  Citation: Kalaitzidou, K.; Zouboulis, A.; Mitrakas, M. Thermodynamic Study of Phosphate Adsorption and Removal from Water Using Iron Oxyhydroxides. Water 2022, 14, 1163. https://doi.org/10.3390/w14071163 Academic Editor: Gaurav Sharma Received: 10 February 2022 Accepted: 4 April 2022 Published: 5 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). water Article Thermodynamic Study of Phosphate Adsorption and Removal from Water Using Iron Oxyhydroxides Kyriaki Kalaitzidou 1, * , Anastasios Zouboulis 2 and Manassis Mitrakas 1 1 Department of Chemical Engineering, School of Engineering, Aristotle University of Thessaloniki, 54124 Thesaloniki, Greece; mmitraka@auth.gr 2 Department of Chemistry, School of Sciences, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece; zoubouli@chem.auth.gr * Correspondence: kalaitzidou@cheng.auth.gr Abstract: Iron oxyhydroxides (FeOOHs) appear to be the optimal group of materials among inorganic adsorbents for the removal of phosphates from water, providing significant adsorption capacities. This research work presents a thermodynamic study of phosphate adsorption by examining five different FeOOHs sorbent nanomaterials. The otablebtained results indicated that the adsorption process in these cases was spontaneous. When the experiments were performed using distilled water, akageneite (GEH), schwertmannite, and tetravalent manganese feroxyhyte (AquAsZero), displaying ΔH ◦ values of 31.2, 34.7, and 7.3 kJ/mole, respectively, presented an endothermic adsorption process, whereas for goethite (Bayoxide) and lepidocrocite, with ΔH ◦ values of −11.4 and −7.7 kJ/mole, respectively, the adsorption process proved to be exothermic. However, when an artificial (according to NSF) water matrix was used, GEH, schwertmannite, lepidocrocite, and AquAsZero presented ΔH ◦ values of 13.2, 3.3, 7.7, and 3.3 kJ/mole, respectively, indicative of an endothermic process, while only for Bayoxide, with ΔH ◦ of −17 kJ/mole, the adsorption remained exothermic. The adsorption enthalpy values generally decreased with the NSF water matrix, probably due to the competition for the same adsorption sites by other co-existing anions as well to the possible formation of soluble phosphate complexes with calcium; however, an overall positive effect on the uptake of phosphates was observed. Keywords: phosphate adsorption; iron oxyhydroxides; thermodynamic parameters; adsorption isotherms; adsorption capacity 1. Introduction The release of phosphorus, mainly through wastewaters in the aqueous environment, results in the acceleration/increase of eutrophication problems in aquatic ecosystems. Con- sidering the lack of phosphorus in the soil (where it is applied as a fertilizer) and, on the other hand, its occasionally excessive content in water sources, which raises the eutrophica- tion problem [1], it is essential to remove and recover phosphorus from alternative sources, considered nowadays as “wastes”, in a reusable form [2–5]. In this regard, the adsorption of anions, especially of phosphates, onto inorganic adsorbent materials is generally of great importance for regulating their concentrations in natural waters. In this case, the adsorption capacity is mainly regulated by the affinity of anions with the surface of the adsorbent media, the relative concentration of the anions, as well as the pH and the temperature of the process [6–8]. Granular solids formed by the spontaneous secondary aggregation of nanoparticles are widely applied in adsorption processes for the removal of pollutants [9]. Iron oxyhy- droxides (FeOOHs) nanomaterials, with building units in the range of 2–10 nm [10–13], have attracted the attention of several researchers, due to their strong affinity to phosphates, along with other, mainly anionic aquatic hazardous species, such as arsenic oxy-anions. Water 2022, 14, 1163. https://doi.org/10.3390/w14071163 https://www.mdpi.com/journal/water