Effect of f-element complexation on the radiolysis of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP])

A systematic study of the impact on the chemical reactivity of the oxidising n-dodecane radical cation (RH˙+) with f-element complexed 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) has been undertaken utilizing time-resolved electron pulse radiolysis/transient absorption spectroscopy and high-level quantum mechanical calculations. Lanthanide ion complexed species, [Ln((HEH[EHP])2)3], exhibited vastly increased reactivity (over 10× faster) in comparison to the non-complexed ligand in n-dodecane solvent, whose rate coefficient was k = (4.66 ± 0.22) × 109 M-1 s-1. Similar reactivity enhancement was also observed for the corresponding americium ion complex, k = (5.58 ± 0.30) × 1010 M-1 s-1. The vastly increased reactivity of these f-element complexes was not due to simple increased diffusion-control of these reactions; rather, enhanced hole transfer mechanisms for the complexes were calculated to become energetically more favourable. Interestingly, the observed reactivity trend with lanthanide ion size was not linear; instead, the rate coefficients showed an initial increase (Lu to Yb) followed by a decrease (Tm to Ho), followed by another increase (Dy to La). This behaviour was excellently predicted by the calculated reaction volumes of these complexes. Complementary cobalt-60 gamma irradiations for select lanthanide complexes demonstrated that the measured kinetic differences translated to increased ligand degradation at steady-state timescales, affording ∼38% increase in ligand loss of a 1 : 1 [La((HEH[EHP])2)3] : HEH[EHP] ratio system.

Radiolytic evaluation of a new technetium redox control reagent for advanced used nuclear fuel separations

Technetium is a problematic radioisotope for used nuclear fuel (UNF) and subsequent waste management owing to its high environmental mobility and coextraction in reprocessing technologies as the pertechnetate anion (TcO4-). Consequently, several strategies are under development to control the transport of this radioisotope. A proposed approach is to use diaminoguanidine (DAG) for TcO4- and transuranic ion redox control. Although the initial DAG molecule is ultimately consumed in the redox process, its susceptibility to radiolysis is currently unknown under envisioned UNF reprocessing conditions, which is a critical knowledge gap for evaluating its overall suitability for this role. To this end, we report the impacts of steady-state gamma irradiation on the rate of DAG radiolysis in water, aqueous 2.0 M nitric acid (HNO3), and in a biphasic solvent system composed of aqueous 2.0 M HNO3 in contact with 1.5 M N,N-di-(2-ethylhexyl)isobutyramide (DEHiBA) dissolved in n-dodecane. Additionally, we report chemical kinetics for the reaction of DAG with key transients arising from electron pulse radiolysis, specifically the hydrated electron (eaq-), hydrogen atom (H˙), and hydroxyl (˙OH) and nitrate (NO3˙) radicals. The DAG molecule exhibited significant reactivity with the ˙OH and NO3˙ radicals, indicating that oxidation would be the predominant degradation pathway in radiation environments. This is consistent with its role as a reducing agent. Steady-state gamma irradiations demonstrated that DAG is readily degraded within a few hundred kilogray, the rate of which was found to increase upon going from water to HNO3 containing solutions and solvents systems. This was attributed to a thermal reaction between DAG and the predominant HNO3 radiolysis product, nitrous acid (HNO2), k(DAG + HNO2) = 5480 ± 85 M-1 s-1. Although no evidence was found for the radiolysis of DAG altering the radiation chemistry of the contacted DEHiBA/n-dodecane phase in the investigated biphasic system, the utility of DAG as a redox control reagent will likely be limited by significant competition with its degradation by HNO2.

Chemical Kinetics for the Oxidation of Californium(III) Ions with Select Radiation-Induced Inorganic Radicals (Cl2•- and SO4•-)

Despite the availability of transuranic elements increasing in recent years, our understanding of their most basic and inherent radiation chemistry is limited and yet essential for the accurate interpretation of their physical and chemical properties. Here, we explore the transient interactions between trivalent californium ions (Cf 3 + ) and select inorganic radicals arising from the radiolytic decomposition of common anions and functional group constituents, specifically the dichlorine (Cl2•-) and sulfate (SO4•-) radical anions. Chemical kinetics, as measured using integrated electron pulse radiolysis and transient absorption spectroscopy techniques, are presented for the reactions of these two oxidizing radicals with Cf 3 + ions. The derived and ionic strength-corrected second-order rate coefficients (k) for these radiation-induced processes are k(Cf 3 + + Cl2•-) = (8.28 ± 0.61) × 105 M-1 s-1 and k(Cf 3 + + SO4•-) = (9.50 ± 0.43) × 108 M-1 s-1 under ambient temperature conditions (22 ± 1 °C).

Methyl nitrate as a byproduct in advanced water treatment systems: Liquid chromatographic determination method and cause of formation

Neutral low-molecular-weight organics such as methyl nitrate that can readily pass through reverse osmosis (RO) membranes employed in potable water reuse facilities attract interest owing to public health considerations. In this study, a novel determination method based on high-performance liquid chromatography, online photochemical conversion to peroxynitrite, and luminol chemiluminescence detection was developed for methyl nitrate measurement in treated water. The maximum photochemical conversion efficiency of methyl nitrate to peroxynitrite was found to be 6.5% using a 222-nm excimer lamp. The calibration curve for the developed method was linear between 1.0 × 10-9 and 1.0 × 10-7 M, and the limit of detection was 0.3 nM (0.03 μg/L) given an injection volume of 200 μL. The methyl nitrate concentrations in RO permeate from reclaimed wastewater and product water after subsequent treatment by a UV/H2O2 advanced oxidation process (AOP) were 2.2 and 22.5 nM (0.17 and 1.7 μg/L), respectively. UV irradiation of RO permeate in the laboratory using a low-pressure Hg lamp confirmed the formation of methyl nitrate in the permeate in the absence of H2O2 and residual chloramines. This chemiluminescent detection method for methyl nitrate will promote a greater understanding of the origin and formation of this treatment byproduct in reclaimed wastewater.

