Photocatalytic degradation of Janus Green Blue dye in wastewater by green synthesised reduced graphene oxide-silver nanocomposite

ABSTRACT In this study, green synthesis of reduced graphene oxide-Ag (rGO-Ag) nanocomposite was carried out via bioreduction of Ag salt and graphene oxide (GO) using waste dry cell battery rod and Corchorus olitorius extract as GO precursor and reducing agent, respectively. Characterisation of rGO-Ag nanocomposite was achieved by UV/Vis spectroscopy, XRD, SEM, FTIR and BET analyses. Photocatalytic degradation of Janus Green blue (JGB) dye was conducted in batch mode. Effects of irradiation time, photocatalyst composition, temperature and recyclability were investigated. BET surface area of the nanocomposite was 532.914 m2/g with pore size of 2.136 nm. Optimal percentage photo-removal and equilibrium adsorption capacity of JGB dye by the nanocomposite achieved were 95.72% and 120.48 mg/g, respectively. Photocatalytic adsorption process followed more accurately Freundlich model implying multilayer adsorption onto rGO-Ag surface. Adsorption kinetics fitted perfectly into pseudo-second-order model while the enthalpy change is endothermic. The photocatalyst exhibited considerable stability, efficiency and was successfully applied for the removal of JGB in wastewater samples.


Introduction
Due to ever-growing human population, industrialisation and globalisation, there has been a sporadic increase in the demand for dyes and dye products in food, textile and leather, pharmaceutical, cosmetics, photovoltaic solar cells, and so on.Consequently, large volumes of dye laden effluents are being generated and released into the environment.Indiscriminate discharge of these effluents into water bodies has resulted in the disruption of food chain, objectionable colour and odour, cytotoxicity, carcinogenicity, mutagenicity, and so on.[1].Although various water treatment methods such as adsorption, ion exchange, coagulation, reverse osmosis, physical precipitation, and so on have been widely reported, these methods have often been proven to be ineffective in terms of cost, time, and so on.[2][3][4].
Advanced Oxidation Processes (AOPs) involving H 2 O 2 , ozone, fenton, UV/Visible light, homogeneous/heterogeneous photocatalysts, and so on have been recognised as reliable, scalable and effective waste water treatment techniques [5].AOPs are based on the in-situ generation of highly reactive transitory species capable of removing and mineralising refractory organic compounds, dyes, water pathogens, and so on.[6].In particular, heterogeneous photocatalysis relies on the interaction between a light source and a solid semiconductor in an aqueous matrix.Different nanomaterials have been reported for photocatalytic degradation of pollutants such as TiO 2 , ZnO, ZrO 2 , CdS, Ag, and so on.[7][8][9].Silver nanoparticles (Ag NPs) display considerable UV light absorption owing to the interband transition of 4d electrons to the 5sp and are thus potential photocatalysts capable of utilising the full solar spectrum [10].Ag NPs on the surface of semiconductors and electron-donor substances cause charge separation of photogenerated electron-hole pairs.In addition, Ag NPs remain arguably the cheapest of the nobel metals and have been widely applied in antibacterial agent, anticancer agent, electrochemical analysis, sensing, catalysis, and so on.[11][12][13][14].However, some challenges often encountered include narrow absorption range, photocorrosion, recombination of electron-hole pair, and so on.
Reduced graphene oxide-metal/metal oxide (rGO-M/MO) nanocomposites possess unique features such as extended light absorption ranges, higher conducting properties, greater porosity for improved interfacial contact with adsorbents, efficient charge transportation and separation, thereby resulting in an overall improvement in photocatalytic activity and stability [15,16].For instance, Wang et al. [17] investigated the photocatalytic degradation of methyl orange by porous graphene/ZnO nanocomposite.The nanocomposite showed a remarkable (100%) degradation of methyl orange in 150 min and a much improved performance compared to ZnO alone under similar conditions.The nanocomposite equally exhibited excellent photocatalytic repeatability and stability over five runs.Abbasi et al. [18] reported the synthesis of GO-Fe 3 O 4 -ZnO nanocomposite in the presence of sodium hydroxide at 90°C in an ultrasound bath using zinc chloride precursor.The photocatalytic activities of the nanocomposite towards methyl orange photodegradation were extensively studied.Higher photocatalytic degradation efficiency was achieved with increased GO-Fe 3 O 4 -ZnO catalyst dosage and irradiation time.
Dry cell batteries are used as an alternative power source in portable devices such as torches, transistor radio, medical and security gadgets, and so on.However, in developing countries, large amount of these batteries are indiscriminately disposed into dumpsite soils and waterways where they constitute environmental threat due to their low biodegradability.Utilisation of waste graphite rods (electrodes) from the discarded batteries as a suitable precursor in the synthesis of graphene oxide (GO) would not only minimise environmental pollution but also be of great economic importance.Thus, the objective of this study was to carry out green synthesis, characterisation of reduced graphene oxidesilver nanocomposite (rGO-Ag NCP) via simultaneous bio-reduction of GO (obtained from waste dry cell batteries) and Ag + ion using Corchorus olitorius aqueous extract as a reducing agent and its application in the photocatalytic degradation of Janus Green Blue dye.Detailed investigation of parameters such as effect of pH, irradiation time, dosage, concentration, temperature, variation in the composition of photocatalyst, regeneration efficiency, mechanism of photocatalysis, kinetic models and thermodynamics of the degradation process are presented.

