Pentachlorophenol radical cations generated on Fe(III)-montmorillonite initiate octachlorodibenzo-p-dioxin formation in clays: density functional theory and fourier transform infrared studies

Iron redox cycling in layered clay minerals and its impact on contaminant dynamics: A review

A majority of clay minerals contain Fe, and the redox cycling of Fe(III)/Fe(II) in clay minerals has been extensively studied as it may fuel the biogeochemical cycles of nutrients and govern the mobility, toxicity and bioavailability of a number of environmental contaminants. There are three types of Fe in clay minerals, including structural Fe sandwiched in the lattice of clays, Fe species in interlayer space and adsorbed on the external surface of clays. They exhibit distinct reactivity towards contaminants due to their differences in redox properties and accessibility to contaminant species. In natural environments, microbially driven Fe(III)/Fe(II) redox cycling in clay minerals is thought to be important, whereas reductants (e.g., dithionite and Fe(II)) or oxidants (e.g., peroxygens) are capable of enhancing the rates and extents of redox dynamics in engineered systems. Fe(III)-containing clay minerals can directly react with oxidizable pollutants (e.g., phenols and polycyclic aromatic hydrocarbons (PAHs)), whereas structural Fe(II) is able to react with reducible pollutants, such as nitrate, nitroaromatic compounds, chlorinated aliphatic compounds. Also structural Fe(II) can transfer electrons to oxygen (O₂), peroxymonosulfate (PMS), or hydrogen peroxide (H₂O₂), yielding reactive radicals that can promote the oxidative transformation of contaminants. This review summarizes the recent discoveries on redox reactivity of Fe in clay minerals and its links to fates of environmental contaminants. The biological and chemical reduction mechanisms of Fe(III)-clay minerals, as well as the interaction mechanism between Fe(III) or Fe(II)-containing clay minerals and contaminants are elaborated. Some knowledge gaps are identified for better understanding and modelling of clay-associated contaminant behavior and effective design of remediation solutions.

Dioxin-like compounds formation mediated by Fe3+-montmorillonite: The substituent effects of halophenols

Toxic dioxin or/and dioxin-like compounds could be naturally formed from the reaction of halophenols on Fe³⁺-montmorillonite minerals under ambient conditions. Given that the toxicities and productions of dioxin or/and dioxin-like compounds are largely determined by the number, species, and position of the carried halogen atoms, it is necessary to explore the substituent effects on the reaction of halophenols with Fe³⁺-montmorillonite. Herein, Fe³⁺-montmorillonite catalyzed polymerizations of six halophenols were examined in a wide range of relative humidity (10%∼80%) using combinations of mass spectrometry identifications and density functional theory calculations. Results show that both the position and species of the substituents substantially impact the reaction rate, product species, and transformation pathways. In general, regardless of humidity ortho-substituted chlorophenols are more reactive than meta-substituted chlorophenols, which is also supported by the density functional theory calculations indicating that the ortho positions are more likely to be attacked. Regarding substituent species, bromophenols are slightly more reactive and also more easily affected by humidities than chlorophenols, which is due to the weaker electron absorbing ability of the bromine atom than the chlorine atom. Hydroxylated polyhalogenated diphenyl ethers are more frequently detected polymerization products, although hydroxylated polyhalogenated biphenyls are greater quantity of products. Overall, this study provides useful information for understanding the natural formation of dioxin or/and dioxin-like compounds mediated by clay minerals and underlying reaction mechanisms.

Humidity induces the formation of radicals and enhances photodegradation of chlorinated-PAHs on Fe(III)-montmorillonite

Chlorinated-PAHs (ClPAHs) are widely detected in the soil surface and atmospheric particles. However, the underlying mechanisms of their photodegradation are not well understood. In the present study, the formation of radicals on ClPAHs-contaminated clay minerals was quantitatively monitored via electron paramagnetic resonance (EPR) spectroscopy, and the impact of relative humidity (RH) was systematically explored. ClPAHs removal (> 75%) was attributed to electron transfer and •OH attack. The degradation easiness of ClPAHs follows: 2-ClNAP >2-ClANT >9-ClPHE >1-ClPYR. Light irradiation significantly improved the generation of reactive oxygen species (ROS, such as •OH and •O₂^-), and further generate a series of hydroxylated products of ClPAHs. Persistent free radicals (PFRs) were only detected on clay minerals contaminated with 2-ClANT and 1-ClPYR. RH 10-80%, the concentration of •OH and •O₂^- increased by 1.07 and 62.79 times respectively, which facilitated transformation of PFRs and ClPAHs degradation. The results of quantum chemical calculations indicate that the initial reaction of ClPAHs photodegradation is mediated by the substitution of •OH for chlorine groups. The present work implies that higher humidity may decrease the generation of PFRs on clay minerals and help mitigate the threats of PFRs and ClPAHs to human health.

