Experimental and Computational Study of Pyrogenic Carbonaceous Matter Facilitated Hydrolysis of 2,4-Dinitroanisole (DNAN)

This study investigated the reaction pathway of 2,4-dinitroanisole (DNAN) on the pyrogenic carbonaceous matter (PCM) to assess the scope and mechanism of PCM-facilitated surface hydrolysis. DNAN degradation was observed at pH 11.5 and 25 °C with a model PCM, graphite, whereas no significant decay occurred without graphite. Experiments were performed at pH 11.5 due to the lack of DNAN decay at pH below 11.0, which was consistent with previous studies. Graphite exhibited a 1.78-fold enhancement toward DNAN decay at 65 °C and pH 11.5 relative to homogeneous solution by lowering the activation energy for DNAN hydrolysis by 54.3 ± 3.9%. This is supported by our results from the computational modeling using Car-Parrinello simulations by ab initio molecular dynamics/molecular mechanics (AIMD/MM) and DFT free energy simulations, which suggest that PCM effectively lowered the reaction barriers by approximately 8 kcal mol-1 compared to a homogeneous solution. Quaternary ammonium (QA)-modified activated carbon performed the best among several PCMs by reducing DNAN half-life from 185 to 2.5 days at pH 11.5 and 25 °C while maintaining its reactivity over 10 consecutive additions of DNAN. We propose that PCM can affect the thermodynamics and kinetics of hydrolysis reactions by confining the reaction species near PCM surfaces, thus making them less accessible to solvent molecules and creating an environment with a weaker dielectric constant that favors nucleophilic substitution reactions. Nitrite formation during DNAN decay confirmed a denitration pathway, whereas demethylation, the preferred pathway in homogeneous solution, produces 2,4-dinitrophenol (DNP). Denitration catalyzed by PCM is advantageous to demethylation because nitrite is less toxic than DNAN and DNP. These findings provide critical insights for reactive adsorbent design that has broad implications for catalyst design and pollutant abatement.

Modeling the Role in pH on Contaminant Sequestration by Zerovalent Metals: Chromate Reduction by Zerovalent Magnesium

The role of pH in sequestration of Cr(VI) by zerovalent magnesium (ZVMg) was characterized by global fitting of a kinetic model to time-series data from unbuffered batch experiments with varying initial pH values. At initial pH values ranging from 2.0 to 6.8, ZVMg (0.5 g/L) completely reduced Cr(VI) (18.1 μM) within 24 h, during which time pH rapidly increased to a plateau value of ∼10. Time-series correlation analysis of the pH and aqueous Cr(VI), Cr(III), and Mg(II) concentration data suggested that these conditions are controlled by combinations of reactions (involving Mg0 oxidative dissolution and Cr(VI) sequestration) that evolve over the time course of each experiment. Since this is also likely to occur during any engineering applications of ZVMg for remediation, we developed a kinetic model for dynamic pH changes coupled with ZVMg corrosion processes. Using this model, the synchronous changes in Cr(VI) and Mg(II) concentrations were fully predicted based on the Langmuir-Hinshelwood kinetics and transition-state theory, respectively. The reactivity of ZVMg was different in two pH regimes that were pH-dependent at pH < 4 and pH-independent at the higher pH. This contrasting pH effect could be ascribed to the shift of the primary oxidant of ZVMg from H+ to H2O at the lower and higher pH regimes, respectively.

Effect of Interfacial Action on the Generation and Transformation of Reactive Oxygen Species in Tripolyphosphate-Enhanced Heterogeneous Fe3O4/O2 Systems

It has been reported that tripolyphosphate (TPP) can enhance the oxygenation of natural Fe(II)-containing minerals to produce reactive oxygen species (ROS). However, the molecular structure of the TPP-Fe(II) mineral surface complex and the role of this complex in the generation and transformation of ROS have not been fully characterized. In the present study, a heterogeneous magnetite (Fe3O4)/O2/TPP system was developed for the degradation of p-nitrophenol (PNP). The results showed that the addition of TPP significantly accelerated the removal of PNP in the Fe3O4/O2 system and extended the range of effective pH to neutral. Experiments combined with density functional theory calculations revealed that the activation of O2 mainly occurs on the surface of Fe3O4 induced by a structural Fe(II)-TPP complex, where the generated O2•- (intermediate active species) can be rapidly converted into H2O2, and then the •OH generated by the Fenton reaction is released into the solution. This increases the concentration of •OH produced and the efficiency of •OH produced relative to Fe(II) consumed, compared with the homogeneous system. Furthermore, the binding of TPP to the surface of Fe3O4 led to stretching and even cleavage of the Fe-O bonds. Consequently, more Fe(II)/(III) atoms are exposed to the solvation environment and are available for the binding of active O2 and O2•-. This study demonstrates how common iron minerals and O2 in the natural environment can be combined to yield a green remediation technology.

Modeling the Partitioning of Anionic Carboxylic and Perfluoroalkyl Carboxylic and Sulfonic Acids to Octanol and Membrane Lipid

Perfluoroalkyl carboxylic and sulfonic acids (PFCAs and PFSAs, respectively) have low acid dissociation constant values and are, therefore, deprotonated under most experimental and environmental conditions. Hence, the anionic species dominate their partitioning between water and organic phases, including octanol and phospholipid bilayers which are often used as model systems for environmental and biological matrices. However, data for solvent-water (SW) and membrane-water partition coefficients of the anion species are only available for a few per- and polyfluoroalkyl substances (PFAS). In the present study, an equation is derived using a Born-Haber cycle that relates the partition coefficients of the anions to those of the corresponding neutral species. It is shown via a thermodynamic analysis that for carboxylic acids (CAs), PFCAs, and PFSAs, the log of the solvent-water partition coefficient of the anion, log KSW (A- ), is linearly related to the log of the solvent-water partition coefficient of the neutral acid, log KSW (HA), with a unity slope and a solvent-dependent but solute-independent intercept within a PFAS (or CA) family. This finding provides a method for estimating the partition coefficients of PFCAs and PFSAs anions using the partition coefficients of the neutral species, which can be reliably predicted using quantum chemical methods. In addition, we have found that the neutral octanol-water partition coefficient, log KOW , is linearly correlated to the neutral membrane-water partition coefficient, log KMW ; therefore, log KOW , being a much easier property to estimate and/or measure, can be used to predict the neutral log KMW . Application of this approach to KOW and KMW for PFCAs and PFSAs demonstrates the utility of this methodology for evaluating reported experimental data and extending anion property data for chain lengths that are unavailable. Environ Toxicol Chem 2023;42:2317-2328. © 2023 SETAC.

pKa prediction of per- and polyfluoroalkyl acids in water using in silico gas phase stretching vibrational frequencies and infrared intensities

To successfully understand and model the environmental fate of per- and polyfluoroalkyl substances (PFAS), it is necessary to know key physicochemical properties (PChPs) such as pKa; however, measured PChPs of PFAS are scarce and of uncertain reliability. In this study, quantitative structure-activity relationships (QSARs) were developed by correlating calculated (M062-X/aug-cc-pVDZ) vibrational frequencies (VF) and corresponding infrared intensities (IRInt) to the pKa of carboxylic acids, sulfonic acids, phosphonic acids, sulfonamides, betaines, and alcohols. Antisymmetric stretching VF of the anionic species were used for all subclasses except for alcohols where the OH stretching VF performed better. The individual QSARs predicted the pKa for each subclass mostly within 0.5 pKa units from the experimental values. The inclusion of IRInt as a pKa predictor for carboxylic acids improved the results by decreasing the root-mean-square error from 0.35 to 0.25 (n > 100). Application of the developed QSARs to estimate the pKa of PFAS within each subclass revealed that the length of the perfluoroalkyl chain has minimal effect on the pKa, consistent with other models but in stark contrast with the limited experimental data available.

