In this work, halloysite nanotubes (HNTs) without modification were used as an efficient adsorbent to explore its natural adsorption capability, which showed excellent adsorption ability for low-concentration ciprofloxacin (CIP). The physicochemical properties of HNTs before and after adsorption were investigated by several characterization techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), N 2 adsorption-desorption analysis, X-ray diffractometer (XRD), and zeta potential analysis. The influences of temperature, initial CIP concentration, adsorbent dosage, and pH value on CIP adsorption performance were also studied. The kinetics analysis revealed that CIP adsorption on HNTs was a kind of monolayer adsorption process and followed a pseudo-second-order rate equation. The zeta potential result indicated that electrostatic interaction between HNTs and CIP molecules was possibly responsible for the adsorption performance. Moreover, HNTs showed no apparent loss in CIP adsorption capability after five cycles, exhibiting potential applications in wastewater treatment. excellent adsorption characteristics is widely used as an adsorbent. However, its high cost restricts its commercial application. In contrast to active carbon, clays are considered to be promising adsorbent candidates because of their lost cost, wide availability, and high adsorption efficiency [14].Halloysite [Al 2 Si 2 O 5 (OH) 4 ·2H 2 O], a type of natural aluminosilicate clay mineral that is widely widespread in China [15], has a similar structure to kaolin, except that a monolayer of water molecules intercalates between the adjacent clay layers [16]. The sizes and polydispersity of halloysite depends on the specific geological deposit [17]. Halloysite nanotubes (HNTs) with negative charges on outer surface are a kind of outstanding hollow tubular material with an internal diameter of 20-30 nm and an external diameter from 30 to 70 nm [18,19], which exhibits excellent physical and chemical properties due to their large specific surface areas, abundant hydroxyl groups, porous structure, and great biocompatibility. HNTs are not toxic to humans [20,21], which represents a prerequisite for pharmaceutical and biomedical applications [22,23]. Most importantly, HNTs have been widely investigated as adsorbents for the removal of dyes and heavy metals in wastewater [24][25][26].Herein, we used HNTs without any modification as adsorbent to investigate its natural adsorption performance toward CIP. The effects of various experiment conditions on adsorption capacity were thoroughly investigated, and pH value played an important role in regulating the adsorption efficiency. Furthermore, the adsorption kinetics and adsorption characteristics were discussed. The pseudo-second-order kinetic model fitted the data well and the equilibrium adsorption data were well described by the Langmuir isotherm model. It was found that the structure and surface property of HNTs were the k...
