The formation of sulfate-green rust through Fe(II) sorption to montmorillonite: Impacts on abiotic nitrate reduction.

Green rust (GR) minerals are generally considered to be effective reductants of pollutants and the electron transfer from aqueous Fe(II) to structural Fe(III) in montmorillonite has recently been discovered to be a pathway to GR formation at pH ∼7.8. In this study, we have further delineated the pH conditions and examined the effect of aqueous sulfate concentrations that allow for the formation of sulfate-GR through this unique pathway. Iron(II) sorption experiments demonstrated that the amount of 'sorbed' Fe(II) on montmorillonite semi-quantitatively transformed into sulfate-GR at pH values ≥7.5 in the presence of environmentally-relevant sulfate concentrations (i.e., 10 mM). However, excess sulfate concentrations (100 mM) resulted in comparatively less Fe(II) sorption and sulfate-GR was only observed to form at pH 8. As such, it was concluded that the degree of Fe(II) sorption to montmorillonite is critical to GR formation when aqueous Fe(II) and montmorillonite co-exist. In contrast to sulfate-GR minerals formed through other pathways (e.g., co-precipitation of dissolved Fe(II) and Fe(III) species), this montmorillonite-synthesized GR was significantly less reactive towards nitrate reduction, with <2.5 % of 0.2 mM nitrate being reduced over a 6-day period. This behaviour was correlated to reduction potential and it was, therefore, concluded that the relatively high reduction potential that occurs in the presence of montmorillonite exerts a significant influence on the rate of nitrate reduction by sulfate-GR to the point that it may not be a competitive process to microbiological nitrate denitrification. As such, the environmental relevance of green rust to nitrate reduction cannot be inferred simply by its presence, but rather the reduction potential of the environmental system in which it is found.

HearNPV susceptibility in Helicoverpa armigera and Helicoverpa punctigera strains resistant to Bt toxins Cry1Ac, Cry2Ab, and Vip3Aa.

Genetically engineered crops expressing insecticidal toxins from Bacillus thuringiensis (Bt) have improved the management of targeted lepidopteran pests and reduced the use of insecticide sprays. These benefits explain an increasing adoption of Bt crops worldwide, intensifying the selection pressure on target species and the risk of resistance. Nucleopolyhedroviruses (NPVs) are effective bioinsecticides against numerous important lepidopteran pests. If Bt-resistant insects are shown to be susceptible to NPVs then these bioinsecticides could be a valuable component of Insecticide Resistance Management (IRM) strategies for Bt crops. We assessed the effectiveness of a Helicoverpa nucleopolyhedrovirus (HearNPV) against several different Bt-resistant strains. Utilising a droplet feeding bioassay we confirmed susceptibility to HearNPV in Helicoverpa punctigera and Helicoverpa armigera larvae resistant to the Bt toxins Cry1Ac, Cry2Ab, and Vip3A. Dual resistant H. punctigera, (Cry1Ac/Cry2Ab, and Cry2Ab/Vip3A) and dual resistant H. armigera (Cry2Ab/Vip3A) were also susceptible to HearNPV. Regardless of their specific resistance profile, Bt-resistant larvae displayed statistically similar lethal concentration (LC50) and lethal time (LT50) responses to HearNPV when compared to Bt-sensitive control insects. These results indicate that Bt-resistant H. armigera and H. punctigera are not cross-resistant to HearNPV. Consequently, the use of HearNPV against these pests may be a valuable tool to an IRM strategy for controlling Bt-resistant populations.

Flow-Electrode CDI Removes the Uncharged Ca-UO2-CO3 Ternary Complex from Brackish Potable Groundwater: Complex Dissociation, Transport, and Sorption.

Unacceptably high uranium concentrations in decentralized and remote potable groundwater resources, especially those of high hardness (e.g ., high Ca2+, Mg2+, and CO32- concentrations), are a common worldwide problem. The complexation of alkali earth metals, carbonate, and uranium(VI) results in the formation of thermodynamically stable ternary aqueous species that are predominantly neutrally charged (e.g ., Ca2(UO2)(CO3)30). The removal of the uncharged (nonadsorbing) complexes is a problematic issue for many water treatment technologies. As such, we have evaluated the efficacy of a recently developed electrochemical technology, termed flow-electrode capacitive deionization (FCDI), to treat a synthetic groundwater, the composition of which is comparable to groundwater resources in the Northern Territory, Australia (and elsewhere worldwide). Theoretical calculations and time-resolved laser fluorescence spectroscopy analyses confirmed that Ca2(UO2)(CO3)30 was the primary aqueous species followed by Ca(UO2)(CO3)32- (at circumneutral pH values). Results under different operating conditions demonstrated that FCDI is versatile in reducing uranium concentrations to <10 µg L-1 with low electrical consumption (e.g ., ∼0.1 kWh m-3). It is concluded that the capability of FCDI to remove uranium under these common conditions depends on the dissociation kinetics of the Ca2(UO2)(CO3)30 complex in the electrical field. The subsequent formation of the negatively charged Ca(UO2)(CO3)32- species results in the efficient transport of uranium across the anion exchange membrane followed by immobilization on the positively charged flow (anode) electrode.

