Biomedical subjects

T David Waite

Publications and source records attributed to T David Waite.

At least 19 recordsLinked to original sources

PubMed2006

Process optimization of fenton oxidation using kinetic modeling.

In the remediation, water, and wastewater industries, an appropriate understanding of the chemical reactions governing the Fenton system allows the development of kinetic models to help design and optimize the performance and efficiency of treatment processes. In this work a rigorous kinetic model describing substrate oxidation by Fenton's reagent, following validation by comparison with experimental data, is extended and applied to provide insight and gain information regarding optimum initial conditions, solution environment, and operating regimes for the decomposition of a target contaminant. The effect of variables such as initial molar ratios of H202 to Fe(II), H202 dosing regimes, solution pH, and the presence or absence of oxygen on the rate and efficiency of contaminant degradation is presented and discussed in light of the reactions involved. Model simulations of the oxidation of various organic species demonstrate the significant role organic radicals and oxidation byproducts can have on treatment performance. An appropriate understanding of the oxidation pathway of the target organic and the reactions of degradation products is essential for the accurate application and use of the kinetic model for design and optimization purposes.

Ferrous Compounds↗

PubMed2006

Optimized parameters for fluorescence-based verification of ballast water exchange by ships.

Mid-ocean ballast water exchange is mandatory for ships discharging foreign ballast in US territorial waters in order to reduce the risk of biological invasions. However, a reliable tool for determining whether the procedure took place is lacking. We investigated chromophoric dissolved organic matter (CDOM) fluorescence as a tracer of mid-ocean exchange on nine research cruises out of Asia, Europe, and the USA, focusing on challenging source conditions (high salinity, low CDOM). Using parallel factor analysis, we identified nine independent fluorescent components present in varying concentrations in the ocean and in ballast water. One component was sufficient for predicting the coastal vs oceanic source of most ballast water samples. Across nine cruises, thresholds (1.7 and 0.7 ppb quinine sulfate equivalent units) at two fixed wavelength pairs (lambda(ex)/lambda(em) = 320/414 and 370/496 nm, respectively) discriminated coastal from oceanic ballast water in > 95% of samples (N = 514). Our results suggest that single- and dual-channel fluorometers could be optimized for verifying ballast water exchange.

Fluorescence↗

PubMed2006

Superoxide-mediated dissolution of amorphous ferric oxyhydroxide in seawater.

We have investigated the kinetics of superoxide-mediated dissolution of amorphous ferric oxyhydroxide (AFO) in seawater by spectrophotometrically examining the rate of formation of a ferrous-ferrozine complex (Fe(II)(FZ)3) with particular attention given to the effect of aging and iron concentration on the rate of superoxide-mediated dissolution of AFO. The production rates of Fe(II)(FZ)3 decreased with aging of AFO for iron concentrations from 50 to 500 nM, indicating that changes to the chemical and physical properties of AFO affected the reactivity of inorganic ferric iron species with superoxide. A kinetic model developed by assuming that Fe(II)' formation is preceded by the thermal dissolution of AFO provided a good description of the Fe(II)(FZ)3 production rates over time. First-order rate constants for Fe(II)' formation were found to depend on the total iron concentration, suggesting that superoxide-mediated Fe(II) reduction is affected not only by the rate of thermal dissolution of AFO but also by the rate of AFO precipitation. The reported high rates of superoxide production by both photochemical and biotic pathways in aquatic systems coupled with the ability of superoxide to dissolve freshly formed ferric oxides suggest that this process may have a significant impact on the biogeochemical cycling of iron, especially if organisms have an affinity for ferrous iron.

Ferric Compounds↗

PubMed2005

Sonolysis of 4-chlorophenol in aqueous solution: effects of substrate concentration, aqueous temperature and ultrasonic frequency.

The sonolysis of 4-chlorophenol (4-CP) in O2-saturated aqueous solutions is investigated for a variety of operating conditions with the loss of 4-CP from solution following pseudo-first-order reaction kinetics. Hydroquinone (HQ) and 4-chlorocatechol (4-CC) are the predominant intermediates which are degraded on extended ultrasonic irradiation. The final products are identified as Cl-, CO2, CO, and HCO2H. The rate of 4-CP degradation is dependent on the initial 4-CP concentration with an essentially linear increase in degradation rate at low initial 4-CP concentrations but with a plateauing in the rate increase observed at high reactant concentrations. The results obtained indicate that degradation takes place in the solution bulk at low reactant concentrations while at higher concentrations degradation occurs predominantly at the gas bubble-liquid interface. The aqueous temperature has a significant effect on the reaction rate. At low frequency (20 kHz) a lower liquid temperature favours the sonochemical degradation of 4-CP while at high frequency (500 kHz) the rate of 4-CP degradation is minimally perturbed with a slight optimum at around 40 degrees C. The rate of 4-CP degradation is frequency dependent with maximum rate of degradation occurring (of the frequencies studied) at 200 kHz.

