Adsorption Mechanisms Of Ciprofloxacin Onto Iron (Hydro-) Oxides

Iron(hydro-) oxide minerals are rich in soil and sediment. They play an important role in the migration of pollutants in the environment because of their large specific surface area. The adsorption of antibiotics on iron minerals has been studied, but there are few studies on the comparison adsorption of antibiotics on different structure forms of iron minerals. In this study, goethite, hematite and magnetite which widely exist in the environment, were chosen as the typical adsorbents. Ciprofloxacin(CIP) was selected as the representative of fluoroquinolone antibiotics. The adsorption characteristics of the ciprofloxacin on iron minerals were studied by batch experiments and microscopic characterization under different conditions. The main results are as follows:(1) The adsorption of CIP on natural iron minerals was in accordance with the pseudo second-order equation. Due to smaller average pore diameter(13.19 nm) and the existence of microporous structure of natural goethite, the adsorption of CIP occurred mainly on the outer surface. Pore diffusion was not obvious. However, the average pore diameters of natural magnetite and hematite(31.08 nm and 15.22 nm) were relatively large, and there was no microporous structure which mainly existed in the forms of large pore. So, the adsorption of CIP included surface diffusion and pore diffusion, and it occurred relatively slowly.(2) Adsorption of CIP on three kinds of natural iron minerals was greatly influenced by solution pH because of the change of the surface charge property of iron minerals and the existence forms of CIP. The initial pH of CIP solution was 3.0, 6.0 and 8.0, and the adsorption under pH of 6.0 was the strongest. Adsorption thermodynamics(16℃, 25℃ and 35℃) showed that the adsorption of CIP on natural iron minerals was a spontaneous exothermic process and adsorption on natural hematite and magnetite was a spontaneous endothermic process.(3) Infrared spectroscopy results showed that the adsorption peak of hydroxyl and hydroxyl groups on the surface of iron minerals was weakened after adsorption, and the increase of ionic strength did not decrease the adsorption of CIP on natural iron minerals. It showed that the complexation reaction that CIP substituted the position of hydroxyl or hydrated hydroxyl groups on the surface of iron minerals was the main adsorption mechanism.(4) Low concentrations(0~0.1 mmol/L) of small molecular organic acids competed the same hydroxyl sites with CIP on synthetic goethite surface and inhibited the adsorption of CIP. With the increase of organic acids concentration(0.1~6 mmol/L), their adsorption on synthetic goethite were gradually saturated. Partial functional groups of small molecule organic acids were not involved in the adsorption on synthetic goethite surface, which bonded with ciprofloxacin in solution and formed CIP-organic acid-synthetic goethite complexes, thereby inhibiting effect gradually reduced or even disappeared.(5) Over the whole pH range(3.0~9.5), small molecule organic acids(oxalic acid, citric acid and citric acid) inhibited the adsorption of CIP on the synthetic goethite, and had a strong inhibition effect at low pH. This was related to that the repulsion between positively charged CIP and synthetic goethite which was not favorable for adsorption. In addition, low pH was conducive for small molecular organic acids to form inner-sphere complex on the surface of the synthetic goethite.