A Meta-Analysis of Influencing Factors on the Activity of BiVO4-Based Photocatalysts

With the continuous advancement of global industrialization, a large amount of organic and inorganic pollutants have been discharged into the environment, which is essential for human survival. Consequently, the issue of water environment pollution has become increasingly severe. Photocatalytic technology is widely used to degrade water pollutants due to its strong oxidizing performance and non-polluting characteristics, and BiVO4-based photocatalysts are one of the ideal raw materials for photocatalytic reactions. However, a comprehensive global analysis of the factors influencing the photocatalytic performance of BiVO4-based photocatalysts is currently lacking. Here, we performed a meta-analysis to investigate the differences in specific surface area, kinetic constants, and the pollutant degradation performance of BiVO4-based photocatalysts under different preparation and degradation conditions. It was found that under the loading condition, all the performances of the photocatalysts can be attributed to the single BiVO4 photocatalyst. Moreover, loading could lead to an increase in the specific surface area of the material, thereby providing more adsorption sites for photocatalysis and ultimately enhancing the photocatalytic performance. Overall, the construct heterojunction and loaded nanomaterials exhibit a superior performance for BiVO4-based photocatalysts with 136.4% and 90.1% improvement, respectively. Additionally, within a certain range, the photocatalytic performance increases with the reaction time and temperature.


Introduction
Due to the growing population, the rise in global industrialization, and the overconsumption of natural resources, environmental pollution problems are becoming increasingly serious, of which water pollution is one of the major global environmental problems [1]. China is one of the world's largest producers and consumers of pesticides, with an annual use of more than 20,000 tons [2]. Moreover, agricultural production heavily relies on chemical fertilizers [3]. Large amounts of organic pollutants from pesticides and fertilizers mix with agricultural runoff under rainwater, exacerbating the pollution of water bodies [4]. Additionally, Mining activities and the industrial production of paper, textile, leather, iron and steel, and petroleum discharge a large amount of printing and dyeing wastewater. Electroplating and metallurgical wastewater contain a large amount of organic pollutants [5,6], including detergents, organic dyes, surfactants, curing agents, etc. [7]. Furthermore, inorganic pollutants such as Cr(VI) are also contained in wastewater released results can provide guidance and assistance for the practical application of BiVO4-based composites in the field of environmental pollution remediation.

Literature Search
In this paper, the peer-reviewed papers so far were searched using Web of Science and ScienceDirect databases. The search was conducted with "BiVO4" OR "'bismuth vanadium tetraoxide' OR 'bismuth vanadate'" AND "Photocatalysis" AND "degradation" AND "'contaminants' OR 'Dyes' OR 'Antibiotics' OR 'Surfactant' OR 'Pesticides'" OR "'BiVO4' OR 'bismuth vanadium tetraoxide' OR 'bismuth vanadate'" AND "Photocatalysis" AND "reduction" AND "Heavy Metal" searched as keywords to compare the pollutant degradation and removal efficiency of single BiVO4 as a photocatalyst and composite photocatalytic material. The initial search resulted in 322 papers. Since the beginning of research in this field in 2005, the number of articles published and the frequency of citations in this field are shown in Figure 1. Subsequently, a secondary screening of the selected literature was further conducted based on the following factors.

Criteria for Data Inclusion
This study included articles in the meta-analysis based on the following criteria: (1) original, peer-reviewed articles published in English; (2) all studies compared the efficiency of single BiVO4 photocatalysts with composite photocatalyst materials for the degradation of pollutants; and (3) studies that were not meta-analyses, conference proceedings, reports, etc., to prevent the duplication of data. After applying the above criteria and excluding duplicate articles, 228 papers were screened from the initial database. Subsequently, 187 articles were excluded based on the relevance of their titles and keywords. Afterward, we excluded seven of these articles after reviewing the selected articles due to missing or quality issues of the main data of the meta-analysis, etc. Finally, 34 most relevant articles were selected for data extraction and analysis after a rigorous screening review process (Figure 2).

Criteria for Data Inclusion
This study included articles in the meta-analysis based on the following criteria: (1) original, peer-reviewed articles published in English; (2) all studies compared the efficiency of single BiVO 4 photocatalysts with composite photocatalyst materials for the degradation of pollutants; and (3) studies that were not meta-analyses, conference proceedings, reports, etc., to prevent the duplication of data. After applying the above criteria and excluding duplicate articles, 228 papers were screened from the initial database. Subsequently, 187 articles were excluded based on the relevance of their titles and keywords. Afterward, we excluded seven of these articles after reviewing the selected articles due to missing or quality issues of the main data of the meta-analysis, etc. Finally, 34 most relevant articles were selected for data extraction and analysis after a rigorous screening review process ( Figure 2).

Data Extraction and Grouping
The data extracted from each article include the preparation conditions of the material (coupling type, preparation time, and reaction temperature) and the degradation conditions (contaminant type, contaminant concentration, photocatalyst dosing, and solution pH). Additionally, the specific surface area, degradation efficiency, and cycling performance of the materials in the single BiVO4 photocatalyst (control) and the composite photocatalyst coupled with other materials (treatment group), as well as their mean and standard deviation, were also counted. The BiVO4-based composites obtained under different preparation methods have different morphologies, properties and advantages (Table 1).

Data Extraction and Grouping
The data extracted from each article include the preparation conditions of the material (coupling type, preparation time, and reaction temperature) and the degradation conditions (contaminant type, contaminant concentration, photocatalyst dosing, and solution pH). Additionally, the specific surface area, degradation efficiency, and cycling performance of the materials in the single BiVO 4 photocatalyst (control) and the composite photocatalyst coupled with other materials (treatment group), as well as their mean and standard deviation, were also counted. The BiVO 4 -based composites obtained under different preparation methods have different morphologies, properties and advantages ( Table 1).

