Organic Chemical Reactions in Supercritical Water

The volatile organic compounds generated from the Maillard reaction between l-ascorbic acid and l-cysteine in hot compressed water

BACKGROUND

Hot compressed water (HCW), also known as subcritical water (SCW), refers to high-temperature compressed water in a special physical and chemical state. It is an emerging technology for natural product extraction. The volatile organic compounds (VOCs) generated from the Maillard reaction between l-ascorbic acid (ASA) and l-cysteine (Cys) have attracted significant interest in the flavor and fragrance industry. This study aimed to explore the formation mechanism of VOCs from ASA and Cys and examine the effects of reaction parameters such as temperature, time, and pH in HCW.

RESULTS

The identified VOCs were predominantly thiophene derivatives, polysulfides, and pyrazine derivatives in HCW. The findings indicated that thiophene derivatives were formed under various pH conditions, with polysulfide formation favored under acidic conditions and pyrazine derivative formation preferred under weak alkaline conditions, specifically at pH 8.0.

CONCLUSION

The Maillard reaction between ASA and Cys mainly produced thiophene derivatives, polysulfides, and pyrazine derivatives in HCW. The generation mechanism was significantly dependent on the surrounding pH conditions. © 2024 Society of Chemical Industry.

Supercritical Water Liquefaction of Mixed Waste Polystyrene, Polypropylene, and Polyethylene for Production of High Yield Oils

Supercritical water liquefaction of different plastic wastes has been investigated under high-temperature and high-pressure conditions. The supercritical water liquefaction of commonly used plastic types, comprising polystyrene (PS), polypropylene (PP), and low-density polyethylene (LDPE) as well as their mixtures, is reported. The experiments were carried out at varying feedstock-to-water ratios with a residence time of 60 min under supercritical water reaction conditions. The process produced high oil yields of over 97 wt %, with the highest yields obtained at a plastic:water ratio of 1:3; at higher levels of input water, the yield of oil decreased slightly. The gas phase mainly consisted of light hydrocarbons such as methane, ethane, propane, and butane, with propane found to be the most abundant gas component. Aromatic hydrocarbons and alicyclic hydrocarbons were the major products in the product oil from the supercritical water liquefaction of polystyrene and polypropylene, whereas alkanes were predominant in the oil obtained from LDPE. Analysis of the oil obtained from binary (1:1) and ternary (1:1:1) plastic mixtures showed it exhibited aromatic hydrocarbons as the major constituent, indicating synergistic interaction. It was found that the incorporation of PP in the mixture facilitated the production of cyclic compounds and suppressed the production of alkanes. Supercritical water liquefaction offers an effective solution to plastic pollution, producing valuable products without the need for catalysts.

Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy

Supercritical water provides distinctly different solvation properties compared to what is known from liquid water. Despite its prevalence deep in the Earth's crust and its role in chemosynthetic ecosystems in the vicinity of hydrothermal vents, molecular insights into its solvation mechanisms are still very scarce compared to what is known for liquid water. Recently, neutral metal particles have been detected in hydrothermal fluids and proposed to explain the transport of gold species to ore deposits on Earth. Using ab initio molecular dynamics, we elucidate the solvation properties of small gold species at supercritical conditions. The neutral metal clusters themselves contribute enormous THz intensity not because of their intramolecular vibrations, but due to their pronounced electronic polarization coupling to the dynamical supercritical solvent, leading to a continuum absorption up to about 1000 cm-1. On top, long-lived interactions between the gold clusters and solvation water leads at these supercritical conditions to a sharp THz resonance that happens to be close to the one due to H-bonding in liquid water at ambient conditions. The resulting distinct resonances can be used to analyse the solvation properties of neutral metal particles in supercritical aqueous solutions.

Mechanistic insights and catalytic enhancement of phenolic wastewater supercritical water gasification: A combined experiment and density functional theory study

Supercritical water gasification technology provides a favorable technology to achieve pollution elimination and resource utilization of phenolic wastewater. In this study, the reaction mechanism of phenolic wastewater supercritical water gasification was investigated using a combination of experimental and computational methods. Five reaction channels were identified to elucidate the underlying pathway of phenol decomposition. Importantly, the rate-determining step was found to be the dearomatization reaction. By integrating computational and experimental analyses, it was found that phenol decomposition via the path with the lowest energy barrier generates cyclopentadiene, featuring a dearomatization barrier of 70.97 kcal/mol. Additionally, supercritical water plays a catalytic role in the dearomatization process by facilitating proton transfer. Based on the obtained reaction pathway, alkali salts (Na2CO3 and K2CO3) are employed as a catalyst to diminish the energy barrier of the rate-determining step to 40.00 kcal/mol and 37.14 kcal/mol. Alkali salts catalysis significantly improved carbon conversion and pollutant removal from phenolic wastewater, increasing CGE from 58.44% to 93.55% and COD removal efficiency from 94.11% to 99.79%. Overall, this study provides a comprehensive understanding of the decomposition mechanism of phenolic wastewater in supercritical water.

Towards Negative Emissions: Hydrothermal Carbonization of Biomass for Sustainable Carbon Materials

The contemporary production of carbon materials heavily relies on fossil fuels, contributing significantly to the greenhouse effect. Biomass is a carbon-neutral resource whose organic carbon is formed from atmospheric CO2. Employing biomass as a precursor for synthetic carbon materials can fix atmospheric CO2 into solid materials, achieving negative carbon emissions. Hydrothermal carbonization (HTC) presents an attractive method for converting biomass into carbon materials, by which biomass can be transformed into materials with favorable properties in a distinct hydrothermal environment, and these carbon materials have made extensive progress in many fields. However, the HTC of biomass is a complex and interdisciplinary problem, involving simultaneously the physical properties of the underlying biomass and sub/supercritical water, the chemical mechanisms of hydrothermal synthesis, diverse applications of resulting carbon materials, and the sustainability of the entire technological routes. This review starts with the analysis of biomass composition and distinctive characteristics of the hydrothermal environment. Then, the factors influencing the HTC of biomass, the reaction mechanism, and the properties of resulting carbon materials are discussed in depth, especially the different formation mechanisms of primary and secondary hydrochars. Furthermore, the application and sustainability of biomass-derived carbon materials are summarized, and some insights into future directions are provided.

