1. Introduction The rapidly ascending crisis of bacterial resistance to antibiotics is threatening the amazing enhancement of human health that has been accomplished in the last century. European centre for disease prevention and control (ECDC) reported that more than 25,000 people die from resistant bacteria each year in Europe [1]. This issue is currently of a global concern as it lays a huge health and economic burden on the healthcare systems. The bacterial antibiotic resistance has been principally imputed to the overuse and misuse of these medications [2-4]. Meanwhile, the development of new antibiotics by the pharmaceutical industry has drastically dropped due to the reduced economic benefits and rigid regulatory requirements. Accordingly, 15 of the 18 largest pharmaceutical companies has forsaken the antibiotic field [2, 5]. Moreover, the overuse of antibiotics is

not limited to human consumption only but exceed it to the extensive use for livestock and agricultural treatments which seems to be more unorganized [6, 7]. As a consequence of continuous exposure to antibiotics residues by environmental microorganisms, many types of bacteria have developed resistance genes to antibiotic confirmed by the discovery of resistant bacteria in some livestock facilities and their surrounding environment [8]. Alarmingly, an increasing number of studies have supported the idea that resistance genes in human and animal pathogens may have been transferred from the mutated environmental microorganisms as they were detected in locations with no history of antibiotic pollution [9, 10]. Besides the resistance issue, antibiotics have also been evidenced to influence aquatic and terrestrial ecosystems. For instance, shifts in algae and bacteria communities with the probability to incite ecological cascade effects on higher trophic levels that depend on them as a food source was reported [11]. It is unfortunate to mention here that antibiotics removal from aquatic environments have received a relatively slight consideration comparing to other organic pollutants in spite of all the above-mentioned risks they may hold [12]. Thus, we believe that antibiotics removal must be of emerging concern by both researchers and regulatory authorities. Over the large range of used techniques for organic pollutants degradation, semiconductor heterogeneous photocatalysis has aroused a tremendous attention due to the high potentials for pollutants degradation and thereafter mineralization into carbon dioxide and water. An important member of this group is CdS which is considered a promising material for visible light harnessing owing to its direct band gap of 2.42 eV. Consequently, it has been increasingly studied for several catalytic reactions such as water splitting and pollutants degradation [13]. However, pure CdS has a weak stability

due to its self-photocorrosion mediated by the photogenerated holes beside the short lifetime of its charge carriers [14]. Therefore, it is extremely recommended to improve the properties of this material to overcome the aforementioned drawbacks. Several modifications can be implemented in this regard such as compositing with other semiconductors or polymers, doping with metal or nonmetal ions and incorporating into high surface area materials such as graphene or carbon nanotubes [15]. Advantageously, doping a small fracture of metal cation into a host crystal can boost impressively its catalytic activity by distorting chemical bonds on the semiconductor surface to create more defects and shift the optical absorption to the visible region. As a matter of fact, there is no perfect crystalline semiconductor. Defects always exist and their presence often has a profound impact on the electronical and optical properties of a semiconductor [16]. On the other hand, the advent of graphene has opened a new horizon for structural design and surface modification in materials science. The astounding characteristic of this material such as high thermal stability, mechanical strength, and high intrinsic carrier mobility due to its monolayer honeycomb carbon atoms structure helped greatly in improving the performance and stability of photocatalytic materials [17]. A considerable number of reports can be found in the literature regarding the study of improved activity of photocatalysts by the doping technique [15, 18, 19] or compositing with graphene or other carbons [20-22]. However, there is still limited works focused on using dopants together with graphene oxide sheets to structure graphene/metal doped catalysts [23-25] or metal doped graphene/catalysts [26-28] for visible light driven photocatalytic applications. Herein, we employed a facile and rapid microwave-furnace assisted method to prepare CdS, Ni doped CdS (NiCdS), graphene-CdS composite (G-

CdS), and graphene-nickel doped CdS composite (G-NiCdS) using ethylene glycol (EG) as a solvent and reductant that facilitated the formation of highly pure narrow size distributed nanoparticles/nanocomposites without hiring other catalysts, surfactants, or complexants. The synthesis steps were deliberately designed to assure doping of Ni in CdS lattice prior to graphene oxide addition. Ni2+ was used in particular as it owns a smaller radius than that of Cd2+ and exists in a singlet ground state having tetrahedral coordination environment which would facilitate its introduction into the host CdS by the strong coupling between Ni d-orbitals and the conduction band of the CdS [29, 30]. Best Ni percentage and initial graphene oxide amount were determined based on the degradation of O-nitrophenol (O-NP) as a model phenolic compound. Afterwards, adsorption and decomposition of cephalexin (CLX) and sulfamethoxazole (SMX) antibiotics were investigated. Results indicated that G-NiCdS was superior due to advantageous combination of both techniques used to modify CdS. Accordingly, a reasonable mechanism was further proposed to elucidate the role of each modification in G-NiCdS functionality.

