Equilibrium Sorption Studies of Cobalt (II) Ions Removal from Aqueous Solution

338 Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 338-352 p ISSN: 2635-3342; e ISSN: 2635-3350 Original Research Article Equilibrium Sorption Studies of Cobalt (II) Ions Removal from Aqueous Solution *1Lemeh N.N., 1Ekabafe L.O., 2Linda N.C. and 1Mahmud H. 1Department of Pure and Industrial Chemistry, University of Lagos, Nigeria. 2Department of Chemistry, University of Port Harcourt, Nigeria. *[email protected] http://doi.org/10.5281/zenodo.18061074 ARTICLE INFORMATION ABSTRACT Article history: Received 04 Sep. 2025 Revised 12 Oct. 2025 Accepted 19 Oct. 2025 Available online 30 Dec. 2025 This study assessed the equilibrium sorption for Co²⁺ uptake from aqueous solutions using modified biochars from palm shells (Elaeis guineensis Jacq.) and sugarcane peels (Saccharum officinarum). KMnO₄/HNO₃ oxidation was used for modification due to their high oxidative nature, increasing biochar surface area. Unmodified and modified biochars were characterized for functional groups, morphology, elemental analysis, and surface area using Fourier transform infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM), energy-dispersive spectroscopy (EDS), and Single/Multi-Point Brunauer-EmmettTeller (BET/MBET). Batch equilibrium sorption for Co²⁺ uptake was analyzed using atomic absorption spectroscopy. Results showed that the KMnO₄/HNO₃-modified palm shell (AP1) and sugarcane peel (BS2) biochars had higher surface areas (MBET: 450.0 m²/g, SBET: 256.0 m²/g for AP1; MBET: 392.5 m²/g, SBET: 241.9 m²/g for BS2) compared to unmodified biochars (MBET: 356.76 m²/g, SBET: 244.2 m²/g for palm shells; MBET: 418.33 m²/g, SBET: 227.2 m²/g for sugarcane peels). Sorption data fit the Freundlich model (R²: 0.9976 for AP1, 0.9426 for BS2). AP1 showed the highest Co²⁺ adsorption with maximum adsorption capacities of 1.49×10¹⁸ mg/g and reusability of 83.1%. KMnO₄:HNO₃-modified palm shell biochar had more and better adsorption sites for Co²⁺, indicating potential for superior heavy metal adsorption compared to unmodified and modified sugarcane peel biochar. © 2025 RJEES. All rights reserved. Keywords: Cobalt Modified biochar Potassium permanganate Hydrochloric acid Adsorption 1. INTRODUCTION Cobalt usage and accidental environmental release have grown dramatically in recent decades, primarily due to the industry's need for the metal for technological growth (Golroudbary et al., 2022). Using chemical additives to clean up contaminated soil reduces the environmental risk that heavy metals provide through adsorption, precipitation, and complexation, among other chemical processes (Beesley, 339 N.N. Lemeh et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 338-352 2011; Chao et al, 2018). Prior studies have demonstrated the effectiveness of inorganic materials such as phosphate, zeolite, and lime in immobilizing heavy metals (Karczewska et al., 2017). Biochar is the ideal option for the most effective cobalt remediation material due to its highly porous nature (Zhang et al., 2018). The substantial affinity between several functional groups in biochar, such as metal cations, carboxyl, phenolic, and hydroxyl groups, is highlighted by Dai et al. (2018).The sorption capacities of this material are determined by its stability and composition. Important characteristics for heavy metal sorption, such as pH, surface negative charge, cation exchange, and metal surface complexation potential, are mainly influenced by the surface functional groups in organic carbonaceous materials (Mukherjee et al., 2011). Calcite, quartz, sylvite, whitlockite, and periclase are among the organic and mineral components found in biochars (Qi et al., 2017). The mineral components in biochars promote ion exchange and surface complexation, increase the number of metal sorption sites, and aid in the formation of metal precipitates by releasing soluble ions such phosphates, sulphates, and carbonates (Xia et al., 2019). Compared to carbon-based sorbents, biochar is a more promising and reasonably priced adsorbent for removing metals from water. In recent years, extensive research has been conducted on the