Dye Contaminants Removal via the Photocatalytic Activity of Metal Oxides-Supported Ag and AgCl Under Visible Light Irradiation

Abstract

One of the major groups of pollutants is formed by dyes existent in industrial wastewater and with increasing worldwide concerns on environmental issues, dye removal has gained significant attention. The Ag/AgCl (silver and silver chloride) counts as a suitable photocatalyst because of its surface plasmon resonances band. In this study, Ag/AgCl nanoparticles were loaded on various supports including TiO₂ (titanium dioxide), La₂O₃ (lanthanum oxide), and ZnO (zinc oxide) using deposition–precipitation and photoreduction method. The photocatalytic activity of composites was evaluated by removing dye solutions under irradiation of visible light. The results displayed that the photocatalytic activity of Ag/AgCl/TiO₂ and Ag/AgCl/ZnO is higher than Ag/AgCl/La₂O₃, owing to TiO₂ and ZnO partial photocatalytic activity under the radiation of visible light. The Ag/AgCl/TiO₂ also displayed remarkable photocatalytic activity compared with AgCl/TiO₂ and Ag/TiO₂ due to the coexistence of both active species Ag⁰ and AgCl in the composite. The nanocomposites' physicochemical properties were analyzed by X-ray diffraction, Fourier-transform infrared spectroscopy, Brunauer–Emmett–Teller, scanning electron microscope, and UV-vis diffusive reflectance spectra mode. This study provides a promising photocatalyst for dye treatment in wastewater using visible light.

Introduction

Organic dyes in industrial wastewater induce great environmental concerns as dyes bring pollution because of improper discharge and other reasons (Zhao et al., 2015). Therefore, dye pollutants replace one of the most severe environmental problems that threaten human health. Nowadays, various techniques are implemented to withdraw dye pollutants from wastewater, for example, biodegradation, adsorption, and photocatalytic oxidation degradation (Jie et al., 2014). A considerable number of studies have revealed that photocatalytic oxidation is a hopeful method for having numerous potential applications in treating most organic pollutants (Li et al., 2017). TiO₂ is widely in use for photocatalytic removal of pollutants because of its outstanding characteristics, such as nontoxicity, low cost, and thermal stability (Stucchi et al., 2015). Yet, TiO₂ does not have an extensive practical application because of low visible-light absorption and consequently, low photocatalytic performance in visible light.

For TiO₂ photocatalyst, various modification methods have been explored and one of the most important technologies to improve the photocatalytic activity of TiO₂ under irradiation of visible light is noble metal modified TiO₂. It is well known that noble metal nanoparticles (NPs), for example, Pt, Au, and Ag have strong UV-vis absorption because of surface plasmon resonance (SPR) nanostructures (Zhou et al., 2012). Silver-based NPs have gained much attention because of their ability to remove dye contaminants, but silver NPs alone do not have an appropriate band gap to make them a practical photocatalyst. Instead, Ag/Ag halides (e.g., Ag/AgBr or Ag/AgCl) are appropriate photocatalysts owing to the SPR band (Das et al., 2017; Sharma et al., 2017). Researches have shown that Ag/AgX (X = Cl, Br) NPs' loading onto suitable supports efficiently prevent the NP aggregation, minimize the photoinduced electron and hole pairs recombination, and therefore, the photocatalytic performance increases (Wang et al., 2017; Liang et al., 2018; Yang et al., 2018).