Selection of Models
Two types of models used in the Meta-analysis are fixed effects models and random effects models. In the fixed effects model, each independent study is from the same overall sample, while in the random effects model, the studies are from different overall samples [78]. The random effects model was chosen as the statistical model for analysis in this study as it more accurately captures the variation in true effect sizes [79]. In this study, samples were taken from photocatalytic degradation studies of BiVO 4 under different experimental designs, and the normal effect sizes of the selected samples logically had different properties, such as the methodological background of the study, the loaded compounds, and the conditions of degradation, all with different effect values. Therefore, the random effects model is more applicable to this study.

Calculation of the Effect Size
In this study, the effects of material preparation conditions and degradation conditions on BiVO 4 single photocatalysts and BiVO 4 coupled with composite photocatalysts of other materials were investigated using the effect size method. The standard measure of effect size is expressed as the logarithmic response ratio (lnRR). The logarithmic response ratio was calculated as follows [80]: where W t and W c are the mean values of each test variable in the experimental and control groups, respectively. A positive effect size (lnRR > 0) indicates an increase in the tested variables (degradation efficiency, BET, and kinetic constants) of the composite photocatalyst made of BiVO 4 coupled with other materials compared to the control BiVO 4 single photocatalyst, while a negative effect size (lnRR < 0) indicates a decrease in the tested variables of the experimental group compared to the control group. An effect size of 0 (lnRR = 0) indicates that there is no significant change in the experimental group compared to the control group. The percentage of the magnitude of the effect size was calculated according to the following formula [48]: The variance of the effect size is calculated by the following equation [81]: where, SD t , and SD c represent the standard deviation of each test variable in the experimental and control groups, respectively, and n t and n c represent the number of repetitions in the experiments of the experimental and control groups, respectively. The weighted response ratio to (RR ++ ) is used to express the overall response effect of variable conditions such as reaction time on the experimental and control groups, which is calculated as follows [82]: where n denotes the number of experimental groups, the weighting factor is represented by the inverse of the variance of the effect size 1/v, i and j represent the i-th and k-th treatments, respectively, and k is the observed value in the observed group. The standard deviation of RR ++ (s) is: The 95% confidence interval (CI) in the study was calculated according to the following formula [83]:

Subgroup Analysis
In this study, the material preparation conditions as well as the degradation conditions listed in Section 2.3 were used as subgroups for a subgroup analysis of each test variable in the experimental as well as the control groups. Because this study used a randomized model for analysis, the selected trials were measured inconsistently and in different units of measure, so standardized mean difference (SMD) was used to express the results of each test variable between the experimental and control groups [84]. SMD is a relative indicator and is independent of baseline risk and has good consistency. The effect size in this study is reported as SMD. The indicator was calculated by the following equation: where, MD and SD denote the mean and standard deviation of each test variable in the experimental and control groups, respectively.

Data Analysis
The meta-analysis in this study was performed by applying the meta package in R language (4.2.3) together with Review Manager 5.3 software. The final result plots were plotted using the ggplot2 package in the R language (4.2.3).

Effects of Different Preparation Conditions on BET of BiVO 4 -Based Composites
The specific surface area of the composites is one of the main factors affecting their photocatalytic activity; therefore, understanding the specific surface area of photocatalytic materials is essential to grasp the performance of photocatalysts. A higher specific surface area provides more active sites [44], which in turn increases the photocatalytic activity of the material [85]. The current method of measuring the specific surface area and pore volume of photocatalytic materials mainly uses the nitrogen adsorption Brunauer-Emmett-Teller (BET) method [86]. In this study, the effect of BET on the BiVO 4 -based composites was counted in the collected 34 related papers ( Figure 3 and Table S1). In the listed preparation conditions, for coupling type, the effect size of BET is maximized when composited with nanomaterials (12.23, 95% CI = 11.65, 12.80), followed by loaded oxides (6.22, 95% CI = 5.80, 6.64) and heterojunctions (5.98, 95% CI = 4.46, 7.51). In this study, the coupling-type loadings with the highest removal efficiency for all pollutants were screened (Table S2). Meanwhile, for the preparation time, the effect size of the BET method of BiVO 4 -based composites gradually decreases with increasing preparation time. Specifically, the highest effect size is observed for a preparation time of 1-6 h (9.8, 95% CI = 9.1, 10.49), followed by preparation for 6-24 h (3.45, 95% CI = 3.2, 3.7) and preparation for more than 24 h (0.67, 95% CI = −0.04, 1.37). The preparation temperature also has different effects on the rate of the BET method effect of BiVO 4 -based composites. The effect size of the BET of the composites was greatest when the preparation temperature was in the mid-temperature range of 100-200 • C (5.87, 95% CI = 5.56, 6.18), while the effect size of the BET of the composites was lowest at low temperature conditions (<100 • C) (1.88, 95% CI = 1.50, 2.25). At the same time, the pH of the solution during the preparation process is one of the main factors affecting the BET of BiVO 4 -based composites. According to the results of the meta-analysis, the effect size of the BET of BiVO 4 -based composites decreases with increasing pH, with the highest effect size under acidic conditions (12.06, 95% CI = 11.40, 12.73) and the lowest under alkaline conditions (3.30, 95% CI = 2.50, 4.10), indicating that the preparation in acidic environment has a greater potential for enhancing the specific surface area of the composites.
2.25). At the same time, the pH of the solution during the preparation process is one of the main factors affecting the BET of BiVO4-based composites. According to the results of the meta-analysis, the effect size of the BET of BiVO4-based composites decreases with increasing pH, with the highest effect size under acidic conditions (12.06, 95% CI = 11.40, 12.73) and the lowest under alkaline conditions (3.30, 95% CI = 2.50, 4.10), indicating that the preparation in acidic environment has a greater potential for enhancing the specific surface area of the composites. All the preparation conditions listed in this study (including material coupling type, material preparation time, material preparation temperature, and pH at preparation) have a positive effect on the specific surface area effect of BiVO4-based composites, indicating that BiVO4-based composites have a larger specific surface area than single materials regardless of the preparation conditions, and the composites can provide more adsorption and active sites for photocatalytic degradation and reduction in environmental pollutants.

