Rates of Abiotic MnII Oxidation by O2: Influence of Various Multidentate Ligands at High pH.

Oxidation of manganous manganese (MnII) is an important process driving manganese cycles in natural aquatic systems and leading to the formation of solid-phase MnIII,IV (hydr)oxide products. Previous research has shown that some simple ligands (e.g., phosphate, sulfate, chloride, fluoride) can bind with MnII to make it unreactive to oxidation by dissolved oxygen. However, there is little to no understanding of the role played by stronger, complex-forming ligands in MnII oxidation reactions. The objective of this study was to evaluate the rates of abiotic MnII oxidation by O2 in the presence of low concentrations of several complex-forming model ligands (pyrophosphate, tripolyphosphate, ethylenediaminetetraacetic acid, oxalate) in bicarbonate-carbonate buffered laboratory solutions of pH 9.42, 9.65, and 10.19. The influence of increasing ligand concentrations on observed autocatalytic profiles of MnII oxidation was investigated, and initial oxidation rates were linked quantitatively to the initial MnII speciation in experimental solutions. Observed rates of MnII oxidation decreased with increasing ligand concentration for all four ligands tested. However, the profiles observed with time and the magnitudes of decrease in initial oxidation rates were different for the different ligands. Likely explanations for these observations include the denticity of the tested ligands, the relative strength of the ligands to complex MnII versus MnIII, and the ability of some ligands to enhance the reduction of MnIII back to MnII on a time scale comparable to the forward homogeneous MnII oxidation reaction.

Iron and Electron Shuttle Mediated (Bio)degradation of 2,4-Dinitroanisole (DNAN).

The Department of Defense has developed explosives with the insensitive munition 2,4-dinitroanisole (DNAN), to prevent accidental detonations during training and operations. Understanding the fate and transport of DNAN is necessary to assess the risk it may represent to groundwater once the new ordnance is routinely produced and used. Experiments with ferrous iron or anthrahydroquinone-2,6-disulfonate (AH2QDS) were conducted from pH 6.0 to 9.0 with initial DNAN concentrations of 100 µM. DNAN was degraded by 1.2 mM Fe(II) at pH 7, 8, and 9, and rates increased with increasing pH. Greater than 90% of the initial 100 µM DNAN was reduced within 10 min at pH 9, and all DNAN was reduced within 1 h. AH2QDS reduced DNAN at all pH values tested. Cells of Geobacter metallireducens were added in the presence and absence of Fe(III) and/or anthraquinone-2,6-disulfonate (AQDS), and DNAN was also reduced in all cell suspensions. Cells reduced the compound directly, but both AQDS and Fe(III) increased the reaction rate, via the production of AH2QDS and/or Fe(II). DNAN was degraded via two intermediates: 2-methoxy-5-nitroaniline and 4-methoxy-3-nitroaniline, to the amine product 2,4-diaminoanisole. These data suggest that an effective strategy can be developed for DNAN attenuation based on combined biological-abiotic reactions mediated by Fe(III)-reducing microorganisms.

Preferential sorption of some natural organic matter fractions to titanium dioxide nanoparticles: influence of pH and ionic strength.

Natural organic matter (NOM) sorption to nanoparticles (NPs) can influence their transport and bioavailability in the aquatic environment. The sorption affinity of NOM to surfaces including NPs is size dependent, and depending on environmental conditions, NOM may enhance or mitigate NPs toxicity. The aim of this study was to investigate the preferential sorption of different-sized fractions of NOM to titanium dioxide (TiO2) NPs. We specifically investigated the influence of pH, ionic strength, and NOM concentration on the extent of this preferential sorption using a constant sorbent concentration (400 mg/L TiO2 NPs). Additionally, sorption of NOM to TiO2 NPs at varying pH was investigated. The nonsorbed NOM was separated from the sorbed, by 50 nm polycarbonate membrane filters and ultracentrifugation. High-performance size exclusion chromatography (HPSEC) was used to determine the average molecular weights of NOM (MWw). Corroborative evidence of preferential sorption of different-sized molecular weight fractions of NOM was obtained from optical techniques such as absorbance and fluorescence spectrophotometry. The total organic carbon was measured by the Total Organic Carbon Analyzer-Shimadzu (TOC-VCPH). The results indicated that there is preferential sorption of larger sized fractions of NOM to TiO2 NPs irrespective of NOM concentration. It was observed that the sorption of larger sized fractions of NOM was much enhanced at lower pH and at higher ionic strength. Both absorbance and fluorescence spectrophotometric techniques gave credible corroborative evidence on the extent of preferential sorption of lager sized fractions of NOM with respect to pH and ionic strength. The sorption results demonstrated higher sorption at lower pH than at higher pH. Overall, the results of this study suggest that the environmental conditions are key factors that can contribute to NOM's fractional preferential sorption to NPs in the aquatic environment.

