Mylène Hiba Aouled Mhemed

Marin Gallego
email: mylene.marin-gallego@imt-atlantique.fr

Jean-François Largeau

Sana Kordoghli

Fethi Zagrouba

Mohand Tazerout

Mylène Marin Gallego

Mylène Marin Gallego

Gas adsorptive desulfurization of thiophene by spent coffee grounds-derived carbon optimized by response surface methodology: Isotherms and kinetics evaluation

Keywords: spent coffee grounds-derived carbon (SCG-AC), RSM, thiophene, desulfurization Highlights

come

Introduction

In the last two decades, environmental regulations to reduce sulfur in fuels have taken effect [START_REF] Saleh | Adsorptive desulfurization of thiophene, benzothiophene and dibenzothiophene over activated carbon manganese oxide nanocomposite: with column system evaluation[END_REF]. Due to the European Commission and the Environmental Protection Agency (EPA) reducing sulfur-containing organic compounds in fluidized catalytic cracking (FCC) gasoline is required [START_REF] Li | A theoretical investigation into the thiophene-cracking mechanism over pure Brønsted acidic zeolites[END_REF]. During the combustion of fuel, sulfur compounds can cause poisoning of the catalyst [START_REF] Saleh | Adsorptive desulfurization of thiophene, benzothiophene and dibenzothiophene over activated carbon manganese oxide nanocomposite: with column system evaluation[END_REF], corrosion of downstream piping [START_REF] Edinger | Adsorption of thiophene by activated carbon: A global sensitivity analysis[END_REF], and the release of SOx. The literature have studied sulfur compound elimination methods. Among the proposed methods, hydrodesulfurization (HDS) is the current desulfurization method in use at oil refineries [START_REF] Betiha | Oxidative desulfurization using graphene and its composites for fuel containing thiophene and its derivatives: An update review, Egypt[END_REF].

The HDS process has seen improvements in catalysis and process design [START_REF] Nunthaprechachan | Adsorptive desulfurization of dibenzothiophene by sewage sludge-derived activated carbon[END_REF]. The HDS reaction varies depending on the different compounds that have sulfur and this lowers its efficiency. HDS tends to be more effective when removing aliphatic sulfuric compounds such as thiols, thioethers, sulfides, and disulfides; and it is less effective for thiophene and its derivatives. This is explained by that their elimination requires much higher pressure and temperature due to the steric hindrance of sulfur atoms [START_REF] Liu | Ion exchange resin immobilised 12tungstophosphoric acid as an efficient and recoverable catalyst for the oxidative removal of organosulfur targetting at clean fuel[END_REF]. HDS is not a cost-effective operation because it requires severe operating conditions, i.e. high pressure and temperature, and a further requirement like the excessive use of hydrogen and catalysts [START_REF] Saleh | Adsorptive desulfurization of thiophene, benzothiophene and dibenzothiophene over activated carbon manganese oxide nanocomposite: with column system evaluation[END_REF]. From this, the HDS process is not cost-effective for the production of low to zero level sulfur [START_REF] Subhan | Enhanced desulfurization characteristics of Cu-KIT-6 for thiophene[END_REF].

Researchers have thought to develop alternative and more cost-effective HDS techniques.

Recently, processes that can be substituted for HDS have been developed. Examples of these alternatives to HDS are as follows: shifting of boiling point by alkylation [START_REF] Pawelec | Retracted article: Towards near zero-sulfur liquid fuels : a perspective review[END_REF], desulfurization via extraction by the transfer of sulfur from oil to solvents [START_REF] Abdullah | NOVEL METHODS OF DEEP DESULFURIZATION: A REVIEW[END_REF], desulfurization via oxidation where the fuel is mixed with an oxidant compound followed by a separation of the formed polar sulfones through extraction or adsorption [START_REF] Safa | Removal of sulfone compounds formed in oxidative desulfurization of middle distillate[END_REF], [START_REF] Pawelec | Retracted article: Towards near zero-sulfur liquid fuels : a perspective review[END_REF], sulphCo desulfurization where sulfur compounds are oxidized in a water-fuel containing H2O2 catalyst via ultrasound [START_REF] Duarte | Sulfur removal from hydrotreated petroleum fractions using ultrasound-assisted oxidative desulfurization process[END_REF], and the oxidative desulfurization process employing photocatalysis where sulfur compounds can be photo-oxidized to CO2, SO4 2-, sulfoxides, and sulfones [START_REF] Shawky | One-pot synthesis of Mn3O4-coupled Ag2WO4 nanocomposite photocatalyst for enhanced photooxidative desulfurization of thiophene under visible light irradiation[END_REF]. Among the proposed alternatives to reduce sulfur levels, adsorptive desulfurization is the most attractive and cost-effective method for removing organosulfur compounds because of low-energy consumption, operational at ambient temperatures and pressures, the regeneration of consumed adsorbent, and the availability of adsorbents [START_REF] Nunthaprechachan | Adsorptive desulfurization of dibenzothiophene by sewage sludge-derived activated carbon[END_REF]. Many adsorbents are commonly used for adsorptive desulfurization, including zeolites [START_REF] Takahashi | New Sorbents for Desulfurization by π-Complexation: Thiophene/Benzene Adsorption[END_REF] silica gel [START_REF] Xun | Synthesis of metal-based ionic liquid supported catalyst and its application in catalytic oxidative desulfurization of fuels[END_REF], and porous organometallic materials (MOFs) [START_REF] Shi | MOF-derived porous carbon for adsorptive desulfurization[END_REF]. Metal oxides (MnO, ZnO, and CaO) are also being researched [START_REF] Saleh | Adsorptive desulfurization of thiophene, benzothiophene and dibenzothiophene over activated carbon manganese oxide nanocomposite: with column system evaluation[END_REF]. Activated carbons (ACs) have been studied as sulfur compound removal for different fuels [START_REF] Saleh | Adsorptive desulfurization of thiophene, benzothiophene and dibenzothiophene over activated carbon manganese oxide nanocomposite: with column system evaluation[END_REF], [START_REF] Edinger | Adsorption of thiophene by activated carbon: A global sensitivity analysis[END_REF], [START_REF] Nunthaprechachan | Adsorptive desulfurization of dibenzothiophene by sewage sludge-derived activated carbon[END_REF], [START_REF] Yu | Adsorption removal of thiophene and dibenzothiophene from oils with activated carbon as adsorbent: effect of surface chemistry[END_REF], because they have a large surface area, well-developed porous structure, various surface functional groups, high numbers of surface functional groups, a high degree of surface reactivity, and good mechanical strength [START_REF] Mahmood | Potential of used Camellia sinensis leaves as precursor for activated carbon preparation by chemical activation with H3PO4; optimization using response surface methodology[END_REF]. From this, precursor type, activating agent, and preparation conditions significantly impact the texture and the chemical characteristics of AC [START_REF] Nunthaprechachan | Adsorptive desulfurization of dibenzothiophene by sewage sludge-derived activated carbon[END_REF]. Traditionally, AC production optimization was conducted through the monitoring of one factor to see its influence from the experimental response. Major issues using this technique include a lack of interactive effects among the studied variables, the increase in the number of experiments, time and consuming reagents and materials. To overcome this issue, optimization was conducted using multivariate statistical techniques [START_REF] Bezerra | Response surface methodology (RSM) as a tool for optimization in analytical chemistry[END_REF]. For the optimization of the preparation process, Response Surface Methodology (RSM) has been shown as an effective statistical tool [START_REF] Das | Box-Behnken statistical design to optimize preparation of activated carbon from Limonia acidissima shell with desirability approach[END_REF]. It combines statistical and mathematical techniques to develop, improve, and optimize processes [START_REF] Khuri | Response surface methodology[END_REF]. This method builds a polynomial equation based on experimental data to define the data set. The objective of this method is to assist with statistical previsions. It is applicable when a response or a set of responses shows influence from several variables. The literature has indicated that RSM is well applied to optimize the production of AC from numerous precursors [21][START_REF] Danish | Optimization of banana trunk-activated carbon production for methylene bluecontaminated water treatment[END_REF][START_REF] Salehi | Optimization of chemical activation of cotton fabrics for activated carbon fabrics production using response surface methodology[END_REF][START_REF] Senthilkumar | Optimization of Activated Carbon Preparation from Pomegranate Peel ( Punica granatum Peel) Using RSM[END_REF].

