Study on the Adsorption of Tetracycline Hydrochloride in Water by Modified Highland Barley Straw Biochar

Global antibiotic pollution (represented by tetracycline hydrochloride, TCH) threatens water environmental safety, and resource recovery of agricultural waste remains a key challenge for sustainable development. Given that utilizing biochar for adsorption is widely recognized as a circular economy-compliant method, this study aimed to verify its applicability in TCH pollution control while recycling agricultural waste by preparing modified biochar from the Xi Zang highland barley straw via chemical activation (KOH, H3PO4, NaHCO3, and ZnCl2) and pyrolysis at 750 °C. Among the products, H3PO4-modified (P-BC) and ZnCl2-modified (Zn-BC) biochars performed best: their abundant micro/mesoporous structures and surface functional groups (–OH/–COOH) enabled excellent TCH adsorption, with the mechanism involving synergy of physical adsorption (dominated by pore filling) and chemical adsorption (hydrogen bonding, electrostatic attraction, cation bridging), alongside multi-layer adsorption. Adsorption was pH-dependent—acidic conditions favored it, while Zn-BC restored efficiency at pH = 9 via Zn2+ bridging. The two biochars were complementary: Zn-BC had higher adsorption capacity, while P-BC showed better stability and ionic interference resistance. Thus, Zn-BC suits high-concentration, low-ionic-strength TCH wastewater, and P-BC is ideal for complex high-ionic-strength water (e.g., industrial/aquaculture wastewater). This study provides theoretical and technical support for high-value utilization of regional agricultural waste and targeted TCH pollution control.