Cu–Fe Incorporated Graphene-Oxide Nanocomposite as Highly Efficient Catalyst in the Degradation of Dichlorodiphenyltrichloroethane (DDT) from Aqueous Solution

Fe/graphene oxide and Cu–Fe/graphene oxide nanocomposite were synthesized by the atomic implantation method to study the photocatalytic degradation of dichlorodiphenyltrichloroethane (DDT). The synthesized nanocomposites were characterized by the XRD, N2 isotherms, SEM with EDX, TEM and XPS analysis. Characterization results have reported that oxides of Cu and Fe were uniformly distributed on graphene oxide and exited in the form of Cu+ and Fe2+ ions in Cu–Fe/graphene oxide nanocomposite. The high photocatalytic DDT removal efficiency 99.7% was obtained for Cu–Fe/graphene oxide under the optimal condition of 0.2 g/L catalyst, 15 mg/L H2O2 and pH 5. It was attributed to the reduction of Fe3+ to Fe2+ by Cu+ ions and –OH radicals formation. However, it was dropped to 90.4% in the recycling study by leaching of iron and without a change in phase structure and morphology.


Introduction
Dichlorodiphenyltrichloroethane (DDT), polychlorinated biphenyl (PCB), Dioxin and Furan are persistent organic pollutants that affect human health and the environment [1]. A few methods have been developed to eliminate/ degrade these pollutants such as adsorption, biodegradation, and chemical degradation [2,3]. Nano-structured zero-valent iron was claimed as a highly active site to degrade DDT in water and soil. Kang et al. have synthesized micro/nano-structured zero-valent iron (MNZVI) by ball-milling industrially reduced iron powder. Plate-like morphology of MNZVI exhibited high degradation performance compared to commercial zero-valent iron powder [4]. Yehia et al. have also synthesized nano-sized zerovalent iron (NZVI) by different methods. It has shown high degradation of DDT in water compared to soil [5]. Later, Tian et al. has synthesized bimetallic Ni/Fe nanoparticles to degrade DDT in aqueous solution. In this catalyst, Fe nanoparticles promoted Fenton reaction and Ni nanoparticles promoted dehydrochlorination [6].
The advanced oxidation process is one of the effective methods to degrade toxic pollutants. This refers to an oxidation process through the formation of hydroxyl radical which promoted the degradation of organic contaminants in water and subsequently it reacts with organic contaminants through a series of multiple steps to form non-toxic products like low molecular weight carboxylic acid, HCl, CO 2 and H 2 O [7]. In the Fenton process, iron-catalyzed hydrogen peroxide (Fe 2+ /H 2 O 2 ) has been used for the removal of pollutants in water. It shows high degradation efficiency, simple operation and low toxicity [8]. However, it has some drawbacks such as lack of recovery of the catalyst, inefficiency in aqueous media (pH 5-9), production of a large amount of sludge and requires further treatment. In order to overcome these drawbacks, a heterogeneous Fenton process using iron-based catalysts such as Fe 2 O 3 , Fe 3 O 4 , FeOOH, Fe powder, Fe-MOF, Fe-Fe 3 O 4 nanoparticles have been developed [9][10][11][12][13]. In this process, oxidation occurs at the solid-liquid interface where iron remains either in the solid phase and/or adsorbed ion.
Few research groups have synthesized composite materials such as Fe 3 O 4 /CeO 2 [12], Fe-g-C 3 N 4 /graphitized mesoporous carbon [14], graphene oxide-Fe 2 O 3 [15], Cu-rGO [16], Fe 3 O 4 -CuO-graphene oxide [17], Fe-Fe 3 O 4 / graphene oxide [18], graphene oxide-amorphous FePO 4 [19] and graphene oxide/Fe-MOF [13] to degrade pollutants in aqueous solution. Among these, supporting materials like graphene and graphene oxide have great interest due to their unique structure, high specific surface area, chemical stability, conductivity and visible light adsorption ability [20,21]. So, graphene oxide was considered as a support to synthesize nanocomposite material. In this article, Fe/graphene oxide and Cu-Fe/graphene oxide nanocomposite were synthesized to study the degradation of DDT under light irradiation. The effect of catalyst concentration, pH and H 2 O 2, , intermediate products formed during the degradation of DDT and recyclability were studied.

