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

The removal of antibiotics from water using sustainable and cost-effective methods remains an environmental challenge. In this study, cotton-stalk biochar (CBC) was used as a substrate and waste eggshells as a calcium source to prepare a β-cyclodextrin-functionalized adsorbent (Ca@CBC/β-CD) via microwave-assisted crosslinking. The obtained material was used for tetracycline (TC) removal from water. Experimental results showed that Ca@CBC/β-CD exhibited the best adsorption performance at approximately pH = 6, and the adsorption kinetics were well described by the pseudo-second-order model. The adsorption isotherm followed the Langmuir model, with the maximum adsorption capacity increasing from 142.36 mg g−1 at 25 °C to 161.91 mg g−1 at 45 °C. The adsorbent also showed good tolerance to common coexisting ions and retained about 84–86% of its initial capacity after five regeneration cycles. Spectroscopic characterization combined with density functional theory (DFT) calculations revealed that TC adsorption was governed by the synergistic contribution of Ca2⁺ inner-sphere complexation/surface bridging, β-CD host–guest inclusion, and multi-point hydrogen bonding. Among the tested machine-learning models, the gradient boosting decision tree showed the best predictive performance (test-set R2 = 0.9914) and identified initial concentration, adsorbent dosage, and contact time as the key adsorption factors. Life-cycle assessment further indicated that the preparation stage generated 5.44 kg CO₂-eq per kg adsorbent, with electricity being the primary hotspot. Overall, Ca@CBC/β-CD represents an efficient, reusable, and relatively sustainable adsorbent for TC removal from water.