Frozen Hydrogen Peroxide and Nitrite Solution: The Acceleration of Benzoic Acid Oxidation via the Decreased pH in Ice

Abstract

We investigated benzoic acid oxidation via the reaction of hydrogen peroxide (H2O2) and nitrite (NO2 –). The oxidation of benzoic acid by reactive nitrous acid (HONO) was negligible, and the reactivity of the H2O2/NO2 – system decreased with a decrease in temperature under aqueous conditions. However, freezing markedly accelerated the chemical reaction. Based on Raman microscope measurements, concentrated species were confirmed in certain regions of the ice. We proposed that the change in nitrite speciation (accordingly, a decrease in the pH below pK a), derived from the freezing concentration effect, was the reason for the accelerated reactions. The oxidation characteristics of the system were monitored under varying conditions, such as initial pH, dosage ratio, benzoic acid concentration, and reaction with various benzene derivatives. The ultrahigh-performance liquid chromatography/electrospray ionization/mass spectrometry (UHPLC/ESI/MS) measurement showed that peroxynitrous acid (HOONO)-mediated oxidation generated hydroxylated and nitrated byproducts. Additionally, decarboxylated products were detected, indicating direct electron transfer from the organic compounds to HOONO. As freezing is a global phenomenon, and H2O2 and NO2 – are ubiquitous in the environment, the transformation of aromatic compounds with H2O2/NO2 – in cold environments must be considered in environmental chemistry.

Uptake of Hydrogen Peroxide from the Gas Phase to Grain Boundaries: A Source in Snow and Ice

“…This work links the direct observation of grain boundary content in ice samples with the diffusive loss. We want to stress the environmental relevance of this direct experimental evidence for H 2 O 2 uptake from the gas phase to the grain boundaries, where H 2 O 2 is a potent oxidant . It is important to note that the relevance of diffusive uptake varies not only with grain boundary content but also from species to species.…”

“…The preference of H 2 O 2 for grain boundaries in environmental snow and ice might result in significant variations in the local concentration of H 2 O 2 . This is relevant as such non-uniform chemical morphologies and highly concentrated patches of reactants at grain boundaries and elsewhere in the ice have recently been shown to impact chemical reactivity in frozen systems strongly. ,, Grain boundaries in natural snowpacks occur at the contact face of individual snow grains . As grain size, shape, and arrangement change with time in snowpacks (metamorphism), detailed studies observing the impact of grain boundaries at different stages of metamorphism are timely to elucidate the mechanism and the environmental impact more extensively.…”

“…Hydrogen peroxide (H 2 O 2 ) is a central inorganic oxidant in the hydrosphere and the atmosphere . In snow, it acts as a chromophore and a precursor for HO x . − Temperature-dependent uptake and emission of H 2 O 2 from snow alter gas-phase concentrations on daily timescales. , Recent interest in the multiphase phase chemistry of H 2 O 2 was sparked by a report on the autonomous formation of H 2 O 2 at the interface of micron-sized droplets. , In ice and snow, grain boundaries and their junctions provide micron-sized confined environments hosting impurities and multiphase reactions. − In particular, oxidation reactions such as that of H 2 O 2 with benzoic acid are highly accelerated in these locations in ice . We, therefore, investigate whether the uptake of H 2 O 2 from the gas phase is a source of H 2 O 2 in grain boundaries of snow.…”

Freezing-enhanced chlorination of organic pollutants for water treatment

“…34,35 Among these mechanisms, the freeze concentration effect has been widely recognized as the predominant driver for the observed enhancement in reactivity under freezing conditions. 16,18,21,[25][26][27][28][29]32,33 Several studies have conclusively demonstrated the signicant impact of this effect on reaction rates during the freezing process. Notably, freezing induces the transportation of solute molecules away from the slowly-moving ice, resulting in their accumulation and concentration in the unfrozen portion of the solution, known as the liquid brine.…”

“…Additionally, agents like peroxymonosulfate, 23,24 IO 4 − , 25 chloride–oxone system, 26 H 2 O 2 /NO 2 − (ref. 27 ) and nitrite 28 have exhibited significantly improved efficiency in oxidizing organic pollutants under freezing conditions compared to in aqueous system. These accelerated reactions are not solely confined to homogeneous reactions in aqueous solutions; freezing has also proven to enhance the efficiency of heterogeneous reactions involving soils 29 and MnO 2 oxides 30 and photochemical reactions under irradiation.…”