Journal Article↗

PubMed2005

Impact of natural organic matter on floc size and structure effects in membrane filtration.

Hematite (10 mg of Fe/L) floc-humic acid assemblages have been formed at pH 4 either by first aggregating hematite particles with salt (100 mM KCl) and then adding humic acid (salt-particle-organic or SPO assemblages) or by suspending the hematite particles in humic acid solutions and then adding salt to induce aggregation (organic-particle-salt or OPS assemblages). The behavior of these assemblages upon deposition on microfiltration (MF) membranes has then been investigated. In the OPS case, the fractal dimension (dF) of the assemblages formed varied dramatically depending upon the extent of charge neutralization by added fulvic acid with dF values typical of diffusion-limited cluster aggregates at low (0.1-0.2 mg/L) humic acid concentrations and dF values typical of reaction-limited cluster aggregates either in the absence of humic acid or concentrations greater than 0.4-0.6 mg/L. In the SPO case, dF values on the order of 2.1 were initially observed and were found to decrease to around 1.8-1.9 for humic acid concentrations greater than 0.6-0.8 mg/L. OPS assemblages with low fractal dimensions were found to be highly compressible once deposited on MF membranes with significantly higher specific cake resistances than was the case for SPO assemblages at transmembrane pressures of 50 kPa and above. These results highlight the importance of both the choice of coagulant (e.g., preformed vs formed in situ) and the transmembrane pressure to which a membrane filtration process might be allowed to rise prior to removal of the fouling layer.

Benzopyrans↗

PubMed2005

Fenton-mediated oxidation in the presence and absence of oxygen.

The increased use of Fenton systems for the treatment of contaminated waters and wastewaters necessitates the development of kinetic models capable of accurately simulating key species concentrations in order to optimize system performance and efficiency. In this work a reaction mechanism in which the hydroxyl radical is nominated to be the active oxidant in Fenton systems is used to describe the oxidation of formic acid (HCOOH) under a variety of experimental conditions. A kinetic model based on this reaction mechanism is shown to adequately describe results of experiments in which starting concentrations of H202 and HCOOH varied over 1 and 4 orders of magnitude, respectively, under both air-saturated and deaerated conditions. The intermediate generated during HCOOH oxidation was observed to increase oxidation efficiency, especially at high initial organic concentrations [relative to Fe(II)], by assisting in the redox cycling of iron. In the presence of oxygen, however, such improvement was attenuated through competition for the organic intermediates. While mechanistic analysis and associated kinetic modeling is invaluable in optimization of Fenton systems, a clear understanding of reaction byproducts and their reactivity toward other species in the system is critical for accurate simulations.

Hydrogen Peroxide↗

PubMed2005

Use of superoxide as an electron shuttle for iron acquisition by the marine cyanobacterium Lyngbya majuscula.

Reduction of iron from the ferric state to the ferrous state is one strategy employed by microorganisms in nearneutral environments to increase its biological availability. In recent years, the existence of mobile reducing agents produced bymicroorganismsto promote iron reduction, known as electron shuttles, has been demonstrated. Production of electron shuttles has been shown for several organisms, employing a variety of mostly organic molecules as the electron carrier. Here we show that the coastal cyanobacterium Lyngbya majuscula produces iron-reducing superoxide radicals (02*-) and that this facilitates increased iron uptake. We suggest that superoxide is a useful electron shuttle because it reacts rapidly and almost indiscriminately with Fe(lll)-organic complexes and its precursor, dissolved oxygen, is ubiquitous in the photic zone. We further suggest that, for these reasons, the generation of superoxide by marine oxygenic photosynthetic microorganisms and its use in facilitating iron uptake may be a reasonably widespread process.

Cyanobacteria↗

PubMed2005

Reduction of organically complexed ferric iron by superoxide in a simulated natural water.

