Preparation, Characterization and Application of a Home–made Graphene for the Removal of Congo Red From Aqueous Solutions

7 Ethylene diaminetetraacetic acid (EDTA) functionalized graphene was synthesized from Nigerian coal using a 8 chemical exfoliation method and the graphene was applied for the removal of Congo red dye from aqueous solutions. 9 The synthesized coal graphene and the raw coal were characterized using Fourier transform infrared (FTIR) 10 spectroscopy, X-ray diffraction (XRD) spectroscopy, Scanning electron microscopy and Energy (SEM) – Energy 11 dispersive X-ray (EDX) spectroscopy. The SEM data revealed surface roughness which is enhanced in the prepared 12 graphene while the EDX revealed an increase in carbon, the main constituent of graphene, from about 26% in the raw 13 coal to about 80% in the prepared graphene. Various adsorption parameters, such as pH, contact time, concentration 14 of Congo red and temperature, were varied for the removal of the dye using raw coal and the synthesized coal 15 graphene. The Liu isotherm gave the best fit of the equilibrium data than the Langmuir, Freundlich and Dubinin- 16 Radushkevich models. The maximum adsorption capacities of the raw coal and synthesized coal graphene at 25 °C 17 are 109.1 mg/g and 129.0 mg/g, respectively. The Avrami fractional order kinetic model was the best model for 18 description of the kinetic data. The model had the lowest values of standard deviation than the pseudo-first order and 19 pseudo-second order models. The adsorption process of the two materials occurred via two stages as proved by 20 intraparticle diffusion model. The adsorption process of the Congo red removal was spontaneous, feasible and 21 endothermic. The study conclusively revealed the graphene nanomaterial to be a viable adsorbent for textile wastewater treatment. on SCG adsorption. Variations in contact time, initial Congo red concentrations and temperature significantly affected the adsorption of Congo red Raw Coal and SCG. Adsorption kinetics was studied using pseudo first order, pseudo-second order, Avrami and intraparticle diffusion models with Avrami fractional model as the most model for description of the kinetic data of the adsorption process. The isotherm parameters were analyzed Freundlich, Lu and Dubinin-Radushkevich models. It observed that the adsorption is favorably fitted to the Liu isotherm model. The maximum adsorption capacities of the raw coal and synthesized coal graphene from the Liu model at 25 are 109.1 mg/g and 129.0 mg/g, respectively. The thermodynamic study showed endothermic and a spontaneous adsorption process for both adsorbent materials. graphene


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Over time, industrialization and population explosion have led to an upward surge in the utilization of dyes for printing 27 and coloring by industries, particularly the textile industry (Hairom et al. 2014) and a lot of wastewater, from plastic, 28 paper, printing, textile and dyeing industries, is generated during printing, production and dyeing processes (Chong et

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Congo red (CR) is the disodium salt of the most widely used direct dye in the textile industry due to its chromaticity 35 (Yao et al. 2016). Various types of dye, particularly the Congo red, are critical sources of wastewater contamination 36 due to its increased oxygen demand and high biological toxicity after indiscriminate dumping in water bodies 37 (Robinson et al. 2001). The presence of Congo red in water bodies results in unpleasant changes in the color of water 38 and its presence even in trace amounts can adversely affect living things due to inhibitory effects on photosynthesis 39 (Tabrez et. al. 2004). The anaerobic breakdown and incomplete bacterial degradation of dyes often result in the 40 production of toxic amines, which pose serious threat to mankind (Weber and Wolfe 1987). For example, Congo red, 41 a synthetic dye is largely non-biodegradable and carcinogenic. Therefore, its presence in the ecosystem even in trace 42 amounts is of great environmental concern due to adverse effect on human health and the economy. These issues, 43 therefore require prompt and effective remediation of textile wastewater to reduce the levels of pollution of Congo 44 red to permissible limits (Zhang et al. 2014).

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To this end, various methods have been adopted for the effective removal of toxic dyes from solutions, such techniques

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Over the past decade, carbonaceous materials have been of great interest to researchers because of their uniqueness, 50 composition and diversity. A case in point is activated carbon, which has been investigated and shows promising result 51 in the effective treatment of wastewater owing largely to its large surface area and mechanical stability (Suhas et al. 52 2017). However, due to the high energy consumption and greenhouse gas emission of coal during preparation of 53 activated carbon (Alhashimi and Aktars 2017), its application has been limited. It is therefore pertinent to source for 54 suitable alternative routes with less risk. Coal and other derivatives are common adsorbents for the treatment of textile 55 wastewater and as a solid source raw material with high carbon content, it is stable at room temperature and quite easy 56 to transport. Owing largely to environmental concerns of global warming, the primary function of coal as fuel for 57 transport has been jettisoned (Shinn 1996). Presently, alternative uses of coal are being researched daily to cater for 58 its large reserves while also making sure that its use is eco-friendly. The use of coal to synthesize graphene could open 59 a new opportunity for coal as a non-conventional carbon source and provide reliability for large synthesis of graphene.

