Comparative Evaluation of Mediated Electrochemical Reduction and Chemical Redox Titration for Quantifying the Electron Accepting Capacities of Soils and Redox-Active Soil Constituents.

The electron accepting capacity (EAC) of soil plays a pivotal role in the biogeochemical cycling of nutrients and transformation of redox-labile contaminants. Prior EAC studies of soils and soil constituents utilized different methods, reductants, and mediators, making cross-study comparison difficult. This study was conducted to quantify and compare the EACs of two soil constituents (hematite and Leonardite humic acid) and 12 soils of diverse composition, using chemical redox titration (CRT) with dithionite as the reductant and mediated electrochemical reduction (MER) with diquat as the mediator. The EACs of hematite and humic acid measured by CRT (EACCRT) and MER (EACMER) are similar and close to the theoretical/reported values. For soils, EACCRT and EACMER increased with iron and organic carbon (TOC) contents, suggesting iron and carbon were the main contributors to soil EAC. EACCRT > EACMER for all soils, and their difference (ΔEAC = EACCRT - EACMER) increased with TOC, presumably due to the longer contact time in CRT and thus more complete reduction of carbonaceous redox moieties. We propose an equation that relates EACCRT to EACMER (ΔEAC = 1796fTOC + 32) and another that predicts EACCRT from dithionite-reducible Fe and TOC (EACCRT = 2705 µmol e-/g C × fTOC + 17907 µmol e-/g Fe × fFedithionite-reducible). Our results suggest that at least 10-15% of soil organic carbon contributed to EACCRT.

Linear Free Energy Relationship for Predicting the Rate Constants of Munition Compound Reduction by the Fe(II)-Hematite and Fe(II)-Goethite Redox Couples.

Abiotic reduction by iron minerals is arguably the most important fate process for munition compounds (MCs) in subsurface environments. No model currently exists that can predict the abiotic reduction rates of structurally diverse MCs by iron (oxyhydr)oxides. We performed batch experiments to measure the rate constants for the reduction of three classes of MCs (poly-nitroaromatics, nitramines, and azoles) by hematite or goethite in the presence of aqueous Fe2+. The surface area-normalized reduction rate constant (kSA) depended on the aqueous-phase one-electron reduction potential (EH1) of the MC and the thermodynamic state (i.e., pe and pH) of the iron oxide-Feaq2+ system. A linear free energy relationship (LFER), similar to that reported previously for nitrobenzene, successfully captures all MC reduction rate constants that span 6 orders of magnitude: log(kSA)=(1.12±0.04)[0.53EH159mV-(pH+pe)]+(5.52±0.23). The finding that the rate constants of all the different classes of MCs can be described by a single LFER suggests that these structurally diverse nitro compounds are reduced by iron oxide-Feaq2+ couples through a common mechanism up to the rate-limiting step. Multiple mechanistic implications of the results are discussed. This study expands the applicability of the LFER model for predicting the reduction rates of legacy and emerging MCs and potentially other nitro compounds.

Thermodynamic Two-Site Surface Reaction Model for Predicting Munition Constituent Reduction Kinetics with Iron (Oxyhydr)oxides.

Iron (oxyhydr)oxides comprise a significant portion of the redox-active fraction of soils and are key reductants for remediation of sites contaminated with munition constituents (MCs). Previous studies of MC reduction kinetics with iron oxides have focused on the concentration of sorbed Fe(II) as a key parameter. To build a reaction kinetic model, it is necessary to predict the concentration of sorbed Fe(II) as a function of system conditions and the redox state. A thermodynamic framework is formulated that includes a generalized double-layer model that utilizes surface acidity and surface complexation reactions to predict sorbed Fe(II) concentrations that are used for fitting MC reduction kinetics. Monodentate- and bidentate Fe(II)-binding sites are used with individual oxide sorption characteristics determined through data fitting. Results with four oxides (goethite, hematite, lepidocrocite, and ferrihydrite) and four nitro compounds (NB, CN-NB, Cl-NB, and NTO) from six separate studies have shown good agreement when comparing observed and predicted surface area-normalized rate constants. While both site types are required to reproduce the experimental redox titration, only the monodentate site concentration controls the MC reaction kinetics. This model represents a significant step toward predicting the timescales of MC degradation in the subsurface.

