Adsorption of an anionic dye on a novel low-cost mesoporous adsorbent: kinetic, thermodynamic and isotherm studies

Our activated carbon was prepared successfully using phosphoric acid as an activated agent. The activated carbon was characterized by Scanning Electron Micrograph (SEM), Brunauer-EmmettTeller (BET), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). The aim of our study is to evaluate the adsorption capacity of Methyl Orange (MO) on Ziziphus lotus activated carbon. Adsorption isotherms were studied according to Langmuir and Freundlich Model, and adsorption kinetics according to pseudo-first and second-order. Results show that the maximum adsorption was reached in the first 10min at ambient temperature with a yield of 96.31%. The Langmuir isotherm shows a correlation coefficient of 99.4 % higher than the Freundlich model and the adsorption kinetic model follow a pseudo-second-order with a maximum adsorption capacity of 769.23 mg/g. FTIR and X-Ray spectroscopy indicate that our activated carbon has an amorphous structure with the presence of functional groups, where BET analysis revealed a high surface area of 553 mg/g, which facilitate the adsorption process.


Introduction
Numerous industries such as textile, leather, paper, rubber, paint and a plastic used in their application extensive compounds lead to discharges toxic colored effluent, contaminates surface and groundwater.Textile wastewater, generally, contains a big amount of pollutants materials like colored materials or dyes, organic compounds and heavy metals ions, these colored materials can affect the physicochemical and the biological properties of the sea, drinking water and globally the ecosystem.In addition to the undesirable colors of textile effluents, some dyes may degrade to produce carcinogens and toxic products [1].Methyl orange (MO), an anionic dye belongs to the azo and the more toxic group of dyes.MO has been widely used in textile, paper, plastics, rubber, cosmetics, printing, dye manufacturing and pharmaceutical industries [2].
The removal of industrial pollutants containing the aromatic band from water has become increasingly necessary in recent years and determining processes to achieve this is the focus of much scientific research.Researchers are indulging their interest in the wastewater treatment by various processes such as precipitation, ion exchange, reverse osmosis, and adsorption.Among these methods, Adsorption process is recognized as one of the most promising and effective approaches in wastewater treatment because of its lowcost ease of operation and high-efficiency [3].There are currently a number of adsorbents including clay minerals, agricultural solid wastes are economical adsorbents, but they suffer from limited adsorption capacities of color removal.
This study highlights the adsorption activity of Methyl Orange (MO) from an aqueous solution onto Ziziphus Lotus activated carbon.MO is stable, shows solubility in water and low biodegradability; hence it is difficult to remove from aqueous solutions by common water purification or treatment methods [4], several factors affected the biosorption were studied in this work as pH, adsorbent dose, contact time and temperature, to deduce the adsorption thermodynamic and kinetic behavior process.The experimental results revealed a high adsorption capacity of MO adsorption on our sorbent.

Material and reagents:
To test the capacity of Ziziphus Lotus sorption, diverse laboratories materials were used in this work: UV visible spectroscopy, multi-parameter Consort C3040 was used for pH measurements, the pH values were adjusted using HCl (0.1N) or NaOH (0.1N) buffer solutions, multi agitator.Methyl orange (MO) (C 14 H 14 N 3 NaO 3 S), obtained from Sigma Aldrich, with a molecular weight of 327.32 g/mol and λ max of 464 nm.Solutions were prepared by dilution with distilled water from the stock solution of MO with the initial concentration of 1000 ppm to reach the desired concentration.

Characterization of the activated carbon:
Ziziphus leaves are very abundant in Morocco and not valorized were collected from Settat-Casablanca, Morocco, washed several times to eliminate the impurities and then activated with the phosphoric acid to obtain the activated carbon.The physicochemical properties of our activated carbon were determined by several analysis methods such as the (BET) Surface Area Analysis using a micrometric machine model 3 FLEX 3500, the surface morphology and the elemental composition was executed by SEM and Energy Dispersive X-Ray (EDX) using an FEI Quanta 450 FEG Environmental Scanning Electron Microscope (ESEM), the X-ray diffraction using a D2 PHASER-BREKUR and FT-IR spectroscopy analyses were performed using a Transform Spectrometer (SHIMADZU FTIR-8400S).

Adsorption experiments:
The adsorption of Methyl Orange on the activated carbon was investigated as an effect of pH, initial dye concentration, contact time, and temperature.Adsorption experiments were reached after stirring 100mL of MO solution with different adsorbent doses and contact time at ambient temperature and agitation speed of 300 Rpm.The suspensions were collected then centrifuged and MO equilibrium concentrations were determined by spectrophotometry UV-VIS at 464 nm.The amount of MO adsorbed at equilibrium noted q e in mg•g −1 and R the yield of adsorption in (%), respectively, were calculated by the following equations (Eq.1 and Eq.2): Where: C 0 : the initial dye concentration (mg•L −1 ), C t : the equilibrium dye concentration (mg•L −1 ), V: the volume of the solution, m: the mass of the adsorbent (g).
Kinetic experiments were carried out by agitating 100 ml of MO solution at a constant dye concentration of 100ppm with 60 mg the activated carbon at fixed agitation speed, ambient temperature and pH = 2. Agitation was made from 5 to 120 min.Adsorption isotherm was obtained by varying the initial MO concentration between 10 to 100 mg/L to provide us several informations about the adsorption behavior using two famous models: Langmuir and Freundlich to fit the equilibrium experimental data of MO adsorption into our activated carbon.Thermodynamic behavior was also studied by varying the temperatures (20-60°C) using 60 mg of adsorbent mixed with 1000 mL of the MO dye solution at pH=2.

