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Article

Polymeric Floating Photocatalysts Based on PE/TiO2 Composites for the Removal of Organic Pollutants in Water

by
Valentina N. Panchenko
,
Mikhail A. Matsko
*,
Dmitry S. Selishchev
and
Denis V. Kozlov
Boreskov Institute of Catalysis SB RAS (BIC SB RAS), 630090 Novosibirsk, Russia
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2023, 7(8), 318; https://doi.org/10.3390/jcs7080318
Submission received: 7 June 2023 / Revised: 17 July 2023 / Accepted: 25 July 2023 / Published: 2 August 2023
(This article belongs to the Section Polymer Composites)
Browse Figures
Versions Notes

Abstract

:
Polymeric floating photocatalysts based on polyethylene (PE)/TiO2 compositions were synthesized by in situ ethylene polymerization in the presence of a titanium–magnesium catalyst synthesized by the sequential deposition of a magnesium–aluminum complex and TiCl4 on commercial TiO2 P25. The optical band gap of the synthesized PE/TiO2 composites was shown to be 3.1–3.3 eV, which allowed for their use as photocatalysts for the utilization of solar light. The photocatalytic activity of the PE/TiO2 composites was studied for the degradation of methyl orange (MO) under irradiation with UV light (λmax = 384 nm). The composites containing 20–50 wt.% of PE were found to have an optimum combination between floatability and photocatalytic activity. The maximum photodegradation rate was observed at an MO concentration below 5 ppm. The polymeric PE/TiO2 floating photocatalysts could be used repeatedly, but the long-term exposure of the composites to UV radiation was accompanied by oxidation of the polymer.

Graphical Abstract

1. Introduction

Dyes are widely used in the textile, paper, and food industries. A large amount of waste makes it necessary to develop efficient methods for their utilization. Photocatalytic degradation using solar light is a promising and environmentally friendly method for removing dyes from industrial wastewater. The development of photocatalysts for the efficient degradation of dyes under solar light is an important task.
Polymeric floating photocatalysts (FPCs), which consist of a floating matrix and TiO2, ZnO, or other semiconductors, have been reported recently for the remediation of wastewater [1,2]. Various materials, namely, tree bark, pearlite, vermiculite, and polymers, such as polyethylene (PE) and polythiophene (PTP), have been suggested for use as the floating matrix. These composites can improve semiconducting photocatalysts and enhance their efficiency. The anchoring of a photocatalyst on a floating substrate decreases its agglomeration, thus increasing the working surface of a catalyst. Floating photocatalysts are located at the water–air interface; therefore, maximum oxygenation can be reached due to the high power of solar radiation. In addition, FPCs do not require mixing, are reusable, and can easily be removed from the water surface after the remediation cycle. These features make it possible to employ FPCs in industrial sediment basins and reservoirs for contaminated wastewater, as well as for the removal of organic pollutants (oil and petrochemical spills) in the regions of natural water bodies enclosed with harbor booms. The most attractive substrates are polymers, particularly PE and PP. They are the most widespread materials; their easy recycling makes them relatively pure materials in ecological terms.
FPCs can be obtained by mixing the melt or solution of a polymer with a photocatalyst, for example, PE [3,4,5] and PP [6], or by the in situ polymerization of the corresponding monomer, for example, pyrrole [7,8] or thiophene [9]. Composite photocatalysts have been found to be highly active in the photocatalytic degradation of dyes and exhibit antibacterial activity toward Escherichia coli [3,6].
The first two methods are quite simple and do not require special equipment. However, some features hinder their wide application, including the agglomeration of photocatalyst nanoparticles; the high temperatures required for obtaining the polymer melt, which often leads to polymer destruction; the limited solubility of polymers in many solvents; and the necessity to regenerate solvents. However, there are some publications on PP-nano-ZnO composites obtained by extrusion [6,10], mixing a cyclohexane solution of low-density PE with nano-TiO2 [11], and mixing linear low-density PE (LLDPE) in a xylene solution with TiO2 P25 [4].
The third method used to obtain composites—polymerization filling—is preferable. Three main approaches to the synthesis of such composites are known. The first approach involves the synthesis of a polymer in the presence of a photocatalyst. This technique was used, for example, to obtain polypyrrole/TiO2 [8]; PE/TiO2 and ethylene-octadecene/TiO2 composites were synthesized in the presence of ethenyl(bisindenyl)zirconium dichloride (rac-EtInd2ZrCl2) as a catalyst and methylaluminoxane (MAO) [3]. The second approach is multistep: an active component (catalyst) is formed on the photocatalyst surface for further participation in polymer synthesis. This technique was employed to obtain TiO2/polyacrylamide composites [12]. The third approach involves the simultaneous synthesis of a photocatalyst and a polymer. It was used to immobilize TiO2 nanoparticles on mulberry wood flour [13].
Previously, we showed that PE/nano-Al2O3 and PE/Y2O3(Eu) composites can be obtained by using in situ ethylene polymerization [14,15]. To this end, a titanium–magnesium catalyst for ethylene polymerization was formed by the sequential deposition of a magnesium–aluminum complex (MAC) and TiCl4 on a nano-Y2O3(Eu) surface. Such catalysts are highly active toward ethylene polymerization and allow the polymer molecular weight to vary across a wide range. In this study, a similar approach was applied to synthesize polymeric floating photocatalysts using a commercially available TiO2 Aeroxide® P25.
The synthesized composites were studied in the photocatalytic degradation of methyl orange (MO). The effects of the MO concentration, the concentration of the composite, and the content of polymeric filler on the rate of MO degradation were investigated.