Degradation of Chloroform by Zerovalent Iron: Effects of Mechanochemical Sulfidation and Nitridation on the Kinetics and Mechanism

Chloroform (CF) is a widely used chemical reagent and disinfectant and a probable human carcinogen. The extensive literature on halocarbon reduction with zerovalent iron (ZVI) shows that transformation of CF is slow, even with nano, bimetallic, sulfidated, and other modified forms of ZVI. In this study, an alternative method of ZVI modification─involving simultaneous sulfidation and nitridation through mechanochemical ball milling─was developed and shown to give improved degradation of CF (i.e., higher degradation rate and inhibited H₂ evolution reaction). The composite material (denoted as S-N(C)-ZVI) gave synergistic effects of nitridation and sulfidation on CF degradation. A complete chemical reaction network (CRN) analysis of CF degradation suggests that O-nucleophile-mediated transformation pathways may be the main route for the formation of the terminal nonchlorinated products (formate, CO, and glycolic polymers) that have been used to explain the undetected products needed for mass balance. Material characterizations of the ZVI recovered after batch experiments showed that sulfidation and nitridation promoted the formation of Fe₃O₄ on the S-N(C)-ZVI particles, and the effect of aging on CF degradation rates was minor for S-N(C)-ZVI. The synergistic benefits of sulfidation and nitridation on CF degradation were also observed in experiments performed with groundwater.

Electron exchange capacity of pyrogenic dissolved organic matter (pyDOM): complementarity of square-wave voltammetry in DMSO and mediated chronoamperometry in water

Pyrogenic dissolved organic matter (pyDOM) is derived from black carbon, which is important in the global carbon cycle and other biogeochemical redox processes. The electron-exchange capacity (EEC) of pyDOM has been characterized in water using mediated chronoamperometry (MCA), which gives precise results under specific operational conditions, but the broader significance of these EECs is less clear. In this study, we described a novel but complementary electrochemical approach to quantify EECs of pyDOM without mediation using square-wave voltammetry (SWV) in dimethyl sulfoxide (DMSO). Using both the SWV and MCA methods, we determined EECs for 10 pyDOMs, 6 natural organic matter (NOM) samples, and 2 model quinones. The two methods gave similar EECs for model quinones, but SWV gave larger EECs than MCA for NOM and pyDOM (by several-fold and 1-2 orders of magnitude, respectively). The differences in the EECs obtained by SWV and MCA likely are due to multiple factors, including the potential range of electrons sampled, kinetics of electron transfer from (macro)molecular structures, and coupling of electron and proton transfer steps. Comparison of the results obtained by these two methods should provide new insights into important environmental processes such as carbon-cycling, wildfire recovery, and contaminant mitigation using carbon-based amendments.

Computational Predictions of the Hydrolysis of 2,4,6-Trinitrotoluene (TNT) and 2,4-Dinitroanisole (DNAN)

Hydrolysis is a common transformation reaction that can affect the environmental fate of many organic compounds. In this study, three proposed mechanisms of alkaline hydrolysis of 2,4,6-trinitrotoluene (TNT) and 2,4-dinitroaniline (DNAN) were investigated with plane-wave density functional theory (DFT) combined with ab initio and classical molecular dynamics (AIMD/MM) free energy simulations, Gaussian basis set DFT calculations, and correlated molecular orbital theory calculations. Most of the computations in this study were carried out using the Arrows web-based tools. For each mechanism, Meisenheimer complex formation, nucleophilic aromatic substitution, and proton abstraction reaction energies and activation barriers were calculated for the reaction at each relevant site. For TNT, it was found that the most kinetically favorable first hydrolysis steps involve Meisenheimer complex formation by attachment of OH^- at the C1 and C3 arene carbons and proton abstraction from the methyl group. The nucleophilic aromatic substitution reactions at the C2 and C4 arene carbons were found to be thermodynamically favorable. However, the calculated activation barriers were slightly lower than in previous studies, but still found to be ΔG^‡ ≈ 18 kcal/mol using PBE0 AIMD/MM free energy simulations, suggesting that the reactions are not kinetically significant. For DNAN, the barriers of nucleophilic aromatic substitution were even greater (ΔG^‡ > 29 kcal/mol PBE0 AIMD/MM). The most favorable hydrolysis reaction for DNAN was found to be a two-step process in which the hydroxyl first attacks the C1 carbon to form a Meisenheimer complex at the C1 arene carbon C1-(OCH₃)OH^-, and subsequently, the methoxy anion (-OCH₃) at the C1 arene carbon dissociates and the proton shuttles from the C1-OH to the dissociated methoxy group, resulting in methanol and an aryloxy anion.

Generation of Reactive Oxygen Species and Degradation of Pollutants in the Fe2+/O2/Tripolyphosphate System: Regulated by the Concentration Ratio of Fe2+ and Tripolyphosphate

Tripolyphosphate (TPP) has many advantages as a ligand for the optimization of the Fe²⁺/O₂ system in environmental remediation applications. However, the relationship between remediation performance and the Fe²⁺/TPP ratio in the system has not been previously described. In this study, we report that the degradation mechanism of p-nitrophenol (PNP) in Fe²⁺/O₂ systems is regulated by the Fe²⁺/TPP ratio under neutral conditions. The results showed that although PNP was effectively degraded at different Fe²⁺/TPP ratios, the results of specific reactive oxygen species (ROS) scavenging experiments and the determination of PNP degradation products showed that the mechanism of PNP degradation varies with the Fe²⁺/TPP ratio. When C_Fe²⁺ ≥ C_TPP, the initially formed O₂^•- is converted to •OH and the •OH degrades PNP by oxidation. However, when C_Fe²⁺ < C_TPP, the O₂^•- persists long enough to degrade PNP by reduction. Density functional theory (DFT) calculations revealed that the main reactive species of Fe²⁺ in the system include [Fe(TPP)(H₂O)₃]^- and [Fe(TPP)₂]^4-, whose content in the solution is the key to achieve system regulation. Consequently, by controlling the Fe²⁺/TPP ratio in the solution, the degradation pathways of PNP can be selected. Our study proposed a new strategy to regulate the oxidation/reduction removal of pollutants by simply varying the Fe²⁺/TPP ratio of the Fe²⁺/O₂ system.

Advances in metal(loid) oxyanion removal by zerovalent iron: Kinetics, pathways, and mechanisms

Metal(loid) oxyanions in groundwater, surface water, and wastewater can have harmful effects on human or ecological health due to their high toxicity, mobility, and lack of degradation. In recent years, the removal of metal(loid) oxyanions using zerovalent iron (ZVI) has been the subject of many studies, but the full scope of this literature has not been systematically reviewed. The main elements that form metal(loid) oxyanions under environmental conditions are Cr(VI), As(V and III), Sb(V and III), Tc(VII), Re(VII), Mo(VI), V(V), etc. The removal mechanisms of metal(loid) oxyanions by ZVI may involve redox reactions, adsorption, precipitation, and coprecipitation, usually with one of these mechanisms being the main reaction pathway and the other playing auxiliary roles. However, the removal mechanisms are coupled to the reactions involved in corrosion of Fe(0) and reaction conditions. The layer of iron oxyhydroxides that forms on ZVI during corrosion mediates the sequestration of metal(loid) oxyanions. This review summarizes most of the currently available data on mechanisms and performance (e.g., kinetics) of removal of the most widely studies metal(loid) oxyanion contaminants (Cr, As, Sb) by different types of ZVI typically used in wastewater treatment, as well as ZVI that has been sulfidated or combination with catalytic bimetals.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Abiotic Transformation of Nitrobenzene by Zero Valent Iron under Aerobic Conditions: Relative Contributions of Reduction and Oxidation in the Presence of Ethylene Diamine Tetraacetic Acid

