Researchers and environmentalists have been working on alternative solutions to the concerns listed above and explored various approaches. Another critical thing to be discussed here is the NO solubility problem in water. Some other toxic gases such as CO₂ and SO₂ are easily treated via the alkali absorption method [46,47]^[1][2]. Since the solubility problem NO can be treated such as other gases, it required some combined or integrated system applied for NO reduction [48]^[3]. This solubility problem indicated the NO_x removal would be easier if chemical modifications were used for its absorption and reduction. Many ways came to notice, but most practice has been undertaken by wet scrubbing using Fe-EDTA. There are several reasons for considering flue gas treatment and reduction techniques, which are discussed herein. The MnO_x/CNT_s catalysts were found to have unusual SCR activity at low temperatures when they were first synthesized. When using the optimal 1.2 percent MnO_x/CNT_s catalyst at 80–180 °C, the NO conversion ranged between 57.4 and 89.2 percent. This occurred from the use of amorphous MnO_x catalysts, which have a higher ratio of Mn⁴⁺ to Mn³⁺ and O_S to (O_S + O_L) than the crystalline MnO_x catalysts [49]^[4]. By impregnation and in situ deposition methods, the same Ce/Mn molar ratio was achieved in the preparation of Ce (1.0) Mn/TiO₂ catalysts. In comparison to the impregnation-prepared Ce(1.0)Mn/TiO₂-IP catalyst, the in situ deposition-prepared Ce(1.0)Mn/TiO₂-SP catalyst demonstrated superior catalytic activity throughout a wide temperature range (150–300 °C) and at high-gas hourly-spaced velocities ranging from 10,500 to 27,000 h⁻¹. Furthermore, the Ce(1.0)Mn/TiO₂-SP catalyst produced by the in situ deposition approach has superior sulphur resistance to the Ce(1.0)Mn/TiO₂-IP catalyst [50,51]^[5][6]. By using the citric acid–ethanol dispersion method, a variety of Gadolinium (Gd)-modified MnO_x/ZSM-5 catalysts were produced and assessed using a low-temperature NH₃-SCR reaction. Of them, the GdMn/Z-0.3 catalyst, which had a molar ratio of 0.3 for Gd to Mn, had the maximum catalytic activity, and it was capable of achieving a 100 percent NO conversion in the temperature range of 120–240 degrees Celsius. Furthermore, when tested in the presence of 100 ppm SO₂, GdMn/Z-0.3 demonstrated superior SO₂ resistance when compared to Mn/Z. It was demonstrated that such catalytic efficacy was primarily driven by surface chemisorbed oxygen species, a wide surface area, an abundance of Mn⁴⁺ and, a proper acidity and reducibility, and the of the catalyst, among other factors [52]^[7].

Among transition metal chelating complexes, Fe-EDTA is the most favorable and stable chelate against the long-range of pH. Comparison for the metal chelate system are also shown in Figure 3. On top of that, ferrous (Fe(II)) has the great affinity towards the NO compared to other d-block elements. By taking advantage of this Fe-EDTA stability and greatest affinity, it has been used for NO_x removal from flue gas. Another advantage of this process (wet scrubbing) is the simultaneous removal of NO_x and SO₂ under ambient conditions. Moreover, the NO_x interaction with Fe(II) is direct chemically binding and forms a very stable bond among other transition metals [12,53]^[8][9].

Basically, this concept is composed of two steps, (i) Fe-EDTA solution absorbs NO_x molecule via making metal-nitrosyl-complex and then (ii) this NO_x reduced into N₂O, N₂ and N-S compounds by utilizing the SO_2, which is a compulsory part of flue gas. This SO₂ transformed into SO₃²⁻ by alkali absorption and reduced NO_x by converting itself into SO₄²⁻ ions. During the reduction of NO_x, the scrubber (Fe-EDTA) also regenerated irrespective the valence form of iron, Fe(II) or Fe(III). As Fe(III) is more stable due to more stability than Fe(II), it cannot be restored without the help of the electron donor externally. The number of electron donors used to reduce Fe(III) back to Fe(II) depends upon the source, condition and more important the NO_x reduction process.