Z-Scheme Photocatalytic Degradation of Potassium Butyl Xanthate by a 2D/2D Heterojunction of Bi₂WO₆ Using MoS₂ as a Co-Catalyst

Abstract

Xanthate pollution during mineral processing is currently a major global challenge. Solar energy-induced photocatalysis is an effective and sustainable solution to this problem. The synthesis of specially designed photocatalysts is a critical step in this field of research. Even though a large number of structures have been developed in this regard, they are limited by low visible-light absorption and the fast recombination of photoinduced electron–hole pairs. To address these barriers, a novel nano-hybrid was developed with Bi₂WO₆ nanoflakes using MoS₂ nanosheets as the co-catalyst. The as-prepared hybrid was critically examined to confirm its successful formation and determine its inherent exclusive characteristics. Finally, the photocatalytic ability for xanthate removal from wastewater was measured. The bandgap energy of Bi₂WO₆ and MoS₂ was 2.85 eV and 1.87 eV, respectively, while that of the synthesized heterostructure was 2.54 eV. The photocatalytic ability of this novel structure is 6.65 and 9.65 times that of Bi₂WO₆ and MoS₂, respectively.

1. Introduction

Every year, approximately two billion tons of minerals are processed, accounting for 10% of global industrial wastewater. Water consumption in the mineral processing sector can vary widely depending on the specific process and the type of mineral being processed. On average, 5–50 m³ of water is required to process one ton of ore [1]. Flotation, in which a slurry of finely ground ore particles is mixed with water and chemical collectors, is an effective method for recovering a wide range of minerals. The choice of the collector depends on the type of mineral being processed and the specific flotation process. Xanthates are a common type of collector that are widely used because of their strong selectivity, high recoverability, low cost, versatility, and ease of use. Therefore, its consumption is increasing and is expected to reach 3.7 billion tons by 2025 [2].

Only 50% of xanthates are consumed during the process, while the remainder is discharged to tailing ponds. They cannot be recovered completely, and their residues drain into the effluent. They are stable in basic media; however, under acidic conditions, they degrade to CS₂, a highly toxic and flammable compound. Even in small quantities, xanthates deteriorate water quality and induce a malodor. Xanthates also have a high bioaccumulation factor in plants, which can affect algal metabolism even at concentrations of less than 1 mg/L. Xanthates disrupt the metabolism of human and animal cells, resulting in damage to the nervous system, liver, and consequently, other organs. In addition, they easily interact with other heavy metals in the environment and complex pollutants, ultimately leading to enzyme disruption and genetic mutations in organisms [3]. Therefore, the effective removal of xanthates from water effluents is crucial.

Chemical precipitation, adsorption, and biological and advanced oxidation methods are generally used to remove xanthates from water bodies [2]. Simple operation and easy control are the advantages of chemical precipitation methods; however, they require large amounts of chemicals and are restricted by large volumes of sludge. In addition, this pH-sensitive process requires large amounts of acids and alkalis [4]. Adsorption is a simple, efficient, and economical method for removing xanthate from water effluents. Its applicability is limited to very low concentrations, in addition to its regeneration and re-usability issues. The complete elimination of pollutants is impossible using this method [5]. Economic, eco-friendly, and simple operations are attractive features of the biological processes in which microorganisms are utilized to decompose xanthates. The major limitations of this process are its low efficiency and long duration. In addition, microbes decompose xanthate into alcohol and CS₂, which are toxic to microbes and trigger secondary contamination [6].

