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

Excessive emissions of gaseous pollutants such as SO2, NOx, 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, NOx, 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.

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.

Removal of Carbon Monoxide from Simulated Flue Gas Using Two New Fenton Systems: Mechanism and Kinetics.

Two novel removal processes of carbon monoxide using two new Fenton systems (i.e., Cu2+/Fe2+ and Mn2+/Fe2+ coactivated H2O2 systems) were developed. The effect of several process parameters (concentrations of H2O2, Fe2+, Cu2+, and Mn2+, reagent pH value, solution temperature, and simulated flue gas components) on CO removal was studied in a bubbling reactor. The mechanism and kinetics of CO removal were also revealed. Results show that adding Cu2+ or Mn2+ obviously enhances the removal process of CO in new Fenton systems. The measured results of free radical yield demonstrate that the enhancing role is derived from producing more ·OH (they are produced due to the synergistic activation role of Cu2+/Fe2+ or Mn2+/Fe2+ in new Fenton systems. The removal efficiency of CO is raised by increasing concentrations of Fe2+, Cu2+, and Mn2+ and is reduced by raising concentrations of CO, NO, and SO2. Increasing H2O2 concentration, reagent pH, and solution temperature demonstrates a dual impact on CO absorption. Three oxidation pathways are found to be responsible for CO removal in new Fenton systems. Results of mass-transfer reaction kinetics reveal that CO removal processes are located in a fast-speed reaction kinetics region (the CO removal process is controlled by the mass transfer rate).

Oxidation Removal of Nitric Oxide from Flue Gas Using UV Photolysis of Aqueous Hypochlorite.

The oxidation removal of nitric oxide (NO) from flue gas using UV photolysis of aqueous hypochlorite (Ca(ClO)2 and NaClO) in a photochemical spraying reactor was studied. The key parameters (e.g., light intensity, hypochlorite concentration, solution temperature, solution pH, and concentration of NO, SO2, O2, and CO2), mechanism and kinetics of NO oxidation removal were investigated. The results demonstrate that UV and hypochlorite have a significant synergistic role for promoting the production of hydroxyl radicals (·OH) and enhancing NO removal. NO removal was enhanced with the increase of light intensity, hypochlorite concentration, or O2 concentration but was inhibited with the increase of NO or CO2 concentration. Solution temperature, solution pH, and SO2 concentration have double the effect on NO removal. NO is oxidized by ·OH and hypochlorite, and ·OH plays a key role in NO oxidation removal. The rate equation and kinetic parameters of NO oxidation removal were also obtained, which can provide an important theoretical basis for studying the numerical simulation of NO absorption process and the amplification design of the reactor.

Novel Process of Simultaneous Removal of Nitric Oxide and Sulfur Dioxide Using a Vacuum Ultraviolet (VUV)-Activated O2/H2O/H2O2 System in A Wet VUV-Spraying Reactor.

A novel process for NO and SO2 simultaneous removal using a vacuum ultraviolet (VUV, with 185 nm wavelength)-activated O2/H2O/H2O2 system in a wet VUV-spraying reactor was developed. The influence of different process variables on NO and SO2 removal was evaluated. Active species (O3 and ·OH) and liquid products (SO32-, NO2-, SO42-, and NO3-) were analyzed. The chemistry and routes of NO and SO2 removal were investigated. The oxidation removal system exhibits excellent simultaneous removal capacity for NO and SO2, and a maximum removal of 96.8% for NO and complete SO2 removal were obtained under optimized conditions. SO2 reaches 100% removal efficiency under most of test conditions. NO removal is obviously affected by several process variables. Increasing VUV power, H2O2 concentration, solution pH, liquid-to-gas ratio, and O2 concentration greatly enhances NO removal. Increasing NO and SO2 concentration obviously reduces NO removal. Temperature has a dual impact on NO removal, which has an optimal temperature of 318 K. Sulfuric acid and nitric acid are the main removal products of NO and SO2. NO removals by oxidation of O3, O·, and ·OH are the primary routes. NO removals by H2O2 oxidation and VUV photolysis are the complementary routes. A potential scaled-up removal process was also proposed initially.

Removal of elemental mercury from flue gas by thermally activated ammonium persulfate in a bubble column reactor.

In this article, a novel technique on removal of elemental mercury (Hg(0)) from flue gas by thermally activated ammonium persulfate ((NH4)(2)S(2)O(8)) has been developed for the first time. Some experiments were carried out in a bubble column reactor to evaluate the effects of process parameters on Hg(0) removal. The mechanism and kinetics of Hg(0) removal are also studied. The results show that the parameters, (NH4)(2)S(2)O(8) concentration, activation temperature and solution pH, have significant impacts on Hg(0) removal. The parameters, Hg(0), SO2 and NO concentration, only have small effects on Hg(0) removal. Hg(0) is removed by oxidations of (NH4)(2)S(2)O(8), sulfate and hydroxyl free radicals. When (NH4)(2)S(2)O(8) concentration is more than 0.1 mol/L and solution pH is lower than 9.71, Hg(0) removal by thermally activated (NH4)(2)S(2)O(8) meets a pseudo-first-order fast reaction with respect to Hg(0). However, when (NH4)(2)S(2)O(8) concentration is less than 0.1 mol/L or solution pH is higher than 9.71, the removal process meets a moderate speed reaction with respect to Hg(0). The above results indicate that this technique is a feasible method for emission control of Hg(0) from flue gas.

A review on removal of mercury from flue gas utilizing existing air pollutant control devices (APCDs).

Mercury is a highly toxic heavy metal pollutant. It is of great significance to develop cost-effective mercury pollution control technologies of coal-fired flue gas. Among various mercury from flue gas removal methods, the application of existing air pollution control devices (APCDs) to remove mercury from flue gas is one of the most valuable methods because it doesn't need to install additional mercury removal equipment, reducing the cost of mercury removal. This review summarizes the recent progress of mercury from flue gas removal by APCDs (e.g., SCR denitration device, WFGD system and dust removal device). SCR denitration device can achieve partial removal of mercury in flue gas through combined with WFGD system, but easy inactivation and poor sulfur/water/heavy metals resistance of SCR catalyzers are still the main problems. WFGD systems can remove most of Hg2+ (80%-95%), but have low treatment ability for Hg0. Various oxidants can effectively oxidize Hg0 into Hg2+. However, traditional oxidants have high prices and secondary pollution due to the formation of by-products. Fabric filters (FFs), electrostatic precipitators (ESPs) and hybrid fabric filters (HFs) can all control the emission of mercury in the flue gas to a certain extent, especially can effectively remove most of HgP and part of Hg2+, but has low removal capacity for Hg0. Compared with ESP, FF has better capture efficiency for Hg2+ and Hg0, and a combination of ESP and FF, that is HF, can effectively improve the mercury removal capacity.

Enhancement in the selectivity of O2/N2 via ZIF-8/CA mixed-matrix membranes and the development of a thermodynamic model to predict the permeability of gases.

Zeolitic imidazolate framework-8 (ZIF-8) has a sodalite topology. ZIF-8 is composed of zinc ion coordinated by four imidazolate rings. The pore aperture of ZIF-8 is 3.4 Å, which readily retains large gas molecules like N2. In this work, mixed-matrix membranes (MMMs) have been fabricated by utilizing ZIF-8 and pristine cellulose acetate (CA) for O2/N2 separation. Membranes of pristine CA and MMMs of ZIF-8/CA at various ZIF-8 concentrations were prepared in tetrahydrofuran (THF). Permeation results of the fabricated membranes revealed increasing selectivity for O2/N2 with increasing pressure as well as ZIF-8/CA concentration up to 5% (w/w). The selectivity of O2/N2 increased 4 times for MMMs containing 5% (w/w) of ZIF-8/CA as compared with the pristine CA membrane. A thermodynamic model has also been developed to predict the permeability of gases through polymeric membranes. The results were compared with literature data as well as the pristine CA membrane produced in this work for model validation.

Preparation of magnetic Co-Fe modified porous carbon from agricultural wastes by microwave and steam activation for mercury removal.