The photodegradation processes and mechanisms of polyvinyl chloride and polyethylene terephthalate microplastic in aquatic environments: Important role of clay minerals

It is well known that microplastics (MPs) may experience weathering and aging under ultraviolet light (UV) irradiation, but it remains unclear if these processes are impacted by natural components, such as clay minerals. In this study, we systematically investigated the photodegradation behaviors of polyvinyl chloride (PVC) and poly (ethylene terephthalate) (PET), two utmost used plastics, in the presence of clay minerals (kaolinite and montmorillonite). The results demonstrated that the clay minerals, particularly kaolinite, significantly promoted the MPs photodegradation, and the aging of PET was more prominent. The photodegradation was the most distinct at pH 7.0, regardless of the presence or absence of the clay minerals. The results of electron paramagnetic resonance and inhibition experiments of reactive oxygen species indicated that the minerals, particularly kaolinite, remarkably facilitated production of •OH, which was the key species contributing to the photodegradation of MPs. Specifically, UV irradiation facilitated the photo-ionization of MPs, producing hydrated electrons and MP radical cations (MP⁺). The Lewis base sites prevalent on the clay siloxane surfaces could stabilize the MP radical cations and prevent their recombination with hydrated electrons, which promoted the generation of •OH under aerobic conditions, and facilitated the degradation of MP. Two-dimensional (2D) Fourier transformation infrared (FTIR) correlation spectroscopy (COS) analysis and ultra-high-performance liquid chromatography coupled to a Q Exactive Orbitrap HF mass spectrometer were used to identify the sequential changes of functional groups, and the degradation products of the MPs. This study improves our understanding on the aging of MPs in the complex natural environment.

Surface catalyzed hydrolysis of chloramphenicol by montmorillonite under limited surface moisture conditions

Phyllosilicates possess high surface acidity under limited surface moisture conditions and are thus able to mediate the abiotic transformation of antibiotics. This route of abiotic transformation has long been ignored given that most of the studies carried out in aqueous phase. In this study, the catalytic performance of cation-exchanged montmorillonites (Mⁿ⁺-Mts) to the hydrolysis of chloramphenicol (CAP) was investigated under different moisture conditions. Montmorillonite exchanged with Fe³⁺ and Al³⁺ show the greatest catalytic activities. Multiple spectroscopic techniques and theoretical calculations indicate that the surface Brønsted- and Lewis-acid properties are sensitive to surface wetting. At lower moisture level (<10%, wt/wt), the strong Brønsted-acid catalysis predominates the hydrolysis of CAP. Attributing to the strong Lewis-acidities, Fe³⁺-Mt and Al³⁺-Mt could perform high catalytic activities over a wider moisture range (10- 100%, wt/wt). However, such hydrolysis reaction was almost suppressed at water content >400%. In addition, the presence of natural organic matter (NOM, 1%, wt/wt) had little impact on the catalytic activities of Fe³⁺-Mt and Al³⁺-Mt. The results of this study highlight the environmental significance of dry surface reaction by clay minerals as an effective abiotic transformation pathway to the elimination of antibiotics in natural field soil, which is commonly partly hydrated.

Oxidative Oligomerization of Phenolic Endocrine Disrupting Chemicals Mediated by Mn(III)-L Complexes and the Role of Phenoxyl Radicals in the Enhanced Removal: Experimental and Theoretical Studies

Soluble manganese(III), stabilized by ligands as Mn(III)-L complexes, are ubiquitous in natural waters and wastewaters and can potentially serve as both the oxidant and reductant in one-electron transfer reactions with organic contaminants. In this study, the oxidative transformations of 14 phenolic endocrine disrupting chemicals (EDCs) by in situ-formed Mn(III)-L complexes, generated from irradiated water containing Mn(II) and humic acid, were investigated. The pseudo-first-order rate constants (k_obs, h^-1) of these phenols varied from 1.0 × 10^-4 to 5.9 × 10^-2. A quantitative structure-activity relationship model was developed, which suggests that the electron-donating ability (E_HOMO) of phenolic chemicals was the most important molecular characteristic for the Mn(III)-L-mediated oxidative transformation. Phenol transformation was initiated by the generation of a phenoxyl radical through electron transfer to Mn(III)-L. Subsequent self-coupling reactions between phenoxyl radicals resulted in the formation of self-coupling dimers and trimers. With the addition of simple phenol as a cosubstrate, enhanced transformations of these phenolic EDCs were clearly observed, and cross-coupling products of simple phenol and the substrates were also detected. In addition, a reaction activation energy calculation based on the transition-state theory indicated that the cross-coupling reaction was more likely than the self-coupling reaction to occur in the presence of phenol. This work provides new insights into the environmental fate of phenolic compounds.

Transformation and toxicity of environmental contaminants as influenced by Fe containing clay minerals: a review

Clay minerals are ubiquitous in soil and sediments, and play important roles in environmental processes. Virtually all clay minerals contain some Fe, either in the crystal structure or adsorbed at the surface of the clay minerals, which could promote many reactions on the clay minerals. The interaction between Fe containing clay mineral and its surrounding contaminants would greatly affect the transformation and toxicity of the contaminants. While it is certain that the Fe plays important roles in the fate of the contaminants, relative qualitative and quantitative assessment of these processes is limited. Based on the overall toxicity variation, this paper reviews the active areas of the researches on the interactions between Fe containing clay mineral and contaminants, and provides perspectives for future work.

