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Effects of pyrolysis temperature on chemical composition of coconut-husk biochar for agricultural applications: a characterization study

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  • Coconut husk, a plentiful agricultural waste, rich in cellulose and lignin, is abundant in tropical and subtropical regions worldwide. The emergence of new green energy technologies harnessing coconut husk has intensified interest in biochar production due to its affordability and low energy requirements. The effectiveness of biochar varies based on the raw materials and production process. Hence, this study aimed to evaluate the chemical and structural properties of coconut-husk biochar produced at different pyrolysis temperatures, focusing on its agricultural benefits. In this research, biochar derived from coconut husk was generated at varying pyrolysis temperatures 325, 350, 400, 500, 600, and 700 °C under limited oxygen supply and a heating rate of 7 °C/min for 3 h. The chemical and structural properties of the produced biochar were meticulously examined using Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectroscopy (EDX) techniques. The findings underscored the significant impact of pyrolysis temperature on the chemical properties and structure of coconut-husk biochar, especially at lower heating rates. Remarkably, the highest yield, recorded at 42.79% (at p < 0.05), was achieved at 325 °C, emphasizing the suitability of lower pyrolysis temperatures for biochar production using coconut husk. Furthermore, alterations in pyrolysis temperature resulted in notable differences in elemental concentrations and significant changes in the biochar structure. These modifications enhance plant and leaf water use efficiencies, boost plant photosynthesis efficiency, modify soil properties, reduce greenhouse gas emissions, and contribute to mitigating global warming.
  • Starting in the early 2000s, China has experienced rapid growth as an emerging wine market. It has now established itself as the world's second-largest grape-growing country in terms of vineyard surface area. Furthermore, China has also secured its position as the sixth-biggest wine producer globally and the fifth-most significant wine consumer in terms of volume[1]. The Ningxia Hui autonomous region, known for its reputation as the highest quality wine-producing area in China, is considered one of the country's most promising wine regions. The region's arid or semiarid climate, combined with ample sunlight and warmth, thanks to the Yellow River, provides ideal conditions for grape cultivation. Wineries in the Ningxia Hui autonomous region are renowned as the foremost representatives of elite Chinese wineries. All wines produced in this region originate from grapes grown in their vineyards, adhering to strict quality requirements, and have gained a well-deserved international reputation for excellence. Notably, in 2011, Helan Mountain's East Foothill in the Ningxia Hui Autonomous Region received protected geographic indication status in China. Subsequently, in 2012, it became the first provincial wine region in China to be accepted as an official observer by the International Organisation of Vine and Wine (OIV)[2]. The wine produced in the Helan Mountain East Region of Ningxia, China, is one of the first Agricultural and Food Geographical Indications. Starting in 2020, this wine will be protected in the European Union[3].

    Marselan, a hybrid variety of Cabernet Sauvignon and Grenache was introduced to China in 2001 by the French National Institute for Agricultural Research (INRA). Over the last 15 years, Marselan has spread widely across China, in contrast to its lesser cultivation in France. The wines produced from Marselan grapes possess a strong and elegant structure, making them highly suitable for the preferences of Chinese consumers. As a result, many wineries in the Ningxia Hui Autonomous Region have made Marselan wines their main product[4]. Wine is a complex beverage that is influenced by various natural and anthropogenic factors throughout the wine-making process. These factors include soil, climate, agrochemicals, and human intervention. While there is an abundance of research available on wine production, limited research has been conducted specifically on local wines in the Eastern Foot of Helan Mountain. This research gap is of significant importance for the management and quality improvement of Chinese local wines.

    Ion mobility spectrometry (IMS) is a rapid analytical technique used to detect trace gases and characterize chemical ionic substances. It achieves this through the gas-phase separation of ionized molecules under an electric field at ambient pressure. In recent years, IMS has gained increasing popularity in the field of food-omics due to its numerous advantages. These advantages include ultra-high analytical speed, simplicity, easy operation, time efficiency, relatively low cost, and the absence of sample preparation steps. As a result, IMS is now being applied more frequently in various areas of food analysis, such as food composition and nutrition, food authentication, detection of food adulteration, food process control, and chemical food safety[5,6]. The orthogonal hyphenation of gas chromatography (GC) and IMS has greatly improved the resolution of complex food matrices when using GC-IMS, particularly in the analysis of wines[7].

    The objective of this study was to investigate the changes in the physicochemical properties of Marselan wine during the winemaking process, with a focus on the total phenolic and flavonoids content, antioxidant activity, and volatile profile using the GC-IMS method. The findings of this research are anticipated to make a valuable contribution to the theoretical framework for evaluating the authenticity and characterizing Ningxia Marselan wine. Moreover, it is expected that these results will aid in the formulation of regulations and legislation pertaining to Ningxia Marselan wine in China.

    All the grapes used to produce Marselan wines, grow in the Xiban vineyard (106.31463° E and 38.509541° N) situated in Helan Mountain's East Foothill of Ningxia Hui Autonomous Region in China.

    Folin-Ciocalteau reagent, (±)-6-Hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), 2,20-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), 2,4,6-tris (2-pyridyl)-s-triazine (TPTZ), anhydrous methanol, sodium nitrite, and sodium carbonate anhydrous were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Reference standards of (+)-catechin, gallic acid, and the internal standard (IS) 4-methyl-2-pentanol were supplied by Shanghai Yuanye Bio-Technology Co., Ltd (Shanghai, China). The purity of the above references was higher than 98%. Ultrapure water (18.2 MΩ cm) was prepared by a Milli-Q system (Millipore, Bedford, MA, USA).

    Stage 1−Juice processing: Grapes at the fully mature stage are harvested and crushed, and potassium metabisulfite (5 mg/L of SO2) was evenly spread during the crushing process. The obtained must is transferred into stainless steel tanks. Stage 2−Alcoholic fermentation: Propagated Saccharomyces cerevisiae ES488 (Enartis, Italy) are added to the fresh must, and alcoholic fermentation takes place, after the process is finished, it is kept in the tanks for 7 d for traditional maceration to improve color properties and phenolics content. Stage 3−Malolactic fermentation: When the pomace is fully concentrated at the bottom of the tanks, the wine is transferred to another tank for separation from these residues. Oenococcus oeni VP41 (Lallemand Inc., France) is inoculated and malic acid begins to convert into lactic acid. Stage 4−Wine stabilization: After malolactic fermentation, potassium metabisulfite is re-added (35 mg/L of SO2), and then transferred to oak barrels for stabilization, this process usually takes 6-24 months. A total of four batches of samples during the production process of Marselan wine were collected in this study.

    Total polyphenols were determined on 0.5 mL diluted wine sample using the Folin-Ciocalteu method[8], using gallic acid as a reference compound, and expressed as milligrams of gallic acid equivalents per liter of wine. The total flavonoid content was measured on 0.05 mL of wine sample by a colorimetric method previously described[9]. Results are calculated from the calibration curve obtained with catechin, as milligrams of catechin equivalents per liter of wine.

    The antioxidative activity was determined using the ABTS·+ assay[10]. Briefly, the ABTS·+ radical was prepared from a mixture of 88 μL of potassium persulfate (140 mmol/L) with 5 mL of the ABTS·+ solution (7 mmol/L). The reaction was kept at room temperature under the absence of light for 16 h. Sixty μL samples were mixed with 3 mL of ABTS·+ solution with measured absorption of 0.700 ± 0.200 at 734 nm. After 6 min reaction, the absorbance of samples were measured with a spectrophotometer at 734 nm. Each sample was tested in triplicate. The data were expressed as mmol Trolox equivalent of antioxidative capacity per liter of the wine sample (mmol TE/L). Calibration curves, in the range 64.16−1,020.20 μmol TE/L, showed good linearity (R2 ≥ 0.99).

