Effect Of Solution Ionic Strength Research Articles

Adsorption characteristics of Congo Red on coal-based mesoporous activated carbon

Adsorption of Congo Red dye (CR) on bituminous coal-based mesoporous activated carbon (AC) from aqueous solutions was studied. The ACs used differed significantly in terms of total surface area, pore volume distribution and surface charge properties. The mesopore contribution to the total pore volume ranged from 52 to 83%. The adsorption tests were performed under static conditions at solution pH 7.8–8.3. The pH at the point of zero charge (pH PZC) for ACs used was over 10. It was found that the higher the fraction of mesopores with a size between 10 and 50 nm, the shorter the time to achieve the equilibrium stage for CR adsorption. The kinetics of adsorption in view of three kinetic models, i.e. the first-order Lagergren model, the pseudo-second-order model and the intraparticle diffusion model, was discussed. The pseudo-second-order kinetic model describes the adsorption of CR on mesoporous activated carbon very well. The correlation coefficients ranged from 0.980 to 0.991. The intraparticle diffusion into small mesopores was found to be the rate-limiting step in the adsorption process. The equilibrium adsorption data were interpreted using Langmuir and Freundlich models. The adsorption of CR was better represented by the Langmuir equation. The monolayer adsorption capacity of ACs was found to increase with increasing both the mesopore volume and the mesopore contribution to their porous texture. The effect of solution ionic strength on the uptake of CR by two different mesoporous carbons was also investigated.

Mechanical Properties of Interfacial Films Formed by Lysozyme Self-Assembly at the Air−Water Interface

We present the first characterization of the mechanical properties of lysozyme films formed by self-assembly at the air-water interface using the Cambridge interfacial tensiometer (CIT), an apparatus capable of subjecting protein films to a much higher level of extensional strain than traditional dilatational techniques. CIT analysis, which is insensitive to surface pressure, provides a direct measure of the extensional stress-strain behavior of an interfacial film without the need to assume a mechanical model (e.g., viscoelastic), and without requiring difficult-to-test assumptions regarding low-strain material linearity. This testing method has revealed that the bulk solution pH from which assembly of an interfacial lysozyme film occurs influences the mechanical properties of the film more significantly than is suggested by the observed differences in elastic moduli or surface pressure. We have also identified a previously undescribed pH dependency in the effect of solution ionic strength on the mechanical strength of the lysozyme films formed at the air-water interface. Increasing solution ionic strength was found to increase lysozyme film strength when assembly occurred at pH 7, but it caused a decrease in film strength at pH 11, close to the pI of lysozyme. This result is discussed in terms of the significant contribution made to protein film strength by both electrostatic interactions and the hydrophobic effect. Washout experiments to remove protein from the bulk phase have shown that a small percentage of the interfacially adsorbed lysozyme molecules are reversibly adsorbed. Finally, the washout tests have probed the role played by additional adsorption to the fresh interface formed by the application of a large strain to the lysozyme film and have suggested the movement of reversibly bound lysozyme molecules from a subinterfacial layer to the interface.

Permeability and Electrokinetic Characterization of Poly(ethylene terephthalate) Capillary Pore Membranes with Grafted Temperature-Responsive Polymers

Poly(ethylene terephthalate) (PET) track-etched membranes with average pore diameters of 692 and 1629 nm were functionalized using the monomer N-isopropylacrylamide (NIPAAm) and a photoinitiated "grafting-from" approach in which a surface-selective reaction has been most efficiently achieved by combinations of the unmodified PET surface with benzophenone and, alternatively, of an aminated PET surface with benzophenone carboxylic acid. Consistent estimations of the pore diameters of the base PET membranes and of the effective grafted polyNIPAAm layer thicknesses on the PET pore walls were possible only on the basis of the permeabilities measured with aqueous solutions of higher ionic strength (e.g., 0.1 M NaCl). However, the permeabilities measured with ultrapure water indicated that the "electroviscous effect" was significant for both base membranes. The influences of membrane pore diameter, surface charge, and solution ionic strength could be interpreted in the framework of the space-charge model. Functionalized membranes with collapsed grafted polymer hydrogel layer thicknesses of a few nanometers exhibited almost zero values of the zeta potential estimated from the trans-membrane streaming potential measurements. This was caused by a "hydrodynamic screening" of surface charge by the neutral hydrogel. Very pronounced changes in permeability as a function of temperature were measured for PET membranes with grafted polyNIPAAm layers, and the effective layer thickness in the swollen state--here up to approximately 300 nm--correlated well with the degree of functionalization. The subtle additional effects of solution ionic strength on the hydrodynamic layer thickness at 25 degrees C were different from the effects for the base PET membranes and could be explained by a variation in the degree of swelling, resembling a "salting-out" effect. Overall, it had been demonstrated that the functionalized capillary pore membranes are well suited for a detailed and quantitative evaluation of the relationships between the synthesis, the structure, and the function of grafted stimuli-responsive polymer layers.

