Matthew A. Tarr

Quantitation of hydroxyl radical during Fenton oxidation following a single addition of iron and peroxide

Chemical probes were used to study the formation of hydroxyl radical in aqueous iron-hydrogen peroxide reaction. Hydroxyl radical formation rate and time dependent concentration were determined in pure water, in aqueous fulvic acid (FA) and humic acid (HA) solutions, and in natural surface waters. Indirect determinations of hydroxyl radical were made by quantitating hydroxyl radical reactions with probe compounds under controlled conditions. High probe concentrations were used to determine radical formation rates and low probe concentrations were used to determine time dependent radical concentration. Two independent probes were used for intercomparison: benzoic acid and 1-propanol. Good agreement between the two probes was observed. Natural water matrices resulted in lower radical formation rates and lower hydroxyl radical concentrations, with observed formation rate and yield in natural waters up to four times lower than in pure water. HA and FA also reduced hydroxyl radical formation under most conditions, although increased radical formation was observed with FA at certain pH values. Hydroxyl radical formation increased linearly with hydrogen peroxide concentration.

Enhanced Fenton degradation of hydrophobic organics by simultaneous iron and pollutant complexation with cyclodextrins

The effectiveness and selectivity of Fenton degradation of hydrophobic organic compounds (HOCs) can be improved by simultaneous complexation of Fe(2+) and the organic compound with a cyclodextrin or derivatized cyclodextrin. Such selective complexation of a target substrate and a catalytic metal is a crude mimic of enzyme systems. Both beta-cyclodextrin and carboxymethyl-beta-cyclodextrin (CMCD) were able to simultaneously complex Fe(2+) and an aromatic hydrocarbon, such as phenol, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls (PCBs). Degradation of compounds included in cyclodextrins was unaffected by hydroxyl radical scavengers, indicating that the radical was formed at the ternary complex (HOC-cyclodextrin-iron) and in close proximity to the included molecule. Without cyclodextrins, humic acid (HA) decreased degradation efficiency. However, in the presence of CMCD, HA did not inhibit degradation of the target compound. CMCD is capable of removing HOCs from HA binding sites while at the same time complexing Fe(2+). PCBs sorbed to glass were resistant to Fenton degradation, but were significantly degraded using a cyclodextrin modified Fenton system. In all of these systems, the ternary HOC-cyclodextrin-iron complexes effectively direct hydroxyl radical reaction toward the HOC, increasing the efficiency of Fenton degradation. One potential application of such targeted degradation systems is the in situ remediation of hydrophobic organic pollutants in contaminated soil and groundwater or in industrial waste streams.

Effect of fire on soil-atmosphere exchange of methane and carbon dioxide in Canadian boreal forest sites

During the spring and summer of 1994 we monitored soil-atmosphere exchanges of methane and carbon dioxide at upland sites in the Canadian boreal forest near the northern study area (NSA) of the Boreal Ecosystem-Atmosphere Study (BOREAS). The effects of fire on methane and carbon dioxide exchange in black spruce stands developed on clay soils were evaluated by measuring fluxes with dark chambers in unburned stands and stands burned in 1994, 1992, and 1987. Similar measurements were made in jack pine stands developed on sandy soils, one unburned and the other burned in 1989. All of the sites were net sinks of atmospheric methane with median fluxes ranging from −0.3 to −1.4 mg CH4-C m−2 d−1. Median fluxes of carbon dioxide from the forest floor to the atmosphere ranged between 1 and 2 g C m−2 d−1. Both ecosystem characteristics (e.g., soil and vegetation type) and burning history (time since burn and fire intensity) appear to have some effect on atmospheric methane consumption and carbon dioxide emission by these forest soils. In general, the jack pine sites were stronger methane sinks and had lower carbon dioxide emissions than the black spruce sites. After a few years of recovery, the burned sites tended to be slightly stronger methane sinks than unburned controls. Our results suggest that soil CO2 effluxes from upland black spruce stands may not be immediately impacted by fire, possibly maintained at preburn levels by microbial decomposition of labile compounds released as a result of the fire. By 2 years postfire there appears to be a significant reduction in soil CO2 flux, due to the loss of tree root and moss respiration and possibly to the depletion of fire-related labile compounds. The observed recovery of soil respiration rates to preburn levels by 7 years postburn is probably due to the respiration of regrowing vegetation and the combined effects of elevated soil temperatures (about 4° to 5°C warmer than unburned sites) and improved litter quality on soil microbial activities. We estimate that soil CO2 emissions from recently burned boreal forest soils in the northern hemisphere could be of the order of 0.35 Pg C yr−1, which is in good agreement with a previous estimate that was derived in a different manner.

