Comparison of two-stage acid-alkali and alkali-acid pretreatments on enzymatic saccharification ability of the sweet sorghum fiber and their physicochemical characterizations

Jahim J, Sajab MS, Ibrahim Z. Synergistic sequential oxidative extraction for nanofibrillated cellulose isolated from oil palm empty fruit bunch

This research presents a comprehensive study of sequential oxidative extraction (SOE) consisting of alkaline and acidic oxidation processes to extract nanocellulose from plant biomass. This proposed process is advantageous as its operation requires a minimum process with mild solvents, and yet successfully isolated high-quality nanofibrillated cellulose (NFC) from raw OPEFB. The SOE involved ammonium hydroxide (NH4OH, 2.6 M) and formic acid (HCOOH, 5.3 M) catalyzed by hydrogen peroxide (H2O2, 3.2 M). This approach was used to efficiently solubilize the lignin and hemicellulose from Oil Palm Empty Fruit Bunch (OPEFB) at the temperature of 100°C and 1 h extraction time, which managed to retain fibrous NFC. The extracted solid and liquor at each stage were studied extensively through physiochemical analysis. The finding indicated that approximately 75.3%dwb of hemicellulose, 68.9%dwb of lignin, and 42.0%dwb of extractive were solubilized in the first SOE cycle, while the second SOE cycle resulted in 92.3%dwb, 99.6%dwb and 99.8%dwb of solubilized hemicellulose, lignin, and extractive/ash, respectively. High-quality NFC (75.52%dwb) was obtained for the final extracted solid with 76.4% crystallinity, which is near the crystallinity of standard commercial NFC. The proposed process possesses an effective synergy in producing NFC from raw OPEFB with less cellulose degradation, and most of the degraded hemicellulose and lignin are solubilized in the liquor.

Bioaugmented ensiling of sweet sorghum with Pichia anomala and cellulase and improved enzymatic hydrolysis of silage via ball milling

Sweet sorghum, as a seasonal energy crop, is rich in cellulose and hemicellulose that can be converted into biofuels. This work aims at investigating the effects of synergistic regulation of Pichia anomala and cellulase on ensiling quality and microbial community of sweet sorghum silages as a storage and pretreatment method. Furthermore, the combined pretreatment effects of ensiling and ball milling on sweet sorghum were evaluated by microstructure change and enzymatic hydrolysis. Based on membership function analysis, the combination of P. anomala and cellulase (PA + CE) significantly improved the silage quality by preserving organic components and promoting fermentation characteristics. The bioaugmented ensiling with PA + CE restructured the bacterial community by facilitating Lactobacillus and inhibiting undesired microorganisms by killer activity of P. anomala. The combined bioaugmented ensiling pretreatment with ball milling significantly increased the enzymatic hydrolysis efficiency (EHE) to 71%, accompanied by the increased specific surface area and decreased pore size/crystallinity of sweet sorghum. Moreover, the EHE after combined pretreatment was increased by 1.37 times compared with raw material. Hence, the combined pretreatment was demonstrated as a novel strategy to effectively enhance enzymatic hydrolysis of sweet sorghum.

In vivo cadmium-assisted dilute acid pretreatment of the phytoremediation sweet sorghum for enzymatic hydrolysis and cadmium enrichment

Phytoremediation with energy crops is considered an integrated technology that provides both environment and energy benefits. Herein, the sweet sorghum cultivated on Cd-contaminated farmland (1.21 mg/kg of Cd in the soil) showed promising phytoremediation potential, and the approach for utilizing sorghum stalks was explored. Sweet sorghum bagasse with Cd contamination was pretreated with dilute acid in order to improve enzymatic saccharification and achieve Cd recovery, resulting in harmless and value-added utilization. After pretreatment, hemicelluloses were dramatically degraded, and the lignocellulosic structures were partially deconstructed with xylan removal up to 98.1%. Under the optimal condition (0.75% H₂SO₄), the highest total sugar yield was 0.48 g/g of raw bagasse; and nearly 98% of Cd was enriched in the liquid phase. Compared with normal biomass, Cd reduced the biomass recalcitrance and further facilitated the deconstruction of biomass under super dilute acid conditions. This work provided an example for the subsequent valorization of Cd-containing biomass and Cd recovery, which will greatly facilitate the development of phytoremediation of heavy metal contaminated soil.