    The FRAP assay was conducted according to a previous study[11]. The FRAP reagent was freshly prepared and mixed with 10 mM/L TPTZ solution prepared in 20 mM/L FeCl3·6H2O solution, 40 mM/L HCl, and 300 mM/L acetate buffer (pH 3.6) (1:1:10; v:v:v). Ten ml of diluted sample was mixed with 1.8 ml of FRAP reagent and incubated at 37 °C for 30 min. The absorbance was determined at 593 nm and the results were reported as mM Fe (II) equivalent per liter of the wine sample. The samples were analyzed and calculated by a calibration curve of ferrous sulphate (0.15−2.00 mM/mL) for quantification.

    The volatile compounds were analyzed on a GC-IMS instrument (FlavourSpec, GAS, Dortmund, Germany) equipped with an autosampler (Hanon Auto SPE 100, Shandong, China) for headspace analysis. One mL of each wine was sampled in 20 mL headspace vials (CNW Technologies, Germany) with 20 μL of 4-methyl-2-pentanol (20 mg/L) ppm as internal standard, incubated at 60 °C and continuously shaken at 500 rpm for 10 min. One hundred μL of headspace sample was automatically loaded into the injector in splitless mode through a syringe heated to 65 °C. The analytes were separated on a MxtWAX capillary column (30 m × 0.53 mm, 1.0 μm) from Restek (Bellefonte, Pennsylvania, USA) at a constant temperature of 60 °C and then ionized in the IMS instrument (FlavourSpec®, Gesellschaft für Analytische Sensorsysteme mbH, Dortmund, Germany) at 45 °C. High purity nitrogen gas (99.999%) was used as the carrier gas at 150 mL/min, and drift gas at 2 ml/min for 0−2.0 min, then increased to 100 mL/min from 2.0 to 20 min, and kept at 100 mL/min for 10 min. Ketones C4−C9 (Sigma Aldrich, St. Louis, MO, USA) were used as an external standard to determine the retention index (RI) of volatile compounds. Analyte identification was performed using a Laboratory Analytical Viewer (LAV) 2.2.1 (GAS, Dortmund, Germany) by comparing RI and the drift time of the standard in the GC-IMS Library.

    All samples were prepared in duplicate and tested at least six times, and the results were expressed as mean ± standard error (n = 4) and the level of statistical significance (p < 0.05) was analyzed by using Tukey's range test using SPSS 18.0 software (SPSS Inc., IL, USA). The principal component analysis (PCA) was performed using the LAV software in-built 'Dynamic PCA' plug-in to model patterns of aroma volatiles. Orthogonal partial least-square discriminant analysis (OPLS-DA) in SIMCA-P 14.1 software (Umetrics, Umeă, Sweden) was used to analyze the different volatile organic compounds in the different fermentation stages.

    The results of the changes in the antioxidant activity of Marselan wines during the entire brewing process are listed in Table 1. It can be seen that the contents of flavonoids and polyphenols showed an increasing trend during the brewing process of Marselan wine, which range from 315.71−1,498 mg CE/L and 1,083.93−3,370.92 mg GAE/L, respectively. It was observed that the content increased rapidly in the alcoholic fermentation stage, but slowly in the subsequent fermentation stage. This indicated that the formation of flavonoid and phenolic substances in wine mainly concentrated in the alcoholic fermentation stage, which is consistent with previous reports. This is mainly because during the alcoholic fermentation of grapes, impregnation occurred to extract these compounds[12]. The antioxidant activities of Marselan wine samples at different fermentation stages were detected by FRAP and ABTS methods[11]. The results showed that the ferric reduction capacity and ABST·+ free radical scavenging capacity of the fermented Marselan wines were 2.4 and 1.5 times higher than the sample from the juice processing stage, respectively, indicating that the fermented Marselan wine had higher antioxidant activity. A large number of previous studies have suggested that there is a close correlation between antioxidant activity and the content of polyphenols and flavonoids[1315]. Previous studies have reported that Marselan wine has the highest total phenol and anthocyanin content compared to the wine of Tannat, Cabernet Sauvignon, Merlot, Cabernet Franc, and Syrah[13]. Polyphenols and flavonoids play an important role in improving human immunity. Therefore, Marselan wines are popular because of their high phenolic and flavonoid content and high antioxidant capacity.

