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| Preparation of rice husk biochar and adsorption characteristics of urea nitrogen |
| Received:March 15, 2021 |
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| KeyWord:pyrolysis;rice husk biochar;urea nitrogen;adsorption kinetics;adsorption characteristics |
| Author Name | Affiliation | E-mail | | ZHONG Xuan | Foshan Nanhai Guangdong Technology University CNC equipment Cooperative Innovation Institute, Guangdong, Foshan 528225, China
College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China | | | JIANG Enchen | College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China | | | LU Luying | College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China | | | GAO Zhennan | College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China | | | WANG Mingfeng | College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China | wangmingfeng@scau.edu.cn |
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| Abstract: |
| This study investigated the adsorption characteristics of urea nitrogen by using continuous pyrolysis biochar. Rice husk biochar (RHB)was prepared at 350, 450, 550℃, and 650℃ in a home-made shaftless spiral continuous pyrolysis device. The effects of pyrolysis temperature on the physical and chemical properties of RHB were studied, along with the adsorption capacity of urea nitrogen in an aqueous solution. The adsorption of urea nitrogen was fit using adsorption kinetic and adsorption isotherm models. The mechanism governing the adsorption of urea nitrogen by RHB was explored in combination with the micro-morphology characteristics of RHB before and after adsorption. Brunauer-Emmett-Teller specific surface area and pore volume of RHB increased with a rise in the pyrolysis temperature, while the average pore size decreased gradually. Compared with RHB prepared at 550℃ and 650℃, RHB prepared at 350℃ and 450℃ retained more acidic oxygen-containing organic functional groups. The adsorption capacity of RHB prepared at 650℃ for urea nitrogen was stronger(the equilibrium adsorption capacity of RHB at 350℃ and 650℃ was 30.59 mg·g-1 and 33.16 mg·g-1, respectively). The results of isothermal adsorption model and adsorption kinetics fittings showed that the adsorption of urea nitrogen by RHB could be described by the Langmuir Freundlich and Elovich models. The adsorption of urea nitrogen by RHB was simultaneously affected by physical adsorption and chemical adsorption. This study revealed the following adsorption process:Urea molecules first penetrate the surface of the liquid film through free diffusion to reach the surface of RHB particles. The molecules chemically adsorb with the adsorption sites of functional groups on the RHB surface. The urea molecules then move from the surface of the RHB particles to the inner complex porous structure and are "blocked" inside the pores. Ultimately, a dynamic equilibrium is established. Furthermore, the adsorption mechanism of RHB varied at different pyrolysis temperatures. At a low pyrolysis temperature, RHB forms hydrogen bonds with urea molecules through surface oxygen-containing functional groups and chemical adsorption. At a high pyrolysis temperature, RHB physically adsorbs to urea molecules through the formation of more complex pore structures. |
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