Synthesis of Graphene Oxide
Graphene oxide was synthesized by the modified Hummer's method [22]. About, 1 g of graphite flakes were transferred into a mixture of 150 mL concentrated H 2 SO 4 and H 3 PO 4 (4:1, v/v (%)), stirred at room temperature for 1 h then 6 g of KMnO 4 was added slowly. The mixture was kept at room temperature for 3 days. It was turned dark brown from dark green. The oxidation reaction was quenched using 600 mL ice-cold water and 3 mL H 2 O 2 . The color of the solution was changed into bright yellow then it was centrifuged and washed with 1 M HCl to remove excess manganese salt. Finally, it was washed with distilled water until neutral pH obtained then dried at 100 °C overnight. The dried compound graphene oxide was labeled as GO.

Synthesis of Fe/GO and Cu-Fe/GO Nanocomposite
Fe/GO and Cu-Fe/GO nanocomposite were synthesized by the atomic implantation method using the equipment shown in Fig. 1. For the synthesis of Fe/GO, a desired quantity of iron (III) chloride hexahydrate and graphene oxide (Fe:GO = 20:80 wt%) were mixed and loaded at the center of the tubular furnace. It was heated at 500 °C for 30 min with a heating rate of 5 °C/min in a nitrogen atmosphere then cooled to room temperature slowly. Similarly, Cu-Fe/GO was prepared using the procedure the same as that of Fe/GO synthesis with precursors composition ratio (Cu:Fe:GO = 2:18:80 wt%).

Characterization
X-ray diffractions were recorded using a D8 Advance X-ray diffractometer having Ni filtered Cu K α radiation in the scan range from 2θ = 5° to 70°. Trista Plus II gas adsorption analyzer was used to measure adsorption-desorption isotherms at 77 K. Before the study of N 2 isotherms, 0.1 g of sample was degasified at 200 °C for 2 h under vacuum. The BET method was used to calculate the specific surface area and the t-plot method was used to calculate micropore volume. The total pore volume was calculated at a relative pressure of 0.99. FT-IR spectra were recorded using JACOS 4700 FT-IR spectrometer in the scan range from 4000 to 400 cm −1 with a resolution of 4 cm −1 using KBr. TEM image was obtained on a JEOL JEM 1400 transmission electron microscopy instrument operated at a voltage of 100 kV. Thermo Fisher K-alpha X-ray photoelectron spectrometer (ESCALAB MKII) was used to record the spectrum. SEM with Energy Dispersive X-ray spectroscopy (EDX) mapping was performed using a HITACHI S-4800.

Photocatalytic Degradation Experiment
Photocatalytic degradation of dichlorodiphenyltrichloroethane (DDT) in aqueous solution was studied using synthesized composite materials by irradiation of simulated sunlight UV-A 315-400 nm (4 lamps, power 15 W). The distance of the tested sample from the source of irradiated light was 20 cm and brightness was 2.03.10 4 lx (M6M PRO Lux Meter LX 1010 BS-Taiwan). The experiment was performed in a Pyrex glass contacting 100 mL DDT solution at a concentration of 10 mg/L. The reaction parameters such as catalyst dosage 0.05-0.4 g/L, effect of pH 3-8 and concentration of H 2 O 2 10-100 mg/L were also studied. Firstly, the reaction mixture was stirred in a dark condition to measure adsorption-desorption equilibrium on the surface of the catalyst. Secondly, it was exposed to sunlight with continuous stirring. The mixture was collected at regular intervals of time, centrifuged and analyzed using GC-MS Agilent GC 7890A, MS 5975C spectrometer. The intermediate products were analyzed using LC-MS HRAM (Thermo Fisher) instrument with a column of Ultra aqueous C18.5 μm. The DDT degradation efficiency (%) was calculated using the following equation.
where C 0 is the initial concentration and C t is the concentration at time t.

XRD and FT-IR Analysis
The crystalline structure and functional groups of synthesized composite materials were analyzed by XRD and FT-IR techniques. The XRD patterns of GO, Fe/GO and Cu-Fe/ GO were shown in Fig. 2a. Pristine graphene oxide has shown a single diffraction peak at 2θ = 10.6° with a reflection plane (001). The diffraction pattern of GO was matched with previously reported one [23].  [24]. Moreover, Cu-Fe/GO has also shown diffraction peaks similar to Fe/GO. The copper oxide peaks have not appeared due to low particle size and/ or undetectable by the XRD [25]. In both Fe/GO and Cu-Fe/ GO nanocomposite materials, the GO peak at 2θ = 10.6° was absent which indicated that oxides of Fe and Cu were intercalated within graphene oxide layer [26]. Moreover, the crystalline GO was converted into amorphous GO during the synthesis of nanocomposite material.