Probing the Stability of Biocompatible Sodium Hyaluronate/Chitosan Nanocoatings Against Changes in Salinity and pH

The stability of polyelectrolyte multilayer assemblies was investigated with emphasis on the effects of solution ionic strength, pH, and polymer molecular weight on the film thickness and surface topography. The multilayers consisting of two polysaccharides, the polyanion sodium hyaluronate (HA) and the polycation chitosan (CH) were studied using surface plasmon resonance (SPR) spectroscopy, impedance quartz crystal microbalance (QCM), and atomic force microscopy (AFM). SPR/QCM experiments show that coatings consisting of four HA/CH bilayers assembled at pH 4.5 in the presence of 0.15 M NaCl are stable in NaCl solutions of concentration less than 0.8 M. These multilayers are stable when placed in contact with aqueous solutions ranging in pH from 3.5 to 9. The molecular weight of the polysaccharides has only a marginal effect on the stability of the films in the range explored here (HA: Mn = 360,000 or 31,000 g/mol; CH: Mn = 160,000 or 30,000 g/mol). AFM imaging reveals that different mechanisms may account for the multilayers stability versus salt and pH treatments. While increasing the ionic strength induces reorganization of the surface topography from isolated spherical islets to elongated worm-like features, changes in pH have no appreciable effects on the coating topography prior to complete disintegration.

Ion Adsorption at the Rutile−Water Interface: Linking Molecular and Macroscopic Properties

A comprehensive picture of the interface between aqueous solutions and the (110) surface of rutile (alpha-TiO2) is being developed by combining molecular-scale and macroscopic approaches, including experimental measurements, quantum calculations, molecular simulations, and Gouy-Chapman-Stern models. In situ X-ray reflectivity and X-ray standing-wave measurements are used to define the atomic arrangement of adsorbed ions, the coordination of interfacial water molecules, and substrate surface termination and structure. Ab initio calculations and molecular dynamics simulations, validated through direct comparison with the X-ray results, are used to predict ion distributions not measured experimentally. Potentiometric titration and ion adsorption results for rutile powders having predominant (110) surface expression provide macroscopic constraints of electrical double layer (EDL) properties (e.g., proton release) which are evaluated by comparison with a three-layer EDL model including surface oxygen proton affinities calculated using ab initio bond lengths and partial charges. These results allow a direct correlation of the three-dimensional, crystallographically controlled arrangements of various species (H2O, Na+, Rb+, Ca2+, Sr2+, Zn2+, Y3+, Nd3+) with macroscopic observables (H+ release, metal uptake, zeta potential) and thermodynamic/electrostatic constraints. All cations are found to be adsorbed as "inner sphere" species bonded directly to surface oxygen atoms, while the specific binding geometries and reaction stoichiometries are dependent on ionic radius. Ternary surface complexes of sorbed cations with electrolyte anions are not observed. Finally, surface oxygen proton affinities computed using the MUSIC model are improved by incorporation of ab initio bond lengths and hydrogen bonding information derived from MD simulations. This multitechnique and multiscale approach demonstrates the compatibility of bond-valence models of surface oxygen proton affinities and Stern-based models of the EDL structure, with the actual molecular interfacial distributions observed experimentally, revealing new insight into EDL properties including specific binding sites and hydration states of sorbed ions, interfacial solvent properties (structure, diffusivity, dielectric constant), surface protonation and hydrolysis, and the effect of solution ionic strength.