Effects of Different Preparation Conditions on Kinetic Constant of BiVO4-Based Composites
The kinetic constant is a parameter that describes the rate of reaction and the concentration between reactants, representing the rate of degradation per unit time of the reactant concentration. A higher kinetic constant value indicates that the photocatalytic material has a faster rate and higher efficiency in the degradation of or reduction in the pollutant [87]. The effects of the preparation conditions and degradation conditions selected in this study on the kinetic constants are shown in Figure 4. For the preparation conditions, the highest effect sizes of kinetic constants among the coupling types were found for carbon materials (2.4, 95% CI = 0.48, 4.  All the preparation conditions listed in this study (including material coupling type, material preparation time, material preparation temperature, and pH at preparation) have a positive effect on the specific surface area effect of BiVO 4 -based composites, indicating that BiVO 4 -based composites have a larger specific surface area than single materials regardless of the preparation conditions, and the composites can provide more adsorption and active sites for photocatalytic degradation and reduction in environmental pollutants.

Effects of Different Preparation Conditions on Kinetic Constant of BiVO 4 -Based Composites
The kinetic constant is a parameter that describes the rate of reaction and the concentration between reactants, representing the rate of degradation per unit time of the reactant concentration. A higher kinetic constant value indicates that the photocatalytic material has a faster rate and higher efficiency in the degradation of or reduction in the pollutant [87]. The effects of the preparation conditions and degradation conditions selected in this study on the kinetic constants are shown in Figure 4. For the preparation conditions, the highest effect sizes of kinetic constants among the coupling types were found for carbon materials (2.4, 95% CI = 0.48, 4.32), followed by oxides (0.58, 95% CI = −0.1, 1.26), sulfides (0.43, 95% CI = −2.89, 3.75), and nanomaterials (0.26, 95% CI = −0.94, 1.47); the effect size of the kinetic constants of metal couples was negative for all coupling types (−0.81, 95% CI = −1.46, −0.15), indicating that the coupling of composites prepared from heavy metals may lead to a decrease in the kinetic constants of the composites. There was little difference in the effect of preparation time on the kinetic constant effect size, which increased with preparation time, with the highest kinetic constant effect size for preparation time >24 h (0.83, 95% CI = −1.46, 3.12). In addition, the composite kinetic constants had the highest effect sizes when the preparation temperature was in the range of 100-200 • C (0.76, 95% CI = 0.06, 1.45) and the lowest effect sizes when the preparation temperature was above 200 • C (0.36, 95% CI = −0.98, 1.7). Finally, the preparation pH also influenced the kinetic constants of the composites, with the highest effect sizes of kinetic constants for pH-neutral conditions (1.71, 95% CI = −0.02, 3.44), while there was minimal variation in the effect sizes of kinetic constants for acidic and basic conditions ( Figure 4 and Table S3). metals may lead to a decrease in the kinetic constants of the composites. There was little difference in the effect of preparation time on the kinetic constant effect size, which increased with preparation time, with the highest kinetic constant effect size for preparation time >24 h (0.83, 95% CI = −1.46, 3.12). In addition, the composite kinetic constants had the highest effect sizes when the preparation temperature was in the range of 100-200 °C (0.76, 95% CI = 0.06, 1.45) and the lowest effect sizes when the preparation temperature was above 200 °C (0.36, 95% CI = −0.98, 1.7). Finally, the preparation pH also influenced the kinetic constants of the composites, with the highest effect sizes of kinetic constants for pH-neutral conditions (1.71, 95% CI = −0.02, 3.44), while there was minimal variation in the effect sizes of kinetic constants for acidic and basic conditions ( Figure 4 and Table S3). The effect size of the reaction kinetic constants was also affected by the different pollutant concentrations in the degradation conditions. The effect size of the reaction kinetics of BiVO4-based composites was highest when the contaminant concentration was 20-200 mg/L (0.97, 95% CI = −1.17, 3.11), followed by the case of contaminant concentration <20 mg/L (0.46, 95% CI = −0.45, 1.37), whereas when the contaminant concentration was higher than 200 mg/L, the kinetic constants of effect was the lowest (0.05, 95% CI = −0.90, 1.01), almost coinciding with the baseline, indicating that the composite material at this concentration has little effect on the kinetic constants. The amount of photocatalyst dosing also has an effect on the rate of kinetic constant effect of the composite. Specifically, the highest effect size of the kinetic constants of the composites was observed when 0.5-1 g/L of photocatalyst was injected (0.77, 95% CI = 0.23, 1.32), the investigation has demonstrated that The effect size of the reaction kinetic constants was also affected by the different pollutant concentrations in the degradation conditions. The effect size of the reaction kinetics of BiVO 4 -based composites was highest when the contaminant concentration was 20-200 mg/L (0.97, 95% CI = −1.17, 3.11), followed by the case of contaminant concentration < 20 mg/L (0.46, 95% CI = −0.45, 1.37), whereas when the contaminant concentration was higher than 200 mg/L, the kinetic constants of effect was the lowest (0.05, 95% CI = −0.90, 1.01), almost coinciding with the baseline, indicating that the composite material at this concentration has little effect on the kinetic constants. The amount of photocatalyst dosing also has an effect on the rate of kinetic constant effect of the composite. Specifically, the highest effect size of the kinetic constants of the composites was observed when 0.5-1 g/L of photocatalyst was injected (0.77, 95% CI = 0.23, 1.32), the investigation has demonstrated that the precise selection of the optimal photocatalyst dosage holds the key to maximizing the rate of photocatalytic reactions. However, both an excessive and an inadequate dosage of photocatalyst proved to be insufficient in achieving the desired outcome. Specifically, a lower dosage results in a diminished number of available sites for photocatalytic reactions, whereas an elevated photocatalyst concentration diminishes the solution's light transmittance, consequently exerting a notable influence on the reaction rate. In addition, for the solution pH at degradation, the highest effect sizes of kinetic constants for composites were found for pH-neutral conditions (0.46, 95% CI = −0.28, 1.21), while the effect sizes for acidic and basic conditions did not differ significantly, which is consistent with the results of the effect of pH on the effect sizes of kinetic constants in the preparation conditions.

