Sorption and Photocatalysis of Dyes on an Oil-Based Composite Enriched with Nanometric ZnO and TiO₂

Abstract

Sustainable development and environmental protection are among the most important challenges facing humanity today. One important issue is the problem of groundwater and surface water pollution which can lead to the degradation of aquatic ecosystems and negatively affect human health. As a result, new methods and materials are being sought that can help remove contaminants from water in an efficient and environmentally friendly manner. In recent years, there has been increasing interest in composite materials made from used cooking oil. This paper presents attempts to obtain composite materials with the addition of nano-sized zinc oxide and titanium oxide. The characterization of the composite materials was performed using FTIR, XRD, and SEM-EDS; their sorption and photocatalytic abilities were studied using batch mode experiments. The materials obtained exhibited sorption and photocatalytic properties. The highest value of photodegradation efficiency of more than 70% was recorded for the oil composite containing 20% zinc oxide. Composites containing 10% zinc oxide and titanium oxide had comparable sorption efficiencies of about 45% but different photodegradation efficiencies of 0.52% and 15.42%, respectively.

1. Introduction

Rapid economic development is driven by the need to raise living standards. However, it comes with heavy environmental interference. There are more than 350,000 industrial chemicals in circulation that are being modified [1]. These substances can be found in air, water, and soil and the difficulty in their removal is due to the fact that they are subject to biological, chemical, physical, and photochemical processes. A high load of organic substances is found in municipal wastewater, wastewater from the paper industry, pharmaceutical industry, tanning industry, food industry and agriculture, among others [2,3,4]. Of particular importance is wastewater from the pharmaceutical industry as it may contain a mixture of active substances that have strong effects on the health and life of living organisms, and, in addition, mixtures of these substances may have different effects from individual forms. In addition, metabolites of pharmaceuticals that have made their way into the wastewater through excretion in urine or feces are present in the wastewater. The toxicity of such mixtures is high and the effects are difficult to remove [3,4]. Another group of substances found in wastewater that are difficult to remove are dyes. Currently, more than 100,000 types of commercial dyes can be distinguished; global annual production exceeds 7 × 10⁵ tons per season [5]. The high wastewater load of these compounds is due to their widespread use in the textile, food, cosmetics, paper, etc., industries [6,7,8].

Organic substances are removed from wastewater by, among other means, flotation, coagulation, membrane separation, ion exchange, adsorption, and advanced oxidation and biodegradation processes. The effectiveness of these methods varies; moreover, most of them generate high costs [9,10,11]. Adsorption is one of the most important and effective methods used in the treatment of wastewater from both organic and inorganic substances. This is mainly due to its simple design and low investment costs which are significant advantages over other treatment methods. In addition, the adsorption process can be planned and implemented achieving high efficiency of pollutant removal. An interesting trend in recent years is the search for low-cost adsorbents that have a high pollutant-binding capacity. Increasingly, scientific interest is focusing on the use of locally available materials such as natural materials, agricultural waste, or industrial waste as low-cost adsorbents [12]. Recently, researchers’ attention has been also focused on heterogeneous photocatalysis using nanoparticles which appears to be a highly effective technique for degrading dyes in aqueous solutions [13,14,15]. Organic pollutants are broken down into simple substances such as water, carbon dioxide, and mineral acids [9]. However, the purification efficiency is affected by the catalyst mass, dye concentration, pH, light intensity, type of photocatalyst, and temperature [10,11]. The study showed that photocatalytic treatment can be successfully applied as a pre-treatment method, that is, before the biological treatment of raw industrial wastewater. This increased their biodegradability and reduced the toxicity of the wastewater [16,17].

Photocatalysis using semiconductor oxide material is a widely used technique in which semiconductors, such as the following, are used: CdS, ZnS, WS₂, CdSe, and CdTe as well as the metal oxides SnO₂, ZnO, α-Fe₂O₃, WO₃, ZrO₂, and TiO₂ [18,19].

