IODINE NUMBER OF HYDROTHERMAL BIOCHAR FROM PLANE TREE LEAVES

ABSTRACT

This study investigates the surface area and adsorption capacity of biochar derived from Platanus orientalis leaves through hydrothermal carbonization (HTC) at varying temperatures and solvent media. Twelve biochar samples were synthesized at 180°C, 200°C, 220°C, and 240°C using three different reaction media: distilled water, a 1:1 mixture of distilled water and 0.1 M citric acid, and a 1:1 mixture of distilled water and 0.05 M citric acid. The iodine number was determined according to ASTM D4607, using a 0.1 N iodine solution and a biochar mass range of 1.75–2.5 g due to low adsorption capacity. The iodine number increased with rising HTC temperature and citric acid concentration. The highest iodine number (483 mg/g) was observed at 240°C in the 0.1 M citric acid medium, indicating enhanced porosity and surface area.

АННОТАЦИЯ

Данное исследование посвящено изучению площади поверхности и адсорбционной способности биоугля, полученного из листьев Platanus orientalis путём гидротермальной карбонизации (ГТК) при различных температурах и в разных растворителях. Двенадцать образцов биоугля были синтезированы при 180°C, 200°C, 220°C и 240°C с использованием трёх различных реакционных сред: дистиллированной воды, смеси дистиллированной воды и 0,1 М лимонной кислоты (1:1), а также смеси дистиллированной воды и 0,05 М лимонной кислоты (1:1).  Йодное число определяли в соответствии с ASTM D4607, используя 0,1H раствор йода и массу биоугля в диапазоне 1,75–2,5 г из-за низкой адсорбционной способности. Было установлено, что йодное число увеличивается с повышением температуры ГТК и концентрации лимонной кислоты. Наибольшее йодное число (483 мг/г) было зафиксировано при 240°C в среде 0,1 М лимонной кислоты, что указывает на улучшение пористости и увеличение площади поверхности.

Keywords: Adsorption, hydrothermal carbonization, Platanus orientalis, biochar, iodine number.

Ключевые слова: Адсорбция, гидротермальная карбонизация, Platanus orientalis, биоуголь, йодное число.

Introduction. The increasing demand for low-cost and effective adsorbents has prompted researchers to explore waste biomass as raw material for biochar production. Agricultural residues, tree leaves, and food waste offer sustainable feedstocks, especially in regions with abundant plant material. For example, Sultanov (2023) highlights the potential of agricultural waste for energy and material recovery, noting that thermochemical methods such as pyrolysis and hydrothermal carbonization (HTC) can convert organic residues into functional carbon-rich products [1].

Platanus orientalis, a deciduous tree commonly found in Fergana City, Uzbekistan, provides a suitable lignocellulosic precursor for biochar due to its availability and high carbon content. Furthermore, the relevance of biochar in water treatment is reinforced by Abdullaev (2024), who emphasizes the critical role of total dissolved solids (TDS) as an indicator of water quality. He argues that controlling TDS levels through adsorption-based technologies can significantly improve drinking water and industrial wastewater quality [2]. Given this context, optimizing biochar for maximum adsorption—reflected by higher iodine numbers—can contribute to sustainable environmental engineering in urban areas like Fergana. Biochar, especially when optimized through surface activation, presents a promising low-cost solution to these challenges.

Biochar, a carbon-rich material produced from biomass under limited oxygen conditions, has received growing attention due to its utility in water purification, soil amendment, and carbon sequestration. One important parameter used to evaluate its adsorptive capacity is the iodine number, which correlates with the material's microporosity and surface area. The iodine number is defined as the milligrams of iodine adsorbed per gram of biochar when the residual iodine concentration in the filtrate falls within the range of 0.008 to 0.040 N (ASTM D4607-94) [3].