Study on the Degradation Efficiency of BiVO 4 -Based Composites for Environmental Pollutants
Environmental pollutants, including organic and inorganic pollutants, pose a serious threat to the natural environment and human health [88]. BiVO 4 and BiVO 4 -based composites, which are non-toxic to the environment, can effectively degrade various organic and inorganic pollutants in water bodies [89,90]. In this study, the efficiency of BiVO 4 -based composites in degrading environmental pollutants under various conditions was investigated in order to better understand the photocatalytic performance of various composites. The degradation efficiency of BiVO 4 -based composites for environmental pollutants is affected by a combination of the material preparation conditions (including coupling type, preparation time, preparation temperature, and pH) and degradation conditions (pollutant concentration, photocatalyst dosing, and solution pH during degradation). The effect sizes of the degradation efficiency of BiVO 4 -based composites under each condition are shown in Figure 3. Regardless of the preparation and degradation conditions, the degradation rate of BiVO 4 -based composites was improved compared to that of single BiVO 4 material ( Figure 5 and Table 2).

Study on the Degradation Efficiency of BiVO4-Based Composites for Environmental Pollutants
Environmental pollutants, including organic and inorganic pollutants, pose a serious threat to the natural environment and human health [88]. BiVO4 and BiVO4-based composites, which are non-toxic to the environment, can effectively degrade various organic and inorganic pollutants in water bodies [89,90]. In this study, the efficiency of BiVO4-based composites in degrading environmental pollutants under various conditions was investigated in order to better understand the photocatalytic performance of various composites. The degradation efficiency of BiVO4-based composites for environmental pollutants is affected by a combination of the material preparation conditions (including coupling type, preparation time, preparation temperature, and pH) and degradation conditions (pollutant concentration, photocatalyst dosing, and solution pH during degradation). The effect sizes of the degradation efficiency of BiVO4-based composites under each condition are shown in Figure 3. Regardless of the preparation and degradation conditions, the degradation rate of BiVO4-based composites was improved compared to that of single BiVO4 material ( Figure 5 and Table 2).  Among the preparative conditions, the highest effect sizes for the degradation efficiency of BiVO 4 -based composites were found when heterojunctions were constructed (13.93, 95% CI = 12.73, 15.13), followed by coupling nanomaterials (13.76, 95% CI = 12.96, 14.56). On the other hand, the lowest effect size of composite degradation efficiency was observed for all types of coupling conditions when coupling oxides (4.33, 95% CI = 3.94, 4.73). The effect size of the degradation efficiency of BiVO 4 -based composites increased with longer preparation time; and the highest effect size of degradation efficiency of composites was observed when the preparation time exceeded 24 h (15.19, 95% CI = 14.26, 16.12), while the lowest effect size of degradation efficiency was observed for a degradation time of 1-6 h (6.96, 95% CI = 6. 72, 7.20). Similarly, the effect size of the degradation efficiency of the composites increased with a higher preparation temperature. The highest effect size of degradation efficiency (11.97, 95% CI = 11.31, 12.62) was observed for high temperature preparation conditions (>200 • C) and the lowest effect size of degradation efficiency (4.89, 95% CI = 4.65, 5.12) was observed for low temperature preparation conditions (<100 • C). In addition, the pH at the time of preparation also had an effect on the effect size of the composite degradation efficiency, which was the highest when the pH was 7, i.e., under neutral conditions (11.64, 95% CI = 10.92, 12.36), followed by basic (10.92, 95% CI = 10.33, 11.52) and acidic conditions (8.87, 95% CI = 8. 33, 9.41). For the degradation conditions, different pollutant concentrations have varying effects on the effect sizes. The highest effect sizes for degradation efficiency were found for pollutant concentrations of 20-200 mg/L (11.18, 95% CI = 10.66, 11.69), which coincided with the results of the reaction kinetic constants. The lowest effect size of composite degradation efficiency was observed when the pollutant was at a low concentration (<20 mg/L) (7.08, 95% CI = 6.84, 7.33). The photocatalyst dosage emerged as a pivotal factor within the degradation conditions. The analysis revealed a discernible trend wherein the magnitude of the degradation efficiency's effect size for BiVO4-based composites grew proportionately with the photocatalyst dosage within a specific range. Notably, the effect size reached its zenith at a dosage of 0.5-1 g/L (7.26, 95% CI = 7.00, 7.52), resulting in an incremental degradation efficiency of 112.02% relative to that of the solitary BiVO 4 material. This augmentation can be attributed to the enhanced provision of reactive sites brought forth by a greater photocatalytic concentration, propelling the photocatalytic reaction. However, when the dosage exceeded 1 g/L, the contribution of loading to the photocatalytic performance of the composites was reduced (4.88, 95% CI = 4.56, 5.21). This attenuation can be attributed to higher concentrations, leading to a reduction in solution transmittance, thereby adversely affecting the photocatalytic performance. The solution pH during degradation also influenced the effect size of the composite degradation efficiency, and similar to the results of the reaction kinetic constants, the highest effect size of the degradation rate was observed for neutral pH conditions (14.81, 95% CI = 13.79, 15.82), followed by acidic (9.47, 95% CI = 9.02, 9.91) and alkaline conditions (4.6, 95% CI = 4.02, 5.19).
The above results show that coupling is a highly effective method to improve the degradation or reduction in environmental pollutants by BiVO 4 photocatalysts, and the various coupling materials prepared by different methods exhibit diverse morphological characteristics and degradation advantages ( Table 1). The photocatalytic degradation and reduction in environmental pollutants are influenced and controlled by several direct and indirect factors, such as the type of material coupling, preparation pH, preparation time, preparation temperature, as well as the concentration of pollutants in the degradation conditions, photocatalyst dosage, and solution pH.

