Adsorption performance on tetracycline by novel magnetic adsorbent derived from hydrochar of low-rank coal and sewage sludge

Herein, a novel magnetic tetracycline (TC) adsorbent (S3C7-Fe) was prepared by co-hydrothermal carbonization (HTC) of sewage sludge (SS) and low-rank coal (LC) combining with K2FeO4 modification. The adsorption performance at different conditions, adsorption kinetics, isotherms and thermodynamics were investigated. The results showed that S3C7-Fe exhibited high TC removal efficiency over a wide pH range (2∼11) and a high coexisting ion concentration range (50∼200 mg/L), and the maximum equilibrium adsorption capacity could reach to 884.04 mg/g. TC adsorption process by S3C7-Fe fitted the Langmuir and pseudo-second-order models, and the adsorption process was homogeneous and dominated by chemisorption. The π-π EDA interaction between the S3C7-Fe and TC molecular was the main mechanism, and pore filling, hydrogen bonding and cation-π complexation also contributed to the adsorption. Besides that, the microscopic interface interaction mechanism between the adsorbent and TC was analyzed both at molecular and electronic levels through theoretical calculations by density functional theory and frontier orbital theory. Interestingly, the results indicated that the oxygen and nitrogen containing functional groups in the adsorbent surface facilitated TC adsorption, and increasing the relative content of COOH and pyrrolic N could be the most effective pathway for improving adsorption capacity.