Zhoua et al. have studied the effect of Ag/AgCl loading on the photocatalytic performance of TiO₂ in removing 4-chlorophenol pollution. The composite proved to have an effective photocatalytic performance to remove 4-chlorophenol from water (Zhou et al., 2011). Wang et al. (2017) prepared Ag/AgCl@chiral TiO₂ nanofibers, which showed very high photocatalytic efficiency for the decomposition of 17-b-ethinylestradiol under irradiation of visible light. Tiana et al. (2014) reported the synthesis of Ag/AgCl/TiO₂ plasmonic photocatalyst through a simple multistage method, and nanocomposite indicated a very high photocatalytic activity to remove the orange 7, 2, and 4-dichlorophenol acid, and even to inactivate Escherichia coli. Tao et al. (2018) synthesized Ag/AgCl/ZnO composites under irradiation of visible light in a continuous microfluidic system and the results showed that the performance of Ag/AgCl/ZnO composites is better than both AgCl/ZnO and Ag/ZnO in the decomposition of methyl orange (MO). Xie et al. (2020) prepared graphite carbon nitride/La₂O₃ (denoted as CN/La) using hydrothermal method and the results showed that CN/La-6% displayed the most photodegradation activity.

Finally, Ag/AgCl composite is highly effective in reaction photocatalytic application, and many studies have focused on Ag/AgCl photocatalytic application. However, to the best of the authors' knowledge, no articles related to the support characteristic effect on photocatalytic performance of supported Ag/AgCl composite have been published. Therefore, this work compares the photocatalytic performance of Ag/AgCl NPs loaded on various supports such as La₂O₃, TiO₂, and ZnO in removing various dyes. The TiO₂ and ZnO are active photocatalysts and lanthanide perovskites also have photocatalytic activity (Nakhostin Panahi et al., 2019, 2020). Consequently, they were selected as support for the Ag/AgCl loading. This study explores the effect of support characteristics on the photocatalytic performance of supported Ag/AgCl composite.

Materials and Methods

Materials

Silver nitrate (AgNO₃), hydrochloric acid (HCl), zinc oxide, and titan dioxide (TiO₂) used for the preparation of nanocomposites were of analytical grade (Merck), were purchased from Mehr Azmon Co. (Iran), and used without any additional purification. Double-distilled water was used as a solvent for washing samples and preparing all necessary solutions.

Synthesis of nanocomposites

The Ag/AgCl/TiO₂ nanocomposites with different loadings of Ag/AgCl (30%wt and 70%wt) were synthesized by deposition–precipitation and photoreduction method. In brief, TiO₂ (1 g) was added to 50 mL of distilled water and ultrasonicated to disperse evenly. Then, AgNO₃ (0.33 g) and HCl (0.13 mL) were added to the suspension, and white precipitation of AgCl was formed. After that, the resulting suspension was subjected to ultrasonic waves and then located under irradiation of ultraviolet light (30 min) so that a few of Ag⁺ to Ag⁰ was reduced. Finally, the resultant product (Ag/AgCl/TiO₂ nanocomposite) was collected by centrifugation and dried at 60°C. Ag/AgCl/ZnO and Ag/AgCl/La₂O₃ nanocomposites were synthesized by a similar method. Ag/AgCl and AgCl/TiO₂ and Ag/TiO₂ with 70%wt were also synthesized by the same method and without TiO₂, without ultraviolet light irradiation, and without adding HCl, respectively (Zhang et al., 2011; Guo et al., 2012).

Characterization of nanocomposites

Information on the crystalline phases of the synthesized nanocomposites was characterized by X-ray diffraction (D500 Ziemence) using Cu Kα radiation. Morphological analysis was performed with a scanning electron microscope (SEM; VEGA, TESCAN-XMU). The textural property was obtained using a Micromeritics Asap 2000 Analyser. The Fourier-transform infrared spectroscopy was carried out with a spectrophotometer (Nicolet isIQ; United Kingdom) in the range of 4,000–400 cm⁻¹ with four wavenumber resolutions. To study the light absorption, the UV-visible light diffusive reflectance spectra (DRS) of the nanocomposites were measured using a UV-vis spectrophotometer (S4100-00-0701001U; Scinco, Korea).

