Graphene Oxide Immobilized Binuclear Biomimetic Catalyst for Triclosan Degradation in Aqueous Solution

Biomimetic catalysts, Zn(salen), Zn(Phe-TPP), ZnPSC 6 and graphene oxide (GO) immobilized ZnPSC 6 (ZnPSC 6 /GO), were fabricated. These biomimetic catalysts were tested for catalytic decomposition of emerging contaminant triclosan (TCS) . Catalysts were characterized using Brunauer-Emmett-Teller (BET) , X-ray Diffraction (XRD) , Fourier Transform infrared (FTIR) , and Raman spectra. The reaction processes were studied using HPLC , and the treated solutions were tested based on total carbon content (TCC) analysis and toxicity analysis. The impacts of catalysts on TCS degradation were investigated through testing removal efficiency, total carbon content (TCC) and ecotoxicity. The results showed that ZnPSC 6 /GO displayed high activity due to salen-porphyrin binuclear active sites and immobilization into GO compared with Zn(salen), Zn(Phe- TPP ), and ZnPSC 6 . Finally, the reaction conditions of TCS oxidation catalyzed by ZnPSC 6 /GO has been preliminarily discussed. In these conditions, high removal efficiency was observed (96.7%; [ZnPSC 6 /GO] = 3.0 ppm, [TCS] = 0.02 mmol/L, [H 2 O 2 ] = 0.60 mmol/L, T = 70 °C, t = 90 min, pH = 8).


INTRODUCTION
In the field of personal care products such as toothpaste, soap, and food packaging, triclosan (TCS, C 12 H 7 Cl 3 O 2 ), as a synthetic chlorinated aromatic compound and a typical antibacterial agent, has found an increasingly concern in recent years. Because TCS is chemical potentially toxic compound to the environment and biohealth owing to its properties being lipophilic, accumulative, toxic, and persistent. Further, chlorine as disinfectant in water could react with TCS to form chlorinated triclosan derivatives (CTDs) and CTDs further transform to polychlorinated dioxin upon the exposure of solar irradiation (Szychowski et al. 2021;Wilburn et al. 2021). Classified as an emerging contaminant, the TCS concentration found in wastewater and surface water are generally in the μg/L and ng/L range. However, in human milk and urine samples, the concentration of TCS is respectively found from 20 to 300 ng/g and of 0.01 to 38 ng/mL (Naidu et al. 2016;Pintado-Herrera et al. 2014). It is thus difficult to degrade this class of contaminants by conventional water treatment processes such as physicochemical and biological processes due to their low concentrations in the environment (Mulla et al. 2020;Savunthari et al. 2021;Wang & Wang 2018;Wang et al. 2021). For this reason, biomimetic complexes may be considered to be used as catalysts as they combine chemo-enzymatic performance (Guo et al. 2021). Kaur et al. (2019) reported that in removal of triclosan from water, the removal rate of triclosan was different when different activated carbons were employed, suggesting that biomimetic modified activated carbon exhibited the maximum adsorption capacity of 395.2 mg/g in contrast to 71.5 mg/g obtained for unmodified activated carbon. Further, a biomimetic hollow fiber module comprising an active layer of polyamide thin film composite (TFC) with integrated aquaporin proteins was used to reject triclosan, the rejection obtained for triclosan was higher than 93% (Salamanca et al. 2021). In dechlorination of triclosan by using biomimetic or enzymatic systems, it was found that high dechlorination selectivity was only attained by using enzyme, while both selectivity and efficiency were obtained by using biomimetic systems (Souchier et al. 2016). In addition, a study suggested that metal salen complexes were effective catalysts used in triclosan oxidation reactions, however, immobilization of complex into graphene oxide may be effective method for improving removal of triclosan in aqueous solution (Qian et al. 2019). In this study, the catalytic performance can be improved by the condensation reaction of 3,5-dibromosalicylaldehyde with ArGO (aminated reduced graphene oxide) followed by the complexing reaction with copper acetate monohydrate (DArGO-Cu). The mechanism for DArGO-Cu to activate oxidant was the more generation of radicals. Catalytic degradation of triclosan by hydrogen peroxide in the presence of the copper salen Schiff base was evaluated under various conditions. The results showed that TCS was removed by 80.5% within 30 min at optimal reaction conditions. The dominant reactive oxygen species (ROS) involved in the reaction was identified as OH radical. The results suggested that the Schiff base is a promising catalyst for catalyzing oxidative degradation of TCS by H 2 O 2 in aqueous solution under neutral condition (Peng et al. 2019). In contrast to metallosalen complex, metalloporphyrin affords high efficiency but low selectivity in degradation of pollutants (Zhao et al. 2019). High efficiency of TCS degradation was obtained (greater than 90%) using metalloporphyrin. The results showed that the immobilized metalloporphyrin has practically the high performance compared to the free form. Activation by metalloporphyrin for triclosan (TCS) degradation was investigated. Concentration of Cl − showed a dual effect on the TCS degradation. Hydroxyl radicals, superoxide and non-radical single oxygen contributed to the degradation of TCS. Dichlorination, hydroxylation, cyclization, and other bond breaking reaction were inferred as the TCS degradation pathways (Banfi et al. 2006).
