Effect of alkali treatment on microstructure and thermal stability of parenchyma cell compared with bamboo fiber

Highlights

• Parenchyma cell and fiber were isolated mechanically from bamboo culm.

• Alkali treatment affected parenchyma cell and fiber differently.

• Starch in parenchyma cell can be extracted by alkali solution.

Abstract

This study aimed to examine and compare alkali treatment influences on the microstructure, chemical composition, and thermal properties of parenchyma cells and fibers in bamboo. The parenchyma cells and fibers were isolated mechanically from the same bamboo and were subsequently treated with sodium hydroxide (NaOH) at various concentrations (2, 5, 10, 15, and 25 %) for 2h followed by air-drying. The alkali-treated parenchyma cells and fibers were characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), chemical composition analysis, and thermogravimetric analysis (TGA). The results showed that the parenchyma cells collapsed and fibers separated when treated by NaOH with a concentration of greater than 10 %. The starch in parenchyma cells could be extracted as individualized granules by alkali solution, particularly at 2% NaOH. The alkali treatment could partly remove lignin from parenchyma cells but almost did not affect hemicellulose content. In comparison, both lignin and hemicellulose in fiber were removed after alkali treatment. Cellulose I was transformed to cellulose Ⅱ in parenchyma cell after treated by 15 % and 25 % NaOH solutions, while the transformation of cellulose I to cellulose II occurred in fibers only at a concentration of 25 %. Parenchyma cells had lower thermal stability than fibers. The alkali treatment influenced the parenchyma cells’ thermal stability more pronouncedly than that of fibers.

Introduction

Bamboo has long been regarded as a composite material comprising reinforcing component fibers and the matrix parenchyma cells, as shown in Fig. 1a,b. In general, bamboo culm consists of about 50 % parenchyma cells, 40 % fibers, and 10 % vascular bundles (vessels, sieve tubes with companion cells) (Liese, 2015). The chemical composition and structure were different between parenchyma cells and fibers (Jin et al., 2019; Lian et al., 2020; Liese, 2015; Zhang et al., 2018). Compared with fibers, parenchyma cells had lower cellulose and lignin contents but had higher hemicellulose content. The parenchyma cell had thin cell walls with a large lumen, but the fibers had thick cell walls with a small lumen, as shown in Fig. 1 c,d. Usually, there was starch in parenchyma cells. Meanwhile, parenchyma cells' mechanical property was also different from that of fibers (Wang et al., 2020).

Bamboo fibers have been extensively studied, including microstructure, mechanical and physical properties, etc. (Shah et al., 2019; Wang et al., 2015a; Yu et al., 2014). Bamboo fibers, possessed excellent mechanical properties, have been used in a wide range of fields, such as textile (Nayak and Mishra, 2016; Yueping et al., 2010), composites (Fang et al., 2020; Huang and Young, 2019; Liu et al., 2012; Ren et al., 2017), pulp and paper (Suzuki et al., 2008), and so on. However, parenchyma cells with relatively low mechanical properties were often ignored in research and applications, even though they accounted for about 50 % of tissues in bamboo. Moreover, the processing residues in the bamboo fiber industry were mainly parenchyma cells with a few fibers, which were usually regarded as wastes (Zhang et al., 2015). In recent decades, parenchyma cells have attracted growing attention from researchers. The microstructure and function of parenchyma cells in different positions and growing stages of bamboo were studied as well as the mechanical properties (Crow and Murphy, 2000; Dixon et al., 2018; Gritsch and Murphy, 2005; He et al., 2002; Hu et al., 2017; Lian et al., 2020; Lybeer et al., 2006). At present, parenchyma cells were isolated from bamboo to prepare cellulose nanofibrils (Abe and Yano, 2010; Wang et al., 2015b; Zhang et al., 2020, 2015), biofuel (Jin et al., 2019), and biocomposites (Ren et al., 2020). These researches suggested that it was much more efficient to prepare cellulose nanofibrils or biofuel from parenchyma cells than that from the mixture of fibers and parenchyma cells (Lin et al., 2020; Xie et al., 2016), as the structure and chemical composition of parenchyma cells were different from those in fibers. Most of the researches about parenchyma cell foucused on the physiologic function and mechanical properties, some were about parechyam cell’s application. While there is little known about pretreatment or modification which help to make the parenchyma cell into high valued products as well as the difference between the effect of the same pretreatment or modification on parenchyma cell and fiber.

Alkali pretreatment is one of the most common and cost-effective methods used for producing cellulose nanofibrils, biofuel, textile, and biocomposites, etc. The effect of alkali treatment on bamboo fiber has been extensively studied in previous researches (Chen et al., 2018, 2017; Liu and Hu, 2008). However, the effect of alkali treatment on parenchyma cells and the comparison of the alkali-treated parenchyma cells and fibers in terms of microstructure, chemical composition, and thermal properties have not been reported, which may be of great importance for future applications of parenchyma cells. In this research, parenchyma cells and fibers, isolated simultaneously from the same bamboo, were treated by alkali solutions at various concentrations. The microstructure, chemical composition, and thermal stability of the alkali-treated parenchyma cells and fibers were examined by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA).