Simultaneous absorption–oxidation of nitric oxide and sulfur dioxide using ammonium persulfate synergistically activated by UV-light and heat

Synergistic degradation of Congo Red by hybrid advanced oxidation via ultraviolet light, persulfate, and hydrodynamic cavitation

In the present study, hybrid activation of sodium peroxydisulfate (PS) by hydrodynamic cavitation and ultraviolet radiation was investigated for Congo Red (CR) degradation. Experiments were conducted using the Box-Benken design on inlet pressure (2-6 bar), PS concentration (0-50 mg. L-1) and UV radiation power (0-32 W). According to the results, at the optimum point where the pressure, PS concentration and UV radiation power were equal to 4.5 bar, 30 mg. L-1 and 16 W respectively, 92.01% of decolorization was achieved. Among the investigated processes, HC/UV/PS was the best process with the rate constant and synergetic coefficient of 38.6 × 10-3 min-1 and 2.76, respectively. At the optimum conditions, increasing the pollutant concentration from 20 mg. L-1 to 80 mg. L-1 decrease degradation rate from 92.01 to 45.21. Presence of natural organic mater (NOM) in all concentrations inhibited the CR degradation. Quenching experiments revealed that in the HC/UV/PS hybrid AOP free radicals accounted for 63.4% of the CR degradation, while the contribution of sulfate (SRs) and hydroxyl radicals (HRs) was 53.1% and 46.9%, respectively.

Computational Approach to the Surface-Crosslinking Process of Superabsorbent Polymer via Central Composite Design

The improvement of gel strength and absorption properties through the surface-crosslinking of superabsorbent polymers (SAPs) is essential for sanitary industry applications. We prepared core-SAP via aqueous solution copolymerization, and then surface-crosslinked the core-SAP under various conditions. The structure of the SAP was characterized using Fourier transform infrared (FT-IR) spectroscopy. Central composite design (CCD) of response surface methodology (RSM) has been applied to determine the optimum surface-crosslinking conditions such as surface-crosslinker content, reaction temperature, and reaction time. The optimal surface-crosslinking conditions were identified at a surface-crosslinker content of 2.22 mol%, reaction temperature of 160 °C, and reaction time of 8.7 min. The surface-crosslinked SAP showed excellent absorbency under load of 50 g/g with a permeability of 50 s. Other absorption properties were also evaluated by measuring the free absorbency and centrifuge retention capacity in saline solution.

A Critical Review on Removal of Gaseous Pollutants Using Sulfate Radical-based Advanced Oxidation Technologies

Excessive emissions of gaseous pollutants such as SO₂, NO_x, heavy metals (Hg, As, etc.), H₂S, VOCs, etc. have triggered a series of environmental pollution incidents. Sulfate radical (SO₄•^-)-based advanced oxidation technologies (AOTs) are one of the most promising gaseous pollutants removal technologies because they can not only produce active free radicals with strong oxidation ability to simultaneously degrade most of gaseous pollutants, but also their reaction processes are environmentally friendly. However, so far, the special review focusing on gaseous pollutants removal using SO₄•^--based AOTs is not reported. This review reports the latest advances in removal of gaseous pollutants (e.g., SO₂, NO_x, Hg, As, H₂S, and VOCs) using SO₄•^--based AOTs. The performance, mechanism, active species identification and advantages/disadvantages of these removal technologies using SO₄•^--based AOTs are reviewed. The existing challenges and further research suggestions are also commented. Results show that SO₄•^--based AOTs possess good development potential in gaseous pollutant control field due to simple reagent transportation and storage, low product post-treatment requirements and strong degradation ability of refractory pollutants. Each SO₄•^--based AOT possesses its own advantages and disadvantages in terms of removal performance, cost, reliability, and product post-treatment. Low free radical yield, poor removal capacity, unclear removal mechanism/contribution of active species, unreliable technology and high cost are still the main problems in this field. The combined use of multiactivation technologies is one of the promising strategies to overcome these defects since it may make up for the shortcomings of independent technology. In order to improve free radical yield and pollutant removal capacity, enhancement of mass transfer and optimization design of reactor are critical issues. Comprehensive consideration of catalytic materials, removal chemistry, mass transfer and reactor is the promising route to solve these problems. In order to clarify removal mechanism, it is essential to select suitable free radical sacrificial agents, probes and spin trapping agents, which possess high selectivity for target specie, high solubility in water, and little effect on activity of catalyst itself and mass transfer/diffusion parameters. In order to further reduce investment and operating costs, it is necessary to carry out the related studies on simultaneous removal of more gaseous pollutants.

