T. David Waite

Kinetics of iron complexation by dissolved natural organic matter in coastal waters

Abstract In coastal waters, where conditions are variable and can change rapidly, the kinetics of iron complexation by NOM is a major factor affecting its speciation. In this study we have examined the kinetics of complexation of ferrous and ferric iron by terrigenous NOM in terms of formation rate constants and dissociation rate constants by employing competitive ligand methods with visible spectrophotometry for determination of the complexed iron. Rate constants for organic ferrous complex formation ranged from 500 to 7.5×10 4 M −1 s −1 , and rate constants for complex dissociation ranged from 1×10 −6 to 3.6×10 −3 s −1 . Formation rate constants for organic ferric complexes were comparable to those determined for strong iron binding ligands found in the open oceans, and from 2.1×10 5 to 9.6×10 7 M −1 s −1 . Ferric complex dissociation data were fitted by a two ligand model, with rate constants for both ligand classes typically higher than those for the strong ligands in the open ocean. Rate constants varied from 2×10 −4 to 4×10 −3 s −1 for the ‘weak’ ligand class, and from 1.0×10 −6 to 1.3×10 −4 s −1 for the ‘strong’ class. All rate constants varied by several orders of magnitude between NOM samples of different origin, reflecting the highly variable composition of these substances. Calculated conditional stability constants for organic complexes with ferric species in seawater, Fe′, were generally similar to those measured in the open oceans. Conditional stability constants for the ferrous complexes were less than those for the ferric complexes by two to four orders of magnitude. There was a weak positive correlation between the conditional stability constants for ferrous and ferric complexes, suggesting that similar functional groups are involved in binding each of the two forms of iron. One of the samples of NOM had a conditional stability constant with Fe′ that was comparable with those for siderophores and similar strong iron binding compounds. These results suggest that in the absence of oxidants, complexes between ferrous iron and NOM may be relatively long-lived. Our work also suggests that organic complexes between iron and terrigenous NOM may be quite strong, and will have a major effect on iron solubility in coastal waters.

Silver-modified mesoporous TiO2 photocatalyst for water purification

Mesoporous anatase (TiO(2)) was modified with silver (Ag) nanoparticles using a photoreduction method. Performance of the resulting TiO(2)-Ag nanocomposites for water purification was evaluated using degradation of Rhodamine B (RhB) and disinfection of Escherichia coli (E. coli) under ultraviolet (UV) irradiation. The composites with different Ag loadings were characterized using physical adsorption of nitrogen, X-ray diffraction, X-ray photoelectron spectroscopy and UV-Visible diffuse reflectance spectroscopic techniques. The results showed that metallic Ag nanoparticles were firmly immobilized on the TiO(2) surface, which improved electron-hole separation by forming the Schottky barrier at the TiO(2)-Ag interface. Photocatalytic degradation of RhB and inactivation of E. coli effectively occurred in an analogical trend. The deposited Ag slightly decreased adsorption of target pollutants, but greatly increased adsorption of molecular oxygen with the latter enhancing production of reactive oxygen species (ROSs) with concomitant increase in contaminant photodegradation. The optimal Ag loadings for RhB degradation and E. coli disinfection were 0.25 wt% and 2.0 wt%, respectively. The composite photocatalysts were stable and could be used repeatedly under UV irradiation.

Methods for reactive oxygen species (ROS) detection in aqueous environments

This review summarizes direct and indirect analytical methods for the detection and quantification of the reactive oxygen species (ROS): 1O2, O2·−/HOO·, H2O2, HO·, and CO3·− in aqueous solution. Each section briefly describes the chemical properties of a specific ROS followed by a table (organized alphabetically by detection method, i.e., absorbance, chemiluminescence, etc.) summarizing the nature of the observable (associated analytical signal) for each method, limit of detection, application notes, and reaction of the probe molecule with the particular ROS.

A changing framework for urban water systems.

