Study of the chemical characterization and pore distribution of activated carbons synthesized from local agri-food waste (Niger)

 Corresponding author: Mahamane Nassirou AMADOU KIARI Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution Liscense 4.0. Study of the chemical characterization and pore distribution of activated carbons synthesized from local agri-food waste (Niger) Ousmaila SANDA MAMANE 1, 2, Rabilou SOULEY MOUSSA 2, Mahamane Nassirou AMADOU KIARI 2, 3, 4, *, Maman Hamissou IBRAHIM GREMA 3, 4, Ali SANDA BAWA 1, 2, Ousmane MAHAMAN SANI 1, 2, Maman Mousbahou MALAM ALMA 2 and Ibrahim NATATOU 1, 2 1 National School of Engineering and Energy Sciences, University of Agadez, Niger. 2 Department of Chemistry, Faculty of Science and Technology, Materials/ Water and Environment Laboratory/ Abdou Moumouni University, Niger. 3 Laboratory of Industrial Processes of Synthesis, the Environment and New Energies (LAPISEN), Institut National Polytechnique Félix Houphouët-Boigny, BP 1093, Yamoussoukro, Côte d’Ivoire. 4 African Centre of Excellence for the Recovery of Waste into High Value-Added Products (CEA-VALOPRO), Yamoussoukro, Côte d’Ivoire. GSC Advanced Research and Reviews, 2025, 24(03), 028–044 Publication history: Received on 26 July 2025; revised on 03 September 2025; accepted on 05 September 2025 Article DOI: https://doi.org/10.30574/gscarr.2025.24.3.0263 Abstract The objective of this study is the chemical characterization and pore distribution of activated carbons (ACs) synthesized from local agri-food waste (Niger) by chemical means. The selected waste materials are the shells of Balanites aegyptiaca (BA) (L.) Del. and Hyphaene thebaica (HT) (L.) Mart. Orthophosphoric acid (H3PO4) and sulfuric acid (H2SO4) are used as impregnation agents. Next, the mass yields of ACs after pyrolysis, iodine (I2) adsorption capacities, and methylene blue (MB) adsorption capacities are determined on the one hand, and total pore volume, cumulative pore volume (BJH), microporous volume, and average BET and BJH pore diameters are determined. Analysis of the results shows that the selected agri-food waste is a good precursor for the production of high-quality AC; it has very high cellulose content (67.6857% and 57.0571% for BA and HT, respectively). The pyrolysis temperature, concentration of the activating agent, and nature of the biomass have very significant effects on the yield of synthesized ACs. Activated carbons prepared by H3PO4 activation developed the best adsorption capacities. The total pore volumes range from CAHT-1 (0.306531 cm³ g⁻¹) < CA-HT-2 (0.507390 cm³ g⁻¹) < CA-C (0.529036 cm³ g⁻¹) < CA-BA-1 (0.677426 cm³ g⁻¹) < CABA-2 (0.889648 cm³ g⁻¹). The average pore diameters range from CA-HT-1 (16.9788 Å) < CA-BA-1 (17.3617 Å) < CAHT-2 (17.6995 Å) < CA-C (22.3516 Å) < CA-BA-2 (22.5136 Å). The pore diameters are essentially mesopores. Keywords: Agri-Food Waste; Activated Carbon; Characterization; Porous 1. Introduction In recent years, adsorption has remained one of the most widely used techniques for removing organic pollutants due to its effectiveness, ease of implementation, and affordable investment cost [1, 2,3]. This method requires the selection of an adsorbent with good characteristics (high adsorption capacity, availability, low cost, etc.). Thus, microporous adsorbents are widely used in the extraction of chemical species in aqueous or gaseous phases due to their excellent adsorption capacity a capacity linked to their large specific surface area and the development of their porosity [4, 5, 6]. To this end, the use of activated carbons (ACs) as adsorbents is of interest in the treatment of industrial wastewater. This is justified by the large exchange surface area developed by this material [7]. To date, numerous studies have been