Pure Subcritical Water Research Articles

Effect and control of energy input on tissue and cell dissociation and chemical depolymerization in pure subcritical water autohydrolysis of naked oat stem

Pure subcritical water autohydrolysis is an economical and green biorefinery method and potentially applicable technology.

Efficient aramid fiber monomerization using alkaline subcritical water

To establish the chemical recycling of copoly(p-phenylene-3,4′-oxydiphenylene terephthalamide) fibers (i.e., Technora®), their hydrolysis in pure supercritical, pure subcritical, and alkaline subcritical water was investigated, and an efficient purification method to recover the constituent monomers in high yields and > 99% purity was developed. Technora® hydrolysis in pure supercritical water required high temperatures, thus leading to monomer degradation and hence low monomer yields. Similarly, Technora® hydrolysis to monomers was incomplete in pure subcritical water, which also resulted in low monomer yields. In contrast, the addition of NaOH to subcritical water promoted fiber decomposition, rendered hydrolysis irreversible, and prevented degradation of the produced monomers by allowing the reaction to proceed at lower temperature. Thus, when subcritical water containing 10 equivalents of NaOH per Technora® unit was used at 250°C and 4 MPa for 3 h, the fibers decomposed with 100% efficiency, and the monomers were recovered in 88–99% yields. The rate of Technora® hydrolysis in a neutral medium of either pure supercritical or subcritical water was represented well using the cylindrical-surface reaction model. This model was adjusted to describe the reaction in an alkaline medium, i.e., subcritical water containing NaOH, by considering the decrease in both NaOH concentration and Technora® surface area.

Decomposition of environmentally persistent cyclic methylsiloxanes in subcritical water

The cyclic methylsiloxanes octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) were reacted in subcritical water at temperatures of 150–350 °C under argon, under O2, or in the presence of H2O2. When the reactions were performed in pure subcritical water (under argon) at 300 °C for 6 h, both D4 and D5 were almost completely hydrolyzed to water-soluble and hexane-non-extractable compounds such as dimethylsilanediol (DMSD). However, the high percentages of total organic carbon (TOC) remained in the aqueous phase [87% and 81% of the carbon atoms in the initial D5 (26.6 μmol) and D4 (30.6 μmol), respectively] and the low silicate yields (both 3% of the silicon atoms in the initial D5 and D4) indicated that little mineralization had occurred. Although the presence of O2 had no significant effect on the reactivity of these substrates in subcritical water at temperatures up to 300 °C, the presence of H2O2 enhanced the decomposition of these substrates. Raising the temperature to 350 °C greatly enhanced the mineralization. Specifically, when the reactions were performed in 2.0 M H2O2 for 2 h, the percentages of TOC that remained in the aqueous phase decreased markedly (to 2% and 10% of the carbon atoms in the initial D5 and D4, respectively) and IR spectral measurements indicated that most of the substrates had been oxidized to silicate.

Research of sulfadiazine using subcritical water and water + alcohol mixtures as the solvent: Solubility and thermodynamic property

Sulfonamides are drugs extensively used for the treatment of infections caused by gram-positive and gram-negative microorganisms, some fungi, and certain protozoa. They are also used in agriculture to prevent and treat diseases in livestock. Sulfadiazine is a member of the sulfonamide family. In this study, the solubility of sulfadiazine in pure subcritical water and water + alcohol mixtures has been measured using a homemade system at (298 to 403) K under a constant pressure of 5.1 MPa. Our results revealed that the solubility of sulfadiazine was enhanced with increasing temperature and higher percentage of ethanol or propanol in water. Modified Apelblat equation and CNIBS/R-K model were used to correlate and predict the solubility of sulfadiazine in pure subcritical water as well as in subcritical water + alcohol mixtures. The predicted solubility values are in good correlation with the experimental data. In addition, “Apparent thermodynamic analysis” was applied to calculate the dissolution enthalpy property of the solution process.

Efficient decomposition of perchlorate to chloride ions in subcritical water by use of steel slag.

Decomposition of perchlorate (ClO4-) in subcritical water in the presence of steel slag, a by-product of the steel industry, was investigated. Reactivity of ClO4- was low in pure subcritical water state up to 300°C, whereas adding steel slag efficiently accelerated the decomposition of ClO4- to Cl-, with no leaching of heavy metals such as chromium and other environmentally undesirable elements (boron and fluorine). When the reaction was performed in subcritical water at a relatively low temperature (250°C) for 6h, virtually all ClO4- ions were removed from the reaction solution. The concentration of Cl- after the reaction was well accounted for by the sum of the amount of Cl- ascribed to the decomposition of ClO4- and the amount of Cl- leached from the slag. This method was successfully applied to decompose ClO4- in water samples collected from a man-made reflection pond following a fireworks display, even though these samples contained much higher concentrations of Cl- and SO42- than ClO4-.

