Mechanism Of Radical Formation Research Articles

Direct Determination of Absolute Radical Quantum Yields in Hydroxyl and Sulfate Radical-Based Treatment Processes.

The absolute radical quantum yield () is a critical parameter to evaluate the efficiency of radical-based processes in engineered water treatment. However, measuring is fraught with challenges, as current quantification methods lack selectivity, specificity, and anti-interference capabilities, resulting in significant error propagation. Herein, we report a direct and reliable time-resolved technique to determine at pH 7.0 for commonly used radical precursors in advanced oxidation processes. For H2O2 and peroxydisulfate (PDS), the values of •OH and at 266 nm were measured to be 1.10 ± 0.01 and 1.46 ± 0.05, respectively. For peroxymonosulfate (PMS), we developed a new approach to determine with terephthalic acid as a trap-and-trigger probe in the nonsteady state system. For the first time, the value was measured to be 0.56 by the direct method, which is stoichiometrically equal to (0.57 ± 0.02). Additionally, radical formation mechanisms were elucidated by density functional theory (DFT) calculations. The theoretical results showed that the highest occupied molecular orbitals of the radical precursors are O-O antibonding orbitals, facilitating the destabilization of the peroxy bond for radical formation. Electronic structures of these precursors were compared, aiming to rationalize the tendency of the values we observed. Overall, this time-resolved technique with specific probes can be used as a reliable tool to determine , serving as a scientific basis for the accurate performance evaluation of diverse radical-based treatment processes.

Recent Advances in Radical Transformations of Propargylic Alcohols

AbstractPropargylic alcohols are regarded as a class of readily available and versatile synthons in organic synthesis. Due to their unique bifunctional character, the transformation of propargylic alcohols has been widely utilized as a powerful strategy for the construction of various functionalized frameworks over the past few decades. Recently, the radical transformation of propargylic alcohols has attracted considerable attention in synthetic chemistry. This review summarizes recent progress in the radical transformation of propargylic alcohols. Based on the different mechanisms of radical formation underlying these transformations, this review has been divided into four parts: (1) transition‐metal catalyzed radical transformation; (2) photo‐induced radical transformation; (3) electro‐induced radical transformation; and (4) metal‐free or oxidant‐mediated radical transformation of propargylic alcohols.

Both experimental and molecular dynamics approaches highlight the central role of interfacial water for radical production by irradiated gold nanoparticles

Nanoparticles devoted to improve radiotherapy treatments are an efficient tool if they can induce the formation of deleterious species in the tumor. Their interaction with radiation is responsible for radical production but in spite of the numerous studies mostly with cells, no consensus has been reached about radical formation mechanism. In order to gain knowledge in the physico-chemical step of this phenomenon, we applied a very sensitive test to quantify hydroxyl radicals and electrons produced when gold atoms, organized as nanoparticles or as a salt in solution, are irradiated by keV and MeV photons (x- and γ- rays). The crucial role of interfacial water is suggested to explain the high quantity of radicals measured for nanoparticles. These experimental data were supplemented by classical molecular dynamics simulations, revealing a specific organization of the water hydrogen bonding network at the nanoparticle surface which could be a key component in the mechanism of radical production by irradiated colloidal suspensions.

