Comparative study of the adsorption properties of several micropollutants on natural phosphate from Tahoua/Niger and activated carbon based on Hyphaene thebaica shells

 Corresponding author: AMADOU KIARI Mahamane Nassirou 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. Comparative study of the adsorption properties of several micropollutants on natural phosphate from Tahoua/Niger and activated carbon based on Hyphaene thebaica shells AMADOU KIARI Mahamane Nassirou 1, *, SANDA MAMANE Ousmaila 2, I. HIMA Halidou 1, ZANGUINA Adamou 1, MALAM ALMA Maman Mousbahou 1 and NATATOU Ibrahim 1 1 Laboratory of Materials, Water and Environment, Science and Technology Faculty, Abdou Moumouni University, BP: 10662 Niamey, Niger. 2 National School of Engineering and Energy Sciences, University of Agadez, B.P: 199 Agadez, Niger. World Journal of Advanced Research and Reviews, 2025, 27(02), 1972-1983 Publication history: Received on 28 May 2025; revised on 20 July 2025; accepted on 27 July 2025 Article DOI: https://doi.org/10.30574/wjarr.2025.27.2.3024 Abstract This study compares the adsorption properties of dyes on natural phosphate from Tahoua and activated carbon. The phosphate used is natural phosphate powder from Tahoua calcined at 850°C for two hours to increase the orthophosphate (P2O5) content. The activated carbons used were obtained from the shell of the Hyphaene thebaica (HT) nut by chemical activation with orthophosphoric acid (H3PO4) at 10% and 40% at a pyrolysis temperature of 450°C with an isothermal plateau of 1 hour 30 minutes. The shell was first characterized. The moisture content obtained was 5.002 %, the ash content was 5.828 % and the volatile matter content (loss on ignition) was 98.9587 %. The activated carbons produced (ACEs) have mass yields of around 42.85 % and 47.05 % when impregnated at 10% and 40%, respectively. Next, tests were conducted to determine the adsorption capacities of diiodine (I2), methylene blue (MB), and methyl orange (MO) on the activated carbons produced (ACPs) and on natural phosphate from Tahoua (NPT). The results obtained show that the adsorption equilibrium time for methyl orange is 40 minutes and around 25 minutes for the developed activated carbon (CAE) and Tahoua natural phosphate (PNT), respectively. Finally, the surface functions and pH at zero charge point were determined using the Boehm method and the first bisector method, respectively. The surface functions of CAEs are acidic in nature and the pH at zero charge point is below neutrality. For Tahoua natural phosphate (PNT), the pH at zero charge point is slightly above neutrality. Keywords: Adsorption; Activated Carbon; Iodine Index; Methylene Blue Index; Methyl Orange; Natural Phosphate from Tahoua 1. Introduction In recent years, awareness of the danger posed by pollution of various environmental components (water, air, soil, etc.) has become a reality that concerns all social and industrial actors. In Niger, the dyeing techniques used in the textile industry have contributed significantly to water pollution. This pollution is due to the discharge of organic and inorganic contaminants that are harmful to human health and ecosystems. To this end, effluent discharges from the textile industry are known for their intense coloration due to the presence of dyes, which are never 100 % exhausted from the dye baths; a quantity always remains in the effluent. The discharge of these effluents poses a major danger to humans and the environment. It is therefore essential to limit this pollution by implementing a dye removal technique adapted to our country (Niger). There are various methods for removing chemicals (heavy metals, dyes, phenols, etc.) from effluents. Among these methods, adsorption on porous World Journal of Advanced Research and Reviews, 2025, 27(02), 1972-1983 1973 materials is one of the most widely used techniques [1–9]. It is a solid-fluid extraction process that requires an adsorbent (solid) and an adsorbate (liquid or gas). Microporous adsorbents are widely used in the extraction of chemical species in aqueous or gaseous phases due to their excellent adsorption capacity. This capacity is linked to the large specific surface area and porosity of the adsorbents [10,11]. Natural phosphates and activated carbons have been the subject of much research in the search for adsorbent materials. They are of interest in the treatment