Kinetics of the Temperature-Dependent eaq - and ⋅OH Radical Reactions with Cr(III) Ions in Aqueous Solutions

The reactivity of chromium(III) species with the major oxidizing and reducing radiolysis products of water was investigated in aqueous solutions at temperatures up to 150 °C. The reaction between the hydrated electron (eaq - ) and Cr(III) species showed a positive temperature dependence over this temperature range. The reaction was also studied in pH 2.5 and 3.5 solutions for the first time. This work also studied the reaction between acidic Cr(III) species and the hydroxyl radical (⋅OH). It was found that Cr3+ did not react significantly with the ⋅OH radical, but the first hydrolysis species, Cr(OH)2+ , did with a rate coefficient of k= (7.2±0.3)×108  M-1  s-1 at 25 °C. The oxidation of Cr(OH)2+ by the ⋅OH radical formed an absorbing product species that ultimately oxidized to give Cr(VI). These newly measured reaction rates allow for the development of improved models of aqueous chromium speciation for the effective remediation of liquid high-level nuclear waste via vitrification processes.

Impact of lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the dodecane radical cation

The impact of trivalent lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the n-dodecane radical cation (RH˙⁺) has been measured by electron pulse radiolysis and evaluated by quantum mechanical calculations. Additionally, Arrhenius parameters were determined for the reaction of the non-complexed TODGA ligand with the RH˙⁺ from 10-40 °C, giving the activation energy (E_a = 17.43 ± 1.64 kJ mol^-1) and pre-exponential factor (A = (2.36 ± 0.05) × 10¹³ M^-1 s^-1). The complexation of Nd(III), Gd(III), and Yb(III) ions by TODGA yielded [Ln^III(TODGA)₃(NO₃)₃] complexes that exhibited significantly increased reactivity (up to 9.3× faster) with the RH˙⁺, relative to the non-complexed ligand: k([Ln^III(TODGA)₃(NO₃)₃] + RH˙⁺) = (8.99 ± 0.93) × 10¹⁰, (2.88 ± 0.40) × 10¹⁰, and (1.53 ± 0.34) × 10¹⁰ M^-1 s^-1, for Nd(III), Gd(III), and Yb(III) ions, respectively. The rate coefficient enhancement measured for these complexes exhibited a dependence on atomic number, decreasing as the lanthanide series was traversed. Preliminary reaction free energy calculations-based on a model [Ln^III(TOGDA)]³⁺ complex system-indicate that both electron/hole and proton transfer reactions are energetically unfavorable for complexed TODGA. Furthermore, complementary average local ionization energy calculations showed that the most reactive region of model N,N,N',N'-tetraethyl diglycolamide (TEDGA) complexes, [Ln^III(TEGDA)₃(NO₃)₃], toward electrophilic attack is for the coordinated nitrate (NO₃^-) counter anions. Therefore, it is possible that radical reactions with the complexed NO₃^- counter anions dominate the differences in rates seen for the [Ln^III(TODGA)₃(NO₃)₃] complexes, and are likely responsible for the reported radioprotection in the presence of TODGA complexes.

Reactivity of Dissolved Organic Matter with the Hydrated Electron: Implications for Treatment of Chemical Contaminants in Water with Advanced Reduction Processes

Advanced reduction processes (ARP) have garnered increasing attention for the treatment of recalcitrant chemical contaminants, most notably per- and polyfluoroalkyl substances (PFAS). However, the impact of dissolved organic matter (DOM) on the availability of the hydrated electron (eaq-), the key reactive species formed in ARP, is not completely understood. Using electron pulse radiolysis and transient absorption spectroscopy, we measured bimolecular reaction rates constant for eaq- reaction with eight aquatic and terrestrial humic substance and natural organic matter isolates ( kDOM,eaq-), with the resulting values ranging from (0.51 ± 0.01) to (2.11 ± 0.04) × 108 MC-1 s-1. kDOM,eaq- measurements at varying temperature, pH, and ionic strength indicate that activation energies for diverse DOM isolates are ≈18 kJ mol-1 and that kDOM,eaq- could be expected to vary by less than a factor of 1.5 between pH 5 and 9 or from an ionic strength of 0.02 to 0.12 M. kDOM,eaq- exhibited a significant, positive correlation to % carbonyl carbon for the isolates studied, but relationships to other DOM physicochemical properties were surprisingly more scattered. A 24 h UV/sulfite experiment employing chloroacetate as an eaq- probe revealed that continued eaq- exposure abates DOM chromophores and eaq- scavenging capacity over a several hour time scale. Overall, these results indicate that DOM is an important eaq- scavenger that will reduce the rate of target contaminant degradation in ARP. These impacts are likely greater in waste streams like membrane concentrates, spent ion exchange resins, or regeneration brines that have elevated DOM concentrations.

Radiolytic Evaluation of 3,4,3-LI(1,2-HOPO) in Aqueous Solutions

The octadentate hydroxypyridinone ligand 3,4,3-LI(1,2-HOPO) (abbreviated as HOPO) has been identified as a promising candidate for both chelation and f-element separation technologies, two applications that require optimal performance in radiation environments. However, the radiation robustness of HOPO is currently unknown. Here, we employ a combination of time-resolved (electron pulse) and steady-state (alpha self-radiolysis) irradiation techniques to elucidate the basic chemistry of HOPO and its f-element complexes in aqueous radiation environments. Chemical kinetics were measured for the reaction of HOPO and its Nd(III) ion complex ([Nd^III(HOPO)]^-) with key aqueous radiation-induced radical transients (e_aq^-, H^• atom, and ^•OH and NO₃^• radicals). The reaction of HOPO with the e_aq^- is believed to proceed via reduction of the hydroxypyridinone moiety, while transient adduct spectra indicate that reactions with the H^• atom and ^•OH and NO₃^• radicals proceeded by addition to HOPO's hydroxypyridinone rings, potentially allowing for the generation of an extensive suite of addition products. Complementary steady-state ²⁴¹Am(III)-HOPO complex ([²⁴¹Am^III(HOPO)]^-) irradiations showed the gradual release of ²⁴¹Am(III) ions with increasing alpha dose up to 100 kGy, although complete ligand destruction was not observed.

Gamma Radiation-Induced Degradation of Acetohydroxamic Acid (AHA) in Aqueous Nitrate and Nitric Acid Solutions Evaluated by Multiscale Modelling

Acetohydroxamic acid (AHA) has been proposed for inclusion in advanced, single-cycle, used nuclear fuel reprocessing solvent systems for the reduction and complexation of plutonium and neptunium ions. For this application, a detailed description of the fundamental degradation of AHA in dilute aqueous nitric acid is required. To this end, we present a comprehensive, multiscale computer model for the coupled radiolytic and hydrolytic degradation of AHA in aqueous sodium nitrate and nitric acid solutions. Rate coefficients for the reactions of AHA and hydroxylamine (HA) with the oxidizing nitrate radical were measured for the first time using electron pulse radiolysis and used as inputs for the kinetic model. The computer model results are validated by comparison to experimental data from steady-state gamma ray irradiations, for which the agreement is excellent. The presented model accurately predicts the yields of the major degradation products of AHA: acetic acid, HA, nitrous oxide, and molecular hydrogen.

