Enhancing photodegradation of 2,4,6 trichlorophenol and organic pollutants in industrial effluents using nanocomposite of TiO2 doped with reduced graphene oxide

Egyptian Journal of Aquatic Research xxx (xxxx) xxx

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Enhancing photodegradation of 2,4,6 trichlorophenol and organic pollutants in industrial effluents using nanocomposite of TiO2 doped with reduced graphene oxide Mohamed H.H. Ali a,⇑, Khairia M. Al-Qahtani b, Siliem M. El-Sayed a a b

National Institute of Oceanography & Fisheries, Cairo, Egypt Chemistry Department, Faculty of Science, Princess Nourah Bint Abdulrhaman University, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 17 May 2019 Revised 13 August 2019 Accepted 19 August 2019 Available online xxxx Keywords: Reduced graphene oxide Photodegradation Industrial effluents TCP TiO2

a b s t r a c t The present study investigated the applicability of synthesized TiO2@rGO photocatalyst for degradation of organic pollutants from industrial wastewater and photodegradation of 2,4,6 trichlorophenol (2,4,6 TCP) under UV irradiation. The results showed a high reduction in COD and TOC values of industrial effluents reaching 85% and 82%, respectively. Effects of several factors on the photodegradation process such as pH of the solution, TCP initial concentration, irradiation time and catalyst dose were studied. The results proved a high relationship between pH of the solution and the adsorption capacity. The maximum degradation ratio reached up to 90% at pH 6 at the initial concentration of TCP of 50 mg/l when exposed to UV irradiation for 180 min and 0.4 g/L catalyst dose. Kinetic studies indicated that the adsorption process obeyed pseudo-second order model with qe values ranging between 22.0 and 22.7 mg/L and R2 > 0.99. Isotherm model studies showed a good fit with the Langmuir model better than Freundlich model. Ó 2019 National Institute of Oceanography and Fisheries. Hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction Chlorophenols belong to the aromatic compounds which are introduced to the surrounding environment through different industrial wastewater, originating from different industrial processes, e.g. paper and pulp bleaching, wood, pharmaceutical factories, dyeing and textile, petrochemicals and pesticide manufacturing (Krishnaiah et al., 2013). These compounds are found in a wide scale in the environment as a result of industrial wastes. The existence of these compounds in the environment causes dangerous toxic and carcinogenic effects for the aquatic organisms, plants, animals and humans even at low concentrations (Kusmierek, 2016). Their toxic effects varied widely according to the chlorine substitution degree in the phenol ring, generally the high the chlorination degree, the high the toxic effects to aquatic life. Moreover, 2,4,6 trichlorophenol (TCP) compound was classified as carcinogenic 2B Group according to the International Agency for Research on Cancer (IARC) (Khorsandi et al., 2018).

Peer review under responsibility of National Institute of Oceanography and Fisheries. ⇑ Corresponding author at: Chemistry Department, National Institute of Oceanography & Fisheries, Cairo, Egypt. E-mail address: [email protected] (M.H.H. Ali).

On the other hand, the decreasing of water pH leads to a considerable increase of their toxicity. Aquatic animals are more sensitive to toxicity than aquatic plants. Even at very low concentrations of TCP, their specific odors were produced in water and the fish flesh were also contaminated (Virtanen and Hattula, 1982). Furthermore, TCP (2,4,6-trichlorophenol) was selected in this study according to its considerable resistance as organochlorine species in the aquatic ecosystems, so there is a necessity to find a breakdown process to convert it to harmless compounds (Bashiri and Rafiee, 2016). The phenolic ring of TCP has three attached chlorine atoms, TCP was found in natural environment in two species types, ionized phenolate and nonionized phenol. Thus, TCP existence in the aquatic ecosystem highly depends on pH variations of the water, both the equilibrium and rate constants are widely different between the two species (Khan et al., 2011). Different traditional techniques which include bio-degradation methods, coagulation, oxidation, adsorption and advanced oxidation process (AOP) were used for the degradation of chlorinated phenols compounds from environmental ecosystems through physical, thermal, chemical, or biological basis (Bashiri and Rafiee, 2016; Yang et al., 2017). Some disadvantages had appeared for these traditional techniques, e.g. in cases of adsorption onto active carbon, most pollutants do not degrade to harmless species. Also, many biological techniques failed to degrade the chlorinated