Photocatalytic activity test

The photocatalytic activity of the synthesized nanocomposites was determined by studying the removal of dyes under irradiation of visible light. The widely used dyes in the industry, including MO, methylene blue (MB), basic red (BR), direct red (DR), and Congo red (CR), were chosen as the model. Properties of dyes are presented in Table 1. The details of the experiment are as follows: The nanocomposite was distributed into 100 mL of dye solution and a light-emitting diode lamp (60 W) was used as an irradiation source (light spectrum of the lamp is given in Supplementary Figure S1) and distance was 10 cm between the surface of the liquid and the light source. The temperature was also controlled by a cooling fan (26°C) throughout the removal experiments. The removal tests were conducted in ultrapure water with neutral pH. To achieve the adsorption–desorption equilibrium of dye on the photocatalyst, the suspension was stirred in the dark for 30 min before irradiation. Every 1 h, 5 mL of suspension was taken out of the reaction beaker and then centrifuged with a g-force equal to 2,332 to separate the photocatalyst particles. The dye solution's absorbance was measured by a UV-vis spectrophotometer (DR 2800; Shimadzu) and the dye concentration was determined according to Beer–Lambert law. Finally, the percentage (%) of dye removal was calculated by the following Equation (1):

Table 1.

Properties of Dyes

Properties of dyes
Congo red	Chemical formula	C₃₂H₂₂N₆Na₂O₆S₂
Structure
	Molar mass	696.66 g/mol
	λ _max	493 nm
	Type of dye	Cationic, acidic, secondary diazo dye
Direct red 23	Chemical formula	C₃₅H₂₅N₇Na₂O₁₀S
Structure
	Molar mass	813.72 g/mol
	λ _max	510 nm
	Type of dye	Anionic, acidic, secondary diazo dye
Basic red	Chemical formula	C₁₉H₂₅Cl₂N₅O₂
Structure
	Molar mass	426.34 g/mol
	λ _max	489 nm
	Type of dye	Cationic, basic, single azo dye
Methylene blue	Chemical formula	C₁₆H₁₈ClN₃S
Structure
	Molar mass	319.85 g/mol
	λ _max	665 nm
	Type of dye	Cationic, basic, thiazine dye
Methyl orange	Chemical formula	C₁₄H₁₄N₃NaO₃S
Structure
	Molar mass	327.33 g/mol
	λ _max	464 nm
	Type of dye	Anionic, acidic, single azo dye

(%) = [(C_{0} - C_{t}) ∕ C_{0}] \times 100

(1)

where C₀ and C_t are defined as the initial dye concentration and the dye concentration after irradiation, respectively (Luo et al., 2014).

Results and Discussions

Characterization

X-ray diffraction analysis

Figure 1 displays X-ray diffraction (XRD) patterns taken from Ag/AgCl, Ag/AgCl/TiO₂ (70%wt), and Ag/AgCl/ZnO (70%wt) powders. At the XRD pattern of Ag/AgCl/TiO₂, the diffraction peaks at 2θ = 25.3°, 37.8°, 48.1°, 55.1°, 62.7°, 68.7°, 70.3°, and 75.1° are assigned to anatase TiO₂ (JCPDS No. 21-1272), which are perfectly indexed to (101), (004), (200), (211), (204), (116), (220), and (215) crystalline planes, respectively. At the XRD pattern of Ag/AgCl/ZnO, the characteristic diffraction peaks at 2θ = 31.772°, 34.420°, 36.256°, 47.603°, 56.627°, and 62.901° are also assigned to ZnO (JCPDS No. 3614-51). The diffraction peaks located at 2θ = 27.8°, 32.2°, 46.2°, 54.9°, 57.6°, 67.4°, 74.5°, and 76.6° of XRD patterns of Ag/AgCl, Ag/AgCl/ZnO, and Ag/AgCl/TiO₂ are assigned to cubic AgCl (JCPDS No. 31-1238), which are indexed to (111), (200), (220), (311), (222), (400), and (331) crystalline planes, respectively. Finally, a peak at 38.2° that exists at all XRD patterns is assigned to metallic AgNP (JCPDS No. 65-2871). All the diffraction peaks in the XRD patterns were marked as the TiO₂, ZnO, AgCl, and Ag. Besides, the gray color of nanocomposites proved that metallic silver exists on the surface of Ag/AgCl, Ag/AgCl/ZnO, and Ag/AgCl/TiO₂ (Guo et al., 2012; Yin et al., 2016; Liang et al., 2018; Yu et al., 2018).