Advanced oxidation processes (AOPs) use reactive oxygen species (ROS) to effectively oxidize xanthates. These methods are relatively fast and efficient [7]. Strong oxidizing agents, such as hydroxyl, sulfate, superoxide, and hydroperoxyl free radicals, oxidize xanthates into completely harmless smaller molecules such as CO₂ and H₂O. Fenton oxidation is a relatively simple and cost-effective process that uses an Fe catalyst. This process requires additional chemicals, such as H₂O₂, for activation. The handling and disposal of hazardous byproducts, such as iron sludge, is also a major hurdle [8]. Ozone oxidation is a fast and effective process for xanthate removal. However, ozone is hazardous to workers; its generation is expensive, and it requires extensive pre-treatment [9]. Hydrogen peroxide is another strong oxidizing agent that degrades xanthates in wastewater. However, careful control of the pH and other water chemistry parameters is required. In addition to the high treatment costs, activation and sludge handling are other serious limitations [10].

Photocatalytic oxidation is more cost-effective than other advanced oxidation processes; it requires less chemical input and produces less waste. It uses light and a catalyst to oxidize organic pollutants in water effluents. The light-activated catalyst generates electron–hole pairs that react with water and oxygen to produce free radicals, which in turn oxidize xanthates [11]. Many of the well-established conventional catalysts are activated under UV light, which is only 3–5% of the solar spectrum. To overcome this serious limitation, a lot of attention has been paid to tailoring the photocatalyst to work effectively in visible light, which accounts for 42–43% of the solar spectrum [12]. The wide bandgap energy and rapid recombination of photogenerated electron–hole pairs greatly impede the practical implementation of this technology in the current industrial scenario.

Doping and semiconductor coupling are widely adopted techniques to improve the activity, selectivity, and stability of the photocatalyst and reduce its bandgap. Doping can be realized by embedding metal ions, such as copper, manganese, and iron, nonmetal elements, such as carbon, nitrogen, and sulfur, or popular noble metals, such as silver and gold [13]. For example, Bian et al. [14] significantly reduced the bandgap of TiO₂ to 2.24 eV by doping N and Ce into it. They removed 96.3% of sodium isobutyl xanthate under visible light irradiation. However, doping does not enhance the properties of all photocatalysts. Only a limited amount can be doped, and some dopants, such as heavy metals, are toxic. Scaling up the production of doped photocatalysts is difficult. Semiconductor coupling offers greater flexibility in controlling the bandgap and energy levels of materials. It offers greater stability and durability compared to doping; therefore, it is the most effective method for fabricating tailor-made photocatalysts with enhanced optical properties. These semiconductor-coupled nanocomposites exhibit enhanced photocatalytic activity due to their synergistic effects and the broadening of the wavelength range of light absorption [15].

Bismuth-based metal oxides, mainly Bi₂WO₆, have attracted the attention of researchers due to their composition, structure, and properties. They have a strong photo response, tunable structure, and high quantum yield. The perovskite-type layered configuration promoted good electronic conductivity. Its narrow bandgap (2.6–2.9 eV) facilitates visible-light absorption. Owing to its low cost, nontoxicity, and excellent thermal and chemical stabilities, it has become a prominent photocatalyst. However, the photocatalytic efficiency is poor owing to the limited response to visible light and sluggish transport. Small surface areas and faster recombination are additional disadvantages for its direct use [16,17]. The two-dimensional nanomaterial molybdenum disulfide (MoS₂) has narrow bandgap energy, excellent photo absorptivity, high charge-carrier mobility, and low toxicity and cost. It can utilize solar energy with a large absorption coefficient in the 400–500 nm wavelength range [18]. Based on these advantages, MoS₂ can serve as a co-catalyst to enhance the photocatalytic activity of Bi₂WO₆ for xanthate degradation.

Bi₂WO₆ is a high bandgap material while MoS₂ is a low bandgap material. This significant difference ensures that each semiconductor absorbs photons within distinct energy ranges. MoS₂ is considered to be a reducing semiconductor, while Bi₂WO₆ is its counterpart oxidizing semiconductor. In general, there is a high possibility of forming an effective Z-Scheme heterostructure with the combination of oxidizing and reducing semiconducting materials. Therefore, it is possible to fabricate a unique 2D/2D heterostructure with strong interfacial interactions and a large number of active sites with the combination of Bi₂WO₆ and MoS₂. This type of heterostructure is expected to promote rapid charge separation, extend light-harvesting ability, and suppress charge–carrier recombination.

