Processes 2022, 10, 1029. https://doi.org/10.3390/pr10051029 www.mdpi.com/journal/processes Article Improvement of As(V) Adsorption by Reduction of Granular to Micro-Sized Ferric Hydroxide Vicenç Martí 1, *, Irene Jubany 2 , Lidia Fernández-Rojo 2 , David Ribas 2 , José Antonio Benito 3 , Brian Diéguez 1 and Ada Ginesta 1 1 Barcelona Research Center in Multiscale Science and Engineering-EEBE, Department of Chemical Engineering, Technical University of Catalonia (UPC), Av. Eduard Maristany 16, 08019 Barcelona, Spain; dieguez.brian.6881@eoivallesoriental.com (B.D.); adaginesta@gmail.com (A.G.) 2 Eurecat, Centre Tecnològic de Catalunya, Water, Air and Soil Unit, Pça. de la Ciència 2, 08243 Manresa, Spain; irene.jubany@eurecat.org (I.J.); lidia.fernandez@eurecat.org (L.F.-R.); davidrrff@gmail.com (D.R.) 3 Department of Materials Science and Metallurgical, EEBE, Technical University of Catalonia (UPC), Av. Eduard Maristany 16, 08019 Barcelona, Spain; jose.a.benito@upc.edu * Correspondence: vicens.marti@upc.edu; Tel.: +34-93-401-0957 Abstract: The remediation of groundwater containing arsenic is a problem that has been addressed using adsorption processes with granulated materials in columns, but the remediation itself could be improved by using micro-sized adsorbents in stirred systems. In this study, arsenate (As(V)) batch adsorption experiments were performed using granular ferric hydroxide (GFH) and two de- rived micro-sized materials. Reduced-size adsorbents were produced by energetic ball milling, giv- ing final sizes of 0.1–2 µm (OF-M samples) and ultra-sonication, producing final sizes of 2–50 µm (OF-U samples). Equilibrium isotherm studies showed that the Langmuir model was a good fit for the three sorbents, with the highest maximum adsorption capacity (qmax) for OF-U and the lowest for OF-M. The adsorption of the two groundwater samples occurred according to the obtained equi- librium isotherms and indicated the absence of interfering agents for the three adsorbents. Batch kinetics tests in stirred beakers followed a pseudo second-order model and indicated that the kinet- ics of the OF-U sorbent was faster than the kinetics of the GFH sorbent. The tests also showed an increase in the qe values for the reduced-size sorbent. The application of ultrasonication to the GFH produced an increase of 23% in the qmax and b term and an increase of 34-fold for the kinetic constant (k2) in the stirred batch systems tested. These results suggest that this new approach, based on ultra- sonication, has the potential for improving the adsorption of arsenic in groundwater. Keywords: arsenic adsorption; ultra-sonication; ferric hydroxide; micro-sized adsorbents; adsorp- tion kinetics 1. Introduction The presence of arsenic in groundwater is a worldwide problem [1–4] and is related to geological factors [5], acid mine drainage (AMD) generation from mining activities [1,6], or to spent pyrites in brownfields [7]. With the possible exception of the latter case, where contamination sites can be located, arsenic contamination is diffuse and its effects extend to groundwater bodies. The most important arsenic removal methods can be di- vided into technologies that are integrated into the current water treatment processes and need to be optimized and specific technologies addressed to arsenic removal. The first group of methods includes oxidation of arsenic (to ensure arsenic as As(V) species); en- hanced coagulation/filtration [1,2,8,9]; and enhanced lime softening [2].The last two meth- ods produce metal hydroxides as secondary products from coagulants (ferric and alumi- num salts) or softening reactants (lime) that finally sorb As(V) species. The arsenic is Citation: Martí, V.; Jubany, I.; Fernández-Rojo , L.; Ribas, D.; Benito, J.A.; Diéguez, B.; Ginesta, A. Improvement of As(V) Adsorption by Reduction of Granular to Micro-sized Ferric Hydroxide. Processes 2022, 10, 1029. https://doi.org/10.3390/pr10051029 Academic Editors: Yanju Liu, Bhaba Biswas and Ravi Naidu Received: 24 April 2022 Accepted: 20 May 2022 Published: 22 May 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).