https://doi.org/10.1016/j.ejar.2019.08.003 1687-4285/Ó 2019 National Institute of Oceanography and Fisheries. Hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: M. H. H. Ali, K. M. Al-Qahtani and S. M. El-Sayed, Enhancing photodegradation of 2,4,6 trichlorophenol and organic pollutants in industrial effluents using nanocomposite of TiO2 doped with reduced graphene oxide, Egyptian Journal of Aquatic Research, https://doi.org/10.1016/j. ejar.2019.08.003

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phenols because of the high resistivity of these compounds to the biodegradable processes (Khan et al., 2011). On the other hand, highly efficient and safer photocatalytic process is used to eliminate the chlorinated phenolic compounds through photocatalysis under specific conditions e.g. temperature, pressure and concentrations. Mostly, the by-products that are arisen from this photocatalytic process are harmless or even have much low toxicity (Cui et al., 2016). The photocatalytic process depends on the releasing of promoted electrons (e
CB ) from the VB (valence band) to CB (conduction band) forming a positive hole þ (hVB ). This hole either oxidizes the organic matter directly or reacts with an electron donor species as water forming OH free radicals. These free radicals react with organic compounds as dyes, chlorophenols and any other compounds (Goel et al., 2010). Furthermore, the promoted electron (e
CB ) may react with O2 forming peroxide radical (HO:2 ) (Garcia-Segura and Brillas, 2017). In these processes, organic matter is completely degraded producing harmless by-products such as CO2 and H2O (Jafari et al., 2016). Titanium dioxide doped with various metals, nonmetals or carbonaceous materials to enhance their degradation activities of different organic compounds especially for chlorinated phenols (Ormad et al., 2001; Xu et al., 2003). The fabrication of graphene (GR) – semiconductor composites has numerous interests to improve the solar energy conversion, the photogeneration pf electrons from these semiconductors (e.g., TiO2, CdS) can be shuttled in the twodimensional (2D) GR sheet (Yang et al., 2014). Furthermore, the optimization photocatalytic efficiency of graphene has been recognized as an ideal co-catalyst (Han et al., 2016). Graphene is used as a doping material to TiO2 since it is characterized by a high surface area as well as great chemical stability and electron transportability (Luo and Zafeiratos, 2017; Xiang et al., 2012). Therefore, TiO2@reduced graphene oxide (rGO) nanocomposites photocatalysts attract the attention of many researchers due to their effectiveness in improving the photocatalytic effects (Divya et al., 2017; Khalid et al., 2016; Zhao et al., 2018). The present work aimed to provide an environmentally friendly process to eliminate TCP pollutants from industrial wastes in addition to describe the main factors affecting the photocatalytic activities such as pH of the solution, adsorbent dosage effect, TCP initial concentration and contact time. Materials and methods 2,4,6 trichlorophenol and all other used chemicals with analytical grade were purchased from Sigma-Aldrich, TiO2 was received from Degussa Company (Germany).

