Iron and aluminum oxides and oxyhydroxides rich soils as eco‐friendly adsorbents for ciprofloxacin in aqueous media

The extensive use of ciprofloxacin (CIP) and subsequent environmental release led to its accumulation in aquatic environments, causing antibiotic resistance among microorganisms. Fe-based compounds are known to be good adsorbents for organic compounds. The complexation between the functional groups in antibiotics and the metal ions and hydroxyl groups in Fe and Al oxides and oxyhydroxides can facilitate the adsorption. However, the interactions between CIP and naturally occurring Fe-Al oxides and oxyhydroxides have not been extensively investigated. Hence, the present study aims to apply laterite (LT) and red earth (RE), two natural soils rich in Fe and Al oxides/oxyhydroxides, to remove CIP in aqueous media. Batch adsorption experiments were conducted to determine the optimum adsorbent dose, initial adsorbate concentration, contact time, and pH. The CIP concentration was measured by a UV spectrophotometer at 273.1 nm (λ max). RE exhibited higher CIP removal (12.6 mg/g maximum adsorption capacity) compared to LT (4.3 mg/g maximum adsorption capacity), at an optimal dose of 10 g/L, 30-min contact time. As RE is rich in ferrihydrite and LT is enriched with goethite and hematite, the two soils showed different interactions with CIP. The Freundlich model explained the CIP adsorption process (R² = 0.9937 for RE and R² = 0.9558 for LT) as multilayer adsorption occurred by electrostatic attraction, followed by chemisorption via COOH groups of CIP and OH groups of soils. Under all tested conditions, negligible CIP desorption (<0.5%) is observed with RE, while LT showed <5% desorption. Desorption increased to 12% at pH 10 for RE and to 46% at pH 3-4 for LT. Even though both RE and LT effectively removed CIP from aqueous media, RE outperforms due to its effectiveness across a broader spectrum of environmental conditions.