XPS Analysis
The oxidation state of Fe and Cu in Cu-Fe/GO nanocomposite can be known by XPS analysis. Figure 3 shows the XPS spectra of Cu-Fe/GO. The survey spectrum has shown binding energies at 285 eV, 530 eV, 711 eV, and 934 eV which correspond to C1s, O1s, Fe2p and Cu2p (Fig. 3a). It indicated that these elements were present in synthesized nanocomposite material Cu-Fe/GO. By deconvolution of Fe2p peak obtained binding energies at 710 eV (Fe2p 3/2 ) and 724 eV (Fe2p 1/2 ). It reported that the existence of Fe 2 O 3 [31]. Low-intensity peaks at 715 eV and 730 eV correspond to the presence of Fe(II)O in the synthesized material [32]. It is important to note that in the case of Fe/GO only the presence of Fe 3+ is could be observed. Copper peak on deconvolution obtained binding energies at 931 eV and 943 eV and 951 eV which described the formation of Cu(II)O material [29]. The peak obtained at 931 eV indicated the presence of

TEM and SEM with EDX Analysis
TEM and SEM with EDX mapping of Cu-Fe/GO were shown in Fig. 4. TEM image has reported that oxides of Cu and Fe were highly dispersed on graphene oxide and particle size was in the nano range (Fig. 4a). Porous network graphene oxide layers were appeared in the SEM image (Fig. 4b). EDX mapping of Cu and Fe has confirmed that CuO and Fe 2 O 3 were uniformly dispersed by the atomic implantation method (Fig. 4c, d). The chemical composition of Fe/GO and Cu-Fe/GO was presented in Fig. 4e. The wt% of copper and iron obtained experimentally was near to theoretically loaded amount. Figure 5 shows the N 2 adsorption-desorption isotherms and pore size distribution of GO and Cu-Fe/GO. The textural properties were presented in Table 1. The pristine graphene oxide has shown a hysteresis loop between the relative pressure P/P 0 = 0.45-1. According to the classification of porous materials, the isotherm pattern of GO is similar to type-IV with the H3 hysteresis loop which indicated that the presence of mesopores [33]. The BET surface area, total pore volume, and pore size of GO were 331 m 2 g −1 , 1.719 cm 3 g −1 and 20.7 nm respectively. Cu-Fe/GO nanocomposite material has also shown an isotherm pattern similar to that of GO. But, the amount of N 2 adsorption was decreased. Hence, there was a change in the textural properties has been

Parameters Influence on DDT Degradation
Effect of catalyst dosage The effect of Cu-Fe/GO dosage for the photocatalytic degradation of DDT was studied using 10 mg/L DDT solution, 15 mg/L H 2 O 2, and pH 5 for 3 h. The DDT removal efficiency values were shown in Fig. 6. At the low dosage of catalyst 0.05 g/L, DDT removal efficiency was 53%. With an increase in catalyst dosage from 0.05 g/L to 0.2 g/L, the removal efficiency was increased and reached to 99.7%. Beyond 0.2 g/L of catalyst dosage, there was no change in degradation efficiency. Hence, the optimum amount of the catalyst for the degradation of DDT was 0.2 g/L for 10 mg/L of DDT solution. It was due to that 0.2 g/L of the catalyst has enough active adsorption sites to degrade the DDT solution. For high concentrated DDT solution, a high catalyst dosage might be needed [34]. Effect of pH The photocatalytic degradation of pollutant in the polluted water depends on the pH of the solution because pollutant has different pH values in aqueous solution [35]. Figure 7 shows the photocatalytic degradation of DDT at different pH of DDT solution using 0.2 g/L of Cu-Fe/GO, 15 mg/L H 2 O 2 for 3 h. The DDT removal efficiency was 99.8% at pH 3 and 5. However, at higher pH > 5 the DDT removal efficiency was decreased and reached 91% (pH 8). The change in the photocatalytic activity of Cu-Fe/GO nanocomposite was due to the decomposition of H 2 O 2 . At pH > 5, H 2 O 2 was decomposed into H 2 O then active centers CuO and Fe 2 O 3 were converted into the corresponding hydroxide [36,37]. Hence, the photocatalytic activity of Cu-Fe/GO was decreased. At pH 3-5, hydroxy radicals were formed during the reaction which enhanced photocatalytic activity of Cu-Fe/GO to degrade DDT [38].
Effect of H 2 O 2 dosage The effect of H 2 O 2 concentration in photocatalytic degradation of DDT using Cu-Fe/GO was also studied. The conditions were 10 mg/L of DDT solution, 0.2 g/L of catalyst and pH 5, for 3 h (Fig. 8). With increase in concentration of H 2 O 2 from 10 to 15 mg/L, DDT removal/degradation efficiency was increased from 85.6 to 99.2%. At a high dosage of H 2 O 2 (30 mg/L, 60 mg/L, and 100 mg/L) removal efficiency was decreased. The change in DDT degradation efficiency was due to the formation of an optimal amount of hydroxy radicals in low concentration of H 2 O 2 whereas at high concentration excess amount of H 2 O 2 reduced hydroxy radicals (Eqs. 1 and 2) [34].