Suppression of Chemical Degradation By Gas Barrier Polymer Electrolyte Membranes

[Introduction]Polymer electrolyte fuel cell (PEFC) is one of the promising technologies for decarbonization. Especially, PEFC is suitable to power heavy-duty vehicles (HDV) because of its high efficiency and high fuel capacity. However, there are some serious technical challenges for PEFC application for HDV such as the durability. When we focus on the chemical degradation of polymer electrolyte membranes (PEMs), it happens due to attack by radical species. One of the radical formation mechanisms is the reaction at the anode side by the penetrated oxygen from cathode. Therefore, a promising mitigation strategy against chemical degradation is the suppression of oxygen permeability through the PEM.To verify this concept, we developed high gas barrier PEM. As a model gas barrier PEM, we made a sandwich type PEM with high oxygen barrier property. The sandwich PEM was prepared by depositing a thin interlayer consisting blended poly(vinyl alcohol) (PVA) as a gas barrier material and poly(vinyl sulfonic acid) (PVS) as a proton conductor in between two layers of Nafion 211 membranes. We evaluate chemical durability using the sandwich gas barrier PEM and discuss about the mechanism.[Experimental]To fabricate thin sandwich PEMs (15-20 µm), ethanol diluted Nafion dispersion solution (20 mg/ml) and PVA/PVS solution (10 mg/ml) were sprayed on a polytetrafluoroethylene (PTFE) sheet by a spray gun (Tamiya HG wide airbrush). The thickness of each layer was controlled by controlling the deposited weight. Then, membrane characterization procedures such as surface roughness, proton conductivity, and dimensional stability were performed. To evaluate fuel cell performance, the PEMs were mounted to a JARI cell with 1 cm2 active area. Chemically accelerated stress test was performed by open circuit voltage (OCV) holding test following NEDO protocol.[Results and Discussion]Several areal densities of PVA/PVS interlayer were successfully incorporated into Nafion membrane to make sandwich PEM. The sandwich PEM shows considerable fuel cell performance, and thin sandwich PEMs show higher performance than the 50 µm sandwich PEM, although it is lower than the thin sprayed Nafion. For example, the peak power density of 15 µm sandwich PEM was 0.44 W cm-2 compared to 0.30 W cm-2 and 0.50 W cm-2 for 50 µm sandwich PEM and 15 µm Nafion, respectively. Furthermore, the incorporation of interlayer to thin PEM can suppress the hydrogen crossover current density from 7.8 mA cm-2 to 5.5 mA cm-2 indicating superior gas barrier property. From the higher gas barrier property, it is predicted thin sandwich PEMs will have higher chemical durability than Nafion with similar thickness.

Mechanisms of Radical Formation on Chemically Modified Graphene Oxide under Near Infrared Irradiation.

In this work, the mechanisms of radical generation on different functionalized graphene oxide (GO) conjugates under near-infrared (NIR) light irradiation are investigated. The GO conjugates are designed to understand how chemical functionalization can influence the generation of radicals. Both pristine and functionalized GO are irradiated by a NIR laser, and the production of different reactive oxygen species (ROS) is investigated using fluorimetry and electron paramagnetic resonance to describe the type of radicals present on the surface of GO. The mechanism of ROS formation involves a charge transfer from the material to the oxygen present in the media, via the production of superoxide and singlet oxygen. Cytotoxicity and effects of ROS generation are then evaluated using breast cancer cells, evidencing a concentration dependent cell death associated to the heat and ROS. The study provides new hints to understand the photogeneration of radicals on the surface of GO upon near infrared irradiation, as well as, to assess the impact on these radicals in the context of a combined drug delivery system and phototherapeutic approach. These discoveries open the way for a better control of phototherapy-based treatments employing graphene-based materials.

Dynamics and mechanism of radical formation in a highly sensitive oxime photoinitiator as revealed by time-resolved absorption and EPR measurements

Reaction dynamics and mechanism of radical formation of a photoinitiator, Irgacure OXE01, were investigated by transient absorption spectroscopy and time-resolved EPR measurements. Femtosecond transient spectroscopy revealed that the S1 state underwent ultrafast intersystem crossing and N-O bond cleavage with a time constant of 1.6 ps, resulting in the production of benzoyloxyl and iminyl radicals. The T1 state also yielded benzoyloxyl and iminyl radicals with a time constant of 300 ps. These radicals decreased in several ns time range by the geminate recombination in the radical pair in competition with the diffusional dissociation into free radicals. Time-resolved EPR measurement indicated that the radical pair produced from the T1 state effectively led to the production of the free radicals. These results strongly suggested the attractive potential via the exchange interaction between the two radicals in the pair suppressed the dissociation process of the radical pair.