Trends in low temperature and non-thermal technologies for the degradation of persistent organic pollutants

The daunting effects of persistent organic pollutants on humans, animals, and the environment cannot be overemphasized. Their fate, persistence, long-range transport, and bioavailability have made them an environmental stressor of concern which has attracted the interest of the research community. Concerted efforts have been made by relevant organizations utilizing legislative laws to ban their production and get rid of them completely for the sake of public health. However, they have remained refractive in different compartments of the environment. Their bioavailability is majorly a function of different anthropogenic activities. Landfilling and incineration are among the earliest classical means of environmental remediation of waste; however, they are not sustainable due to the seepage of contaminants in landfills, the release of toxic gases into the atmosphere and energy requirements during incineration. Other advanced waste destruction technologies have been explored for the degradation of these recalcitrant pollutants; although, some are efficient, but are limited by high amounts of energy consumption, the use of organic solvents and hazardous chemicals, high capital and operational cost, and lack of public trust. Thus, this study has systematically reviewed different contaminant degradation technologies, their efficiency, and feasibility. Finally, based on techno-economic feasibility, non-invasiveness, efficiency, and environmental friendliness; radiation technology can be considered a viable alternative for the environmental remediation of contaminants in all environmental matrices at bench-, pilot-, and industrial-scale.

Theoretical study on the degradation mechanism of perfluoro-ethanesulfonic acid under subcritical hydrothermal alkaline conditions

Perfluorosulfonic acid, a widely recognized persistent organic pollutant, has attracted significant attention due to its severe environmental contamination, necessitating urgent resolution. To discover effective degradation strategies, this study implemented density functional theory, utilizing Gaussian 09 software with the WB97XD/6-311++G(2d,2p)//CCSD(T)/6-311++G(2df,2p) computational approach to conduct an in-depth reaction pathway analysis of perfluoroethane sulfonic acid (PFEtS) under subcritical hydrothermal alkaline conditions. It was revealed that PFEtS exhibits an uneven electron density distribution along the carbon chain backbone, with the bond energy of the C2-F4 bond being the lowest, followed by the C1-F1 bond, and the S-C1 bond energy being lower than those of C1-C2 and C-F bonds, rendering it susceptible to breakage. Based on these observations, seven potential degradation pathways of PFEtS were proposed under subcritical hydrothermal alkaline conditions, following optimization, and five reaction pathways have been identified. The degradation process unfolds in two stages. Initially, hydroxyl groups replace the sulfonate in PFEtS to form perfluoroethanol. Subsequently, full mineralization is achieved under alkaline conditions. The most probable reaction pathway involves hydroxyl groups attacking the C1 position, resulting in the generation of CO2 and inorganic fluoride ions. The first step of the reaction is the rate-determining step, with a theoretical rate constant calculated to be 8.41 × 10-5 L mol-1 s-1. This theoretical value is in close agreement with the experimentally determined degradation rate constant of perfluorooctane sulfonate under identical conditions, which is 8.67 × 10-4 L mol-1 s-1. This finding corroborates the experimental observation that longer-chain perfluoro-sulfonates degrade faster than their shorter-chain counterparts.

Complex phase diagram and supercritical matter

The supercritical region is often described as uniform with no definite transitions. The distinct behaviors of the matter therein, e.g., as liquidlike and gaslike, however, suggest "supercritical boundaries." Here we provide a mathematical description of these phenomena by revisiting the Yang-Lee theory and introducing a complex phase diagram, specifically a four-dimensional (4D) one with complex T and p. While the traditional 2D phase diagram with real temperature T and pressure p values (the physical plane) lacks Lee-Yang (LY) zeros beyond the critical point, preventing the occurrence of criticality, the off-plane zeros in this 4D scenario still induce critical anomalies in various physical properties. This relationship is evidenced by the correlation between the Widom line and LY edges in van der Waals, 2D Ising model, and water. The diverged supercritical boundaries manifest the high-dimensional feature of the phase diagram: e.g., when LY zeros of complex T or p are projected onto the physical plane, boundaries defined by isobaric heat capacity C_{p} or isothermal compression coefficient K_{T} emanates. These results demonstrate the incipient phase transition nature of the supercritical matter.

Experimental Investigation on the Pyrolysis and Conversion Characteristics of Organic-Rich Shale by Supercritical Water

Organic-rich shale oil reservoirs with low-medium maturity have attracted increasing attention because of their enormous oil and gas potential. In this work, a series of experiments on pyrolysis of the particle and core samples were carried out in a self-made supercritical water pyrolysis apparatus to evaluate the feasibility and benefits of supercritical water in promoting the transformation efficiency and oil yield of the low-medium maturity organic-rich shale. Core samples had a mass loss of 8.4% under supercritical water pyrolysis, and many microcracks were generated, which increased the pyrolysis efficiency substantially. The oil yield of shale pyrolysis could reach 72.40% under supercritical water conditions at 23 MPa and 400 °C, which was 53.02% higher than that under anhydrous conditions. In supercritical water conditions, oxygen-containing compounds are less abundant than in anhydrous conditions, suggesting that supercritical water can inhibit their formation. Also, supercritical water conditions produced higher yields for light fraction, medium fraction, and heavy fraction shale oil than those under anhydrous conditions. These results indicate that supercritical water pyrolysis is feasible and has excellent advantages for low-medium maturity organic-rich shale.