Urban water infrastructure and the institutions responsible for its management have gradually evolved over the past two centuries. Today, they are under increasing stress as water scarcity and a growing recognition of the importance of factors other than the cost of service provision are forcing a reexamination of long-held ideas. Research and development that supports new technological approaches and more effective management strategies are needed to ensure that the emerging framework for urban water systems will meet future societal needs.

Superoxide-mediated formation and charging of silver nanoparticles.

Contemporary studies indicate that reactive oxygen species (ROS) such as superoxide play a key role in the toxicity and behavior of silver nanoparticles (AgNPs). While there have been suggestions that superoxide is able to reduce silver(I) ions with resultant production of AgNPs, no experimental evidence that this process actually occurs has been produced. Here we present definitive experimental evidence for the reduction of silver(I) by superoxide. A second-order rate constant of 64.5 ± 16.3 M(-1)·s(-1) is determined for this reaction in the absence of AgNPs. The overall rate constant, however, increases by at least 4 orders of magnitude in the presence of AgNPs. A model based on electron charging and discharging of AgNPs satisfactorily describes the kinetics of this process. The ability for AgNPs to undergo catalytic cycling provides a pathway for the continual generation of ROS and the regeneration of AgNPs following oxidation.

Iron uptake and toxin synthesis in the bloom-forming Microcystis aeruginosa under iron limitation

Toxin production during cyanobacterial blooms poses a significant public health threat in water bodies globally and requires the development of effective bloom management strategies. Previously, synthesis of the hepatotoxin microcystin has been proposed to be regulated by iron availability, but the contribution of the toxin to the adaptation of cyanobacteria to environmental stresses, such as changing light intensity and nutrient limitation, remains unclear. The aim of this study was to compare the iron stress response in toxic and non-toxic strains of Microcystis aeruginosa subjected to moderate and severe iron limitation. The transcription of a number of genes involved in iron uptake, oxidative stress response, toxin synthesis and transcriptional control of these processes was accessed by quantitative real-time PCR (qRT-PCR). The process of adaptation of M. aeruginosa to iron stress was found to be highly dynamic and strain-specific. Toxin production in PCC 7806 increased in an iron-dependent manner and appeared to be regulated by FurA. The inability to produce microcystin, either due to natural mutations in the mcy gene cluster or due to insertional inactivation of mcyH, affected the remodelling of the photosynthetic machinery in iron-stressed cells, the transport of Fe(II) and transcription of the Fur family of transcriptional regulators. The presence of the toxin appears to give an advantage to microcystin-producing cyanobacteria in the early stages of exposure to severe iron stress and may protect the cell from reactive oxygen species-induced damage.

Silver Nanoparticle—Algae Interactions: Oxidative Dissolution, Reactive Oxygen Species Generation and Synergistic Toxic Effects

The short-term toxicity of citrate-stabilized silver nanoparticles (AgNPs) and ionic silver Ag(I) to the ichthyotoxic marine raphidophyte Chattonella marina has been examined using the fluorometric indicator alamarBlue. Aggregation and dissolution of AgNPs occurred after addition to GSe medium while uptake of dissolved Ag(I) occurred in the presence of C. marina. Based on total silver mass, toxicity was much higher for Ag(I) than for AgNPs. Cysteine, a strong Ag(I) ligand, completely removed the inhibitory effects of Ag(I) and AgNPs on the metabolic activity of C. marina, suggesting that the toxicity of AgNPs was due to the release of Ag(I). Synergistic toxic effects of AgNPs/Ag(I) and C. marina to fish gill cells were observed with these effects possibly attributable to enhancement in the generation of reactive oxygen species by C. marina on exposure of the organism to silver.

