Photocatalytic degradation of 2,4-dichlorophenoxyacetic acid from aqueous solutions by Ag3PO4/TiO2 nanoparticles under visible light: kinetic and thermodynamic studies

Between the countless chemical substances applied in agriculture, 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide is considered as a toxic and carcinogenic pollutant which is difficult to remove from water due to its biological and chemical stability and high solubility. The goal of this study was photocatalytic degradation of 2,4-D, using Ag3PO4/TiO2 nanoparticles under visible light. The Ag3PO4/TiO2 nanoparticles were characterized using XRD, FESEM and EDS analysis to investigate its crystal structure and elemental compounds. The effect of operating parameters such as pH, contact time, catalyst dose, and initial concentration of herbicide on the efficiency of the process was studied. Increasing the pH and initial concentration of herbicide led to the reduction of the efficiency of removing the herbicide, while increasing contact time and catalyst dose increased the efficiency. The best result (98.4% removal efficiency) was achieved at pH1⁄4 3, 1 g/L catalyst dose, 60 min contact time, and 10 mg/L initial concentration of 2,4-D. According to the results, 2,4-D removal efficiency with Ag3PO4/TiO2 photocatalyst reached 96.1% from 98.4% after 5 cycles of reaction. The pseudofirst-order kinetics was the best fit for the 2,4-D degradation by Ag3PO4/TiO2 with correlation coefficients (R1⁄4 0.9945). The results demonstrated that the photocatalytic process using Ag3PO4/TiO2 nanoparticles in the presence of visible light had a relatively good efficiency in removing 2,4-D. Moreover, Ag3PO4/TiO2 can be used as a reusable photocatalyst for the degradation of such toxins from polluted water and wastewater.


INTRODUCTION
Water contamination has received much attention due to its harmful effects on human health and the whole ecology (de Souza et al. ). The growth of agricultural activities causes the wide use of many different types of herbicides by farmers, leading to the existence of their residues in water (Matias et al. ). More than 1,500 herbicide brand products comprise 2,4-dichlorophenoxyacetic acid (2,4-D) as the active ingredient (Li et al. ). 2,4-D is the most common systemic herbicide, which is applied to control broad-leaf weeds; it has also been used for many years because of its very low price and selectivity (Nunes et al. ). 2,4-D has a long half-life, which creates enough time for it to be transferred to surface and underground water and to create kidney and liver issues in animals and humans (Arroyave et al. ). Considering its high solubility and its link to many diseases including cancer and endocrine disorders, it is vital to remove the concentration of 2,4-D from water (Xu et al. ). The structure and properties of 2,4-D are shown in Figure 1 and Table 1.
Until now, various techniques have been used to remove 2,4-D from aqueous solutions such as adsorption (Trivedi et al. ), biological degradation (Quan et al. ), electrochemical (Fontmorin et al. ), irradiation, Fenton (Chen et al. ), ozonation (Piera et al. ), chemical oxidation (Kwan et al. ), photocatalytic degradation (Baloochi et al. ), and many others (Youssef et al. ). Photolysis has attracted many considerations in the removal of 2,4-D in recent years (Bian et al. ). The use of photocatalyst in  Titanium dioxide (TiO 2 ) is known as a superb choice for photocatalytic degradation. This is because TiO 2 is a low cost, non-toxic, stable, and highly efficient semiconductor (Olama et al. ; Heydari et al. ; Rahmat et al. ). In addition, TiO 2 has a relatively large band gap (about 3.2 eV) that requires ultraviolet (UV) light (about 4% of the solar spectrum) for activation, limiting its application in the visible light region (Cui et al. ). Recent studies have focused on the modified photocatalysts that can be activated in the presence of visible light, which contains the larger portion (46%) of the solar light (De la Cruz et al. ). Ag 3 PO 4 has been reported to be successfully utilized under visible light for water treatment and organic contaminant photodegradation (Huang et al. ; Bian et al. ). Photocatalytic degradation is one of the most advanced oxidation processes, causing the degradation of pollutants by producing hydroxyl free radicals. This process begins with exposing a heterogeneous photocatalyst to the photons of a light source. With light radiation, electrons in the valence layer are transferred to the conduction layer, and electron holes are created; these holes react with the water molecules on the surface and produce hydroxyl radicals that can oxidize the contaminant. Finally, the organic compounds will be degraded into minerals according to the following equations (Dos Santos et al. ).
In this study, the photocatalytic activity of Ag 3 PO 4 /TiO 2 for degradation of 2,4-D under visible light was evaluated. The effects of the practical parameters, including pH, initial 2,4-D concentration, time, catalyst dosage, temperature, and water matrices, on the degradation of 2,4-D were further investigated. Therefore, the stability and reusability of the photocatalyst were also checked.

