Waste Eggshells and Cotton Stalks Turned Into Antibiotic-Capturing Biochar

A new adsorbent built from two waste streams—cotton stalks and eggshells—can pull tetracycline, one of the world’s most heavily used antibiotics, out of water with a maximum capacity of 161.91 milligrams per gram. The material, called Ca@CBC/β-CD, combines calcium-rich biochar with β-cyclodextrin, a sugar-derived molecule that acts as a molecular host, and was prepared using microwave-assisted crosslinking. Researchers report that the adsorbent retained about 84–86% of its initial capacity after five regeneration cycles and tolerated common coexisting ions, while a life-cycle assessment put the preparation-stage footprint at 5.44 kilograms of CO₂-equivalent per kilogram of adsorbent, with electricity as the dominant hotspot.

The work, published as open-access original research in the journal Biochar, addresses a persistent environmental problem. Tetracycline is a broad-spectrum antibiotic used extensively in livestock and aquaculture because it is inexpensive and effective against bacterial infections. Animals excrete the drug and its transformation products directly in feces and urine, contaminating surface water, groundwater, and soils. One recent study cited by the authors found average tetracycline contamination in European waters ranging from 0 to 20 nanograms per liter. Although adsorption is attractive for water treatment because it is simple, low-cost, low-energy, and carries minimal risk of secondary pollution, existing adsorbents often fall short on cost, regeneration, or capacity, keeping the search for better materials alive.

The synthesis route is deliberately built around waste. Cotton stalk powder was rinsed, dried at 105 °C for 12 hours, ground, and sieved, then carbonized in a tube furnace under nitrogen flow at 600 °C for 3 hours to produce cotton-stalk biochar, or CBC. Waste eggshells collected from dining facilities were stripped of their inner membrane, washed, dried, and ground the same way. Eggshell powder and cotton stalk powder were blended at a 1:1 mass ratio and co-carbonized under identical conditions, yielding the calcium-rich material Ca@CBC. In the final step, β-cyclodextrin and sodium hydroxide were dissolved in water, mixed with the biochar and a small amount of glutaraldehyde crosslinker, and irradiated in a microwave at 350 watts for just 15 minutes to graft the cyclodextrin onto the carbon surface.

The microwave step proved decisive in a side-by-side comparison. Using the same batch of Ca@CBC precursor and the same grafting formulation, the researchers varied only the heating mode: the microwave route finished the functionalization in 15 minutes, while a conventional water bath needed 120 minutes. Electricity consumption rose from roughly 0.09 kilowatt-hours for the microwave route to about 0.60 kilowatt-hours for the water-bath route, and the microwave-grafted sample also showed a higher adsorption capacity under standard conditions. Because the precursor biochar was made identically in both cases, the differences trace to the heating mode during β-CD immobilization rather than to the initial carbonization, making microwave irradiation a clear time- and energy-saving intensification strategy.

Characterization revealed why the combined modification works so well. Pristine CBC had a modest BET surface area of 15.720 square meters per gram, but adding eggshell-derived calcium raised it to 28.789 square meters per gram while shrinking the average pore diameter from 11.460 to 2.780 nanometers, indicating the generation of micropores. Grafting β-cyclodextrin onto CBC increased the surface area to 44.312 square meters per gram and enlarged pore volume, while the full combination, Ca@CBC/β-CD, achieved the highest surface area of all four materials at 83.869 square meters per gram. X-ray diffraction showed that calcium remained as calcite-type CaCO₃ after carbonization at 600 °C under nitrogen, with no detectable CaO or Ca(OH)₂, and that this crystalline phase persisted through grafting. Scanning electron microscopy showed a rough surface studded with bright calcium-containing particulates after co-carbonization, and a polymeric coating anchored on those particulates after cyclodextrin grafting.

Batch adsorption tests pinpointed the operating window. The material performed best at approximately pH 6, where tetracycline exists mainly in forms favorable for both host–guest inclusion with β-cyclodextrin and bridging or inner-sphere complexation with the material’s functional groups and Ca²⁺ sites. Capacity stayed high between pH 3 and 7 but dropped significantly at higher pH, where electrostatic repulsion between the negatively charged adsorbent surface and anionic tetracycline species intensifies. Kinetics followed a rapid initial uptake that approached equilibrium over roughly 3 hours, and the pseudo-second-order model fit the data better than the pseudo-first-order model, with R² values of 0.9755 versus 0.9514, suggesting the rate is governed largely by chemical interactions and active site occupancy. Segmented Weber-Morris analysis confirmed a multi-stage process in which film diffusion gives way to intra-particle diffusion and finally a reaction-coupled diffusion equilibrium, so intra-particle diffusion is not the sole rate-limiting step.