of industrial wastewater [1]. Phosphates have a wide range of physical, chemical, and textural properties. They are capable of forming bonds with organic molecules of various sizes. Recent studies have shown that these materials, whether natural or synthetic, can remove organic compounds from wastewater [1]. Activated carbon consists mainly of carbonaceous material with a porous structure. It can be produced from any carbon-rich raw material [12]. Nowadays, various studies have focused on the production and characterization of activated carbons from several types of plant-based materials such as: the shell of Balanites aegyptiaca, the shell of Zizyphus mauritiana, the shell of Hyphaene thebaica [13], the branches or petioles of the rônier palm [8,9], bamboo stems or canes [14], coconut [15], eucalyptus wood [16], the shells of Tieghmelia, known as makoré, and Delonix, known as flamboyant [17], the shells of Parinari macrophylla [18], and coffee residues [19]. It is in this context that our work fits in, the overall objective of which is to conduct a comparative study of the dye adsorption properties of natural phosphate from Tahoua and activated carbons. The latter are produced from the shell of the Hyphaene thebaica kernel by chemical activation with orthophosphoric acid. 2. Materials and methods 2.1. Phosphate Figure 1 shows the raw natural phosphate powder from Tahoua, and Figure 2 shows the processed natural phosphate powder from Tahoua. Figure 1 Raw natural phosphate powder from Tahoua Figure 2 Processed natural phosphate powder from Tahoua 2.2. Shells of biomass used The shells of Hyphaene thebaica (Figure 3a) were used to prepare activated carbon. These were collected from the waste dump of a local market called Katako (Niamey, Niger). They were crushed, washed with tap water to remove dust, rinsed with distilled water, and then dried in an oven at 105 °C for 24 hours (Figure 3b). World Journal of Advanced Research and Reviews, 2025, 27(02), 1972-1983 1974 (a) (b) Figure 3 Hyphaene thebaica shells (a) and crushed shells (b) 2.3. Reagents used The reagents used in this work are presented in Table 1. Table 1 Chemicals use Chemicals Mollir mass (g/mol) Put (%) Sources Orthophosphorique Acid (H3PO4) 98 85 % Merck Methylene bleu (C16H18Cl3S) 319.86 - Sigma-Aldrich Sodium thiosulfate (Na2S2O3.5H2O) Iodine 248.18 126.9 99 % - Sigma-Aldrich Sigma-Aldrich 3. Methodology 3.1. Phosphate processing Calcination is used to enrich the orthophosphate (P2O5) content and reduce the organic carbon and carbon dioxide content in phosphates. The principle of this process is to heat the natural phosphate powder from Tahoua to a high temperature of 850 °C for 2 hours [20]. 3.2. Characterization of biomass 3.2.1. Moisture content and volatile matter content Moisture content and volatile matter content were determined in accordance with AFNOR XP CEN/TS 14774 and AFNOR XP CEN/TS 15148 standards, respectively [6,21]. First, porcelain crucibles of mass P containing 2 g of biomass were dried in an oven at 105 °C for 24 hours. After cooling in a desiccator, the crucibles were weighed again. To determine the volatile matter content, the dried samples were placed in a muffle furnace at 1000°C for 3 hours. After cooling in a desiccator, the whole batch was weighed. The moisture content (H) and volatile matter content (PF) are given by expressions 1 and 2 respectively. (H)= P1− P2 P1− P × 100 … .. (1) (P. F )= P2− P3 P2− P ×100 … …. (2) Where P1 and P2 are the masses of the crucible + sample before and after drying in the oven; P3 is the mass of the whole after carbonization in the furnace. World Journal of Advanced Research and Reviews, 2025, 27(02), 1972-1983 1975 3.2.2. Ash content The ash content was determined in accordance with standard AFNOR XP CEN/TS 14775 [6,21]. In a porcelain crucible with an empty mass m1, 1 g of biomass ground to a particle size of less than 200 μm was introduced. The sample was mineralized at 815 °C using a muffle furnace for 2 hours. After cooling in a desiccator, the whole thing (sample + crucible) was weighed to get the mass m3. The ash content is given by the following equation: (%C)= m3 − m1 m2− m1 × 100 … ….. (3) With m2, the mass of the crucible + sample before mineralization. 