Hydrothermal Liquefaction: How the Holistic Approach by Nature Will Help Solve the Environmental Conundrum

Hydrothermal liquefaction (HTL) represents a beacon of scientific innovation, which unlocks nature's alchemical wonders while reshaping the waste-to-energy platform. This transformative technology offers sustainable solutions for converting a variety of waste materials to valuable energy products and chemicals-thus addressing environmental concerns, inefficiencies, and high costs associated with conventional waste-management practices. By operating under high temperature and pressure conditions, HTL efficiently reduces waste volume, mitigates harmful pollutant release, and extracts valuable energy from organic waste materials. This comprehensive review delves into the intricacies of the HTL process and explores its applications. Key process parameters, diverse feedstocks, various reactor designs, and recent advancements in HTL technology are thoroughly discussed. Diverse applications of HTL products are examined, and their economic viability toward integration in the market is assessed. Knowledge gaps and opportunities for further exploration are accordingly identified, with a focus on optimizing and scaling up the HTL process for commercial applications. In conclusion, HTL holds great promise as a sustainable technology for waste management, chemical synthesis, and energy production, thus making a significant contribution to a more sustainable future. Its potential to foster a circular economy and its versatility in producing valuable products underscore its transformative role in shaping a more sustainable world.

Pressure-Modulated Dissolution Behavior of LLM-105 Crystals in High-Temperature Water

The exploration of the microstructural evolution and reaction kinetics of energetic materials with high-temperature and high-pressure water contributes to the understanding of their microscopic physicochemical origin, which can provide critical experimental data for the use of energetic materials. As a promising high-energy and insensitive energetic material, LLM-105 has been investigated under extreme conditions such as high pressure and high temperature. However, little information is available about the effect of water on LLM-105 under high pressure and high temperature. In this work, the interaction between LLM-105 and water under HP-HT was investigated in detail. As a result, the dissolving behavior of LLM-105 in water under high pressure and high temperature is related to the initial pressure. When the initial pressure is less than 1 GPa, LLM-105 crystals are dissolved in high-temperature water; when the initial pressure is above 1 GPa, LLM-105 particles are only decomposed in high-temperature water. When the solution is saturated at a high temperature, recrystallization of the LLM-105 sample appears in the solution. High pressure hindered the dissolution process of the sample in HP-HT water because the interaction between the solute and the solvent was weakened by high pressure. The initial pressure is one of the significant parameters that determines whether LLM-105 crystals can be dissolved in high-temperature water. More importantly, water under high pressure and high temperature can not only act as a solvent when dissolving the samples but also act as a catalyst to accelerate the decomposition process. In addition, the HP-HT water reduced the decomposition temperature of the LLM-105 crystal to a large extent. The research in this paper not only provides insights into the interaction between LLM-105 and water but also contributes to the performance of energetic materials under extreme conditions and their practical applications in complex conditions.

Destruction and defluorination of PFAS matrix in continuous-flow supercritical water oxidation reactor: Effect of operating temperature

Cleanup and disposal of stockpiles and waste streams containing per- and polyfluoroalkyl substances (PFAS) require effective end-of-life destruction/mineralization technologies. Two classes of PFAS, perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs), are commonly found in legacy stockpiles, industrial waste streams, and as environmental pollutants. Continuous flow supercritical water oxidation (SCWO) reactors have been shown to destroy several PFAS and aqueous film-forming foams. However, a direct comparison of the SCWO efficacy for PFSAs and PFCAs has not been reported. We show the effectiveness of continuous flow SCWO treatment for a matrix of model PFCAs and PFSAs as a function of operating temperature. PFSAs appear to be significantly more recalcitrant than PFCAs in the SCWO environment. The SCWO treatment results in a destruction and removal efficiency of 99.999% at a T > 610 °C and at a residence time of ∼30 s. Fluoride recovery lags destruction PFAS at 510 °C and reaches >100% above 610 °C, confirming the formation of liquid and gaseous phase intermediate product during lower temperature oxidation. This paper establishes the threshold for destroying PFAS-containing liquids under SCWO conditions.

Evolution of elemental nitrogen involved in the carbonization mechanism and product features from wet biowaste

Hydrothermal carbonization (HTC) represents elegant thermochemical conversion technology suitable for energy and resource recovery from wet biowaste, while the elemental nitrogen is bound to affect the HTC process and the properties of the products. In this review, the nitrogen fate during HTC of typical N-containing-biowaste were presented. The relationship between critical factors involved in HTC like N/O, N/C, N/H, solid ratio, initial N in feedstock, hydrothermal temperature and residence time and N content in hydrochar were systematic analyzed. The distribution and conversion of N species along with hydrothermal severity in hydrochar and liquid phase was discussed. Additionally, the chemical forms of nitrogen in hydrochar were elaborated coupled with the role of N element during hydrochar formation mechanism and the morphology features. Finally, the future challenges of nitrogen in biowaste involved in HTC about the formation and regulation mechanism of hydrochar were given, and perspectives of more accurate regulation of the physicochemical characteristics of hydrochar from biowaste based on the N evolution is expected.

