Optimization of Activated Carbons Prepared from Parinari macrophylla Shells

Plant matter constitutes an important source for producing carbonaceous materials. This work deals with the preparation of active carbons from shells of Parinari macrophylla (agricultural waste in Niger). Physical, chemical and mixed activations are considered. Several parameters of preparation are optimized, as the nature of the activation gas (N 2 or CO 2 , dry and wet), the concentration of the activating agent (H 3 PO 4 ), the time of impregnation and the pyrolysis temperature program. The active carbons are characterized through their iodine numbers, their specific surface areas and their porous volumes. Active carbons, produced from shells of Parinari macrophylla display iodine numbers up to 599 mg I 2 /g and specific surface areas up to 727 m 2 /g. They also show microporous characteristics, with a mean pore diameter, usually, lower than 20 Å and a microporous surface percentage up to 88.7% and a microporous volume percentage up to 82.1%. The microporosity is far more developed for the active carbons produced by chemical activation.

), but the active carbons, synthetized that way, are free from impurities left by the activating agents used in chemical activations [5] [6] [7] [8]. Thus, meeting the requirements of sustainable development, active carbons are produced from agricultural by-products, such as coconut shells [5] [6], date stones [2], coffee grounds [8], Balanites aegyptiaca shells [7], sugarcane bagasse and sunflower seed hulls [9]. Several works have already studied the production and the characterization (specific surface areas, pores size distributions, crystalline state, surface functions) of A.C. from various biomasses [1] [2] [3] [4] [5] [8] [9] [10] [11] [12]. These characteristics are important for the use of A.C. as adsorbents in purification and depollution applications [3] [9] as well as catalyst supports [13] [14]. As the raw materials show differences in composition (carbon percentage), they do not behave the same way towards the activating agents and/or the temperature. The nature of biomass has thus a major impact on the characteristics of the end product.
The shell of Parinari macrophylla is a waste hardly biodegradable which represents an environment problem. Parinari macrophylla (a.k.a. Neocarya macrophylla or cayor apple tree) is a tree whose height ranges from 6 m to 10 m. It belongs to the Chrysobalancea family and to the Parinari genus. The plant is native to West and Central Africa [15] [16] [17]. This study aims to valorize these shells by producing active carbons. We intend to optimize the activation procedure and to characterize the end products regarding their adsorption capacities, their specific surfaces areas and their porous textures. The use of this raw material meets two goals, producing cheap quality AC from local agro-sourced material and valorizing wastes so that they gain an added value [7].

Production of the Active Carbons
The Parinari macrophylla (PM) shells were ground and sieved. The active carbons were produced using the granulometric fraction ranging from 2.5 mm to 5 mm.

Physical Activation
The physical activation was carried out in a horizontal furnace (Lenton Thermal Design, CSC 12/600H), under a flow of nitrogen or of carbon dioxide (80 mL/min). The temperature was slowly risen with a ramp of 5˚C/min up to the final temperature of 800˚C/min which was maintained for 2 hrs. During the heating, two temperature plateaus were kept at 150˚C for 30 min. and at 400˚C for 1 hr (Figure 1). The goal of the step at 150˚C was to dry the biomass and was always performed under a flow of nitrogen. The flowing gas was changed to carbon dioxide, if needed, after this drying step. The step at 400˚C was used to  help the decomposition of the organic matter of the biomass. Its other purpose was to slow down the temperature rise, which is known to favour the production of more porous active carbons [18] [19] [20]. After the heating at 800˚C, the furnace was let to cool down to room temperature, always under a flow of nitrogen.
Following the same operating conditions, as described above, we have pro-

Mixed Activation
The mixed activation consists in the impregnation of a chemical activating agent (H 3 PO 4 ), followed by a pyrolysis, carried out in the same conditions as a physical activation.
We have studied the 24 hrs impregnation of the orthophosphoric acid at three concentrations (1.5 mol/L, 3 mol/L and 5 mol/L). As for the chemical activation, the ground biomass was put into contact with H 3 PO 4 , with a ratio biomass (g)/acid solution (mL) of 1:3. After the impregnation, the samples were filtered and dried overnight at 110˚C. The pyrolysis was carried out following the same procedure as the physical activation, described above. The A.C. thus obtained were thoroughly washed with deionised water in a Soxhlet extractor, until the pH of the water, in contact with the sample, reached a value of 7.
Samples are identified as follows: Mx (for mixed activation)-H 3 PO 4 as the chemical activating agent (H 3 PO 4 concentration)-pyrolysis gas (N 2 or CO 2 ). For 1.5M, whose pyrolysis was performed under a flow of nitrogen.