2. Experimental

2.1. Preparation of samples All chemicals used in this study were analytical grade and used without further purification.

Natural

graphite

flakes,

cadmium

chloride

hemipentahydrate

(CdCl2·2.5H2O), nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O), and thiourea (CH4N2S) were used as source materials for photocatalysts synthesis. The preparation of graphene oxide (GO) was carried out by the modified Hummers method reported elsewhere [31] with the addition of ultrasonication step for 60 min at 30 °C prior to reaction termination by H2O2 in order to allow for GO exfoliation.

CdS nanoparticles were prepared by the following procedure: 5 mM of cadmium chloride and 6 mM of thiourea were added into Pyrex beaker of 250 ml containing 35 ml of EG and stirred at room temperature for 120 min. The beaker was then placed in a microwave oven operated at 720 W using a pulse regime with 20% power for 4 min. The formed precipitate was centrifuged (3000 rpm, 10 min) and washed several times with distilled water and absolute ethanol. Then was dried in an oven at 65 °C for 12 h and afterwards calcined at 450 °C for 60 min. NixCd1-xS nanoparticles were prepared similarly by firstly adding a mixture of cadmium source and nickel source in the appropriate stoichiometric amounts into the solvent and then continuing the above procedure. For G-CdS and GNiCdS nanocomposites, different amounts of graphene oxide were dissolved in the 120 min mixed precursors-EG solutions and then ultrasonicated for 60 min to guaranty well dispersion of GO. Subsequently, the microwave and calcination steps were conducted as above.

2.2. Optimizing synthesis conditions In order to find the best optimization for photocatalysts synthesization, degradation percentage of the O-NP (5 mg.L-1) after 90 min of visible light irradiation was chosen as the determining factor. The two variables analyzed were Ni percentage (0-4%) and the initial amount of graphene oxide (0-25 mg). Thereafter, CLX and SMX degradation was assessed employing the fine-tuned photocatalysts in addition to pure CdS.

2.3. Characterization X-ray diffraction (XRD) diagrams of all samples were measured using Bruker AXS D8Advance diffractometer fitted with a )Cu Kα λ = 1.5418 Å) radiation tube. The data were analyzed on PANalytical X’Pert High Score Plus 2.2 software. Fourier transform infrared (FTIR) spectra in the range of 400-4000 cm-1 were recorded in transmission mode using

(Thermo Nicolet 8700), the powder samples were grounded with KBr and compressed into a pellet prior to analysis. The morphologies and structures were observed by scanning electron microscopy (SEM, KYKY-EM3200) and high-resolution transmission electron microscopy (HRTEM, 300 kV Philips CM30). UV-Vis absorption spectra were measured using a UV-Vis double-beam spectrophotometer (PG Instruments T80+). Finally, the amounts of ions leached into water during the recycled photocatalytic experiments were measured using inductively coupled plasma optical emission spectrometry (ICP-OES, Varian Vista MPX).

2.4. Adsorption isotherms of the antibiotics Batch equilibrium experiments were performed in the dark over a 120 min period. The effect of the initial antibiotic concentration on its adsorption was determined by fixing the amount of the photocatalyst at 50 mg and varying the concentrations of CLX and SMX in the range 5-20 mg.L-1. The equilibrium concentration of substrates was then calculated for each photocatalyst by measuring the absorbance with UV-Vis spectrophotometer.