Zero valent iron (ZVI) applications to remediation of shallow groundwaters can be affected by dissolved oxygen (DO) and organic ligands. To explore the intersection between these complicating factors, this study thoroughly characterized the reactions of nitrobenzene (NB) with ZVI in the presence DO and the model ligand ethylene diamine tetraacetic acid (EDTA). The results showed that NB is degraded by both ZVI reduction and ZVI-induced advanced oxidation under oxygen-limited conditions. The contribution of ·OH to the degradation of NB increased with time so that nearly 39% of NB was oxidized by ·OH at 15 min (pH = 3), but reduction was still the main pathway of NB transformation throughout. NB reduction products, such as aniline (AN), were also oxidized by ·OH. The lower the pH, the greater the contribution of advanced oxidation, but DO was the limiting factor for ·OH generation. Only 4.7% NB was fully degraded by ring opening and/or mineralization because the production of •OH was limited by low DO. After the transformation of NB and AN, other benzene ring and nitrogen-containing intermediates were identified (e.g., p-nitrophenol, p-aminophenol, hydroquinone, and p-benzoquinone). The removal of total organic carbon and total organic nitrogen was minimal. The results suggested that the relative doses of ZVI, DO, and iron-complexing ligands can be balanced for the optimal (rapid and deep) removal of organic contaminants.

FeNX(C)-Coated Microscale Zero-Valent Iron for Fast and Stable Trichloroethylene Dechlorination in both Acidic and Basic pH Conditions

FeN_X in Fe single-atom catalysts can be the active site for adsorption and activation of reactants. In addition, FeN_X species have been shown to facilitate electron transfer between Fe and the carbon supports used in newly developed metal-air batteries. We hypothesized that the combination of FeN_X species with granular zero-valent iron (ZVI) might result in catalyzed reductive decontamination of groundwater contaminants such as trichloroethylene (TCE). Here, such materials synthesized by ball milling microscale ZVI with melamine and the resulting N species were mainly in the form of pyridinic, pyrrolic, and graphitic N. This new material (abbreviated as N-C-mZVI^bm) dechlorinated TCE at higher rates than bare mZVI^bm (about 3.5-fold) due to facilitated electron transfer through (or around) the surface layer of iron oxides by the newly formed Fe-N_X(C). N-C-mZVI^bm gave higher k_TCE (0.4-1.14 day^-1) than mZVI^bm (0-0.4 day^-1) over a wide range of pH values (4-11). Unlike most ZVI systems, k_TCE for N-C-mZVI^bm increased with increasing pH values. This is because the oxide layer that passivates Fe⁰ at a high pH is disrupted by Fe-N_X(C) formed on N-C-mZVI^bm, thereby allowing TCE dechlorination and HER under basic conditions. Serial respike experiments gave no evidence of decreased performance of N-C-mZVI^bm, showing that the advantages of this material might remain under field applications.

Quantitative structure activity relationships (QSARs) and machine learning models for abiotic reduction of organic compounds by an aqueous Fe(II) complex

Due to the increasing diversity of organic contaminants discharged into anoxic water environments, reactivity prediction is necessary for chemical persistence evaluation for water treatment and risk assessment purposes. Almost all quantitative structure activity relationships (QSARs) that describe rates of contaminant transformation apply only to narrowly-defined, relatively homogenous families of reactants (e.g., dechlorination of alkyl halides). In this work, we develop predictive models for abiotic reduction of 60 organic compounds with diverse reducible functional groups, including nitroaromatic compounds (NACs), aliphatic nitro-compounds (ANCs), aromatic N-oxides (ANOs), isoxazoles (ISXs), polyhalogenated alkanes (PHAs), sulfoxides and sulfones (SOs), and others. Rate constants for their reduction were measured using a model reductant system, Fe(II)-tiron. Qualitatively, the rates followed the order NACs > ANOs ≈ ISXs ≈ PHAs > ANCs > SOs. To develop QSARs, both conventional chemical descriptor-based and machine learning (ML)-based approaches were investigated. Conventional univariate QSARs based on a molecular descriptor E_LUMO (energy of the lowest-unoccupied molecular orbital) gave good correlations within classes. Multivariate QSARs combining E_LUMO with Abraham descriptors for physico-chemical properties gave slightly improved correlations within classes for NCs and NACs, but little improvement in correlation within other classes or among classes. The ML model obtained covers reduction rates for all classes of compounds and all of the conditions studied with the prediction accuracy similar to those of the conventional QSARs for individual classes (r² = 0.41-0.98 for univariate QSARs, 0.71-0.94 for multivariate QSARs, and 0.83 for the ML model). Both approaches required a scheme for a priori classification of the compounds for model training. This work offers two alternative modeling approaches to comprehensive abiotic reactivity prediction for persistence evaluation of organic compounds in anoxic water environments.

Sulfidation of Zero-Valent Iron by Direct Reaction with Elemental Sulfur in Water: Efficiencies, Mechanism, and Dechlorination of Trichloroethylene

Sulfidation can enhance both the reactivity and selectivity (i.e., electron efficiency, ε_e) of zero-valent iron (ZVI) in contaminant removal, which may make this technology cost-effective for a wider range of water treatment applications. However, current sulfidation methods involve either hazardous or unstable sulfidation agents (e.g., Na₂S, Na₂S₂O₃, and Na₂S₂O₄) or energy-intensive preparations (e.g., mechanochemical sulfidation with elemental sulfur). In this study, we demonstrate that very efficient sulfidation of microscale ZVI (mZVI) can be achieved at all S/Fe molar ratios (∼100% sulfidation efficiency, ε_s) simply by direct reaction between elemental sulfur (S⁰) and ZVI in an aqueous suspension at ambient temperature. In comparison, the ε_s values obtained using Na₂S, Na₂S₂O₃, or Na₂S₂O₄ as the sulfidation agents were only ∼23, ∼75, and ∼38%, respectively. The sulfidated mZVI produced using the new method reacts with trichloroethylene (TCE) with very high rates and electron efficiencies: rate constants and electron efficiencies were 800- and 79-fold higher than those of the unsulfidated mZVI. The enhanced performance of this material, together with the operational advantages of S⁰ for sulfidation (including safety, stability, and cost), may make it a desirable product for full-scale engineering applications.

Role of complexation in the photochemical reduction of chromate by acetylacetone

Organic ligands can alter the redox behavior of metal species through the generation of metal-ligand complexes. Photo-induced complexation between ligands and metals is an important, but under-appreciated, aspect of process. Acetylacetone (AA) is a good chelating agent due to keto-enol tautomerization. In the presence of AA, photoreduction of Cr(VI) is accelerated; however, it is unclear exactly how complexation is involved in UV/AA mediated Cr(VI) reduction. On the basis of spectral and kinetic analyses, this study shows that the formation of {Cr(VI)-AA}* complexes is the main mechanism of Cr(VI) reduction by UV/AA. Evidence for this includes (1) the formation rate constant of Cr(III)-AA complexes in the UV system was 2-3 orders of magnitude greater than that in the thermal system; (2) there was a linear relationship between the photons absorbed by AA and the reduction rate constants of Cr(VI); and (3) the reaction appeared initially zero-order in Cr(VI) and turned to first-order as the pool of available Cr(VI) ran out. The results presented here are not only important for the better understanding of the complexation effects in the reduction of Cr(VI), but also crucial for the possible application of the UV/AA process in many other scenarios.

Environmental occurrence, fate, effects, and remediation of halogenated (semi)volatile organic compounds

Guest editors Elizabeth Edwards, Lucy Carpenter, Sarah Blossom and Paul Tratnyek introduce the Halogenated (semi)volatile organic compounds themed issue of Environmental Science: Processes & Impacts.