    Table 1.  GC-IMS integration parameters of volatile compounds in Marselan wine at different fermentation stages.
    No. Compounds Formula RI* Rt
    [sec]**
    Dt
    [RIPrel]***
    Identification
    approach
    Concentration (μg/mL) (n = 4)
    Stage 1 Stage 2 Stage 3 Stage 4
    Aldehydes
    5 Furfural C5H4O2 1513.1 941.943 1.08702 RI, DT, IS 89.10 ± 4.05c 69.98 ± 3.22c 352.16 ± 39.06b 706.30 ± 58.22a
    6 Furfural dimer C5H4O2 1516.6 948.77 1.33299 RI, DT, IS 22.08 ± 0.69b 18.68 ± 2.59c 23.73 ± 2.69b 53.39 ± 9.42a
    12 (E)-2-hexenal C6H10O 1223.1 426.758 1.18076 RI, DT, IS 158.17 ± 7.26a 47.57 ± 2.51b 39.00 ± 2.06c 43.52 ± 4.63bc
    17 (E)-2-pentenal C5H8O 1129.2 333.392 1.1074 RI, DT, IS 23.00 ± 4.56a 16.42 ± 1.69c 18.82 ± 0.27b 18.81 ± 0.55b
    19 Heptanal C7H14O 1194.2 390.299 1.33002 RI, DT, IS 17.28 ± 2.25a 10.22 ± 0.59c 14.50 ± 8.84b 9.11 ± 1.06c
    22 Hexanal C6H12O 1094.6 304.324 1.25538 RI, DT, IS 803.11 ± 7.47c 1631.34 ± 19.63a 1511.11 ± 26.91b 1526.53 ± 8.12b
    23 Hexanal dimer C6H12O 1093.9 303.915 1.56442 RI, DT, IS 588.85 ± 7.96a 93.75 ± 4.67b 92.93 ± 3.13b 95.49 ± 2.50b
    29 3-Methylbutanal C5H10O 914.1 226.776 1.40351 RI, DT, IS 227.86 ± 6.39a 33.32 ± 2.59b 22.36 ± 1.18c 21.94 ± 1.73c
    33 Dimethyl sulfide C2H6S 797.1 193.431 0.95905 RI, DT, IS 120.07 ± 4.40c 87.a02 ± 3.82d 246.81 ± 5.62b 257.18 ± 3.04a
    49 2-Methylpropanal C4H8O 828.3 202.324 1.28294 RI, DT, IS 150.49 ± 7.13a 27.08 ± 1.48b 19.36 ± 1.10c 19.69 ± 0.92c
    Ketones
    45 3-Hydroxy-2-butanone C4H8O2 1293.5 515.501 1.20934 RI, DT, IS 33.20 ± 3.83c 97.93 ± 8.72b 163.20 ± 21.62a 143.51 ± 21.48a
    46 Acetone C3H6O 836.4 204.638 1.11191 RI, DT, IS 185.75 ± 8.16c 320.43 ± 12.32b 430.74 ± 3.98a 446.58 ± 10.41a
    Organic acid
    3 Acetic acid C2H4O2 1527.2 969.252 1.05013 RI, DT, IS 674.66 ± 46.30d 3602.39 ± 30.87c 4536.02 ± 138.86a 4092.30 ± 40.33b
    4 Acetic acid dimer C2H4O2 1527.2 969.252 1.15554 RI, DT, IS 45.25 ± 3.89c 312.16 ± 19.39b 625.79 ± 78.12a 538.35 ± 56.38a
    Alcohols
    8 1-Hexanol C6H14O 1365.1 653.825 1.32772 RI, DT, IS 1647.65 ± 28.94a 886.33 ± 32.96b 740.73 ± 44.25c 730.80 ± 21.58c
    9 1-Hexanol dimer C6H14O 1365.8 655.191 1.64044 RI, DT, IS 378.42 ± 20.44a 332.65 ± 25.76a 215.78 ± 21.04b 200.14 ± 28.34b
    13 3-Methyl-1-butanol C5H12O 1213.3 414.364 1.24294 RI, DT, IS 691.86 ± 9.95c 870.41 ± 22.63b 912.80 ± 23.94a 939.49 ± 12.44a
    14 3-Methyl-1-butanol dimer C5H12O 1213.3 414.364 1.49166 RI, DT, IS 439.90 ± 29.40c 8572.27 ± 60.56b 9083.14 ± 193.19a 9152.25 ± 137.80a
    15 1-Butanol C4H10O 1147.2 348.949 1.18073 RI, DT, IS 157.33 ± 9.44b 198.92 ± 3.92a 152.78 ± 10.85b 156.02 ± 9.80b
    16 1-Butanol dimer C4H10O 1146.8 348.54 1.38109 RI, DT, IS 24.14 ± 2.15c 274.75 ± 12.60a 183.02 ± 17.72b 176.80 ± 19.80b
    24 1-Propanol C3H8O 1040.9 274.803 1.11042 RI, DT, IS 173.73 ± 4.75a 55.84 ± 2.16c 80.80 ± 4.99b 83.57 ± 2.34b
    25 1-Propanol dimer C3H8O 1040.4 274.554 1.24784 RI, DT, IS 58.20 ± 1.30b 541.37 ± 11.94a 541.33 ± 15.57a 538.84 ± 9.74a
    28 Ethanol C2H6O 930.6 231.504 1.11901 RI, DT, IS 5337.84 ± 84.16c 11324.05 ± 66.18a 9910.20 ± 100.76b 9936.10 ± 101.24b
    34 Methanol CH4O 903.6 223.79 0.98374 RI, DT, IS 662.08 ± 13.87a 76.94 ± 2.15b 61.92 ± 1.96c 62.89 ± 0.81c
    37 2-Methyl-1-propanol C4H10O 1098.5 306.889 1.35839 RI, DT, IS 306.91 ± 4.09c 3478.35 ± 25.95a 3308.79 ± 61.75b 3313.85 ± 60.88b
    48 1-Pentanol C5H12O 1257.6 470.317 1.25222 RI, DT, IS 26.13 ± 2.52c 116.50 ± 3.71ab 112.37 ± 6.26b 124.17 ± 7.04a
    Esters
    1 Methyl salicylate C8H8O3 1859.6 1616.201 1.20489 RI, DT, IS 615.00 ± 66.68a 485.08 ± 31.30b 470.14 ± 23.02b 429.12 ± 33.74b
    7 Butyl hexanoate C10H20O2 1403.0 727.561 1.47354 RI, DT, IS 95.83 ± 17.04a 62.87 ± 3.62a 92.59 ± 11.88b 82.13 ± 3.61c
    10 Hexyl acetate C8H16O2 1298.6 524.366 1.40405 RI, DT, IS 44.72 ± 8.21a 33.18 ± 2.17d 41.50 ± 4.38c 40.89 ± 4.33b
    11 Propyl hexanoate C9H18O2 1280.9 499.577 1.39274 RI, DT, IS 34.65 ± 3.90d 70.43 ± 5.95a 43.97 ± 4.39b 40.12 ± 4.05c
    18 Ethyl hexanoate C8H16O2 1237.4 444.749 1.80014 RI, DT, IS 55.55 ± 5.62c 1606.16 ± 25.63a 787.24 ± 16.95b 788.91 ± 28.50b
    20 Isoamyl acetate C7H14O2 1127.8 332.164 1.30514 RI, DT, IS 164.22 ± 1.00d 243.69 ± 8.37c 343.51 ± 13.98b 365.46 ± 1.60a
    21 Isoamyl acetate dimer C7H14O2 1126.8 331.345 1.75038 RI, DT, IS 53.61 ± 4.79d 4072.20 ± 11.94a 2416.70 ± 49.84b 2360.46 ± 43.29c
    26 Isobutyl acetate C6H12O2 1020.5 263.605 1.23281 RI, DT, IS 101.65 ± 1.81a 15.52 ± 0.67c 44.87 ± 3.21b 45.96 ± 1.41b
    27 Isobutyl acetate dimer C6H12O2 1019.6 263.107 1.61607 RI, DT, IS 34.60 ± 1.05d 540.84 ± 5.64a 265.54 ± 8.31c 287.06 ± 3.66b
    30 Ethyl acetate dimer C4H8O2 885.2 218.564 1.33587 RI, DT, IS 1020.75 ± 6.86d 5432.71 ± 6.55a 5052.99 ± 9.65b 5084.47 ± 7.30c
    31 Ethyl acetate C4H8O2 878.3 216.574 1.09754 RI, DT, IS 215.65 ± 3.58a 38.29 ± 2.37c 71.59 ± 2.99b 69.32 ± 2.85b
    32 Ethyl formate C3H6O2 838.1 205.127 1.19738 RI, DT, IS 175.48 ± 3.79d 1603.20 ± 13.72a 1472.10 ± 5.95c 1509.08 ± 13.26b
    35 Ethyl octanoate C10H20O2 1467.0 852.127 1.47312 RI, DT, IS 198.86 ± 36.71b 1853.06 ± 17.60a 1555.51 ± 24.21a 1478.05 ± 33.63a
    36 Ethyl octanoate dimer C10H20O2 1467.0 852.127 2.03169 RI, DT, IS 135.50 ± 13.02d 503.63 ± 15.86a 342.89 ± 11.62b 297.28 ± 14.40c
    38 Ethyl butanoate C6H12O2 1042.1 275.479 1.5664 RI, DT, IS 21.29 ± 2.68c 1384.67 ± 8.97a 1236.52 ± 20.21b 1228.09 ± 5.09b
    39 Ethyl 3-methylbutanoate C7H14O2 1066.3 288.754 1.26081 RI, DT, IS 9.70 ± 1.85d 200.29 ± 4.21a 146.87 ± 8.70b 127.13 ± 12.54c
    40 Propyl acetate C5H10O2 984.7 246.908 1.48651 RI, DT, IS 4.57 ± 1.07c 128.63 ± 4.28a 87.75 ± 3.26b 88.49 ± 1.99b
    41 Ethyl propanoate C5H10O2 962.1 240.47 1.46051 RI, DT, IS 10.11 ± 0.34d 107.08 ± 3.50a 149.60 ± 5.39c 167.15 ± 12.90b
    42 Ethyl isobutyrate C6H12O2 971.7 243.229 1.56687 RI, DT, IS 18.29 ± 2.61d 55.22 ± 1.07c 98.81 ± 4.67b 104.71 ± 4.73a
    43 Ethyl lactate C5H10O3 1352.2 628.782 1.14736 RI, DT, IS 31.81 ± 2.91c 158.03 ± 2.80b 548.14 ± 74.21a 527.01 ± 39.06a
    44 Ethyl lactate dimer C5H10O3 1351.9 628.056 1.53618 RI, DT, IS 44.55 ± 2.03c 47.56 ± 4.02c 412.23 ± 50.96a 185.87 ± 31.25b
    47 Ethyl heptanoate C9H18O2 1339.7 604.482 1.40822 RI, DT, IS 39.55 ± 6.37a 38.52 ± 2.47a 28.44 ± 1.52c 30.77 ± 2.79b
    Unknown
    1 RI, DT, IS 15.53 ± 0.18 35.69 ± 0.80 12.70 ± 0.80 10.57 ± 0.86
    2 RI, DT, IS 36.71 ± 1.51 120.41 ± 3.44 198.12 ± 6.01 201.19 ± 3.70
    3 RI, DT, IS 44.35 ± 0.88 514.12 ± 4.28 224.78 ± 6.56 228.32 ± 4.62
    4 RI, DT, IS 857.64 ± 8.63 33.22 ± 1.99 35.05 ± 5.99 35.17 ± 3.97
    * Represents the retention index calculated using n-ketones C4−C9 as external standard on MAX-WAX column. ** Represents the retention time in the capillary GC column. *** Represents the migration time in the drift tube.
     | Show Table
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    This study adopted the GC-IMS method to test the volatile organic compounds (VOCs) in the samples from the different fermentation stages of Marselan wine. Figure 1 shows the gas phase ion migration spectrum obtained, in which the ordinate represents the retention time of the gas chromatographic peaks and the abscissa represents the ion migration time (normalized)[16]. The entire spectrum represents the aroma fingerprints of Marselan wine at different fermentation stages, with each signal point on the right of the relative reactant ion peak (RIP) representing a volatile organic compound detected from the sample[17]. Here, the sample in stage 1 (juice processing) was used as a reference and the characteristic peaks in the spectrum of samples in other fermentation stages were compared and analyzed after deducting the reference. The colors of the same component with the same concentration cancel each other to form a white background. In the topographic map of other fermentation stages, darker indicates higher concentration compared to the white background. In the 2D spectra of different fermentation stages, the position and number of peaks indicated that peak intensities are basically the same, and there is no obvious difference. However, it is known that fermentation is an extremely complex chemical process, and the content and types of volatile organic compounds change with the extension of fermentation time, so other detection and characterization methods are needed to make the distinction.