effectiveness of biochar in eliminating heavy metals, especially cobalt, from aqueous solutions (Sun et al., 2019). Even though much of the current research has focused on sorption strategies for metals in general, evaluating biochar's potential for heavy metal sorption requires comparing various metal removal mechanisms. Biochars (BCs) are recognized as environmentally friendly and adaptable materials with significant potential for the remediation of contaminated soils, while modified biochars (MBCs), which have the advantages of a large specific surface area and high porosity, are a commonly used microbialimmobilization carrier (Kim et al., 2018). Biochar offers microorganisms a conducive environment, produced from natural biomass resources like sawdust and agricultural waste. Additionally, biochar can improve the interaction between pollutants and microorganisms due to its great potential for adsorbing petroleum toxins. Biochar, for instance, is a material with significant promise for treating contaminated soil that is both cost-effective and environmentally friendly. It is produced when biomass is pyrolyzed and has been demonstrated to improve soil properties (porosity, water holding capacity, aeration, and aggregate stability), raise soil pH and cation exchange capacity, and increase productivity while adsorbing hydrophobic organics (Sizmur et al., 2016). Biochars' unique physicochemical properties and adsorption capabilities allow for additional functionalization to enhance their attributes and promote environmental and agricultural sustainability. To improve the rate of pollutant removal, the properties of the biochar can be altered using a variety of physical, chemical, and biological methods (Xiao et al., 2018; Wang et al., 2017). Among these treatments are clays, metal oxides, biofilms, alkali or acid alterations, steam activation, and organic compounds. These methods are used to add extra materials for specific purposes, change the surface properties of pure biochar, and expand its surface area. Since different metals exist in distinct species or valence states under various pH or redox circumstances, they might have variable primary sorption processes (Li et al., 2017). Although cobalt is frequently found in soil as Co2+ and Co3+ ions, these species' bioavailability varies, and soil may contain environmental risks. The behavior of Co in soils is greatly influenced by Fe and Mn oxides, which are known to have a remarkable affinity for Co. This is due to the fact that the majority of Cobalt (up to 79%) has been shown to be strongly coupled to soil-based Mn and Fe oxyhydroxides. Leyssens et al. (2017) state that Co (II) is highly soluble in water, bioavailable, and probably very mobile. One important mechanism that reduces soil bioavailability and co-mobility of Co is the surface oxidation of Co 2+ on oxyhydroxide minerals, which mostly generates Co (III). This is one significant mechanism that reduces the bioavailability and mobility of Co in soils (Ma et al., 2010). Metal ore mining and smelting activities (mostly of copper, zinc, lead, and cobalt) as well as cobaltcontaining alloys and compounds are the main causes of cobalt pollution in the environment (Gál et al., 2008). Furthermore, the main sources of cobalt pollution have been determined to be sewage effluents, urban runoff, and agricultural runoff (from phosphate fertilizers and pesticides). Since rechargeable lithium-ion batteries absorb half of the world's cobalt supply (Alves Dias et al., 2018), the cobalt 340 N.N. Lemeh et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 338-352 dependence of the electronic device industry should be acknowledged as a growing environmental concern. However, how much cobalt was released into the environment is still unclear. The problem appears to be limited since elevated levels of cobalt in soil and groundwater are localized and may be caused by local geological factors, air deposition at smelting sites, or metal ore mining. However, cobalt can readily seep into groundwater or infiltrate soil by air deposition, posing a risk to food safety and agricultural quality (Liu et al., 2019). Despite being a necessary component of