Removal of 1,4-Naphthoquinone by Birnessite-Catalyzed Oxidation: Effect of Phenolic Mediators and the Reaction Pathway.

This study investigated the birnessite (δ-MnO2) catalyzed oxidative removal of 1,4-naphthoquinone (1,4-NPQ) in the presence of phenolic mediators; specifically, the kinetics of 1,4-NPQ removal under various conditions was examined, and the reaction pathway of 1,4-NPQ was verified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The removal rate of 1,4-NPQ by birnessite-catalyzed oxidation (pH = 5) was faster in the presence of phenolic mediators with electron-donating substituents (pseudo-first-order initial stage rate constant (k1) = 0.380-0.733 h-1) than with electron-withdrawing substituents (k1 = 0.071-0.244 h-1), and the effect on the substituents showed a positive correlation with the Hammett constant (Σσ) (r2 = 0.85, p < 0.001). The rate constants obtained using variable birnessite loadings (0.1-1.0 g L-1), catechol concentrations (0.1-1.0 mM), and reaction sequences indicate that phenolic mediators are the major limiting factor for the cross-coupling reaction of 1,4-NPQ in the initial reaction stages, whereas the birnessite-catalyzed surface reaction acts as the major limiting factor in the later reaction stages. This was explained by the operation of two different reaction mechanisms and reaction products identified by LC-MS/MS.

Evaluating spectroscopic and chromatographic techniques to resolve dissolved organic matter via end member mixing analysis.

Real-time or near real-time in-situ monitoring of dissolved organic matter (DOM) composition in natural waters and engineered treatment systems provides critical information to water quality scientists and engineers, particularly when the monitoring techniques can provide some information about the chemical nature of DOM. The efficacy of various indices derived from rapid, low-cost spectroscopic and chromatographic techniques to discriminate DOM composition was tested for samples prepared from well-defined mixtures of purified Aldrich humic acid (PAHA) and Suwannee River fulvic acid (SRFA). Sensitivities of the discrimination indices were examined by comparing (1) the differences between measured values and those predicted based from mass balance and the end member characteristics, and (2) the linear correlations between index values and mass ratios of the DOM mixtures. Size exclusion chromatography (SEC) results revealed that the weight-average molecular weight (MW(w)) may be a useful approach for tracking DOM mixing processes, although the number-average molecular weight (MW(n)) may be better for distinguishing different DOM compositions. Specific ultraviolet absorbance measured at 254 nm (SUVA(254)) performed better as a discrimination index than did two previously recommended absorbance ratios, both in terms of making better predictions of intermediate compositions and in exhibiting a more linear correlation with PAHA mass ratio. Several well-defined peaks in the derivative absorption spectra (301 and 314 nm for the first derivative, 217 nm for the third derivative, and 211 and 224 nm for the fourth derivative) also were found to be promising potential DOM discrimination indices. Finally, a fluorescence ratio based on humic- versus fulvic-like fluorescence proved to be a superior DOM discrimination index for the two DOM end members studied here. In general, this study illustrates the evaluation process that should be followed to develop rapid, low-cost discrimination indices to monitor DOM compositions based on end member mixing analyses.

Dynamic exchanges between DOM and POM pools in coastal and inland aquatic ecosystems: A review.

