SYNTHESIS AND CHARACTERIZATION OF CdS/UiO-66/Ag3PO4 NANOCOMPOSITE FOR PHOTOCATALYTIC DEGRADATION OF METHYL ORANGE UNDER VISIBLE LIGHT IRRADIATION

Single, binary and ternary nanomaterials were synthesized by precipitation, solvothermal, simple solution and impregnation methods to serve as photocatalysts. The crystal structure, morphology, band gap energy, functional groups and optical properties of these materials were characterized by XRD, SEM-EDX, UVVis, FTIR, and PL instrumental techniques, respectively. Photocatalaytic degradation performances of all the assynthesized photocatalysts were investigated under visible light irradiation using MO as a model organic pollutant. The photocatalytic degradation performances of all the photocatalysts were evaluated on aqueous solution of the model pollutant dye as well as on a real sewage sample solution collected from Bahir Dar Textile Share Company. Results suggested that the optimized ternary nanocomposite photocatalyst exhibited a relatively higher efficiency towards the photodegradation of both the methyl orange (MO) dye solution (90%) and the real sewage sample solution (71.2%). The effect of operational parameters such as pH (4), initial dye concentration (10 mg/L) and photocatalyst load (0.2 g/L) in MO dye degradation were investigated by using the ternary CdS/UiO66/Ag3PO4 (R4) nanocomposite.


INTRODUCTION
Water contamination by organic pollutants such as dyes is a serious environmental problem for human society. In addition, dyed (colored) wastewater usually consists of a number of contaminants including acids, bases, dissolved solids, toxic compounds, and other colored materials [1]. Many dyes are considered to be toxic and even carcinogenic [2]. Dye contaminated wastewaters mostly enter the environment as discharges from textile, leather, paper, printing, plastic, and food industries [3]. The presence of small amounts of dyes in water (even < 1 mg/L) is highly visible; hence it affects the pleasant appearance and causes significant loss in luminosity apart from depletion of oxygen in the aquatic system [4]. Thus, the presence of dye materials greatly influences the quality of water and the removal of these kinds of pollutants from the environment is indispensible [5].
In the past decades, various physical, chemical, and biological techniques have been developed to remove organic pollutants from wastewater. Among these efforts, semiconductor based photodegradation is seen as one of the most advantageous strategies since organic pollutants can be directly degraded by this method [6][7][8]. Particularly, the photocatalysis process promoted by visible-light irradiation is more appealing due to its effective utilization of the solar energy [9,10]. Metal oxides such as Fe 2 O 3 and Ag 3 PO 4 as well as metal sulfides like CdS have been studied as efficient photocatalysts for the degradation of various organics [11]. Most importantly, porous solids based heterogeneous photocatalytic technology has been developed recently as a much better solution to this challenge. In this regard, metal organic frameworks (MOFs) have attracted attention as catalysts in the degradation of dyes under light irradiation owing to their high surface areas, high porosity, high thermal stability, low density and 3.91 g/12.14 mmol of zirconium oxychloride octahydrate (ZrOCl 2 .8H 2 O) salt was dissolved in 50 mL of DMF and stirred for 30 min. Then 2.01 g/12.26 mmol of 1,4-benzendicarboxylic acid was dissolved in 50 mL of DMF and stirred for 30 min. The metal salt solution was added to the linker solution slowly and stirred for 24 h. The process continued for 24 h in an oven at 120 o C for a precipitation to form. The precipitate so formed was centrifuged at 2500 rpm for 30 min and the solid part was washed three times with DMF and four times with methanol. Finally, the solid [S 1 ] was dried in open air at room temperature and weighed [22]. It is expected to have structural formula of Zr 6 O 4 (OH) 4 [C 6 H 4 (CO 2 ) 2 ] 12 [18]. Silver orthophosphate (Ag 3 PO 4 ) was prepared by chemical precipitation method in the dark using silver nitrate and disodium hydrogen phosphate as raw materials [23]. To this effect first, a solution of AgNO 3 was prepared by adding 30 mmol/5.1 g of AgNO 3 powder in 150 mL deionized water. Likewise, a solution of Na 2 HPO 4 was prepared by dissolving 10 mmol/1.42 g of Na 2 HPO 4 powder in 100 mL deionized water. Then after, AgNO 3 solution was added drop by drop into the Na 2 HPO 4 solution with continuous stirring for 3 h in the dark room and the precipitation so generated was made to settle down for 12 h. The resulting precipitate was filtered from the mother liquor and washed with double deionized water four times followed by ethanol two times. It was then dried at 60 o C for 12 h, and finally calcined at 300 o C for 2 h to get Ag 3 PO 4 nanoparticles labeled as S 2 . In this study, we have used cadmium acetate tetra hydrate (Cd(CH 3 COO) 2 .4H 2 O) as a Cd +2 ion source and disodium sulfide nanohydrate (Na 2 S.9H 2 O) as S -2 ion source, respectively. For the synthesis of CdS nanoparticle using precipitation method, equal molarities of 0.1 M/12.78 g/41.4 mmol of Cd(CH 3 COO) 2 .4H 2 O and 9.95 g/41.4 mmol of Na 2 S.9H 2 O solution were prepared in separate beakers. Then the sodium sulfide solution was added drop wise into the cadmium acetate solution with a continuous stirring for 2 h at room temperature to form a clear solution. During the reaction N 2 gas was continuously purged through the suspension. Then the resulting yellow precipitate was collected by centrifugation and washed three times with deionized water and ethanol to remove the residue. Finally, the obtained CdS precipitate was further dried at 70 for 5 h [24]. The clean solid precipitate was calcined at 300 o C for 2 h in a furnace, ground into powder and kept in a container by labeling it as CdS nanoparticle; S 3 .