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Nanomaterials, such as graphene, have been a point of focus for researchers because of their outstanding properties 61 and diversity. The presence of various functional groups, binding sites, and large surfaces on these materials are the 62 main characteristics that make graphene and graphene derivatives excellent adsorbents for the removal of countless 63 pollutants, including toxic dyes, from aqueous effluents (Galashey and Polukhin 2014). The large specific surface area 64 of graphene makes bonding sites available for modification by other compounds such as ethylenediaminetetraacetic 65 acid (EDTA) (Cui et al. 2015). Ethylenediaminetetraacetic acid is an excellent precursor that serves many functions 66 in the industrial sphere such as an auxiliary chemical in dyeing, as a stabilizer, a softener or even a metal complex in 67 coordination titration (Repo et al. 2013). Ethylenediaminetetraacetic acid is a favourable option for modifying composites with other materials such as graphene to enhance the adsorption capacity (Ali, 2012). A lot of 70 nanomaterials, functionalized with EDTA, have been reported (Pang and Wilson 1991;Pang and Wilson 1993) and 71 showed substantial efficiency adsorption towards dyes and heavy metals in aqueous solution but not much effort has 72 been geared towards using EDTA as a precursor for graphene synthesis. Rehman et al. (2019) reported the synthesis 73 of nanocrystalline Hematite using EDTA as a precursor which considers the feasibility of EDTA as a precursor rather 74 than a modifier.

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The current study, therefore, provided a pathway to prepare graphene from coal through a rapid and non-combustible 76 method using EDTA as a precursor and chelating agent for the growth process and then investigate its effectiveness 77 in the removal of Congo red from solution.    Energy-dispersive X-Ray (EDX) spectroscopy techniques were used to characterize the Raw Coal and SCG. The FTIR 91 spectra were recorded in the spectral range of 4000 -400 cm -1 using a FTIR spectrometer (Agilent Technologies,

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Germany). The XRD patterns were obtained using X-ray diffractometer (D8 advance, Bruker) with Cu Ka radiation    The Energy-dispersive X-ray data of the Raw Coal and SCG are presented in    The XRD spectra presented in Figure 3 revealed that the diffractogram of the raw coal has 2θ° values stretching 147 between 11° to 65°. The raw coal displayed peaks of high intensity compared to the SCG, which is an indication of 148 its higher crystallinity than the graphene precursor (Vassilev 1994   39.55 mg/g observed at pH 3 while adsorption capacity of Raw Coal for removal of Congo red decreased as we moved 182 from acidic to alkaline region with maximum removal (24.77 mg/g) at pH 3. Congo red is dipolar and therefore it is 183 an anionic and cationic in alkaline medium and acidic medium, respectively. However, as the pH of the Congo red 184 dye decreases, the color of the solution changes from orange to dark blue (Stephen, 2000). This phenomenal color 185 change, which solely depends on the pH of the dye solution, is a pointer to the ionic character of the Congo red 186 molecule. This is because of the lone pair transition that happens upon acidification leading to a change in the 187 wavelength of the red dye (Stephen, 2000). From Figure 5, the quantity of Congo red removed varied only slightly,

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The rate expressions from which the rate constants were obtained are presented in Equations 4 -7 for pseudo-first 225 order, pseudo-second order, Avrami fractional order and intraparticle diffusion models, respectively.

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where the quantity of dye removed at equilibrium is given as Qe (mg/g) and the quantity adsorbed at a given time, t, 231 is given as Qt (mg/g), kf (1/min) is the pseudo-first order rate constant, ks (g/mg min) is the pseudo-second order rate 232 constant, kAv (1/min) represents the Avrami fractional order rate constant, nAv is the Avrami fractional kinetic order 233 that is related to the mechanism of adsorption, kipd (mg/g min 0.5 ) is the intraparticle mass transfer constant, and C 234 (mg/g) represents the boundary layer.  Table 2 presents the parameters of the models. From the table, SD values are 1.856 mg/g and 1.521 mg/g (pseudo-237 first order), 0.7212 mg/g and 1.143 mg/g (pseudo-second order), and 0.2199 mg/g and 0.3433 mg/g (Avrami model)

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for Raw Coal and SCG, respectively. It is evident that the kinetic profile of the adsorption process did not follow 239 pseudo-first order and pseudo-second order kinetic model but followed the Avrami fractional order kinetic model.

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To investigate the mechanism of the adsorption process, the intraparticle diffusion model was used to interpret the 249 kinetic data. The intraparticle plots, Figure 6c

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The Liu model is a three-parameter isotherm model that assumes that the active binding sites of the adsorbent cannot 288 possess the same energy (Liu et al. 2003). The adsorbent surface will therefore present active sites that the adsorbate 289 molecules can preferentially occupy, hence, the preferred active sites will be saturated with the adsorbate molecules 290 (Liu et al. 2003). Since the Liu model is the best model for description of the equilibrium adsorption data, then the

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Kg was converted from L/mg to mol/g.

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The ΔG° values ranged between -23.73 kJ/mol and -30.33 kJ/mol for Raw Coal and between -26.20 kJ/mol and -331 31.67 kJ/mol for SCG ( intercept, respectively, of the van't Hoff plot (Figure 8a). The values of ΔH° are positive, which is an indication that 334 the adsorption process was endothermic, this only reflects in the values of equilibrium constant (Figure 8a)      Fourier transform infrared spectra of (a) SCG before adsorption of Congo red and (b) SCG after adsorption of Congo red Effect of initial pH on the removal of Congo red dye by raw coal and SCG