conducted on the transformation of solid waste into porous materials, particularly activated carbon (AC) [8]. These GSC Advanced Research and Reviews, 2025, 24(03), 028–044 29 materials have always played a major role in both domestic and industrial activities. Indeed, AC is by far the most widely used porous material, consisting mainly of carbonaceous material with a porous structure. It is a structure generally obtained after a high-temperature carbonization stage of lignocellulosic biomass (carbon-rich raw materials such as wood, peat, coal, lignite, coconut shells, peach pits, etc.), which has a very large specific surface area that gives it a high adsorption capacity [9]. ACs with very high adsorption capacity are particularly useful in water purification [6, 10], sugar decolorization [4, 11], volatile solvent recovery [4, 12], dye fixation, gas treatment [4, 13, 14], and in the process of extracting precious metals (gold and silver) from leaching solutions [10]; gold particles or nanoparticles being adsorbed in the porous matrix of the AC. Despite the availability of biomass in the subregion, our countries continue to import activated carbon for various applications, particularly in ore processing [15,16]. That is why, in this study, activated carbons are synthesized from local lignocellulosic biomass, notably the shells of Balanites aegyptiaca (L.) Del. and Hyphaene thebaica (HT) (L.) Mart. kernels, using chemical methods in order to study their chemical characteristics and pore distributions. Niger has abundant supplies of this biomass, which constitutes agri-food waste that is difficult to biodegrade in tropical countries [17,18]. Furthermore, two (2) activating agents are used in this study: orthophosphoric acid (H3PO4) and sulfuric acid (H2SO4). Next, the mass yields of the ACs after pyrolysis, the iodine (I2) adsorption capacities, and the methylene blue (MB) adsorption capacities are determined on the one hand, and the total pore volume, cumulative pore volume (Vp) (BJH), micropore volume, and average pore diameter (BET and BJH) are determined. 2. Materials and methods 2.1. Thermal decomposition of Balanites aegyptiaca (L.) Del. and Hyphaene thebaica (HT) (L.) Mart. shells The composition and physical properties of biomass are parameters that influence the conversion of lignocellulosic biomass into activated carbon. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on the shells of Balanites aegyptiaca (L.) Del. and Hyphaene thebaica (L.) Mart. to determine the mass decomposition of these materials. They were determined using a brand thermogram (Lab: METTLER STARe SW 8.10). Figure 1 shows the sample of Balanites aegyptiaca (L.) Del. shells after crushing and sieving. Figure 1 Shells of Balanites aegyptiaca (L.) Del. after crushing and sieving 2.2. Synthesis of activated carbons After conditioning the raw materials, the activated carbons are produced in three stages [4, 19]: • impregnation of the biomass in solutions of the activating agent; GSC Advanced Research and Reviews, 2025, 24(03), 028–044 30 • pyrolysis of the impregnated biomass; • purification of the product obtained. In this work, two activating agents are used, namely orthophosphoric acid (H3PO4) and sulfuric acid (H2SO4). To optimize the performance of activated carbons synthesized from Hyphaene thebaica (L.) Mart., the effects of certain activation and pyrolysis parameters were studied. Table 1 shows the effect of activation and pyrolysis parameters. Table 1 Optimization parameters for the production of ACEs Biomass Parameters Hyphaene thebaica (L.) Mart. Concentration of the activating agent (%) 10 20 30 40 50 Pyrolysis temperature (°C) 300 400 500 600 700 The samples of activated carbon synthesized following this optimization are used to determine the mass yields after pyrolysis, the iodine (I₂) adsorption capacities, and the methylene blue (MB) adsorption capacities. 