Synthesis of Ag 3 PO 4 /TiO 2 photocatalyst
Synthesis of silver phosphate deposed onto Degussa titania (P25) was carried out using the in situ precipitation method reported by Yao et al. (). 1.6 g of titania (P25 with 80% anatase and a surface area of 50 m 2 g À1 ) was dispersed in 50 ml of distilled water and sonicated for 5 min. After sonication, 3.05 g of AgNO 3 was added to the titania dispersed water and the resulting solution was magnetically stirred for 10 min at 200 rpm. Sodium phosphate was previously dispersed in 50 mL distilled water and was then added dropwise to the prepared solution. The final solution was magnetically stirred for 300 min. Then, the color changed from white to yellow in the solution. The Ag 3 PO 4 /TiO 2 nanocomposites were then filtered and washed with ethanol and water and dried at 60 C for 12 h. The photocatalyst was prepared and then was used for photocatalytic reaction and characterization (Taheri et al. ).

Characterization of Ag 3 PO 4 /TiO 2 nanocomposite
In order to characterize the physicochemical properties of the Ag 3 PO 4 /TiO 2 nanocomposite, the X-ray diffraction (XRD) pattern technique was applied. XRD analysis was employed to investigate the existence of Ag, phosphate, and TiO 2 nanoparticles in this structure. The crystalline size of Ag 3 PO 4 /TiO 2 nanocomposite was calculated applying the Debye-Scherer relation given in Equation (7) D ¼ Kλ βcosθ (7) where D is the crystalline size (nm), λ is the wave-length of the X-ray radiation, θ is the diffraction angle, K is the Scherer constant (0.94), and β is the full width at half maximum (FWHM) of main intensity peak (Orooji et al. ). Field emission scanning electron microscopy (FESEM) images were obtained by using a sigma vp scanning electron microscope equipped with an Energy Dispersive Spectrometer (EDS) for the determination of the elemental compounds distribution, surface morphology, and size of the nanoparticles.

Photocatalytic degradation procedures
One factor at a time was used to obtain the optimized conditions. The type and value of operational parameters in the degradation of 2,4-D are presented in Table 2. During our photocatalytic experiment, 100 ml of the solution containing the desired concentration of 2,4-D was loaded in the open glass reactor under continuous stirring. The appropriate amount of photocatalyst was dispersed to the solution and was magnetically stirred for 15 min in the fully dark environment to ensure adsorption/desorption equilibration. Then, a 300 W xenon (ozone free) lamp with a UV cut off filter (λ < 420 nm) was turned on. After that, 2 ml samples were filtered with a syringe filter and analyzed with HPLC.

Analytical methods
HPLC was employed to monitor the concentration of 2,4-D. The separation was achieved on a C18 (OD) column. The mobile phase consisted of 80:20 UPW: acetonitrile eluted at 0.8 ml/min and 30 C, while the injection volume was 20 μl. Detection was achieved through a UV detector. The limit of detection (LOD) and limit of quantification (LOQ) for 2,4-D are 15 and 50 μg/L, respectively. The excitation wavelength was 280 nm. Under these conditions, the retention time was 2.45 min. The 2,4-D removal efficiency was calculated with Equation (8) where C o demonstrates initial concentration of 2,4-D (mg/L), C e denotes residual concentration of 2,4-D (mg/L) after degradation. Also, the adsorption capacity of 2,4-D by Ag 3 PO 4 /TiO 2 was calculated according to the following equation (Mazloomi et al. ; Zakeri et al. ).
where q e is the amount of 2,4-D adsorbed per mass unit of photocatalyst in milligrams per gram, C i and C e are the initial and equilibrium concentration of 2,4-D (mg/L), respectively. V is the volume of the solution (L), and M is the amount of nanoparticle used (g).