Dynamic exchanges between dissolved organic matter (DOM) and particulate organic matter (POM) plays a critical role in organic carbon cycling in coastal and inland aquatic ecosystems, interactions with aquatic organisms, mobility and bioavailability of pollutants, among many other ecological and geochemical phenomena. Although DOM-POM exchange processes have been widely studied from different aspects, little to no effort has been made to date to provide a comprehensive, mechanistic, and micro-spatial schema for understanding various exchange processes occurring in different aquatic ecosystems in a unified way. The phenomena occurring between DOM and POM were explained here with the homogeneous and heterogeneous mechanisms. In the homogeneous mechanism, the participating components are only organic matter (OM) constituents themselves with aggregation and dissolution involved, whereas OM is associated with other components such as minerals and particulate colloids in the heterogeneous counterpart. Besides the generally concerned processes of aggregation/dissolution and adsorption/desorption, other ecological factors such as sunlight and organisms can also participate in DOM-POM exchanges through altering the chemical nature of OM. Despite the limitation of current analytical technologies, many unknown and/or unquantified processes need to be identified to unravel the complicated exchanges of OM between its dissolved and particulate states. Based on the review of several previous mathematical models, we proposed a unified conceptual model to describe all major dynamic exchange mechanisms on the basis of exergy theory. More knowledge of dynamic DOM-POM exchanges is warranted to overcome the potential problems arising from a simple division of OM into dissolved versus particulate states and to further develop more sophisticated mathematic models.

Solubilization of polycyclic aromatic hydrocarbons by perfluorinated surfactant micelles.

Due to their chemical and thermal stability, perfluorinated surfactants (PFSs) are promising materials for the development of novel environmental remediation applications. This stability also leads to the persistence of PFS in the environment; therefore, their properties and behavior should be well understood. This study focused on polycyclic aromatic hydrocarbon (PAH) and PFS interactions, particularly the solubilization of PAHs by PFS micelles. Naphthalene. phenanthrene, and pyrene were selected as representative PAHs and an anionic PFS, ammonium perfluorooctanoate (APFO) was used. Critical micelle concentration (CMC) values of APFO measured by surface tension, fluorescence probe, and solubility enhancement methods fell in the range of 20-30 mM at 22 +/- 1 degrees C. Apparent solubilities of molecular oxygen and PAHs in APFO micellar solutions depended linearly on the APFO concentration. Molar solubilization ratio (MSR) values were determined to be 9.50 x 10(-4), 4.17 x 10(-3), 2.31 x 10(-4), and 4.09 x 10(-5) and mole fraction micellar partition coefficient (Kmic) values were found to be 1.89 x 10(2), 9.50 x 10(2), 2.12 x 10(3), and 3.79 x 10(3) for oxygen, naphthalene, phenanthrene, and pyrene, respectively at 22 +/- 1 degrees C. log Kmic values for three PAHs were shown to be linearly correlated with the log values of octanol-water partition coefficients (log Kow).

Testing a surface tension-based model to predict the salting out of polycyclic aromatic hydrocarbons in model environmental solutions.

Molar-based Setschenow constants (Ks) for six alkali and alkaline earth metal-based inorganic salts were determined at 20 degrees C to evaluate their influence on the solubilities, and thus the aqueous activity coefficients, of three polycyclic aromatic hydrocarbons (PAHs). The six salts tested exhibited a wide range of Ks values, varying from 0.105 +/- 0.009 M(-1) (for NaClO4 and pyrene) to 1.29 +/- 0.17 M(-1) (for K2SO4 and perylene). In general, salting out effects with these electrolytes were observed in the order Ca2+ > Na+ > K+ and SO4(2-) > Cl- > ClO4-, consistent with previous reports. However, the expected salting out trend of perylene > pyrene > naphthalene was only observed with K2SO4. In CaCl2 solutions, the Ks value of pyrene was significantly lower than that of naphthalene. For NaCl, KCl and NaClO4, pyrene Ks values were found to be lower than, but not significantly different from, those of naphthalene. Setschenow constants for all six salts were predicted using a semi-empirical, thermodynamically-based equation that relates the standard free energy change associated with transferring solutes from water to a salt solution to the difference in surface tensions between the two solutions. With this equation, predicted Ks values were in reasonable agreement with observed Ks values (generally within +/- 50%). Lack of better agreement between predicted and observed values likely reflects the inability of the simple surface tension model to account for all interactions among the cations, anions, PAH molecules and water molecules in the respective systems.

Effects of pH and phosphate on the adsorptive fractionation of purified Aldrich humic acid on kaolinite and hematite.

