Mechanical insights into desulfurization by peroxymonosulfate oxidation via a non-reactive oxygen species pathway

Air pollution by sulfur dioxide (SO2) remains a pressing concern for both the environment and human health. Desulfurization enhanced by persulfate based advanced oxidation processes (PS-AOPs) has been proven to be a feasible method. However, the inherent contradiction between the rapid diffusion mass transfer of SO2 in the "gas-liquid-gas" phase and the limited lifespan of reactive oxygen species (ROS) can not be ignored. Excessive investment in PS is required to sustainably generate ROS to achieve continuous desulfurization performance, which may lead to excessive PS consumption. To address this issue, whether PS can achieve the oxidation absorption of SO2 via a non-reactive oxygen species pathway was investigated. Experimental and computational results demonstrated that peroxymonosulfate (PMS) instead of peroxydisulfate (PDS) had a great SO2 removal performance, the utilization of PS could be effectively achieved by maintaining a 1:1 molar ratio of PMS and removed SO2. The presence of HOO bonds in the PMS introduced a partial positive charge to the oxygen atom, making the PMS polar and more susceptible to be attacked by the nucleophile HSO3-. So SO2 underwent a series of processes including dissolution, dissociation, one-oxygen atom transfer, and ionization before ultimately being converted into SO42- ions, effectively achieving its removal from flue gas. This study may presents a novel approach for achieving high-efficiency flue gas desulfurization.

Enhancement of norfloxacin degradation by citrate in S-nZVI@Ps system: Chelation and FeS layer

The degradation of organic pollution by sulfur-modified nano zero-valent iron(S-nZVI) combined with advanced oxidation systems has been extensively studied. However, the low utilization of nZVI and low reactive oxygen species (ROS) yield in the system have limited its wide application. Herein, a natural organic acid commonly found in citrus fruits, citric acid (CA), was combined with the conventional S-nZVI@Ps system to enhance the degradation of norfloxacin (NOR). The addition of CA increased the NOR removal by about 31% compared with the conventional S-nZVI@Ps system under the same experimental conditions. Among them, the enhanced effect of CA is mainly reflected in its ability to promote the release of Fe2+ and accelerate the cycling of Fe2+ and Fe3+ to further improve the utilization of nZVI and the generation of ROS; it also promotes the dissolution of the active substance (FeS) on the surface of S-nZVI to further improve the degradation rate of NOR. More importantly, the chelate of CA and Fe2+ (CA-Fe2+) had higher reactivity than alone Fe2+. Free radical quenching and electron spin resonance (ESR) experiments indicated that the main ROS for the degradation of NOR in the CA/S-nZVI@Ps system were SO4•- and OH•. CA-bound sulfur-modifying effects on NOR degradation was systematically investigated, and the degradation mechanism of NOR in CA/S-nZVI@Ps system was explored by various techniques. Additionally, the effect of common anions in water matrix on the degradation of NOR in CA/S-nZVI@Ps system and its degradation of various pollutants were also studied. This study provides a new perspective to enhance the degradation of pollutants by S-nZVI combined with advanced oxidation system, which can help to solve the application boundary problem of S-nZVI.

Radical-induced oxidation removal of multi-air-pollutant: A critical review

Sulfur dioxide (SO₂), nitric oxide (NO) and elemental mercury (Hg⁰) are three common air pollutants in flue gas. SO₂ and NO are the main precursors for chemical smog and Hg⁰ is a bio-toxicant for human. Cooperative removal of multi-air-pollutant in flue gas using radical-induced oxidation reaction is considered as one of the most promising methods due to the high removal efficiency, low cost and less secondary environmental impact. The common radicals used in air pollution control can be classified into four types: (1) hydroxyl radical (OH), (2) sulfate radical (SO₄^-), (3) chlorine-containing radicals (Cl, ClO₂, ClO, HOCl^-, etc.) and (4) ozone. This review summarizes the generation methods and mechanism of the four kinds of radicals, as well as their applications in the removal of multi-air-pollutant in flue gas. The reactivity, selectivity and reaction mechanism of the four kinds of radicals in multi-air-pollutant removal were comprehensively described. Finally, some future research suggestions on the development of new technique for cooperative removal of multi-air-pollutant in flue gas were provided.