Quantifying the efficiency and selectivity of organohalide dechlorination by zerovalent iron

The efficiency and selectivity of zerovalent iron-based treatments for organohalide contaminated groundwater can be quantified by accounting for redistribution of electrons derived from oxidation of Fe⁰. Several types of efficiency are reviewed, including (i) the efficiency of Fe(0) utilization, ε_Fe(0), (ii) the electron efficiency of target contaminant reduction, ε_e, and (iii) the electron efficiency of natural reductant demand (NRD) involving H₂O, O₂, and co-contaminants such as nitrate, ε_NRD. Selectivity can then be calculated by using ε_e/ε_NRD. Of particular interest is ε_e and the key to its determination is measuring the total quantity of electrons provided by Fe⁰ oxidation, which can be based on either the loss of Fe(0), the formation of Fe(ii)/Fe(iii), or the composition of the total reaction products. Recently, many data have accumulated on ε_e for the treatment of various chlorinated solvents (esp. trichloroethylene, TCE) by zerovalent iron (ZVI), and analysis of these data shows that ZVI particle properties (e.g., stabilization with polymers, bimetallic modification, sulfidation, etc.) and other operational factors have variable effects on ε_e. Of particular interest is that pre-exposure of ZVI to reduced sulfur species (i.e., sulfidation) consistently improves the ε_e of contaminant reduction, mainly by suppressing the reduction of water.

Reduction of 1,2,3-trichloropropane (TCP): pathways and mechanisms from computational chemistry calculations

The characteristic pathway for degradation of halogenated aliphatic compounds in groundwater or other environments with relatively anoxic and/or reducing conditions is reductive dechlorination. For 1,2-dihalocarbons, reductive dechlorination can include hydrogenolysis and dehydrohalogenation, the relative significance of which depends on various structural and energetic factors. To better understand how these factors influence the degradation rates and products of the lesser halogenated hydrocarbons (in contrast to the widely studied per-halogenated hydrocarbons, like trichloroethylene and carbon tetrachloride), density functional theory calculations were performed to compare all of the possible pathways for reduction and elimination of 1,2,3-trichloropropane (TCP). The results showed that free energies of each species and reaction step are similar for all levels of theory, although B3LYP differed from the others. In all cases, the reaction coordinate diagrams suggest that β-elimination of TCP to allyl chloride followed by hydrogenolysis to propene is the thermodynamically favored pathway. This result is consistent with experimental results obtained using TCP, 1,2-dichloropropane, and 1,3-dichloropropane in batch experiments with zerovalent zinc (Zn⁰, ZVI) as a reductant.

Electrochemical characterization of natural organic matter by direct voltammetry in an aprotic solvent

The complex and indeterminant composition of NOM makes characterization of its redox properties challenging. Approaches that have been taken to address this challenge include chemical probe reactions, potentiometric titrations, chronocoulometry, and voltammetry. In this study, we revisit the use of direct voltammetric methods in aprotic solvents by applying an expanded and refined suite of methods to a large set of NOM samples and model compounds (54 NOM samples from 10 different sources, 7 NOM model compounds, and 2 fresh extracts of plant materials that are high in redox-active quinonoid model compounds dissolved in DMSO). Refinements in the methods of fitting the data obtained by staircase cyclic voltammetry (SCV) provided improved definition of peaks, and square wave voltammetry (SWV), performed under the same conditions as SCV, provided even more reliable identification and quantitation of peaks. Further evidence is provided that DMSO improves the electrode response by unfolding some of the tertiary structure of NOM polymers, thereby allowing greater contact between redox active functional groups and the electrode surface. We averaged experimental peak potentials for all NOM compounds and calculated potentials in water. Average values for Epa1, Epc1, and Ep1 in DMSO were -0.866 ± 0.069, -1.35 ± 0.071, and -0.831 ± 0.051 V vs. Ag/Ag+, and -0.128, -0.613, and -0.0930 V vs. SHE in water. In addition to peak potentials, the breadth of SCV peaks was quantified as a way to characterize the degree to which the redox activity of NOM is due to a continuum of contributing functional groups. The average breadth values were 1.63 ± 0.24, 1.28 ± 0.34, and 0.648 ± 0.15 V for Epa1, Epc1, and Ep1 respectively. Comparative analysis of the overall dataset-from SCV and SWV on all NOMs and model compounds-revealed that NOM redox properties vary over a narrower range than expected based on model compound properties. This lack of diversity in redox properties of NOM is similar to conclusions from other recent work on the molecular structure of NOM, all of which could be the result of selectivity in the common extraction methods used to obtain the materials.

Enhanced Photooxidation of Hydroquinone by Acetylacetone, a Novel Photosensitizer and Electron Shuttle

Quinones are important electron shuttles as well as micropollutants in the nature. Acetylacetone (AA) is a newly recognized electron shuttle in aqueous media exposed to UV irradiation. Herein, we studied the interactions between AA and hydroquinone (QH₂) under steady-state and transient photochemical conditions to clarify the possible reactions and consequences if QH₂ and AA coexist in a solution. Steady-state experimental results demonstrate that the interactions between AA and QH₂ were strongly affected by dissolved oxygen. In O₂-rich solutions, the phototransformation of QH₂ was AA-independent. Both QH₂ and AA utilize O₂ as the electron acceptor, but in O₂-insufficient solutions, AA became an important electron acceptor for the oxidation of QH₂. In all cases, the coexistence of AA increased the phototransformation of QH₂, whereas the decomposition of AA in O₂-saturated and oversaturated solutions was inhibited by the presence of QH₂. The underlying mechanisms were investigated by a combination of laser flash photolysis (LFP) and reduction potential analysis. The LFP results show that the excited AA serves as a better electron shuttle than QH₂. As a consequence, AA might regulate the redox cycling of quinones, leading to significant effects on many processes, ranging from photosynthesis and respiration to photodegradation.

Effects of Sulfidation and Nitrate on the Reduction of N-Nitrosodimethylamine by Zerovalent Iron

Competition among oxidizing species in groundwater and wastewater for the reductive capacity of zerovalent iron (ZVI) makes the selectivity of ZVI for target contaminant degradation over other reduction pathways a major determinant of the feasibility of ZVI-based water treatment processes. The selectivity for reduction of contaminants over water is improved by sulfidation, but the effect of sulfidation on other competing reactions is not known. The interaction between these competing reactions was investigated using N-nitrosodimethylamine (NDMA) as the target contaminant, nitrate as a co-contaminant, and micrometer-sized ZVI with and without sulfidation. Unsulfidated ZVI reduced NDMA to dimethylamine via N,N-dimethylhydrazine, but the addition of nitrate decreased the rate of NDMA reduction and increased the quantity of intermediate observed. With sulfidated ZVI, the kinetics and products of NDMA reduction were similar to those with unsulfidated ZVI, but no inhibitory effect of nitrate was observed. Conversely, the reduction of nitrate-which dominated NDMA reduction in unsulfidated ZVI systems-was strongly inhibited by sulfidation. H₂ and Fe²⁺ generation by sulfidated ZVI was almost independent of nitrate concentration. Therefore, sulfidation improved the efficiency of NDMA reduction by ZVI in the presence of nitrate mainly by inhibiting nitrate reduction. The shift in selectivity of ZVI for NDMA over nitrate upon sulfidation was due to replacement of Fe⁰/Fe_xO_y surface sites with FeS.

