Microwave Pretreatment Research Articles

Investigating the Effect of Microwave Pretreatment on Bastnasite Grinding for Comminution Energy Reduction and Rare Earth Recovery

Investigating the Effect of Microwave Pretreatment on Bastnasite Grinding for Comminution Energy Reduction and Rare Earth Recovery

Microwave-Assisted Acid and Alkali Pretreatment of Napier Grass for Enhanced Biohydrogen Production and Integrated Biorefinery Potential

Napier grass, a promising lignocellulosic energy crop, presents a complex composition that limits its bioconversion into fermentable products. To address this challenge, we applied microwave (MW) pretreatment assisted by acid and alkali, using varying chemical concentrations (0.5–1% w/v) and pretreatment times (3–10 min). Acid-catalyzed MW pretreatment achieved a maximal hemicellulose removal of 69.8%, while alkali-catalyzed MW pretreatment resulted in significant lignin removal of 65.5%. Without chemical catalysis, the pretreated hydrolysate significantly increased hydrogen yield to 38.0 ± 2.9 mL H2/g volatile solid (VS), five times greater than that obtained from untreated biomass. Hydrogen yield was further enhanced when the MW-pretreated solid underwent simultaneous saccharification and fermentation. The highest hydrogen yield of 89.2 ± 7.2 mL H2/g VS was achieved from alkali-catalyzed MW pretreated solid (0.5% w/v NaOH, 5 min), with a chemical oxygen demand (COD) solubilization of 62.6%. Increasing the NaOH concentration to 1% (w/v) slightly decreased hydrogen yield but significantly increased COD solubilization to 85.8%. The high carbohydrate content facilitated rapid cellulase hydrolysis, producing and accumulating a high concentration of fermentable sugars. However, this accumulation subsequently led to a shift towards lactic acid formation. The improved hydrogen yield and increased COD solubilization, along with the shift towards lactic acid production, suggest the possibility of optimizing this process for simultaneous production of multiple valuable products in an integrated biorefinery approach, potentially enhancing the economic viability of biomass conversion.

The Research on Temperature Variation Characteristic of Wood during High-intensity Microwave Pretreatment

The Research on Temperature Variation Characteristic of Wood during High-intensity Microwave Pretreatment

Physicochemical, techno-functional, and thermal properties of Kabuli chickpea's aquafaba extracted by optimized microwave-assistive process

Aquafaba, a liquid by-product from the soaking and cooking of legumes, is often discarded in canning processes despite its rich protein, carbohydrate content, and functional properties. This study investigates the impact of microwave irradiation pretreatment on the extraction efficiency and the physicochemical, techno-functional, and thermal properties of Kabuli chickpea aquafaba. The findings revealed that increased microwave irradiation time and power led to a reduction in foaming ability and extraction efficiency but significantly enhanced the soluble protein content, density, dry matter, and foam stability of aquafaba. Optimal extraction was achieved with 5 min of irradiation at 900 W, yielding the highest extraction efficiency (175.3 %), protein concentration (1779.12 mg kg-1), density (1.019 g/mL), dry matter (4.11 %), foaming ability (341.21 %), and foam stability (49.56 %). Thermogravimetric analysis and FT-IR spectroscopy confirmed that microwave pretreatment improved the thermal stability of aquafaba without altering its chemical structure. Microwave-assisted extraction improves the quality and functionality of aquafaba, offering a sustainable alternative to waste disposal in the food industry. While the initial investment in microwave technology may be high, it transforms aquafaba from a waste by-product into a valuable resource, leading to significant profitability for canning industries. Given the high techno-functional, physicochemical, and thermal properties of aquafaba, it appears to be a suitable alternative to more expensive ingredients like egg whites and a promising substitute in vegan products.

