Elucidating the Role of Electron Shuttles in Reductive Transformations in Anaerobic Sediments

Model studies have demonstrated that electron shuttles (ES) such as dissolved organic matter (DOM) can participate in the reduction of organic contaminants; however, much uncertainty exists concerning the significance of this solution phase pathway for contaminant reduction in natural systems. To compare the identity and reactivity of ES in anaerobic sediments with those in model systems, two chemical probes (4-cyano-4‘-aminoazobenzene (CNAAzB) either free or covalently bound to glass beads) were synthesized that allowed for differentiation between surface-associated and solution-phase electron-transfer processes. The feasibility of these chemical probes were demonstrated in abiotic model systems (Fe(II)/Fe(III) oxide) and biotic model systems (Fe(II)/Fe(III) oxide or river sediment amended with S. putrefaciens strain cells). Experiments in the abiotic systems revealed that the addition of model hydroquinones and chemically reduced DOM increased reduction rates of free CNAAzB, whereas no enhancement in reactivity was observed with the addition of model quinones or DOM. Bound CNAAzB was also reduced by model hydroquinones and reduced DOMbut not by model quinones and untreated DOMin the abiotic model systems, indicating that Fe(II)/Fe(III) oxides do not function as a bulk reductant for the reduction of ES. Addition of model quinones or untreated DOM to the biotic models systems with sediment increased reduction rates of bound CNAAzB, which correlated well with the dissolved organic carbon content. In natural sediment slurries, reduction rates of bound CNAAzB correlated well with parameters for organic carbon (OC) content of both sediments and supernatants. Our results support a scenario in which reducible organic contaminants will compete with iron oxides for the electron flow generated by the microbially mediated oxidation of organic carbon and subsequent reduction of quinone functional groups associated with DOM

Nitroaromatic Reduction Kinetics as a Function of Dominant Terminal Electron Acceptor Processes in Natural Sediments

The reductive transformation of p-cyanonitrobenzene (pCNB) was investigated in laboratory batch slurries exhibiting dominant terminal electron accepting processes (TEAPs). Pseudo-first-order rate constants (kobs) were measured for the reduction of pCNB in nitrate-reducing, iron-reducing, sulfate-reducing, and methanogenic sediment slurries. Reduction was extremely slow in nitrate-reducing slurries but increased in slurries exhibiting TEAPs with significant concentrations of solution phase Fe(II). As the reduction of pCNB progressed in the Fe(II) rich systems, significant but nonstoichiometric decreases in aqueous Fe(II) concentration were measured. Normalization of kobs to initial aqueous Fe(II) concentrations (kobs/[Fe(II)]t=0) gave values ranging from 0.0040 to 0.0052 d-1 μM-1 for nitrate-reducing, iron-reducing, and methanogenic sediment slurries as well as sulfate-reducing sediment slurries in which lactate served as a source of organic carbon. The kobs/[Fe(II)]t=0 ratios were 1-fold greater for sulfate-reducing batch slurries amended with acetate and iron-reducing slurries equilibrated with a 3% H2 atmosphere indicating that the electron source and system parameters such as pH play a determinant role in the reaction kinetics. Although these data demonstrate that aqueous phase Fe(II) must be present for significant reduction to occur, a limited role for aqueous phase Fe(II) as a quantitative indicator of reactivity is suggested

Identifying Indicators of Reactivity for Chemical Reductants in Sediments

To conduct site-specific exposure assessments for contaminants containing reducible functional groups, it is imperative to know the identity and reactivity of chemical reductants in natural sediments and to associate their reactivity with easily measurable sediment properties. For this purpose the reactivity, as defined by pseudofirst order reduction rate constants for <i>p</i>-cyanonitrobenzene (pCNB), was measured in twenty-one natural sediments of different origins that were incubated to attain both anoxic (less reducing) and anaerobic (microbially reducing) conditions. The reactivity of the anoxic sediments increased with pH and an increasing amount of Fe­(II) added. A good electron balance between pCNB reduction and Fe­(II) consumption was observed for anaerobic sediments of high solids loading (50 g/L), but not when solids loading was 5 g/L. Based on cluster and regression analysis, pCNB reactivity in the anaerobic sediments correlates strongly with aqueous Fe­(II) concentrations for sediments with low organic carbon (OC) content (<4.2%), but with dissolved OC concentrations (DOC) for the sediments with high OC content (>6.4%). These observations indicate surface-associated Fe­(II) and reduced DOC are the predominant reductants in the anaerobic sediments, and that aqueous Fe­(II) and DOC will serve as readily measurable indicators of pCNB reactivity in these systems

Influence of Dissolved Organic Matter and Fe(II) on the Abiotic Reduction of Pentachloronitrobenzene