Therefore, it is highly desirable that metallosalen complex and metalloporphyrin should be combined, such as binuclear salen-porphyrin complex with two active sites within one complex. The catalytic activity of binuclear complex [Fe(III)(BPMP)Cl(mu-O)Fe(III) Cl -3 ], as a functional biomimetic analog to methane monooxygenase, was evaluated in cyclohexane oxidation using H 2 O 2 as oxidant. Conversions up to 19.2% were observed (12.6 and 6.6% yields for cyclohexanol and cyclohexanone, respectively) (Esmelindro et al. 2005). Both binuclear copper complexes [{CuI(L 1 )} 2 ][OTf] 2 (1) and [{CuII(L 2 )Cl} 2 ] (2) have been shown to be excellent catalysts for the cyclopropanation of various olefins with ethyl diazoacetate (EDA), with catalyst 1 found to be marginally superior to 2 (Das et al. 2012). Porphyrin-salen compound can be obtained by covalently linking by a flexible alkoxy chain . Previous studies in our laboratory have shown that the immobilization of Co(salen) on graphene oxide is able to increase the oxidation of lignin (Zhou & Lu 2016); The combination of laccase and Cu(salen) did extend degradation of refractory lignin units (Zhou 2020).
Further, based on the literatures, it is found that no research is conducted on the influence of binuclear salen-porphyrin complex for triclosan degradation. Hence, in this paper, an attempt is made to investigate the influence of the combination of metallosalen complex and metalloporphyrin in the salen-porphyrin complex and the immobilization of salen-porphyrin complex into GO towards triclosan degradation in aqueous solution.

MATERIALS AND METHODS
Chloroform (Sinopharm Chemical Reagent Co. Ltd., China) was dried with NaHCO 3 and CaCl 2 , and distilled prior to use. Triclosan (TCS > 98%) and graphene oxide (GO, 99.0%) were acquired from Sigma-Aldrich and stored at 4 °C. All other chemicals (Sinopharm Chemical Reagent Co. Ltd., China) were of analytical grade unless specifically noted otherwise. Milli-Q-Plus ultrapure water was used in all experiments.
The sixth step was to prepare GO-immobilized ZnPSC 6 (ZnPSC 6 /GO) as described in literature by Cooke and Smith (1994). ZnPSC 6 (0.04 g) was fully dispersed in N, N-dimethylformamide (DMF, 20 mL) with the help of ultrasonication. The suspension containing 620 mL water and 16 g GO was then added. The mixture was stirred under ultrasonication at room temperature for 5 h. Then, the mixture was filtered to obtain the solid. The solid was washed with water and vacuum dried at 50 °C. The resulting GO immobilized ZnPSC 6 was denoted ZnPSC 6 / GO. The loading capacity was ~0.6 g/mg, which was based on the zinc analysis (PerkinElmer Optima 8300 ICP-AES). The specific surface area was characterized with Brunauer-Emmett-Teller (BET) equation with a Micromeritics ASAP-2020 (USA). The XRD pattern was recorded with Rigaku Dmax X-ray diffractometer (Nifiltered, Cu Kα radiation, 40 kV and 30 mA, 2θ, 5-40°, scanning speed 6°/min). FTIR spectra were recorded in the range of 400-4000 cm -1 on a Nicolet Impact 410 spectrometer. Raman spectra was obtained at an excitation wavelength laser of 530 nm on confocal micro-Raman spectrometer (Almega XR).