Collection and treatment of industrial wastewater samples Industrial effluents samples were collected during winter 2019 from the drains of some factories including; cement, petrochemicals, juices, milk and cheese, paper and pulp as well as textile factories. The samples were kept in clean 2-litre polyethylene containers. Some chemical characteristics of samples were determined to represent the status of water quality criteria, e.g. pH, DO, BOD, NO2, NO3, NH4, PO4 and TP according to the standard methods (APHA, 2005). On the other hand, chemical oxygen demand (COD) was determined for selected wastes samples before and after treatment with prepared nanocomposite photocatalyst using closed reflux method (Boyles, 1997), while total organic carbon (TOC) was determined using TOC-analyzer (Shimadsu 5000A). Different doping ratio of the synthesized TiO2@rGO photocatalyst was added to each wastewater sample under specific conditions (pH = 6, 0.4 g/L photocatalyst dose and 180 min of UV irradiation time), mixed samples were centrifuged at 12000– 15000 rpm for 20 min, then COD and TOC were re-evaluated again. Batch adsorption study Photocatalytic degradation of 2,4,6-TCP with TiO2@rGO nanoparticles was carried out by UV light using 60 cm mercury lamps (six lamps of 11 Watt for each lamp). The transmission wavelength was adjusted at k > 300 nm. Different factors such as pH of the solution, catalyst dose effect, UV irradiation time and initial TCP concentrations were assessed. The mixture of TCP solution with the photocatalyst was magnetically stirred in the dark for one hour to obtain the sorption-desorption equilibrium, after that, the mixture was subjected to UV irradiation with continuous magnetic stirring. An aliquot of each sample was centrifuged at 12000 rpm to remove the nanocomposite catalyst. The concentrations of TCP were photometrically estimated at wavelength k = 315 nm using UV–VIS spectrophotometer Jenway 6800UV/Vis double beam. The photocatalysis capacity (qe) was calculated by the difference between initial (Co) and the equilibrium (Ce) TCP concentrations according to the following equation;

qe ¼

ðC 0
C e ÞxV M

where; V is the used volume of solution (L) and M is catalyst mass (g).

Results and discussion

Preparation and characterization of TiO2@rGO nanocomposites

Batch experiments

Preparation and physical characterization of used nanocomposites catalyst with different doping ratio (1%, 3% and 5% TiO2@rGO) were described in details in Ali et al. (2018). In Brief, graphite powder (30 g) was added to 15 g of NaNO3 to 360 mL of concentrated sulfuric acid, an ice bath was used with stirring the mixture for 30 min. Then, 90 g of KMnO4 was slowly added. Temperature was kept under 14 °C for 13 h. Thereafter, about 410 mL of deionized water was carefully added while raising the temperature of the mixture to 50 °C for another 12 h., H2O2 (125 mL) was slowly added with stirring for 3 h. TiO2 nanoparticles were pretreated in furnace at 500 °C for 2 h. Reduced GO was added to absolute ethanol with a concentration of 0.5 mg/ml. Weight ratio of rGO to TiO2 of 1, 3 and 5 wt% were regulated in rGO/ethanol solution. This mixture was treated by ultrasound for 30 min. Finally, the obtained mixture was dried at 60 °C for 12 h. to obtain the TiO2@rGO nanocomposites

Effect of pH To study the pH effect on photocatalytic degradation of 2,4,6 trichlorophenol, a series solution of 50 mg/L of 2,4,6 TCP with different pH values ranging from 1 to 9 were mixed with 0.4 g/L of prepared catalyst. The experimental mixtures were left in the dark under magnetic stirring for half-hour to attain the sorption–desorption equilibrium. After that, they were exposed to UV irradiation for 180 min (Fig. 1). The results showed that there was an obvious increase of 2,4,6 TCP photodegradation with the increase of pH. The maximum value of degradation (93.6%) was obtained at pH 6. Then a noticeable decrease of degradation rate with the increasing of pH values reached 75% at pH 9. Saritha et al. (2009) reported that the maximum photodegradation of 2,4,6 TCP occurred at medium pH values. The present results showed that TiO2@rGO 5% have the highest degradation than TiO2@rGO 1% and 3% catalyst (Fig. 1).

Please cite this article as: M. H. H. Ali, K. M. Al-Qahtani and S. M. El-Sayed, Enhancing photodegradation of 2,4,6 trichlorophenol and organic pollutants in industrial effluents using nanocomposite of TiO2 doped with reduced graphene oxide, Egyptian Journal of Aquatic Research, https://doi.org/10.1016/j. ejar.2019.08.003

M.H.H. Ali et al. / Egyptian Journal of Aquatic Research xxx (xxxx) xxx

Fig. 1. Effect of pH on the photodegradation of 2,4,6 TCP (C0 = 50 mg/L, T = 25 °C).