2. Results and Discussion

2.1. Characterization of the Photocatalysts

2.1.1. Phase Analysis

Phase analysis by X-ray diffraction is important for examining the crystal structure, composition, and phase of the as-synthesized pristine and novel hybrid materials. The XRD pattern of pristine Bi₂WO₆ (Figure 1) indicates that the main diffraction peaks (∗) at 28.23°, 32.79°, 47.12°, and 55.86° are due to diffraction of the crystal faces (113), (200), (220), and (313), respectively. JCPDS card No. 73-1126 indicates the successful formation of Bi₂WO₆ in the orthorhombic phase [19]. In Figure 1, the peaks (♠) at 14.03°, 33.31°, and 58.91° indicate (002), (101), and (110) planes of pure MoS₂. This matches well with the hexagonal phase of MoS₂ (JCPDS card No. 37-1492) [20]. The Bi₂WO₆/MoS₂ composite profile was also similar to that of Bi₂WO₆, indicating that MoS₂ insertion did not disturb the crystal structure of Bi₂WO₆ [21]. However, slight peak shifts were observed at 14.07°, 28.34°, 32.83°, 47.09°, 55.81°, and 58.72°, indicating the successful formation of the composite.

2.1.2. Morphology Analysis

The surface morphology was identified using SEM analysis. Figure 2a,b shows the Bi₂WO₆ nanoflake bunches under 1 µm and 500 nm magnifications, respectively. The small flakes agglomerated into spherical flowers with different orientations. MoS₂ formed small flower petal-like sheets without clustering (Figure 2c,d). Figure 2e,f shows the morphology of the as-prepared nanocomposite under 500 nm magnification. A major portion of the nanocomposite is composed of Bi₂WO₆ flakes with MoS₂ petals between them. This indicates the successful formation of the composite. The samples were subjected to elemental analysis (EDS). A summary of this analysis is presented in Figure 3. Figure 3a shows an image of the sample being analyzed; Figure 3b–f indicate the presence of each element in the composition and its uniform distribution throughout the sample, and Figure 3g shows the quantitative analysis of each element in terms of the weight and atomic percentage. Bismuth and tungsten were the major elements because their input was 10 times greater than that of MoS₂. This agrees with the results shown in Figure 2e,f. TEM can provide high-resolution images with fine details. Electron penetration provides a high-dimensional image of the sample. Figure 4a–d show the TEM images of the prepared samples at resolutions of 100, 50, 20, and 10 nm, respectively. The Bi₂WO₆ flakes and MoS₂ sheets became distinctly visible as the magnification increased. The lattice fringes (Figure 4e) with a d spacing of 0.315 nm and 0.62 nm represent (113) lattice plane of orthorhombic Bi₂WO₆ and hexagonal plane (002) of MoS₂. The diffraction spots of the corresponding SAED pattern in Figure 4f indicate the simultaneous presence of Bi₂WO₆ and MoS₂.

2.1.3. Elemental Analysis

X-ray photoelectron spectroscopy (XPS) is a powerful analytical tool for detecting the chemical composition and electronic structure of materials at the atomic level. The peaks listed in the survey spectrum (Figure 5a) indicate the presence of all constituent elements (that is, Bi, W, O, Mo, and S) in the composite. High-resolution Bi 4f, W 4f, O 1s, Mo 3d, S 2s, and S 2p spectra are presented in Figure 5b–e. The peak positions, attributing elements, and corresponding electronic states are summarized in

Table 1. X-ray photoelectron spectroscopy analysis of Bi₂WO₆/MoS₂ nanocomposite.