Efficiency of Degradation of Organic Pollutants
The types and amounts of organic pollutants in the water environment are increasing due to the growing pollution caused by industrial, agricultural, and household production activities, etc. [91]; for example, dyeing and textile wastewater, organic pesticide wastewater and industrial wastewater from food, pharmaceuticals, electroplating, metallurgy, etc., which contain significant amounts of organic pollutants [92]. Photocatalysis is currently a low-cost, efficient, and environmentally friendly means of degrading organic pollutants in water bodies [26], among the 34 relevant literatures screened in this study, as many as 32 focused on the degradation of organic pollutants in the environment. Consistent with the analysis of the overall degradation efficiency, the BiVO 4 -based composites outperformed the unloaded single BiVO 4 photocatalysts under various preparation conditions as well as degradation conditions ( Figure 6 and Table S4).
The highest effect size of BiVO 4 -based composites on the degradation efficiency of organic pollutants (14.29, 95% CI = 13.33, 15.25) was observed when coupling nanomaterials in the preparation conditions, because the porous structure of nanomaterials can greatly increase the specific surface area of the composites, thus improving the photocatalytic performance of BiVO 4 -based composites [93]. This was followed by construct heterojunctions (11.92, 95% CI = 10.86, 12.97) as well as metals (11.52, 95% CI = 10.67, 12.34). The effect size of the composite degradation efficiency of organic pollutants increased with longer material preparation time, and the highest effect size of composite degradation efficiency was observed when the preparation time was greater than 24 h (14.61, 95% CI = 13. 71, 15.5). Similarly, the effect size of the degradation efficiency of BiVO 4 -based composites increased with a higher preparation temperature, with the highest effect size of the composite degradation efficiency when the preparation temperature was higher than 200 • C (13.01, 95% CI = 12. 30, 13.72). It is shown that the longer preparation time and high temperature conditions have the highest degree of optimization for the degradation efficiency of the composites to degrade organic pollutants within a certain range. The effect of solution pH at preparation on the effect size of composite degradation efficiency was consistent with the overall results, with the highest effect size of composite degradation efficiency when the pH was neutral (13.01, 95% CI = 11. 83, 14.19), followed by alkaline (10.92, 95% CI = 10.33, 11.52) and acidic conditions (8.87, 95% CI = 8. 33, 9.41). It is shown that the BiVO 4 -based composites have a better degradation efficiency for environmental organic pollutants than single photocatalysts when the pH at the time of preparation is around 7. Nanomaterials 2023, 13, x FOR PEER REVIEW 15 of 26 The highest effect size of BiVO4-based composites on the degradation efficiency of organic pollutants (14.29, 95% CI = 13.33, 15.25) was observed when coupling nanomaterials in the preparation conditions, because the porous structure of nanomaterials can greatly increase the specific surface area of the composites, thus improving the photocatalytic performance of BiVO4-based composites [93]. This was followed by construct heterojunctions (11.92, 95% CI = 10.86, 12.97) as well as metals (11.52, 95% CI = 10.67, 12.34). The effect size of the composite degradation efficiency of organic pollutants increased with longer material preparation time, and the highest effect size of composite degradation efficiency was observed when the preparation time was greater than 24 h (14.61, 95% CI = 13. 71, 15.5). Similarly, the effect size of the degradation efficiency of BiVO4-based composites increased with a higher preparation temperature, with the highest effect size of the composite degradation efficiency when the preparation temperature was higher than 200 °C (13.01, 95% CI = 12. 30, 13.72). It is shown that the longer preparation time and high temperature conditions have the highest degree of optimization for the degradation efficiency of the composites to degrade organic pollutants within a certain range. The effect of solution pH at preparation on the effect size of composite degradation efficiency was consistent with the overall results, with the highest effect size of composite degradation efficiency when the pH was neutral (13.01, 95% CI = 11. 83, 14.19), followed by alkaline (10.92, 95% CI = 10.33, 11.52) and acidic conditions (8.87, 95% CI = 8. 33, 9.41). It is shown that the BiVO4-based composites have a better degradation efficiency for environmental organic pollutants than single photocatalysts when the pH at the time of preparation is around 7.
For degradation conditions, the highest effect sizes for composite degradation efficiency were found for pollutant concentrations of 20-200 mg/L (15.65, 95% CI = 14.94, . It was shown that the prepared composites degraded environmental organic pollutants in the medium concentration range with more positive excursions and a better degradation efficiency than the uncoupled single photocatalyst. The findings of this study illuminate a noteworthy trend, wherein the most pronounced effect size pertaining to the enhancement of the degradation efficiency of environmental organic pollutants (7.50, 95% CI = 7.23, 7.76) is discernible within the photocatalyst dosage range of 0.5-1 g/L. Evidently, augmenting the photocatalyst quantity within a certain range amplifies the photocatalytic prowess of the composites to a greater degree. However, an excessive photocatalyst dosage tends to mitigate the extent of enhancement in the photocatalytic efficiency of the composites. The excessive dosage appears to be associated with a decreased solution transmittance, which consequently engenders a reduction in photocatalytic performance. This observation resonates harmoniously with the outcomes of earlier analyses. In addition, the pH of the solution during the degradation of environmental organic pollutants had an effect on the rate of effect on degradation efficiency, and calculations showed that acidic (13.31, 95% CI = 12.70, 13.93) and neutral (12.74, 95% CI = 11.86, 13.62) conditions contributed more to the degradation efficiency of organic pollutants. In contrast, under alkaline degradation conditions (3.41, 95% CI = 3.11, 3.71), although the effect size increased, the increment was smaller compared to acidic and neutral conditions.