The selection of a proper adsorbent to eliminate sulfur compounds needs to consider the following parameters like cost-effectiveness, availability, and adsorptive properties. Many adsorbents were used in various adsorptive desulfurization studies. Many of them have successfully retained sulfur compounds such as H2S but failed to eliminate heterocyclic sulfur compounds [START_REF] Edinger | Adsorption of thiophene by activated carbon: A global sensitivity analysis[END_REF]. In this study, a cost-effective and sustainable material like spent coffee grounds (SCG) makes a good precursor for the preparation of AC. The produced adsorbent was then applied to adsorptive desulfurization. SCG residue is a good choice as an adsorbent for the following reasons: its abundant production by the coffee industry; the production rate goes as high as 748,000 tons per year [START_REF] Yuliusman | Production of Activated Carbon from Coffee Grounds Using Chemical and Physical Activation Method[END_REF]; and AC prepared from SCG has been proved to be a good candidate for H2S removal. One study revealed that the functional groups of nitrogen present on the activated carbon derived spent coffee grounds surface have an important catalytic role in the oxidation of hydrogen sulfide [START_REF] Kante | Spent coffee-based activated carbon: Specific surface features and their importance for H2S separation process[END_REF]. Nevertheless, no study has been conducted to exploit its ability to eliminate heterocyclic sulfur compounds. As thiophene compound exist in a great fraction of the entire sulfur content in FCC gasoline [START_REF] Li | A theoretical investigation into the thiophene-cracking mechanism over pure Brønsted acidic zeolites[END_REF], the proposed challenge was to prepare AC from SCG-AC and test its performance towards thiophene removal as a heterocyclic sulfur compounds model present in FCC gasoline.

Activated carbons are commonly used for emission control at low concentrations. While the adsorption of thiophene on AC is well known and researched for the deep desulfurization of fuels in liquid phase [START_REF] Saleh | Adsorptive desulfurization of thiophene, benzothiophene and dibenzothiophene over activated carbon manganese oxide nanocomposite: with column system evaluation[END_REF][START_REF] Yu | Adsorption removal of thiophene and dibenzothiophene from oils with activated carbon as adsorbent: effect of surface chemistry[END_REF][START_REF] Baeza | Adsorption of thiophene and dibenzothiophene on highly dispersed Cu/ZrO2 adsorbents[END_REF], the literature is more scarce concerning the gas phase adsorption [START_REF] Edinger | Adsorption of thiophene by activated carbon: A global sensitivity analysis[END_REF]. This research examined the systematic optimization of the activated carbon production from spent coffee grounds (SCG-AC), with phosphoric acid (H3PO4) activation and exploited its ability for thiophene removal from the gas phase at ambient conditions. To optimize the AC production process, the Box-Behnken design in the response surface methodology (RSM) was investigated. The following factors were chosen: acid concentration, impregnation ratio, and activation temperature for use in this study. Tow responses including iodine number (IN) and methylene blue number (MB) were explored. IN and MB were selected for the measurement of micropore content and qualitative mesopore structure, respectively [START_REF] Nunthaprechachan | Adsorptive desulfurization of dibenzothiophene by sewage sludge-derived activated carbon[END_REF]. Since there is insufficient information on the kinetic and isotherm studies for gas-solid adsorption systems, kinetic and isotherm analysis of thiophene adsorption was discussed. Fig. 1 shows the experimental approach used in this study.

Materials and Methods

Materials

Spent coffee grounds used in this study was locally collected from the university cafeteria of IMT Atlantique, Nantes (France). Phosphoric acid 85% (purity 98%), thiophene (purity 98%), iodine (purity 99%), and methylene blue (technical quality) were purchased from Sigma-Aldrich Co. Hydrochloric acid (purity 99.5%), Sodium Chloride (purity 99.5%), were purchased from Acros Chemicals. Sodium hydroxide ( analytical quality) was obtained from Fischer Scientific.

Preparation of the precursor and the activated carbon

Samples of spent coffee grounds were first dried in an oven at 50 °C for 48 hours. A quantity of 100 g of dried spent coffee grounds (SCG) was mixed with a solution of phosphoric acid H3PO4 at different concentrations (85 wt%, 50 wt%, and 67.5 wt%). The impregnation ratio, defined as the weight of impregnant (H3PO4)/weight of precursor (spent coffee grounds), was 0.5, 1 and 1.5. The mixture was left at room temperature for 24 h and then dried in an oven at 105 °C for 2 h. Then the mixture was introduced into a horizontal tubular furnace and heated from room temperature to desired temperature (400-500 °C) at a heating rate of 5 °C.min -1 for 120 min. The obtained products were cooled to room temperature inside the reactor in the existence of a flow of nitrogen and then washed numerous times with distilled water until neutralization of the pH of the rinse water. SCG-AC was finally dried at 105 °C for 24 h. The steps of the activated carbon derived-spent coffee grounds production were presented in Fig. 1.

Characterization of the SCG and SCG-AC

Several analyses were performed to characterize the materials.

The proximate analysis of the precursor material (SCG) and SCG-AC (activated carbon obtained on optimal conditions) were determined by a FLASH EA 1112 Series CHNS-O analyzer (Thermo-finnigne instrument, Netherlands). The mineral matter content (EDX) was measured using X-ray Fluorescence model EDX-800HS2 (Shimadzu, USA). Thermogravimetric analysis (TGA) were carried out using a STSYS Evolution 1750 (SETARAM, France). For this analysis, 6 mg of powder samples were thermally treated under an inert atmosphere of N2 (20 ml/min) at 5 °C/min. The Sample was placed in an alumina crucible and heated from room temperature to 800 °C to evaluate the thermal decomposition of the lignocellulosic material and measure the mass loss.

In order to have a general look at the effect of the presence of phosphoric acid on the spent coffee grounds pyrolysis process and compare it with the thermogravimetric results, two experiments were performed. The first test consists on pyrolyzing the spent coffee grounds (SCG) in the same conditions as the TGA in a rotary quartz reactor. The second experiment was to add phosphoric acid to the SCG whilst preserving the same operating conditions as before. In order to manipulate the gases produced during the pyrolysis process in these two experiments, a Micro GC R-3000 (SRA INSTRUMENTS, France) was established at the outlet of the horizontal rotary quartz reactor. The gas samples were taken automatically every three minutes.

The micropore content of the SCG-AC was determined in terms of the iodine number (IN).

The iodine number was deduced due to the ASTM D4607-14 method [START_REF] Tran | Activated carbons from golden shower upon different chemical activation methods: synthesis and characterizations[END_REF]. The qualitative mesopore structure of SCG-AC was evaluated by methylene blue (MB) adsorption at room temperature. The MB concentration was calculated using a JENWAY6300 UV-Vis spectrophotometer at a maximum wavelength of 665 nm [START_REF] Tran | Activated carbons from golden shower upon different chemical activation methods: synthesis and characterizations[END_REF].

