Thermodynamics of nitroaromatic compound adsorption from water by smectite clay

Nitroaromatic compounds enter the environment through their use as explosives, pesticides, solvents, and synthetic intermediates in the manufacturing of dyes, perfumes, and drugs. Recent studies have found that many nitroaromatic compounds are strongly retained by smectites, especially K+-saturated smectites. Sorption occurs when nitroaromatic compounds replace water associated with the clay and form complexes between K+ and -NO2 groups. This study seeks to further understand nitroaromatic-clay interactions from the viewpoint of energetics. Adsorption isotherms of 1,3-dinitrobenzene, 1,4-dinitrobenzene, and 1,3,5-trinitrobenzene from aqueous solution by K+- and Ca2+-saturated smectite (SWy-2) were measured at several temperatures between 4 degrees C and 37 degrees C to determine the molar differential adsorption enthalpies. Adsorption was found to be an exothermic process on both homoionic K+- and Ca2+-smectite. The smaller adsorption enthalpy on Ca-SWy-2 was consistent with its much smaller adsorption capacity for nitroaromatics compared to K-SWy-2. Our best estimate forthe enthalpy of 1,3,5-trinitrobenzene interactions with K-SWy-2 is -124 kJ/mol, which is referenced to gas-phase 1,3,5-trinitrobenzene, corrected forthe displacement of interlayer water, and can be directly compared with quantum chemical enthalpies from the literature. Our comparable estimates for 1,3- and 1,4-dinitrobenzene interaction enthalpies are near -90 kJ/mol. We conclude that our adsorption enthalpy results are consistent with the hypothesis that nitroaromatic compounds are sorbed strongly by K-smectites because they form inner- and/or outer-sphere complexes with K+ cations in clay interlayers. Indeed, the basal spacings of rewetted clay films in the presence of nitroaromatic compounds imply that water molecules cannot effectively compete with the adsorbed nitrobenzenes for reactive sites on K-SWy-2.     

Geochemical modulation of pesticide sorption on smectite clay

Pesticide adsorption by soil clays can be dramatically influenced by the exchangeable cations present. Among the common exchangeable base cations in soils (Ca2+, Mg2+, K+, and Na+), K+-saturated clays frequently demonstrate the strongest affinity for pesticides. In the presence of multiple exchangeable cations in the system, we hypothesize that the magnitude of pesticide sorption to soil minerals is proportional to the fraction of clay interlayers saturated with K+ ions. To test this hypothesis, we measured sorption of three pesticides with different polarities (dichlobenil, monuron, and biphenyl) by homoionic K- and Ca-smectite (SWy-2) in KCl/CaCl2 aqueous solutions. The presence of different amounts of KCl and CaCl2 resulted in varying populations of K+ and Ca2+ on the clay exchange sites. The sorption of dichlobenil and, to a lesser extent monuron, increased with the fraction of K+ on clay mineral exchange sites. Ca- and K-SWy-2 displayed the same sorption capacities for nonpolar biphenyl. X-ray diffraction patterns indicated that at lower fractions of K+-saturation, exchangeable K+ ions were randomly distributed in clay interlayers and did not enhance pesticide sorption. At higher populations of K+ (vs Ca2+), demixing occurred causing some clay interlayers, regions, or tactoids to become fully saturated by K+, manifesting greatly enhanced pesticide sorption. The forward and reverse cation exchange reactions influenced not only K+ and Ca2+ populations on clays but also the nanostructures of clay quasicrystals in aqueous solution which plays an important, if not dominant, role in controlling the extent of pesticide sorption. Modulating the cation type and composition on clay mineral surfaces through cation exchange processes provides an environmental-safe protocol to manipulate the mobility and availability of polar pesticides, which could have applications for pesticide formulation and in environmental remediation.     

