Researchers in Brazil have demonstrated that a hybrid advanced oxidation process can transform pharmaceutical-laden wastewater into an effluent that is dramatically safer for plants, earthworms, onion root cells, and beneficial microorganisms, offering a rigorous ecotoxicological case for reusing treated sewage in agriculture. The study, published in Environmental Science and Pollution Research, addresses a long-standing blind spot in water treatment: proving that a chemically “clean” effluent is also biologically compatible.

Conventional wastewater treatment plants were never designed to strip out recalcitrant organic micropollutants such as pharmaceuticals, and trace amounts routinely slip through into receiving waters. Advanced oxidation processes (AOPs) attack these stubborn compounds by generating highly reactive free radicals that fragment contaminant molecules. But chemical degradation alone does not guarantee safety. As contaminants break down, they spawn transformation products (TPs) that can, in some cases, be as toxic as—or more toxic than—the parent compounds. The research team, led by Lucas Gustavo da Costa and Alam Gustavo Trovó of the Federal University of Uberlândia, together with collaborators at the Oswaldo Cruz Institute, set out to answer a deceptively simple question: does measurable chemical removal of micropollutants actually translate into reduced biological harm?

The centerpiece of their work is the H₂O₂/S₂O₈²⁻/UVC process, a hybrid system that combines hydrogen peroxide and persulfate with short-wave ultraviolet C radiation. UVC photons cleave both oxidants simultaneously, generating hydroxyl radicals (HO•) and sulfate radicals (SO₄•⁻) in the same reaction volume. This dual-radical strategy outperforms systems relying on either oxidant alone, or on peroxymonosulfate (HSO₅⁻), which is costlier and demands more energy for activation. The hybrid route also resists interference from inorganic species commonly found in real effluents, making it an economically attractive candidate for deployment at full-scale treatment plants. The process had already been chemically optimized in the team’s earlier work using multivariate mixture design and rotatable central composite design experiments; the new study is its first integrated biological stress test.

The experiments used real municipal effluent from a wastewater treatment plant in Uberlândia, Minas Gerais, sampled after the plant’s final tertiary treatment stage of coagulation-flocculation with ferric chloride and flotation. The effluent was enriched with three pharmaceuticals representing different therapeutic classes: colchicine (COL, an antimitotic agent), nitazoxanide (NTZ, an antiparasitic), and sulfamethoxazole (SMX, a widely detected antibiotic), each at 325 nmol L⁻¹—equivalent to 130, 100, and 82 micrograms per liter, respectively. That concentration was deliberately chosen as high enough for direct HPLC–DAD analysis without preconcentration, yet low enough to be environmentally representative. Treatment was carried out in an amber glass reactor irradiated by two 8-watt UVC mercury lamps emitting at 254 nm, with a measured irradiance of 4.9 W m⁻². Residual oxidants were neutralized with sodium thiosulfate before any biological testing, ensuring that observed effects could not be attributed to leftover peroxide chemistry.

The ecotoxicological battery spanned multiple trophic levels and levels of biological organization. Oxidative stress was assessed in the earthworm Eisenia andrei by measuring malondialdehyde (MDA), a marker of lipid peroxidation, and protein carbonylation, an irreversible oxidative modification of proteins. Phytotoxicity was evaluated using lettuce (Lactuca sativa) seeds and a growth index combining germination rate and root elongation. Cytotoxicity and genotoxicity were quantified in onion (Allium cepa) root meristems through the mitotic index and the frequency of chromosomal and nuclear aberrations—micronuclei, chromosome breaks, stickiness, bridges, and nuclear buds—across 5,000 cells per sample. Finally, growth inhibition was tested in two environmentally relevant microbes: Azospirillum brasilense, a plant growth-promoting bacterium used in soybean cultivation, and Saccharomyces cerevisiae, a yeast that persists in soils and fermentative niches.

The untreated enriched effluent told a worrying story. It exhibited substantial genotoxicity in onion cells—21%, approaching the 24% seen with the positive control, methyl methanesulfonate—and severe phytotoxicity, with lettuce growth indices far below the 80% threshold that signals absence of toxicity. Even the unspiked effluent itself suppressed lettuce growth, achieving a growth index of only 47%, evidence that bioactive compounds survive conventional treatment. In earthworms, individual aqueous solutions of NTZ and SMX raised MDA levels by 37.2% and 23.5% respectively, while the three-compound mixture increased lipid peroxidation by 38.0%, pointing to additive or synergistic oxidative stress mechanisms involving reactive oxygen species and Fenton-type chemistry within cells.