2. Materials and Methods

2.1. Reagents

A 0.84 M heptane solution of triethylaluminum (Al(C3H5)3, TEA), TiCl4, and a 0.5 heptane solution of dibutylmagnesium (Mg(C4H9)2, MgBu2) were purchased from Acros Organics (Geel, Belgium) and used for the catalytic polymerization of ethylene, as received without additional purification. Before use, the reagents were stored in an inert atmosphere. A commercially available TiO2 Aeroxide P25 (TiO2) from Evonik Ind. (Essen, Germany) was used as the photocatalytically active component. TiO2 P25 had a rutile (20%) and anatase (80%) phase composition, a specific surface area of 56.3 m2/g, a crystal size of 30 nm, a pore volume of 0.42 cm3/g, and a bulk density of 0.150 g/cm3.

2.2. Synthesis of PE/TiO2(P25) by In Situ Ethylene Polymerization

The synthesis of PE/TiO2 composites was performed by in situ ethylene polymerization according to a previously published technique [14,15]. In the first step, the catalytic system TMC(P25) was prepared on the surface of a TiO2 photocatalyst by sequential deposition of an organomagnesium compound, MgBu2, and TiCl4, onto TiO2(P25). Before use, TiO2 was dehydroxylated by calcination in air at 450 °C for 3 h, followed by treatment at the same temperature for another 3 h under vacuum. The final suspension of the polymerization catalyst was ultrasonicated for 30 min and then transferred to an autoclave for polymerization. The suspension polymerization of ethylene was carried out in a 0.7 L stainless steel autoclave at an ethylene pressure of 4 atm in the presence of TEA as a co-catalyst (1.2 mmol/L) in heptane (250 mL). For some composites, hydrogen was introduced during polymerization at a pressure of 1 atm. The PE content in the PE/TiO2 composites was controlled by varying the polymerization time.
The composites synthesized without hydrogen are referred to in the text as PE/TiO2, whereas those formed in the presence of hydrogen are denoted as H-PE/TiO2. All composites were stored in the dark to prevent oxidation of the polymer.

2.3. Characterization of Samples

2.3.1. TG Analysis of Composites

Thermal analysis of the samples was performed using an STA 449 C Jupiter simultaneous thermal analyzer (NETZSCH Group, Selb, Germany). The samples were placed in corundum crucibles for examination. Air was fed into a chamber with the sample at a rate of 30 mL/min. The sample was heated from room temperature to 30 °C at a rate of 2 °C/min. This was followed by temperature-programmed heating at a rate of 10 °C/min to 1000 °C. The experimental data were analyzed using the NETZSCH Proteus Thermal Analysis software package.

2.3.2. Scanning Electron Spectroscopy (SEM)

The SEM images of the PE/TiO2 samples were obtained using a JSM 6460 LV scanning electron microscope (JEOL, Tokyo, Japan).

2.3.3. A DRIFTS Study

DRIFT spectra were taken on an FTIR-8400S instrument (Shimadzu, Kyoto, Japan) using a DRS-8400S attachment in the region of 400–6000 cm−1 with a 4 cm−1 step and accumulation over 100 scans. The spectra were expressed in absorption mode using the Kubelka–Munk function (Equation (1)):
Jcs 07 00318 i001
where R is the reflectance

2.3.4. UV-Vis Spectroscopy

Powdered samples of TiO2 and PE/TiO2 were placed in a quartz cuvette (l = 2 mm) to measure the UV-vis diffuse reflectance spectra. The measurements were performed on a UV-2501 PC Shimadzu instrument using an IRS-250A diffuse reflectance attachment in the region of 190–900 nm with a step size of 2 nm. According to ref. [16], the optical bandgap (Eg) was estimated by approximating the Tauc plots using Equation (2):
F(R∞) h [ν]1/2 = B (hν − Eg),