    Figure 1.  2D-topographic plots of volatile organic compounds in Marselan wine at different fermentation stages.

    To visually display the dynamic changes of various substances in the fermentation process of Marselan wine, peaks with obvious differences were extracted to form the characteristic fingerprints for comparison (Fig. 2). Each row represents all signal peaks selected from samples at the same stage, and each column means the signal peaks of the same volatile compound in samples from different fermentation stages. Figure 2 shows the volatile organic compounds (VOCs) information for each sample and the differences between samples, where the numbers represent the undetermined substances in the migration spectrum library. The changes of volatile substances in the process of Marselan winemaking is observed by the fingerprint. As shown in Fig. 2 and Table 2, a total of 40 volatile chemical components were detected by qualitative analysis according to their retention time and ion migration time in the HS-GC-IMS spectrum, including 17 esters, eight alcohols, eight aldehydes, two ketones, one organic acid, and four unanalyzed flavor substances. The 12 volatile organic compounds presented dimer due to ionization of the protonated neutral components before entering the drift tube[18]. As can be seen from Table 2, the VOCs in the winemaking process of Marselan wine are mainly composed of esters, alcohols, and aldehydes, which play an important role in the construction of aroma characteristics.

    Figure 2.  Fingerprints of volatile organic compounds in Marselan wine at different fermentation stages.
    Table 2.  Antioxidant activity, total polyphenols, and flavonoids content of Marselan wine at different fermentation stages.
    Winemaking stage TFC (mg CE/L) TPC (mg GAE/L) FRAP (mM FeSO4/mL) ABTs (mM Trolox/L)
    Stage 1 315.71 ± 0.00d 1,083.93 ± 7.79d 34.82c 38.92 ± 2.12c
    Stage 2 1,490.00 ± 7.51c 3,225.51 ± 53.27c 77.32b 52.17 ± 0.95b
    Stage 3 1,510.00 ± 8.88a 3,307.143 ± 41.76b 77.56b 53.04 ± 0.76b
    Stage 4 1,498.57 ± 6.34b 3,370.92 ± 38.29a 85.07a 57.46 ± 2.55a
    Means in the same column with different letters are significantly different (p < 0.05).
     | Show Table
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    Esters are produced by the reaction of acids and alcohols in wine, mainly due to the activity of yeast during fermentation[19], and are the main components of fruit juices and wines that produce fruit flavors[20,21]. In this study, it was found that they were the largest detected volatile compound group in Marselan wine samples, which is consistent with previous reports[22]. It can be observed from Table 2 that the contents of most esters increased gradually with the extension of fermentation time, and they mainly began to accumulate in large quantities during the stage of alcohol fermentation. The contents of ethyl hexanoate (fruity), isoamyl acetate (banana, pear), ethyl octanoate (fruity, pineapple, apple, brandy), ethyl acetate (fruity), ethyl formate (spicy, pineapple), and ethyl butanoate (sweet, pineapple, banana, apple) significantly increased at the stage of alcoholic fermentation and maintained a high level in the subsequent fermentation stage (accounting for 86% of the total detected esters). These esters can endow a typical fruity aroma of Marselan wine, and played a positive role in the aroma profiles of Marselan wine. Among them, the content of ethyl acetate is the highest, which is 5,153.79 μg/mL in the final fermentation stage, accounting for 33.6% of the total ester. However, the content of ethyl acetate was relatively high before fermentation, which may be from the metabolic activity of autochthonous microorganisms present in the raw materials. Isobutyl acetate, ethyl 3-methyl butanoate, propyl acetate, ethyl propanoate, ethyl isobutyrate, and ethyl lactate were identified and quantified in all fermentation samples. The total contents of these esters in stage 1 and 4 were 255.28 and 1,533.38 μg/mL, respectively, indicating that they may also have a potential effect on the aroma quality of Marselan wine. The results indicate that esters are an important factor in the formation of flavor during the brewing process of Marselan wine.

    Alcohols were the second important aromatic compound in Marselan wine, which were mainly synthesized by glucose and amino acid decomposition during alcoholic fermentation[23,24]. According to Table 2, eight alcohols including methanol, ethanol, propanol, butanol, hexanol, amyl alcohol, 3-methyl-1-butanol, and 2-methyl-1-propanol were detected in the four brewing stages of Marselan wine. The contents of ethanol (slightly sweet), 3-methyl-1-butanol (apple, brandy, spicy), and 2-methyl-1-propanol (whiskey) increased gradually during the fermentation process. The sum of these alcohols account for 91%−92% of the total alcohol content, which is the highest content of three alcohols in Marselan wine, and may be contributing to the aromatic and clean-tasting wines. On the contrary, the contents of 1-hexanol and methanol decreased gradually in the process of fermentation. Notably, the content of these rapidly decreased at the stage of alcoholic fermentation, from 2,026.07 to 1,218.98 μg/mL and 662.08 to 76.94 μg/mL, respectively, which may be ascribed to volatiles changed from alcohols to esters throughout fermentation. The reduction of the concentration of some alcohols also alleviates the strong odor during wine fermentation, which plays an important role in the improvement of aroma characteristics.

    Acids are mainly produced by yeast and lactic acid bacteria metabolism at the fermentation stage and are considered to be an important part of the aroma of wine[22]. Only one type of acid (acetic acid) was detected in this experiment, which was less than previously reported, which may be related to different brewing processes. Acetic acid content is an important factor in the balance of aroma and taste of wine. Low contents of volatile acids can provide a mild acidic smell in wine, which is widely considered to be ideal for producing high-quality wines. However, levels above 700 μg/mL can produce a pungent odor and weaken the wine's distinctive flavor[25]. The content of acetic acid increased first and then decreased during the whole fermentation process. The content of acetic acid increased rapidly in the second stage, from 719.91 to 3,914.55 μg/mL reached a peak in the third stage (5,161.81 μg/mL), and decreased to 4,630.65 μg/mL in the last stage of fermentation. Excessive acetic acid in Marselan wine may have a negative impact on its aroma quality.