The hydroformylation of 4-penten-1-ol and 3-buten-1-ol in water with HRh(CO)(TPPTS)3 and the effects of solution ionic strength

Abstract The reaction chemistry and kinetics of the low temperature hydroformylation of the water-soluble substrates, 4-penten-1-ol and 3-buten-1-ol, in aqueous solution utilizing the catalyst HRh(CO)(TPPTS)3 is reported where, TPPTS is trisulphonated triphenylphosphine. The reactions were carried out at relatively low temperature and high stirring rates (1700 rpm) to maintain similar rates of mass transfer of gases into the aqueous phase. Activation parameters and reaction selectivity for the hydroformylation of 4-penten-1-ol are found to be dependent on solution ionic strength. At [Rh]=5×10−4 M, an activation energy for the hydroformylation of 4-penten-1-ol of 23 kcal mol−1 is estimated. As sodium sulfate is added to the catalytic phase the activation energy increases. The dependence of reaction selectivity on ionic strength is unexpected. The reaction can be directed to yield a product distribution of modest linearity (75%) or an exceptionally high ratio of the branched product, observed exclusively as a cyclic 2-hydroxy-3-methyltetrahydropyran (98%), by control of solution ionic strength and temperature. A wider range of selectivities can be obtained for 4-penten-1-ol in water with HRh(CO)(TPPTS)3 than can be obtained in toluene with HRh(CO)(PPh3)3. The hydroformylation of 3-buten-1-ol is less sensitive to reaction conditions. An activation energy of 25 kcal mol−1 is estimated at [Rh]=2.5×10−4 M. Reaction selectivity favors in this case the formation of a six-membered over a five-membered cyclic acetal and is not influenced by temperature or solution ionic strength. It appears that the hydroxy group can direct the site of CO addition to alkenols, perhaps by chelate formation, and that solution ionic strength affects its ability to do so.

Interfacial Dissociation and Unfolding of Glucose Oxidase

The effect of solution ionic strength on the dissociation and unfolding of glucose oxidase (GOx) adsorbed at the air/water interface has been studied by specular neutron reflection. The structure and composition of the GOx layers was determined at pH 7 under null reflecting water (NRW) comprised of D2O and H2O with a molar ratio close to 1:11. Under this isotopic contrast, the specular reflectivity arises entirely from the adsorbed layer. At the lower ionic strength of 4.5 mM, GOx adsorption forms a uniform layer of some 30 Å thick. The layer thickness tends to increase with bulk GOx concentration and eventually reaches the short axial length of the enzyme's globular monomer structure of 37 Å. These results suggest that while the enzyme is dissociated into monomers it does not unfold further into the distributions characteristic of random polypeptide segments. At the higher ionic strength of 0.45 M, the thickness of the GOx layers is between 40 and 50 Å when the bulk GOx concentration is below 0.4 μM. This dimension is close to the short axial length of the globular dimer of 52 Å, suggesting that at the higher ionic strength the enzyme adsorbs with its long 80 Å axis parallel to the surface of water. As the concentration is increased above 1 μM, a secondary layer starts to form on the solution side of the main surface layer, leading to a bilayer adsorption of some 50 Å for each layer. Although the inner solution layer only contains some 5−8% of the adsorbed material, neutron reflection is sufficiently accurate to detect its structural dimension. Further neutron measurements under D2O suggest that the dissociated monomers and the dimers have different extent of immersion into the aqueous subphase. This observation is consistent with the different surface activities of the two species, which are related to the distributions of amino acid groups on their outer surfaces.

Mixed salts—scaling limits and propensity

The main aim of desalination is removal of dissolved species from water either by thermal or membrane processes. In either process, the saline solution becomes concentrated, and once the concentration of salts exceeds its saturation limit, they form an unwanted deposit referred to as a fouled or scale layer. Even though precipitating salts coexist in industrial water systems, due to the complexity of the fouling process, research has primarily been concentrated on a single salt precipipation. The effect of other ionic species has been taken into account through the solution's ionic strength and use of the Debye Huckle Theory or a modified version of that. This paper critically assesses the current practices and consolidates the recent and ongoing work done by the author and her students [1–5] in thermodynamics and scaling limits in mixed salt solutions. The results indicate that the thermodynamics of single salt crystallization are not applicable to practical conditions of feed water when mixtures of salts are present. It is shown in various systems that the interactive effects of coexisting salts and the presence of a common ion cannot be ignored. Co-precipitation of salts with or without a common ion affects the solubility constants of the salts, and therefore application of the solubility data obtained under a single salt precipitation are questionable in mixed salt precipitation. Even the presence of minute amounts of another precipitating salt affects the thermodynamics and the scale structure and strength, which renders inapplicable the solubility data for the pure salts. The effect depended on both the dominance and type of salt. It should be emphasized that these results were obtained despite the fact that these assessments took into account the effect of solution ionic strength. Thus, considering the effect of ionic strength—though necessary—is not sufficient in mixed solutions. The presence of one salt does indeed affect the structure of the other salt. The effect of other salts on the scale morphology should not be ignored, and the author hypothesizes that to be the reason for inapplicability of the pure salt thermodynamics to that of mixed solutions. This paper establishes through thermodynamic relationships and through the Gibbs free energy of formation that the thermodynamic solubility constant of a salt in pure solution is different from that in a mixed solution if the morphology is affected by the presence of other salts.