2.4. Photodegradation of MO

Methyl orange was dissolved in distilled water to prepare the MO solution with a concentration of 1–20 ppm. Photodegradation experiments were performed in a batch reactor containing 60 mL of the MO solution and 0.1 g of the PE/TiO2 or n-PE/TiO2 composite.
Before irradiation, the reaction mixture was kept in the dark for 1 h to establish the adsorption–desorption equilibrium of MO on the surface of the floating composite. After that, a UV lamp (λUV ≈ 384 nm, 15 W) was switched on, and the reactor was exposed to UV light for 2–5 h. The UV lamp was placed on top at a distance of 10 cm from the surface of the solution. The experiments were carried out without stirring the MO solution (as a model of a floating photocatalyst in the environment).
After the first run, the composite was washed with deionized water and dried for further reuse.
To investigate the kinetics of MO adsorption and photodegradation, a probe for analysis using UV-vis spectroscopy was sampled from the reaction mixture every 15–30 min. UV-vis spectra were measured on a UV-2501 PC Shimadzu instrument using a quartz cuvette (l = 10 mm).
Kinetic plots of MO adsorption and degradation were calculated using Equation (3) based on the optical density at 464 nm, which was attributed to the characteristic absorption of MO:
η = (1 − D/D0) × 100%
where D0 is the initial optical density of the MO solution and D is the optical density of the solution measured during the reaction.

3. Results

3.1. Synthesis of the PE/TiO2 Material

The PE/TiO2 composites were synthesized using TiO2 P25, which consisted of 80 wt.% anatase and 20 wt.% rutile. In the first step, the catalytic system TMC(P25) was prepared on the surface of a TiO2 photocatalyst by the sequential deposition of an organomagnesium compound, MgBu2, and TiCl4, onto TiO2(P25), which was preliminarily dehydroxylated at 450 °C. In the second step, ethylene polymerization was performed.
Our earlier study, in which IR spectroscopy was used [16], revealed that the sequential deposition of MAC and TiCl4 on aluminum nano-oxide Nafen results in the formation of a titanium–magnesium active component with a composition close to that of known titanium–magnesium catalysts (TMCs) containing TiCl3 and MgCl2. The main reaction is the interaction of MgBu2 with the OH groups of Al2O3 (reaction (4)), leading to the anchoring of the organomagnesium compound on the surface of the aluminum nano-oxide Nafen, and the subsequent interaction of TiCl4 with surface organomagnesium groups (reaction (5)). This interaction was accompanied by the reduction in TiCl4 and the formation of a catalyst containing magnesium oxychloride and titanium trichloride, which were anchored on the aluminum oxide surface.
Jcs 07 00318 i002
Jcs 07 00318 i003
Presumably, similar reactions occurred on the TiO2 surface during the sequential deposition of MAC and TiCl4.
Under polymerization conditions, the interaction of compound (A) with the TEA co-catalyst results in the formation of active sites containing alkylated Ti3+ compounds, which are involved in ethylene polymerization and lead to the formation of PE on the TiO2 surface.
Chemical analysis data showed that TMC(P25) contains 1.13 wt.% of Mg. According to ref. [16], MAC interacts predominantly with isolated μ1-OH groups. Under the assumption of the specific interaction of MAC with the μ1-OH groups of titanium oxide, their amount in TiO2 was 765 µmol/g.
It is known [17] that TiO2 nanoparticles tend to form agglomerates and associates due to their high surface energies. To ensure a uniform distribution of TiO2 in the polymer matrix, enhance the dispersion stability of TiO2 nanoparticles, and create interfaces between the organic and inorganic phases, the surface of the filler is commonly modified with alkoxy silanes, fluoro-silane, and glycidyl methacrylate. Presumably, the anchoring of TMC on the surface of the TiO2 photocatalyst also decreases the agglomeration of nanoparticles. However, this effect was observed during the polymer synthesis (Scheme 1). Indeed, our TEM study [15] demonstrated that Y2O3(Eu) filler particles were uniformly distributed in PE/Y2O3(Eu) composites.