Binary nanocomposites (Ag 3 PO 4 /UiO-66, CdS /UiO-66 and CdS/UiO-66/Ag 3 PO 4 )
Ag 3 PO 4 /UiO-66 nanocomposite was prepared by a simple solution method based on methods described in [20] with some modifications. The Ag 3 PO 4 /UiO-66 composite was prepared based on the molar ratio of Ag:Zr. Typically, for the preparation of Ag 3 PO 4 /UiO-66 composite with Ag:Zr molar ratio of ( 0.5:1), 4.36 mg/0.0307 mmol of Na 2 HPO 4 was dissolved in 9.5 mL of deionized water. Then after, 3.564 g/0.5474 mmol of the as synthesized UiO-66 were added to the Na 2 HPO 4 solution. After stirring for 1 h, 15.64 mg/0.092 mmol of AgNO 3 dissolved in 28.48 mL of deionized water was added to the UiO-66 mixture drop wise. The total reaction mixture was about 38 mL in volume, and was stirred vigorously at room temperature for 12 h. Finally, the product was collected by filtration, thoroughly washed with deionized water for several times, and dried in vacuum oven at 80 o C (B 1 ).
CdS/UiO-66 nanocomposite was prepared following the procedure described by [21]. Accordingly, 0.133 g/0.45 mmol of Cd(CH 3 COO) 2 ·4H 2 O and 0.5 g/1.55 mmol of the assynthesized UiO-66 were dissolved in 50 mL DMSO/dimethyl sulfoxide/(CH 3 ) 2 SO. After magnetic stirring for 60 min, the obtained suspension was placed in an oven and heated to 180 o C for 12 h. The reaction mixture was cooled to room temperature. The products were collected and rinsed through centrifugation followed by redispersion processes using water and ethanol three times. The CdS loaded UiO-66 hybrids were obtained after drying the washed powders in vacuum oven overnight at 80 o C (B 2 ). For the synthesis of CdS/Ag 3 PO 4 nanostructures (Ag 3 PO 4 /CdS ratio of 3), 2.07 mmol/0.3 g of CdS nanoparticles was dispersed in 50 mL of deionized water and stirred for 2 h. To the above dispersion 6.21 mmol/2.37 g of Na 2 HPO 4 was added with vigorous stirring for 1 h. Finally, 18.63 mmol/3.16 g of Ag 3 PO 4 was added to the reaction mixture. Formation the composite material proceeded for 2 h at room temperature at pH of 4. The resulting yellowish precipitate was dried for 12 h in a vacuum oven and calcined at 300 o C for 3 h (B 3 ) [25].  3.06 and 6.10 mg, corresponding to Ag/Cd mole ratio of 4, 3, 2, and 1) was added to the above dispersion and sonicated further for 2 h. Finally, stoichiometric amount of Na 2 S.9H 2 O (1.20, 1.61, 2.40 and 4.75 mg) dissolved in 50 mL of DIW was added to the above mixture and stirred vigorously for 1 h. The precipitate obtained as such was filtered and washed four times by deionized water (DIW) and two times with ethanol to remove impurity. The product was dried at 80 o C for 12 h in vacuum oven. The obtained powder was then ground to get fine particles and then labeled as R 1 , R 2 , R 3 and R 4 for each molar ratios (1:1, 2:1, 3:1 and 4:1), respectively.