FIG. 1.

XRD pattern of Ag/AgCl/TiO₂ (70%wt), Ag/AgCl/ZnO (70%wt) and Ag/AgCl.

FTIR analysis

The FTIR spectra belonging to the TiO₂, Ag/AgCl/TiO₂ (70%wt), ZnO, and Ag/AgCl/ZnO (70%wt) samples are given in Supplementary Figure S2. The vibrations at 500–700 cm⁻¹ for TiO₂ and Ag/AgCl/TiO₂ correspond to stretching vibrations of Ti–O bond, and at the FTIR spectra of ZnO and Ag/AgCl/ZnO, the vibrations within 400–600 cm⁻¹ are indexed to the characterized vibrations of the Zn–O bond. The vibration ∼3,400 cm⁻¹ can be assigned to the hydroxyl groups of TiO₂ or ZnO. At all samples, the vibration at ∼1,630 cm⁻¹ is due to the bending vibration of OH bond of chemisorbed and/or physisorbed water molecules (Liu et al., 2017; Seirafi et al., 2018). For Ag/AgCl/TiO₂ and Ag/AgCl/ZnO, a new peak appeared at ∼1,385 cm⁻¹ attributed to the interaction between AgNP and TiO₂ or ZnO that caused by Ag/AgCl deposited on the TiO₂ or ZnO surface (García-Serrano et al., 2009).

Brunauer–Emmett–Teller surface area and SEM and EDAX analysis

The particle size and morphology of Ag/AgCl/TiO₂ (70%wt) and Ag/AgCl/ZnO (70%wt) were examined by SEM, and the results are given in Fig. 2. The SEM images of Ag/AgCl/TiO₂ imply that this composite is uniform and spherical. According to SEM images of Ag/AgCl/ZnO, the Ag/AgCl particles were distributed as nanosize on the ZnO surface.

FIG. 2.

SEM images of (a, b) Ag/AgCl/TiO₂ (70%wt) and (c, d) Ag/AgCl/ZnO (70%wt). SEM, scanning electron microscope.

The Brunauer–Emmett–Teller (BET) surface area was explored by nitrogen adsorption–desorption experiments. The BET-specific surface area of TiO₂ and Ag/AgCl/TiO₂ (70%wt) were 33.7 and 35.1 cm²/g, respectively. The BET-specific surface area of ZnO and Ag/AgCl/ZnO (70%wt) also were 21.2 and 23.4 cm²/g, respectively. Therefore, it is concluded that the BET surface area of the nanocomposites is similar to those of the TiO₂ and ZnO pristine.

Additional information obtained from SEM–EDAX (energy dispersive X-ray) profiles regarding the presence of elements in Ag/AgCl/TiO₂ and Ag/AgCl/ZnO are given in Supplementary Figure S3. The Ag peak that appeared in the EDAX profiles of Ag/AgCl/TiO₂ and Ag/AgCl/ZnO, indicates that it is present in the nanocomposites. Finally, the spectrums confirmed the existence of the corresponding chemical elements (Ag, Cl, Ti, and O for Ag/AgCl/TiO₂ and Ag, Cl, Zn, and O for Ag/AgCl/ZnO), and also no other elements were detected.

UV-vis diffuse reflectance spectra

Figures 3 and 4 provide the UV-vis diffuse-reflectance spectra of the TiO₂, Ag/AgCl, Ag/AgCl/TiO₂ (70%wt), and Ag/AgCl/ZnO (70%wt), respectively. It is clear from Fig. 3 that the visible light absorption of Ag/AgCl/TiO₂ was increased compared with TiO₂ and the Ag/AgCl and Ag/AgCl/ZnO also showed absorption at visible light region regarding SPR of AgNP. It is proof of the fact that Ag/AgCl, Ag/AgCl/TiO₂, and Ag/AgCl/ZnO have optical activity in the visible region (Guo et al., 2012; Liang et al., 2018).