3.2. Characterization of FPCs

Figure 1 shows the SEM images of TiO2 and two H-PE/TiO2 composites obtained by TMC(P25), with 28.5 and 85.4 wt.% PE content, respectively. The initial TiO2 consisted of fine spherical particles with a diameter smaller than 50 nm (according to the XRD data, the average size of the TiO2 crystallites was 30 nm). The H-PE/TiO2 composites also consisted of spherical particles, but they were larger in size. The diameter of the particles increased with increasing PE content (Figure 1b,c). The SEM data suggest that PE was formed around the TiO2 crystallites (Scheme 1).
The PE content in the synthesized PE/TiO2 composites was estimated from the polymer yield, measured as the difference between the masses before and after polymerization (Table 1). The thermogravimetric differential thermal analysis (TG-DTA) was used for the analysis of the PE content. Figure 2 shows the TG curves for the PE/TiO2 and H-PE/TiO2 composites, in which the estimated content of the polymer was 38.2 and 40.5 wt.%, respectively. Three regions could be distinguished in the TG curves from the studied samples. The first region, at 30–200 °C, corresponded to the removal of physically adsorbed water. The second region, at 200–500 °C, was related to the degradation of the polymer in the composite [18]. The third region, at 500–1000 °C, was the phase transformation of anatase to rutile. The n-PE/TiO2 composite, which was obtained by polymerization in the absence of hydrogen and hence contains higher molecular weight PE, decomposed at a higher temperature compared to the H-PE/TiO2 sample (Figure 2).
According to the TG-DTA data, the total weight losses in the PE/TiO2 and H-PE/TiO2 samples were 34.8 and 39.3 wt.%, which corresponded to the PE content of 29.6 and 34.4 wt.%, respectively. These data were close to the polymer content measured by the gravimetric method (Table 1). A slight difference between the data of gravimetry and TG-TGA analysis was because the result of the gravimetric method included the weight of physically adsorbed water. Further in the text, the polymer content measured using the gravimetric method is indicated.
The main characteristic of a photocatalyst is the band gap (Eg). Figure 3 shows the electronic spectra of TiO2 and the two PE/TiO2 composites with PE content of 26.1 and 60.0 wt.%. Figure 4 shows the electronic spectra of TiO2 and two H-PE/TiO2 composites with PE content of 55.8 and 85.4 wt.%. Equation (2) was used to fit the Tauc plots and calculate the bandgap (Eg). The initial TiO2 photocatalyst had a gap Eg of 3.0 eV. The gap Eg calculated for the TiO2 sample coincided with the literature data for rutile [19]. Coating the TiO2 photocatalyst with the polymer led to an increase in Eg. With an increasing PE content, Eg gradually increased. For the 26.1PE/TiO2 and 60PE/TiO2 samples, Eg was equal to 3.2 and 3.3 eV, whereas, for the H-55.8PE/TiO2 and H-85.4PE/TiO2 samples, it was 3.1 and 3.2 eV, respectively. According to ref. [18], LDPE has an optical energy gap between the valence band and the conduction band (Egopt) of 3.1–3.6 eV. Thus, the increase in Eg in the PE/TiO2 and H-PE/TiO2 composites can be attributed to an increase in the PE layer thickness in the samples under consideration.
Mixing TiO2 with water leads to the formation of a suspension, which rapidly segregates (Figure 5a,b). Upon the addition of H-17.5PE/TiO2 powder to water, only 1–3 wt.% of the sample settles down. The PE/TiO2 and H-PE/TiO2 samples that contain more than 20 wt.% of PE sit in the water well and do not sink for a long time (Figure 5c,d). These results show that the PE content in the PE/TiO2 composites determines their floatability.

3.3. PECs Performance in Photocatalytic Degradation of Methyl Orange

To study the properties of photocatalysts for the decomposition of organic substances, simple model compounds were employed [20,21,22,23,24,25,26,27,28,29,30,31]. We used methyl orange (MO), which is one of the most studied compounds. This made it possible to compare the performance of the new photocatalysts with those already known.

3.3.1. Correlation between Adsorption and Degradation of MO

It is known [1,2] that the amount of adsorbed dye is an important parameter affecting its photodegradation rate. Figure 6 illustrates the kinetics of MO adsorption onto the 26.1PE/TiO2 and H-28.5PE/TiO2 composites. One can see that the adsorption–desorption equilibrium in the system was established in 6 h. A possible reason for this is that water is a non-wetting liquid for PE, and the study was performed without stirring the mixture.
It should be noted that the amount of dye adsorbed by the 26.1PE/TiO2 composite, which was obtained without hydrogen, was two times smaller than that adsorbed by H-28.5PE/TiO2.
Figure 7 illustrates the change in the concentration of MO under UV irradiation in the absence and presence of the PE/TiO2 composite, as well as the adsorption of MO in the presence of the composite. It can be seen that MO decomposition practically did not occur in the absence of the catalyst (Figure 7a, curve 1). In the presence of PE/TiO2, the adsorption of MO was observed (Figure 7a, curve 2). When the UV irradiation was turned on in the presence of the PE/TiO2 composite, the decomposition of MO increased significantly (Figure 7a, curve 3 and Figure 7b).

