Degradation of Sulfamethoxazole Using a Hybrid CuO_x–BiVO₄/SPS/Solar System

Abstract

In recent years, advanced oxidation processes (AOPs) demonstrated great efficiency in eliminating emerging contaminants in aqueous media. However, a majority of scientists believe that one of the main reasons hindering their industrial application is the low efficiencies recorded. This can be partially attributed to reactive oxygen species (ROS) scavenging from real water matrix constituents. A promising strategy to cost-effectively increase efficiency is the simultaneous use of different AOPs. Herein, photocatalysis and sodium persulfate activation (SPS) were used simultaneously to decompose the antibiotic sulfamethoxazole (SMX) in ultrapure water (UPW) and real water matrices, such as bottled water (BW) and wastewater (WW). Specifically, copper-promoted BiVO₄ photocatalysts with variable CuO_x (0.75–10% wt.) content were synthesized in powder form and characterized using ΒΕΤ, XRD, DRS, SEM, and HRTEM. Results showed that under simulated solar light irradiation alone, 0.75 Cu.BVO leads to 0.5 mg/L SMX destruction in UPW in a very short treatment time, whereas higher amounts of copper loading decreased SMX degradation. In contrast, the efficiency of all photocatalytic materials dropped significantly in BW and WW. This phenomenon was surpassed using persulfate in the proposed system resulting in synergistic effects, thus significantly improving the efficiency of the combined process. Specifically, when 0.75 Cu.BVO was added in BW, only 40% SMX degradation took place in 120 min under simulated solar irradiation alone, whereas in the solar/SPS/Cu.BVO system, complete elimination was achieved after 60 min. Moreover, ~37%, 45%, and 66% synergy degrees were recorded in WW using 0.75 Cu, 3.0 Cu, and 10.0 Cu.BVO, respectively. Interestingly, experimental results highlight that catalyst screening or process/system examination must be performed in a wide window of operating parameters to avoid erroneous conclusions regarding optimal materials or process combinations for a specific application.

1. Introduction

In the last decade, scientists have pointed out the urgent need to develop and apply new wastewater treatment technologies able to eliminate emerging contaminants such as pharmaceuticals and endocrine-disrupting compounds (EDCs) detected at trace levels in environmental systems [1,2,3,4]. To this end, the European Commission is currently reviewing urban wastewater treatment and is considering including, where possible, requirements for elimination of micropollutants from domestic wastewater. Specifically, in early 2022, the Commission stated that drinking water across the EU should be monitored more closely for the possible presence of two endocrine disruptors (β-estradiol and nonylphenol) throughout the water supply chain, with more chemical substances expected to be added to this “watch-list” soon.

To address this challenge, the application of advanced oxidation processes (AOPs) involving the in situ production of reactive species, which degrade organic matter in efficient and usually non-selective ways, has attracted researchers’ interest. AOPs constitute a family of similar (but not identical) processes including heterogeneous photocatalysis, electrochemical oxidation, (photo) Fenton, UV/oxidants, ozonation/H₂O₂, and ultrasound [1,5].

Among AOPs, heterogeneous photocatalysis is probably considered the greenest technology, as it has the potential to use sunlight to initiate reactions for micropollutant degradation. Moreover, it eliminates the requirement for a constant supply of precursor chemicals, making it particularly attractive for applications in remote places [6,7]. Sulfamethoxazole (SMX), a well-known antibiotic identified in surface water, has been the subject of many studies regarding the efficiency of photocatalytic systems [8,9]. Parabens, another class of emerging contaminants found in various environmental matrices, have been photocatalytically treated with promising results [10,11].