Photocatalytic activity
Photocatalytic activities of the as-synthesized photocatalysts were studied through comparison of methyl orange (MO) degradation under dark and visible light using each of the as-synthesized photocatalysts at a time. For studying the methyl orange degradation, 0.2 g/L of each assynthesized photocatalyst powder and 10 mg/L of 100 mL aqueous solution of MO was taken in a reactor tube. The suspension of catalyst and MO may be expected to achieve adsorptiondesorption equilibration after the reactor was kept for 1 h under dark environment [26]. Then the suspension was subjected to visible light irradiation with continuous stirring using a magnetic stirrer. In the event of doing the stirring air/oxygen was purged into the solution as required. Then absorbance of MO aqueous solution was measured in 20 min time interval by taking each time 10 mL of the solution. In other words, 10 mL the MO aqueous suspension was withdrawn at 20 min time interval, centrifuged at 3000 rpm for 10 min and then filtered off; the clear solution was taken in cuvette and its absorbance measured at λ max of MO. The percentage degradation was calculated for each case using the formula: where A 0 is absorbance of dye at initial stage, A t is absorbance of dye at time "t". Rate of degradation of methyl orange by the photocatalysts was expressed by the Langmuir-Hinshelwood model. When the initial concentration of the dye is lower, the reaction rate can be expressed as [27]: where k (min -1 ) is the apparent pseudo-first order reaction rate constant, C o is absorbance of MO dye in solution at the initial stage, C t is the absorbance at a given time and t (min) is the reaction time. Adsorption tests in dark were carried out in order to evaluate the adsorption of dye on catalyst surface and C t /C o versus irradiation time was plotted for comparison.

Effects of operational parameters under visible light Photocatalyst load and initial MO concentration
The effect of photocatalyst load, CdS//UiO-66/Ag 3 PO 4 (R4), was investigated to find the optimum amount of the catalyst at which the degradation was higher that prevents excess use of photocatalyst. To this effect, the catalyst dose was varied from 0.01 to 0.4 g/L keeping other parameters such as initial dye concentration and pH constant (10 mg/L and pH 4). The effect of concentration of MO solution was studied by varying the initial MO concentration keeping pH = 4 and photocatalyst load 0.2 g/L.

Initial pH and point of zero charge (PZC)
The effect of pH of the aqueous MO solution was determined by adjusting the pH of the initial MO solution at 2, 4, 6, 8, 10 and 12 using 1, 0.1 and 0.01 M HCl and NaOH solutions each time for the optimum initial MO concentration and adsorbent dose of the photocatalyst in 100 mL of solution. The pH with higher degradation efficiency was obtained. The pH of the PZC of CdS/UiO-66/Ag 3 PO4 (R4) was determined following the procedure described in [28].