3.3.2. Effect of the Polymer Content in the PE/TiO2 Composite on Catalyst Performance

Figure 8 shows the kinetics of MO photodegradation over the H-PE/TiO2 composites containing 17.5, 42.8, 55.8, and 85.4 wt.% of PE. Table 2 lists the amounts of MO adsorbed in 1 h and decomposed after 4 h of exposure to UV radiation. It can be seen that the amounts of adsorbed MO and decomposed MO (CMO) decreased with increasing polymer content in the composite (Table 2), which may be related to an increase in the accessibility of TiO2 with a decrease in the thinkness of the polymer layer.
The linear dependence of ln(C/C0)-t shows that the degradation of MO follows a pseudo-first-order (Figure 8b).
Note that in the presence of the H-17.5PE/TiO2 composite, cloudiness of the MO solution is observed after 2.5 h of exposure to UV radiation, and after 5 h, the composite settles down. This may be due to the partial degradation of the polymer under UV radiation. Indeed, it is known [20,30,31,32] that the irradiation of PE with UV or solar light leads to its oxidation. This issue is discussed in detail below.
In the presence of the H-42.8PE/TiO2 and H-55.85PE/TiO2 composites, the CMO is not high and constitutes 33.0 and 22.4 wt.%, respectively (Table 2, exps. 2 and 3). Nevertheless, these values are comparable with the data reported in ref. [20], which was devoted to the photodegradation of methylene blue, indigo carmine, and drimaren red under the action of UV (Hg, 254 nm) or solar irradiation in the presence of a floating photocatalyst based on TiO2 P25 grafted on expanded polystyrene (EPS) with a TiO2 content of 1, 7, and 18 wt.%. The maximum activity (40–55% degradation at an FPC loading of 1 g) was observed for 18%TiO2/EPS.
It is known [22,23,24] that the photodegradation of MO is a complicated multistep process. The final products of photodegradation are carbon dioxide and aliphatic acids. This means that, at the end of the MO photodegradation, the acidity of the medium should change. Indeed, the pH of the MO solution decreases at the end of the reaction from pH0 = 6.7–6.9 to pHe = 5.4–5.8 (Table 2).

3.3.3. Effect of the MO Concentration

Figure S2 and Table 3 (exps. 1, 2, 5, and 6) list the amounts of MO adsorbed in 1 h in the dark. One can see that a decrease in the MO concentration was accompanied by an increase in the amount of adsorbed MO.
The results obtained were consistent with the data reported in ref. [25], which demonstrated that the adsorption equilibrium for the PTP/TiO2 composites depends on the MO concentration. At MO concentrations of 1 and 10 ppm, the adsorption ratio of the MO solution was equal to 28 and 8%, respectively.
The amount of MO that decomposed during 4 h of exposure to UV radiation in the presence of the 26.1PE/TiO2 composite also depended on the concentration of the MO solution. One can see that a decrease in the MO concentration led to an increase in CMO.
Our results agree with the data reported for TiO2 [26,27,28] and PTP/TiO2 composites [30]. In these studies, the maximum rate of MO photodegradation for all the photocatalysts was observed at concentrations below 5 ppm (1.6 mM). This effect was attributed to the blocking of those active sites on the photocatalyst surface, which were involved in the formation of radicals. This effect could also be the main cause of the deterioration of the degradation activity in the presence of the 26.1PE/TiO2 composite.

3.3.4. Effect of the PE/TiO2 Composite Concentration

Table 4 lists the dependence of CMO on the loading of the 38.2PE/TiO2 and H-40.5PE/TiO2 composites. One can see that the composite loading changes symbiotically with the amount of decomposed MO. An increase in the loading of 38.2PE/TiO2 and H-40.5PE/TiO2 from 0.1 g to 0.5 g leads to an increase in CMO from 12.5% to 77.7 wt.% and from 9.0% to 68.8 wt.%, respectively. CMO normalized to the FPC weight also changes for 38.2PE/TiO2 and H-40.5PE/TiO2 from 12.5% to 15.5% and from 9.0% to 13.8%, respectively.
It is known [21,33,34] that the amount of photocatalyst is an essential parameter determining the photodegradation rate of dyes because it determines the number of active surface sites involved in the formation of OH radicals. At the same time, ref. [21] studied the photodegradation of dyes in the presence of a floating photocatalyst based on TiO2 P25 grafted on expanded polystyrene (EPS), and it was noted that the degradation rate nonlinearly depends on the photocatalyst concentration. At certain concentrations (depending on the catalyst type), the rate plateaued. This result could be related to the effect of oxygenation; dye diffusion may also control the reaction rate. For TiO2, which is not a floating photocatalyst, its high concentration can lead to a decrease in the photodegradation rate, which is caused by the blocking of UV radiation due to the formation of a cloudy suspension [33,34].
Figure 9 illustrates the changes in the UV-vis spectra of MO solutions during the adsorption and photodegradation with loaded 0.5 g of the 38.2PE/TiO2 and H-40.5PE/TiO2 composites. It is seen that the changes in the UV-vis spectra depend on the polymer structure.
During the MO degradation in the presence of the 38.2PE/TiO2 composite, the intensities of the bands at 270 and 464 nm decrease, which testifies to the adsorption of MO, and the intensity of the absorption band in the region of 210–300 nm increases, which indicates the appearance of MO degradation products.
When the H-40.5PE/TiO2 composite is used as a photocatalyst, photodegradation is also accompanied by a decrease in the intensity of the absorption bands at 270 and 464 nm. At the same time, new bands appear in the regions of 210–300, 300–400, and 550–700 nm, the intensity of which increases with time. The appearance of these bands indicates that the MO degradation products pass into the solution. This may be related to the structural characteristics of the polymers in the 38.2PE/TiO2 and H-40.5PE/TiO2 composites. In the case of the polymer obtained without hydrogen, the molecular weight of the polymeric part is higher and diffusion limitations may occur.