2.5. Photoreactor setup and analysis procedure Experiments were conducted using a home-made photoreactor described elsewhere [32] and shown in Fig. 1. The photocatalyst-substrate dispersions consisted of 50 mg photocatalyst in 50 ml of 10 mg.L-1 substrate at the pristine pH values without further adjustment (i.e. 6.8 for CLX and 6.9 for SMX). The light source was a 650 W halogen lamp (OSRAM 64540, λmax=465 nm) with 14.7 mW.cm-2 visible light intensity measured by (Lux-UV-IR meter, Leybold Didactic GMBH 666 230) radiometer under simulated conditions to our photoreactor system. The photocatalytic reactions were initiated by illuminating the dispersions after being equilibrated for 120 min in the dark. Aliquots were withdrawn after different intervals of illumination followed by centrifugation to

separate solid particles. The quantitative analysis was performed by measuring the absorbance and the degradation efficiency was calculated by the following formula: D% 

A  Ae C  Ce  Ae Ce

(1)

Where Ce and Ae are the equilibrium concentration (mg.L-1) and absorbance, respectively. C and A are the concentration and absorbance after intervals of illumination (t).

3. Results and discussion

3.1. Best synthesis conditions optimization The optimal amount of GO has the common percent of maximum 5% to enhance the catalytic activity of anchored semiconductors as indicated insistently in the literature [28, 33, 34]. Nevertheless, this issue can be only experimentally decisively judged, especially in such cases like ours where nanoparticles yield cannot be accurately predicted. Detecting the initial amount of GO should be added is a crucial step for this large surface material can play a double-edged sword role. It can improve significantly the potency of catalytic materials by the efficient separation of generated carriers on one hand while a high load of GO can deprive the attached nanoparticles of efficient light harvesting by forming an opaque layer that limits light diffusion into the reaction solution on the other hand. In our study, the best amount of GO to boost the activity was found to be 10 mg as seen in Fig. 2. The dopant amount did not alter the best amount of GO, i.e. 10 mg showed better catalytic activity among all Ni percentages tested. Moreover, 1% of Ni doping demonstrated to be the optimal fraction for catalytic enhancement. This can be ascribed to three probable causes (i) drastic decrease of band gap energy with increased dopant amount more than 1%, this possibly has decreased the energy of photo-induced electrons and the efficiency of the photocatalytic system [35], (ii) with high doping concentration,

Ni might have played the role of recombination center for photo-induced electrons and holes and thus diminished the photocatalytic efficiency [36, 37], and (iii) increasing in dopant percentage might has allowed for NiS, the weaker photocatalyst, to be formed. Hence, photocatalysts with 10 mg initial GO amount and/or 1% dopant percent were adopted for the subsequent studies.

3.2. Structure, morphology, chemical analysis and optical properties XRD was utilized to examine the crystallinity, phase, and size of the various samples. Results are depicted in Fig. 3. To start with CdS, the crystallinity was improved to a great extent after calcination (64.43 to 75.3%). Recrystallization process was accompanied by grain growth (24.91 to 35.95 nm) reflected by the decreased width and increased intensity of peaks after calcination. The diffraction peaks of all powders correlated well with the highly pure single phase hexagonal structure of CdS (JCPDS card number 01-077-2306). The two nanocomposites patterns were identical to that of the calcined CdS with no typical peaks relate to GO or other carbon species attributed to (i) the wrecked GO structure by nanoparticles anchored on its surface, (ii) the reduction of its functional groups by EG and/or (ii) the overwhelming of the relatively weak peak of graphene at 26° by CdS peak (0 0 2) [20, 38]. Furthermore, no nickel-derived peaks could be detected in the doped samples implying that Ni2+ was incorporated by substituting Cd2+ in CdS lattice and not by attaching directly to the graphene surface or forming NiS particles which can be credited to the very small fraction of nickel used and the delayed addition of GO during synthesis. The doping with Ni was further confirmed by the slight shift of diffraction peaks of NiCdS and G-NiCdS to a higher angle due to the damage in CdS crystal lattice induced by the dopant. The average crystallite sizes given in Table 1 were

