Use Of Subcritical Water Research Articles

Exploring the green frontier: Subcritical water chromatography for sustainable analytical practices.

Water in the subcritical state is characterized by properties significantly different from water under standard conditions. These include low viscosity, low surface tension, and a much lower dielectric constant, increasing the solubility of nonpolar substances. For this reason, it can provide an alternative solvent and be used in chromatographic techniques-subcritical water chromatography (SBWC). SBWC appears to be one of the greenest analytical techniques until we unravel chromatography with pure water at room temperature. The versatility of SBWC is explored through its applications in the separation and analysis of a wide range of compounds, including pharmaceuticals, natural products, etc. The use of subcritical water as a mobile phase requires suitable stable stationary phases and special apparatus. Still, it makes it possible to conduct analyses without using organic solvents. When using this technique, it is important to remember that it suits the analysis of thermally stable substances. The following work is a critical review of developments in SBWC.

Immunostimulatory effects of a subcritical water extract of Ganoderma.

Ganoderma, a medicinal mushroom with various physiological activities, has been extensively investigated regarding its effectiveness. The aim of the present study was to examine the effects of a subcritical water extract of Ganoderma (SWEG) on the immune system. The use of subcritical water with a higher temperature and pressure than hot water allows efficient elution of components from natural products. As an evaluation of the effectiveness of SWEG, a cell proliferation and a cell differentiation test were carried out using A-6 cells, a model of hematopoietic stem cells. Furthermore, an oral administration test in mice was conducted to examine the effects of SWEG on the number and function of immune cells. As a result, SWEG was revealed to promote both self-renewal and differentiation into immune cells such as T cells and natural killer (NK) cells in experiments with A-6 cells. These results were not obtained in experiments using hot water extract of Ganoderma lucidum and Ganoderma sinense. The oral administration test in mice demonstrated that SWEG increased hematopoietic precursor cells, immature B cells, and NK cells in the bone marrow, and T cells in the thymus. In addition, SWEG enhanced the immune functions in the spleen by promoting granzyme B expression and NK cell activity. SWEG was demonstrated to be a food material that acts on HSCs and regulates immunity in vivo.

Potential of subcritical water hydrolyzed soybean husk as an alternative biosorbent to uptake basic Red 9 dye from aqueous solutions

Bioethanol produced from lignocellulosic sources still faces problems related to the feasibility of this technological route. Within the biorefinery concept and clean technology, subcritical water hydrolysis (SWH) is efficient for dissociating lignocellulosic biomass. The solid co-products can be used for other applications to become SWH a more feasible process. The potential use of subcritical water hydrolyzed soybean husks (SWHSH) as a biosorbent to remove basic Red 9 dye (BR9) from aqueous solutions was evaluated in this study. SWHSH was efficient in the uptake of BR9, mainly at a pH of 8.0. The Langmuir model satisfied the biosorption equilibrium profile with a biosorption capacity of 56.8 mg g−1. The thermodynamic parameters indicate that the biosorption is spontaneous, with the ΔG0 ranging from − 22.08 to − 24.88 kJ mol1, with an endothermic nature (ΔH0 = 5.59 kJ mol−1). The biosorption equilibrium was in 60 min for all the initial concentrations studied. The Linear Driving Force (LDF) model fitted the data well, furnishing diffusivity values from 1.41 to 2.00 × 10−8 cm2 s−1. Desorption was also possible under acid conditions, and SWHSH could be effectively used 3 times. Last, the fixed-bed biosorption showed that the SWHSH could remove the BR 9 dye up to 180 min without regeneration, presenting a biosorption capacity of 46.1 mg g1 for 900 mL of treated effluent with an initial concentration of 200 mg L1. The characterization and biosorption results indicate that the BR9 tends to be adsorbed by physical forces, possibly by hydrogen bonds, electrostatic interaction, ππ interaction, and cation-π interaction. Overall, the SWHSH demonstrated potential application as a biosorbent for the removal of BR9.

Design of highly active Ni catalysts supported on carbon nanofibers for the hydrolytic hydrogenation of cellobiose.