vitamin B12, excessive cobalt intake has been associated with several adverse health outcomes, such as asthma-like allergies (Leyssens et al., 2017). Future exposure to cobalt may result in asthma attacks, manifesting as shortness of breath, chest tightness, wheezing, and/or coughing. Additionally, cobalt exposure can harm the thyroid, liver, kidneys, and heart. Long-term or frequent exposure to cobalt dust can result in lung fibrosis or scarring, even without early symptoms. The existence of xenobiotics, which have unknown effects on the environment and human health, is one of the new challenges in reducing the danger of elemental transfer into food systems. One of the most significant challenges currently facing the treatment of heavy metal pollution in water is the development of adsorption materials with high adsorption capabilities for cobalt (Co). By producing biochar from a variety of two materials (in this study, sugarcane peel and palm shell), researchers and practitioners can improve the sustainability and efficacy of heavy metal removal from water, lower costs, and optimize the qualities of biochar for specific uses (Nan et al., 2018). Adsorption is the primary method of removing heavy metals from water, and it is characterized by its low cost, straightforward processes, and renewal (Foo and Hameed, 2009). Chemical oxidation methods that use H2O2, KMnO4, HNO3, or an HNO3/H2SO4 mixture are frequently employed to achieve surface oxidation (Godwin et al., 2019). Since both methods have strong oxidative qualities and won't react, KMnO4/HNO3 oxidation is used after pyrolysis to help further alter the biochar. A prior study found that when yak dung biochar was treated with HNO3, the adsorption capacity of Co2+ rose from 76.4 to 169.6 mg/g (Wang et al., 2018).The C=C on the surface of the biochar was oxidized to C=O by HNO3, which is responsible for eliminating Co2+, according to their FTIR data. The results of this study address the pressing need for efficient Cobalt adsorption technologies and open the door to a new strategy for the sustainable management of invasive species. This study demonstrates for the first time the highly effective adsorption of Cobalt (Co (ii)) by biochar made from sugarcane peel and palm shell, presenting a novel method for repurposing invasive biomass into useful sorbents. 2. MATERIALS AND METHODS 2.1. Materials The Palm shells were purchased from Boundary Market Ajegunle, Lagos, Nigeria and the Sugarcane Peels were collected at Park Lane Apapa, Lagos, Nigeria. Cobalt chloride hexahydrate (CoCl2·6H2O) was obtained from Ace Chemicals and allied Products Ojota market Lagos, Nigeria. Prior to use, diluted standard Co (II) solutions were made right away. Hydrochloric acid (HCl), sodium hydroxide (NaOH), potassium permanganate (KMnO4), and nitric acid (HNO3) were all obtained from the University of Lagos Laboratory's Chemistry Department. They were manufactured by Kernel, Qualikems, and Merck chemicals in Darmstadt, Germany. The sugarcane peels and palm shells were cleaned with deionized water, dried in an ovum at 103°C to a consistent weight, ground into a powder, and then placed in a crucible. After that, the palm shells and sugarcane peels were carbonized for six hours at 500–550°C and two hours at 600–650°C, respectively, in a muffle furnace (QXL-201, CN). After bringing the products to room temperature, they were taken out and pulverized using a pestle and mortar to create BC, which was then dried and put away. After being weighed, 60 g of palm shell biochar and 50 g of sugarcane peel biochar were impregnated into 1000 mL of a solution containing a specific concentration of potassium permanganate and nitric acid. The mixture was then kept at 25 oC for 24 hours on a constant-temperature shaker. 