One-Step or Two-Step Acid/Alkaline Pretreatments to Improve Enzymatic Hydrolysis and Sugar Recovery from Arundo Donax L.

Abstract

Energy crops are not easily converted by microorganisms because of their recalcitrance. This necessitates a pretreatment to improve their biodigestibility. The effects of different pretreatments, as well as their combination on the enzymatic digestibility of Arundo donax L. were systematically investigated to evaluate its potential for bioconversion. Dilute alkaline pretreatment (ALP) using 1.2% NaOH at 120 °C for 30 min resulted in the highest reducing sugar yield in the enzymatic hydrolysis process because of its strong delignification and morphological modification, while ferric chloride pretreatment (FP) was effective in removing hemicellulose and recovering soluble sugars in the pretreatment stage. Furthermore, an efficient two-step ferric chloride-alkaline pretreatment (FALP) was successfully developed. In the first FP step, easily degradable cellulosic components, especially hemicellulose, were dissolved and then effectively recovered as soluble sugars. Subsequently, the FP sample was further treated in the second ALP step to remove lignin to enhance the enzymatic hydrolysis of the hardly degradable cellulose. As a result, the integrated two-step process obtained the highest total sugar yield of 420.4 mg/g raw stalk in the whole pretreatment and enzymatic hydrolysis process; hence, the process is a valuable candidate for biofuel production.

Comparative Analysis of Acidic and Alkaline Pretreatment Techniques for Bioethanol Production from Perennial Grasses

“…While FeCl 3 effectively dissolves hemicellulose and recovers soluble sugars, it falls short in enhancing enzymatic hydrolysis. Further treatment with NaOH in the second step enhanced delignification, improving the efficiency of enzymatic hydrolysis [29]. Similarly in two-stage pretreatment of Miscanthus using acid and lime, higher recoveries of both glucose (>80%) and xylose (>70%) were observed, outperforming the results of single-stage pretreatments [121].…”

“…The two-stage acid-alkaline pretreatment exhibited the highest reducing sugar yields, 56.9 g/L glucose and 4.2 g/L xylose, followed by NaOH pretreatment with reducing sugar yields of 51.6 g/L glucose and 13.5 g/L xylose, and finally, H 2 SO 4 pretreatment with yields of 29.2 g/L glucose and 5.1 g/L xylose [26]. In a comparable context of sequential acid-alkaline pretreatments involving H 2 SO 4 , FeCl 3 , NaOH, and a two-stage FeCl 3 -NaOH process for Arundo donax by [29], the combined acid-alkaline treatment outperformed individual acid and alkaline pretreatments. The total soluble sugars obtained from H 2 SO 4 and FeCl 3 pretreatments were in the lower range of 236.2 to 255.9 mg/g of reducing sugar, as opposed to NaOH treatment, which achieved a maximum of 379.7 mg/g of reducing sugar.…”

Advanced Bioethanol Production: From Novel Raw Materials to Integrated Biorefineries

“…Combined pretreatments seem to increase the efficiency of the process compared to the individual technologies [81,82]; they allow the obtention of high sugar yields with milder operation conditions and require lower solvent and enzyme amounts, thus improving the cost and sustainability of the final product [83][84][85]. However, Galbe and Wallberg [45] have some doubts about the real applicability of combined pretreatments because of their elevated costs and complexity.…”

“…The most basic configuration for a two-step chemical pretreatment is to recover the hemicellulosic fraction through a low pH pretreatment and then apply an alkaline method to the remaining fiber to separate the lignin from the cellulose [81,83,90].…”

LPMO as a Key Enzyme in the Sustainable Conversion of Lignocellulosic Biomass

“…The purpose of pretreatment is to open the structure of a complex matrix such as lignocellulose, thereby reducing the crystallinity of the material and making the cellulose accessible, separating the components of the lignocellulosic biomass, and removing the lignin. 2,3 The advantages of biologically mediated pretreatment using enzymes are high specificity of enzymes for cellulosic substrates, and carrying out the process in aqueous media. In addition, biological pretreatment is more environmen-tally friendly, and does not require the use of toxic and corrosive chemicals.…”