Complexation studies of Eu(III) with alumina-bound polymaleic acid: effect of organic polymer loading and metal ion concentration

To contribute to the comprehension of the metal ion sorption properties in mixed mineral-organic matter systems, interaction studies between Eu(III) and polymaleic acid (PMA)-coated alumina colloids were carried out at pH 5 in 0.1 M NaClO4. The studied parameters were the metal ion concentration (1 x 10(-10) to 1 x 10(-4) M) and PMA loading on alumina (gamma = 10-70 mg/g). The data were described by a surface complexation model. The constant capacitance model was used to account for electrostatic interactions. The results showed thattwo sites were necessary to explain the sorption data. At high Eu loading, Eu is surrounded by one carboxylate group and one aluminol group. The existence of this ternary surface site was in agreement with time-resolved laser-induced fluorescence spectroscopy measurements. At low metal ion concentrations, a surface site corresponding to a 1:1 Eu/COO-stoichiometry was deduced from modeling. Spectroscopic data did not corroborate the existence of this latter site. This discrepancy was explained by postadsorption kinetic phenomenon: a migration of the metal ion within the adsorbed organic layer.     

Colloidal α-Al2O3 Europium(III) and humic substances interactions: a macroscopic and spectroscopic study

Eu(III) sorption onto α-Al(2)O(3) in the presence of purified Aldrich humic acid (PAHA) is studied by batch experiments and time-resolved laser-induced luminescence spectroscopy of Eu(III). Experiments are conducted at varying pH, at 0.1 mol/L NaClO(4), 10(-6) mol/L Eu(III), 1 g/L α-Al(2)O(3) and 28 mg/L PAHA, which assured a complete Eu(III)-PAHA complexation. Adsorption of Eu(III) presents the expected pH-edge at 7, which is modified by addition of PAHA. Presence of Eu(III) slightly increases PAHA sorption throughout the pH range. The evolutions of luminescence spectra and decay times of the binary systems, that is, Eu(III)/α-Al(2)O(3) and Eu(III)/PAHA, indicate a progressive surface- and humic-complexation with increasing pH. The typical biexponential luminescence decay in Eu(III)/PAHA system is also recorded; the fastest deactivation depending barely on pH. In ternary Eu(III)/PAHA/α-Al(2)O(3) system, the existence of a luminescence biexponential decay for all pH means that Eu(III) is always in the direct neighborhood of the humic substance. Below pH 7, the spectra of the ternary system (Eu(III)/PAHA/α-Al(2)O(3)) are not different from the ones of Eu(III)/PAHA system, implying the same complex symmetry. Nevertheless, the increase of luminescence decay time points to a change in PAHA conformation onto the surface.     

Interaction of Eu(III)/Cm(III) with alumina-bound poly(acrylic acid): sorption, desorption, and spectroscopic studies

This paper contributes to the comprehension of kinetic and equilibrium phenomena governing trace metal ion sorption on organic matter coated mineral particles. Sorption and desorption experiments were carried out with trivalent metal ions M(III) (M = Eu, Cm) and poly(acrylic acid) (PAA)-coated alumina colloids at pH 5 in 0.1 M NaClO4. Under these conditions, M(III) interaction with the solid is governed by sorbed PAA. The results were compared with spectroscopic data obtained by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Within less than 30 s, a state of local equilibrium is reached between M(III) and adsorbed poly(acrylic acid). M(II) bound to the organic-mineral surface and to dissolved PAA keeps five water molecules in its first hydration sphere. Interaction of M(III) with alumina-bound PAA appears to be strongerthan with dissolved PAA. With increasing contact time, a change of the metal ion speciation occurs at the organic-mineral surface. This change is explained quantitatively by kinetically controlled reactions, which succeed a rapid local equilibrium. The experimental findings suggest, in agreement with model calculations, that a part of the initially sorbed M(III) is slowly converted to a kinetically stabilized species, thereby losing water molecules from the first coordination sphere as indicated by TRLFS. This species might be assigned as a ternary Al2O3-M(III)-PAA complex. The second part of the initially bound M(II) appears to experience as well kinetically controlled reactions, however, without showing changes in the first coordination sphere. We assume that the kinetic stabilization is the consequence of rearrangement processes of the PAA at the alumina surface.     

