Synthesis of Novel Bismuth-Based Catalysts for the Degradation of Microplastics in Aquatic Matrices

Abstract

Microplastics are one of the most widely studied and concerning contaminants, as they are present in all environmental compartments, especially water bodies. Wastewater treatment plants are one of the most important pathways through which these pollutants enter the environment. Currently, novel techniques are being developed to eliminate microplastics from wastewater, including heterogeneous photocatalysis. In this study, two bismuth-based photocatalysts were synthesized using different methods: BiPO₄ by the solvothermal method, and Bi₂O₃/TiO₂ by the wet impregnation method. These were morphologically and structurally characterized. Both photocatalysts were then tested in laboratory-scale experiments to evaluate their effectiveness on the degradation of polypropylene microplastics under UV irradiation in ultrapure water. The effectiveness of the treatment was estimated by measuring the reduction in the area of each of the microplastics, and structural changes were assessed using infrared spectroscopy (FTIR). It was found that the BiPO₄ catalyst was more effective when applying UV-B radiation, achieving a reduction in the area of microplastics of up to 10.81%, while the Bi₂O₃/TiO₂ catalyst showed a higher efficiency when applying UV-A, achieving a reduction in the area of microplastics of up to 9.15%. The study of microplastics by ATR-FTIR revealed the appearance and modification of some absorption bands, which indicates incipient degradation. The application of both catalysts in real wastewater showed a reduction in the efficiency of the treatment; hence, further studies should be conducted to determine the influence of other variables in the photocatalytic process. In the current context of growing environmental concern, the development of new, easily synthesized catalysts represents a key strategy for reducing or eliminating MPs. This study presents significant advances in the formulation and evaluation of innovative catalysts, demonstrating their potential as effective and accessible tools for mitigating one of the most pressing global pollution challenges.

1. Introduction

Plastics are synthetic polymeric materials that are extensively used in many aspects of daily life. Due to their key characteristics, such as light weight, relatively inexpensive cost, high flexibility, and durability, the global consumption of plastic materials has increased, especially during recent decades [1]. According to a new OECD report [2], the world is producing twice as much plastic waste as it was two decades ago, with most of it ending up in landfill, incinerated, or leaked into the environment, and only 9% being successfully recycled. Inadequate management of plastic recycling results in approximately 13 million tons of plastic waste entering aquatic environments each year [3]. Increased production together with the poor management of plastic waste makes plastic pollution in aquatic and terrestrial ecosystems a global-scale problem; furthermore, it will increase as synthetic polymers continue to be produced, used, and disposed [4]. The following plastics are the most commonly produced: mainly polyethylene (PE), which accounts for 29.8%, followed by polypropylene (PP) (19.4%), polyvinyl chloride (PVC) (10%), polyethylene terephthalate (PET) (7.9%), polyurethane (PUR) (7.9%), polystyrene (PS) (6.2%), and other plastics (18.8%) [5].

Microplastics (MPs) are plastic particles ranging in size from 1 µm to 5 mm. They appear in regular or irregular shapes and originate either from primary sources (originally manufactured) or secondary sources (resulting from the degradation and fragmentation of larger plastics due to physical, chemical, and biological processes) [6].

One of the largest inputs of MPs into the environment is from wastewater treatment plants (WWTPs) [7]. Many authors have presented results on the detection and quantification of MPs in WWTP effluents [8,9], while others have presented removal efficiencies of MPs in WWTPs. Although conventional WWTPs can remove MPs efficiently (64–99%) [10], this percentage is not sufficient when considering the daily discharge rate. [8] presented a review comparing MPs removal rates across 21 studies. On average, the secondary and tertiary treatment stages of WWTPs removed 88% and 94% of microplastics, respectively. Additionally, the majority of MPs (approximately 72%) were removed during preliminary and primary treatment. However, the total amount of MPs would still be discharged daily into the environment, meaning that the final effluent could act as one of the main routes by which MPs enter aquatic environments [11]. It is estimated that European wastewater treatment plants discharge nearly up to 520,000 tonnes of MPs into freshwater streams [12]. PP is one of the most frequently detected polymers and results in the highest concentrations in most studies [13]. PP (Equations (1)–(3)) is an unsaturated hydrocarbon containing only carbon and hydrogen atoms [14]. In this mechanism (Equations (1)–(3)), M* refers to the radical initiator, whereas M designates the growing polypropylene chain that carries the radical site at its terminal carbon. Through successive additions of propylene units, the chain propagates until termination occurs by recombination or disproportionation of radicals.

$$\begin{array}{cc}{M}^{*}+{CH}_2& =CH\to \\ & |\\ & {CH}_3\end{array}$$

(1)

$$\begin{array}{ccc}M-{CH}_2{CH}_2+& {CH}_2& =CH\to \\ |& & |\\ {CH}_3& & {CH}_3\end{array}$$

(2)

$$\begin{array}{cc}M {CH}_2{CHCH}_2{CH}_2& =CH\to etc.\\ | |& \\ {CH}_3 {CH}_3& \end{array}$$

(3)

Advanced oxidation processes (AOPs) have been extensively studied in recent decades as a chemical treatment of emerging contaminants such as pharmaceuticals and pesticides [15,16]. By using alone or combining various methods, such as light, heat, plasma and catalysts, reactive oxygen species (ROS), known as radicals, are effectively produced during the treatment process. Since these radicals produced have a high redox potential, they can easily break down various contaminants [17]. Advanced oxidation processes are classified into two main groups depending on their mode of activation. Photochemical methods employ light radiation in the process, in contrast to non-photochemical methods. Examples of advanced oxidation processes include ozonation, Fenton, photo-Fenton, photocatalysis, radiation, sonolysis, and electrochemical oxidation [15].

