The effect of ultrasonic intensity on physicochemical properties of Chinese fir
Introduction
Owing to their renewability, biomass resources are among the potential alternatives to fossil fuels [1]. Lignocellulosic materials are a type of biomass resource consisting mainly of cellulose, hemicellulose, lignin, and small amounts of extractives [2]. Wood is a lignocellulosic material with a relatively long growth cycle and is closely related to human life. As strategic sources of energy, wood resources have increasingly attracted worldwide attention. Global wood resources have exhibited a major strategic shift from natural forests to plantations. Fast-growing trees have a relatively short growth cycle and provide a large reserve, but the disadvantage is the low utilization rate due to wood defects.
China and Vietnam are the only two producers of Chinese fir (Cunninghamia lanceolata) [3]. Currently, there are more than 10.96 million ha2 of Chinese fir forests that have the potential to yield 7.26 billion m3 of wood [4]. Chinese fir can be used in the construction industry and in the production of wood-based panels, furniture, and paper [5]. According to the existing literature, numerous approaches have been developed to improve heat treatment (steam and high temperature) [6], [7], [8], immersion modification treatment (phenolic resin, furfuryl alcohol resin and alkyl ketene dimer) [9], [10], as well as densification treatment [11] and other treatments. However, the residue that remains after processing has rarely been considered, resulting in energy-related concerns and environmental implications. In addition, most products made from Chinese fir are of low value [12].
Ultrasound is defined as mechanical sound waves with a frequency above 20 kHz. Cavitation can be generated by the ultrasound [13]. Macromolecular substances can be degraded selectively by cavitation [14]. In addition, the cavitation effect can be applied to both homogeneous and heterogeneous reaction systems [15]. In general, ultrasonic pretreatment is applied to optimize the use of biomass resources. It is also used to enhance the added value of products and ensure the integrity of the resources. Moreover, ultrasonic pretreatment has been increasingly applied to wood. Ultrasonic intensity (UI) refers to the energy per square centimeter per second that is perpendicular to the direction of the traveling wave; it is a specific value that can be measured macroscopically [16]. Studies have shown that ultrasonic waves play a role in improving the extraction yield of biomass materials and shortening the extraction time [17], [18], [19], enhancing the permeability of wood [20], [21], [22], influencing the thermal decomposition characteristics of wood components [23], [24], [25], and affecting the crystallinity of cellulose [26], [27]. Ultrasound has also been used for the nondestructive testing of the wood properties (defects and mechanical properties) of Chinese fir [28], shortening the drying process, and enhancing the acoustic performance of wood [29].
To the best of our knowledge, a large number of studies have focused on the ultrasonic treatment of small-sized materials, such as powders and particles, as well as the property assessment of ultrasound-assisted acid and alkali-treated materials. However, the researches rarely work on large-sized materials treated with deionized water. The relationship between UI and the physicochemical properties of wood has been rarely reported. In this study, the UI, chemical components, chemical structure, crystallinity, morphology, and extractives of Chinese fir were determined. The relationship between UI and the physicochemical properties of wood was investigated. The influence of the instantaneous ultrasonic intensity (IUI) and the integrated area under the curve of the ultrasonic intensity (UIA) were also analyzed.
Section snippets
Materials preparation
Chinese fir samples measuring 20 cm × 10 cm × 2 cm (L × T × R) were obtained from air-dried timber provided by Xianglong Forestry Co. Ltd., in Lipu County, Guangxi Province, China. The moisture content was 10–12%, and the basic density was 0.34 g/cm3. All samples were free of knots and lacked visible evidence of infection by mold, stain, or fungi. The samples were soaked in deionized water for 5 d to obtain samples with a moisture content of 90 (±5) % prior to the ultrasonic treatment. The
Ultrasonic intensity
Ultrasound is a sine sound wave. It can be generated by an ultrasonic generator. The UI is considered as the main parameter of ultrasound. It refers to the energy per square centimeter per second that is perpendicular to the direction of the traveling wave. Fixed-point detection can probe the UI value. Generally, a higher UI value indicates more and larger bubbles, resulting in increased overall cavitation activity. However, the collapse intensity of the cavitation bubbles not only depends on
Conclusion
In this study, the effects of ultrasonic treatments on the physicochemical properties of Chinese fir were investigated. The IUI changed throughout the 35 min experiment, and the maximum value was 9 W/cm2. A correlation was observed between the UI and the changes in the physicochemical properties of the wood; the larger the UIA, the stronger the cavitation effect was. The largest UIA was observed at 25 kHz, followed by those at 40, 59, and 28 kHz. The same order of the ultrasonic frequency was
CRediT authorship contribution statement
Jing Qian: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Project administration. Yajing Li: Investigation. Jingjing Gao: Investigation. Zhengbin He: Supervision. Songlin Yi: Resources, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We gratefully acknowledge the financial support of the National Science Foundation of China: the Mechanism of Ultrasound Promotion Heat and Mass Transfer within Boundary Layer during Wood Drying (31600457) and Study on the Moisture Migration Mechanism during Ultrasonic-Assisted Wood Vacuum Superheated Steam Drying (31270604).
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