Uptake of Cm(III) and Eu(III) by calcium silicate hydrates: a solution chemistry and time-resolved laser fluorescence spectroscopy study

The interaction of the two chemical homologues [Cm(III) and Eu(III)] with calcium silicate hydrates (CSH phases) at pH 13.3 has been investigated in batch-type sorption studies using Eu(III) and complemented with time-resolved laser fluorescence spectroscopy (TRLFS) using Cm(III). The sorption data for Eu(III) reveal fast sorption kinetics and a strong uptake by CSH phases with distribution ratios of (6 +/- 3) x 10(5) L kg(-1). Three different Cm(III) species have been identified: A nonfluorescing species, which was identified as a curium hydroxide (surface) precipitate, and two fluorescing Cm(III)/CSH-sorbed species. The fluorescing sorbed species have characteristic emission spectra with main peak maxima at 618.9 and 620.9 nm and fluorescence emission lifetimes of 289 +/- 11 and 1482 +/- 200 micros, respectively. From the fluorescence lifetimes, it was calculated that the two fluorescing Cm(III) species have one or two and no water molecules left in their first coordination sphere, suggesting that these species are incorporated into the CSH structure. A structural model for Cm(III) and Eu(III) incorporation into CSH phases is proposed based on the substitution for Ca at two different types of sites in the CSH structure.     

U(VI)-kaolinite surface complexation in absence and presence of humic acid studied by TRLFS

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) was applied to study the U(VI) surface complexes on kaolinite in the presence and absence of humic acid (HA). Two uranyl surface species with fluorescence lifetimes of 5.9 +/- 1.4 and 42.5 +/- 3.4 micros and 4.4 +/- 1.2 and 30.9 +/- 7.2 micros were identified in the binary (U(VI)-kaolinite) and ternary system (U(VI)-HA-kaolinite), respectively. The fluorescence spectra of adsorbed uranyl surface species are described with six and five fluorescence emission bands in the binary and ternary system, respectively. The positions of peak maxima are shifted significantly to higher wavelengths compared to the free uranyl ion in perchlorate medium. HA has no influence on positions of the fluorescence emission bands. In the binary system, both surface species can be attributed to adsorbed bidentate mononuclear surface complexes, which differ in the number of water molecules in their coordination environment. In the ternary system, U(VI) prefers direct binding on kaolinite rather than via HA, but it is sorbed as a uranyl-humate complex. Consequently, the hydration shell of the U(VI) surface complexes is displaced with complexed HA, which is simultaneously distributed between kaolinite particles. Aluminol binding sites are assumed to control the sorption of U(VI) onto kaolinite.     

Pyrene and phenanthrene sorption to model and natural geosorbents in single- and binary-solute systems

Sorption of pyrene and phenanthrene to model (illite and charcoal) and natural (Yangtze sediment) geosorbents were investigated by batch techniques using fluorescence spectroscopy. A higher adsorption of phenanthrene was observed with all sorbents, which is related to the better accessibility of smaller molecules to micropores in the molecular sieve sorbents. In addition, pyrene sorption in binary-solute systems with a constant initial concentration of phenanthrene (0.1 μmol L(-1) or 2 μmol L(-1)) was studied. A 0.1 μmol L(-1) concentration of phenanthrene causes no competitive effect on the pyrene sorption. A 2 μmol L(-1) concentration of phenanthrene significantly suppresses the sorption of pyrene, especially in the low concentration range; nonlinearity of the pyrene sorption isotherms thus decreases. The competitive effect of 2 μmol L(-1) phenanthrene on the pyrene sorption is overestimated by the ideal adsorbed solution theory (IAST) using the fitted single sorption results of both solutes. An adjustment of the IAST application by taking into account the molecular sieve effect is proposed, which notably improves the IAST prediction for the competitive effect.     

pH dependence of Fenton reagent generation and As(III) oxidation and removal by corrosion of zero valent iron in aerated water