Facile nanoplastics formation from macro and microplastics in aqueous media

The immense production of plastic polymers combined with their discordancy with nature has led to vast plastic waste contamination across the geosphere, from the oceans to freshwater reservoirs, wetlands, remote snowpacks, sediments, air and multiple other environments. These environmental pollutants include microplastics (MP), typically defined as small and fragmented plastics less than 5 mm in size, and nanoplastics (NP), particles smaller than a micrometer. The formation of micro and nanoplastics in aqueous media to date has been largely attributed to fragmentation of plastics by natural (i.e., abrasion, photolysis, biotic) or industrial processes. We present a novel method to create small microplastics (≲ 5 μm) and nanoplastics in water from a wide variety of plastic materials using a small volume of a solubilizer liquid, such as n-dodecane, in combination with vigorous mixing. When the suspensions or solutions are subjected to ultrasonic mixing, the particle sizes decrease. Small micro- and nanoparticles were made from commercial, real world and waste (aged) polyethylene, polystyrene, polycarbonate and polyethylene terephthalate, in addition to other plastic materials and were analyzed using dark field microscopy, Raman spectroscopy and particle size measurements. The presented method provides a new and simple way to create specific size distributions of micro- and nanoparticles, which will enable expanded research on these plastic particles in water, especially those made from real world and aged plastics. The ease of NP and small MP formation upon initial mixing simulates real world environments, thereby providing further insight into the behavior of plastics in natural settings.

Multiscale modelling of the radical-induced chemistry of acetohydroxamic acid in aqueous solution

Acetohydroxamic acid (AHA) is a small organic acid with a wide variety of industrial, biological, and pharmacological applications. A deep fundamental molecular level understanding of the mechanisms responsible for the radical-induced reactions of AHA in these environments is necessary to predict and control their behaviour and elucidate their interplay with other attendant chemical species, for example, the oxidative degradation products of AHA. To this end, we present a comprehensive, multiscale computer model for interrogating the radical-induced degradation of AHA in acidic aqueous solutions. Model predictions were critically evaluated by a systematic experimental radiation chemistry investigation, leveraging time-resolved electron pulse irradiation techniques for the measurement of new radical reaction rate coefficients, and steady-state gamma irradiations for the identification and quantification of AHA degradation products: acetic acid, hydroxylamine, nitrous oxide, and molecular hydrogen, with formic acid and methane as minor products. Excellent agreement was achieved between calculation and experiment, indicating that this fundamental model can accurately predict the degradation pathways of AHA under irradiation in acidic aqueous solutions.

A comprehensive multiscale model determines the fundamental reaction mechanisms of the radical-induced degradation of acetohydroxamic acid in acidic aqueous solutions.

Transient Radiation-Induced Berkelium(III) and Californium(III) Redox Chemistry in Aqueous Solution

Despite the significant impact of radiation-induced redox reactions on the accessibility and lifetimes of actinide oxidation states, fundamental knowledge of aqueous actinide metal ion radiation chemistry is limited, especially for the late actinides. A quantitative understanding of these intrinsic radiation-induced processes is essential for investigating the fundamental properties of these actinides. We present here a picosecond electron pulse reaction kinetics study into the radiation-induced redox chemistry of trivalent berkelium (Bk(III)) and californium (Cf(III)) ions in acidic aqueous solutions at ambient temperature. New and first-of-a-kind, second-order rate coefficients are reported for the transient radical-induced reduction of Bk(III) and Cf(III) by the hydrated electron (e_aq^-) and hydrogen atom (H^•), demonstrating a significant reactivity (up to 10¹¹ M^-1 s^-1) indicative of a preference of these metals to adopt divalent states. Additionally, we report the first-ever second-order rate coefficients for the transient radical-induced oxidation of these elements by a reaction with hydroxyl (^•OH) and nitrate (NO₃^•) radicals, which also exhibited fast reactivity (ca. 10⁸ M^-1 s^-1). Transient Cf(II), Cf(IV), and Bk(IV) absorption spectra are also reported. Overall, the presented data highlight the existence of rich, complex, intrinsic late actinide radiation-induced redox chemistry that has the potential to influence the findings of other areas of actinide science.

Critical Review of UV-Advanced Reduction Processes for the Treatment of Chemical Contaminants in Water

UV-advanced reduction processes (UV-ARP) are an advanced water treatment technology characterized by the reductive transformation of chemical contaminants. Contaminant abatement in UV-ARP is most often accomplished through reaction with hydrated electrons (e_aq ^-) produced from UV photolysis of chemical sensitizers (e.g., sulfite). In this Review, we evaluate the photochemical kinetics, substrate scope, and optimization of UV-ARP. We find that quantities typically reported in photochemical studies of natural and engineered systems are under-reported in the UV-ARP literature, especially the formation rates, scavenging capacities, and concentrations of key reactive species like e_aq ^-. The absence of these quantities has made it difficult to fully evaluate the impact of operating conditions and the role of water matrix components on the efficiencies of UV-ARP. The UV-ARP substrate scope is weighted heavily toward contaminant classes that are resistant to degradation by advanced oxidation processes, like oxyanions and per- and polyfluoroalkyl substances. Some studies have sought to optimize the UV-ARP treatment of these contaminants; however, a thorough evaluation of the impact of water matrix components like dissolved organic matter on these optimization strategies is needed. Overall, the data compilation, analysis, and research recommendations provided in this Review will assist the UV-ARP research community in future efforts toward optimizing UV-ARP systems, modeling the e_aq ^--based chemical transformation kinetics, and developing new UV-ARP systems.

