Increasing demand for energy is predominantly achieved through the utilization of fossil fuels in the past decades. However, the depletion of fossil resources coupled with several environmental challenges necessitates the exploration of alternative and sustainable energy [1]. Therefore, production of advanced biofuels by fermentation from renewable biomass has attracted extensive attention [2]. Among the array of renewable resources, lignocellulosic biomass stands out as an ideal substitute for fossil fuels owing to its abundance, cost-effectiveness, and remarkable sustainability [3, 4]. In general, depolymerizing carbohydrates (cellulose and hemicellulose) into fermentable monosaccharides (glucose, xylose) is essential for biofuels production [5]. However, cellulose macromolecules intertwine with hemicellulose and lignin, forming crystalline regions by hydrogen bonds, thereby impeding direct contact between cellulose and cellulase [6]. Therefore, using pretreatment to reduce the crystallinity index of cellulose for increasing enzymatic efficiency is the focus of overcoming energy utilization of lignocellulose [7].
Numerous pretreatment methods for lignocellulose have been developed, encompassing physical, chemical, physicochemical, and biological approaches. Among them, hydrothermal pretreatment (HTP), a physicochemical method, is considered one of the most promising technologies due to its simplicity, cost-effective, and minimal production of inhibitors after-pretreatment [8]. Moreover, the disruption of cellulose’s crystalline structure and the enhanced enzymatic hydrolysis for fermentable sugar production have been effectively achieved via organic acid pretreatment [9, 10]. Recently, Tang et al. (2023) demonstrated that tartaric acid-assisted HTP resulted in the removal of 39.9% lignin and 90.2% xylan from sunflower straw [11]. Citric acid (CA), one of the most widely produced organic acids globally, is derived through microbial fermentation using renewable resources [12] and show potential in biomass conversion applications [13, 14]. Furthermore, CA is a metabolic intermediate formed in the TCA cycle of microorganisms, which might not inhibit the metabolic activity when using the CA-contained biomass hydrolysates as substrates. Gomes et al. (2020) observed that citric acid pretreatment (CAP) could reduce the lignin content of bagasse and increase the sugars yield [15]. The advantages and application potential of CAP, providing a sustainable bioconversion solution and showcasing versatility in mild organic acid pretreatment, are noteworthy.
Common reed (Phragmites australis), a prominent herbaceous species, emerges as a candidate for bioenergy production because of its robust growth kinetics, high biomass productivity, and notable resilience to a spectrum of environmental stress factors [16]. Recently, some pretreatment methods, such as H2SO4- or NaOH-assisted choline chloride (ChCl)/glycerol DES pretreatment [17, 18], microwave-assisted ChCl/p-toluene sulfonic acid DES pretreatment [19], and ammonia/sodium sulfite pretreatment [20], have been explored to increase the enzymatic hydrolysis yield of ~ 85–88%. However, the use of inorganic alkaline reagent has corrosive to pretreatment equipment, and the cost of DES is relative higher. Thus, investigation of HTP and CAP to reed biomass still should be explored to improve the fermentable sugars production.
To this end, in this study, the changes of chemical components in reed biomass and enzymatic hydrolysis properties by HTP and CAP were firstly investigated. By varying pretreatment conditions, the optimized pretreatment methods were developed. In addition, the effect of Tween-80 addition on enzymatic hydrolysis of CA-pretreated reed was analyzed. Finally, characterization techniques such as FTIR, SEM, and XRD were utilized to elucidate the properties and structure of reed biomass, thereby substantiating the improvements in enzymatic efficiency achieved through different pretreatments.