The direct transformation of cellulose into sugar alcohols (one-pot conversion) over supported nickel catalysts represents an attractive chemical route for biomass valorization, allowing the use of subcritical water in the hydrolysis step. The effectiveness of this process is substantially conditioned by the hydrogenation ability of the catalyst, determined by design parameters such as the active phase loading and particle size. Herein, mechanistic insights into catalyst design to produce superior activity were outlined using the hydrolytic hydrogenation of cellobiose as a model reaction. Variations in the impregnation technique (precipitation in basic media, incipient wetness impregnation, and the use of colloidal-deposition approaches) endowed carbon-nanofiber-supported catalysts within a wide range of Ni crystal sizes (5.8–20.4 nm) and loadings (5–14 wt%). The link between the properties of these catalysts and their reactivity has been established using characterization techniques such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma-optical emission spectroscopy (ICP-OES). A fair compromise was found between the Ni surface area (3.89 m2/g) and its resistance against oxidation for intermediate crystallite sizes (∼11.3 nm) loaded at 10.7 wt%, affording the hydrogenation of 81.2% cellobiose to sorbitol after 3 h reaction at 190°C and 4.0 MPa H2 (measured at room temperature). The facile oxidation of smaller Ni particle sizes impeded the use of highly dispersed catalysts to reduce the metal content requirements.

The use of subcritical water as a mobile phase in HPLC analysis

The use of subcritical water as a mobile phase in HPLC analysis

Protein Hydrolysis by Subcritical Water: A New Perspective on Obtaining Bioactive Peptides.

The importance of bioactive peptides lies in their diverse applications in the pharmaceutical and food industries. In addition, they have been projected as allies in the control and prevention of certain diseases due to their associated antioxidant, antihypertensive, or hypoglycemic activities, just to mention a few. Obtaining these peptides has been performed traditionally by fermentation processes or enzymatic hydrolysis. In recent years, the use of supercritical fluid technology, specifically subcritical water (SW), has been positioned as an efficient and sustainable alternative to obtain peptides from various protein sources. This review presents and discusses updated research reports on the use of subcritical water to obtain bioactive peptides, its hydrolysis mechanism, and the experimental designs used for the study of effects from factors involved in the hydrolysis process. The aim was to promote obtaining peptides by green technology and to clarify perspectives that still need to be explored in the use of subcritical water in protein hydrolysis.

Implementation of subcritical water extraction with natural deep eutectic solvents for sustainable extraction of phenolic compounds from winemaking by-products

In attempt to recover valuable phenolic compounds from grape pomace by-products, a new implemented methodology has been proposed. The subcritical water extraction technique has been implemented with the use of emerging eco-solvents such as natural deep eutectic solvents (NADESs). The extraction efficiency of different NADESs were screened. Among them, choline chloride containing urea as hydrogen bond donor was selected (ChClU). However, due to pressurised system and the viscosity of NADESs, ChClU could not be used as truly solvent but as a solvent modifier in an promising percentage of 30%. The optimum extraction temperature for the isolation of phenolic compounds was 100 °C since extractions above this temperature implied the loss of the original composition of grape pomace due to thermaldegradation reactions and neoformation of Maillard compounds. The employ of subcritical water combined with 30% of ChClU led to a significant rise of extracted phenolic compounds quantities in comparison with the solely use of subcritical water. Among them, the extraction of catechin and epicatechin suffered an increase of 45.05 and 47.98% respectively, so this implementation led to highly efficient and truly eco-friendly extraction method. The implementation of subcritical water with NADESs to extract valuable compounds from winery by-products has not been applied until now.

Subcritical water and supercritical carbon dioxide: efficient and selective eco-compatible solvents for coffee and coffee by-products valorization

This review aims to establish the state of the art of the existing literature on the valorization of coffee and coffee by-products such as spent coffee grounds through the use of subcritical water (SCW) and supercritical carbon dioxide (SC-CO2).

Cellulose Conversion Into Hexitols and Glycols in Water: Recent Advances in Catalyst Development.

Conversion of biomass cellulose to value-added chemicals and fuels is one of the most important advances of green chemistry stimulated by needs of industry. Here we discuss modern trends in the development of catalysts for two processes of cellulose conversion: (i) hydrolytic hydrogenation with the formation of hexitols and (ii) hydrogenolysis, leading to glycols. The promising strategies include the use of subcritical water which facilitates hydrolysis, bifunctional catalysts which catalyze not only hydrogenation, but also hydrolysis, retro-aldol condensation, and isomerization, and pretreatment (milling) of cellulose together with catalysts to allow an intimate contact between the reaction components. An important development is the replacement of noble metals in the catalysts with earth-abundant metals, bringing down the catalyst costs, and improving the environmental impact.

Preconcentration of Organic Substances on Low-Polar Adsorbents in the Flow Systems of Analysis

A review of works performed by the authors and devoted to the development of new approaches to the preconcentration of organic compounds (phenols, phthalates, polycyclic aromatic hydrocarbons, and other toxic substances) on low-polar adsorbents is presented. Possibilities of the application of calculation methods to the selection of effective adsorbents and optimal concentration conditions and to the prediction of the behavior of analytes in flow adsorption chromatography systems of analysis are discussed. New methods of the adsorption preconcentration of analytes on fluoroplastic and carbon adsorbents, including the use of subcritical water for desorption, are proposed. A short list of procedures developed for water analysis using these preconcentration methods is presented.