Spectroscopic study of dinitrophenol herbicide sorption on smectite

Sorption of two dinitrophenolic herbicides, 4,6-dinitro-o-cresol (DNOC) and 4,6-dinitro-2-sec-butylphenol (DINOSEB) to smectite was studied using FTIR, HPLC, and quantum chemical methods. The high affinity of DNOC and DINOSEB for smectite surfaces was attributed to site-specific interactions with exchangeable cations and nonspecific van der Waals interactions with the siloxane surface. The positions of the nu(asym)(NO) and nu(sym)(NO) vibrational modes were perturbed by the exchangeable cations with similar changes occurring for both alkali and alkaline earth cations as a function of ionic potential. The cation-induced changes to the vibrational bands of the NO2 groups indicate that exchangeable metal cations are coordinated to -NO2 groups. Quantum chemical methods predicted a red-shift of the nu(asym)(NO) band and a corresponding blue-shift of the nu(sym)(NO) modes, as was observed experimentally. The nature of the smectite surface itself did not strongly influence the vibrational modes of sorbed DNOC or DINOSEB on the basis of a comparison of DNOC sorbed to three different smectites (K-SWy-2, K-SAz-1, and K-SHCa-1). FTIR spectra of DNOC and DINOSEB sorbed to a K-SWy-2 smectite were studied quantitatively using a modified form of Beers law. The FTIR-derived sorption isotherm of DNOC sorbed to K-SWy-2 was in good agreement with the isotherm derived from HPLC measurements. The molar absorptivity value of DNOC sorbed to K-SWy-2 smectite was 1.43 x 10(7) cm2/mol in good agreement with literature values for nitroaromatics (average value of 1.72 x 10(7) +/- 0.3 cm2/mol). On the basis of this value, the limit of detection using the FTIR method of approximately 5 microgDNOC g(clay) was determined. These two observations (sorption isotherms and molar absorptivity) provide a direct link between the macroscopic sorption results and the FTIR spectra.     

Spectroscopic study of carbaryl sorption on smectite from aqueous suspension

Sorption of carbaryl (1-naphthyl-N-methyl-carbamate) from aqueous suspension to smectite was studied using Fourier transform infrared (FTIR), high-performance liquid chromatography (HPLC) (for batch sorption), and quantum chemical methods. The amount of carbaryl sorbed was strongly dependent on the surface-charge density of the smectite with more sorption occurring on the two "low" surface-charge density smectites (SHCa-1 and SWy-2) compared to that of the high surface-charge SAz-1 smectite. In addition, the amount of carbaryl sorbed was strongly dependent on the nature of the exchangeable cation and followed the order of Ba approximately Cs approximately Ca > Mg approximately K > Na approximately Li for SWy-2. A similartrend was found for hectorite (SHCa-1) of Cs > Ba > Ca > K approximately Mg > Na approximately Li. Using the shift of the carbonyl stretching band as an indicator of the strength of interaction between carbaryl and the exchangeable cation, the observed order was Mg > Ca > Ba approximately K > Na > Cs. The position of the carbonyl stretching band shifted to lower wavenumbers with increasing ionic potential of the exchangeable cation. Density functional theory predicted a cation-induced lengthening of the C=O bond, resulting from the carbonyl group interacting directly with the exchangeable cation in support of the spectroscopic observations. Further evidence was provided by a concomitant shift in the opposite direction by several vibrational bands in the 1355-1375 cm(-1) region assigned to stretching bands of the carbamate N-Ccarbonyl and Oether-Ccarbonyl bonds. These data indicate that carbaryl sorption is due, in part, to site-specific interactions between the carbamate functional group and exchangeable cations, as evidenced by the FTIR data. However, these data suggest that hydrophobic interactions also contribute to the overall amount of carbaryl sorbed. For example, the FTIR data indicated thatthe weakest interaction occurred when Cs+ was the exchangeable cation. In contrast, the highest amount of carbaryl sorption was observed on Cs-exchanged smectite. Of all the cations studied, Cs has the lowest enthalpy of hydration. It is suggested that this low hydration energy provides the carbaryl with greater access to the hydrophobic regions of the siloxane surface.     

