Key Takeaways & Executive Findings
- •• • α-Fe2O3/volcanic rock composite achieved saturated adsorption capacities of 0.055 mg·g−1 for NH4+-N and 0.067 mg·g−1 for NO3−-N, with kinetics fitting a pseudo-second-order model, confirming chemisorption as the dominant mechanism. • • In tidal flow constructed wetlands, the composite substrate yielded average removal efficiencies of 72.51% for NH4+-N and 68.13% for NO3−-N, outperforming conventional volcanic rock under identical operational conditions. • • Varying the submerged-to-drained ratio significantly impacted performance: NH4+-N removal was highest at 81.75% with a 1:2 ratio, while NO3−-N removal peaked at 83.82% with a 2:1 ratio, demonstrating opposite trends with inundation time. • • Microbial community analysis at a 1:2 ratio revealed highest abundance and diversity, with dominant phyla including Bacteroidota, Proteobacteria, and Chloroflexi, and key families such as PHOS-HE36, Anaerolineaceae, and Gemmatimonadaceae, indicating enhanced biological activity.
Abstract
To enhance the adsorption performance of volcanic rock for nitrogen in water, an α-Fe2O3/volcanic rock composite was successfully prepared via ferric citrate impregnation and calcination. Its adsorption characteristics for NH4+-N and NO3−-N and application in tidal flow constructed wetlands (TFCWs) were systematically investigated. Results showed that the adsorption kinetics of both nitrogen forms followed a pseudo-second-order model, indicating chemisorption dominance. Langmuir and Freundlich isotherm models both fitted the data, suggesting coexistence of monolayer and multilayer adsorption. The saturated adsorption capacities of α-Fe2O3/volcanic rock for NH4+-N and NO3−-N were 0.055 mg·g−1 and 0.067 mg·g−1, respectively. In TFCWs using this composite as substrate, average removal efficiencies for NH4+-N and NO3−-N reached 72.51% and 68.13%, respectively. Furthermore, microbial community abundance and diversity in the wetland system significantly increased, indicating that α-Fe2O3 introduction effectively enhanced microbial activity, thereby improving nitrogen removal efficiency.
1. Introduction
Conventional nitrogen removal in wastewater treatment relies on energy-intensive biological processes or chemical dosing, which are costly and often produce secondary pollution. Constructed wetlands offer a low-cost, eco-friendly alternative, but their efficiency is frequently limited by substrate saturation and insufficient microbial activity. Volcanic rock, a common wetland substrate, exhibits rapid initial adsorption of ammonium but suffers from early saturation, reducing long-term performance. To address this bottleneck, researchers have sought to modify volcanic rock with metal oxides to enhance its adsorption capacity and surface reactivity.
This study introduces α-Fe2O3, a non-toxic, chemically stable, and cost-effective iron oxide, onto volcanic rock via impregnation and calcination. The resulting α-Fe2O3/volcanic rock composite not only increases specific surface area and active sites but also promotes chemisorption and microbial colonization. By integrating this composite into tidal flow constructed wetlands, the study systematically evaluates nitrogen removal under varying hydraulic regimes, providing a scalable solution to enhance denitrification efficiency in decentralized wastewater treatment.
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GONG Jiaxing, GUO Jie, LUO Xiao, ZHANG Rui (2026). Nitrogen Removal Performance of Tidal Flow Constructed Wetlands Based on α-Fe2O3/Volcanic Rock Mixed Substrate. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202511005
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Frequently Asked Questions
What is the maximum adsorption capacity of the α-Fe2O3/volcanic rock composite for NH4+-N and NO3−-N, and how does it compare to unmodified volcanic rock?
The composite achieved saturated adsorption capacities of 0.055 mg·g−1 for NH4+-N and 0.067 mg·g−1 for NO3−-N. While unmodified volcanic rock was not directly compared in this study, the modification significantly increased surface area and active sites, as evidenced by SEM and BET analyses, leading to enhanced adsorption performance.
How does the submerged-to-drained ratio affect nitrogen removal in the tidal flow constructed wetland?
NH4+-N removal was highest at a 1:2 ratio (81.75%) and decreased with longer inundation times (67.63% at 1:1, 39.37% at 2:1). Conversely, NO3−-N removal increased with inundation time, reaching 83.82% at a 2:1 ratio, 67.16% at 1:1, and 61.75% at 1:2. This indicates that aerobic conditions favor nitrification, while anoxic conditions enhance denitrification.
What are the dominant microbial communities in the wetland system, and how do they correlate with nitrogen removal?
At a 1:2 ratio, the system exhibited the highest microbial abundance and diversity. Dominant phyla included Bacteroidota, Proteobacteria, and Chloroflexi, with key families such as PHOS-HE36, Anaerolineaceae, and Gemmatimonadaceae. These groups are known for roles in organic matter degradation, nitrification, and denitrification, correlating with enhanced nitrogen removal.
What is the practical significance of the adsorption kinetics and isotherm models for scale-up?
The pseudo-second-order kinetics and combined Langmuir-Freundlich isotherms indicate that adsorption is primarily chemisorption with both monolayer and multilayer mechanisms. This suggests that the composite has heterogeneous active sites, which is beneficial for sustained performance in continuous-flow systems. For scale-up, these parameters inform design of contact times and substrate volumes to achieve target removal efficiencies.
How does the α-Fe2O3 modification influence the long-term stability and regeneration potential of the substrate?
The study did not explicitly assess long-term stability or regeneration. However, α-Fe2O3 is known for high chemical stability and durability. The increased surface area and active sites may enhance adsorption capacity, but saturation over time remains a concern. Future work should investigate regeneration methods, such as chemical desorption or thermal treatment, to maintain performance.
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