3.3.5. Re-Use of PE/TiO2 Composites

The possibility to reuse FPCs is also one of the important characteristics of photocatalysis. Table 3 (exps. 2–4) lists the changes in the MO amount after three cycles in the presence of the n-26.1PE/TiO2 composite. One can see that CMO in the first two cycles is close and equal to 12.5 and 12.8 wt.% (Table 3, exps. 2,3). After the third cycle, CMO sharply increases to 38.9 wt.% (Table 3, exp. 4).
It is known [7,30,31,32] that exposure to UV or solar light leads to the oxidation of PE, which may result in its degradation. This suggests that UV radiation is accompanied by both the degradation of the basic MO dye and the decomposition of the polymeric substrate. To verify this hypothesis, we measured the surface area of the 26.1PE/TiO2 composite after each cycle (Table 3). It is seen that after each cycle, the surface area increases (32.6, 36.3, and 45.2 m2/g) and approaches that of the initial TiO2 (56.3 m2/g). An increase in the SBET may testify to the appearance of pores in the composite.
Figure 10 shows the DRIFT spectra in the vibrational region of the carbonyl groups for the initial composite 26.1PE/TiO2 and after the third cycle. A broad absorption band with a maximum of 1640 cm−1 (δ of the OH groups of adsorbed water) is observed in the spectrum of the composite in the region of 1500–1800 cm−1 (Figure 9, spectrum 1). After the third cycle, the band in this region broadens (Figure 9, spectrum 2). The deconvolution of the spectrum made it possible to distinguish the bands at 1590, 1640, and 1695 cm−1. The appearance of bands at 1590 and 1695 cm−1 indicates the formation of ketone, carbonyl, and carboxyl groups [35]. Thus, the data obtained demonstrate that UV irradiation is accompanied not only by the degradation of MO but also by the oxidation of the polymer matrix.

4. Conclusions

For the first time, floating PE/TiO2 photocatalysts with a polymer content of 17.5–85.4 wt.% were synthesized by the in situ polymerization method using TMC(P25) as a polymerization catalyst. TMC(P25) was synthesized by sequential deposition of the organomagnesium compound, MgBu2, and TiCl4 on the surface of a TiO2 photocatalyst and was used for ethylene polymerization in suspension.
The SEM study revealed that the particles of the initial TiO2 photocatalyst and those of the composites had a similar spherical shape but different sizes. The diameters of the particles in the composites depended on the content of the polymer. For example, at PE contents of 28.5 and 85.4%, the diameters of the particles were 80–100 and 150-–200 nm, respectively. The polymer replication effect suggests that the filler (TiO2 photocatalyst) was uniformly distributed in the polymer matrix.
One of the main characteristics of photocatalysts is the band gap (Eg). The gap Eg of the PE/TiO2 composites was equal to 3.1–3.3 eV, which allowed their application as photocatalysts for the utilization of solar light. Under UV radiation (λmax = 384 nm), the synthesized composites decomposed the MO. As the MO concentration increased, the efficiency of photocatalytic degradation decreased, which was primarily related to the blocking of the sites for the formation of OH radicals. The maximum photodegradation rate was observed at MO concentrations below 5 ppm.
The content of the polymer matrix in the composite affects its floatability and MO photodegradation rate. The best characteristics were obtained for the composites containing 20–50 wt.% of PE. This was primarily associated with the accessibility of MO to the surface of the TiO2 photocatalyst because the polymer was formed around the TiO2 particle; thus, an increase in the PE content in the composite led to an increase in the thickness of the polymer matrix (Scheme 1).
The efficiency of photodegradation also depended on the amount of the applied PE/TiO2 composite. When the loading of the 38.2PE/TiO2 and H-40.5PE/TiO2 composites increased from 0.1 to 0.5 g, CMO increased from 12.5 to 77.7 wt.% and from 9 to 68.8 wt.%, respectively.
The composites could be reused at least three times. However, the process of decomposition of the polymer matrix is clearly observed (TiO2 gradually decomposed the polymer under UV radiation). These are the main advantages of floating photocatalysts. After the removal of organic pollutants from the environment, the PE layer is likely to be decomposed completely by UV radiation and the only byproduct is an inert TiO2.