Effect of time The irradiation time is considered as one of the important factors that affect the photocatalytic degradation process (Ilyas et al., 2011). As shown in Fig. 2, the photodegradation rate has a remarkable increase with irradiation time increase, which reached maximum degradation of 83.5%, 84.6% and 87.6% for TiO2@rGO 1%, 3% and 5% respectively. Yehia et al. (2015) declared that the highest degradation of phenol reached to 75% using 1 g/L of nano-sized zero-valent iron (nZVI)) dosage after 160 min. Effect of dose To determine the optimum TiO2@rGO dose in photodegradation of 2,4,6 TCP, serial standard solutions of 2,4,6 TCP with 50 mg/L concentration were mixed with different photocatalyst dose varying between 0.05 and 0.5 g/L with continuous stirring for 180 min under UV irradiation. The results showed a remarkable increase of photodegradation efficiency with the increase of photocatalytic dose reaching the maximum values of 88%, 89.5% and 91.3% at 0.4 g/L of 1%, 3% and 5% TiO2@rGO, respectively (Fig. 3). The enhancing of photocatalytic efficiency as a result of increasing photocatalyst dose could be explained on the basis of increasing the available surface area of the photocatalyst and the formation of more active radicals (MirzaHedayat et al., 2018). On the other hand, Zhang et al. (2013) illustrated that the enhanced photocatlytic effect of RGO/ZnO-S1 composite depend on the particle size of ZnO, the smaller the ZnO size, the highest the photocatalytic effect. Furthermore, the present results showed a relative decrease of photocatalytic efficiency that was observed at dose 0.5 g/L due

Fig. 2. Effect of irradiation time on photocatalytic degradation of 2,4,6 TCP (C0 = 50 mg/L, pH = 6 and T = 25 °C).

3

Fig. 3. Effect of photocalyst dose on the photodegradation of 2,4,6 TCP (C0 = 50 mg/l; t = 180 min, pH = 6 and T = 25 °C).

to the lack of light penetration and the photocatalytic cumulative effect in the solution (Dixit et al., 2010). Effect of initial 2,4,6 TCP concentration The initial 2,4,6 TCP concentrations’ effect on photodegradation reaction was carried out using serial concentrations ranging between 5 and 50 mg/L with photocatalyst dose of 0.4 g/L (Fig. 4). The results revealed a gradual decrease of photodegradation activities with increasing the concentration of 2,4,6 TCP. Low concentrations of TCP (5 and 10 mg/L) have high removal efficiency reaching 88% and 82%. This is due to the presence of sufficient surface area of the provided catalyst and low intermediate production at low TCP concentration (Ahmed et al., 2010). However, the rich formation of intermediate products at high concentration of TCP at constant photocatalyst dose would be competing with the TCP molecules themselves onto the photocatalytic surface, which might explain the decreased degradation of TCP (Kashif and Ouyang, 2009). Furthermore, more oxidant radicals such as OH and O
2 were required to enhance the decomposition process of certain substrates (Chiou and Juang, 2007). Thus, the optimum concentration of 2,4,6 TCP is 5 mg/L. However, the concentration of 50 mg/L was chosen in order to evaluate the maximum reaction rate. A simple summarized comparison between the current data and some previous studies that focused on the photodegradation of different phenolic compounds is tabulated in Table 1. This comparison in addition to knowing the newest progress to improve the degradation efficiency and goals of such treatments help the researchers to design and provide more efficient photocatalysts.

Fig. 4. Effect of initial 2,4,6 TCP concentration on the photodegradation process (dose = 0.4 g/L, t = 180 min, pH = 6 and T = 25 °C).

Please cite this article as: M. H. H. Ali, K. M. Al-Qahtani and S. M. El-Sayed, Enhancing photodegradation of 2,4,6 trichlorophenol and organic pollutants in industrial effluents using nanocomposite of TiO2 doped with reduced graphene oxide, Egyptian Journal of Aquatic Research, https://doi.org/10.1016/j. ejar.2019.08.003

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Table 1 List of various photocatalysts and their degradation efficiencies of different phenolic compounds. Mass and morphology of catalyst

Nature of lamp, power and intensity

pH

Kinetics

Time

Efficiency %

Refs.

