Enhanced Tetracycline Hydrochloride Removal by Ultra-microporous Phosphorus-Doped KOH-Activated Microalgal Biochar: Adsorption Performance and Mechanistic Insights

The overuse of the broad-spectrum antibiotic tetracycline hydrochloride (TCH) in humans and animals poses serious risks to environmental safety and human health. Adsorption offers a convenient yet effective way to lower its concentration. The KOH-activated Chlorella vulgaris -derived biochar (CVBC) was co-doped with Ca or P using a simple impregnation-pyrolysis method to modify its physicochemical properties. The P-doped KOH-activated Chlorella vulgaris -derived biochar (P-KOH/CVBC) achieved a maximum TCH adsorption capacity of 662.91 mg/g (Langmuir isotherm model, initial TCH concentration 100 mg/L, 303 K, dosage of 200 mg/L), which is higher than most of the adsorbents. Moreover, P-KOH/CVBC exhibited a superior pH buffering capacity and enhanced resistance to ionic interference. This is because the adsorption mechanism of TCH onto P-KOH/CVBC is dominated by π-π EDA interactions, whereas that for Ca-KOH/CVBC is primarily through metal complexation. The π-π EDA interactions not only enhance the adsorption capacity of P-KOH/CVBC for TCH but also contribute to its strong resistance to ionic interference. Notably, this study is the first to reveal the indispensable role of ultra-microporosity (pore size ≤ 0.7 nm) in the TCH adsorption process. Density functional theory calculations revealed that the ultra-micropore structure significantly enhanced the interaction between the biochar and TCH, as compared with super-micropores (0.7 nm < pore size < 2 nm), thereby facilitating the adsorption process. Therefore, this research demonstrated that P doping enhances TCH adsorption on KOH-activated biochar and elucidated the ultra-microporosity-mediated adsorption pathways.