Superoxide (and potentially its conjugate acid hydroperoxyl) is unique among the reactive oxygen species in that its standard redox potential in circumneutral natural waters potentially allows it to reduce ferric iron to the more soluble ferrous state. Here we have observed the superoxide/ hydroperoxyl-mediated reduction of ferric complexes with a variety of synthetic organic ligands and several complexes with natural organic matter (NOM), as well as freshly precipitated amorphous ferric oxyhydroxide, in bicarbonate buffered solutions at pH 8.1. From measurements of superoxide decay in the presence of the complexes, we calculated second-order rate constants for superoxide/ hydroperoxyl-mediated reduction that vary from (9.3+/-0.2) x 10(3) M(-1) s(-1) for the complex between Fe(III) and desferrioxamine B up to (1.9+/-0.2) x 10(5) M(-1) s(-1) for Fe(III)-salicylate and (2.3+/-0.1) x 10(5) M(-1) s(-1) for one of the Fe(III)-NOM complexes. We also verified that ferrous iron was produced from superoxide/hydroperoxyl-mediated Fe(III) reduction using ferrozine to trap free Fe(II). Low yields of the ferrozine complex when compared to the measured rates of superoxide decay suggest that ferric complexes are reduced directlyto corresponding ferrous complexes, with much of the ferrous complex reoxidizing before it is able to release free ferrous iron. This is an important consideration for microorganisms, as the kinetics of trace metal uptake is typically governed by free ion activity.

Deferoxamine↗

PubMed2005

Quantification of the oxidizing capacity of nanoparticulate zero-valent iron.

Addition of nanoparticulate zero-valent iron (nZVI) to oxygen-containing water results in oxidation of organic compounds. To assess the potential application of nZVI for oxidative transformation of organic contaminants, the conversion of benzoic acid (BA) to p-hydroxybenzoic acid (p-HBA) was used as a probe reaction. When nZVI was added to BA-containing water, an initial pulse of p-HBA was detected during the first 30 min, followed by the slow generation of additional p-HBA over periods of at least 24 h. The yield of p-HBA increased with increasing BA concentration, presumably due to the increasing 'ability of BA to compete with alternate oxidant sinks, such as ferrous iron. At pH 3, maximum yields of p-HBA during the initial phase of the reaction of up to 25% were observed. The initial rate of nZVI-mediated oxidation of BA exhibited a marked reduction at pH values above 3. Despite the decrease in oxidant production rate, p-HBA was observed during the initial reaction phase at pH values up to 8. Competition experiments with probe compounds expected to exhibit different affinities for the nZVI surface (phenol, aniline, o-hydroxybenzoic acid, and synthetic humic acids) indicated relative rates of reaction that were similar to those observed in competition experiments in which hydroxyl radicals were generated in solution. Examination of the oxidizing capacity of a range of Fe0 particles reveals a capacity in all cases to induce oxidative transformation of benzoic acid, but the high surface areas that can be achieved with nanosized particles renders such particles particularly effective oxidants.

Hydrogen-Ion Concentration↗

PubMed2004

Oxidative degradation of the carbothioate herbicide, molinate, using nanoscale zero-valent iron.

Degradation of the carbothiolate herbicide, molinate, has been investigated in oxic solutions containing nanoscale zero-valent iron particles and found to be effectively degraded by an oxidative pathway. Both ferrous iron and superoxide (or, at pH < 4.8, hydroperoxy) radicals appearto be generated on corrosion of the zero-valent iron with resultant production of strongly oxidizing entities capable of degrading the trace contaminant.

Azepines↗

PubMed2003

Effect of dissolved natural organic matter on the kinetics of ferrous iron oxygenation in seawater.

We have investigated the kinetics of Fe(II) oxygenation in seawater in the presence of a variety of natural organic materials obtained from vegetation near Moreton Bay, Queensland. Natural organic matter (NOM) was observed mostly to accelerate Fe(II) oxygenation, but in some cases oxidation was retarded. We fitted a previously developed kinetic model to the experimental data to determine the critical rate constants, kf for the formation of Fe(II)--NOM complexes and kox for the oxygenation of the Fe(II)--NOM complexes, when assumed to be first order with respect to both the concentration of Fe(II) and the dissolved O2. Analysis of the critical model reactions indicated that the process is in general non-pseudo-first-order but approaches pseudo-first-order under certain conditions. These limiting conditions are rapidly approached, which makes it difficult to determine unique values of kf and kox from oxidation data alone. Both parameters varied considerably between the different samples of NOM, with the value of kox ranging from about 2 up to 1000 M(-1) s(-1) as compared with 13 M(-1) s(-1) for inorganic Fe(II) in seawater. Despite large assumptions, the values of kox calculated were consistent with a linear free energy relation.

Iron↗

PubMed2003

Kinetics of hydrolysis and precipitation of ferric iron in seawater.

