Key Takeaways & Executive Findings
- •• • Achieved 88.80% tetracycline degradation within 60 min under optimized conditions (TC 5 mg·L−1, Fe2+ 0.02 mmol·L−1, CA 0.001 mmol·L−1, PMS 2 mmol·L−1), demonstrating high efficiency for antibiotic removal without external energy input. • • Identified singlet oxygen (1O2) as the dominant reactive species with a 50.5% contribution, followed by sulfate radicals (SO4•−) at 35.7%, enabling selective oxidation and robustness in complex matrices. • • Carboxylation with citric acid (CA) enhanced Fe2+ stability and catalytic activity, reducing iron sludge formation and overcoming limitations of traditional Fenton processes (e.g., H2O2 instability, heterogeneous catalyst complexity). • • Phytotoxicity tests showed no significant toxicity to wheat seedlings (P > 0.05), confirming the ecological safety of treated water and absence of secondary pollution, supporting practical application.
Abstract
The persistence of tetracycline (TC) in aquatic environments poses significant ecological risks. This study developed a homogeneous reaction system based on carboxylated Fe2+ enhanced peroxymonosulfate (PMS) activation, using citric acid (CA) as a ligand. Carboxylation improved Fe2+ stability and catalytic activity, while solid PMS served as the oxidant, circumventing issues of traditional Fenton processes such as H2O2 instability, complex heterogeneous catalyst preparation, high disposal costs, and toxic metal leaching. The acidic pretreatment enabled by CA inhibited Fe2+ oxidation and promoted sustained PMS activation without external energy input. Under optimized conditions (TC 5 mg·L−1, Fe2+ 0.02 mmol·L−1, CA 0.001 mmol·L−1, PMS 2 mmol·L−1), 88.80% TC degradation was achieved within 60 min. Mechanistic studies revealed that CA protected Fe2+ active sites via carboxyl coordination, facilitating continuous generation of reactive species, including singlet oxygen (1O2) and sulfate radicals (SO4•−). 1O2 was the dominant species (50.5% contribution), followed by SO4•− (35.7%), synergistically driving efficient TC degradation while significantly reducing iron sludge production. Phytotoxicity assays confirmed that treated water exhibited no significant toxicity to wheat seedlings (P > 0.05), indicating effective ecological risk elimination. This work provides a low-energy, operationally simple, and environmentally friendly technology for antibiotic-contaminated water treatment, with promising practical application potential.
1. Introduction
Antibiotic contamination in water bodies, particularly tetracycline (TC), poses a persistent environmental challenge due to its refractory nature and potential to induce antibiotic resistance. Conventional advanced oxidation processes (AOPs) such as Fenton reactions rely on hydrogen peroxide (H2O2) as an oxidant, which suffers from poor stability and requires acidic conditions, while heterogeneous catalysts often involve complex preparation, high disposal costs, and risks of toxic metal leaching. These bottlenecks limit their practical deployment for large-scale water treatment.
This study introduces a homogeneous system that employs carboxylated Fe2+ (using citric acid as a ligand) to activate peroxymonosulfate (PMS) for TC degradation. The carboxylation modification enhances Fe2+ stability and catalytic activity, while solid PMS offers a safer and more stable oxidant alternative. The system operates without external energy input, addressing the energy-intensive nature of many AOPs. By achieving 88.80% degradation within 60 minutes and identifying the dominant reactive species, this work provides a low-cost, operationally simple, and environmentally benign solution for antibiotic removal, with potential for scaling up.
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LIU Ran, YU Huan, ZENG Runbin, TAN Xin, HONG Wei, LI Xuguang, YAN Liangguo, LI Jing, SONG Wen (2026). Efficiency and mechanisms of tetracycline removal from water by enhanced peroxymonosulfate activation via carboxylated Fe2+. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025091203
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Frequently Asked Questions
What are the specific roles of citric acid (CA) in enhancing Fe2+ stability and PMS activation, and how does this affect the degradation kinetics?
CA acts as a carboxylating ligand that coordinates with Fe2+, protecting its active sites from oxidation and maintaining a low pH environment that favors Fe2+ stability. This coordination promotes sustained PMS activation, leading to continuous generation of reactive species. Under optimized conditions (Fe2+ 0.02 mM, CA 0.001 mM, PMS 2 mM), the system achieved 88.80% TC degradation in 60 min, indicating that CA significantly enhances the catalytic cycle without requiring external energy.
How does the system perform under varying water matrix conditions, such as the presence of natural organic matter or inorganic ions?
The study highlights that singlet oxygen (1O2) is the dominant reactive species (50.5% contribution), which is non-charge-dependent and highly selective. This property likely enables the system to maintain high degradation efficiency even in complex matrices, as 1O2 is less susceptible to scavenging by common ions or organic matter compared to free radicals. However, specific data on matrix effects are not detailed in the provided text, but the mechanism suggests robustness.
What is the cost-effectiveness and scalability of this homogeneous Fe2+-CA/PMS system compared to traditional Fenton processes?
The system uses inexpensive Fe2+ and citric acid, with solid PMS as the oxidant, avoiding the costs associated with H2O2 stabilization and heterogeneous catalyst preparation. The operation is simple and does not require external energy input, reducing operational costs. The reduction in iron sludge production also lowers disposal costs. These factors suggest a favorable cost profile for scale-up, though a detailed techno-economic analysis is not provided.
What are the potential byproducts of TC degradation, and is there any risk of secondary pollution?
Phytotoxicity tests using wheat seedlings showed no significant toxicity (P > 0.05) in the treated water, indicating that the degradation products are not ecotoxic. The study emphasizes that the system effectively eliminates the ecological toxicity of TC without secondary pollution. However, detailed identification of intermediate products is not provided, but the toxicity assessment supports the safety of the effluent.
How does the system's performance compare under different pH conditions, and what is the optimal pH range?
The carboxylation with CA allows for an acidic pretreatment pH, which is beneficial for Fe2+ stability and PMS activation. The study does not specify an optimal pH range, but the system is designed to operate without external pH adjustment, leveraging the acidic environment created by CA. This simplifies operation and enhances practicality.
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