Peak Position (eV)	Attributed to	Electronic State	References
159.93	Bi 4f7/2	Bi3+	[23]
165.38	Bi 4f5/2
35.12	W 4f7/2	W6+	[24]
37.27	W 4f5/2
529.8	O 1s	Bi-O	[25]
530.7		W-O
228.64	Mo 3d5/2	Mo4+	[26]
231.73	Mo 3d3/2
162.17	S 2p3/2	S2−	[27]
163.43	S 2p1/2

. As per the results portrayed in the table, the chemical states of bismuth, tungsten, molybdenum, and sulfur are Bi³⁺, W⁶⁺, Mo⁴⁺, and S²⁻, respectively. Moreover, the new peaks at 227.93 eV and 230.97 eV attributed to Mo⁺⁵ oxidation state, which may occur due to S-Mo charge transfer but not due to MoO₃ [22]. The peaks of O 1s at 529.8 eV and 530.7 eV correspond to W–O and Bi–O bonds, respectively.

2.1.4. Bandgap Analysis

The photocatalytic performance of a material depends on its optical properties, which are usually expressed in terms of bandgap energy. The bandgap energy of the material was calculated based on the UV–vis diffuse reflectance spectra (DRS). The DRS spectra of pristine and composite materials are shown in Figure 6a. The light-harvesting capacity of Bi₂WO₆ is poor in the visible region. However, when coupled with MoS₂, the capacity considerably improved. The bandgap energies of pure Bi₂WO₆ is 2.85 eV (Figure 6b), and that of MoS₂ is 1.87 eV (Figure 6c). The value for the composite was 2.54 eV.

2.2. Photocatalytic Activity

As described in the previous section, the Bi₂WO₆/MoS₂ nanocomposites were successfully prepared to enhance the light-harvesting capacity of its basic material Bi₂WO₆. In this section, the photocatalytic abilities are analyzed. The relevance and significance of xanthate removal were explained in the Introduction. The feed concentration, volume, catalyst dosage, and sampling procedure are described in the experimental section. The photocatalytic ability of the composite was tested along with that of pristine materials. A blank experiment was conducted to account for the self-degradation of PBX. The experiments began with dosage effects. Three experiments were conducted using 5, 10, and 15 mg of as-prepared composites. The results were calculated as a percentage removal and are presented in Figure 7a. The highest reduction occurred at a dose of 10 mg; therefore, this dose was used as the model for further experiments. Figure 7b shows the absorption profiles of PBX at different time intervals during photocatalysis using 10 mg of the synthesized nanocomposite. It showed distinct absorption at a wavelength of approximately 300 nm. No significant reduction in peak intensity was observed for the first 30 min of agitation under dark conditions. This indicates a mild effect of adsorption on the process. However, a significant difference was observed during the first 10 min of solar light irradiation. Following the same trend, complete mineralization was observed within 50 min. This is primary evidence of the catalytic activity of the prepared composites. Moreover, its performance was compared with that of pristine materials. As shown in Figure 7c, the results of the blank experiment indicate no self-degradation of the feed solution during the investigation. It also portrays the removal trends of pristine Bi₂WO₆ and MoS₂, which could remove approximately 40% and 35% of xanthate (PBX) in the same period in which the as-prepared composite degraded almost one hundred percent. These results indicate the success of this nanocomposite with an upgraded light-harvesting capacity compared with its pristine counterparts. The kinetic parameters are an additional measure of the composite caliber. They play critical roles in the design of photo-reactors. As proven by many researchers, pseudo-first-order kinetics accurately represent the photocatalytic degradation of organic pollutants in water effluents. The same is true in this case, as per Figure 7d.