estimated based on the broadening of (1 0 0), (0 0 2), and (1 0 1) peaks using the DebyeScherrer equation. Typical SEM images of CdS and NiCdS and HRTEM images of GO, G-CdS, and GNiCdS are shown in Fig. 4. SEM images show the CdS and NiCdS samples calcined at 450 °C as clusters of (~ 40 nm) nanoparticles with more uniform particle size distribution in the case of NiCdS. Pure GO had an apparent crumpled silk-like carbon nanosheets morphology with a flat surface. The sheets were almost transparent implying the monolayer structure of the prepared GO powder and its applicability in the photocatalytic system as no great hindrance for light penetration in the solution is expected [24]. In nanocomposites images, graphene nanosheets seem to be shielded homogeneously with a high load of nanoparticles. The nanoparticles in both products were connected together at the edges in network form but not agglomerated which demonstrates the role of graphene in inhibiting clusters formation by establishing separate nucleation centers for nanoparticles growth on its surface. Moreover, no nanoparticles were observed outside the graphene nanosheets which ascertains the ideal optimization of GO amount utilized and the perfect attachment of nanoparticles on its surface. Utilization of EG as a dispersing medium in a microwave route is believed to be an important factor that prevented the aggregation and thus facilitated the graphene-nanoparticles connection [39]. Nanoparticles sizes calculated from these images (Table 1) were a little bit bigger than that obtained by XRD which was expected as XRD investigates only the size of coherent scattering domains of the crystallites and not the size of the whole nanoparticle. It is worth noting here that the presence of graphene has not affected the nucleation speed and thus the variation of size between products was minimal presumably due to the short

time of reaction. Finally, the lattice fringes with an interplanar distance of (3.578 Å) shown in G-NiCdS image (Fig. 4g) can be assigned to the (1 0 0) plane of the hexagonal CdS in accordance with XRD. FTIR spectroscopy was utilized to determine the functional groups and types of bonds present in the system and to investigate the fate of the oxygen-containing functional groups of GO following the synthesis process. Fig. 5 shows the spectra of the synthesized nanoparticles/nanocomposites and the pure GO. Unexpectedly, the peaks fall at around 2355 and 3398 cm-1 in GO spectrum were noticed also in CdS and NiCdS spectra. The interpretation we suggest here is that photocatalyst surface has adsorbed both H2O and CO2 [40]. Therefore, adsorbed CO2 reacted with the co-adsorbed water to yield carbonate species. Hence, the occurrence of some peaks at about 1110, 1450, 2355, and 3398 cm-1 in most samples is characteristic to the bending vibration of (C-OH), symmetric and asymmetric stretching vibrations of (O-C-O), and stretching vibration of (O-H) of adsorbed bicarbonate and carbonate, respectively. This incidence can be considered as an indicator of the excellent adsorption efficacy of the prepared photocatalysts. The presence of the metallic compound was confirmed by the peak falls at the beginning of all spectra at about 400-410 cm-1 except in GO spectrum. In GO spectrum, the intense peak at 1043 is ascribed to the C-O stretching vibration of the epoxy group. The peaks at 1220 and 1407 cm-1 correspond to the C-OH stretching and C-O-H deformation vibrations, respectively. The peak at 1611 cm−1 is assigned to the C=C stretching vibration. The peak with high intensity at 1719 cm-1 is ascribed to the C=O bond stretching of COOH group. In comparison with the nanocomposites spectra, the dramatic decrease and even the disappear of most GO functional groups confirms the successful reduction of GO to

graphene. Only the peak ascribed to the epoxy group still shown after reduction, which may help to enhance the dispersion of nanocomposites in water and further promotes the stability of nanoparticles attached to the surface of graphene nanosheets as suggested by Hu et. al [22]. Besides the utilization of the reductant EG in a microwave route, the calcination was also believed to be a crucial step that induced the perfect reduction of GO and its attachment to the CdS nanoparticles. The absorbance spectra of different photocatalysts are shown in Fig. 6. It can be seen that absorption edge of the pure CdS appeared at ~525 nm which can be assigned to the intrinsic band gap absorption of CdS due to the electron transitions from the valence band to the conduction band. The spectra also revealed that absorption edges of all photocatalysts were well in the visible light region between 475-550 nm. NiCdS, in comparison with pure CdS, presented a notable red shift. Such enhancement in visible light absorption is generally interpreted by the formation of impurity levels by the doped Ni ion due to increased defect sites in the crystal structure [41].This red shift further confirms the substitution of Ni2+ into CdS structure lattice leading to a narrower band gap. On the other hand, the addition of graphene has also red shifted the absorption edge to visible region owing to coupling effect originated from the interaction between the nanoparticles and graphene which has modified the electron-hole recombination of CdS and graphene during irradiation [42, 43]. The band gap energies of the photocatalysts were estimated from the optical absorption spectra using the Tauc relation. The values were calculated to be 2.38, 2.36, 2.31, and 2.28 eV for CdS, G-CdS, NiCdS, and GNiCdS, respectively. Accordingly, it was speculated that G-NiCdS may have the highest visible light catalytic activity.