Fomration of hydroxylated polychlorinated diphenyl ethers mediated by Structural Fe(III) in smectites

Fe(III)-bearing clay minerals are ubiquitous in the environment. However, the fate of organic contaminants mediated by structural Fe(III) in clays was rarely reported. Here we demonstrated that hydroxylated polychlorinated diphenyl ethers (HO-PCDEs) could be spontaneously formed from the reaction of 2,4,6-trichlorophenol (2,4,6-TCP) with three native smectites: SWy-2, NAu-1, and NAu-2. Further research demonstrated that the structural Fe(III) in smectite is indispensable for the mediation of 2,4,6-TCP to produce chlorophenoxy radical for the subsequent dimerization. The reaction is highly dependent on the relative humidity of the system and the site occupancy of structural Fe(III). Active structural Fe(III) in NAu-2 that played a significant role in the dimerization reaction is relatively more distorted, which would interact strongly with 2,4,6-TCP under low humidity and be inhibited by water molecules. Hence reaction on NAu-2 is suppressed as relative humidity increases. Whereas, water molecules would reduce the activation and reaction energies via forming a hydrogen bond with reaction intermediates, thus enhancing the reactions on SWy-2 and NAu-1 with less water sensitive structural Fe(III). Considering the wide distribution of Fe(III)-bearing smectites in the environment, the contribution of structural Fe(III) for the formation of more toxic dioxin-like compounds from chlorophenols might need to be taken into consideration to evaluate their potential environmental risks.

Transformation of gaseous 2-bromophenol on clay mineral dust and the potential health effect

Iron-bearing clays are ubiquitously distributed as mineral dusts in the atmosphere. Bromophenols were reported as the major products from thermal decomposition of the widely used brominated flame retardants (BFRs). However, little information is available for the reactivity of iron associated with mineral dusts to interact with the atmospheric bromophenols and the subsequent toxic effects. Herein, three common clay minerals (montmorillonite, illite and kaolinite) were used to simulate mineral dusts, and the reactions with gaseous 2-bromophenol were systematically investigated under environmentally relevant atmospheric conditions. Our results demonstrate that structural Fe(III) in montmorillonite and Fe(III) from iron oxide in illite mediated the dimerization of 2-bromophenol to form hydroxylated polybrominated biphenyl and hydroxylated polybrominated diphenyl ether. The surface reaction is favored to occur at moisture environment, since water molecules formed complex with 2-bromophenol and the reaction intermediates via hydrogen bond to significantly lower the reaction energy and promote the dimerization reaction. More importantly, the formed dioxin-like products on clay mineral dust increased the toxicity of the particles to A549 lung cell by decreasing cell survival and damaging cellular membrane and proteins. The results of this study indicate that not only mineral dust itself but also the associated surface reaction should be fully considered to accurately evaluate the toxic effect of mineral dust on human health.

Interfacial formation of environmentally persistent free radicals-A theoretical investigation on pentachlorophenol activation on montmorillonite in PM2.5

Environmentally persistent free radicals (EPFRs) in atmospheric fine particulate matters (PM_2.5) possess high bioactivity and result in severe health problems. The facile transformation of aromatic pollutants into EPFRs on montmorillonite (MMT), an important solid component in PM_2.5, is an activation of air pollutants into more toxic chemical species and also attributes to the secondary source of EPFRs in PM_2.5. In this study, the interfacial reactions of pentachlorophenol (PCP), a typical EPFR precursor in air pollution, on the Fe(III)-, Ca- and Na-MMT surfaces have been explored by the density functional theory (DFT) calculations using the periodic slab models. The PCP molecule is found to be exothermically adsorbed on the three MMT surfaces. Moreover, significant charge transfer from PCP to Fe takes place and finally leads to the surface-bound phenoxyl radical formation on the Fe(III)-MMT surface since the half-filled 3d orbital of Fe³⁺ in Fe(III)-MMT could act as electron acceptor allowing the electron transferring from the 2p orbital of the phenolic O in PCP to Fe ion. However, similar charge transfer is not found in the Ca- and Na-MMTs, and the PCP transformation reaction is hindered on the Ca- and Na-MMT surfaces. Namely, the PCP activation to the corresponding EPFRs is impossible on the Ca-MMT and Na-MMT surfaces, while the catalytically active Fe(III)-MMT in PM_2.5 can transform the chlorinated phenols into more toxic phenoxy-type EPFRs at ambient temperatures. Accordingly, more attention should be paid on the effect of MMT with catalytical capacity on the toxicity of PM_2.5.

Formation of environmentally persistent free radicals during the transformation of anthracene in different soils: Roles of soil characteristics and ambient conditions

Transformation of PAHs, i.e., anthracene, and production of environmentally persistent free radicals (EPFRs) on seven types of representative soils were investigated, focusing on the influences of soil characteristics and ambient conditions on these reactions. The transformation rate of anthracene exhibits the order of red earth > yellow earth > latosol ∼ fluvo-aquic soil > brown earth > chernozem > calcic brown soil, which is positively correlated with Fe content in soils. Afterwards, batch reactions on pure representatives of soil minerals, including Fe₂O₃, Fe₃O₄, FeOOH, and MnO₂, demonstrate that anthracene is prone to mineral-promoted transformation. The presence of higher amount of organic carbon lower the transformation rate of anthracene, whereas the formed EPFRs can be stabilized for a longer time. Subsequent experiments associated with the influences of environmental conditions on mineral-promoted reactions suggest that both anthracene transformation and EPFRs generation readily occur under dry condition. Light irradiation not only promotes the formation of EPFRs, but also greatly accelerates the decay of EPFRs and the 1/e lifetime decreases from 5 to 20 d in dark to approximately 1 d. Meanwhile, the anoxic condition is favorable for the persistence of EPFRs. The obtained results suggested the potential environmental risks association with EPFRs in PAHs-contaminated soils.