Simultaneous NO and SO2 removal by aqueous persulfate activated by combined heat and Fe2+: experimental and kinetic mass transfer model studies

This study evaluates the chemistry, kinetics, and mass transfer aspects of the removal of NO and SO₂ simultaneously from flue gas induced by the combined heat and Fe²⁺ activation of aqueous persulfate. The work involves experimental studies and the development of a mathematical model utilizing a comprehensive reaction scheme for detailed process evaluation, and to validate the results of an experimental study at 30-70 °C, which demonstrated that both SO₂ and Fe²⁺ improved NO removal, while the SO₂ is almost completely removed. The model was used to correlate experimental data, predict reaction species and nitrogen-sulfur (N-S) product concentrations, to obtain new kinetic data, and to estimate mass transfer coefficient (K_La) for NO and SO₂ at different temperatures. The model percent conversion results appear to fit the data remarkably well for both NO and SO₂ in the temperature range of 30-70 °C. The conversions ranged from 43.2 to 76.5% and 98.9 to 98.1% for NO and SO₂, respectively, in the 30-70 °C range. The model predictions at the higher temperature of 90 °C were 90.0 and 97.4% for NO and SO₂, respectively. The model also predicted decrease in K_La for SO₂ of 1.097 × 10^-4 to 8.88 × 10^-5 s^-1 (30-90 °C) and decrease in K_La for NO of 4.79 × 10^-2 to 3.67 × 10^-2 s^-1 (30-50 °C) but increase of 4.36 × 10^-2 to 4.90 × 10^-2 s^-1 at higher temperatures (70-90 °C). This emerging sulfate-radical-based process could be applied to the treatment of flue gases from combustion sources. Graphical abstract.

Removal of multi-pollutant from flue gas utilizing ammonium persulfate solution catalyzed by Fe/ZSM-5

A nano-sized iron loaded ZSM-5 zeolite (Fe/ZSM-5) catalyst was firstly used to activate (NH₄)₂S₂O₈ solution for the simultaneous removal of multi-pollutant from flue gas. The simultaneous removal efficiencies 100% of SO₂, 72.6% of NO and 93.4% of Hg° were achieved under the condition that the catalyst dose was 0.8 g/L, concentration, pH and temperature of (NH₄)₂S₂O₈ solution were 0.03 mol/L, 5 and 65 °C, respectively. The stability of catalyst was checked by a continuous test, proving that the catalytic activity was maintained for 4 h and the leached iron reached low levels. Based on the catalyst characterizations, product analysis and literatures, the removal mechanism was speculated preliminarily, during which, OH and SO₄^- played key roles for oxidizing NO and Hg° into NO₃^- and Hg²⁺.

Simultaneous removal of NO and SO2 from flue gas using vaporized H2O2 catalyzed by nanoscale zero-valent iron

To remove NO and SO₂ from flue gas simultaneously, a heterogeneous catalytic oxidation system was developed with the nanoscale zero-valent iron (nZVI), vaporized H₂O₂, and sodium humate (HA-Na) acting as the catalyst, oxidant, and absorbent, respectively. The experimental results indicated that the desulfurization was mainly influenced by the absorption, and the denitrification was significantly affected by the catalytic oxidation parameters. Under the optimal conditions, the simultaneous removal efficiencies of SO₂ and NO were 100 and 88.4%, respectively. The presence of ^·OH during the removal process was proved by the scavenger tests, and the production of ^·OH with and without nZVI was indirectly evaluated by the electron paramagnetic resonance (EPR) and methylene blue experiments. Moreover, the fresh and aged nZVI were characterized by a series of techniques and the results suggested that the redox pair Fe²⁺/Fe³⁺ released by nZVI could react with H₂O₂ to provide the sustainable ^·OH, which was important for the oxidation from NO and SO₂ to NO₃^- and SO₄^2-. The removal mechanism was proposed preliminarily based on the correlative experiments, characterizations, and references.

Simultaneous removal of NO and SO2 from flue gas by combined heat and Fe2+ activated aqueous persulfate solutions

The use of advanced oxidation processes (AOPs) to integrate flue gas treatments for SO₂, NO_x and Hg⁰ into a single process unit is rapidly gaining research attention. AOPs are processes that rely on the generation of mainly the hydroxyl radical. This work evaluates the effectiveness of the simultaneous removal of NO and SO₂ from flue gas utilizing AOP induced by the combined heat and Fe²⁺ activation of aqueous persulfate, and elucidates the reaction pathways. The results indicated that both SO₂ in the flue gas and Fe²⁺ in solution improved NO removal, while the SO₂ is almost completely removed. Increased temperature led to increase in NO removal in the absence and presence of both Fe²⁺ and SO₂, and in the absence of either SO₂ or Fe²⁺, but the enhanced NO removal due to the presence of SO₂ alone dominated at all temperatures. The removal of NO increased from 77.5% at 30 °C to 80.5% and 82.3% at 50 °C and 70 °C in the presence of SO₂ alone, and from 35.3% to 62.7% and 81.2%, respectively, in the presence of Fe²⁺ alone. However, in the presence of both SO₂ and Fe²⁺, NO conversion is 46.2% at 30 °C, increased only slightly to 48.2% at 50 °C; but sharply increased to 78.7% at 70 °C compared to 63.9% for persulfate-only activation. Results suggest NO removal in the presence of SO₂ is equally effective by heat-only or heat-Fe²⁺ activation as the temperature increases. The results should be useful for future developments of advanced oxidation processes for flue gas treatments.