However, photocatalytic industrial applications in wastewater treatment are still minimal. The already reported low quantum yields are mainly attributed to the small portion of reactive oxygen species (ROS) generated in a photocatalytic system that eventually participates in the degradation reactions. This observation is strongly related to scavenging phenomena due to the presence of a significant variety of organic and inorganic species in real water matrices not considered in most studies carried out in UPW [2,6]. The majority of photocatalytic systems exhibit significantly lower performance in water matrices of environmental concern than in UPW; therefore, the effect of the water matrix is a crucial criterion that should be taken into account to make photocatalysis more attractive for commercial applications. Another factor that suppresses photocatalytic efficiency is the high recombination rate of photogenerated species that characterizes most visible light active semiconductors used as photocatalysts. In attempts to deal with this, many new materials configurations, such as heterojunctions, doped samples, and so on, were introduced as a promising approach to enhance photocatalytic quantum yield [12,13,14,15]. However, the photocatalytic materials with improved properties were not necessarily accompanied by higher performance regarding pollutant decomposition.

To overcome these limitations, some researchers suggested that photocatalysis could be applied together with other AOPs, such as persulfate oxidation [3]. In persulfate oxidation, sulfate radicals (SO₄^●−) are produced after persulfate activation. Typically, heat or UV irradiation is used to break O-O bonds in the persulfate structure and the formation of SO₄^●− [16]. However, to decrease the energy requirements, an alternative activation method involving reactions with transition metals has been proposed in recent years [17]. The benefits of that process are multiplied when the system transition metal/persulfate works heterogeneously, as it avoids secondary pollution due to metal leaching. At first, the most promising results were from using copper or cobalt-based oxides as heterogeneous activators [18]. In recent studies, other configurations, such as phosphides [19] or perovskites [20], have shown high catalytic performance in micropollutant degradation systems. For example, diclofenac degradation in water was studied in a peroxymonosulfate/LaFeO₃ heterogeneous system [21], and lanthanum cobaltite perovskite was proposed as a promising peroxymonosulfate activator for carbamazepine degradation [22].

The addition of persulfate in a photocatalytic system has a potential dual benefit. Persulfate can react with photogenerated electrons, thus suppressing the recombination rate and enhancing photocatalytic reaction rate. At the same time, it can be activated by light irradiation and the appropriate photocatalytic material, producing highly reactive sulfate radicals that can participate in micropollutant degradation. However, the maximum benefit is gained only if the interaction between the two systems results in a synergistic, rather than cumulative, effect.

Taking the aforementioned into consideration, in the present study, SMX degradation was tested in a hybrid solar/SPS/CuO_x/BiVO₄ system in real water matrices. Copper-promoted BiVO₄ samples were selected as photocatalysts based on our previous studies that showed, on the one hand, their high photocatalytic efficiency for micropollutant degradation in ultrapure water (UPW). However, the oxidation of pollutants was inhibited in secondary effluent (WW) [23]. On the other hand, previous studies showed the demonstrated activity of copper configurations as efficient heterogeneous persulfate activators [19,24]. Specifically, excellent results have been reported for a CuO/persulfate system for 2,4-dichlorophenol degradation and a magnetic CuO-Fe₃O₄/persulfate system for phenol degradation [25,26]. Furthermore, apart from CuO_x formulations, an up-to-date work proved the applicability of Cu₃P as a heterogeneous sodium persulfate (SPS) activator for the degradation of the antibiotic agent sulfamethoxazole (SMX) [19]. Based on these findings, and considering the low cost and toxicity of copper, CuO_x was adopted in the present system. Μonoclinic bismuth vanadate (mBiVO₄) has been recognized as one of the most promising photocatalysts. This is due to its narrow band gap of 2.3–2.5 eV, low synthesis cost and low toxicity [23,27]; mBiVO₄ has been used in various photocatalytic applications such as decomposition of micropollutants, production of hydrogen through water splitting, and elimination of pathogenic microorganisms [28,29]. However, the photocatalytic efficiency of mBiVO₄ is very low, due to the high recombination rate of photogenerated electrons and holes [30]. Kanigaridou et al. [23] mentioned the beneficial role of copper oxide coupling with BiVO₄ in the photocatalytic destruction of the endocrine disruptor bisphenol A (BPA). They reported that the apparent rate constant (k_app) in 0.75 wt% CuO_x/BiVO₄ was almost twice the k_app of pure BiVO₄ [23].