Photocatalysis refers to reactions that occur under the action of light or general phenomena related to photochemical and catalytic processes [18]. This process causes the acceleration of the reaction through the use of a suitable photocatalyst, allowing for the mineralization of the organic compounds present in the study medium to CO₂ and H₂O using radiation. The radiation used is ultraviolet or visible radiation, since both have sufficient energy to interact with the material and produce highly reactive radicals [19]. This radiation excites the electrons in the valence band, which move to the conduction band, leaving holes in it [20]. These electron-hole pairs move to the surface of the photocatalyst, where they initiate oxidation-reduction reactions with the adsorbed pollutants [21]. A semiconductor material is used as a photocatalyst, since it offers the ability to separate its charges (electrons and holes) and its chemical energy can be used to carry out redox reactions [22]. The radicals generated can attack microplastics, causing the polymer chains to fracture, resulting in mineralization [23,24]. In general terms, the degradation process of microplastics follows specific routes that have already been described by other authors in former studies (Equations (4)–(11)) [25,26]:

$$Photocatalyst+hv\to h^{+}+e^{-}$$

(4)

$$h^{+}+H_{2}O \left(or {OH}^{-}\right)\to ·OH+H^{+} \left(or·OH\right)$$

(5)

$$e^{-}+O_2\to ·O_2^{-}$$

(6)

$$O_2+e^{-}+{2H}^{+}\to H_{2}O$$

(7)

$$H_{2}O+hv\to 2·OH$$

(8)

$$H_2{O}_2+{·O}_2^{-}\to ·OH+O_2+{OH}^{-}$$

(9)

$$e^{-}+H_2{O}_2\to ·OH+{OH}^{-}$$

(10)

$$Microplastic+ROSs\to C{O}_2+H_{2}O$$

(11)

The catalytic degradation of MPs has been subject of extensive research involving a broad spectrum of materials. TiO₂ and ZnO are the most widely reviewed catalysts, primarily due to their low cost. However, other materials such as graphene oxide, CdS, MOFs, and bismuth-based compounds have also been studied. Among these, bismuth-based catalysts emerge as a particularly attractive option, offering both cost-effectiveness and reduced toxicity [27]. According to the literature, bismuth phosphate (BiPO₄) is a photocatalyst that shows one to two times higher photocatalytic activity than titanium dioxide (TiO₂) in studies of carbamazepine degradation [28] or dye degradation [29]. It is mainly selected for its high photocatalytic activity, as well as its low cost and low toxicity [30]. Morphology is mainly responsible for the activity of this compound, as the BET surface can be lower than that of TiO₂, depending on the synthesis method. In addition, BiPO₄ presents a high energy band (3.81 eV) compared to TiO₂ (3.2 eV) [31,32]. Two of the parameters that influence the photocatalytic activity of a semiconductor are the BET surface area and the energy band (band-gap). A high surface area implies higher photocatalytic activity, while a small energy band favours the production of active species [33]. However, in this photocatalyst, the opposite is observed; in this case, the wide energy band allows for a higher oxidation capacity and, with it, a higher capacity to mineralize pollutants [34], mainly due to the phosphate, which prevents the recombination of the electron-hole pair.

The second photocatalyst, Bi₂O₃/TiO₂ was selected to improve the optical properties of TiO₂ by enhancing light absorption [35]. Unlike the previous bismuth-based compound (Bi₂O₃), it possesses a stable phase with a band-gap energy significantly lower than the previous compound (2.85 eV) [36]. This photocatalyst has shown to effectively degrade alizarin red S dye [35] due to the incorporation of Bi³⁺ ions into the TiO₂ lattice, which inhibits electron-hole pair recombination and facilitates the reaction.

Amid escalating environmental concerns, the pursuit of novel catalysts with straightforward synthesis routes emerges as a pivotal approach to tackling MP contamination. This research introduces meaningful progress in the design and testing of such catalytic systems, underscoring their promise as practical and efficient solutions to one of today’s most critical pollution challenges. The main objective is to study the degradation of microplastics by photocatalysis, specifically, the degradation of polypropylene (PP) by photocatalysis under ultraviolet light UV-A and UV-B, and two different catalysts based on bismuth and titanium (BiPO₄ and Bi₂O₃/TiO₂).

2. Materials and Methods

2.1. Reagents and Materials

Filters (0.8 µm polycarbonate filters PC membrane 47 mm) were supplied by IsoporeTM (Darmstadt, Germany). Bismuth (III) nitrate pentahydrate (Bi(NO₃)₃)-5H₂O, 99%), PP ((C₃H₆)_n, 99%) and Ethylene glycol (H(OCH₂CH₂)nOH) were obtained from Sigma Aldrich (Madrid, Spain). Iron II sulphate 7-hydrate purissimum, sulphuric acid 95–98%, hydrogen peroxide 30% v/v (pure, pharma grade), and Sodium phosphate dodecahydrate (Na₃PO₄-12H₂O) were purchased from Panreac (Barcelona, Spain). Absolute ethanol (CH₃CH₂OH) was marketed by Supelco (Madrid, Spain). Titanium dioxide (TiO₂, P25) was supplied by Evonik Industries (Essen, Germany). Sodium chloride extra-pure was purchased from Scharlab, and Acetone (C₃H₆O, Ph. Eur.) from VWR Chemicals (Barcelona, Spain).

2.2. Experimental Procedure

2.2.1. Catalyst Synthesis

BiPO₄ was synthesized using the solvothermal method [37]. There are several synthesis methods (sonochemical method or chemical vapour deposition), but the solvothermal is the simplest method and the one that allows us to control the morphology and structure of the products in a better way [31]. Bi₂O₃/TiO₂ was synthesized by the incipient wetting impregnation method in order to observe how the deposition of a bismuth oxide on a known photocatalyst, such as titanium oxide, would mainly affect its optical properties.

