The moisture content was determined by simple oven drying according to the standard method (ASTM D1763-84). An empty crucible was weighed (m_o). A mass of 8 g (m_i) of sample was weighed into a crucible and the whole was placed in the oven at 110˚C for 24 h. After removing from the oven, the assembly was cooled to room temperature in a desiccator and reweighed (m₁). Weighing was carried out until a constant mass was obtained. The moisture content was calculated from Equation (1):

Moisturecontent ( % ) = ( m i − ( m 1 − m o ) ) × 100 m i (1)

m_o: mass of the empty crucible before drying (g); m_i: mass of the sample before drying (g); m₁: mass of the whole (crucible + sample) after drying (g).

The ash is the grey or whitish residue from the complete incineration of solid biofuels to a constant mass. The ash content was determined according to the standard method (ASTM D2866-11). An empty crucible with lid was weighed (m_o). A mass of 1 g (m_i) of sample was weighed into the crucible and the assembly was placed in an oven set at 650˚C for 4 h. After cooling to room temperature, the assembly was weighed again (m₁). The ash content was calculated from Equation (2):

Ashcontent ( % ) = ( m 1 − m o ) × 100 m i (2)

m_o: mass of the empty crucible before carbonization (g); m_i: mass of the sample before carbonization (g),

m₁: mass of the whole (crucible with lid + sample) after carbonization (g).

The volatile matters are the gases and vapors released from the sample during the heating process. The volatile matters content was determined according to the standard method (ASTM D5832-98). An empty crucible with lid was weighed (m_o). A mass of 1 g (m_i) of sample was weighed into a crucible and the whole was placed in an oven set at 900˚C for 7 min. After cooling to room temperature, the assembly was reweighed (m₁). The volatile matter content was calculated from Equation (3):

Volatilematterscontent ( % ) = ( m i − ( m 1 − m o ) ) × 100 m i (3)

m_o: mass of empty crucible before carbonization (g); m_i: mass of the sample before carbonization (g); m₁: mass of the whole (crucible with lid + sample) after carbonization (g).

The fixed carbon content was calculated from Equation (4):

Fixedcarbon ( % ) = 100 − Moisturecontent − Volatilematters − Ashcontent (4)

The iodine value is an important parameter for characterizing the adsorption capacity of carbons. It evaluates the adsorption capacity of very small molecules and is defined as the quantity, in milligrams, of iodine adsorbed by 1 g of activated carbon in an iodine solution whose normality is 0.02 N [9] . Thus, 0.1 g of activated carbon was brought into contact with 20 mL of a 0.01 M iodine solution in a 250 mL Erlenmeyer flask. The mixture was stirred for 30 min and then filtered. After filtration, a 10 mL volume of this filtrate was titrated with a 5 × 10⁻³ M sodium thiosulphate solution (Na₂S₂O₃∙5H₂O) in the presence of starch as an end-of-reaction indicator. The iodine number (IN) was obtained from Equation (5):

IN ( mg / g ) = ( C i − C f ) × V m (5)

C_i: concentration of iodine before adsorption (mol/L); C_f: concentration of iodine after adsorption (mol/L); V: volume of the solution (L); m: mass of activated carbon (g).

The methylene blue number, expressed in mg/g, is defined as the quantity in mg of methylene blue adsorbed by 1 g of activated carbon. It indicates the capacity of activated carbon to adsorb medium-sized molecules [9] . Thus, 0.1 g of activated carbon was brought into contact with 30 mL of a 500 mg/L methylene blue solution in a 250 mL Erlenmeyer flask. The mixture was stirred for 4 h and then filtered. After filtration, the solution was determined by UV-visible spectrophotometer at a maximum wavelength of 664 nm. The value of the methylene blue number (MBN) was obtained from Equation (6):

MBN ( mg / g ) = ( C o − C e ) × V m (6)

C_o: initial concentration of the methylene blue solution (mg/L); C_e: residual concentration of methylene blue at equilibrium (mg/L); V: volume of methylene blue solution (L); m: mass of activated carbon (g).

