Activated Carbons Based on Shea Nut Shells ( Vitellaria paradoxa ): Optimization of Preparation by Chemical Means Using Response Surface Methodology and Physicochemical Characterization

In this study, shea residues (Vitellaria paradoxa) dumped in the wild by the units processing almonds into butter were used in the production of activated carbons. Shea nut shells harvested in the locality of Baktchoro, West Tandjile Division of Chad were used as a precursor for the preparation of activated carbons by chemical activation with phosphoric acid (H 3 PO 4 ) and sulphuric acid (H 2 SO 4 ). Central Composite Design (CCD) was used to optimize the preparation conditions, and the factors used were concentration of activating agent (1 - 5 M), carbonization temperature (400˚C - 700˚C) and residence time (30 - 120 min). The studies showed that at optimal conditions the yield was 51.45% and 42.35%, while the iodine number (IN) was 709.45 and 817.36 mg/g for CAK-P (phosphoric acid activated carbon) and CAK-S (sulphuric acid activated carbon) respectively. These two activated carbons (ACs) which were distinguished by their considerable iodine number, were variously characterized by elementary analysis, pH at the point of zero charge (pHpzc), bulk density, moisture content,


Introduction
A major environmental problem facing agro-food industries is their inability to efficiently manage the residues and by-products generated during the primary processing of raw materials. On the industrial scale, the processing of agricultural commodities generates huge quantities of lignocellulosic residues that are disposed of by land filling or incineration. These practices have a negative effect on man and his environment. For example, open-air incineration generates substances which are toxic to humans and constitute greenhouse gases that have damaging effects on the climate [1]. It is because they are wrongly considered as wastes that these agricultural residues are treated in this way.
A large quantity of these lignocellulose residues can be recycled by fermenting them to generate methane which is a fuel, and ethanol which is both a solvent and a fuel [2]. They can also be composted to obtain an enriched soil conditioner, which can improve soil fertility. Furthermore, a controlled pyrolysis of these lignocellulosic wastes produces activated carbons [3]. Of these alternative treatments of the lignocellulosic residues, the transformation into activated carbons is more flexible; with chemical activation significantly reducing the energy cost of the process.
Activated carbons are carbon-based materials, found in granulated or black powdered form, and having a porous structure with very large surface areas.
It is in the quest for sustainable development that the recycling of agricultural and industrial waste, especially shea residues (Vitellaria paradoxa), arouses particular interest. Socioeconomically, shea is valuable in that its butter has medicinal, food and cosmetic values [11], and it is relatively abundant in many African countries. In the Republic of Chad, for example, shea industries produce 556,000 tonnes of almonds per year [12]. While the nuts are used for butter production, nut shells, the more woody part of the fruit could be used as a raw material for the production of activated carbons. This alternative use of the nut shells has the advantage that, on the economic domain, value has been added onto the wastes; on the perspective of environmental pollution, the generation of greenhouse gases is avoided or highly minimised; and finally concerning sanitation the generation of pollutants harmful to man is avoided. The objective of this research was to optimize the preparation of activated carbons using shea hulls as precursor. The process optimization was carried out using Response Surface Methodology (RSM), applying the Central Composite Design (CCD) in order to obtain maximum information with minimal experiments [13].

Materials
The shea nut shells (CNK) (Figure 1) collected in Baktchoro in the West Tandjilé Division in Chad were washed with tap water to remove the remaining pulp and other forms of attached impurities. They were dehydrated by drying in an oven set at 105˚C for 24 hours.
Finally, they were crushed and sieved to obtain particles of size 0.5 -2 mm that were preserved for use in this study.
Preparation of activated carbons 20 g of CNK was impregnated by mixing it with 40 mL solutions of phosphoric acid (H 3 PO 4 ) or sulphuric acid (H 2 SO 4 ) of concentrations ranging from 1 to 5 mol·L −1 . After stirring for 24 hours to ensure reaction between the activating agent and the CNK, the mixture was then filtered and the filtrate dried in an oven at 105˚C for 1 hour to obtain the impregnated material. In each case, the impregnated materials was pyrolysed in a furnace of mark HERAEUS that was set at temperatures ranging from 400˚C to 700˚C for residence times of 30 to 120 minutes, and a constant heating rate of 6˚C·min −1 . The residue from filtering the mixture was thoroughly washed with distilled water to remove the residual acid and then dried in a Binder oven set at 105˚C to obtain the activated carbon.

