Carbon Sequestration Potential of Coconut Shell Biochar Produced by Slow Pyrolysis

297 Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 297-304 p ISSN: 2635-3342; e ISSN: 2635-3350 Original Research Article Carbon Sequestration Potential of Coconut Shell Biochar Produced by Slow Pyrolysis *Ejimadu, M.C., Okuo, J.M. and Okieimen, F.E. Centre for Biomaterials Research, Department of Chemistry, University of Benin, Benin City, Nigeria. *[email protected] http://doi.org/10.5281/zenodo.18060952 ARTICLE INFORMATION ABSTRACT Article history: Received 25 Jun. 2025 Revised 20 Sep. 2025 Accepted 01 Oct. 2025 Available online 30 Dec. 2025 Coconut shell biochar samples were subjected to slow pyrolysis at various temperatures: 350 - 700°C, and characterized in terms of physicochemical properties: yield, pH, cation exchange capacity, elemental composition (C, H, N, S, and O), and biochar stability using the KMnO4 oxidation method. The biochar yield decreased markedly (by about 47.17%) with an increase in pyrolysis temperature from 350 – 700 °C, while the values of pH and cation exchange capacity increased with an increase in pyrolysis temperature. The carbon content (mol %) of the biochar samples increased from 4.50 for the unpyrolysed coconut shell to 5.72 mol (%) for the biochar sample obtained at 700 °C. The O, N, and O contents decreased with an increase in pyrolysis temperature. The H:C and O:C ratios, indices obtained, and values that are indicative of the carbon sequestration potential of coconut shell-derived biochar, decreased with an increase in pyrolysis temperature. The accelerated chemical oxidative test with KMnO4 gave values for biochar stability that increased with an increase in pyrolysis temperature, consistent with the suggestion that coconut shell biochar prepared by slow pyrolysis at high temperatures (≥ 600) has potential in carbon sequestration applications. © 2025 RJEES. All rights reserved. Keywords: Biochar stability Carbon sequestration Coconut shell biochar Physicochemical properties Pyrolysis Ultimate composition 1. INTRODUCTION Biochar, the carbon rich product of pyrolysis of biomass plays an important role and transformation of chemical elements and compounds between living organisms, the atmosphere and Earth's crest (biogeochemical circle) has attracted much attention in recent years (Lehmann et al., 2006; Harvey et al., 2012). It is widely recognized that biochar is an important carbon sink in the global carbon cycle in addition to its role as effective sorbent and viable soil amendments (Woolf et al., 2010). The high recalcitrance resistance of biochar to abiotic and biotic degradation is generally regarded as responsible for it potential as carbon sink (carbon sequestration) (Cheng et al., 2006; Harvey et al., 2012). The high carbon content 298 M.C. Ejimadu et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 297-304 of biochar and its low turnover rates is responsible for the increased interest in land application of biochar as an important aspect of the strategy for carbon sequestration and global climate change mitigation. The physicochemical properties of biochar depend on the biomass feedstocks, pyrolysis process; hydrothermal, thermochemical; slow, fast or flash pyrolysis, and process conditions; heat treatment temperature; duration of heat treatment, and heating rate (Enders et al., 2012). The processes define biochar functional properties to include physicochemical and textural properties, stability/ recalcitrance and carbon sequestration potential. A number of methods have emerged for evaluating biochar stability; proximate analysis (Enders et al., 2012) O:C or H:C molar ratios (Spokas, 2010; Enders et al., 2012) International Biochar Institute Guidelines (IBI, 2015), and chemical oxidation (Cross and Sohig, 2013). Coconut shell and husks constitute a major waste stream in the coconut industry with worldwide annual production exceeding 124 million tons (Ajien et al., 2023). These waste materials are often disposed directly into the environment or through open-air burning with potential environmental and public health concerns (Jain et al., 2024). Previous studies on pyrolysis of coconut biomass (husk and shell), its activation (by chemical and physical methods), surface modification and temperature effect on the textural and physicochemical properties of the derived biochar were reviewed in a recent publication (Ejimadu et al., 2025). The growing interest in the multifunctional properties of biochar relevant to its soil application requires that locally available biomass materials be included in the resource base for biochar production. This study examined the effect of heat treatment temperature on carbon sequestration potential of coconut shell biochar produced by slow pyrolysis; heat treatment temperature ≤ 700 oC; elevation of heat treatment temperature ≤ 1hr; heating rate 1-10oC.min-1. 2. MATERIALS AND METHODS 2.1. Materials Coconut shells were collected from a local market in Benin City. They were washed with tap water and rinsed with distilled water to remove physical impurities, then air-dried at room temperature for seven days. The dried coconut shells were ground to 2 mm before being converted into biochar through pyrolysis at different temperatures (350, 400, 450, 500, 600, and 700 °C) and held at the highest heating temperature for 30 mins. The resulting biochars were further crushed, sieved through a 250 µm sieve, then stored in a plastic container and labeled CSB350, CSB400, CSB450, CSB500, CSB600 and CSB700, respectively. All chemicals used in this study were of analytical grade. 