    It was also found that the composition and content of aldehydes produced mainly through the catabolism of amino acids or decarboxylation of ketoacid were constantly changing during the fermentation of Marselan wines. Eight aldehydes, including furfural, hexanal, heptanal, 2-methylpropanal, 3-methylbutanal, dimethyl sulfide, (E)-2-hexenal, and (E)-2-pentenal were identified in all stage samples. Among them, furfural (caramel bread flavor) and hexanal (grass flavor) are the main aldehydes in Marselan wine, and the content increases slightly with the winemaking process. While other aldehydes such as (E)-2-hexenal (green and fruity), 3-methylbutanol (fresh and malt), and 2-methylpropanal (fresh and malt) were decomposed during brewing, reducing the total content from 536.52 to 85.15 μg/mL, which might potently affect the final flavor of the wine. Only two ketones, acetone, and 3-hydroxy-2-butanone, were detected in the wine samples, and their contents had no significant difference in the fermentation process, which might not affect the flavor of the wine.

    To more intuitively analyze the differences of volatile organic compounds in different brewing stages of Marselan wine samples, principal component analysis was performed[2628]. As presented in Fig. 3, the points corresponding to one sample group were clustered closely on the score plot, while samples at different fermentation stages were well separated in the plot. PC1 (79%) and PC2 (18%) together explain 97% of the total variance between Marselan wine samples, indicating significant changes in volatile compounds during the brewing process. As can be seen from the results in Fig. 3, samples of stages 1, 2, and 3 can be distinguished directly by PCA, suggesting that there are significant differences in aroma components in these three fermentation stages. Nevertheless, the separation of stage 3 and stage 4 samples is not very obvious and both presented in the same quadrant, which means that their volatile characteristics were highly similar, indicating that the volatile components of Marselan wine are formed in stage 3 during fermentation (Fig. S1). The above results prove that the unique aroma fingerprints of the samples from the distinct brewing stages of Marselan wine were successfully constructed using the HS-GC-IMS method.

    Figure 3.  PCA based on the signal intensity obtained with different fermentation stages of Marselan wine.

    Based on the results of the PCA, OPLS-DA was used to eliminate the influence of uncontrollable variables on the data through permutation test, and to quantify the differences between samples caused by characteristic flavors[28]. Figure 4 revealed that the point of flavor substances were colored according to their density and the samples obtained at different fermentation stages of wine have obvious regional characteristics and good spatial distribution. In addition, the reliability of the OPLS-DA model was verified by the permutation method of 'Y-scrambling'' validation. In this method, the values of the Y variable were randomly arranged 200 times to re-establish and analyze the OPLS-DA model. In general, the values of R2 (y) and Q2 were analyzed to assess the predictability and applicability of the model. The results of the reconstructed model illustrate that the slopes of R2 and Q2 regression lines were both greater than 0, and the intercept of the Q2 regression line was −0.535 which is less than 0 (Fig. 5). These results indicate that the OPLS-DA model is reliable and there is no fitting phenomenon, and this model can be used to distinguish the four brewing stages of Marselan wine.

    Figure 4.  Scores plot of OPLS-DA model of volatile components in Marselan wine at different fermentation stages.
    Figure 5.  Permutation test of OPLS-DA model of volatile components in Marselan wine at different fermentation stages (n = 200).

    VIP is the weight value of OPLS-DA model variables, which was used to measure the influence intensity and explanatory ability of accumulation difference of each component on classification and discrimination of each group of samples. In previous studies, VIP > 1 is usually used as a screening criterion for differential volatile substances[2830]. In this study, a total of 22 volatile substances had VIP values above 1, indicating that these volatiles could function as indicators of Marselan wine maturity during fermentation (see Fig. 6). These volatile compounds included furfural, ethyl lactate, heptanal, dimethyl sulfide, 1-propanol, ethyl isobutyrate, propyl acetate, isobutyl acetate, ethanol, ethyl hexanoate, acetic acid, methanol, ethyl formate, ethyl 3-methylbutanoate, ethyl acetate, hexanal, isoamyl acetate, 2-methylpropanal, 2-methyl-1-propanol, and three unknown compounds.

    Figure 6.  VIP plot of OPLS-DA model of volatile components in Marselan wine at different fermentation stages.

    This study focuses on the change of volatile flavor compounds and antioxidant activity in Marselan wine during different brewing stages. A total of 40 volatile aroma compounds were identified and collected at different stages of Marselan winemaking. The contents of volatile aroma substances varied greatly at different stages, among which alcohols and esters were the main odors in the fermentation stage. The proportion of furfural was small, but it has a big influence on the wine flavor, which can be used as one of the standards to measure wine flavor. Flavonoids and phenols were not only factors of flavor formation, but also important factors to improve the antioxidant capacity of Marselan wine. In this study, the aroma of Marselan wines in different fermentation stages was analyzed, and its unique aroma fingerprint was established, which can provide accurate and scientific judgment for the control of the fermentation process endpoint, and has certain guiding significance for improving the quality of Marselan wines (Table S1). In addition, this work will provide a new approach for the production management of Ningxia's special wine as well as the development of the native Chinese wine industry.

  • The authors confirm contribution to the paper as follows: study conception and design: Gong X, Fang L; data collection: Fang L, Li Y; analysis and interpretation of results: Qi N, Chen T; draft manuscript preparation: Fang L. All authors reviewed the results and approved the final version of the manuscript.

  • The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

  • This work were supported by the project of Hainan Province Science and Technology Special Fund (ZDYF2023XDNY031) and the Central Public-interest Scientific Institution Basal Research Fund for Chinese Academy of Tropical Agricultural Sciences in China (Grant No. 1630122022003).

  • The authors declare that they have no conflict of interest.

  • Supplemental Fig. S1 Variation patterns of measured elements with the increasing pyrolysis temperature.
    Supplemental Fig. S2 Scree plot analysis showing percentage of explained variations in response to number of principle components for coconut husk biochar production.
    Supplemental Fig. S3 PCA for the parameters influencing coconut husk biochar production; relationship between principle components -I and II.
    Supplemental Fig. S4 PCA for the parameters influencing coconut husk biochar production; relationship between principle components -I and III.
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  • Cite this article

    Dissanayaka DMNS, Udumann SS, Nuwarapaksha TD, Atapattu AJ. 2023. Effects of pyrolysis temperature on chemical composition of coconut-husk biochar for agricultural applications: a characterization study. Technology in Agronomy 3:13 doi: 10.48130/TIA-2023-0013
    Dissanayaka DMNS, Udumann SS, Nuwarapaksha TD, Atapattu AJ. 2023. Effects of pyrolysis temperature on chemical composition of coconut-husk biochar for agricultural applications: a characterization study. Technology in Agronomy 3:13 doi: 10.48130/TIA-2023-0013

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Effects of pyrolysis temperature on chemical composition of coconut-husk biochar for agricultural applications: a characterization study

Technology in Agronomy  3 Article number: 13  (2023)  |  Cite this article

Abstract: Coconut husk, a plentiful agricultural waste, rich in cellulose and lignin, is abundant in tropical and subtropical regions worldwide. The emergence of new green energy technologies harnessing coconut husk has intensified interest in biochar production due to its affordability and low energy requirements. The effectiveness of biochar varies based on the raw materials and production process. Hence, this study aimed to evaluate the chemical and structural properties of coconut-husk biochar produced at different pyrolysis temperatures, focusing on its agricultural benefits. In this research, biochar derived from coconut husk was generated at varying pyrolysis temperatures 325, 350, 400, 500, 600, and 700 °C under limited oxygen supply and a heating rate of 7 °C/min for 3 h. The chemical and structural properties of the produced biochar were meticulously examined using Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectroscopy (EDX) techniques. The findings underscored the significant impact of pyrolysis temperature on the chemical properties and structure of coconut-husk biochar, especially at lower heating rates. Remarkably, the highest yield, recorded at 42.79% (at p < 0.05), was achieved at 325 °C, emphasizing the suitability of lower pyrolysis temperatures for biochar production using coconut husk. Furthermore, alterations in pyrolysis temperature resulted in notable differences in elemental concentrations and significant changes in the biochar structure. These modifications enhance plant and leaf water use efficiencies, boost plant photosynthesis efficiency, modify soil properties, reduce greenhouse gas emissions, and contribute to mitigating global warming.

