Hydrothermal Pretreatment of Lignocellulosic Materials for Improving Bioethanol Production

With the continued depletion of non-renewable energy resources,it is essential to seek new methods of harnessing clean and renewable energy.In this regard,second-generation bioethanol derived from lignocellulosic biomass has attracted increasing attention in recent years.The choice of the pretreatment method of lignocellulose is critical to the subsequent bioconversion processes.Compared with other conventional chemical pretreatment methods,hydrothermal pretreatment is a simple,low-cost,and economically feasible process that requires water as the only reagent.This paper reviews the research efforts that have been made toward hydrothermal pretreatment of lignocellulosic biomass and focuses on the transformations involving cellulose,hemicellulose,and lignin during this process.

Lower temperature hydrothermal pretreatment improves the anaerobic digestion performance of spent cow bedding

The anaerobic digestion(AD)performance of spent cow bedding was investigated with different hydrothermal pretreatment(HP)conditions.Spent cow bedding was pretreated with low temperatures(50,70,and 90℃)and different pretreatment times(2-72 h)with ammonia and without ammonia.The results showed that spent cow bedding was a good raw material for AD.After pretreatment,the concentration of volatile fatty acids(VFAs)in the group of hydrothermal pretreatments with ammonia(HPA)was higher than that in the HP group at the same pretreatment temperature and time.The optimal pretreatment condition was achieved with an HPA of 50℃ holding for 72 h.At the optimal condition,the highest concentration of VFAs was 1.58-10.85 times higher than that of the other pretreated groups.The highest hemicellulose and lignin removal rates were 58.07%and 10.32%,respectively.The highest methane yield was 163.0 ml(g· VS)^(-1),which was 48.9%higher than that of the untreated group.The VFAs,pH,and reducing sugars showed positive relationships with the methane yield.Therefore,HP at low temperature can enhance the AD performance of spent cow bedding.

Comparative Study of Water-soluble and Alkali-soluble Hemicelluloses Extracted by Hydrothermal Pretreatment

The purpose of this study was to explore the differences of the hemicelluloses extracted by hydrothermal pretreatment using water and alkaline solutions(Na OH or KOH). The physicochemical properties and structural characteristics of two water-soluble and four alkali-soluble hemicelluloses extracted from the triploid of Populus tomentosa Carr. through the hydrothermal pretreatment were comparatively studied. It was observed that the alkalis(Na OH and KOH) were more effective than distilled water as extractants. Sugar analysis showed that xylose(66.83%~86.49%) was the major constituent, followed by glucose(6.83%~18.49%). Mannose(1.40%~8.42%), galactose(2.17%~4.05%), and arabinose(0.21%~2.26%) were also detected in the hemicellulosic fractions. The results of gelpermeation chromatography(GPC) indicated that the hemicelluloses extracted using the alkaline solutions had relatively higher molecular weights than those solubilized in distilled water. Further, based on spectroscopic ~1Hnuclear magnetic resonance(~1H-NMR) and two-dimensional heteronuclear singular quantum correlation(2D-HSQC) analyses, it was confirmed that the hemicellulosic fractions had a major structure of(1→4)-b-D-xylan and a minor structure of(1→4)-a-D-glucan with small amounts of substituted sugars and glucuronic acid attached.

The effect of hydrothermal pretreatment on the structure and fast pyrolysis behaviors of Sheng Li lignite

The structure and composition of coal determine its fast pyrolysis characteristics,and the study of the relationship between them can play an important role in the efficient and clean utilization of coal.So,in this work,hydrothermal pretreatment was used to artificially change the structure and composition of Sheng Li(SL)lignite,which was used to investigate the influence of structural changes on pyrolysis.The physicochemical structure and composition of samples were characterized by X-ray diffraction,specific surface area and porosity analyzer,solid-state 13C nuclear magnetic resonance,Fourier transform infrared spectroscopy,and elemental analyzer.Pyrolysis experiments were carried out in a powderparticle fluidized bed reactor,and the distribution and composition of the pyrolysis products were analyzed.The gasification activity of char was investigated by thermogravimetric analysis with a CO_(2) atmosphere.The results show that hydrothermal pretreatment(HTP)can destroy the cross-linking structure of SL lignite,and affect its aromaticity,pore structure,functional group,and carbon structure to change the distribution and composition of pyrolysis products of SL lignite,especially the composition of tar.Finally,the structure–activity relationship between the structure,composition,and pyrolysis characteristics of coal was comprehensively studied.

Hydrothermal pretreatment of protein-rich substrate:Modified physicochemical properties and consequent responses in its anaerobic digestion

Wasted tofu rich in protein was subject to hydrothermal pretreatment(HTPT)under different conditions(at 120,140,160 and 180℃;for 0,30,60 and 90 min)followed by biochemical methane potential(BMP)tests,and 140℃and 0 min were found to be respectively the best temperature and duration for HTPT of tofu in terms of its biogas production.Under the under the optimal conditions(140℃,0 min)the accumulative methane yield reached up to 510.9 mL⋅(gVS)-1,which was 26.98%higher than that without HTPT(402.3 mL⋅(gVS)-1).The start-up process of continuous anerobic digestion(AD)of the tofu before and after hydrothermal treated(HT)at the optimal HTPT conditions(140℃,0 min)was examined,to investigate and compare how their consequent AD responded to HTPT.It was found that,for start-up of continuous AD,the HT tofu delivered more balanced nutrients and thus led to more stable AD and quicker biogas production.Unavoidably,HTPT generated products refractory to biodegradation,to slightly decrease the total biogas production.During AD of HT tofu some weak ammonia-tolerant microbes,such as methylotrophic methanogens,survived and played indispensable roles.Analyses of living microbial community structure indicated that,some hydrolytic acidification bacteria intolerant to ammonia nitrogen(such as Proteobacteria)were always active and appeared at high proportion.The viable methylotrophic methanogens,e.g.RumEn M2,took obvious responsibilities in start-up of the AD for HT tofu.

