare the functionalizing sol, both TEOS and ETUS were subjected to preliminary acidic hydrolysis. For TEOS hydrolysis, it was mixed with an equal volume of ethanol (1.12 mL) at constant stirring (500 rpm) followed by the addition of 1 mL of hydrochloric acid (0.0024 М) as a catalyst. The hydrolysis was carried out at 70–80°С for 30 min until the formation of transparent solution. Separately, the hydrolysis of trifunctional silane (ЕТUS) was carried. Similarly, equal volumes of ЕТUS and ethanol (4 mL each) were mixed in a glass with the addition of 0.9 mL of hydrochloric acid (0.0024 M) as a catalyst at room temperature (20-25°C). The mixture was stirred until the formation of transparent sol (3 min). The sols of TEOS and ЕТUS were mixed at TEOS/ЕТUS molar ratio 3/1 and constant stirring at 500 rpm. Afterwards, the resulting TEOS/ЕТUS sol was diluted with ethanol and deposited onto the membranes.
Before functionalization, the tubular membrane supports were activated as described in [
Thereafter, the functionalized membranes were dried for 2 h in air (at ambient temperature), for 2 h at 30°C and 50°C, and 20 h at 80°C. The samples were named as
Scanning electron microscopy (SEM) was used to investigate the porous structure of the membranes. The images were obtained with an electron microscope (JSM 6060 LA, Jeol, Japan) using secondary electrons at an accelerating voltage of 30 kV. The surface of the samples was covered with a thin continuous layer of gold by cathodic sputtering in vacuum.
FT-IR spectra were recorded on a Thermo Nicolet Nexus FTIR spectrometer using diffuse reflection mode “SMART Collector” in the 4000–400 cm-1 range, with a resolution of 4 cm-1. The samples were ground with KBr (Fluka, spectranal) at the ratios of
For contact angle measurements, a membrane was mounted on a horizontal stage and a water drop (10
The filtration of water and aqueous solutions of heavy metal ions (Cu(II), Ag(I), and Cd(II)) through the planar membranes was carried out using an Amicon stirred ultrafiltration cell (Millipore Corporation, USA) in the dead-end mode (Figure
Amicon stirred ultrafiltration cell (Millipore Corporation, USA) (a) and experimental setup for studying transport characteristics of tubular membranes in the circulation mode (b): 1—feed solution, 2—retentate, 3—membrane cell, 4—pump, 5—container with purified permeate, and 6—container with feed solution.
Working aqueous heavy metal ion solutions were prepared from the batches of the corresponding nitrate salts of these metals: Cu(NO3)2·3H2O solution (pH ~5.8), AgNO3 solution (pH ~2), and Cd(NO3)2·4H2O solution (pH ~5.7). The samples of the permeate and concentrate were tested for the content of Cu(II), Ag(I), and Cd(II) ions.
Membrane regeneration for copper(II) ion desorption was carried out in the filtration unit in the dynamic mode by pumping 0.1 N HCl solution through the membranes for 30 min, followed by washing with distilled water (still in the filtration unit) till neutral pH values [
Heavy metal ion concentrations in the probes were determined by atomic absorption spectrophotometer C-115-M1 in depleted (oxidative) flame (acetylene/air mixture) using the resonance signals at 328.1 nm (Ag), 324.7 nm (Cu), and 228.3 nm (Cd). The source of resonance radiation was a spectral lamp LS-2 with a detection limit of 0.01
The retention (
Since porous materials with thiourea groups in the surface layer are proved to be perspective for the sorption of heavy metal ions from aqueous solutions, we made an attempt of introduction these groups in the structure of the functionalizing layer of planar ceramic membranes. However, the procedure for the surface layer deposition was similar to the functionalization of membranes with mercapto groups, as described in [
Scheme of the reaction of hydrolytic polycondensation of alkoxysilanes.
Because of sufficient viscosity of the original sol, it was diluted with ethanol to facilitate the formation of a homogeneous polysiloxane layer on the membrane surface (Table
Functionalizing membrane supports with thiourea groups.