Effects of potassium on hydrothermal carbonization of sorghum bagasse

Hydrothermal carbonization (HTC) reacts with biomass in water at a high temperature and pressure to produce hydrochar with a higher heating value (HHV) and lower ash content than dry torrefaction. The high potassium content in biomass can promote thermochemical conversion; however, it lowers the melting temperature of the ash, causing slugging and fouling. Therefore, this study, investigated the effect of potassium on the HTC of sorghum bagasse by comparing the removal of potassium by washing with the addition of K2CO3. Consequently, the ash content was the highest in the potassium-added hydrochar and was 3.81% at a reaction time of 2 h. Elemental analysis showed that the lower the potassium content, the higher the carbon content, and the hydrochar with potassium removed by water washing at a reaction time of 3 h had the highest carbon content at 68.3%. Fourier transform infrared spectrometer showed dehydration and decarboxylation reactions due to HTC, but no significant differences were observed between the potassium concentrations. The mass yield decreased with increasing potassium content, and was 27.2% for the potassium-added hydrochar after 3 h. This trend was more pronounced with increasing reaction temperature. On the other hand, HHV was not affected by the potassium content. Therefore, the energy yield was similar to the weight yield. Thermal gravimetry and derivative thermal gravimetry (TG-DTG) analysis showed that higher potassium tended to accelerate the decomposition of lignin and decrease the oxidation temperature.

Production and Characterization of Hydrothermal Extracts of the Needles from Four Conifer Tree Species: Scots Pine, Norway Spruce, Common Juniper, and European Larch

Coniferous trees are the most dominant trees in Finland with a great economic value for pulp, paper, and timber making. Thus, their utilization also results in large quantities of residues, especially bark and needles. Tree needles are a rich source of bioactive compounds, which have a considerable utilization potential in different pharmaceutical or techno-chemical applications. In this study, hydrothermal extraction (HTE) of the needles from four conifer tree species, namely, Scots pine, Norway spruce, common juniper, and European larch, was performed. Besides water, ethanol was also used as a solvent to enhance extraction efficiency and selectivity. All of the HTE experiments were conducted with a customized high-pressure reactor operated at 120 °C and 5 bar. The obtained needle extracts were then analyzed using a direct-infusion ultrahigh-resolution Fourier transform ion cyclotron (FT-ICR) mass spectrometry. The FT-ICR analysis of water and ethanol extracts allowed identification of over 200 secondary plant metabolites, including monosaccharides, organic acids, terpenoids, a variety of phenolic compounds, and nitrogen alkaloids. The use of ethanol as the extraction solvent considerably enhanced the recovery of lipids, especially terpenoids, some polyphenols, and other unsaturated hydrocarbon species.

Fate of phthalate esters in landfill leachate under subcritical and supercritical conditions and determination of transformation products

The hypothesis of this study is that the complex organic load of landfill leachate could be reduced by supercritical water oxidation (SCWO) in a single stage, but this operation could lead to the formation of some undesired by-products of phthalate esters (PAEs). In this context, the fate of selected PAEs, butyl benzyl phthalate (BBP), di-2-ethylhexyl phthalate (DEHP) and di-n-octyl phthalate (DNOP), was investigated during the oxidation of leachate under subcritical and supercritical conditions. Experiments were conducted at various temperatures (250-500 °C), pressures (10-35 MPa), residence times (2-18 min) and dimensionless oxidant doses (DOD: 0.2-2.3). The SCWO process decreased the leachate's chemical oxygen demand (COD) from 34,400 mg/L to 1,120 mg/L (97%). Removal efficiencies of DEHP and DNOP with longer chains were higher than BBP. The DEHP, DNOP and BBP compounds were removed in the range of -35 to 100%, -18 to 92%, and 28 to 36%, respectively, by the SCWO process. Many non-target PAEs were qualitatively detected in the raw leachate apart from the selected PAEs. Besides, 97% of total PAEs including both target and non-target PAEs was mineralized at 15 MPa, 300 °C and 5 min. Although PAEs were highly mineralized during SCWO of the leachate, aldehyde, ester, amide and amine-based phthalic substances were frequently detected as by-products. These by-products have transformed into higher molecular weight by-products with binding reactions as a result of complex SCWO process chemistry. It has also been determined that some non-target PAEs such as 1,2-benzenedicarboxylic acid bis(2-methylpropyl)ester and bis(2-ethylhexyl) isophthalate can transform to the DEHP. Therefore, the suggested pathway in this study for PAEs degradation during the SCWO of the leachate includes substitution and binding reactions as well as an oxidation reaction.

Subcritical Water Hydrolysis of Fresh and Waste Cooking Oils to Fatty Acids Followed by Esterification to Fatty Acid Methyl Esters: Detailed Characterization of Feedstocks and Products

In this present work, the hydrolysis of a sample of rapeseed oil (RSO) and two waste cooking oil (WCO) samples in subcritical water has been carried out in a stirred batch stainless-steel reactor to produce fatty acids. Using RSO as a model triglyceride, the effects of reaction parameters on the yields of fatty acids were investigated to determine the optimum set of hydrolysis conditions to be a temperature of 300 °C, a reaction time of 60 min, and a vegetable oil-water mass ratio of 1:2. The set of optimum conditions was applied to the hydrolysis of the WCOs. Oleic acid was the dominant fatty acid with yields of 74.4 wt % from RSO and 57.5 and 72.4 wt % from the two WCOs, respectively, while palmitic acid was the second most abundant fatty acid with yields of up to 31 wt %. The feedstocks and fatty acid products were characterized by acid-base titration and thermogravimetric analysis (TGA). Thereafter, the hydrolysis products from the optimum conditions were esterified to their fatty acid methyl esters (FAMEs), which were further characterized by gas chromatography-mass spectrometry (GC/MS) and TGA. With RSO at the optimum hydrolysis conditions, acid-base titration gave a fatty acid yield of 97.2 wt %, while TGA gave 86 wt %. Under the same conditions, the yield of FAMEs from GC/MS analysis was 88.6 wt %, while TGA gave a FAMEs' yield of 91 wt %. This study showed that the simple TGA method provided detailed and complete characterization of lipid feedstocks and their conversion products. In addition, subcritical water hydrolysis can be used to valorize WCOs to fatty acids, with little or no extensive feedstock purification, for various applications including biodiesel production.

