Enhanced removal of estradiol by Twice-ball milled magnetic biochar: Mechanisms and efficiency

Steroid estrogens (SEs) threaten ecosystems and human health. Biochar is an efficient adsorbent for SEs removal due to its porous structure, yet single-modified biochar's capacity is unstable and its mechanisms are poorly understood. Therefore, this study prepared Twice-ball milled magnetic biochar (TmMBC) through a secondary ball milling combined with potassium ferrate (K₂FeO₄) modification. By integrating adsorption kinetics, isothermal experiments, and soil column migration tests, this study systematically investigated the removal performance and mechanisms of TmMBC for estradiol (E2). The results indicated that TmMBC has a specific surface area of 683.51 m²/g, with a micropore area increased to 659.70 m²/g. At pH 7 and 15 °C, the maximum adsorption capacity for E2 (1 mg/L) was 56.30 mg/g, which is a 40.75% enhancement compared to Ball milled biochar (BmBC). Kinetic studies demonstrated that the adsorption process of TmMBC conforms to a pseudo-second-order kinetic model (R² > 0.98), indicating that chemical adsorption is predominant. Additionally, the analysis of environmental factors showed that TmMBC maintains efficient adsorption (>45 mg/g) within the pH range of 3-9, and the presence of Ca²⁺ can further enhance its adsorption capacity by 37.6%. In the soil column experiment, the addition of 0.1 mg TmMBC can delay the penetration time of E2 to over 40 pore volumes (pv), achieving a retention rate of 100%. In contrast, the addition of microplastics (PE, PLA) and cow dung affects the migration behavior of E2 to varying degrees. The study confirms that TmMBC significantly enhances the adsorption and fixation capacity of E2 through the synergistic effects of graded pore structure, Fe-O active sites, and surface functional groups. This research provides theoretical support for the development of efficient and low-cost technologies for E2 pollution remediation, while also offering new insights into the application of biochar in agricultural pollution remediation.