Efficient FeO-induced decomposition of the fluorinated room-temperature ionic liquid [Me 3 PrN][(CF 3 SO 2 ) 2 N] to fluoride ions in subcritical and supercritical water

Decomposition of [Me3PrN][(CF3SO2)2N], a typical room-temperature ionic liquid used for electrochemical devices, in subcritical and supercritical water was investigated with the aim of developing a technique to recover the fluorine component from ionic liquid wastes. When the reaction was carried out in pure subcritical water at 342 °C for 6 h, 99% of the anionic moiety, [(CF3SO2)2N]−, remained, and the F− yield was only 1%. In contrast, in the presence of zerovalent iron, the yield of F− increased to 52%. Although the reactivity of FeO toward the anionic moiety in subcritical water was lower than that of zerovalent iron, the reactivity of FeO increased dramatically when the water was heated to the supercritical state, under which conditions FeO underwent in situ disproportionation to form zerovalent iron and Fe3O4. Specifically, when the reaction was carried out at 376 °C for 18 h, the F− yield reached 90%, which was the highest yield obtained under the tested conditions. In addition, under these conditions, the formation of environmentally undesirable CF3H was suppressed.

Iron-induced efficient mineralization of a cyclic perfluoroalkyl surfactant in subcritical and supercritical water

The decomposition of [CF2(CF2SO2)2N]−, a typical cyclic perfluoroalkyl surfactant, in subcritical and supercritical water was investigated. This is the first report on the decomposition of cyclic perfluoroalkyl surfactants, which are being developed as greener alternatives to environmentally persistent and bioaccumulative perfluoroalkyl surfactants. [CF2(CF2SO2)2N]− decomposition was negligible in either pure subcritical water under argon or in subcritical water in the presence of O2. In contrast, the mineralization of [CF2(CF2SO2)2N]− to F− ions was accelerated when a zerovalent metal (copper, zinc, or iron) was used in the reaction system under argon, and iron led to the most efficient mineralization. When the reaction was performed at 350°C for 6h, the F− yield was 70.6 times that in the absence of iron. Using supercritical water enhanced the reactivity: when [CF2(CF2SO2)2N]− was heated at 380°C with iron for 18h, 82.5% of the fluorine content in the initial [CF2(CF2SO2)2N]− was transformed into F− ions. The mineralization of [CF2(CF2SO2)2N]− was accompanied by transformation of the zerovalent iron to Fe3O4.

Decomposition of Perfluorinated Ion-Exchange Membrane to Fluoride Ions Using Zerovalent Metals in Subcritical Water

The decomposition of Nafion NRE-212, a typical perfluorinated ion-exchange membrane used for fuel cells, in subcritical water was investigated. This is the first report of the decomposition of a perfluorinated ion-exchange membrane aimed at the development of a technique to recover the fluorine component for waste treatment. Although the membrane showed little decomposition in pure subcritical water, the addition of several zerovalent metals to the reaction system accelerated the membrane decomposition to F− ions, and the acceleration increased in the order Al < no metal < Zn < Cu ≪ Fe. When the membrane and iron powder were heated in subcritical water at 350 °C for 17 h, 73.2% of the fluorine content in the initial membrane was successfully transformed into F− ions. In addition to F− ions, trifluoroacetic acid and HCF(CF3)OC2F4SO3− were also detected in the reaction solutions as intermediates, and CO2 and CF3H were detected in the gas phase. Time profiles of the products suggest that one pathway of the decomposition of the membrane proceeded by decomposition of the pendant-chain part, followed by decomposition of the polymer backbone.

Iron-induced decomposition of perfluorohexanesulfonate in sub- and supercritical water

Decomposition of perfluorohexanesulfonate (PFHS), a bioaccumulative analogue of perfluorooctanesulfonate (PFOS), in sub- and supercritical water was investigated. Although PFHS was only slightly reactive in pure subcritical water at 350 °C, it decomposed to F − and SO 4 2 - ions when the temperature was increased to 380 °C, at which temperature the water became supercritical state. Addition of zerovalent iron to the reaction system dramatically accelerated PFHS decomposition to F − ions in both sub- and supercritical water: for example, when the initial PFHS concentration was 741 μM, the F − yields at 350 °C were 4.13–16.0 times as high as those in the absence of iron, depending on the amount and the particle size of the iron powder. After the reactions, small amounts of CO 2 and CF 3H were also detected in the gas phase; these increased with temperature, and the amount of CF 3H increased markedly when the reaction was carried out in supercritical water. Increasing the specific surface area of the iron powder markedly increased PFHS consumption and F − formation in the aqueous phase, which indicates that the reactions occurred on the iron surface and that the increased specific surface area was a key factor in accelerating the decomposition of PFHS to F − ions.