Hydrothermal pretreatment with microwaves: A new strategy for ensuring the sustainability of biorefineries

Lignocellulosic biomass (LCB) includes agricultural residues, crop wastes, and food wastes that are ubiquitously present and can be used as renewable resources to generate bioenergy or biofuels. Still, direct utilization of LCB to produce biofuel is not an easy task due to the recalcitrance property of LCB. The pretreatment strategy is an important step in a biorefinery to reduce biomass recalcitrance and to use it for bioenergy including value-added product generation. Microwave irradiation is procuring interest regarding LCB and waste material pretreatment over time before moving to biological transformation to biofuels. The coupling of microwave heating with water has made it the most effective treatment for biomass valorization to generate biofuel and other value-added products, turning it into the most cost-effective and eco-friendly approach. Despite its numerous advantages, scaling up microwave pretreatment from laboratory experiments to industrial applications faces several challenges. These limitations become significant hurdles in scaling up. As a result, it is necessary to focus research on cost evaluations and life cycle assessment (LCA) studies for commercialization that will benefit the bioeconomy and the environment. This review discusses microwave-assisted pretreatment significance in valorizing LCB for biofuel generation including the cost evaluation and LCA studies that have the most important role in commercializing the microwave-assisted pretreatment from lab scale to industrial scale.

Enhancing Iron Ore Grindability through Hybrid Thermal-Mechanical Pretreatment

Grinding is an important process of ore beneficiation that consumes a significant amount of energy. Pretreating ore before grinding has been proposed to improve ore grindability, reduce comminution energy, and enhance downstream operations. This paper investigates hybrid thermal mechanical pretreatment to improve iron ore grinding behavior. Thermal pretreatment was performed using conventional and microwave approaches, while mechanical pretreatment was conducted with a pressure device using a piston die. Results indicate that conventional (heating rate: 10 °C; maximum temperature: 400 °C), microwave (2.45 GHz, 1.7 kW, 60 s), and mechanical (14.86 MPa, zero delay time) pretreatments improved the studied iron ore grindability by 4.6, 19.8, and 15.4%, respectively. Meanwhile, conventional-mechanical and microwave-mechanical pretreatments enhanced the studied iron ore grindability by 19.2% and 22.6%, respectively. These results suggest that stand-alone mechanical pretreatment or microwave pretreatment may be more beneficial in improving the grinding behavior of the studied fine-grain iron ore sample. The results of the mechanical pretreatment obtained in this study may be used in a simulation of the HPGR system for grinding operations of similar iron ore

Change in components by pretreatments alters the antimicrobial activity of Strobilanthes cusia biomass

The plant Strobilanthes cusia contains various antibacterial components, but the mechanisms behind its antimicrobial activity are not yet fully understood. This study demonstrates that alterations in components due to different pretreatments significantly impacted the antimicrobial activity of S. cusia biomass. Biomass dried using microwave pretreatment, which contained a high amount of indican, exhibited the lowest antibacterial activity, whereas heat-dried biomass without indican showed the highest antibacterial activity. High-performance liquid chromatography analysis indicated that pretreatments influenced the ratios of indican, indoxyl, indigo, and indirubin in S. cusia biomass, likely due to the effect of pretreatments on β-glucosidase activity. Extracts with higher levels of indoxyl, an intermediate product of indican hydrolysis, exhibited stronger inhibitory effects against Staphylococcus aureus. Indoxyl increased the NAD+/NADH ratio, which led to elevated hydrogen peroxide generation and subsequent cell damage. Furthermore, transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy revealed distinct morphological changes in cells exposed to indoxyl. Finally, leveraging the water-soluble properties of indoxyl, we propose a novel approach using microwave drying assisted by β-glucosidase to enhance the antimicrobial activity of S. cusia biomass. These findings provide a theoretical and methodological foundation for the evaluation and future application of S. cusia's bioactive substances.

Remarkable improvement in the NOx storage ability of Pd/SSZ-13 zeolite PNA via a facile pretreatment strategy