The picture changed decisively after oxidation. Following 10 minutes of UVC-driven treatment—the point at which 80% chemical degradation had been achieved, matching the minimum removal target proposed in the European Union’s COM(2022)541 directive—genotoxicity fell to 10%, and after 20 minutes, corresponding to the limit of quantification for the parent compounds, it dropped further to 6%, a statistically significant reduction. The plant growth index climbed above 80%, crossing from toxic territory into biocompatibility. The mitotic index in onion cells remained statistically indistinguishable from the deionized-water control (around 39–40% versus 40%), demonstrating that the process generated no cytotoxic byproducts capable of arresting cell division. And crucially for agricultural applications, neither the treated effluent nor its transformation products inhibited growth of A. brasilense or S. cerevisiae.

The mechanistic details are instructive. In the untreated enriched effluent, micronuclei—membrane-bound DNA fragments expelled from the nucleus during flawed cell division—were among the most frequent aberrations, indicating clastogenic and aneugenic damage. Colchicine exposure predictably produced C-metaphase accumulation, a signature of its tubulin-binding, spindle-disrupting mechanism first described in Allium nearly a century ago. SMX depressed the mitotic index and induced chromosomal damage, consistent with prior findings in Vicia faba. NTZ, whose genotoxicity had never before been reported, produced chromosome breaks, stickiness, and nuclear buds. After oxidation, the overall aberration burden fell sharply, particularly micronuclei and stickiness, although a few bridges and polyploid cells persisted even at 20 minutes—a reminder that some transformation products or resistant residuals may linger.

The microbial results came with a subtlety. Reduced A. brasilense growth after treatment initially looked like a red flag, but the team attributes it to depletion of assimilable organic matter, which serves as radical scavenger during oxidation and as a nutrient source afterward—not to toxicity. The negative control likewise grew slowly. This distinction matters because AOPs are designed to mineralize organic carbon, and a nutrient-poorer medium should not be misread as a more toxic one. For S. cerevisiae, no inhibition occurred at any stage of treatment, although NTZ alone significantly depressed yeast optical density, reinforcing the compound’s cell-cycle interference potential even at nanomolar-scale exposures.

The study’s broader significance lies in its methodological stance. By adopting an “effect-driven approach,” the team evaluated the toxicity of whole reaction mixtures without needing to isolate and identify individual transformation products—an impractical task given that many degradation byproducts are not commercially available. Their findings align with a growing consensus that treatment efficacy must be judged not merely by parent-compound removal but by the nature and reactivity of the resulting transformation products. Biochemical biomarkers such as MDA and protein carbonylation detected sublethal disturbances that mortality-based endpoints would have missed entirely, providing early warnings of cellular distress at environmentally relevant concentrations.

One caveat deserves attention: while lipid peroxidation declined after treatment, protein carbonylation rose slightly—1.05-fold above control—suggesting some transformation products may still provoke protein oxidation even as lipid damage subsides. The authors flag this discrepancy and call for future identification of the specific TPs responsible, alongside expanded organism panels, longer-term exposure studies, and pilot-scale validation across different environmental matrices.

The regulatory context sharpens the urgency. The EU’s proposed urban wastewater treatment directive mandates at least 80% removal of specified organic micropollutants and microbiological control for agricultural reuse, yet it conspicuously omits eco-compatibility assessment of the treated water itself. This study supplies exactly that missing dimension, showing that a process satisfying the chemical benchmark also delivers measurable biological benefit—lower genotoxicity, restored plant growth, intact cell division, and unharmed beneficial microbes. As water scarcity intensifies globally and reuse becomes less optional, the Brazilian team’s integrated bioassay framework offers a template for ensuring that the water farmers irrigate with is not merely chemically compliant, but genuinely ecologically safe.

Antibiotic contamination in natural water sources has emerged as a pressing environmental crisis, posing significant risks to both ecosystems and public health worldwide. Conventional water treatment methodologies often fall short of efficiently eliminating persistent antibiotic residues, which frequently enter water bodies through human and animal excretion. These residual contaminants, such as levofloxacin and other widely used antibiotics, resist degradation and can perpetuate antimicrobial resistance, thereby escalating the urgent need for innovative and sustainable remediation technologies.

In a groundbreaking development, a team of researchers led by Dr. Jiafang Xie at the Institute of Urban Environment, Chinese Academy of Sciences, has engineered a novel biochar-based catalyst derived from rice husks — an abundant agricultural byproduct — that demonstrates unprecedented efficacy in the rapid degradation of antibiotics under environmentally benign conditions. Their study, published in Biochar, reveals that the cobalt oxide-loaded biochar catalyst can instantaneously activate peroxymonosulfate to achieve complete breakdown of levofloxacin in merely four minutes at neutral pH, marking a transformative leap in wastewater treatment science.

The synthesis of this biochar catalyst, designated as RHBA800@25Co3O4, involved the meticulous preparation of oxygen-rich activated biochar from rice husk biomass followed by the strategic dispersion of cobalt oxide (Co3O4) nanoparticles across its porous matrix. This synergistic structural design produces a highly reactive and accessible catalytic surface, effectively combining the high surface area and functional group abundance of biochar with the potent oxidative capabilities of cobalt oxide nanostructures. This optimization paves the way for superior interaction with peroxymonosulfate, an oxidant known for its potential in advanced oxidation processes.