Solvothermal synthesis is a method used to obtain crystals of insoluble substances under high-temperature and -pressure conditions, which promotes the formation and growth of crystalline phases. It is performed in high-pressure autoclaves, where an organic solution is sealed and heated at a temperature range between 100 and 1000 °C, with pressures from 1 to 100 MPa. These conditions help to dissolve the precursors with low solubility or reactivity under normal conditions, thanks to the combination of temperature, pressure, and solvent [38,39]. Photocatalyst Bi₂O₃/TiO₂ was prepared by the wet impregnation method considering a final bismuth oxide content of 10% by weight with respect to titanium oxide. This method involves placing dry titanium oxide in contact with an impregnation solution that is equal to the pore volume of the support [40]. The single or successive incipient wet impregnation method, followed by a heating process at 180 °C, proved to be an efficient method for the preparation of photocatalysts [41]. For the deposition of bismuth oxide on the TiO₂ catalyst surface, wet impregnation of the commercial P25 precursor (TiO₂), with the appropriate bismuth precursor (Bi(NO₃)₃-5H₂O), was carried out.

Further on, for the preparation of the catalyst, 22.5 mmoles of Bi(NO₃)₃-5H₂O and 22.5 mmoles of Na₃PO₄-12H₂O were dissolved under vigorous stirring separately in 115 mL of ethylene glycol. For this purpose, exactly 10.91 g and 8.55 g, respectively, were weighed. With a Pasteur pipette, the Na₃PO₄-12H₂O solution was added slowly on the Bi(NO₃)₃-5H₂O solution and kept in vigorous agitation for 1h. Next, the solvothermal treatment was carried out after 48 h at 140 °C in a muffle furnace. The solution obtained in the previous step was introduced in the solvothermal reactor, hermetically closed, and introduced into the oven. After 48 h, the products were washed in a centrifuge three times with ethanol and dried at 80 °C for 12 h. Finally, they were calcined again for 4 h at 450 °C (heating ramp of 2 °C/min). Finally, the product obtained was sieved with a 75 µm mesh.

A solution of 2.013 g of Bi(NO₃)₃ in 10 mL of distilled water was dissolved for the synthesis of Bi₂O₃/TiO₂, which was added dropwise over 9 g of (P25) TiO₂ powder and stirred manually with a stirring rod, allowing for the pores of the precursor to fill with bismuth oxide. Once the impregnation wet point was observed, the product obtained was dried for 24 h at 105 °C (this is known as an impregnation cycle). Until the Bi(NO₃)₃ solution was exhausted, three impregnation cycles were carried out. After impregnation, the sample was sieved with a 75 µm mesh and then calcined in a muffle furnace (5 °C/min up to 500 °C maintaining the treatment for 60 min) to decompose the bismuth nitrate.

2.2.2. Microparticle Characterization

A Carl Zeiss Axio Imager M1m (Carl Zeiss, Jena, Germany) optical microscope was used to characterize the polypropylene particles before and after exposure to photocatalysis. The polypropylene particles were first characterized by diameter and area measurements by taking a picture of them individually and determining their area and longest diameter using specific tools in the software included with the microscope (Zeiss ZEN^®, ver. 3.7). Commercial PP particles were sieved to select a size between 400 and 600 µm, and, subsequently, particles comprising a range between 500 and 550 µm were selected. To determine the size of the sample population, a total of 70 particles with an average area of 0.190 ± 0.052 (μm²) + standard deviation (SD), respectively, was studied. All particles used in each test were studied individually.

2.2.3. FT-IR Analysis

Once the microparticles were characterized before and after exposure to photocatalysis, they were analyzed by Fourier Transform Infrared Spectroscopy (FT-IR). A Perkin Elmer Spectrum 3 in Attenuated Total Reflectance (ATR) mode was used. A baseline polypropylene spectrum was generated to serve as a reference for comparing the polypropylene particles after photocatalytic exposure with those prior to treatment (Figure 1). It is necessary to know the initial structure of the PP particles in order to understand if there is a change in them once the photocatalysis process is applied. In the case of the PP particles under study, the bands identified are vas and vs of -CH₃ and -CH₂ at 2950, 2917, 2868, 2837 cm⁻¹. Bands of δas and δs were in the C-H plane at 1457 and 1376 cm⁻¹ and the δ balance was below 1167 cm⁻¹.

Figure 1. Reference PP microparticle FTIR-ATR spectrum.

Each microparticle was analyzed by performing 4 scans between wavelengths of 4000 and 650 cm⁻¹, with a spectral resolution of 4 cm⁻¹. During the analysis, a background was performed every 5 microparticles analyzed, depending on the laboratory conditions.

2.2.4. Photocatalytic Activity Tests on Polypropylene Microparticles

In order to study the degradation process, a total of eight tests were conducted. Two of the tests were performed for the catalysts (BiPO₄ and Bi₂O₃/TiO₂), considering two types of light: UV-A and UV-B. Additionally, two more tests were carried out, one for each light type, to evaluate the potential degradation caused by UV-A or UV-B in aqueous media without the influence of the synthesized catalysts. All of the samples were analyzed in duplicate. Finally, the catalysts were tested in the presence of urban wastewater and the polypropylene (PP) particles under investigation.

Initial control of the luminous intensity of the lamps used to generate the radiation was carried out. A Radiometer UVP intensity metre with an associated sensor was used to approach the radiation source, and provided data at 20 mW/cm².

The concentration of PP particles was selected based on data from WWTPs located in the province of Cadiz, studied by [10], in which they were found to be between 16.40 ± 7.85 MPs/L and 131.35 ± 95.36 MPs/L. Specifically, ten previously characterized polypropylene particles were selected in 200 mL of ultrapure water with a concentration of 2.5 g/L catalyst. These samples were placed in an agitator at 200 rpm for 48 h in contact with irradiation (UV-A or UV-B).

2.2.5. Wastewater Treatment Plant

Wastewater samples were obtained from the Medina Sidonia WWTP. The treatment processes implemented at the WWTP have been previously described [42]. In summary, the water line includes a primary treatment stage, comprising coarse and fine screening units along with a degreasing system, followed by a secondary treatment stage involving an extended aeration biological reactor and a secondary clarifier for water purification. Samples were collected after the secondary treatment and just before being discharged.