Subcritical water: Use in chemical analysis

The review shows prospects of the use of subcritical water instead of organic solvents and aqueous–organic mixtures at different stages of analysis. Subcritical water was applied to the extraction of target compounds from natural samples, such as soils, sand, and plant raw materials. The use of subcritical water expands possibilities of HPLC. The use of subcritical water as an eluent in HPLC is complicated by the possible destruction of the adsorbent and the decomposition of substances to be determined at elevated temperatures. Adsorbents based on zirconium and titanium oxides, some polymeric adsorbents, and porous graphitized carbon are stable in the medium of subcritical water. Subcritical water can be used at several stages of analysis, for example, for the extraction and subsequent chromatographic separation of analytes.

Measurement of As species in rice by HPLC-ICPMS after extraction with sub-critical water and hydrogen peroxide

The use of sub-critical water and hydrogen peroxide provides a fast, clean and efficient means of extracting arsenic from rice.

Valorization of coffee industry residues by subcritical water hydrolysis: Recovery of sugars and phenolic compounds

Two abundant coffee waste residues (powder and defatted cake) were treated using subcritical water (SubCW) for hydrolysis and extraction of reducing sugars (RS), total reducing sugars (TRS), and total phenolic compounds (TPC) under semi-continuous flow conditions. The flow-through process was carried out at 150, 175, 200 and 250°C, with a water flow of 10mL/min and reaction pressures of either 22.5 or 30MPa. For treated coffee powder, the maximum observed sugar recovery was 6.3% for RS (150°C and 30MPa) and 9.0% for TRS (150°C and 30MPa). The maximum TPC recovery was 26.64mg GAE (Gallic Acid Equivalent)/g powder coffee, observed at 200°C and 22.5MPa. For the defatted coffee cake, the maximum sugar yields were 8.79% and 17.23% for RS and TRS; both observed at a treatment temperature of 175°C. The highest TPC yield was 55.31mg TPC GAE/g defatted coffee cake, also at 175°C. HPLC was used to quantify specific carbohydrates (arabinose, cellobiose, glucose, and xylose), 5-hydroxy-methyl-furfural (5-HMF) and furfural in both coffee waste hydrolyzates, providing evidence of thermal degradation of the coffee carbohydrates. Scanning electron microscopy of the treated samples revealed particles deposited on the surface and other signs of physical degradation of the biomass structure. Fourier Transform Infrared Spectroscopy of the residues revealed that the density of surface bound acid groups increased with increasing treatment temperature. The results presented here provide a basis for the use of subcritical water to obtain reducing sugars and phenolic compounds from coffee residue.

Subcritical Water: A Method for Green Production of Cellulose Nanocrystals

In the present study, an innovative method to produce cellulose nanocrystals is proposed. The conventional production of nanocrystals uses concentrated solutions of strong acids to promote the hydrolysis of cellulose amorphous regions and hemicelluloses. However, in the conventional method, long duration washing steps and the nanocrystals low temperature resistance still limit their larger industrialization and some applications in processes or end-uses that require heat resistance, like extrusion. In this context, the use of subcritical water (120 °C and 20.3 MPa for 60 min) allows higher diffusion, activity, and ionization of water. With that, partial hydrolysis of cellulose can be attended (with 21.9 wt % NCC yield). The cellulose source, the hydrolyzed cellulose, and a commercial nanocellulose were submitted to different analytical techniques to evaluate their morphology and physicochemical characteristics. The obtained cellulose nanocrystals presented a high crystallinity index (79.0% by XRD), rod-li...

Applicability of Subcritical Water Treatment on Oil Seeds to Enhance Extractable Lipid

Subcritical water (SCW) has been widely studied for its unique properties both as catalyst and solvent in various chemical processes. The use of SCW to pretreat agricultural products and waste has been extensively studied for producing fermentable sugars. In this study, SCW pretreatment was carried out to increase and/or improve the extractability of oils from oil seeds like Datura stramonium, Jatropha curcas, and sunflower seeds. SCW pretreatment of D. stramonium seeds resulted in 50 % increase of oil yield (from 17.16 to 28.25 %). Although negligible increases were observed from both J. curcas and sunflower seeds, SCW pretreatment allowed full extraction of the oils without grinding and/or dehulling of the seeds. This pretreatment process caused insignificant changes in the composition and quality of the oils extracted. Efficient SCW treatment can be accomplished under mild conditions (175 °C, 3.5 MPa) in a short time (15 min).

