Adsorption of enrofloxacin by cherry stone-based biochar modified with zinc chloride

Abstract: Enrofloxacin (ENR) is one type of widely-used potent antibacterial drug to treat the bacterial infections of cultured animals. However, the ingested ENR cannot be fully metabolized or absorbed. About 15%-50% of the original ENR is discharged into the external environment and wastewater in the form of urine, resulting in serious antibiotic pollution. Furthermore, the low removal performance of traditional sewage treatment plants has led to the widespread antibiotic residues in surface water, underground water, and even drinking water. And drug contamination caused by these residues and the development of drug-resistance genes can pose serious ecological risks. Biochar (a new type of carbon material adsorbent) can be expected to serve as antibiotic removal, due to the extremely low entry conditions, extensive and cost-free raw materials, and renewable properties. Tianshui City of Gansu Province is one of the main cherry-producing areas in China. The resulting cherry stone has been a solid waste. A few cherry stones have been used for pillow primary processing and utilization. But most of them have been discarded, leading to a waste of resources and great pollution to the environment. As a result, there is a high demand for the processing and utilization of cherry stone. Taking the cherry stone (an agricultural solid waste) as the raw material, this study aims to prepare the biochar composites using high-temperature pyrolysis. Zinc chloride was also used to modify the property. N2 adsorption-desorption experiment and scanning electron microscopy (SEM) were used to determine the specific surface area, porosity, and morphology of unmodified biochar (PBC) and modified biochar (Zn-BC). Fourier infrared spectroscopy (FT-IR) and X-ray electron spectroscopy (XPS) were used to characterize the composition of active groups and functional groups before and after the adsorption of ENR by PBC and Zn-BC, respectively. Adsorption kinetics and isothermal adsorption experiments were used to clarify the adsorption mechanism of ENR by the two kinds of carbon. The results show that the surface aperture of PBC was relatively less, and the surface was mostly porous and smooth. Zn-BC exhibited a large pore size and layered structure, where each layer was distributed with a large number of honeycomb pore sizes, and the surface was irregular with the larger specific surface area and more pore structure. The specific surface area was 1 148.79 m²/g, which was 2.5 times that of PBC. The total pore volume increased by nearly 85 times. Zinc chloride changed the structure of biochar by etching holes on the biochar skeleton. The modified biochar shared a better adsorption effect on ENR, indicating a higher adsorption capacity than PBC. IR and XPS analysis confirmed that Zn-BC also introduced more oxygen-containing functional groups. The adsorption was attributed to the formation of complexes between ENR and oxygen-containing functional groups. As such, the adsorption mechanism was related to the pore filling, π-π bond interaction, and hydrogen bond interaction. The adsorption kinetics and adsorption isothermal model fitted better with the quasi-second-order kinetic model and Langmuir isothermal adsorption model. The chemical adsorption of ENR was dominated by the two biochar types. The adsorption was mainly controlled by pore filling, π-π bond interaction and hydrogen bond, indicating the single-layer adsorption. There was better consistency with the previous analysis of IR and XPS. The particle diffusion model showed that the ENR was firstly adsorbed by the outer surface of Zn-BC during the whole adsorption. And then ENR was adsorbed by the inner surface in the Zn-BC micropore. This adsorption was controlled by intra-particle diffusion. Thermodynamic analysis showed that gibbs free energy (_∆G⁰) and enthalpy change (_∆H⁰) of both biochars were negative, but entropy change (_∆S⁰) of both biochars was positive, indicating the spontaneous adsorption ENR by both biochars, increasing entropy and exothermic reaction. Therefore, Zn-BC-modified biochar can be used as a better material to remove ENR in water, particularly for the water treatment and remediation caused by antibiotic pollution.