Overlooked Role of Peroxides as Free Radical Precursors in Advanced Oxidation Processes

Research efforts on advanced oxidation processes (AOPs) have long been focused on the fundamental chemistry of activation processes and free radical reactions. Little attention has been paid to the chemistry of the precursor oxidants. Herein, we found that the precursor oxidants could lead to quite different outcomes. A counterintuitive result was observed in the photoreduction of bromate/iodate: the combination of H₂O₂ and UV enhanced the reduction of bromate/iodate, whereas the addition of persulfate to the UV system led to an inhibitory effect. Thermodynamic and kinetic evidence suggests that the reduction of bromate in UV/H₂O₂ was attributable to the direct reaction between HOBr and H₂O₂. Both experimental determination and kinetic simulation demonstrate that the reaction between HOBr and H₂O₂ dominated over the ^•OH-mediated reactions. These results suggest that H₂O₂ possesses some particular redox properties that distinguish it from other peroxides. The prototypical UV/H₂O₂ process is not always an AOP: it can also be an enhanced reduction process for chemicals with intermediates that are reducible by H₂O₂. Considering the increasing interest in persulfate-based AOPs, the results of this study identify some novel advantages of the classical H₂O₂-based AOPs.

Unique Structural Characteristics of Catalytic Palladium/Gold Nanoparticles on Graphene

Adding Au to Pd nanoparticles (NPs) can impart high catalytic activity with respect to hydrogenation of a wide range of substances. These materials are often synthesized by reducing metallic precursors; hence, sonochemical and solvothermal processes are commonly used to anchor these bimetals onto thin supports, including graphene. Although similar NPs have been studied reasonably well, a clear understanding of structural characteristics relative to their synthesis parameters is lacking, due to limitations in characterization techniques, which may prevent optimization of this very promising catalyst. In this report, a strategic approach has been used to identify this structural and material synthesis correlation, starting with controlled sample preparation and followed by detailed characterization. This includes advanced scanning transmission electron microscopy and electron energy loss spectroscopy; the latter using a state-of-the-art instrumentation to map the distribution of Pd and Au, and to identify chemical state of the Pd NPs, which has not been previously reported. Results show that catalytic bimetal NP clusters were made of small zero-valent Pd NPs aggregating to form a shell around an Au core. Not only can the described characterization approach be applied to similar material systems, but the results can guide the optimization of the synthesis procedures.

Modeling the Kinetics of Hydrogen Formation by Zerovalent Iron: Effects of Sulfidation on Micro- and Nano-Scale Particles

The hydrogen evolution reaction (HER) that generates H₂ from the reduction of H₂O by Fe⁰ is among the most fundamental of the processes that control reactivity in environmental systems containing zerovalent iron (ZVI). To develop a comprehensive kinetic model for this process, a large and high-resolution data set for HER was measured using five types of ZVI pretreated by acid-washing and/or sulfidation (in pH 7 HEPES buffer). The data were fit to four alternative kinetic models using nonlinear regression analysis applied to the whole data set simultaneously, which allowed some model parameters to be treated globally across multiple experiments. The preferred model uses two independent reactive phases to match the two-stage character of most HER data, with rate constants ( k's) for each phase fitted globally by iron type and phase quantities ( S's) fitted as fully local (independent) parameters. The first, faster stage was attributed to a reactive mineral intermediate (RMI) phase like Fe(OH)₂, which may form in all experiments during preequilibration, but is rapidly consumed, leaving the second, slower stage of HER, which is due to reaction of Fe⁰. In addition to providing a deterministic model to explain the kinetics of HER by ZVI over a wide range of conditions, the results provide an improved quantitative basis for comparing the effects of sulfidation on ZVI.

Planetary Health thematic web collection

Associate Editor Paul Tratnyek and Guest Editor Joe Needoba introduce the Environmental Science: Processes & Impacts Planetary Health collection.

Dynamic interactions between sulfidated zerovalent iron and dissolved oxygen: Mechanistic insights for enhanced chromate removal

Recent research on contaminant removal by zerovalent iron (ZVI) has evolved from investigating simple model systems to systems that encompass increased dimensions of complexity. Sulfidation and aerobic conditions are two of the most broadly relevant complications. Combining these two, this study investigated the dynamic interactions between sulfidated microscale ZVI and dissolved O₂, for removal of Cr(VI), a model contaminant for metals and metalloids. The results show that the coupling of sulfidation and oxygenation significantly improves Cr removal, which is attributed to enhanced Fe(II) production that resulted from accelerated corrosion of Fe(0). The Cr(VI) removal rate increased with increasing O₂ saturation from 0% to 100% but showed a bimodal dependence on the S/Fe ratio. At the optimal S/Fe ratio, the ZVI exhibits a highly porous surface morphology, which, according to prior literature on sulfur induced corrosion, promotes corrosion. In addition, a novel time series correlation was developed between aqueous Fe(II) and Cr(VI) based on data collected in the presence and absence of 1,10-phenanthroline, to probe for changes of reductants during the reaction time course. The analysis indicated that Fe(0) was responsible for the initial small amount of Cr(VI) removal, which then transitioned to a phase controlled by surface Fe(II). The slopes of the time series correlations during the latter phase of the reaction vary with experimental conditions but are mostly much higher than the theoretical stoichiometric ratio between Cr(VI) and Fe(II) (i.e., 0.33), indicating that Fe(II) regeneration contributes significantly to Cr removal.

Sulfidation of Iron-Based Materials: A Review of Processes and Implications for Water Treatment and Remediation

Iron-based materials used in water treatment and groundwater remediation-especially micro- and nanosized zerovalent iron (nZVI)-can be more effective when modified with lower-valent forms of sulfur (i.e., "sulfidated"). Controlled sulfidation for this purpose (using sulfide, dithionite, etc.) is the main topic of this review, but insights are derived by comparison with related and comparatively well-characterized processes such as corrosion of iron in sulfidic waters and abiotic natural attenuation by iron sulfide minerals. Material characterization shows that varying sulfidation protocols (e.g., concerted or sequential) and key operational variables (e.g., S/Fe ratio and sulfidation duration) result in materials with structures and morphologies ranging from core-shell to multiphase. A meta-analysis of available kinetic data for dechlorination under anoxic conditions, shows that sulfidation usually increases dechlorination rates, and simultaneously hydrogen production is suppressed. Therefore, sulfidation can greatly improve the efficiency of utilization of reducing equivalents for contaminant removal. This benefit is most likely due to inhibited corrosion as a result of sulfidation. Sulfidation may also favor desirable pathways of contaminant removal, such as (i) dechlorination by reductive elimination rather than hydrogenolysis and (ii) sequestration of metals as sulfides that could be resistant to reoxidation. Under oxic conditions, sulfidation is shown to enhance heterogeneous catalytic oxidation of contaminants. These net effects of sulfidation on contaminant removal by iron-based materials may substantially improve their practical utility for water treatment and remediation of contaminated groundwater.

Mechanochemically Sulfidated Microscale Zero Valent Iron: Pathways, Kinetics, Mechanism, and Efficiency of Trichloroethylene Dechlorination

In water treatment processes that involve contaminant reduction by zerovalent iron (ZVI), reduction of water to dihydrogen is a competing reaction that must be minimized to maximize the efficiency of electron utilization from the ZVI. Sulfidation has recently been shown to decrease H₂ formation significantly, such that the overall electron efficiency of (or selectivity for) contaminant reduction can be greatly increased. To date, this work has focused on nanoscale ZVI (nZVI) and solution-phase sulfidation agents (e.g., bisulfide, dithionite or thiosulfate), both of which pose challenges for up-scaling the production of sulfidated ZVI for field applications. To overcome these challenges, we developed a process for sulfidation of microscale ZVI by ball milling ZVI with elemental sulfur. The resulting material (S-mZVI^bm) exhibits reduced aggregation, relatively homogeneous distribution of Fe and S throughout the particle (not core-shell structure), enhanced reactivity with trichloroethylene (TCE), less H₂ formation, and therefore greatly improved electron efficiency of TCE dechlorination (ε_e). Under ZVI-limited conditions (initial Fe⁰/TCE = 1.6 mol/mol), S-mZVI^bm gave surface-area normalized reduction rate constants (k'_SA) and ε_e that were ∼2- and 10-fold greater than the unsulfidated ball-milled control (mZVI^bm). Under TCE-limited conditions (initial Fe⁰/TCE = 2000 mol/mol), sulfidation increased k_SA and ε_e ≈ 5- and 50-fold, respectively. The major products from TCE degradation by S-mZVI^bm were acetylene, ethene, and ethane, which is consistent with dechlorination by β-elimination, as is typical of ZVI, iron oxides, and/or sulfides. However, electrochemical characterization shows that the sulfidated material has redox properties intermediate between ZVI and Fe₃O₄, mostly likely significant coverage of the surface with FeS.