3.2.3. Apparent density To determine the apparent density, a 50 cm3 test tube with an empty mass m1 was filled with biomass ground to a particle size of less than 200 μm, and the mass m2 was recorded [6,21]. The density is given by the equation ρ = m2 − m1 V … … (4) 3.3. Production of activated carbon 3.3.1. Preparation of activated carbon The method used to prepare activated carbon is chemical activation [22–25]. The pretreated biomass is impregnated with two solutions of H3PO4 at 10 % and 40 %, at a rate of 20 g of biomass per 100 ml of solution. Impregnation is carried out under magnetic stirring for 24 hours at atmospheric pressure and room temperature. The samples were then filtered on ashless Büchner filter paper, washed with distilled water, and dried in an oven at 105 °C for 24 hours. After removal from the oven, the samples were cooled in a desiccator for 15 minutes. Once dried, each impregnated sample was placed in a crucible with a lid, and the assembly was placed in a muffle furnace at a temperature of 450 °C, with a heating rate of 2.66 °C/min and an isothermal plateau of 90 min at the end of pyrolysis. After the 90-minute plateau at the final pyrolysis temperature, the crucible was removed from the oven and cooled in a desiccator for 15 minutes. After cooling, the charcoal was washed thoroughly with distilled water to remove impurities until the pH was close to neutral, then dried in an oven at 105°C. The activated carbons thus obtained were ground and then sieved to obtain grains of a size less than or equal to 500 μm. 3.4. Characterization 3.4.1. Mass yield after pyrolysis This is an important quantitative characteristic of activated carbon that reflects the loss of biomass mass during pyrolysis. The following formula gives the expression for mass yield Yield = mf mi × 100 … … (5) final mass (mf) and initial mass (mi). 3.4.2. Surface function The acid and base functions on the surface of activated carbons were determined and quantified using the Boehm titration method [26]. (Boehm, 1994). To determine these functions, 0.2 g of AC were brought into contact with 20 mL of each of the following solutions: NaOH, Na2CO3, NaHCO3, C2H5ONa, and HCl at 0.1 M. Each solution was stirred for 24 hours to ensure that a maximum amount of the surface groups of AC reacted, then filtered; after filtration, 10 mL of each of the five solutions was measured. The basic solutions are titrated with 0.1 M hydrochloric acid using three drops of World Journal of Advanced Research and Reviews, 2025, 27(02), 1972-1983 1976 bromothymol blue, phenolphthalein, bromocresol green, and helianthin as color indicators, respectively, and the acid solution is titrated with 0.1 M sodium hydroxide using bromothymol blue as the color indicator. The quantification of acid and base groups is performed using the following formula néqR = NiVi− Nf Vf … .. (6) nor is the number of equivalent grams that reacted; Nivi is the number of equivalent grams before the reaction; NV is the number of equivalent grams after the reaction. 3.4.3. pH at zero charge point (pHPZC) The pH at zero charge point (pHPZC), which allows the net charge of the material surface to be determined, was calculated using the method described by Ousmaila et al., [5]. This method involves preparing 0.1 M sodium chloride (NaCl) solutions at different pH values, namely 2, 4, 6, 8, and 10. The pH values were adjusted with a pH meter using sodium hydroxide and hydrochloric acid solutions. In a 50 mL beaker, 0.1 g of CA was mixed with 20 mL of each solution. The mixture was stirred magnetically for 72 hours. The suspension was filtered through filter paper and the pH of the filtrate was measured. Then, the pHf = f (pHi) curve was plotted. The intersection point between this curve and the first bisector gives the pH at the zero-loading point of the activated carbon in question. pHf: final pH and pHi: initial pH. 3.5. Adsorption of micropollutants 3.5.1. Iodine index The iodine index is an important characteristic in the evaluation of the micropores of activated carbon. In this study, it was determined using the method applied by GUEYE [16]. Thus, a volume of 20 mL of a 0.02 N iodine solution was brought into contact with 0.2 g of activated carbon for 4 min. The treated solution was filtered, and then 10 mL of filtrate was titrated with a 0.1 N sodium thiosulfate solution in the presence of a few drops of starch paste. Finally, the iodine index (Id) expressed in mg/g was calculated using equation (7). 