2.2.6. Physicochemical Analysis

The parameters determined were pH, Total Organic Carbon (TOC), and Total Nitrogen (TN). pH was measured using a Crison model pH metre (Crison, Barcelona, Spain). TOC and TN measurements were performed using a Shimadzu TOC-L analyser (CPH) (Shimadzu, Kioto, Japan) equipped with ASI-L and TNM-L modules, located at INMAR (University Institute of Marine Research), the University of Cádiz. TOC concentration was calculated as the difference between CIT and CT after filtering the wastewater samples through 0.45 µm glass fibre filters (Merck, Darmstadt, Germany). Additionally, ion concentrations were measured in the samples before and after the photocatalytic treatment using a Metrohm 881 Compact IC Pro ion chromatography module (Metrohm, Herisau, Switzerland). All analyses were conducted following the standard method [43].

The structural study of the synthesized photocatalysts was carried out using X-Ray Diffraction, Electron Microscopy, and Fourier Transform Infrared Spectroscopy techniques. The X-ray diffraction diagrams were obtained in a Bruker AXS model D8 Advance (Bruker, MA, USA) powder diffractometer installed in the Central Services of Science and Technology of the University of Cadiz. The radiation used was Cu Kα, operating the X-ray tube at 40 KV and 40 mA, and a scanning range (2θ) of 3–75°. PowderCell2.4 software was used for the identification.

Electron microscopy was performed by the Central Science and Technology Services of the University of Cadiz using a Thermo ScientificTm Talos F200X (Thermo Fisher Scientific, MA, USA) scanning/transmission microscope (S/TEM) that combines high-resolution S/TEM and TEM imaging with X-ray scattering spectroscopy signal detection and 3D chemical characterization with compositional mapping. The sample was deposited on a grid with a Lazzy Carbon film. The technique used is the High Anular Angle Dark File (HAADF) technique. The analytical characterization, on the other hand, was carried out using the characteristic X-Ray Emission Spectroscopy (XEDS). The conditions used were as follows: (a) Magnification: 79,000×; (b) Mapping time: 7 min. Once the mapping was obtained, high-resolution photos (HRTEM) were taken in transmission mode by only using the light beam.

The textural characterization of the synthesized photocatalysts was carried out using the N₂ adsorption–desorption technique at liquid nitrogen temperature (−196 °C). This study allowed us to determine the adsorption isotherm [44] characteristic according to IUPAC, the specific surface area of the photocatalysts by applying the BET model (Brunauer, Emmett and Teller), and the total pore volume by the BJH method (Barret, Joyner and Halenda). Nitrogen volumetric adsorption measurements were carried out using a commercial automatic analyser, Micromeritics, model ASAP 2020. The automatic analysers were controlled by a computer using the 2020C software. Before starting the analysis, the sample was evacuated under vacuum at 200 °C for two hours to eliminate possible species adsorbed to the surface.

The catalyst compositions were analyzed using X-ray fluorescence (XRF) with a Bruker S4 Pioneer spectrophotometer and by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) employing a Thermo Elemental Iris Intrepid instrument.

Ultraviolet-visible spectroscopy was used to study the band-gap, which is a technique that allowed us to observe the transition of electrons from the valence band (BV) to the conduction band (BC) in order to calculate the band energy. The technique was carried out with a scanning range of 200–800 nm. The hydrodynamic size (HD) was obtained using a Malvern Zetasizer Nano-ZS (Malvern Instruments, Worcestershire, UK).

2.3. Quality Assurance and Quality Control

Regarding the characteristics of the chosen polymer, its shape and size make cross-contamination difficult. However, to avoid contamination throughout the process, all equipment was washed several times with distilled water and covered with aluminum foil. Cotton gowns and gloves were worn during sampling and analysis. All reagents were pre-filtered through a 0.45 µm glass fibre filter to prevent contamination.

3. Results and Discussion

3.1. Catalyst Synthesis

The catalysts studied were BiPO₄ and Bi₂O₃/TiO₂. Once synthesized, they were subjected to compositional characterization by XRF and ICP-AES, structural analysis, band-gap determination, electron microscopy (HAADF-XEDS and HRTEM), and Fourier Transform Attenuated Total Reflectance Infrared Spectroscopy (FTIR-ATR).

3.1.1. Compositional Characterization by XRF and ICP-AES

XRF and ICP techniques provide results in terms of the mass percentage of photocatalyst composition ([wt]%). Therefore, the compositional analysis of both photocatalysts can be determined. The XRF technique enables the execution of a semi-quantitative analysis of all elements present on the catalyst. In contrast, the ICP-AES measurements provide quantitative results, thereby yielding exact percentages by the mass of the element measured (in this case, bismuth). This is due to the fact that it is the semiconductor that will produce the photocatalytic process. It is evident that both techniques facilitate the determination of the elemental composition of the catalyst in congruence with its nominal composition; this is particularly evident in the case of ICP-AES. In addition, XRF allows for the detection of impurities in any of the prepared catalysts. As illustrated in Table 1, the results for BiPO₄ and Bi₂O₃/TiO₂ photocatalysts were obtained using XRF and ICP-AES. For both of the prepared catalysts, the bismuth content was found to be lower than the nominal value. The nominal value is defined as 69 wt% for the BiPO₄ catalyst and 10 wt% for the Bi₂O₃/TiO₂ catalyst. The detection of sodium by XRF may indicate the presence of impurities, possibly originating from the precursors used during the synthesis of the BiPO₄ catalyst. These sodium-containing impurities have the potential to modify the overall elemental composition of the sample. This, in turn, affects the measured bismuth content and cause it to deviate from the nominal value expected for pure BiPO₄.

Table 1. Compositional characterization by XRF and ICP-AES for BiPO₄ and Bi₂O₃/TiO₂ photocatalysts.