We have investigated the kinetics of iron hydrolysis and precipitation and dissolution of the solid formed via a novel chemical method, namely accessibility of iron to the fungal siderophore desferrioxamine B (DFB), with spectrophotometric detection of the ferrioxamine complex. Our results support a mechanism in which precipitation of dissolved inorganic iron in seawater is first order with respect to total (dissolved and precipitated) iron, with a second-order rate constant of (4.1 +/- 1.1) x 10(7) M(-1) s(-1) at pH 8.1. The rate of dissolution was first order with respect to the total iron concentration and the first-order rate constant decreased from 2.3 x 10(-4) s(-1) after aging for 1 min to 4.8 x 10(-6) s(-1) after aging for 1 week. The proposed reaction mechanism simulated a solubility limit condition in agreement with experimental observations, from which we calculated the solubility of ferric iron to be 1.2 x 10(-13) M when the precipitate had been aged for 1 week. This is approximately 2 orders of magnitude less than reported in previous studies, possibly due to the chemically based method for dissolved iron determination used here compared with traditional physical separation methods. Our results confirm that Fe(III) hydrolysis in seawater is fast and show thatthe precipitated solid is quite labile initially but rapidly becomes much less so, with important implications for sequestration by organic ligands such as siderophores.

Chemical Precipitation↗

PubMed2003

Adsorption of the endocrine-active compound estrone on microfiltration hollow fiber membranes.

Results of studies reported here show that adsorption could result in considerable accumulation of hormones on hydrophobic hollow fiber membrane surfaces during filtration of trace-hormone-containing feed solutions with a linear adsorption isotherm applicable over the majority of the estrone concentration range examined (2.6-154 ng/L). Models based on both diffusion and surface reaction limitation were used to describe the kinetics of estrone adsorption to the membranes tested. Results indicate that the rate of adsorption of estrone to the hollow fiber membranes was limited principally by surface reaction rate rather than the rate of diffusive transport to membrane surface sites. Both adsorption and desorption kinetics were satisfactorily described by pseudo-first-order expressions. These results are of environmental significance, especially in drinking water applications, where contaminants such as natural and synthetic hormones may accumulate on the membranes and desorb during backwashing and membrane cleaning.

Adsorption↗

PubMed2003

Kinetic modeling of TiO2-catalyzed photodegradation of trace levels of microcystin-LR.

A kinetic model has been developed to investigate the relative importance of major pathways for the photocatalytic degradation of trace levels of the cyanobacterial toxin microcystin-LR (MLR) in solutions containing a complex suite of dissolved organic matter and to test the sensitivity of MLR degradation to rate constants of the key processes. The kinetic model incorporates adsorption of the trace contaminant, other organics and oxygen on the particle surface, surface reactions between adsorbed radical and nonradical species, desorption of surface radical species, solution phase radical reactions, and radical termination pathways. Under conditions where the contaminant adsorbs strongly to semiconductor surface sites, rapid degradation is observed, and a primary degradation step appears to involve reaction between surface-located long-lived organic radicals (formed from hydroxyl radical scavenging by the bulk organic) and adsorbed trace contaminant. MLR degradation is relatively insensitive to changes in light intensity under these strongly adsorbing conditions but highly dependent under weakly adsorbing conditions and when solution phase degradation is important. While not verified independently, desorption of surface bound superoxide appears to lead to the production of organic peroxyl radicals through reaction of superoxide with the bulk organic. These solution phase organic peroxyl radicals are highly reactive and appear to be the primary source of trace contaminant degradation under conditions where the trace contaminant shows no observable adsorption and surface degradation is negligible. Under alkaline conditions, adsorption of carbonate onto the particle surface results in scavenging of surface hydroxyl radicals to form surface carbonate radicals that rapidly quench surface bound superoxide. This prevents organic peroxyl production, the primary agent of solution-phase trace contaminant degradation.

Adsorption↗

PubMed2002

Charge effects in the fractionation of natural organics using ultrafiltration.

Comparison of two commonly used techniques for molecular weight determination of natural organics, ultrafiltration (UF) fractionation and high-performance size exclusion chromatography (SEC), shows that neither technique gives absolute measures of molecular weight. Investigations of International Humic Substances Society standard humic and fulvic acids as well as natural organic matter concentrated from surface freshwaters show that charge effects and solution conditions are important in both SEC and UF fractionation with various components of the natural organics being affected differently. Membranes with a smaller molecular weight cutoff (MWCO) produce permeates with a lower UV/DOC ratio, suggesting that the more aromatic components of natural organics are removed by the lower molecular weight cutoff membranes. Variation in ionic strength has little effect on the rejection of humic acid fractions but does significantly influence the rejection of low molecular weight acids. pH and organic concentration do not affect DOC rejection significantly over the pH range of 4.5-10 and the DOC concentration range of 15-60 mgL(-1). These results indicate that UF should not be applied for quantitative "size" analysis unless performed under well-defined conditions. If performed under conditions appropriate to water treatment, UF fractionation can give information of direct applicability to treatment such as the MWCO required to achieve significant organics removal.