Influence of smectite hydration and swelling on atrazine sorption behavior

Smectites, clay minerals commonly found in soils and sediments, vary widely in their ability to adsorb organic chemicals. Recent research has demonstrated the importance of surface charge density and properties of exchangeable cations in controlling the affinity of smectites for organic molecules. In this study, we induced hysteresis in the crystalline swelling of smectites to test the hypothesis that the extent of crystalline swelling (or interlayer hydration status) has a large influence on the ability of smectites to adsorb atrazine from aqueous systems. Air-dried K-saturated Panther Creek (PC) smectite swelled less (d(001) = 1.38 nm) than never-dried K-PC (d(001) = 1.7 nm) when rehydrated in 20 mM KCl. Correspondingly, the air-dried-rehydrated K-PC had an order of magnitude greater affinity for atrazine relative to the never-dried K-PC. Both air-dried-rehydrated and never-dried Ca-PC expanded to approximately 2.0 nm in 10 mM CaCl2 and both samples had similar affinities for atrazine that were slightly lower than that of never-dried K-PC. The importance of interlayer hydration status in controlling sorption affinity was confirmed by molecular modeling, which revealed much greater interaction between interlayer water molecules and atrazine in a three-layer hydrate relative to a one-layer hydrate. The entropy change on moving atrazine from a fully hydrated state in the bulk solution to a partially hydrated state in the smectite interlayers is believed to be a major factor influencing sorption affinity. In an application test, choice of background solution (20 mM KCl versus 10 mM CaCl2) and air-drying treatments significantly affected atrazine sorption affinities for three-smectitic soils; however, the trends were not consistent with those observed for the reference smectite. Further, extending the initial rehydration time from 24 to 240 h (prior to adding atrazine) significantly decreased the soil's sorption affinity for atrazine. We conclude that interlayer hydration status has a large influence on the affinity of smectites for atrazine and that air-drying treatments have the potential to modify the sorption affinity of smectitic soils for organic molecules such as atrazine.     

Enhanced sorption of trichloroethene by smectite clay exchanged with Cs+

Trichloroethene (TCE) is one of the most common pollutants in groundwater, and Cs+ can be a cocontaminant at nuclear facilities. Smectite clays have large surface areas, are common in soils, have high affinities for some organic contaminants, and hence can potentially influence the transport of organic pollutants entering soils and sediments. The exchangeable cations present near smectite clay surfaces can radically influence the sorption of organic pollutants by soil clays. This research was undertaken to determine the effect of Cs+, and other common interlayer cations, such as K+ and Ca2+, on the sorption of TCE by a reference smectite clay saponite. Cs-saturated clay sorbed the most TCE, up to 3500 mg/kg, while Ca-saturated smectite sorbed the least. We hypothesize that the stronger sorption of TCE by the Cs-smectite can be attributed to the lower hydration energy and hence smaller hydrated radius of Cs+, which expands the lateral clay surface domains available for sorption. Also, Cs-smectite interlayers are only one or two water layers thick, which may drive capillary condensation of TCE. Our results implicate enhanced retention of TCE in aquifer materials containing smectites accompanied by Cs+ cocontamination.     

Probing the specific sorption sites on montmorillonite using nitroaromatic compounds and hexafluorobenzene