Enhanced Removal of Chlorophene and 17β-estradiol by Mn(III) in a Mixture Solution with Humic Acid: Investigation of Reaction Kinetics and Formation of Co-oligomerization Products

Reaction with soluble Mn(II) has been considered as a main decay pathway for superoxide in natural waters, accompanied by an important Mn redox cycling. In this study, the interaction of Mn(II) and humic acid (HA) was investigated in visible light irradiated water. Our results indicate that HA may play a dual role to act as a photosensitizer to produce superoxide anions (O₂^-) and as a strong ligand to stabilize the Mn(III), forming soluble Mn(III)_L species for substrate transformation. Furthermore, the reaction kinetics, products, and mechanisms of chlorophene (CP) and estradiol (E2) mixture in the Mn(II)/HA/visible light reaction systems were assessed. The removal of CP and E2 was enhanced by 24.3% and 13.2%, respectively, in mixture solution at initial concentration of 1.0 μM for each target contaminant, as compared to the case of single-compound degradation. Product identification and density functional theory calculations indicated that cross-coupling reaction of CP and E2 radicals was more likely to occur than the self-coupling reaction in mixture solution. In addition, estrogenic activities of initial reaction solution were also effectively decreased during the transformation process. These findings provide new insights into Mn(III)-mediated reactions to better understand the environmental fate of organic contaminant mixture in waters.

National surveillance of 2,3,7,8-substituted polychlorinated dibenzo-p-dioxins/furans in soil in Taiwan

In this study, the polychlorinated dibenzo-p-dioxin/furan (PCDD/F) levels in 381 soil samples coming from different background areas (n = 238) and contaminated areas (n = 143) in Taiwan were investigated from 2011 to 2015 using high resolution gas chromatograph/high resolution mass spectrometry (HRGC/HRMS). The contaminated areas showed higher PCDD/F contamination as compared to the background areas (1230 vs 749 pg/g dry weight (dw)); 14.0 vs 6.25 pg WHO₂₀₀₅-TEQ/g dw). The lowest levels were recorded in the outlying islands (1.28 pg WHO₂₀₀₅-TEQ/g dw). PCDD/F congener distribution profiles in the background and contaminated areas as well as on the islands varied. OCDD was the dominant congener in almost all locations, including the northern background (87.7%) and central contaminated (74.2%) areas. Other dominant species included OCDF and 1,2,3,4,6,7,8-HpCDD. Levels of PCDD/F-TEQs in Taiwanese soils, including the background areas, were higher than in some developing countries and regions with global background levels due to high industrialization except for the contaminated areas. Geographic differences in dioxin-contaminated soils were also shown in this study. Higher soil dioxin-TEQs were observed in locations with high populations and population densities. Despite the limitations, the economic status of Taiwan represented by the annual averaged family income (AAFI) was positively correlated to soil dioxin-TEQs.

Transformation of Polycyclic Aromatic Hydrocarbons and Formation of Environmentally Persistent Free Radicals on Modified Montmorillonite: The Role of Surface Metal Ions and Polycyclic Aromatic Hydrocarbon Molecular Properties

This paper presents the transformation of PAHs (phenanthrene, anthracene, benzo[a]anthracene, pyrene, and benzo[a]pyrene) on montmorillonite clays that are modified by transition-metal ions [Fe(III), Cu(II), Ni(II), Co(II), or Zn(II)] at room temperature (∼23 °C). The decay of these PAHs follows first-order kinetics, and the dependence of the observed rate constants ( k_obs, day^-1) on the presence of metal ions follows the order Fe(III) > Cu(II) > Ni(II) > Co(II) > Zn(II). The values of k_obs show reasonable linear relationships with the oxidation potentials of the PAHs and the redox potentials of the metal ions. Notably, transformation of these PAHs results in the formation of environmentally persistent free radicals (EPFRs), which are of major concern due to their adverse effects on human health. The potential energy surface (PES) calculations using density functional theory were performed to understand the trends in k_obs and the plausible mechanisms for radical formation from the PAHs on modified clays. The yields and stability of these EPFRs from anthracene and benzo[a]pyrene on clay surfaces varies with both the parent PAH and the metal ion. The results demonstrated the potential role of metals in the formation and fate of PAH-induced EPFR at co-contaminated sites.

A novel method for synthesis of polyaniline and its application for catalytic degradation of atrazine in a Fenton-like system

Recently, polyaniline (PANI) has received widespread attention for the free volume, optical transmittance and electrical conductivity. In this study, a chemical vapor deposition method was developed to synthesize the conductive PANI-clay composite catalyzed by Fe(III)-saturated attapulgite (Fe(III)-ATTP). The reaction is initiated by the electron transfer from aniline (ANI) to Fe(III), subsequently generating ANI radical cation. The radical could further polymerize and form PANI in the constrained micropore structure of ATTP. The Raman, Fourier transform infrared and X-ray photoelectron spectra confirmed the formation of PANI on Fe(III)-ATTP surface by comparison with the PANI standard. The newly synthesized Fe(III)-ATTP-PANI composite exhibited superior reactivity as indicated by the efficient dissipation of atrazine in the presence of hydrogen peroxide (H₂O₂), and the degradation rate increased up to almost 150 times compared to Fe(III)-ATTP. The higher reactivity of Fe(III)-ATTP-PANI/H₂O₂ system was attributed to the accelerated electron transfer, the formation of ferrous ions, and the enhanced adsorption of atrazine onto attapulgite. Furthermore, our experimental results demonstrated that Fe(III)-ATTP-PANI showed good stability and it could be reused for several reaction cycles with high reactivity. This new material could act as an environmental-friendly catalyst in Fenton-like reaction system and show promising potential to effectively eliminate many persistent organic contaminants.