Study on sub-critical water mediated hydrolysis of Miscanthus a lignocellulosic biomass

The present study was undertaken to evaluate the impact of sub-critical water (Sub-CW) processing parameters, such as temperature and biomass:Sub-CW:ethanol:CO 2 ratio, in order to optimise the delignification of the Miscanthus χ giganteus, a lignocellulosic biomass. The percentage lignin solubilised (delignification) was calculated by measuring the amount of lignin, using the Klason assay, that remained in the insoluble fraction. The amount of biomass solubilised was determined by calculating the difference between the initial weight of biomass and the weight of the residual insoluble fraction. Experimental results showed a maximum solubilisation and delignification of 53% and 86% respectively at 200 °C and biomass/solvent ratio of 1:100, i.e., 2.5 g in 250 ml of water:ethanol mixture (50:50). Scanning Electron Microscopy (SEM) images were taken to analyse the effect of hydrolysis on cellulose fibres structure. The results showed the residual fibres appeared intact, with some lignin globules attached to them. Consequently, it was concluded that the use of Sub-CW:ehanol:CO 2 mediated hydrolysis under the stated operating conditions supported delignification without destroying the cellulose fibres.

Subcritical Water Assisted Clean Cross-Aldol Reactions

This study investigates the use of subcritical water in cross-aldol reactions containing both organic materials, benzaldehyde/acetone and acetophenone/1,3,5-trioxane, and an inorganic material, ZnCl2. The results are compared to those obtained under solvent-free conditions. It is found that water suppresses the loss of organic materials and accelerates the formation of cross-aldol reaction products. Satisfactory yields (46–63%) were obtained with ZnCl2.

Properties of subcritical water as an eluent for reversed-phase liquid chromatography—Disruption of the hydrogen-bond network at elevated temperature and its consequences

The use of subcritical water as an eluent for reversed-phase liquid chromatography is further explored. Shape selectivity as well as thermodynamic values for solute transfer were measured and compared to those seen with traditional ambient methanol/water and acetonitrile/water mobile phases. Linear solvation energy analysis was also used to analyze extrapolated values of the retention factor in pure water at ambient temperatures ( k ′ w ) for subcritical water and ambient hydroorganic mobile phases. Results indicate that it is likely that a large disruption in the hydrogen-bonding network of water at high temperatures causes unique chromatographic selectivity, as well as prohibits accurate extrapolation from high temperature to ambient conditions using pure water. Additionally, subcritical water was not found to be a suitable mobile phase for determining k ′ w for use in estimating octanol/water partition coefficients.

Evaluation of sub-critical water as an extraction fluid for model contaminants from recycled PET for reuse as food packaging material

Recycling of plastics for food-contact packaging is an important issue and research into meaningful and cost-effective solutions is in progress. In this paper, the use of sub-critical water was evaluated as an alternative way of purifying poly(ethylene terephthalate) (PET) flakes for direct food contact applications. The effects of temperature, pressure and flow rate were assessed on the extraction efficiency of two of the most challenging classes of contaminants (toluene and benzophenone) from PET by sub-critical water using a first-order fractional experimental design. Extraction yield was quantified using GC/FID. The most important parameter was flow rate, indicating that the decrease in sub-critical water polarity with temperature was insufficient to eliminate partition effects. Temperature was also important, but only for the optimization of toluene extraction. These results may be explained by the poor solubility of higher molar mass compounds in sub-critical water compared to lower molar mass compounds under the same conditions, and the small decrease in dielectric constant with temperature under the experimental conditions evaluated. As cleaning efficiency is low and PET is very susceptible to hydrolysis, which limits the use of higher temperatures vis-à-vis physical recycling, the proposed extraction is unsuitable for a standalone super-clean process but may be a step in the process.

Continuous Flow Hydrolysis of Sunflower Oil for Biodiesel

Free fatty acids are an important intermediate for several industrial applications, particularly for production of biodiesel via methanolysis. The use of subcritical water as both solvent and reactant for the hydrolysis of vegetable oil to generate fatty acids has recently been proven to be a successful medium for hydrolysis without employing acid or alkali catalysts, while allowing for a simple process and high yield. Continuous flow hydrolysis of sunflower oil in subcritical water to obtain FFA was investigated in a tubular reactor at 10 to 20 MPa, 270 to 350°C and water/oil ratios of 80:20 and 50:50 v:v%. The rate of the hydrolysis was enhanced significantly by increasing reaction temperature end decreasing the water/oil ratio.