Review: Hydrothermal treatment of per- and polyfluoroalkyl substances (PFAS)

PER: and polyfluoroalkyl substances (PFAS) are a concerning and unique class of environmentally persistent contaminants with biotoxic effects. Decades of PFAS discharge into water and soil resulted in PFAS bioaccumulation in plants, animals, and humans. PFAS are very stable, and their treatment has become a global environmental challenge. Significant efforts have been made to achieve efficient and complete PFAS mineralization using existing and emerging technologies. Hydrothermal treatments in subcritical and supercritical water have emerged as promising end-of-life PFAS destruction technologies, attracting the attention of scholars, industry, and key stakeholders. This paper reviews the state-of-the-art research on the behavior of PFAS, PFAS precursors, PFAS alternatives, and PFAS-containing waste in hydrothermal processes, including the destruction and defluorination efficiency, the proposed reaction mechanisms, and the environmental impact of these treatments. Scientific literature shows that >99% degradation and >60% defluorination of PFAS can be achieved through subcritical and supercritical water processing. The limitations of current research are evaluated, special considerations are given to the challenges of technology maturation and scale-up from laboratory studies to large-scale industrial application, and potential future technological developments are proposed.

Supercritical water co-oxidation behavior in the different monohydric alcohol-ammonia reaction environment

The degradation of ammonia is a key rate-limiting step during the supercritical water oxidation of nitrogen-containing organics. This paper studied the co-oxidation behavior between different ammonia-alcohol environments, including the influence of reaction parameters and the co-oxidation mechanism. The results showed that increasing temperature, oxidation coefficient, residence time, and alcohol concentration significantly promoted the degradation of NH₃-N and TOC, while rising the ammonia concentration enhanced the NH₃-N destruction but inhibited the TOC degradation. Alcohols were oxidized first in the co-oxidation system to produce more OH* and HO₂* radicals. Ethanol generated the highest concentration of HO₂* in the shortest time, leading to more significant ammonia removal than isopropanol and methanol; however, the produced intermediate products like aldehydes and ketones reacted with residual ammonia to generate a small amount of organics at lower temperatures, inhibiting the degradation of alcohols slightly, and combined catalyst or nitrate in the batch reactor or used continuous supercritical water oxidation or supercritical hydrothermal combustion system without controlling the exotherm of fuels could improve this.

Recovery of Terephthalic Acid from Densified Post-consumer Plastic Mix by HTL Process

In this study, we investigate the hydrothermal liquefaction (HTL) of PET separated from a densified postconsumer plastic mix, with the aim of recovering its monomer. This second raw material is made up of 90% polyolefin, while the remaining 10% is made up of PET, traces of metals, paper, and glass. After preliminary separation by density in water, two batch experiments were performed on the sunken fraction (composed mainly of PET) in a stainless steel autoclave at 345 °C for 30 and 20 min. Both trials resulted in similar yields of the three phases. In particular, the solid yield is around 76% by weight. After a purification step, this phase was analyzed by UV-Vis, ¹H-NMR, and FTIR spectroscopy and resulted to be constituted by terephthalic acid (TPA), a product of considerable industrial interest. The study proved that the hydrothermal liquefaction process coupled with density separation in water is effective for obtaining TPA from a densified postconsumer plastic mix, which can be used for new PET synthesis.

Double universality of the transition in the supercritical state

Universality aids consistent understanding of physical properties and states of matter where a theory predicts how a property of a phase (solid, liquid, and gas) changes with temperature or pressure. Here, we show that the matter above the critical point has a remarkable double universality. The first universality is the transition between the liquid-like and gas-like states seen in the crossover of the specific heat on the dynamical length with a fixed inversion point. The second universality is the operation of this effect in many supercritical fluids, including N₂, CO₂, Pb, H₂O, and Ar. Despite different structure and chemical bonding, the transition has the same fixed inversion point deep in the supercritical state. This advances our understanding of the supercritical state previously considered to be a featureless area on the phase diagram and a theoretical guide for improved deployment of supercritical fluids in green and environmental applications.

Ferric Oxide Colloid: Towards Green Nano-Fertilizer for Tomato Plant with Enhanced Vegetative Growth and Immune Response Against Fusarium Wilt Disease

Global food crisis due to climate change, pandemic COVID-19 outbreak, and Russia-Ukraine conflict leads to catastrophic consequences; almost 10 percent of the world’s population go to bed hungry daily. Narrative solution for green agriculture with high vegetation and crop yield is mandatory; novel nanomaterials can improve plant immunity and restrain plant diseases. Iron is fundamental nutrient element; it plays vital role in enzyme activity and RNA synthesis; furthermore it is involved in photosynthesis electron-transfer chains. This study reports on the facile synthesis of colloidal ferric oxide nanoparticles as novel nano-fertilizer to promote vegetation and to suppress Fusarium wilt disease in tomato plant. Disease index, protection percent, photosynthetic pigments, and metabolic indicators of resistance in plant as response to induction of systemic resistance (SR) were recorded. Results illustrated that Fe₂O₃ NPs had antifungal activity against F. oxysporum. Fe₂O₃ NPs (at 20 µg/mL) was the best treatment and reduced percent disease indexes by 15.62 and gave highly protection against disease by 82.15% relative to untreated infected plants. Fe₂O₃ NPs treatments in either (non-infected or infected) plants showed improvements in photosynthetic pigments, osmolytes, and antioxidant enzymes activity. The beneficial effects of the synthesized Fe₂O₃ NPs were extended to increase not only photosynthetic pigments, osmolytes contents but also the activities of peroxidase (POD), polyphenol oxidase (PPO), catalase (CAT) and superoxide dismutase (SOD), enzymes of the healthy and infected tomato plants in comparison with control. For, peroxidase and polyphenol oxidase activities it was found that, application of Fe₂O₃ NPs (10 µg/mL) on challenged plants offered the best treatments which increased the activities of POD by (34.4%) and PPO by (31.24%). On the other hand, application of Fe₂O₃ NPs (20 µg/mL) on challenged plants offered the best treatments which increased the activities of CAT by (30.9%), and SOD by (31.33%).

