Influence of Anionic Cosolutes and pH on Nanoscale Zerovalent Iron Longevity: Time Scales and Mechanisms of Reactivity Loss toward 1,1,1,2-Tetrachloroethane and Cr(VI)

Nanoscale zerovalent iron (NZVI) was aged over 30 days in suspension (2 g/L) with different anions (chloride, perchlorate, sulfate, carbonate, nitrate), anion concentrations (5, 25, 100 mN), and pH (7, 8). During aging, suspension samples were reacted periodically with 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) and Cr­(VI) to determine the time scales and primary mode of NZVI reactivity loss. Rate constants for 1,1,1,2-TeCA reduction in Cl^–, SO₄^2–, and ClO₄^– suspensions decreased by 95% over 1 month but were generally equivalent to one another, invariant of concentration and independent of pH. In contrast, longevity toward 1,1,1,2-TeCA depended upon NO₃^– and HCO₃^– concentration, with complete reactivity loss over 1 and 14 days, respectively, in 25 mN suspensions. X-ray diffraction suggests that reactivity loss toward 1,1,1,2-TeCA in most systems results from Fe(0) conversion into magnetite, whereas iron carbonate hydroxide formation limits reactivity in HCO₃^– suspensions. Markedly different trends in Cr­(VI) removal capacity (mg Cr/g NZVI) were observed during aging, typically exhibiting greater longevity and a pronounced pH-dependence. Notably, a strong linear correlation exists between Cr­(VI) removal capacities and rates of Fe­(II) production measured in the absence of Cr­(VI). While Fe(0) availability dictates longevity toward 1,1,1,2-TeCA, this correlation suggests surface-associated Fe­(II) species are primarily responsible for Cr­(VI) reduction

Chlorinated Solvent Transformation by Palladized Zerovalent Iron: Mechanistic Insights from Reductant Loading Studies and Solvent Kinetic Isotope Effects

Palladized nanoscale zerovalent iron (Pd/NZVI) has been utilized for source zone control, yet the reductant responsible for pollutant transformation and the optimal conditions for subsurface application remain poorly understood. Here, trends in Pd/Fe reactivity toward 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) and <i>cis</i>-dichloroethene (<i>cis</i>-DCE) were compared in H₂O and D₂O batch systems as a function of pH, chlorinated solvent concentration, Pd surface loading, Pd/Fe mass loading, Pd/Fe aging time, and zerovalent iron [Fe(0)] particle size. For Pd/NZVI, the solvent kinetic isotope effect [i.e., <i>k</i>_obs(H₂O)/<i>k</i>_obs(D₂O) or SKIE] for 1,1,1,2-TeCA and <i>cis</i>-DCE reduction increased substantially with Pd loading and Pd/NZVI concentration, evidence that multiple pathways exist for chlorinated solvent reduction. At low Pd loadings and Pd/NZVI concentrations with relatively small SKIEs (less than ∼5), we propose that modest reactivity enhancements (≤10-fold) reflect more efficient electron transfer to 1,1,1,2-TeCA from Fe(0) facilitated by Pd nanodeposits. Much larger SKIEs (e.g., exceeding 100 for <i>cis</i>-DCE) imply the involvement of atomic hydrogen in more reactive systems with high Pd loadings and Pd/NZVI concentrations. Generally, evidence of SKIEs supporting a dominant role for atomic hydrogen was not observed for Pd/Fe prepared from micrometer-sized Fe(0), or for any size of nonpalladized Fe(0). During anaerobic aging of Pd/NZVI, decreases in the SKIE for 1,1,1,2-TeCA reduction suggest that atomic hydrogen will contribute to reactivity for only approximately 1 week after application

Sorption and Mineral-Promoted Transformation of Synthetic Hormone Growth Promoters in Soil Systems

