Activated carbons (ACs) were prepared using palm nut shells collected in the city of Franceville, in the Haut-Ogooué region of Gabon. The entire preparation procedure of the ACs was reported in a recent study [8] . However after impregnation at a ZnCl₂ solution to 1 M (1:1 ratio (ZnCl₂: Acros Organics, purity > 98.5%) and drying, the solids were activated at 400˚C, 500˚C, and 600˚C for 1 h and 30 minutes (min) with a heating rate of 5˚C·min⁻¹. The activated carbons obtained were cooled, rinsed with 0.1 M HCl (HCl: Emsure, purity: 33%) and washed with distilled water until the pH of the residual water being equal to 6.5. Then dried in the oven for 48 h at 100˚C. The activated carbons obtained at 400˚C, 500˚C, and 600˚C were indexed AC-400, AC-500, and AC-600, respectively ( Figure 1 ).

1) N₂ physisorption

Data on the surface areas and pore volumes were carried out using a Micromeritics TRISTAR 3000 instrument following method described in the work of Belin et al. [2] . About 100 mg of activated carbon sample was pretreated during 1 h at 90˚C, and 10 h at 350˚C successively. Physisorption isotherms of nitrogen (N₂) were carried out at −196˚C. The specific surface areas of samples were evaluated by means of the Brunauer, Emmett and Teller (BET) theory [23] . Microporous and mesoporous volumes were determined by t-plot method of De Boer and Dubinin-Radushkevich equations respectively [24] - [26] .

2) Iodine number

The iodine number test was performed on prepared activated carbons based on the ASTM D4607-94 method [27] . 10 mL of hydrochloric acid (1 N) was mixed with different masses of activated carbon (0.1, 0.3, and 0.5 g), stirred and boiled for 30 seconds. Once the mixture cooled, 50 mL of an iodine solution (0.02 N (Fisher Scientific)) was added. The mixture was then stirred vigorously for 30 seconds and then vacuum-filtered. The filtrate was then titrated with a sodium thiosulfate solution, Na₂S₂O₃ (0.1 N (Na₂S₂O₃: Labosi, purity > 99%)), until stained. 2 mL starch (1 g·L⁻¹) were then added to the titrated filtrate and then the titration was continued until the solution was completely discolored.

The residual concentration of iodine is given by the following relationship (2):

$C_r=\frac{C\cdot V_{eq}}{V}$ (1)

With, C_r: residual concentration of I₂ (N); C: concentration of Na₂S₂O₃ (N); V_eq: volume of Na₂S₂O₃ (mL) and V: volume of titrated filtrate (mL).

The residual concentration of I₂ should be between 16 × 10⁻⁴ N and 8 × 10⁻³ N. Thus, the amount of iodine adsorbed is given by the following formula (2):

$Q_{I_2}=\frac{\left[A-\left(DF\cdot B\cdot V_{eq}\right)\right]}{m_{CA}}$ (2)

where, A is the concentration of I₂ (mol·L⁻¹) × Molar mass of iodine (g·mol⁻¹); B is the concentration of Na₂S₂O₃ (mol·L⁻¹) × Molar Mass of Iodine (g·mol⁻¹), DF is the dilution factor ( $\frac{\left(V_{I_2}+V_{HCl}\right)}{V_{\text{Filtrate}}}$) and m_AC is activated carbon mass (g).

By plotting the curve $Q_{I_2}$ = f(C_r), the iodine number is then the value of $Q_{I_2}$ (mg·g⁻¹) corresponding to a residual iodine concentration of 0.04 N.

3) Methylene blue number

The method used was the same as that proposed by the European Centre of Chemical Industry Federations (ECCIF) [28] . 25 mL of methylene blue (M.B) solution (120 mg·L⁻¹ (M.B: Alp’Osmose, purity > 98.2%)) were put in contact with 0.1 g of activated carbon and then stirred (200 rpm) for 20 minutes. The mixture was then filtered on Büchner and the residual concentration of methylene blue was determined by colorimetry using a Thermo Scientific Evolution 60S spectrophotometer, using a previously established methylene blue calibration curve, at a wavelength λ = 664 nm.

The methylene blue number is given by the following relationship (3):

$Q_{BM}=\frac{\left(C_0-C_f\right)V}{m_{AC}}$ (3)

where, Q_BM is the methylene blue number (mg·g⁻¹); C₀ is the initial concentration of methylene blue (mg·L⁻¹); C_f is the residual concentration of methylene blue (mg·L⁻¹); m_AC is the activated carbon mass (g) and V is the volume of the methylene blue solution in (L).