Accurate identification of radicals by in-situ electron paramagnetic resonance in ultraviolet-based homogenous advanced oxidation processes

Accurate identification of radicals in advanced oxidation processes (AOPs) is important to study the mechanisms on radical production and subsequent oxidation-reduction reaction. The commonly applied radical quenching experiments cannot provide direct evidences on generation and evolution of radicals in AOPs, while electron paramagnetic resonance (EPR) is a cutting-edge technology to identify radicals based on spectral characteristics. However, the complexity of EPR spectrum brings uncertainty and inconsistency to radical identification and mechanism clarification. This work presented a comprehensive study on identification of radicals by in-situ EPR analysis in four typical UV-based homogenous AOPs, including UV/H2O2, UV/peroxodisulfate (and peroxymonosulfate), UV/peracetic acid and UV/IO4− systems. Radical formation mechanism was also clarified based on EPR results. A reliable EPR method using organic solvents was proposed to identify alkoxy and alkyl radicals (CH3C(=O)OO·, CH3C(=O)O· and ·CH3) in UV/PAA system. Two activation pathways for radical production were proposed in UV/IO4− system, in which the produced IO3·, IO4·, ·OH and hydrated electron were precisely detected. It is interesting that addition of specific organic solvents can effectively identify oxygen-center and carbon-center radicals. A key parameter in EPR spectrum for 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin adduct, AH, is ranked as: ·CH3 (23 G) >·OH (15 G) >IO3· (12.9 G) >O2·− (11 G) ≥·OOH (9–11 G) ≥IO4· (9–10 G) ≥SO4·− (9–10 G) >CH3C(=O)OO· (8.5 G) > CH3C(=O)O· (7.5 G). This study will give a systematic method on identification of radicals in AOPs, and shed light on the insightful understanding of radical production mechanism.

The role of hydroxylation on·OH generation for enhanced ozonation of benzoic acids: Reactivity, ozonation efficiency and radical formation mechanism

• Hydroxyl groups activated benzoic acid for enhanced reactivity with ozone. • Possible pathways for the generation of O 2 ·- and·OH in ozonation of benzoic acids were proposed. • Plausible mechanism for the activated ozonation of benzoic acid was elucidated. • Synergistic effect in co-ozonation and reduced superiority in catalytic ozonation were observed with extra·OH production.

Proton-Transfer Reactions in One-Electron-Oxidized G-Quadruplexes: A Density Functional Theory Study.

Recently, G-quadruplexes (Gq) formed in B-DNA as secondary structures are found to be important therapeutic targets and material for developing nanodevices. Gq are guanine-rich and thus susceptible to oxidative damage by producing short-lived intermediate radicals via proton-transfer reactions. Understanding the mechanisms of radical formation in Gq is of fundamental interest to understand the early stages of DNA damage. Herein, we used density functional theory including aqueous phase (ωB97XD-PCM/6-31++G**) and considered single layer of Gq [G-quartets (G4): association of four guanines in a cyclic Hoogsteen hydrogen-bonded arrangement (Scheme 1)] to unravel the mechanisms of formation of intermediates by calculating the relative Gibbs free energies and spin density distributions of one-electron-oxidized G4 and its various proton-transfer states: G•+, G(N1-H)•, G(N2-H')•, G(N2-H″)•, G(N1-H)•-(H+O6)G, and G(N2-H)•-(H+N7)G. The present calculation predicts the formation of G(N2-H)•-(H+N7)G, which is only ca. 0.8 kcal/mol higher in energy than the initially formed G•+. The formation of G(N2-H)•-(H+N7)G plays a key role in explaining the formation of 8-OG along with G(N1-H)• formation via tautomerization from G(N2-H)•, as proposed recently.

Molecular dynamic study on mechanisms of polyvinylidene fluoride decomposition by using supercritical water

As a thermoplastic fluoropolymer, polyvinylidene fluoride (PVDF) has been widely used as a binder in lithium batteries. However, PVDF is hard to be degraded naturally. Due to the unique properties of supercritical water (SCW), SCW conversion of polymers is considered as a clean and efficient conversion technology. The effects of different reaction parameters on the decomposition of PVDF in SCW were analyzed by the molecular dynamics (MD) method. The results show that higher reaction temperature, longer reaction time, lower raw material concentration, and the addition of oxidant are more conducive to the decomposition of PVDF in SCW. In addition, the molecular structure of products, product yield, and reaction rate of PVDF in SCW under different conditions were analyzed. The decomposition path, HO2 radical formation mechanism, and F element migration path of PVDF in SCW were also studied by using the MD visualization tool. Compared with SCWO, SCWG has a lower reaction rate and the products from SCWG has a longer molecular chain. The main path for HF generation is the direct interaction between the free F atom and H atom in the water molecules.