1 g/L TiO2 0.02 g TiO2 nanoparticles

200 W Hg-Xe lamp, 10 mW/cm2 Four UVA 6 W tubes with 20–1.5 cm

5 6

1st order *

15 h 48 h

99 100

0.5 g/L C-N-doped TiO2

Flexible strips of visible light emitting diode (VisLED 20 W UV lamp

7

Pseudo-1st order *

5h

99

(Ohko et al., 2001) (Yeo and Kang, 2006) (Wang et al., 2010)

9h

80

(Luo et al., 2015)

400 W metal halide lamp

*

30 min

87.2

(Chen et al., 2017)

0.5 g/L G/Bi2Fe4O9

150 W Xe arc lamp

5

6h

100

(Hu et al., 2015)

0.4 g/l TiO2@rGO

6 * 11 W Hg Lamp

6

Pseudo-2nd order Pseudo-1st order Pseudo-2nd order

180 min

90

Present study

0.5 g/L of TiO2-wood charcoal composites 0.1 g AgBr@ rGO

6

Table 2 Constants of Langmuir and Freundlich isotherms for 2,4,6 TCP adsorption using TiO2@rGO photocatalysts. Langmuir Isotherm

Freundlich Isotherm 2

metal

b

qmax

RL

R

TiO2@rGO 1% TiO2@rGO 3% TiO2@rGO 5%

0.03 0.04 0.06

68.50 33.30 25.00

0.400 0.330 0.250

0.99 0.99 0.99

Fig. 5. Langmuir isotherm plot for adsorption of TCP by TiO2@rGO photocatalysts.

Kf

n

R2

1.68 1.40 1.45

0.90 0.81 0.80

0.97 0.92 0.91

Fig. 6. Freundlich isotherm plot for adsorption of TCP by TiO2@rGO photocatalysts.

Please cite this article as: M. H. H. Ali, K. M. Al-Qahtani and S. M. El-Sayed, Enhancing photodegradation of 2,4,6 trichlorophenol and organic pollutants in industrial effluents using nanocomposite of TiO2 doped with reduced graphene oxide, Egyptian Journal of Aquatic Research, https://doi.org/10.1016/j. ejar.2019.08.003

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Adsorption isotherms studies Isotherms models of Langmuir and Freundlich were used to check the suitability of the adsorption process. Langmuir models postulate homogenous adsorption with sites of adsorption, where Qmax refers to the maximum adsorption capacity, and b is Langmuir constant, which is related to the affinity of adsorption. On the other hand, Freundlich assumed a heterogenous multilayer adsorption process. Langmuir and Freundlich isotherms are expressed as follows: max bC e Equation qe ¼ q1þbC , Freundlich equation e 1=n
1 qe ¼ K f C e where: qmax (mg g ) is the maximum sorbate uptake; b (L mg
1); qe (mg g
1) is the amount of adsorbed substrate; Ce (g L
1) the adsorbate concentration at equilibrium; Kf is the adsor-

Langmuir

5

and 0.40 (Table 2). The same observation was obtained by Ali et al. (2019) for methylene blue degradation dye using TiO2@rGO nanocomposites. Kinetics studies Pseudo-first order Eq. (1) and pseudo-second order Eq. (2) models are the commonly used among these models due to their great applicability in comparison with other adsorption models (Ghasemi et al., 2014). Pseudo-first order model assumes that there is a direct proportion between the occupation rate of sorption sites and the unoccupied sites.

logðqe
qt Þ ¼ logqe


k1 t 0:203

ð1Þ

bent capacity and n is the adsorption intensity. The calculated constants of Langmuir and Freundlich isotherms are shown in Table 2 and Figs. 5 and 6. The results confirmed that Langmuir isotherm model is well-fitting more than Freundlich model, with R2 = 0.99 for all the doping ratios of TiO2@rGO used (Table 2 & Figs. 5 and 6). Wang et al. (2018) reported that the adsorption isotherms of some phenolic compounds on the surface of graphene oxide and reduced graphene oxide were also fitted with the Langmuir model. To detect the mode type of Langmuir isotherm, the dimensionless constant (RL) was calculated where irreversible mode (RL = 0), linear mode (RL = 1), unfavorable mode (RL > 1), and the favorable mode if (0 < RL < 1) (McKay et al., 1982). The present findings confirmed that the favorable adsorption of 2,4,6 TCP onto the surface of all used catalysts has occurred since RL varied between 0.25