In this article, a magnetic cobalt-iron modified porous carbon derived from agricultural wastes by microwave and steam activation was developed to remove elemental mercury in coal-fired flue gas. The effects of operating parameters on Hg0 capture were discussed. Reaction mechanism and regeneration performance were also studied. Results show that the activation of microwave and steam significantly improves the pore structure of the porous carbon. The ultrasound-assisted impregnation promotes the dispersion of cobalt oxides and iron oxides on the samples. The Co0.4Fe12/RSWU(500) sorbent exhibits highest Hg0 removal efficiency at 130 °C. The characterization analysis shows that cobalt oxides and iron oxides are the main active components for Hg0 removal. The XPS analysis suggests that the chemisorption oxygen and the lattice oxygen (derived from Co3+/Co2+ and Fe3+/Fe2+) participate in the Hg0 capture process. Moreover, the cobalt-iron mixed oxide modified porous carbon has a good regeneration performance, which is conductive to reduce the costs in the future application.

Preparation of microwave-activated magnetic bio-char adsorbent and study on removal of elemental mercury from flue gas.

In this study, novel activated magnetic bio-char adsorbents were proposed to remove the element mercury (Hg0) from flue gas. Microwave activation and Mn-Fe mixed oxides impregnation assisted by ultrasound treatment were applied on the modification of renewable cotton straw chars. The influence of different preparation methods, loading value of Mn-Fe, molar ratio of Mn/Fe, calcining temperature, reaction temperature and individual flue gas ingredients (O2, NO, SO2 and H2O) on removal of Hg0 was investigated in a fixed bed system. The characterization results reveal that microwave activation is advantageous for the development of the pore structure, and ultrasound treatment can optimize the dispersion of Mn and Fe active ingredients. MnFe4%(3/10)/CSWU700 adsorbent exhibits the optimal Hg0 removing performance. O2, NO, low concentration of SO2 (<600 ppm) and low concentration of H2O (<2%) are found to be favourable for the capture of Hg0, while high concentrations of SO2 and H2O inhibit the removal of Hg0. Chemical adsorption acts a pivotal part in the process of Hg0 removal. Mn and Fe active ingredients are consumed in large quantities during the Hg0 capture. In addition, chemisorbed oxygen (Oß) also plays an indispensable in the oxidation process of Hg0. Furthermore, the magnetic adsorbent MnFe4%(3/10)/CSWU700 presents a good regeneration performance and adsorption capacity.

Gas-phase elemental mercury removal using ammonium chloride impregnated sargassum chars.

In this article, pyrolyzed bio-chars derived from a kind of macroalgae, sargassum, were modified by ammonium chloride (NH4Cl) impregnation, and were applied to remove Hg0 from flue gas. The characteristics of sorbents were investigated by the Brunauer-Emmett-Teller, X-ray photoelectron spectroscopy, scanning electron microscopy and ultimate and proximate analysis. The key parameters (e.g. loading value, reaction temperature and concentration of O2, NO, SO2 and water vapor), kinetics analysis and reaction mechanism of Hg0 removal were investigated. The results show that increasing loading value, reaction temperature, O2 concentration and NO concentration enhance Hg0 removal. The increase in SO2 concentration or water vapor concentration has a dual effect on Hg0 removal. The C-Cl groups and C=O groups play an important role in the process of Hg0 removal. The Hg0 removal process of modified samples meets the pseudo-second-order kinetic model.

Simultaneous removal of NO and SO2 using vacuum ultraviolet light (VUV)/heat/peroxymonosulfate (PMS).

Simultaneous removal process of SO2 and NO from flue gas using vacuum ultraviolet light (VUV)/heat/peroxymonosulfate (PMS) in a VUV spraying reactor was proposed. The key influencing factors, active species, reaction products and mechanism of SO2 and NO simultaneous removal were investigated. The results show that vacuum ultraviolet light (185 nm) achieves the highest NO removal efficiency and yield of and under the same test conditions. NO removal is enhanced at higher PMS concentration, light intensity and oxygen concentration, and is inhibited at higher NO concentration, SO2 concentration and solution pH. Solution temperature has a double impact on NO removal. CO2 concentration has no obvious effect on NO removal. and produced from VUV-activation of PMS play a leading role in NO removal. O3 and ·O produced from VUV-activation of O2 also play an important role in NO removal. SO2 achieves complete removal under all experimental conditions due to its very high solubility in water and good reactivity. The highest simultaneous removal efficiency of SO2 and NO reaches 100% and 91.3%, respectively.