341 N.N. Lemeh et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 338-352 After impregnation, the product was carefully rinsed with deionized water until the pH reached a neutral level. Then, it was dried in an oven set at 105°C until it reached a consistent weight, producing modified Sugarcane Bagasse BC and modified Palm Shell Biochar (BC). The dry material was then crushed into a fine powder for use in ensuing adsorption tests as shown in Figure 1. To optimize the unmodified palm shell biochar (OP) modification procedure and unmodified sugarcane peel biochar (OS), several potassium permanganate and nitric acid ratios were developed. AP1, AS2, BP1, BS2, and AP1 were modified palm shell and sugar peel biochars with KMnO4:HNO3 ratios of 0.3:2.0 and 0.8:2.0 mol.L-1, respectively. The optimal conditions for modified biochars were estimated using the Co2+ elimination rate as a reference index. Figure 1: Pictorial view of biochar formation process 2.2. Biochar Characterization Using the methodology described by Jindo et al. (2014), the biochars were assessed for pH, total ash content, liming potentials, specific surface area, biochar yield, and shape. pH Determination: The potentiometric method, an instrumental technique that involves measuring potential, was used to determine the pH of the biochar sample. We calibrated the pH meter using standard buffers 4, 7, and 10. Five grams (5 g) of biochar were placed in a 1:2 ratio beaker, and ten millilitres (ml) of deionized water were added. After stirring the sample for 30 minutes, the pH electrode was immediately placed in the slurry, and the pH was measured immediately (Qin H et al., 2023). Total Ash: Dry combustions at 720 °C for three hours in a muffle furnace were used to determine the total ash content. Jindo et al. (2014) were followed in calculating the mineral element content. Biochar liming potential: Depending on the feedstock and procedure, the ash component of biochar, which contains carbonates and alkaline oxides, can considerably raise alkalinity (Vassilev et al. 2013a) (Xie et al. 2015). As a result of this, biochars can be utilized as liming agents in acidic soils and acquire liming capabilities (Novak et al. 2009). It is not possible to propose lime until the soil's pH-buffering capacity (pH-BC) and the biochar's liming potential have been established. The carbonate content of soil was determined by modifying the Rayment and Higginson (1992) method. Liming equivalent, or calcium carbonate equivalent, was calculated. This is accomplished by combining 0.5 g of ground biochar with 10.0 mL of a standardized 1 M HCl solution, stirring the combination for two hours. After that, it is left on an end-over-end shaker for sixteen hours. The slurry is then titrated without the need for a separate process using an auto titrator that vigorously swirls a standardized 0.5 M NaOH solution until a neutral pH (~7.0) is reached. 342 N.N. Lemeh et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 338-352 It is stated how many milliliters of NaOH solution were used. After performing a blank titration (i.e., without biochar) using 10.0 mL of standardized 1 M HCl, the volume of NaOH solution utilized is measured (b mL). The batch also includes a reference sample of CaCO3 powder that was dried for an hour at 105°C. The liming equivalence (% CaCO₃-equivalent) is then obtained using the following formulas: Chemical reaction and calculations: CaCO3 + 2 HCl → CaCl2 + H2O + CO2 (g) (1) In equation (1) the reaction above, one mole of CaCO3 (about 100.09 g of CaCO3) is dissolved using two moles of HCl. Tiration is utilized to find unreacted H+. The amount of H+ neutralized by the biochar is then calculated using the difference between the biochar sample and the blank. Thus, the % CaCO3 -eq is calculated in Equation (2): % CaCO₃ equivalent = ( M × (b−a) × 10−3×100.09×100) (2 × W ) (2) Where: M = mol L⁻¹, the standardized molarity of NaOH b = the blank's NaOH consumption volume (mL) a = amount of NaOH that the biochar sample consumed (mL) To convert volume from mL to L 100, use the conversion factor 10⁻³.09 = CaCO₃ molar mass 100 = multiplier to represent the outcome as a percentage W = biochar mass (g) Since 1 mole of CaCO3 reacts with 2 moles of H⁺, 2 = factor accounted for the stoichiometry Biochar Yield: The biochar yield was calculated using the weight ratio of the pyrolysis product to the original ingredients as described in Equation (3). % Yield (dry weight basis) of processed material based on weight differential before and after pyrolysis. % Yield = Actual yield nitial weight of