FIG. 3.

UV-vis diffuse reflectance spectra of Ag/AgCl, TiO₂ and Ag/AgCl/TiO₂ (70%wt). (a) Plot of adsorption versus wavelength of light. (b) Plot of (αhυ)² versus the energy of light (hυ) afford the band gap energy.

FIG. 4.

UV-vis diffuse reflectance spectra of Ag/AgCl/ZnO (70%wt). (a) Plot of adsorption versus wavelength of light. (b) Plot of (αhυ)² versus the energy of light (hυ) afford the band-gap energy.

The band-gap energy (E_bg) could be estimated by the following Equation (2):

where α, h, υ, A, and E_bg are absorption coefficient, Planck constant, light frequency, constant, and band-gap energy, respectively. The n is 2 or 1/2 for indirect and direct transitions, respectively, and n = 2 was used to determine the band-gap energy of nanocomposites (Yin et al., 2016). The value of the band-gap energy can be obtained from the (αhυ)²-hυ plot by extrapolating the linear portion of (αhυ)² to 0. According to Figs. 3b and 4b, the band-gap energy of TiO₂, Ag/AgCl, Ag/AgCl/TiO₂, and Ag/AgCl/ZnO were estimated to be 3.42, 2.9, 3.0, and 3.08 eV, respectively.

This means that Ag/AgCl loading can make the band gap narrow. Consequently, more efficient use of visible light can be achieved, and Ag/AgCl/TiO₂ show more removal efficiency of pollution compared with TiO₂ under visible light radiation.

Photocatalytic property

Ag/AgCl/TiO₂ nanocomposite

The Ag/AgCl/TiO₂ (30%wt) photocatalytic activity was evaluated by removing different dyes under irradiation of visible light. According to results given in Fig. 5a, Ag/AgCl/TiO₂ (30%wt) has a high photocatalytic activity for removal of MB, MO, CR, and BR dyes under irradiation of visible light.

FIG. 5.

Removal of (a) methylene blue, methyl orange and basic red (15 ppm) and direct red and Congo red (40 ppm) by Ag/AgC/TiO₂ (30% wt) and (b) methylene blue, methyl orange and basic red (30 ppm) and direct red and Congo red (70 ppm) by Ag/AgCl/TiO₂ (70% wt) under visible light irradiation (photocatalyst dose = 70 mg/100 mL).

For further improvement of photocatalytic activity, Ag/AgCl was loaded with more amount (70%wt) on TiO₂, and Ag/AgCl/TiO₂ (70%wt) was tested to remove high concentrations of organic dyes. The removal results are given in Fig. 5b. According to this Figure, the Ag/AgCl/TiO₂ (70%wt) can remove high concentrations of dyes under irradiation of visible light. To differentiate between the absorption and the photocatalysis of dyes, experiments were also carried out in the absence of irradiation of visible light and under identical conditions. The results of dye removal through adsorption over the Ag/AgCl/TiO₂ (70%wt) nanocomposite (dark test) are given in Supplementary Figure S4, and it was found to be slight.

Owing to the more use of the CR in the industry, this dye was chosen as a model for further study on the photocatalytic properties of Ag/AgCl/TiO₂ nanocomposite.

Ag/TiO_2, AgCl/TiO₂, AgNP, Ag/AgCl and TiO₂ components

The Ag/TiO₂ (70%wt), AgCl/TiO₂ (70%wt), and Ag/AgCl/TiO₂ (70%wt) nanocomposites' photocatalytic activity in removing CR under irradiation of visible light is given in Fig. 6a. It could be found that discoloration percentage by Ag/TiO₂ and AgCl/TiO₂ is less compared with Ag/AgCl/TiO₂, and this implies the necessity of the coexistent active species Ag⁰ and AgCl for high photocatalytic performance.

FIG. 6.