Reaction kinetics
Equations (10) and (11)  Where C t and C 0 are herbicide concentrations at t and zero times, respectively, k is the degradation rate constant (min À1 ).
The pseudo-first-order and pseudo-second-order kinetic model equations for 2,4-D degradation by Ag 3 PO 4 /TiO 2 are represented as follows (Sharifi et al. ; Tapouk et al. ).
According to these equations, the q e and q t parameters are the adsorption capacity at the equilibrium time and time t (mg/g), respectively. Also, k 1 and k 2 are the adsorption rate constant (min À1 ) in pseudo-first-order (Dehghani

Thermodynamic studies
In order to study the thermodynamics of the adsorption process, the thermodynamic parameters, including changes in enthalpy (ΔH ), changes in entropy (ΔS ), and Gibb's free energy (ΔG ), were obtained using the following equations (Dehghania et al. ).
In this equation, R represents the global constant of gases (8.314 J/mol.K) and T is the temperature in Kelvin.
To determine the enthalpy (ΔH ) and entropy (ΔS ) parameters, Ln (q e m/C e ) vs. 1/T was plotted. Where m is the photocatalyst dosage (g/L), q e was the amount of 2,4-D adsorbed per unit mass of the photocatalyst (mg/g), and C e was the equilibrium concentration of 2,4-D (mg/L).

Characterization of the synthesized photocatalysts
The XRD patterns of Ag 3 PO 4 /TiO 2 composite photocatalyst ( Figure 2) showed a remarkable peak at 25.3 , which could be indexed to the (101) plane of TiO 2 (P25). For the Ag 3 PO 4 /TiO 2 , except the peak at 25.3 indexed to the (101) plane of TiO 2 (P25), further peaks at 2θ values of 20. 93 , 29.78 , 33.39 , 36.68 , 47.94 , 52.85 , 55.19 and 57.64 could be indexed to the (110) According to the figure it is clear that the structure of Ag 3 PO 4 is cube-shaped and TiO 2 particles are composed of anatase phase. The sharp and strong XRD peaks demonstrate the extremely crystallized structure of Ag 3 PO 4 /TiO 2 nanoparticles. These results affirmed that Ag 3 PO 4 /TiO 2 particles had been successfully synthesized and Ag 3 PO 4 nanoparticles have been perfectly placed on the surface of the TiO 2 . This pattern showed complete integrity without impurities.
The morphology of the synthesized samples evaluated in and 4 show the FESEM images and the EDS spectrum of the synthesized Ag 3 PO 4 /TiO 2 heterostructure photocatalyst. As observed in these images, the synthesized Ag 3 PO 4 /TiO 2 consists of agglomerations of spherical shape nanoparticles. The energy dispersive X-ray spectrum (EDS) analysis of this photocatalyst proves the existence of Ag, Ti, P, and O elements. Moreover, the particle size appears to be less than 81 nm.

Effect of solution pH
The pH of a solution plays a key role in the photocatalytic degradation of an organic pollutant from aqueous solutions. In this work, the effect of solution pH in the range of 3-11 was investigated on the photocatalytic process efficiency for 2,4-D degradation. These experiments were conducted at initial 2,4-D concentration of 20 mg/mL, catalyst dosage of 0.5 g/L during 30 min. The temperature was maintained at 25 C with a magnetic stirrer equipped with a thermostat. Figure 5 shows the efficiency of the photocatalytic degradation under several pH values. According to the results, the phtocatalytic degradation efficiency of 2,4-D increased with decreasing pH. The maximum photocatalytic degradation efficiency of 2,4-D was obtained at pH ¼ 3 (acidic pH). This can be related to P Ka of the 2,4-D herbicide and pH pzc of the photocatalyst. The P Ka factor for the 2,4-D herbicide is 2.64 (Tang et al. ). Above this pH, the herbicide exists in the anion form. On the other hand, pH pzc for TiO 2 and Ag 3 PO 4 nanoparticles has been reported to be 6.9 and 6.55, respectively (Gaya & Abdullah ). The surface charge of the photocatalyst is positive at a lower pH than pH pzc (acidic pH). Therefore, the pH should be lower than the pH pzc and higher than the pollutant p Ka