Complete Defluorination of Perfluorinated Compounds by Hydrated Electrons Generated from 3-Indole-acetic-acid in Organomodified Montmorillonite

Here we describe a unique process that achieves complete defluorination and decomposition of perfluorinated compounds (PFCs) which comprise one of the most recalcitrant and widely distributed classes of toxic pollutant chemicals found in natural environments. Photogenerated hydrated electrons derived from 3-indole-acetic-acid within an organomodified clay induce the reductive defluorination of co-sorbed PFCs. The process proceeds to completion within a few hours under mild reaction conditions. The organomontmorillonite clay promotes the formation of highly reactive hydrated electrons by stabilizing indole radical cations formed upon photolysis, and prevents their deactivation by reaction with protons or oxygen. In the constrained interlayer regions of the clay, hydrated electrons and co-sorbed PFCs are brought in close proximity thereby increasing the probability of reaction. This novel green chemistry provides the basis for in situ and ex situ technologies to treat one of the most troublesome, recalcitrant and ubiquitous classes of environmental contaminants, i.e., PFCs, utilizing innocuous reagents, naturally occurring materials and mild reaction conditions.

Bioremediation of polychlorinated-p-dioxins/dibenzofurans contaminated soil using simulated compost-amended landfill reactors under hypoxic conditions

Compost-amended landfill reactors were developed to reduce polychlorinated-p-dioxins and dibenzofurans (PCDD/Fs) in contaminated soils. By periodically recirculating leachate and suppling oxygen, the online monitoring of the oxidation reduction potential confirmed that the reactors were maintained under hypoxic conditions, with redox levels constantly fluctuating between -400 and +80mV. The subsequent reactor operation demonstrated that PCDD/F degradation in soil could be facilitated by amending compost originating from the cow manure and waste sludge and that the degradation might be affected by the availability of easily degradable substrates in the soil and compost. The pyrosequencing analysis of V4/V5 regions of bacterial 16S rRNA genes suggested that species richness of the soil microbial community was increased by a factor of 1.37-1.61. Although the bacterial community varied with the compost origin and changed markedly during reactor operation, it was dominated by Alphaproteobacteria, Gammaproteobacteria, Actinobacteria, and Firmicutes. The aerotolerant anaerobic Sedimentibacter and Propionibacterium spp., and the uncultured Chloroflexi group could be temporarily induced to a high abundance by amending the cow manure compost; the bacterial growths were associated with the rapid degradation of PCDD/Fs. Overall, the novel bioremediation method for PCDD/F-contaminated soils using hypoxic conditions was effective, simple, energy saving, and thus easily practicable.

Model System Study of Environmentally Persistent Free Radicals Formation in a Semiconducting Polymer Modified Copper Clay System at Ambient Temperature

This paper systematically investigates how environmentally persistent free radicals (EPFRs) are formed in a phenol contaminated model soil. Poly-p-phenylene (PPP) modified and copper-loaded montmorillonite (MMT) clays were developed and used as models of soil organic matter and the clay mineral component, respectively, with phenol being employed as a precursor pollutant. The polymer modification of the clays was carried out via surface-confined Kumada catalyst-transfer chain-growth polymerization. The presence and location of the polymer were confirmed by a combination of thermogravimetric analysis (TGA), Raman spectroscopy, and X-ray diffraction data. EPFRs were formed by the Cu(II)-clay (Cu(II)CaMMT) and poly-p-phenylene-Cu(II)clay (PPP-Cu(II)CaMMT) composite systems under environmentally relevant conditions. The g-factor and concentration of EPFRs formed by the Cu(II)CaMMT and PPP-Cu(II)CaMMT systems were found to be 2.0034 and 1.22 × 1017 spins/g and 2.0033 and 1.58 × 1017spins/g, respectively. These g-factors are consistent with the formation of phenoxyl radicals. Extended X-Ray absorption fine structure (EXAFS) analysis shows that there are distinct differences in the local stuctures of the phenoxyl radicals associated with only the Cu(II) redox centers and those formed in the presences of the PPP polymer. X-ray absorption near edge spectroscopy (XANES) results provided evidence for the reduction of Cu(II) to Cu(I) in the EPFR forming process. The 1/e lifetimes of the formed EPFRs revealed a decay time of ~20 h for the Cu(II)CaMMT system and a two-step decay pattern for the PPP-Cu(II)CaMMT system with decay times of ~13.5 h and ~55.6 h. Finally, the generation of reactive oxygen species (hydroxyl radical; •OH) by these clay systems was also investigated, with higher concentrations of •OH detected for the phenol-dosed Cu(II)CaMMT and PPP-Cu(II)CaMMT systems, compared to the non-EPFR containing undosed PPP-Cu(II)CaMMT system.