The textural properties of optimal SCG-AC were determined by N2 adsorption at 77 K using Micromeritics Smart VacPrep. The surface area (SBET) of the optimal SCG-AC was measured with a surface area analyzer. The total pore volume was deduced from the quantity of nitrogen adsorbed at P/P° = 0.95, by applying the Barret-Joiner-Halenda method (BJH) [START_REF] Mahmood | Potential of used Camellia sinensis leaves as precursor for activated carbon preparation by chemical activation with H3PO4; optimization using response surface methodology[END_REF].

The point of zero charge pHpzc characterizes the charge at the surface of the SCG-AC. If pH > pHpzc the adsorbent has a negative surface charge. If pH< pHpzc the adsorbent has a positive surface charge. It is defined as the pH of the aqueous solution in which the solid exists under a neutral electrical potential. Various pH solutions ranging from 2 to 10 were prepared by mixing 0.1 g of activated carbon with 20 ml of 0.1M NaCl solution. The pH value was adjusted by adding an aqueous solution of NaOH or HCl [START_REF] Bouhamed | Adsorptive removal of copper(II) from aqueous solutions on activated carbon prepared from Tunisian date stones: Equilibrium, kinetics and thermodynamics[END_REF]. The acidic and basic functional groups on the surface of the optimal SCG-AC were checked by Boehm's titration method [START_REF] Boehm | Surface oxides on carbon and their analysis: a critical assessment[END_REF].

All analyzes were performed in three replicates.

Design of experiments

The SCG-derived carbon production was studied with the Response Surface Methodology using the Box Behnken Design (BBD). BBD requires three levels for each factor [START_REF] Khuri | Response surface methodology[END_REF]. The variables studied were acid concentration (A), impregnation ratio (B) and activation temperature (C). These three variables with their corresponding ranges were selected based on the literature and preliminary studies [START_REF] Bouhamed | Adsorptive removal of copper(II) from aqueous solutions on activated carbon prepared from Tunisian date stones: Equilibrium, kinetics and thermodynamics[END_REF].

The goal of the experimental design is to obtain a relation y= f (xi) between the response of the system and the factors xi. The interest of modeling the response by a polynomial equation is to predict all the answers of the field of study, with a limited number of experiments. The different levels for each process variable (low, middle and high levels) were designated as -1, 0 and +1, which led to 17 experimental runs. Replication at the center points was applied to estimate experimental errors. The experimental design chosen involved three-level and threefactors. The work domain coding for these three factors is presented in Table 1.

In our case, the mathematical model is a polynomial model of second order, which takes into account the main effects βi and the interactions βii and βij. The equation y = f (xi) will be written as [START_REF] Mahmood | Potential of used Camellia sinensis leaves as precursor for activated carbon preparation by chemical activation with H3PO4; optimization using response surface methodology[END_REF]:

1 2 0 1 1 1 1 n n n n i i ii i ij i j i i i j i y x x x x β β β β - = = = = + = + + + ∑ ∑ ∑ ∑ (1)
Where β0 is the coefficient of intercept. βi, βii and βij are the linear, quadratic and interaction coefficients respectively; xi are the coded variables.

Expert design 11.0.1.0 (Stat-Ease, Minneapolis, USA) was used for the statistical analysis of the experimental data. The quality of the fit of the polynomial model was expressed by the correlation coefficient (R 2 ). The significance and adequacy of the used model was further explained using F-value (Fisher variation ratio), probability value (Prob > F), and adequate precision (AP) [START_REF] Machrouhi | Statistical optimization of activated carbon from Thapsia transtagana stems and dyes removal efficiency using central composite design[END_REF].

5.Adsorption of sulfur compound

The optimum SCG-derived carbon (from RSM) was dried in an oven at 105 °C for 2 hours before use. A common isotherm method was applied. Fixed known weighed amounts of the SCG-AC were suspended in identical batch reactors of 2 liters (see Fig. 2). Different weighed amounts of liquid thiophene were added and volatilized. The thiophene's initial concentration was kept lower than the saturating vapor pressure to avoid condensation in the reactors. The volumes injected vary between 20 and 300 μl at 25 °C ± 0.2 °C for 500 min. Once equilibrium was reached, the remaining pollutant concentration was measured by gas chromatograph (Chromatography Perkin Elmer Clarus 500) equipped with a flame photometric detector. To study the adsorption kinetics on the SCG-AC, 100 ppm of thiophene was injected in the batch reactor and was controlled until equilibrium is reached.

The amount of adsorbed thiophene is calculated using the following equation:

(2)

Where qe: amount of solute adsorbed at equilibrium per unit weight of adsorbent (mmol/g); Ce: thiophene concentration at equilibrium (mmol/l); m: mass of adsorbent (g); C0: initial concentration of thiophene (mmol/l); V: volume of the reactor (l).

Fig. 2. Adsorption experiments

The removal efficiency of the SCG-AC was evaluated using the following mathematical equation:

% Adsorption C C C 100 (3)
where Ci (ppm): initial adsorbate (thiophene) concentration and Ce (ppm): equilibrium adsorbate concentration.

Adsorption isotherms

Different models have been used in the literature to describe the adsorption isotherms. The Langmuir, Freundlich, and Langmuir-Freundlich models are the most frequently used models for the adsorption process. In this work, we used these isotherm models to fit the experimental data for the adsorption of thiophene from gas phase onto the SCG-AC at ambient conditions.

The Langmuir isotherm is [START_REF] Abdul Manap | Adsorption isotherm and kinetic study of gas-solid system of formaldehyde on oil palm mesocarp bio-char: Pyrolysis effect[END_REF]:

(4)
Where qm is the saturation value of qe and KL is the Langmuir constant. In this model, it is assumed that the number of adsorption sites on the surface of the AC is fixed and that each site can adsorb only one particle. This model neglects the interaction between adsorbed particles [34].

The Freundlich isotherm is:

(5)

Where KF and 1/n are the Freundlich constants. Freundlich model can be applied in many cases, especially in the case of multilayer adsorption with possible interactions between the adsorbed molecules at low concentrations. Although its accuracy, this model has the disadvantage of not providing an upper limit to the solid phase concentrations [START_REF] Nunthaprechachan | Adsorptive desulfurization of dibenzothiophene by sewage sludge-derived activated carbon[END_REF].

The Langmuir-Freundlich isotherm is [START_REF] Fallah | Effect of aromatics on the adsorption of thiophenic sulfur compounds from model diesel fuel by activated carbon cloth[END_REF]:

(6)
Where KLF and n are constants. The value of n indicates the heterogeneity of the site energies.

The Langmuir-Freundlich is able to model the distribution adsorption isotherm of an adsorbate at high and low concentrations [START_REF] Jeppu | A modified Langmuir-Freundlich isotherm model for simulating pH-dependent adsorption effects[END_REF].

Kinetic models

The adsorption kinetic of thiophene was investigated using three model: the pseudo-first order, pseudo-second order, and the Elovich model.

The pseudo-first-order kinetic model is mainly used to describe the initial stages of the adsorption process [START_REF] Saleh | Adsorptive desulfurization of dibenzothiophene from fuels by rubber tyres-derived carbons: Kinetics and isotherms evaluation[END_REF]. It is defined according to the Lagergren's (1898) relation by: ln ln (7) Where, qe (mmol/g) and qt (mmol/g) are the amounts of adsorbed thiophene at equilibrium and time t (min), respectively; k1 (min -1 ) is the rate constant of the Lagergren-first-order kinetics model.