    • Agricultural waste generation and mismanagement have emerged as serious concerns in the continuous multiple cropping systems found across the globe, this escalating issue places significant strain on the environment and the economy of countries[13]. However, there is a glimmer of hope in the form of re-utilizing agricultural waste, offering a promising solution to multiple environmental challenges. This approach not only supports sustainable agriculture but also plays a crucial role in mitigating the impacts of climate change, making it a win-win situation benefiting the environment, farmers, and society as a whole. As the global population continues to grow, there is an ever-increasing demand for agricultural products, which subsequently leads to a rapid rise in waste production[4]. To tackle this escalating problem, the concept of re-utilizing agricultural waste to create beneficial new produce has gained significant traction and popularity in recent times. Coconut (Cocos nucifera L.) husk is one of the abundant agricultural wastes produced in almost all coconut states and industries as a by-product of crushing raw coconut nuts[5]. It is a carbonaceous mixture of cellulose (38%–39%), hemicellulose (17.33%), and lignin (46%–53%)[6].

      The Sri Lankan coconut industry is blooming rapidly, with nearly around 400,000 hectares across the island giving 3,086 million nut yields in 2019, making it the world's fourth-largest producer[7]. This will give context to the scale of the annual coconut husk generation in Sri Lankan. Even though the majority of the coconut husks are burned or dumped in waste-lands, they can be used for bioenergy production such as second-generation ethanol[8], producing nano-crystalline cellulose for biodegradable films (by polymers)[6], extracting value-added organic acids such as formic acid and acidic acid[5], and isolating phenolic compounds (protein cross-linker agents) for strengthening protein gel[9]. In addition, coconut husks are utilized as mulching material in agricultural crops to conserve soil and moisture in the field. It can be either in the raw form of coconut husk or as a modified product such as coco-nets, and mats[10]. In addition to those uses, coconut husk has been identified as an excellent source of biochar production subjecting to thermochemical processes[11].

      Biochar is an easy and cost-effective way of transferring nutrients in coconut husk into the cropping cycle. Simply, biochar can be defined as 'one of the products of pyrogenic carbonaceous materials through thermochemical conversion of biomass or biodegradable materials in a limited or absence of oxygen content'[12].

      The re-utilization of agricultural waste through coconut-husk biochar production offers significant environmental and agricultural benefits. The porous structure and absorption properties of biochar might transform soil into an excellent habitat for soil macro and micro fauna populations as well as diversity, thereby increasing bio-enzymatic activities[13]. Field application may increase the overall plant root growth and its performance[14]. According to the literature, that could happen as stimulation for alkaline biochar applications, which have a higher specific surface area, water holding capacity, and release nutrients slowly[15,16]. Biochar increases plant performance by enhancing water use efficiency but relying on (1) soil factors like pH variation and soil texture, (2) biochar production parameters, and qualities, and (3) crop management practices especially irrigation[17]. Other than that, biochar can increase photosynthetic efficiency by modifying tissue chlorophyll concentration, stomatal conductance, transpiration rate, water use efficiency, and increasing shoot biomass[18]. He et al. reported that the response of C3 plants to biochar application was much higher than that of C4 plants[18]. It enhances some soil properties like porosity, aggregate stability, available water-holding capacity, and saturated hydraulic conductivity while negatively affecting soil bulk density[19,20]. Biochar increases the soil organic carbon content by increasing carbon sequestration and forming beneficial soil carbon sinks[21]. Thereby it ultimately helps to reduce atmospheric carbon dioxide content, alleviating greenhouse gases and mitigating global warming[22]. However, that effect is dependent on pyrolysis temperature, qualities of raw materials, soil pH, irrigation activities and fertilization practices, and surface area of biochar[23]. Pyrolysis temperature or the temperature at which biomass is pyrolyzed plays a crucial role in determining biochar quality than other factors. Excessive temperatures can lead to the loss of beneficial properties and the formation of undesired compounds. Optimal pyrolysis conditions need to be determined for each feedstock to achieve the desired biochar characteristics. This research was conducted to determine the effect of pyrolysis temperature on the chemical and physical composition of coconut-husked biochar in terms of agricultural benefits.

    • Coconut husks were collected from Rathmalagara sub-research station, Coconut Research Institute, Sri Lanka. Biomass was chopped and initially air-dried for 2 d before being oven-dried at a temperature of 60 °C for 24 h. Oven-dried biomass was tightly packed into metal containers and covered by a lid to facilitate the limited oxygen environment. Then samples were pyrolyzed in Muffle Furnace (Model P330, Nabertherm, Germany) at six different temperatures; 325, 350, 400, 500, 600, and 700 °C, maintaining 7 °C/min heating rate for 3 h until wood gas flow stopped. The labeled carbonaceous biochar was stored under room conditions until further studies.

    • The structural and chemical properties of produced biochar were analyzed using Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectroscopy (EDX) technique (Model Zeiss EVO LS15) at the laboratory, at the Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka. The samples were observed from the SEM at the same magnitude as the initial step and then the measurements were taken in percentage in weight basis. Three replicates were used for the evaluation.

    • All the statistical analysis was carried out using R statistical software (version 4.1.3). Finally, the mean values of the data were statistically compared using the One-way Analysis of variance (ANOVA) at 5% significance and Tukey's pairwise comparison test to determine whether there are any statistically significant differences between the means of biochar yield/nutrient content with six temperature levels.

    • A range of pyrolysis temperatures was chosen to encompass various thermochemical processes, including hydrothermal liquefaction (250−400 °C), pyrolysis (400−800 °C), and gasification (700−1,300 °C)[24]. This broad temperature selection aimed to determine the optimal temperature for producing coconut-husk biochar by gaining comprehensive insights into these processes. As mentioned by Xu et al., the pyrolysis temperature has a greater influence on the chemical and physical properties of coconut-husk biochar[25]. Therefore, SEM with EDX was employed for elemental analysis due to its effectiveness in comparison to other elemental digestion methods such as atomic absorption spectroscopy, inductively coupled plasma mass spectrometry, or wet chemical digestion considering several reasons; (1) allows for non-destructive analysis of samples; (2) high-resolution imaging capabilities; (3) detects a wide range of elements, from carbon to uranium, depending on the instrument's capabilities; (4) high sensitivity to detect elements present in very low concentrations; and (5) relatively fast compared to other elemental digestion methods. Supplemental Fig. S1 summarised the variation patterns of measured elements with the increasing pyrolysis temperature. A scree plot was developed to determine the number of principal components to retain in Principal Component Analysis (PCA) depending on the amount of variation they cover (Supplemental Fig. S2). Based on its results, PCA plots were designed with identified three major principal components to explain 78.6 % of the total variability (Supplemental Figs. S3 & S4). A small angle between two vectors implies a positive correlation between two variabilities while the formation of a large angle between two vectors suggests a negative correlation. A 90° angle indicates no correlation between the two characteristics.

    • Figure 1a illustrates the variation in biochar yield across different pyrolysis temperatures. The highest biochar yield (42.79%) was recorded at 325 °C, whereas the minimum biochar yield (26.97%) was observed at 600 °C. According to these results, a significant decrement in biochar yield could be observed with the increment of pyrolysis temperature (at p < 0.05). That is mainly due to the increased primary and secondary decomposition of coconut husk structural components such as lignocelluloses and the greater release of volatile components with increasing temperatures[26]. As mentioned in previous literature, hemicellulose and cellulose complete their decomposition at 220–315 °C and 315–400 °C temperatures respectively[27]. Beyond 400 °C, crystalline celluloses begin to decompose, leaving lignin as the only remaining organic substance in the production process[28]. Notably, bio-oil is the predominant product after reaching 500 °C[24]. Similar results were recorded by Suman & Gautam[11].