The main goal of the present study is to add important information regarding the efficiency of the hybrid photocatalytic/persulfate system for effective micropollutant degradation in water to the literature. As far as we know, this is the first time that SMX degradation has been studied in real water matrices in the proposed hybrid system. The novelty of the present study also lies in the fact that photocatalysts were tested in all water matrices, thus avoiding misleading conclusions arising from the fact that in most reported studies the screening of photocatalytic materials takes place only in ultrapure water.

Copper-promoted BiVO₄ samples were synthesized using a polyol-reduction method and characterized utilizing BET, XRD, DRS, TEM/HRTEM, and SEM/EDS. The effectiveness of the hybrid solar/SPS/CuO_x/BiVO₄ system for SMX degradation was studied in ultrapure water (UPW), bottled water (BW), and secondary treated wastewater (WW).

2. Results and Discussion

According to XRD analysis, all photocatalysts used in the present study are characterized by the scheelite-monoclinic phase of BiVO₄ (JCPDS No. 14-0688) with no additional peaks due to copper species being discerned in copper-promoted samples. Characteristic XRD patterns of pure BiVO₄ and 3.0 Cu.BVO are shown in Figure 1.

Their specific surface area, as determined utilizing the BET method, was found to be <2 m²/g in accordance with previous studies [31]. The samples’ optical properties were examined employing UV-Vis DRS, as shown in Figure 2; it is observed that pure BiVO₄ can absorb light at wavelengths < 550 nm, corresponding to an optical band gap of 2.25–2.30 eV. The addition of copper does not significantly alter the position of the absorption edge, but it leads to increased absorption at longer wavelengths, which increases as Cu content increases. Their morphology was further examined through SEM/EDS and TEM/HRTEM analysis, as shown in Figure 3. SEM images of the 0.75 Cu.BVO and 3.0 Cu.BVO catalysts with EDS mapping of Cu (Figure 3A–D) show that copper is homogeneously distributed on the surface of BiVO₄. Similar results were obtained for all copper-promoted samples (not shown for brevity). The HR-TEM image of pure BiVO₄ (Figure 3F) shows the interplanar spacings of 0.31 nm and 0.47 nm, which correspond to the (121) and (110) planes of monoclinic BiVO₄, respectively. The presence of the CuO phase with a crystal size of less than 5 nm was confirmed for the 3.0 Cu.BVO sample. As shown in Figure 3E, CuO nanoparticles are uniformly dispersed on the BiVO₄ surface and are in intimate contact with the BiVO₄ nanocrystals. Qualitatively similar results were obtained for xCu.BVO samples with lower Cu loading. It should be noted, however, that the presence of Cu₂O nanoparticles should not be excluded. This is because of the low crystallinity of the observed CuO_x nanoparticles and the quite similar (0.25 nm and 0.24 nm) d-spacing values of the (111) planes of the CuO and Cu₂O phases. Details concerning their physicochemical characteristics can be found elsewhere [23,32].

In order to assess the photocatalytic efficiency of the samples, a set of experiments dealing with 0.5 mg/L SMX photodecomposition under solar irradiation in UPW was undertaken. As shown in Figure 4A, 0.75 Cu.BVO is the most photocatalytically active copper-promoted sample; its k_app is equal to 0.0991 min⁻¹. Further addition of copper species did not favor SMX degradation, resulting in lower k_apps equal to 0.0062 and 0.0079 min⁻¹ in 3.0 Cu.BVO and 10.0 Cu.BVO, respectively. The observed lowering of photocatalytic activity at higher copper oxide loadings can be attributed to the formation of bulk agglomerates which act as recombination centers and hinder the irradiation of BiVO₄, thus lowering the number of photogenerated species [33,34,35].