𝐼𝑑= (C0− CthioVthio 2VI2 )MI2Vads mca … … (7) Where, Id: adsorption capacity of I2; C0: initial concentration of the I2 solution (in mol/L); Cthio: concentration of Na2S2O3 (in mol/L); Vthio: volume of Naso₃ at equivalence (in mL); VI₂: volume of iodine titrated (in mL); MI₂: molar mass of I₂ (in g/mol); Vads: adsorption volume (in mL); mca: mass of adsorbent used (in g). adsorption volume (in mL); mica: mass of adsorbent used (in g). 3.5.2. Methylene Blue (MB) Index The MB index, expressed in mg. g-1, represents the adsorption capacity of medium-sized molecules for the purpose of evaluating mesopores and macrospores. a) Preparation of standard MB test solution Place 1.2 g of MB in a 1 L volumetric flask. Fill the flask with distilled water to the mark. The solution is left to stand for approximately one night. The solution is then tested by adding 0.25 % acetic acid (i.e., 5.0 ml per 1 L of solution). For this purpose, a test solution was prepared by dilution corresponding to an absorbance of 0.840 ± 0.01 at a wavelength of 620 nm. b) Procedure for determining the MB index Methylene blue adsorption tests were performed using the CEFFIC protocol [5,6]. In a 250 mL Erlenmeyer flask, we added 100 mg of adsorbent to 100 mL of standard MB solution. The mixture was stirred for 20 min. After stirring, the mixture was filtered and the residual concentration of methylene blue was measured at 𝞴 = 620 nm using a UV-visible spectrophotometer. World Journal of Advanced Research and Reviews, 2025, 27(02), 1972-1983 1977 The methylene blue index is given by: IBM =(𝐶𝑖− 𝐶𝑟)𝑉𝑀 𝑚 × 100 … .. (8) With IBM: adsorption capacity of CA (in mg/g); Ci: initial concentration of BM solution (in mol/L); Cr: residual concentration of BM solution (in mol/L); V: volume of BM solution (in mL); M: molar mass of BM; m: mass of adsorbent used (in g) 3.6. Methyl orange (MO) adsorption kinetics In a 500 mL beaker, 200 mL of methyl orange (MO) solution with an initial concentration of 50 mg/L and 80 mg of adsorbent are added. This mixture is stirred at 25 °C. At regular intervals, 1 mL of the mixture was sampled and filtered using a 0.45 μm pore size filter paper. A JENWAY 6305 UV-visible spectrophotometer was used to measure the residual MO concentration at a wavelength of 520 nm. The quantities adsorbed at equilibrium (Qe) were calculated using equation 9. Qe=(Ci− Cr) m × V … .. ( 9) Where Ci and Cr are the initial and residual concentrations of methyl orange (in mg. L-1); V is the volume (in L) of the solution used for the adsorption tests; and m is the mass (in g) of adsorbent. 4. Results and discussion 4.1. Characterization of biomass The composition and physical properties of biomass are parameters that influence its conversion into activated carbon [13]. Table 2 summarizes the results of the immediate analysis performed on the shell of Hyphaene thebaica (HT). Table 2 Immediate analysis results Biomass Moisture content (%) Ash content (%) Carbon content (%) Actual density g/cm3 Hyphaene thebaic (HT) 5.0026 5.8286 74.850 0.3220 Table 2 shows that the ash content of biomasses is less than 6 %. This indicates that these biomasses can be used to produce activated carbons with large specific surface areas [8] .This parameter has a significant effect on the quality of activated carbon. It is also known that high ash content reduces the specific surface area of activated carbons [13,16]. In fact, ash intercalates into the carbon structure, clogs the pores, and reduces the porosity of the activated carbon. Unlike the moisture and ash content, which are relatively low, the carbon content is very high, at around 74.85 %. Biomass is therefore mainly composed of organic matter. Such volatile matter values are a good indicator for obtaining activated carbon with a high degree of graphitization, a high gross calorific value (GCV), and sufficient functional groups [13,16]. World Journal of Advanced Research and Reviews, 2025, 27(02), 1972-1983 1978 4.1.1. Mass yield The mass yield results after pyrolysis are shown in Table 3. Table 3 Mass yield results after pyrolysis of CAs Biomass H3PO4 concentration (%) Impregnation time (h) Pyrolysis temperature (℃) Heating rate °C/min Carbonization time (h) Mass yield (%) Hyphaene thebaica 10 24 450 2.66 1 h 30 42.85 40 24 450 2.66 1 h 30 47.05 These results show that increasing the concentration of the activating agent H3PO4 increases the mass yields after pyrolysis of CAs. This can be explained by the fact that H3PO4, as an activating agent, is a dehydrating agent that delays thermal decomposition, limits the loss of volatile matter, and leads to the formation of a rigid carbon matrix [5,21]. 