Kinetics and mechanisms of ultrasonic degradation of volatile chlorinated aromatics in aqueous solutions

Ultrasonic decompositions of chlorobenzene (ClBz), 1,4-dichlorobenzene and 1-chloronaphthalene were investigated at 500 kHz in order to gain insight into the kinetics and mechanisms of the decomposition process. The disappearance of ClBz on sonication is almost simultaneously accompanied by the release of chloride ions as a result of the rapid cleavage of carbon-chlorine bonds with a concomitant release of CO, C2H2, CH4 and CO2. The intermediates resulting from attack of HO. radicals were detected but in a quite low yield (less than 2 microM). The generation of H2O2 on sonolysis is not significantly affected by the presence of aqueous ClBz while the generation of NO2- and NO3- is inhibited initially due to the presence of ClBz which diffuses into the gas-bubble interfaces and inhibits the interactions between free radicals and nitrogen. Moreover, brown carbonaceous particles are present throughout the ultrasonic irradiation process, which are consistent with soot formation under pyrolytic conditions. These important features suggest that, at the relatively high initial substrate concentrations used in this study, ultrasonic degradation of ClBz takes place predominantly both within the bubbles and within the liquid-gas interfaces of bubbles where it undergoes high-temperature combustion. Under these conditions, the oxidation of ClBz by free radical HO. outside of bubbles is a minor factor (though results of recent studies suggest that attack by HO. is more important at lower initial substrate concentrations). The sonochemical decomposition of volatiles follows pseudo-first-order reaction kinetics but the degradation rates are affected by operating conditions, particularly initial substrate concentration and ultrasonic intensity.

H2O2-Mediated Oxidation of Zero-Valent Silver and Resultant Interactions among Silver Nanoparticles, Silver Ions, and Reactive Oxygen Species

The H(2)O(2)-mediated oxidation of silver nanoparticles (AgNPs) over a range of pH (3.0-14.0) is investigated here, and an electron charging-discharging model capable of describing the experimental results obtained is developed. AgNPs initially react with H(2)O(2) to form Ag(+) and superoxide, with these products subsequently reacting to reform AgNPs (in-situ-formed AgNPs) via an electron charging-discharging mechanism. Our experimental results show that the AgNP reactivity toward H(2)O(2) varies significantly with pH, with the variation at high pH (>10) due particularly to the differences in the reactivity of H(2)O(2) and its conjugate base HO(2)(-) with AgNPs whereas at lower pH (3-10) the pH dependence of H(2)O(2) decay is accounted for, at least in part, by the pH dependence of the rate of superoxide disproportionation. Our results further demonstrate that the in-situ-formed AgNPs resulting from the superoxide-mediated reduction of Ag(+) have a different size and reactivity compared to those of the citrate-stabilized particles initially present. The turnover frequency for AgNPs varies significantly with pH and is as high as 1776.0 min(-1) at pH 11.0, reducing to 144.2 min(-1) at pH 10.0 and 3.2 min(-1) at pH 3.0.

Role of Gelling Soluble and Colloidal Microbial Products in Membrane Fouling

The mechanism underlying gel layer formation on membrane surfaces from soluble and colloidal microbial products (SCMPs) produced under unfavorable operational conditions for membrane bioreactors (MBRs) has been investigated using supernatants from a bench-scale MBR. SCMPs can be grossly classified into gelling and nongelling SCMPs with the gelling fraction associated mostly with the polysaccharide content. The significant role played by multivalent metals in gel formation through metal-ligand complexation has been confirmed. Functional groups of the gelling SCMPs were determined by pH titration and zeta potential measurement as amine/phenolic sites (pK(a) 9.3 and 8.0), carboxylic sites (pK(a) 6.6, 4.9, and ca. 4.0), and phosphoric sites (pK(a) ca. 2.5). The carboxylic sites were more directly involved with multivalent cation complexation; however, the gelling propensity of the SCMP dispersion was minimally affected by pH change in the circum-neutral pH range, suggesting that the strong carboxylic sites were principally responsible for gel formation. The SCMPs demonstrated a high potential for gel formation given the high density of the strong carboxylic acid groups of about 0.44 mmol/g-TOC and a moderate calcium binding stability constant of about 4.9 x 10(3) M(-1).