The pseudo-second-order model is based on the assumption that the limiting stage of the reaction rate can be the chemisorption involving valence forces by electron exchange between the adsorbent and the adsorbate [START_REF] Saleh | Adsorptive desulfurization of dibenzothiophene from fuels by rubber tyres-derived carbons: Kinetics and isotherms evaluation[END_REF]. The equation of the psudo-second-order model is defined by [START_REF] Ho | Pseudo-second order model for sorption processes[END_REF]:

1 + (8)
Where qt is the amount of thiophene (mmol/g) at a time (min) and k2 (g/(mmol min)) is the adsorption rate constant.

Elovich model is introduced to further study the adsorption process. Elovich model is one of the most often used models to describe chemisorption kinetics. It is also successfully applied to describe the initial adsorption step and multilayer adsorption on a high heterogeneous surface area [START_REF] Abdul Manap | Adsorption isotherm and kinetic study of gas-solid system of formaldehyde on oil palm mesocarp bio-char: Pyrolysis effect[END_REF].

1 ! ln " 1 ! ln ( 9
)
where α is is equilibrium rate constant, mmol g -1 min -1 , and β is a constant referred to surface coverage and activation energy of chemisorption, g.mmol -1 . In the case when the correlation between the adsorption capacity and the constant β is negative it is assumed that the adsorbent does not retain the adsorbate.

Adsorption mecanism

The kinetic experimental results were also applied to the Weber intraparticle diffusion mechanism to include the stages touching the adsorption kinetics. The equation is defined by Weber and J.C [START_REF] Weber | Kinetics of Adsorption on Carbon from Solution[END_REF]:

#$ %.' + ( (10
) Where C is the intercept (mmol/g) which is related to the boundary layer thickness and kid is the slope which represents the intraparticle diffusion rate constant (mmol/g min 1/2 ).

Results and discussion

Characterization of the precursor (SCG) and the influence of the active agent (H3PO4)

Table 2 shows both proximate and ultimate analysis of raw material. SCG contained a good percentage of carbon (50.40 wt%) and oxygen (38.50 wt%). This could be due to the higher lignin contents in the spent coffee grounds. However, ultimate analysis showed a high levels of nitrogen (~2.40 wt%) in the studied precursor, certainly due to its high protein content [START_REF] Fermoso | Thermochemical decomposition of coffee ground residues by TG-MS: A kinetic study[END_REF].

The elemental analysis of the SCG showed a low ash content of 1.8 % with a volatile content of 77.90 wt% hence a lower fixed carbon content (20.3 wt%). The basic elemental constituents of biomass minerals are Si, Ca, K, and Mg with smaller amounts of S, P, Fe, and Cu. The existence of inorganic minerals, especially the alkali (K, Na, etc.) and alkaline-earth metals (Mg, Ca, etc.) influence the biomass pyrolysis mechanism. For example, K in the biomass mineral matter catalytically favors char formation [START_REF] Eom | Effect of essential inorganic metals on primary thermal degradation of lignocellulosic biomass[END_REF]. In our case, spent coffee grounds contains a high percentage in K; therefore, it is suitable for char production.

Contrariwise biomass with high percentages of alkali metals and Cl are less desirable as fuel because it causes fouling, deposition, corrosion, plugging, and agglomeration [START_REF] Vassilev | Ash contents and ashforming elements of biomass and their significance for solid biofuel combustion[END_REF]. According to the EDX analysis the amount of alkali metals and Cl in spent coffee grounds is low (less than 10 wt%). The thermogravimetric analysis (TGA) of the SCG was conducted in absence of H3PO4 and is presented in the Fig. 3a. Additionally, pyrolysis experiments involving SCG were done on a large scale in a rotary reactor with both the presence and the absence of the activating agent H3PO4 (Fig. 3b and Fig. 3c). During the pyrolysis experiments in the rotary reactor the CO and CO2 content of the pyrolysis gases were followed by micro-GC analysis. The results identified the optimal pyrolysis temperature and defined the decomposition scheme of the functional groups on the surface of the SCG-AC. Weight loss occurred when the temperature was between 25-150 °C and represents water evaporation. Mass loss occurred between 150-450 °C and is characterized by the decomposition of hemicellulose and cellulose; and the desorption of CO2 and some CO (Fig. 3(a,b)). This phenomenon is described as the decomposition of the following: carboxyl, lactone, and lactol groups [START_REF] Hotová | Oxygen surface groups analysis of carbonaceous samples pyrolysed at low temperature[END_REF]. When the temperature is above 400 °C, then lignin decomposition and CO desorption occur (Fig. 3 b).

Desorption is caused by the decomposition of the following: carbonyl, ether, quinine, and phenol groups on carbon surface. One study revealed that the rise of the release of the CO, in the decomposition process, means the relative richness of carbonyl and phenol groups on the surface of the synthesized AC [START_REF] Saleh | Adsorptive desulfurization of dibenzothiophene from fuels by rubber tyres-derived carbons: Kinetics and isotherms evaluation[END_REF]. The CO is primarily decomposed on temperatures above 600 °C. Phosphoric acid (H3PO4) when used as an activating agent (Fig. 3c) will lower desorption of CO2 on the AC surface, which led to the growth in the number of carboxylic and lactonic groups. Further H3PO4 mitigates and forces the percentage by volume of CO released from 12 to 6% which is considered an important reduction of the carbonyl and phenol groups on the surface of AC. So our activated carbon prepared in temperature ranging from 400 to 500 °C must be richer in carboxylic and lactonic functions than in carbonyl and phenol functions. The hypothesis of this research has been proven from the results of Boehm titration.

3.2.Experimental results of the Design of experiments (DOE)

The experimental results obtained at the designed experimental conditions according to the Box Behnken design are presented in Table 3. From this table, we noticed that the SCG-AC sample activated at 400 °C in an acid concentration of 67.5 wt% with an impregnation ratio of 1.5 g/g gives the optimum iodine adsorption value (1133 mg/g). The greater methylene blue number of 93 mg/g is obtained by the activated carbon prepared at 500 °C in an acid concentration of 85 wt% with an impregnation ratio of 1 g/g. According to this table the relative error between the experimental and the predicted values is less than 5 %.

Statistical analysis

The analysis of variance (ANOVA) established the model of this research as well as to identify the relationship among the variables and the responses [START_REF] Senthilkumar | Optimization of Activated Carbon Preparation from Pomegranate Peel ( Punica granatum Peel) Using RSM[END_REF]. Table 4 research [START_REF] Mahmood | Potential of used Camellia sinensis leaves as precursor for activated carbon preparation by chemical activation with H3PO4; optimization using response surface methodology[END_REF]. Typically, if a value "Prob > F" is less than 0.05, then the terms of models are significant. For this research, all F-values are equal to 242.28, and 73.77 for IN and MB responses, respectively; and the Prob > F less than 0.0001 shows that the models used in this research are significant.

Iodine number (IN)

According to the ANOVA analysis for the iodine number, the substantial model terms for iodine adsorption are the acid concentration (A), impregnation ratio (B), interaction between acid concentration and impregnation ratio (AB), interaction between impregnation ratio and temperature (BC) and quadratic term of the primary factors of interest in the experiment A

The acid concentration, impregnation ratio, and interaction between activation temperature and impregnation ratio exhibited a positive impact on the iodine adsorption response.

However, the activation temperature is the only quadratic term that presented a positive incidence on the iodine number. According to Table 4, we noticed that the impregnation ratio has the most obvious effect on the iodine number based on the highest F-value of 398.69.

Hence, it could be perceived that the number of micropores were developed with the impregnation ratio of 1.5 g/g in the studied domain. In fact, at the high level of the significant model terms, the activation reaction may take place rapidly producing a development of porosity of the obtained activated carbons and an increase in the microporosity [START_REF] Machrouhi | Statistical optimization of activated carbon from Thapsia transtagana stems and dyes removal efficiency using central composite design[END_REF].

