Eggshells and Cotton Stalks Converted Into Antibiotic-Capturing Biochar

Researchers have developed an innovative adsorbent material, Ca@CBC/β-CD, using waste cotton stalks and eggshells, capable of removing tetracycline from water with a maximum capacity of 161.91 milligrams per gram. This material combines calcium-rich biochar with β-cyclodextrin, a sugar-derived molecule, through microwave-assisted crosslinking. The adsorbent retains 84–86% of its initial capacity after five regeneration cycles and withstands common coexisting ions. Its life-cycle assessment reveals a preparation-stage footprint of 5.44 kilograms of CO₂-equivalent per kilogram, primarily due to electricity use.

Published in the journal Biochar, this research addresses the environmental issue of tetracycline contamination, a broad-spectrum antibiotic widely used in livestock and aquaculture. Animals excrete tetracycline, leading to contamination of water and soil. Adsorption is a promising water treatment method due to its simplicity and low cost, but existing adsorbents often lack in cost-effectiveness, regeneration, or capacity.

The synthesis of Ca@CBC/β-CD focuses on waste utilization. Cotton stalk powder is processed and carbonized to create cotton-stalk biochar (CBC). Eggshells are similarly processed and combined with cotton stalk powder in a 1:1 ratio, then co-carbonized to produce the calcium-rich Ca@CBC. β-cyclodextrin is grafted onto this biochar using a microwave at 350 watts for 15 minutes, significantly reducing the time and energy compared to conventional methods.

The microwave-assisted method enhances adsorption capacity and efficiency. The precursor biochar is identical in both microwave and conventional methods, but the microwave route uses less electricity and achieves higher adsorption capacity. This suggests that the heating mode during β-CD immobilization is crucial.

Characterization of the material shows that adding eggshell-derived calcium increases the BET surface area from 15.720 to 28.789 square meters per gram and reduces pore diameter, creating micropores. Grafting β-cyclodextrin further increases the surface area to 44.312 square meters per gram. The final product, Ca@CBC/β-CD, achieves the highest surface area of 83.869 square meters per gram. X-ray diffraction confirms the presence of calcite-type CaCO₃, and scanning electron microscopy reveals a rough surface with calcium particulates.

Batch adsorption tests indicate the material performs best at pH 6, where tetracycline is most effectively captured through host–guest inclusion and complexation with functional groups and Ca²⁺ sites. The adsorption capacity remains high between pH 3 and 7 but decreases at higher pH levels. Kinetic studies show a rapid initial uptake, with equilibrium reached in about 3 hours, and the pseudo-second-order model fits the data best.

Isotherm data supports these findings, with the Langmuir model fitting better than the Freundlich model at pH 6. The maximum adsorption capacity increases with temperature, indicating an endothermic process. The adsorbent tolerates common ions well, though phosphate and certain cations reduce capacity through competitive interactions.

Spectroscopy and quantum calculations reveal the adsorption mechanism, involving Ca²⁺ inner-sphere complexation, β-CD host–guest inclusion, and hydrogen bonding. Calcium leaching tests show minimal dissolved Ca²⁺, indicating surface-confined coordination.

Machine learning enhances predictive capabilities. Six algorithms were tested, with gradient boosting decision tree performing best. SHAP analysis highlights initial tetracycline concentration as the most influential factor. A Python-based interface allows users to predict capacity based on various parameters.

The life-cycle assessment shows electricity as the main contributor to the carbon footprint, followed by transport and β-cyclodextrin. Economic estimates place the cost at $1.2 per kilogram, aligning with commercial biochar prices. The study serves as a proof-of-concept, with future work needed on continuous-flow testing and optimization of synthesis conditions.