Nitroaromatic pesticides (NAPs) are hydrophobic contaminants that can accumulate in sediments by the deposition of suspended solids from surface waters. Fe(II) and dissolved organic matter (DOM), present in suboxic and anoxic zones of freshwater sediments, can transform NAPs in natural systems. We studied the reduction of pentachloronitrobenzene (PCNB) to pentachloroaniline (PCA) in controlled studies using Fe(II) and surface water DOM isolates from Pony Lake, Antarctica, and Suwannee River, GA, in unfiltered and 0.45 μm filtered solutions. We observed rapid reduction of PCNB to PCA in the presence of Fe(II) and DOM (t1/2 ≈ 30 min to 4 h) and very limited reduction in DOM-only systems. DOM in unfiltered systems inhibited iron colloid formation and potentially limited the formation of reactive Fe(II)−iron colloid surface complexes, causing reductive transformation in Fe(II)−DOM media to be slower in some cases relative to Fe(II)-only controls. Conversely, in 0.45 μm filtered solutions, PCNB reduction in Fe(II)−DOM media was faster than the Fe(II)-only controls, suggesting that DOM enhances the reductive capacity of Fe(II) in the absence of iron colloids. This work shows that DOM may significantly affect the reactivity of Fe(II) toward NAPs under suboxic and anoxic conditions in natural wetland sediments

QSAR Study of the Reduction of Nitroaromatics by Fe(II) Species

The development of predictive models for the reductive transformation of nitroaromatics requires further clarification of the effect of environmentally relevant variables on reaction kinetics and the identification of readily available molecular descriptors for calculating reactivity. Toward these goals, studies were performed on the reduction of a series of monosubstituted nitrobenzenes in Fe(II)-treated goethite suspensions. The energy of the lowest unoccupied molecular orbital, ELUMO (B3LYP/6-31G*,water), of the nitrobenzenes was capable of explaining 99% of the variability in the rates. Results of experiments in which the surface area loading of ferric oxides was systematically varied indicate that (i) the reactivity of mineral-surface-associated Fe(II), Fe(II)surf, toward the reduction of p-cyanonitrobenzene (CNNB) decreased in the order hematite > goethite > lepidocrocite > ferrihydrite and (ii) the surface density of Fe(II)surf did not play a crucial role in determining the observed reactivity trend. CNNB was reduced in Fe(II)-only control experiments in a pH range of 7.28−7.97 with a pH dependency consistent with the transformation of Fe(II) to Fe(OH)3 or related oxides. The pH dependency of the reduction of CNNB in Fe(II)-treated ferric oxide suspensions (pH 6.1−7.97) could be accounted for by the oxidation of Fe(II)surf, forming an Fe(III) oxide

Effect of Natural Organic Matter on the Reduction of Nitroaromatics by Fe(II) Species

Uncertainty still exists regarding the role(s) of natural organic matter in the reduction of chemicals in anoxic environments. This work studied the effect of Suwannee river humic acid (SRHA) on the reduction of nitrobenzenes in goethite suspensions by Fe(II) species. The pseudo-first-order rate constant for the reduction of p-cyanonitrobenzene (kCNNB) was different for the first 3 half-lives in systems where Fe(II)aq and dissolved SRHA were equilibrated in reverse orders with goethite in suspensions. kCNNB and the reduction capacity of the system having SRHA added after Fe(II)aq was equilibrated with goethite was lower than that of the system for which the components were added in the reverse order. SRHA decreased the reduction capacity of the former system by oxidizing and/or complexing the surface-associated Fe(II), Fe(II)surf, and/or hindering the access of CNNB to Fe(II)surf. The log kCNNB increased linearly with increasing concentrations of Fe(II)aq, which decreased as a result of increasing concentrations of SRHA in the system. Different kCNNBʼs were observed for systems in which Fe(II)aq was equilibrated with goethite/SRHA suspensions for 24 and 48 h, suggesting sorbed SRHA oxidized and/or complexed Fe(II)aq. Findings suggest the concentration of Fe(II)aq and accessible Fe(II)surf will influence the reduction rates of nitroaromatics in anoxic environments

Identification of Unsaturated and 2H Polyfluorocarboxylate Homologous Series and Their Detection in Environmental Samples and as Polymer Degradation Products