5. Concluding Remarks and Further Work

The first results of the use of PE/TiO2 composites obtained by in situ polymerization for the decomposition of organic waste (for example, MO) allowed us to identify a number of important correlations that are necessary to continue the work. First, it is necessary to test this method for the decomposition of hydrocarbons (oil wastes are the most probable). Second, it is necessary to choose the optimal content of the polymer part. On the one hand, the polymer part is necessary for the floating ability of the photocatalyst; on the other hand, the loss of TiO2 activity due to its complete encapsulation by the polymer layer should be avoided. It is also necessary to optimize the morphology (porosity) and structure of the polymer matrix (for example, by obtaining copolymers of ethylene with hexene-1 or propylene) to accelerate the self-decomposition of the polymer part of the composites after the removal of organic waste from the water surface. Third, it seems promising to continue the search for a more active form of TiO2 to improve catalyst performance.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jcs7080318/s1, Figure S1: (A) Changes in the concentration of MO (CMO = 5 ppm, pH = 6.7): (1) UV irradiation without composite, (2) adsorption of MO in the presence of 26.1PE/TiO2, and (3) UV irradiation in the presence of 26.1PE/TiO2 ((A) UV radiation and (B) degradation of MO: the difference between curve (3) and curve (2)); Figure S2: Effect of concentration of MO on the discoloration of MO dye in the presence of 26.1PE/TiO2: Concentration of MO (1) 1.7 ppm, (2) 5 ppm, and (3) 15 ppm; Figure S3: UV-vis spectra of MO during adsorption and the discoloration in the presence of 26.1PE/TiO2: (A) 1 cycle, (B) 3 cycles.

Author Contributions

Conceptualization, V.N.P., D.V.K. and M.A.M.; methodology, V.N.P.; validation, V.N.P., D.S.S. and M.A.M.; formal analysis, M.A.M. and V.N.P.; investigation V.N.P. writing—original draft preparation, V.N.P. and D.S.S.; writing—review and editing, D.V.K. and M.A.M.; supervision, D.V.K. and M.A.M.; project administration, D.V.K. and M.A.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Higher Education of the Russian Federation (project AAAA21-121011490008-3).

Data Availability Statement

Data are contained within the paper and Supplementary Materials.

Acknowledgments

The authors thank V.A. Zakharov for some important notes.

Conflicts of Interest

The authors declare no conflict of interest.