The molecular weight (MW) fractionation of purified Aldrich humic acid (PAHA) resulting from adsorption on kaolinite and hematite was investigated for different solution pH and phosphate conditions. Adsorption was highly pH-dependent, with higher uptake at lower pH values. For all pH conditions, the weight-average MW (MWw) of residual PAHA remaining in solution after adsorption deviated from the original MWw, indicating that preferential adsorption of certain MW components occurred. The extent of preferential adsorption depended on the percent carbon adsorption at a given pH condition. For similar percent carbon adsorption ranges, a greater extent of preferential adsorption of the higher MW PAHA components was observed with higher pH values as demonstrated by the lowest residual MWw value occurring at pH 9. Detailed analyses of selected residual PAHA samples clearly showed that adsorption selectivity for particular MW components was strongly influenced by solution pH. The extent of preferential adsorption of lower MW PAHA components decreased in the presence of a small amount of phosphate. This effect was more evident for hematite than kaolinite, and became greater with lower solution pH irrespective of the mineral type. The different fractionation patterns observed for PAHA were reasonably well explained by the physicochemical trends occurring in its MWw fractions and the underlying sorption processes.

Molecular weight fractionation of humic substances by adsorption onto minerals.

Molecular weight (MW) fractionation of Suwannee River fulvic acid (SRFA) and purified Aldrich humic acid (PAHA) by adsorption onto kaolinite and hematite was investigated in equilibrium and rate experiments with a size-exclusion chromatography system using ultraviolet (UV) light detection. The extent of adsorptive fractionation based on UV detection was positively correlated with the percent carbon adsorption for both humic substances (HS), although the specific fractionation pattern observed depended on the particular HS and mineral used. Higher MW fractions of SRFA, an aquatic HS, were preferentially adsorbed to both kaolinite and hematite whereas the fractionation trends for PAHA, a terrestrial peat HS, differed for the two minerals. The contrasting fractionation patterns for SRFA versus PAHA can be explained reasonably well by the different structural trends that occur in their respective MW fractions and the underlying adsorption processes. Rate studies of adsorptive fractionation revealed an initial rapid uptake of smaller HS molecules by the mineral surfaces, followed by their replacement at the surface by a much slower uptake of the larger HS molecules present in aqueous solution.

Cosolvent effect on the catalytic reductive dechlorination of PCE.

Reductive dechlorination of chlorinated organic contaminants is an effective approach to treat this widespread group of environmentally hazardous substances. Metalloporphyrins can be used to catalyze reduction reactions by shuttling electrons from a reducing agent (electron donor) to chlorinated organic contaminants, thus rendering them to non-chlorinated acetylene, ethylene or ethane as major products. Iron, nickel and vanadium oxide tetraphenyl porphyrins (TPPs) were used as models of non-soluble metalloporphyrins that are common in subsurface environments, and hence may inflect on the ability to use natural ones. The effect of cosolvents on metalloporphyrins is demonstrated to switch the reduction of tetrachlorethylene (PCE) from no reaction to complete PCE transformation within 24 h and the production of final non-chlorinated compounds. Variations in product distributions for the different metalloporphyrins indicate that changes in the core metal can influence reaction rates and effective pathways. Furthermore, different cosolvents can generate varied product distributions, again suggesting that different pathways and/or rates are operative in the reduction reactions. Comparison of different cosolvent effects on PCE reduction using vitamin B12--a soluble natural metalloporphyrinogen--as the catalyst shows less pronounced differences between reactions in various cosolvent solutions versus only aqueous solution.

Role of metalloporphyrin core metals in the mediated reductive dechlorination of tetrachloroethylene.

A promising approach to abiotically dechlorinate a variety of chlorinated organic contaminants under reducing conditions is to utilize porphyrins or other tetrapyrrole macrocycles as electron transfer mediators/shuttles for catalyzing their reduction. In this study, various experimental approaches were used to elucidate the role of porphyrin core metals in the reductive dechlorination of tetrachloroethylene (PCE). The importance of specific core metals for the reactivity of a porphyrin and its mediated reaction mechanisms was demonstrated by inserting different metals into metallo tetrakis (N-methyl-4-4 pyridiniumyl) porphyrin (TMPyP). No PCE dechlorination was observed when the free-base (i.e., no core metal) and iron core metal forms of TMPyP were utilized. When using nickel or cobalt TMPyP, reductive dechlorination of PCE occurred but appeared to follow different pathways for the two metals based on product analyses. Physical (e.g., steric) considerations suggest that direct contact between a porphyrin core metal and PCE may be limited and therefore that the entire metalloporphyrin molecule should be viewed as a functional system in which the organic macrocycle has an active part in reductive dechlorination reactions. This view is supported by the fact that slight changes in the functional groups on a porphyrin macrocycle, particularly those far removed from the core metal itself, greatly affected the reactivity and mechanism of the porphyrin. Solution conditions also had a major effect on porphyrin reactivities, to the extent that a nonreactive metalloporphyrin could be activated merely by adjusting the pH of the solution or by adding a small amount of cosolvent. The collective results of this study suggest that fine tuning of naturally occurring metalloporphyrin complexes and/or their environments can enhance the catalyzed detoxification of chlorinated contaminants in many natural and engineered environmental systems.