Ligand-mediated contaminant degradation by bare and carboxymethyl cellulose-coated bimetallic palladium-zero valent iron nanoparticles in high salinity environments.

The application of nanoscale zero-valent iron (nZVI) for the degradation of contaminants has been extensively investigated, however, few studies have focused on degradation in high salinity environments. In this study, the ability of bare and carboxymethyl cellulose (CMC)-coated bimetallic Pd-nZVI particles to degrade 33'44'-tetrachlorobiphenyl in high saline water (SW) is examined with particular attention given to the effects of ethylenediaminetetraacetic acid (EDTA) on the rate of degradation. EDTA enhances the reactivity of Pd-nZVI in SW, with evidence provided to link this to the removal of the passivating layer. Additionally, a conceptual model is proposed which provides a quantitative description of the removal of these iron oxide layers in the presence of EDTA. An optimum EDTA to bare Pd-nZVI molar ratio of 0.1 exists, with insufficient EDTA unable to remove the passivating layer whilst excess EDTA results in Fe loss and enhanced agglomeration due to magnetic attraction of the bare Fe(0) particles. In contrast, CMC-coating of Pd-nZVI assemblages actually impedes degradation, despite the coated particles displaying a smaller average size compared to uncoated particles, with even the presence of EDTA in this case not significantly improving degradation. The reduced reactivity in the presence of CMC is primarily attributed to the effect of CMC on the association of Pd with nZVI particles. In particular, the presence of CMC reduced the total amount of Pd incorporated with the stabilized particles compared to the non-stabilized particles. Additionally, the presence of CMC results in less Pd present in its reactive zero-valent oxidation state.

Oxidant Generation Resulting from the Interaction of Copper with Menadione (Vitamin K3)-a Model for Metal-mediated Oxidant Generation in Living Systems.

The oxidation of hydroquinones is of interest both due to the generation of reactive oxygen species (ROS) and to the implications to trace metal redox state. Menadione (MNQ), a typical toxicant quinone used extensively for studying the mechanisms underlying oxidative stress, is known to be an effective source of exogenous ROS. In this study, the kinetics and mechanism of the oxidation of menadiol (MNH2Q, the reduced form of MNQ) in the absence and presence of copper (Cu) over the pH range 6.0-7.5 was examined. The autoxidation rate increased with increasing pH and concentration of O2 and also slightly increased with increasing concentration of MNH2Q and MNQ with Cu shown to play a significant role in catalysing the oxidation of MNH2Q. A kinetic model showed that the mono-deprotonated menadiol, MNHQ-, accounted for the pH dependence of the autoxidation rate. In this proposed mechanism, both MNH2Q and MNHQ- species were oxidized quickly by Cu(II), generating menadione semiquinone (MNSQ•-) and superoxide (O2•-) and the reduced form of Cu, Cu(I). Oxygen not only facilitated the catalytic role of Cu(II) by rapidly regenerating Cu(II) but also effectively removed MSNQ•-, generating the important chain-propagating species O2•-. The model demonstrated that Cu(I) was a significant sink of O2•- resulting in the generation of H2O2 with subsequent generation of highly oxidative intermediates including Cu(III). These results provide considerable insight into the clinical significance of the biological activation and detoxification of MNQ with the kinetic model developed of use in identifying key processes in the generation of harmful oxidants in living systems.

Investigating the effect of ascorbate on the Fe(II)-catalyzed transformation of the poorly crystalline iron mineral ferrihydrite.