feedstock)× 100 (3) The morphologies and structural composition of the biochars were examined using scanning electron microscopy (SEM). A Phenom-World scanning electron microscope was used to conduct energydispersive spectroscopy (EDX). The Brunauer-Emmett-Teller (BET) method (Quantachrome NovaWin – Data collection and Reduction for NOVA instrument version 11.03, Ahmadu Bello University, Zaria) was used to measure the biochars' specific surface area before and after the alteration. The adsorbent's functional groups' surface was verified using Agilent's Fourier transform infrared (FT-IR) spectroscopy. The collected spectra ranged from 4000 to 650 cm-1. 2.3. Adsorption Experiment The pollutant used in the experiment was a stock solution of Co (II). In order to do adsorption investigations, 1g of adsorbent was added to the appropriate concentration of various components. An atomic absorption spectrophotometer was used to quantify the concentration of Co (II) in the suspensions following a 12-hour shake period. Equations (4) and (5) were used to determine the amount of Co (II) adsorbed on the biochars when the system was in equilibrium (Hou et al. 2016). The Langmuir and Freundlich isothermal adsorption models were examined in Equations (6) and (7). qe = (C0−Ce) V m (4) Adsorption (%) = (C0−Ce) C0 × 100 % (5) lnqe = lnKT + 1 n ln Ce (6) Ce qe = 1 qmCe + 1 qmKL (7) 343 N.N. Lemeh et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 338-352 qe (mg.g-1) in the equations represents the adsorbent's equilibrium adsorption capability on cobalt. C0 (mg.L-1) is the cobalt concentration in the initial solution, whereas Ce (mg.L-1) is the cobalt concentration in the solution when adsorption reaches equilibrium. The adsorbent's mass is denoted by M (mg), the aqueous solution's volume by V (mL), and the theoretical maximum adsorption capacity for cobalt is qm (mg•g-1). KL is the Langmuir model constant (L.mg-1). Impact of dosage on sorption: To generate the sorption dosage data, the following requirements have to be fulfilled: 0.1 to 1.0 g of the adsorbent per 100 mL of solution; 2 ppm for metal ion concentrations; 7.85 pH for all sorption investigations; and 12 hours of shaking time. Impact of pH on sorption: To provide sorption pH data, the following parameters are used: dose of the adsorbent, 0.5 g/100 ml of solution; metal ion concentrations for Co2+ range from 2 ppm, and all sorption tests have a pH of 2; 4, 6, 8, and 10; The sorption test would be carried out at 27.5 oC for a duration of 12 hours of shaking. Impact of Cd (II) ions concentration on sorption: When producing data, the following standards are used: All sorption tests use a sorbent dose of 0.5 g per 100 mL of metal ion solution, a metal ion concentration ranging from 5 to 7 ppm, a pH of 7.85 for Co2+, and a 12-hour shaking duration. The sorption tests were conducted at a temperature of 27.5oC. Impact of contact time on sorption: The impact of contact time on sorption was studied using 0.5 g of adsorbent per 100 mL of solution, a starting metal concentration of 2 ppm for Co²⁺, and contact times ranging from 2 to 12 h under continuous shaking. The pH was 7.85. The temperature of the mixtures was maintained at 27.5oC. Effect of temperature on sorption: The following settings were applied in order to produce sorption thermodynamic data: The sorption tests would be conducted using an adsorbent dosage of 0.5 g/100 mL of solution, metal ion concentration ranges of 2 ppm Co2+, pH 7.85, and a 12-hour shaking period. Temperatures of 10, 20, 30, 40, and 50 degrees would be used for the sorption tests. The pseudo-first-order (8) and pseudo-second-order (9) models were employed to match the sorption kinetics. Qt = Qe (1 − 𝔢−𝑘1𝑡 ) (8) Where k1 is the pseudo-first-order rate constant (1/min), Qt (mg/g) is the sorption amount at time t, and Qe (mg/g) is the sorption amount at equilibrium. Qt = 𝑘2𝑄𝑒2𝑡 (𝑘2𝑄𝑒𝑡 + 1) (9) The sorption amount at equilibrium is Qe (mg/g), the sorption amount at time t is Qt (mg/g), and the pseudo-second-order rate constant is k2 (g/(mg•min)). Equations (6) and (7) were used to calculate the Co ion sorption capacity and the ion adsorption efficiency (η), respectively. By averaging the three times the same processes were performed, the adsorbent's ultimate capacity was ascertained. The ions concentration at the adsorption equilibrium and the initial concentration are denoted by Ce (mg/L) and C0 (mg/L), respectively; the volume of the ions solution used in the experiment is denoted by V (L); and the dry weight of the biochar added to the ions solution is denoted by m (mg). The sorption capacity of modified adsorbent ions is denoted by Qe (mg/g). 