The activated carbons yield reflects the mass loss of the precursor during the preparation of the activated carbon. It was determined from Equation (7):

ACYield ( % ) = ( massofactivatedcarbon ) × 100 massofprecursor (7)

The pH of zero-point charge (pHzpc) defines the pH at which the overall surface charge of a solid material is zero. Below the pHzpc value, the surface carries a positive charge and above the pHzpc the surface is negatively charged [8] . Solution of 0.1 M sodium chloride was prepared at different pH values (2 to 10). The pH values were adjusted with a pH meter using 0.1 M sodium hydroxide and 0.1 M hydrochloric acid solutions. Then 0.1 mg of activated carbon was mixed with 30 mL of each solution in 250 mL Erlenmeyer flasks. The mixture was stirred for 72 h and then filtered with filter paper. After filtration, the pH was measured again. The pHzpc value was determined by plotting ΔpH = pH_f − pH_i = f (pH_i).

The determination of the surface functional groups was carried out by the Boehm method, which corresponds to the acid-base titration. The basic groups are determined as a whole, while the acid groups are determined separately. A mass of 0.1 g of activated carbon was mixed with 30 mL of each of the aqueous solutions of sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), potassium bicarbonate (KHCO₃) and 0.1 N hydrochloric acid (HCl). Each solution was stirred for 72 h and then filtered with filter paper. Then, 5 mL of the filtrate of each solution was determined. The basic solutions were determined by hydrochloric acid and the acidic solution by sodium hydroxide. Phenolphthalein and helianthin were used as indicators.

In addition, Table 1 and Table 2, below, present respectively the chemical reagents and the apparatus used for the realization of the various experiments.

The Fourier transform infrared spectroscopy is a very important analysis for the study of functional groups on the surface of a material. It is based on the irradiation of a sample with infrared (IR) radiation whose absorbed adsorption wavelengths are characteristic of the different chemical groups or atoms present on its surface. The IR analyses were carried out using a BRUKER infrared spectrometer with retention of 4 cm⁻¹ in the 4000 - 400 cm⁻¹ range. The samples and the potassium bromide (KBr) were pre-dried at 100˚C for 24 hours before analysis. The KBr and samples were prepared using the “disc technique” by using a finely ground mixture of 0.25 mg sample with 100 mg KBr. The pellets thus formed by uniaxial compression were placed in the sample holder of the apparatus for

analysis. The results of the transmittance versus wavenumbers are read directly from the screen. Subsequently, the spectrogram obtained was analyzed to identify the functional groups on the surface of the samples.

The influence of the pyrolysis temperature (Figure 2(a)) and the concentration (Figure 2(b)) on the activated carbons yield is shown in Figure 2.

Figure 2 shows that the yield values range from 43.35% to 39.7% for 10% activated ACP, from 27% to 21.5% for 20% activated ACP, from 29.5% to 22.5%

for 30% activated ACP and from 33% to 26% for 40% activated ACP when the temperature varies from 400˚C to 700˚C. In all cases, the percentage of activated carbons yield decreases with increasing pyrolysis temperature and the high yield values were obtained at low temperature (10˚C). This decrease could be explained by the loss of volatile matter under the effect of temperature, in the presence of H₃PO₄. Indeed, the transformation of precursors into activated carbon involves the loss of oxygen (O) and hydrogen (H) atoms in the form of volatile compounds (H₂O, CO, CO₂, CH₄) [10] . In addition, the low rate of decrease in activated carbons yield could mean the formation and stabilization of activated carbons [10] . On the other hand, increasing the concentration of phosphoric acid increases the activated carbons yield. This can be explained by the fact that H₃PO₄ is a dehydrating agent that delays thermal decomposition, limiting the loss of volatile matter and leading to the formation of a rigid carbon matrix [17] . The difference observed could be explained by a certain loss of mass of activated carbons during washing.

The influence of the pyrolysis temperature (Figure 3(a)) and the impregnation rate (Figure 3(b)) on the activated carbons yield is shown in Figure 3.

Figure 3 shows that the activated carbons yield values range from 11.4% to 8.85% for ACK activated at 1, from 9.25% to 5.9% for ACK activated at 1.5, from 6.9% to 4.8% for ACP activated at 2 when the temperature is increased from 400˚C to 700˚C. It is observed that the percentage of activated carbons yield decreases with the increase of the pyrolysis temperature and the impregnation ratio. This decrease could be explained by the loss of volatile matter under the effect of temperature in the presence of KOH, as previously with H₃PO₄. Furthermore, the activated carbons yield decreases with the increase of the impregnation ratio. This decrease could be explained by a continuous release of tars inside the pores. These results are consistent with the results obtained by other authors [14] .