Methylene blue number (MB)

The greatest important effects on the methylene blue adsorption are the three primary factors, interaction between acid concentration and activation temperature (AB), interaction between impregnation ratio and activation temperature (BC) and the quadratic term of the primary factors of interest in the research A 2 , B 2 , and C 2 (see eq. 12).

:; ℎ=>;?; @>A; ?AB@;C .

The positive factor coefficients were indicators of a synergistic effects and negative factor coefficients showed an antagonistic effect on these responses. The acid concentration, activation temperature, interaction between acid concentration and activation temperature, interaction between impregnation ratio and activation temperature presented a positive effect on the MB response. Although, the quadratic term of the acid concentration showed a negative effect on the expansion of mesopores. According to Table 4, among the primary factors of interest the activation temperature has the most important effect on the methylene blue number due to the higher F-value (99.33). Hence, the number of mesopores are developed with the activation temperature of 500 °C in the studied field.

Diagnostic model

An equation's fitness is apparent from correlation coefficient, adequate precision and standard deviation. The following values all indicate the responses are well correlated by the quadratic model: Equation [START_REF] Duarte | Sulfur removal from hydrotreated petroleum fractions using ultrasound-assisted oxidative desulfurization process[END_REF] showed correlation coefficient of (R 2 = 0.9877); and equation [START_REF] Shawky | One-pot synthesis of Mn3O4-coupled Ag2WO4 nanocomposite photocatalyst for enhanced photooxidative desulfurization of thiophene under visible light irradiation[END_REF] showed correlation coefficient of (R 2 = 0.9917). Adequate precision is a signal-to-noise ratio. The Design-Expert software provides a diagnostic of the Box-Cox plot (see Fig. 6). This plot provides a guideline for selecting the correct power law transformation by the choice of a suitable λ value. The best value for λ is that corresponding to the minimum value for ln residual SS. This value is bounded by upper and lower 95 % confidence interval. If the 95 % confidence interval around this λ includes 1 then no specific transformation is recommended.

Interaction effects on the responses

The contour plots were illustrated using Design-Expert® software, by varying two parameters and keeping the other variable constant at the "zero" level. Fig. 7a shows the contour plot of the combined effect of acid concentration (A) and impregnation ratio (B) on the iodine number response at constant temperature (450 °C). The impregnation ratio defined as the amount of phosphorous incorporated into the precursor is a parameter that controls the development of the microporosity as well as the mesoporosity in the produced adsorbent [START_REF] Molina-Sabio | Role of chemical activation in the development of carbon porosity[END_REF].

The outline illustrates that both variables have a significant effect on the iodine number. A maximum iodine number of about 1120 mg.g -1 was attained with an impregnation ratio of 1.50 g/g and an acid concentration of 68.7 wt%. A further increase in the acid concentration and the level of impregnation ratio leads to a considerable reduction in the adsorption of iodine molecules. This can be explained by the production of heterogeneity on the micropore size and later a mesoporosity of the SCG-AC.

Fig. 7b illustrates the shared effect of the impregnation ratio (B) and the activation temperature (C) on the iodine number. The acid concentration is set at zero levels (A = 67.5 wt%). The adsorption of iodine increases from 1100 to 1140 mg/g with the increment of the impregnation ratio from 0.5 to 1.5 g/g and temperature from 400 to 500 °C. A possible explication for this phenomenon is that an increase of temperature can raise the degree of the reaction H3PO4-precursor, thus resulting in increasing devolatilization, the development of the pore structure, and the formation of a large number of active sites. Md-Desa et al. [START_REF] Md-Desa | OPTIMIZATION OF ACTIVATED CARBON PREPARATION FROM SPENT MUSHROOM FARMING WASTE (SMFW) VIA BOX[END_REF] reported similar observations using AC from spent mushroom farming.

Fig. 7c illustrates the significant common interaction between acid concentration (A) and activation temperature (C) on the methylene blue number at constant impregnation ratio (B = 1 g/g). The adsorption of methylene blue is improved when acid concentration increased to reach a maximum adsorption value of 80 mg. g -1 . From the contour curve, it was noted that when the temperature increased from 400 to 500 °C, the MB is enhanced by 33.33 %. Thus, the aromatic condensation reaction between adjacent molecules supports the development of the porous structure. Hence due to a higher temperature, the number of micropores decreases compared to the mesopores [START_REF] Das | Box-Behnken statistical design to optimize preparation of activated carbon from Limonia acidissima shell with desirability approach[END_REF].

Process optimization

This study purposes to control the optimal conditions to obtain an SCG-AC with a high content of both mesopores and micropores. In order to find a compromise between the adsorption of iodine and the methylene blue, the desirability function was applied using the Expert Design software. Box-Behnken design had been applied with the desirability approach in order to optimize the preparation of activated carbon from SCG where the responses such as iodine and methylene blue adsorption value were desired to be maximum [START_REF] Das | Box-Behnken statistical design to optimize preparation of activated carbon from Limonia acidissima shell with desirability approach[END_REF]. In this optimization analysis, the operational variables were selected to remain within the value range. The most suitable experimental conditions were selected for verification. Thus, the optimal conditions of the SCG-AC preparation were: an activation temperature of 500 °C, an acid concentration of 68.7 wt% and an impregnation ratio of 1.50 g/g with predictive values

for the number of iodine and methylene blue of 1148 mg.g -1 and 86 mg.g -1 respectively. The experimental values obtained for the adsorption of iodine and methylene blue are in good agreement with the predicted one (see Table 5). The relative low error percentage (< 5 %) between the experimental and predicted values indicates the accuracy of the optimization process. According to the literature, the adsorption capacity of an adsorbent is related to its adsorption capacity for smaller nonpolar molecules (iodine), and large molecules (methylene blue) [START_REF] Jiang | Activated carbons chemically modified by concentrated H2SO4 for the adsorption of the pollutants from wastewater and the dibenzothiophene from fuel oils[END_REF]. Based on the optimization results of the experimental design, the SCG-AC is both a microporous and mesoporous material. Therefore, it is suitable for the adsorption of small and large molecules.

Characterization of the SCG-AC prepared under the optimal conditions

The results of elemental analyzers for the optimal SCG-AC are presented in Table 6.

According to the elemental analysis, activated carbon derived from SCG contains 2.06 wt% of nitrogen. Nitrogen is expected to be incorporated into the carbon matrix as a result of the presence of caffeine in the organic precursor [START_REF] Kante | Spent coffee-based activated carbon: Specific surface features and their importance for H2S separation process[END_REF]. A significant amount of oxygen is also detected due to the oxygen surface functional groups. The number of oxygen surface functional groups is an imperative characteristic of the AC. The presence of oxygen functional groups is essential to ensure the development of a high microporosity structure on the AC [49]. Phosphorus species are not removed from the carbon surface and their content reaches 6.84 wt%. According to the EDX data, the significant proportions of P, Ca and K can affect the acidic and basic properties of the carbon surface and thus influence the adsorption behavior [START_REF] Sorokhaibam | Development of Newer Adsorbents: Activated Carbons Derived from Carbonized Cassia fistula[END_REF]. In this respect, it was revealed that the existence of phosphorus in the carbon matrix (due to the chemical activation by phosphoric acid) increases its capacity and selectivity for the elimination of thiophene compounds. Indeed, these phosphorus species have a strong acidity that enhances the attraction of thiophene compounds whose character is slightly basic [START_REF] Seredych | Role of phosphorus in carbon matrix in desulfurization of diesel fuel using adsorption process[END_REF]. Thus the occurrence of these species on the surface of the SCG-AC resulting the use of phosphoric acid as an activating agent can improve the adsorption of thiophene [START_REF] Kante | Spent coffee-based activated carbon: Specific surface features and their importance for H2S separation process[END_REF][START_REF] Seredych | Role of phosphorus in carbon matrix in desulfurization of diesel fuel using adsorption process[END_REF].