Electron Transfer Energy and Hydrogen Atom Transfer Energy-Based Linear Free Energy Relationships for Predicting the Rate Constants of Munition Constituent Reduction by Hydroquinones.

No single linear free energy relationship (LFER) exists that can predict reduction rate constants of all munition constituents (MCs). To address this knowledge gap, we measured the reduction rates of MCs and their surrogates including nitroaromatics [NACs; 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), 2-amino-4,6-dinitrotoluene (2-A-DNT), 4-amino-2,6-dinitrotoluene (4-A-DNT), and 2,4-dinitrotoluene (DNT)], nitramines [hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroguanidine (NQ)], and azoles [3-nitro-1,2,4-triazol-5-one (NTO) and 3,4-dinitropyrazole (DNP)] by three dithionite-reduced quinones (lawsone, AQDS, and AQS). All MCs/NACs were reduced by the hydroquinones except NQ. Hydroquinone and MC speciations were varied by controlling pH, permitting the application of a speciation model to determine second-order rate constants (k) from observed pseudo-first-order rate constants. The intrinsic reactivity of MCs (oxidants) decreased upon deprotonation, while the opposite was true for hydroquinones (reductants). The rate constants spanned ∼6 orders of magnitude in the order NTO ≈ TNT > DNP > DNT ≈ DNAN ≈ 2-A-DNT > DNP- > 4-A-DNT > NTO- > RDX. LFERs developed using density functional theory-calculated electron transfer and hydrogen atom transfer energies and reported one-electron reduction potentials successfully predicted k, suggesting that these structurally diverse MCs/NACs are all reduced by hydroquinones through the same mechanism and rate-limiting step. These results increase the applicability of LFER models for predicting the fate and half-lives of MCs and related nitro compounds in reducing environments.

Modeling the Reduction Kinetics of Munition Compounds by Humic Acids.

Dissolved organic matter (DOM) comprises a sizeable portion of the redox-active constituents in the environment and is an important reductant for the abiotic transformation of nitroaromatic compounds and munition constituents (NACs/MCs). Building a predictive kinetic model for these reactions would require the energies associated with both the reduction of the NACs/MCs and the oxidation of the DOM. The heterogeneous and unknown structure of DOM, however, has prohibited reliable determination of its oxidation energies. To overcome this limitation, humic acids (HAs) were used as model DOM, and their redox moieties were modeled as a collection of quinones of different redox potentials. The reduction and oxidation energies of the NACs/MCs and hydroquinones, respectively, via hydrogen atom transfer (HAT) reactions were then calculated quantum chemically. HAT energies have been used successfully in a linear free energy relationship (LFER) to predict second-order rate constants for NAC reduction by hydroquinones. Furthermore, a linear relationship between the HAT energies and the reduction potentials of quinones was established, which allows estimation of hydroquinone reactivity (i.e., rate constants) from HA redox titration data. A training set of three HAs and two NACs/MCs was used to generate a mean HA redox profile that successfully predicted reduction kinetics in multiple HA/MC systems.

Reductive Transformation of 3-Nitro-1,2,4-triazol-5-one (NTO) by Leonardite Humic Acid and Anthraquinone-2,6-disulfonate (AQDS).