Scavengers
To evaluate the predominant active species responsible for MO degradation, photocatalytic experiments were undertaken in the presence of scavengers. To this effect, the active species such as h + , *O 2 and •OH were deliberately scavenged using scavengers NaHCO 3 , AgNO 3 and methanol, respectively [29]. Typically, 0.2 g/L CdS/UiO-66/Ag 3 PO 4 (R 4 ) (4:1) and 0.1 M of 10 mL of each scavenger were placed into 100 mL of 10 mg/L of dye solution, then, the suspension was irradiated using visible light irradiation for the same time. Then 10 mL suspension was withdrawn at 20 min interval and centrifuged at 3000 rpm for 10 min. Absorbance of the dye was determined using UV/Visible spectrophotometer at the λ max of MO solution. Finally, the degradation efficiency of the photocatalyst over the dye was calculated to evaluate the main role of active species.

Real sample
The wastewater collected from Bahir Dar Textile Share Company was treated using ternary CdS /UiO-66/Ag 3 PO 4 (R 4 ) (4:1) nanocomposite under optimized conditions upon shining visible light irradiation. The experiment was conducted by taking 100 mL of sample of wastewater in 250 mL beaker and its pH was adjusted at 4. Then 0.2 g/L of the photocatalyst was added into the solution and the suspension was kept in the dark for 1h with continuous stirring to make sure the establishment of adsorption/desorption equilibrium. Finally, the suspension was irradiated with visible light upon continuous stirring and its absorbance was measured at 20 min interval by withdrawing 10 mL suspension for 180 min. Then the plot of C t /C o versus irradiation time was plotted.

RESULTS AND DISCUSSION
Characterization of the as-synthesized photocatalysts XRD patterns. Figure 1 shows the XRD patterns of the as-synthesized UiO-66 and the corresponding hybrids with Ag 3 PO 4 and CdS. For UiO-66(S 1 ) the intensive peaks appearing at small 2θ angles in the XRD pattern are characteristics of porous materials, which possess abundant pores or cavities. The most probable diffraction peaks of the MOFs were found below the scattering angle (2θ) of 10 o and this may be due to the inverse relation of 2θ and porosity of the photocatalyst [30]. For Ag 3 PO 4 (S 2 ), the peaks identified at 2θ equal to 33 diffuse absorption spectra. UV-Vis diffuse absorption spectroscopic data of UiO 66, CdS/UiO-66, Ag 3 PO 4 /CdS and CdS/UiO-66/Ag 3 PO 4 (R 1 , R ) obtained from the plots of absorbance against wavelength are presented in Figure 2 intercept of the tangent line on descending part of the absorption peak at the wavelength axis gives the value of absorption maximum λ max . Estimation of band gap energy using UV absorption spectroscopy in dispersed samples may not sometimes provide clear tangential line as the peak may not be well resolved for a given sample. To avoid this, we use Tauc plot of absorbance against wavelength using equation [38]: here α, hν, A, and Eg are optical absorption coefficient, the photonic energy, proportionality constant, and band gap enery, respectively. The direct band gaps of the prepared photocatalysts were determined from the plot of (αhν) 2 vs hν as indicated in Figure 2. The calculated band gaps 66, Ag 3 PO 4 and CdS are found to be 3.45, 2.43, and 2.44 eV. These ngs are similar with previous reports made on these nanoparticles [26,39]. The binary 1 ), CdS/UiO-66 (B 2 ) and CdS/Ag 3 PO4 (B 3 ) have exhibited calculated band gaps of 2.6, 2.9 and 2.5 eV, respectively. The results obtained as such ar good agreement with results of the previously reported works [21,40]. In the ternary systems the calculated band gap energies were recorded as 2.14, 2.14, 2.13 and 2.12 respectively. As can be noted here, formation of the ternary systems resulted in even lower which resulted from the interfacial combination and matched band edges formed between the three individual synthesized nanocomposite (CdS/UiO-66/Ag 3 PO 4 ). Based on the calculated band gaps and preliminary photocatalysis result (Data not shown) R 4 was selected for isible diffuse reflectance spectra of S 1 , S 2 , S 3 , B 1 , B 2 , B 3 , R 1 , R 2 , R 3 and R 4 . morphologies of the as-synthesized nanomaterials namely, UiO 66, CdS/UiO-66, CdS/Ag 3 PO 4 and CdS/UiO-66/Ag 3 PO 4 (R 1 , ) were characterized by SEM-EDX. Figure 3 depicted only the ternary nanocomposite 577 Vis diffuse absorption spectroscopic data of UiO-66, , R 2 , R 3 , Figure 2. The intercept of the tangent line on descending part of the absorption peak at the wavelength axis gy using UV-Vis absorption spectroscopy in dispersed samples may not sometimes provide clear tangential line as the peak may not be well resolved for a given sample. To avoid this, we use Tauc plot of In the ternary systems were recorded as 2.14, 2.14, 2.13 and 2.12 respectively. As can be noted here, formation of the ternary systems resulted in even lower which resulted from the interfacial combination and matched band edges formed Based on the was selected for . synthesized nanomaterials namely, UiO-66, R 2 , R 3 EDX. Figure 3 depicted only the ternary nanocomposite selected for photocatalytic application. Despite this, from the SEM images, one can discern that all the single and binary systems have irregular shapes with no distinct morphologies (data not shown). In contrast to this result, almost all the ternary systems appeared to be rod like structures. The elemental compositions of the as-synthesized nanomaterials were determined by EDX. The ternary systems revealed the presence of silver (0.4-6%; 0-2.7%; 0-3.4%; 0-5.6%) and zirconium (62.5-98.9%; 92.5-100%; 96.2-100%; 93.7-99.2%) for R 1 , R 2 , R 3 and R 4 , respectively, evidencing the heterogeneity of the ternary systems.   Shifting the peaks to lower wave number benefits the photo generated electron transfer and hence enhances the photocatalytic activity of composites. The characteristic peak at 3413 cm -1 could be attributed to the O-H stretching vibration. Band at 746 cm -1 can be ascribed to Cd-S stretching of synthesized nano-composite [42].