Corrosion of zerovalent iron (ZVI) in oxygen-containing water produces reactive intermediates that can oxidize various organic and inorganic compounds. We investigated the kinetics and mechanism of Fenton reagent generation and As(III) oxidation and removal by ZVI (0.1m2/g) from pH 3-11 in aerated water. Observed half-lives for the oxidation of initially 500 microg/L As(III) by 150 mg Fe(0)/L were 26-80 min at pH 3-9. At pH 11, no As(III) oxidation was observed during the first two hours. Dissolved Fe(III) reached 325, 140, and 6 microM at pH 3, 5, and 7. H2O2 concentrations peaked within 10 min at 1.2, 0.4, and < 0.1 microM at pH 3, 5, and 7, and then decreased to undetectable levels. Addition of 2,2'-bipyridine (1-3 mM), prevented Fe(II) oxidation by O2 and H2O2 and inhibited As(III)oxidation. 2-propanol (14 mM), scavenging OH-radicals, quenched the As(III) oxidation at pH 3, but had almost no effect at pH 5 and 7. Experimental data and kinetic modeling suggest that As(III) was oxidized mainly in solution by the Fenton reaction and removed by sorption on newly formed hydrous ferric oxides. OH-radials are the main oxidant for As(III) at low pH, whereas a more selective oxidant oxidizes As(III) at circumneutral pH.     

Adsorption of humic substances on goethite: comparison between humic acids and fulvic acids

The adsorption of humic acids (HA) to goethite (at pH 3-11) and the proton co-adsorption (at pH 4.0, 5.5, and 7.0) were measured, and the results were compared to those of fulvic acids (FA). Compared to FA, the adsorption of HA is stronger and more ionic strength dependent. The adsorption of both HA and FA decreases with increasing pH. The relative change of the adsorption with pH is bigger for HA than for FA at relatively low pH. At relatively high pH, it is the opposite. Protons are released at pH 4.0 and co-adsorbed at pH 5.5 and 7.0 upon the adsorption of both HA and FA. The observed pH dependency of HA and FA adsorption is in agreement with the proton co-adsorption data. Model calculations show that the adsorbed FA particles are on average located in the Stern layer, whereas the adsorbed HA particles protrude beyond the Stern layer. The closer location to the surface of the adsorbed FA leads to stronger electrostatic interactions between the FA particles and the surface, which explains the larger amount of protons released at low pH and co-adsorbed at high pH with each mass unit of FA adsorbed than that with HA adsorbed. The model also revealsthatfor FA a mean-field (smeared-out) approximation is reasonable, but for HA a patchwise approach is more appropriate at relatively low loading.     

Time-resolved laser fluorescence spectroscopy study on the interaction of curium(III) with Desulfovibrio äspöensis DSM 10631T

The influence of microorganisms on migration processes of actinides has to be taken into account for the risk assessment of potential high-level nuclear waste disposal sites. Therefore it is necessary to characterize the actinide-bacteria species formed and to elucidate the reaction mechanisms involved. This work is focused on the sulfate-reducing bacterial (SRB) strain Desulfovibrio ?sp?ensis (D. ?sp?ensis) DSM 10631T which frequently occurs in the deep granitic rock aquifers at the Asp? Hard Rock Laboratory (Asp? HRL), Sweden. We chose Cm(III) due to its high fluorescence spectroscopic sensitivity as a model system for exploring the interactions of trivalent actinides with D. ?sp?ensis in the trace concentration range of 3 x 10(-7) mol/L. A time-resolved laser fluorescence spectroscopy (TRLFS) study has been carried out in the pH range from 3.00 to 7.55 in 0.154 mol/L NaCl. We interpret the pH dependence of the emission spectra with a biosorption forming an inner-sphere surface complex of Cm(III) onto the D. ?sp?ensis cell envelope. This Cm(III)-D. ?sp?ensis-surface complex is characterized by its emission spectrum (peak maximum at 600.1 nm) and its fluorescence lifetime (162 +/- 5 micros). No evidence was found for incorporation of Cm(III) into the bacterial cells under the chosen experimental conditions.     