Efficiency of Degradation of Inorganic Pollutants
Agricultural and industrial activities emit a large amount of wastewater containing heavy metal pollutants and greenhouse gas emissions into the environment [94]. In addition, photocatalytic means can effectively remove inorganic pollutants such as NO, NO x , and CO from air pollutants [95]. In contrast to the extensive research on the photocatalytic removal of environmental organic pollutants, there are fewer studies on inorganic pollutant removal. After conducting a thorough screening process, only two relevant papers on the removal of environmental inorganic pollutants by BiVO 4 -based composites were identified. The meta-analysis of these selected studies revealed that the removal efficiency of inorganic pollutants (mainly the reduction in heavy metals in wastewater) increased after the coupling of a single BiVO 4 material compared to the case without coupling (Table S5).
For the preparation conditions of the composites, the main types of coupling used for the reduction in inorganic pollutants in the environment were nanostructures and heterojunctions, where the BiVO 4 -based composites showed a higher rate of reduction efficiency effect on inorganic pollutants after loading nanomaterials (10.16, 95% CI = 8.96, 11.36) and a better coupling effect. For the preparation time of the selected composites in the literature, similar to the effect size results for the overall degradation efficiency, the longer the preparation time, the larger the positive offset in the effect size for the inorganic pollutant reduction efficiency, and for the reduction in inorganic pollutants, the effect size was higher for 6-24 h of preparation (11.11, 95% CI = 9. 81, 12.42). Similarly, as the preparation temperature increases, the rate of the effect of the composite on the reduction efficiency of environmental inorganic pollutants rises. The highest effect size of reduction efficiency was observed when the preparation temperature was higher than 100 • C (11.69, 95% CI = 10.32, 13.06), as shown in the analytical results. For the solution pH at preparation, the effect size increment on the removal efficiency of environmental pollutants was lower for the removal of inorganic pollutants (9.04, 95% CI = 8. 27, 9.81) than for the removal of organic pollutants under neutral conditions with higher effect size increments, indicating that the BiVO 4 -based composites are more efficient in degrading organic pollutants under the same conditions, but there are fewer original data on the reduction in inorganic pollutants, and so the conclusion may not be generalizable.
For each reduction condition of the composites, similar to the above analysis, the photocatalyst dosing was proportional to the effect size of the inorganic pollutant reduction efficiency, with the highest effect size of the reduction efficiency of the BiVO 4 -based composites seen for the reduction in environmental inorganic pollutants when the dosing was 0.5-1 g/L (5.63, 95% CI = 5.11, 6.16), which is consistent with the above findings. Regardless of the pollutant concentration and the pH of the solution adjusted for the reduction in inorganic pollutants, the results of the analysis showed that the reduction in environmental inorganic pollutants was enhanced by the loading of BiVO 4 materials, with a greater increase in the effect of the efficiency of the reduction in inorganic pollutants under alkaline conditions compared to the degradation of organic pollutants (8.12, 95% CI = 7.14, 9.10). Due to the small amount of raw data, further analysis will not be performed here.

Multiple Factors Jointly Determine the Photocatalytic Activity of BiVO 4 -Based Photocatalysts
Through a diligent review of the meticulously selected literature and an exhaustive meta-analysis, we have acquired an initial grasp of the efficacy exhibited by BiVO 4 -based composites in eliminating environmental pollutants across varying conditions. The interplay of factors such as coupling type, preparation duration, reaction temperature, and degradation pH collectively contributes to shaping the ultimate degradation performance of these composites. Notably, all of the aforementioned conditions exert a beneficial influence on elevating the removal efficiency of environmental pollutants.

