光化学降解黄腐酸及其对黄腐酸-苯并(α)芘结合性质的影响

在不同照射时间、pH和溶解氧浓度下,研究了从加拿大魁北克省Rimouski河提取的黄腐酸(RRFA)在模拟太阳光照射过程中对苯并(α)芘结合性质的影响.结果表明,RRFA经模拟太阳光照射后,其与苯并(α)芘的结合系数KOC下降.光化过程中RRFA的相对分子质量或SUVA270与KOC之间有良好的指数回归关系.在pH 4.1～8.0范围内,以光照后的pH条件下测定照射样品的KOC,KOC平均下降47.4%±17.4%;以光照前的pH条件下测定照射样品的KOC,KOC平均下降48.7%±16.9%.溶解氧浓度的升高能加速RRFA的光氧化降解过程,与相应的未照射样品相比,空气饱和与氧气饱和条件下照射后的KOC分别下降30%和74%,氮气饱和条件下照射后的KOC升高122%.     

Lead(II) complexation by fulvic acid: how it differs from fulvic acid complexation of copper(II) and cadmium(II)

Pb²⁺, like Cu²⁺, forms strong complexes with fulvic acids (Cd²⁺-fulvate complexes are much weaker), but Pb-fulvate precipitates at a much lower mole ratio of metal ion to fulvic acid than either Cu-fulvate or Cd-fulvate does. Physical association of Pb²⁺ with Pb-fulvate solids as well as complexation by sites still available in the precipitates probably causes the increased removal of free Pb²⁺ from solution after precipitation begins.     

Effect of monochloramine on isolated fulvic acid

Monochloramine is interesting both as a selective oxidant of fulvic acid and as a drinking water disinfectant. In this study, the controlled reaction of aquatic fulvic acid with monochloramine did not result in products detectable by ether extraction-gas chromatography-flame ionization detection techniques. Evidence of a reaction was shown by bleaching of the fulvic acid solution (decreased absorbance at 465 nm) and chlorine substitution. Chlorine-containing products were quantified by the total carbon adsorbable organic halide (TOX) parameter. Bleaching and organic chlorine formation were much less extensive than in the reaction of free chlorine with fulvic acid. Monochloramine was shown to produce an organic chlorine fraction more hydrophilic and with higher molecular size than that produced by chlorine and fulvic acid. Results suggest that monochloramine may be useful tool for the investigation of certain humic functionalities because it reacts rather selectively and to a small extent with aquatic fulvic acid.     

Effect of groundwater fulvic acid on the adsorption of arsenate by ferrihydrite and gibbsite

Adsorption studies have been conducted at pH 4, 6 and 8 to assess the effect of fulvic acid on arsenate adsorption to ferrihydrite and gibbsite. The studies compared the adsorption of arsenate on the mineral surfaces in the absence of fulvic acid, to those cases where increasing concentrations of fulvic acid (0.3–60 μM) were added to the mineral–arsenate suspensions. Experiments where arsenate was added to mineral–fulvate suspensions were also conducted. The results suggest that arsenate adsorption on both gibbsite and ferrihydrite decreases with increasing concentrations of fulvic acid. This effect was highest at pH 4, and decreased at pH 6 and 8. Ferrous ion concentrations were very low during the ferrihydrite experiments and support the view that fulvic acid can both displace arsenate from and inhibited its adsorption to mineral surfaces. The experiments also indicated that the amount of arsenate adsorbed was lower if fulvic acid was added before rather than after arsenate. This may reflect the relative size of arsenate and fulvic acid molecules and their ability to penetrate the crystal matrices of the minerals.     

