nanomaterials Article Methionine-Functionalized Graphene Oxide/Sodium Alginate Bio-Polymer Nanocomposite Hydrogel Beads: Synthesis, Isotherm and Kinetic Studies for an Adsorptive Removal of Fluoroquinolone Antibiotics Sushma Yadav 1 , Anupama Asthana 1 , Ajaya Kumar Singh 1, * , Rupa Chakraborty 1 , S. Sree Vidya 2 , Ambrish Singh 3 and Sónia A. C. Carabineiro 4   Citation: Yadav, S.; Asthana, A.; Singh, A.K.; Chakraborty, R.; Sree Vidya, S.; Singh, A.; Carabineiro, S.A.C. Methionine-Functionalized Graphene Oxide/Sodium Alginate Bio-Polymer Nanocomposite Hydrogel Beads: Synthesis, Isotherm and Kinetic Studies for an Adsorptive Removal of Fluoroquinolone Antibiotics. Nanomaterials 2021, 11, 568. https://doi.org/10.3390/ nano11030568 Academic Editor: Athanasios C. Mitropoulos Received: 2 December 2020 Accepted: 20 February 2021 Published: 25 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Chemistry, Govt. V.Y.T. PG Autonomous College, Durg 491001, India; sushmabhilai80@gmail.com (S.Y.); anurakeshbhilai@gmail.com (A.A.); roopachakraborty1991@gmail.com (R.C.) 2 Department of Chemistry, Kalyan PG College, Durg 490006, India; vidsan1987@gmail.com 3 School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China; drambrishsingh@gmail.com 4 LAQV-REQUIMTE, Departmentof Chemistry, NOVA School of Science and Technology, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal; sonia.carabineiro@fct.unl.pt * Correspondence: ajayaksingh_au@yahoo.co.in or ajayaksingh@govtsciencecollegedurg.ac.in; Tel.: +91-940-6207572; Fax: +91-788-2211688 Abstract: In spite of the growing demand for new antibiotics, in the recent years, the occurrence of fluoroquinolone antibiotics (as a curative agent for urinary tract disorders and respiratory problems) in wastewater have drawn immense attention. Traces of antibiotic left-overs are present in the water system, causing noxious impact on human health and ecological environments, being a global concern. Our present work aims at tackling the major challenge of toxicity caused by antibiotics. This study deals with the efficient adsorption of two commonly used fluoroquinolone (FQ) antibiotics, i.e., Ofloxacin (OFX) and Moxifloxacin (MOX) on spherical hydrogel beads generated from methionine- functionalized graphene oxide/ sodium alginate polymer (abbreviated Met-GO/SA) from aqueous solutions. The composition, morphology and crystal phase of prepared adsorbents were charac- terized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HR-TEM) and thermogravimetric analysis/differential thermogravimetry (TGA/DTG). Batch adsorption tests are followed to optimize the conditions required for adsorption process. Both functionalized and non-functionalized adsorbents were compared to understand the influence of several experimental parameters, such as, the solution pH, contact time, adsorbent dosage, temperature and initial con- centration of OFX and MOX on adsorption. The obtained results indicated that the functionalized adsorbent (Met-GO/SA) showed a better adsorption efficiency when compared to non-functionalized (GO/SA) adsorbent. Further, the Langmuir isotherm was validated as the best fitting model to de- scribe adsorption equilibrium and pseudo second-order-kinetic model fitted well for both types of adsorbate. The maximum adsorption capacities of Met-GO/SA were 4.11 mg/g for MOX and 3.43 mg/g for OFX. Thermodynamic parameters, i.e., ΔG ◦ , ΔH ◦ and ΔS ◦ were also calculated. It was shown that the overall adsorption process was thermodynamically favorable, spontaneous and exothermic in nature. The adsorbents were successfully regenerated up to four cycles with 0.005 M NaCl solutions. Overall, our work showed that the novel Met-GO/SA nanocomposite could better contribute to the removal of MOX and OFX from the liquid media. The gel beads prepared have adequate features, such as simple handling, eco-friendliness and easy recovery. Hence, polymer gel beads are promising candidates as adsorbents for large-scale water remediation. Keywords: methionine functionalized; graphene oxide; polymer nanocomposite; hydrogel beads; fluoroquinolones antibiotics; adsorption; isotherms; kinetics; thermodynamics Nanomaterials 2021, 11, 568. https://doi.org/10.3390/nano11030568 https://www.mdpi.com/journal/nanomaterials