Insights into manganese(VII) enhanced oxidation of benzophenone-8 by ferrate(VI): Mechanism and transformation products

Benzophenones (BPs) are commonly used as UV filters in cosmetics and plastics products and are potentially toxic to the environment. This paper presents kinetics and products of BPs oxidation by ferrate(VI) (FeO₄^2-, Fe(VI)) promoted by permanganate (Mn(VII)) . Degradation of 10.0 µM 2,2'-dihydroxy-4-methoxybenzophenone (BP-8)were determined under different experimental conditions ([Mn(VII)] = 0.5-1.5 µM, [Fe(VI)] = 50-150 µM, and pH = 7.0-10.0). The addition of Mn(VII) traces to Fe(VI)-BP-8 solution enhanced kinetics and efficiency of the removal. Similar enhanced removals were also seen for other BPs (BP-1, BP-3, and BP-4) under optimized conditions. The second-order rate constants (k, M^-1s^-1) of the degradation of BPs showed positive relationship with the energy of the highest occupied orbital (E_HOMO). The possible interaction between Mn(VII) and BP-8 and the enhanced generation of Fe(V)/Fe(IV) and •OH was proposed to facilitate the oxidation of the target benzophenone, supported by in-situ electrochemical measurements, theoretical calculations and reactive species quenching experiments. Thirteen oxidation products of BP-8 suggested hydroxylation, bond breaking, polymerization and carboxylation steps in the oxidation. Toxicity assessments by ECOSAR program showed that the oxidized intermediate products posed a tapering ecological risk during the degradation process. Overall, the addition of Mn(VII) could improve the oxidation efficiency of Fe(VI).

High efficient removal of 4-aminophenylarsonic acid from aqueous solution via enhanced FeOOH using Mn(VII)

Efficient removal of 4-aminophenylarsonic acid from contaminated water sources is essential to mitigate arsenic pollution. We proposed a competent technique to achieve 4-aminophenylarsonic acid removal via adsorption on enhanced α-FeOOH using various concentrations of Mn(VII). The elimination rate of 4-aminophenylarsonic acid applying FeOOH with Mn(VII) was dependent on acidic conditions. More than 99.9% of 4-aminophenylarsonic acid was eliminated in a 6-min reaction time under acidic conditions. The reaction of 4-aminophenylarsonic acid was fast at 4.0 and 5.0 pH, with its complete oxidation into arsenate and the liberation of manganese Mn(II) in the initial stage of the reaction. Similarly, the reaction rate constant (k_obs) decreased from 0.7048 ± 0.02 to 0.00155 ± 0.00007 as the pH increased from 4.0 to 9.0. Oxidation capacity was considerably enhanced via the removal of electrons from 4-aminophenylarsonic acid to Mn(VII) after the creation of its radical intermediate and further change in Mn(III) to Mn(II) in the solution. The results showed that Mn(VII) played a crucial role in 4-aminophenylarsonic acid degradation at a low pH (e.g., 4.0), and the oxidation process proceeded in different manners, namely, electron transfer, hydroxylation, and ring-opening. These results illustrated that Mn(VII) is an effective, economic purification process to mitigate 4-aminophenylarsonic acid generated from poultry waste.

The role of source emissions in sulfate formation pathways based on chemical thermodynamics and kinetics model

Sulfate is a major PM_2.5 constituent and poses a significant threat to ecosystems and human health, which has attracted lots of attention to the sulfate formation mechanism. In recent years, there has been great scientific interest in the multiphase oxidation of SO₂ in aqueous aerosol particles. Many factors are involved in the reaction process, including precursor SO₂, oxidants/catalysts, and aerosol acidity, which are three channels closely related to the source emission. The conjoint analysis of source emissions and sulfate aqueous formation can provide a scientific basis for designing effective strategies, though the related research is extremely limited. Here, we applied an improved solute strength-dependent chemical Thermodynamics & Kinetics model (for aqueous pathway contribution) and the Partial Target Transformation-Positive matrix factor model (for source apportionment) to explore the role of source emission in sulfate aqueous formation. The results indicated H₂O₂ aqueous oxidation was the dominant pathway (65.9 %), and secondary nitrate source may grow together with sulfate formation from H₂O₂ pathway. H₂O₂ and TMI pathways were related to higher SOR (sulfur oxidation rate). TMI pathway was significant in summer (54.6 %) and increased with secondary sources and vehicle exhaust. NO₂ pathway was more significant at low secondary source and high coal combustion (higher contribution of NO₂ pathway appeared in winter, 24.7 %). While high formation rate of the O₃ pathway always occurred at low source levels. Coal combustion and vehicle exhaust showed obvious effects on sulfate aqueous formation. Notably, aerosol acidity is a significant factor related to sources and plays a key role in sulfate formation. The result also suggested aerosol pH may be more important than the amounts of substances involved in the oxidation reaction. The findings in this work can provide useful information for better understanding sulfate aqueous formation and offer a scientific basis for designing strategies for air pollution control and sulfate mitigation.