The objective of this work was to test two possible sorption mechanisms of organic chemicals to montmorillonite: n-π electron-donor-acceptor (EDA) interaction with lone electron pairs of siloxane oxygens (n-donors) and complexation with exchangeable cations. Batch sorption experiments were performed for 1,3-dinitrobenzene, 1,4-dinitrobenzene (π-electron acceptors and cation binders), and hexafluorobenzene (π-electron acceptor only) to homoionic montmorillonites in water or hexane. For all three sorbates, the aqueous sorption affinity showed large cation dependency (Cs(+) > K(+) > Na(+)), wherein sorption of hexafluorobenzene to Cs(+)-montmorillonite was the strongest (K(d) in the order of 10(4) L/kg). Change of the solvent media from water to hexane generally favored sorption, indicating suppressive effect by cation hydration. Cosorption of 1,4-dinitrobenzene prominently decreased sorption of 1,3-dinitrobenzene to all cation-exchanged montmorillonites; however, hexafluorobenzene caused strong competition only to Cs(+)-montmorillonite. Furthermore, complexation of exchangeable cations by 18-crown-6 ether dramatically suppressed sorption of 1,3-dinitrobenzene to K(+)-montmorillonite in water and all cation-exchanged montmorillonites in hexane, but not to the rest. The contrast patterns of binary competitive sorption between nitroaromatics and hexafluorobenzene indicated they sorbed to different sites on montmorillonite. It was proposed that sorption of hexafluorobenzene was dominated by n-π EDA interaction, while sorption of nitroaromatics was dominated by cation-polar interaction.     

Selenoprotein deficiency enhances radiation-induced micronuclei formation

The availability of selenium and the levels of specific selenoproteins might affect cancer risk by influencing the ability of DNA damaging agents to cause genomic instability and mutations. Transgenic mice that express reduced levels of selenoproteins and previously shown to be more susceptible to pathology associated with cancer development were used to study this possibility. These mice were exposed to X-rays and DNA damage assessed in the erythrocytes, where micronuclei formation was higher compared to the same cells obtained from irradiated wild-type controls. To determine whether the selenoprotein glutathione peroxidase-1 (GPx-1) might be involved in this protection, its levels were reduced by siRNA targeting in LNCaP human prostate cells. UV-induced micronuclei frequency was higher in these cells compared to control-transfected cells. These results indicate a role for selenoproteins in protecting DNA from damage and support human data implicating GPx-1 as a possible target of the chemoprotective effect of selenium.     

Polyfunctional ionogenic compound sorption: challenges and new approaches to advance predictive models

Polyfunctional ionogenic compounds are unique in that they sorb to environmental solids at multiple receptor sites via multiple interaction mechanisms. However, existing sorption models fail to accommodate: (i) sorption via a single mechanism (e.g., cation exchange) at one sorbent receptor site type (e.g., exchange site) distributed across multiple soil components (e.g., organic matter and aluminosilicates); and (ii) sorption at a specific sorbent receptor site (e.g., exchange site) involving distinct sorbate structural moieties (e.g., -NH(3)(+) and -COOH) and distinct interaction mechanisms (e.g., cation exchange and cation bridging). In response, this study offers a mechanism-based framework for conceptualizing the equilibrium solid-water sorption coefficient, K(d), with particular emphasis on the mechanisms of cation exchange and surface complexation/cation bridging. The unique mapping of sorbate structural moieties, sorbent receptor sites, and sorption mechanisms is used to advance mechanism-specific probe compounds for cation exchange and surface complexation/cation bridging for quantifying the relevant site abundance and baseline sorption free energy. Existing literature studies point to the feasibility of developing mechanism-specific structural corrections to "adjust" mechanism-specific probe sorption measures to estimate the magnitude of sorption for any polyfunctional ionogenic compound of interest. Advancement of our conceptual framework to a quantitative K(d) model requires more extensive evaluation of ionogenic compound sorption under consistent experimental conditions.     

复合型调味料的形成和发展

当今调味品市场上复合型调味料的优势越来越显著。从理论上阐述了单一味与复合味的关系，复合型调味料的形成和应用，并对复合型调味料的发展进行了展望。     

GC evaluation of flavour compound sorption from water solutions by corn starch cryotextures obtained by freezing

Sorption of essential oil aroma components, n-alcohols and linalool by starch corn cryotexture was studied. Results show that terpene hydrocarbons of Rosmarinus officinalis L. essential oil are sorbed quantitatively from 0.05% water solution by cryotexture due to hydrophobic interactions with starch polysaccharides. Aroma compounds with oxygen atoms within the molecule are sorbed two times less. A templating effect with glucose, sucrose, maltose and some essential oil components was observed. Sorption of n-alcohols C₄, C₆, C₈, linalool and their mixture with alkyl acetates in concentration 0.5 — 15 mmol/l was carried out. Individual octanol is sorbed by cryotexture on 88%, hexanol on 20%, linalool on 25%. Butanol is not sorbed at the chosen concentration. Synergism in sorption of hexanol from mixture with octanol and alkyl acetates was observed, while a suppression effect of octanol sorption was found for the same mixture.     