      Figure 1. 

      Variation of coconut-husk biochar properties with different pyrolysis temperatures.(a) Biochar yield. (b) Variation of biochar-C content. (c) Variation of biochar-O content. (d) Variation of biochar-N content. (e) Variation of biochar-P content. (f) Variation of biochar-K content. (g) Variation of biochar-Na content. (h) Variation of biochar-Ca content. (i) Variation of biochar-Mg content. (j) Variation of biochar-S content. (k) Variation of biochar-Cl content. (l) Variation of biochar-Si content. Means with different letters represent significant differences at p < 0.05 level.

    • As shown in Fig. 1b, the C content showed an increment from 52.03% to 78.71% as a response to increasing pyrolysis temperature from 325 °C to 600 °C. However, that increment was not statistically different (at p < 0.05) among treatments. Elevated pyrolysis temperatures typically result in biochar with heightened carbon content, lower volatile matter, increased stability, and augmented resistance to decomposition[29]. Furthermore, the observed phenomenon suggests that the polymerization of organic substances may account for the elevated C yield at higher temperatures[28]. These findings are in line with the observations made by Brassard and colleagues, reinforcing the understanding of the relationship between pyrolysis temperature and carbon content[30]. Furthermore, a study investigated the use of SEM-EDX to effectively characterize biochar produced through different pyrolysis methods and temperatures showed that SEM-EDX can provide consistent measurements of carbon (C), oxygen (O), and C/O ratios in biochar which can be further explored as an operational tool for biochar characterization[31].

    • Figure 1c shows a decreasing trend with increasing production temperature could be observed in O content produced from coconut-husk biochar. The results indicated that the production temperature at 325 °C had the highest O yield which was recorded as 25.68%. Even though, there was no significant difference among the 325−400 °C range, after that a significant difference was identified. As shown by Sarkar & Wang, raising the pyrolysis temperature encourages the volatilization process leading low yield of O in the final product[32]. A similar result was obtained in rice-straw-based biochar production[33]. Since raising the pyrolysis temperature influences the dehydration, aromatization, decarboxylation, condensation, and polymerization processes, it will change the affinity level in chemical bonds between C, H, and O elements[33]. This will facilitate the volatilization of the above-mentioned nutrients leading to low H/C, and O/C ratios in biochar (Table 1). Biochar with lower values for these ratios indicates higher biochar C stability and maximum carbon sequestration ability[27,30]. According to that, it can be suggested that biochar produced at 600 and 700 °C are more suitable for carbon sequestration purposes and biochar produced at low temp would serve more soil nutrient retention by enhancing soil cation exchange capacity.

      Table 1.  Variation of O/C ratio at different pyrolysis temperatures.

      Pyrolysis temperature (°C)O/C ratio of coconut-husk biochar
      3250.49
      3500.46
      4000.32
      5000.22
      6000.08
      7000.10
    • N is the fourth most abundant element in plants after C, H, and O. It plays a significant role as a major structural component in various organic compounds found in plants, including proteins, nucleotides, porphyrins, and alkaloids. These compounds are essential for plant growth and function, contributing to processes such as enzyme activity, genetic information storage, photosynthesis, and various physiological functions in plants[34]. By adding N-rich biochar materials, chemical fertilizer requirements for agriculture can be minimized. Even though raising the pyrolysis temperature did not show a significant tendency of an increase in N yield in biochar production, it reported an unexpected increase at 700 °C (19.5%) (Fig. 1d). Generally N content in biochar shows a decreasing trend with the increasing pyrolysis temperature due to volatilization of N as Hydrogen cyanide and Ammonia in the un-condensable gas mixture and leaching of N as organic-N in the condensable liquid phase[35]. Dehydration and de-carbonization occurring in the production process can increase the N yield in biochar[25].

    • P is a vital nutrient in agriculture, supporting plant growth, development, energy transfer, and various physiological processes such as photosynthesis, stress tolerance and resilience, and reproduction, ultimately contributing to improved crop productivity and quality[36]. According to Fig. 1e, increasing pyrolysis temperature from 350 to 700 °C has led to the reduction of P content by 0.06% to 0.02% respectively. However, a significant difference between the six different treatments was not observed. According to previous findings, inherent P concentration in biochar and P sorption capacity of biochar have a positive relationship leading to more P available for plants[37]. By applying those findings to this experiment, it can be concluded that biochar pyrolyzed at 350–400 °C could be used to increase the P availability as well as P sorption capacity.

    • K also serves a crucial function in imparting resilience to plants when they encounter a range of biotic and abiotic stresses, which encompass diseases, pests, drought, salinity, cold temperatures, frost, and waterlogging[34]. Other than that, regulation of water balance, enzyme activation, and metabolism also can be maintained well by monitoring K requirement. With the increment of pyrolysis temperature, a significant decreasing trend in the K content of biochar from coconut husk was observed (Fig. 1f). The highest K yield (6.83%) was recorded at the temperature of 350 °C. According to previous studies, it had confirmed that biochar prepared at a lower pyrolysis temperature (around 400 °C) is the best based on the K specification[38]. At around lower production temperatures (around 400 °C), Potassium chloride (KCl), complex-K, and graphite layer-K (K in HOPG intercalation compounds) were formed, and at around increased temperatures (around 800 °C), Potassium sulfate was formed with the above-mentioned K forms. With the increasing pyrolysis temperature, complex-K and graphite layer-K starts to decompose and KCl will be volatilized leading to lower K concentration at the end of the process[38].

    • While Na is an essential nutrient for many living organisms, it is generally not considered a vital nutrient for most plants. In fact, excessive Na can be detrimental to plant growth and health. As shown in Fig. 1g, biochar produced from coconut husk showed a tendency to increase the yield of Na content with rising pyrolysis temperature due to minimum volatilization losses[39]. The maximum Na yield (0.54%) was observed around the temperature at 600 °C. Na compounds in the coconut-husk feedstock might have relatively high boiling points. At higher pyrolysis temperatures, these compounds may not undergo significant volatilization losses and, instead, remain within the biochar matrix. However, further research may need to confirm this. Similar results were obtained by Rafiq et al. concluding that crop-based biomass produces higher Na yield at higher pyrolysis temperatures[40].

    • Ca serves an electrochemical function by acting as a counter ion for the anions found in both inorganic and organic acids[34]. With the increment of pyrolysis temperature from 325−700 °C, Ca content varied from 0.22%−0.07% range in a descending manner (Fig. 1h). That will indicate the less availability of Ca2+ ions as a plant nutrient. However, there was no significant difference among the six pyrolysis temperatures. Ca compounds present in the feedstock undergo thermal decomposition at elevated temperatures. This decomposition can result in the release of gaseous compounds or the formation of stable compounds that do not contribute to the Ca content in the biochar at elevated temperatures. Similar behavior was observed in different types of biochar produced by vine pruning, poultry litter, orange pomace, and seaweed[41].

    • Mg is an essential nutrient in agriculture, playing a vital role in chlorophyll synthesis, photosynthesis, enzyme activation, nutrient uptake, stress tolerance, and overall crop productivity[36]. Proper magnesium management is crucial for promoting healthy plant growth and maximizing agricultural yields. Figure 1i reveals that the maximum Mg content of 0.19% was obtained at a production temperature of 350 °C, whereas the minimum Mg content (0.02%) was recorded at a temperature of 325 °C. With the increment of pyrolysis temperature, a descending trend in the Mg element of biochar from coconut husk was observed indicating the less availability of Mg2+ ions for plant absorption. Tag et al. observed a similar variation of Mg yield with biochar pyrolysis temperature[42].