Hydrothermal carbonization (HTC) is a promising low-temperature method for synthesizing biochar in aqueous environments. The present study explores how temperature and reaction medium composition affect the iodine number of biochar synthesized from the leaves of Platanus orientalis, a common tree species in Fergana City, Uzbekistan. While numerous studies have examined HTC of agricultural residues [5-6], there is limited data on biochar from Platanus orientalis leaves, an abundant urban biomass in Fergana City, Uzbekistan. Moreover, the role of organic acids, such as citric acid, in modifying the porosity during HTC remains underexplored. Citric acid can donate protons and chelate metal ions, thus catalyzing dehydration and aromatization reactions that contribute to pore formation [6]. This work investigates the effect of temperature and citric acid concentration on the iodine number of P. orientalis leaf biochar synthesized by HTC.

Materials and Methods. 

Feedstock Preparation. Dried leaves of P. orientalis were collected from Fergana City. The leaves were ground to a fine powder and thoroughly washed with distilled water to remove decomposed organic residues. After washing, they were oven-dried at 105 °C for 12 hours. A 5 g portion of this raw material (not hydrothermally treated) yielded an iodine number of 205 mg/g, serving as a baseline for comparison..

Hydrothermal Treatment. Biochar samples were synthesized at 180 °C, 200 °C, 220 °C, and 240 °C in a polypropiolactone-lined stainless-steel autoclave. For each temperature, three reaction media were used: Medium A: 20 mL distilled water, Medium B: 10 mL water + 10 mL of 0.05 M citric acid, and Medium C: 10 mL water + 10 mL of 0.1 M citric acid. Each reaction was run for 6 hours. After cooling, the biochar was separated by filtration, thoroughly washed with distilled water, and oven-dried at 105 °C for 12 hours. The HTC method was adapted based on carbonization protocols outlined in regional biomass valorization studies [5,6].

Iodine number determination. The iodine number was measured in accordance with ASTM D4607-94 [1]. A standard 0.1 N iodine solution was used. Due to the low adsorption capacity of the biochar, 1.75–2.5 g of biochar was used in each test to ensure the residual iodine concentration in the filtrate fell within the specified range (0.008–0.040 N). The amount of iodine adsorbed was calculated by titration with standardized 0.1 N sodium thiosulfate using starch as an indicator.

Results and discussion. Table 1 summarizes the iodine numbers for all twelve samples. A monotonic increase in iodine number with temperature is observed across all media. At a given temperature, samples treated with higher citric acid concentration exhibit greater iodine numbers, confirming the activating role of citric acid in enhancing microporosity. Table 1 presents the iodine number values for the 12 biochar samples synthesized under different conditions.

Table 1.

Iodine numbers of P. orientalis biochar synthesized by HTC under various conditions

The data reveal that both HTC temperature and citric acid concentration positively influence the iodine number. This trend is attributed to increased degradation of lignocellulosic material and enhanced carbonization, leading to the formation of more micropores at higher temperatures and acidic conditions. Citric acid likely aids in partial hydrolysis of cellulose and hemicellulose, producing porous structures during HTC.  The iodine number of the raw, dried powder (205 mg/g) is lower than all samples treated at ≥200 °C in acidified media, confirming that HTC enhances the microporous surface area of the biomass. The maximum increase (483 mg/g at 240 °C in 0.1 M citric acid) reflects a 135% improvement over the untreated raw powder.

Conclusion. Biochar synthesized from Platanus orientalis leaves through hydrothermal treatment demonstrates significantly enhanced iodine adsorption capacity with increasing temperature and citric acid concentration. The optimal condition (240 °C, 0.1 M citric acid) yielded a high iodine number of 483 mg/g, indicating considerable microporosity. These findings suggest that modified hydrothermal conditions can be tailored to produce efficient biochar adsorbents from local biomass waste in Fergana City, contributing to sustainable environmental management.

References:

1. Sultanov B.S. Utilization of agricultural waste in the production of biogas // Ekonomika i sotsium. – 2023. – No. 6-1(109). – P. 428–429.
2. Abdullaev S.S. The importance of total dissolved solids in assessing water quality // Universum: tekhnicheskie nauki: elektron. nauch. zhurn. – 2024. – No. 6(123). – URL: https://7universum.com/ru/tech/archive/item/17858 (accessed: 28.05.2025).
3. ASTM D4607–94. Standard Test Method for Determination of Iodine Number of Activated Carbon. – West Conshohocken, PA: ASTM International, 1994. – 6 p.
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