Effect of Coupling Type on Photocatalytic Activity of Composites
For the coupling type, the rate of effect on pollutant removal efficiency can be maximized when BiVO 4 -based heterojunctions are constructed successfully (13.93, 95% CI = 12.73, 15.13), followed by coupled nanomaterials (13.76, 95% CI = 12.96, 14.56) and sulfides (8.68, 95% CI = 8.00, 9.36) ( Figure 5 and Table 2). By examining the screened literature, it was found that when heterojunctions are successfully constructed, the pollutant removal efficiency of the composites can be better improved. The formation of heterojunctions can increase the specific surface area of the composites and effectively narrow the band gap width of BiVO 4 , which can accommodate a larger wavelength range of visible light [49]. In addition, the formation of heterojunctions can inhibit the compounding of photogenerated electron-hole pairs by promoting the transfer of photogenerated carriers, which improves the degradation efficiency of BiVO 4 -based composites to a certain extent [96]. In the realm of photocatalytic reactions, heterojunctions are commonly classified into three primary types: type II heterojunctions, Ztype heterojunctions, and S-type heterojunctions. In the context of a type II heterojunction, the semiconductors' band gaps are ingeniously interleaved, engendering disparate valence band energies that prompt charge carriers to migrate in opposite directions, thereby enhancing charge separation effectiveness [97]. Conversely, the Z-type heterojunction's surface features a modest polarization effect, leading to the robust trapping of photogenerated electron pairs, consequently impeding their recombination. Remarkably, the Z-type heterojunction showcases a heightened redox activity, effectively prolonging the migration cycle of photogenerated electron pairs. This temporal extension ultimately bolsters the efficiency of photocatalytic reactions by elongating the lifespan of photogenerated carriers [98,99]. On another front, the S-type heterojunction exerts adept control over carrier transfer pathways, selectively retaining valuable electrons within the conduction band (CB). This strategic curation facilitates the proficient separation of electrons, culminating in an elevated photocatalytic performance [100]. The porous structure of the nanomaterials enables a strong interaction between BiVO 4 and the nanomaterials, which enhances the separation of photogenerated carriers [56], leading to the enhancement of the degradation properties of the composites. Sulfide coupling can inhibit carrier binding to a certain extent and accelerate electron-hole separation, leading to an increase in photocurrent density and thus enhancing the photocatalytic performance of BiVO 4 -based composites [101].
Relatively speaking, the positive shifts in the photocatalytic efficiency effect sizes of BiVO 4 -based composites coupled with metals (8.09, 95% CI = 7.49, 8.69), oxides (4.33, 95% CI = 3.94, 4.73), and carbon materials (7.49, 95% CI = 7.02, 7.96) are somewhat smaller ( Table 2 and Figure 5). The amalgamation of plasma metal nanoparticles with semiconductor photocatalytic materials introduces lattice distortion and assumes the role of electron traps on the semiconductor, thereby fostering electron-hole pair separation and interfacial electron transfer, ultimately heightening photocatalytic efficacy [102]. However, the photocatalytic performance of the composites is easily affected by the structure of noble metal ions such as Au, Ag, and their positions on the semiconductor photocatalysts, which makes the photocatalytic performance under metal-loaded conditions less stable [103]. The oxidebased composites primarily revolve around semiconductor combinations, exemplified by BiVO 4 -integrated oxides, which enable the efficient segregation of electrons and holes in opposite directions. Concurrently, the synergy of semiconductors with varying bandgap widths extends the light absorption spectrum of the composites, bolstering photocatalytic efficiency [104]. However, as a trade-off, coupling with narrow-bandwidth semiconductors often engenders challenges concerning stability, cost, and preparatory complexity. Similarly, the incorporation of carbon materials contributes to an enhanced photocatalytic efficacy by adeptly suppressing electron-hole recombination. Nonetheless, these composites also contend with stability issues [105].

Effect of Preparation Time and Preparation Temperature on Photocatalytic Activity of Composites
The preparation time of the composites also has an effect on the effect size of the degradation efficiency. In the literature selected for this study, the preparation time was categorized into three fractions, 1-6 h, 6-24 h, and more than 24 h, and the effect size of the degradation efficiency of the composites increased with longer preparation time of the materials, with the effect size reaching its maximum at more than 24 h of preparation (15.19, 95% CI = 14.26, 16.12) ( Figure 5 and Table 2). A longer preparation time provides more contact time for BiVO 4 -based composites for photocatalytic performance enhancement [105]. The composite preparation temperatures were categorized into three types according to the literature selected for this study, low temperature (≤100 • C), medium temperature (100-200 • C), and high temperature (≥200 • C), in which the effect size of the pollutant degradation efficiency of the composites was the largest when high temperature preparation was used (11.97,95% CI = 11.31,12.62), while that of the pollutant degradation efficiency for low temperature preparation was the lowest (4.89, 95% CI = 4.65, 5.12) ( Figure 5 and Table 2). It is shown that the amount of effect on the degradation efficiency of environmental pollutants of BiVO 4 -based composites increases with the temperature of material preparation within a certain range. A higher preparation temperature allows BiVO 4 to obtain better-ordered mesoporous conditions, and the crystallinity of BiVO 4 increases with increasing temperature, thus increasing the rate of the effect of the composites in degrading pollutants [106]. The effect of preparation conditions on the specific surface area of the composites also showed that preparation at medium and high temperatures had a better promotion for the increment of the specific surface area effector of BiVO 4 -based composites (Figure 3), and this results in an increase in the specific surface area of the composite, which in turn increases the degradability of the composite.

Effect of Degradation pH on Photocatalytic Activity of Composites
The pH value of a solution is considered a significant factor affecting photocatalytic activity because it induces changes in the characteristics of both pollutants and photocatalysts. In the case of ciprofloxacin (CIP), when the pH of the solution is above 8.9, the CIP molecules exist in the form of anions. Conversely, when the pH is below 3.6, the CIP molecules exist in the form of cations [107,108]. Furthermore, an excess of H + and OH − ions can react with superoxide anion radicals (O 2 − ) and/or compete for adsorption, leading to a reduced removal efficiency of CIP in acidic and alkaline solutions. In contrast, within the pH range of 3.6 to 8.9, CIP adopts a neutral form. In this neutral form, the CIP ring possesses a higher electron density, making it more susceptible to photo-degradation by the reactive species generated in the photocatalytic system. Therefore, under neutral conditions, the photocatalyst can more effectively degrade CIP [108].
As delineated by our aforementioned meta-analysis, the most substantial effect size concerning the photocatalytic efficacy of BiVO 4 -based composites in pollutant degradation was observed (11.64, 95% CI = 10.92, 12.36) when operating within neutral pH conditions (refer to Figure 5 and Table 2). Notably, the utilization of BiVO 4 matrix composites within this neutral milieu resulted in pronounced positive shifts in the effect sizes of photocatalytic performance for both organic (13.01, 95% CI = 11.83, 14.19) and inorganic (9.04, 95% CI = 8.27, 9.81) pollutants within the environmental spectrum (as demonstrated in Figure 6 and detailed in Table S5).