DEGRADATION OF TCS
TCS was degraded in a flask under magnetic stirring at different reaction conditions using hydrogen peroxide as oxidant in the presence of catalyst. At the same time, the pH of the solution was regulated with buffer solution. 2 mL samples were collected from the flask via the syringe at the different reacting time, then filtered and saved for further analysis. The content of TCS was analyzed with HPLC equipped with Zorbax Eclipse Plus C18 column (250 mm × 4.6 mm × 5 μm) using methanol/ water mixture (4:1 v/v) as mobile phase. The flow rate of mobile phase was 1.0 mL/min. The total carbon content (TCC) analysis was performed using Vario EL Elemental Analyzer. The toxicity of the solutions was obtained by a Microtox Model 500 Toxicity Analyzer, using a freezedried preparation of marine bacterium Vibrio fischeri (Photobacterium phosphoreum) (Thongrom et al. 2014).

CHARACTERIZATION OF ZnPSC 6 /GO
The surface areas of GO and ZnPSC 6 /GO were 627.1 m 2 /g and 583.4 m 2 /g, respectively. The low surface area of ZnPSC 6 /GO compared to GO indicated the presence of ZnPSC 6 /GO within the supercages of GO (Mohamad et al. 2021). Figure 2 shows the XRD patterns of GO and ZnPSC 6 /GO. As shown in Figure 2, due to oxidation of graphite (26.5°, Figure 2(a)), the diffraction peak of GO at 10.8° may be assigned to the reflection of GO (001) plane (Figure 2(b)), demonstrating that some oxygencontaining groups were introduced between the graphite layers (Stobinski et al. 2014). After the immobilization of ZnPSC 6 into GO, the diffraction peak of GO at 10.8° was observed but its intensity decreased. This suggested that the immobilization did not destroy the sheet structure of GO. Meanwhile, in XRD analysis, the peaks at 2θ = 22.5° of GO and ZnPSC 6 /GO indicated the amorphous silicon derived from 3-aminopropyltrimethoxysiloxane after graphene oxidation ). FIGURE 2. XRD spectra of (a) graphene, (b) GO and (c) ZnPSC 6 /GO It can be observed by comparing the FTIR spectra of GO and ZnPSC 6 /GO in Figure 3 that the change occurred after immobilization was applied. The broad peak at 3400 cm -1 corresponded to hydroxyl groups in GO. The peaks corresponding to the absorptions at 1724 cm -1 and 1626 cm -1 were the characteristic peaks of the stretching vibration of C=O and C=C double bond of GO, respectively, and the peaks at 1217 cm -1 and 1050 cm -1 indicated the C-OH and C-O bonds of GO, respectively. In addition, the FTIR spectra in Figure 3 showed the existence of functional groups on Si-O-Si and Si-O-C, as indicated by the peaks at 1104 cm -1 and 1022 cm -1 , indicating that ZnPSC 6 was linked with GO. As for the existence of ZnPSC 6 within GO, additional peaks appeared at wavenumber values of 3052, 2922/2850, 1598, 1486, 1268, 1166, 945, 798/752, 528  Raman spectroscopy is often used as a principal tool to investigate the ordered and disordered structure of carbon materials such as GO (Abouelsayed et al. 2017). Shown in Figure 4 were the Raman spectra of GO and ZnPSC 6 /GO. The Raman plots of GO and ZnPSC 6 /GO showed two distinct bands at about 1332 and ~1600 cm -1 , which were the D and G bands, respectively. It was found that compared to the G band of GO at 1597 cm -1 , that of ZnPSC 6 /GO blue shifted to 1610 cm -1 , plausibly due to local stress caused by the bond between GO and complex, confirming that oxygen-containing groups of GO were introduced into the ZnPSC 6 and confirming that ZnPSC 6 was immobilized into GO (Beams et al. 2015). The first step was to evaluate and compare the catalytic activity of different catalysts during catalytic oxidation of TCS. As shown in Figure 5(a), the removal efficiency of TCS was 30.6, 34.8, and 38.5% for Zn(salen)/H 2 O 2 , Zn(Phe-TPP)/H 2 O 2 , and ZnPSC 6 /H 2 O 2 systems at 40 °C in 90 min, respectively. The results showed that chemical structure and active sites did contribute to the effect on catalytic activity of complexes. The