Oily sludge treatment in subcritical and supercritical water: A review

Oily sludge, an inherent byproduct of the petroleum industry, presents dual characteristics of petroleum resources and hazardous waste. Owing to the unique physicochemical properties of sub-/supercritical water, hydrothermal technologies have been increasingly used for oily sludge treatment. This review is the first to focus on oily sludge treatment using sub-/supercritical water. Eight hydrothermal technologies used for different purposes are summarized herein: pressurized hot water extraction (PHWE) for hydrocarbon separation, thermal hydrolysis (TH) for dewaterability improvement, hydrothermal carbonization (HTC) for hydrochar production, wet air oxidation (WAO) for biodegradability improvement, hydrothermal liquefaction (HTL) for bio-oil production, supercritical water upgrading (SCWU) for light oil production, supercritical water oxidation (SCWO) for complete degradation, and supercritical water gasification (SCWG) for H₂-rich syngas production. Moreover, a general reaction pathway for sub-/supercritical water treatment of oily sludge is presented, with a particular focus on the chemical mechanism at temperatures above 350 °C. Lastly, two reaction maps are included to illustrate the reaction pathways of two groups of identifiable model compounds in oily sludge: aliphatic and aromatic hydrocarbons. This review provides detailed information that can promote a better understanding of various hydrothermal technologies, a guideline for selecting the suitable hydrothermal process for a particular oily sludge, and recommendations for further researches.

Partial oxidation of methane to methanol on boron nitride at near critical acetonitrile

Direct catalytic conversion of methane to methanol with O₂ has been a fundamental challenge in unlocking abundant natural gas supplies. Metal-free methane conversion with 17% methanol yield based on the limiting reagent O₂ at 275 °C was achieved with near supercritical acetonitrile in the presence of boron nitride. Reaction temperature, catalyst loading, dwell time, methane–oxygen molar ratio, and solvent-oxygen molar ratios were identified as critical factors controlling methane activation and the methanol yield. Extension of the study to ethane (C2) showed moderate yields of methanol (3.6%) and ethanol (4.5%).

Degradation and partial oxidation of waste plastic express packaging bags in supercritical water: Resources transformation and pollutants removal

Millions of waste plastic express packaging bags (PEPBs) were generated with the rapid development of the express delivery industry due to the boom of electronic commerce. Waste PEPBs contain polyethylene (PE) material and large number of pollutants such as plasticizers and flame retardants. In this study, two effective and environmental-friendly methods were proposed to produce valuable products and remove pollutants from waste PEPBs by supercritical water degradation (SCWD) and supercritical water partial oxidation (SCWPO) treatments. Both SCWD and SCWPO treatments could effectively obtain valuable products (wax, liquid oil, CaCO₃) and remove bisphenol A (BPA) and di-(2-ethylhexyl) phthalate (DEHP) from waste PEPBs. No obvious difference about the conversion could be found between SCWD and SCWPO treatments. 425 °C, 60 min, solid-to-liquid ratio of 1:20 g/mL, and V(H₂O₂):V(H₂O) ratio of 1:3 mL/mL were the optimal conditions for the conversion of waste PEPBs by SCWD and SCWPO treatments. The maximum conversion could reach 98.13%. The produced wax and liquid oil were easily separated from each other. The produced wax mainly included long-chain olefins or long-chain alkanes, and a small amount of alcohols, ethers and aldehydes. SCWD treatment was favorable for obtaining long-chain alkenes, while SCWPO treatment was favorable for obtaining long-chain alkanes. The main chemical compounds contained in the produced liquid oil were decomposed from DEHP and BPA. DEHP was decomposed to produce 2-ethyl-1-hexanol and acetophenone. BPA was decomposed to produce 4-tert-butylphenol and other alkylated derivatives of benzene and phenol. In comparison with SCWD treatment, DEHP and BPA could be decomposed more thoroughly by SCWPO treatment.

Singular value decomposition analysis of the electron density changes occurring upon electrostatic polarization of water

In-depth elucidation of how molecules are electrically polarized would be one key factor for understanding the properties of those molecules under various thermodynamic and/or spatial conditions. Here this problem is tackled for the case of hydrogen-bonded water by conducting singular value decomposition of the electron density changes that occur upon electrostatic polarization. It is shown that all those electron density changes are approximately described as linear combinations of ten orthonormal basis “vectors”. One main component is the interatomic charge transfer through each OH bond, while some others are characterized as the atomic dipolar polarizations, meaning that both of these components are important for the electrostatic polarization of water. The interaction parameters that reasonably well reproduce the induced dipole moments are derived, which indicate the extent of mixing of the two components in electrostatic polarization.

The main features of the electron density changes that occur upon electrostatic polarization of water are elucidated by conducting singular value decomposition analysis of those changes.