Influence of uranyl complexation on the reaction kinetics of the dodecane radical cation with used nuclear fuel extraction ligands (TBP, DEHBA, and DEHiBA)

Specialized extractant ligands - such as tri-butyl phosphate (TBP), N,N-di-(2-ethylhexyl)butyramide (DEHBA), and N,N-di-2-ethylhexylisobutryamide (DEHiBA) - have been developed for the recovery of uranium from used nuclear fuel by reprocessing solvent extraction technologies. These ligands must function in the presence of an intense multi-component radiation field, and thus it is critical that their radiolytic behaviour be thoroughly evaluated. This is especially true for their metal complexes, where there is negligible information on the influence of complexation on radiolytic reactivity, despite the prevalence of metal complexes in used nuclear fuel reprocessing solvent systems. Here we present a kinetic investigation into the effect of uranyl (UO₂²⁺) complexation on the reaction kinetics of the dodecane radical cation (RH˙⁺) with TBP, DEHBA, and DEHiBA. Complexation had negligible effect on the reaction of RH˙⁺ with TBP, for which a second-order rate coefficient (k) of (1.3 ± 0.1) × 10¹⁰ M^-1 s^-1 was measured. For DEHBA and DEHiBA, UO₂²⁺ complexation afforded an increase in their respective rate coefficients: k(RH˙⁺ + [UO₂(NO₃)₂(DEHBA)₂]) = (2.5 ± 0.1) × 10¹⁰ M^-1 s^-1 and k(RH˙⁺ + [UO₂(NO₃)₂(DEHiBA)₂]) = (1.6 ± 0.1) × 10¹⁰ M^-1 s^-1. This enhancement with complexation is indicative of an alternative RH˙⁺ reaction pathway, which is more readily accessible for [UO₂(NO₃)₂(DEHBA)₂] as it exhibited a much larger kinetic enhancement than [UO₂(NO₃)₂(DEHiBA)₂], 2.6× vs. 1.4×, respectively. Complementary quantum mechanical calculations suggests that the difference in reaction kinetic enhancement between TBP and DEHBA/DEHiBA is attributed to a combination of reaction pathway (electron/hole transfer vs. proton transfer) energetics and electron density distribution, wherein attendant nitrate counter anions effectively 'shield' TBP from RH˙⁺ electron transfer processes.

Curium(iii) radiation-induced reaction kinetics in aqueous media

Insight into the effects of radiolytic processes on the actinides is critical for advancing our understanding of their solution chemistry because the behaviour of these elements cannot be easily separated from the influence of their inherent radiation field. However, minimal information exists on the radiation-induced redox behaviour of curium (Cm), a key trivalent transuranic element present in used nuclear fuel and frequently used as an alpha radiation source. Here we present a kinetic study on the aqueous redox reactions of Cm(iii) with radicals generated through the radiolysis of aqueous media. In particular, we probe reaction kinetics in nitric acid solutions that are used as the aqueous phase component of used nuclear fuel reprocessing solvent systems. Second-order rate coefficients (k) were measured for the reaction of Cm(iii) with the hydrated electron (e_aq^-, k = (1.25 ± 0.03) × 10¹⁰ M^-1 s^-1), hydrogen atom (H˙, k = (5.16 ± 0.37) × 10⁸ M^-1 s^-1), hydroxyl radical (˙OH, k = (1.69 ± 0.24) × 10⁹ M^-1 s^-1), and nitrate radical (NO₃˙, k = (4.83 ± 0.09) × 10⁷ M^-1 s^-1). Furthermore, the first-ever Cm(ii) absorption spectrum (300-700 nm) is also reported. These kinetic data dispel the status quo notion of Cm(iii) possessing little to no redox chemistry in aqueous solution, and suggest that the resulting Cm(ii) and Cm(iv) transients could exist in irradiated aqueous solutions and be available to undergo subsequent redox chemistry with other solutes.

Radiation-induced effects on the extraction properties of hexa-n-octylnitrilo-triacetamide (HONTA) complexes of americium and europium

The candidate An(iii)/Ln(iii) separation ligand hexa-n-octylnitrilo-triacetamide (HONTA) was irradiated under envisioned SELECT (Solvent Extraction from Liquid waste using Extractants of CHON-type for Transmutation) process conditions (n-dodecane/0.1 M HNO3) using a solvent test loop in conjunction with cobalt-60 gamma irradiation. The extent of HONTA radiolysis and complementary degradation product formation was quantified by HPLC-ESI-MS/MS. Further, the impact of HONTA radiolysis on process performance was evaluated by measuring the change in 243Am and 154Eu distribution ratios as a function of absorbed gamma dose. HONTA was found to decay exponentially with increasing dose, affording a dose coefficient of d = (4.48 ± 0.19) × 10-3 kGy-1. Multiple degradation products were detected by HPLC-ESI-MS/MS with dioctylamine being the dominant quantifiable species. Both 243Am and 154Eu distribution ratios exhibited an induction period of ∼70 kGy for extraction (0.1 M HNO3) and back-extraction (4.0 M HNO3) conditions, after which both values decreased with absorbed dose. The decrease in distribution ratios was attributed to a combination of the destruction of HONTA and ingrowth of dioctylamine, which is capable of interfering in metal ion complexation. The loss of HONTA with absorbed gamma dose was predominantly attributed to its reaction with the n-dodecane radical cation (R˙+). These R˙+ reaction kinetics were measured for HONTA and its 241Am and 154Eu complexes using picosecond pulsed electron radiolysis techniques. All three second-order rate coefficients (k) were essentially diffusion limited in n-dodecane indicating a significant reaction pathway: k(HONTA + R˙+) = (7.6 ± 0.8) × 109 M-1 s-1, k(Am(HONTA)2 + R˙+) = (7.1 ± 0.7) × 1010 M-1 s-1, and k(Eu(HONTA)2 + R˙+) = (9.5 ± 0.5) × 1010 M-1 s-1. HONTA-metal ion complexation afforded an order-of-magnitude increase in rate coefficient. Nanosecond time-resolved measurements showed that both direct and indirect HONTA radiolysis yielded the short-lived (<100 ns) HONTA radical cation and a second long-lived (μs) species identified as the HONTA triplet excited state. The latter was confirmed by a series of oxygen quenching picosecond pulsed electron measurements, affording a quenching rate coefficient of k(3[HONTA]* + O2) = 2.2 × 108 M-1 s-1. Overall, both the HONTA radical cation and triplet excited state are important precursors to the suite of measured HONTA degradation products.