A pair of homologous series of polyfluorinated degradation products have been identified, both having structures similar to perfluorocarboxylic acids but (i) having a H substitution for F on the α carbon for 2H polyfluorocarboxylic acids (2HPFCAs) and (ii) bearing a double bond between the α–β carbons for the unsaturated PFCAs (2uPFCAs). Obtaining an authentic sample containing 2uPFOA and 2HPFOA, we optimized a mass-spectrometric multiple-reaction-monitoring (MS/MS) technique and then identified uPFCA and HPFCA homologous series in sludge-applied agricultural soils and fodder grasses for cattle grazing. Analysis of samples from a degradation experiment of commercial fluorotelomer-based polymers (FTPs), the dominant product of the fluorotelomer industry, confirmed that commercial FTPs are a potential source of uPFCAs and HPFCAs to the environment. We further confirmed the identity of the uPFCAs by imposing high-energy ionization to decarboxylate the uPFCAs then focused on the fluorinated chains in the first MS quadrupole. We also employed this high-energy ionization to decarboxylate and analyze PFCAs by MS/MS (for the first time, to our knowledge). In exploratory efforts, we report the possible detection of unsaturated perfluorooctanesulfonate in environmental samples, having a conceptual double-bond structure analogous to uPFOA. Using microcosms spiked with fluorotelomer compounds, we found 2uPFOA and 2HPFOA to be generated from unsaturated 8:2 fluorotelomer acid (8:2 FTUCA) and propose β- and α-oxidation mechanisms for generation of these compounds from 8:2 FTUCA. In light of these experimental results, we also reexamined the proposed biodegradation pathways of 8:2 fluorotelomer alcohol

Reduction of Nitrosobenzenes and <i>N</i>-Hydroxylanilines by Fe(II) Species: Elucidation of the Reaction Mechanism

Although there has been a substantial effort toward understanding the reduction of nitroaromatics in Fe(II)-treated ferric oxide systems, little has been done to gain insight into the factors controlling the transformation of their reaction intermediates, nitrosobenzenes and N-hydroxylanilines, in such systems. Nitrosobenzenes, the first intermediates, were reduced by Fe(II) solutions as well as by Fe(II)-treated goethite suspensions at pH 6.6. Experimental observations indicate a reactivity trend in which the presence of electron-withdrawing groups in the para position increased the rate of reduction of the nitrosobenzenes. N-Hydroxylanilines, the second intermediates, were reduced in Fe(II)-treated goethite suspensions but were not reduced by Fe(II)aq. Their reactivity trend indicates that electron-withdrawing groups in the para position decreased their rate of reduction. The bond dissociation enthalpy of the N−O linkage was the most useful molecular descriptor for predicting the rates of reduction of N-hydroxylanilines in Fe(II)-treated goethite suspensions, suggesting that the cleavage of the N−O bond is the rate-determining step for reduction. The rate of reduction of p-cyano-N-hydroxylaniline showed a linear relationship against the concentration of surface-associated Fe(II) in hematite, goethite, and lepidocrocite suspensions, while having a relatively low sensitivity toward changes in pH within the near-neutral range in hematite suspensions

Factors Controlling Regioselectivity in the Reduction of Polynitroaromatics in Aqueous Solution

Regioselectivities in the bisulfide reduction of 10 polynitroaromatics (PNAs) to monoamine products have been determined; four of these compounds have also been reduced by anoxic sediments in heterogeneous aqueous solution, and the same regioselectivities are observed. Analyses of Austin Model 1−Solvation Model 2 electrostatic potential surfaces for the radical anions of these polynitroaromatic compounds provides a reliable method of predicting the regioselectivity of their reduction. In particular, at their minimum-energy geometries in aqueous solution, it is the more negative nitro group that is selectively reduced. This is consistent with a mechanism where regioselection occurs upon kinetic protonation at the site of maximum negative charge in the radical anion formed after the first electron transfer to the neutral PNA. Inclusion of solvation effects is critical in order to confidently predict the electrostatic prefer ence for the reduction of one nitro group over the others. Sterically uncongested nitroaromatic radical anions have gas-phase geometries in which the nitro group is coplanar with the aromatic ring. However, ortho substituents and solvation effects both oppose this tendency and can lead to nitro groups that are rotated out of the ring plane and pyramidalized

Environmental Fate of Cl-PFPECAs: Predicting the Formation of PFAS Transformation Products in New Jersey Soils

Although next-generation per- and polyfluorinated substances (PFAS) were designed and implemented as safer and environmentally degradable alternatives to “forever” legacy PFAS, there is little evidence to support the actual transformation of these compounds and less evidence of the safety of transformed products in the environment. Multiple congeners of one such PFAS alternative, the chloro-perfluoropolyether carboxylates (Cl-PFPECAs), have been found in New Jersey soils surrounding a manufacturing facility. These compounds are ideal candidates for investigating environmental transformation due to the existence of potential reaction centers including a chlorinated carbon and ether linkages. Transformation products of the chemical structures of this class of compounds were predicted based on analogous PFAS transformation pathways documented in peer-reviewed literature. Potential reaction products were used as the basis for high-resolution mass-spectrometric suspect screening of the soils. Suspected transformation products of multiple congeners, the Cl-PFPECAs, including H-PFPECAs, epox-PFPECAs, and diOH-PFPECAs, were tentatively observed in these screenings. Although ether linkages have been hypothesized as potential reaction centers under environmental conditions, to date, no documentation of ether scission has been identified. Despite exhaustive scrutiny of the high-resolution data for our Cl-PFPECA-laden soils, we found no evidence of ether scission