Key Takeaways & Executive Findings
- •• • Optimal Mn loading: 10 wt% MnO2 on γ-Al2O3 achieves the highest gaseous Tl capture capacity; increasing to 15 wt% reduces performance, indicating an optimal dispersion–activity trade-off. • • Redox mechanism: Mn species oxidize Tl+ to Tl3+, enhancing immobilization; H2-TPR and O2-TPD confirm improved redox properties of MnO2/γ-Al2O3 compared to bare γ-Al2O3. • • DFT insights: TlCl adsorption on MnO2/γ-Al2O3 involves stronger Mn–Cl bond formation and higher adsorption energy than Al–Cl on γ-Al2O3, with greater charge transfer, explaining enhanced capture. • • Industrial relevance: Placing 10MnO2/γ-Al2O3 upstream of SCR catalysts can intercept gaseous Tl, reducing Tl flux to the catalyst and mitigating poisoning, thereby extending catalyst lifetime and ensuring stable cement kiln operation.
Abstract
Thallium (Tl) is a highly toxic trace heavy metal, posing severe risks to human health and the environment. Cement kilns are significant sources of gaseous Tl emissions, with concentrations up to 25 μg·m−3, which can poison SCR catalysts and cause environmental contamination. This study developed MnO2/γ-Al2O3 adsorbents via wet impregnation with varying Mn loadings (0–15 wt%) to capture gaseous TlCl. Fixed-bed adsorption experiments at 300 °C with 20% O2 revealed that capture capacity initially increased with Mn loading, peaking at 10 wt% MnO2 (10MnO2/γ-Al2O3), then declined at 15 wt%. Characterization (XRD, O2-TPD, H2-TPR) indicated that Mn species enhanced redox properties, oxidizing Tl+ to Tl3+ and immobilizing it on the surface. DFT calculations showed that TlCl forms stronger Al–Cl and Mn–Cl bonds on MnO2/γ-Al2O3 than on γ-Al2O3, with higher adsorption energy and greater charge transfer, corroborating experimental results. The optimal adsorbent, 10MnO2/γ-Al2O3, demonstrates superior Tl capture performance, offering a promising upstream solution for protecting SCR catalysts and reducing atmospheric Tl emissions from cement kilns.
1. Introduction
Cement kilns are a major source of gaseous thallium (Tl) emissions, with concentrations reaching up to 25 μg·m−3. These emissions not only pose severe health risks to nearby populations but also cause rapid deactivation of selective catalytic reduction (SCR) catalysts used for NOx control. For instance, field samples from a cement plant showed Tl accumulation of 6.4% on the catalyst after 5,000 hours, leading to near-complete loss of activity. Existing control strategies, such as using TiO2 as a protective layer, have demonstrated some efficacy but suffer from limited long-term stability and suboptimal capture efficiency under varying flue gas conditions.
This study addresses the bottleneck by developing MnO2/γ-Al2O3 adsorbents that combine the structural stability of γ-Al2O3 with the strong redox activity of MnO2. The Mn species facilitate the oxidation of Tl+ to Tl3+, enhancing chemical fixation on the surface. Systematic experiments and DFT calculations reveal that a 10 wt% MnO2 loading provides the optimal balance between active site availability and dispersion, achieving superior Tl capture performance. This material can be deployed upstream of SCR reactors to intercept Tl, thereby protecting the catalyst and reducing environmental emissions, offering a practical solution for the cement industry.
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XING Jiaying, WANG Jiawang, WANG Chunbo, CHEN Jianjun, LI Junhua (2026). Performance and Mechanism of MnO2/γ-Al2O3 for Gaseous Thallium Capture from Cement Kiln Flue Gas. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202506080
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Frequently Asked Questions
What is the optimal Mn loading for maximum Tl capture, and why does performance decline at higher loadings?
The optimal Mn loading is 10 wt% (10MnO2/γ-Al2O3), which achieves the highest Tl capture capacity. At 15 wt%, performance declines, likely due to agglomeration of MnO2 particles, reducing the number of accessible active sites and lowering dispersion, as supported by XRD and TPR data.
How does the presence of O2 affect the Tl capture mechanism?
Experiments were conducted under 20% O2, which is typical for cement flue gas. O2 participates in the redox cycle, regenerating Mn species and facilitating the oxidation of Tl+ to Tl3+. This enhances Tl immobilization, as confirmed by the higher capture efficiency compared to inert conditions.
What is the thermal stability of the adsorbent under typical cement kiln flue gas temperatures (around 300 °C)?
The adsorbent was tested at 300 °C, showing stable performance over 2 hours. The γ-Al2O3 support provides thermal stability, and MnO2 remains active at this temperature. No significant deactivation was observed, indicating suitability for upstream placement before SCR.
Can this adsorbent be regenerated or disposed of safely after saturation?
The study does not detail regeneration procedures. However, since Tl is immobilized as Tl2O3 or TlCl, the spent adsorbent must be treated as hazardous waste. Safe disposal or potential recovery of Tl could be explored, but further research is needed to assess long-term stability and leaching behavior.
How does the cost of MnO2/γ-Al2O3 compare to existing commercial adsorbents like activated carbon or TiO2?
γ-Al2O3 is a low-cost, widely available support, and Mn(NO3)2 precursor is relatively inexpensive. The synthesis method (wet impregnation) is scalable. Compared to TiO2, γ-Al2O3 offers similar semiconductor properties at lower cost, making MnO2/γ-Al2O3 a cost-effective alternative for Tl capture.
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