2.2. Physicochemical Characterization 2.2.1. Determination of yield The biochar yield was determined using the formula as described by Naeem et al. (2014): Biochar yield (wt%) = 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑏𝑖𝑜𝑐ℎ𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑓𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘 𝑢𝑠𝑒𝑑 x 100 (1) 2.2.2. Determination of pH A mixture of biochar and deionized water (1:10 wt/wt) was prepared, and the slurry was equilibrated for 1 hour before measuring the pH using a precalibrated digital pH meter (Jenway 3020, Dunmow Essex, Jenway Ltd., England). 2.2.3. Cation exchange capacity (CEC) The CEC was determined using the modified ammonium acetate displacement method as described by Othugile et al. (2022). Samples (0.2g each) were leached five times with 20 mL deionized water to reduce interference from soluble salts. The samples were then leached five times with 20 mL of 1M sodium acetate (pH 7), to remove or extract exchangeable cations. The samples were later washed with 20ml of ethanol five 299 M.C. Ejimadu et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 297-304 times to remove the excess sodium ion (Na+). The Na+ on the exchangeable sites of the material was then displaced five times using 100ml of 1M Ammonium acetate (pH 7). The CEC was calculated from the Na⁺ displaced by NH₄⁺, measured using a flame photometer. 2.3. Ultimate Composition The percentages of C, H, S, and N in biochar were determined with an elemental analyzer (Elemental Vario EL III, Germany), and the O content was calculated by subtracting C, H, N, and S from the total (Gazulla et al., 2016). 2.4. Chemical Oxidation Stability The KMnO4 method (Tirol-Padre and Ladha, 2004) was used to assess the easily oxidizable fraction of the studied biochars. Biochar samples were acid pretreated to remove inorganic carbonates following the method described by Fidel et al. (2013). Typically, the biochar was added to a solution of KMnO4 (33mM, pH 7.2) in a 1:50 w/v biochar: KMnO4 ratio. It was agitated for 60mins, heated at about 60° C for 60mins and thereafter left overnight at 25 oC. The solid residue was separated, thoroughly rinsed with distilled water, and dried at 80°C for 4 days. The mass loss from the oxidation treatment was measured gravimetrically, and the dried sample was subsequently analyzed for its elemental composition (C, H, N, O and S). An indication of the stability of biochar to oxidative degradation AE may be obtained from Equation 2. AE = Brx BrC Bt x BrC x 100 (2) Where BrC and BtC represent the carbon content in the residual and total biochar respectively, and Br and Bt are the mass after and before treatment (Cross and Sohi, 2013). 3. RESULTS AND DISCUSSION 3.1. Yield, pH and Cation Exchange Capacity The trend that can be readily observed in the data shown in Figure 1 includes: decrease in biochar yield with increase in pyrolysis temperature. The decrease in biochar yield from 67.70 ± 0.56% for CSB350 to 35.76 ± 0.42% for CSB700 is consistent with those reported in literature (Mimmo et al., 2014; Masek et al., 2013; Wang et al., 2024; Handiso et al., 2024; Chen et al., 2012, 2014). Figure 1: Effect of heat treatment temperature on the biochar yield, pH and CEC of coconut shell biochar Biochar pH varied from 7.90 ± 0.90 for CSB350 to 10.70 ± 0.77 for CSB700. Increase in pH of biochar with increase in pyrolysis temperature has been attributed to accumulation of inorganic salts in the biochar 0 10 20 30 40 50 60 70 300 400 500 600 700 800 Yield pH CEC Pyrolysis temperature (oC) 300 M.C. Ejimadu et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 297-304 resulting from high degree of carbonization (Yuan et al., 2011). High pH is generally considered to have ameliorating effect on soil properties relevant to agronomic benefits. The cation exchange capacity (cmol.kg1) of the biochar samples increased steadily from 11.47 ± 2.40 for CSB350 to 19.60 ± 2.26 for CSB500 and thereafter declined somewhat, reaching 14.11 ± 2.50 for CSB700. Cation exchange capacity (CEC) is the amount of exchangeable cation; Na+, NH+4, Ca2+, K+ and Mg2+ that a material can retain. It depends on presence of surface functional groups (charges) that can attract and retain cation and on the textural properties; surface area and porosity (pore size and pore volume) of a material (Liang et al., 2006). CEC is a useful indicator of soil fertility (Robertson et al., 1999). The higher values of CEC associated with moderate temperatures of pyrolysis is consistent with the suggestion of the retention of surface functional groups, which are loss at high pyrolysis temperatures. Textural characteristics; specific surface area, pore size and pore volume, have been shown to follow similar trend with increase in pyrolysis temperature (Brown et al., 2006; Yang et al., 2016). 