Effect of catalyst dosage
The effect of the amount of catalyst in the limited area of 0.1-3 g/L was examined on the photocatalyst performance for 2,4-D degradation under the following conditions, pH ¼ 3, initial 2,4-D concentration ¼ 20 mg/L at a temperature of 25 C during 30 minutes. The results, as shown in Figure 6, demonstrated that the photocatalytic efficiency of Ag 3 PO 4 /TiO 2 increased with increasing catalyst amount from 0.1 to 1 g/L. This can be attributed to the increase in the surface area and active sites available in the photocatalyst for photocatalytic degradation of the pollutant. After 1 g/L, excessive amounts of catalyst reduced the degradation efficiency. This is due to the additional increase in catalyst amounts, which leads to the agglomeration of catalyst nanoparticles, decrease in the active sites on catalyst surface, prevention of light penetration (Chun et al. ; Singh & Muneer ; Gaya & Abdullah ), and eventually reduction in the removal efficiency.

Effect of contact time
The results of investigating the effect of contact time on 2,4-D removal with Ag 3 PO 4 /TiO 2 under visible light are shown in Figure 7. In this diagram, the increase of removal efficiency is clearly seen with increasing contact time. By increasing the contact time to 60 minutes, the removal efficiency increases with a relatively steep slope, which can be due to the presence of unsaturated active sites on the outside surface of the nanocomposite. After this time, the trend of   removal efficiency was approximately constant, which can be ignored. Therefore, 60 minutes was considered as the optimum contact time. The optimum contact time of 2,4-D removal was considered 90 minutes with MIEX resin in Diang et al. () study and 60 minutes in Dehghani et al. () study by using modified granular activated carbon. In the study conducted by Tang et al. (), the maximum contact time for removal of 2,4-D with a concentration above 100 mg/L from the solution was 5 minutes. This may be due to the high tendency of 2,4-D for the carboxyl groups that exist on the Fe/OMC nanoparticles. The reason for the difference in equilibrium time in different studies can be attributed to the type of nanoparticles that were used for the removal of 2,4-D and the difference in the initial concentrations of the desired contaminant.

Effect of initial 2,4-D concentration
The effect of initial concentration of 2,4-D in the range of 10-50 mg/L was evaluated on the photocatalyst efficiency under the given conditions, pH ¼ 3, catalyst dosage ¼ 1 g/L at a temperature of 25 C during 60 minutes. As shown in Figure 8, the photocatalyst efficiency decreased with increasing 2,4-D concentration. These results are obviously related to occupying the empty active site from the photocatalyst, catalyst surface becomes saturated and inactivated eventually, therefore, less degradation efficiency is achieved (Arana et al. ). The results of Orooji et al. () study are consistent with the present study.

Photocatalytic activity
Ag 3 PO 4 /TiO 2 composite under visible light represented high photocatalytic activity in the degradation of 2,4-D. Figure 9 shows the results of the photocatalytic activity of TiO 2 , Ag 3 PO 4 and Ag 3 PO 4 /TiO 2 composite with and without visible light for 2,4-D degradation. As shown in the Figure 9, titanium dioxide had a low removal efficiency because titanium dioxide is active only in the presence of UV light radiation due to its wide band gap (3.2ev). The Ag 3 PO 4 substantially increased the photocatalytic degradation activity of Ag 3 PO 4 /TiO 2 , which indicates the existence of a synergistic effect between silver and titanium phosphate nanoparticles. Furthermore, degradation activity of Ag 3 PO 4 /TiO 2 composite investigated in the presence of visible light compared with the absence of visible light irradiation. The results showed the photocatalytic degradation activity of Ag 3 PO 4 /TiO 2 composite under visible light irradiation was much greater than Ag 3 PO 4 /TiO 2 composite without light, which can be attributed to the photo generated active species, this subject denoting the role of light irradiation.