Isotherm data told a consistent story. The Langmuir model, which assumes predominantly monolayer coverage on approximately homogeneous sites, fit better than the Freundlich model at pH 6 and a dosage of 0.5 grams per liter. The maximum adsorption capacity rose monotonically with temperature, from 142.36 milligrams per gram at 25 °C to 153.11 at 35 °C and 161.91 at 45 °C, consistent with an endothermic process in which desolvation and conformational matching promote Ca²⁺-bridging complexation and cyclodextrin inclusion. The separation factor of 0.0331 at an initial concentration of 50 milligrams per liter indicated favorable adsorption. The adsorbent also showed good tolerance to common coexisting ions, though inhibition followed clear orders: phosphate suppressed uptake most among anions because it forms sparingly soluble salts with Ca²⁺ and directly occupies calcium sites, while among cations the order was Ca²⁺ > Mg²⁺ > Na⁺ ≈ K⁺, with divalent ions reducing capacity through electrical double-layer compression and competitive complexation. Humic acid, Fe³⁺, and Al³⁺ caused stronger suppression still, attributed to solution complexation with tetracycline, competitive coordination at calcium sites, and partial masking of accessible adsorption domains.

Spectroscopy and quantum-level calculations converged on the mechanism. X-ray photoelectron spectroscopy showed the low-binding-energy O 1s component, associated with inner-sphere Ca–O coordination, growing from 64.5% to 79.5% of the envelope after adsorption, while Ca 2p peaks shifted only modestly by about 0.13 electron-volts with unchanged spin–orbit splitting, indicating calcium remained as Ca²⁺ and adsorption altered its coordination environment rather than its valence. FT-IR spectra showed carbonate peaks at 1420 and 880 wavenumbers attenuating after adsorption and the broad O–H band near 3400 wavenumbers broadening and red-shifting, signaling reinforced hydrogen bonding. Density functional theory calculations at the dispersion-corrected ωB97X-D/def2-TZVP level with a water solvation model, combined with independent gradient model and Hirshfeld surface analyses, visualized the contacts directly: electrostatic potential maps showed highly negative pockets on tetracycline’s carbonyl and phenolate oxygens opposing the strongly positive region around calcium, and interaction isosurfaces revealed compact Ca–O basins in chelating conformations and double hydrogen bonds at the β-cyclodextrin rim in a third conformation. Together, the evidence points to synergy among Ca²⁺ inner-sphere complexation and surface bridging, β-CD host–guest inclusion, and multi-point hydrogen bonding, with π–π stacking and hydrophobic enrichment as secondary contributions. Calcium leaching tests showed dissolved Ca²⁺ stayed well below 1 milligram per liter during the 3-hour equilibrium period, indicating the dominant calcium contribution is surface-confined coordination rather than homogeneous complexation in the bulk solution.

Machine learning added a predictive layer. The researchers evaluated six supervised algorithms on 68 experimental data points spanning pH, initial concentration, dosage, temperature, and contact time, using Bayesian optimization with five-fold cross-validation. The gradient boosting decision tree performed best on the test set, with R² = 0.9914 and the lowest root-mean-square error at 3.8439 milligrams per gram, outperforming random forest, LightGBM, CatBoost, Elastic Net, and TabPFN. SHAP analysis attributed 68.7% of the model’s explanatory power to initial tetracycline concentration, followed by dosage at 13.5% and contact time at 12.4%, with pH at 3.8% and temperature at 1.6% playing secondary roles. The team also deployed the trained model in a Python-based graphical interface that lets users adjust the five parameters and obtain real-time capacity predictions. On the sustainability side, the cradle-to-gate life-cycle assessment, modeled in SimaPro with the ecoinvent database under the CML-IA baseline method, found electricity accounted for 62.4% of the 5.44-kilogram CO₂-equivalent footprint, followed by transport at 13.6% and β-cyclodextrin at 10.4%, while the midpoint profile was led by marine ecotoxicity, then human toxicity and freshwater ecotoxicity, burdens traced mainly to the coal-heavy electricity mix, chlor-alkali sodium hydroxide, and upstream reagent inventories. An economic estimate placed the unit direct cost at roughly $1.2 per kilogram under optimized conditions, within the reported price range for commercial biochar of about $0.449 to $1.847 per kilogram. The authors frame the work as a proof-of-concept under a fixed preparation route and note that continuous-flow fixed-bed column testing and systematic optimization of the eggshell-to-stalk ratio, β-CD dosage, and grafting conditions remain for future work.

Subject of Research: Development of a β-cyclodextrin-functionalized calcium-rich biochar from cotton stalks and eggshells for tetracycline removal from wastewater, evaluated with adsorption experiments, DFT calculations, machine learning, and life-cycle assessment

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

Article References: Liu, C., Crini, G., Bello-Mendoza, R., Wilson, L. D., Jawad, A. H., Balasubramanian, P., Nguyen, X. C., Zheng, Q., & Li, F. (2026). Microwave-assisted β-cyclodextrin modified calcium-rich biochar for tetracycline removal from wastewater: mechanistic, machine learning, density functional theory calculations and life cycle assessment. Biochar, 8(1), Article 124. https://doi.org/10.1007/s42773-026-00640-w

Image Credits: AI Generated

DOI: 10.1007/s42773-026-00640-w

Keywords: biochar, tetracycline, adsorption, β-cyclodextrin, eggshells, cotton stalks, microwave synthesis, machine learning, density functional theory, life cycle assessment, water treatment, antibiotic pollution