3. RESULT AND DISCUSSION 3.1. Surface Characteristics Analysis of the Biochar The adsorbent's surface chemistry and morphology significantly impact adsorption processes (Ghanim et al., 2020). Table 1 lists the volume, DR micropore width, and BET N2 surface area of the modified (AP1, AS2, BP1, and BS2) and unmodified (OP and OS) biochars. Compared to AS2, which has values of 229.8m2/g and 371.390m2/g, respectively, the modified biochar AP1 has a superior single and multi-point 344 N.N. Lemeh et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 338-352 BET surface area of 256.0m2/g and 450.023m2/g. Both Chars were altered using a ratio of 0.3:2 KMnO4:HNO3. In comparison to BP1, this also had 0.8:2 KMnO4:HNO3 changed; BS2 has a better single and multi-point BET surface area of 241.9m2/g and 392.508m2/g, with values of 220.2m2/g and 345.364m2/g. In contrast to OS, which has single and multi-point BET surface areas of 227.2m2/g and 418.333m2/g, respectively, OP has single and multi-point BET surface areas of 244.4m2/g and 356.276m2/g. Following treatment with KMnO4 and HNO3, the surface area of the modified biochars rose. The higher BET may accelerate the adsorption capacity (Tang et al., 2018). Following characterizations and batch adsorption tests, AP1 and BS2 of the modified biochar were utilized, and they were subsequently contrasted with the unmodified biochar. Table 1: BET surface area and DR micro pore values of the modified and unmodified biochar Samples SBET(m2/g) MBET(m2/g) DR micro pore width (nm) DR micro pore volume (cc/g) AP1 256.0 450.023 6.381 0.1626 AS2 229.8 371.390 6.195 0.1418 BP1 220.2 345.364 6.083 0.1350 BS2 241.9 392.508 6.214 0.1502 OP 244.4 356.276 5.839 0.1454 OS 227.2 418.333 6.538 0.1475 When combined with energy dispersive X-ray spectroscopy (EDS or EDX), scanning electron microscopy (SEM) offers a comprehensive view of the biochar's morphological features and elemental makeup. The SEM results for the modified and unmodified biochar with large surface areas (AP1, BS2, OP, and OS) at 500x, 1000x, and 2000x are displayed in Figure 2. The cellulose structure of sugarcane peel, which may be divided into fibrous, prismatic, and spherical forms, is likely the source of the noticeable longitudinal fibrous structures seen in the biochar OS and BS2 (Joseph et al, 2010). Due to the raw material's dehydration, a small number of pores of varying sizes developed in the biochar. Additionally, AP1 and BS2 are clean in the SEM image, indicating that the KMnO4/HNO3 oxidant has largely or completely removed the ashes from the unmodified biochar. One characteristic of biochar is that its surface is flat and moderately porous. The largest pore distribution was found on the surface of biochar AP1 and BS2, which may provide more adsorption sites and increase the biochar's adsorption capacity. Figure 2: SEM images comparing (a) OP with AP1 and (b) OS with BS 3.2. Elemental Analysis of Biochar According to Table 2, the non-metallic elements with the highest amounts were C, N, and Si. In contrast to the modified biochar (AP1 and BS2), the unmodified biochar (OP and OS) had higher C and Si 345 N.N. Lemeh et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 338-352 elemental concentrations. In contrast to OP and OS, AP1 and BS2 have a very high N elemental concentration. This is because of the KMnO4/HNO3 alteration, which demonstrates the reduction in ash brought on by the loss of silicon and carbon (Fan et al., 2018). Except Na, which is nearly equal in OP (0.25%) and AP1 (0.27%) and decreases from OS to BS2, the unmodified biochar (OP and OS) has a significant amount of the metallic elements Mg and K. Table 2: EDS elemental composition of the