Revista Română de Materiale / Romanian Journal of Materials 2020, 50 (1), 3 – 9 3

EFECTUL NANOCOMPOZITELOR DE SILICE MEZOPOROASĂ CU TiO2 / ZnO ÎN EPURAREA APELOR UZATE

EFFECT OF MESOPOROUS SILICA WITH TiO2 / ZnO NANOCOMPOSITES IN WASTEWATER TREATMENT

FLORINA-DIANA DUMITRU, MIHAELA-ANDREEA MONCEA, ANDREEA GEORGIANA BARAITARU , ANA-MARIA PANAIT, MARIUS VIOREL OLTEANU, GYÖRGY DEÁK

National Institute for Research and Development in Environmental Protection, Splaiul Independenței no. 294, Bucharest, Romania

Municipal and industrial waste discharges along

with agricultural activities determines an increase of the toxic pollutants in the aquatic environments, affecting thus the living organisms. Due to the high porosities and large specific surface areas resulted from nanoscale size, mesoporous materials can be used for persistent pollutants removal. Therefore, SiO2-TiO2 and SiO2-ZnO mesoporous composites were synthesized using three different routes, to investigate their potential application as nanofilters for wastewaters. The obtained materials were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscopy (SEM). To assess the heavy metals retention efficiency, the nanopowders were stirred in a laboratory synthesized water enriched with cadmium, nickel and lead. The initial and at a certain time concentration (after 1, 2, 3, and 24h) being evaluated through atomic absorption spectroscopy (AAS). The results showed good retention efficiencies of the obtained nanocomposites for the investigated heavy metals.

Eliminarea deșeurilor municipale și industriale,

împreună cu activitățile agricole determină o creștere a poluanților toxici din mediile acvatice, afectând astfel organismele vii. Datorită porozităților ridicate și a suprafețelor specifice mari rezultate ca urmare a dimensiunilor nanometrice, materialele mezoporoase pot fi utilizate pentru îndepărtarea poluanților persistenți. Prin urmare, compozitele mezoporoase SiO2-TiO2 și SiO2-ZnO au fost sintetizate folosind trei rute diferite, pentru a investiga posibilitatea utilizării lor ca nanofiltre pentru ape uzate. Materialele obținute au fost caracterizate prin fluorescență cu raze X (XRF), difracție de raze X (XRD) și microscopie electronică cu scanare (SEM). Pentru a evalua eficiența de retenție a metalelor grele, nanopulberile au fost agitate într-o apă sintetizată în laborator, îmbogățită cu cadmiu, nichel și plumb. Concentrația inițială și la un anumit timp (după 1, 2, 3 și 24 de ore) fiind evaluată prin spectroscopie de absorbție atomică (AAS). Rezultatele au arătat eficiențe bune de reținere a metalelor grele investigate în cazul nanocompozitelor obținute.

Keywords: mesoporous composites, heavy metals removal, structural characterization 1. Introduction

Heavy metal loading of waters causes

serious damages on the aquatic ecosystems (flora and fauna) and further to the human health. As a consequence, the disturbance induced by heavy metal pollution produces the loss of biodiversity and increases the bioaccumulation of pollutants in the food chain. There are different methods to remove pollutants by using nanomaterials since the traditional materials and treatment technology (e.g. activated carbon, oxidation, reverse osmosis etc.) are insufficient to treat wastewaters contaminated with complex pollutants. Thus, in the recent years the researches were conducted towards developing cost effective and reliable nanomaterials for which the mesoporous SiO2 has become the favorite component [1, 2].