Sample collection and treatment
C. olitorius leaves were purchased from a local market in Oye-Ekiti.Waste dry cell batteries were collected from a dumpsite in Oye-Ekiti, Ekiti State, Nigeria.The graphite electrodes were carefully removed from the waste batteries and ground with a mechanical grinder.The pulverised graphite rod was kept in a glass vial and stored at room temperature (30 ± 2°C) till further use.JGB dye stock solution (10 g/L) was prepared in distilled water, and serial standard solutions (1-10 g/L) were prepared by corresponding dilution of the stock solution with distilled water.

Preparation of graphene oxide from powdered graphite rods
Graphene oxide (GO) was prepared from powdered graphite rod by modified Hummer's method [22].Briefly, 1 g of powdered graphite rod was carefully weighed into a 250 mL flat bottom flask placed in an ice-water bath.A total of 23 mL concentrated sulphuric acid was added and stirred for 10 min.Three grams of KMnO 4 was added and the solution was stirred for 10 min and left inside the water bath for 1 h.The temperature was increased to 30°C and stirred for 1 h.A total of 46 mL distilled water was then added dropwise and the temperature raised to 96°C for 30 min.A total of 10 mL H 2 O 2 was added to the solution followed by 140 mL distilled water.GO was obtained by centrifugation at 3500 rpm for 10 min and washed with methanol.The GO residue was oven dried at 80°C for 4 h and kept in a sealed glass vial until further use.

Preparation of aqueous extract of C. olitorius
C. olitorius leaves were air-dried in the room and washed thoroughly with distilled water.Ten grams of finely cut leaves were boiled in 150 mL distilled water for 20 min and extract filtered with a filter paper.The filtrate was cooled to room temperature and refrigerated at 4°C.

Preparation of rGO-Ag NCP using aqueous extract of C. olitorius
One gram of GO was treated with 100 mL 5 mM AgNO 3 solution and 100 mL of freshly prepared aqueous extract of C. olitorius (as the reducing agent) in a 500 mL flat bottom flask.The mixture was heated at 80°C on a magnetic stirrer with constant stirring at 300 rpm for 45 min.The resulting rGO-Ag NCP was cooled to room temperature and centrifuged at 3500 rpm for 10 min.The nanocomposite was washed with methanol, oven dried at 80°C for 4 h and kept in a sealed glass vial until further use.

UV-Vis spectroscopy
UV-Vis absorption spectra of GO, rGO and rGO-Ag nanocomposites were obtained using Perkin Elmer Lambda 20 in the wavelength range of 200-800 nm.Aliquots of GO, rGO and rGO-Ag NCPs were placed in a quartz cuvette (1 cm path length) and the absorbance recorded.

Scanning Electron Microscopy (SEM) analysis
Surface granule morphology each of GO, rGO and rGO-Ag NCPs was examined using scanning electron microscopic tool.A thin layer of sample granule was placed on aluminium specimen holder by double-sided tape.The specimen holder was loaded in a polaron SC 7610 sputter coater and coated with gold to a thickness of about 30 nm to prevent charging.The specimen holder was transferred to XL-20 series.Scanning electron microscope of rGO-Ag nanocomposite was examined using a JEOL USA Model: JSM-7900F at an accelerating voltage of 15-20 kV.