Simultaneous absorption of SO2 and NO from flue gas using ultrasound/Fe2+/heat coactivated persulfate system.

A novel process on simultaneous absorption of SO2 and NO from flue gas using ultrasound (US)/Fe2+/heat coactivated persulfate system was proposed. The influencing factors, active species, products and mechanism of SO2 and NO removal were investigated. The results indicate that US enhances NO removal due to enhancement of mass transfer and chemical reaction. US of 28kHz is more effective than that of 40kHz. NO removal efficiency increases with increasing persulfate concentration, ultrasonic power density and Fe2+ concentration (at high persulfate concentration). Solution pH, solution temperature and Fe2+ concentration (at low persulfate concentration) have double effect on NO removal. SO2 is completely removed in most of tested removal systems, except for using water absorption. US, Fe2+ and heat have a synergistic effect for activating persulfate to produce free radicals, and US/Fe2+/heat coactivated persulfate system achieves the highest NO removal efficiency. ·OH and SO4-· play a leading role for NO oxidation, and persulfate only plays a complementary role for NO oxidation.

Removal of elemental Mercury from flue gas using wheat straw chars modified by K2FeO4 reagent.

In this article, wheat straw (WS) char, a common agricultural waste and renewable biomass, was pyrolyzed and then modified by K2FeO4 reagent to develop an efficient sorbent for removal of Hg0 from flue gas. Brunauer-Emmett-Teller, scanning electron microscopy with energy spectrum and X-ray diffraction (XRD) were employed to characterize the sorbents. The effects of K2FeO4 loading, reaction temperature, Hg0 inlet concentration and concentrations of gas mixtures O2, NO and SO2 in flue gas on Hg0 removal were investigated in a fixed-bed reactor. The results show that K2FeO4-impregnation can improve pore structure of WS char and produce new active sites, which significantly enhance Hg0 removal. Increasing Hg0 inlet concentration significantly decreases Hg0 removal efficiency. O2 in flue gas promotes Hg0 oxidation by replenishing the oxygen groups on the surface of modified chars. The presence of NO obviously promotes Hg0 removal since it can oxidize Hg0 to Hg(NO3)2. SO2 in flue gas significantly decreases Hg0 removal efficiency due to the competition adsorption between SO2 and Hg0. The increase in reaction temperature has a dual impact on Hg0 removal.

A study on removal of elemental mercury in flue gas using fenton solution.

A novel technique on oxidation-separation of elemental mercury (Hg(0)) in flue gas using Fenton solution in a bubbling reactor was proposed. The effects of several process parameters (H2O2 concentration, Hg(0) inlet concentration, Fe(2+) concentration, solution temperature, solution pH, gas flow) and several flue gas components (NO, SO2, O2, CO2, inorganic ions and particulate matters on Hg(0) removal were studied. The results indicate that H2O2 concentration, Fe(2+) concentration, solution pH and gas flow have great effects on Hg(0) removal. Solution temperature, Hg(0), NO, SO2, CO3(2-) and HCO3(-) concentrations also have significant effects on Hg(0) removal. However, Cl(-), SO4(2-), NO3(-), O2 and CO2 concentrations only have slight effects on Hg(0) removal. Furthermore, reaction mechanism of Hg(0) removal and simultaneous removal process of Hg(0), NO and SO2 were also studied. Hg(0) is removed by oxidation of OH and oxidation of H2O2. The simultaneous removal efficiencies of 100% for SO2, 100% for Hg(0) and 88.3% for NO were obtained under optimal test conditions. The results demonstrated the feasibility of Hg(0) removal and simultaneous removal of Hg(0), SO2 and NO using Fenton solution in a bubbling reactor.