Removal of Congo red (70 ppm) by (a) Ag/TiO₂ (70% wt), AgCl/TiO₂ (70% wt), and Ag/AgCl/TiO₂ (70% wt) and (b) Ag, TiO₂, Ag/AgCl and Ag/AgCl/TiO₂ (70% wt) under visible light irradiation (photocatalyst dose = 70 mg/100 mL).

The comparison between the photocatalytic activity of TiO₂, Ag/AgCl, and AgNP with Ag/AgCl/TiO₂ nanocomposite was also performed, and results of the removal efficiency are given in Fig. 6b. It is obvious that the photocatalytic activity of AgNP and TiO₂ is very low in visible light, and the photocatalytic activity of Ag/AgCl is also less than Ag/AgCl/TiO₂. The TiO₂ substrate has a large surface area and can uniformly disperse Ag/AgCl NPs upon the surface. Therefore, the loading of Ag/AgCl onto TiO₂ efficiently prevents particle aggregation, and consequently, photocatalytic activity increases (Wang et al., 2017; Liang et al., 2018).

According to dark tests (given in Supplementary Figure S5), the adsorption of CR over Ag/TiO₂, AgCl/TiO₂, AgNP, Ag/AgCl, and TiO₂ was slight.

Photocatalytic mechanism

To explore the photocatalytic mechanism of Ag/AgCl/TiO₂ and to verify the specific reactive species, several sacrificial agents were used for scavenging the active species during the photocatalytic reaction. Hence, isopropyl alcohol was used for quenching •OH, benzoquinone (BQ) for quenching • $O_{2}^{-}$ , and also ammonium oxalate (AO) for quenching h⁺, which are generated during the visible light irradiation process and playing a critical role in degrading dyes. As given in Fig. 7, the addition of isopropyl alcohol, ammonium oxalate, and benzoquinone inhibit CR degradation, suggesting that h⁺, •OH, and • $O_{2}^{-}$ all play significant roles in the decomposition of CR over the Ag/AgCl/TiO₂ nanocomposite. Moreover, the obvious decrease of the degradation efficiency after adding ammonium oxalate and benzoquinone indicates that h⁺ and • $O_{2}^{-}$ are the chief active species of the photocatalytic process of Ag/AgCl/TiO₂ toward CR. In addition, the h⁺ in the valence bond can oxide chloride ion (Cl⁻) of AgCl to chlorine atom (Cl⁰). This chlorine atom is a reactive radical species that oxidizes CR and then again is reduced to chloride ion (Shu et al., 2014).

FIG. 7.

Effects of various scavengers on the photocatalytic activity of Ag/AgCl/TiO₂ (70% wt) toward Congo red under visible light irradiation (photocatalyst dose = 70 mg/100 mL).

Therefore, based on the above observations, a feasible mechanism was proposed for the decomposition of CR over the Ag/AgCl/TiO₂ nanocomposite under irradiation of visible light. A schematic of the electron-hole separation and also transfer path of charges in the Ag/AgCl/TiO₂ is given in Supplementary Figure S6 (a). Regarding SPR, electron-hole pairs are photoinduced in AgNP under irradiation of visible light. The photogenerated electrons at the AgNP are easily transferred to the conduction band of TiO₂. Then, they are trapped by oxygen molecules (O₂) that were adsorbed on the catalyst's surface, and finally, superoxide radicals (• $O_{2}^{-}$ ) are generated. Meanwhile, the plasmon-induced holes at the AgNP are transferred to the AgCl surface and cause Cl⁻ and H₂O oxidation to Cl⁰ and •OH, respectively. Finally, the produced reactive radical species oxidize pollutant molecules (Wang et al., 2016; Yin et al., 2016; Wang et al., 2017; Gan et al., 2019). The low photocatalytic activity of AgCl at the visible light is because of the wide band gap. However, AgCl has an essential role in visible light-driven photocatalysis (Liu et al., 2015). The CR solution before and after removal test and also absorbance spectra of the CR solution before and after the photocatalysis process are given in Supplementary Figure S6 (b) and (c).