A facile strategy was employed to pretreat Pd/SSZ-13 zeolite obtained via the initial-impregnated method under a microwave field, aiming to greatly enhancing the NOx storage-release ability. By investigating the effect of microwave treatment parameters (power, atmosphere and time) on the improvement of PNA performance, the optimal conditions were determined (175 W, 10 % O2/N2, 15 min). Under these conditions, the Pd/SSZ-13 exhibited an enhanced NOx adsorption and desorption amount to 2.60 and 2.38 times, respectively, compared to the untreated sample after three cycles of NO-TPD. Deeper insights into the variations in Pd/SSZ-13 properties induced by the microwave radiation were clarified through a series of characterizations. The micropore surface area of Pd/SSZ-13 zeolite activated by microwave increased by 3.91 %, and the micropore volume also improved by 3.85 %, which might facilitate the adhesion of more active Pd species to the materials surface, and aid in the dispersion of PdOx clusters into the zeolite channels. After microwave activation, the average particle size of Pd decreased from 10.09 nm to 7.08 nm. Particularly, the content of Pd(II) species such as Pd2+, [Pd(OH)]+ and PdO as active NOx adsorption sites enhanced from 66.68 % to 68.93 %. Additionally, the microwave treatment not only increased the number of Lewis and Brønsted acid sites, but also strengthened the interaction between NOx molecules and the zeolite skeleton. The above reasons collectively promoted the NOx storage-release properties of Pd/SSZ-13 zeolite, showing that the microwave pretreatment is a highly efficient strategy for improving the performance of PNA materials.

Enhancing thermophilic methane production from oil palm empty fruit bunches through various pretreatment methods: A comparative study

This study investigated the effects of various pretreatment methods on the anaerobic digestibility of oil palm empty fruit bunches (EFB) for methane production. Pretreatment methods included weak alkaline (2 % Ca(OH)2), weak acid (2 % acetic acid), acidified palm oil mill effluent (POME), biogas effluent, hydrothermal (180 °C, 190 °C, and 200 °C), and microwave pretreatments. All pretreatment methods enhanced methane yield compared to untreated EFB (189.45 mL-CH4/g-VS), with weak alkaline pretreatment being the most effective (277.11 mL-CH4/g-VS), followed by hydrothermal pretreatment at 180 °C (244.33 mL-CH4/g-VS) and biogas effluent pretreatment (238.32 mL-CH4/g-VS). The enhanced methane yield was attributed to increased cellulose content (45.5 % for weak alkaline pretreatment), reduced hemicellulose (18.0 % for hydrothermal pretreatment at 200 °C), and lignin contents (19.0 % for hydrothermal pretreatment at 200 °C), decreased crystallinity index (40.0 % for hydrothermal pretreatment at 200 °C), and increased surface area. Weak alkaline pretreatment also showed the highest net energy balance (8.73 kJ/g-VS) and a short break-even point (2 years). Microbial community analysis revealed that weak alkaline pretreatment favored the growth of syntrophic acetate-oxidizing bacteria and hydrogenotrophic methanogens, contributing to improved methane yield. This study demonstrates the potential of EFB pretreatment, particularly weak alkaline and biogas effluent pretreatment, for enhancing methane production and sustainable management of palm oil mill waste.

Optimization of Microwave-Assisted Inorganic Salt Pretreatment for Production of Fermentable Sugars from Spent Coffee Ground

Spent coffee ground (SCG) is a solid waste that is generated in the coffee brewing process for coffee beverage production. SCG hold a great potential in reducing sugar production as it consists of high amount of carbohydrates. However, SCG is a lignocellulosic biomass (LCB) which requires pretreatment to degrade the lignocellulosic structure and enhance enzyme accessibility during saccharification process. Hence, this research aims to study the performance of microwave-assisted inorganic salt pretreatment for generation of reducing sugars. Different concentrations of NaCl was applied to determine their effects on reducing sugar produced as well as pH, and solid recovery. Pretreatment with 3% NaCl was found to yield the highest reducing sugar concentration of 43.56 mg/mL, hence it was selected for the subsequent optimization study. Process optimization was designed by using the Response Surface Methodology approach (RSM) with Central Composite Design (CCD), based on three pretreatment parameters; solid to liquid ratio (1:50 - 8:50), microwave power (300W – 800W) and irradiation time (1-10 minutes). The optimized conditions were achieved at solid to liquid of 8:50, microwave power of 800 watt and irradiation time of 10 minutes for a maximum response of 40.89 mg/mL. Moreover, it was observed that microwave-assisted NaCl pretreatment had significantly caused surface morphological changes of the SCG and the removal of functional groups in lignin, resulted in increment of the crystallinity value. In conclusion, microwave pretreatment is a promising green technology for fermentable sugar production from SCG.