Characterization of the biosorbent:
The total surface area (or BET surface area S BET ) is one of the most widely known parameters associated with activated carbons [5].The Table 1 presents the textural properties of Ziziphus activated carbon.The S BET of this low-cost activated carbon is 553 m 2 .g−1 , this value is higher than 377 m 2 .g−1 reported by Wang et  The SEM analysis was also performed.The Fig. 1 shows the SEM images of the activated carbon at 2400 magnifications.This result confirms those obtained using BET analysis and indicated a large development of pores that are clearly found on the surface of the Ziziphus Lotus activated carbon, this could be thanks to the activation process using the phosphoric acid (H 3 PO 4 ) as chemical activating agent [12].Also, we notice the presence of heterogeneous pores which are essential for the adsorption of different types of pollutants  FT-IR analysis of the bio-carbon was performed to determine the functional groups, and then evaluate the involvement of these functional groups in the adsorption of MO dye onto Ziziphus Lotus leaves activated carbon.The Fig. 2 (a) display the spectra of the activated carbon, the band located between 3200 and 3600 cm -1 can be attributed to the hydroxyl groups -OH to lignin [16] and the hydrogen-bonded OH vibration of the cellulosic structure.The band observed around 1610 cm −1 could be attributed to the stretching of the aromatic rings (C=C).The band at 1223 cm −1 is partly associated with C-O stretching and O-H bending modes in the functional group, the band at around 1080 cm −1 can be attributed to the C-OH stretching of phenolic groups, also it can be seen a band around 640 cm -1 that justify the presence of the out-of-plane deformation mode of O-H of cellulose [17].The presence of lignin and cellulose structure suggest a lignocellulosic structure of active and carbonized Ziziphus Lotus, this structure is also observed for other carbon, such as Brazilian coconut shell and Tunisian olive-waste cakes [18].It is well acknowledged that the polyphenolic compounds like lignin groups present in lignocellulosic materials are the active sites for attachment of ions, the use of these materials in the natural state, without any treatment, can cause problems of lower durability, leaching of soluble organic components at strong acidic and basic media, and lower adsorption capacity.

Kinetic studies:
The first and second-order kinetic models were plotted (Fig. 3); Table 2 resumes the obtained results.The validity of each model was evaluated by the fitness of the straight line (R 2 ), from the results we observed that the adsorption of MO was better fitted with the pseudo-second-order with a R²=0.99,The Table .2shows the kinetic models parameters; the high equilibrium adsorption q e (769.23mg/g) at an equilibrium time of 10min indicates a high degree of affinity existing between MO and our activated carbon.

Table 2. Adsorption kinetic parameters of Methyl Orange on Ziziphus leaves adsorbent
These results indicate that the kinetic model of the adsorption is based on the assumption that the rate-limiting step is a chemical adsorption involving valance force through sharing or exchange of electrons between adsorbent and adsorbate.A similar result was also obtained by

Adsorption isotherm study:
Freundlich and Langmuir isotherm models explain the capacity of the adsorbent, the interaction between the solute-solution and the nature of adsorbed accumulation materials on the surface.The Fig. 4 shows the plot of Langmuir and Freundlich isotherm models.The results shown in Fig. 4

Thermodynamic studies:
The effect of temperature (20, 40, 50 and 60 °C) on the adsorption of MO by our activated carbon was studied.The values of ΔH 0 and ΔS 0 can be calculated from the slope and the intercept of the plot of ln K L versus 1/T, respectively, when the values of ΔG 0 were calculated using the thermodynamic equations (Table 3), that indicates a physisorption type of the MO sorption.The negative values of overall free energy changes during the adsorption process indicate the spontaneous nature of the adsorption process.The positive value of ΔS 0 (72.74J/mol.K) resulted from the increased randomness due to the adsorption of MO, similar thermodynamic behavior was obtained by Subbaiah and Kim, (2016) [23] using aminated pumpkin seed powder.

Conclusion
The present work shows that a new activated carbon can be prepared from Ziziphus leaves through phosphoric acid activation.Ziziphus leaves were selected as a suitable agriculture product thanks to its abundance in Morocco and its disposal in the environment without any treatment.Several parameters were studied to deduce their effect on Methyl Orange adsorption.Adsorption of Methyl orange onto our activated carbon increases with decreasing temperature, with decreasing pH and with increasing concentration, as expected from the law of mass action, in the range studied.The results show that the pH =2, adsorbent dose of 60mg and very low contact time of 10min are sufficient to eliminate 97% of MO.

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. Thus, it can be claimed that the surface of Ziziphus leaves activated carbon has a nanoporous structure.Similar observations were reported by Ricou-Hoeffer et al., (2001) [14] and Hameed and Daud (2008) [15] using fly ash and agricultural waste activated carbon, respectively.

Fig. 2 .
Fig.2.(a) FTIR and (b) XRD spectra of Ziziphus lotus activated carbon The XRD spectrum of activated carbon was also performed in the Fig.2 (b).It indicates the presence of broad diffraction background and the absence of a sharp peak reveal a predominantly of amorphous structure [19].The amorphous structure of the activated carbon was revealed by several products such as oil palm, empty fruit brunch and coconut shell [20].

Table 3 .
Thermodynamic parameters of adsorption of MO onto Ziziphus lotus adsorbent