Radiolytic degradation of formic acid and formate in aqueous solution: modeling the final stages of organic mineralization under advanced oxidation process conditions

The successful use of advanced oxidation processes to treat aqueous solutions containing undesirable organic species requires the degradation of these species to lower molecular weight, lower hazard compounds. Safe application of this technology requires a thorough understanding of the mechanisms of degradation. These oxidative transformations are mainly initiated by the reactions of reactive oxygen species, particularly hydroxyl radicals. These react with organic molecules to generate carbon-centered radicals. In the presence of dissolved oxygen, the carbon-centered radicals are next converted to peroxyl radicals, which then decay to lower molecular weight species by multiple mechanistic pathways. Formic acid and its conjugate base formate are the last stable chemical species produced immediately before the complete mineralization of any organic molecule undergoing oxidative degradation in aqueous solution. Once understood, the radical-induced chemistry of formic acid/formate under these conditions has wide applicability in all advanced oxidation technologies. To develop this quantitative knowledge, we have performed a series of ⁶⁰Co gamma irradiation studies on aqueous formic acid/formate over different pH and solution conditions. The measured species concentration changes, as a function of applied dose, are compared with the predictions of a kinetic computer model constructed from literature reactions and reported rate coefficients. The excellent agreement found between the results and modeling gives confidence in the mechanism presented here and provide the first complete computer model for the radiolytic degradation of formic acid in water.

Does addition of 1-octanol as a phase modifier provide radical scavenging radioprotection for N,N,N′,N′-tetraoctyldiglycolamide (TODGA)?

The incorporation of 1-octanol as a phase modifier in TODGA solvent system formulations promotes TODGA radiolysis under organic-only conditions, and radioprotection under biphasic nitric acid conditions.

In-situ chemical oxidation of chlorendic acid by persulfate: Elucidation of the roles of adsorption and oxidation on chlorendic acid removal

The oxidation of chlorendic acid (CA), a polychlorinated recalcitrant contaminant, by heat-, mineral-, and base-activated persulfate was investigated. In pH 3-12 homogeneous (i.e., solid-free) solutions, CA was oxidized by ^•OH and SO₄^•- radicals, resulting in a nearly stoichiometric production of Cl^-. The rate constants for the reaction between these radicals and CA were measured at different temperatures by electron pulse radiolysis, and were found to be k_OH = (8.71 ± 0.17) × 10⁷ M^-1s^-1 and k_SO4 = (6.57 ± 0.83) × 10⁷ M^-1s^-1 at 24.5 °C for ^•OH and SO₄^•-, respectively. CA was oxidized at much slower rates in solutions containing iron oxyhydroxide or aquifer soils, partially due to the adsorption of CA on these solids. To gain further insight into the effect of solids during in-situ remediation of CA, the adsorption of CA onto iron (hydr)oxide, manganese dioxide, silica, alumina, and aquifer soils was investigated. The fraction of CA that was adsorbed on these materials increased as the solution pH decreased. Given that the solution pH can decrease dramatically in persulfate-based remedial systems, adsorption may reduce the ability of persulfate to oxidize CA. Overall, the results of this study provide important information about how persulfate can be used to remediate CA-contaminated sites. The results also indicate that the groundwater pH and geology of the subsurface can have a significant influence on the mobility of CA.

Effect of chemical environment on the radiation chemistry of N,N-di-(2-ethylhexyl)butyramide (DEHBA) and plutonium retention

N,N-di-(2-ethylhexyl)butyramide (DEHBA) has been proposed as part of a hydro-reprocessing solvent extraction system for the co-extraction of uranium and plutonium from spent nuclear fuel, owing to its selectivity for hexavalent uranium and tetravalent plutonium. However, there is a critical lack of quantitative understanding regarding the impact of chemical environment on the radiation chemistry of DEHBA, and how this would affect process performance. Here we present a systematic investigation into the radiolytic degradation of DEHBA in a range of n-dodecane solvent system formulations, where we subject DEHBA to gamma irradiation, measure reaction kinetics, ligand integrity, degradation product formation, and investigate solvent system performance through uranium and plutonium extraction and strip distribution ratios. The rate of DEHBA degradation in n-dodecane was found to be slow (G = -0.31 ± 0.02 μmol J^-1) but enhanced upon contact with the oxidizing conditions of the investigated solvent systems (organic-only, or in contact with either 0.1 or 3.0 M aqueous nitric acid). Two major degradation products were identified in the organic phase, bis-2-ethylhexylamine (b2EHA) and N-(2-ethylhexyl)butyramide (MEHBA), resulting from the cleavage of C-N bonds, and could account for the total loss of DEHBA up to ∼300 kGy for organic-only conditions. Both b2EHA and MEHBA were also found to be susceptible to radiolytic degradation, having G-values of -0.12 ± 0.01 and -0.08 ± 0.01 μmol J^-1, respectively. Solvent extraction studies showed: (i) negligible change in uranium extraction and stripping with increasing absorbed dose; and (ii) plutonium extraction and retention exhibits complex dependencies on absorbed dose and chemical environment. Organic-only conditions afforded enhanced plutonium extraction and retention attributed to b2EHA, while acid contacts inhibited this effect and promoted significant plutonium retention for the highest acidity. Overall it has been demonstrated that chemical environment during irradiation has a significant influence on the extent of DEHBA degradation and plutonium retention.

Effect of Ionizing Radiation on the Redox Chemistry of Penta- and Hexavalent Americium

The recent development of facile methods to oxidize trivalent americium to its higher valence states holds promise for the discovery of new chemistries and critical insight into the behavior of the 5f electrons. However, progress in understanding high-valent americium chemistry has been hampered by americium's inherent ionizing radiation field and its concomitant effects on americium redox chemistry. Any attempt to understand high-valent americium reduction and/or disproportionation must account for the effects of these radiolytic processes. Therefore, we present a complete, quantitative, mechanistic description of the radiation-induced redox chemistry of the americyl oxidation states in aerated, aqueous nitric acid, as a function of radiation quality (type and energy) and solution composition using multiscale modeling calculations supported by experiment. The reduction of Am(VI) to Am(V) was found to be most sensitive to the effects of ionizing radiation, undergoing rapid reductions with the steady-state products of aqueous HNO₃ radiolysis, i.e., HNO₂, H₂O₂, and HO₂^•, which dictated its practical lifetime under acidic conditions. In contrast, Am(V) is only susceptible to radiolytic oxidation, mainly through its reactions with NO₃^•, and is notably radiation-resistant with respect to direct one-electron reduction to produce Am(IV). Our multiscale modeling calculations predict that the lifetime of Am(V) is dictated by its rate of disproportionation, 2AmO₂⁺ + 4H_aq⁺ → AmO₂²⁺ + Am⁴⁺ + 2H₂O, with a fourth-order dependence on [H_aq⁺] in agreement with previous experimental findings, giving an optimized rate coefficient of k = 2.27 × 10^-6 M^-5 s^-1. This disproportionation initially produces Am(IV) and Am(VI) species, but the lack of any spectroscopic evidence in our study for Am(IV) suggests that solvent reduction of this cation occurs rapidly. The ultimate product of all the Am(VI)/Am(V) irradiations is Am(III), which shows great stability in an irradiation field.