1) pH at point of zero charge (pH_pzc)

The pH at point of zero charge (pH_pzc) of activated carbons were obtained based on the acid-base titration method described by Amola [29] and reproduced in in a recent study [8] . The pH_pzc of activated carbons are given by the point of intersection between the experimental curves and the theoretical curves of pH_final = f(pH_initial).

2) Surface Functional Groups

The quantification of surface functional groups of activated carbons was performed using the Boehm method [30] , with little modifications applied in a recent study [8] . The number of equivalents (m_eq·g⁻¹) or concentrations (mmol·g⁻¹) of acid or basic surface functional groups were calculated using the following Equation (4):

$n\left(\text{mmol}\cdot {\text{g}}^{-1}\right)=\frac{C\times \left(V_b-V_s\right)\times 1000}{m}$ (4)

where C is the concentration of NaOH (Emsure, purity > 99%) or HCl (mol·L⁻¹); V_b and V_s are the equivalent volumes of the blank and sample (L) respectively; m is the mass of activated carbon (g) and 1000 is the conversion factor (mmol).

The study of the effect of calcination temperature on porosity was conducted based only on data from the methylene blue and iodine numbers and those obtained from the N₂ physisorption study of prepared ACs.

Iodine number is an important parameter in the characterization of activated carbons [23] . Indeed, it provides information on the ability of activated carbons to trap or adsorb small molecules and therefore provides information on the microporosity of the material [31] [32] . Figure 2 shows the iodine number values obtained on activated carbons calcined to 400˚C, 500˚C, and 600˚C (AC-400, AC-500, and AC-600 respectively). Regardless of the calcination temperature applied, the prepared ACs developed micropores in their structures. The iodine number obtained vary between 360 - 680 mg·g⁻¹. In contrast to the work of Huang et al. [31] , the results in Figure 2 indicate an increase in the iodine number as the calcination temperature (T_cal) increases from 400˚C to 500˚C and a decrease as T_cal increases from 500 ˚C to 600˚C. Indeed, the values of the iodine number obtained on AC-400 and AC-500 (460 and 680 mg·g⁻¹ respectively) are higher than those obtained on AC-600 (360 mg·g⁻¹).

In our experimental conditions, it seems that at 500˚C (AC-500) we develop more adsorption sites favorable to interactions with iodine and/or microporosity likely to trap/adsorb small molecules, compared to AC-400. In addition, when T_cal increases from 500˚C to 600˚C, a loss of adsorption sites or microporous surface is observed. This loss of microporous surface could be related to collapse of the microporous structure at 600˚C or clogging of the microporous due to ash formation during calcination at this temperature [31] .

Figure 3 shows the methylene blue number values obtained on adsorbents studied. Prepared ACs appear to have developed mesopores within their structures but very negligible compared to micropores ( Figure 2 ). Indeed, the values of the methylene blue number range between 5.72 and 8.90 mg·g⁻¹. As in the case of the iodine number, the methylene blue number also increases as the calcination temperature (T_cal) increases from 400˚C to 500˚C and decreases from 500˚C to 600˚C ( Figure 3 ). There is also a loss of adsorption sites or mesoporous surfaces, on AC-600, favorable to the trapping of big molecules, such as methylene blue, when T_cal = 600˚C.

In our experimental conditions it appears that, regardless of T_cal, the prepared ACs seem to develop mainly a microporous structure. This parameter appears to be related to the activation agent (ZnCl₂) used in the ACs activation process. Although little is known about the mechanism of this process, it is known in the literature that ZnCl₂ used as an activating agent promotes pore creation during the calcination process in the preparation of ACs [33] - [35] .

The results obtained are in agreement with the work of Molina-Sabio et al. [36] , who used ZnCl₂ as an activating agent in the preparation of activated carbons. Similarly, they observe a loss of porosities when the carbonization temperature is above 500˚C.

Figure 4 shows the N₂ physisorption isotherms obtained on the prepared ACs. The isotherms obtained are type I, characteristic of microporous materials [37] , regardless of T_cal.

Table 1 summarizes the specific surface areas (S_BET), average diameters and pore volumes obtained after exploitation of isotherms of N₂ physisorption of studied adsorbents.

S_BET: specific surface BET. V micro: microporous volume. V_meso: mesoporous volume.