		Nominal Value [wt]%	XRF [wt] %	ICP-AES [wt] %
BiPO4	Bismuth	69	60.9	64.4
	Phosphorus	10	7.5	*
	Sodium		1.0	*
Bi2O3/TiO2	Bismuth	10	8.6	9.2
	Titanium	50	54.7	*

*: not measured/not detected/not applicable.

3.1.2. Structural Study

X-ray diffraction allows for the study of the crystal structure of the synthesized photocatalysts. Figure 2 shows the experimental XRD for BiPO₄ and Bi₂O₃/TiO₂. In the case of BiPO₄ catalysis, the structure obtained by XRD offered a greater similarity in its diffraction peaks in comparison to the JCPDS chip of the monoclinic phase of this compound (JCPDS 15-0767), which has a symmetry space group of P21/n (14).

Figure 2. Experimental XRD for BiPO₄ and Bi₂O₃/TiO₂ catalysts.

The obtained peaks were associated with the monoclinic phase of BiPO₄, assigning the planes to the most intense peaks for 19.0° (011), 21.4° (−111), 25.3° (111), 27.1° (200), 20.1° (120), 31.2° (012), 34,5° (−202), 36.9° (−212), 42.7° (−111), 46.3° (212), and 52.7° (040). A structural refinement was then performed using PowderCell2.4 software, with the resulting lattice parameters shown in Table 2.

Table 2. Grid parameters correspond to the monoclinic BiPO₄ phase and the tetragonal TiO₂ and cubic Bi₂O₃ phases.

Lattice Parameters (nm)
BiPO4	a	b	c
Monocyclic phase (BiPO4)	6.752	6.933	6.468
Bi2O3/TiO2	a	b	c
Tetragonal phase (TiO2)	9.785	*	9.513
Cubic phase (Bi2O3)	10.245	*	*

a, b, c: cell edge length; *: not measured/not detected/not applicable.

BiPO₄ crystallizes in three different structures: namely hexagonal, with a P3121 space group; low-temperature monoclinic with a space group P21/n; and high-temperature monoclinic, P 21/m. It is important to note that the low-temperature monoclinic phase is the most stable [45]. Concerning the hexagonal phase, the products synthesized for 1 h at 200 °C are indicative of the hexagonal phase. However, when the solvothermal residence time is extended for 3 h, they are indicative of the monoclinic phase. These observations reveal that time plays a crucial role in the selective formation of the BiPO₄ phase [46].

Ref. [45] states that another determining parameter for synthesis is the pH of the solution, which was the relevant factor in obtaining the low-temperature monoclinic phase. In the presence of strong acidic conditions (pH < 1), the product obtained a low-temperature monoclinic phase (space group P21/m). By contrast, under neutral or weakly acidic conditions (3 < pH < 7), a high-temperature monoclinic phase (space group P21/n) is obtained, the latter being in accordance with the solvent used in the solvothermal reaction (ethylene glycol).

The diffraction pattern of Bi₂O₃/TiO₂ catalysts is shown in Figure 2. TiO₂ has three main crystalline polymorphs: anatase, rutile, and brookite (being rutile the most common metastable polymorph [32]). Conversely, the bismuth oxide Bi₂O₃ would manifest a monoclinic structure, whose corresponding space group is P21/c (14), and a cubic structure, whose corresponding space group is I*3* (229). A correlation analysis of the XRD diffraction peaks with the corresponding JCPDS patterns was conducted, revealing that TiO₂ crystallizes in the anatase phase (JCPDS 021-1272), while Bi₂O₃ adopts a cubic structure with space group I*3* (229) (JCPDS 27-0050). The diffraction peaks of the anatase phase, which is a tetragonal phase, are assigned to the planes, from highest to lowest angle: (101), (004), (200), (105), and (313). From all of the different possibilities for the bismuth oxide, it was determined that the bismuth oxide in the samples shows a symmetry corresponding to the I*3* (229) space group. This is the only plane observed and is characteristic of the structure, in agreement with the experimental XRD (311), whose angle is described above.

After indexing the phases of this compound, we proceeded to determine the lattice parameters for the TiO₂ phase and the Bi₂O₃ phase (Table 2).

3.1.3. Band-Gaps

The band-gaps for the photocatalysts object of this study were obtained using UV-Vis absorption data. In this way, diffuse reflectance data was obtained and converted to apparent absorption spectra using the Kubelka–Munk function. The bang-gap energy (Eg) of the materials was determined from the Tauc curve by plotting (F(R) x hv)ⁿ versus hv(eV), with n = 2. In this way, the energy of the forbidden band is obtained by extrapolating the linear part of this plot to the x-axis (eV).

Initially, a simple absorption profile was observed for both photocatalysts, in which there was negligible absorption from approximately 850 nm up to the absorption edge. In the case of BiPO₄ (Figure 3a), absorption occurred between 238 and 357 nm. In addition, the Tauc diagram for BiPO₄ was plotted and linearly fitted from the value of (F(R) × hv)ⁿ versus the energy in eV raised to 2 (Figure 3b). By extrapolating the linear trend line, the band-gap value was estimated to be 3.7 eV.

Figure 3. BiPO₄ absorption profile (a) and linearly fitted Tauc plot for BiPO₄ (b).

For Bi/Ti, absorption occurred between 291 and 433 nm (Figure 4a). The Band-Gap value was obtained in the same way described above (Figure 4b). Band-Gap values were 3.7 and 3.05 for BiPO₄ and Bi₂O₃/TiO₂, respectively. These results show that the BiPO₄ photocatalyst exhibits a larger bang-gap, therefore requiring a strong light irradiation for the process activation to occur. When compared to former data, the deposition of bismuth on P25 produces alteration of the band-gap energy compared to experimental values of P25, 3.25 eV [32]. There is an enhanced light response due to the bismuth oxide–titanium oxide coupling.

Figure 4. Bi₂O₃/TiO₂ photocatalyst absorption profile (a) and linearly fitted Tauc plot for Bi₂O₃/TiO₂ (b).