The Impact of pH and Irradiation Wavelength on the Production of Reactive Oxidants during Chlorine Photolysis

Chlorine photolysis is an advanced oxidation process which relies on photolytic cleavage of free available chlorine (i.e., hypochlorous acid and hypochlorite) to generate hydroxyl radical, along with ozone and a suite of halogen radicals. Little is known about the impact of wavelength on reactive oxidant generation even though chlorine absorbs light within the solar spectrum. This study investigates the formation of reactive oxidants during chlorine photolysis as a function of pH (6-10) and irradiation wavelength (254, 311, and 365 nm) using a combination of reactive oxidant quantification with validated probe compounds and kinetic modeling. Observed chlorine loss rate constants increase with pH during irradiation at high wavelengths due to the higher molar absorptivity of hypochlorite (p K_a = 7.5), while there is no change at 254 nm. Hydroxyl radical and chlorine radical steady-state concentrations are greatest under acidic conditions for all tested wavelengths and are highest using 254 and 311 nm irradiation. Ozone generation is observed under all conditions, with maximum cumulative concentrations at pH 8 for 311 and 365 nm. A comprehensive kinetic model generally predicts the trends in chlorine loss and oxidant concentrations, but a comparison of previously published kinetic models reveals the challenges of modeling this complex system.

The Impact of pH and Irradiation Wavelength on the Production of Reactive Oxidants during Chlorine Photolysis

Chlorine photolysis is an advanced oxidation process which relies on photolytic cleavage of free available chlorine (i.e., hypochlorous acid and hypochlorite) to generate hydroxyl radical, along with ozone and a suite of halogen radicals. Little is known about the impact of wavelength on reactive oxidant generation even though chlorine absorbs light within the solar spectrum. This study investigates the formation of reactive oxidants during chlorine photolysis as a function of pH (6-10) and irradiation wavelength (254, 311, and 365 nm) using a combination of reactive oxidant quantification with validated probe compounds and kinetic modeling. Observed chlorine loss rate constants increase with pH during irradiation at high wavelengths due to the higher molar absorptivity of hypochlorite (p K_a = 7.5), while there is no change at 254 nm. Hydroxyl radical and chlorine radical steady-state concentrations are greatest under acidic conditions for all tested wavelengths and are highest using 254 and 311 nm irradiation. Ozone generation is observed under all conditions, with maximum cumulative concentrations at pH 8 for 311 and 365 nm. A comprehensive kinetic model generally predicts the trends in chlorine loss and oxidant concentrations, but a comparison of previously published kinetic models reveals the challenges of modeling this complex system.

Time-resolved and steady-state irradiation of hydrophilic sulfonated bis-triazinyl-(bi)pyridines - modelling radiolytic degradation

Efficient separation of the actinides from the lanthanides is a critical challenge in the development of a more sophisticated spent nuclear fuel recycling process. Based upon the slight differences in f-orbital distribution, a new class of soft nitrogen-donor ligands, the sulfonated bis-triazinyl-(bi)pyridines, has been identified and shown to be successful for this separation under anticipated, large-scale treatment conditions. The radiation robustness of these ligands is key to their implementation; however, current stability studies have yielded conflicting results. Here we report on the radiolytic degradation of the sulfonated 2,6-bis(1,2,4-triazin-3-yl)pyridine (BTP(S)) and 6,6'-bis(1,2,4-triazin-3-yl)-2,2'-bipyridine (BTBP(S)) in aerated, aqueous solutions using a combination of time-resolved pulsed electron techniques to ascertain their reaction kinetics with key aqueous radiolysis products (eaq-, H˙, ˙OH, and ˙NO3), and steady state gamma radiolysis in conjunction with liquid chromatography for identification and quantification of both ligands as a function of absorbed dose. These data were used to construct a predictive deterministic model to provide critical insight into the fundamental radiolysis mechanisms responsible for the ligands' radiolytic stability. The first-order decays of BTP(S) and BTBP(S) are predominantly driven by oxidative processes (˙OH and, to a lesser extent, H2O2), for which calculations demonstrate that the rate of degradation is inhibited by the formation of ligand degradation products that undergo secondary reactions with the primary products of water radiolysis. Overall, BTP(S) is ∼20% more radiolytically stable than BTBP(S), but over 90% of either ligand is consumed within 1 kGy.

31P NMR study of the activated radioprotection mechanism of octylphenyl-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) and analogues

Complexation of nitric acid by ligands containing conjugated aromatic phosphine oxide functionalities affords activated radioprotection through quenching n-dodecane excited states originating from gamma radiolysis.

Gamma radiolysis of hydrophilic diglycolamide ligands in concentrated aqueous nitrate solution

Advanced analytical techniques and predictive multi-scale modeling calculations show that gamma radiolysis of hydrophilic diglycolamides in concentrated, aqueous nitrate solutions is significantly slower and less structurally sensitive than under pure water conditions.

Elucidating the Elementary Reaction Pathways and Kinetics of Hydroxyl Radical-Induced Acetone Degradation in Aqueous Phase Advanced Oxidation Processes

Advanced oxidation processes (AOPs) that produce highly reactive hydroxyl radicals are promising methods to destroy aqueous organic contaminants. Hydroxyl radicals react rapidly and nonselectively with organic contaminants and degrade them into intermediates and transformation byproducts. Past studies have indicated that peroxyl radical reactions are responsible for the formation of many intermediate radicals and transformation byproducts. However, complex peroxyl radical reactions that produce identical transformation products make it difficult to experimentally study the elementary reaction pathways and kinetics. In this study, we used ab initio quantum mechanical calculations to identify the thermodynamically preferable elementary reaction pathways of hydroxyl radical-induced acetone degradation by calculating the free energies of the reaction and predicting the corresponding reaction rate constants by calculating the free energies of activation. In addition, we solved the ordinary differential equations for each species participating in the elementary reactions to predict the concentration profiles for acetone and its transformation byproducts in an aqueous phase UV/hydrogen peroxide AOP. Our ab initio quantum mechanical calculations found an insignificant contribution of Russell reaction mechanisms of peroxyl radicals, but significant involvement of HO₂^• in the peroxyl radical reactions. The predicted concentration profiles were compared with experiments in the literature, validating our elementary reaction-based kinetic model.