3-Nitro-1,2,4-triazol-5-one (NTO) is a major and the most water-soluble constituent in the insensitive munition formulations IMX-101 and IMX-104. While NTO is known to undergo redox reactions in soils, its reaction with soil humic acid has not been evaluated. We studied NTO reduction by anthraquinone-2,6-disulfonate (AQDS) and Leonardite humic acid (LHA) reduced with dithionite. Both LHA and AQDS reduced NTO to 3-amino-1,2,4-triazol-5-one (ATO), stoichiometrically at alkaline pH and partially (50-60%) at pH ≤ 6.5. Due to NTO and hydroquinone speciation, the pseudo-first-order rate constants (kObs) varied by 3 orders of magnitude from pH 1.5 to 12.5 but remained constant from pH 4 to 10. This distinct pH dependency of kObs suggests that NTO reactivity decreases upon deprotonation and offsets the increasing AQDS reactivity with pH. The reduction of NTO by LHA deviated continuously from first-order behavior for >600 h. The extent of reduction increased with pH and LHA electron content, likely due to greater reactivity of and/or accessibility to hydroquinone groups. Only a fraction of the electrons stored in LHA was utilized for NTO reduction. Electron balance analysis and LHA redox potential profile suggest that the physical conformation of LHA kinetically limited NTO access to hydroquinone groups. This study demonstrates the importance of carbonaceous materials in controlling the environmental fate of NTO.

Reduction of 3-Nitro-1,2,4-Triazol-5-One (NTO) by the Hematite-Aqueous Fe(II) Redox Couple.

3-Nitro-1,2,4-triazol-5-one (NTO) is an insensitive munition compound (MC) that has replaced legacy MC. NTO can be highly mobile in soil and groundwater due to its high solubility and anionic nature, yet little is known about the processes that control its environmental fate. We studied NTO reduction by the hematite-Fe2+ redox couple to assess the importance of this process for the attenuation and remediation of NTO. Fe2+(aq) was either added (type I) or formed through hematite reduction by dithionite (type II). In the presence of both hematite and Fe2+(aq), NTO was quantitatively reduced to 3-amino-1,2,4-triazol-5-one following first-order kinetics. The surface area-normalized rate constant (kSA) showed a strong pH dependency between 5.5 and 7.0 and followed a linear free energy relationship (LFER) proposed in a previous study for nitrobenzene reduction by iron oxide-Fe2+ couples, i.e., log kSA = -(pe + pH) + constant. Sulfite, a major dithionite oxidation product, lowered kSA in type II system by ∼10-fold via at least two mechanisms: by complexing Fe2+ and thereby raising pe, and by making hematite more negatively charged and hence impeding NTO adsorption. This study demonstrates the importance of iron oxide-Fe2+ in controlling NTO transformation, presents an LFER for predicting NTO reduction rate, and illustrates how solutes can shift the LFER by interacting with either iron species.

Experimental Validation of Hydrogen Atom Transfer Gibbs Free Energy as a Predictor of Nitroaromatic Reduction Rate Constants.

Nitroaromatic compounds (NACs) are a class of prevalent contaminants. Abiotic reduction is an important fate process that initiates NAC degradation in the environment. Many linear free energy relationship (LFER) models have been developed to predict NAC reduction rates. Almost all LFERs to date utilize experimental aqueous-phase one-electron reduction potential ( EH1) of NAC as a predictor, and thus, their utility is limited by the availability of EH1 data. A promising new approach that utilizes computed hydrogen atom transfer (HAT) Gibbs free energy instead of EH1 as a predictor was recently proposed. In this study, we evaluated the feasibility of HAT energy for predicting NAC reduction rate constants. Using dithionite-reduced quinones, we measured the second-order rate constants for the reduction of seven NACs by three hydroquinones of different protonation states. We computed the gas-phase energies for HAT and electron affinity (EA) of NACs and established HAT- and EA-based LFERs for six hydroquinone species. The results suggest that HAT energy is a reliable predictor of NAC reduction rate constants and is superior to EA. This is the first independent, experimental validation of HAT-based LFER, a new approach that enables rate prediction for a broad range of structurally diverse NACs based solely on molecular structures.

Validation of Cu toxicity to barley root elongation in soil with a Terrestrial Biotic Ligand Model developed from sand culture.