A review on physico-chemical delignification as a pretreatment of lignocellulosic biomass for enhanced bioconversion

Effective pretreatment of lignocellulosic biomass (LCB) is one of the most important steps in biorefinery, ensuring the quality and commercial viability of the overall bioprocess. Lignin recalcitrance in LCB is a major bottleneck in biological conversion as the polymerization of lignin with hemicellulose hinders enzyme accessibility and further bioconversion to fuels and chemicals. Therefore, there is a need to delignify LCB to ease further bioprocessing. The efficiency of delignification, quality and quantity of the desired products, and generation of inhibitors depend upon the type of pretreatment employed. This review summarizes different single and integrated physicochemical pretreatments for delignification. Additionally, conditions required for effective delignification and the advantages and drawbacks of each method were evaluated. Advances in overcoming the recalcitrance of residual lignin to saccharification and the methods to recover lignin after delignification are also discussed. Efficient lignin recovery and valorization strategies provide an avenue for the sustainable lignocellulose biorefinery.

Sweet sorghum for phytoremediation and bioethanol production

As an energy crop, sweet sorghum (Sorghum bicolor (L.) Moench) receives increasing attention for phytoremediation and biofuels production due to its good stress tolerance and high biomass with low input requirements. Sweet sorghum possesses wide adaptability, which also has high tolerances to poor soil conditions and drought. Its rapid growth with the large storage of fermentable saccharides in the stalks offers considerable scope for bioethanol production. Additionally, sweet sorghum has heavy metal tolerance and the ability to remove cadmium (Cd) in particular. Therefore, sweet sorghum has great potential to build a sustainable phytoremediation system for Cd-polluted soil remediation and simultaneous ethanol production. To implement this strategy, further efforts are in demand for sweet sorghum in terms of screening superior varieties, improving phytoremediation capacity, and efficient bioethanol production. In this review, current research advances of sweet sorghum including agronomic requirements, phytoremediation of Cd pollution, bioethanol production, and breeding are discussed. Furthermore, crucial problems for future utilization of sweet sorghum stalks after phytoremediation are combed.

Graphical Abstract

Electrocatalysis degradation of tetracycline in a three-dimensional aeration electrocatalysis reactor (3D-AER) with a flotation-tailings particle electrode (FPE): Physicochemical properties, influencing factors and the degradation mechanism

Novel particle electrodes, i.e. flotation tailings particle electrode (FPE), were prepared using flotation tailings, garden soil, and soluble starch with a mass ratio of 16:3:1, and then used in tetracycline wastewater treatment. The physicochemical properties of FPE were systematically characterized using SEM, XRD, FT-IR and XRF. Tetracycline adsorption and its adsorption mechanism onto FPE was explored for the first time. Parameters affecting FPE's degradation efficiency and energy consumption such as current density, electrolysis time, initial concentration, initial pH and aeration rate were examined. The electrocatalytic degradation of tetracycline shows that the degradation of tetracycline meets the pseudo-first-order kinetics. Moreover, the numbers of •OH produced on the surfaces of the cathode, anode and particle electrode were compared. Results showed that the adsorption-saturated FPE can be regenerated by electrochemical action to induce further absorption and form in-situ electrocatalysis. In order to find out the transformation products in water and degradation pathways of Tetracycline, UHPLC method was used to obtain the degradation pathways for Tetracycline. So, this work could provide a fabrication of high-efficiency and low-cost electrocatalytic for removal of pharmaceuticals pollutants from waste water as well as deeper insight into electrocatalytic mechanism, transformation products, and degradation pathways of Tetracycline in water.

The key role of delignification in overcoming the inherent recalcitrance of Chinese fir for biorefining

The enzymatic digestibility of softwood is hindered for its highly recalcitrant nature to enzymatic attack. In this study, the effects of dilute sulfuric acid pretreatment (DSAP), acidic sodium chlorite pretreatment (SCP), and their combined pretreatments (DSA-SCP and SC-DSAP) on Chinese fir sawdust were investigated, respectively. Results demonstrated that lignin was the most important obstacle, and digestibility increased linearly with lignin removal yield. Furthermore, the results revealed that the order of sequential pretreatment significantly affected the delignification, and hemicellulose should be removed first. Compared to SC-DSAP, DSA-SCP involving the hemicellulose-removal-first strategy exhibited higher delignification efficiency. DSA-SCP caused lignin removal of 92.3% and the enzymatic hydrolysis was high of 97.9%. Finally, a regression model with high reliability was established to quickly evaluate pretreatment process. In summary, this study highlighted the importance of delignification for saccharification of softwood and unveiled the effect of hemicellulose on delignification.