The inorganic core of the iron storage protein, ferritin, is recognized as being analogous to the poorly crystalline iron mineral, ferrihydrite (Fh). Fh is also abundant in soils where it is central to the redox cycling of particular soil contaminants and trace elements. In geochemical circles, it is recognized that Fh can undergo Fe(II)-catalyzed transformation to form more crystalline iron minerals, vastly altering the reactivity of the iron oxide and, in some cases, the redox poise of the system. Of relevance to both geochemical and biological systems, we investigate here if the naturally occurring reducing agent, ascorbate, can effect such an Fe(II)-catalyzed transformation of Fh at 25 °C and circumneutral pH. The transformation of ferrihydrite to possible secondary Fe(III) mineralization products was quantified using Fourier transform infrared (FTIR) spectroscopy, with supporting data obtained using X-ray absorbance spectroscopy (XAS) and X-ray diffraction (XRD). Whilst the amount of Fe(II) formed in the presence of ascorbate has resulted in Fh transformation in previous studies, no transformation of Fh to more crystalline Fe(III) (oxyhydr)oxides was observed in this study. Further experiments indicated this was due to the ability of ascorbate to inhibit the formation of goethite, lepidocrocite and magnetite. The manner in which ascorbate associated with Fh was investigated using FTIR and total organic carbon (TOC) analysis. The majority of ascorbate was found to adsorb to the Fh surface under anoxic conditions but, under oxic conditions, ascorbate was initially adsorbed then became incorporated within the Fe(III) (oxyhydr)oxide structure (i.e., co-precipitated) over time.

Effect of Shewanella oneidensis on the Kinetics of Fe(II)-Catalyzed Transformation of Ferrihydrite to Crystalline Iron Oxides.

Iron (oxyhydr)oxides are widespread in natural and engineered systems, potent adsorbents of contaminants and a source of energy for iron-reducing bacteria. Microbial reduction of iron (oxyhydr)oxides results in the formation of Fe(II) which can induce the transformation of these iron minerals, typically from less crystalline to more crystalline forms, affecting the biogeochemical cycling of iron and the behavior of any species adsorbed to the iron (oxyhydr)oxides. Factors influencing the transformation rate of the poorly crystalline iron (oxyhydr)oxide, ferrihydrite, to more crystalline forms in the presence of the iron reducing bacterium Shewanella oneidensis MR-1 are investigated under controlled laboratory conditions in this work. In particular, the amount of Fe(II) produced increased the transformation rate while increasing concentrations of the electron donor, lactate, decreased the rate. Using kinetic parameters determined from abiotic controls, the results of transformation experiments in the presence of Shewanella oneidensis were modeled with this exercise revealing that less goethite and more lepidocrocite formed than expected. Conversely, studies using the Shewanella exudate only, containing biogenic Fe(II), displayed rates of transformation that were satisfactorily modeled using these abiotic control kinetic parameters. This result suggests that the physical presence of the microbes is pivotal to the reduction in ferrihydrite transformation rate observed in the biotic experiments relative to the analogous abiotic controls.

Fe(II) Interactions with Smectites: Temporal Changes in Redox Reactivity and the Formation of Green Rust.

In this study, temporal changes in the redox properties of three 0.5 g/L smectite suspensions were investigated-a montmorillonite (MAu-1) and two nontronites (NAu-1 and NAu-2) in the presence of 1 mM aqueous Fe(II) at pH 7.8. X-ray absorption spectroscopy revealed that the amount of Fe(II) added quantitatively transformed into chloride-green rust (Cl-GR) within 5 min and persisted over 18 days. Over the same time, the reduction potential of all three suspensions increased by 50 to 150 mV to equilibrate at approximately -100 mV vs SHE. The reduction of a model organic contaminant, 4-chloronitrobenzene (4-CINB), also became increasingly slower over time with virtually no 4-CINB reduction being observed after 18 days. There was a strong correlation between reduction potential and the quantity of 4-ClNB reduced by MAu-1, although other factors were likely involved in the decreased redox reactivity observed in the nontronites. It is hypothesized that the temporal increase in reduction potential results from clay mineral dissolution resulting in increased Fe(III) contents in the Cl-GR. These results demonstrate that long-term studies are recommended to accurately predict contaminant management strategies.

Use of fourier transform infrared spectroscopy to examine the Fe(II)-Catalyzed transformation of ferrihydrite.