Formation of environmentally persistent free radical (EPFR) in iron(III) cation-exchanged smectite clay

Environmentally persistent free radicals (EPFRs) have been found at a number of Superfund sites, with EPFRs being formed via a proposed redox process at ambient environmental conditions. The possibility of such a redox process taking place at ambient environmental conditions is studied utilizing a surrogate soil system of phenol and iron(III)-exchanged calcium montmorillonite clay, Fe(III)CaM. Sorption of phenol by the Fe(III)CaM is demonstrated by Fourier-transformed infra-red (FT-IR) spectroscopy, as evidenced by the peaks between 1345 cm(-1) and 1595 cm(-1), and at lower frequencies between 694 cm(-1) and 806 cm(-1), as well as X-ray diffraction (XRD) spectroscopy, as shown by an increase in interlayer spacing within Fe(III)CaM. The formation and characterization of the EPFRs is determined by electron paramagnetic resonance (EPR) spectroscopy, showing phenoxyl-type radical with a g-factor of 2.0034 and ΔHP-P of 6.1 G at an average concentration of 7.5 × 10(17) spins per g. EPFRs lifetime data are indicative of oxygen and water molecules being responsible for EPFR decay. The change in the oxidation state of the iron redox center is studied by X-ray absorption near-edge structure (XANES) spectroscopy, showing that 23% of the Fe(III) is reduced to Fe(II). X-ray photoemission spectroscopy (XPS) results confirm the XANES results. These findings, when combined with the EPFR concentration data, demonstrate that the stoichiometry of the EPFR formation under the conditions of this study is 1.5 × 10(-2) spins per Fe(II) atom.

Assessment of PCDD/Fs formation in the Fenton oxidation of 2-chlorophenol: Influence of the iron dose applied

Toxic polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) may be formed during remediation of chlorinated phenols via Fenton oxidation. To highlight the need for monitoring the production of toxic byproducts in these reactions, this work assessed the influence of iron dose (0.09-0.36 mM) on the Fenton oxidation of 2-chlorophenol (2-CP, 15.56 mM), a potential precursor of PCDD/Fs, by quantifying 2-CP removal and mineralization rates as well as byproducts yields, including PCDD/Fs. Although the increase in the iron dose showed positive contribution to 2-CP oxidation, under the operating conditions of the current study (H2O2 at 20% of the stoichiometric dose and 20 °C), there was no effect on the mineralization rate, and TOC and chlorine balances were far to be closed, depicting the presence of chlorinated organic byproducts in the reaction medium. After 4 h of treatment, the total PCDD/Fs concentrations increased by 14.5-39 times related to the untreated sample when the iron doses tested decreased from 0.36 to 0.09 mM, with preferential formation of PCDFs over PCDDs and dominance of lower chlorinated congeners such as tetra and penta-PCDD/Fs. The treatment with the highest iron dose (0.36 mM) exhibited the lowest PCDD/Fs yields and was thus most successful at mitigating toxic byproducts of the Fenton oxidation, leading to lower sample toxic equivalence (TEQ) value.

Enhanced Photoreduction of Nitro-aromatic Compounds by Hydrated Electrons Derived from Indole on Natural Montmorillonite

A new photoreduction pathway for nitro-aromatic compounds (NACs) and the underlying degradation mechanism are described. 1,3-Dinitrobenzene was reduced to 3-nitroaniline by the widely distributed aromatic molecule indole; the reaction is facilitated by montmorillonite clay mineral under both simulated and natural sunlight irradiation. The novel chemical reaction is strongly affected by the type of exchangeable cation present on montmorillonite. The photoreduction reaction is initiated by the adsorption of 1,3-dinitrobenzene and indole in clay interlayers. Under light irradiation, the excited indole molecule generates a hydrated electron and the indole radical cation. The structural negative charge of montmorillonite plausibly stabilizes the radical cation hence preventing charge recombination. This promotes the release of reactive hydrated electrons for further reductive reactions. Similar results were observed for the photoreduction of nitrobenzene. In situ irradiation time-resolved electron paramagnetic resonance and Fourier transform infrared spectroscopies provided direct evidence for the generation of hydrated electrons and the indole radical cations, which supported the proposed degradation mechanism. In the photoreduction process, the role of clay mineral is to both enhance the generation of hydrated electrons and to provide a constrained reaction environment in the galley regions, which increases the probability of contact between NACs and hydrated electrons.

PCDD/Fs in air and soil around an e-waste dismantling area with open burning of insulated wires in south China

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in ambient air and farmland soil sampled in 2006 around an e-waste dismantling area with open burning of insulated wires in Longtang in south China were investigated. The total toxic equivalent concentrations of PCDD/Fs were 3.2-31.7 pg/m(3) in air and 5.8 12.4 ng/kg in farmland soil at an e-waste site and 0.063-0.091 pg/m(3) in air at a background site. PCDD/Fs in the air at the e-waste site were characterized with dominant 1,2,3,4,6,7,8-HpCDF and OCDF and higher concentrations of furans than dioxins, suggesting open burning of insulated wires was likely to be the main source of PCDD/Fs. Compared with the results in this study, the level of PCDD/F tended to lessen with the average TEQ concentration decreasing by 41 % and the pattern changed to be dominated by OCDD in the air of Longtang in 2010 when insulated wires were openly burned in only a small scale. Our results indicate that the lower chlorinated congeners with higher vapor pressures have enhanced atmospheric transport tendencies.