UV Photolysis of Chloramine and Persulfate for 1,4-Dioxane Removal in Reverse-Osmosis Permeate for Potable Water Reuse

A sequential combination of membrane treatment and UV-based advanced oxidation processes (UV/AOP) has become the industry standard for potable water reuse. Chloramines are used as membrane antifouling agents and therefore carried over into the UV/AOP. In addition, persulfate (S₂O₈^2-) is an emerging oxidant that can be added into a UV/AOP, thus creating radicals generated from both chloramines and persulfate for water treatment. This study investigated the simultaneous photolysis of S₂O₈^2- and monochloramine (NH₂Cl) on the removal of 1,4-dioxane (1,4-D) for potable-water reuse. The dual oxidant effects of NH₂Cl and S₂O₈^2- on 1,4-D degradation were examined at various levels of oxidant dosage, chloride, and solution pH. Results showed that a NH₂Cl-to-S₂O₈^2- molar ratio of 0.1 was optimal, beyond which the scavenging by NH₂Cl of HO^•, SO₄^•-, and Cl₂^•- radicals decreased the 1,4-D degradation rate. At the optimal ratio, the degradation rate of 1,4-D increased linearly with the total oxidant dose up to 6 mM. The combined photolysis of NH₂Cl and S₂O₈^2- was sensitive to the solution pH due to a disproportionation of NH₂Cl at pH lower than 6 into less-photoreactive dichloramine (NHCl₂) and radical scavenging by NH₄⁺. The presence of chloride transformed HO^• and SO₄^•- to Cl₂^•- that is less-reactive with 1,4-D, while the presence of dissolved O₂ promoted gaseous nitrogen production. Results from this study suggest that the presence of chloramines can be beneficial to persulfate photolysis in the removal of 1,4-D; however, the treatment efficiency depends on a careful control of an optimal NH₂Cl dosage and a minimal chloride residue.

Kinetic studies of the AOP radical-based oxidative and reductive destruction of pesticides and model compounds in water

Absolute second-order rate constants for hydroxyl radical (HO) reaction with four organophosphorus pesticides, malathion, parathion, fenthion and ethion, and a suite of model compounds of structure (EtO)₂P(S)-X (where X = Cl, F, SH, SEt, OCH₂CF₃, OEt, NH₂, and CH₃) were measured using electron pulse radiolysis and transient absorption techniques. Specific values were determined for these four pesticides as k = (3.89 ± 0.28) x 10⁹, (2.20 ± 0.15) x 10⁹, (2.02 ± 0.15) x 10⁹ and (2.93 ± 0.10) x 10⁹ M^-1 s^-1, respectively, at 20 ± 2 °C. The corresponding Brönsted plot for all these compounds demonstrated that the HO oxidation reaction mechanism for the pesticides was consistent with the model compounds, attributed to initial HO-adduct formation at the P(S) moiety. For malathion, steady-state ⁶⁰Co radiolysis and ³¹P NMR analyses showed that hydroxyl radical-induced oxidation produces the far more potent isomalathion, but only with an efficiency of 4.9 ± 0.3%. Analogous kinetic measurements for the hydrated electron induced reduction of these pesticides gave specific rate constants of k = (3.38 ± 0.14) x 10⁹, (1.38 ± 0.10) x 10⁹, (1.19 ± 0.12) x 10⁹ and (1.20 ± 0.06) x 10⁹ M^-1 s^-1, respectively, for malathion, parathion, fenthion and ethion. Model compound measurements again supported a single reduction reaction mechanism, proposed to be electron addition at the PS bond to form the radical anion. These results demonstrate, for the first time, that the radical-based treatment of organophosphorus contaminated waters may present a potential toxicological risk if advanced oxidative processes are used.

Temperature dependence of hydroxyl radical reactions with chloramine species in aqueous solution

The absolute temperature-dependent kinetics for the reaction between hydroxyl radicals and the chloramine water disinfectant species monochloramine (NH₂Cl), as well as dichloramine (NHCl₂) and trichloramine (NCl₃), have been determined using electron pulse radiolysis and transient absorption spectroscopy. These radical reaction rate constants were fast, with values of 6.06 × 10⁸, 2.57 × 10⁸, and 1.67 × 10⁸ M^-1 s^-1 at 25 °C for NH₂Cl, NHCl₂, and NCl₃, respectively. The corresponding temperature dependence of these reaction rate constants, measured over the range 10-40 °C, is well-described by the transformed Arrhenius equations:giving activation energies of 8.57 ± 0.58, 6.11 ± 0.40, and 5.77 ± 0.72 kJ mol^-1 for these three chloramines, respectively. These data will aid water utilities in predicting hydroxyl radical partitioning and chemical contaminant removal efficiencies under real-world advanced oxidation process treatment conditions.

Indirect photodegradation of the lampricides TFM and niclosamide

3-Trifluromethyl-4-nitrophenol (TFM) and 2',5-dichloro-4'-nitrosalicylanilide (niclosamide) are lampricides used in tributaries of the Great Lakes to kill the invasive parasitic sea lamprey (Petromyzon marinus). Although the lampricides have been applied since the late 1950s, their photochemical behavior in natural environments is still not well understood. This study examines the indirect photodegradation of these two compounds and the resulting yields of organic and inorganic photoproducts in water samples collected from five tributaries of Lake Michigan. The tributaries were selected to span the length of Lake Michigan and its natural carbonate geologic gradient. In the presence of dissolved organic matter (DOM), the niclosamide photodegradation rate triples, while the rate of TFM photodegradation is unchanged. Additionally, the yield of lampricide organic products is influenced by DOM because many of the organic photoproducts themselves are prone to DOM-mediated indirect photodegradation. The indirect photodegradation of niclosamide is primarily mediated by reaction with singlet oxygen, which accounts for more than 50% of the increased photodegradation rate. Additionally, hydroxyl radicals and carbonate radicals (CO₃^-˙) influence niclosamide indirect photolysis, and their contribution is dependent on the specific river water chemistry. For example, CO₃^-˙ contribution to niclosamide photodegradation, while small, is greater in southern tributaries where there is higher carbonate alkalinity.