Degradation pathways of pentachlorophenol by photo-Fenton systems in the presence of iron(III), humic acid, and hydrogen peroxide

The degradation characteristics and pathways of pentachlorophenol (PCP) by the photo-Fenton systems were studied in H2O2 aqueous solutions, which contained Fe(III) only [H2O2/Fe(III) system] and Fe(III) + humic acid (HA) [H2O2/Fe(III)/HA system] at pH 5.0. Although 40% of the PCP was degraded after 5 h of irradiation in the H2O2/Fe(III) system, more than 90% was degraded in the H2O2/Fe(III)/HA system. This shows that at pH 5.0 the degradation of PCP is clearly enhanced by the presence of HA in the photo-Fenton system. In the H2O2/Fe(III) system, the production of octachlorodibenzo-p-dioxin (OCDD) was detected, and 2-hydroxy nonachlorodiphenyl ether was also identified as a precursor of OCDD. However, no OCDD production was observed in the H2O2/Fe(III)/HA system. This indicates that the presence of HA represses the production of OCDD during the degradation of PCP by the photo-Fenton system. Such an effect by HA can be attributed to a reaction sequence wherein reaction intermediates derived from PCP, such as PCP., are incorporated into HA. This was verified by 13C NMR and pyrolysis-GC/MS studies.     

Incorporation of Eu(III) into hydrotalcite: a TRLFS and EXAFS study

The behavior of radionuclides in the environment (geo-, hydro-, and biosphere) is determined by interface reactions like adsorption, ion exchange, and incorporation processes. Presently, operational gross parameters for the distribution between solution and minerals are available. For predictive modeling of the radionuclide mobility in such systems, however, individual reactions and processes need to be localized, characterized, and quantified. A prerequisite for localization and clarification of the concerned processes is the use of modern advanced analytical and speciation methods, especially spectroscopy. In this study, Eu(III) was chosen as an analogue for trivalent actinides to identify the different species that occur by the Ln(III)/hydrotalcite interaction. Therefore, Eu(III) doped Mg-Al-Cl-hydrotalcite was synthesized and investigated by TRLFS, EXAFS, and XRD measurements. Two different Eu/hydrotalcite species were obtained. The minor part of the lanthanide is found to be inner-sphere sorbed onto the mineral surface, while the dominating Eu/hydrotalcite species consists of Eu(III) that is incorporated into the hydrotalcite lattice. Both Eu/hydrotalcite species have been characterized by their fluorescence emission spectra and lifetimes. Structural parameters of the incorporated Eu(III) species determined by EXAFS indicate a coordination number of 6.6 +/- 1.3 and distances of 2.41 +/- 0.02 A for the first Eu-OH shell.     

Investigation of interparticle forces in natural waters: effects of adsorbed humic acids on iron oxide and alumina surface properties

The nature of interparticle forces acting on colloid particle surfaces with adsorbed surface films of the internationally used humic acid standard material, Suwannee River Humic Acid (SHA), has been investigated using an atomic force microscope (AFM). Two particle surfaces were used, alumina and a hydrous iron oxide film coated onto silica particles. Adsorbed SHA dominated the interactive forces for both surface types when present. At low ionic strength and pH > 4, the force curves were dominated by electrostatic repulsion of the electrical double layers, with the extent of repulsion decreasing as electrolyte (NaCl) concentration increased, scaling with the Debye length (kappa(-1)) of the electrolyte according to classical theory. At pH approximately 4, electrostatic forces were largely absent, indicating almost complete protonation of carboxylic acid (-COOH) functional groups on the adsorbed SHA. Under these conditions and also at high electrolyte concentration ([NaCl] > 0.1 M), the absence of electrostatic forces allowed observation of repulsion forces arising from steric interaction of adsorbed SHA as the oxide surfaces approached closely to each other (separation < 10 nm). This steric barrier shrank as electrolyte concentration increased, implying tighter coiling of the adsorbed SHA molecules. In addition, adhesive bridging between surfaces was observed only in the presence of SHA films, implying a strong energy barrier to spontaneous detachment of the surfaces from each other once joined. This adhesion was especially strong in the presence of Ca2+ which appears to bridge SHA layers on each surface. Overall, our results show that SHA is a good model for the NOM adsorbed on colloids.     