Formation of Environmentally Persistent Free Radicals (EPFRs) on the Phenol-Dosed α-Fe2O3(0001) Surface.

Environmentally persistent free radicals (EPFRs) are a class of toxic air pollutants that are found to form by the chemisorption of substituted aromatic molecules on the surface of metal oxides. In this study, we employ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) to perform a temperature-dependent study of phenol adsorption on α-Fe2O3(0001) to probe the radical formation mechanism by monitoring changes in the electronic structure of both the adsorbed phenol and metal oxide substrate. Upon dosing at room temperature, new phenol-derived electronic states have been clearly observed in the UPS spectrum at saturation coverage. However, upon dosing at high temperature (>200 °C), both photoemission techniques have shown distinctive features that strongly suggest electron transfer from adsorbed phenol to Fe2O3 surface atoms and consequent formation of a surface radical. Consistent with the experiment, DFT calculations show that phenoxyl adsorption on the iron oxide surface at RT leads to a minor charge transfer to the adsorbed molecule. The experimental findings at high temperatures agree well with the EPFRs' proposed formation mechanism and can guide future experimental and computational studies.

Mechanistic Studies on Photocatalytic Overall Water Splitting over Ga2O3-Based Photocatalysts by Operando MS-FTIR Spectroscopy.

Photocatalytic water splitting on semiconductor photocatalysts is one of the most important physichemical processes, but its surface reaction mechanisms are not fully understood. Based upon the ATR-FTIR investigations combining with the mass spectroscopy (MS) analysis, a direct hydroxyl radical formation mechanism that is different from those observed for other semiconductor photocatalysts is proposed. This study provides the insight into overall water splitting mechanism on Ga2O3-based photocatalysts at a molecular level, and it helps one to further understand the photocatalysis on semiconductor photocatalysts.

Formation of persistent free radicals in sludge biochar by hydrothermal carbonization

The uses of biochar as soil fertilizer can offset global warming and reduce dependence on limited mineral resources in the future circular economy, yet biochar may contain contaminants that can ultimately enter the food chain. In particular, persistent free radicals are emerging contaminants previously detected in biochar but underlying mechanisms of radical formation are not yet established. Here we studied radical generation during hydrothermal carbonization of waste sludge at 160–220 ºC for 0.5–2 h with solid weight ratios of 10%w–40%w using electron paramagnetic resonance and Fourier transform infrared spectrometry. Results reveal that radical concentration increases with temperature, reaction time, and weight ratio in sludge biochars, reaching a content of 47.2 × 1015 spins/g for 220 ºC, 2 h heating, and 40%w solid ratio. Moreover, low temperature of about 160 ºC favors the production of oxygen-centered radicals, whereas higher temperature of 220 ºC produces carbon-centered radicals. Our findings imply that biochar ecotoxicity should be assessed prior applications to prevent adverse health effects.

Revealing a key inhibitory mechanism of 2‐amino‐3,8‐dimethylimidazo[4,5‐f] quinoxaline via trapping of methylglyoxal

The inhibitory effects of vitamins (nicotinic acid, pyridoxamine [PM], and l-ascorbic acid) and phenolic acids (ferulic acid and p-coumaric acid) on the formation of 2-amino-3,8-dimethylimidazo [4,5-f] quinoxaline (MeIQx) were studied in a glycine/glucose/creatinine model system and fried tilapia cakes. The results showed that PM was the most potential inhibitor and the inhibition rates reached 82.72% and 78.54% in model system and fried tilapia cakes, respectively. Detailed formation mechanism of MeIQx was put forward to find the inevitable species in the non-free radical formation mechanism of MeIQx. Dose-dependent analysis of PM on methylglyoxal (MGO ) and MeIQx formation were studied by using model systems and the results showed that MGO and MeIQx were both reduced about 60% in reaction mixtures when the molar ratio of PM to glycine was 1:16, which indicated that MGO is a key intermediate on the pathway of MeIQx formation. Quantum chemistry calculations showed that PM can act as a useful inhibitor to inhibit the formation of MeIQx and react with MGO to form new compounds. A pathway for the inhibitory activity of PM against MeIQx formation was proposed. PRACTICAL APPLICATION: Pyridoxamine was the most effective inhibitor against heterocyclic aromatic amines (HAAs) and could be applied to a variety of food systems. While the inhibitory mechanism is still unclear. Detailed formation mechanism of MeIQx was put forward first and suggested methylglyoxal as an inevitable species in the non-free radical formation mechanism of MeIQx in this study. Pyridoxamine trapping methylglyoxal is likely a key mechanism against the generation of MeIQx was demonstrated by quantum chemistry calculation and experimental demonstration. These findings may provide effective suggestions for reducing HAAs and similar toxicants in daily cuisine.