Prediction of the speciation of alkaline earths adsorbed on mineral surfaces in salt solutions

Despite the fact that the bulk compositions of most low temperature natural surface waters, groundwaters, and porewaters are heavily influenced by alkaline earths, an understanding of the development of proton surface charge in the presence of alkaline earth adsorption on the surfaces of minerals is lacking. In particular, models of speciation at the mineral-water interface in systems involving alkaline earths need to be established for a range of different minerals. In the present study, X-ray standing wave results for Sr²⁺ adsorption on rutile as a tetranuclear complex [Fenter, P., Cheng, L., Rihs, S., Machesky, M., Bedyzk, M.D., Sturchio, N.C., 2000. Electrical double-layer structure at the rutile-water interface as observed in situ with small-period X-ray standing waves. J. Colloid Interface Sci.225, 154-165] are used as constraints for all the alkaline earths in surface complexation simulations of proton surface charge, metal adsorption, and electrokinetic experiments referring to wide ranges of pH, ionic strength, surface coverage, and type of oxide. The tetranuclear reaction

4>SOH+M²⁺+H₂O=(>SOH)₂(>SO^-)₂_M(OH)⁺+3H⁺     

Electrochemical studies of copper and lead complexation by fulvic acid. I. Potentiometric measurements and a critical comparison of metal binding models

Copper and lead ion-selective electrodes have been used to monitor titrations of Pb and Cu with fulvic acid extracted from the River Tamar, Southwest England. The titrations were carried out in 0.1 M NaClO₄ at 25°C at pH values in the range 6.0 to 6.5. The titration data were analysed using five different models; two assuming small values of discrete binding sites (multi-site and multidendate models), and three assuming continua of binding sites (electrostatic, normal distribution and affinity spectrum models). On both statistical and practical grounds the multi-site model was identified as the most effective in modelling the data. A synthetic data set proposed by Fish and Morel (1984) was used to examine the effect of random errors on the various data analysis procedures.     

The reaction of ozone with isolated aquatic fulvic acid

Aquatic fulvic acid, isolated from a North Carolina bay lake, was reacted with ozone, an alternative oxidant in drinking water treatment. Ozonated samples were acidified, extracted with ether, dried, concentrated, and methylated prior to GC-MS analysis. Identified reaation products include mono-, di- and tribasic aliphatic acids and benzene tricarboxylic acids. The products with the highest relative concentrations were succinic and malonic acid, although all products were at relatively low concentration levels. Many of the products identified in this study have also been seen among the reaction products of fulvic acid and other oxidants, such as Cl₂, CIO₂ and KMnO₄.     

Deprotonation energies of a model fulvic acid. I. Carboxylic acid groups

A model Suwannee fulvic acid (SFA [Leenheer, J.A., 1994. In: Baker, L.A. (Ed.), Chemistry of Dissolved Organic Matter in Rivers, Lakes and Reservoirs. Advances in Chemistry Series, vol. 237. American Chemical Society]) was energy minimized in various deprotonation states using semi-empirical methods. The structures were minimized in the isolated SFA phase and SFA with 60 water molecules to mimic the first solvation sphere. The relative energies of deprotonation were calculated at four carboxylic acid sites with Hartree-Fock (HF/6-31G(d)) and density functional theory (B3LYP/6-31G(d)) methods. Comparisons were made between the theoretical methods and states of solvation. Isolated and solvated models resulted in different relative deprotonation orders. The energy changes calculated for removing a H⁺ from a given carboxylic acid group as a function of overall model molecule charge are large enough to explain the large variations of carboxyl group pK_as in dissolved natural organic matter. Analysis of the SFA structure as a function of molecular charge is also discussed.     