Mass Yield of the Pyrolysis
The pyrolysis yield (y) is related to the activation mode and to the source material. It is defined as the ratio between the final A.C. mass (m f ) and the initial mass of the biomass (m i ). The mass loss, caused by the activation process, also called burn-off, is given in Equation (1). The sum of the yield and the burn-off

Iodine Number
The adsorption capacity of active carbons may be evaluated through their iodine numbers. The iodine number measures the quantity of iodine (in mg) adsorbed by 1 g of the A.C., in fixed conditions. The method used in this work is based on the ASTM norm D4607-94 [21]. The sample is firstly boiled for 30 seconds in 10 mL HCl 5% to get rid of any sulfur compounds that might be present in the solid and that would interfere with the analysis. 100 mL of I 2 0.1 N is then put into contact with the sample during 30 seconds and the suspension is filtered immediately. The iodine in the filtrate is titrated by a solution of sodium thiosulfate 0.1 N. Normally, the ASTM norm requires the titration of the residual iodine after adsorption on 3 different masses of sample. The iodine number is, by definition, the quantity of adsorbed iodine (in mg/g) corresponding to a residual iodine concentration of 0.02 N. Nevertheless, it does exist tables that give the iodine number based on the volume of thiosulfate necessary to titrate the residual iodine after adsorption on a single mass of solid. These tables exist for sample masses of 1 g, 1.5 g, 2 g and 3 g.

Nitrogen Adsorption at 77 K
The isotherms of the adsorption of nitrogen at 77 K were recorded using a Micromeritics Gemini VII device. The BET treatment of the isotherms led to the determination of the specific surface areas, the mean pore diameters and the total pore volumes (measured at P/P˚ = 0.99). The t-plot treatment of the data enabled the determination of the microporous surface areas and volumes.

Mass Yield of the Pyrolysis
The mass yield ranges are presented, for the three activation modes, in Table 2.
These results do not allow to prefer the physical activation over the other activation modes (mean yield of 25%  the loss of volatile matters, and it leads to a rigid carbonaceous matrix, as stated by Zhao et al. [22]. In the case of the chemical activation, a decrease of the pyrolysis yields is logically observed when the temperatures get higher (from 45% at 300˚C to 38% at 400˚C and 34% at 500˚C, all other parameters being held at their reference values).

Iodine Number
The A.C. adsorption capacity, including the microporosity, in a condensed phase (aqueous) is estimated through the iodine number. Figure

Nitrogen Adsorption at 77 K
The nitrogen adsorption at 77 K has enabled to determine the BET specific surface area, BET microporous surface area, total and microporous volumes of the produced A.C. All these characteristics are listed in Table 3.   13 and 14). The micropores remain the major porosity type of all the A.C. produced, even for the solids with the lowest specific surface areas (entries 12 and 15). Indeed, all the chemically activated carbons display a microporous surface area representing more than 74% of the total area, a microporous volume representing more than 63% (sample which has the lowest porosity is entry 12) of the total pore volume and a mean pore diameter lower than 20 Å. The same observations can be made for the mixed activations when considering the phosphoric acid concentrations (entries 21, 22 and 23 as well as entries 24, 25 and 26). Excepted for the lowest phosphoric concentration, nitrogen seems to lead to lower specific surface areas than carbon dioxide (entries 22 vs. 25 as well as entries 23 vs. 26). Microporous surface and microporous volumes are often, higher than 80%. This activation mode has allowed to produce active carbons displaying specific surface area values similar to the values obtained for the A.C. produce by chemical activation, but while using lower concentrations in H 3 PO 4 . It is to be noted that the mixed activations, using CO 2 , give A.C. with higher specific surface areas than when N 2 is used. Chemical and mixed activations generally lead to active carbons with similar porosities whereas the ones produced by physical activation rather develop a lower porosity. Indeed, the orthophosphoric acid, used during the chemical and mixed activations, plays a major role in the pore formation. It takes part in the depolymerization reactions involving the macromolecules of the biomass (degraded in CO, CO 2 and CH 4 ), as well as in the condensation and cyclization reactions of the polyaromatic structures [24], which lead to the formation of a porous and rigid carbonaceous material. On the contrary, during the physical activation, the reactions at high temperature (up to 800˚C), under flows of CO 2 or N 2 alone, do not allow to get the same level of pore formation [24]. Furthermore, it is to be noted that the best active carbons, produced from the pyrolysis of Parinari macrophylla shells, have characteristics similar to these of some active carbons produced from others agricultural wastes [1] [2] [12] [20] [25] and of some commercial active carbons, which have generally higher pore volumes though (frequently close to 0.4 cm 3 /g or higher, while the active carbons produced for this work reach 0.3 cm 3 /g at best). For instance, the active carbon supplied by Merck (ref. 2184) show a specific surface area of 469 m 2 /g and a pore volume of 0.38 cm 3 /g, or the active carbon Lurgi Hydrafin show a specific surface area of 762 m 2 /g and a pore volume of 0.40 cm 3 /g.
Nevertheless, the nature of the pyrolysis gas has no effect on the percentage of