Contribution of lactic acid bacteria to flavour compound formation in dairy products

Cheese flavour cannot be produced without starter bacteria. Lactic acid bacteria convert lactose to lactic acid and this together with their production of diacetyl and acetaldehyde are their main contributions to the flavour of cultured milks and fresh cheeses. In matured cheeses, the starter bacteria die out quickly and the rate at which they lyse and release their enzymes into the system has an influence on the rate at which free amino acids are formed. Rennet alone is mainly responsible for the formation of large, medium and small peptides but, without interaction with other enzymes, is capable of producing only methionine, histidine, glycine, serine and glutamic acid at quantifiable levels. Free amino acids in Cheddar cheese are mainly the result of microbial peptidase activity. These amino acids, together with the products of glycolysis, form substrates for secondary flora, the nature of which, in many cases, determines the cheese variety. They also form substrates for enzymes from the milk, e.g. the production of H₂S appears to be dependent on milk enzymes. Methionine, which is released by rennet, is further metabolized by starter enzymes with the production of methanethiol which plays a major role in cheese flavour possibly as a potentiator for other flavours. —Dicarbonyls, particularly methylglyoxal and diacetyl, and bacteria which can produce them, appear to play a crucial role in the formation of cheese flavour, both the desirable flavour of full-fat cheese and the meaty-brothy off-flavour of low-fat cheese. Although, theoretically, there are many compounds in cheese which could react purely chemically to form flavour compounds, these reactions are also mediated by enzymes in the cheese system and it seems unlikely that straight out chemical reactions play a major role in the production of cheese flavour. The role of the secondary flora is likely to be much more important than that of chemical reactions. Particularly in Cheddar and Emmental it has been shown that good quality cheeses have a low oxidation-reduction potential. This is more likely to be an indicator for the establishment of the anaerobic conditions required for the flavour forming reactions to proceed than an active causal agent of flavour formation. The function of glutathione is more likely to be as some sort of facilitator in enzyme reactions than as an agent for the reduction of oxidation-reduction potential. The ability of bacteria to accumulate glutathione from their media is likely to be one of the indicators of flavour generating capacity. Suitable selected strains of adjunct bacteria increase the rate and intensity of formation of Cheddar cheese flavour but unsuitable adjuncts can also cause off-flavours.     

Effect of the withering process on volatile compound formation during black tea manufacture

The amount and composition of volatile compounds in the volatile oils were compared between non-withered tea and withered tea. Important differences in the aroma constituents were found in the amounts of E-2-hexenal, Z-3-hexanol ester, linalool and its oxides, and methylsalcylate. In withered teas, the amounts of Z-3-hexenol ester, linalool and its oxides, and methylsalcylate were much greater than in non-withered tea, but the E-2-hexenal content was higher in non-withered tea. From the results it was thought that the withering process has an important role in the formation of volatile compounds in tea shoots during black tea fermentation.     