3.1.4. Electron Microscopy (HAADF-XEDS and HRTEM)

Regarding the photocatalyst BiPO₄, in HRTEM images, BiPO₄ particles are homogeneous and exhibit a round morphology. They form small clusters with an average size of 5 nm (Figure S1).

Regarding the Bi₂O₃/TiO₂ photocatalyst, the distribution of bismuth and titanium can be observed in Figure S2 using HAADTF-XEDS. Compared to the HAADF-XEDS images, it can be observed that the smallest proportion of the element in the sample corresponds to bismuth. This outcome is consistent with the anticipated lower percentage in comparison with titanium. It is evident that bismuth is distributed uniformly across the entire surface of the titanium oxide. The EDX composition allows for the determination of the composition of the Bi₂O₃/TiO₂ catalyst at the nanoscale level, which corresponds to the nominal composition (titanium 69.6% and bismuth 5.2%). At the nanoscale, the bismuth content is slightly lower than the bulk values determined by XRF and ICP (Table 1), which may be attributed to heterogeneity in the dispersion of bismuth oxide on the titanium oxide surface.

3.1.5. Fourier Transformed Infrared Spectroscopy in Attenuated Total Reflectance (FTIR-ATR)

The results of the structural characterization of both photocatalysts by FTIR-ATR show that, for the BiPO₄ photocatalyst, a broad band with four peaks was observed (Figure S3a). It is notable that this spectrum lacks the presence of hydroxyl group (OH) bands; according to [47], hydroxyl defects can be detected and estimated by IR spectra and can also significantly affect photocatalytic activity [45]. The absence of peaks at 3480 cm⁻¹ and 1620 cm⁻¹ indicates the non-existence of OH-related defects in our sample. The bands present at 1067, 987, and 950 cm⁻¹ are assigned to the vPO₄ vibration. No significant band was observed in the FTIR-ATR spectrum for the Bi₂O₃/TiO₂; only one dip can be observed at the end of the spectrum at 650 cm⁻¹ (Figure S3b). However, it is known that the peaks appearing around 549 and 625 cm⁻¹ are attributed to Bi-O-Bi and Bi-O stretching vibrations [48].

3.2. Textural Properties

The textural characterization of the synthesized photocatalysts was carried out by performing volumetric adsorption–desorption isotherms of N₂ at 77K, together with the application of the BET method (Brunauer, Emmett and Teller), to determine the specific surface area of each of the catalysts. In addition, the BJH method (Barret, Joyner, and Halenda) was applied to obtain the pore volume of the catalysts surface. The adsorption/desorption isotherm of both photocatalysts is represented in Figure S5. Both correspond to an isotherm type IV, according to the isotherm classification recommended by IUPAC [44].

This type of isotherm is characteristic of mesoporous and macroporous solids. In conditions of low pressure, it exhibits a concave configuration in relation to the relative pressure axis (P/Po). The predominant characteristic is the hysteresis loop, which is associated with capillary condensation (typical of adsorption/desorption in mesopores). In medium-pressure conditions, capillary condensation occurs [49]. The specific surface area of the catalysts, as well as the pore volume, are shown in Table 3. The Bi₂O₃/TiO₂ catalyst exhibits a surface area four times greater than that of BiPO₄.

Table 3. Specific surface area and pore volume for BiPO₄ and Bi₂O₃/TiO₂ photocatalysts.

	Surface BET (m2/g) a	Pore Volume (cm3/g) b
BiPO4	11.4	0.1
Bi2O3/TiO2	43.3	0.4

a: result obtained with the BET method; b: result obtained with the BJH method.

In addition to the specific surface area, it is also important to understand the aggregation state of the catalysts within the reaction medium, especially in the context of reactions that are carried out in the aqueous phase. Therefore, Dynamic Light Scattering (DLS) measurements were conducted in ultrapure water (Figure S4). Both catalysts exhibited a heterogeneous hydrodynamic size distribution ranging from 100 to 6000 nm. In the case of the Bi₂O₃/TiO₂ catalyst, most particles agglomerated above the micrometre scale.

3.3. Photocatalytic Activity for Degradation of Polypropylene Microplastics

In order to study the effect of UV radiation on the degradation of microplastics in the absence of photocatalysts, the area of the ten PP particles was characterized. The initial average area was compared with the final one obtained. This approach allowed us to understand how the different radiation applied in this study would behave in the absence of the catalyst, obtaining a ratio in % degradation. Regarding UV-A, a degradation media in area of 2.23 and 2.13% for replicates 1 and 2, respectively, were obtained, while in the case of UV-B, a degradation of 3.13% and 3.64% was obtained for replicates 1 and 2, respectively. FTIR-ATR spectra corresponding to the UV-A and UV-B radiation test show that UV alone is not capable of producing changes in the PP structure since no banding was observed. This phenomenon has been reported in the literature [50], with no changes observed in microplastic samples exposed to sunlight in the absence of a catalyst (50 h) when compared to control.

Once the activity of UV light on the PP particles under study was evaluated, the effect of each of the synthesized catalysts on the polymer was studied (at a concentration of 2.5 g/L and 10 PP particles in 200 mL of ultrapure water in stirring for 48 h). A degradation media percentage of 3.32% (BiPO₄UV-A), 9.15% (Bi₂O₃/TiO₂UV-A), 10.81% (BiPO₄UV-B), and 5.79% (Bi₂O₃/TiO₂UV -B) was obtained. Therefore, we can conclude that the photocatalyst that showed the greatest PP degradation with respect to its area was BiPO₄ in UV-B. Bi₂O₃/TiO₂ photocatalyst showed a better performance in UV-A, being slightly inferior to BiPO₄ in UV-B.

The band-gap of a catalyst is strongly influenced by its crystalline structure, which determines its response to light irradiation [34]. Accordingly, each catalyst exhibits a characteristic band-gap and distinct photocatalytic behaviour. BiPO₄, owing to its relatively large band-gap, displays superior photocatalytic activity in the degradation of MPs under the more energetic UV-B light. Conversely, the Bi₂O₃/TiO₂ catalyst, with a narrower band-gap, performs more effectively under UV-A irradiation. It is worth noting that these catalysts not only differ in crystalline structure, but also in chemical composition, consistent with the observed band-gap values. As a result, a straightforward correlation between structure and catalytic activity cannot be established.