Constants for a Terrestrial Biotic Ligand Model (TBLM) to predict the Cu toxicity to barley root elongation (RE) were developed from controlled sand culture experiments. These constants were used to predict RE in soil culture. The competition of H+, Ca2+, and Mg2+ to Cu2+ toxicity were studied individually and independently, and linear relationships between EC50 free Cu2+ and H+, Ca2+, and Mg2+ activities were found, meaning that the cations H+, Ca2+, and Mg2+ will alleviate the toxicity of Cu2+ in solutions. Toxicity accompanying increasing concentration of solution ions other than Cu2+ was observed and modeled as an osmotic effect which improved soil culture toxicity prediction. The Root Mean Square Error (RMSE) of %RE and EC50 (50% effective concentration) for soil toxicity prediction using TBLM parameters developed from sand culture are 13.0 and 0.23 respectively, which are as good as that of 14.0 and 0.24 using parameters that developed from soil culture itself. A model including the activity at the root plasma membrane surface was tested and found not to provide improvement over the use of bulk solution activity to predict metal toxicity. TBLM parameters obtained from water solution culture were unable to accurately predict the EC50s in soils whereas the parameters obtained from sand culture were able to predict the toxicity in soils. Including the toxicity of CuOH+ was found to improve the toxicity prediction slightly.

Toll-Like Receptor 2 and Its Relationship With Streptococcus in Psoriasis.

The authors present the immunopathologic findings of Toll-like receptor 2 in psoriasis. This novel work shows positive staining within the dermal capillaries in psoriatic lesions. Neither normal skin nor lesional skin in eczema showed similar staining. The authors postulate how the activation of this innate immune system reactant plays a significant role in psoriasis and show how it may be associated with a cascade of events that begins with streptococcus and ends with psoriasis.

A metabolomic study on the responses of daphnia magna exposed to silver nitrate and coated silver nanoparticles.

We examined the short-term toxicity of AgNPs and AgNO3 to Daphnia magna at sublethal levels using (1)H NMR-based metabolomics. Two sizes of polyvinylpyrrolidone-coated AgNPs (10 and 40nm) were synthesized and characterized and their Ag(+) release was studied using centrifugal ultrafiltration and inductively coupled plasma mass spectrometry. Multivariate statistical analysis of the (1)H NMR spectra showed significant changes in the D. magna metabolic profiles following 48h exposure to both AgNP particle sizes and Ag(+) exposure. Most of the metabolic biomarkers for AgNP exposure, including 3-hydroxybutyrate, arginine, lysine and phosphocholine, were identical to those of the Ag(+)-exposed groups, suggesting that the dominant effects of both AgNPs were due to released Ag(+). The observed metabolic changes implied that the released Ag(+) induced disturbance in energy metabolism and oxidative stress, a proposed mechanism of AgNP toxicity. Elevated levels of lactate in all AgNP-treated but not in Ag(+)-treated groups provided evidence for Ag-NP enhanced anaerobic metabolism. These findings show that (1)H NMR-based metabolomics provides a sensitive measure of D. magna response to AgNPs and that further targeted assays are needed to elucidate mechanisms of action of nanoparticle-induced toxicity.

A general model for kinetics of heavy metal adsorption and desorption on soils.

In this study, we propose a general kinetics model for heavy metal adsorption and desorption reactions in soils when soil organic matter (SOM) is the dominant adsorbent. The kinetics model, integrated with the equilibrium speciation model WHAM VI, specifically considers metal reactions with SOM and dissolved organic matter (DOM) and accounts for the variations of solution chemistry. Metal reactions with SOM are associated with two groups of sites, one from the monodentate sites and another one from the bidentate and tridentate sites. There are three model parameters, desorption rate coefficients of the two groups of SOM sites for each metal and reactive organic carbon (ROC) for each soil. The applicability of the kinetics model was mainly examined with three elements, Cu, Pb, and Zn, which demonstrate different binding ability with organic matter. The kinetic data were collected with a stirred-flow reactor covering a wide range of experimental conditions, including varying SOM, DOM, Ca, and metal concentrations, reaction pHs, and different flow rates. The kinetics model has been successfully applied to describe heavy metal adsorption and desorption on soils under various reaction conditions.

Coma blisters sans coma.

