Autocatalytic hydrothermal pretreatment by recycling byproduct organic acids to directionally depolymerize cassava straw
Graphical abstract
Introduction
Traditional fossil resources cannot satisfy increasing demands owing to their limited and nonrenewable nature, which has aggravated energy scarcity [1,2]. Lignocellulosic materials are considered to be an alternative resource for bioethanol production. Cassava, the world’s sixth most important crop, has been gradually valued as a highly attractive and competitive resource for its low cost, high availability and lack of competition with grain crops [3,4]. In accordance with the relevant reports, more than 7 million tons of cassava will be in demand for use by cassava ethanol enterprises within two years [5]. With the development of cassava-based enterprises, cassava straw will be produced simultaneously and will account for approximately 70–80% of the total cassava production [6]. The common forms of cassava straw use, such as direct incineration, have led to issues relating to environmental protection and energy conservation. For this reason, transforming cassava straw into bioethanol has become a promising tactic. Nevertheless, cassava straw is made up of three primary components, namely, cellulose, hemicellulose and lignin. The presence of hemicellulose and lignin in a complex spatial structure may result in restrictions between cellulose and enzymes [[7], [8], [9], [10]]. The minimization of this presence can increase the porosity of raw materials and the accessibility of the enzyme. Therefore, exploring an effective way to pretreat cassava straw has become imperative.
Many different pretreatment methods have been studied, including physical, chemical and biological pretreatments [11,12]. Among these methods, hydrothermal pretreatment is recognized as an environmentally friendly method that only uses water. During the process, hydronium ion (H3O+) is produced for catalyzing hydrolysis [13,14]. However, mild hydrothermal pretreatments cannot loosen structure from deep within the samples, which may inhibit the efficiency of the enzymatic hydrolysis. In addition, certain deficiencies were also noted, such as high water consumption and low monosaccharide production rate. In view of these challenges, optimizing the typical hydrothermal pretreatments is of great importance.
Scholars have gradually shifted from the broad study of cellulose to studying co-utilization between cellulose and hemicellulose [15]. Considering the various severities of pretreatment needed to produce maximum C-5 and C-6 sugar yields, synchronously employing cellulose and hemicellulose during one stage is difficult [16]. Hence, a two-stage hydrothermal pretreatment was adopted in which lower-severity conditions are primarily used during the first stage for acquiring C-5 sugars, and during the second stage, more severe conditions can increase the accessibility of cellulose during enzyme hydrolysis. Kumar [17] and Karcher [18] studied a two-stage pretreatment using additional diluted acid to recover high sugar yields. Yuan [19] combined two-stage pretreatment with alkaline pre-extraction and an acid-catalyzed stream to pretreat wheat straws. Adding extra chemical reagents (such as H2SO4 and HCl) will introduce S/Cl, which can increase the difficulty of subsequent wastewater disposal and result in energy consumption caused by incineration residues. Compared with chemical reagents, byproduct organic acids can be biodegraded and converted into biogas, with the added merits of reducing wastewater and making maximum use of the carbon in the byproduct. Additionally, byproduct organic acids are able to strengthen depolymerization without corroding the equipment. Rich byproduct organic acids are produced in the pretreated liquid, including acetic acid, lactic acid and formic acid. Acetic acid and lactic acid are formed by the degradation of acetyl groups and xylose, respectively. Similarly, formic acid is the degradation product of furfural and 5-hydroxymethylfurfural (5-HMF) [20]. Some studies have reported that the hydronium ions generated from byproduct organic acids are more significant than those generated from water to catalyze the pretreatment process [21]. In view of these advantages, recycling byproduct organic acids in pretreated liquid during two-stage hydrothermal pretreatment is an alternative autocatalytic method to reduce water consumption, increase the pretreatment intensity with no additional chemical acids and obtain higher C-5 and C-6 sugar yields at different stages. Compared with traditional acids-added pretreatment, recycling byproduct organic acids can not only enhance pretreatment intensity but also control the acid concentration below the level of inhibiting the following enzymatic hydrolysis and fermentation, which shows great potentialities as an enhanced autocatalytic pretreatment method.
The objective of this study was to recover the maximum C-5 and C-6 sugar from cassava straw using the above two-stage autocatalytic hydrothermal pretreatment. The two-stage hydrothermal pretreatment without byproduct organic acids recycling was investigated at different intensities to determine the suitable treatment time and temperature. During the two-stage autocatalytic process, the pretreated liquid, which is rich in byproduct organic acids, was recycled at the two stages under optimal time and temperature. First, the byproduct organic acids in the pretreated liquid were recycled primarily to strengthen the conversion of hemicellulose to C-5 sugars (xylose and arabinose). Then during the second stage, the proportion of first-stage pretreated liquid was also explored to maximize the C-6 sugar (glucose) yield in the following enzymatic hydrolysis. The effects of the organic acid concentrations were the focus of this autocatalytic process.
Section snippets
Materials
The cassava straw feedstock was procured from Guangxi Cofco bioenergy Co. LTD. The primary straw constituents were cellulose, hemicellulose and lignin, accounting for 30.28%, 18.51% and 16.28% of the material, respectively. Besides, ashes and other soluble substance was 3.94% and 4.57%, respectively, and the rest was water. After the naturally dried cassava straw was crushed in a grinder, 40–60 mesh size fractions were screened out and stored in the dryer for subsequent experiments.
Two-stage hydrothermal pretreatment
The
The two-stage hydrothermal pretreatment
Hemicellulose accounts for approximately 1/3 of the dry raw materials and its effective utilization is indispensable to the feasibility of cellulose ethanol production [23]. During the first stage of the two-stage hydrothermal pretreatment, the cellulose and hemicellulose were partially decomposed. Changes in the C-5 sugar yield and cellulosic loss were depicted in Fig. 2. The maximum yield for the C-5 sugars was 68.49% following treatment at 180 °C for 60 min. Compared with the C-5 sugar
Conclusions
The strengthening effects of recycling byproduct organic acids during a two-stage hydrothermal pretreatment were investigated. For the first stage, the maximum yield of C-5 sugars was 81.12% with the 0.93 wt% byproduct organic acid after treating at 180 °C for 60 min. However, the effects of the organic acids on the second stage would excessively strengthen the pretreatment, resulting in a relatively high cellulosic loss. Thus, the optimum condition of the second stage was 200 °C for 20 min
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgement
The authors sincerely express thanks to the financial support provided by Tianjin University.
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The first two authors contributed equally to this work.