Application Prospect of K Used for Catalytic Removal of NOx, COx, and VOCs from Industrial Flue Gas: A Review

NOx, COx, and volatile organic compounds (VOCs) widely exist in motor vehicle exhaust, coke oven flue gas, sintering flue gas, and pelletizing flue gas. Potassium species have an excellent promotion effect on various catalytic reactions for the treatment of these pollutants. This work reviews the promotion effects of potassium species on the reaction processes, including adsorption, desorption, the pathway and selectivity of reaction, recovery of active center, and effects on the properties of catalysts, including basicity, electron donor characteristics, redox property, active center, stability, and strong metal-to support interaction. The suggestions about how to improve the promotion effects of potassium species in various catalytic reactions are put forward, which involve controlling carriers, content, preparation methods and reaction conditions. The promotion effects of different alkali metals are also compared. The article number about commonly used active metals and promotion ways are also analyzed by bibliometric on NOx, COx, and VOCs. The promotion mechanism of potassium species on various reactions is similar; therefore, the application prospect of potassium species for the coupling control of multi-pollutants in industrial flue gas at low-temperature is described.

Review on the NO removal from flue gas by oxidation methods

Due to the increasingly strict emission standards of NOx on various industries, many traditional flue gas treatment methods have been gradually improved. Except for selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) methods to remove NOx from flue gas, theoxidation method is paying more attention to NOx removal now because of the potential to simultaneously remove multiple pollutants from flue gas. This paper summarizes the efficiency, reaction conditions, effect factors, and reaction mechanism of NO oxidation from the aspects of liquid-phase oxidation, gas-phase oxidation, plasma technology, and catalytic oxidation. The effects of free radicals and active components of catalysts on NO oxidation and the combination of various oxidation methods are discussed in detail. The advantages and disadvantages of different oxidation methods are summarized, and the suggestions for future research on NO oxidation are put forward at the end. The review on the NO removal by oxidation methods can provide new ideas for future studies on the NO removal from flue gas.

A novel combined system using Na2S2O8/urea to simultaneously remove SO2 and NO in marine diesel engine exhaust

The novel combined system using Na₂S₂O₈/urea was used to simultaneously absorb nitric oxide and sulfur dioxide emissions from marine diesel engines as well as inhibit the formation of nitrate in cleaning wastewater to meet the increasingly stringent requirements of regulations. The influences of reaction temperature, Na₂S₂O₈ concentration, urea concentration, SO₂ concentration, NO concentration and pH value on SO₂ removal efficiency, NO removal efficiency and nitrate concentration were investigated. The experimental results showed that different reaction temperatures had different influences on SO₂ removal efficiency, NO removal efficiency and nitrate concentration. An increase in Na₂S₂O₈ could improve the absorption of NO. The addition of urea could effectively improve the removal efficiency of NO and reduce the nitrate concentration. The removal efficiencies of 1000 ppm NO and 1000 ppm SO₂ achieved 100 % with 0.2 mol/L Na₂S₂O₈ and 2 mol/L urea at 70℃, and the nitrate content was 8.56 mg/L which was far lower than the regulatory requirement of 60 mg/L. The acidic condition (pH ≤ 5.5) not only facilitated the absorption of NO but also reduced the generation of nitrate. According to the experimental results, the novel combined system was promising to be applied to the control technology of marine diesel engine exhaust.