The most commonly studied photocatalyst is titanium dioxide due to its chemical inertness, biological properties, non-toxicity, and low price [20]. Photocatalysis using TiO2 is an advanced oxidation technique that plays a key role in the effective removal of impurities from aqueous solutions in many different fields of science and industry. Thanks to its versatility, it is used in catalysis, materials chemistry, energy chemistry, environmental chemistry, and surface chemistry as well as processing chemistry and chemical engineering. [21]. The range of photoactivity of titanium dioxide is between 300 and 390 nm which allows the use of the ultraviolet range of sunlight. Excitation of the photocatalyst’s activity in visible light is a result of doping and surface modification, for example, by combining titanium (IV) oxide with tungsten (VI) oxide which is characterized by a lower energy gap of 2.8 eV. The combination of these two semiconductors allows for the formation of a photocatalyst with a wider range of absorbance of radiation in both the UV and the visible range [22,23]. Zinc (II) oxide has equally favorable photocatalytic properties, a lack of toxicity, a low price, and antibacterial properties. At room temperature, ZnO has a wide energy gap of 3.4 eV that is suitable for short-wavelength optoelectronic applications and a high binding energy (60 meV) enabling efficient exciton emission at room temperature. The ZnO nanostructure is used in the catalytic reaction process due to its large surface area and high photocatalytic activity [24]. Photocatalysts undergo numerous modifications to improve their performance properties, such as high or low photocatalytic activity [25,26,27].

Over the past decade, much work has been performed to develop nanocomposites produced by the action of a modified inorganic carrier with polymer matrices. Such processes make it possible to produce new classes of polymeric materials that combine the properties of both inorganic particles and organic polymer matrices which affects their processability and flexibility. The composites produced in this way have unique electrical, thermal, and optical properties, allowing the expansion of their applications in many industries [28,29,30].

This paper presents a method for obtaining composite materials based on used cooking oil, also called waste cooking oil (WCO), the surface of which has been modified with zinc and titanium oxides; preliminary tests have been carried out to determine their sorption and photocatalytic capacities. Methylene blue was selected for the study of sorption and photocatalytic capacity. This was due to the fact that it is a substance widely used in the textile industry, has a well-defined chemical structure, and is relatively safe. In addition, methylene blue has a high binding capacity to various materials such as porous bodies and organic compounds, making it a versatile model substance. Its sorption properties depend on various factors, such as pH, temperature, and the presence of other compounds which makes it possible to study various experimental conditions and their impact on sorption processes. A great advantage is also the fact that methylene blue shows visible color changes during sorption which facilitates the monitoring and quantitative assessment of the sorption process, both by visual observation and spectroscopic techniques.

2. Materials and Methods

For this study, canola oil (WCO) after frying (its physicochemical properties are described in [31]), concentrated sulfuric (VI) acid of p.a. purity that was used as the acid catalyst, sand with a grain size of up to 1.4 mm in the air-dry state, commercial zinc oxide, and titanium oxide were used. The zinc oxide was purchased as a p.p.a. chemical reagent from Merck Company (Darmstadt, Germany) while the titanium oxide was in the form of a commercial dye called titanium white. The chemical composition of the oxides was determined using a PW4025/00 MiniPal compact energy-dispersive X-ray spectrometer from PANalytical B.V. with an integrated rhodium lamp (for this reason a rhodium-derived peak is visible on each spectrum). In addition, using the XRD method, the size of the zinc–titanium oxide crystallites was determined using the calculated full width at half maximum of the reflections and the Debye–Scherrer method using the relationship between the broadening of the diffraction line profile and the size of the crystallites [32], described by the Formula (1):

𝐵𝑘 = 𝐾∙𝜆 𝐷ℎ𝑘𝑙∙𝑐𝑜𝑠Θ

(1)

where Bk—the width of the reflectance depending on the size of the crystallites, [rad]; K—a constant related to the shape, a dimensionless value for which 0.9 was adopted [33]; λ—the wavelength of radiation, [Å]; D—the size of the crystallites in the direction perpendicular to (hkl), [Å]; and θ—the Bragg angle, [°]. Surface analysis of the oxides was performed using an Apreo 2 S LoVac (Thermo Fisher Scientific, Waltham, MA, USA) scanning electron microscope.

Composite materials modified with zinc oxide (CM–ZnO) or titanium oxide (CM–TiO₂) were obtained by two methods. The first involved mixing WCO with sulfuric acid and then with a mixture of sand and oxide so that the oxides were distributed throughout the block. In the second method, WCO was mixed with sulfuric acid, then sand was added and oxide was applied to the surface of the mixture. The samples were annealed in aluminum molds in a laboratory muffle furnace from Nabertherm B180.

The process parameters for obtaining composite materials with photocatalytic properties (1-oxide in the entire volume of the block and 2-oxide on the surface of the block) are shown in

Table 1. Process parameters for obtaining photocatalytic composite materials.