On the contrary, as shown in Figure 4B, apparent kinetic constants for all copper loadings tested were some orders of magnitude lower in real water matrices due to the presence of co-existing inorganic and organic ions competing with SMX for ROS [36] and catalyst surface. Specifically, in BW, 0.75 Cu.BVO was also the best performing photocatalytic material; however, its k_app was found to equal 0.0433 min⁻¹. Interestingly, its k_app decreased to 0.0191 min⁻¹ for 3.0 Cu.BVO, whereas it was found to equal 0.0227 min⁻¹ for 10.0 Cu.BVO. This observation implies that the optimum loading of a co-catalyst, such as metal oxides, is closely connected with the experimental parameters, such as water matrix, in agreement with other studies [37]. This behavior was further confirmed by experiments carried out in WW. As shown in Figure 4B, SMX degradation was faster when a copper-promoted sample with higher copper loading, 10.0 Cu.BVO, was used. In addition, a 0.75 Cu.BVO sample that showed the best photocatalytic results in UPW and BW was now characterized by the lowest k_app. In general, retarding phenomena were more intense in WW than in BW, making SMX degradation practically non-viable.

In order to moderate hindering phenomena in real water matrices, SPS was added to the photocatalytic system, and the degree of synergy was quantified. At first, the efficiency of the hybrid system solar/SPS/catalyst was studied in UPW; results are shown in Figure 5.

Figure 5A illustrates that 0.75 Cu.BVO is characterized by high photocatalytic activity which resulted in complete 0.5 mg/L SMX decomposition after 30 min of irradiation (solar/0.75 Cu.BVO system). Considering its activity toward SPS activation (SPS/0.75 Cu.BVO), SMX degradation did not exceed 60% in 60 min. SPS activation was more restricted using solar irradiation alone (solar/SPS). When solar irradiation and SPS were applied together, complete SMX removal was achieved in 30 min, resulting in a zero synergy degree. In contrast, Figure 5B and

Table 1. Apparent kinetic constants and synergy degree for all CuO_x loadings examined in UPW, BW, and WW.

	Matrix	ktotal, 10−2 min−1	S (%)	R2
0.75 Cu.BVO	UPW	11.33	0	0.998
	BW	4.33	75.5	0.966
	WW	0.67	37	0.984
3.0 Cu.BVO	UPW	9.95	71.6	0.989
	BW	2.27	61.1	0.968
	WW	0.92	45	0.997
10.0 Cu.BVO	UPW	8.45	68.2	0.975
	BW	1.91	70.4	0.932
	WW	0.85	66.5	0.951

show that for 3.0 Cu.BVO the combined process resulted in 71% synergy.

Considering the individual processes, 3.0 Cu.BVO was less photocatalytically active and moderately activated SPS. Qualitatively similar results were obtained for 10.0 Cu.BVO (Figure 5C). Moreover, the amount of copper leached in the liquid phase in the solar/CuO_x/SPS system was determined to be very low at the end of the run, corresponding to only 0.2% of the Cu contained in the 500 mg/L of the sample with the higher copper content (10.0 Cu.BVO).

Results for the combined processes are summarized in Figure 6; complete SMX degradation occurred in 30 min of irradiation for all catalysts tested. This is attributed to the high photocatalytic activity of 0.75 Cu.BVO and the existence of powerful synergistic effects in samples with higher copper loading.

In order to gain insight into the efficacy of the present system towards SMX degradation,

Table 2. SMX degradation in different solar/SPS/photocatalyst systems.

	SMX Concentration (Mg/L)	Catalyst Concentration (Mg/L)	Persulfate Concentration (Mg/L)	k (Min−1)	Time Period for Complete Degradation (Min)	Ref.
Solar/SPS/Biochar (BC)	0.25	90	250	0.065	90	[38]
Solar/SPS/Cu3P	0.5	40	100	0.114	20	[19]
vis/PDS/MIF-100(Fe)	10	500	1000	0.012	180	[39]
solar/PDS/g-C3N4	1	500	120	0.068	60	[40]
Solar/SPS/3.0 Cu.BVO	0.5	500	100	0.099	25	Present study