Using nitrogen isotope fractionation to assess abiotic reduction of nitroaromatic compounds

Compound-specific isotope analysis (CSIA) is an increasingly important tool for the qualitative and quantitative assessment of transformations of organic compounds in contaminated environments. To date, the use of CSIA has been mainly restricted to the elements C and H, although Nconstitutes a very important reactive centerfor many priority contaminants. To evaluate the potential use of N isotope effects in the fate assessment of organic contaminants, we investigated the N isotope enrichment during the abiotic reduction of 4 substituted nitroaromatic compounds (NACs), using two abiotic model reductants, namely Fe(ll) sorbed to goethite (alpha-FeOOH) and juglone (8-hydroxy-1,4-naphthoquinone) in the presence of H2S. Substantial and virtually identical isotope enrichment factors, EpsilonN, of about -30%, indicative of the breaking of one N-O bond, were found for all NACs, regardless of the reductant involved and the substitution of the NAC. These results indicate that the EpsilonN-values determined in our study could be representative for the reduction of aromatic NO2-groups and thus be used to assess the abiotic transformation of NACs qualitatively and quantitatively in complex anoxic environments.     

Preventive effect of adding antioxidants on mutagenic compound formation in fumes of cooking oil

The preventive effect of various antioxidants on the formation of mutagenic compounds such as trans,trans‐2,4‐decadienal in degummed peanut oil (DPO) fumes was investigated. The mutagenicity of the DPO fumes was significantly reduced by antioxidants added before heating. The addition of antioxidants increased the smoke point and oxidative stability of DPO and decreased the yield of oil fumes and the amount of mutagens. Butylated hydroxyanisole and butylated hydroxytoluene were more effective than natural antioxidants in reducing the amount of four enal compounds in fumes from DPO. Thus edible cooking oil with a high smoke point, less fume and lower mutagenicity might be developed with an appropriate antioxidant. Copyright © 2004 Society of Chemical Industry     

Geochemical modulation of bioavailability and toxicity of nitroaromatic compounds to aquatic plants

Nitroaromatic compounds (NACs) are prominent soil and sediment contaminants that are strongly adsorbed by smectites at extents that depend on hydration properties of the exchangeable cation. Potassium smectites adsorb nitroaromatics much more strongly than calcium smectites, so that adjustment of K+ versus Ca2+ occupation on cation exchange sites in smectites can be used to modulate the retention and release of nitroaromatics. We suggest that this modulation can be used to advantageously manage the bioavailability and toxicity of NACs during bioremedation. We have measured the toxicity of 2,4-dinitrotoluene (2,4-DNT) to duckweed grown in smectite suspensions and utilized Ca2+/K+ exchange to retain or release 2,4-DNT. Retention by potassium smectite reduced bioavailability and hence toxicity to duckweed. Addition of Ca2+ to replace K+ by ion exchange released adsorbed 2,4-DNT, which is toxic to duckweed. So smectites can be used to sequester or release 2,4-DNT predictably and provide means to control bioavailability and environmental toxicity.     

论川味半固态复合调味料风味成分的来源和形成

为给研发新型半固态复合调味料提供参考,通过介绍川菜及用料特点,分析了川味半固态复合调味料中的麻味、成味、辣味、鲜味和香味等风味成分的来源和形成条件,认为不同的烹调方法,使用的调味料不同,复合调味料的配比也就不同,并提出了今后半固态复合调味料的开发方向：广泛使用天然物质、产品口味多样化、生产工艺采用高科技和包装礼品化等。     

Quantification and reduction of organic sulfur compound formation in a commercial wood pulping process

This study experimentally quantified the formation of organic sulfur compounds in a commercial SuperBatch kraft pulping process using a laboratory pilot-scale digester. The results indicate that wood chips not only can adsorb HS- but also methyl mercaptan (MM) in the black liquor used for pretreatment during the two pretreatment stages. The absorption rate of MM is much faster than that of HS-. In the third stage of SuperBatch pulping, the rate of formation of MM and dimethyl sulfide (DMS) is very similar to that found in conventional batch kraft pulping processes. The study examined the applicability of the phase transition cooking (PTC) concept for reducing organic sulfur compounds in pulping, previously developed in laboratory batch pulping using a bomb-type digester, in a SuperBatch process. It was confirmed that there is a phase transition point (PTP) corresponding to PTC in SuperBatch pulping beyond which further delignification significantly increases the formation of volatile organic sulfur compounds. The results indicate that a 40% reduction of TRS formation can be achieved by using PTC.