NOx attenuation in flue gas by •OH/SO4•--based advanced oxidation processes

The combustion of fossil fuels has resulted in rapidly increasing emissions of nitrogen oxide (NO_x), which has caused serious human health and environmental problems. NO capture has become a research focus in gas purification because NO accounts for more than 90% of NO_x and is difficult to remove. Advanced oxidation processes (AOPs), features the little secondary pollution and the broad-spectrum strong oxidation of hydroxyl radicals (^•OH), are effective and promising strategies for NO removal from coal-fired flue gas. This review provides the state of the art of NO removal by AOPs, highlighting several methods for producing ^•OH and SO₄^•-. According to the main radicals responsible for NO removal, these processes are classified into two categories: hydroxyl radical-based AOPs (HR-AOPs) and sulfate radical-based AOPs (SR-AOPs). This paper also reviews the mechanisms of NO capture by reactive oxygen species (ROS) and SO₄^•- in various AOPs. A HiGee (high-gravity) enhanced AOP process for improving NO removal, characterized by intensified gas-liquid mass transfer and efficient micro-mixing, is then proposed and discussed in brief. We believe that this review will be useful for workers in this field. Graphical abstract.

Simultaneous desulfurization and denitrification from flue gas by catalytic ozonation combined with NH3/(NH4)2S2O8 absorption: Mechanisms and recovery of compound fertilizer

An integrated method of simultaneous desulfurization and denitrification from flue gas by catalytic ozonation combined with NH₃/(NH₄)₂S₂O₈ absorption was developed for the first time. It consisted of two parts: (1) the catalytic ozonation of NO over FeO_x/SAPO-34 to study the effects of the various influencing factors, and (2) the absorption-oxidation of NO_x and SO₂ induced by ozone combined with a NH₃/(NH₄)₂S₂O₈ solution in a bubble column reactor. In Part 1, results showed that under the optimal condition of a molar ratio of 0.5 for O₃/NO, a residence time of 3 s, a water vapor volume fraction of 4%, a NO initial concentration of 536 mg/m³, and a SO₂ initial concentration of 343 mg/m³, the oxidation rate of NO was 55%. The characterizations of poisoned catalyst are briefly discussed. In Part 2, as the gas passed sequentially through the ozonizing reactor and the absorber (NH₃/(NH₄)₂S₂O₈ solution of 0.8% ammonia and 0.2 mol/L (NH₄)₂S₂O₈), a NO conversion rate of approximately 92.6% and SO₂ conversion rate of 100% were obtained. The pH of the NH₃/(NH₄)₂S₂O₈ solution had a significant impact on the NO conversion. According to the analysis of the composition of products under different pHs, a mechanism of desulfurization and denitrification based on NH₃/(NH₄)₂S₂O₈ solutions was proposed. The reaction product as a compound fertilizer contained up to 24.5% nitrogen.

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.

Novel Simultaneous Removal Technology of NO and SO2 Using a Semi-Dry Microwave Activation Persulfate System

As it has a simple system and a small floor area, flue gas simultaneous desulfurization and denitrification technology has a good development prospect, and related research has become a hot topic in the field of flue gas purification. In this work, a novel simultaneous removal technology of NO and SO₂ from flue gas using a semi-dry microwave activation persulfate system was developed for the first time. A series of experiments and characterization analyses had been implemented to research the feasibility of this new flue gas purification technology. The oxidation products, free radicals, and mechanism of NO and SO₂ simultaneous removal were revealed. The effect of the main technological parameters on NO and SO₂ simultaneous removal was also studied. Relevant results demonstrated that an increase in the microwave radiation power, persulfate concentration, and O₂ concentration enhanced NO and SO₂ simultaneous removal. The increase of NO and SO₂ concentrations weakened NO and SO₂ simultaneous removal. The reagent dosage, pH value of the solution, and reaction temperature showed a dual influence on NO and SO₂ simultaneous removal. Free-radical capture experiments revealed that both SO₄^-• and ^•OH that were produced by microwave activation of persulfate were the major active species and played very key roles in NO and SO₂ simultaneous removal. The main products (sulfate and nitrate) and byproducts (NO₂) in the tail gas were found. The process application and product post-treatment routes were also proposed. The result may provide the necessary inspiration and guidance for the development and application of microwave-activated advanced oxidation technology in the flue gas treatment area.

Integrated leaching–carbonation kinetic model on CO2 mineralization of alkaline solid wastes in a high-gravity rotating packed bed

Ca²⁺ leaching and carbonation kinetics associated with liquid side mass transfer for CO₂ mineralization of alkaline solid wastes in a RPB were determined.