where: qt is the amount of TCP (mg/g) adsorbed at time t, qe is the amount of TCP (mg/g) adsorbed at equilibrium, and k1 (min
1) and k2 (mg/g.min) are the rate constants of pseudo-first order and pseudo-second-order reactions. The calculated kinetic constants for both the pseudo-first and the pseudo-second order reactions are shown in Table 4, while the linear relations are shown in Figs. 7 and 8. The results confirmed that the adsorption of 2,4,6 TCP obeyed pseudo-second order reaction with qe ranging between 22.0 and 22.7 mg/l and corresponding to R2 = 0.99 (Table 3). These results are in agree-

Fig. 7. Pseudo-first order reaction plot for TCP adsorption TiO2@rGO photocatalysts.

Fig. 8. Pseudo-second order reaction plot for TCP adsorption TiO2@rGO photocatalysts.

t 1 1 ¼ þ t qt k2 q2e qe

ð2Þ

Please cite this article as: M. H. H. Ali, K. M. Al-Qahtani and S. M. El-Sayed, Enhancing photodegradation of 2,4,6 trichlorophenol and organic pollutants in industrial effluents using nanocomposite of TiO2 doped with reduced graphene oxide, Egyptian Journal of Aquatic Research, https://doi.org/10.1016/j. ejar.2019.08.003

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Table 3 Constants of Pseudo-first order and pseudo-second order reactions for 2,4,6 TCP degradation using TiO2@rGO photocatalysts. Pseudo-first order reaction catalyst

Pseudo-second order reaction qe

TiO2@rGO 1% TiO2@rGO 3% TiO2@rGO 5%

K1

R
3

8.5  10 8.5  10
3 7.1  10
3

1.91 2.16 2.6

2

qe

0.82 0.80 0.87

R2

K2
3

4  10 4  10
3 4  10
3

22.0 22.2 22.7

0.99 0.99 0.99

Table 4 Some chemical characteristics of selected industrial wastewaters effluents.

pH DO mg/l BOD mg/l COD mg/l TOC mg/l HCO3 mg/l NO3 mg/l NO2 mg/l NH4 mg/l PO4 mg/l TP* mg/l

Cement Factory

Petrochemical Factory

Juices Factory

Milk and Cheese Factory

Paper and pulp Factory

Textile factory

7.50 1.1 95.6 631 212 243 824.1 447.8 1778 164.4 1315

7.40 1.0 37.2 1050 318 496 1256.1 0.0 1719 512.1 2155

11.12 0.0 215.0 895 175 0 1513.7 94.0 2350 561.2 1831

12.20 0.0 220.0 656 196 0 1631.7 95.8 2264 936.7 1356

5.75 0.0 91.0 358 138 1316 944.4 76.9 1342 265 965

8.56 2.2 79.0 1580 547 355 2450 125 4563 516 1478

*TP: Total Phosphorus.

Fig. 9. Removal of COD in wastewaters effluents using TiO2@rGO with various rGO doping ratio.

Fig. 11. Removal of COD in 2,4,6 TCP solution using TiO2@rGO with various rGO doping ratio.

Fig. 10. Removal of TOC in wastewaters effluents using TiO2@rGO with various rGO doping ratio.

ment with the results of Anirudhan and Ramachandran (2014) who reported that the removal of 2,4,6 TCP from the petroleum refinery industry effluents using a catalyst of surfactant-modified bentonite obeyed pseudo-second order reaction.