Predicting Biogas Yield after Microwave Pretreatment Using Artificial Neural Network Models: Performance Evaluation and Method Comparison

Predicting Biogas Yield after Microwave Pretreatment Using Artificial Neural Network Models: Performance Evaluation and Method Comparison

Sustainable biofuels from corncobs: optimization of microwave pretreatment for enhanced fermentable sugar yield

Microwave-Assisted Alkali (MAA) pretreatment of corncob biomass was optimized for maximum fermentable sugar yield using dilute sulfuric acid hydrolysis. A Central Composite Design (CCD) investigated the effects of microwave power (450–9000 W), NaOH concentration (1.5 to 4.5% wt./v), and irradiation time (10–20 min) on lignin removal. The optimal MAA pretreatment conditions were found to be 691.73 W, 3.4% NaOH, and 15.5 min, resulting in the selective removal of 73.36% lignin with high retention of cellulose (71.69%) and hemicellulose (22.14%). Characterization techniques (FTIR, SEM, XRD, TGA) confirmed substantial changes in surface morphology, functional groups, crystallinity, and thermal stability of the pretreated corncobs. Dilute sulfuric acid hydrolysis (0.5–2.5%, 110130 °C, 3060 min) of the optimized sample yielded a maximum reducing sugar output of 537.42 mg/g, achieved at 0.534% H2SO4, 122.3 °C, for 58 min. Optimizing Microwave-Assisted Alkali pretreatment process for corncobs could boost biofuel production efficiency, sustainably utilize agricultural waste, and advance a more sustainable energy landscape.

Effect of microwave and ultrasonic pre-treatments on anaerobic co-digestion of orange wastes and municipal sewage sludge: A case study

The application of orange wastes (OW) in anaerobic digestion process still constitutes a technological challenge. One of the possibilities to overcome this issue is application of pre-treatment strategy. In this study, the effect of microwave (MW) and ultrasonic (US) pre-treatments on OW and sewage sludge (SS) co-digestion was examined. Firstly, the selection of optimal pre-treatment conditions in terms of solubilisation and generation of inhibitors (phenol and limonene contents) was conducted. For MW, irradiation time of 10 min and specific energy input of 28 089 kJ/kgTS were chosen as optimal. For US pre-treatment, specific energy input of 8914 kJ/kgTS and time of 20 min were adopted. Then, the mesophilic anaerobic co-digestion in batch mode was performed. However, neither of the two proposed pre-treatments had a significant impact on methane production, as compared to the untreated sample.

The comparative techno-economic and life cycle assessment for multi-product biorefinery based on microwave and conventional hydrothermal biomass pretreatment

Microwave as an efficient and clean energy, is easy integrated with DES pretreatment and biorefinery process to achieve process intensification for cleaner production and sustainable development. However, there are worries about its economic and environmental feasibility for application in biorefinery plant. This work conducted the techno-economic (TEA) and life cycle assessment (LCA) of the entire biorefinery process based on DES pretreatment by microwave heating (MH) for producing glucose, furfural and lignin from lignocellulose, of which the results were compared with conventional heating (CH). For pretreating equivalent biomass, the total yield of products obtained from MH-assisted process was 8.53% higher than that of CH-assisted process due to microwave intensification effect. From the perspective of cradle-to-gate LCA, the MH-assisted process saved energy demand of 5.82%, reducing the environmental impacts global warming (GW) and fossil resource scarcity (FRS) by 4.85% and 5.69%, respectively, relative to CH-assisted process. The main environmental hotspots of both two processes concentrated on the raw material extraction (RME), followed by product separation and solvent recovery (PSSR) stages. Based on economic evaluation, the specific values of TAC regarding one-ton product for MH- and CH-assisted processes were 3194.35 and 3400.37 $/(y⋅t), respectively. Therefore, the MH-assisted pretreatment possessed certain environmental and economic advantages due to product increasement, energy consumption reduction in the DES pretreatment, and burden reduction for subsequent enzyme hydrolysis. Compared to the single-product biorefinery, the multi-product biorefinery strategy was more sustainable and could maximize the advantages of microwave intensification. However, some issues about the design of microwave reactors and temperature monitoring must be considered to pursue scale-up biorefinery plant.