3.2. Elemental Characteristics Table 1 gives the elemental composition (mol%) of raw coconut shell (CS) and the biochar samples obtained at different pyrolysis temperatures. Table 1: Ultimate composition and molecular H/C, O/C and (N+O)/C ratios of coconut shell biochar prepared at different pyrolysis temperatures Sample Elemental composition (mol%) Atomic mole ratio C (%) H (%) O (%) N (%) (H/C) (O/C) CS 4.50 3.22 2.61 0.04 0.72 0.58 CSB350 5.02 3.03 2.26 0.03 0.60 0.45 CSB400 5.12 2.99 2.12 0.03 0.58 0.41 CSB500 5.33 2.53 2.11 0.04 0.47 0.40 CSB600 5.42 2.43 2.09 0.07 0.45 0.38 CSB700 5.72 2.30 1.71 0.07 0.40 0.30 CS - coconut shell; CSB - coconut shell biochar It can be seen that the carbon content of the biochar samples was lower, but H and O content were higher in the unpyrolyzed coconut shell than in the biochar samples. An increase in pyrolysis temperature was associated with an increase in the C content of the biochar samples. The reduced O and H contents of the biochar samples with an increase in pyrolysis have been explained in terms of dehydration and deoxygenation reactions leading to the elimination of H and O and resulting in the accumulation of C in biochar residue (Chen et al., 2012; Chen et al., 2014; Demirbas, 2004). The contents and ratios of the elements are important factors that influence the stability or carbon sequestration potential of biochar. Nitrogen is an important nutrient, and its presence in biochar can have agronomic relevance. The results in Table 1 show that N content of coconut shell (CS) is low, 0.04 mol%, and is consistent with the value reported for similar high-lignin biomass (Gaskin et al., 2008). The N content decreased somewhat from 0.04 mol% for CS to 0.03 for CSB350 and thereafter increased with an increase in pyrolysis temperature to 0.07 mol% for CSB700 similar to the trend reported by Gaskin et al. (2008). The H/C and O/C ratios of coconut shell biochar samples obtained at different pyrolysis temperatures are given in Table 1 and shown in Figure 2 as a van Krevelen graph. It can be seen that the values are highest for CS; 0.71 and 0.58, and decrease with an increase in pyrolysis temperature to 0.40 and 0.30, respectively for CSB700. The H/C and O/C ratios are indicative of the structures of the biochar and are useful indicators of biochar stability. 301 M.C. Ejimadu et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 297-304 Figure 2: Van Krevelen graph of coconut shell biochar prepared at different temperatures The H/C molecular ratios obtained in this study can be considered a basic proxy for the average number of H - C bonds per carbon atom, which provides an estimate of the average size of polyaromatic graphene clusters in the biochar, and indication of the overall stability of biochar in soil (Chu et al., 2019). The values of the H/C given in Table 1 are lower than 0.9 and indicates that they are stable structure and can find use in carbon sequestration (Crombie et al., 2013). The O/C ratios given in Table 1 decreased from 0.58 for CS to 0.30 for CSB700. It has been shown that biochars half-life is related to O/C molecular ratios; a longer than 1000 years half-life is expected for biochar with O/C molecular ratio lower than 0.2; a half-life range of 100 - 1000 years for biochar with O/C molecular ratio of between 0.2 and 0.6 and half-life of shorter than 100 years for biochar with O/C molecular ratio higher than 0.6 (Tu et al., 2022). The results from this show that increase in pyrolysis temperature could increase the stability of coconut shell biochar and enhance its potential in carbon sequestration application. 3.3. Chemical Oxidation Stability of Biochar Pyrolytic biochars possess carbons of different stability; recalcitrant (stable) and labile (unstable) forms. Pyrolysis conditions, particularly temperature, in addition to the biomass type plays important role in the relative stability of biochars. The stable forms of carbon in biochars are of direct relevance to carbon sequestration potential of biochar. Chemical oxidation of coconut shell biochar produced by slow pyrolysis at various temperatures was determined by the KMnO4 oxidation method. The stability of the biochars obtained (%) is shown in Figure 3. It can be seen that the chemical oxidation stability of the biochars was pyrolysis temperature dependent and varied from 58.63% for CS to 64.84% for CSB700. The correlation of biochar stability with H/C and O/C molecular ratios (Figure 4) is consistent with reports from previous studies (Malghani et al., 2013; Wei et al., 2020; Leng et al., 2019) that implicates low values of H/C and O/C with increased biochar stability. Figure 4 correlated the major indicators of biochar stability and their implications in carbon sequestration. The results corroborate the suggestion from this study that high treatment temperature will be required to produce biochar of the requisite carbon sequestration potential. 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.25 0.35 0.45 0.55 0.65 CS CSB350 CSB400 CSB500 CSB600 CSB700 O/C H/C 302 M.C. Ejimadu et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 297-304 Figure 3: Effect of pyrolysis temperature on oxidative stability of coconut shell biochar Figure 4: Correlation of H/C and O/C molecular ratio with biochar stability 4. CONCLUSION This study examined the effect of slow pyrolysis at different temperatures on carbon sequestration potential measured by H/C and O/C molecular ratios and oxidation stability, of coconut shell biochars. The results were convergent and showed the positive effect of pyrolysis temperature on the indices of carbon sequestration. 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