summarizes the main characteristics of other solar/SPS/photocatalyst systems demonstrating SMX degradation. Kemmou et al. [38], who studied the degradation of 0.5 mg/L SMX using -activated persulfate, reported longer time periods required for complete SMX degradation than those reported in Figure 5. Alexopoulou et al. [19] used Cu₃P as a heterogeneous persulfate activator. They found that SMX degrades quickly in UPW; the degradation rate depends on parameters such as SMX, SPS, and catalyst concentrations. Their results are of the same order of magnitude as the present study. Furthermore, Yin et al. [39] found that SMX was rapidly eliminated in the vis/PDS/MIF-100(Fe) system, and Song et al. [40] investigated the degradation of SMX in a solar/PDS/g-C₃N₄ system.

SMX degradation profiles in BW under solar/SPS, solar/0.75 Cu.BVO, SPS/0.75 Cu.BVO, and solar/SPS/0.75 Cu.BVO systems are shown in Figure 7A. In the presence of only 100 mg/L SPS under simulated solar irradiation, 40% SMX degradation was observed after 120 min, showing the limited activation of SPS by sunlight. Similar SMX removal was recorded in the solar/0.75 Cu.BVO system. Furthermore, 0.75 Cu.BVO showed restricted activity as a SPS activator, resulting in only 30% SMX removal in the same period. However, when 0.75 Cu.BVO and SPS were added to the system, complete 0.5 mg/L SMX degradation was obtained in 60 min. The synergy, as quantified by Equation (2), was 75%. Qualitatively similar results were recorded for the 3.0 Cu.BVO sample.

Specifically, despite the higher amount of copper species on the surface of BiVO₄ that could lead to increased efficiency towards persulfate activation, SMX degradation was less than 20% in the SPS/3.0 Cu.BVO system, as shown in Figure 7B. However, the combination with simulated solar irradiation strongly enhanced SMX degradation, resulting in complete degradation after 120 min of irradiation. The degree of synergy was 61%. Considering the efficiency of the sample with the highest copper loading towards SMX degradation in BW (Figure 7C), it is observed that it was practically inactive in both systems examined (solar/10.0 Cu.BVO and solar/SPS/0.75 Cu.BVO), resulting in less than 10% SMX removal. In BW, all materials tested led to complete SMX degradation in the hybrid system, whereas SPS activation due to sunlight was lower than 40%. In addition, copper-promoted BiVO₄ samples showed low efficiency toward persulfate degradation.

Figure 8 outlines results obtained in BW in hybrid systems for all photocatalytic materials tested; SMX degradation was favored in the 0.75 Cu.BVO sample, but increasing copper loading slowed down the degradation kinetics. In contrast, the degree of synergy was similar in all cases (Figure 8B), implying that it is independent of the process’s overall performance.

Significantly lower yields were recorded in WW (Figure 9). It should be noted that in that case, 500 mg/L SPS were added to the photocatalytic reactor. Specifically, as shown in Figure 9A, practically no SMX degradation occurred in the solar/0.75 Cu.BVO system and less than 20% in the SPS/0.75 Cu.BVO. SMX degradation increased to 50% in the combined solar/SPS/0.75 Cu.BVO, resulting in 37% synergy. Synergistic rather than cumulative effects were also observed in WW for the solar/SPS/3.0 Cu.BVO system, as shown in Figure 9B and Table 2, with the degree of synergy increasing to 45%.

In agreement with results obtained in BW, 3.0 Cu.BVO showed very low efficiency towards the activation of SPS (SPS/3.0 Cu.BVO). Figure 9C shows results obtained for the 10.0 Cu.BVO catalyst; SPS cannot practically be activated either by sunlight or by 10.0 Cu.BVO in WW. However, SMX removal reaches 60% after 120 min of irradiation in the hybrid system. In general, the SMX degradation rate was lower in WW than in BW, as expected, due to the increased complexity of the water matrix. Interestingly, in that case (Figure 10) 3.0 CuBVO and 10.0 Cu.BVO showed similar catalytic activity in the combined system, whereas both the k_app and synergy degree were lower for 0.75 Cu.BVO.