3.3. Mechanism of photocatalysts formation Based on the characterization results, the mechanism of the various photocatalysts formation is as follows: cadmium chloride and thiourea were readily dissolved in EG and reacted to give thiourea-cadmium complex [Cd(SCN2H4)2]2+. Once the irradiation with microwaves started, the complex underwent a thermal decomposition to yield cadmium sulfide nanoparticles and some gaseous products. During the reaction, EG changed first to glycolaldehyde, then to a series of carboxylic acids and finally to CO2 and H2O. This mechanism has been described widely in the literature and can be considered as a general approach for metal sulfides formation in such conditions [44, 45]. Illustration of the formation process is shown in Scheme 1. In the presence of GO, ethylene glycol reduced cadmium salt and then GO successively as GO was added later. Then, the thioureacadmium complex formed in the solution was attached to graphene surface via the electrostatic attraction. Next, G-nanoparticles composites were formed after microwave irradiation due to thermal decomposition of the complex. Here, three steps assured the reduction of GO to G including EG reduction, microwave heating and finally, calcination. Furthermore, the incorporation of Ni2+ was not expected to be difficult as the ion radius of Ni2+ (0.69 Å) is smaller than that of Cd2+ (0.96 Å) [30]. Hence, Ni2+ smoothly substituted Cd2+ into CdS crystal lattice as the appropriate molar ratio was employed which was confirmed by XRD and optical results.

3.4. Adsorption isotherms Since the adsorption of pollutants on the catalyst surface is a critical step to initiate the photocatalytic mediated degradation, the adsorptive properties of the photocatalysts have been studied to investigate especially the role of graphene in this process. Langmuir and Freundlich isotherms, in this prospective, are the most commonly used adsorption

isotherms to describe sorption from aqueous media. These isotherms are represented by the equations (2) and (3), respectively: Ce C 1   e Qe KQ m Q m

(2)

logQe  logK f 

1 logC e n

(3)

Where Qe is the amount of substrate adsorbed at equilibrium (mg.g-1), Qm is the maximum Langmuir adsorption capacity (mg.g-1), K is the Langmuir equilibrium adsorption constant (L.mg-1), Kf is the Freundlich capacity factor (mg.g-1.(L.mg-1)1/n) and (1⁄n) is the Freundlich intensity parameter. The linear plot of Ce/Qe versus Ce suggests the applicability of Langmuir isotherm whereas the linear plot of log Qe versus log Ce suggests the applicability of Freundlich isotherm. The obtained data shown in Table 2 indicate that CLX adsorption fitted to both isotherms while SMX adsorption fitted to Freundlich but not to Langmuir isotherm. The applicability of Freundlich isotherm in both cases suggests the multilayer adsorption and the heterogeneous nature of adsorbents [46] while for CLX the applicability of Langmuir model is presumably due to good adsorption of the small range of concentrations studied comparing to the large adsorption capacity of the photocatalysts that allowed for both equations to reach a linear form. SMX has shown small adsorption in all cases that may has induced better differentiation. What was actually essential for us is to compare the adsorption behavior of the four photocatalysts. The experimental data of Table 2 clearly indicate the superiority of nanocomposites in antibiotics adsorption. We assume here that graphene introduction helped to increase adsorption by concentrating the substrates near the attached CdS

owing to its large surface. The direct adsorption of antibiotics by graphene is improbable as most functional groups were reduced or substituted by nanoparticles.