The data following the linear trend show the goodness of fit for this model. Rate constants were estimated based on these linear fits. The values for Bi₂WO₆, MoS₂, and the Bi₂WO₆/MoS₂ composite were 0.01135, 0.00782, and 0.07549 min⁻¹, as shown in Figure 7e, and the rate constant value of the composite was very high compared to the other two. The performance of the composite was 6.65 times superior to that of Bi₂WO₆ and 9.65 times that of MoS₂. Therefore, the results prove the hypothesis proposed in the Introduction: semiconductor coupling by joining Bi₂WO₆ with MoS₂ can result in a highly efficient photocatalyst. This has turned out to be true, with strong evidence of successful composite formation. Catalyst stability and recyclability are also important in the era of economic and stringent environmental constraints. From this perspective, the catalyst was reused five times consecutively, and the PBX removal efficiency was calculated. As per the results shown in Figure 7f, no significant decline in the catalyst performance was observed, indicating the stability and reusability of the as-prepared nanocomposite.

2.3. Plausible Photocatalytic Mechanism

The intrinsic mechanism of the process was investigated. When light is incident on a photocatalyst, the electrons are excited and move to the conduction band, leaving behind holes in the valence band. These electrons and holes interact with the surrounding water molecules and generate free radicals, which in turn degrade organic pollutants. This is the typical mechanism for pristine photocatalysts. However, in the composites, these electrons and holes migrate from one energy level to another to balance the overall electric potential. Based on their migration path and final positions, they generate free radicals that actively participate in the reaction. These free radicals can be identified by examining the various trapping agents. In this study, triethanolamine (TEA), p-benzoquinone (BQ), and isopropyl alcohol (IPA) were used to trap holes and superoxide and hydroxyl free radicals. The degradation studies were repeated by adding these agents to determine their effects. Figure 8 shows the results of these studies. The addition of BQ and TEA did not have a considerable effect on the process, whereas the impact of IPA is distinctly visible in Figure 8. This indicates the critical role of hydroxyl free radicals in xanthate degradation. Hydroxyl is the most reactive species compared to others due to its high reduction potential of 2.8 V. It attacks rapidly with a high rate constant [28]. Therefore, the degradation rate was extremely high, resulting in complete removal within 50 min.

The identification of the exact band-edge position is also important for investigating the electron and hole migration pathways and the corresponding free-radical generation. The band-edge positions were calculated using the standard procedure presented in the literature [29]. Additionally, the valence band (VB) and conduction band (CB) potentials are measured using the empirical formulae: E_VB = χ − E_e + (0.5) E_g; E_CB = E_VB − E_g. Where E_VB and E_CB are the VB and CB potentials, respectively. χ is the electronegativity of the semiconductor; for Bi₂WO₆ and MoS₂, the χ values are 6.23 and 5.35 eV, respectively. E_g and E_e refer to the bandgap energy and energy of free electrons on the hydrogen scale (~4.5 eV), respectively. The values for pristine Bi₂WO₆ are 0.3012 and 3.1511 eV, and those for MoS₂ are −0.0873 and 1.7826 eV. As seen from the band-edge positions depicted in Figure 9, the highest conduction band value is for MoS₂, that is, −0.0873 eV. Since it is less than −0.33 eV vs. NHE (standard reduction potential), the production of superoxide radicals is not possible; however, it is sufficient to produce hydroxyl radicals [30]. This is in agreement with the results of the scavenger studies. Similarly, only Bi₂WO₆ exhibits a redox potential (E_VB = 3.1511 eV; E(OH⁻/^•OH = 1.96 eV) to produce ^•OH radicals. If the charge transfer is a Type II mechanism, holes must migrate from the VB of Bi₂WO₆ to that of MoS₂ and directly participate in the degradation process; however, this contradicts the results from the scavenger studies. In addition, the energy gap between the valence band of MoS₂ and the conduction band of Bi₂WO₆ was small (1.4815 eV). When Bi₂WO₆ and MoS₂ join, the free electrons of MoS₂ flow to those of Bi₂WO₆ to balance the Fermi level [31]. Therefore, the MoS₂ side was positively charged, and the Bi₂WO₆ side was negatively charged at the interface, resulting in a built-in electric field. The photogenerated electrons in the CB of Bi₂WO₆ could easily join the holes in the VB of MoS₂. This Z-scheme improves the separation efficiency of electrons and holes, producing highly active catalysts. This extended life of the free radicals improves the effective breakdown of PBX. Therefore, the as-prepared heterojunction completely degraded the PBX in 50 min.