FTIR spectroscopic analysis
FTIR spectra of the C. olitorius aqueous extract, GO, rGO and rGO-Ag NCPs were run as KBr pellets on Shimadzu Spectrum TwoTM spectrometer in the frequency range 4000-400 cm −1 .

Wide-angle X-ray diffractometry
Wide-angle X-ray diffraction patterns of powdered rGO-Ag NCP sample were obtained using Empyrean XRD diffractometer at 40 mA and 45 kV with Cu Kα (1.5418 Å) radiation at an angular incidence of 10-75°.

Brunauer-Emmett-Teller (BET) surface area analysis
The specific surface area (BET) of rGO-Ag NCP was measured with a Quantachrome instrument using the adsorption of N 2 at the temperature of liquid nitrogen.Prior to the analysis, the sample was degassed at 523 K for 3 h.The pore size and pore volume were estimated using Barrett-Joyner-Halenda (BHJ) theory.BET surface area was about 532.914 m 2 /g, while the BJH surface area was measured to be about 617.3 m 2 /g.The BJH pore volume was calculated to be about 0.3043 cc/g with an average pore size of 2.136 nm (see S1). the resulting mixture was filtered to separate the photocatalyst and absorbance of the filtrate measured at 660 nm using a T-60 UV-Visible spectrophotometer.The photo-removal efficiency experiments were carried out for three repeated cycles to study the recyclability and stability of the rGO-Ag nanocomposite sorbent.After each cycle, the nanocomposite catalyst was filtered, washed with distilled water and dried in the oven at 105°C before reuse.In order to fully understand the influence of various components involved in the photodegradation process, the experiment was conducted using different photocatalyst combinations viz., rGO-Ag/UV/H 2 O 2 , Ag/UV/H 2 O 2 , UV/H 2 O 2 and UV light alone under the same conditions and the % photo-removal efficiency determined.The photo-removal efficiency percentage was calculated using [23] where C 0 is the initial concentration of JGB dye and C f is the concentration of dye after photo-irradiation.The amount of JGB dye adsorbed; q e (mg of dye per g rGO-Ag) was calculated according to Vanderborght and Van Grieken [24].

Adsorption isotherm
For this study, Langmuir and Freundlich adsorption isotherm models were adopted.The Langmuir isotherm is based on the theoretical principle that the adsorption sites are of equal energy and the coverage of adsorbate molecules on a solid surface occurs only in a monolayer.The linearised form of the Langmuir model as described by Igwe and Abia [25] is shown in Eq. ( 3) .
where Ce is the equilibrium concentration of dye solution (mg/L), qe is the amount of pollutant adsorbed per unit mass of the rGO-Ag NCP photocatalyst (mg/g), qm is the Langmuir constant representing adsorption capacity (mg/g) and K L is Langmuir constant representing energy of adsorption (L/mg).A plot of 1/qe against 1/Ce is linear for a sorption process obeying the basis of this equation with k L and qm obtained from the slope and intercept respectively.The Freundlich isotherm proposed that the analyte uptake occurs on a heterogeneous surface by multilayer adsorption and non-uniform distribution of the heat of adsorption over the adsorbent surface.The linear form of the Freundlich equation as described by Igwe and Abia [25].
where C e is the equilibrium concentration of dye solution (mg/L), q e is the amount of pollutant adsorbed per unit mass of the nanocomposite photocatalyst (mg/g), n is the number of layers and K F is the Freundlich constant.A straight line plot of log q e against log C e has K F and n as the intercept and slope, respectively.For practical application, wastewater containing JGB dye was analysed before and after treatment with rGO-Ag NCP (0.75 g photocatalyst contacted with 40 mL effluent at 300 rpm, pH 2, 40°C for 30 min) and the % photo-removal efficiency determined.The chemical oxygen demand (COD) was analysed before and after treatment using the standard method [26].