Towards a Miniaturized Culture Screening for Cellulolytic Fungi and Their Agricultural Lignocellulosic Degradation

The substantial use of fungal enzymes to degrade lignocellulosic plant biomass has widely been attributed to the extensive requirement of powerful enzyme-producing fungal strains. In this study, a two-step screening procedure for finding cellulolytic fungi, involving a miniaturized culture method with shake-flask fermentation, was proposed and demonstrated. We isolated 297 fungal strains from several cellulose-containing samples found in two different locations in Thailand. By using this screening strategy, we then selected 9 fungal strains based on their potential for cellulase production. Through sequence-based identification of these fungal isolates, 4 species in 4 genera were identified: Aspergillus terreus (3 strains: AG466, AG438 and AG499), Penicillium oxalicum (4 strains: AG452, AG496, AG498 and AG559), Talaromyces siamensis (1 strain: AG548) and Trichoderma afroharzianum (1 strain: AG500). After examining their lignocellulose degradation capacity, our data showed that P. oxalicum AG452 exhibited the highest glucose yield after saccharification of pretreated sugarcane trash, cassava pulp and coffee silverskin. In addition, Ta. siamensis AG548 produced the highest glucose yield after hydrolysis of pretreated sugarcane bagasse. Our study demonstrated that the proposed two-step screening strategy can be further applied for discovering potential cellulolytic fungi isolated from various environmental samples. Meanwhile, the fungal strains isolated in this study will prove useful in the bioconversion of agricultural lignocellulosic residues into valuable biotechnological products.

Effect of sequential pretreatment combinations on the composition and enzymatic hydrolysis of hazelnut shells

Hazelnut shells, a high lignin containing biomass, were subjected to individual and sequential liquid hot water (LHW), alkaline (AP) and dilute acid pretreatments (DAP). Among the single pretreatments, LHW demonstrated the highest cellulose recovery of 98.1%, DAP resulted in the highest hemicellulose solubilization of 56.0%, and AP of the highest lignin removal of 49.6%. Employing two-step pretreatment on hazelnut shells, in general, demonstrated an enhanced action of the second pretreatment; therefore, the sequence of the pretreatment methods had a significant impact on both substrate characteristics and enzymatic hydrolysis efficiency of biomass. In terms of delignification, AP-LHW achieved 60.7% lignin removal, while LHW-DAP showed the highest hemicellulose removal of 93.8% and DAP-LHW resulted in the highest cellulose recovery of 94.0%. Structural properties of raw and pretreated hazelnut shells were observed by FTIR. The maximum glucose recovery of 54.9% was observed in DAP-LHW pretreated samples. For this pretreatment combination, almost 1.8 MJ total energy was required to recover 10.2 g glucose. The findings indicated that complete removal of the physical barrier of lignin and hemicellulose might not be essential; partial relocation of lignin and alteration of cellulose structure may also be efficient in increasing the sugar recovery from the lignocellulosic biomass.

Integrating bran starch hydrolysates with alkaline pretreated soft wheat bran to boost sugar concentration

Soft wheat bran (SWB), one of the most abundant byproducts from the wheat milling industry, is a potential candidate for biofuel production. In this study, bran starch hydrolysates were separately integrated with dilute acid pretreated SWB and alkaline pretreated SWB to boost fermentable sugar concentration. Alkaline pretreatment showed higher sugar recoveries than acid pretreatment. Significant sugar degradation for acid pretreatment was observed when pretreatment temperature higher than 170 ℃. The optimal pretreatment condition was 15% solid loading with 0.08 mol/L NaOH at 150 ℃ for 20 min. The neutralization reaction between dilute alkaline and released acids reduced sugar decomposition and inhibitors formation. Integrating bran starch hydrolysates with alkaline pretreated SWB yielded the highest glucose concentration of 50.91 g/L and a total sugar concentration of 101.29 g/L.