Integration of ultrasound and microwave pretreatments with solid-state fermentation enhances the release of sugars, organic acids, and phenolic compounds in wheat bran

Wheat bran (WB), a byproduct of milling, is rich in bioactive compounds with significant health benefits. This study aimed to enhance the release of phenolic compounds, sugars, and organic acids from WB by integrating ultrasound (UsP) and microwave (MWP) pretreatments with solid-state fermentation (SSF). UsP and MWP disrupted WB cell walls, followed by SSF with Aspergillus niger. UsP increased total phenolic content by 21.30 % on day 1 of SSF. UsP and MWP boosted the availability of bound phenolic compounds like vanillic acid and dihydroxybenzoic acid. Both pretreatments enhanced antioxidant activity compared to untreated fermented WB, with peak activity on day 5 of fermentation at 1411 ± 5.156 μM Trolox/100 g DW for UsP WB and 291.6 ± 1.092 μM Trolox/100 g DW for MWP WB. This integrated approach improved the extraction efficiency of fermentable monosaccharides, particularly glucose and xylose, offering a sustainable bioprocessing strategy for WB valorization and supporting the circular bioeconomy.

Enhancing fruit quality, antioxidant profile and sensory characteristics of Phoenix dactylifera L. through microwave based processing

Date fruit, Phoenix dactylifera L. is an excellent source of nutrients, but a major portion of this fruit is wasted due to climatic conditions, microbial infestation, and many other factors. These factors are responsible for the uneven ripening of date fruits, due to which major proportions are wasted each year. Thus, there is a need to process un-ripened date fruits for quality enhancement. In this regard, the influence of novel microwave pretreatment for the ripening of three different date fruit varieties (Khidrawi, Halawi, and Dhakki) was analyzed and compared with different conventional (hot water, acetic acid 0.5%, sun dried, and hot air dried) processing methods. The results indicated that in all three date fruit varieties, microwave processed fruits showed significant improvement in proximate composition including, carbohydrates (75–79 g/100 g fruits), moisture (13–18 g/100 g fruit), and dietary fiber contents (6.96–7.78 g/100 g) in comparison to control. Moreover, significant improvement in antioxidant capacity i.e. DPPH inhibition (82–91%), total phenolic contents (286–299 mgGAE/100 g), and total flavonoids contents (49–66 mg QE/100 g) was observed in microwave processed samples as compared to other treatments. Additionally, significant reduction in total bacterial count was observed in microwave processed samples (0.90 ± 0.03, 0.69 ± 0.02, 0.79 ± 0.02 in Khidrawi, Halawi and Dhakki, respectively) as compared to other treatments. In three different varieties, Dhakki was observed to be more acceptable among consumers with respect to sensory attributes as compared to other varieties. Hence, the use of microwave processing was observed to be better for the ripening of date fruits.

Mechanisms and advancements in microwave-enhanced CO2 mineralization of lightweight porous concrete

Lightweight porous concrete (LPC) typically exhibits low early strength, which can be addressed by enhancing early strength through carbon dioxide (CO2) mineralization curing while achieving permanent CO2 sequestration. The efficacy of this process relies on maintaining optimal moisture levels. This study investigates the influence of moisture content on the performance of solid waste-based lightweight porous concrete blocks (RSFAC) subjected to microwave heating pretreatment. Moisture levels were adjusted to 10%, 20%, 30%, and 40% (initial water-to-cement ratio of 45.4%) before undergoing mineralization curing within an environment set at a temperature of 20 °C, relative humidity 75%, and a CO2 concentration of 40% for a duration of 0.5 h. The impact of moisture content on RSFAC's compressive strength, carbon sequestration rate, mineralization products, and microstructure was evaluated. Results indicate that compared to conventional oven pretreatment methods requiring nearly 10 h, microwave pretreatment technology achieves equivalent water content within just 50 min, significantly reducing the time needed for mineralization pretreatment. RSFAC, when pretreated with microwave heating, outperforms untreated samples (Ref-0 and Ref-1) in terms of compressive strength and carbon sequestration efficiency. Optimal performance was observed at a moisture content level of 30%, with the highest recorded compressive strength achieved at 1, 3, and 7 d respectively. At the curing age of 7 d, the compressive strength reached up to 2.32 MPa while the carbon sequestration rate reached approximately 11.61%. Furthermore, aragonite and calcite (tetrahedral and hexahedral), without poorly crystalline spherulites, were identified as the main crystalline CaCO3 generated by RSFAC under this specific moisture content condition; pores larger than 50 nm served as primary reaction sites for RSFAC mineralization curing, whereas there was a significant increase in pore proportion with sizes less than 20 nm after filling with mineralization products.