Sample | Sol/EtOH ratio | Volume of deposited sol (mL) | ∆m (g) | MD ( |
Thiourea groups content (theor.) (mmol/g) |
---|---|---|---|---|---|
1/4 | 0.025 | 0.0014 | 335 | 2.65 | |
1/8 | 0.025 | 0.0017 | 410 | 2.65 | |
1/8 | 0.025 | 0.0006 | 145 | 2.65 | |
1/8 | 0.025 | 0.0099 | 2385 | 2.65 | |
1/8 | 0.010 | 0.0013 | 315 | 2.65 | |
1/8 | 0.025 | 0.0022 | 530 | 2.65 | |
1/2 | 15 | 0.4529 | 8876 | 2.65 |
Figure
SEM images of the membranes with thiourea functional groups:
The method of FTIR spectroscopy was used to verify the functionalization, namely, the formation of polysiloxane network, and the incorporation of thiourea groups. For this reason, there were recorded FTIR spectra of thiourea-functionalized silica particles (Figure
FTIR spectra of polysiloxane spheres with thiourea groups (1), membrane functionalized with thiourea groups
Finally, it should be mentioned that the IR spectra of functionalized membranes contain the most intense absorption band in the range of 1059–1200 cm-1 with a high-frequency shoulder. Its presence is associated with the formation of a three-dimensional polysiloxane network [
To evaluate the changes in surface hydrophilicity, we conducted the measurement of water contact angles of the membrane supports after functionalization. The water contact angle of the initial membrane support was measured and described earlier [
Contact angle measurements for
Water flux through planar membranes, initial (a) and functionalized with thiourea groups (b).
It appears that the flux through the membranes functionalized with sol nine times diluted with ethanol (
To evaluate the performance of functionalized membranes in retention of heavy metal ions, we conducted the experiments with the filtration of model solutions of Ag(I), Cd(II), and Cu(II) nitrate salts (Figure
Silver(I) (a) and cadmium(II) (b) ions retention by membranes with thiourea groups (
However, for the Cu(II) ions, there is observed increasing retention during the filtration which suggests the involvement of size-exclusion membrane mechanism. An attempt was made to reuse functionalized membranes with thiourea groups in the second cycle after the regeneration procedure. The regeneration was carried out with 0.1% HCl acid. Interestingly, that the membranes with thiourea groups showed improved retention of copper(II) ions in the second cycle, comparing with the first (see Figure
Copper(II) ions retention by membranes with thiourea groups during first cycle of filtration (a) and second cycle (b) (
When analyzing the performance of tubular ceramic membranes in the filtration of water, it was witnessed that water flux decreases strongly after functionalization with polysiloxane layers containing thiourea groups (see Figure
Water flux through tubular membranes, initial (a) and functionalized with thiourea groups (b).
Similar to planar membranes, there were conducted experiments on heavy metal ion retention by tubular alumina membranes with thiourea functions, using copper(II) nitrate model solutions (see Figure
Retention (a) and water flux (b) for tubular membrane with thiourea groups (
Moreover, the retention of copper(II) ions appeared to be the same before and after regeneration. However, a substantial decrease in water flow through the membrane after the regeneration was observed. This, similar to the functionalized planar membranes, may suggest some changes in the properties of the functionalizing layer during the regeneration procedure.
The Sol-gel method based on the reaction of hydrolytic polycondensation of alkoxysilanes was used for creating selective layers with amino and thiourea groups on the surfaces of planar and tubular ceramic membranes. It was shown that such approach allows influencing the degree of hydrophobicity of the created surface layer. Additionally, it allows controlling the porous structure of the surface layer at the stage of formation and ripening of silica sol and its transition to gel, by varying the ratio of reactants, the nature of the solvent, and the drying mode.
The identification of functional coatings on the surface of planar ceramic membranes was carried out by means of IR spectroscopy, while their morphology was analyzed with SEM. It is necessary to use diluted sols for the formation of monolayer-like coating and better interaction of the particles with the surface of the support. It will ensure better fixation of the active layer and contribute to the membrane exploitation in several cycles. It was shown that the deposited layers adsorb heavy metal ions, such as copper(II), silver(I), and cadmium(II). There possibility of membrane regeneration with hydrochloric acid and reuse in several cycles of copper(II) ion sorption/desorption was also shown. An important feature of the proposed functionalization technique is the possibility of introducing other functional groups in the surface of ceramic membranes, i.e., depending on the practical task; there is the possibility of preparing sols with different silanes.
The SEM, FTIR, and metal ion solution filtration data used to support the findings of this study are included within the article.
The authors declare no conflict of interest.
The research was funded by NATO Science for Peace and Security Program SPS.NUKR.SFP 984398 (2012-2017).