Sample	ZnO [%]	TiO2 [%]	Cat/Oilcat	Annealing Temperature [°C]	Annealing Time [h]
Control	0	0	0.27
1–5 ZnO	5	0	0.27
1–10 ZnO	10	0	0.27
1–20 ZnO	20	0	0.27
1–10 TiO2	0	10	0.27	220	20
1–5 ZnO-5 TiO2	5	5	0.27
1–2.5 ZnO-2.5 TiO2	2.5	2.5	0.32
2–10 ZnO	10	0	0.27
2–5 TiO2	0	5	0.27

. The content of catalyzed oil in the material was constant at 20% by weight of the solid components in the sample.

Thermal analysis of the composites was performed using SDT 650 apparatus from TA Instruments in the temperature range of 25–1000 °C and a temperature increment of 10 °C/min. The molecular structure characterization of the solids was carried out by Fourier-transform infrared spectroscopy using a Nicolet iS5 FT-IR spectrometer from Thermo Scientific in the wavelength range of 500–4000 cm⁻¹.

The sorption and photodegradation of methylene blue (MB) using the materials was studied using batch mode experiments. The composite material was placed in the MB solution and then left for 30 min with continuous stirring. After this time, the concentration of the MB solution in the system was tested and the amount of MB adsorbed by the composite material was determined. The photocatalytic activities of the photocatalyst were investigated by measuring the degradation of the MB aqueous solutions under UV light irradiation using a UV lamp (wavelength = 365 nm). A certain amount of the material was added while mixing 50 mL of the solution C₀ = 100 mg/L of dye with a magnetic stirrer for a specified period of time. The amounts of the degraded dye R_D (mg/g) (2) and efficiency of photocatalytic degradation of MB (3) were calculated from the following dependencies:

$$R_D(\mathrm{m}\mathrm{g}/\mathrm{g})=\frac{\left(C_0-C_t\right)·\mathrm{V}}{\mathrm{m}}$$

(2)

$$E(\%)=\frac{\left({\mathrm{C}}_0-C_t\right)}{C_0}·100$$

(3)

where $C_0$ and $C_t$ are the initial and final concentrations of the dye solution (mg/dm³) at time t, V is the solution volume (dm³), and m is the mass of the material (g).

Approximation profiles were created to determine the values of independent parameters to obtain the most favorable estimated values of the outcome factor. The approximated values of the output factor for combinations of input factor values were converted to a usability scale. The approximation profile approach can potentially reduce the need for extensive experimental testing. The usability values of the dependent variable can vary from 0.0 (undesirable) to 1.0 (highly desirable). Statistical analysis was conducted in version 10 of STATISTICA by StatSoft^® [34].

3. Results and Discussion

3.1. Characterisation of Zinc Oxide and Titanium Oxide

Figure 1 shows an SEM microphotograph of pure zinc oxide and titanium oxides.

Titanium oxide is characterized by its homogeneous structure and fine crystallites with a spherical structure. The ZnO surfaces are characterized by numerous corners and edges and thus have potential reactive surface sites. The defects significantly change the grain boundary properties and characteristics. These defects introduce imperfections that affect the mechanical, electrical, and optical properties of the materials. Additionally, the presence of defects in the grain boundaries can influence the recombination and diffusion of charge carriers [35]. The finely crystalline nature of its lattice arrangement enhances its structural integrity and promotes efficient electron transport. These characteristics make titanium oxide an ideal candidate for various applications, including photocatalysis. The diffractogram of the ZnO and TiO₂ is shown in Figure 2.

The ZnO diffractogram is typical of crystals with a hexagonal structure; intense and narrow diffraction peaks are visible which correspond to different crystallographic planes. The most important peaks can be observed around the values of the diffraction angle (2θ) around 31.8°, 34.4°, 36.2°, 47.5°, and 56.6°. These peaks are characteristic of zinc oxide in the wurtzite (W-ZnO) phase. The wurtzite ZnO phase is characterized by polar crystal surfaces which makes it not centrosymmetric [36]. The TiO₂ diffractogram shows diffraction peaks that correspond to different crystallographic planes. However, the width of the peaks tends to be greater than that of ZnO which is related to the smaller crystal sizes in the titanium oxide under study. XRD diffraction peak positions and Miller indices values of titanium oxide and zinc oxide are shown in

Table 2. XRD diffraction peak positions and Miller indices values of titanium oxide and zinc oxide.