Efficient Decomposition of Environmentally Persistent Perfluorooctanesulfonate and Related Fluorochemicals Using Zerovalent Iron in Subcritical Water

Decomposition of perfluorooctanesulfonate (PFOS) and related chemicals in subcritical water was investigated. Although PFOS demonstrated little reactivity in pure subcritical water, addition of zerovalent metals to the reaction system enhanced the PFOS decomposition to form F-ions, with an increasing order of activity of no metal approximately equal Al < Cu < Zn << Fe. Use of iron led to the most efficient PFOS decomposition: When iron powder was added to an aqueous solution of PFOS (93-372 microM) and the mixture was heated at 350 degrees C for 6 h, PFOS concentration in the reaction solution fell below 2.2 microM (detection limit of HPLC with conductometric detection), with formation of F-ions with yields [i.e., (moles of F- formed)/(moles of fluorine content in initial PFOS) x 100] of 46.2-51.4% and without any formation of perfluorocarboxylic acids. A small amount of CHF3 was detected in the gas phase with a yield [i.e., (moles of CHF3)/(moles of carbon content in initial PFOS) x 100] of 0.7%, after the reaction of PFOS (372 microM) with iron at 350 degree C for 6 h. Spectroscopic measurements indicated that PFOS in water markedly adsorbed on the iron surface even at room temperature, and the adsorbed fluorinated species on the iron surface decomposed with rising temperature, with prominent release of F- ions to the solution phase above 250 degrees C. This method was also effective in decomposing other perfluoroalkylsulfonates bearing shorter chain (C2-C6) perfluoroalkyl groups and was successfully applied to the decomposition of PFOS contained in an antireflective coating agent used in semiconductor manufacturing.

Separation of nonpolar analytes using methanol–water mixtures at elevated temperatures

Pure subcritical water has been found to be an efficient mobile phase for reversed-phase separations of both polar and moderately polar compounds. However, subcritical water must be modified with organic solvent in order to elute nonpolar analytes in an efficient manner. In this study, the separation of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and benzene, toluene, and p-xylene (BTX) was performed by using heated methanol–water mixtures as the mobile phase. Temperatures employed in this study ranged from 21 to 140 °C, while the percentage of methanol in the mobile phase ranged from 52 to 90%. The retention times of analytes were matched under different mobile phase conditions by increasing the temperature and decreasing the percentage of methanol in the mobile phase.

Temperature effect on peak width and column efficiency in subcritical water chromatography.

Subcritical water has been recently employed as the mobile phase to eliminate the use of organic solvents in reversed-phase liquid chromatography. Although the influence of temperature on retention in subcritical water chromatography has been reported, the temperature effect on peak width and column efficiency has not yet been quantitatively studied. In this work, several polar and chlorinated compounds are separated using pure subcritical water on Zorbax RX-C8, PRP-1 (polystyrene-divinylbenzene), Hypersil ODS, and ZirChrom-polybutadiene columns. Isothermal separations are performed at temperatures ranging from 60 degrees C to 160 degrees C. The retention time and peak width of analytes are reduced with increasing temperature. However, the column efficiency is either improved or almost unchanged with the increasing temperature in the low-temperature range (lower than the 100 degrees C to 120 degrees C range), but it is decreased when temperature is further raised in the high-temperature range (higher than the 100 degrees C to 120 degrees C range). Therefore, a maximum in column efficiency is obtained at temperatures within the 100 degrees C to 120 degrees C range in most cases.

Retention Behavior of Phenols, Anilines, and Alkylbenzenes in Liquid Chromatographic Separations Using Subcritical Water as the Mobile Phase

The unique characteristic of subcritical water is its widely tunable physical properties. For example, the polarity (measured by dielectric constant) of water is significantly decreased by raising water temperature. At temperatures of 200-250 °C (under moderate pressure to keep water in the liquid state), the polarity of pure water is similar to that of pure methanol or acetonitrile at ambient conditions. Therefore, pure subcritical water may be able to serve as the mobile phase for reversed-phase separations. To investigate the retention behavior in subcritical water separation, the retention factors of BTEX (benzene, toluene, ethylbenzene, and m-xylene), phenol, aniline, and their derivatives have been determined using subcritical water, methanol/water, and acetonitrile/water systems. Subcritical water separations were also performed using alumina, silica-bonded C18, and poly(styrene-divinylbenzene) columns to study the influence of the stationary phase on analyte retention under subcritical water conditions.