Kinetic model and thermodynamics studies
In order to investigate the adsorption kinetics of JGB dye onto the rGO-Ag NCP surface, the pseudo-first [27] and second-order [28] kinetic models were used as described in as described in Equations (5a) and (5b), respectively.where q t and q e are the amount of pollutant adsorbed (mg/g) at time t and equilibrium, respectively.k 1 (min −1 ) is the rate constant of the pseudo-first-order kinetic model, and k 2 (g.mg −1 .min −1 ) is the rate constant of the pseudo-second-order kinetic model.
where C o (mg/L) is the concentration after pollutant adsorption and C e (mg/g) is the equilibrium concentration.
ΔH o and ΔS o are deduced from the slope and intercept of the plot of In K against 1/T.R is the molar gas constant (8.314J/mol/K) and T is the temperature in Kelvin.

UV/Visible and FTIR spectra
Figure 3 shows the UV/Vis spectra of GO, rGO and rGO-Ag NCP.The prominent peak at 231 and a small peak 314 nm correspond to the GO bands.This is attributed to the π-π* transition of aromatic C-C and the n-π* transition of C=O, respectively, [30].The new peak observed around 346 nm is an indication of the formation of rGO, i.e., reduction of GO to rGO.This attested to the restoration of the electronic conjugation of the graphene sheets occurs by bioreduction process using C. olitorius aqueous extract [31].The appearance of a new peak around 451 nm is due to the surface plasmon resonance (SPR) of Ag nanoparticles which is due to the collective oscillation of free conduction electrons induced by an interacting electromagnetic field [32].
The result of the FTIR analysis of the C. olitorius aqueous extract, GO, rGO and rGO-Ag NCP is shown in Figure 4.For the C. olitorius aqueous extract (Figure 4a)), the peaks at 715.61 and 866.07 cm −1 revealed bending vibrations of C=C, while the peaks at 1211.34 and 1126.47 cm −1 showed the stretching vibration of C−O.The peaks observed at 1635.69 and 3444.12cm −1 correspond to C=O carbonyl and -OH stretching vibrations, respectively.These functional groups are responsible for the co-reduction of GO and Ag + to rGO-Ag NCP and its stabilisation under prevailing condition.A summary of the list of major peaks and their corresponding functional groups is shown in the supplementary list (S2).The peaks around 3500, 2923.39,1579.33,1132.1 cm −1 correspond to -OH (stretching), -C-H (stretching), C=C (aromatic) and C-O-C groups in the GO, rGO and rGO-Ag NCP (Figure 4b)).The remarkable decrease in the intensity of the -C=O (carboxylic acid) stretching band around 1640 cm −1 for rGO and rGO-Ag NCP confirmed the bioreduction of GO by the aqueous extract of C. olitorius.

SEM analysis of rGO-Ag NCP
SEM is a useful microscopic tool to characterise surface feature and evaluate morphological changes in materials.The SEM image of rGO-Ag NCP revealed a dense, coarse, ruptured reduced graphene oxide sheets interspersed with Ag nanoparticles (Figure 5).The coarse layer facilitated facile adsorption of components onto its surface via physical and chemical interactions.

XRD pattern of rGO-Ag NCP
The XRD patterns of rGO and rGO-Ag NCP are shown in Figure 6.The prominent, broad peak at 2θ = 25.8 o correspond to the 002 facet of rGO.The interlayer distance of rGO is 0.361 nm.This is in close agreement with the value reported by Cui et al. [33] in the one-pot reduction of GO at subzero temperatures.The diffraction peaks observed at 2θ = 32.

Effect of temperature
The effect of temperature on the photo-removal efficiency of rGO-Ag NCP is presented in Figure 7a).Increasing the temperature of the medium from 30°C to 70°C resulted in a corresponding increase in the % photo-removal efficiency of JGB dye.Maximum % photo-removal efficiency value of 90.6% was attained at 70°C.This might be attributed to a linear decrease in bandgap of Ag (2.51 eV) as a function of temperature that facilitated rapid transportation and separation of the photogenerated electron-hole pairs and higher diffusion rate of the JGB dye onto the nanocomposite photocatalyst surface [36].Chong et al. [37] reported a temperature range between 20°C and 80°C as the ideal temperature for effective photomineralization of organic content using activated titania.An increase in the apparent activation energy was observed around 0°C, while temperature above 80°C promoted the recombination of charge carriers.