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Jcs 07 00318 sch001
Scheme 1. Scheme of the synthesis of a floating PE/TiO2 photocatalyst.
Scheme 1. Scheme of the synthesis of a floating PE/TiO2 photocatalyst.
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Figure 1. SEM images of (a) TiO2, (b) H-28.5PE/TiO2, (c) and H-85.4PE/TiO2.
Figure 1. SEM images of (a) TiO2, (b) H-28.5PE/TiO2, (c) and H-85.4PE/TiO2.
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Figure 2. DSC and TG curves for (a) 38.2PE/TiO2 and (b) H-40.5PE/TiO2 composites.
Figure 2. DSC and TG curves for (a) 38.2PE/TiO2 and (b) H-40.5PE/TiO2 composites.
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Figure 3. Kubelka–Munk (a) and Tauc (b) plots for (1) TiO2, (2) 26.1PE/TiO2, and (3) 60.0PE/TiO2 samples.
Figure 3. Kubelka–Munk (a) and Tauc (b) plots for (1) TiO2, (2) 26.1PE/TiO2, and (3) 60.0PE/TiO2 samples.
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Figure 4. Kubelka–Munk (a) and Tauc (b) plots for (1) TiO2, (2) H-55.8PE/TiO2, and (3) H-85.4PE/TiO2 samples.
Figure 4. Kubelka–Munk (a) and Tauc (b) plots for (1) TiO2, (2) H-55.8PE/TiO2, and (3) H-85.4PE/TiO2 samples.
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Figure 5. Photographs of TiO2 (a) suspended in water and (b) stored for 24 h; (c) polymeric H-28.5PE/TiO2 composite added to water and (d) stored for 30 days.
Figure 5. Photographs of TiO2 (a) suspended in water and (b) stored for 24 h; (c) polymeric H-28.5PE/TiO2 composite added to water and (d) stored for 30 days.
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Figure 6. Kinetic plots of MO adsorption (CMO = 5 ppm, pH = 6.7) on the surface of (1) 26.1PE/TiO2; (2) H-28.5PE/TiO2.
Figure 6. Kinetic plots of MO adsorption (CMO = 5 ppm, pH = 6.7) on the surface of (1) 26.1PE/TiO2; (2) H-28.5PE/TiO2.
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Figure 7. (a) Changes in the concentration of MO (CMO = 5 ppm, pH = 6.7). (1) UV irradiation without the addition of PE/TiO2; (2) adsorption of MO in the presence of 26.1 PE/TiO2; (3) UV irradiation in the presence of 26.1 PE/TiO2; (a) UV radiation; (b) degradation of MO: curve (2) subtracted from curve (3).
Figure 7. (a) Changes in the concentration of MO (CMO = 5 ppm, pH = 6.7). (1) UV irradiation without the addition of PE/TiO2; (2) adsorption of MO in the presence of 26.1 PE/TiO2; (3) UV irradiation in the presence of 26.1 PE/TiO2; (a) UV radiation; (b) degradation of MO: curve (2) subtracted from curve (3).
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Figure 8. Effect of PE content in the H-PE/TiO2 composites on the rate of MO photodegradation: (a) linear scale and (b) logarithmic scale. (1) 17.5 wt.%, (2) 42.8 wt.%, (3) 55.8 wt.%, and (4) 85.4 wt.%.
Figure 8. Effect of PE content in the H-PE/TiO2 composites on the rate of MO photodegradation: (a) linear scale and (b) logarithmic scale. (1) 17.5 wt.%, (2) 42.8 wt.%, (3) 55.8 wt.%, and (4) 85.4 wt.%.
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Figure 9. Changes in UV-vis spectra of a reaction solution during adsorption and degradation of MO over (a) H-40.5PE/TiO2 and (b) 38.2PE/TiO2.
Figure 9. Changes in UV-vis spectra of a reaction solution during adsorption and degradation of MO over (a) H-40.5PE/TiO2 and (b) 38.2PE/TiO2.
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Figure 10. DRIFT spectra of 26.1PE/TiO2 composite before (1) and after 3 cycle (2).
Figure 10. DRIFT spectra of 26.1PE/TiO2 composite before (1) and after 3 cycle (2).
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Table 1. Weight change of the H-PE/TiO2 and PE/TiO2 composites according to TG-DTA analysis.
Table 1. Weight change of the H-PE/TiO2 and PE/TiO2 composites according to TG-DTA analysis.
SampleChanging Weight at Definite Temperatures,
% wt.		Total Changing Weight
% wt.		Content of PE,
% wt.
100–160 °C200–290 °C290–400 °C400–500 °CTG-TGATG-TGAFor yield of PE
PE/TiO25.22.215.711.734.829.638.2
H-PE/TiO24.92.718.313.439.334.442.8
Table 2. Effect of PE content in the H-PE/TiO2 composites on the discoloration of MO.
Table 2. Effect of PE content in the H-PE/TiO2 composites on the discoloration of MO.
SamplePE, wt%MO
ppm		pH0pHeChanging of MO
%
for 1 h
Adsorption		for 4 h
UV-Radiation
117.556.75.412.155.5
242.856.95.49.233.0
355.856.95.85.522.4
485.456.76.61.30.5
Table 3. Effect of the concentration of MO on pH, and degradation of MO in the presence 26.1PE/TiO2 composite.
Table 3. Effect of the concentration of MO on pH, and degradation of MO in the presence 26.1PE/TiO2 composite.
SampleConcentration of MO,
ppm		SBET, m2/gpH0pHeChanging of MO
%
for 1 h
Adsorption		for 4 h
UV-Radiation
11.7-6.76.48.416.7
25.032.66.76.44.512.5
3 15.036.36.76.44.312.8
4 25.045.26.76.64.338.9
515.0-6.96.94.05.5
620.0-6.96.94.02.0
1 2 cycles, 2 3 cycles.
Table 4. Effect of the load of 38.2PE/TiO2 and H-40.5PE/TiO2 composites on pH, and the degradation of MO.
Table 4. Effect of the load of 38.2PE/TiO2 and H-40.5PE/TiO2 composites on pH, and the degradation of MO.
SampleC(MO),
ppm		Load of FPC,
g		pH0pHeChanging of MO for 4 h UV Radiation,
%wt.
Totalfor Load of Catalyst
38.2PE/TiO2
150.16.76.412.512.5
250.26.76.424.012.0
350.56.74.077.715.5
H-40.5PE/TiO2
450.17.26.19.09.0
550.56.75.768.813.8
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Panchenko, V.N.; Matsko, M.A.; Selishchev, D.S.; Kozlov, D.V. Polymeric Floating Photocatalysts Based on PE/TiO2 Composites for the Removal of Organic Pollutants in Water. J. Compos. Sci. 2023, 7, 318. https://doi.org/10.3390/jcs7080318

AMA Style

Panchenko VN, Matsko MA, Selishchev DS, Kozlov DV. Polymeric Floating Photocatalysts Based on PE/TiO2 Composites for the Removal of Organic Pollutants in Water. Journal of Composites Science. 2023; 7(8):318. https://doi.org/10.3390/jcs7080318

Chicago/Turabian Style

Panchenko, Valentina N., Mikhail A. Matsko, Dmitry S. Selishchev, and Denis V. Kozlov. 2023. "Polymeric Floating Photocatalysts Based on PE/TiO2 Composites for the Removal of Organic Pollutants in Water" Journal of Composites Science 7, no. 8: 318. https://doi.org/10.3390/jcs7080318

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