In silico environmental chemical science: properties and processes from statistical and computational modelling

Quantitative structure-activity relationships (QSARs) have long been used in the environmental sciences. More recently, molecular modeling and chemoinformatic methods have become widespread. These methods have the potential to expand and accelerate advances in environmental chemistry because they complement observational and experimental data with "in silico" results and analysis. The opportunities and challenges that arise at the intersection between statistical and theoretical in silico methods are most apparent in the context of properties that determine the environmental fate and effects of chemical contaminants (degradation rate constants, partition coefficients, toxicities, etc.). The main example of this is the calibration of QSARs using descriptor variable data calculated from molecular modeling, which can make QSARs more useful for predicting property data that are unavailable, but also can make them more powerful tools for diagnosis of fate determining pathways and mechanisms. Emerging opportunities for "in silico environmental chemical science" are to move beyond the calculation of specific chemical properties using statistical models and toward more fully in silico models, prediction of transformation pathways and products, incorporation of environmental factors into model predictions, integration of databases and predictive models into more comprehensive and efficient tools for exposure assessment, and extending the applicability of all the above from chemicals to biologicals and materials.

Oxidation potentials of phenols and anilines: correlation analysis of electrochemical and theoretical values

Phenols and anilines have been studied extensively as reductants of environmental oxidants (such as manganese dioxide) and as reductates (e.g., model contaminants) that are transformed by environmental oxidants (ozone, triple organic matter, etc.). The thermodynamics and kinetics of these reactions have been interpreted using oxidation potentials for substituted phenols and anilines, often using a legacy experimental dataset that is of uncertain quality. Although there are many alternative oxidation potential data, there has been little systematic analysis of the relevance, reliability, and consistency of the data obtained by different methods. We have done this through an extensive correlation analysis of kinetic data for phenol or aniline oxidation by manganese oxide-compiled from multiple sources-and oxidation potentials obtained from (i) electrochemical measurements using cyclic and square wave voltammetry and (ii) theoretical calculations using density functional theory. Measured peak potentials (E_p) from different sources and experimental conditions correlate very strongly, with minimal root mean squared error (RMSE), slopes ≈ 1, and intercepts indicative of consistent absolute differences of 50-150 mV; whereas, one-electron oxidation potentials (E₁) from different sources and theoretical conditions exhibit large RMSE, slopes, and intercepts vs. measured oxidation potentials. Calibration of calculated E₁ data vs. measured E_p data gave corrected values of E₁ with improved accuracy. For oxidation by manganese dioxide, normalization of rate constants (to the 4-chloro congener) allowed correlation of phenol and aniline data from multiple sources to give one, unified quantitative structure-activity relationship (QSAR). Comparison among these QSARs illustrates the principle of matching the observational vs. mechanistic character of the response and descriptor variables.

QSARs and computational chemistry methods in environmental chemical sciences

Guest editors Kathrin Fenner and Paul Tratnyek introduce the themed issue on “QSARs and computational chemistry methods in environmental chemical sciences” of Environmental Science: Processes & Impacts.

Effects of Sulfidation, Magnetization, and Oxygenation on Azo Dye Reduction by Zerovalent Iron

Applications of zerovalent iron (ZVI) for water treatment under aerobic conditions include sequestration of metals (e.g., in acid mine drainage) and decolorization of dyes (in wastewaters from textile manufacturing). The processes responsible for contaminant removal can be a complex mixture of reduction, oxidation, sorption, and coprecipitation processes, which are further complicated by the dynamics of oxygen intrusion, mixing, and oxide precipitation. To better understand such systems, the removal of an azo dye (Orange I) by micron-sized granular ZVI at neutral pH was studied in open (aerobic) stirred batch reactors, by measuring the kinetics of Orange I decolorization and changes in "geochemical" properties (DO, Fe(II), and Eh), with and without two treatments that might improve the long-term performance of this system: sulfidation by pretreatment with sulfide and magnetization by application of a weak magnetic field (WMF). The results show that the changes in solution chemistry are coupled to the dynamics of oxygen intrusion, which was modeled as analogous to dissolved oxygen sag curves. Both sulfidation and magnetization increased Orange I removal rates 2.4-71.8-fold, but there was little synergistic benefit to applying both enhancements together. Respike experiments showed that the enhancement from magnetization carries over from magnetization to sulfidation, but not the reverse.

Sulfidation of Nano Zerovalent Iron (nZVI) for Improved Selectivity During In-Situ Chemical Reduction (ISCR)

The high reactivity of nano zerovalent iron (nZVI) leads to inefficient treatment due to competition with various natural reductant demand (NRD) processes, especially the reduction of water to hydrogen. Here we show that this limitation can be alleviated by sulfidation (i.e., modification by reducing sulfur compounds). nZVI synthesized on carboxylmethylcelluose (CMC-nZVI) was sulfidated with either sulfide or dithionite. The reactivity of the resulting materials was examined with three complementary assays: (i) direct measurement of hydrogen production, (ii) reduction of a colorimetric redox probe (indigo disulfonate, I2S), and (iii) dechlorination of trichloroethylene (TCE). The results indicate that sulfidation at S/Fe molar ratios of ≥0.3, effectively eliminates reaction with water, but retains significant reactivity with TCE. However, sulfidation with sulfide leaves most of the nZVI as Fe(0), whereas dithionite converts a majority of the nZVI to FeS (thus consuming much of the reducing capacity originally provided by the Fe(0)). Simplified numerical models show that the reduction kinetics of I2S and TCE are mainly dependent on the initial reducing equivalents and that the TCE reduction rate is affected by the aging of FeS. Overall, the results suggest that pretreatment of nZVI with reducing sulfur compounds could result in substantial improvement in nZVI selectivity.

Structure-Activity Relationships for Rates of Aromatic Amine Oxidation by Manganese Dioxide

New energetic compounds are designed to minimize their potential environmental impacts, which includes their transformation and the fate and effects of their transformation products. The nitro groups of energetic compounds are readily reduced to amines, and the resulting aromatic amines are subject to oxidation and coupling reactions. Manganese dioxide (MnO2) is a common environmental oxidant and model system for kinetic studies of aromatic amine oxidation. In this study, a training set of new and previously reported kinetic data for the oxidation of model and energetic-derived aromatic amines was assembled and subjected to correlation analysis against descriptor variables that ranged from general purpose [Hammett σ constants (σ(-)), pKas of the amines, and energies of the highest occupied molecular orbital (EHOMO)] to specific for the likely rate-limiting step [one-electron oxidation potentials (Eox)]. The selection of calculated descriptors (pKa, EHOMO, and Eox) was based on validation with experimental data. All of the correlations gave satisfactory quantitative structure-activity relationships (QSARs), but they improved with the specificity of the descriptor. The scope of correlation analysis was extended beyond MnO2 to include literature data on aromatic amine oxidation by other environmentally relevant oxidants (ozone, chlorine dioxide, and phosphate and carbonate radicals) by correlating relative rate constants (normalized to 4-chloroaniline) to EHOMO (calculated with a modest level of theory).

Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals

Increasing recognition that abiotic natural attenuation (NA) of chlorinated solvents can be important has created demand for improved methods to characterize the redox properties of the aquifer materials that are responsible for abiotic NA. This study explores one promising approach: using chemical reactivity probes (CRPs) to characterize the thermodynamic and kinetic aspects of contaminant reduction by reducing iron minerals. Assays of thermodynamic CRPs were developed to determine the reduction potentials (ECRP) of suspended minerals by spectrophotometric determination of equilibrium CRP speciation and calculations using the Nernst equation. ECRP varied as expected with mineral type, mineral loading, and Fe(II) concentration. Comparison of ECRP with reduction potentials measured potentiometrically using a Pt electrode (EPt) showed that ECRP was 100-150 mV more negative than EPt. When EPt was measured with small additions of CRPs, the systematic difference between EPt and ECRP was eliminated, suggesting that these CRPs are effective mediators of electron transfer between mineral and electrode surfaces. Model contaminants (4-chloronitrobenzene, 2-chloroacetophenone, and carbon tetrachloride) were used as kinetic CRPs. The reduction rate constants of kinetic CRPs correlated well with the ECRP for mineral suspensions. Using the rate constants compiled from literature for contaminants and relative mineral reduction potentials based on ECRP measurements, qualitatively consistent trends were obtained, suggesting that CRP-based assays may be useful for estimating abiotic NA rates of contaminants in groundwater.

Sequestration of Antimonite by Zerovalent Iron: Using Weak Magnetic Field Effects to Enhance Performance and Characterize Reaction Mechanisms

Many oxyanion-forming metals (As, Sb, Se, Tc, etc.) can be removed from water by adsorption and/or redox reactions involving iron oxides, including the oxides associated with zerovalent iron (ZVI). The rate of antimonite (Sb(III) hydrolysis species) removal by ZVI was determined in open, well-mixed batch reactors as a function of experimental factors, including aging of the ZVI, addition of Fe(II), Sb dose, mixing rate, pH, initial concentrations of Sb(III), etc. However, the largest effect observed was the roughly 6-8 fold increase in Sb(III) removal rate due to the application of a weak magnetic field (WMF) during the experiments. The WMF effect on Sb removal arises from stimulated corrosion and delayed passivation of the ZVI, as evidenced by time series correlation analysis of "geochemical" properties (DO, Fetot, Eh, and pH) measured synchronously in each experiment. The removal of Sb under the conditions of this study was mainly due to oxidation of Sb(III) to Sb(V) and adsorption and coprecipitation onto the iron oxides formed from accelerated corrosion of ZVI, as evidenced by Sb K-edge XANES, EXAFS, and XPS. The degree of the WMF enhancement for Sb(III) was found to be similar to the WMF effect reported previously for Sb(V), As(III), As(V), and Se(VI).

Activation of Manganese Oxidants with Bisulfite for Enhanced Oxidation of Organic Contaminants: The Involvement of Mn(III)

MnO4(-) was activated by HSO3(-), resulting in a process that oxidizes organic contaminants at extraordinarily high rates. The permanganate/bisulfite (PM/BS) process oxidized phenol, ciprofloxacin, and methyl blue at pHini 5.0 with rates (kobs ≈ 60-150 s(-1)) that were 5-6 orders of magnitude faster than those measured for permanganate alone, and ∼5 to 7 orders of magnitude faster than conventional advanced oxidation processes for water treatment. Oxidation of phenol was fastest at pH 4.0, but still effective at pH 7.0, and only slightly slower when performed in tap water. A smaller, but still considerable (∼3 orders of magnitude) increase in oxidation rates of methyl blue was observed with MnO2 activated by HSO3(-) (MO/BS). The above results, time-resolved spectroscopy of manganese species under various conditions, stoichiometric analysis of pH changes, and the effect of pyrophosphate on UV absorbance spectra suggest that the reactive intermediate(s) responsible for the extremely rapid oxidation of organic contaminants in the PM/BS process involve manganese(III) species with minimal stabilization by complexation. The PM/BS process may lead to a new category of advanced oxidation technologies based on contaminant oxidation by reactive manganese(III) species, rather than hydroxyl and sulfate radicals.

Methods for characterizing the fate and effects of nano zerovalent iron during groundwater remediation

The emplacement of nano zerovalent iron (nZVI) for groundwater remediation is usually monitored by common measurements such as pH, total iron content, and oxidation-reduction potential (ORP) by potentiometry. However, the interpretation of such measurements can be misleading because of the complex interactions between the target materials (e.g., suspensions of highly reactive and variably aggregated nanoparticles) and aquifer materials (sediments and groundwater), and multiple complications related to sampling and detection methods. This paper reviews current practice for both direct and indirect characterizations of nZVI during groundwater remediation and explores prospects for improving these methods and/or refining the interpretation of these measurements. To support our recommendations, results are presented based on laboratory batch and column studies of nZVI detection using chemical, electrochemical, and geophysical methods. Chemical redox probes appear to be a promising new method for specifically detecting nZVI, based on laboratory tests. The potentiometric and voltammetric detections of iron nanoparticles, using traditional stationary disc electrodes, rotating disc electrodes, and flow-through cell disc electrodes, provide insight for interpreting ORP measurements, which are affected by solution chemistry conditions and the interactions between iron nanoparticles and the electrode surface. The geophysical methods used for characterizing ZVI during groundwater remediation are reviewed and its application for nZVI detection is assessed with results of laboratory column experiments.

Predicting reduction rates of energetic nitroaromatic compounds using calculated one-electron reduction potentials

The evaluation of new energetic nitroaromatic compounds (NACs) for use in green munitions formulations requires models that can predict their environmental fate. Previously invoked linear free energy relationships (LFER) relating the log of the rate constant for this reaction (log(k)) and one-electron reduction potentials for the NAC (E1NAC) normalized to 0.059 V have been re-evaluated and compared to a new analysis using a (nonlinear) free-energy relationship (FER) based on the Marcus theory of outer-sphere electron transfer. For most reductants, the results are inconsistent with simple rate limitation by an initial, outer-sphere electron transfer, suggesting that the linear correlation between log(k) and E1NAC is best regarded as an empirical model. This correlation was used to calibrate a new quantitative structure-activity relationship (QSAR) using previously reported values of log(k) for nonenergetic NAC reduction by Fe(II) porphyrin and newly reported values of E1NAC determined using density functional theory at the M06-2X/6-311++G(2d,2p) level with the COSMO solvation model. The QSAR was then validated for energetic NACs using newly measured kinetic data for 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), and 2,4-dinitroanisole (DNAN). The data show close agreement with the QSAR, supporting its applicability to other energetic NACs.