Benzopyrans↗

PubMed2002

Kinetic model for Fe(II) oxidation in seawater in the absence and presence of natural organic matter.

A detailed kinetic model has been developed to describe the oxidation of Fe(II) in seawater in both the absence and the presence of natural organic material. Experimental data were collected using a luminol chemiluminescence-based method to measure Fe(II), assuming that both the inorganic and the organically complexed species were detected. In the absence of organic matter, the data were modeled based on the Haber-Weiss mechanism with the inclusion of a back-reaction of Fe(III) with superoxide and precipitation of Fe(OH)3. Both reactions were found to be significant using sensitivity analysis. When organic matter is present, the model was extended by organic complexation of Fe(II) and Fe(III) with the creation of a parallel oxidation pathway for Fe(II). Fe(II) oxidation at natural (nanomolar) concentrations was accurately predicted for a range of organic concentrations. The model also accounted for scavenging of superoxide by sub-nanomolar levels of dissolved copper and by organic matter when present. The presence of a relatively strong Fe(III) binding ligand was observed to significantly increase the rate of Fe(II) oxidation, while ultimately retaining most of the iron in the system in dissolved (organically complexed) form. The complexation reactions and reaction of inorganic and organically bound Fe(II) with oxygen were found to be critical reactions in the system, while Fe(III) hydrolysis became unimportant even at low organic concentrations. The superoxide radical was also observed to have a major role in the cycling of iron due to its ability to act as both an oxidant and a reductant. The model indicates that the rate constant for the reaction of Fe(II) with O2 has generally been underestimated in previous work and that the secondary oxidation of Fe(II) by H2O2 and subsequently OH* plays a relatively minor role in these systems.

Hydroxyl Radical↗

PubMed2002

Photo-Fenton degradation of dichloromethane for gas phase treatment.

A continuous photo-Fenton process has been used for the degradation of gaseous dichloromethane (DCM). By absorbing gaseous DCM into a reactive Fenton mixture, the scrubbing and degradation processes could be completed in the one reactor. Operating with a Dark Fenton solution did not result in removal of DCM any better than simply using MilliQ water. This was because the Fe(II) quickly converted to Fe(III) but was unable to regenerate. After a short time, the Fenton process was no longer operating and the DCM quickly accumulated in the reaction solution, preventing further accumulation due to a decreasing concentration gradient in the reactive solution. However, by using UV light and increasing the retention time from 20 to 50 s, there was sufficient time for the reactive solution to regenerate and continuous operation could achieve at least 65% removal of DCM from the gaseous phase at ambient temperature.

Air Pollution↗

PubMed2002

Effect of pH on the ultrasonic degradation of ionic aromatic compounds in aqueous solution.

The sonolysis of 4-nitrophenol (4-NP) and aniline in O2-saturated aqueous solutions was performed at 610 kHz with ultrasonic power of 25 W and aqueous temperature of 15 +/- 1 degrees C. The initial rate of degradation of both 4-NP and aniline in sonolysis of aqueous media follows pseudo-first-order reaction kinetics. Investigation of the H2O2 generation rate in phosphate buffer media (0.01 M) over the range of pH 2-9 revealed a maximum yield at pH approximately 3.2. The pH, which results in modification of the physical properties (including charge) of molecules with ionisable functional groups, plays an important role in the sonochemical degradation of chemical contaminants. For hydrophilic substrates, the neutral species more easily diffuse to and accumulate at the hydrophobic interface of liquid-gas bubbles in comparison with their corresponding ionic forms. As a consequence, the degradation rate of 4-NP under ultrasonic irradiation decreases with increasing pH. In contrast, the disappearance rate of aniline exhibits a maximum under alkaline conditions due to the high solubility of the ionic anilinium ion and the (potentially) preferential movement of the uncharged form to the interface. Additionally, the rate of reaction of the uncharged aniline molecule (which dominates at pH > 4.6) with hydroxyl radicals is reported to be about three times as fast as the rate of reaction of the cationic anilinium species.

Journal Article↗