Effect of variation in pH
The effect of pH was studied between pH 2-10 at 40°C, 0.1 g rGO-Ag photocatalyst dosage for 40 min (Figure 7b) ()).Increasing the pH of the medium led to a gradual decrease in the % photo-removal efficiency of the JGB dye.Maximum % photo-removal efficiency of 86% was attained at pH 2. This might be due to electrostatic attraction between negatively charged (electron rich) OH − group on the rGO-Ag NCP surface (adsorbent) and positively (cationic) charged JGB dye in an acidic medium.The progressive reduction in the photo-removal efficiency at higher pH (alkaline) was as a result of the increase in the population of OH − groups resulting in electrostatic repulsion and their subsequent migration from adsorbent surface in order to minimise repulsion.Hence, lesser active sites are available for photocatalytic adsorption of JGB dye at the surface.A similar observation on the influence of pH on removal efficiency was reported from previous studies [38,39].

Effect of variation in adsorbate concentration
Photocatalytic degradation experiments were conducted between 1 and 10 g/L JGB concentration with rGO-Ag photocatalyst dosage of 0.1 g at pH 2, 40°C and for 40 min (Figure 7c)()).Maximum % photo-removal efficiency (95.72%) was achieved at 2.5 g/L.At this concentration, the active sites on the rGO-Ag NCP photocatalyst surface became fully saturated with JGB dye molecules.Also, there is a strong UV light interaction with the catalyst and JGB dye, considerable photogenerated electron-hole pairs for maximum photodegradation of the dye.Beyond 2.5 g/L, % photo-removal efficiency decreased steadily to 88% at 10 ppm, implying a relative supersaturation state and reduced interaction between the catalyst and UV light.

Effect of variation in adsorbent dosage
The study of adsorbent dosage effect was necessary to determine the amount required to attain optimum % photo-removal efficiency.The dosage effect was studied within the range of 0.1-1.0g at 40°C, 0.1% 40 mL JGB solution for 40 min.A steady increase in % photo-removal efficiency was observed as the rGO-Ag NCP dosage amount increased gradually from 82.6% (at 0.1 g) to its maximum value of 91.8% at 0.75 g (Figure 7d) ()).The increment might be attributed to the increase in the active sites population on the rGO-Ag NCP resulting in the generation of greater amount of electron-hole pairs and hence the free radical species (*OH) necessary for the photocatalytic degradation of JGB dye.Roozban et al. [40] reported over 90% removal efficiency for methyl orange using 0.5% wt multi-walled carbon nanotube-ZnO nanocomposite.

Effect of variation in irradiation time
Irradiation time is a function of the duration of exposure of the reaction vessel to the UV radiation at a specific wavelength (254 nm).For this study, the exposure time was 100 min at 40°C, 0.1 g dosage and 0.1% 40 mL JGB solution.From Figure 7e,), a gradual increase in % photo-removal efficiency was noticed from 74% after 20 min to 89% after 100 min.Prolonged exposure to UV radiation resulted in an increase in the population of available photogenerated electron-hole pairs and subsequent interaction with JGB, resulting in its mineralisation under prevailing conditions to CO 2 and water.Abbasi reported a similar enhancement in the photocatalytic removal efficiency of methyl orange upon increasing the irradiation time using magnetic(Fe 3 O 4 ) graphene oxide (GO) and Fe 3 O 4 -GO-ZnO nanocomposites [38].

Effect of variation in photocatalyst composition
In order to fully understand the influence of different components of the photocatalyst on the overall degradation process and hence its mechanism, the process was conducted in the presence of rGO-Ag/UV/H 2 O 2 , Ag/UV/H 2 O 2 , UV/H 2 O 2 and UV light alone under same conditions.From Figure 7f,), the order of % photoremoval efficiency is as follows: rGO-Ag/UV/H 2 O 2 > Ag/UV/H 2 O 2 > UV/H 2 O 2 > UV light alone.The highest % photodegradation efficiency (95.7%) was achieved with the rGO-Ag/UV/H 2 O 2 combination.This was closely followed by Ag/UV/H 2 O 2 combination (76.50%), while treatment with UV light alone gave the lowest value of 49.37%.The result indicated a near perfect mineralisation of the organic JGB dye and affirmed enhanced photocatalytic efficiency of rGO-Ag/UV/H 2 O 2 nanocomposite in an aqueous medium.It also showed the important role of reduced graphene in the photodegradation process by ensuring facile generation of reactive oxygen species, improved and efficient charge transportation and separation (zero band gap), extended UV light absorption range, enhanced pore size and better conducting properties [10,11].