Lignin extraction and recovery in hydrothermal pretreatment of bamboo

A significant amount of lignin and hemicellulose are dissolved in the hydrothermal treatment of biomass.The hemicellulose can be recovered and utilized for value-added products.The dissolved lignin can undergo depolymerization and condensation reactions,and interferes with the separation and purification process for hemicellulose recovery.This paper investigated the behavior of the lignin extracted from the hydrothermal pretreatment of bamboo and its contributions to the physical characteristics of the hydrolysate.It was found that the turbidity of the hydrolysate was strongly associated with the lignin fragments and suspended long chain hemicelluloses.As the lignin depolymerization and condensation reactions occurred simultaneously,the dissolved lignin fractions in the hydrolysate increased first and then decreased.The molecular weight(MW)of the dissolved lignin fragments ranged from 3342~5611 g/mol,with mainly guaiacyl(G)and syringyl(S)unit in the structure.

Hydrothermal carbonization pretreatment makes a remarkable difference in activation of rice and lettuce in food waste

Cooked rice and the vegetables like lettuce are common kitchen waste,which are carbonaceous materials and have the potential as feedstock for the production of activated carbon.Cooking is similar to hydrothermal treatment(HTC),which might impact the subsequent activation of kitchen waste.In this study,the HTC of lettuce,rice,or their mixture and the activation of the resulting hydrochars were conducted.The results indicated that cross-polymerization between the N-containing organics from lettuce and the sugar derivatives from rice took place in their co-HTC,which significantly increased the hydrochar yield.Activation of the hydrochar from the coHTC generated the AC with a yield of 2 times that from direct activation of mixed lettuce/rice.However,the coHTC facilitated aromatization,reducing reactivity with K2C2O4in activation and producing the AC with main micropores and low specific surface area.Activation of the hydrochar from HTC of rice followed the above trend,while that from lettuce was the opposite.The organics in lettuce were thermally unstable and could not undergo sufficient aromatization.The activation of hydrochar from HTC of lettuce thus generated the AC with the lowest yield,but the highest specific surface area(1684.9 m2/g),abundant mesopores,and superior capability for adsorption of tetracycline.However,the environmental impacts and energy consumption for the production of AC from the hydrochar of lettuce were higher than that from hydrochar of co-HTC.

Efficient Photocatalytic Degradation of the Persistent PET Fiber‑Based Microplastics over Pt Nanoparticles Decorated N‑Doped TiO_(2) Nanoflowers

Fiber-based microplastics(FMPs)are highly persistent and ubiquitously exist in the wastewater of textile industry and urban sewage.It remains challenging to completely remove such newly emerged organic pollutants by the predominant physical techniques.In this work,we investigated a photocatalytic degradation catalyzed by TiO_(2) catalyst to demonstrate the feasibility of implementing efficient chemical protocol to fast degrading polyethylene terephthalate(PET)-FMPs(a major FMP type existing in environment).The result shows that a hydrothermal pretreatment(180℃/12 h)is necessary to induce the initial rough appearance and molecular weight reduction.With the comprehensive action of the nano-flower shaped N doped-TiO_(2) catalyst(Pt@N-TiO_(2)-1.5%)on the relatively low molecular weight intermediates,an approximate 29%weight loss was induced on the pretreated PET-FMPs,which is about 8 times superior to the untreated sample.This work not only achieves a superior degradation effect of PET-FMPs,but also provides a new inspiration for the proposal of reduction strategies in the field of environmental remediation in the future.

Characterization of value-added chemicals derived from the thermal hydrolysis and wet oxidation of sewage sludge

We evaluated the effect of hydrothermal pretreatments,i.e.,thermal hydrolysis(TH)and wet oxidation(WO)on sewage sludge to promote resource recovery.The hydrothermal processes were performed under mild temperature conditions(140°C–180°C)in a high pressure reactor.The reaction in acidic environment(pH=3.3)suppressed the formation of the color imparting undesirable Maillard’s compounds.The oxidative conditions resulted in higher volatile suspended solids(VSS)reduction(~90%)and chemical oxygen demand(COD)removal(~55%)whereas TH caused VSS and COD removals of~65%and~27%,respectively at a temperature of 180°C.During TH,the concentrations of carbohydrates and proteins in treated sludge were 400–1000 mg/L and 1500–2500 mg/L,respectively.Whereas,WO resulted in solids solubilization followed by oxidative degradation of organics into smaller molecular weight carboxylic acids such as acetic acid(~400–500 mg/L).Based on sludge transformation products generated during the hydrothermal pretreatments,simplified reaction pathways are predicted.Finally,the application of macromolecules(such as proteins),VFAs and nutrients present in the treated sludge are also discussed.The future study should focus on the development of economic recovery methods for various value-added compounds.