Relation between the Iodine Number and the BET Specific Surface Area
Globally, the iodine numbers change alongside the BET surface areas. Although the two measures do not characterize exactly the same property of the solids, they both give an indication of the adsorption capacity for small size molecules. Indeed, both analyses are based on the adsorption of small molecules, N 2 having a molecular radius of 2.1 Å and a cross-sectional area of 16.2 Å 2 , and I 2 having a molecular radius of 3.3 Å and a cross-sectional area of 22.6 Å 2 . These dimensions give them a good accessibility to the majority of the active carbon porosity, including the microporosity (pore diameters lower than 20 Å) despite the fact that the N 2 adsorption occurs in gas phase and the I 2 adsorption occurs in condensed aqueous phase. It is thus legit and interesting to plot the graph of specific surface area vs iodine number and micropore surface area vs iodine number. These plots are presented in Figures 5-7 for the A.C. produced by physical, chemical and mixed activations, respectively.   For a given activation mode, the linear regressions "total surface area vs iodine number" and "micropore surface area vs iodine number" are basically parallel, which indicates that there is, as well, a good correlation between the specific surface area and the micropore surface area. It is thus possible to estimate, roughly, the specific surface area or the iodine number if the other one is known, and it is also possible to estimate the micropore surface area from either the total surface area or the iodine number.

Relation between the Pyrolysis Yield, the Iodine Number and the Specific Surface Area
The porosity of the A.C. is created simultaneously with the decomposition of the biomass, during the pyrolysis and the activation processes. In the case of Parinari macrophylla as the source material, the production of the A.C. goes along with a mass loss of 55% to 77%, corresponding to pyrolysis yields of 45% and 23% respectively (Table 2). However, the highest values of surface area or iodine number are not connected to the highest mass losses, and this observation is true for all the activation modes.
Moreover, the chemical activation with H 3 PO 4 leads to higher values of specific surface areas and iodine numbers, albeit with a higher pyrolysis yield (thus less mass loss), than the physical activation. This confirms that the orthophosphoric acid, as an activating agent, delays the decomposition of the biomass while it contributes to the formation of a rigid carbonaceous network [22], which can be explained by the creation of P-O-C bonds [12] [28] [29]. H 3 PO 4 hence takes part in the biomass decomposition and also in the pore creation process. Marsh and Rodriguez-Reinoso [24] have suggested a reaction mechanism for the effect of H 3 PO 4 on lignocellulosic biomass. It implies the formation of phosphoric acid esters at temperatures lower than 450˚C, the elimination of the acid at temperatures above 450˚C and the creation of ink bottle shaped pores as H 3 PO 4 is released. However, a fraction of the acid remains in the A.C., as proved by the presence of FTIR signals corresponding to P-O-C bonds.

Conclusions
The present study proves that one can produce active carbons from Parinari macrophylla shells. The analysis of the isotherms of nitrogen adsorption shows At this stage of the research, the solids obtained by physical activation alone are not really interesting, although they develop some porosity. Indeed, they show the lower pyrolysis yields and their specific surface areas, as well as their iodine numbers, are too low to consider them as proper active carbons.
Concerning the chemical activation, this method has given the porous solids displaying the highest specific surface areas, iodine numbers and pore volumes, characteristics that fit more what can be expected from active carbons. However, a major drawback of this activation mode is the use of chemicals (orthophosphoric acid in this case), often at high concentration, which can be harmful to the environment and to the properties of the active carbon surface.
The mixed activation has led to obtain porous solids with physico-chemical properties similar to the ones observed after chemical activations, but with the benefit of using a more dilute activating agent. Therefore, it would be worthwhile to carry on investigating this activation method.
The usefulness of the active carbons prepared from Parinari macrophylla shells has to be evaluated through their performances as adsorbents or as catalyst supports. This could be of high economic and ecological interest as this could be an outlet to recycle an agricultural waste.