The interaction between the hydrogen atoms of the carboxylic groups and the sulfur atom has been proved to be favorable for the adsorption of sulfur compounds, thanks to the increase in the adsorption energies involved for these molecules [START_REF] Yu | Adsorption removal of thiophene and dibenzothiophene from oils with activated carbon as adsorbent: effect of surface chemistry[END_REF]. Hence the carboxylic functional groups contribute favorably to the adsorption of sulfur molecules. Table 6 summarizes the amount of acidic and basic groups obtained from Boehm's titration method. The surface of the SCG-derived carbon is rich on acidic groups and more precisely on carboxylic groups (0.32 mmol.g -1 ). The acidic groups may be derived from the reaction between H3PO4 and precursor material during the activation [START_REF] Mahmood | Potential of used Camellia sinensis leaves as precursor for activated carbon preparation by chemical activation with H3PO4; optimization using response surface methodology[END_REF]. Whence the acidic nature of the SCG-AC. The presence of oxygen-containing functional groups on the surface of the AC enhances the exchanges between the SCG-AC and thiophene and then the enhancement of its elimination.

The point of zero charges is the pH at which the surface of an adsorbent is neutral. Thus, the AC contains as much positively charged as negatively charged surface functions. The pHpzc of the optimized SCG-AC assert the dominance of the acidic groups on the AC surface. The specific surface area (SBET) of the SCG-derived carbon was determined from N2 adsorption isotherms (see Fig. 8). The plot of the optimal SCG-AC exhibits a typical hybrid type I-IV isotherm. The observed hysteresis is due to the capillary condensation taking place in mesopores, and the limiting uptake over a range of high P/P0 [START_REF] Sing | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Provisional)[END_REF]. The hysteresis can be classified as type H2 according to the IUPAC classification. It was found that adsorbent that gives rise to H2 hysteresis is often disordered and the distribution of pore size and shape is not well defined [START_REF] Sing | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Provisional)[END_REF]. The SCG-AC hysteresis loop is more pronounced than those founded by Ramasahayam et al. [START_REF] Ramasahayam | Spent coffee grounds derived P, N co-doped C as electrocatalyst for supercapacitor applications[END_REF] for the same precursor which is a result of pore propagation and mesopores widening in the presence of the activating agent H3PO4. An interesting observation was also detected: the non-closure point of the desorption branch at low relative pressures.

This can be due to irreversible chemical interaction between N2 and adsorbent [START_REF] Ramasahayam | Spent coffee grounds derived P, N co-doped C as electrocatalyst for supercapacitor applications[END_REF]. The results of the structural characterization of the SCG-AC sample by scanning electron microscopy and X-Ray diffraction will be reported in future work. The SBET and the total pore volume are 410 m 2 /g and 0.25 cm 3 /g respectively. Similar results for the specific surface area (405.68 m 2 /g) of the spent coffee grounds derived carbon prepared by chemical activation with the ZnCl2 were found by Yuliusman and al. [START_REF] Yuliusman | Production of Activated Carbon from Coffee Grounds Using Chemical and Physical Activation Method[END_REF].

AC

To assess the influence of adsorbate dose on the thiophene removal, different initial concentrations of thiophene were used (Fig. 9a). We noticed that the increase of adsorbate initial concentration led to the decrease in percent removal. The SCG-AC has achieved more than 96% and 91% thiophene removal at concentrations of 10 ppm and 25 ppm, respectively in 100 min. At extremely higher initial concentrations the thiophene removal efficiency of the SCG-AC declined to 33 % removal at 200 ppm initial thiophene concentration. The influence of residence time on the removal performance of the thiophene has also been investigated.

Adsorption was studied at various time intervals (0-1600 min) at a fixed initial concentration of adsorbates (100 ppm). The obtained removal percentages were plotted against the equilibrium time to show clearly the effect of contact time for kinetic modelling. Fig. 9b reveals the impact of contact time on the elimination of thiophene by the SCG-AC. The adsorption was fast within the first 500 min for thiophene but slowing down between 500 and 1600 min as equilibrium was attained. The adsorption capacity of thiophene was 57.80 %.

Isotherms of thiophene adsorption at ambient conditions

The relationship between the adsorbed amount of thiophene and its equilibrium concentration at a constant temperature can be represented by an adsorption isotherm. The adsorption isotherm is the most employed method for representing the equilibrium state of an adsorption system. From Fig. 10, the adsorption capacity of thiophene (qe) increases with the progression of the equilibrium concentration (Ce). Therefore, the equilibrium adsorption isotherm can be used to describe the interactivity behavior between the adsorbate and the adsorbent as well as the design of the adsorption system [START_REF] Nunthaprechachan | Adsorptive desulfurization of dibenzothiophene by sewage sludge-derived activated carbon[END_REF].

Fig.10. Thiophene adsorption isotherms at 25 °C on the optimal SCG-AC

The calculated adsorption capacities (from Fig. 10) are listed in Table 8. It is clear from the coefficients of correlations (R 2 ) that the Langmuir-Freundlich isotherm fits the present data much better than Langmuir and Freundlich isotherm. These results show that the Langmuir-Freundlich isotherm represents the adsorption data of thiophene on SCG-AC in the range of the studied concentration. According to this model, the adsorption of a molecule is not done on one adsorption site but on 1/ n site. This is reliable with the fact that Langmuir-Freundlich isotherm is based on the hypothesis that an adsorbent has a highly heterogeneous surface with different sites for adsorption or different functional groups. 9).

Although our SCG-AC provided a better adsorption capacity than other adsorbents from the works of Edinger et al. [START_REF] Edinger | Adsorption of thiophene by activated carbon: A global sensitivity analysis[END_REF] and Prajapati et al. [START_REF] Prajapati | Fixed bed adsorptive desulfurization of thiophene over Cu/Nidispersed carbon nanofiber[END_REF], however it still less performant than those of Ag-Y and Cu-Y [START_REF] Takahashi | New Sorbents for Desulfurization by π-Complexation: Thiophene/Benzene Adsorption[END_REF]. This can be described by the variance in surface chemistry, textural properties as well as the bonding strength between the thiophene and the AC [START_REF] Boulinguiez | Adsorption/Desorption of Tetrahydrothiophene from Natural Gas onto Granular and Fiber-Cloth Activated Carbon for Fuel Cell Applications[END_REF]. A comparative study with tetrahydrothiophene under the same operating conditions showed a close adsorption capacity between SCG-AC and the Fiber-cloth material (ACFC).

Kinetic of thiophene adsorption

Kinetic studies and isotherm models are a used tools to simulate and describe experimental equilibrium data of an adsorption process [START_REF] Abdul Manap | Adsorption isotherm and kinetic study of gas-solid system of formaldehyde on oil palm mesocarp bio-char: Pyrolysis effect[END_REF]. Experimental data were fitted with kinetic models and the results were presented in Fig. 11(a,b,c). The adsorption kinetics are analyzed according to the pseudo-first-order model (equation 7) pseudo-second-order (equation 8), and Elovich model (equation 9). The results of the kinetic parameters were collected in Table 10.