Concerning the degradation observed in the BiPO₄UV-B spectra (Figure 5a) using FTIR-ATR, a reduction in intensity resulted evident in the vas and vs -CH₃ and -CH₂- bands, as well as in δas and δs in the C-H plane. In the region between 650 and 1250 cm⁻¹ a broadening and dropping of the roll bands was observed (Figure 5a). In addition, another low-transmittance band appeared at 1200 cm⁻¹. For this same catalyst in UV-A, new bands appeared in the region between 650 and 1250 cm⁻¹ (Figure 5b).

Figure 5. Comparison of reference PP spectrum (▬) to PP subjected to BiPO₄UV-B (a) and BiPO₄UV-A (b).

For Bi₂O₃/TiO₂UV-A, the most characteristic bands undergo an intense decrease in transmittance. An intense band was formed at approximately 1200 cm⁻¹ and 1150 cm⁻¹. It is possible to see the elimination of bands below 1000 cm⁻¹. A band appeared, which dropped to 650 cm⁻¹, corresponding to the Bi2O₃/TiO₂ photocatalyst (Figure 6a). At UV-B for the same catalyst, this decrease in transmittance did not appear as intense. Nevertheless, a band at 1200 cm⁻¹ and 1150 cm⁻¹, in addition to another band falling at 650 cm⁻¹, was observed, corresponding once more to the Bi₂O₃/TiO₂ photocatalyst (Figure 6b).

Figure 6. Comparison of reference PP spectrum (▬) to PP subjected to Bi₂O₃/TiO₂UV-A (a) and Bi₂O₃/TiO₂UV-B (b).

PP photodegradation can be identified by the presence of new absorption bands, which are associated with carbonyl (-C=O) and hydroxyl/hydroperoxyl (-OH, -OOH) groups that are formed as by-products of the reaction [51] whose absorbances would be in the region of 1725 and 3500cm⁻¹, respectively. As can be observed in the spectra (Figure 5 and Figure 6), no absorption is observed in these areas, indicating a low degree of oxidation in the molecule. This is further confirmed by comparing the two spectra, which reveals that the carbonyl band may be limited in FTIR-ATR. Two factors contribute to this variation in the baseline: reflectance infrared spectra, which varies with respect to the initial PP, and the roughness of the PP surface [52].

In a limited number of the spectra of the particles analyzed, a small band associated with these two groups could be observed in some cases. There was a band around 1200 cm⁻¹, which may be indicative of the presence of reaction by-products such as the -C-O group. Significant changes in the absorbance peaks of the groups below 1167 cm⁻¹ may be due to the exposure of the carbon chain skeleton, which may be caused by surface photocatalytic degradation [53]. This implies that possibly not all PP microparticles undergo the same photodegradation, that the reduction in transmittance is indicative of size decrease in the microparticle [54], and that mainly surface photocatalytic degradation occurs.

Regarding the efficiency of bismuth-based photocatalysts in the degradation of microplastics, other authors have previously addressed this topic. They have proven that catalysts based on this element can be used for microplastic treatment in water matrices (Table 4). Ref. [55] studied the effectiveness of copper oxide/bismuth vanadate-based catalysts for degradation of LDPE, PP, and PA microplastics, achieving noticeable surface degradation and variations in the carbonyl and vinyl indices. Other authors such as [56] synthesized bismuth-based catalysts and tested their performance on degradation of PE microplastics. After 6 h of reaction, fragmentation of MPs occurred, leading to the formation of smaller micro-nanoplastics. In comparison with the commercial TiO₂ P25 catalyst [57,58], our catalysts display a similar performance. For example, photodegradation resulted in a 6% weight loss of polyamide (PA) after 105 h and a 13% weight loss of polyester after 48 h.

Table 4. Efficiency of bismuth-based photocatalysts for the degradation of microplastics.

Catalysts	MPs	Findings	References
BiPO4	PP	10.81% degradation	Present study
Bi2O3/TiO2	PP	9.15% degradation	Present study
Copper oxide/bismuth vanadate	LDPE, PP, PA	Noticeable surface degradation and variations in the carbonyl and vinyl indices	[55]
Synthesized bismuth-based	PE	Fragmentation of MPs to NPs occurred	[56]
Commercial TiO2 P25	PA	A 6% weight loss of PA after 105 h	[57]
Commercial TiO2 P25	PA	Decrease PA thickness with a weight loss of 13.5% in 48 h	[58]

As is the case of other contaminants, microplastic photocatalytic degradation generates intermediates and other products that need to be considered during the process. These compounds can be as harmful to the environment as microplastics themselves [59]. Therefore, it is essential to evaluate their presence and quantify them in order to avoid the negative consequences caused by microplastic degradation. However, these degradation by-products are produced only after a sufficient degradation of microplastics has been achieved. In this study, surface and incipient degradation was achieved, but it would be recommended to follow future studies including the presence of by-products derived from polypropylene degradation, such as alcohols, ketones, or esters, previously reported in similar studies [60].

In order to establish the possibility of catalyst leaching into the medium, the Bi and Ti content was analyzed by ICP once the catalyst was removed. The results obtained from the ICP-AES measurement of the reaction medium once the catalyst was removed were [Bi] less than 0.020 mg/L for BiPO₄ and Bi₂O₃/TiO₂. Bi₂O₃/TiO₂ had [Ti] under 0.005 mg/L. The results obtained are lower than the detection limit of the analytical method. The results indicate that there is no leaching of the elements present in the synthesized photocatalysts. This is an optimal result for future environmental applications.