According to results, a decrease in S_BET and porous volumes is observed when the T_cal increases from 500˚C to 600˚C and an increase from 400˚C to 500˚C. Indeed, the S_BET and microporous volume obtained on AC-600 (116.2 m²·g⁻¹ and 0.046 cm³·g⁻¹ respectively) are lower than those obtained on AC-400 (182.4 m²·g⁻¹ and 0.077 cm³·g⁻¹ respectively) and AC-500 (210.3 m²·g⁻¹ and 0.085 cm³·g⁻¹ respectively). The results of the N₂ physisorption study are in agreement with those obtained in the quantification of the methylene blue and iodine number. Indeed, prepared ACs have predominantly microporous structures with negligible mesoporosity. On the other hand, the average pore sizes obtained on the adsorbents studied are comparable ( Table 1 ). Therefore, there is no effect of calcination temperature on pore size in prepared ACs.

Based on the study of the porosities of prepared ACs, adsorption being a surface phenomenon, AC-500 adsorbent has the best surface and pore volume properties ( Table 1 ) for use in solid adsorption experiments. It appears, in our experimental conditions, that 500˚C is the maximum calcination temperature to be applied in the preparation of ACs. Above 500˚C, a loss of surface area and micropores volume would be possible.

In order to evaluate the effect of calcination temperature on the chemical properties of prepared ACs, acidic and basic surface functional groups have been quantified and pH at point of zero charge (pH_pzc) determined.

The pH_pzc is used to determine the acid-base character of prepared ACs and to evaluate the overall charge of the material according to the pH of the media [38] [39] . The surfaces of the activated carbons will be charged + or – when the pH of the treated media is < and/or > at pH_pzc [18] [29] . Figure 5 shows the pH_pzc values determined on prepared activated carbons.

The pH_pzc values obtained vary between 5.60 - 6.85. The pH_pzc increases as T_cal increases and tends towards neutral pH values. Indeed, the pH_pzc obtained on AC-500 and AC-600 (6.85 and 6.70 respectively) are close to the pH value = 7 and higher than the pH_pzc of the AC-400 adsorbent (5.60). Based on these results and in our experimental conditions, the pH_pzc of prepared ACs tends towards neutrality as the calcination temperature increases. However, it appears that regardless of T_cal, prepared ACs retain an acidic character, in particular the adsorbent AC-400 ( Figure 5 ).

Figure 6 shows the concentrations of the acidic (carboxylic, phenolic, etc.) and basic (pyrones, chromenes, etc.) surface functional groups quantified on AC-400, AC-500, and AC-600. Acidic functional groups are predominant on the surface of prepared ACs. Indeed, their concentrations vary between 2.80 - 3.58 mmol·g⁻¹ and those of the basic functional groups between 0.21 - 0.83 mmol·g⁻¹. These results are in agreement with those of pH_pzc ( Figure 5 ). Prepared adsorbents are mainly acidic in character. In the work of Zue Mve et al. [40] , by preparing of activated carbons with ZnCl₂ as an activating agent, they also obtained the majority of groups of acidic functions on the surface of the solids. Therefore, it appears that ZnCl₂ promotes the activation of acidic functional groups on the surfaces of prepared ACs. The works of Thouraya et al. [41] has also revealed an increase in acidic surface functional groups when the activating agent is acidic. This study also that showed that the results of acidic and basic functional groups on the surface of ACs obtained by Boehm titration were in agreement with those obtained by FTIR spectroscopy. According to results of this study, the wide transmittance band appearing at 3423 cm⁻¹ is related to the OH stretching vibration mode in alcohol and phenol. The peak at 2359 cm⁻¹ could be attributed to CN stretching. Bands appearing between 1622 and 1543 cm⁻¹ are ascribed to C–C vibrations in aromatic rings. The band between 1340 - 1360 cm⁻¹ may be attributed to the aromatic CH and carboxyl–carbonate structures. The bands located at 1111 cm⁻¹ and 1052 cm⁻¹ are related to C–O stretching in alcohols and phenols. The bands observed between 1000 cm⁻¹ and 500 cm⁻¹ are due to the out-of-plane deformation mode of C–H for alkene aromatic rings.

The results do not show significant variation in the acid and basic functional groups of prepared ACs as T_cal increases ( Figure 6 ). However, on AC-500, there was a slight decrease in the amounts of acid surface functional groups (2.80 mmol·g⁻¹), compared to amounts obtained on AC-400 and AC-600 (3.52 and 3.23 mmol·g⁻¹ respectively).

Thus, in our experimental conditions, the calcination temperature has a negligible effect on acidic and basic surface functional groups of studied materials. This parameter appears to be related to the activation agent used.