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

To measure the bulk density, a mass of activated carbon was carefully poured into a graduated and previously weighed 25 cm³ volumetric flask. The difference in mass m_AC compared to the occupied volume (V_oc) was used to calculate the apparent density as follows:

d a p p = m A C V o c (7)

To determine the moisture level, an empty and well dried porcelain crucible was weighed (m₀) using a precision scale of 1/1000b and 1 g of activated carbon was properly distributed on it and weighed again (m₁). 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 (m₂). The humidity, T was calculated by the relationship (8) [22]:

T % = m 1 − m 2 m 1 − m 0 × 100 (8)

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: pH f = f ( pH i ) and the bisector of the first quadrant.

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/P₀) 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].

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 NaHCO₃ (0.1 mol·L⁻¹), or Na₂CO₃ (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 NaHCO₃ neutralizes carboxylic acid groups, Na₂CO₃ 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]:

η e q g = N 0 V 0 − N f V 0 (9)

where, η e q g is the number of gram equivalents that reacted, while N 0 V 0 and N f V 0 are the number of gram equivalents before and after reaction.

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.

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.

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 Y₂ and Y₄ for CAK-P and CAK-S respectively (Equations (10) and (11)):

^aExp. val.: experimental values; ^bPred. val.: predicted values; IN = iodine number.

Y 2 = 329.721 − 11.2223 X 1 + 0.162766 X 2 − 1.05419 X 3     + 9.0987 X 1 2 − 0.1395 X 1 X 2 + 0.966389 X 1 X 3     + 0.000760 X 2 2 − 0.004348 X 2 X 3 + 0.001745 X 3 2 (10)

Y 4 = 66.049 − 29.0745 X 1 + 1.32658 X 2 + 2.32076 X 3     − 5.46871 X 1 2 + 0.093858 X 1 X 2 − 0.007277 X 1 X 3     − 0.000707 X 2 2 − 0.002319 X 2 X 3 − 0.000098 X 3 2 (11)

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 R² of this are close to unity for the two activated carbons (R² = 91.64% for CAK-P and R² = 96.57 for CAK-S).

*Significant; Df = degree of freedom; MS = mean square; IN = iodine number; adjR² = adjusted R².

Referring to the probable p-values ≤ 0.05, the terms X₁, X₂, X₁X₂ and X₁X₃ for CAK-P and X₁, X₂, X₃ and X₁X₂ 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(c) and Figure 2(d) show the response surface obtained according to concentration and temperature with a fixed time of 75 minutes. The examination of this figure 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.

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 (Y₁) and CAK-S (Y₃) are given by Equations (12) and (13).

Y 1 = 72.0488 − 3.299650 X 1 − 0.030175 X 2 − 0.215706 X 3     + 0.322817 X 1 2 + 0.001054 X 1 X 2 + 0.013986 X 1 X 3     − 0.000023 X 2 2 − 0.000288 X 2 X 3 − 0.000199 X 3 2 (12)

Y 3 = 40.604 − 0.024779 X 1 + 0.007747 X 2 + 0.001594 X 3     + 0.219102 X 1 2 − 0.002458 X 1 X 2 − 0.001083 X 1 X 3     − 0.000006 X 2 2 − 0.000097 X 2 X 3 + 0.000203 X 3 2 (13)

The results of the ANOVA show that the terms X₂, X₃, X₁X₃ and X₂X₃ are significant for the CAK-P and X₂, X₃, X₁X₂ and X₂X₃ are significant for CAK-S. According to Figure 3(c) and Figure 3(d), an increase in carbonization temperature leads to a decrease in yield, while the concentration of activating agents and the residense time have a less significant effect on the response. This decrease in yield can be attributed to the fact that an increase in the calcination temperature increases the amount of carbon combusted and the continued release of volatile material, as is reflected by the loss of material from the precursor. The maximum yield was achieved when the carbonization temperature and residence time were maintained at their lowest values. The results obtained are consistent with those of the literature [18] [26] and it can be concluded that temperature has a greater impact on yield compared to the concentration of activating agents and the time of calcination on the performance of CAK-P and CAK-S.

The optimal conditions for the production of the activated carbons were deduced from the maximum yield values and the iodine number. The experimental values of the responses were analyzed by Statgraphic plus 5.0 software and the results are presented in 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 H₃PO₄ and 400˚C carbonization temperature for CAK-P; and 3.5 mol·L⁻¹ of H₂SO₄ and 700˚C for CAK-S.

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.

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.

The textural structure of the activated carbons was determined by the BET method, using relative pressure (P/P₀) from 0.005 to 0.4. Figure 6 and Figure 7 show the analyses on porosity for CAK-P and CAK-S respectively. According to the classification of IUPAC and taken up by Francisco et al. (2018) [29], the adsorption and desorption isotherm of N₂ is Type I and H4 hysteresis illustrates the coexistence of micropores and mesopores. The result shows that these two activated carbons have more micropores than mesopores. The results are summarized in Table 7. The measurement of specific surfaces according to the Brunauer-Emmett-Teller analysis (BET) yielded 457.38 m²·g⁻¹ and 490.62 m²·g⁻¹ respectively for CAK-P and CAK-S. Thus the microporous surfaces are 522.55 m²·g⁻¹ and 570.56 m²·g⁻¹ and the mesoporous ones are 9.1974 m²·g⁻¹ and 10.779 m²·g⁻¹ respectively for CAK-P and CAK-S. Therefore, the two activated carbons 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].

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

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.