Photoluminescence (PL) study
The photoluminescence's spectra ofthe nanocomposite materials were shown in Figure 5. It was observed that PL intensity of the binary and ternary nanocomposites is much lower as compared to their respective single nanomaterials. Photoluminescence effect is present as the result of direct recombination, lower recombination of generated carriers causing the decrease of light emission intensity. The order of intensity is UiO-66 (S 1 ) > CdS (S 3 ) > Ag 3 PO 4 (S 2 ) > UiO-66/CdS (B 2 ) > UiO-66/Ag 3 PO 4 (B 1 ) > Ag 3 PO 4 /CdS (B 3 ) > R 1 > R 2 > R 3 > R 4 . It is in good agreement with the results obtained for photocatalytic degradation curves presented in the Figure 6. In binary and ternary nanocomposite systems the photoinduced electrons and holes can be effectively separated and hence excitonic PL intensity goes down. This is because, the lower the excitonic PL intensity, the stronger the capacity of the coupled materials to capture photoinduced electrons, the higher the separation rate of photo induced electrons and holes, and hence the higher the photocatalytic activity. It is evidently observed from photoluminescence spectra that the ternary (R 4 ) nanocomposite prepared in the present work showed relatively higher efficiency for photocatalytic degradation that concurs the result obtained from XRD and UV-Vis results. This finding is also supported from the photocatalytic activity of this composite compared with its single and binary congeners.