Surface catalyzed oxidative oligomerization of 17β-estradiol by Fe(3+)-saturated montmorillonite

With widespread detection of endocrine disrupting compounds including hormones in wastewater, there is a need to develop cost-effective remediation technologies for their removal from wastewater. Previous research has shown that Fe(3+)-saturated montmorillonite is effective in quickly transforming phenolic organic compounds such as pentachlorophenol, phenolic acids, and triclosan via surface-catalyzed oligomerization. However, little is known about its effectiveness and reaction mechanisms when reacting with hormones. In this study, the reaction kinetics of Fe(3+)-saturated montmorillonite catalyzed 17β-estradiol (βE2) transformation was investigated. The transformation products were identified using liquid chromatography coupled with mass spectrometry, and their structures were further confirmed using computational approach. Rapid βE2 transformation in the presence of Fe(3+)-saturated montmorillonite in an aqueous system was detected. The disappearance of βE2 follows first-order kinetics, while the overall catalytic reaction follows the second-order kinetics with an estimated reaction rate constant of 200 ± 24 (mmol βE2/g mineral)(−1) h(–1). The half-life of βE2 in this system was estimated to be 0.50 ± 0.06 h. βE2 oligomers were found to be the major products of βE2 transformation when exposed to Fe(3+)-saturated montmorillonite. About 98% of βE2 were transformed into βE2 oligomers which are >10(7) times less water-soluble than βE2 and, therefore, are much less bioavailable and mobile then βE2. The formed oligomers quickly settled from the aqueous phase and were not accumulated on the reaction sites of the interlayer surfaces of Fe(3+)-saturated montmorillonite, the major reason for the observed >84% βE2 removal efficiency even after five consecutive usages of the same of Fe(3+)-saturated montmorillonite. The results from this study clearly demonstrated that Fe(3+)-saturated montmorillonite has a great potential to be used as a cost-effective material for efficient removal of phenolic organic compounds from wastewater.

Adsorption of diethyl phthalate ester to clay minerals

Phthalate esters are a group of plasticizers, which have been widely detected in China's agricultural and industrial soils. In this study, batch adsorption experiments were conducted to investigate the environmental effects on the adsorption of diethyl phthalate ester (DEP) to clay minerals. The results showed that DEP adsorption isotherms were well fitted with the Freundlich model; the interlayer spacing of K(+) saturated montmorillonite (K-mont) was the most important adsorption area for DEP, and di-n-butyl ester (DnBP) was limited to intercalate into the interlayer of K-mont due to the bigger molecular size; there was no significant effect of pH and ionic strength on DEP adsorption to K-mont/Ca-mont, but to Na-mont clay. The adsorption to kaolinite was very limited. Data of X-ray diffraction and FTIR spectra further proved that DEP molecules could intercalate into K-/Ca-mont interlayer, and might interact with clay through H-bonding between carbonyl groups and clay adsorbed water. Coated humic acid on clay surface would enhance DEP adsorption at low concentration, but not at high concentration (eg. Ce>0.26 mM). The calculated adsorption enthalpy (ΔHobs) and adsorption isotherms at varied temperatures showed that DEP could be adsorbed easier as more adsorbed. This study implied that clay type, compound structure, exchangeable cation, soil organic matter and temperature played important roles in phthalate ester's transport in soil.

Transformation of anthracene on various cation-modified clay minerals

In this study, anthracene was employed as a probe to explore the potential catalytic effect of clay minerals in soil environment. Clay minerals saturated with various exchangeable cations were tested. The rate of anthracene transformation follows the order: Fe-smectite >> Cu-smectite > Al-smectite ≈ Ca-smectite ≈ Mg-smectite ≈ Na-smectite. This suggests that transition-metal ions such as Fe(III) play an important role in anthracene transformation. Among Fe(III)-saturated clays, Fe(III)-smectite exhibits the highest catalytic activity followed by Fe(III)-illite, Fe(III)-pyrophyllite, and Fe(III)-kaolinite, which is in agreement with the interlayer Fe(III) content. Moreover, effects by two common environmental factors, pH and relative humidity (RH), were evaluated. With an increase in pH or RH, the rate of anthracene transformation decreases rapidly at first and then is leveled off. GC-MS analysis identifies that the final product of anthracene transformation is 9,10-anthraquinone, a more bioavailable molecule compared to anthracene. The transformation process mainly involves cation-π bonding, electron transfer leading to cation radical, and further oxidation by chemisorbed O2. The present work provides valuable insights into the abiotic transformation and the fate of PAHs in the soil environment and the development of contaminated land remediation technologies.

Use of Cl and C isotopic fractionation to identify degradation and sources of polychlorinated phenols: mechanistic study and field application

The widespread use of chlorinated phenols (CPs) as a wood preservative has led to numerous contaminated sawmill sites. However, it remains challenging to assess the extent of in situ degradation of CPs. We evaluated the use of compound-specific chlorine and carbon isotope analysis (Cl- and C-CSIA) to assess CP biotransformation. In a laboratory system, we measured isotopic fractionation during oxidative 2,4,6-trichlorophenol dechlorination by representative soil enzymes (C. fumago chloroperoxidase, horseradish peroxidase, and laccase from T. versicolor). Using a mathematical model, the validity of the Rayleigh approach to evaluate apparent kinetic isotope effects (AKIE) was confirmed. A small but significant Cl-AKIE of 1.0022 ± 0.0006 was observed for all three enzymes, consistent with a reaction pathway via a cationic radical species. For carbon, a slight inverse isotope effect was observed (C-AKIE = 0.9945 ± 0.0019). This fractionation behavior is clearly distinguishable from reported reductive dechlorination mechanisms. Based on these results we then assessed degradation and apportioned different types of technical CP mixtures used at two former sawmill sites. To our knowledge, this is the first study that makes use of two-element CSIA to study sources and transformation of CPs in the environment.