S. No.	TiO2		ZnO
	2 Theta (Degrees)	hkl	2 Theta (Degrees)	hkl
1	25.30	011	31.77	100
2	27.42	110	34.41	002
3	36.08	101	36.25	101
4	36.98	013	47.53	012
5	37.84	004	56.59	110
6	39.30	200	62.87	013
7	41.24	111	66.37	200
8	44.08	120	67.95	112
9	48.06	020	69.09	201
10	53.96	015	72.57	004
11	55.06	121	76.97	202
12	56.66	220
13	62.70	024
14	64.06	130
15	68.88	116
16	70.30	220
17	75.14	125
18	76.03	031
19	76.54	202

. The peaks, corresponding to planes <110>, <101>, <200>, <120>, <220>, <024>, <130>, <116>, <202>, and <111>, are characteristic of the rutile phase (R-TiO₂) while the other peaks are characteristic of the anatase phase (A-TiO₂). The rutile and anatase phases of titanium dioxide differ in crystal structure, thermodynamic stability, physical properties, light absorption, and applications. Both anatase and rutile exhibit a tetragonal crystal structure. However, anatase possesses octahedrons that are interconnected through the sharing of four edges, forming the characteristic four-fold axis. Moreover, each of the phases has its own unique features that determine their use in various fields and technologies. The rutile phase exhibits a wider range of UV absorption than the anatase phase; therefore, it is more desirable for photocatalytic applications [37]. The average size of the crystallites based on calculations according to the Debye–Scherrer equation was 24.15 nm. Zinc oxide crystallites varied in size and shape, with an average size of 110.68 nm (crystallite size calculations can be found in the supplementary materials).

3.2. Sorption and Photocatalytic Effect of Composite Materials

Figure 3 shows the resulting composite materials containing zinc oxide dispersed throughout the sample; SEM-EDS microphotographs are shown in Figure 4. The composites are characterized by a solid and porous texture and dark brown color.

On the surface of the samples, silicon, calcium, and aluminum were identified as sand components as well as carbon as a component of the WCO. In addition, metals (zinc and titanium) or their mixture, introduced in the form of oxides, were identified in each composite.

Figure 5 shows an FT-IR spectrum of oil composites containing zinc oxide, titanium oxide, and a mixture of these oxides.

In the case of ZnONP, a broad peak observed at 3467 cm⁻¹ is attributed to the (-OH) group present in this material. Weak peaks around 2900 cm⁻¹ correspond to the stretching vibrations of the C-H bonds in the methylene functional group and hydroxyl groups [38,39]. Bands around 1730 cm⁻¹ can be assigned to the stretching vibrations of the C=O bond in the ester group, around 1450 cm⁻¹ to the stretching vibrations of the C=C bond in the alkenyl group, and 1100–1060 cm⁻¹ from the stretching vibrations of the C-O bond. C-H deformation vibrations occur at wave numbers of 776–781 cm⁻¹. The peaks observed around 690 cm⁻¹ originate from the bending vibrations of C-H bonds in mono-substituted aromatic compounds [40,41,42]. Peaks in the range of 570–620 cm⁻¹ originate from the asymmetric and symmetric stretching vibrations of the Zn-O bonds in the structure of zinc oxide. Peaks around 470 cm⁻¹ are due to the presence of TiO₂, while peaks around 450 cm⁻¹ are characteristic of the Zn-O-Zn network vibrations associated with the trigonal crystal lattice of zinc oxide [43,44,45].

Thermal analysis of the oil composite with zinc oxide and titanium oxide (Figure 6) showed that the initial weight loss of the oil block occurred in the temperature range from 25 to 100 °C which is due to the loss of water absorbed by this material from the environment. The highest weight loss associated with the decomposition of the sample components is observed in the temperature range 250–600 °C. In the temperature range of 200–350 °C, polyesters are degraded. Mass loss at a temperature of about 300 °C can be caused by the decomposition of Zn(OH)₂ [44]. The peak about 470 °C is due to the slow dehydration process of TiO₂ [46]. Degradation at around 600 °C can be attributed to the oxidation reaction of double bonds in long chains of fatty acids; above this temperature, the weight of the sample stabilizes [47,48].

3.3. Photocatalytic Effect of Composite Materials

The efficiency of the photodegradation process and the accompanying sorption process are shown in

Table 3. Photodegradation and sorption efficiency for composite materials based on waste cooking oil.

	Sorption [%]	Sorption + Photodegradation [%]	Photodegradation [%]
Control sample	27.35	27.46	0.10
1–5 ZnO	41.88	41.99	0.10
1–10 ZnO	45.49	46.01	0.52
1–20 ZnO	25.63	96.04	70.41
1–10 TiO2	46.85	62.27	15.42
1–5 ZnO + 5 TiO2	38.43	39.69	1.25
1–2.5 ZnO + 2.5 TiO2 *	31.53	36.50	4.97
2–10 ZnO	42.61	44.65	2.04
2–5 TiO2	21.71	27.14	5.44

(*) the composite has disintegrated in MB solution.

.