Oxidative remobilization of technetium sequestered by sulfide-transformed nano zerovalent iron

Our previous study showed that formation of TcS2-like phases is favored over TcO2 under sulfidic conditions stimulated by nano zerovalent iron. This study further investigates the stability of Tc(IV) sulfide upon reoxidation by solution chemistry, solid phase characterization, and X-ray absorption spectroscopy. Tc dissolution data showed that Tc(VII) reduced by sulfide-transformed nZVI has substantially slower reoxidation kinetics than Tc(VII) reduced by nZVI only. The initial inhibition of Tc(IV) dissolution at S/Fe = 0.112 is due to the redox buffer capacity of FeS, which is evidenced by the parallel trends in oxidation-reduction potentials (ORP) and Tc dissolution kinetics. The role of FeS in inhibiting Tc oxidation is further supported by the Mössbauer spectroscopy and micro X-ray diffraction data at S/Fe = 0.112, showing persistence of FeS after 24-h oxidation but complete oxidation after 120-h oxidation. X-ray absorption spectroscopy data for S/Fe = 0.011 showed significantly increasing percentages of TcS2 in the solid phase after 24-h oxidation, indicating stronger resistance of TcS2 to oxidation. At S/Fe = 0.112, the XAS results revealed significant transformation of Tc speciation from TcS2 to TcO2 after 120-h oxidation. Given that no apparent Tc dissolution occurred during this period, the speciation transformation might play a secondary role in hindering Tc oxidation. Collectively, the results indicate that sequestrating Tc as TcS2 under stimulated sulfate reduction is a promising strategy to improve the long-term stability of reduced Tc in subsurface remediation.

Coupled effects of aging and weak magnetic fields on sequestration of selenite by zero-valent iron

The sequestration of Se(IV) by zero-valent iron (ZVI) is strongly influenced by the coupled effects of aging ZVI and the presence of a weak magnetic field (WMF). ZVI aged at pH 6.0 with MES as buffer between 6 and 60 h gave nearly constant rates of Se(IV) removal with WMF but with rate constants that are 10- to 100-fold greater than without. XANES analysis showed that applying WMF changes the mechanism of Se(IV) removal by ZVI aged for 6-60 h from adsorption followed by reduction to direct reduction. The strong correlation between Se(IV) removal and Fe2+ release suggests direct reduction of Se(IV) to Se(0) by Fe0, in agreement with the XANES analysis. The numerical simulation of ZVI magnetization revealed that the WMF influence on Se(IV) sequestration is associated mainly with the ferromagnetism of ZVI and the paramagnetism of Fe2+. In the presence of the WMF, the Lorentz force gives rise to convection in the solution, which narrows the diffusion layer, and the field gradient force, which tends to move paramagnetic ions (esp. Fe2+) along the higher field gradient at the ZVI particle surface, thereby inducing nonuniform depassivation and eventually localized corrosion of the ZVI surface.

Effects of metal ions on the reactivity and corrosion electrochemistry of Fe/FeS nanoparticles

Nano-zerovalent iron (nZVI) formed under sulfidic conditions results in a biphasic material (Fe/FeS) that reduces trichloroethene (TCE) more rapidly than nZVI associated only with iron oxides (Fe/FeO). Exposing Fe/FeS to dissolved metals (Pd(2+), Cu(2+), Ni(2+), Co(2+), and Mn(2+)) results in their sequestration by coprecipitation as dopants into FeS and FeO and/or by electroless precipitation as zerovalent metals that are hydrogenation catalysts. Using TCE reduction rates to probe the effect of metal amendments on the reactivity of Fe/FeS, it was found that Mn(2+) and Cu(2+) decreased TCE reduction rates, while Pd(2+), Co(2+), and Ni(2+) increased them. Electrochemical characterization of metal-amended Fe/FeS showed that aging caused passivation by growth of FeO and FeS phases and poisoning of catalytic metal deposits by sulfide. Correlation of rate constants for TCE reduction (kobs) with electrochemical parameters (corrosion potentials and currents, Tafel slopes, and polarization resistance) and descriptors of hydrogen activation by metals (exchange current density for hydrogen reduction and enthalpy of solution into metals) showed the controlling process changed with aging. For fresh Fe/FeS, kobs was best described by the exchange current density for activation of hydrogen, whereas kobs for aged Fe/FeS correlated with electrochemical descriptors of electron transfer.

Disinfection of ballast water with iron activated persulfate

The treatment of ballast water carried onboard ships is critical to reduce the spread of nonindigenous aquatic organisms that potentially include noxious and harmful taxa. We tested the efficacy of persulfate (peroxydisulfate, S2O8(2-), PS) activated with zerovalent iron (Fe(0)) as a chemical biocide against two taxa of marine phytoplankton grown in bench-scale, batch cultures: the diatom, Pseudonitzshia delicatissima and the green alga, Dunaliella tertiolecta . After testing a range of PS concentrations (0-4 mM activated PS) and exposure times (1-7 days), we determined that a dosage of 4 mM of activated PS was required to inactivate cells from both species, as indicated by reduced or halted growth and a reduction in photosynthetic performance. Longer exposure times were required to fully inactivate D. tertiolecta (7 days) compared to P. delicatissima (5 days). Under these conditions, no recovery was observed upon placing cells from the exposed cultures into fresh media lacking biocide. The results demonstrate that activated PS is an effective chemical biocide against species of marine phytoplankton. The lack of harmful byproducts produced during application makes PS an attractive alternative to other biocides currently in use for ballast water treatments and merits further testing at a larger scale.

Mechanisms and kinetics of alkaline hydrolysis of the energetic nitroaromatic compounds 2,4,6-trinitrotoluene (TNT) and 2,4-dinitroanisole (DNAN)

The environmental impacts of energetic compounds can be minimized through the design and selection of new energetic materials with favorable fate properties. Building predictive models to inform this process, however, is difficult because of uncertainties and complexities in some major fate-determining transformation reactions such as the alkaline hydrolysis of energetic nitroaromatic compounds (NACs). Prior work on the mechanisms of the reaction between NACs and OH(-) has yielded inconsistent results. In this study, the alkaline hydrolysis of 2,4,6-trinitrotoluene (TNT) and 2,4-dinitroanisole (DNAN) was investigated with coordinated experimental kinetic measurements and molecular modeling calculations. For TNT, the results suggest reversible formation of an initial product, which is likely either a Meisenheimer complex or a TNT anion formed by abstraction of a methyl proton by OH(-). For DNAN, the results suggest that a Meisenheimer complex is an intermediate in the formation of 2,4-dinitrophenolate. Despite these advances, the remaining uncertainties in the mechanisms of these reactions-and potential variability between the hydrolysis mechanisms for different NACs-mean that it is not yet possible to generalize the results into predictive models (e.g., quantitative structure-activity relationships, QSARs) for hydrolysis of other NACs.

Reductive sequestration of pertechnetate (⁹⁹TcO₄⁻) by nano zerovalent iron (nZVI) transformed by abiotic sulfide

Under anoxic conditions, soluble pertechnetate (⁹⁹TcO₄⁻) can be reduced to less soluble TcO₂·nH₂O, but the oxide is highly susceptible to reoxidation. Here we investigate an alternative strategy for remediation of Tc-contaminated groundwater whereby sequestration as Tc sulfide is favored by sulfidic conditions stimulated by nano zerovalent iron (nZVI). nZVI was pre-exposed to increasing concentrations of sulfide in simulated Hanford groundwater for 24 h to mimic the onset of aquifer biotic sulfate reduction. Solid-phase characterizations of the sulfidated nZVI confirmed the formation of nanocrystalline FeS phases, but higher S/Fe ratios (>0.112) did not result in the formation of significantly more FeS. The kinetics of Tc sequestration by these materials showed faster Tc removal rates with increasing S/Fe between 0 and 0.056, but decreasing Tc removal rates with S/Fe > 0.224. The more favorable Tc removal kinetics at low S/Fe could be due to a higher affinity of TcO₄⁻ for FeS than iron oxides, and electron microscopy confirmed that the majority of the Tc was associated with FeS phases. The inhibition of Tc removal at high S/Fe appears to have been caused by excess HS(-). X-ray absorption spectroscopy revealed that as S/Fe increased, the pathway for Tc(IV) formation shifted from TcO₂·nH2₂ to Tc sulfide phases. The most substantial change of Tc speciation occurred at low S/Fe, coinciding with the rapid increase in Tc removal rate. This agreement further confirms the importance of FeS in Tc sequestration.