Photocatalytic studies
Comparison of photocatalytic activities of as synthesized photocatalysts. It is known that the photocatalytic redox reaction mainly takes place on the surface of the photocatalysts and the improved surface properties significantly influence the efficiency of the photocatalysts.  Figure 6. Figure 6. Comparison of photocatalytic activities of as-synthesized single, binary and ternary photocatalysts (Sorption-desorption equilibrium is done for 1 h before the photocatalysis is initiated).
In general, all the as-synthesized materials performed better as compared to the pristine MOF (UiO-66). This possibly happened as the result of poor absorption in the visible irradiation due to its wide band gap. The relatively higher photocatalytic performance of CdS and Ag 3 PO 4 could possible be due to their sensitivity to visible irradiation as evidenced in their narrow band gap. Despite this finding, the performance of Ag 3 PO 4 was even better as the result of its more oxidizing valence band that involves holes in the photo-oxidation process. Contrary to the host MOF, the MOF composited with narrow band semiconductors showed better photocatalytic activities evidencing the sensitizing ability of the narrow band semiconductors in both respects. As reported by [31][32][33] single photocatalyst UiO-66. This may be explained in terms of the possible retardation of the photochemical process due to some likely back reaction taking place between CdS or Ag 3 PO 4 with UiO-66 there by producing more number of charge carriers that would consequently increase the degradation efficiency. Thus, the efficiency of photogenerated electron-hole in the binary systems CdS/UiO-66 and Ag 3 PO 4 /UiO-66 was higher than that of the pristine UiO-66.
When it comes to the ternary photocatalysts (CdS/UiO-66/Ag 3 PO 4 ), they exhibit the highest photocatalytic activity compared with their binary congeners; Ag 3 PO 4 /UiO-66, CdS/UiO-66 and CdS/Ag 3 PO 4 . The pronounced enhancement of the photocatalaytic activity by the ternary nanocomposites may be attributed for their having more than one paths to form electron-hole pairs because of the existence of three different interfaces, and the electron-hole recombination prevented to the maximum extent in such system. Among the ternary systems, the nanocomposite R 4 (4:1) showed highest photocatalytic activity as compared with the other percentage ratios and hence selected for the subsequent study. Enhanced photocatalytic activity of R 4 could be due to effective loading of Ag 3 PO 4 and CdS on UiO-66 nanocomposite to create visible sensitive heterojunction which increases its photo absorption capacity in the visible region.

Effect of experimental parameters on MO degradation under visible light pHpzc and Effect of pH
The pHpzc of the photocatalyst was found to be 5.89 (data not shown), which is expected to be the point at which the surface charge of R 4 is neutral. The photodegradation result shown below revealed that higher adsorption and higher photocatalysis were obtained at pH less than the PZC of the catalyst, which is pH = 4.0. This is due to the higher interaction between the positively charged surface of the photocatalyst particles and the negatively charged (anionic) MO molecules at this pH. To study the effect of pH on the degradation of MO, the experiments were carried out at various pH values, ranging from 2 to 12 by adjusting with 0.1 M each of NaOH or HCl, the catalyst load 0.2 g/L and at 10 mg/L dye concentration (Figure 7).  The percent degradation increased considerably with an increase in pH from 2 to 4 for assynthesized nanocomposite and this indicates the percentage decolorization/degradation is higher at acidic condition than the alkaline condition. The photocatalyst exhibited maximum rate of degradation (90%) at pH = 4 at 180 min. Unlike others, the percent degradation declined beyond this pH. The general trend is, however, similar to previous studies done for photocatalytic degradation of MO dyes [43,44].

Effect of initial dye concentration
The photocatalaytic degradation of dye was carried out by varying the initial concentration of dye from 10 to 30 mg/L in order to determine the appropriate dose of dye solution. It was found that nanocomposites exhibited a higher photocatalytic activity at a low dye concentration (10 mg/L) but showed decreased degradation efficiency at high initial dye concentration (data not shown). This is because with increase in the dye concentration, the solution becomes more intense in color and the path length of the photons entering the solution is decreased resulting in only a few photons reaching the catalyst surface. Hence, the productions of hydroxyl radicals are reduced. Therefore, the degradation efficiency is reduced [45]. This negative effect is because of the following reasons: the number of dye molecules that are adsorbed on the surface of photocatalyst increase because of the increment in dye concentration. In addition, at a high dye (MO) concentration a significant amount of visible light is absorbed by the dye molecules [46]. The generation of OH• radicals on the surface of catalyst is likely to be reduced since active sites on the surface of the catalyst are occupied by the dye ions.