The maximum photodegradation effect of more than 70% was obtained using a composite material containing 20% zinc oxide. Comparing the materials containing 10% oxides, it can be seen that in both cases the sorption efficiency is comparable (about 45%) while the photodegradation efficiency is significantly higher when using a material enriched with titanium oxide and amounts to 15.42% as opposed to 0.52% for zinc oxide. This fact may be due to the particle size of the titanium oxide which is in line with the scientific results of other researchers that suggest that the particle size of photocatalytic nanomaterials such as TiO₂ can have a significant effect on their photocatalytic activity [49,50]. Smaller particles typically exhibit higher photocatalytic activity as the larger particle surface area allows for greater adsorption of contaminants and increases the efficiency of solar capture. During the first stage of sorption, the adsorbate is transferred to the surface of the sorbent, usually by diffusion. Then, diffusion occurs on the outer surface of the sorbent and the adsorbate migrates into the pores. On the inner surface of the pores, there is a mutual interaction between the adsorbate and the free active sites. These steps or their various combinations have a significant impact on the speed of the sorption process [51]. Understanding these sorption steps and mechanisms is extremely important for optimizing sorption processes, designing effective sorbents, and using appropriate operating conditions. Results from other researchers [52] also indicate that the photocatalytic activity of TiO₂ is mostly higher than ZnO which may be due to TiO₂′s better ability to generate electron–hole pairs. It is also worth noting that the sorption efficiency was similar for the two cases in which 10% oxides were used, suggesting that the chemical composition of the material (titanium oxide vs. zinc oxide) may not have a significant effect on the sorption process. However, the differences in photodegradation efficiency between the two materials may be related to their photocatalytic properties, such as the energy gap, ability to generate electron–hole pairs, and impurity reactivity. This underscores the importance of choosing the right photocatalytic material depending on the specific research conditions and application goals. Composite materials containing ZnO and TiO₂ can find application in the production of protective coatings on various types of surfaces, such as metal, glass, ceramics, etc. Thanks to their antibacterial, anti-corrosive, and UV-protective properties, such coatings can protect surfaces from harmful external influences.

Based on the empirical data obtained, an approximation profile was created and the utility function was determined (Figure 7). In the case of using an amount of 20% zinc oxide, the utility is 0.92. This profile illustrates how the utility decreases when changing the amount of oxides used.

When designing new photocatalytic materials for industrial applications, a utility function approximation profile can help scale up the manufacturing process, guiding the selection of optimal formulations and compositions. The use of approximation profiles and utility functions in the study of composite materials enriched with zinc and titanium oxides provides an effective approach to analyzing and comparing different combinations of materials.

The technology for producing composites based on used cooking oil enriched with nanometric zinc and titanium oxides is simple and does not require complicated equipment. Moreover, it makes it possible to obtain composites of any shape and size, which broadens its spectrum of applications.

4. Conclusions

Composite materials derived from used cooking oil and zinc and titanium oxides show potential for use as photocatalysts. Their solid and porous structure increases the active surface area. Zinc-containing materials showed photocatalytic efficiency up to about 70% and sorption efficiency up to about 46%. Composites containing a mixture of zinc oxide and titanium (in the amount of 5% each) showed comparable sorption capacity to composites containing only zinc oxide in the amount of 5%. The removal of dyes from the solution on an oil-based composite enriched with zinc and titanium nanoxides occurred through the process of photodegradation and sorption. Photodegradation was a result of the properties of ZnO and TiO₂, such as the ability to generate electron–hole pairs, energy bandgaps, and reactivity towards pollutants. On the other hand, sorption occurred due to the diffusion of the adsorbate onto the surface of the sorbent and migration into the pores of the composite. Changing the process parameters allows the modification of the properties of CM–ZnO and CM–TiO₂ which affects their sorption and the photocatalytic capacities of the materials. This is a very important aspect as it allows the modelling of the properties of the sorbent/photocatalyst for its application. The research falling within the scope of this article covers only a very narrow slice of research, enabling the conclusion that the studied material is characterized by dye removal capabilities by sorption and photocatalysis. Further research is envisaged to investigate the main factors affecting adsorption and photocatalysis as well as strategies for improving the performance of the obtained materials. Investigation of the occurring mechanisms of adsorption and photocatalysis, as well as the relationship between the adsorption process and photocatalysis, may contribute to the development of knowledge with the use of composites based on used cooking oil having additional and very important characteristics such as the ability to reduce the amount of hazardous substances from the environment. The technology for obtaining CM–oxide is in line with the tenets of clean technologies, the goal of which is to transform waste into a fully-fledged and useful product.