Temperature Dependent Absorption Spectra of Br−, Br2•−, and Br3− in Aqueous Solutions

Treating disinfection byproducts with UV or solar irradiation and in UV advanced oxidation processes: A review

This review focuses on the degradation kinetics and mechanisms of disinfection byproducts (DBPs) under UV and solar irradiation and in UV-based advanced oxidation processes (AOPs). A total of 59 such compounds are discussed. The processes evaluated are low pressure, medium pressure and vacuum UV irradiation, solar irradiation together with UV/hydrogen peroxide, UV/persulfate and UV/chlorine AOPs. Under UV and solar irradiation, the photodegradation rates of N-nitrosamines are much higher than those of halogenated DBPs. Among halogenated DBPs, those containing iodine are photodegraded more rapidly than those containing bromine or chlorine. This is due to differences in their bond energies (E_N-N < E_C-I < E_C-Br < E_C-Cl). Molar absorption coefficients at 254 nm and energy gaps can be used to predict the photodegradation rates of DBPs under low pressure UV irradiation. But many DBPs of interest cannot be degraded to half their original concentration with less than a 500 mJ cm^-2 dose of low pressure UV light. HO^• generally contributes to less than 30% of the degradation of DBPs except iodo-DBPs in UV/H₂O₂ AOPs. Reaction mechanisms under UV irradiation and in HO^•-mediated oxidation are also summarized. N-N bond cleavage initiates their direct UV photolysis of N-nitrosamines as C-X cleavage does among halogenated compounds. HO^• generally initiates degradation via single electron transfer, addition and hydrogen abstraction pathways. Information on the reaction rate constants of SO₄^•- and halogen radicals with DBPs is rather limited, and little information is available about their reaction pathways. Overall, this review provides improved understanding of UV, solar and AOPs.

Halide Photoredox Chemistry

Halide photoredox chemistry is of both practical and fundamental interest. Practical applications have largely focused on solar energy conversion with hydrogen gas, through HX splitting, and electrical power generation, in regenerative photoelectrochemical and photovoltaic cells. On a more fundamental level, halide photoredox chemistry provides a unique means to generate and characterize one electron transfer chemistry that is intimately coupled with X-X bond-breaking and -forming reactivity. This review aims to deliver a background on the solution chemistry of I, Br, and Cl that enables readers to understand and utilize the most recent advances in halide photoredox chemistry research. These include reactions initiated through outer-sphere, halide-to-metal, and metal-to-ligand charge-transfer excited states. Kosower's salt, 1-methylpyridinium iodide, provides an early outer-sphere charge-transfer excited state that reports on solvent polarity. A plethora of new inner-sphere complexes based on transition and main group metal halide complexes that show promise for HX splitting are described. Long-lived charge-transfer excited states that undergo redox reactions with one or more halogen species are detailed. The review concludes with some key goals for future research that promise to direct the field of halide photoredox chemistry to even greater heights.

Visible Light Driven Bromide Oxidation and Ligand Substitution Photochemistry of a Ru Diimine Complex