The applicability of pseudo-first-order, pseudo-second-order, and Elovich models to the kinetics of thiophene sorption by SCG-AC were evaluated by comparing the value of the correlation coefficients, R 2 . The best kinetic model describing the thiophene adsorption was the Elovich model (R 2 =0.9507). The Elovich model is a well-known model that described the adsorption of gas onto solid systems [START_REF] Abdul Manap | Adsorption isotherm and kinetic study of gas-solid system of formaldehyde on oil palm mesocarp bio-char: Pyrolysis effect[END_REF]. This means that the rate limiting step in the process is the chemical sorption between the thiophene and the active sites of the SCG-AC.

According to the Elovich model assumptions, there are probably two types of sorption sites implying in this sorption process. One with the oxygen surface groups by complexation and the other one with the π electrons of the graphene layers of the activated carbon by electrostatic interaction [START_REF] Largitte | A review of the kinetics adsorption models and their application to the adsorption of lead by an activated carbon[END_REF].

Table 10. Kinetic parameters obtained for thiophene adsorption on SCG-AC.

SCG-AC qe, exp (mmol g -1 )

1.65

Pseudo-first-order qe (mmol g -1 ) 1.99 K1 (min -1 ) 0.80 ×10 -3 R 2 0.8893 Pseudo-second-order qe (mmol g -1 ) 0.77 K2 (g mmol -1 min -1 )

3.80 ×10 -3 R 2 0.7920 Elovich model β (mmol g -1 )

1.63 α (mmol g -1 min -1 ) 6.19 ×10 -3 R 2 0.9507

Adsorption mechanism

The influence of mass transfer resistance on binding of thiophene on the SCG-AC was verified by exploring Weber and Morris intra-particle diffusion resistance model using equation 10. According to the intraparticle diffusion model, the curve representing the amount adsorbed as a function of t 1/2 must be linear and pass through the origin if the intraparticle diffusion is involved in the adsorption process. In this case, intraparticle diffusion should be the step that controls the adsorption rate [START_REF] Saleh | Adsorptive desulfurization of thiophene, benzothiophene and dibenzothiophene over activated carbon manganese oxide nanocomposite: with column system evaluation[END_REF]. In our case, the deviation of the line of the intraparticle diffusion model from the origin represents the effect of the diffusion through the film surrounding the particles of the SCG-AC. The greater the deviation, the more significant will be the effect of the mass transfer in the film on the kinetics of the adsorption process [START_REF]Adsorptive desulphurization of model oil by Ag nanoparticles-modified activated carbon prepared from brewer's spent grains[END_REF]. The Table 11 illustrates the intra-particle diffusion model parameters. We note that the representative line of the model (Fig. 11d) does not go through the origin and that the correlation coefficient (R 2 =0.8768) is weak. This result shows that intraparticle diffusion is not the only rate-controlling phase [START_REF] Saleh | Adsorptive desulfurization of dibenzothiophene from fuels by rubber tyres-derived carbons: Kinetics and isotherms evaluation[END_REF]. Therefore, the adsorption proceeds via a complex mechanism consisting of both kinetic and intra-particle transport within the pores of adsorbent [START_REF]Adsorptive desulphurization of model oil by Ag nanoparticles-modified activated carbon prepared from brewer's spent grains[END_REF]. This is supported by a finding shown in Fig. 12 where the adsorption of thiophene occurred in two phases: rapid and slow phases. This is due to the great availability of vacant active sites on the SCG-AC surface. As the time increased slower step took place where the active sites on which thiophene binds become fewer. The formation of monolayer of thiophene at the exterior surface became saturated as the amount of thiophene molecules was higher than the available sites (see Fig. 12). The saturation of thiophene forced the system to prevent the changes in order to remain in equilibrium state which also contributed to a slow phase [START_REF] Abdul Manap | Adsorption isotherm and kinetic study of gas-solid system of formaldehyde on oil palm mesocarp bio-char: Pyrolysis effect[END_REF].

Table 11. Intra-particle diffusion parameters for adsorption of thiophene

Kid (mmol g -1 min -0.

Conclusions

The AC prepared from SCG by chemical activation with H3PO4 was optimized using RSM.

Process optimization of SCG-based AC was successfully explored by BBD of RSM. The ANOVA of the primary factors indicated that the impregnation ratio and acid concentration have the most significant effect on the iodine adsorption, whereas the acid concentration, impregnation ratio, and activation temperature have the most substantial impact on the methylene blue adsorption. The optimal conditions for maximum iodine and methylene blue adsorption have been found for an activation temperature of 500 °C with an impregnation ratio of 1.5 g/g, and phosphoric acid concentration of 68.7 wt%. The Boehm results confirm that carboxylic acids are the main functional groups found on the surface of the optimal SCG-AC. The nitrogen adsorption-desorption isotherms of the produced material exhibit a typical hybrid type I-IV isotherm with hysteresis type H2 and the BET surface area was 410 m 2 /g.

The optimized SCG-AC was used in adsorptive desulfurization of thiophene from the gas at ambient temperature. A good agreement between experimental results and Langmuir-Freundlich isotherm was obtained. According to this model, the adsorption of thiophene takes place via 1/ n site. The best kinetic model describing the thiophene adsorption was the Elovich model which suggests that there are probably two types of sorption sites implying in this sorption process: one with the oxygen surface groups by complexation and the other one with the π electrons of the graphene layers of the activated carbon by electrostatic interaction.

According to the proposed mechanism, the adsorption of thiophene on the SCG-AC proceeds via a complex mechanism consisting of both kinetic and intra-particle transport within the pores of the adsorbent. This study was established in order to get first trends about the adsorption capacities of activated carbon prepared from spent coffee grounds, towards thiophene adsorption. Though reported sorption capacities are promising, experiments were performed at ambient conditions while the envisaged application of SCG-AC in fluidized catalytic cracking (FCC) gasoline treatment demands elevated ones (around 500 °C). The effect of temperature will be the subject of future work.

Fig. 1 .

 1 Fig. 1. Experimental process setup.

Fig. 3 .

 3 Fig. 3. TGA analysis (a) and Micro-GC analysis of SCG pyrolysis without phospohoric

  lists the ANOVA for the quadratic models of the iodine number (IN) and methylene blue number (MB) responses. The significance of each parameter and interaction between different parameters is evaluated considering p-value and F-value. Commonly, large F-value and small p-value would prove the statistical implication of the model and the model terms. An, F-value and p-value > F were used to verify the significance levels for each coefficient used in this

2 A: 3 . 5 .

 235 Fig.4(a,b) represents the actual values versus the predicted values for iodine number and methylene blue number responses, respectively. From those plots, most of the data points were well dispersed near to the straight line. This is explicated by an excellent relationship between the experimental and predicted values of the responses. Thus, these statistical tests showed that the developed quadratic models could successfully correlate between the variables of AC production and responses.

Fig. 4 .Fig. 5 (

 45 Fig. 4. The experimental data versus predicted value plot for the (a) IN and (b) MB

Fig. 5 .

 5 Fig. 5. The residuals data versus predicted value plot for the model (a) IN and (b) MB

Fig. 6 (

 6 Fig.6(a,b) represent the Box-Cox plot for iodine and methylene blue number transformation, respectively. We noticed that a 95 % confidence interval around the minimum point of the curve contains 1. So, no specific transformation is recommended for the two models.

Fig. 6 .

 6 Fig.6. Box-Cox plot for the model (a) IN (b) MB responses.

Fig. 7d illustratesFig. 7 .

 7 Fig.7dillustrates the surface plot of MB adsorption as a function of impregnation ratio and activation temperature. From the plot, it is obvious that the MB adsorption increased with the increase in temperature from 440 to 500 °C and impregnation ratio from 0.8 to 1.5 g/g. The curve segment of the plot indicated that the MB adsorption was particularly touched by the interactive effects of impregnation ratio and activation temperature. The highest methylene blue number was reached at 500 °C and an impregnation rate of 1.5 g/g.