Determination of Yield and Iodine Number (IN) of Activated Carbons
The yield of activated carbon was determined as the mass of the dried activated carbon relative to the mass of the precursor, and given by the Equation (1):

Iodine Number (IN)
The iodine number of the activated carbons was determined using the procedure established by the American Society for Testing and Materials (ASTM) [14]. This was done by mixing 100 mg of activated carbon with 30 mL of iodine solution of concentration 0.01 mol·L −1 in a sealed bottle. The mixture was stirred away from the sun rays at a speed of 200 rpm for 3 hours. The resulting mixture was quickly filtered using filter paper, and 10 mL of filtrate was titrated with a solution of sodium thiosulfate pentahydrate (Na 2 S 2 O 3 ·5H 2 O), of concentration 0.005 N, using starch solution as indicator. From the chemical reaction given in Equation (2)

Optimizing the Preparation of Activated Carbons
Experimental design: response surface methodology The preparation of ACs was optimized through Response Surface Methodology. Central Composite Design was chosen to generate the experimental matrix in order to reduce the number of experiments to be performed while simultaneously studying the effects of several factors [15] [16]. Thus, the total number of experiments performed was calculated from Equation (4) given below [17].
N is the total number of experiments, 2 indicates the number of levels per factor, k the number of factors studied, 2 k the number of factor points, 2k the number of axial points and k c the number of points in the center that determined errors or reproducibility of the data.
Based on literature [18] and on previously conducted tests in our laboratory [19], the factors that significantly influence the physical properties of activated carbons are the concentration of activating agents (X 1 ), the carbonization temperature (X 2 ) and residence time (X 3 ). In this work, H 3 PO 4 and H 2 SO 4 were used as activating agents. Table 1 presents the independent experimental variables used as calculated from Equation (5) [17] [20]. The values of the experimental variables were coded as follows: −1 for the small value, 0 the value at the centre and +1 the high value. To assess quantitatively and qualitatively the activated carbons obtained, the mass yield and iodine number were used as responses or performance indicators.
The second order polynomial Equation (6)   Validating the statistical model The empirical model obtained was validated using Statgraphic plus 5.0 software as the evaluation instrument. The model was regarded as good when the experimental values obtained were close to those predicted by the t-test and p-value. When the t-test value was high and the p-value small, then, the correlation coefficient was high. In general, a model was valid when R 2 was greater than 75% and the p-value was 5% or less (with 95% confidence level) [16].

Characterization of Activated Carbons Prepared at the Optimal Conditions
The activated carbons prepared at the optimal conditions were characterized by various physicochemical methods. These optimal conditions were those provided by the Statgraphic plus 5.0 software; and they were the concentration of activating agents, the carbonization temperature and residence time, that led to high values of iodine number.

EDX Analysis
Elementary analysis using energy dispersion X-ray spectrometry was applied to investigate the presence and percentage of atoms that made up the activated carbons.