4.1.2. Surface functions Table 4 shows the mEq/g content of the adsorbents' functions. Table 4 Surface function of adsorbents Adsorbent carboxylic lactone phenol carbonyl Total acid Basic total CA-HT 10 % 1.2 1.3 3.2 2.4 8.1 - CA-HT 40 % 1.5 2.1 2 2.1 7.7 - As expected, with this acid chemical activation, the CA-HT 10% and CA-HT 40% activated carbons produced contain a large amount of acid groups and no basic groups. This can be explained by the fact that the ACEs were not exposed to oxygen below 200°C or above 700°C, they did not undergo hydrogen treatment, and they were not degassed at room temperature, as this is the stage at which basic functions are introduced. Similar results have been reported in the literature [5,21]. These acidic surface groups are probably due to polyphosphates bound to the surface of the carbon and to carboxylic groups. Orthophosphoric acid acts as a catalyst by promoting bond cleavage reactions and facilitating cross-linking via cyclization, condensation, and the formation of phosphate and polyphosphate bonds [27]. Carbonyl and phenolic groups predominate on their surface. 4.1.3. pH at zero charge point (pHPCN) Figures 4, 5, and 6 show the pH determination curves at zero charge point for activated carbons activated with orthophosphoric acid and natural phosphate from Tahoua, respectively. World Journal of Advanced Research and Reviews, 2025, 27(02), 1972-1983 1979 Figure 4 Determination of the pHPCN of CA HT 10% activated with H3PO4 Figure 5 Determination of the pHPCN of CA HT 40 % activated with H3PO4 Figure 6 Determination of the pHPCN of PNT The pH values at the zero charge point of activated carbons are all below neutrality (pH < 7). They range from 2.54 to 4.55. Similar results have been observed in other studies [27,28]. At a pH higher than these values, the surface of the activated carbon is negatively charged, whereas it is positively charged at a pH lower than the pHpcn. As for PNT, the 7.32 World Journal of Advanced Research and Reviews, 2025, 27(02), 1972-1983 1980 pH value at the zero-charge point is close to neutrality at 7.32. At a pH higher than this value, the surface of PNT is negatively charged, whereas it is positively charged at a pH lower than the pHpcn. 4.2. Adsorption of micropollutants 4.2.1. Iodine index The iodine index results for the activated carbons produced and the natural phosphate from Tahoua are shown in Table 5. Table 5 Iodine index results in mg/g PNT CA HT 10 % CA HT 40 % Vthio (mL) 3 0.7 0.1 QI2 126.90 418.77 494.91 In the case of CAs, the results show that the parameter influencing the iodine index is the concentration of the activating agent (H3PO4). Several authors have shown that the iodine index increases with the concentration of the activating agent [4,21,29]. These results are similar to those obtained by [4,21]. For phosphate, the adsorption capacity of I2 is lower than that of the two (2) ACs. This could be due to the fact that raw natural phosphate from Tahoua has been found to be a material with moderate adsorption affinity compared to activated carbons for iodine removal. 4.2.2. Determination of the Methylene Blue (MB) index The MB results for the activated carbons produced and the natural phosphate from Tahoua are shown in Table 6 Table 6 Methylene blue index results in mg/g Adsorbent PNT CA HT 10% CA HT 40% BM index 548.879 810.01 886.72 It can be seen that the adsorption capacity of BM by CA increases with the concentration of the activating agent (H3PO4). These results are similar to those obtained by [4,21]. For PNT, the adsorption capacity value obtained for BM is relatively low compared to those obtained by ACs. This could be due to the fact that raw Tahoua natural phosphate has been found to be a material with moderate adsorption affinity compared to activated carbons for BM removal. 4.2.3. Methyl orange contact time The kinetic study allows us to determine the contact time required to reach adsorption equilibrium. To do this, we monitored the adsorption kinetics of MO for an initial concentration of 50 mg/L, with a mass of 80 mg of adsorbent. The results of the adsorption kinetics of MO on the adsorbents are presented in Figures 7