Effect of photocatalyst loading
Influence of photocatalyst dose on the degradation of organic pollutants was carried out by varying the catalyst dose from 0.01 to 0.4 g/L ( Figure 8). In this study, the degradation of MO initially increases with increasing in photocatalyst load from 0.01 g/L to 0.2 g/L. However, further increase of the catalyst loading from 0.2 g/L to 0.4 g/L results in decreased the degradation of MO. This may be due to non uniform light intensity distribution apparently resulting from overloading. As the result, the reaction rate would become lower with increased photocatalyst dosage. At the same time, at lower photocatalyst loading, the degradation of the organic molecule; (MO) should be lower implying that more light is transmitted through the reactor and lesser transmitted radiation would only be utilized in the photocatalytic reaction [47]. The optimum catalyst dose for synthesized nanocomposite was found to be 0.2 g/L. The result was similar to that of [44], who demonstrated that the effect of catalyst loading would be more on the solar photocatalytic degradation for the range 0.01 to 0.4 g/L of MO. A number of studies have indicated that the photocatalytic degradation rate initially increased with catalyst loading and then decreases at higher values because of light scattering and screening effects. The tendency towards agglomeration (particle-particle interaction) also increases at high solids concentration, resulting in a reduction in catalyst surface area available for light absorption and hence a drop in the photocatalytic degradation rate. Although the number of active sites in solution will increase with catalyst loading, a point appears to be reached where light penetration is to be compromised because of excessive particle concentration. The trade off between these two opposing phenomena results in an optimum catalyst loading for the photocatalytic reaction [48].

Effect of scavengers
To evaluate the mechanism of photocatalytic degradation of CdS/UiO-66/Ag 3 PO 4 (R4) over methyl orange, the influences of active species such as superoxide radical ( _ *O 2 ), hole (h + ) and hydroxyl radical (*OH) on the photo degradation process were evaluated using 10 mg/L of MO and 0.2 g/L of the photocatalyst at pH = 4 ( Figure 9). Different scavengers were used to remove the corresponding active species so that the function of different active species in the photocatalytic activities process based on the change of photocatalaytic conversion of targeted pollutant methyl orange could be understood. The scavengers used in this reaction were NaHCO 3 for h+, CH 3 OH/H 2 O for *OH and AgNO 3 for superoxide radical *O 2 [49]. The photocatalytic conversion of MO without scavenger was 88.2%. When AgNO 3 was added, the photocatalaytic degradation of MO decreased to 63% where as when CH 3 OH/H 2 O and NaHCO 3 were added, the photocatalaytic conversion of MO become 64.8% and 74.08%, respectively. The results indicated that all the scavengers considered have suppressed the photocatalaytic degradation efficiency of the photocatalyst although the effect of AgNO 3 and CH 3 OH/H 2 O are more pronounced. Therefore, the main active species in this photo degradation reaction are the superoxide radical *O 2 and hydroxyl radicals. The direct involvement of holes appeared to be restricted. Rather the holes involve indirectly via the reaction of these species with water molecules to create very reactive hydroxyl radicals. This experiment evidences the involvement of the conduction band electrons and the valence band holes in the redox process with the highest photocatalaytic efficiency observed with the ternary systems as compared with their binary congeners.

Analysis of real sample
The efficiency of the as-synthesized R 4 photocatalyst was studied on the degradation of the real sample under visible light irradiation by using 0.2 g/L photocatalyst load at pH = 4, whose result is shown in Figure 10. The degradation efficiency in the real wastewater sample which was 71.22% found to be lower than that of the model MO solution (90%) due to the complexity of the wastewater which includes not only mixture of organic dyes but also other chemicals from the bleaching steps [50]. Figure 9. Degradation rate of MO as function of irradiation time in the absence and presence of scavengers (sorption-desorption equilibrium is done 1 h before the photocatalysis is initiated). Figure 10. Photocatalytic degradation of MO and real textile wastewater using R 4 (sorptiondesorption equilibrium is done for 1 h before the photocatalysis is initiated).

CONCLUSION
The crystal structures, band gaps, surface morphology, thermal stability, bond vibration (stretching), optical properties and specific surface area of all nano-composite were studied Results suggested that the optimization of ternary CdS/UiO-66/Ag 3 PO 4 (R 4 ) nanocomposite photocatalyst exhibited a relatively higher efficiency on the degradation of both MO and real sewage sample which are about 90% and 71.2%, respectively. All the results confirmed that the photocatalytic degradation of optimize ternary, R 4 (CdS/UiO-66/Ag 3 PO 4 ) system was comparatively higher than its ternary (R 1 , R 2 , R 3 ), binary and single forms.