3.5. Photodegradation of CLX and SMX As seen in Fig. 7, The photocatalytic degradation of CLX and SMX was enhanced according to the following order CdS< G-CdS< NiCdS< G-NiCdS. CLX could be almost eliminated within 180 min (~95%) using G-NiCdS. Meanwhile, pure CdS induced only (~60%) removal. SMX was also removed to a high level reached (~95%) within 240 min by G-NiCdS in comparison with (~54%) of pure CdS. These results indicate undoubtedly the advantages of combining doping technique with graphene anchoring for improving the photoactivity of semiconductors. To further understand the reason behind this enhancement, mechanism of catalytic reactions on G-NiCdS was proposed after detecting the primary reactive species that participated the CLX and SMX oxidation. In order to do so, control experiments were carried out in the presence of isopropanol (IP, 1.3×10-3 mol.L-1) as a scavenger for HO•, benzoquinone (BQ, 0.74×10-3 mol.L-1) as a scavenger for O2-•, potassium persulfate (K2S2O8, 0.74×10-3 mol.L-1) as electrons scavenger, and potassium iodide (KI, 1.2×10-3 mol.L-1) as holes scavenger. Calculated rate constants and degradation percents shown in Fig. 8 revealed that CLX degradation was almost unaffected when e- or O2-• were quenched. On the other hand, h+ quenching moderately decreased the degradation. However, HO• scavenging induced a profound impact on reaction rate. Surprisingly, SMX degradation has not followed the pattern of CLX. the reaction was not greatly affected by incapacitating h+ or HO• functionality implying that HO• has a minimal contribution to SMX degradation. Whereas O2-• production initiated by molecular oxygen

acceptation of the generated electron has shown to be the dominant mechanism for SMX oxidation. Based on the above observations, a suitable mechanism for photocatalytic degradation of CLX and SMX over the G-NiCdS composite can be proposed as illustrated in Scheme 2. The NiCdS (HO• based photocatalyst) was anchored on graphene (O2-• based photocatalyst) by surface charges attraction. Upon illumination with visible light, electrons are photoexcited from the valence band of CdS to the new energy level produced by the dopant in the conduction band of CdS which serves as a charge trap to hinder carriers recombination and afterwards intermediates the transport of interfacial charges to the attached graphene layer. Here, graphene does not participate the generation of electrons and holes by absorption of visible light as it does not possess an energy gap. Favorably, the function of graphene is to serve as an electron conductor due to its πconjugation and two-dimensional planar structure which offers a high aptitude to accept and transport the electrons reserved into the trapping sites to react with dissolved oxygen molecules in the solution to form superoxide anions that contribute the degradation process (mainly for SMX). On the other hand, the improved lifetime of hydroxyl radicals allow them to contribute effectively in the oxidation of substrate molecules adsorbed on the catalyst surface (mainly for CLX).

3.6. Reusability and leaching studies In order to verify the photostability and to test the reusing feasibility of G-NiCdS photocatalyst, recycled experiments for the photodegradation of CLX and SMX were performed. After the first cycle of catalytic reactions, suspensions were filtered, washed with water and ethanol, and then dried for the second cycle. A third cycle was carried out in the same procedure. Results depicted in Fig. 9a indicate that G-NiCdS could be

recycled comfortably without substantial reduce in its activity (>4%) during the subsequent cycles for both antibiotics. The inhibition of photocorrosion for the modified CdS was further investigated by comparing the XRD patterns and TEM images of the recycled catalyst before and after the photocatalytic reactions. It can be observed from Fig. 9b and c that the morphology and size of the particles have not changed appreciably and the crystal structure remained intact after three cycles supported by the retained catalytic activity. The strong link between NiCdS and graphene and the efficient separation and transfer of carriers were the key factors that promoted the stability and unaltered photoactivity. Lately, the filtrates of the recycled experiments were isolated and subjected to the ICPOES in order to evaluate the leaching behavior of the catalyst. The high release of ions into the treated solutions is generally the major obstacle for the actual application of CdS based photocatalysts as the exposure of ions such as cadmium and nickel may induce an environmental toxicity influence on aquatic organisms besides their direct effect on the human health. Fortunately, obtained results illustrated in Table 3 indicate that a small fracture of Cd2+ was released within all cycles and no detectable leaching of Ni2+ was observed. The leached amounts of Cd2+ in our work were approximate or even less than some reported amounts from modified CdS catalysts. For instance, Liu et al. [47] prepared CdS/N-doped TiO2 loaded on activated carbon fibers for methyl orange degradation and obtained 7% Cd2+ leaching. Gao et al. [20] synthesized graphene oxideCdS composite and yielded 3.5% Cd2+ leaching after the catalytic process. It is possible to postulate here that the secondary pollution attributed to leaching is of a minor influence. The slight catalyst leaching is imputed to the graphenic shield that restrained

S2- oxidation by the photogenerated holes/ HO• and secured, to a great extent, the NiCdS structure.