Ag/AgCl/ZnO and Ag/AgCl/La₂O₃

In the next step, Ag/AgCl NPs upon ZnO and La₂O₃ were also loaded (Ag/AgCl/ZnO [70%wt] and Ag/AgCl/La₂O₃ [70%wt]). The removal efficiency of CR by Ag/AgCl/ZnO (70%wt) and Ag/AgCl/La₂O₃ (70%wt) under irradiation of visible light is given in Fig. 8a. The Congo red removal by Ag/AgCl/TiO₂ (removal 88.5%) and Ag/AgCl/ZnO (removal 88%) nanocomposites is higher than Ag/AgCl/La₂O₃ (removal 45%), and this indicates that the type of support in composite affects photocatalytic performance. Therefore, the photocatalytic activity of TiO₂, ZnO, and La₂O₃ supports in the removal of CR was also investigated, and results are given in Fig. 8b. It was found that TiO₂ and ZnO have partial photocatalytic activity (removal 19% and 25%, respectively) under the radiation of visible light (Oda et al., 2015; Ullah et al., 2018) and La₂O₃ has no photocatalytic activity for removal of CR. Therefore, because TiO₂ and ZnO are partially visible light active photocatalysts, there can be a synergistic effect between TiO₂ or ZnO and Ag/AgCl. The loading of Ag/AgCl onto TiO₂ or ZnO presents an adding second semiconductor function way for improving the photocatalytic activity, and consequently, the CR removal increases.

FIG. 8.

Removal of Congo red (70 ppm) by (a) Ag/AgCl/TiO₂ (70% wt), Ag/AgCl/ZnO (70% wt) and Ag/AgCl/La₂O₃ (70% wt) and (b) TiO₂, ZnO and La₂O₃ under visible light irradiation (photocatalyst dose of Ag/AgCl/TiO₂, Ag/AgCl/ZnO, TiO₂ and ZnO = 70 mg/100 mL and Ag/AgCl/La₂O₃ and La₂O₃ = 10 mg/100 mL).

The CR removal through adsorption, using the Ag/AgCl/ZnO (70%wt) and Ag/AgCl/La₂O₃ (70%wt) nanocomposites (dark test), were also carried out, and results are given in Supplementary Figure S7 that was slight.

Ag/AgCl/TiO₂ recycling

The photocatalyst recovery for reuse is one of the most significant characteristics in practical application for wastewater treatment (Yang et al., 2017). For this purpose, the cycle removal experiments of CR were performed under identical conditions to investigate the photostability of Ag/AgCl/TiO₂ (70%wt). The TiO₂ was more available and cheaper than ZnO, and therefore, we selected Ag/AgCl/TiO₂ for cycle removal experiments. After each removal cycle, the Ag/AgCl/TiO₂ was filtered and washed with water, and then dried at 70°C (Feng et al., 2018). The results of CR removal using the regenerated Ag/AgCl/TiO₂ after four cycles are given in the Supplementary Figure S8. It was found that the CR removal efficiency slightly decreases (∼10%) after the fourth cycle of the experiment. It is because of the covered active surface sites with intermediates. Consequently, the Ag/AgCl/TiO₂ photocatalyst's high performance during regeneration examinations makes it an excellent photocatalyst for wastewater treatment.

In the next step, the Ag/AgCl/TiO₂ (70%wt) performance for decolorization of real textile industry's wastewater was evaluated. This wastewater mainly contains nonbiodegradable dyes, which are highly resistant to conventional oxidizing agents. The wastewater had a maximum absorption at 520 nm wavelength. Therefore, this wavelength was chosen to measure decolorization. The results of decolorization are given in Supplementary Figure S9 that was less than pure water. The real dye-contaminated wastewater has a very complex matrix compared with a dye in ultrapure water; therefore, the applicability of this approach to treating real dye-contaminated wastewater is low.