Additional experiments using appropriate scavengers for different reactive species were performed to investigate the mechanism of SMX decomposition; results are shown in Figure 11. More specifically, EDTA was used to bind photogenerated holes, tert butanol (which reacts mainly with hydroxyl radicals), and methanol (which reacts at a similar rate with hydroxyl radicals and sulfate radicals). From Figure 11, it is evident that the presence of t-butanol does not cause a significant inhibition. In contrast, SMX degradation was hindered in the presence of methanol; the inhibition becomes significant when EDTA is present in the solution, which indicates the dominant role of photogenerated holes.

To shed light on the contribution of sulfate radicals, the consumption of SPS was measured in the presence and absence of the photocatalyst or solar irradiation. The consumption of SPS was significantly higher in the hybrid system than in the individual processes, as shown in Figure 12.

For example, after 60 min the consumption of persulfate was 7%, 14%, and 4% for SPS/Solar, 3.0 Cu.BVO/SPS, and the hybrid 3.0 Cu.BVO/SPS/solar system, respectively.

Taking the aforementioned into consideration, the SMX degradation mechanism in the combined process can be described as follows: Under solar light irradiation, photoproduced pairs of electrons-holes are formed in copper-promoted BiVO₄ samples. SPS can now react with electrons in the semiconductor’s conduction band, thus forming SO₄^●− which can participate in SMX degradation. In addition, the photogenerated holes in the valence band are now “free” to decompose SMX. Additional SO₄^●− are also available in the system due to SPS activation by sunlight. The proposed degradation mechanism is shown in Scheme 1.

3. Materials and Methods

3.1. Chemical and Water Matrices

Sulfamethoxazole (C₁₀H₁₁N₃O₃S, CAS: 723-46-6), sodium persulfate (Na₂S₂O₈, CAS: 7775-27-1), t-butanol (C₄H₁₀O, CAS: 75-65-0), methanol (CH₃OH, CAS: 67-56-1), humic acid (HA, CAS: 1415-93-6), sodium chloride (NaCl, CAS: 7647-14-5), sodium bicarbonate (NaHCO₃, CAS: 144-55-8), and acetonitrile (CH₃CN, CAS: 75-05-8, for HPLC analysis) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Chemicals used for catalyst preparation can be found elsewhere [23].

Apart from ultrapure water (UPW, pH = 6.5), secondary effluent from the University of Patras campus wastewater treatment plant (WW) and commercially bottled water were used in degradation experiments. Their main characteristics are shown in

Table 3. Real water matrices’ characteristics.

Parameter	WW	BW
pH	8.0	7.4
Conductivity (μs/cm)	300	350
Sulfate (mg/L)	35	10
Chloride (mg/L)	75	8
Nitrate (mg/L)	60	10
Bicarbonate (mg/L)	180	240

.

3.2. Catalyst Preparation and Characterization

Copper-promoted BiVO₄ photocatalysts (0.75, 3.0, and 10.0 wt.% Cu) were prepared using a polyol-reduction method with ethylene glycol as a reductant [41]. The preparation method is described in detail in our previous work [23,32]. Photocatalysts thus prepared are denoted in the following as xCu.BVO, where x represents the copper loading (wt.%).

Photocatalysts were characterized through physisorption at the temperature of liquid nitrogen (77 K) (BET method) using a Micromeritics (Gemini III 2375, Norcross, GA, USA), X-ray diffraction (XRD), (Brucker D8 Advance, Billerica, MA, USA), diffuse reflectance spectra (DRS) (Varian Cary 3E, Palo Alto, CA, USA) high resolution transmission electron microscopy (TEM) and scanning electron microscopy with an energy dispersive spectrometer (SEM/EDS) (JEOL 6300, Peabody, MA, USA). High-resolution transmission electron microscopy (HR-TEM) images were obtained using a JEOL JEM-2100 (Peabody, MA, USA) system operated at 200 kV (resolution: point 0.23 nm, lattice 0.14 nm). Details regarding the methodologies and procedures are available in former studies [42].