Fig. 12. Removal of TOC in 2,4,6 TCP solution using TiO2@rGO with various rGO doping ratio

Please cite this article as: M. H. H. Ali, K. M. Al-Qahtani and S. M. El-Sayed, Enhancing photodegradation of 2,4,6 trichlorophenol and organic pollutants in industrial effluents using nanocomposite of TiO2 doped with reduced graphene oxide, Egyptian Journal of Aquatic Research, https://doi.org/10.1016/j. ejar.2019.08.003

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Some chemical criteria of wastewater samples

References

Table 4 illustrates some chemical criteria of selected wastewater effluents. Generally, the results revealed bad water quality status of the selected samples with an elevation of most estimated parameters. Effluents of juice and milk factories have extremely high pH values reaching 12.2, while paper factory showed acidic condition with pH = 5.75. Dissolved oxygen is depleted completely in some drains. Meanwhile, COD, BOD and TOC showed extremely high values (Table 1). These data are in accordance with those obtained by Al-Ibrahem (2014) who reported that water quality index has a bad rating for several industrial wastewaters effluents from similar factories in KSA.

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Degradability of industrial effluents Chemical oxygen demand (COD) and total organic carbon (TOC) were measured after the treatment of wastes by the prepared photocatalyst to assess the efficiency of organic matter degradability. The used TiO2@rGO photocatalyst exhibited a reasonable success in organic matter removal. Figs. 9 and 10 showed the decrease of COD and TOC values in industrial wastes, with ranges of 71–85% and 51–82%, respectively. The removal efficiency results of the used photocatalyst revealed that increasing the doping ratio of rGO to TiO2 enhanced the photocatalyst’s activities to degrade the organic matter found in the wastewaters effluents, since TiO2@rGO 5% has more reduction activities than TiO2@rGO 3% and TiO2@rGO 1% (Figs. 9 and 10). To compare between the organic matter degradability in wastewaters and pure solution of 2,4,6 TCP, 200 mL of different concentrations of 2,4,6 TCP standard solution (20–100 mg/l) was mixed with 0.4 g/L TiO2@rGO for 180 min of UV irradiation. After that, COD and TOC were determined (Figs. 11 and 12). The results showed a high removal percent ranging between 79–95% and 71– 91% for COD and TOC, respectively. These results were in agreement with those of Benbachir et al. (2017) as they reported that a rapid decrease in COD values reached up to 96.6% for 2,4,6 TCP using chloride of zirconium oxide photocatalyst. The generation of active oxygen during the photocatalytic process oxidized 2,4,6 TCP by opening the aromatic ring and the production of CO2 and H2O2 (Kieber and Helz, 1986). Conclusion The development of several photocatalysts has been used for applications of wastewater treatment. Reduced graphene oxide dope with pare TiO2 nanocomposites were prepared and used for the elimination of organic matter from industrial effluents and also for photodegradation of 2,4,6 TCP compound. The used TiO2@rGO photocatalyst has a great affinity to the removal of organic matter through wastewaters since the decrease of COD and TOC values in industrial wastes were found in the ranges of 71% –85% and 51– 82%, respectively. The maximum 2,4,6 TCP photodegradation (up to 90%) was achieved in slight neutral media (pH = 6) during 180 min of UV irradiation. Kinetic studies proved that the adsorption obeyed pseudo-second order reaction model with qe values ranging between 22.0 and 22.7 mg/L. Furthermore, Langmuir isotherm model provided the best model for the photodegradation of TCP compound. Acknowledgement This research was supported by the Environmental Pollution Research Chair at Princess Nourah bint Abdulrahman University (Grant no. EP-02).

Please cite this article as: M. H. H. Ali, K. M. Al-Qahtani and S. M. El-Sayed, Enhancing photodegradation of 2,4,6 trichlorophenol and organic pollutants in industrial effluents using nanocomposite of TiO2 doped with reduced graphene oxide, Egyptian Journal of Aquatic Research, https://doi.org/10.1016/j. ejar.2019.08.003

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Please cite this article as: M. H. H. Ali, K. M. Al-Qahtani and S. M. El-Sayed, Enhancing photodegradation of 2,4,6 trichlorophenol and organic pollutants in industrial effluents using nanocomposite of TiO2 doped with reduced graphene oxide, Egyptian Journal of Aquatic Research, https://doi.org/10.1016/j. ejar.2019.08.003