The Fe(II)-catalyzed transformation of the poorly crystalline Fe(III) oxyhydroxide mineral, ferrihydrite (Fh), to more crystalline Fe(III) mineral species such as magnetite, goethite, and lepidocrocite has been quantitatively evaluated under various conditions using X-ray adsorption spectroscopy (XAS) and Fourier transform infrared (FTIR) spectroscopy. Using the peak height of signature FTIR peaks of sub-micron sized lepidocrocite and goethite references minerals, the FTIR results were comparable to the XAS results within experimental error. This was independent of whether the Fe(II)-catalyzed transformation was initiated by the Fe(III)-reducing bacterium Shewanella oneidensis MR-1 or by added ferrous ammonium sulfate in the presence or absence of lactate. Whilst the use of FTIR has not been previously employed to follow this transformation process, it has advantages relative to XAS including a lower sample requirement (approximately 30-fold lower), greater accessibility and greater safety of operation. Whilst problems with quantifying magnetite in the presence of lepidocrocite were identified in this study using reference Fe(III) oxyhydroxide suspensions, large amounts of magnetite were not produced during transformation under the conditions employed in this study. Reference spectra of lath-like nano-goethite particles (with dimensions of approx. 10 × 50nm) also resulted in higher IR absorbance and a slight red-shift in signature peak positions relative to sub-micron sized goethite particles with this shift potentially affecting the reliable quantification of samples of unknown size. Despite this, good agreement between the XAS and FTIR data for samples containing iron oxides undergoing continuous transformation was obtained suggesting that FTIR may be a convenient, inexpensive means of following such mineral transformations.

Influence of Dissolved Silicate on Rates of Fe(II) Oxidation.

Increasing concentrations of dissolved silicate progressively retard Fe(II) oxidation kinetics in the circum-neutral pH range 6.0-7.0. As Si:Fe molar ratios increase from 0 to 2, the primary Fe(III) oxidation product transitions from lepidocrocite to a ferrihydrite/silica-ferrihydrite composite. Empirical results, supported by chemical kinetic modeling, indicated that the decreased heterogeneous oxidation rate was not due to differences in absolute Fe(II) sorption between the two solids types or competition for adsorption sites in the presence of silicate. Rather, competitive desorption experiments suggest Fe(II) was associated with more weakly bound, outer-sphere complexes on silica-ferrihydrite compared to lepidocrocite. A reduction in extent of inner-sphere Fe(II) complexation on silica-ferrihydrite confers a decreased ability for Fe(II) to undergo surface-induced hydrolysis via electronic configuration alterations, thereby inhibiting the heterogeneous Fe(II) oxidation mechanism. Water samples from a legacy radioactive waste site (Little Forest, Australia) were shown to exhibit a similar pattern of Fe(II) oxidation retardation derived from elevated silicate concentrations. These findings have important implications for contaminant migration at this site as well as a variety of other groundwater/high silicate containing natural and engineered sites that might undergo iron redox fluctuations.

The reduction of 4-chloronitrobenzene by Fe(II)-Fe(III) oxide systems - correlations with reduction potential and inhibition by silicate.

Recent studies have demonstrated that the rate at which Fe(II)-Fe(III) oxyhydroxide systems catalyze the reduction of reducible contaminants, such as 4-chloronitrobenzene, is well correlated to their thermodynamic reduction potential. Here we confirm this effect in the presence of Fe(III) oxyhydroxide phases not previously assessed, namely ferrihydrite and nano-goethite, as well as Fe(III) oxyhydroxide phases previously examined. In addition, silicate is found to decrease the extent of Fe(II) sorption to the Fe(III) oxyhydroxide surface, increasing the reduction potential of the Fe(II)-Fe(III) oxyhydroxide suspension and, accordingly, decreasing the rate of 4-chloronitrobenzene reduction. A linear relationship between the reduction potential of the Fe(II)-Fe(III) oxyhydroxide suspensions and the reduction rate of 4-chloronitrobenzene (normalized to surface area and concentration of sorbed Fe(II)) was obtained in the presence and absence of silicate. However, when ferrihydrite was doped with Si (through co-precipitation) the reduction of 4-chloronitrobenzene was much slower than predicted from its reduction potential. The results obtained have significant implications to the likely effectiveness of naturally occurring contaminant degradation processes involving Fe(II) and Fe(III) oxyhydroxides in groundwater environments containing high concentrations of silicate, or other species which compete with Fe(II) for sorption sites.

Donnan membrane speciation of Al, Fe, trace metals and REEs in coastal lowland acid sulfate soil-impacted drainage waters.