Either used as such or functionalized with other components, the so called molecular sieves based on SiO2 have gained great interest in their use for purifying contaminated water [3, 4] due to their special properties such as high porosities and

large specific surface areas (BET) [5]. Due to the high density of silanol groups from the pore channels, various functional silica-based materials can be synthesized and applied in many areas of science and technology. Thus, the high retention capacity of SBA-15 (well-ordered hexagonal mesoporous silica) for Cu2+, Zn2+, Ni2+, Co2+ and Pb2+ ions under its functionalization with salicylaldehyde and - NH2 groups was demonstrated [6, 7] as well as its catalytic activity on dyes removal from wastewaters at neutral pH in terms of TiO2 versus ZnO doping [8]. Mercury has been removed from wastewater using hexagonal mesoporous silica functionalized with ethylenediamine showing that this material has high activity towards the removal of Hg2+ from contaminated water [9].

Taking into account that the attentions is mainly focused on heavy metals removal from wastewaters, the present work highlights the retention efficiency of a synthetized mesoporous material (SBA - 15), combined with TiO2 and ZnO from a distilled water enriched with Cd2+, Ni2+ and

Autor corespondent/Corresponding author, E-mail: baraitaru_andreea@yahoo.com

4 F-D. Dumitru, M-A. Moncea, A.G. Baraitaru, A-M. Panait, M. V. Olteanu, G. Deák / Effect of mesoporous silica with TiO2 / ZnO nanocomposites in wastewater treatment

Pb2+ ions, demonstrating its applicability in environmental protection field. This aspect contributes to the widening of the spectrum regarding the currently used methods in applications corresponding to the field of environmental protection, more specifically in wastewater treatment applications. 2. Materials and methods

The bottom-up synthesis method was applied in order to obtain SBA-15 nanocomposite materials. For the nanostructured silica-based composites it was used a type of mesoporous silica, previously developed, characterized and tested [4, 5], further mixed with TiO2 and ZnO by different methods, to obtain complex materials with specific characteristics for environmental applications. 2.1.Z1/T1 synthesis (nanocomposite materials –

first method) The surfactant, Pluronic P123 Poly

(ethylene glycol) – block – poly (propylene glycol) – block – poly (ethylene glycol)), was dissolved in 2M HCl solution (according to [4]) at room temperature. After 30 minutes of stirring, the TEOS (tetraethyl orthosilicate) silica precursor was added and the mixture was continuously stirred for 1 hour at 40 °C. Separately, zinc oxide (previously synthesized [10] with an avverage cristalite size of 48 nm) and titanium dioxide (anatase form acquired from Merck with an average particle size of 25 nm) nano powders were dispersed in 8 ml ethanol at room temperature, with a mass ratio powder/ethanol of 1: 5. The resulted solutions were then combined and vigorously stirred for 4 hours obtaining thus two different mixtures. The resulted solution was aged for 24 hours at 100 °C in a sealed container, under autogenous pressure. The obtained products were washed with distilled water and filtered, dried overnight and calcined at 550 °C for 5 hours and 30 minutes resulting thus SBA-15/ZnO (Z1) and SBA-15/TiO2 (T1) nanostructured silica-based composites. 2.2.Z2/T2 and Z3/T3 synthesis (nanocomposite

materials – second method) The calcinated mesoporous silica [4], was

functionalized with ZnO / TiO2 in accordance with the next procedure: 1 g of the calcinated mesoporous silica was firmly mixed with 0,08 g ZnO (for Z2 synthesis) and TiO2 (for T2 synthesis) respectively after which the powders mixtures were thermally treated at 550 °C for 5 hours and 30 minutes obtaining thus Z2 and T2 composites. The same procedure was used to obtain Z3 / T3 nanostructured silica-based composites, the difference consisting of using uncalcinated mesoporous silica. During the final step of the mesoporous silica synthesis which consists of calcination process at 550 °C, the surfactant is

completely removed from the porous matrix of silica. For the case where uncalcinated silica was used, the surfactant was removed after doping stage, a process that could influence the structure and characteristics of the finished product.