Decomposition of Iodinated Pharmaceuticals by UV-254 nm-assisted Advanced Oxidation Processes

Iodinated pharmaceuticals, thyroxine (a thyroid hormone) and diatrizoate (an iodinated X-ray contrast medium), are among the most prescribed active pharmaceutical ingredients. Both of them have been reported to potentially disrupt thyroid homeostasis even at very low concentrations. In this study, UV-254 nm-based photolysis and photochemical processes, i.e., UV only, UV/H₂O₂, and UV/S₂O₈^2-, were evaluated for the destruction of these two pharmaceuticals. Approximately 40% of 0.5μM thyroxine or diatrizoate was degraded through direct photolysis at UV fluence of 160mJcm^-2, probably resulting from the photosensitive cleavage of C-I bonds. While the addition of H₂O₂ only accelerated the degradation efficiency to a low degree, the destruction rates of both chemicals were significantly enhanced in the UV/S₂O₈^2- system, suggesting the potential vulnerability of the iodinated chemicals toward UV/S₂O₈^2- treatment. Such efficient destruction also occurred in the presence of radical scavengers when biologically treated wastewater samples were used as reaction matrices. The effects of initial oxidant concentrations, solution pH, as well as the presence of natural organic matter (humic acid or fulvic acid) and alkalinity were also investigated in this study. These results provide insights for the removal of iodinated pharmaceuticals in water and/or wastewater using UV-based photochemical processes.

Oxidative remediation of 4-methylcyclohexanemethanol (MCHM) and propylene glycol phenyl ether (PPh). Evidence of contaminant repair reaction pathways

Rate constants and reaction efficiencies were determined for HO˙-mediated reactions for 4-methylcyclohexanemethanol and propylene glycol phenyl ether to simulate contaminant remediation in surface waters.

Investigation of the Coupled Effects of Molecular Weight and Charge-Transfer Interactions on the Optical and Photochemical Properties of Dissolved Organic Matter

We studied the formation of photochemically produced reactive intermediates (RI) from dissolved organic matter (DOM). Specifically, we focused on the effects of variable molecular weight and chemical reduction on the optical properties of DOM (absorbance and fluorescence) and the formation of singlet oxygen ((1)O2), DOM triplet excited states ((3)DOM*), and the hydroxyl radical ((•)OH). The data are largely evaluated in terms of a charge-transfer (CT) model, but deficiencies in the model to explain the data are pointed out when evident. A total of two sets of samples were studied that were subjected to different treatments; the first set included secondary-treated wastewaters and a wastewater-impacted stream, and the second was a DOM isolate. Treatments included size fractionation and chemical reduction using sodium borohydride. Taken as a whole, the results demonstrate that decreasing molecular weight and borohydride reduction work in opposition regarding quantum efficiencies for (1)O2 and (3)DOM* production but in concert for fluorescence and (•)OH production. The optical and photochemical data provide evidence for a limited role of CT interactions occurring in lower-molecular-weight DOM molecules. In addition, the data suggest that the observed optical and photochemical properties of DOM are a result of multiple populations of chromophores and that their relative contribution is changed by molecular-weight fractionation and borohydride reduction.

Isotope Dependence and Quantum Effects on Atomic Hydrogen Diffusion in Liquid Water

Relative diffusion coefficients were determined in water for the D, H, and Mu isotopes of atomic hydrogen by measuring their diffusion-limited spin-exchange rate constants with Ni(2+) as a function of temperature. H and D atoms were generated by pulse radiolysis of water and measured by time-resolved pulsed EPR. Mu atoms are detected by muonium spin resonance. To isolate the atomic mass effect from solvent isotope effect, we measured all three spin-exchange rates in 90% D2O. The diffusion depends on the atomic mass, demonstrating breakdown of Stokes-Einstein behavior. The diffusion can be understood using a combination of water "cavity diffusion" and "hopping" mechanisms, as has been proposed in the literature. The H/D isotope effect agrees with previous modeling using ring polymer molecular dynamics. The "quantum swelling" effect on muonium due to its larger de Broglie wavelength does not seem to slow its "hopping" diffusion as much as predicted in previous work. Quantum effects of both the atom mass and the water librations have been modeled using RPMD and a qTIP4P/f quantized flexible water model. These results suggest that the muonium diffusion is very sensitive to the Mu versus water potential used.

Experimental and theoretical studies on aqueous-phase reactivity of hydroxyl radicals with multiple carboxylated and hydroxylated benzene compounds

In this study, we shed light on the initial addition of hydroxyl radicals (HO˙) to multiple carboxylated and hydroxylated benzene compounds in aqueous-phase advanced oxidation processes (AOPs).

Development of linear free energy relationships for aqueous phase radical-involved chemical reactions

Aqueous phase advanced oxidation processes (AOPs) produce hydroxyl radicals (HO•) which can completely oxidize electron rich organic compounds. The proper design and operation of AOPs require that we predict the formation and fate of the byproducts and their associated toxicity. Accordingly, there is a need to develop a first-principles kinetic model that can predict the dominant reaction pathways that potentially produce toxic byproducts. We have published some of our efforts on predicting the elementary reaction pathways and the HO• rate constants. Here we develop linear free energy relationships (LFERs) that predict the rate constants for aqueous phase radical reactions. The LFERs relate experimentally obtained kinetic rate constants to quantum mechanically calculated aqueous phase free energies of activation. The LFERs have been applied to 101 reactions, including (1) HO• addition to 15 aromatic compounds; (2) addition of molecular oxygen to 65 carbon-centered aliphatic and cyclohexadienyl radicals; (3) disproportionation of 10 peroxyl radicals, and (4) unimolecular decay of nine peroxyl radicals. The LFERs correlations predict the rate constants within a factor of 2 from the experimental values for HO• reactions and molecular oxygen addition, and a factor of 5 for peroxyl radical reactions. The LFERs and the elementary reaction pathways will enable us to predict the formation and initial fate of the byproducts in AOPs. Furthermore, our methodology can be applied to other environmental processes in which aqueous phase radical-involved reactions occur.