Reusability of photocatalyst
One of the most critical issues for practical applications in photocatalytic processes is the investigation of the long term stability and reusability of photocatalysts in photocatalytic degradation processes. To evaluate the reuse of the photocatalyst, 1 g/L of Ag 3 PO 4 /TiO 2 photocatalyst was added to 10 mg/L 2,4-D in 100 ml of aqueous solution under the visible light for five consecutive cycles. After each step, the catalyst was centrifuged, washed with water and ethanol, and dried in the oven at a temperature of 60 C; the catalyst was used again for degradation purposes. As shown in Figure 10, the efficiency of degradation of 2,4-D decreased slightly from 98.4% to 96.1% after five cycles; this may be related to the inactivation of some Ag 3 PO 4 /TiO 2 photocatalyst (Xin et al. ). It could be concluded that recycling of Ag 3 PO 4 /TiO 2 show high efficiency for 2,4-D removal. This is consistent with the Liu et al. () study.

Reaction kinetics
In the present study, the kinetics of 2,4-D degradation by Ag 3 PO 4 /TiO 2 was investigated using two common models, pseudo-first-order and pseudo-second-order kinetic models (Figures 11 and 12). The values of the kinetic parameters are shown in Table 3. Figure 11 demonstrates that the degradation rate of 2,4-D conformed with pseudo-first-order kinetic model. Hence, the high value of correlation coefficient (R 2 ¼ 0.9908) approved that the data were matched well with pseudo-first-order kinetic for 2,4-D degradation. Moreover, Sandeep et al. (), Meenakshi et al. (Meenakshi & Sivasamy ) have reported the same results regarding that photocatalytic degradation of 2,4-D by TiO 2 and zinc oxide nanorods respectively, followed the first-order kinetics.
Thermodynamic study Figure 13 shows Ln (q e m/C e ) diagram versus 1/T. Slope and y-intercept of this diagram were used to determine the values of ΔH and ΔS . According to Table 4, we observed  that the values of ΔH and ΔS were 75.581 kJ/mol and 0.289 kJ/mol K, respectively. The positive ΔH value confirmed that the reaction was endothermic in nature. The positive ΔS value showed a high tendency to adsorb 2,4-D to the catalyst and therefore high degradation efficiency. The values of ΔG was obtained at temperatures of 293.15 K, 303.15 K, and 313.15 K equal to À9.189 kJ/mol, À12.084 kJ/mol, and À14.975 kJ/mol, respectively ( Figure 14). The negative value of ΔG demonstrated the feasibility and automatic nature of the process (Dehghania et al. ). Also in the Nejati et al. study on the 2,4-D removal from aqueous solution by Cu-Fe-layered double hydroxide, ΔS , ΔH and ΔG values were obtained as positive, negative and negative respectively (Nejati et al. ).

CONCLUSION
In this work, a novel Ag 3 PO 4 /TiO 2 heterostructure photocatalyst was successfully prepared via in-situ deposition of Ag 3 PO 4 nanoparticles onto TiO 2 (P25) and was applied under the visible light for degradation 2,4-D from aqueous solution. The operation parameters on the degradation efficiency were evaluated by one-factor-at-a-time (OFAT) method. The results showed that the maximum degradation efficiency of 2,4-D was attained at the optimum condition of the process (pH ¼ 3, catalyst dosage ¼ 1 g/L, contact time ¼ 60 min, initial 2,4-D concentration ¼ 10 mg/L). 2,4-D degradation efficiency with Ag 3 PO 4 /TiO 2 photocatalyst reached 96.1% from 98.4% after 5 cycles. The results demonstrated that the photocatalytic process by using Ag 3 PO 4 /TiO 2 nanoparticles in the presence of visible light have a relatively    Pseudo-first-order Ln (q e -q t ) ¼ Ln q e -k 1 t 0.994 0.028 (min À1 ) Pseudo-secondorder t/q t ¼ 1/k 2 q e 2 þ t/q e 0.853 0.052 (g/mg.min) Figure 14 | ΔG values at different temperatures.
good efficiency in removing 2,4-D. Ag 3 PO 4 /TiO 2 can be used as a reusable photocatalyst for degradation of such toxins from polluted water and wastewater.