Opportunities and challenges for flow-through hydrothermal pretreatment in advanced biorefineries

Hydrothermal pretreatment (HTP) using only water offers great potential to reduce the overall cost of the bioconversion process. However, traditional HTP performed in a batch has limitations in removing lignin and often needs to be performed under severe conditions to achieve reasonable pretreatment effects. Lignin left in the pretreated residue at these conditions is also highly condensed, thus possessing an even more adverse impact on the hydrolysis process, which requires high enzyme loadings. To address these technical challenges, HTP performed in a flow-through configuration was developed to simultaneously achieve near-complete hemicellulose recovery, high lignin removal and high sugar release. Despite facing challenges such as potentially large water usage, flow-through HTP still represents one of the most cost-effective and eco-friendly pretreatment methods. This review mainly covers the latest cutting-edge innovations of flow-through HTP along with structural and compositional changes of cellulose, hemicellulose, and lignin before and after pretreatment.

Heat Transfer Limitations in Supercritical Water Gasification

Supercritical water gasification (SCWG) is a promising technology for the valorization of wet biomass with a high-water content, which has attracted increasing interest. Many experimental studies have been carried out using conventional heating equipment at lab scale, where researchers try to obtain insight into the process. However, heat transfer from the energy source to the fluid stream entering the reactor may be ineffective, so slow heating occurs that produces a series of undesirable reactions, especially char formation and tar formation. This paper reviews the limitations due to different factors affecting heat transfer, such as low Reynolds numbers or laminar flow regimes, unknown real fluid temperature as this is usually measured on the tubing surface, the strong change in physical properties of water from subcritical to supercritical that boosts a deterioration in heat transfer, and the insufficient mixing, among others. In addition, some troubleshooting and new perspectives in the design of efficient and effective devices are described and proposed to enhance heat transfer, which is an essential aspect in the experimental studies of SCWG to move it forward to a larger scale.

Review of the destruction of organic radioactive wastes by supercritical water oxidation

Organic materials, such as ion exchange resins, plastic, oils, and solvents, are widely used in the operation and decommission of nuclear facilities. The generated radioactive organic wastes are both radioactive and organic; therefore, the degradation of such wastes becomes more difficult. Due to delays in the disposal of radioactive organic wastes, potential safety risks are increasing. With the advantages of degrading refractory organics rapidly and thoroughly, supercritical water oxidation (SCWO) has become a potential alternative way to degrade radioactive organic wastes. This review focused on the degradation characteristics of different radioactive wastes from the perspective of potential practical applications. Some improved methods for facilitating the degradation of radioactive wastes by SCWO are considered and analyzed. Moreover, the kinetics and intermediate pathways of radioactive organic wastes are further analyzed. The distribution, migration and transformation of radionuclides during the SCWO reaction, as well as the further processing of radionuclides in gas-, liquid- and solid-phase products, were summarized and discussed. Furthermore, some fruitful areas for further work were reviewed for the highly efficient degradation of radioactive organic wastes. This review can provide useful information and guidance for the industrial applications of SCWO treatment for radioactive wastes.

Valorization of Chinese hickory shell as novel sources for the efficient production of xylooligosaccharides

Chinese hickory shell, a by-product of the food industry, is still not utilized and urgent to develop sustainable technologies for its valorization. This research focuses on the systematical evaluation of degraded products and xylooligosaccharide production with high yield from the shell via hydrothermal process. The pretreatment was carried out in a bath pressurized reactor at 140-220 °C for 0.5-2 h. The results indicated that the pretreatment condition strongly affected the chemical structures and compositions of the liquid fraction. The maximum yield of XOS (55.3 wt%) with limitation of by-products formation was achieved at 160 °C for 2 h. High temperature (220 °C) and short time (0.5 h) contributed to hydrolysis of xylooligosaccharide with high DP to yield 37.5 wt% xylooligosaccharide with DP from 2 to 6. Xylooligosaccharide obtained mainly consisted of xylan with branches according to the HSQC NMR analysis. Overall, the production of XOS with a high yield from food waste will facilitate the valorization of food waste in the biorefinery industry.

Investigating active phase loss from supported ruthenium catalysts during supercritical water gasification

Active phase loss mechanisms from Ru/AC catalysts were studied in continuous supercritical water gasification (SCWG) for the first time by analysing the Ru content in process water with low limit-of-detection time-resolved ICP-MS. Ru loss was investigated alongside the activity of commercial and in-house Ru-based catalysts, showing very low Ru loss rates compared to Ru/metal-oxides (0.2–1.2 vs. 10–24 μg g_Ru⁻¹ h⁻¹, respectively). Furthermore, AC-supported Ru catalysts showed superior long-term SCWG activity to their oxide-based analogues. The impact on Ru loss of several parameters relevant for catalytic SCWG (temperature, feed concentration or feed rate) was also studied and was shown to have no effect on the Ru concentration in the process water, as it systematically stabilised to 0.01–0.2 μg_Ru L⁻¹ for Ru/AC. Looking into the type of Ru loss in steady-state operation, time-resolved ICP-MS confirmed a high probability of finding Ru in the ionic form, suggesting that leaching is the main steady-state Ru loss mechanism. In non-steady-state operation, abrupt changes in the pressure and flow rate induced important Ru losses, which were assigned to catalyst fragments. This is directly linked to irreversible mechanical damage to the catalyst. Taking the different observations into consideration, the following Ru loss mechanisms are suggested: 1) constant Ru dissolution (leaching) until solubility equilibrium is reached; 2) minor nanoparticle uncoupling from the support (both at steady state); 3) support disintegration leading to the loss of larger amounts of Ru in the form of catalyst fragments (abrupt feed rate or pressure variations). The very low Ru concentrations detected in process water at steady state (0.01–0.2 μg_Ru L⁻¹) are close to the thermodynamic equilibrium and indicated that leaching did not contribute to Ru/AC deactivation in SCWG.

Ru loss mechanisms were investigated for the first time in SCWG by ICP-MS. Ru leaching at steady state was very low, close to thermodynamic models. Abrupt changes in process conditions must be avoided to prevent catalyst damage and higher Ru loss.