Thermodynamics and kinetics for mixed calcium carbonate and calcium sulfate precipitation

The effects of CaSO 4 on CaCO 3 precipitation were studied in the batch tests at 60, 70 and 80°C in mixtures having calcium carbonate as the dominant salt and at a given total initial calcium concentration of 0.03 M with sulfate concentration ranging from 0 to 0.01 M . Solubility products and rate constants were determined from thermodynamic and kinetic studies and the results indicated that even minute amounts of calcium sulfate affect the thermodynamics, kinetics and the scale structure and no longer the solubility data and rate constants for the pure salt were applicable. Presence of CaSO 4 from 0.002 to 0.01 M increased the calcium carbonate solubility product more than an order of magnitude. The effect of salt mixture on the solubility constant of non-dominant salt (calcium sulfate) was reverse as calcium sulfate solubility increased to its pure value with increases in its molar ratio. In addition, the rate equation suggested in the literature (J. Colloid Interface Sci. 37 (1971) 824; J. Colloid Interface Sci. 36 (1971) 166) for pure salt was not applicable to the experimental data. The general observations indicated that the presence of CaSO 4 had weakened the CaCO 3 scale which is usually very adherent. The experimental results did take into account the effect of solution ionic strength, however, they suggest that data for pure salt precipitation seem not to be extendable to co-precipitation.

Efeito da força iônica da solução de equilíbrio sobre a adsorção/dessorção de chumbo em Latossolos brasileiros

A adsorção/dessorção de metais pelos solos é afetada por vários fatores, dentre os quais, a concentração salina da solução do solo. Este estudo teve o objetivo de avaliar o efeito da variação da força iônica da solução de equilíbrio (I) sobre a adsorção/dessorção de Pb por Latossolos brasileiros. Amostras do horizonte A foram colocadas para reagir com Pb(NO3)2 0,15 mmol L-1 em Ca(NO3)2 5 e 50 mmol L-1 (pH 5,5; I = 15 e 150 mmol L-1; relação solo:solução 1:100), durante 72 horas. Em seguida, as amostras foram colocadas para dessorver em Ca(NO3)2 5 mmol L-1. Dos 17 Latossolos, apenas seis apresentaram diferenças significativas entre as quantidades adsorvidas nos dois valores de I. As correlações significativas obtidas entre as quantidades adsorvidas e alguns atributos dos solos (SiO2 e Fe2O3 do ataque sulfúrico, Fe d e Fe o, caulinita, hematita, CTC e superfície específica) foram as mesmas para os dois valores de I. A variação de I durante a adsorção teve pouco efeito sobre a dessorção de chumbo. A não-variação da quantidade adsorvida com a mudança de I indica que o Pb é adsorvido, na maioria dos Latossolos, como complexo de esfera interna. Conclui-se que, dentro destas condições experimentais, o Pb não fez parte do complexo de troca da maioria dos solos estudados.

Adhesion of waste water bacteria to reverse osmosis membranes

A stirred cell was used to study initial adhesion of three sewage bacteria belonging to the genus Pseudomonas to the three reverse osmosis (RO) membranes BW30, PVD and CAB2, and the nanofiltration membrane NF45. Membranes were immersed in suspensions containing 10 8 bacteria/ml for 10 min. All three strains were capable of rapidly colonising the four membranes, but to different extents. It was found that bacteria would sometimes aggregate upon adhering to particular RO membranes. The effects of solution ionic strength and pH, and conditioning of membranes (by prior exposure to filtrates of treated and untreated sewage) on the number of adherent bacteria were investigated. Minimal bacterial attachment occurred in a very low ionic strength milieu (deionised water). Salt concentrations corresponding to waste water and to twice that concentration resulted in significantly higher but statistically similar numbers of attached microbes. Adhesion of the three isolates was not affected by pH in the range of 4–8. The number of bacteria attaching to the membranes could be increased or reduced by conditioning films of sewage origin, conditioning films could also trigger or inhibit aggregation of adherent cells. Some surface properties of the membranes (roughness, hydrophobicity) and bacterial cells (electrophoretic mobility, functional groups by affinity chromatography) were also investigated.

