Adsorption of ciprofloxacin on iron oxides in the absence or presence of polyethylene microplastic

Abstract

Antibiotics and microplastics are considered emergent pollutants, with soils serving as critical terminal repository. As such, study aimed to evaluate the outer- and inner-sphere adsorptions of ciprofloxacin on synthetic ferrihydrite, goethite and hematite under three pH conditions (4.7, 6.2, 9.0), both in absent and presence of polyethylene microplastic (40–48 µm, 5 % w/w) (novelty by simulating double environmental contamination with ciprofloxacin and microplastic). Another novelty was estimating the adsorption stability of ciprofloxacin using chemical sequential extractions: i) outer-sphere (0.01 mol L⁻¹ NaOH, pH 12); (ii) low energy inner-sphere (0.1 mol L⁻¹ KH₂PO₄, pH 4.5); iii) high energy inner-sphere - residual ciprofloxacin. Total ciprofloxacin adsorption capacity (mg g⁻¹) followed decreasing order: ferrihydrite (18.1–20.8) > hematite (5.7–13.8) > goethite (3.5–12.7). When considering adsorption relative to the specific surface area (mg m⁻²), order shifted to hematite (0.46) > goethite (0.30) > ferrihydrite (0.07) at pH 6.2, where highest adsorption rates were observed. Outer-sphere adsorption, as percentage of total adsorption, was most significant under alkaline conditions (pH 9.0): ferrihydrite (29–55 %) > hematite (8–46 %) > goethite (2–24 %). Conversely, residual adsorption was predominant at pH 4.7 for crystalline iron oxides (hematite, goethite). Maximum ciprofloxacin adsorption capacity on pure microplastic was 7.6 mg g⁻¹, pH 4.7. The inclusion of two crystalline Fe oxides allowed to evaluate the positive effect on ciprofloxacin adsorption of greater exposure of more reactive ferrol groups. Addition of 5 % microplastic to iron oxides did not affect ciprofloxacin adsorption on ferrihydrite, goethite, and hematite. From environmental perspective, Fe oxides, abundant in humid tropical soils, have demonstrated high capacity for ciprofloxacin filtration (characterized by high adsorption energy), even in environments subject to dual contamination with microplastic.