Regeneration efficiency of rGO-Ag NCP photocatalyst
Photocatalysts with higher adsorption-desorption rate are more often economically preferred.)Figure 7g shows the regeneration efficiency of rGO-Ag NCP photocatalyst on the degradation of JGB dye for three consecutive cycles.The result showed a slow, steady reduction in regeneration efficiency of the photocatalyst over the three runs.Percentage regeneration efficiency decreased from 89% after the first repeated cycle to around 65% after the third cycle.The relative stability and regeneration efficiency of the nanocomposite photocatalyst might be due to its relatively narrow pore size (2.136 nm), presence of reduced graphene oxide (for easy charge transfer and separation) and large surface area (532.914m 2 /g) (see S1

Adsorption isotherms and kinetic models of photocatalytic degradation of JGB dye
Figure 8 shows the plots obtained for the Langmuir and Freundlich adsorption isotherm models for the JGB dye.The Freundlich isotherm is a more accurately fitted adsorption model on account of its relatively higher regression value (see S3) implying multilayer adsorption onto the heterogeneous rGO-Ag NCP surface and non-uniform distribution of heat of adsorption.The result of the kinetic model studies is shown in Figure 9.The pseudo-first and pseudo-second rate constants derived from Figure 9 are summarised in S3.The rate of adsorption of JGB dye onto the nanocomposite surface is relatively faster with the pseudo-first order (0.003 min −1 ) than the pseudo-second-order kinetics (0.0024 min −1 ).It is noteworthy that with respect to the regression values, the adsorption processes conformed more to the pseudo-second-order kinetics, implying a favourable chemisorption mechanism for the JGB dye photodegradation.Ayanda et al. [41] reported similar observation in the removal of Congo red dye using termite mound.

Thermodynamic studies of photocatalysis of JGB dye
The Van't Hoff plot for the photocatalytic degradation of JGB dye onto rGO-Ag NCP surface is shown in S4.The standard free energy (ΔG o ), enthalpy (ΔH o ) and entropy (ΔS o ) changes for the rGO-Ag NCP were determined using from the slope and intercept of the plot of In K against1/T (K).The positive value of ΔH o (14.385KJ/mol/K) showed that the adsorption process is endothermic (see S5).The ΔS o value of 18.33 J/mol indicates that degree of randomness or disorderliness (ΔS o ) of the process is favourable.The positive value of free energy change (ΔG o ) at different temperatures shows the non-spontaneous nature of the photocatalytic adsorption process at rGO-Ag NCP surface under prevailing conditions.

Proposed mechanism of rGO-Ag NCP photocatalytic degradation of JGB dye
When the rGO-Ag NCP photocatalyst surface is exposed to high energy UV incident radiation, electron-hole pairs are generated and photoexcitation of the electron from the valence band (vb) to the conduction band (e CB −) leaving behind a positive hole (hVB+) (Figure 10).The hVB+ and eCB− are powerful oxidising and reducing agents, respectively.The hVB+ reacted with H 2 O molecule yielding H + ion and hydroxyl radical.Generation of *OH radical is via the interaction of OH − ions with positive holes (h+) present in the valence band of Ag component of the rGO-Ag nanocomposite photocatalyst.Hydroxyl radical (*OH) generated has the second highest oxidation potential (2.80 V), which is only slightly lower than fluorine, the strongest oxidant.The conductive band electron reacted with O 2 forming an anion radical superoxide.Subsequent reaction led to the formation of hydrogen peroxide, thus increasing its overall population and ultimately resulted in the formation of *OH radical [42].Due to its electrophilic nature, the *OH oxidise organic JGB dye molecules, eventually converting them to CO 2 , water, nitrate and nitrite.