Metalloporphyrin solubility: a trigger for catalyzing reductive dechlorination of tetrachloroethylene.

Metalloporphyrins are well known for their electron-transfer roles in many natural redox systems. In addition, several metalloporphyrins and related tetrapyrrole macrocycles complexed with various core metals have been shown to catalyze the reductive dechlorination of certain organic compounds, thus demonstrating the potential for using naturally occurring metalloporphyrins to attenuate toxic and persistent chlorinated organic pollutants in the environment. However, despite the great interest in reductive dechlorination reactions and the wide variety of natural and synthetic porphyrins currently available, only soluble porphyrins, which comprise a small fraction of this particular family of organic macrocycles, have been used as electron-transfer shuttles in these reactions. Results from the present study clearly demonstrate that metalloporphyrin solubility is a key factor in their ability to catalyze the reductive dechlorination of tetrachloroethylene and its daughter compounds. Additionally, we show that certain insoluble and nonreactive metalloporphyrins can be activated as catalysts merely by changing solution conditions to bring about their dissolution. Furthermore, once a metalloporphyrin is fully dissolved and activated, tetrachloroethylene transformation proceeds rapidly, giving nonchlorinated and less toxic alkenes as the major reaction products. Results from the present study suggest that if the right environmental conditions exist or can be created, specific metalloporphyrins may provide a solution for cleaning up sites that are contaminated with chlorinated organic pollutants.

Effects of mineral surfaces on pyrene partitioning to well-characterized humic substances.

Mineral surfaces can alter the ability of humic substances (HS) to bind hydrophobic organic contaminants. In this study, complete adsorption (i.e., to avoid HS adsorptive fractionation effects) of a small subset of well-characterized terrestrial and aquatic HS on kaolinite and hematite significantly changed their subsequent organic carbon-normalized partition coefficients K(ads)(oc) for pyrene relative to their original respective dissolved organic carbon-normalized partition coefficients K(dis)(oc). Parallel experiments with ultrafiltration (UF) fractions obtained from purified Aldrich humic acid (PAHA) (Aldrich Chemical, Milwaukee, WI) gave similar results. The heterogeneity among the PAHA UF fractions was examined via their mineral surface adsorption characteristics and their subsequent ability to bind pyrene. As expected, variations in maximum adsorption densities (q(max)), Langmuir adsorption constants (K(q)), and pyrene K(ads)(oc) values were observed among the PAHA UF fractions. However, general trends of q(max), K(q), and pyrene log K(ads)(oc) values for the PAHA UF fractions versus the logarithm of their weight-average molecular weights (MW(w)) did not typically match the corresponding trends obtained with the four aquatic and terrestrial HS. In general, an ideal mixture competitive adsorption model gave reasonable predictions for PAHA sorption to kaolinite and hematite based on their corresponding UF isotherm parameters. Ideal mixture predictions of pyrene partitioning to adsorbed PAHA from the corresponding UF fraction results were better for kaolinite versus hematite, indicating that the underlying mineral surface can alter the effects of HS heterogeneity on hydrophobic organic contaminant sorption.

Molecular-level investigation of monoaromatic compound sorption to suspended soil particles by deuterium nuclear magnetic resonance.

Molecular-level sorption behavior of monoaromatic compounds in suspensions of water-dispersable clay components was studied by measuring 2H nuclear magnetic resonance (NMR) spin-spin relaxation times (T2). In general, decreased T2 values indicate stronger solute-sorbent interactions and increased sorption of the solute. A decreasing trend for T2 values in the order benzene > fluorobenzene > toluene (-C6D5 moiety) was observed, which was probably caused by the hydrophobic effect. The T2 values for benzene and the -C6D5 moiety of toluene increased with increasing pH, whereas the trend with pH was much weaker and less consistent for fluorobenzene and the methyl group of toluene. Conversely, no clear relationship was found between T2 values and pH for dichloromethane. These contrasting results cannot be explained by the pH-dependent self-assembly and hydrophobicity of humics. Instead, directed specific forces, including hydrogen bonding, cation-pi interactions, and aromatic-aromatic interactions, are proposed between the benzene ring of monoaromatic solutes and soil organic matter (SOM). Substituents of benzene affect these interactions by varying the pi electron density. When the soil fraction was treated with NaOH to remove humic and fulvic acids, T2 values for the different monoaromatic solutes were surprisingly lower compared with those for the untreated soil fraction. This result is probably caused by the increased ratio of solutes adsorbed to "hard" or "glassy" SOM components, which leads to less mobile sorbed solute molecules, after removing NaOH-extractable humics that contain more "soft" or "rubbery" SOM components.