Performance assessments extended beyond controlled laboratory conditions to real-world aqueous environments, where the catalyst maintained remarkably high degradation efficiencies. Trials conducted in various water samples—ranging from lake and tap water to secondary effluent discharged from municipal sewage treatment plants—confirmed its robust activity and versatility. Furthermore, in a custom-designed fixed-bed reactor, the catalyst demonstrated sustained functionality over 72 consecutive hours, highlighting its stability and practicality for continuous flow water purification systems.

A central scientific breakthrough of this research lies in elucidating the elusive catalytic mechanism underpinning Co3O4-mediated peroxymonosulfate activation, a topic previously clouded in uncertainty. Employing a combination of sophisticated in situ Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations, the investigators identified lattice oxygen within Co3O4 as a pivotal contributor to the generation of reactive intermediates. Notably, under reaction conditions, lattice oxygen induces formation of a novel surface species, Co3O4−α–OH, which exhibits stronger affinity for peroxymonosulfate molecules and facilitates accelerated electron transfer essential for rapid oxidative reactions.

This discovery not only unravels the crucial role of lattice oxygen chemistry in cobalt oxide catalysts but also resonates with the catalyst’s observed ultrafast kinetics. The biochar support enhances cobalt oxide dispersion and prevents nanoparticle aggregation, optimizing active site availability. Meanwhile, the transformation of lattice oxygen into hydroxylated intermediates intensifies catalytic efficiency by promoting quicker and more effective peroxymonosulfate activation through both radical and non-radical pathways, including the generation of sulfate radicals, hydroxyl radicals, and singlet oxygen species.

Importantly, the degradation process exhibited comprehensive antibiotic elimination with reduced toxicity in the resulting solution. Analytical examination of transformation products revealed that the byproducts formed possess significantly lower antimicrobial activity. Correspondingly, bacteriological tests employing Escherichia coli demonstrated that treated levofloxacin solutions lacked antibacterial inhibition zones compared to untreated samples. This validates the catalyst’s dual role in not only cleaving antibiotic molecules but also mitigating potential ecological hazards associated with harmful metabolites.

The implications of this research extend beyond immediate water purification applications, underscoring a sustainable and circular approach to environmental remediation. By valorizing agricultural residues like rice husk into high-performance catalytic materials, the study exemplifies the integration of waste management with advanced chemical technology. This aligns with global efforts to address pollution while fostering resource efficiency and environmental stewardship through biochar innovation.

Dr. Xie emphasizes that the fusion of agriculture-derived biochar and transitional metal oxides represents a promising frontier in catalysis, where material design and mechanistic insights coalesce to solve practical challenges. The precise identification of Co3O4−α–OH intermediates from lattice oxygen transformation not only advances fundamental knowledge but also guides future catalyst development for broader environmental and industrial applications requiring efficient oxidation chemistry.

Looking ahead, the research team envisions deploying this cobalt oxide biochar catalyst in larger-scale water treatment infrastructures to tackle widespread antibiotic contamination issues. The demonstrated durability and performance in mixed and complex water matrices suggest high feasibility for real-world implementation. This technological innovation paves the way for rapid, cost-effective, and environmentally friendly solutions to safeguard water quality and public health on a global scale.

Ultimately, this study heralds a new era where interdisciplinary research marries the principles of materials science, environmental engineering, and catalysis to combat the persistent pollutant load burdening aquatic systems. The remarkable speed and efficacy of the RHBA800@25Co3O4 catalyst in simultaneously activating peroxymonosulfate and degrading antibiotics redefines potential benchmarks for next-generation water treatment technologies, inviting further exploration and adaptation in the fight against pollution and antimicrobial resistance.

Subject of Research: Experimental study on the development and mechanistic analysis of cobalt oxide-loaded rice husk biochar catalyst for rapid antibiotic degradation in water.

Article Title: In situ observation of Co3O4−α–OH formation on optimized biochar for peroxymonosulfate activation and ultrafast antibiotics degradation

News Publication Date: 16 June 2026

Web References:
Journal Biochar: https://link.springer.com/journal/42773
DOI: http://dx.doi.org/10.1007/s42773-026-00634-8

References:
Zhang, J., Xie, J., Zhu, S. et al. In situ observation of Co3O4−α–OH formation on optimized biochar for peroxymonosulfate activation and ultrafast antibiotics degradation. Biochar 8, 113 (2026). https://doi.org/10.1007/s42773-026-00634-8

Image Credits: Jian Zhang, Jiafang Xie, Shuhui Zhu, Jiacheng E. Yang, Bo Weng & Yuming Zheng

Keywords

Biochar, cobalt oxide, Co3O4−α–OH, antibiotic degradation, peroxymonosulfate activation, wastewater treatment, advanced oxidation processes, levofloxacin, rice husk catalyst, environmental remediation, antimicrobial resistance, catalytic mechanism