Study of the Potential Uses of Hydrochar from Grape Pomace and Walnut Shells Generated from Hydrothermal Carbonization as an Alternative for the Revalorization of Agri-Waste in Chile

A myriad of resources and efforts have been devoted to assessing the possibilities of using locally sourced biomass to produce energy, reduce CO2 emissions, and, in turn, lower dependance on petroleum. Grape pomace (GP) and walnut shells (WS) are organic waste generated in Chile. Within the last decade, the potential benefits and application of biomass have received significant attention, both in terms of producing functionalized carbon materials, and the various potential applications in the field of energy storage and environmental protection. The proposed research motivation is on the development of carbonous materials through thermal decomposition processes. Few researchers have addressed the idea of developing a multipurpose carbonaceous matrix from hydrochar, and there remains a need for an efficient method to obtain hydrochar specially from grape pomace. Hence, the general objective of this research is to study the potential of grape pomace and walnut shells treated with hydrothermal carbonization (HTC) as an alternative low-cost and efficient carbonous matrix. Proximate and elemental analysis was determined to distinguish the nature of the feedstock along with the hydrochar produced. Yield and reaction severity were also studied to study the impacts of temperature and residence time for both feedstocks. Successful results from the proposed work have broad applications for increasing the sustainability biomass applications, contributing to a positive economic impact.

Elevating density functional theory to chemical accuracy for water simulations through a density-corrected many-body formalism

Density functional theory (DFT) has been extensively used to model the properties of water. Albeit maintaining a good balance between accuracy and efficiency, no density functional has so far achieved the degree of accuracy necessary to correctly predict the properties of water across the entire phase diagram. Here, we present density-corrected SCAN (DC-SCAN) calculations for water which, minimizing density-driven errors, elevate the accuracy of the SCAN functional to that of "gold standard" coupled-cluster theory. Building upon the accuracy of DC-SCAN within a many-body formalism, we introduce a data-driven many-body potential energy function, MB-SCAN(DC), that quantitatively reproduces coupled cluster reference values for interaction, binding, and individual many-body energies of water clusters. Importantly, molecular dynamics simulations carried out with MB-SCAN(DC) also reproduce the properties of liquid water, which thus demonstrates that MB-SCAN(DC) is effectively the first DFT-based model that correctly describes water from the gas to the liquid phase.

Structure and Properties of Supercritical Water: Experimental and Theoretical Characterizations

Water in the supercritical region of the phase diagram exhibits a markedly different structure and properties from that at ambient conditions, which is useful in controlling chemical reactions. Nonetheless, the experimental, as well as theoretical, characterization of the substance is not easy because the region is next to the critical point. This article reviews the experimental as well as theoretical studies on water in the supercritical region and its properties as a solvent for chemical reactions, as carried out by the authors and based on small-angle X-ray scattering and the statistical mechanics theory of molecular liquids, also known as reference interaction-site model (RISM) theory.

Migration and transformation of heavy metals in hyperaccumulators during the thermal treatment: a review

The pollution of heavy metals (HMs) in the soil has become one of the important factors affecting the national environment and human health. Phytoremediation, as a technology to deal with HM pollution in soil, has been extensively studied and applied due to its sustainability and environmental friendliness. However, hyperaccumulators polluted by HMs need to be properly treated to avoid secondary pollution to the environment. This paper reviews the migration and transformation of HMs during the incineration, pyrolysis, gasification, and hydrothermal treatment of hyperaccumulators; comprehensively evaluates the advantages and disadvantages of each technology in the treatment of HM-enriched hyperaccumulators; and analyzes the current development status and unsolved problems in detail for each technology. Generally speaking, thermal treatment technology can fix most of the HMs of exchangeable fraction in biochar, reducing its bioavailability and biotoxicity. In addition, the application direction and research focus of the target product are discussed, and it is clarified that in the future, it is necessary to further optimize the reaction conditions and explore the mechanism of HM immobilization to maximize the immobilization of HMs and improve the quality and output of the target product.

A Review of Hydrothermal Liquefaction of Biomass for Biofuels Production with a Special Focus on the Effect of Process Parameters, Co-Solvents, and Extraction Solvents

Hydrothermal liquefaction is one of the common thermochemical conversion methods adapted to convert high-water content biomass feedstocks to biofuels and many other valuable industrial chemicals. The hydrothermal process is broadly classified into carbonization, liquefaction, and gasification with hydrothermal liquefaction conducted in the intermediate temperature range of 250–374 °C and pressure of 4–25 MPa. Due to the ease of adaptability, there has been considerable research into the process on using various types of biomass feedstocks. Over the years, various solvents and co-solvents have been used as mediums of conversion, to promote easy decomposition of the lignocellulosic components in biomass. The product separation process, to obtain the final products, typically involves multiple extraction and evaporation steps, which greatly depend on the type of extractive solvents and process parameters. In general, the main aim of the hydrothermal process is to produce a primary product, such as bio-oil, biochar, gases, or industrial chemicals, such as adhesives, benzene, toluene, and xylene. All of the secondary products become part of the side streams. The optimum process parameters are obtained to improve the yield and quality of the primary products. A great deal of the process depends on understanding the underlined reaction chemistry during the process. Therefore, this article reviews the major works conducted in the field of hydrothermal liquefaction in order to understand the mechanism of lignocellulosic conversion, describing the concept of a batch and a continuous process with the most recent state-of-art technologies in the field. Further, the article provides detailed insight into the effects of various process parameters, co-solvents, and extraction solvents, and their effects on the products’ yield and quality. It also provides information about possible applications of products obtained through liquefaction. Lastly, it addresses gaps in research and provides suggestions for future studies.