Bulk Density
To measure the bulk density, a mass of activated carbon was carefully poured into a graduated and previously weighed 25 cm 3 volumetric flask. The difference in mass m AC compared to the occupied volume (V oc ) was used to calculate the apparent density as follows:

Moisture Content
To determine the moisture level, an empty and well dried porcelain crucible was weighed (m 0 ) using a precision scale of 1/1000b and 1 g of activated carbon was properly distributed on it and weighed again (m 1 ). The combination consisting of crucible and activated carbon was then placed in the oven set at 100˚C for 1 hour. After cooling to room temperature, the system is again weighed (m 2 ). The humidity, T was calculated by the relationship (8) [22]:

pH and pH at the Point of Zero Charge (pH PZC )
The pH of the ACs was obtained by putting 30 mL of distilled water and 100 mg of AC in contact in a wash-bottle. The mixture was agitated for 24 hours; decanted, and the pH of the solution was measured using an ATC-mark pH-meter. The pH at the point of zero charge (pH pzc ) was determined according to the protocol described by Tchakala et al., (2012) [7]. In washed bottles containing 30 mL of sodium chloride (NaCl) solution of concentration 0.1 mol·L −1 , the pH was adjusted from 2 to 10 using 0.1 mol·L −1 solution of hydrochloric acid HCl or 0.1 mol·L −1 sodium hydroxide NaOH to obtain the initial values pH i . 100 mg of activated carbon was introduced into each bottle and the mixture stirred for 24 hours. After leaving the mixtures to settle, the final pH values, pH f were measured. The pH pzc was obtained as the intersection of the curve: and the bisector of the first quadrant.

Specific Surface Area
The porous structure of the ACs was determined by nitrogen adsorption at 77.13 K. This measurement was made using a sorptophotometric porosimeter (Thermo Electron Corporation Advanced Data Processing). The B.E.T method (Brunauer, Emmett and Teller) was used to determine the specific surface of the samples. The total pore volume V T is the volume of liquid nitrogen adsorbed at the relative pressure (P/P 0 ) of 0.005 to 0.4 [1]. Thus, from the B.E.T method coupled with the B.J.H. (Barret-Joyner-Halenda) the distribution of pore size, and consequently the microporous volume, V micro , mesoporous volume, V meso and the average pore diameter, D p were calculated [23].

Quantification of Functional Surface Groups
The quantification of surface functional groups was carried out using the Boehm method (1966) [24]. To do this, 0.1 g of activated carbon in 30 mL of a solution of NaHCO 3 (0.1 mol·L −1 ), or Na 2 CO 3 (0.1 mol·L −1 ) or NaOH (0.1 mol·L −1 ) was agitated at room temperature for 48 hours to determine the acid groups, or with 30 mL of HCl (0.1 mol·L −1 ) to determine the basic groups. The filtrates were titrated with HCl (0.1 mol·L −1 ) or NaOH (0.1 mol·L −1 ), depending on whether acid or basics groups were concerned; and in the presence of an indicator. The calculation was based on the principle that NaHCO 3 neutralizes carboxylic acid groups, Na 2 CO 3 neutralizes carboxylic and lactone groups and NaOH neutralizes carboxylic, lactone and phenolic groups and the basic groups are neutralized in their entirety by HCl. The number of equivalents (meq·g −1 ) and the type of functional groups were determined from the relationship (9) [22]: where, eqg η is the number of gram equivalents that reacted, while 0 0 N V and 0 f N V are the number of gram equivalents before and after reaction.

Fourier-Transform Infrared Spectroscopy (IR-TF)
A Thermoscientific Nicolet iS5 brand spectrophotometer was used to investigate the chemical functional groups present on the surface of the activated carbons. The IR-TF spectra were recorded between the wavenumbers 3500 and 580 cm −1 .

Scanning Electron Microscopy (SEM)
In order to study surface morphology and verify the porosity of the adsorbents, samples of the prepared activated carbons were examined by the PHILIPS XL30 S-FEG scanning electron microscope (SEM) equipped with a spectrometer that uses a cathode luminescence detector. Samples of each activated carbon prepared under the conditions of the best iodine number were selected for the SEM analysis.

Experimental Design Using the Central Composite Design
The central composite design was used with the aim of finding the optimal conditions for the preparation of activated carbon in a short time. Mass yield and iodine number were chosen as responses of the factors influencing (X 1 , X 2 and X 3 ) the preparation of the activated carbons. The results of the experimental matrix are presented in Table 2. Repetitive experiments 7, 8 and 11 were used to verify the reproducibility of the data during handling.