2.3. Activation of the biomass sample En 250 mL beakers, 16 g of the pretreated raw material and 100 mL of the activating agent solution (H3PO4 and H2SO4) are mixed together. The mixture obtained is stirred for 15 hours on a magnetic stirrer (Figure 2) at atmospheric pressure and room temperature. The sample is then filtered on ashless filter paper using a Büchner funnel (Figure 3), washed with distilled water, and dried in an oven at 105°C for 24 hours. Figure 2 Impregnation on magnetic stirrers GSC Advanced Research and Reviews, 2025, 24(03), 028–044 31 Figure 3 Filtering the sample on a Büchner funnel 2.3.1. Pyrolysis of impregnated biomass samples The dry sample obtained after impregnation was placed in a programmable high-temperature muffle furnace (Figure 4). The furnace temperature was gradually increased to the pyrolysis temperature (450°C) at a heating rate of 2.5°C min-1, with an isothermal plateau of 1 hour 30 minutes at the end of heating, representing the residence time in the furnace. Figure 4 Muffle furnace Upon removal from the furnace, the charred samples are cooled in a desiccator (Figure 5). GSC Advanced Research and Reviews, 2025, 24(03), 028–044 32 Figure 5 Cooling of samples in a desiccator 2.3.2. Purification At the end of the production process, the cooled activated carbon is washed thoroughly with hot distilled water until the pH reaches between 6.5 and 7 to remove any impurities, then dried in an oven at 105°C for 24 hours. The processed activated carbon (PAC) is then cooled and stored in airtight containers until characterization tests are performed. 2.4. Chemical characterization of activated carbon 2.4.1. Yield calculation The yield values for activated carbon production are determined using the following formula Yield (%)=Massfinal Massinitial ×100 (1) 2.4.2. Determination of iodine adsorption capacities on synthesized activated carbons The iodine index characterizes the areas accessible to any particle smaller than or equal to the size of the I2 molecule, as well as their adsorption capacity, in particular the mini-micropores accessible to small particles responsible for tastes and odors. To determine iodine adsorption capacity, the procedure used by the Waste Study Center, which is an adaptation of the CEFIC 1989 method and AWWA B 600-78 standard, was used [4]. The experimental protocol is as follows: In a 100 mL beaker, 0.2 g of activated carbon powder (particle size less than 100 µm) was mixed with 20 mL of a 0.02 N I2 solution. The mixture was stirred for 4 to 5 minutes and then filtered using a Büchner filter and ashless filter paper. Next, 10 mL of the filtrate was titrated with a 0.1 N Na₂S₂O₃ solution. Phenolphthalein was used as a color indicator. The iodine index QI₂ is determined using the following formula (2) [5] • Qɪ₂: adsorption capacity of I₂ or I₂ index (mg g⁻¹); • C0: initial concentration of the I₂ solution (mol L⁻¹); • Cthio: concentration of Na₂S₂O₃ (mol L⁻¹); • Vthio: volume of Na₂S₂O₃ at equivalence (mL); • VI₂: volume of iodine titrated (mL); • MI₂: molar mass of I₂ (g mol⁻¹); • Vads: the adsorption volume (mL); • mca: mass of activated carbon used (g). GSC Advanced Research and Reviews, 2025, 24(03), 028–044 33 2.4.3. Determination of methylene blue (MB) adsorption capacities on synthesized activated carbons The MB index, expressed in mg g-1, represents the adsorption capacity of medium-sized molecules for the purpose of evaluating mesopores and macropores. MB adsorption was performed by introducing 0.1 g of CA, previously dried in an oven at 105°C, into a 250 mL Erlenmeyer flask containing 100 mL of the standard MB analysis solution. The mixture was stirred for 20 min. After this