3. Material and Methods

3.1. Photocatalysts Synthesis and Characterization

Bi₂WO₆ nanoflakes were prepared using a simple hydrothermal method. A total of 4.365 g of Bi(NO₃)₃·5H₂O was added to 30 mL of Millipore water under constant stirring. Sodium tungstate solution was prepared in the same quantity as the bismuth nitrate solution by adding 1.485 g of sodium tungstate dihydrate (Na₂WO₄·2H₂O) to 30 mL Millipore water. This solution was added dropwise to the bismuth nitrate solution and stirred for 60 min. Subsequently, it was transferred into a 100 mL TLSS (Teflon-lined stainless steel) autoclave, sealed, and heated at 200 °C for 24 h. MoS₂ nanosheets were prepared in the same fashion by mixing 2.48 g of ammonium molybdate tetrahydrate and 4.56 g and thiourea in 70 mL of Millipore water. Both products were cleaned via centrifugation using ethanol and water in an alternating manner and dried at 80 °C for a day to yield the final products. Similarly, the composite was synthesized using the prepared Bi₂WO₆ nanoflakes and MoS₂ nanosheets in a 10:1 ratio. Two solutions were prepared: one by adding 200 mg Bi₂WO₆ to 100 mL and another by adding 20 g MoS₂ to 20 mL ethanol. The MoS₂ solution was then added dropwise to the Bi₂WO₆ solution and mixed thoroughly for two hours. The uniform solution was placed in a 200-mL TLSS, sealed, and heated at 200 °C for 16 h. The final product, that is, the composite, was obtained by the aforementioned drying process. The details of the characterization and instrumentation specifications are reported in our previous studies [12].

3.2. Photocatalysis Experiments

A potassium butyl xanthate (PBX) solution (20 mg/L) was prepared accurately, and 5, 10, and 15 mg of the as-prepared composite were added to 50 mL of this solution to yield three separate solutions. The contents were continuously stirred for 30 min in the dark to determine the effects of adsorption. Therefore, solar light was used to activate the catalyst. PBX samples were withdrawn every 10 min and centrifuged for 5 min. The supernatant was collected, and its concentration was measured using a UV–vis spectrophotometer. Degradation was measured in terms of the concentration ratio; degradation kinetic models were tested, and corresponding parameters were estimated using the results reported in our previous studies [32].

4. Conclusions

Bi₂WO₆ is a low-cost and stable photocatalyst. However, its efficiency is low due to limited photo absorption and sluggish transport of electrons and holes. It was coupled with MoS₂ to adjust its bandgap and enhance its photocatalytic ability. All the characterization studies indicated the successful formation of heterojunctions. The oxidative states of the elements were Bi³⁺, W⁶⁺, Mo⁴⁺, and S^2–. The bandgap energies of Bi₂WO₆, MoS₂, and the composite were 2.85, 1.87, and 2.54 eV, respectively. The band-edge position values for pristine Bi₂WO₆ were 0.30115509 eV and 3.15116 eV, and for MoS₂, they were −0.0873509 and 1.78265 eV. The degradation followed pseudo-first-order kinetics, with a high-rate constant value of 0.07549 min⁻¹. Hydroxyl radicals play a critical role in this process, and their strong reduction potential leads to their rapid degradation. This z-scheme mechanism improves the separation efficiency of electrons and holes, thereby producing a highly active catalyst. Only 10 mg of the as-prepared catalyst completely removed PBX in 50 min. The photocatalytic activity is 6.65 times that of Bi₂WO₆ and 9.65 times that of MoS₂. The prepared catalysts were stable even after the usage of five consecutive cycles.