To characterize the obtained nanoparticles, a series of analyses were performed using high-performance research equipment: X-ray fluorescence measurements (XRF) were performed on a Rigaku Supermini spectrometer in order to obtain the oxide composition; information regarding the crystalline structure and lattice parameters were acquired by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with θ-2θ configuration and Cu Kα1 radiation (λ = 0.154060 Å). The data were obtained by scanning in the range of 2θ = 0,001 - 10° with a step of 0.01°; SEM micrographs were performed on a HITACHI SU-70 FE-SEM at low acceleration voltage (2 kV) to determine the microstructure of SiO2 crystals with no previous coating.

In order to determine the applicability of the obtained mesoporous silica-based materials in the field of environmental protection, cleaning tests of a laboratory synthesized water were carried out. In this context a distilled water with a neutral pH (~7) was enriched with different concentrations of heavy metals (0,3158 mg/L of Cd, 1,0665 mg/L of Ni, 0,5369 mg/L of Pb) and placed in contact with nanopowders up to 24 hours. The retention efficiency of Tn / Zn mesoporous silica-based materials was tested after 1, 2, 3 and 24 hours when samples of filtered water were collected. Cd, Ni and Pb concentrations were determined by atomic absorption spectroscopy using a high-resolution continuum source atomic absorption spectrometer (HRCS-AAS ContrAA 700, Analytik Jena).

Table 1 Oxide composition of synthesized materials, determined by

XRF / Compoziția oxidică a materialelor sintetizate, determinate prin FRX

Mesoporous silica-based

materials

SiO2

[mass %] TiO

2 [mass %]

ZnO [mass %]

Z1 95.1994 - 4.8006

Z2 91.3569 - 8.6431

Z3 84.2842 - 15.7158

T1 92.149 7.851 -

T2 79.4654 20.5346 -

T3 85.0302 14.9698 -

3. Results and discussions 3.1. X-Ray Fluorescence analysis

The percentage oxide composition of the synthesized materials, determined by XRF analyses (Table 1) highlighted an advanced purity of the developed nanopowders, as follows:

F-D. Dumitru, M-A. Moncea, A.G. Baraitaru, A-M. Panait, M. V. Olteanu, G. Deák / Effect of mesoporous silica 5 with TiO2 / ZnO nanocomposites in wastewater treatment

Fig. 1 - XRD spectra of Calcinated and Uncalcinated Mesoporous Silica / Spectrul DRX al Silicei Mezoporoase Calcinată și Necalcinată.

Fig. 2 - XRD spectra of Z1, Z2 and Z3 / Spectrul DRX al Z1, Z2 și Z3.

Fig. 3 - XRD spectra of T1, T2 and T3/ Spectrul DRX al T1, T2 și T3.

6 F-D. Dumitru, M-A. Moncea, A.G. Baraitaru, A-M. Panait, M. V. Olteanu, G. Deák / Effect of mesoporous silica with TiO2 / ZnO nanocomposites in wastewater treatment

Z1, Z2 and Z3 contain mainly SiO2 (over 84 %) and the ZnO doping element (between 4,8 – 16 %), T1, T2 and T3 consist of SiO2 (over 79 %) and TiO2 doping oxide (between 7 – 21).

3.2. X-Ray Fluorescence analysis The X-ray diffraction determinations for the

developed both simple and doped mesoporous silicas nanocomposites (Calcinated and Uncalcinated Mesoporous Silica, Z1, Z2, Z3, T1, T2 and T3) were performed at small angles, as it can be seen from Figures 1,2 and 3.

Following the X-ray diffraction analysis, the hexagonal structure of the mesoporous silica can be observed for all doped samples (Figure 2 and Figure 3), as well as for the calcinated control sample (Figure 1) physically emphasized by the presence of the three intensity peaks: (100), (110), (200), as observed by other researchers when investigating mesoporous silica functionalization [11, 12]. The high intensity (100) diffraction peak, observed at 2θ = 1,4°, indicates an ordered pore structure.