    • S is an essential macro-nutrient in plant growth, playing a vital role in protein synthesis, chlorophyll formation, nutrient uptake, and disease resistance[34]. The S element exhibited a slow descending trend with the increment in production temperatures (Fig. 1j). Even though it varied from 0.16%−0.01% in response to temperature change from 325–700 °C, there was no significant difference (at p < 0.05) among treatments. These variations might happen due to increased volatilization of water-extractable sulfate and organosulfur present in biochar[42]. Similar findings were reported by Zhao et al. for corn-straw biochar by observing a reduction in total S content and water-soluble sulfate with increasing HCl and NaH2PO4-extractable sulfate with increasing pyrolysis temperature[41]. Furthermore, biochar rich with various elements such as Na, Ca, K, Mg, P, and S showed a greater potential for immobilizing heavy metals in soil[43].

    • The presence of sufficient Cl content in plants will help the maintenance of turgor and osmoregulation functions[34]. Figure 1k, reveals that the highest Cl content of 4.35% was obtained at a temperature of 600 °C, whereas the lowest Cl content of 0.14% was recorded at 500 °C. A significant increase of this element could be recorded at 600 °C temperature range when compared to other treatments. Raising pyrolysis temperature causes an increase the heavy metal chlorination, especially with Zn, Cu, Ni, and Mn leading to low heavy metal concentration and Cl yield at the end product[44].

    • Si shows beneficial effects on plant growth and development, yield maximizing, and disease resistance[45]. As described in Fig. 1l, there is a significant difference between the treatments when considering the Si content with the changes in the production temperature. It showed an increasing trend with respect to the increasing pyrolysis temperature recording maximum Si content at the temperature of around 700 °C. Similar observations were recorded by Qian & Chen concluding higher quartz (SiO2) appeared at around 400 °C while higher kalsilite (KAlSiO4) and diopside (CaMgSi2O6) formed around 700 °C[46]. According to previous findings, Si-rich biochar is more active in balancing soil Si content, increasing the Si availability for plant functioning, remediation of some inorganic pollutants such as Al, As, Cd, and Cr, and increasing biotic stress (pest and diseases) tolerance. Nonetheless, Si is an essential plant nutrient for several major crops grown worldwide such as maize, wheat, and rice[47].

    • Correlation analysis of measured characteristics for coconut-husk biochar is shown in Fig. 2. As shown in the graphical illustration, hidden positive and negative correlations among variables can be identified in different absolute values of corresponding correlation coefficients. Pyrolysis temperature shows the strongest positive relationship with Na. Other than that, it shows fewer corresponding correlations with Si > C > N > Cl content. O is the element that shows the strongest negative relationship with pyrolysis temperature while reducing the strength of the negative relationship with S, Mg, P, Ca, and K content respectively. Biochar yield of produced coconut-husk biochar shows negative correlations with Na content, Si content, Cl content, N content, C content, K content, and Mg content while O content, S content, Ca content, and P content show positive correlations.

      Figure 2. 

      Graphical display of the correlation matrix, using the 'corrplot' package of R-software. Red color indicates the negative correlations between shown variables; blue color indicates the positive correlations; white color indicates no correlation. The areas of the squares show the absolute value of corresponding correlation coefficients. Parameters covered by blue lines show similar correlation variation patterns.

      Other than the chemical properties of biochar, structural characteristics were significantly influenced by the production temperature[40]. Xu et al. confirmed the significance (p < 0.001) of pyrolysis temperature on the importance of biochar's sorption capacity[25]. Using SEM technique, surface morphology and changes in structural properties could be clearly investigated.

      As shown in Fig. 3ae, biochar derived from coconut husk has an uneven, heterogeneous porous-rich surface giving increased surface area and facilitating higher absorption capacity[48]. Volatilization of structural components will lead to the shrinkage of coconut fiber resulting in this uneven surface morphology. Pores are irregular and longitudinal in shape[49]. According to Dhar et al. surface porosity of biochar made from coconut husk will increase with the increasing pyrolysis temperature leading to lesser strength[29]. Those pores show 1–3 µm in length at lower pyrolysis temperatures (350–450 °C) and 3–10 µm in length at higher pyrolysis temperatures (500–600 °C) under x 2,000 magnification. The surface morphologies of coconut-husk biochar at 400 °C were examined with SEM techniques (Fig. 3).

      Figure 3. 

      Scanning Electron Microscopy (SEM) micrographs of coconut-husk biochar produced at 400 °C. (a) Under 3.00 K X of magnification. (b) Under 1.00 K X of magnification. (c) Under 500 X of magnification. (d) Under 250 X of magnification. (e) Under 100 X of magnification.

    • The present study investigated the influence of pyrolysis temperature on coconut-husk biochar production with the purpose of optimizing biochar yield and its characteristics. The results demonstrated the profound influence of pyrolysis temperature on biochar characteristics and its usefulness as an agricultural input. The yield of biochar derived from coconut husk varied significantly with the changes in pyrolysis temperature. The maximum yield, which was recorded as 42.79%, was produced at 325 °C highlighting the suitability of low pyrolysis temperature for producing coconut-husk biochar. Raising pyrolysis temperature activates or deactivates the decomposition, volatilization, polymerization, chlorination, dehydration, and de-carbonization processes that can influence the biochar characteristics. As a result, O, Na, K, Mg, and Si yield in the biochar showed significant differences between the treatments. Notable changes were observed in structural properties as well. Biochar production by coconut husk resulted in a nutrient-rich source that could be used as a greater soil amendment. It could increase the soil nutrient content and water-holding capacity while reducing agricultural waste generation. Future studies are essential to observe the effect of pyrolysis time period, nutrient transferring capacity, and heavy metal composition of coconut-husk biochar.

    • Study conception and design: Dissanayaka DMNS, Atapattu AJ; data collection: Udumann SS; analysis and interpretation of results: Dissanayaka DMNS, Nuwarapaksha TD; draft manuscript preparation: Dissanayaka DMNS, Udumann SS, and Atapattu AJ. All authors reviewed the results and approved the final version of the manuscript.

    • The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

    • We would like to express our appreciation to the technical staff of the Agronomy Division of the Coconut Research Institute.

      • The authors declare that they have no conflict of interest.

      • Supplemental Fig. S1 Variation patterns of measured elements with the increasing pyrolysis temperature.
      • Supplemental Fig. S2 Scree plot analysis showing percentage of explained variations in response to number of principle components for coconut husk biochar production.
      • Supplemental Fig. S3 PCA for the parameters influencing coconut husk biochar production; relationship between principle components -I and II.
      • Supplemental Fig. S4 PCA for the parameters influencing coconut husk biochar production; relationship between principle components -I and III.
      • Copyright: © 2023 by the author(s). Published by Maximum Academic Press, Fayetteville, GA. This article is an open access article distributed under Creative Commons Attribution License (CC BY 4.0), visit https://creativecommons.org/licenses/by/4.0/.
    Figure (3)  Table (1) References (49)
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    Dissanayaka DMNS, Udumann SS, Nuwarapaksha TD, Atapattu AJ. 2023. Effects of pyrolysis temperature on chemical composition of coconut-husk biochar for agricultural applications: a characterization study. Technology in Agronomy 3:13 doi: 10.48130/TIA-2023-0013
    Dissanayaka DMNS, Udumann SS, Nuwarapaksha TD, Atapattu AJ. 2023. Effects of pyrolysis temperature on chemical composition of coconut-husk biochar for agricultural applications: a characterization study. Technology in Agronomy 3:13 doi: 10.48130/TIA-2023-0013

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