Nickel sequestration in a kaolinite-humic acid complex

Incorporation of first row transition metals into stable surface precipitates can play an important role in reducing the bioavailability of these metals in neutral and alkaline soils. Organic coatings may interfere with this sorption mechanism by changing the surface characteristics and by masking the mineral surface from metal sorptives. In this study, kinetic sorption and desorption experiments were combined with extended X-ray absorption fine structure (EXAFS) spectroscopy to elucidate the effect of humic acid (HA) coatings on the formation and stabilization of nickel precipitates at the kaolinite-water interface. Initial Ni uptake (pH 7.5, [Ni]i = 3 mM, and I = 0.02 M NaNO3) increased with greater amounts of HA coated onto the kaolinite surface. Ni uptake continued over an extended period of time without reaching an apparent equilibrium. EXAFS analysis of the Ni sorption complex structures formed over time (up to 7 months) revealed the formation of a Ni-Al layered double hydroxide (LDH) precipitate at the kaolinite surface in the absence of HA. HA alone formed an inner-sphere complex with Ni (with 2 carbon atoms at an average radial distance of 2.85 A). A Ni-Al LDH precipitate phase was formed at the kaolinite surface in the presence of a 1 wt % HA coating. However, with 5 wt % HA coated at the kaolinite surface, the formation of a surface precipitate was slowed significantly, and the precipitate formed was similar in structure to Ni(OH)2(s). The Ni(OH)2 precipitate was not resistant to proton dissolution, while the Ni-Al LDH precipitate was. These results augment earlier findings that the incorporation of Ni and other first row transition metals into stable surface precipitates is an important sequestration pathway for toxic metals in the environment, despite the presence of ubiquitous coating materials such as humic acids.     

Use of spectroscopic techniques for uranium(VI)/montmorillonite interaction modeling

To experimentally identify both clay sorption sites and sorption equilibria and to understand the retention mechanisms at a molecular level, we have characterized the structure of hexavalent uranium surface complexes resulting from the interaction between the uranyl ions and the surface retention groups of a montmorillonite clay. We have performed laser-induced fluorescence spectroscopy (LIFS) and X-ray photoelectron spectroscopy (XPS) on uranyl ion loaded montmorillonite. These structural results were then compared to those obtained from the study of uranyl ions sorbed onto an alumina and also from U(VI) sorbed on an amorphous silica. This experimental approach allowed for a clear determination of the reactive surface sites of montmorillonite for U(VI) sorption. The lifetime values and the U4f XPS spectra of uranium(VI) sorbed on montmorillonite have shown that this ion is sorbed on both exchange and edge sites. The comparison of U(VI)/clay and U(VI)/oxide systems has determined that the interaction between uranyl ions and montmorillonite edge sites occurs via both [triple bond]AlOH and [triple bond]SiOH surface groups and involves three distinct surface complexes. The surface complexation modeling of the U(VI)/montmorillonite sorption edges was determined using the constant capacitance model and the above experimental constraints. The following equilibria were found to account for the uranyl sorption mechanisms onto montmorillonite for metal concentrations ranged from 10(-6) to 10(-3) M and two ionic strengths (0.1 and 0.5 M): 2[triple bond]XNa + UO2(2+) <==> ([triple bond]X)2UO2 + 2Na+, log K0(exch) = 3.0; [triple bond]Al(OH)2 + UO2(2+) <==> [triple bond]Al(OH)2UO2(2+), log K0(Al) = 14.9; [triple bond]Si(OH)2 + UO2(2+) <==> [triple bond]SiO2UO2 + 2H+, log K0(Si1) = -3.8; and [triple bond]Si(OH)2 + 3UO2(2+) + 5H2O <==> [triple bond]SiO2(UO2)3(OH)5- + 7H+, log K0(Si2) = -20.0.     

X-ray absorption spectroscopy study of the effects of pH and ionic strength on Pb(II) sorption to amorphous silica

Pb(III) sorption to hydrous amorphous SiO2 was studied as a function of pH and ionic strength using XAS to characterize the sorption products formed. Pb sorption increased with increasing pH and decreasing ionic strength. The XAS data indicated that the mechanism of Pb(II) sorption to the SiO2 surface was pH-dependent. At pH < 4.5, a mononuclear inner-sphere Pb sorption complex with ionic character dominated the Pb surface speciation. Between pH 4.5 and pH 5.6, sorption increasingly occurred via the formation of surface-attached covalent polynuclear Pb species, possibly Pb-Pb dimers, and these were the dominant Pb complexes at pH > or = 6.3. Decreasing ionic strength from I = 0.1 to I = 0.005 M NaClO4 significantly increased Pb sorption but did not strongly influence the average local coordination environment of sorbed Pb at given pH, suggesting that the formation of mononuclear and polynuclear Pb complexes at the surface were coupled; possibly, Pb monomers control the formation of Pb polynuclear species by diffusion along the surface, or they act as nucleation centers for additional Pb uptake from solution. This study shows that the effectiveness of SiO2 in retaining Pb(II) is strongly dependent on solution conditions. At low pH, Pb(II) may be effectively remobilized by competition with other cations, whereas sorbed Pb is expected to become less susceptible to desorption with increasing pH. However, unlike for Ni(II) and Co(II), no lead phyllosilicates are formed at these higher pH values; therefore, SiO2 is expected to be a less effective sink for Pb immobilization than for these other metals.     