Donnan dialysis has been applied to forty filtered drainage waters collected from five coastal lowland acid sulfate soil (CLASS) catchments across north-eastern NSW, Australia. Despite having average pH values<3.9, 78 and 58% of Al and total Fe, respectively, were present as neutral or negatively-charged species. Complementary isotope dilution experiments with (55)Fe and (26)Al demonstrated that only soluble (i.e. no colloidal) species were present. Trivalent rare earth elements (REEs) were also mainly present (>70%) as negatively-charged complexes. In contrast, the speciation of the divalent trace metals Co, Mn, Ni and Zn was dominated by positively-charged complexes and was strongly correlated with the alkaline earth metals Ca and Mg. Thermodynamic equilibrium speciation calculations indicated that natural organic matter (NOM) complexes dominated Fe(III) speciation in agreement with that obtained by Donnan dialysis. In the case of Fe(II), however, the free cation was predicted to dominate under thermodynamic equilibrium, whilst our results indicated that Fe(II) was mainly present as neutral or negatively-charged complexes (most likely with sulfate). For all other divalent metals thermodynamic equilibrium speciation calculations agreed well with the Donnan dialysis results. The proportion of Al and REEs predicted to be negatively-charged was also grossly underestimated, relative to the experimental results, highlighting possible inaccuracies in the stability constants developed for these trivalent Me(SO4)2(-) and/or Me-NOM complexes and difficulties in modeling complex environmental samples. These results will help improve metal mobility and toxicity models developed for CLASS-affected environments, and also demonstrate that Australian CLASS environments can discharge REEs at concentrations an order of magnitude greater than previously reported.

Reductive reactivity of borohydride- and dithionite-synthesized iron-based nanoparticles: A comparative study.

In this study sodium dithionite (NaS2O4) and sodium borohydride (NaBH4) were employed as reducing agents for the synthesis of nanosized iron-based particles. The particles formed using NaBH4 (denoted nFe(BH4)) principally contained (as expected) Fe(0) according to XAS and XRD analyses while the particles synthesized using NaS2O4, (denoted nFe(S2O4)) were dominated by the mixed Fe(II)/Fe(III) mineral magnetite (Fe3O4) though with possible presence of Fe(0). The ability of both particles to reduce trichloroethylene (TCE) under analogous conditions demonstrated remarkable differences with nFe(BH4) resulting in complete reduction of 1.5mM of TCE in 2h while nFe(S2O4) were unable to effect complete reduction of TCE in 120 h. Moreover, acetylene was the major reaction product formed in the presence of nFe(S2O4) while the major reaction product formed following reaction with nFe(BH4) was ethylene, which was further reduced to ethane as the reaction proceeded. Considering that effective Pd reduction to Pd(0) requires the presence of Fe(0), this is consistent with our finding that Fe(0) is not the dominant phase formed when employing dithionite as a reducing agent under the conditions employed in this study.

Ultrasonic vocalizations during intermittent swim stress forecasts resilience in a subsequent juvenile social exploration test of anxiety.

Current behavioral paradigms of stress resilience traditionally employ forms of prior manipulation or subsequent testing. Recent work has reported adult rat ultrasonic vocalizations (USVs) emitted during intermittent swim stress (ISS) may serve as a predictor of resilience. ISS-induced USVs predicted resilience on several endpoints of behavioral depression and may be considered a forecast of innate resilience. However, a potential problem for these previous findings is the lack of generalizability to other contexts, because both the stress induction and post-stress testing occur in water. The current study tests the generalizability of USVs as a predictor of stress resilience in a non-water-based post-test, the juvenile social exploration test of anxiety. The results provide further support that USVs emitted during ISS predict resilience to depression- and anxiety-like behaviors. Extensions of this work to examine the neurobiology of innate resilience associated with ISS-induced USVs are discussed with comparisons to extant models of learned resilience.

The impacts of low-cost treatment options upon scale formation potential in remote communities reliant on hard groundwaters. A case study: Northern Territory, Australia.

The majority of small, remote communities within the Northern Territory (NT) in Central Australia are reliant on groundwater as their primary supply of domestic, potable water. Saturation indices for a variety of relevant minerals were calculated using available thermodynamic speciation codes on collected groundwater data across the NT. These saturation indices were used to assess the theoretical formation of problematic mineral-scale, which manifests itself by forming stubborn coatings on domestic appliances and fixtures. The results of this research show that 63% of the measured sites within the NT have the potential to form calcium carbonate (CaCO(3)) scale, increasing to 91% in arid, central regions. The data also suggests that all groundwaters are over-saturated with respect to amorphous calcium-bridged ferric-silica polymers, based on the crystalline mineral index (Ca(3)Fe(2)Si(3)O(12)), although the quantitative impact of this scale is limited by low iron concentrations. An assessment of possible low-cost/low-technology management options was made, including; lowering the temperature of hot-water systems, diluting groundwater with rainwater and modifying the pH of the source water. Source water pH modification (generally a reduction to pH 7.0) was shown to clearly alleviate potential carbonate-based scale formation, over and above the other two options, albeit at a greater technical and capital expense. Although low-cost/low-technology treatment options are unlikely to remove severe scale-related issues, their place in small, remote communities with minor scale problems should be investigated further, owing to the social, technical and capital barriers involved with installing advanced treatment plants (e.g. reverse osmosis) in such locations.

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.