Fig. 8 .

 8 Fig. 8. N2 adsorption-desorption isotherm of SCG-AC prepared at optimized conditions.

Fig. 9 .

 9 Fig. 9. Effect of Adsorbate dose on the removal capacity of SCG-AC (b) Effect of contact

Fig. 11 .

 11 Fig. 11. Plots of pseudo-first order kinetic (a) pseudo-second order adsorption kinetic

Fig. 12 .

 12 Fig. 12. Schematic diagram of thiophene adsorption process on the SCG-AC.

Table 1 . Association of factors with coded variables in the experimental design

 1

	Factor	Code	Units	-1	Coded variable level 0	+1
	Acid concentration (A)	A	wt%	50	67.5		85
	Impregnation ratio (B)	B	g/g	1:2	1:1	1.5 :1
	Activation temperature (C)	C	°C	400	450		500
	2.

Table 2 . Proximate and ultimate analysis of the raw material Ultimate analysis (wt%)

 2

	C	H	N	S	O*
	50.40	6.90	2.40	<0.06	38.50
		Proximate analysis (wt%)
	Volatils yields db	Fixed carbon	Ash
	77.90		20.30	1.80
			EDX analysis (wt%)
	K	Cl	Ca	P	Fe
	0.76	2.10	1.23	0.046	0.05

*Obtained by difference db dry basis

  2 , B2 , and C 2 . The equation below[START_REF] Duarte | Sulfur removal from hydrotreated petroleum fractions using ultrasound-assisted oxidative desulfurization process[END_REF] indicates the quadratic regression model that is defined by all coded factors once all insignificant terms are excluded:

	Iodine number . 7.50 B C 40.75 A /0 0 1 1119 + 11.75 A + 19.12 B 5.50 B + 8 C	16.25 A B +

Table 3 . Independent variables and results for the preparation of activated carbon by the BOX-Behnken Design (BBD)

 3 It compares the range of the predicted values at the design points to the average prediction error. A value greater than 4 is desirable in support of the fitness of the model. The ratio of 43.95 and 25.86 for iodine number and methylene blue number respectively are greater than 4

	0.20	%	and	VC2	=	4.00	%)	were	less	than	10	%.

which indicate an adequate signal. These models can be used to navigate the design space

[START_REF] Safa | Removal of sulfone compounds formed in oxidative desulfurization of middle distillate[END_REF]

. Standard deviation used in this study were equal to 2.21 and 2.27 for equations (

11

) and

[START_REF] Shawky | One-pot synthesis of Mn3O4-coupled Ag2WO4 nanocomposite photocatalyst for enhanced photooxidative desulfurization of thiophene under visible light irradiation[END_REF]

, respectively. A good capability for the developed quadratic models in this study has been shown

[START_REF] Mahmood | Potential of used Camellia sinensis leaves as precursor for activated carbon preparation by chemical activation with H3PO4; optimization using response surface methodology[END_REF]

. To signify the model's replicability, the variance coefficient (VC) was developed from the ratio of standard error for estimates of the average value of the observed responses. A model is considered replicable when the VC value is less than 10 %

[START_REF] Mahmood | Potential of used Camellia sinensis leaves as precursor for activated carbon preparation by chemical activation with H3PO4; optimization using response surface methodology[END_REF]

. Both models used in this research were shown to be replicable because both regressions (VC1 =

Table 4 . ANOVA results

 4

	1
	ANOVA of IN

Table 5 . Optimization of SCG-AC by RSM

 5

	A: Acid concentration (wt%)	B: impregnation ratio	C: Temperature (°C)	Iodine number (mg/g) Pred Exp	error (%)	Methylene blue number (mg/g) error (%) Pred Exp
	68.70	1.50	500	1148	1139	0.80	87	90	3.45

Table 6 . Textural properties, surface functional groups, and pHpzc values of the SCG- AC prepared at optimized conditions.

 6

				Element analysis (wt%)
	C	H	N	S	O	P	Cl	Ca	K	Cu	Fe
	69.70	3.20 2.06 <LD 18.24 6.84 1.72	0.82 0.56 0.04 0.02
			Acidic functional groups (mmol.g -1 )
	carboxylic groups	phenolic groups	lactonic groups	total acidic groups	basic groups	pHpzc
	0.32		0.00		0.18		0.50		0.36	3.60
					Textural properties
	SBET (m 2 /g)	Average pore diameter (nm)		VT (cm 3 /g)		Vmic (cm 3 /g)		Vmeso (cm 3 /g)
	410.00		2.44			0.25		0.05		0.04
	3.9.

Comparison with the iodine and methylene blue adsorption value of activated carbon prepared from various lignocellulosic biomass A

  comparison study has been done between the iodine and methylene blue adsorption value of the SCG-AC with those carbons obtained in the literature prepared from various agricultural wastes is represented in Table7. As is apparent from the table that the iodine and methylene blue adsorption values of SCG-AC is relevant to other studies investigated by other works. Thus this research revealed the usage of the spent coffee grounds as an economical, accessible raw feedstock with great potential of producing activated carbon with high iodine and methylene blue adsorption values.

Table 7 . Comparison of the iodine and methylene blue adsorption value of activated carbon prepared from various lignocellulosic biomass.

 7

	Precursors	Activating agent	IN	MB	Ref
	Pomegranate peels	H3PO4	1455	168	[24]
	Almond shells		791	148	[55]
	Banana trunk		-	64.66	[22]
	Cotton fabrics		595.21	-	[23]
	Spent coffee grounds		1139	90	This work
	3.10.

Adsorptive desulfurization 3.10.1. Effect of adsorbate dose and contact time on the adsorption efficiency of SCG-

Table 8 . The parameters of different isotherms for the adsorption of thiophene in gas phase by SCG-AC

 8 Due to the lack of data from the literature about the adsorption of thiophene in gas phase under ambient conditions, this work exposed a comparison with others reported results to know the capacity range of adsorption of thiophene by different adsorbents (Table

	Langmuir		Freundlich		Langmuir-Freundlich
	qm (mmol/g)	KL (l/mmol)	R 2	KF l 1/n mmol (1-1/n) g -1 1/n	R 2	qm (mmol/g)	KLF (l/mmol)	1/n	R 2
	1.70	9.44	0.9504	1.55	0.33 0.9582	2.23	4.07	0.56 0.9880

Table 9 . Uptake gas desulfurization capacity of different adsorbent materials

 9

	Adsorbents	Conditions	Adsorbate	Temperature	Adsorbent capacity	References
				(°C)	(mmol/g)
	Ag-Y			180	2.602
	Cu-Y	Dynamic		90	3.910	[13]
	Na-ZSM5	gas phase	Thiophene	90	1.023
	Commercial AC			100	0.138	[3]
	Ni/Cu dispersed carbon nanofiber			150	0.006	[56]
	Fiber-cloth material (ACFC)	Static gas phase	Tetrahydrothiophene	25	2.300	[57]
	SCG-AC	Static gas phase	Thiophene	25	2.230	Present study

Acknowledgements

This work has been partly carried-out in the frame of the Partenariat Hubert Curien Program through the PHC UTIQUE 2017 (Project code: 17G1139) and the authors gratefully acknowledge the support of the Comité Mixte Franco-Tunisien (CMCU) and the University

of Gabes (Tunisia). The authors are grateful to Mr. Eric CHEVREL of IMT Atlantique for his 1 assistance in the activated carbon production tests.