Kinetics and mechanistic aspects of As(III) oxidation by aqueous chlorine, chloramines, and ozone: relevance to drinking water treatment

Kinetics and mechanisms of As(III) oxidation by free available chlorine (FAC-the sum of HOCl and OCl-), ozone (O3), and monochloramine (NH2Cl) were investigated in buffered reagent solutions. Each reaction was found to be first order in oxidant and in As(III), with 1:1 stoichiometry. FAC-As(III) and O3-As(III) reactions were extremely fast, with pH-dependent, apparent second-order rate constants, k'app, of 2.6 (+/- 0.1) x 10(5) M(-1) s(-1) and 1.5 (+/- 0.1) x 10(6) M(-1) s(-1) at pH 7, whereas the NH2Cl-As(III) reaction was relatively slow (k'app = 4.3 (+/- 1.7) x 10(-1) M(-1) s(-1) at pH 7). Experiments conducted in real water samples spiked with 50 microg/L As(III) (6.7 x 10(-7) M) showed that a 0.1 mg/L Cl2 (1.4 x 10-6 M) dose as FAC was sufficient to achieve depletion of As(III) to <1 microg/L As(III) within 10 s of oxidant addition to waters containing negligible NH3 concentrations and DOC concentrations <2 mg-C/L. Even in a water containing 1 mg-N/L (7.1 x 10(-5) M) as NH3, >75% As(III) oxidation could be achieved within 10 s of dosing 1-2 mg/L Cl2 (1.4-2.8 x 10(-5) M) as FAC. As(III) residuals remaining in NH3-containing waters 10 s after dosing FAC were slowly oxidized (t1/2 > or = 4 h) in the presence of NH2Cl formed by the FAC-NH3 reaction. Ozonation was sufficient to yield >99% depletion of 50 microg/L As(III) within 10 s of dosing 0.25 mg/L O3 (5.2 x 10(-6) M) to real waters containing <2 mg-C/L of DOC, while 0.8 mg/L O3 (1.7 x 10(-5) M) was sufficientfor a water containing 5.4 mg-C/L of DOC. NH3 had negligible effect on the efficiency of As(III) oxidation by O3, due to the slow kinetics of the O3-NH3 reaction at circumneutral pH. Time-resolved measurements of As(III) loss during chlorination and ozonation of real waters were accurately modeled using the rate constants determined in this investigation.     

Macroscopic studies of the effects of selenate and selenite on cobalt sorption to gamma-Al2O3

Metal ion sorption can be significantly impacted by the presence of other solutes or complexing species. In this research, macroscopic sorption studies were conducted to evaluate the effect of strongly sorbing Se(IV) and weakly sorbing Se(VI) oxyanions on cobalt(II) sorption to gamma-Al2O3. Se(IV) was found to significantly alter Co(ll) sorption as a function of Co(II) surface coverage, while Se(VI) was found to have no effect on Co(II) sorption. Under low Co(II) surface loadings (<0.1 micromol/m2), Se(IV) increased Co(II) sorption as a function of the Se(IV) coverage. At low Se(IV) surface coverages, no change in Co(II) sorption was detectable, while at high Se(IV) loadings Co(II) sorption was significantly increased. The increase in Co(ll) sorption in the bisorbate systems can be explained by either an electrostatic enhancement mechanism or byternary complex formation. Se(IV) decreased Co(II) sorption at high Co(ll) surface loadings (>0.5 micromol/m2) where coprecipitation of Co(II) and A(III) in the form of layered double hydroxides (LDH) is expected to be the dominant sorption mechanism for the single-sorbate case. The extent of the Co(ll) sorption reduction in Co(III)/Se(IV) bisorbate systems compared to the corresponding single-sorbate systems increased with increasing Co(II) surface coverage. The rate of Co(II) desorption was reduced in the presence of Se(IV) compared to the single-sorbate case, indicating a direct interaction between Co(II) and Se(IV). A reaction between Co(II) and Se(IV) is further supported by an increase in Se(IV) sorption in the same bisorbate samples where Co(II) sorption is decreased. Thus, the macroscopic data indicates Se(IV) may be altering the mechanism of Co(II) sorption, potentially forming a ternary surface complex or different surface precipitate.     