Immobilization of facet-engineered Ag3PO4 on mesoporous Al2O3 for efficient industrial waste gas purification with indoor LED illumination

The immobilization of a photocatalyst on a proper support is pivotal for practical environmental applications. Herein, as a rising visible light photocatalyst, different facet-engineered Ag3PO4{111}, Ag3PO4{110}, Ag3PO4{100} and irregular Ag3PO4 were first immobilized on mesoporous Al2O3, then applied for photocatalytic removal of ppm-leveled NO under visible light illumination. Photocatalytic performances for NO removal by these immobilized Ag3PO4 were found to be highly facet-dependent, showing the degradation kinetics in the order of Ag3PO4{111} tetrahedra > Ag3PO4{110} rhombic dodecahedra > Ag3PO4{100} cubes > irregular Ag3PO4. The density functional theory (DFT) calculations were used to investigate the difference in the catalytic activity of Ag3PO4/Al2O3 with exposed {111}, {110}, and {100} facets. The experimental characterization and theoretical calculations all confirm that the {111} facets with high surface energy can not only facilitate adsorption/activation of O2 and H2O but also be beneficial to the generation of •O2− and •OH radicals, which result in significantly enhanced activity for NO oxidation. A modified band diagram showing diff ;erent degrees of band edge bending can explain these observations. A reaction pathway study based on both in situ FT-IR and molecular-level simulation of NO adsorption and transformation indicates that this Ag3PO4{111}/Al2O3 can efficiently adsorb NO and transform it into harmless nitrate products via NO → NO+ and NO2+ → nitrate or nitrite routes. The present work reveals the facet-dependent radical formation mechanisms of Ag3PO4/Al2O3, and also provides new perspectives for promoting environmental applications of immobilized photocatalysts.

Degradation of organic pollutants by peroxymonosulfate activated by MnO2 with different crystalline structures: Catalytic performances and mechanisms

In this study, a novel α-MnO2 (OMS-2) material with long and uniform nanofibers was synthesized by morphological and phase transitions from δ-MnO2 (OL-1) under a hydrothermal reaction. We systematically investigated the catalytic performances of OMS-2 and OL-1 for the activation of PMS (peroxymonosulfate) to degrade 4-nitrophenol (4-NP) in water. According to the results from Brunauer Emmett Teller (BET), thermo gravimetric analyzer (TGA), H2-temperature programmed reduction (H2-TPR), cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and density-functional theory (DFT) calculation, OMS-2 has a larger BET area, more active sites, better adsorption ability, a faster electron transfer rate, and more multiple valence states of Mn than OL-1. These results also well illustrate OMS-2 has much better catalytic performance than OL-1. The results of the electron paramagnetic resonance (EPR) and the radical quantification experiments confirmed that sulfate radicals (SO4−) and hydroxyl radicals (OH) were the main oxidants and OMS-2 has better radical generation capability than OL-1. The LC-MS results indicated that there were two routes for the degradation of 4-NP and the degradation mechanism of 4-NP in the OMS-2/PMS system was similar to that in the OL-1/PMS system. Finally, we proposed the PMS activation mechanism, the formation mechanism of radicals, and the degradation mechanism of 4-NP based on the two different kinds of MnO2 with different morphologies.