Solid-Phase Microextraction of PAHs from Aqueous Samples Using Fibers Coated with HPLC Chemically Bonded Silica Stationary Phases

In this study, porous layer solid-phase microextraction (SPME) was investigated for the extraction of polycyclic aromatic hydrocarbons (PAHs) from water. The porous layer coatings were prepared using silica particles (5 μm diameter) bonded with phenyl, C8, and monomeric and polymeric C18 stationary phases for use in HPLC. It was found that several factors affected the selectivity for extraction of PAHs, including functional group in the bonded phase, alkyl chain length of the bonded phase, and phase type (monomeric or polymeric). The distribution coefficients of PAHs in the porous layer increased with an increasing number of carbon atoms, and greater selectivity toward solute molecular size and shape were obtained using a polymeric C18 porous layer. The effect of solution ionic strength on extraction was also investigated. A phenyl bonded phase porous layer fiber was used to extract PCB congeners from Arochlor 1254, and headspace SPME was applied to the analysis of contaminated soil samples.

Retention of imazaquin in soil

AbstractThe molecular complexity of imazaquin and presence of ionizable functional groups limits the ability to predict sorption behavior from single soil parameters such as organic carbon content. Partition coefficients (Kp) for both neutral and anionic forms of imazaquin as well as the effects of solution ionic strength and composition were investigated to more adequately describe sorption of imazaquin in soil. Soils representing a range of characteristics were evaluated, including soils with permanent negative or variable surface charge. Imazaquin retention resulted from combined sorption for the neutral (Kocn, 1,110 ± 80 L/kg) and anionic (Koc,a, 38 ± 20 L/kg) forms. Imazaquin sorption was best correlated to soil organic carbon content and soil‐solution pH. However, results indicated that positively charged Fe2+ and Al3+ oxyhydroxides contribute to sorption of the organic anion; thus mineral surfaces contributed to sorption in soils with low organic carbon content. The effects of electrolyte matrices on imazaquin sorption were accounted for by concomitant changes in pH. However, enhanced imazaquin sorption was observed with increasing ionic strength for soils where pH‐induced changes in speciation were negligible, indicating the role of mechanisms other than weak hydrophobic interactions. Addition of H2 PO significantly decreased imazaquin sorption, especially in weathered soils.

Gibbs-Donnan-Based Interpretations of the Hydrolysis Behavior of Zeolites

A careful investigation of the effect of solution ionic strength on the ion-exchange behavior of zeolites that was conducted by R. Harjula and co-workers demonstrated that mass-action-based formulations failed to anticipate the equilibria encountered once the ionic strength of the solution in contact with the zeolite was reduced to ≤1.0 x 10 -3 M. The results of this research have been analyzed employing Gibbs-Donnan-based concepts in an attempt to understand better the nature of this phenomenon. Complications introduced by zeolite hydrolysis reactions became amenable to analysis through application of the Donnan model. Whereas this led to a more knowledgeable assessment of the hydrolysis phenomenon than the earlier empirical treatment by Harijula and co-workers, failure to anticipate ion-exchange behavior at low ionic strengths still persisted. Reasonable rationalization of this failure, unavailable to the mass-action-based formulation used earlier by Harijula and co-workers, was, however, provided by the Gibbs-Donnan model.