Sorption of arsenate and arsenite on RuO2 x xH2O: a spectroscopic and macroscopic study

The sorption reactions of arsenate (As(V)) and arsenite (As(III)) on RuO2 x xH2O were examined using macroscopic and spectroscopic techniques. Constant solid:solution isotherms were constructed from batch sorption experiments and sorption kinetics assessed at pH 7. X-ray absorption near edge spectroscopy (XANES) was employed to elucidate the solid-state speciation of sorbed As. At all pH values studied (pH 4-8), RuO2 x xH2O showed a high affinity for As regardless of the initial As species present. Sorption was higher at all pH values when the initial As species was As(III). Oxidation of As(III) (250 mg/L solution) to As(V) was virtually complete (98-100%) within 5 s. XANES results showed the presence of only As(V) on the RuO2 x xH2O regardless of the initial As oxidation state. There was no change in the As oxidation state on the solid phase for 4 weeks in both oxic and anoxic environments. It is speculated that changes in the RuO2 x xH2O structure, due to oxidation reactions, caused the higher total As sorption capacity when As(III) was the initial species. The As sorption capacity of RuO2 x xH2O is greater than that of other metal oxides reviewed in this study. The ability of RuO2 x xH2O to rapidly oxidize As(III) is much greater than other oxides, such as MnO2.     

Simultaneous determination of stevioside, rebaudioside A and glycyrrhizic acid in foods by HPLC

A method for the simultaneous determination of stevioside (Stev), rebaudioside A (RebA) and glycyrrhizic acid (GA) in foods was developed. These sweeteners were extracted from foods, except for dried fishes and shellfishes, by dialysis against Tris-HCl buffer (pH 9.0). Dried fishes and shellfishes were extracted with Tris-HCl buffer--methanol (2:8). The extracts were cleaned up with an Oasis MAX cartridge. The cartridge was washed with 0.05 mol/L sodium acetate (pH 4.0)--methanol (19:1), and the three sweeteners were eluted with 0.1 mol/L phosphoric acid--acetonitrile (1:1). Stev, RebA and GA in the eluate were chromatographed on a Develosil RPAQUEOUS-AR-5 (4.6 mm i.d. x 250 mm) column with 0.02 mol/L phosphoric acid-acetonitrile--methanol (90:55:5) as a mobile phase and monitored at 210 nm for Stev and RebA, and at 254 nm for GA. The recoveries of Stev, RebA and GA from 8 kinds of foods spiked at the level of 0.1 g/kg were 81.7-101%, 81.5-100% and 78.6-95.0%, respectively. The determination limits were 0.01 g/kg in samples.     

可德兰多糖-阿魏酸接枝共聚物的制备工艺优化

采用自由基介导的接枝方法制备可德兰多糖-阿魏酸（FA）接枝共聚物,通过单因素试验考察FA添加量、维生素C（Vc）添加量、过氧化氢（H2O2）浓度和反应时间对接枝率和得率的影响,在此基础上,利用正交试验设计进一步优化接枝共聚物的制备工艺参数。研究表明,可德兰多糖-阿魏酸接枝共聚物制备的最佳工艺条件为：25 mL 5 mg/mL可德兰多糖溶液,FA和Vc添加量分别为250 mg和50 mg,H2O2浓度5 mol/L,反应时间12 h,此时,可德兰多糖-FA接枝共聚物的接枝率和得率均最大,分别为98.62 mg FA/g和40.84%,并通过紫外和红外光谱验证了接枝反应的发生。因此,该方法不仅改善了可德兰多糖的理化特性,而且能够提高可德兰多糖-FA接枝共聚物的接枝率,为其在食品工业中的应用奠定了理论基础。