Formation of dioxins during exposure of pesticide formulations to sunlight

Chlorinated pesticides can contain impurities of dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), and their precursors, as a result of various manufacturing processes and conditions. As precursor formation of PCDD/Fs can also be mediated by ultraviolet light (UV), this study investigated whether PCDD/Fs are formed when currently used pesticides are exposed to natural sunlight. Formulations containing pentachloronitrobenzene (PCNB; n=2) and 2,4-dichlorophenoxyacetic acid (2,4-D; n=1) were exposed to sunlight in quartz tubes, and the concentration of 93 PCDD/F congeners were monitored over time. Considerable formation of PCDD/Fs was observed in both PCNB formulations (by up to 5600%, to a maximum concentration of 57000 μg ∑PCDD/F kg(-1)) as well as the 2,4-D formulation (by 3000%, to 140 μg ∑PCDD/F kg(-1)). TEQ also increased by up to 980%, to a maximum concentration of 28 μg kg(-1) in PCNB, but did not change in the 2,4-D formulation. Assuming similar yields as observed in the present study as a worst case scenario the use of PCNB in Australia may result in the formation of 155 g TEQ annum(-1), contributed primarily by OCDD formation. This warrants detailed evaluations on the contemporary release of PCDD/Fs to the environment after the use of pesticides. Changes in congener profiles (including the ratio of PCDDs to PCDFs (DF ratio)) suggest that pesticide sources of PCDD/Fs after sunlight exposure may not be recognized based on matching source fingerprints established from manufacturing impurities. These changes also provide preliminary insights into the possible formation routes and types of precursors involved.

Distribution, characteristics, and worldwide inventory of dioxins in kaolin ball clays

Distribution, characteristics, and global inventory of dioxins (polychlorinated dibenzo-p-dioxins [PCDDs] and dibenzofurans [PCDFs] and dioxin like polychlorinated biphenyls) in kaolin clays collected from 10 countries were investigated. Dioxins were found in all kaolin clay samples analyzed, at total concentrations ranging from 1.2 pg/g (Brazil) to 520,000 pg/g (USA). Dioxin concentrations in kaolin clays from a few countries (e.g., Brazil and UK) were lower than those reported for background soils in Japan. Dioxin profiles in kaolin clays were characterized by the domination of the congener octachlorodibenzo-p-dioxin (OCDD), and the concentrations of other congeners decreased in the order of reduction in the levels of chlorination. Furthermore, specific distribution of congeners, with predominant proportions of 1,4,6,9-substituted PCDDs within each homologue group, was found in most clay samples. The ratios of concentrations of PCDD to PCDF and 1,2,3,7,8,9-HxCDD to 1,2,3,6,7,8-HxCDD indicated differences in the profiles found for anthropogenic sources (including pentachlorophenol) and kaolin clays. Concentrations of PCDD/Fs in kaolin clays, except for American ball clays, did not exceed the environmental criteria set by the Law Concerning Special Measures against Dioxins in Japan. Based on the average concentrations measured in our study, inventories of PCDD/Fs from the production/usage of ball clays on a global scale were estimated to be 650 kg/yr; the corresponding value on a TEQ basis is 2400 g-TEQ/yr. More than 480 kg of OCDD is estimated to be released annually from the production of kaolin clays worldwide, suggesting that kaolin clays can be a major contributor for additional source of dioxins, especially OCDD, in the environment.

Clay mediated route to natural formation of Polychlorodibenzo-p-dioxins

Recent studies have documented the ubiquitous occurrence of polychlorodibenzo-p-dioxins and dibenzofurans (PCDD/Fs) of unknown origin in soils and clay deposits. Interestingly, the PCDD/F congener profiles do not match any known natural or anthropogenic source, and global PCDD/F budgets fail to account for the observed levels in soils. To reconcile these observations, clay minerals had been hypothesized to play a central role in the natural in situ synthesis of PCDD/Fs. We recently demonstrated the clay-mediated formation of the most prevalent PCDD congener in soils, octachlorodibenzo-p-dioxin (OCDD), supporting this hypothesis. Here we report the formation of the direct precursors ("predioxins") of the most toxic PCDD congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), and of 1,2,4,7,8-pentachlorodizenzodioxin (1,2,4,7,8-PeCDD), and two additional dimers, from the reaction of 2,4,5-trichlorophenol (2,4,5-TCP) with Fe(III)-montmorillonite clay. We propose plausible reaction pathways, each initiated by single electron transfer from 2,4,5-TCP to Fe(III)-montmorillonite forming the 2,4,5-TCP radical cation. The operative reaction mechanisms, inferred from experimental results, are supported by quantum mechanical calculations. The key role of montmorillonite is apparently to stabilize the reactive radical cation intermediate. Fortuitously, PCDD formation reactions on clay surfaces are more facile for less toxic higher chlorinated congeners like OCDD, as predicted by the proposed reaction mechanism and consistent with the observed PCDD congener distributions in soils. Importantly, increasing the toxicity equivalency factor of OCDD would immediately cause many soils to exceed PCDD regulatory levels due to the predominance of this congener.