Key Takeaways & Executive Findings
- •• • Achieved 91.3% tetracycline removal and 71.7% TN removal (83.3% NO3−-N) in Fe-C-Mn CW, versus 27.2% TC and 7.8% TN in gravel control, demonstrating superior simultaneous removal for secondary effluent polishing. • • Fe(0) oxidation to Fe(II)/Fe(III) and Mn(IV) reduction to Mn(II) synergistically enhanced electron transfer, mitigating iron passivation and sustaining autotrophic denitrification and TC degradation over extended operation. • • Selective enrichment of Trichosporon and Bacillota (TC degraders) and unclassified_f_Rhodocyclaceae (denitrifiers) despite lower overall diversity indicates a tailored microbial community driving synergistic pollutant removal. • • TC degradation proceeds via demethylation, hydroxylation, deamination, and ring-opening, converting TC into smaller metabolites, confirming effective mineralization and reduced toxicity.
Abstract
Constructed wetlands (CWs) with conventional substrates often exhibit limited removal of nitrogen and antibiotics from secondary effluent. This study developed an iron-carbon-manganese ore (Fe-C-Mn) composite substrate CW to enhance simultaneous removal of nitrogen and tetracycline (TC). Under influent TC of 2 mg·L−1 and total nitrogen (TN) of 15 mg·L−1, the Fe-C-Mn system achieved average TC removal of 91.3%, significantly higher than the gravel control (27.2%). TN and nitrate nitrogen (NO3−-N) removals reached 71.7% and 83.3%, respectively, versus 7.8% and 1.2% in the control. Substrate analysis revealed increased surface roughness and synergistic generation of active components (Fe(II)/Fe(III) and Mn(II)), driving autotrophic denitrification and TC biodegradation/chemical degradation. Microbial community analysis indicated reduced overall diversity but selective enrichment of potential TC degraders (e.g., Trichosporon, Bacillota) and denitrifiers (e.g., unclassified_f_Rhodocyclaceae). TC degradation pathways included demethylation, hydroxylation, and ring-opening, ultimately yielding small metabolites. These findings provide theoretical and technical support for enhanced removal of antibiotics and nitrogen from secondary effluent using CWs.
1. Introduction
Conventional constructed wetlands (CWs) using gravel or zeolite substrates face critical bottlenecks in polishing secondary effluent: nitrate (NO3−-N) is chemically stable and recalcitrant, while tetracycline (TC) residues persist at μg·L−1 levels, fostering antibiotic resistance genes and ecotoxicity. Traditional substrates provide limited electron transfer and adsorption capacity, resulting in slow degradation kinetics and poor simultaneous nitrogen removal. Iron-based materials, such as zero-valent iron or iron-carbon micro-electrolysis, offer strong reducing power and can generate reactive species (e.g., ·OH, H2O2) to degrade TC and reduce nitrate. However, iron passivation—surface oxide layer formation—blocks electron transfer and diminishes long-term efficiency. Manganese oxides (MnO2) can alleviate passivation by mediating redox reactions and accelerating iron corrosion, while also promoting oxidative degradation of organics and supporting manganese autotrophic denitrification. Yet, the synergistic mechanisms of combined iron-carbon and manganese ore substrates in CWs for simultaneous TC and nitrogen removal remain unexplored.
This study addresses that gap by constructing a CW with a Fe-C-Mn composite substrate and evaluating its performance against a gravel control under realistic secondary effluent conditions (TC=2 mg·L−1, TN=15 mg·L−1). The system achieved 91.3% TC removal and 71.7% TN removal, far exceeding the gravel control (27.2% and 7.8%, respectively). Substrate characterization and microbial analysis reveal that Fe(II)/Fe(III) and Mn(II) generation drives autotrophic denitrification and TC degradation, while specific microbial taxa are enriched. These findings provide a mechanistic basis for designing advanced CW substrates to overcome the dual challenges of nitrogen and antibiotic removal in wastewater polishing.
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CHENG Yuwei, LU Xixin, ZHANG Haiqing, HAO Yinli, QU Dan, LIU Yongze, JIN Rencai, HUANG Jinkun, ZHU Yan (2026). Performance of Iron-Carbon-Manganese Ore Composite Substrate Constructed Wetland for Simultaneous Removal of Nitrogen and Tetracycline. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202512052
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Frequently Asked Questions
What is the long-term stability of the Fe-C-Mn composite substrate regarding iron passivation and sustained removal efficiency?
The study demonstrates that Mn(IV) reduction to Mn(II) mitigates iron passivation by mediating electron transfer, as evidenced by increased surface roughness and active Fe(II)/Fe(III) and Mn(II) generation. While long-term data beyond the experimental period are not provided, the synergistic mechanism suggests sustained performance. However, pilot-scale studies over months are needed to confirm operational lifespan and potential need for substrate regeneration.
How does the Fe-C-Mn system achieve simultaneous removal of nitrate and tetracycline, and what are the dominant pathways?
Nitrate removal is primarily driven by autotrophic denitrification, facilitated by Fe(II) and Mn(II) as electron donors. Tetracycline removal involves both chemical degradation (via reactive oxygen species from iron-carbon micro-electrolysis) and biodegradation by enriched taxa like Trichosporon and Bacillota. The degradation pathway includes demethylation, hydroxylation, and ring-opening, leading to smaller metabolites. The synergy between chemical and biological processes ensures high removal efficiencies (91.3% TC, 83.3% NO3−-N).
What is the cost-effectiveness of the Fe-C-Mn composite substrate compared to conventional gravel, considering material and operational costs?
The Fe-C-Mn substrate uses iron-carbon pellets and manganese ore, which are relatively low-cost industrial materials. While the initial capital cost is higher than gravel, the significantly improved removal efficiencies (TC: 91.3% vs 27.2%; TN: 71.7% vs 7.8%) reduce the need for additional treatment steps, potentially lowering overall lifecycle costs. A detailed techno-economic analysis is not provided, but the enhanced performance justifies the investment for secondary effluent polishing.
How does the reduced microbial diversity in the Fe-C-Mn system affect its resilience to environmental fluctuations or shock loads?
The study shows that while overall diversity decreases, the system selectively enriches functional microorganisms (TC degraders and denitrifiers), which may enhance resistance to specific pollutants. However, reduced diversity could make the system more vulnerable to environmental changes (e.g., temperature, pH). The presence of chemical degradation mechanisms (Fe-C micro-electrolysis) provides a buffer, but further research is needed to assess resilience under dynamic loading conditions.
What are the potential scale-up challenges for implementing Fe-C-Mn CWs in full-scale wastewater treatment plants?
Scale-up challenges include ensuring uniform substrate distribution, preventing clogging due to iron oxide precipitation, and managing pH changes from iron corrosion. The study used a 9.7 L column; pilot-scale tests are necessary to evaluate hydraulic retention time, substrate longevity, and maintenance requirements. Additionally, the high TC concentration (2 mg·L−1) used may not reflect real effluent levels (μg·L−1), so performance at lower concentrations should be validated.
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