This work examines the fate of synthetic growth promoters (trenbolone acetate, melengestrol acetate, and zeranol) in sterilized soil systems, focusing on their sorption to organic matter and propensity for mineral-promoted reactions. In organic-rich soil matrices (e.g., Pahokee Peat), the extent and reversibility of sorption did not generally correlate with compound hydrophobicity (e.g., <i>K</i>_ow values), suggesting that specific binding interactions (e.g., potentially hydrogen bonding through C17 hydroxyl groups for the trenbolone and melengestrol families) can also contribute to uptake. In soils with lower organic carbon contents (1–5.9% OC), evidence supports sorption occurring in parallel with surface reaction on inorganic mineral phases. Subsequent experiments with pure mineral phases representative of those naturally abundant in soil (e.g., iron, silica, and manganese oxides) suggest that growth promoters are prone to mineral-promoted oxidation, hydrolysis, and/or nucleophilic (e.g., H₂O or OH^–) addition reactions. Although reaction products remain unidentified, this study shows that synthetic growth promoters can undergo abiotic transformation in soil systems, a previously unidentified fate pathway with implications for their persistence and ecosystem effects in the subsurface

Phototransformation Rates and Mechanisms for Synthetic Hormone Growth Promoters Used in Animal Agriculture

Trenbolone acetate, melengestrol acetate, and zeranol are synthetic hormones extensively used as growth promoters in animal agriculture, yet despite occurrence in water and soil little is known about their environmental fate. Here, we establish the time scales and mechanisms by which these synthetic growth promoters and their metabolites (SGPMs) undergo phototransformation in sunlit surface waters. The families of trenbolone acetate (including 17β-trenbolone, 17α-trenbolone, and trendione) and melengestrol acetate (including melengestrol) readily undergo direct photolysis, exhibiting half-lives between ∼0.25 and 1 h in both natural and simulated sunlight that were largely insensitive to solution variables (e.g., pH, temperature, and cosolutes). Direct photolysis yielded products that not only are more photostable but also maintain their steroidal ring structure and therefore may retain some biological activity. In contrast, zeranol, β-zearalanol, and zearalanone only exhibited reactivity in irradiated solutions of model humic and fulvic acids, and rates of indirect photolysis increased steadily from pH 7 to 9. Use of selective probe and quencher compounds suggest hydroxyl radical and triplet state dissolved organic matter are responsible for zeranol family decay at neutral pH, although singlet oxygen contributes modestly in more alkaline waters. This observed pH-dependence appears to result from photooxidants reacting primarily with the monodeprotonated form of zeranol (p<i>K</i>_a values of 8.44 and 11.42). This investigation provides the first characterization of the fate of this emerging pollutant class in sunlit surface waters and prioritizes future efforts on the identity, fate, and biological impact of their more persistent phototransformation products

Dissolution of Hematite Nanoparticle Aggregates: Influence of Primary Particle Size, Dissolution Mechanism, and Solution pH

The size-dependent dissolution of nanoscale hematite (8 and 40 nm α-Fe₂O₃) was examined across a broad range of pH (pH 1–7) and mechanisms including proton- and ligand- (oxalate-) promoted dissolution and dark (ascorbic acid) and photochemical (oxalate) reductive dissolution. Empirical relationships between dissolution rate and pH revealed that suspensions of 8 nm hematite exhibit between 3.3- and 10-fold greater reactivity per unit mass than suspensions of 40 nm particles across all dissolution modes and pH, including circumneutral. Complementary suspension characterization (i.e., sedimentation studies and dynamic light scattering) indicated extensive aggregation, with steady-state aggregate sizes increasing with pH but being roughly equivalent for both primary particles. Thus, while the reactivity difference between 8 and 40 nm suspensions is generally greater than expected from specific surface areas measured via N₂–BET or estimated from primary particle geometry, loss of reactive surface area during aggregation limits the certainty of such comparisons. We propose that the relative reactivity of 8 and 40 nm hematite suspensions is best explained by differences in the fraction of aggregate surface area that is reactive. This scenario is consistent with TEM images revealing uniform dissolution of aggregated 8 nm particles, whereas 40 nm particles within aggregates undergo preferential etching at edges and structural defects. Ultimately, we show that comparably sized hematite aggregates can exhibit vastly different dissolution activity depending on the nature of the primary nanoparticles from which they are constructed, a result with wide-ranging implications for iron redox cycling