Treatment of wastewater
The photocatalytic degradation efficiency of rGO-Ag NCP was applied in the treatment of wastewater containing JGB dye.Pphoto-removal efficiency of 91.0% was achieved at optimum conditions of pH 2, 0.75 g adsorbent dosage, 300 rpm and 40°C after 1 h.Analysis of the COD (a measure of the amount of oxygen required for chemical oxidation of pollutants in an effluent) showed a sharp decrease from 445.02 mg/L (before treatment) to 23.15 mg/L after treatment, representing 95% reduction in COD value, thus affirming the effectiveness of rGO-Ag NCP photocatalyst in the degradation of the JGB dye-laden effluent.

Conclusion
We have successfully explored the possibility of ecofriendly synthesis route for rGO-Ag NCP photocatalyst using C. olitorius aqueous extract and waste battery rod powder as a reducing agent and graphene oxide precursor.Optical, morphological, structural crystallinity and functional groups analysis affirmed the presence of the rGO-Ag NCP.Optimal percentage photo-removal and equilibrium adsorption capacity of JGB dye by the rGO-Ag NCP photocatalyst were 95.72% and 120.48 mg/g, respectively, when 40 mL of 2.5 g/L JGB was reacted with 0.1 g dosage for 40 min at 300 rpm.Regarding photocatalyst composition, the order of % photoremoval efficiency is as follows: rGO-Ag/UV/H 2 O 2 > Ag/UV/H 2 O 2 > UV/H 2 O 2 > UV light alone, signifying the importance of reduced graphene oxide in the overall photodegradation process.The photocatalytic adsorption process followed more accurately the Freundlich adsorption model implying multilayer adsorption onto rGO-Ag surface.The adsorption kinetics fitted perfectly into the pseudo-second-order model, while the thermodynamic studies showed a nonspontaneous degradation process.The rGO-Ag NCP displayed excellent photocatalytic degradation efficiency in the practical treatment of real wastewater sample.

Figure 2 Figure 2 .
Figure2shows the schematic set-up for the photocatalytic degradation of JGB dye by rGO-Ag NCP.For the effect of dosage, a suspension was prepared by adding 0.1-1.0g of rGO-Ag NCP to a mixture containing 2 mL 30% hydrogen peroxide and 40 mL 1 g/L JGB dye solution in a 100 mL Pyrex glass beaker.The mixture was stirred continuously on a magnetic stirrer for 30 min at 300 rpm in the dark to ensure equilibration at room temperature.The suspension was quantitatively transferred into a flat dish with a 100 mm diameter and exposed to ultraviolet (UV) radiation using a UVP Compact lamp (model UVG-11, 4 W, 230 V, λ = 254 nm) for 40 min.The distance between the UV source and the photo-reaction vessel was 10 cm.The pH of the mixture was studied between pH 2 and 10 by adjusting with 0.1 M HCl or NaOH.The influence of UV irradiation time was monitored at 20, 40, 60, 80 and 100 min.The JGB dye concentration was investigated at 1.0-10.0g/L.The effect of temperature was evaluated between 30°C and 70°C by heating the suspension on a magnetic stirrer for 30 min at 300 rpm prior to exposure to UV radiation.In each case,

Figure
Figure FTIR spectra of: (a) C. olitorius aqueous extract, (b) GO, rGO and rGO-Ag 4 o , 38.32 o , 44.98 o , 53.53 o , 56.16° are ascribed to the (111), (200), (220), (311) and (222) facets of face centred cubic crystalline phases of Ag component of the nanocomposite.The observed relative decrease in the intensity of 002 rGO crystalline facet of the rGO-Ag NCP might be due to the chemical modification resulting in the incorporation of Ag nanoparticles into its nanocomposite matrix.The result of this study agreed with diffraction patterns observed in Ag nanoparticles and rGO-Ag NCP from earlier studies[30,34,35].

Figure 7 .
Figure 7. (a) Effect of temperature on photo-removal efficiency of rGO-Ag NCP, (b) Effect of pH on photo-removal efficiency of rGO-Ag NCP, (c) Effect of concentration on photo-removal efficiency of rGO-Ag NCP, (d) Effect of adsorbent dosage on photo-removal efficiency of rGO-Ag NCP, (e) Effect of irradiation time on photo-removal efficiency of rGO-Ag NCP, (f) Effect of variation in photocatalyst composition on photo-removal efficiency of rGO-Ag NCP, (g) Regeneration efficiency of rGO-Ag NCP

Figure
Figure Pseudo (a) and second (b) order kinetic plots.