Effect of Solution Ionic Strength on Analyte Charge State Distributions in Positive and Negative Ion Electrospray Mass Spectrometry

The variability of ESMS charge state distributions was investigated for a number of analytes when the ionic strength of their solutions was altered over a wide range. Solution ionic strength was adjusted by adding varying amount of electrolytes (NH 4 OAc, CsCl or (n-C 4 H 9 ) 4 N + Cl - ) to protein solutions of myoglobin and lysozyme. Measured solution QH values were found to be nearly constant over the entire range of CsCl and (n-C 4 H 9 ) 4 N + Cl - addition, and a slight increase in QH was observed upon NH 4 OAc addition. Analyte peak intensities in positive ion ESMS reproducibly decreased as the solution ionic strength increased. However, analyte charge state distributions remained quite constant desite 4 orders of magnitude variation in electrolyte concentration

Effect of Solution Ionic Strength and Iron Coatings on Mineral Grains on the Sorption of Bacterial Cells to Quartz Sand

Understanding the interaction between bacterial cells and solid surfaces is essential to our attempts to quantify and predict the transport of microbes in groundwater aquifers, whether from the point of view of contamination or from that of bioremediation. The sorption of bacterial cells suspended in groundwater to porous medium grains was examined in batch studies. Bacterial sorption to clean quartz sand yielded equilibrium, linear, adsorption isotherms that varied with the bacterial strain used and the ionic strength of the aqueous solution. Values of K(d) (the slope of the linear sorption isotherm) ranged from 0.55 to 6.11 ml g, with the greatest sorption observed for the highest groundwater ionic strength. These findings are consistent with the interpretation that an increasingly compressed electrical double layer results in stronger adsorption between the like-charged mineral surface and the bacterial cells. When iron-oxyhydroxide-coated sand was used, however, all of the added bacteria were adsorbed up to a threshold of 6.93 x 10 cells g of coated sand, beyond which no further adsorption occurred. The irreversible, threshold adsorption is the result of a strong electrostatic attraction between the sesquioxide coating and the bacterial cells. Experimental results of adsorption in mixtures of quartz and Fe(III)-coated sand were successfully predicted by a simple additive model for sorption by the two substrate phases. Even small amounts of Fe(III)-coated sand in a mixture influenced the extent of adsorption of bacterial cells. A quantitative description of adsorption in the mixtures can be realized by using a linear isotherm for reversible adsorption to the quartz grains with a y intercept that represents the number of cells irreversibly adsorbed to the Fe(III)-coated sand.

Effect of ionic environment on BSA filtration and the properties of BSA deposits

Although many current applications of membrane filtration involve the removal, concentration, or exchange of electrolytes, there is still no fundamental understanding of the effects of ionic composition and concentration on the performance of these filtration processes. We have obtained experimental data for the effect of ionic species on the hydraulic permeability of bovine serum albumin deposits formed during filtration. The permeability of the protein deposits decreased with increasing solution ionic strength at pH both above and below the protein isoelectric point but was relatively independent of ionic strength at the isoelectric point. The permeability of the BSA deposits at neutral pH was markedly smaller when measured with solutions of divalent cations than with solutions of monovalent cations but was unaffected by the valence of the anions. This dependence on ionic strength and ion valence was consistent with a simple model in which the protein layer permeability is determined by the balance between the compressive pressure associated with the filtration and the electrostatic repulsion between the negatively charged proteins in the deposit. These results were then combined with available data for the albumin diffusivity and osmotic pressure to examine the effects of solution ionic strength on the flux during protein filtration.

Effect of solution ionic strength on lipid peroxidation initiation by the perhydroxyl (xanthine oxidase-derived) and peroxyl radicals.

The role of solution ionic strength in perhydroxyl (HOO.) and peroxyl (ROO.) radical initiated lipid peroxidation has been defined and investigated. Xanthine oxidase activity was used as the source of superoxide (O2-) and its conjugate acid (HOO.) in these experiments. While the enzyme's activity varied with changes in ionic strength, the effect could be factored out of the lipid peroxidation studies. Both HOO.- and ROO.-initiated peroxidations of linoleic acid were promoted by increases in solution ionic strength: the inclusion of 0.1 M of various alkali metal salts in the reaction resulted in up to a 4-fold increase in the overall peroxidation rate. Significant differences between alkali metal cations (Li+, Na+, K+, Cs+) and halogen anions (F-, Cl-, Br-) were not observed. Thus, the increased rates of lipid peroxidation were attributable to changes in solution ionic strength rather than specific ion-reaction interactions. Ionic stimulation of lipid peroxidation occurred only in the presence of preexisting fatty acid hydroperoxides (LOOHs), which provided additional support for the hydrogen atom transfer mechanism previously proposed [Aikens, J., and Dix, T. A. (1991) J. Biol. Chem. 266, 15091] for the HOO./LOOH initiation process. Physiologically appropriate salt concentrations were used in these studies, hence the results may have biological significance.