Characterization of an NF-κB-regulated, miRNA-146a-mediated down-regulation of complement factor H (CFH) in metal-sulfate-stressed human brain cells

Micro RNAs (miRNAs) represent a family of small ribonucleic acids (RNAs) that are post-transcriptional regulators of messenger RNA (mRNA) complexity. Brain cells maintain distinct populations of miRNAs that support physiologically normal patterns of expression, however, certain miRNA abundances are significantly altered in neurodegenerative disorders such as Alzheimers disease (AD). Here we provide evidence in human neural (HN) cells of an aluminum-sulfate- and reactive oxygen species (ROS)-mediated up-regulation of an NF-kappaB-sensitive miRNA-146a that down-regulates the expression of complement factor H (CFH), an important repressor of inflammation. This NF-kappaB-miRNA-146a-CFH signaling circuit is known to be similarly affected by Abeta42 peptides and in AD brain. These aluminum-sulfate-inducible events were not observed in parallel experiments using iron-, magnesium-, or zinc-sulfate-stressed HN cells. An NF-kappaB-containing miRNA-146a-promoter-luciferase reporter construct transfected into HN cells showed significant up-regulation of miRNA-146a after aluminum-sulfate treatment that corresponded to decreased CFH gene expression. These data suggest that (1) as in AD brain, NF-kappaB-sensitive, miRNA-146a-mediated, modulation of CFH gene expression may contribute to inflammatory responses in aluminum-stressed HN cells, and (2) underscores the potential of nanomolar aluminum to drive genotoxic mechanisms characteristic of neurodegenerative disease processes.

Direct carbon monoxide photoproduction from plant matter

Initial studies to quantify direct carbon monoxide photoproduction from several plant species are reported. In addition to measuring CO emissions from live plant leaves, emission rates from dead leaf matter were also determined. Senescent leaf matter photoproduced CO at rates that ranged from 1.3 to 5.4 times higher per unit area than living leaves, and dead leaves photoproduced CO about an order of magnitude more rapidly than living leaves. It may therefore be necessary to incorporate CO photoproduction from dead plant matter into predictions of global CO emissions from plants. Methods are presented for direct measurement of CO photoproduction from live, intact leaves, from excised leaves, and from fallen leaves. Although these techniques were initially used for laboratory studies, they are directly applicable to field studies. Results of mechanistic studies indicate that oxygen affects CO photoproduction but that carbon dioxide exhibits no direct influence. Formation of CO was shown to be the result of direct photochemical transformation on or in the plant matter. Furthermore, for live plant leaves, CO photoproduction was observed to occur internal to the leaf.

Gold Nanoparticle−Quantum Dot−Polystyrene Microspheres as Fluorescence Resonance Energy Transfer Probes for Bioassays

The paper describes the development of highly sensitive particle-based fluorescence resonance energy transfer (FRET) probes that do not use molecular fluorophores as donors and acceptors. In these probes, CdSe/ZnS luminescent quantum dots (QDs) were capped with multiple histidine-containing peptides to increase their aqueous solubility while maintaining their high emission quantum yield and spectral properties. The peptide-modified QDs (QD-His) were covalently attached to carboxyl-modified polystyrene (PS) microspheres to form highly emitting PS microspheres (QD-PS). Gold nanoparticles (AuNPs) were then covalently attached to the QD-PS surface to form AuNP-QD-PS composite microspheres that were used as FRET probes. Attachment of AuNPs to QD-PS completely quenched the QD emission through FRET interactions. The emission of QD-PS was restored when the AuNPs were removed from the surface by thiol ligand displacement. The new AuNP-QD-PS FRET platform is simple to prepare and highly stable, and it opens many new possibilities for carrying out FRET assays on microparticle-based platforms and in microarrays. The versatility of these assays could be greatly increased by replacing the linkers between the QDs and AuNPs with ones that selectively respond to specific cleaving agents or enzymes.