modified and unmodified biochar’s Samples Non-metallic elements (%) Metallic elements (%) C N Si Cl S P Mg Na K Ca Fe Ti OP 88.32 5.18 4.24 0.12 0.15 0.17 0.30 0.25 4.24 0.38 0 0 OS 89.48 4.24 0.99 0.37 0.26 0.58 0.46 0.37 2.57 0.28 0 0.08 AP1 88.13 8.62 1.42 0.31 0.16 0.14 0.27 0.27 0.19 0 0.12 0.04 BS2 86.78 7.24 0.75 2.51 0.31 0.43 0.37 0.20 0.34 0.23 0.31 0 The FTIR spectra of the modified and unmodified biochars are displayed in Figure 3. Carboxyl, esters, and anhydride groups are among the several active functional groups found in biochar (Ma et al., 2010). It is clear that the oxygen-containing functional groups on the surface of AP1 and BS2 are significantly impacted by the alteration process. The stretching vibration of -OH is responsible for the absorption peak at wavenumber 3380cm-1 for AP1 and 3570cm-1 for BS2 (Chen et al., 2008). The findings imply that the biochar treated with KMnO4/HNO3 has more oxygen-containing functional groups, such as hydroxyl groups. The existence of the aldehyde terminal C-H functional group, which also suggests the presence of oxygen-containing functional groups, is shown by the stretching seen at 2922.2 cm-1 in AP1. The stretching vibrations of different aromatic ethers, or O-H and C-O, are represented by the absorption peak in the biochars between wave numbers 1039.9cm-1 and 11107.7cm-1, suggesting that the biochars have a good aromatic structure (Li B et al., 2017). (a) (b) (c) (d) Figure 3: FTIR spectrum of (a) OP, (b) AP1, (c) OS and (d) BS2 346 N.N. Lemeh et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 338-352 3.3. Properties Analysis Table 3 displays the proximate analysis for the unmodified biochars (OS and OP). The alkaline biochars had pH values of 10.7 and 9.1, CaCO3 equivalents of 48.04% and 26.98%, and total ash contents of 4.5% and 1.8%. The yields for OS and OP were 26.8% and 33.2%, respectively. Table 3: Proximate analysis of unmodified biochar (OS and OP) Samples pH Total ash CaCO3 equi. (%) Yield (%) OS 10.7 4.5 48.04 26.8 OP 9.1 1.8 26.98 33.2 3.4. Adsorption Isotherms of Co(II) According to Wang et al. (2023), Table 4 shows the unmodified (OP and OS) and modified (AP1 and BS2) plots, respectively, contain the linear constant (n), the Langmuir model constant KL(L/mg), the Freundlich model constant Kf(mg/g), and the predicted maximum adsorption capacity for Cobalt qm(mg/g). The interaction between the adsorbent (biochars) and the adsorbate (Co2⁺) could be examined using the Langmuir and Freundlich isotherm models, which provided valuable insights into the adsorption process. Table 4 also shows the experimental parameters of the Langmuir and Freundlich adsorption, while figure 4 and 5 shows the statistical plot Co2+ uptake by AP1, OP, BS2, OS and their fit to the Langmuir model and Freundlich. The adsorption constant of the Langmuir model of AP1 and BS2 is higher compared to OP and OS, showing the affinity of AP1 and BS2 adsorbent with CO2+ in solution, and it is easier to immobilize free CO2+ by the adsorption reaction. AP1 and BS2 are also higher sequencially when compared to OP and OS. The 1/n values (from 0.0257 - 0.847) shows the intensity of the adsorption reaction, and values is less than one, showing that the adsorption of Co2+ is an ideal adsorption process that easily occur (Faust and Scipenbush, 2014). The Freundlich plot for AP1 (R2 = 0.9976) is superior to the Langmuir plot (g) (R2 = 0.9659), while the Freundlich plot for BS2 (R2 = 0.9426) is superior to the Langmuir plot (e) (R2 = 0.914), which indicates that the Freundlich isotherm can better describe the adsorption of Co2+, surfaces having heterogeneous adsorption sites or multi-layer adsorption are better described by the Freundlich model (Liu et al., 2019). Table 4: Shows the Langmuir and Freundlich isotherm parameters for AP1, OP, BS2, OS Samples qe (mg.g1) Langmuir Kl (L.Mg1) R2 Freundlich KF (Mg.g1) 1/n R2 AP1 84.74 0.177 0.9659 1.49x1018 0.0518 0.9976 OP 41.15 0.117 0.9334 1.12x1040 0.0257 0.8734 BS2 714.28 60.75 0.914 9.6x102 0.8473 0.9426 OS 74.07 1.687 0.877 4.3x10-2 0.0436 0.8011 Figure 4: Experimental isotherm data of Co2+ uptake by AP1, OP, BS2, OS and their fit to the Langmuir model