Characterization of cation-pi interactions in aqueous solution using deuterium nuclear magnetic resonance spectroscopy.

Chemical interactions of aromatic organic contaminants control their fate, transport, and toxicity in the environment. Recent molecular modeling studies have suggested that strong interactions can occur between the pi electrons of aromatic molecules and metal cations in aqueous solutions and/or on mineral surfaces, and that such interactions may be important in some environmental systems. However, spectroscopic evidence for these so-called cation-pi interactions has been extremely limited to date. In this paper, cation-pi interactions in aqueous salt solutions were characterized via 2H nuclear magnetic resonance (NMR) spin-lattice relaxation times (T1) and calculations of molecular correlation times (tau(c)) for a series of perdeuterated (d6-benzene) benzene-cation complexes. The T1 values for d6-benzene decreased with increasing concentrations of LiCl, NaCl, KCl, RbCl, CsCl, and AgNO3, with the largest effects observed in the AgNO3 and CsCl solutions. Upon normalizing tau(c) values by solution viscosity effects, an overall affinity trend of Ag+ >> Cs+ > K+ > Rb+ > Na+ > Li+ was derived for the d6-benzene-cation complexes. The ability of Ag+ to complex d6-benzene was significantly reduced upon addition of NH3, which strongly coordinates Ag+ at high pH. Results with d6-benzene, d8-naphthalene, d2-dichloromethane, and d12-cyclohexane in 0.1 M methanolic salt solutions confirmed that spin-lattice relaxation rates are characterizing cation-pi interactions. The relatively strong cation-pi bonding observed between Ag+ and aromatic hydrocarbons probably results from covalent interactions between the aromatic pi electrons and the d orbitals of Ag+, in addition to the normal electrostatic interaction.

Cation-pi bonding: a new perspective on the sorption of polycyclic aromatic hydrocarbons to mineral surfaces.

Recent molecular modeling and spectroscopic studies have suggested that relatively strong interactions can occur between aromatic pi donors and metal cations in aqueous solutions. The objective of this study was to characterize potential cation-pi interactions between pi donors and exchangeable cations accumulated at mineral surfaces via both spectroscopic and batch sorption methods. Quadrupolar splitting in deuterium nuclear magnetic resonance ((2)H NMR) spectroscopy for d(2)-dichloromethane, d(6)-benzene, and d(8)-toluene (C(6)D(5)- moiety) in aqueous suspensions of a Na-saturated reference montmorillonite unambiguously indicated the ordering of solute molecules with respect to the clay surface. The half line broadening (Delta nu(1/2)) of (2)H NMR of d(6)-benzene in montmorillonite suspensions showed that soft exchangeable cations generally resulted in more benzene sorption compared with harder cations (e.g., Ag(+) > Cs(+) > Na(+) > Mg(2+), Ba(2+)). In batch sorption experiments, saturating minerals (e.g., porous silica gels, kaolinite, vermiculite, montmorillonite) with a soft transition metal or softer base cations generally increased the polycyclic aromatic hydrocarbon (PAH) sorption relative to harder cations (e.g., Ag(+) >> Cs(+) > K(+) > Na(+); Ba(2+) > Mg(2+)). Sorption of phenanthrene to Ag(+)-saturated montmorillonite was much stronger compared with 1,2,4,5-tetrachlorobenzene, a coplanar non-pi donor having slightly higher hydrophobicity. In addition, a strong positive correlation was found between the cation-dependent sorption and surface charge density of the minerals (e.g., vermiculite, montmorillonite >> silica gels, kaolinite). These results, coupled with the observations in (2)H NMR experiments with montmorillonite, strongly suggest that cation-pi bonding forms between PAHs and exchangeable cations at mineral surfaces and affects PAH sorption to hydrated mineral surfaces.