α‑Fe₂O₃ Nanoparticles as Oxygen Carriers for Chemical Looping Combustion: An Integrated Materials Characterization Approach to Understanding Oxygen Carrier Performance, Reduction Mechanism, and Particle Size Effects

Through continuous flow reactor experiments, materials characterization, and theoretical calculations, we provide new insights into the reduction of hematite (α-Fe₂O₃) nanoparticles by methane (CH₄) during chemical looping combustion (CLC). Across CLC-relevant temperatures (500–800 °C) and gas flow rates (2.5–250 h^–1), decreasing α-Fe₂O₃ particle size (from 350 to 3 nm) increased the duration over which CH₄ was completely converted to CO₂ (i.e., 100% yield). We attribute this size-dependent performance trend to the greater availability of lattice oxygen atoms in the near-surface region of smaller particles with higher surface area-to-volume ratios. All particle sizes then exhibited a relatively rapid rate of reactivity loss that was size- and temperature-independent, reflecting a greater role for magnetite (Fe₃O₄), the primary α-Fe₂O₃ reduction product, in CH₄ oxidation. Bulk (X-ray diffraction, XRD) and surface (X-ray photoelectron spectroscopy, XPS) analysis revealed that oxygen carrier reduction proceeds via a two-stage solid-state mechanism; α-Fe₂O₃ reduction to Fe₃O₄ followed the unreacted shrinking core model (USCM) while subsequent reduction of Fe₃O₄ to wüstite (FeO) and FeO to iron metal (Fe) followed the nucleation and nuclei growth model (NNGM). Atomistic thermodynamics modeling based on density functional theory supports that reduction initiates via the USCM, as partially reduced α-Fe₂O₃ surfaces exhibited a wide range of stability relative to bulk Fe₃O₄. Reduction and reoxidation cycling experiments were also performed to explore more practical aspects related to the long-term performance of unsupported α-Fe₂O₃ nanoparticles as oxygen carriers for CLC

Lack of Influence of Extracellular Polymeric Substances (EPS) Level on Hydroxyl Radical Mediated Disinfection of <i>Escherichia coli</i>

Photolysis of nitrate, a prevalent constituent in agriculturally impacted waters, may influence pathogen attenuation in such systems through production of hydroxyl radical (^•OH). This study focuses on the efficacy of ^•OH generated during nitrate photolysis in promoting <i>E. coli</i> die-off as a function of extracellular polymeric substances (EPS) coverage. EPS levels of four <i>E. coli</i> isolates were systematically altered through a sonication extraction method and photochemical batch experiments with a solar simulator examined isolate viability loss as a function of time in nitrate solutions. <i>E. coli</i> viability loss over time exhibited two regimes: an initial induction time, <i>t</i>_s, with little decay was followed by rapid exponential decay characterized by a first-order disinfection rate constant, <i>k</i>. Increasing steady-state ^•OH concentrations enhanced <i>E. coli</i> viability loss, increasing values of <i>k</i> and decreasing <i>t</i>_s values, both of which were quantified with a multitarget bacterial disinfection model. Notably, at a given steady-state ^•OH concentration, values of <i>t</i>_s and <i>k</i> were independent of EPS levels, nor did they vary among the different <i>E. coli</i> strains considered. Results herein show that while ^•OH generated via nitrate photolysis enhances rates of disinfection in surface water, the mechanism by which ^•OH kills <i>E. coli</i> is relatively insensitive to common bacterial variables

Microbial Biotransformation Products and Pathways of Dichloroacetamide Herbicide Safeners