well-defined structural ZnPSC 6 had two active sites of salen and porphyrin to activate H 2 O 2 for TCS degradation, thus improving the removal efficiency of TCS (Gangemi et al. 2019). Moreover, the results showed that after the immobilization of ZnPSC 6 into GO, the obtained removal efficiency of TCS increased as a result, which was due to the layer structure of GO affording more voids to provide more accessibilities of the active centre (Gur et al. 2019). Consequently, ZnPSC 6 /GO gave the highest removal efficiency of TCS of 42.3%. The binuclear complex ZnPSC 6 has two active sites of salen and porphyrin and thus has more active sites compared to Zn(salen) and Zn(Phe-TPP) to regulate catalysis by oxo-complex (Li et al. 2018;Maruyama et al. 1991). In addition, ZnPSC 6 is a biomimetic complex mimicking an enzyme resulting in high catalytic selectivity (Verma et al. 2021). Further, the porous property of GO make the substrate easily approach active sites of ZnPSC 6 (Yunarti et al. 2022). The ecotoxicity and TCC of solutions respectively analyzed by Toxicity Analyzer and Elemental Analyzer were shown in Figure 5(b). Low ecotoxicity was observed for the ZnPSC 6 /GO treated solution compared with ZnPSC 6 , Zn(Phe-TPP), Zn(salen), indicating that TCS was deeply mineralized with ZnPSC 6 /GO because TCC conversion of ZnPSC 6 /GO treated solution was highest (Samarghandi et al. 2021).
pH of medium was considered as potential effect in the degradation reaction of organic pollutants by catalysis (Elsayed et al. 2020). A batch of experiments, in which the initial pH was set at 5, 6, 7, 8, 9, and 11, was performed to investigate the pH effect on TCS removal. As shown in Figure 6(a), the TCS removal efficiency was highest at pH 8, and low under acidic conditions. Thus, the optimum degradation of TCS by ZnPSC 6 /GO can be obtained in weak alkali medium. Therefore, weak alkali medium was beneficial to improve the catalytic activity of ZnPSC 6 (Meininger et al. 2017;Vargas et al. 2004). Figure 6(b) demonstrated the temperature effect on the oxidation of TCS. When the reaction temperature increased from 20 to 70 °C, the removal of TCS increased from 37.1 to 65.5%. Further increase of reaction temperature had little effect on TCS degradation. ZnPSC 6 /GO concentration effect was also discussed in the degradation of TCS. The reaction afforded increasing removal of TCS with increasing ZnPSC 6 /GO concentration (Figure 6(c)). This may be explained that increasing ZnPSC 6 /GO concentration resulted in increasing catalytic sites to activate H 2 O 2 (Huang et al. 2021). When hydrogen peroxide was used as oxygen source, the H 2 O 2 concentration exhibited positive effect on TCS degradation (Crincoli & Huling 2021). It can be seen from Figure 6(d) that when H 2 O 2 concentration increased from 0.1 to 0.6 mmol/L, the removal of TCS gradually increased from 68.5 to 96.7% and remained little chang with further increase of H 2 O 2 . Thus, the optimum H 2 O 2 concentration was 0.6 mmol/L.

CONCLUSION
In this work, a biomimetic complex based catalyst structure, ZnPSC 6 , with binuclear salen-porphyrin active sites, and in the form of immobilization into GO (ZnPSC 6 /GO), was fabricated and used to activate H 2 O 2 to degrade TCS in water. The characterization of catalysts indicated that ZnPSC 6 and GO were combined in immobilized catalyst, which produced high activity than ZnPSC 6 , Zn(Phe-TPP) and Zn(salen). Attributed to the improvement of performance from immobilization of ZnPSC 6 into GO and the synergic effect of ZnPSC 6 and GO, the ZnPSC 6 /GO showed excellent performance in H 2 O 2 activation for degrading TCS. Compared with other biomimetic catalysts such as Zn(salen), Zn(Phe-TPP), ZnPSC 6 , the ZnPSC 6 /GO exhibited distinct advantage over activation for the outstanding degradation efficiency of TCS. In addition, removal efficiency of 96.7% was obtained under the optimum reaction conditions.