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Microwave-assisted β-cyclodextrin modified calcium-rich biochar for tetracycline removal from wastewater: mechanistic, machine learning, density functional theory calculations and life cycle assessment

Credit: Chong Liu, Grégorio Crini, Ricardo Bello-Mendoza, Lee D. Wilson, Ali H. Jawad, Paramasivan Balasubramanian, Xuan Cuong Nguyen, Qingfu Zheng & Fayong Li

Antibiotic residues in water are an increasing environmental concern, especially as widely used medicines can pass through human, livestock, and aquaculture systems and enter rivers, groundwater, and soils. Tetracycline, a common broad-spectrum antibiotic, is one such pollutant. Although effective for treating bacterial infections, it can persist in the environment and contribute to ecological risks.

Now, researchers have developed a promising material that turns two abundant wastes, cotton stalks and discarded eggshells, into a high-performance adsorbent for removing tetracycline from contaminated water. The study, published in Biochar, presents a β-cyclodextrin modified calcium-rich cotton-stalk biochar, named Ca@CBC/β-CD, prepared through a microwave-assisted crosslinking process.

“Our goal was to design a practical adsorbent that is not only efficient, but also built from low-cost and waste-derived resources,” said corresponding author Prof. Fayong Li. “By combining cotton-stalk biochar, eggshell-derived calcium, and β-cyclodextrin, we created multiple ways for the material to capture tetracycline molecules.”

The new material works through a combination of physical and chemical interactions. The cotton-stalk biochar provides a porous carbon framework. Eggshell-derived calcium introduces active mineral sites. β-cyclodextrin, a ring-shaped molecule produced from starch, adds cavity-like structures and hydroxyl groups that can help trap organic pollutants.

In laboratory tests, Ca@CBC/β-CD showed its best tetracycline adsorption near pH 6, a condition relevant to many natural and wastewater systems. Its maximum adsorption capacity reached 161.91 mg g⁻¹ at 45 °C, higher than the capacity measured at lower temperature. The material also showed good resistance to common coexisting ions in water and maintained about 84 to 86% of its initial adsorption capacity after five reuse cycles, suggesting potential for repeated operation.

To understand why the adsorbent performed well, the team used spectroscopic analyses and density functional theory calculations. The results showed that tetracycline removal was driven by calcium-mediated inner-sphere complexation and surface bridging, β-cyclodextrin host-guest inclusion, and multi-point hydrogen bonding. In simpler terms, tetracycline molecules are not captured by just one mechanism. They are held by several cooperative interactions across the biochar surface.

The study also incorporated machine learning to predict adsorption behavior under different experimental conditions. Among six tested models, the gradient boosting decision tree model performed best, achieving a test-set R² of 0.9914. The model identified initial tetracycline concentration, adsorbent dosage, and contact time as the most important factors controlling adsorption capacity. The researchers also developed a Python-based graphical interface for rapid prediction and experimental design.

“Machine learning helped us move beyond single-factor experiments,” said co-author Prof. Grégorio Crini. “It provides a faster way to estimate performance and can guide future optimization before costly experiments are performed.”

The researchers further evaluated the environmental footprint of producing the adsorbent. A life cycle assessment found that the preparation stage generated 5.44 kg CO₂-equivalent per kg of adsorbent, with electricity use as the main environmental hotspot. This points to a clear improvement pathway: reducing energy demand during production could make the material more sustainable.

The authors note that the work is a proof of concept and that future studies should optimize the eggshell-to-biochar ratio, β-cyclodextrin dosage, microwave conditions, and test the material in continuous-flow systems.

Together, the findings offer a waste-to-resource strategy for antibiotic pollution control and provide a design framework for developing next-generation biochar adsorbents for real-world wastewater treatment.