Compared to the simple mesoporous silica [5], the 100 peak shifted to a higher 2θ in case of ZnO and TiO2 nanocomposite samples, due to the two doping oxides added to the siliceous matrix. No visible characteristic peaks of ZnO or TiO2 were observed in the obtained patterns, due to the considerable difference between the amount of mesoporous silica and doping oxides used in the synthesis step. Additionally, depending on the synthesis route, it can be observed a slightly shift to the right and an intensity increase of the secondary peaks. In the case of uncalcinated mesoporous silica, the absence of the two low intensity peaks was observed. This behaviour could be explained by the presence of the surfactant in the silica matrix which provides an amorphous character.

3.3. Scanning electron microscopy analyses

The results from the SEM analysis, shown in Figures 4 and 5, revealed the microstructural features of the 6 types of mesoporous SiO2.

a

b

c

Fig. 4 - SEM images for a) Z1; b) Z2; c) Z3. Imagini SEM ale a) Z1; b) Z2; c) Z3

a

b Fig. 5 continues on next page

F-D. Dumitru, M-A. Moncea, A.G. Baraitaru, A-M. Panait, M. V. Olteanu, G. Deák / Effect of mesoporous silica 7 with TiO2 / ZnO nanocomposites in wastewater treatment

c

Fig. 5 - SEM images for a) T1; b) T2; c) T3 Imagini SEM ale a) T1; b) T2; c) T3 The SEM images of Z1, Z2 and Z3

nanopowders presented crystalized structures in shape of cylindrical particles with a unidirectional distribution, clustered in filamentous rods, which is a specific structure for mesoporous silica [13]. In case of T1, T2 and T3 can be observed the same desirable structure consisting of fully transformed rope like particles from the spherical initial phase. Although, the analysed samples display an undefined and quantitatively irregular material loading, distributed throughout the mass of material.

From Figures 4 and 5 it can be observed that the influence of ZnO and TiO2 is not significant in terms of microstructure modifications, compared with the simple mesoporous silica, regardless of the synthesis method used. The compact silica bars have no structural defects, and their porous structure can be clearly observed.

3.4. Binding capacity tests

All heavy metals (Cd, Ni, Pb) determinations were performed according to the guidelines of "Water quality. Determination of trace elements by atomic absorption spectrometry with graphite furnace" (15586: 2004 standard) [14].

Heavy metals concentrations were determined following the next procedure: a 25 mL sub-sample was collected in borosilicate containers, over which concentrated nitric acid was added up to a pH value < 2 for preservation; after stabilizing the concentrations, the probes were analysed through atomic absorption spectrometry with flame.

All six types of filtering materials (Z1, Z2, Z3 and T1, T2, T3) were tested in terms of binding capacity through contact method on a magnetic stirrer. For each sample, 0.5 g of silica composite nanopowder was used for 200 mL synthetic water from which 25 mL solution were extracted at 1, 2, 3 and 24 hours. The same container was used for all four terms of time.

Fig. 6 - The concentration of nickel determined in aqueous solutions,

following the heavy metals binding capacity tests of composites Z1, Z2, Z3, T1, T2, T3 / Concentrația de nichel determinată în soluție apoasă, ca urmare a testelor privind capacitatea de legare a metalelor grele a compozitelor Z1, Z2, Z3, T1, T2, T3

Fig. 7 - The concentration of cadmium determined in aqueous solutions, following the heavy metals binding capacity tests of composites Z1, Z2, Z3, T1, T2, T3 / Concentrația de cadmiu determinată în soluție apoasă, ca urmare a testelor privind capacitatea de legare a metalelor grele a compozitelor Z1, Z2, Z3, T1, T2, T3.

8 - The concentration of lead determined in aqueous solutions,

following the heavy metals binding capacity tests of composites Z1, Z2, Z3, T1, T2, T3 / Concentrația de plumb determinată în soluție apoasă, ca urmare a testelor privind capacitatea de legare a metalelor grele a compozitelor Z1, Z2, Z3, T1, T2, T3.