3.3. Experimental Procedure and Analytical Methods

In a typical experiment, an appropriate amount of a stock SMX solution was added to the photocatalytic reactor, which was filled with UPW, BW, or WW (the reactor’s capacity is 120 mL). After adding the desired amount (typically 500 mg/L) of the photocatalyst, the system was stirred in the dark for 15 min to achieve the adsorption−desorption dynamic equilibrium. Next, the suspension was irradiated by simulated solar irradiation using a solar simulator (Oriel-LCS-100, 100 W Xe ozone-free lamp). The incident intensity in the photoreactor was calculated using chemical actinometry and was found to be equal to 7.6 × 10⁻⁷ einstein/(L.s). Sampling took place at fixed times; after filtration, analysis through high-performance liquid chromatography (HPLC) occurred.

To estimate reaction rates, SMX degradation was considered to follow a pseudo-first-order kinetic expression (Equation (1)) [43]:

$$-\frac{dC}{dt}={{\displaystyle k}}_{app}C\iff \ln \frac{{{\displaystyle C}}_0}{C}={{\displaystyle k}}_{app}t$$

(1)

where C and C₀ correspond to SMX concentration at time t = t and t = 0, respectively, and k_app is the kinetic constant.

Persulfate consumption was measured using a Hach DR5000 (Loveland, CO, USA) spectrophotometer according to the method proposed by Liang et al. [44].

Leaching of copper in the liquid phase was measured via atomic absorption spectrometer (SHIMADZU AA-6800, Kyoto, Japan).

The degree of synergy, S, was quantified according to the following equation (Equation (2)) [45]:

$$S(\%)=\frac{{{\displaystyle k}}_{combined}-{\displaystyle {\sum }_i^n{{\displaystyle k}}_i}}{{{\displaystyle k}}_{combined}}\times 100$$

(2)

$$\mathrm{S}\left(\begin{array}{l}> 0, \mathrm{synergistic} \mathrm{effect}\\ =0, \mathrm{cumulative} \mathrm{effect}\\ < 0, \mathrm{antagonistic} \mathrm{effect}\end{array}\right.$$

4. Conclusions

Summarizing, this work examines the possible synergy of the SPS/CuO_x.BiVO₄ system in the presence of simulated solar irradiation. According to the results, using an oxidant, such as persulfate, to improve the performance of the combined process seems promising, making the combined process more attractive for real aqueous matrices where the separate processes show significantly reduced performance.

In recent years, research has shifted to conditions that simulate real problems. Ideally, the evaluation of both catalytic materials and processes must be performed holistically by including parameters such as performance under different conditions, cost and reusability of the materials, toxicity, and environmental and energy costs.

In this light, and inspired by previous research, this work shows that the simultaneous use of more than one process seems to be a promising solution that overcomes some of the disadvantages of individual processes. However, it is not the de facto optimal solution proposed for all cases.

Since the performance of both materials and processes depends to a vast extent on the conditions, the evaluation of materials or systems must be performed according to the problem in question and in a range of experimental or operational conditions. Uncritical evaluation or adoption of optimal conditions from unrepresentative experiments can lead to incorrect conclusions.

The main conclusions extracted from the present study can be summarized as follows:

(1)

The hybrid system, solar/SPS/CuO_x.BiVO₄, leads to significantly improved photocatalytic yields than the individual systems (solar/CuO_x.BiVO₄ and SPS/CuO_x.BiVO₄), considering SMX degradation in real water matrices.

(2)

In real water matrices, the interaction between the photocatalytic-persulfate system and matrix components results in a synergistic rather than a cumulative effect, with a high degree of synergy. Using combined technologies seems to be preferred in very complex aqueous matrices where the competition for available oxidizing species and the catalyst surface are maximized.

(3)

The complexity of the water matrix determines the best performing catalytic material; 0.75 Cu.BVO showed the highest efficiency in UPW and BW, whereas 3.0 Cu.BVO and 10.0 Cu.BVO were characterized by higher activity in WW.