Inhibited hydroxyl radical degradation of aromatic hydrocarbons in the presence of dissolved fulvic acid

Abstract Degradation of aqueous aromatic compounds with hydroxyl radical produced by Fenton chemistry is inhibited by dissolved fulvic acid. The degree of inhibition is significantly greater than that expected, based on a simple model in which the fraction of aromatic compound bound to fulvic acid is considered to be unreactive. The reaction of three aromatic compounds (phenol, fluorene, and phenanthrene) with hydroxyl radical was monitored as a function of Suwannee River fulvic acid concentration. Steady state hydroxyl radical concentration was produced by Fenton chemistry (H 2 O 2 +Fe 2+ →Fe 3+ +HO − +HO·) with continuous addition of hydrogen peroxide to Fe 2+ solutions. Separation of the hydroxyl radical formation sites from the location of the aromatic compound is believed to be the reason for the observed reduction in rate constants, indicating that these systems are heterogeneous on a microenvironmental scale. The larger than expected inhibition of Fenton degradation by fulvic acid indicates that natural organic matter presents a significant impediment to remediation of pollutants in natural waters and soils.

Mechanisms of ammonia and amino acid photoproduction from aquatic humic and colloidal matter.

Photochemical release of free amino acids was observed from dissolved fulvic acid (Suwannee River) and from colloidal fractions collected from Bayou Trepagnier, LA. Water samples were irradiated with a solar simulator, and free amino acid concentrations were determined using high performance liquid chromatography of the fluorescent derivitized amino acids. Increased concentrations of at least 20 amines were observed upon irradiation of water samples. Among the amino acids identified were alanine, asparagine, citrulline, glutamic acid, histidine, norvaline, and serine. Amino acid concentrations increased in the range of 0.03-9.5 nM h(-1). Studies on the mechanism of photochemical release of ammonia from dissolved natural organic matter (NOM) indicated at least two mechanisms. One mechanism proceeds through an hydroxyl radical intermediate. This mechanism continues in the dark after irradiation through decomposition of photochemically produced H2O2 to form hydroxyl radical. Although NOM photosensitized degradation of amino acids produces ammonia, amino acids do not appear to be an important intermediate in the photochemical formation of ammonia from NOM.

Aqueous sonolytic decomposition of polycyclic aromatic hydrocarbons in the presence of additional dissolved species.

Sonochemical degradation of aqueous polycyclic aromatic hydrocarbons (PAHs) results in a first-order loss of the PAHs (k = 0.010-0.027 s-1). When sonication occurred in the presence of other organic compounds, the degradation rate constant was reduced quite dramatically. This reduction is believed to come about through scavenging of radicals by the matrix chemical. When oxygen was bubbled into the PAH solution before sonication, the degradation rate constant was elevated. Nitrogen purging resulted in decreased rate constants. These results indicate that oxygen was an important precursor in the degradation of the PAHs. Organic compounds, including humic acid, benzoic acid, and sodium dodecyl sulfate, decreased PAH degradation rate constants by scavenging oxygen derived reactive transients.

Dissolved organic matter inhibition of sonochemical degradation of aqueous polycyclic aromatic hydrocarbons

Sonochemical degradation of aqueous polycyclic aromatic hydrocarbons (PAHs) was found to be rapid in the absence of other dissolved compounds (k = 0.006-0.015 s-1). In the presence of 20 mg Cl-1 fulvic acid, first-order PAH degradation rate constants decreased from 2.3- to 3.7-fold. Similar results were obtained with added benzoic acid, a crude analog for fulvic acid. In natural waters, PAH degradation was almost completely inhibited. Analysis of the kinetic behavior and reaction products indicates that PAHs are most likely degraded through a radical cation mechanism. Hydroxyl radical appeared to play an insignificant role in the degradation. Inhibited degradation was probably the result of either altered cavitation processes or isolation of the PAH away from cavitation sites.