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

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.

Evaluation of Macaranga tanarius as a biomass feedstock for fermentable sugars production

Macaranga tanarius is a fast-growing tree species that could be potentially utilized as a biomass feedstock for biorefinery. The average productivity of M. tanarius biomass was estimated to be ~19.2 ton/ha if the above-ground biomass is harvested bi-annually. Different pretreatment approaches were investigated to increase the enzymatic digestibility of foliage and woody biomass. The results indicated that no pretreatment was required for the foliage biomass while sequential acid/alkali pretreatment was necessary for the woody biomass before enzymatic hydrolysis. For the woody biomass, the delignification was 34.5% after sequential dilute acid/alkali pretreatment. The reducing sugar yields from enzymatic hydrolysis of foliage and pretreated woody biomass were 0.31 and 0.42 g/g dry biomass, respectively. The results also showed that both hydrolysates were fermentable by lactic acid bacteria. Overall, the results suggested that M. tanarius could be a potential feedstock for biorefinery based on the findings and processes derived from this study.

Two-Stage Pretreatment to Improve Saccharification of Oat Straw and Jerusalem Artichoke Biomass

Pretreatment is a necessary step when lignocellulosic biomass is to be converted to simple sugars; however single-stage pretreatment is often insufficient to guarantee full availability of polymeric sugars from raw material to hydrolyzing enzymes. In this work, the two-stage pretreatment with use of acid (H2SO4, HNO3) and alkali (NaOH) was applied in order to increase the susceptibility of Jerusalem artichoke stalks (JAS) and oat straw (OS) biomass on the enzymatic attack. The effect of the concentration of reagents (2% and 5% w/v) and the order of acid and alkali sequence on the composition of remaining solids and the efficiency of enzymatic hydrolysis was evaluated. It was found that after combined pretreatment process, due to the removal of hemicellulose and lignin, the content of cellulose in pretreated biomass increased to a large extent, reaching almost 90% d.m. and 95% d.m., in the case of JAS and OS, respectively. The enzymatic hydrolysis of solids remaining after pretreatment resulted in the formation of up to 45 g/L of glucose, for both JAS and OS. The highest glucose yield was achieved after pretreatment with 5% nitric acid followed by NaOH, and 90.6% and 97.6% of efficiency were obtained, respectively for JAS and OS.

Optimization of a pretreatment and hydrolysis process for the efficient recovery of recycled sugars and unknown compounds from agricultural sweet sorghum bagasse stem pith solid waste

Background

Sweet sorghum bagasse (SSB), comprising both a dermal layer and pith, is a solid waste generated by agricultural activities. Open burning was previously used to treat agricultural solid waste but is harmful to the environment and human health. Recent reports showed that certain techniques can convert this agricultural waste into valuable products. While SSB has been considered an attractive raw material for sugar extraction and the production of value-added products, the pith root in the SSB can be difficult to process. Therefore, it is necessary to pretreat bagasse before conventional hydrolysis.

Methods

A thorough analysis and comparison of various pretreatment methods were conducted based on physicochemical and microscopic approaches. The responses of agricultural SSB stem pith with different particle sizes to pretreatment temperature, acid and alkali concentration and enzyme dosage were investigated to determine the optimal pretreatment. The integrated methods are beneficial to the utilization of carbohydrate-based and unknown compounds in agricultural solid waste.

Results

Acid (1.5−4.5%, v/v) and alkali (5−8%, w/v) reagents were used to collect cellulose from different meshes of pith at 25–100 °C. The results showed that the use of 100 mesh pith soaked in 8% (w/v) NaOH solution at 100 °C resulted in 32.47% ± 0.01% solid recovery. Follow-up fermentation with 3% (v/v) acid and 6.5% (w/v) alkali at 50 °C for enzymolysis was performed with the optimal enzyme ratio. An analysis of the surface topography and porosity before and after pretreatment showed that both the pore size of the pith and the amount of exposed cellulose increased as the mesh size increased. Interestingly, various compounds, including 42 compounds previously known to be present and 13 compounds not previously known to be present, were detected in the pretreatment liquid, while 10 types of monosaccharides, including D-glucose, D-xylose and D-arabinose, were found in the enzymatic solution. The total monosaccharide content of the pith was 149.48 ± 0.3 mg/g dry matter.