4 4. Conclusion CdS nanoparticles were synthesized by a simple microwave method and subsequent calcinations at 450 °C. Modifications in CdS structure were conducted by the insertion of 1% Ni ions and/or anchoring the nanoparticles onto graphene surface. The resultant photocatalysts were experienced for the photodegradation of CLX and SMX under visible light irradiation. Remarkable improvement in antibiotics degradation was achieved by the order G-CdS< NiCdS< G-NiCdS in comparison with pure CdS. The results distinctly suggest that G-NiCdS combined the advantages of NiCdS and G-CdS. Thus, we can briefly summarize the factors behind G-NiCdS supremacy in: (a) <(a)**1**>Well crystallization degree of nanoparticles due to calcination process. (b) <(b)**1**>Decreased nanoparticles agglomeration by introducing a matrix to attach on and align horizontally. (c) <(c)**1**>Enhanced adsorption properties due to high surface area presented by graphene which facilitates also the dispersion of product in the reaction solution. (d) <(d)**1**>Efficient visible light harvesting ability of the narrowed band gap photocatalyst resulted from Ni doping and strong attachment of NiCdS to graphene. (e) <(e)**1**>Enhanced stability and catalytic potency due to efficient electron-hole separation by the fashioned defects and effective charge transfer via graphene monolayer. Acknowledgment

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Fig. 1. Photoreactor system

Fig. 2. Degradation of O-NP with different synthesized photocatalysts (synthesis conditions optimization)

Fig. 3. XRD patterns for (A) non-calcined CdS, (B) CdS, (C) G-CdS, (D) NiCdS and (E) G-NiCdS

Fig. 4. SEM images of (a) CdS and (b) NiCdS; Low-magnification HRTEM images of (c) GO, (d) G-CdS, and (e and f) G-NiCdS; High-magnification HRTEM image of (g) G-NiCdS

Fig. 5. FTIR spectra of GO and the prepared photocatalysts

Fig. 6. UV-Vis spectra and Tauk plots of the prepared photocatalysts

Fig. 7. Photodegradation of CLX and SMX using the different photocatalysts (50 mg of catalyst with 50 mL of 10 mg.L-1 antibiotic solutions) and UV-Vis absorption spectral changes as a function of irradiation time in the presence of G-NiCdS

Fig. 8. Effect of different scavengers on reaction rate constants (min-1) and photocatalytic degradation percentages of CLX and SMX (control experiments)

Fig. 9. (a) Cycling tests for CLX and SMX degradation by the G-NiCdS composite under visible light, (b) HRTEM images of G-NiCdS after recycle, and (c) XRD patterns of G-NiCdS before and after recycle

Scheme 1. Formation of the different photocatalysts

Scheme 2. Mechanism of photocatalysis in G-NiCdS nanocomposite under visible light. Tables Table 1. Crystallinity and average crystallite size (nm) estimated by XRD and microscopes CdS (non-calcined)

CdS

NiCdS

G-CdS

G-NiCdS

Crystallinity (%)

64.43

75.36

72.71

74.07

74.43

XRD

24.91

35.95

30.38

34.17

29.19

RSD%

7.17

4.18

6.22

3.41

5.01

SEM/HRTEM*

N/A

41.62

42.37

37.17

35.46

RSD%

N/A

15.12

12.10

13.19

11.12

*Average of 10 measurements

Table 2. Langmuir and Freundlich isotherms of antibiotics adsorption onto photocatalysts Langmuir isotherm Qm CLX

CdS

K

R2

Freundlich isotherm Kf

166.66 0.049 0.960 8.315

n

R2

1.179 0.995

NiCdS

136.98 0.087 0.927 11.816 1.288 0.986

G-CdS

96.15

0.240 0.905 19.279 1.605 0.925

G-NiCdS 92.59

0.355 0.973 24.917 1.843 0.935

CdS

N/A

N/A

0.577 0.903

0.583 0.928

NiCdS

N/A

N/A

0.594 0.605

0.585 0.95

G-CdS

N/A

N/A

0.499 0.972

0.641 0.933

G-NiCdS N/A

N/A

0.523 1.450

0.681 0.956

SMX

Table 3. Leached Cd2+ and degradation percentages of antibiotics during the successive cycles CLX Cycle

1

2

SMX 3

1

2

3

▓ Cd2+ Leach (%) 2.29 1.26

1.92 2.47 1.14

1.99

Degradation

95

90

TDENDOFDOCTD

94

90

94

92

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