The complex [Ru(deeb)(bpz)₂]²⁺ (RuBPZ²⁺, deeb = 4,4'-diethylester-2,2'-bipyridine, bpz = 2,2'-bipyrazine) forms a single ion pair with bromide, [RuBPZ²⁺, Br^-]⁺, with K_eq = 8400 ± 200 M^-1 in acetone. The RuBPZ²⁺ displayed photoluminescence (PL) at room temperature with a lifetime of 1.75 μs. The addition of bromide to a RuBPZ²⁺ acetone solution led to significant PL quenching and Stern-Volmer plots showed upward curvature. Time-resolved PL measurements identified two excited state quenching pathways, static and dynamic, which were operative toward [RuBPZ²⁺, Br^-]⁺ and free RuBPZ²⁺, respectively. The single ion-pair [RuBPZ²⁺, Br^-]⁺* had a lifetime of 45 ± 5 ns, consistent with an electron transfer rate constant, k_et = (2.2 ± 0.3) × 10⁷ s^-1. In contrast, RuBPZ²⁺* was dynamically quenched by bromide with a quenching rate constant, k_q = (8.1 ± 0.1) × 10¹⁰ M^-1 s^-1. Nanosecond transient absorption revealed that both the static and dynamic pathways yielded RuBPZ⁺ and Br₂^•- products that underwent recombination to regenerate the ground state with a second-order rate constant, k_cr = (2.3 ± 0.5) × 10¹⁰ M^-1 s^-1. Kinetic analysis revealed that RuBPZ⁺ was a primary photoproduct, while Br₂^•- was secondary product formed by the reaction of a Br^• with Br^-, k = (1.1 ± 0.2) × 10¹⁰ M^-1 s^-1. Marcus theory afforded an estimate of the formal reduction potential for E⁰(Br^•/-) in acetone, 1.42 V vs NHE. A ¹H NMR analysis indicated that the ion-paired bromide was preferentially situated close to the Ru^II center. Prolonged steady state photolysis of RuBPZ²⁺ and bromide yielded two ligand-substituted photoproducts, cis- and trans-Ru(deeb)(bpz)Br₂. A photochemical intermediate, proposed to be [Ru(deeb)(bpz)(κ¹-bpz)(Br)]⁺, was found to absorb a second photon to yield cis- and trans-Ru(deeb)(bpz)Br₂ photoproducts.

Rate constant for the H˙ + H2O → ˙OH + H2 reaction at elevated temperatures measured by pulse radiolysis

Maintaining the structural integrity of materials in nuclear power plants is an essential issue associated with safe operation. Hydrogen (H₂) addition or injection to coolants is a powerful technique that has been widely applied such that the reducing conditions in the coolant water avoid corrosion and stress corrosion cracking (SCC). Because the radiation-induced reaction of ˙OH + H₂ → H˙ + H₂O plays a crucial role in these systems, the rate constant has been measured at operation temperatures of the reactors (285-300 °C) by pulse radiolysis, generating sufficient data for analysis. The reverse reaction H˙ + H₂O → ˙OH + H₂ is negligibly slow at ambient temperature; however, it accelerates considerably quickly at elevated temperatures. Although the reverse reaction reduces the effectiveness of H₂ addition, reliable rate constants have not yet been measured. In this study, the rate constants have been determined in a temperature range of 250-350 °C by pulse radiolysis in an aqueous I^- solution.

Role of bromide in hydrogen peroxide oxidation of CTAB-stabilized gold nanorods in aqueous solutions

In recent years hydrogen peroxide has often been used as the oxidizing agent to tune the resonance wavelength of gold nanorods (AuNRs) through anisotropic shortening in the presence of cetyltrimethylammonium bromide (CTAB). However, a complete picture of the reaction mechanism remains elusive. In this work, we present a systematic study on the mechanism of the AuNR oxidation by revealing the important role of bromide. Hydrogen peroxide slowly oxidizes bromide into elemental bromine. The latter two form tribromide, which exhibits a characteristic 272 nm absorption peak. The peak intensity, representing the concentration of tribromide, is found to have a linear correlation with the oxidation rate of AuNRs. Tribromide approaches AuNRs through conjugating strongly with CTA cationic micelles, which leads to the oxidation occurring on the surface of AuNRs. In contrast, the CTA micelles protect AuNRs from the direct oxidation by hydrogen peroxide. Our findings are believed to provide new insights into the reaction mechanism occurring in the relevant CTAB-AuNR systems, which can be important for understanding the principles governing the reaction dynamics.

Sequential radiation chemical reactions in aqueous bromide solutions: pulse radiolysis experiment and spur model simulation

Pulse radiolysis experiments were carried out to observe transient absorptions of reaction intermediates produced in N₂O- and Ar-saturated aqueous solutions containing 0.9–900 mM NaBr.