Iodine Number
The iodine number of carbon CAK-P (phosphoric acid activated carbon) and CAK-S (sulphuric acid activated carbon) is an important parameter which provides information on their microporosity. The results of the iodine number are presented in Table 2. Experimental values of the iodine number range from 301.95 to 655.81 mg·g −1 and between 419.10 and 781.05 mg·g −1 respectively for CAK-P and CAK-S. The mathematical models of the quadratic regression used to predict the optimal iodine number values are Y 2 and Y 4 for CAK-P and CAK-S respectively (Equations (10) and (11)): Journal of Materials Science and Chemical Engineering  Table 3 summarizes the analysis of variance (ANOVA) performed to determine the variables that interact significantly during the production of the activated carbons. It appears that the experimental values and predicted values are in agreement, since the determination coefficients R 2 of this are close to unity for the two activated carbons (R 2 = 91.64% for CAK-P and R 2 = 96.57 for CAK-S). Referring to the probable p-values ≤ 0.05, the terms X 1 , X 2 , X 1 X 2 and X 1 X 3 for CAK-P and X 1 , X 2 , X 3 and X 1 X 2 for CAK-S have values less than the said probability p, which shows that all these terms are significant. Therefore, temperature and concentration have a significant effect on the iodine number. This is confirmed by the Pareto diagram analysis shown in Figure 2(a) and Figure 2(b) which clearly leave the corresponding bands cut by a vertical test line. Figure 2 shows that the iodine number increase with the increase in activation temperature. This is because when the temperature rises, the volatile materials evaporate and some pores are enlarged while more pores are created [25], thus increasing the sites for adsorption [18].
In general, the increase in the concentration of activating agents leads to an increase in the iodine number. Tagne et al. (2019) [16] also reported similar results. For soft lignocellulosic precursors, a high concentration of activating agent leads to less porous activated carbon, which leads to a reduction in the iodine number due to the destruction and/or absence of the appearance of pores.
However, the combined action of high concentration and high temperature increase the pores of activated carbon, thereby leading to high iodine number values, as can be seen with experiment 9 in this work.

Activated Carbon Yield
The outcome from the production process of activated carbon is the yield. The performance values shown in Table 2 range from 33.74% -51.45% and 34.08% -41.95% respectively for CAK-P and CAK-S. The yield is seen to decrease as the temperature increases. The yields are also seen to be high at low residence time and high concentration of the activating agent, as evidenced by experiment 15 ( Table 2). The mathematical models of CAK-P (Y 1 ) and CAK-S (Y 3 ) are given by Equations (12) and (13).
The results of the ANOVA show that the terms X 2 , X 3 , X 1 X 3 and X 2 X 3 are significant for the CAK-P and X 2 , X 3 , X 1 X 2 and X 2 X 3 are significant for CAK-S. According to Figure 3

Process Optimization and Test Validation
The optimal conditions for the production of the activated carbons were de-  Table 4. We observe that the experimental values are close to the predicted values, implying that the difference between these two values is not significant. However, it is not easy to simultaneously meet the optimal performance and iodine number requirements provided by this software for the preparation of better activated carbons because the zone of interest of the factors is different. The same remark was made by Lekene et al. (2018) [18]. The activated carbons prepared under optimal conditions using iodine number were consequently characterized. At the residence time of 120 min, the optimal conditions obtained were 5 mol·L −1 of H 3 PO 4 and 400˚C carbonization temperature for CAK-P; and 3.5 mol·L −1 of H 2 SO 4 and 700˚C for CAK-S.

EDX Analysis
EDX analysis of the activated carbons is shown in Figure 4 while Table 5 gives the weight percentages of the elements present in them. This analysis was coupled with the scanning electron microscopy (SEM/EDX) captured on a selected area 1 section of the SEM image. Carbon appears to be the most representative element in CAK-P and CAK-S, this is explained by the increase in the degree of aromatization during carbonization. The small amounts of the other elements originated from the lignocellulosic precursor, and they become perceptible on the departure of volatile substances during carbonization.