Isotopic constraints on the formation pathways and sources of atmospheric nitrate in the Mt. Everest region

Inorganic particulate nitrate (p-NO₃^-), gaseous nitric acid (HNO_3(g)) and nitrogen oxides (NO_x = NO + NO₂), as main atmospheric pollutants, have detrimental effects on human health and aquatic/terrestrial ecosystems. Referred to as the 'Third Pole' and the 'Water Tower of Asia', the Tibetan Plateau (TP) has attracted wide attention on its environmental changes. Here, we evaluated the oxidation processes of atmospheric nitrate as well as traced its potential sources by analyzing the isotopic compositions of nitrate (δ¹⁵N, δ¹⁸O, and Δ¹⁷O) in the aerosols collected from the Mt. Everest region during April to September 2018. Over the entire sampling campaigns, the average of δ¹⁵N(NO₃^-), δ¹⁸O(NO₃^-), and Δ¹⁷O(NO₃^-) was -5.1 ± 2.3‰, 66.7 ± 10.2‰, and 24.1 ± 3.9‰, respectively. The seasonal variation in Δ¹⁷O(NO₃^-) indicates the relative importance of O₃ and HO₂/RO₂/OH in NO_x oxidation processes among different seasons. A significant correlation between NO₃^- and Ca²⁺ and frequent dust storms in the Mt. Everest region indicate that initially, the atmospheric nitrate in this region might have undergone a process of settling; subsequently, it got re-suspended in the dust. Compared with the Δ¹⁷O(NO₃^-) values in the northern TP, our observed significantly higher values suggest that spatial variations in atmospheric Δ¹⁷O(NO₃^-) exist within the TP, and this might result from the spatial variations of the atmospheric O₃ levels, especially the stratospheric O₃, over the TP. The observed δ¹⁵N(NO₃^-) values predicted remarkably low δ¹⁵N values in the NO_x of the sources and the N isotopic fractionation plays a crucial role in the seasonal changes of δ¹⁵N(NO₃^-). Combined with the results from the backward trajectory analysis of air mass, we suggest that the vehicle exhausts and agricultural activities in South Asia play a dominant role in determining the nitrate levels in the Mt. Everest region.

Stable oxygen isotope constraints on nitrate formation in Beijing in springtime

Rapid accumulation of aerosol nitrate (NO₃^-) contributes to haze pollution; however, studies quantifying NO₃^- formation mechanisms remain scarce. To explore aerosol nitrate formation pathways, total suspended particulate (TSP) samples were collected in Beijing during the spring of 2013, and the concentration of NO₃^- and δ¹⁸O- NO₃^- value were analyzed. The NO₃^- concentrations on polluted days (PD) were higher than those on non-polluted days (NPD). Furthermore, higher δ¹⁸O- NO₃^- values were observed on PD (86.8 ± 8.1‰) as compared with NPD (73.7 ± 11.0‰) suggest that more nitrate was produced by pathways with relative high δ¹⁸O-HNO₃ values during PD. Based on the calculated δ¹⁸O-HNO₃ values from different formation pathways and the observed δ¹⁸O- NO₃^- values, the possible fractional contributions of HNO₃ formed via various pathways to TSP NO₃^- were estimated using the Bayesian isotope mixing model. The δ¹⁸O- NO₃^- constrained calculations suggest that the pathways of N₂O₅ + H₂O/Cl^-, NO₃ + VOCs, and ClNO₃ + H₂O possibly contributed 53%-89% to nitrate production during PD. During NPD, the NO₂ + OH pathway produced 37%-69% of the NO₃^-. Using the δ¹⁸O- NO₃^- value combined with the isotope mixing model is a promising approach for exploring NO₃^- formation pathways.

Oxidation of aniline aerofloat in flotation wastewater by sodium hypochlorite solution

Aniline aerofloat (dianilinodithiophosphoric acid (C6H5NH)2PSSH) is a widely used phosphorodithioic organic flotation collector that contains aniline groups and dithiophosphate groups. In the present study, sodium hypochlorite solution was used to oxidize aniline aerofloat. The effect of operational parameters and optimum oxidation conditions on aniline aerofloat was studied, and the oxidation pathway of aniline aerofloat was proposed by analyzing its main oxidation intermediates. The results showed that NaOCl concentration had a significant influence on aniline aerofloat oxidation and at 100 mg/L aniline aerofloat, 84.54% was removed under the following optimal conditions: NaOCl concentration = 1.25 g/L, pH = 4, and reaction time = 60 min. The main reaction of aniline aerofloat by NaOCl included N-P bond cleavage, aniline group oxidation, aniline group chlorination, and dithiophosphate group oxidation. The initial reaction was the N-P bond cleavage and the anilines and dithiophosphate was further oxidized to other intermediates by five parallel reaction pathways.