Effects of ferrous iron and molecular oxygen on chromium(VI) redox kinetics in the presence of aquifer solids.

The kinetics and stoichiometry of the reduction of hexavalent chromium (Cr(VI)) with ferrous iron (Fe(II)) were examined in systems with and without aquifer solids. Cr(VI) reduction was rapid in the absence of solids, but demonstrated slower and more complex kinetics in the presence of aquifer solids. The aquifer solids removed Fe(II) from solution and a portion of the reducing capacity of Fe(II) was transferred to the aquifer solids. The solid phases were then able to continue to remove Cr(VI). This suggests in situ treatment of Cr(VI) by Fe(II) injection would be feasible in the aquifer environment. In general, re-oxidation of reduced chromium by molecular oxygen was not observed in our systems over time periods of nearly 1 year, suggesting that the potential for chromium solubilization under these oxidizing conditions will be low. An empirical model was developed to describe the reduction kinetics of Cr(VI) in the presence of solids. The model assumes that the reaction is brought about by pseudo-species of iron that react instantaneously, rapidly and slowly with Cr(VI). A fourth pseudo-species is assumed to be non-reactive. Model coefficients were determined by non-linear regression. The model was able to describe observed concentrations of chromium well, but analysis of model errors indicated the potential existence of a distribution of species with different reaction rates rather than just three distinct species. Another model was developed to predict concentrations of different pseudo-species depending on the total amount of Fe(II) added and the amount of aquifer solids present. This model assumed that pseudo-species could result from intrinsic characteristics of the aquifer solids as well as being formed by addition of Fe(II), which could sorb to the aquifer solids.

Combined HPLC/HPSEC study of Suwannee River Fulvic Acid adsorptive fractionation on α-aluminum oxide.

A novel liquid chromatographic (LC) method with repeated injections of Suwannee River Fulvic Acid (SRFA) was used to investigate its adsorptive fractionation by synthetic α-Al(2)O(3). Eluent (i.e., non-retained) SRFA for each injection was monitored by two ultraviolet (UV) absorbance detection channels (300 and 365 nm) and one fluorescence detection channel (λ(ex)=350 nm, λ(em)=450 nm). Preferential adsorption of SRFA constituents was revealed by the different responses of the three LC detection channels. Samples of non-retained SRFA from injections of three independent replicate experiments were collected and aggregated for subsequent analysis by steady state ultraviolet-visible (UV/vis) absorption spectrometry and size exclusion chromatography (SEC). The ratio of absorbance at 254 and 204 nm, a surrogate for specific UV absorbance at 254 nm, increased with increasing injection number for the non-retained SRFA, indicating the preferential adsorption of SRFA constituents containing aromatic moieties. SEC analysis confirmed the preferential adsorption of higher molecular weight (MW) SRFA constituents as the non-adsorbed SRFA fractions increased in MW across the series of injections. The SEC results also suggested that certain SRFA constituents in the ca. 2-5 kDa MW range adsorbed in early injections were displaced by higher MW species (ca. 5-10 kDa) in later injections.

Competition between kaolinite flocculation and stabilization in divalent cation solutions dosed with anionic polyacrylamides.

Divalent cations have been reported to develop bridges between anionic polyelectrolytes and negatively-charged colloidal particles, thereby enhancing particle flocculation. However, results from this study of kaolinite suspensions dosed with various anionic polyacrylamides (PAMs) reveal that Ca(2+) and Mg(2+) can lead to colloid stabilization under some conditions. To explain the opposite but coexisting processes of flocculation and stabilization with divalent cations, a conceptual flocculation model with (1) particle-binding divalent cationic bridges between PAM molecules and kaolinite particles and (2) polymer-binding divalent cationic bridges between PAM molecules is proposed. The particle-binding bridges enhanced flocculation and aggregated kaolinite particles in large, easily-settleable flocs whereas the polymer-binding bridges increased steric stabilization by developing polymer layers covering the kaolinite surface. Both the particle-binding and polymer-binding divalent cationic bridges coexist in anionic PAM- and kaolinite-containing suspensions and thus induce the counteracting processes of particle flocculation and stabilization. Therefore, anionic polyelectrolytes in divalent cation-enriched aqueous solutions can sometimes lead to the stabilization of colloidal particles due to the polymer-binding divalent cationic bridges.