Coma blisters (CBs) are self-limited lesions that occur in regions of pressure during unconscious states classically induced by barbiturates. We report a case of CBs sans coma that were histologically confirmed in a 41-year-old woman who developed multiple tense abdominal bullae with surrounding erythema following a transatlantic flight. Interestingly, the patient was fully conscious and denied medication use or history of medical conditions. A clinical diagnosis of CBs was confirmed by histopathologic findings of eccrine gland necrosis, a hallmark of these bulIous lesions.

A new paradigm in the treatment of kerions: treat the inflammation.

Kerions result from a massive delayed-type hypersensitivity reaction to a dermatophyte. Treatment traditionally has been directed primarily toward the dermatophyte. The authors propose, however, that inflammation should be the initial target oftreatment. Clinical findings and treatment outcomes for two patients with kerions, treated with short courses of anti-inflammatory agents, are presented. Earlier studies showing minimal effects with corticosteroid treatment of kerions may have had design flaws. The anti-inflammatory treatment of kerions is both safe and effective and permits the duration of therapy to be shortened dramatically.

A pentad of vancomycin reactions.

Vancomycin is used in the treatment of infections caused by gram-positive bacteria resistant to beta-lactam antibiotics. It is also used in the treatment ofenterococci, although this use has been limited due to resistance. The authors report the clinical and pathological presentations of 5 different vancomycin hypersensitivity reactions, including morbilliform eruption, erythroderma, acute generalized exanthematous pustulosis, linear immunoglobulin A bullous dermatosis, and toxic epidermal necrolysis. With the increased prevalence of beta-lactam-resistant bacteria, reliance on vancomycin continues to increase; thus, the recognition of reactions to this drug will be helpful in caring for patients who require this medication in the future.

A Novel Approach to the Treatment and Prevention of Alzheimer's Disease Based on the Pathology and Microbiology.

Utilizing the pathology and microbiology found in tissue from patients with documented Alzheimer's disease (AD), the pathogenesis of this fateful disorder has been made clear. Borrelia burgdorferi and Treponema denticola spirochetes enter the brain, mostly via neuronal pathways and the entorhinal circulation. These organisms easily pass through the blood-brain barrier and have an affinity for neural tissue. Once in the brain, the spirochetes make intra- and extracellular biofilms, and it is the biofilms that create the pathology. Specifically, it is the intracellular biofilms that are ultimately responsible for neurofibrillary tangles and dendritic disintegration. The extracellular biofilms are responsible for the inflammation that initially is generated by the first responder, Toll-like receptor 2. The hypothesis that arises from this work is two-pronged: one is related to prevention; the other to treatment. Regarding prevention, it is very likely possible that AD could be prevented by periodic administration of penicillin (PCN), which would kill the spirochetes before they made biofilms; this would prevent the disease and would not allow any of the above deleterious changes generated by the biofilms to occur. As regards treatment, it may be possible to slow or prevent further decline in early AD by administration of PCN together with a biofilm disperser. The disperser would disrupt the biofilm coating and enable the PCN to kill the spirochetes. This protocol could be administered in a trial with the control arm utilizing the current treatment. The progress of the treatment could be evaluated by one of the current blood tests that is semi-quantitative. The specific protocols are listed.

Assessment of methods for collecting fallout brake pad wear debris for environmental analysis.

Three methods for collecting or generating fallout brake pad wear debris for environmental analysis were assessed: collection from wheels or hubs of automobiles (natural), generation from an inexpensive sanding process (sanded), and collection of fallout debris from dynamometer tests using the Los Angeles City Traffic protocol (LACT). Brake wear debris was collected from four automobiles with semimetalic brake pads and analyzed for physicochemical properties. For automobiles where all three types of debris were collected, bulk copper mass fractions ranged from 22-23% in sanded particles and 24-27% in LACTparticles, but were reduced to 1-6% in natural debris. The smaller copper mass fraction in natural debris was attributed to contamination with road dust, which was found to comprise 37-97% of the natural particles. The ratio of surface to bulk copper mass fraction was up to five times larger for natural than LACT debris, suggesting that copper may leach into stormwater faster and to a greater extent for natural particles. While the LACT method appears best for collecting only fallout particles, significant differences in copper distributions in the natural and LACT debris suggests that metal distribution in LACT debris may not be representative of fallout particles generated under actual driving conditions, where airborne road dust may play a role. Although dynamometer tests have been the preferred method for generating debris for assessment of metal dissolution from brake particles, data from this study indicate that such samples may result in biased estimates of metal leaching.

