Development of a steam or microwave-assisted sequential salt-alkali pretreatment for lignocellulosic waste: Effect on delignification and enzymatic hydrolysis
Graphical abstract
Introduction
Lignocellulosic biomass represents an extraordinarily large amount of bio-resources suitable for the production of many bio-based products such as fuels and chemicals [20]. Sugarcane is one such biomass, with annual yields reaching 328 Tg [35]. Sugarcane leaf waste (SLW) makes up approximately 40% of the plants total biomass, equating to 131 Tg. Since SLW has no specific use post-harvest, it is either burnt or dumped in landfill sites [38]. Lignocellulose, such as SLW, is composed mainly of the carbohydrates glucan and xylan in the cellulose and hemicellulose layer respectively, indicating its feedstock potential [18]. These polymers are encrusted within a recalcitrant lignin layer. Due to the integral structural complexity of lignocellulose, the conversion of these polymers into monomeric sugars pose significant challenges [20]. The presence of lignin severely constrains enzymatic hydrolysis by irreversibly binding to cellulases [5]. Moreover, enzymatic hydrolysis is restricted by accessibility of the enzyme to cellulose, thus a robust pretreatment is required to delignify and disrupt the lignocellulosic matrix [43]. Some of the main criteria for an effective pretreatment is: (a) minimal energy demand, (b) low-cost, (c) minimal by-products [41].
Numerous pretreatment technologies have been investigated. These include the use of organic acids [26], [34], alkali [40], ionic liquids [4], cold alkali extraction [9], microwave-assisted alkaline glycerol [10], microwave-alkali [21], inorganic salt [17], alkalic salt [30], alkaline peroxide [2] and sequential (two stage) dilute acid-alkali [22], among others.
Alkali pretreatment has emerged as one of the most effective pretreatments, given its low polluting, non-corrosive nature that requires less intensive conditions compared to other technologies. Alkali’s, such as sodium hydroxide, primarily function to remove lignin through fibre swelling [18]. Inorganic salts have also garnered much attention since they are non-polluting, have very low toxicity and show high catalytic activity compared to acids [44]. Inorganic salts such as metal chlorides interact mainly with the ether linkages in xylan, leading to the removal of hemicellulose [16]. Therefore, the synergistic effect of sequential salt-alkali pretreatment could increase the degradation of lignocellulose and enhance enzymatic saccharification. Metal chloride salts in combination with steam, dilute acid, ferric oxide and ionic liquids have been investigated [33], however, there is a dearth of knowledge on the impact of sequential salt-alkali pretreatment on the enzymatic saccharification and delignification of lignocellulosic biomass.
Microwave irradiation has also attracted considerable interest over conventional heating since it causes different chemical changes in lignocellulosic biomass [45]. Microwave heating causes fibre swelling and fragmentation which reportedly enhances enzymatic saccharification. In addition, microwave irradiation breaks down the complex lignocellulosic structure through dielectric polarization causing molecular collisions [10], [45]. Microwave irradiation also offers significant advantages over conventional heating such as: (a) short reaction times, (b) fast heat transfer, (c) energy efficiency, (d) direct heating, and (e) it is considered environmentally friendly [3]. However, there are no reports on microwave-assisted sequential salt-alkali pretreatment of lignocellulosic biomass.
The aim of this study was to develop a steam salt-alkali (SSA) and a microwave-assisted salt-alkali (MSA) pretreatment for SLW. In addition, the delignification and enzymatic saccharification efficiency of these two pretreatments were compared. Furthermore, Fourier Transform Infrared Spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to determine physicochemical changes in the pretreated SLW structure.
Section snippets
Materials
Sugarcane leaf waste (SLW) was collected from a sugarcane plantation located on the North Coast of South Africa (29°42′ 18″S, 31°02′ 44″E). The leaves were dried at 60 °C for 72 h post-collection and milled to a particle size ≤1 mm. SLW fibre composition was determined by the NREL method [37]. The commercial cellulase enzyme preparation, Cellic CTec 2, was generously provided by Novozymes (Novozymes A/S, Denmark).
Preliminary screening
Pretreatments were carried out in 100 ml Erlenmeyer flasks with a solid loading of 10%
Preliminary screening
Among the three different salts evaluated at the initial screening stage, ZnCl2 was found to yield the highest reducing sugar (0.14 g/g) followed by NaCl (0.10 g/g) and NH4Cl (0.09 g/g). NaOH was found to be the most effective alkali, yielding 0.95 g/g reducing sugar while Ca(OH)2 gave 0.65 g/g. ZnCl2 and NaOH have been separately reported for enhancing enzymatic hydrolysis due to their strong solubilization properties [17], [18]. Therefore, ZnCl2 and NaOH were chosen for the sequential
Conclusion
In this study, two different sequential pretreatments were examined- SSA and MSA. These pretreatments were modelled, optimized and validated with R2 > 0.97. Reducing sugar yields of 1.21 and 1.17 g/g were obtained with the SSA and MSA pretreatments respectively. Major structural changes were observed with the pretreated biomass after SEM and FTIR analysis. High delignification was also achieved (80.5 and 73% for SSA and MSA respectively). The developed pretreatment methods demonstrated high
Acknowledgements
The financial assistance of the National Research Foundation of South Africa (NRF) [Grant Number 101275] towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the author and are not necessarily attributed to the NRF.
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