Microwave pre-treatment of niger (Guizotia abyssinica) seeds: Impact on stability, physicochemical characteristics, bioactive and fatty acid profiles of extracted oil

The study explored the impact of microwave pre-treatment (MPT) at different powers (360, 540 and 720 W) and exposure times (2, 4, 6 and 8 min) on the qualitative characteristics of niger seed oil (NgO). The changes in physicochemical characteristics, oxidative stability index (OSI), phenolic constituents, Maillard products, tocopherols, pigments and phytosterols in NgO were elucidated after MPT of seeds. The NgO yield increased from 33.19 % to 38.76 % and OSI augmented from 2.69 to 3.93 h upon MPT at 540 W and 720 W for 8 min, respectively. The tocopherols and phytosterols levels augmented upon MPT at 540 W for 4 and 6 min, respectively while declined by further elevating power (720 W) and exposure time (8 min). The total hydroxycinnamic (27.41 mg/kg) and hydroxybenzoic (1.48 mg/kg) acids and total flavonoids (115.83 mg/kg) were highest upon MPT at 720 W for 6 min while viscosity (56.71 mPa.s), chlorophylls (0.12 mg/kg), carotenoids (0.85 mg/kg) and 5-hydroxymethylfurfural (1.38 mg/kg) were highest upon MPT at 720 W for 8 min. In conclusion, the MPT promoted the release of bioactive compounds and lowered the generation of harmful oxidative by-products. The results highlight the potential of MPT in enhancing quality attributes of NgO for use in the food, cosmetics, and pharmaceutical industries.

Effect of potassium persulphate-induced microwave pretreatment for cost-effective sludge solubilization and bioenergy recovery

The disintegration of municipal waste activated sludge (MWAS) by energy-intensive physical pretreatments is considered expensive. In the present study, an effort was undertaken to disintegrate the sludge by microwave assisted pretreatment (MAP) in an energy-efficient and economical way by combining the microwave with oxidant potassium persulphate. Besides, the sulphate radicals generated through activated persulfate can efficiently disintegrate and solubilize sludge. In this context, microwave pretreatment was combined with potassium persulfate to activate the generation of sulphate radicals for economically enhanced sludge solubilization. MAP was done by varying the microwave power levels (10 to 50 %) and pretreatment time (0 to 30 min). Microwave assisted potassium persulphate pretreatment (MAPPP) was done by varying the potassium per sulphate dosage (0.005 to 0.04 g/g Total solids (TS) and pretreatment time (0 to 14 min). Combining microwave with potassium persulfate brings about a synergistic effect, generating sulphate and hydroxyl radicals that can effectively disintegrate and solubilize biopolymers of sludge. A higher sludge lysis rate of 37.25 % was achieved by this microwave assisted potassium persulfate pretreatment (MAPPP) when compared to microwave assisted pretreatment (MAP) alone (26.27 % after thickening). A higher methane yield of 219.14 mL/g Volatile solids (VS) was obtained through MAPPP compared to MAP (154.22 mL/g VS). The detailed mass, energy, and economic assessment at a large-scale level for MAP and MAPPP revealed that MAPPP was energetically and economically feasible with an energy ratio of 1.22 and net profit of 42.03 USD/Ton compared to MAP. Thus, it can be confirmed that MAPPP was economically viable to implement on a large scale.

Effect of Cold Plasma and Microwave Pretreatments Combined with Hot-Air Drying on Physical Properties, Chemical Composition, and Volatile Profile of Pomegranate Peels

Effect of Cold Plasma and Microwave Pretreatments Combined with Hot-Air Drying on Physical Properties, Chemical Composition, and Volatile Profile of Pomegranate Peels