Key Takeaways & Executive Findings
- •• • A-type zeolite synthesized from fly ash exhibits a specific surface area of 18.59 m2/g and pore size of 3–5 nm, achieving saturated adsorption capacities of 1.07 mmol/g (SO2), 0.26 mmol/g (CO2), and 0.048 mmol/g (NO) at 1000 mg/m3 and 20 °C; these values are critical for designing low-concentration flue gas treatment systems. • • H-type zeolite, with a higher surface area of 22.32 m2/g, outperforms A-type, reaching 1.12 mmol/g (SO2), 0.29 mmol/g (CO2), and 0.053 mmol/g (NO) under identical conditions, indicating that surface area and silanol density are decisive for acidic gas uptake. • • In-situ DRIFTS identifies T–O (T=Si/Al) groups as the primary adsorption sites; adsorption energies for A-type zeolite are -5.11 kJ/mol (SO2), -4.07 kJ/mol (CO2), and -1.41 kJ/mol (NO), confirming physisorption and guiding future modification strategies. • • Kinetic data fit the Arrhenius equation, enabling prediction of temperature-dependent adsorption rates; this is essential for scaling up to industrial flue gas conditions where temperature varies.
Abstract
Fly ash, a byproduct of coal combustion, poses severe environmental challenges. This study synthesizes A- and H-type zeolites from fly ash via hydrothermal treatment and evaluates their adsorption performance for low-concentration acidic gases (SO2, CO2, NO) at 1000 mg/m3. The zeolites exhibited pore sizes of 3–5 nm, with specific surface areas of 18.59 m2/g (A-type) and 22.32 m2/g (H-type). At 20 °C, A-type zeolite achieved maximum saturated adsorption capacities of 1.07 mmol/g for SO2, 0.26 mmol/g for CO2, and 0.048 mmol/g for NO; H-type zeolite showed higher capacities: 1.12, 0.29, and 0.053 mmol/g, respectively. In-situ DRIFTS revealed that T–O (T=Si/Al) groups serve as key active sites, with adsorption energies for A-type zeolite calculated as -5.11 kJ/mol (SO2), -4.07 kJ/mol (CO2), and -1.41 kJ/mol (NO). Kinetic analysis indicated conformity to the Arrhenius equation. The results demonstrate that fly ash-based zeolites are promising adsorbents for acidic gas removal, with H-type outperforming A-type due to larger surface area and more silanol sites. This work provides a theoretical basis for utilizing fly ash in gas purification, contributing to the circular economy.
1. Introduction
Coal combustion in power generation, steel, and cement industries generates vast quantities of fly ash, whose improper disposal contaminates soil, water, and air. Simultaneously, flue gas contains low-concentration acidic gases such as SO2, CO2, and NO, which are harmful to human health and the environment. Conventional adsorbents often fail to efficiently capture these gases at low concentrations, especially under varying temperatures and multi-component conditions. Zeolites, with their high surface area, thermal stability, and tunable pore structure, offer a promising solution. However, current research predominantly focuses on high-concentration pollutant adsorption, leaving a gap in understanding low-concentration acidic gas removal using fly ash-derived zeolites.
This study addresses that gap by synthesizing A- and H-type zeolites from fly ash and systematically evaluating their adsorption performance for SO2, CO2, and NO at 1000 mg/m3. By employing in-situ DRIFTS and kinetic modeling, the work elucidates the adsorption mechanisms, identifying T–O groups as key active sites. The findings not only demonstrate the feasibility of using fly ash for gas purification but also provide quantitative data on adsorption capacities and energies, which are essential for designing efficient, cost-effective flue gas treatment systems. This approach aligns with the circular economy principle of 'waste controlling waste', offering a sustainable pathway for both fly ash utilization and air pollution control.
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DAI Ruijia, ZHAO Yongqi, DOU Jinxiao, YU Jianglong (2026). Synthesis of A- and H-type Zeolites from Fly Ash and Their Adsorption Mechanisms for SO2, CO2, and NO. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202604023
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Frequently Asked Questions
What are the specific surface areas and pore sizes of the synthesized A- and H-type zeolites, and how do they correlate with adsorption performance?
A-type zeolite has a specific surface area of 18.59 m2/g, while H-type has 22.32 m2/g; both exhibit pore sizes in the 3–5 nm range. The higher surface area of H-type directly correlates with its superior adsorption capacities (e.g., 1.12 mmol/g SO2 vs. 1.07 mmol/g for A-type), indicating that surface area is a primary factor in providing active sites for gas uptake.
How do the adsorption energies for SO2, CO2, and NO on A-type zeolite compare, and what does this imply for selectivity?
Adsorption energies on A-type zeolite are -5.11 kJ/mol (SO2), -4.07 kJ/mol (CO2), and -1.41 kJ/mol (NO). The stronger binding of SO2 suggests preferential adsorption over CO2 and NO, which could be exploited for selective removal in mixed gas streams, though competitive adsorption dynamics require further study.
What is the industrial relevance of the adsorption capacities achieved at 1000 mg/m3 and 20 °C?
At 1000 mg/m3 (approximately 350 ppm SO2, 550 ppm CO2, 800 ppm NO), the capacities of 1.07–1.12 mmol/g for SO2 are comparable to commercial zeolites, indicating potential for low-concentration flue gas polishing. However, the low NO capacity (0.048–0.053 mmol/g) suggests limited effectiveness for NO removal alone, necessitating further modification.
How does the kinetic behavior fit the Arrhenius equation, and what are the implications for temperature-dependent performance?
The adsorption kinetics for all gases on both zeolites conform to the Arrhenius equation, allowing prediction of rate constants at different temperatures. This is crucial for industrial applications where flue gas temperatures vary; it enables optimization of operating conditions to maximize adsorption efficiency.
What are the limitations of this study and potential avenues for future research?
The study acknowledges that the adsorption capacities are insufficient for large-scale industrial flue gas treatment. Future work should explore metal modification to enhance adsorption capacity, and extend testing to other pollutants like VOCs and NH3-SCR. Additionally, long-term stability and regeneration cycles need assessment.
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