The influence of the pyrolysis temperature (Figure 4(a)) and of the concentration (Figure 4(b)) on the iodine number is shown in Figure 4.

It can be seen from Figure 4 that in all cases the increase in temperature from 400˚C to 700˚C and in concentration from 10% to 40% leads to an increase in the amount of iodine until it reaches a maximum value and then decreases. In the case of temperature, the increase in the amount of iodine as a function of temperature can be explained by the fact that the increase in temperature leads to the elimination of the volatile materials contained in the precursor [8] . This elimination leads to an increase in the adsorption capacity of the activated carbons and therefore the iodine number. On the other hand, the decrease in the value of the iodine number beyond temperatures of 500˚C and 600˚C can be explained by the fact that part of the micropores formed has been destroyed. This decrease in the number of micropores is accompanied by a reduction in the adsorption capacity of activated carbons [8] . In the case of concentration, the

increase in the quantity of iodine could be attributed to impregnation with phosphoric acid, which inhibits the formation of tar and favors the release of volatile matter to produce more micropores [18] . These micropores favor the adsorption of small molecules like iodine. Hence it increases. Furthermore, all ACP achieved maximum iodine content at 20%. But when the concentration is higher than 20%, the amount of iodine decreases. This decrease can be explained, on the one hand, by excessive dehydration and the destruction of the micropores by making them larger due to the very high oxidizing capacity of H₃PO₄ [19] and, on the other hand, by a possible formation of phosphate, via the interaction between the excess of H₃PO₄ and the inorganic elements present in the precursor, under the action of pyrolysis and would lead to the obstruction of the pores, thus preventing further volatilization [19] . The maximum iodine number (850.26 mg/g) was obtained at a temperature of 500˚C for a H₃PO₄ concentration of 20%.

The influence of the pyrolysis temperature (Figure 5(a)) and the impregnation ratio (Figure 5(b)) on the iodine index is represented in Figure 5.

The results in Figure 5 show that the amount of iodine increases with increasing temperature while with concentration it increases to a maximum value before decreasing. In the case of temperature, the amount of iodine in all activated carbons reaches maximum values at 700˚C. The effect of temperature can be explained by the fact that the increase in temperature leads to the elimination of the volatile matter contained in the precursor [20] . In the case of concentration, this could be explained by the creation of micropores during impregnation with the base, which favors the adsorption of small molecules such as iodine [20] . Moreover, the maximum iodine quantity was obtained at 1.5 for all the carbons. But beyond 1.5, there is an enlargement of the micropores and this enlargement is unfavorable to the adsorption of small molecules [21] . The maximum iodine value (865.49 mg/g) was obtained at a temperature of 500˚C for a H₃PO₄ concentration of 20%.

The influence of the pyrolysis temperature (Figure 6(a)) and of the concentration (Figure 6(b)) on the methylene blue number is represented by Figure 6.

From Figure 6, it can be seen that the amount of methylene blue increases proportionally with increasing temperature and concentration. Increasing the temperature leads to a continuous release of the volatiles contained in the precursor, while increasing the concentration enhances the catalytic oxidation of the precursor causing the micropores to enlarge into mesopores [22] . The higher the percentage of mesopores, the higher the adsorption of methylene blue. These results are in perfect correlation with those found for the iodine value, which showed the appearance of mesopores for temperatures and concentrations higher than 500˚C and 20% respectively.

The influence of the pyrolysis temperature (Figure 7(b)) and the impregnation ratio (Figure 7(a)) on the methylene blue number is represented by Figure 7.

Figure 7 shows that the amount of methylene blue increases proportionally with increasing temperature and concentration. As with H₃PO₄ seen previously, increasing the temperature leads to a continuous release of the volatiles contained in the precursor while increasing the ratio enhances the catalytic oxidation of the precursor causing the enlargement of the micropores into mesopores. The best parameters for the preparation of ACP and ACK are summarized in Table 4 below.