Ayodhya et al. (2016) studied the CR dye degradation under the irradiation of solar, visible, and UV lights using copper sulfide NPs, and 75.47% and 60.35% of dye were degraded at UV and solar lights, respectively. Pathania et al. (2016) synthesized chitosan-g-poly(acrylamide)/ZnS nanocomposite materials through microwave radiations, and their photocatalytic activity was tested for the CR degradation under irradiation of simulated solar. According to the results, 75% of CR was degraded. Therefore, the comparison between the photocatalytic performance of Ag/AgCl/TiO₂ and Ag/AgCl/ZnO nanocomposites with other photocatalysts (given in Table 2) proves that these nanocomposites are more effective in the CR dye degradation.

Table 2.

The Summary of Studies on Photocatalytic Removal of Congo Red

Sample	Light source	% of removal	References
Copper sulfide	Solar	60.35%	Ayodhya et al. (2016)
Chitosan-g-poly(acrylamide)/ZnS	Simulated solar	75%	Pathaniaa et al. (2016)
Zinc(II) metal-organic framework, [Zn(oba)(4-bpmb)0.5]n·(TMU-6)	Visible	48.4%	Masoomi et al. (2016)
Graphene nanoplatelets/doped bismuth ferrite nanoparticle	Visible	74%	Fatima et al. (2020)
Bi_1−xLa_xFeO₃	Visible	75%	Zhang et al. (2012)
Ag/AgCl/TiO₂	Visible	88.5%	Present work

Conclusions

The Ag/AgCl/TiO₂ nanocomposite was successfully synthesized using deposition–precipitation and the photoreduction method. The photocatalytic activity of Ag/AgCl/TiO₂ was superior compared with Ag/TiO₂ and AgCl/TiO₂, and as a consequence, for the high photocatalytic efficiency, the coexistence of both active species Ag⁰ and AgCl in the composite is necessary. The Ag/AgCl/TiO₂ (70%wt) can remove high concentrations of dye pollutants under irradiation of visible light. According to DRS results, loading of Ag/AgCl increases absorption at visible light region owing to SPR of AgNP, and loading of Ag/AgCl also makes the band gap narrow. In the following, Ag/AgCl NPs were loaded on ZnO, and La₂O₃ and photocatalytic results showed that the type of support in composite affects photocatalytic performance. The removal of CR by Ag/AgCl/TiO₂ and Ag/AgCl/ZnO was more than Ag/AgCl/La₂O₃. In the final, Ag/AgCl/TiO₂ nanocomposite for decolorization of real textile industry wastewater was also utilized and according to results, the removal efficiency was less than pure water.

Footnotes

Acknowledgment

The authors thank the University of Zanjan for its financial support.

Author Disclosure Statement

The present research is an independent scientific project resulting from (MSc/PhD) Thesis.

Funding Information

No funding was received for this article.

Supplementary Material

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Supplementary Material

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Dye Contaminants Removal via the Photocatalytic Activity of Metal Oxides-Supported Ag and AgCl Under Visible Light Irradiation

Abstract

Introduction

Materials and Methods

Materials

Synthesis of nanocomposites

Characterization of nanocomposites

Photocatalytic activity test

Results and Discussions

Characterization

X-ray diffraction analysis

FTIR analysis

Brunauer–Emmett–Teller surface area and SEM and EDAX analysis

UV-vis diffuse reflectance spectra

Photocatalytic property

Ag/AgCl/TiO2 nanocomposite

Ag/TiO2, AgCl/TiO2, AgNP, Ag/AgCl and TiO2 components

Photocatalytic mechanism

Ag/AgCl/ZnO and Ag/AgCl/La2O3

Ag/AgCl/TiO2 recycling

Conclusions

Footnotes

Acknowledgment

Author Disclosure Statement

Funding Information

Supplementary Material

References

Supplementary Material

Ag/AgCl/TiO₂ nanocomposite

Ag/TiO_2, AgCl/TiO₂, AgNP, Ag/AgCl and TiO₂ components

Ag/AgCl/ZnO and Ag/AgCl/La₂O₃

Ag/AgCl/TiO₂ recycling