Discussion

An integrated technique for obtaining value-added products from sweet sorghum pith is presented in this work. Based on this technique, lignin and hemicellulose were effectively broken down, amorphous cellulose was obtained and all sugars in the sweet sorghum pith were hydrolysed into monosaccharides. A total of 42 compounds previously found in these materials, including alcohol, ester, acid, alkene, aldehyde ketone, alkene, phenolic and benzene ring compounds, were detected in the pretreatment pith. In addition, several compounds that had not been previously observed in these materials were found in the pretreatment solution. These findings will improve the transformation of lignocellulosic biomass into sugar to create a high-value-added coproduct during the integrated process and to maximize the potential utilization of agricultural waste in current biorefinery processing.

Combined dilute hydrochloric acid and alkaline wet oxidation pretreatment to improve sugar recovery of corn stover

Two-stage dilute hydrochloric acid (DA)/aqueous ammonia wet oxidation (AWO) pretreatment was used to recover the sugars of corn stover. The morphology characterizations of samples were detected by SEM, BET and SXT. The results showed that DA-AWO process demonstrated a positive effect on sugar recovery compared to AWO-DA. 82.8% of xylan was recovered in the first stage of DA-AWO process at 120 °C for 40 min with 1 wt% HCl. The second stage was performed under relative mild reaction conditions (130 °C, 12.6 wt% ammonium hydroxide, 3.0 MPa O₂, 40 min), and 86.1% lignin could be removed. 71.5% of glucan was achieved with a low enzyme dosage (3 FPU·g^-1) in the following enzymatic hydrolysis. DA-AWO pretreatment was effective due to its sufficient hydrolysis of hemicellulose in the first stage and remarkably removal of the lignin in the second stage, resulting in high sugar recovery with a low enzyme dosage.

Comparison of liquid hot water, very dilute acid and alkali treatments for enhancing enzymatic digestibility of hazelnut tree pruning residues

The effect of pretreatments on the composition of the hazelnut tree pruning residue (HTPR) and on the digestibility of the cellulose was investigated. The liquid hot water (LHW) and the very dilute acid (VDA) treatments were effective in solubilizing hemicellulose. The cellulose conversion increased up to around 60% (corresponding to 32-36 g/L glucose) with decreasing hemicellulose concentration in the pretreated HTPR. The alkali treatment provided partial delignification, however, the glucose production was comparably lower. Combining the hemicellulose removal and the delignification effect of different pretreatments in two-stage processes (LHW-alkali and VDA-alkali treatments) enhanced the cellulose concentration in the solids, but not the amount of glucose released in the enzymatic digestion. These results suggested that the hemicellulose was the main barrier against the conversion of cellulose in the LHW and VDA treated HTPR and the glucose in the hydrolysis medium inhibited the cellulase activity, which prevented the complete conversion of cellulose.

Microwave-assisted inorganic salt pretreatment of sugarcane leaf waste: Effect on physiochemical structure and enzymatic saccharification

This paper presents a method to pretreat sugarcane leaf waste using microwave-assisted (MA) inorganic salt to enhance enzymatic saccharification. The effects of process parameters of salt concentration, microwave power intensity and pretreatment time on reducing sugar yield from sugarcane leaf waste were investigated. Pretreatment models based on MA-NaCl, MA-ZnCl₂ and MA-FeCl₃ were developed with high coefficients of determination (R² >0.8) and optimized. Maximum reducing sugar yield of 0.406g/g was obtained with 2M FeCl₃ at 700W for 3.5min. Scanning electron microscopy (SEM), Fourier Transform Infrared analysis (FTIR) and X-ray diffraction (XRD) showed major changes in lignocellulosic structure after MA-FeCl₃ pretreatment with 71.5% hemicellulose solubilization. This regime was further assessed on sorghum leaves and Napier grass under optimal MA-FeCl₃ conditions. A 2-fold and 3.1-fold increase in sugar yield respectively were observed compared to previous reports. This pretreatment was highly effective for enhancing enzymatic saccharification of lignocellulosic biomass.