Degradation of DDT by Adsorption, Fenton Process and Photo-Fenton Process
After tuning the parameters, the optimal condition was used to study the degradation of DDT by adsorption, Fenton process and Photo-Fenton process using Cu-Fe/GO (Fig. 9a). The adsorption process has shown around 40% DDT removal efficiency for 3 h. Moreover, the Fenton process and Photo-Fenton process have shown DDT removal efficiency 85% and 99.2% respectively. High efficiency was obtained by the Photo-Fenton process compared with the Fenton process and adsorption process. It was attributed to the presence of Cu + ions (from XPS) which reduced Fe 3+ to Fe 2+ ion and provided more -OH radicals under visible [36,39]. Figure 9b shows DDT removal efficiency using Fe/GO and Cu-Fe/GO catalysts by the Photo-Fenton process. Cu-Fe/GO has shown high DDT removal efficiency compared with Fe/GO by the promotional effect of copper. Persistent organic pollutants degradation by advanced oxidation process using Photo-Fenton catalyst is one of the effective methods. Our results have shown that DDT was completely oxidized into CO 2 , H 2 O, and HCl. For better understanding, the reaction mechanism and intermediates produced during the reaction were analyzed by LC-MS. From the formed intermediates, the possible reaction pathway was: (i) dechlorination of DDT step by step, (ii) breaking of carbon chains and de-cyclization to form hydrocarbon with shorter chain, (iii) complete oxidation of hydrocarbon to form CO 2 and H 2 O (Fig. 10). The shorter chain hydrocarbons (C 14 , C 12 , C 7 and C 6 ) identified in the DDT degradation process was good agreement with reported literature [40,41].

Stability of Cu-Fe/GO Nanocomposite
Recyclability of the photocatalyst Cu-Fe/GO was studied up to 4 cycles (Fig. 11a). In each cycle, the catalyst was separated from the equilibrium solution by filtration then washed with methanol and dried at 60 °C overnight. In the 1st cycle, DDT removal efficiency was 99.2%. After 4 cycles, it was reached to 90.4%. The used catalyst was characterized by XRD (Fig. 11b). There was a change in the intensity of the peak at 2θ = 24°. It was due to the leaching of iron. However, no change in morphology has been observed (Fig. 11c). From these results, the synthesized Cu-Fe/GO nanocomposite catalyst has shown high photocatalytic degradation of DDT.

Conclusion
In this work, Fe/GO and Cu-Fe/GO nanocomposite were used for the photocatalytic degradation of dichlorodiphenyltrichloroethane in aqueous solution. Characterization results have confirmed that oxides of Cu and Fe were presented as CuO and Fe 2 O 3 on the graphene oxide. Cu-Fe/GO nanocomposite has obtained high DDT removal efficiency at pH of the DDT solution < 5 by the formation -OH radicals which enhanced DDT removal efficiency. Moreover, it has also shown high DDT removal efficiency compared with Fe/ GO by the promotional effect of copper. In the recycle study, Cu-Fe/GO has shown good photocatalytic stability. Hence, graphene oxide supported metal oxides could be considered as an efficient and promising photocatalyst for the degradation of persistent organic pollutants.
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