Other Factors Affecting Photocatalytic Efficiency
Numerous additional factors wield influence over photocatalytic reactions. Among these, the light source, encompassing both duration and intensity, also exerts an impact on photocatalytic efficiency. Within the purview of photocatalytic technology, the array of light sources encompasses sunlight [109], simulated sunlight [110], ultraviolet lamps [111], xenon lamps [112], mercury lamps [109], among others. Notably, roughly half of solar energy constitutes visible light, with a mere 4-6% being near-ultraviolet (UV) light capable of exciting photocatalysts. However, the majority of contemporary photocatalysts exhibit inefficiencies in harnessing sunlight. Consequently, there is an imperative to engineer photocatalysts that display a heightened responsiveness to visible light, a realm in which BiVO 4 occupies a prominent position as a significant class of visible-light-driven photocatalysts. Wang et al. compared the photocatalytic performance of BiVO 4 -based photocatalysts under UV and visible light by evaluating rhodamine B as a target pollutant and found that C-doped BiVO 4 -based photocatalysts had better photocatalytic activity under visible light irradiation [113]. Zhang et al. (2021) found that the degradation performance of photocatalysts for pollutants increased with the increase in light duration and light intensity over a range of that which is found in [114]. This further indicates that the light source is also one of the possible influencing factors for photocatalyst activity.
The exposed crystalline surface of semiconductor photocatalysts is also one of the important factors affecting photocatalytic efficiency. A good crystalline surface structure can enable the photogenerated electrons and holes of the photocatalysts to realize a more efficient separation, and Srinivasan et al. (2022) found that BiVO 4 -based photocatalysts grown along the (121) and (040) crystalline surfaces had a promotive effect on the photocatalytic performance of the photocatalysts by adjusting the various preparation parameters [115]. Tian (2020) et al. found that BiVO 4 -based photocatalysts with (010) and (110) as the main exposed crystalline surfaces have different photocatalytic performances from those found in [116]. All of the above indicate that the crystal surface exposure is closely related to the catalytic performance of photocatalysts.
In addition, the photocatalytic performance of BiVO 4 -based composites is also closely related to their particle size. When the size of the composite particles is as small as the nanometer scale, a quantum size effect occurs. As the size decreases, the forbidden band width of the semiconductor becomes larger and the redox capacity becomes stronger. At the same time, the smaller the size, the shorter the time for electron-hole pairs to reach the surface of the catalyst and the faster the reaction with the adsorbed material on the surface of the catalyst [117,118]. The synergistic coupling of g-C 3 N 4 with BiVO 4 into ultrathin nanosheets has been discerned as a potent enhancer of the photocatalytic performance exhibited by the resulting composites. [119]. A reduced particle size contributes to a heightened photocatalytic reaction performance by amplifying the exposure of active sites [120].

Conclusions
In this study, a systematic evaluation of the performance of BiVO 4 -based composites for the removal of organic and inorganic pollutants from the environment was performed using meta-analysis. The preparation conditions, including coupling type, preparation time, and reaction temperature, as well as the pollutant degradation conditions, including pollutant type, pollutant concentration, photocatalyst dosage, and solution pH, which can affect the pollutant degradation efficiency, were reviewed. This study is the first to analyze the environmental pollution degradation performance of BiVO 4 -based composites using metaanalysis, aiming to find out the optimal conditions for BiVO 4 -based composites to degrade environmental pollutants by photocatalytic technology in practical applications through systematic comparative analysis. BiVO 4 -based composites with different preparation conditions exhibit distinct morphological and performance advantages ( Table 1). The results of the above meta-analysis show that BiVO 4 -based composites exhibit a higher removal efficiency for environmental pollutants compared to single BiVO 4 materials, regardless of whether they are doped with metals, oxides, sulfides, carbon materials, nanomaterials, or construct heterojunctions. Furthermore, other preparation conditions can also improve the performance of the composites under certain conditions. Composites enhance the effectiveness of their removal of environmental organic and inorganic pollutants by increasing the specific surface area of the material and enhancing the reaction kinetic constants.

Outlook and Future Perspective
While numerous studies have documented the pertinent attributes of BiVO 4 -based photocatalysts, there remains a plethora of issues that warrant thorough consideration and resolution in the future: (1) According to the results of our meta-analysis, in order to make the photocatalytic performance of BiVO 4 -based composites as high as possible, the method should be designed to optimize the various preparation and reaction conditions of the materials to construct nanoscale heterojunctions with large specific surface area. And the preparation pH should be controlled to be around neutral, while allowing each preparation material be fully mixed at a higher temperature. (2) BiVO 4 -based photocatalysts have demonstrated a promising capacity for effectively degrading organic pollutants present in wastewater, including substances like rhodamine B and methylene blue. Additionally, these photocatalysts exhibit a notable proficiency in reducing inorganic heavy metals, such as Cr(VI), within aqueous environments. Although current research primarily encompasses laboratory-based simulations and verifications, there exists a substantial opportunity for future endeavors to transition toward large-scale applications in real-world wastewater treatment processes. (3) BiVO 4 -based photocatalysts, responsive to visible light, present a cost-effective alternative to conventional photocatalytic devices, offering greater environmental compatibility. In the material synthesis process, prioritizing the utilization of economically viable and environmentally benign raw materials for coupling with BiVO 4 helps prevent potential secondary pollution. This strategic approach not only ensures the mitigation of material preparation-related environmental impacts, but also paves the way for feasible large-scale industrial applications.
In conclusion, further research is required to identify BiVO 4 -based photocatalysts with optimal photocatalytic performance under specific preparation conditions as well as degradation conditions and incorporate them into large-scale production applications. This involves the continuous optimization of complex types and their loadings to maximize the photocatalytic performance of the composites and to explore their photocatalytic effects on different types of pollutants. In addition, it is essential to develop preparation methods that are cost-effective, energy-efficient, environmentally friendly, and suitable for industrial applications.

Conflicts of Interest:
The authors declare that they have no known competing financial interest or personal relationship that could have appeared to influence the work reported in this paper.