Physico-Chemical Characteristics of the Activated Carbons: pH, pHpzc, Bulk Density, Moisture Content and Boehm Titration
The physico-chemical properties that were determined for the prepared ACs are presented in Table 6. The pH values of 3.34 and 6.91 respectively for CAK-P and CAK-S show that the two activated carbons are acidic. pHpzc values are 2.23 and 6.9 respectively for CAK-P and CAK-S (Table 6, Figure 5). These are the pH values where the two carbons exist at zero electrical potential. This means that for solutions where pH > pHpzc, the electrostatic attraction is favourable for the cationic pollutant because the surface of the material is deprotonated. On the other hand, when pH < pHpzc, the electrostatic attraction is favourable for the anionic pollutant since the surface of the material is protonated [27].    The bulk density of an activated carbon provides information on the microcristalline structure and strong inter-linking bonds [28]. The moisture content of these ACs is in line with the standard of ≤5% as stipulated by the American Work and Water Society.

Specific Surface Area
The textural structure of the activated carbons was determined by the BET method, using relative pressure (P/P 0 ) from 0.005 to 0.4. Figure 6 and Figure 7 show the analyses on porosity for CAK-P and CAK-S respectively. have a similar texture, but the one activated with sulphuric acid has better developed pores. In view of these results it is obvious that these ACs can be used to adsorb small and medium-sized molecules from aqueous solution. Table 7 shows the values obtained.    Figure 8 shows the IR-TF spectra recorded in the 3500 to 580 cm −1 region. Low peaks at 2915.36 and 2861.41 cm −1 on the biomass spectrum (CNK) indicate the symmetric and asymmetric stretching vibration of methyl group [18] [28]. The absence of these peaks on the spectra of the CAK-P and CAK-S activated carbons shows that during the activation with phosphoric and sulphuric acids, there was a reaction resulting in the formation of other oxygenated functional groups. The signal at 2368.65 cm −1 may be due to O-C-O and the peak at 1731.65 cm −1 corresponds to the C-O group of esters and lactones. The appearance of the peak at 1040.36 cm −1 corresponds to the C-O group of primary, secondary, tertiary alcohols and ethers, while that at 1257.96 cm −1 indicates the presence of the same group C-O group of carboxylic acids and esters or the P-O link of ester phosphates or O-C of the P-O-C bridge [1]. Between 800 and 650 cm −1 we have the C-H bond of aromatic polynuclear systems [30].

Scanning Electron Microscopy (SEM) Analysis
The results obtained from the SEM analyses of the activated carbons are presented in Figure 9. These pictures show that the two activated carbons have Journal of Materials Science and Chemical Engineering  almost identical porosity. It should be remembered that a developed porosity increases the specific surface area of the activated carbons [31] and therefore increases the number of sites available for the fixing of pollutants during adsorption. It is also observed that both activated carbons have a heterogeneous surface.

Conclusion
In this study, a triacid and diacid were used as activating agents in the preparation of ACs using shea residues (Vitellaria paradoxa) as precursors. Central composite design was used to study the influence of preparation parameters, concentration of activating agents, carbonization temperature and residence time, on expected responses, yield and iodine number. Under the optimal conditions recommended by Statgraphic plus 5.0, thus, 5 mol·L −1 , 400˚C and 120 min for CAK-P and 3.5 mol·L −1 , 700˚C and 120 min for CAK-S, the iodine numbers were 709.45 mg·g −1 and 817.36 mg·g −1 . The results of pHpzc, FTIR and Boehm titration indicate that CAK-P and CAK-S ACs prepared under these conditions are acidic. They have a microporous surface of 522.55 and 570.65 m 2 ·g −1 respec-tively for CAK-P and CAK-S. Comparative analysis of the physic-chemical characteristics shows that these two ACs are of good quality, the better one being the one activated with sulphuric acid.