Dichloroacetamide safeners are common ingredients in commercial herbicide formulations. We previously investigated the environmental fate of dichloroacetamides via photolysis and hydrolysis, but other potentially important, environmentally relevant fate processes remain uncharacterized and may yield products of concern. Here, we examined microbial biotransformation of two dichloroacetamide safeners, benoxacor and dichlormid, to identify products and elucidate pathways. Using aerobic microcosms inoculated with river sediment, we demonstrated that microbial biotransformations of benoxacor and dichlormid proceed primarily, if not exclusively, via cometabolism. Benoxacor was transformed by both hydrolysis and microbial biotransformation processes; in most cases, biotransformation rates were faster than hydrolysis rates. We identified multiple novel products of benoxacor and dichlormid not previously observed for microbial processes, with several products similar to those reported for structurally related chloroacetamide herbicides, thus indicating potential for conserved biotransformation mechanisms across both chemical classes. Observed products include monochlorinated species such as the banned herbicide CDAA (from dichlormid), glutathione conjugates, and sulfur-containing species. We propose a transformation pathway wherein benoxacor and dichlormid are first dechlorinated, likely via microbial hydrolysis, and subsequently conjugated with glutathione. This is the first study reporting biological dechlorination of dichloroacetamides to yield monochlorinated products in aerobic environments

Environmental Fate and Effects of Dichloroacetamide Herbicide Safeners: “Inert” yet Biologically Active Agrochemical Ingredients

Safeners are included in many commercial herbicide formulations to selectively protect crops from injury induced by active ingredients. Despite their bioactivity, safeners are classified as inert from a regulatory perspective, and as such, safeners have received minimal attention in the peer-reviewed literature regarding their environmental fate and effects. Herein, we review what is known about the uses, physicochemical properties, environmental transformations, and (eco)­toxicological effects of dichloroacetamide safeners, which represent one of the most commonly used safener classes (estimated use of >2 × 10⁶ kg/year in the United States). We particularly highlight transformation pathways that may enhance biological activity and/or persistence; for example, limited studies suggest dichloroacetamides can transform via dechlorination into products with increased bioactivity. We also identify several research needs to improve our understanding of the environmental fate and potential risks of this overlooked agrochemical class, which in turn will enhance the efficacy and safety of future herbicide safener formulations

Hydroxyl Radical Formation during Ozonation of Multiwalled Carbon Nanotubes: Performance Optimization and Demonstration of a Reactive CNT Filter

We explored factors influencing hydroxyl radical (^•OH) formation during ozonation of multiwalled carbon nanotubes (MWCNTs) and assessed this system’s viability as a next-generation advanced oxidation process (AOP). Using standard reactivity metrics for ozone-based AOPs (<i>R</i>_CT values), MWCNTs promoted ^•OH formation during ozonation to levels exceeding ozone (both alone and with activated carbon) and equivalent to ozone with hydrogen peroxide. MWCNTs oxidized with nitric acid exhibited vastly greater rates of ozone consumption and ^•OH formation relative to as-received MWCNTs. While some of this enhancement reflects their greater suspension stability, a strong correlation between <i>R</i>_CT values and surface oxygen concentrations from X-ray photoelectron spectroscopy suggests that surface sites generated during MWCNT oxidation promote ^•OH exposure. Removal of several ozone-recalcitrant species [<i>para</i>-chlorobenzoic acid (<i>p</i>-CBA), atrazine, DEET, and ibuprofen] was not significantly inhibited in the presence of radical scavengers (humic acid, carbonate), in complex aquatic matrices (Iowa River water) and after 12 h of continuous exposure of MWCNTs to concentrated ozone solutions. As a proof-of-concept, oxidized MWCNTs deposited on a ceramic membrane chemically oxidized <i>p</i>-CBA in a flow through system, with removal increasing with influent ozone concentration and mass of deposited MWCNTs (in mg/cm²). This hybrid membrane platform, which integrates adsorption, oxidation, and filtration via an immobilized MWCNT layer, may serve as the basis for future novel nanomaterial-enabled technologies, although long-term performance trials under representative treatment scenarios remain necessary