Increased protons in TNTs exchange with alkaline metal cations, protecting CeO₂ and achieving enhanced alkaline poisoning resistance.

Titanate nanotubes (TNTs) synthesized by hydrothermal method were increasingly used as the catalyst support for the selective catalytic reduction (SCR) of NO with ammonia. This paper reports the critical process of postwashing to prepare satisfactory TNTs for the uses of SCR catalysts. Herein, alkaline TNTs (TNTs‐AL), acidic TNTs (TNTs‐AC), and neutral TNTs (TNTs‐NE) were synthesized by controlling washing pH value. When these TNTs were utilized as the catalyst supports for manganese oxides (Mn/TNTs‐AL, Mn/TNTs‐AC, and Mn/TNTs‐NE), the key role of pH value was found. Titanate nanosheets, titanate nanorods and titanate nanotubes were dominated in Mn/TNTs‐AL, Mn/TNTs‐AC, and Mn/TNTs‐NE, respectively. MnO₂ crystal was observed when using TNTs‐AC or TNTs‐NE as the support. By contrast, Mn₃O₄ and NaNO₃ were observed when using TNTs‐AL as the support. Mn/TNTs‐NE showed the best SCR activity, in line with the largest surface area, the best dispersion, and the most active redox property of manganese oxides. Mn/TNTs‐AL showed negligible SCR activity, resulting from the minimum surface area, the Mn₃O₄‐dominating crystal structure, and the bad dispersion of manganese oxides.

The feasibility of removal of sulfate ions from the sodium alkali FGD wastewater using the ettringite precipitation method was evaluated. Factors affecting the removal of sulfate ions, such as NaAlO₂ dosage, Ca(OH)₂ dosage, solution temperature, anions (Cl⁻, NO₃⁻ and F⁻), and heavy metal ions (Mg²⁺ and Mn²⁺), were investigated, and the optimal experimental conditions for the removal of sulfate ions were determined. Experimental results indicate that the ettringite precipitation method can effectively remove SO₄²⁻ with removal efficiency of more than 98%. All the investigated factors have influences on the removal of sulfate ions, and among them, the dosage of reagents, solution temperature, and fluoride ions have the strongest influence. In addition, the method can effectively synergistically remove F⁻ and heavy metal ions with removal efficiencies of more than 90% and 99%, respectively; meanwhile, Cl⁻ and NO₃⁻ also can be removed minimally by the method. The result of actual wastewater treatment shows that the method is feasible for treating high-concentration sulfate wastewater. The ettringite precipitation method has the potential to be a commercial application in the future.

The desulfurization and denitrification wastewater (DDW) from the wet flue gas treatment project is difficult to be treated and recycled because of high chloride ion (Cl⁻) concentration. Cl⁻ can cause equipment and piping corrosion. However, there is a lack of cost-effective treatment technologies for the removal of Cl⁻ from the DDW. In this research, the feasibility of Cl⁻ removal from the DDW using Friedel’s salt precipitation method was evaluated. Factors affecting the Cl⁻ removal, such as Ca(OH)₂ dosage, NaAlO₂ dosage, solution’s initial pH, solution’s temperature, reaction time, stirring speed, and anions (SO₄²⁻, NO₃⁻, and F⁻), were investigated, and the optimal experimental conditions for Cl⁻ removal were determined. Experimental results showed that Friedel’s salt precipitation method can remove Cl⁻ effectively and can achieve synergistic removal of SO₄²⁻, F⁻, and heavy metal ions. Under the best experimental conditions, the average removal efficiencies of Cl⁻, SO₄²⁻, F⁻, and heavy metal ions reach more than 85%, 98%, 94%, and 99%, respectively. The Cl⁻ removal mechanism studies showed that Cl⁻ can be removed by precipitation as Ca₄Al₂Cl₂(OH)₁₂. The purified wastewater and the precipitated solid can be reused to reduce the consumption of water and alkali. Friedel’s salt precipitation method is an effective control technology for the synergistic removal of Cl⁻, SO₄²⁻, F⁻, and heavy metal ions and has enormous potential to be applied in the industrial wastewater treatment field.

In this work, the feasibility of the simultaneous removal of NO_x and SO₂ through a simple process using a composite absorbent (NaClO₂/Na₂S₂O₈) was evaluated. Factors affecting the removal of NO_x and SO₂, such as NaClO₂ and Na₂S₂O₈ concentrations, solution temperature, the initial pH of solution, gas flow rate, and SO₂, NO, and O₂ concentrations were studied, with a special attention to NO_x removal. Results indicate that a synergistic effect on NO_x removal has been obtained through combination of NaClO₂ and Na₂S₂O₈. NaClO₂ in the solution played a more important role than did Na₂S₂O₈ for the removal of NO_x. The above factors had an important impact on the removal of NO_x, especially the solution temperature, the initial pH of the solution, and the oxidant concentrations. The optimum experimental conditions were established, and a highest efficiency of NO_x removal of more than 80% was obtained. Meanwhile, tandem double column absorption experiments were conducted, and a NO_x removal efficiency of more than 90% was reached, using NaOH solution as an absorbant in the second reactor. A preliminary reaction mechanism for NO_x and SO₂ removal was deduced, based on experimental results. The composite absorbent has the potential to be used in the wet desulfurization and denitration process, to realize the synergistic removal of multi-pollutants.