Molecular modeling of Al and benzene interactions with Suwannee fulvic acid

The effects that Al³⁺ and benzene interactions exhibit on a model fulvic acid were investigated. Energy minimizations of the structures mimicking the interactions of Al³⁺-Suwannee fulvic acid (SFA), benzene-SFA and Al³⁺-benzene-SFA were run with a solvation sphere of 60 H₂O molecules with the semi-empirical methods PM3 and PM5. The semi-empirical method PM3 was run with Gaussian 98 and CAChe Workstation Pro 6.1.1 to compare the results of the energy minimization algorithms in the two programs. PM5 calculations were run with CAChe Workstation Pro 6.1.1. Molecular dynamics (MD) simulations were run in Cerius² (Accelrys Inc., San Diego, CA) using the Universal Force Field (UFF) 1.02 [Rappé A. K., Casewit C., Colwell K., Goddard W., and Skiff W. (1992) UFF, a full periodic-table force-field for molecular mechanics and molecular dynamics simulations. J. Am. Chem. Soc.114(25), 10024-10035] and COMPASS force field [Sun H. (1998) COMPASS: an ab initio force-field optimized for condensed-phase applications—overview with details on alkane and benzene compounds. J. Phys. Chem. B102, 7338-7364]. Single point calculations were run on the minimized structures at the B3LYP/6-31G(d) level to obtain more accurate estimates of the energy on the minimized structures derived from the PM3, PM5, and UFF methods and to normalize energies to the same reference state. This methodology was used as the standard of comparison for all the models to assess whether or not a given configuration was reasonably stable.The PM3/G03 energy minimizations predicted the lowest B3LYP/6-31G(d) potential energies of the methodologies examined in this study. Thus, this method is considered the most reliable of those tested. The PM3/G03 method predicted that there would be aromatic-aromatic interactions between benzene and SFA. The presence of Al³⁺ was predicted not to interfere with aromatic-aromatic interactions between benzene and SFA, but benzene may influence the location of metal complexation to SFA.     

Catalytic ozonation of dimethyl phthalate by Ce-substituted goethite

Dimethyl phthalate (DMP) is ubiquitous in aquatic environments due to extensively used as plasticizer, which has received increasing attention in recent years. In this study, the catalytic ozonation of dimethyl phthalate was performed using Ce-substituted goethite as a novel catalyst, which was prepared by isomorphous substitution method. The specific surface area, pHpzc and surface hydroxyl density of the catalyst were determined. The catalyst was characterized using X-ray diffraction, scanning electron microscope and Fourier transform infrared spectroscopy. The removal efficiency of DMP was almost 100% after 30 min, and about 40% DMP was mineralized after 60 min, which was nearly four times higher than single ozonation. During catalytic ozonation process, anions (PO₄ ^3?, SO₄ ^2?, Cl^?) affected DMP degradation, indicating that surface hydroxyl groups on the surface of catalyst were main active sites. The electron transfer process by redox reaction between Ce³⁺/Ce⁴⁺, Fe²⁺/Fe³⁺ was proposed, and their interaction could also promote the formation of hydroxyl radicals. Ce-substituted goethite was an efficient catalyst for degradation of DMP by catalytic ozonation.     

Modeling of the formation of complex molecules in protostellar objects

The results of molecular composition modeling are presented for the well studied low-mass star-forming region TMC-1 and the massive star-forming region DR21(OH), which is poorly studied from a chemical point of view. The column densities of dozens of molecules, ranging from simple diatomic to complex organic molecules, are reproduced to within an order of magnitude using a one-dimensional model for the physical and chemical structure of these regions. The chemical ages of the regions are approximately 105 years in both cases. The main desorption mechanisms that are usually included in chemical models (photodesorption, thermal desorption, and cosmic-ray-induced desorption) do not provide sufficient gasphase abundances of molecules that are synthesized in surface reactions; however, this shortcoming can be removed by introducing small amount of reactive desorption into the model. It is possible to reproduce the properties of the TMC-1 chemical composition in a standard model, without requiring additional assumptions about an anomalous C/O ratio or the recent accretion of matter enriched with atomic carbon, as has been proposed by some researchers.     

Modeling of cadmium speciation and dynamics in the Seine estuary (France)

A complexation model (MOCO) was used to describe cadmium (Cd) speciation during estuarine transit in the Seine estuary. This model was developed from field data. Laboratory experiments based on the use of¹⁰⁹Cd enabled checking of certain model simplifications and hypotheses and evaluation of parameters which could not be measured directly. MOCO was coupled with a 3D multivariable hydrosedimentary model (SAM3D) to simulate Cd dynamics in the estuary. These results were compared with measurements (dissolved and particulate Cd) obtained during cruises representative of various hydrodynamic conditions. The purpose of this article is to present the modeling approach used, and its expected applications and limits.     