8 F-D. Dumitru, M-A. Moncea, A.G. Baraitaru, A-M. Panait, M. V. Olteanu, G. Deák / Effect of mesoporous silica with TiO2 / ZnO nanocomposites in wastewater treatment

In Figures 6-8 are presented the results of the efficiency tests for the mesoporous silica nanocomposite samples obtained by the 3 methods (Z1, Z2, Z3, T1, T2, T3). For the three Zn composite samples, the following aspects were observed: In case of cadmium (Figure 7), a concentration decrease tendency can be observed for Z2 and Z3 throughout the test range. In the case of Z1, the decreasing tendency is observed only for the first three cleaning tests, while after 24 hours, the concentration of cadmium is slightly increased. Regarding the nickel retention efficiencies (Figure 6), these exhibit lower values (31.7 % for Z1 after 3 h of contact; 33.4 % for Z2 after 24 h of contact; 35.2 % for Z3 after 24 h of contact) compared to the other tested heavy metals, but without a clear trend to decrease or increase depending on the contact time. At the same time, the concentration values are very similar regardless of the testing time or the used synthesis method. In case of lead (Figure 8) impressive retention efficiencies can be observed, with concentrations close to the detection limit (0.015 mg / L), the small differences being most likely due to the number of internal metal bonds formed between the OH groups of the uncalcinated mesoporous silica and the metal oxides used for its functionalization. This capacity of the developed mesoporous silica of binding lead cations is larger compared to nickel due to the electronegativity of the atomic species which is higher in case of lead.

The results of the efficiency tests for the TiO2 composite mesoporous silica samples by the 3 methods (T1, T2, T3) are as follows: The cadmium concentration values (Figure 7) obtained for all 3 tested samples do not vary according to the contact time with the wastewater, but according to the used synthesis method. Thus, T2 shows efficiencies of up to 75.6 %, T3 up to 67.4 %, and T1 values up to 51 %. This fact can also be explained by the clear differences regarding the concentration of doping element, resulted from X-ray fluorescence analysis (7.9 % for T1; 20.5 % for T2, 15 % for T3, in terms of TiO2 concentration). In case of nickel (Figure 6), the situation is similar to that of ZnO functionalized materials. Thus, retention efficiencies of up to 20.1 % for T1, 24.8 % for T2 and 21.8 % for T3 can be observed. In this case too, concerning lead retention (Figure 8), were obtained the best values (95.6 % for T1; 96.2 % for T2; 96.6 % for T3), without significant differences determined by the contact time.

4. Conclusions

The six mesoporous silica-based nanocomposites with hexagonal structure were obtained following three synthesis approaches: one in liquid phase and two in solid phase. The heavy metals retention properties of the obtained materials were assessed in order to determine their

applicability in the field of environmental protection.

The structural characterization of the obtained samples emphasized three intensity peaks (100, 110, 200) which showed a good crystallinity of the synthesized nanoparticles corresponding to the hexagonal structure, specific for this type of silica. Microstructural analysis showed hexagonal porous particles crystallized in the form of cylindrical micelles with a unidirectional distribution, clustered in filamentous rods. From the SEM micrographs achieved for all synthesized samples, it was established that the influence of ZnO and TiO2 is not significant in terms of microstructure modifications, compared with the simple mesoporous silica.

Following the cleaning tests, it was observed that the binding capacity is selective depending on the type of material used for obtaining the nanocomposites but also the adopted synthesis method. Thus, a good efficiency of lead retention from wastewater of over 95 % for all six nanocomposites has been demonstrated. In case of nickel retention efficiencies, lower and similar values for both groups of samples (for Zn – 31-36 % and for Ti – 20-25%) were recorded. However, a considerable influence of the parameters variation can be observed in case of cadmium removal tests.

Acknowledgments

This work was supported by a grant of the Romanian Ministry of Research and Innovation, CCCDI-UEFISCDI, project number 26PCCDI/01.03.2018, “Integrated and sustainable processes for environmental clean-up, wastewater reuse and waste valorisation” (SUSTENVPRO), within PNCDI III.

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