Predicting Cr(vi) adsorption on soils: the role of the competition of soil organic matter.

Cr(vi) has posed a serious risk for the environment and human beings because of its pollution and toxicity. It is essential to understand the equilibrium behavior of Cr(vi) in soils. In this study, the adsorption of Cr(vi) on fourteen soils was studied with batch experiments and quantitative modeling. The batch experiments included the adsorption edge and adsorption isotherm experiments, investigating the adsorption of Cr(vi) with varying soil properties, solution pH, and initial Cr(vi) concentrations. The experimental data were then modeled using the surface complexation models in Visual MINTEQ of CD-MUSIC by considering the adsorption of Cr(vi) and ions onto Fe (hydr)oxides and Al (hydr)oxides, and the Stockholm Humic Model and the fixed charge site model by accounting for the adsorption of the cations to soil organic matter and clay, respectively. Particularly, the modeling method of this study introduced an important parameter RO- to account for the amount of soil organic matter irreversibly adsorbed on soil minerals. Overall, the model predicted reasonably well for the equilibrium partition of Cr(vi) under various conditions with a root-mean-square-error of 0.35 for the adsorption edge data and 0.19 for the adsorption isotherm data. According to the model calculations, ferrihydrite dominated the binding of Cr(vi) at pH of 3.0-7.0. The content of ferrihydrite and reactive soil organic matter was found to be the main factor influencing RO-. The modeling results help to understand and predict Cr(vi) adsorption on different soils and are beneficial to environmental risk assessment and pollution remediation.

A Unified Linear Free Energy Relationship for Abiotic Reduction Rate of Nitroaromatics and Hydroquinones Using Quantum Chemically Estimated Energies.

Determining the fate of nitroaromatic compounds (NACs) in the environment requires the use of predictive models for compounds and conditions for which experimental data are insufficient. Previous studies have developed linear free energy relationships (LFERs) that relate the thermodynamic energy of NAC reduction to its corresponding rate constant. We present a comprehensive LFER that incorporates both the reduction and oxidation half-reactions through quantum chemically calculated energies. Environ Toxicol Chem 2020;39:2389-2395. © 2020 SETAC.

Hydrogen Atom Transfer Reaction Free Energy as a Predictor of Abiotic Nitroaromatic Reduction Rate Constants: A Comprehensive Analysis.

A linear free energy model is presented that predicts the second-order rate constant for the abiotic reduction of nitroaromatic compounds (NACs). Previously presented models use the one-electron reduction potential EH1(ArNO2) of the NAC reaction ArNO2+e-→ArNO2•- . If EH1(ArNO2) is not available, it has been proposed that EH1(ArNO2) be computed directly or estimated from the gas-phase electron affinity (EA). The model proposed uses the Gibbs free energy of the hydrogen atom transfer (HAT) reaction ArNO2+H•→ArNOOH• as the parameter in the linear free energy model. Both models employ quantum chemical computations for the required thermodynamic energies. The available and proposed models are compared using experimentally determined second-order rate constants from 5 investigations from the literature in which a variety of NACs were exposed to a variety of reductants. A comprehensive analysis utilizing all the NACs and reductants demonstrate that the HAT energy model and the experimental one-electron reduction potential model have similar root mean square errors and residual error probability distributions. In contrast, the model using the computed EA has a more variable residual error distribution with a significant number of outliers. The results suggest that a linear free energy model utilizing computed HAT reaction free energy produces a more reliable prediction of the NAC abiotic reduction second-order rate constant than previously available methods. The advantages of the proposed HAT energy model and its mechanistic implications are discussed as well. Environ Toxicol Chem 2020;39:1678-1684. © 2020 SETAC.