Temperature dependence of goethite dissolution promoted by trihydroxamate siderophores

This article reports an investigation of the temperature dependence of goethite dissolution kinetics in the presence of desferrioxamine B (DFO-B), a trihydroxamate siderophore, and its acetyl derivative, desferrioxamine D1 (DFO-D1). At 25 and 40°C, DFO-D1 dissolved goethite at twice the rate of DFO-B, whereas at 55°C, the behavior of the two ligands was almost the same. Increasing the temperature from 25 to 55°C caused little or no significant change in DFO-B or DFO-D1 adsorption by goethite. A pseudo-first-order rate coefficient for dissolution, calculated as the ratio of the mass-normalized dissolution rate coefficient to the surface excess of siderophore, was approximately the same at 25 and 40°C for both siderophores. At 55°C, however, this rate coefficient for DFO-D1 was about half that for DFO-B. Analysis of the temperature dependence of the mass-normalized dissolution rate coefficient via the Arrhenius equation led to an apparent activation energy that was larger for DFO-B than for DFO-D1, but much smaller than that reported for the proton-promoted dissolution of goethite. A compensation law was found to relate the pre-exponential factor to the apparent activation energy in the Arrhenius equation, in agreement with what has been noted for the proton-promoted dissolution of oxide minerals and for the complexation of Fe³⁺ by DFO-B and simple hydroxamate ligands in aqueous solution. Analysis of these results suggested that the siderophores adsorb on goethite with a only single hydroxamate group in bidentate ligation with an Fe(III) center.     

Distribution of barium and fulvic acid at the mica-solution interface using in-situ X-ray reflectivity

The interfacial structures of the basal surface of muscovite mica in solutions containing (1) 5 × 10⁻³ m BaCl₂, (2) 500 ppm Elliott Soil Fulvic Acid I (ESFA I), (3) 100 ppm Elliott Soil Fulvic Acid II (ESFA II), (4) 100 ppm Pahokee Peat Fulvic Acid I (PPFA), and (5) 5 × 10⁻³ m BaCl₂ and 100 ppm ESFA II were obtained with high resolution in-situ X-ray reflectivity. The derived electron-density profile in BaCl₂ shows two sharp peaks near the mica surface at 1.98(2) and 3.02(4) Å corresponding to the heights of a mixture of Ba²⁺ ions and water molecules adsorbed in ditrigonal cavities and water molecules coordinated to the Ba²⁺ ions, respectively. This pattern indicates that most Ba²⁺ ions are adsorbed on the mica surface as inner-sphere complexes in a partially hydrated form. The amount of Ba²⁺ ions in the ditrigonal cavities compensates more than 90% of the layer charge of the mica surface. The electron-density profiles of the fulvic acids (FAs) adsorbed on the mica surface, in the absence of Ba²⁺, had overall thicknesses of 4.9-10.8 Å and consisted of one broad taller peak near the surface (likely hydrophobic and positively-charged groups) followed by a broad humped pattern (possibly containing negatively-charged functional groups). The total interfacial electron density and thickness of the FA layer increased as the solution FA concentration increased. The sorbed peat FA which has higher ash content showed a higher average electron density than the sorbed soil FA. When the muscovite reacted with a pre-mixed BaCl₂-ESFA II solution, the positions of the two peaks nearest the surface matched those in the BaCl₂ solution. However, the occupancy of the second peak decreased by about 30% implying that the hydration shell of surface-adsorbed Ba²⁺ was partially substituted by FA. The two surface peaks were followed by a broad less electron-dense layer suggesting a sorption mechanism in which Ba²⁺ acts dominantly as a bridging cation between the mica surface and FA. When the muscovite reacted first with FA and subsequently with BaCl₂, more Ba²⁺ could be adsorbed on the FA-coated mica surface. The peak closest to the mica included Ba²⁺ ions adsorbed directly on the mica in an amount similar to that in the BaCl₂ solution but more broadly distributed. A second peak observed within the FA layer suggests that the FA coating provides additional sites for Ba²⁺ sorption. The results indicate that enhanced uptake of heavy metals can occur when an organic coating already exists on a mineral surface.