contact time, it was filtered through filter paper and the residual concentration of methylene blue in the solution was determined using a UV-visible spectrophotometer at a wavelength of 620 nm, which is the wavelength at which the adsorption of the MB molecule is maximum. Equation (3) gives the calculation of the Methylene Blue index. • QBM: adsorption capacity of CA (in mg g-1); • Ci: initial concentration of BM solution (in mol L-1); • Cr: residual concentration of BM solution (in mol L-1); • V: volume of the BM solution (in mL); • M: molar mass of BM; • mca: mass of CA used (in g). 2.5. Determination of the pore distributions of synthesized activated carbons The porosities of four (4) samples of the Elaborated Activated Carbons (CA-BA-1; CA-BA-2; CA-HT-1 and CA-HT-2) and a Commercial Activated Carbon (CA-C) were determined. These analyses were carried out at the LABIRIS laboratory of the Université Libre de Bruxelles (ULB) in Belgium using a MICROMERITICS Gemini VII Surface Area and Porosity device by adsorption of liquid nitrogen N2 at 77 K. The surface area occupied by a nitrogen molecule is equal to 0.162 nm². The experimental protocol A mass of 100 to 130 mg of the dry Activated Carbon sample was placed in a BET tube treated at 130°C, under a nitrogen flow, for 30 minutes using a MICROMERITICS FlowPrep 060 Sample Degas System. The sample was then allowed to return to room temperature, still under a nitrogen flow. The exact mass of the dry sample was determined to 1/10 mg just before connecting the tube to the BET measuring device. Total pore volume The total pore volume was determined using formula (4) at a point P/P0 = 0.99. Vads is the amount of N2 adsorbed expressed in cm3 and mCA is the mass of activated carbon in g. The external volumes (mesopores + macropores) of CAEs and CA-C are determined from the total pore volumes minus the micropore volumes. Thus, the external specific surface areas are determined from the BET specific surface areas minus the micropore areas. 2.5.1. Cumulative pore volume (Vp) (BJH) This is also determined by the Barret, Joyner, and Halenda (BJH) model (at adsorption and desorption) between 17,000 Å and 3,000,000 Å. It is obtained by plotting the cumulative pore volume curve (∆Vdesorbed) as a function of pore width (t). The pore volume is thus determined by the derivative of the curve. GSC Advanced Research and Reviews, 2025, 24(03), 028–044 34 2.5.2. Microporous volume Total microporous volumes are determined at the initial partial pressure P/P0 or using the t-curve method (Harkins and Jura). The latter is presented in this paper; it is obtained by plotting the curve of thickness (t) as a function of adsorbed volume. Thus, the linear regression line is plotted between 3.5 and 10 Å. The micropore volume was determined using the t-plot model and at the initial relative pressure. For the t-plot method, it is obtained by plotting the curve of the amount of N2 adsorbed as a function of the statistical thickness t. 2.6. Average BET and BJH pore diameter The average pore size was determined by the BET and BJH models (adsorption and desorption) using formula (5). Where V is the amount of N2 adsorbed expressed in cm³ g⁻¹ and S is the specific surface area in m² g⁻¹. 3. Results 3.1. Characteristics of lignocellulosic biomass Table 2 summarizes the results of immediate and thermal analyses performed on the shells of Balanites aegyptiaca (BA) (L.) Del. and Hyphaene thebaica (HT) (L.) Mart. Table 2 Characteristics of lignocellulosic biomass used Biomass Immediate analysis of biomass (%) Humidity rate Ash content Loss on ignition Balanites aegyptiaca (L.) Del. Hyphaene thebaica (L.) Mart. 4.9914 5.0026 2.9994 5.8286 96.3619 98.9587 Biomasses Biomass constituents(%) Cellulose Hemicellulose Lignin Balanites aegyptiaca (L.) Del. Hyphaene thebaica (L.) Mart. 67.6857 57.0571 5.9369 5.5598 26.3774 37.3831 3.2. Chemical Characterization of Activated Carbons 3.2.1. Mass yield after pyrolysis The results of the mass yields of activated carbons produced by activation with H3PO4 are shown in Table 3. Table 3 Mass yield after pyrolysis/H3PO4 activation Biomass Parameters Mass of CAE (g) Yield (%) Hyphaène Thebeca Concentration (%) 10 20 30 40 50 8.57 8.29 8.18 9.41 9.74 42.85 41.45 40.9 47.05 48.7 Pyrolysis temperature (°C) 300 400 500 600 700 10.64 6.7 6.22 6.26 0.66 53.2 33.5 31.1 31.3 3.3 GSC Advanced Research and Reviews, 2025, 24(03), 028–044 35 Table 4 shows the mass yield results for activated carbons produced by H2SO4 activation. Table 4 Mass yield after pyrolysis/H2SO4 activation Biomass Parameters Mass of CAE (g) Yield (%) Hyphaène Thebeca Concentration (%) 10 20 30 40 50 7.38 6.81 6.59 6.43 5.18 36.9 34.05 32.95 32.15 25.9 Pyrolysis temperature (°C) 300 400 500 600 700 8.82 6.26 2.82 2.34 3.42 44.1 31.3 14.1 11.7 17.1 3.3. Iodine adsorption capacities of synthesized activated carbons The iodine indices of activated carbons produced by H3PO4 activation are shown in Table 5. Table 5 Iodine adsorption capacity (mg g⁻¹) / H₃PO₄ activation Biomass Parameters Volume of Na₂S₂O₃ at equivalence (mL) Iodine adsorption capacity (mg g-1) Hyphaène Thebeca Concentration (%) 10 20 30 40 50 0.7 0.4 0.3 0.1 0.1 838.2 914.4 939.8 990.6 990.6 Pyrolysis temperature (°C) 300 400 500 600 700 1.05 0.4 0.25 0.9 0.25 749.3 914.4 952.5 787.4 952.5 Table 6 shows the iodine index results for activated carbons produced by activation with H2SO4. Table 6 Iodine adsorption capacity (mg g-1) / H2SO4 activation Biomasse Parameters Volume of Na₂S₂O₃ at equivalence (mL) Iodine adsorption capacity (mg g-1) Hyphaène Thebeca Concentration (%) 10 20 30 40 50 0.9 0.9 0.9 0.8 0.7 787.4 787.4 787.4 812.8 838.2 Pyrolysis temperature (°C) 300 400 500 600 700 2.4 1.1 0.8 0.4 0.3 406.4 736.6 812.8 914.4 939.8 GSC Advanced Research and Reviews, 2025, 24(03), 028–044 36 3.3.1. BM index The results of the BM indices for activated carbons produced by H3PO4 activation are summarized in Table 7. Table 7 BM adsorption capacity (mg.100g-1) /H3PO4 activation Biomass Parameters Absorbance of residual solutions BM adsorption capacity Rate (%) Hyphaène Thebeca Concentration (%) 10 20 30 40 50 0.093666667 0.092333333 0.056333333 0.028333333 0.112333333 626.7051144 627.8247305 658.0543653 681.5663035 611.030489 88.86115331 89.01990481 93.30619537 96.63997691 86.63863228 Pyrolysis temperature (°C) 300 400 500 600 700 0.023 0.012 0.012333333 0.037 0.067333333 686.044768 695.2816008 695.0016968 674.2887989 648.8175325 97.27498292 98.58468281 98.54499494 95.60809215 91.99649547 The results of the methylene blue indices of activated carbons produced by activation with H2SO4 are shown in Table 8. Table 8 BM adsorption capacity (mg.100g-1) /Activation Biomass Parameters Absorbance of residual solutions BM adsorption capacity Extraction rate (%) Hyphaène Thebeca Concentration (%) 10 20 30 40 50 0.123333333 0.082666667 0.139333333 0.048 0.103666667 601.7936561 635.9419473 588.3582628 665.051966 618.3079936 85.32893239 90.1708532 83.42391436 94.29839226 87.67051704 Pyrolysis temperature (°C) 300 400 500 600 700 0.187 0.171666667 0.077333333 0.084333333 0.067666667 548.3319871 561.2075723 640.4204117 634.5424272 648.5376285 77.74854816 79.57419043 90.80585921 89.97241382 91.9568076 3.3.2. Microporosity of CAEs and CA-C Microporous volumes The microporous volume corresponds to the volume determined at the origin of the line. The slope of the line gives the specific microporous surface area, i.e., S = Vads,liq/t. Figures 6, 7, 8, 9, and 10 show the t-curves for CAEs and CAC. 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