3.4. Photocatalytic Activity for the Degradation of Polypropylene Microplastics in Wastewater

Considering that WWTPs are one of the main pathways through which MPs enter the aquatic environment [61], this study examines how varying from an ultrapure water matrix to a wastewater matrix influences the photocatalytic activity of photocatalysts on PP microparticles

In order to evaluate the efficiency of the photocatalysts under real conditions, it is necessary to understand the influence of the other possible parameters included in the process. Therefore, prior to the photocatalytic study, a characterization of the wastewater used in this experiment was carried out (Table 5). In this way, TOC measurements allowed for the evaluation of mineralization, and the analysis of anions and cations in the medium were essential to detect the inorganic ions present in the water. Inorganic ions tend to inhibit photocatalytic reactions by being adsorbed on the surface of the photocatalyst, competing with the pollutant species for the oxidizing species [62].

Table 5. Characterization of the principal parameters of wastewater used.

Parameters	Initial
TOC (mg/L)	12.11 ± 0.61
TN (mg/L)	45.68 ± 1.25
Sodium (mg/L)	186.58 ± 12.56
Ammonium (mg/L)	48.20 ± 3.21
Potassium (mg/L)	32.89 ± 1.65
Calcium (mg/L)	123.11 ± 12.21
Fluoride (mg/L)	1.5 ± 0.04
Chloride (mg/L)	252.22 ± 9.3
Nitrite (mg/L)	1.41 ± 0.12
Bromide (mg/L)	0.87 ± 0.02
Sulphate (mg/L)	137.5 ± 12.15
pH (pH units)	8.00 ± 0.03

Additionally, 2.5 g/L of photocatalyst (BiPO₄) was added in 200 mL of wastewater in addition to 10 PP microparticles, previously characterized by their area, and this was then subjected to UV-B radiation. The results show that, when compared to the experiments performed with a 10.81% ultrapure water matrix, the % degradation area was reduced, obtaining an average degradation of 8.24%. Hence, this reveals that the matrix variation reduces the degradation. This is unlike the BiPO₄UV-B test, in which there was a decrease in transmittance and the appearance of new bands in the fingerprint area. In this case, with water originating from the WWTP outlet, there was no modification of the structure of the PP microparticles, except for a small band observed at 1200 cm⁻¹, corresponding to C-O, as mentioned above, in which a small absorption corresponding to the carbonyl group (C=O) was observed (Figure 7a). The results of the area degradation test for the Bi₂O₃/TiO₂ photocatalyst were an average of 7.31% with 2.5 g/L in 200 mL of wastewater and 10 PP particles with UV-A radiation. This percentage is slightly lower than that for the other catalyst (8.24%) and the 9.15% with Bi₂O₃/TiO₂UV-A in ultrapure water. The spectrum showed no variation in the transmittance of vas and vs bands -CH₃ and -CH₂-, nor in δas and δs in the -C-H plane. The appearance of new bands around 1200 and 1000 cm⁻¹ and the drop and broadening of the PP bands in the fingerprint zone was observed. These bands, located below 1167 cm⁻¹, may form due to the exposure of the carbon chain skeleton, which may be caused by surface photocatalytic degradation (Figure 7b).

Figure 7. Comparison of reference PP spectrum (▬) compared to PP subjected to BiPO₄UV-B (a) and Bi₂O₃/TiO₂UV-A (b) in the photocatalytic process of a wastewater effluent.

For the values obtained for TOC and NT in the samples analyzed after the photocatalytic process had been completed in the wastewater matrix, the results were below the detection limit of the equipment (D. L. = 2 mg/L). Among the parameters studied, there was a significant increase in nitrate (144 ± 18 and 175 ± 72 for BiPO₄ and Bi₂O₃/TiO₂UV-A, respectively) and sulphate (296 ± 17 and 411 ± 106 for BiPO₄ and Bi₂O₃/TiO₂UV-A, respectively) concentrations. This is an indicator of the nitrification process and the degradation of organic matter. There is research showing that the presence of sulphate can affect the efficiency of photocatalytic performance, especially in the case of BiPO₄, which exhibits a positive effect [63]. Sulphate ions react with the -OH radical, generating SO₄²⁻, with this being the more oxidizing agent; however, this is not in accordance with the pH of the sample. According to the author, BiPO₄ at alkaline pH is negatively charged, suppressing the generation of SO₄²⁻. In addition, it has been determined that most ions and organic matter can inhibit or negatively affect the photocatalytic process [64].

4. Conclusions

Two bismuth-based photocatalysts have been synthesized to study their photocatalytic activity on polypropylene (PP). BiPO₄ was synthesized by the solvothermal route, resulting in a catalyst with a high band-gap (3.7 eV), which presents a bismuth composition of 64.4% as well as a monoclinic phase (P21/n). The Bi₂O₃/TiO₂ photocatalyst was synthesized by the incipient wet impregnation method; the presence of Bi₂O₃/TiO₂ showed to improve the light absorption in comparison with commercial TiO₂, presenting a composition of 9.2% in bismuth, as well as a cubic phase (I*3*) for Bi₂O₃ and tetragonal phase for TiO₂ P21/c.

The efficiency of the method resulted higher in the case of BiPO₄ in UV-B radiation and for Bi₂O₃/TiO₂ in UV-A radiation. In addition, there was no leaching into the study matrix, which implies an advantage with respect to future environmental applications. Synthesized photocatalysts based on bismuth demonstrated effectiveness in degradation, both in the ultrapure water matrix and in the wastewater matrix, obtaining, in the latter, a lower average degradation percentage. Based on the parameters studied, it was not possible to establish clear relations with respect to the effect of the matrix.

This study provides a solid background for future research on the photocatalytic degradation of microplastics under real conditions. Due to the complexity of real wastewater matrices, future work should aim to figure out the influence of matrix composition on degradation and removal efficiency. It would also be essential to investigate the performance of the most promising catalyst over a wide range of concentrations and under different ultraviolet light sources. Furthermore, exploring alternative catalyst supports and a broader spectrum of plastic polymer types could provide valuable insights into the versatility and scalability of the approach.