Fabrication of Ag3VO4 decorated phosphorus and sulphur co-doped graphitic carbon nitride as a high-dispersed photocatalyst for phenol mineralization and E. coli disinfection

In this work, we have successfully anchored Ag3VO4 (AV) onto P and S co-doped g-C3N4 (PSGCN) to prepare high-dispersible AV/PSGCN photocatalyst via a deposition-precipitation method. The P and S co-doped g-C3N4 was synthesized via thermal polycondensation using hexachlorotriphosphazene (HCCP) and thiourea as precursors. AV/PSGCN was characterized using various spectral techniques. The atomic force analysis indicated that the thickness of AV/PSGCN was less than 3.0 nm. The zeta potential and Tyndall effect experiments ascertained formation of the well-dispersed suspension of AV/PSGCN in water. The co-doping resulted in lowering optical band gap of g-C3N4. The photoluminescence and electrochemical impedance analysis indicated suppression in recombination of photogenerated electron and hole pairs in AV/PSGCN. The photodegradation of phenol followed pseudo-first order kinetics. Hydroxyl radicals and holes were the two main reactive species for photodegradation of phenol. The COD, HPLC and LC-MS analyses confirmed mineralization of phenol in 6 h. Unlike conventional slurry type photo-reactors, AV/PSGCN was not magnetically agitated during photocatalytic reactions. AV/PSGCN exhibited significant antibacterial activity for E. coli disinfection. The photodegradation of phenol and bacterial disinfection occurred through hole and hydroxyl radical formation mechanism.

Mechanism and kinetics of catalytic ozonation for elimination of organic compounds with spinel-type CuAl2O4 and its precursor

CuAl2O4 based mixed oxides were used as heterogeneous catalysts for ozone activation to degrade organics in aqueous solution. The solids were thoroughly characterized by SEM/EDS, N2 physisorption, XRD, FTIR, Pyridine-FTIR, TEM and XPS. We demonstrated that the solid precursor calcined at 300 °C exhibited the best catalytic ozonation activity with respect to CuAl2O4 spinel phase obtained at higher temperatures. Such performance was attributed to the better textural properties and a higher density of active sites (hydroxyl groups and Lewis acidity). Specifically, the mixed oxide/O3 process allows to reach a near complete color removal of the dye solution (100 mg L−1) in 25 min at neutral pH. Corresponding reaction rate value was measured at 0.112 min−1 and was clearly higher compared with the single oxide ozonation process (0.071 min−1 for CuO/O3 and 0.074 min−1 for Al2O3/O3). Then, we proposed that such catalytic performance was related to a synergistic function between ≡Cu2+ and ≡Al3+, which took part of a mechanism of radical formation. In such mechanism, present ≡Al3+ could act as a reservoir for surface active sites such as hydroxyl groups and Lewis acid sites, while ≡Cu2+ could provide the possibility of electron transfer with ozone for the enhancement of radical generation. We suggested that the interaction between chemisorbed ozone and surface hydroxyl groups initially stabilized on ≡Al3+ initiated the generation of reactive radical species. This interaction led as well to the formation of surface adsorbed HO and few O2− on ≡Cu2+ Lewis acid sites. Besides, the interfacial redox reaction with ozone is favored by the presence of ≡Cu2+ following the sequence of ≡Cu2+/≡Cu+/≡Cu2+ redox cycle.

Mechanism of Radical Formation in the H-Bond Network of D1-Asn298 in Photosystem II.

In photosystem II (PSII), redox-active tyrosine Z (TyrZ) forms a low-barrier H-bond with Nε of D1-His190. The PSII crystal structures show that Nδ of D1-His190 donates an H-bond to the carbonyl O of D1-Asn298. However, at a level of ∼2 Å resolution, a clear discrimination between the -NH2 and -C═O groups of the asparagine side chain may not be possible based on the electron density map. Using quantum mechanical/molecular mechanical calculations, we investigated the energetics of the D1-Asn298 conformations. In the D1-Asn298-rotated conformation, where the amide N group donates an H-bond to deprotonated Nδ of D1-His190, oxidation of S2 resulted in formation of a neutral radical, either TyrZ• or D1-His190•. This suggests that in the D1-Asn298-rotated conformation, the redox potential ( Em) values of TyrZ/D1-His190 are lower than the Em of the Mn4CaO5 cluster due to deprotonated D1-His190. The large disorder of a water molecule (water 1117A) at D1-Asn298 in the crystal structure as well as the absence of water 1117A in the Sr2+-substituted crystal structure may be associated with coexistence of the two D1-Asn298 conformations in the crystals.

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 ( kobs, day-1) on the presence of metal ions follows the order Fe(III) > Cu(II) > Ni(II) > Co(II) > Zn(II). The values of kobs 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 kobs 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.