Citation: Paletta, R.; Candamano, S.; Filippelli, P.; Lopresto, C.G. Influence of Fe 2 O 3 Nanoparticles on the Anaerobic Digestion of Macroalgae Sargassum spp. Processes 2023, 11, 1016. https://doi.org/10.3390/ pr11041016 Academic Editor: Jean-Louis Lanoiselle Received: 14 February 2023 Revised: 20 March 2023 Accepted: 23 March 2023 Published: 27 March 2023 Copyright: © 2023 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/). processes Article Influence of Fe 2 O 3 Nanoparticles on the Anaerobic Digestion of Macroalgae Sargassum spp. Rosy Paletta 1 , Sebastiano Candamano 2 , Pierpaolo Filippelli 2 and Catia Giovanna Lopresto 1, * 1 Department of Computer Engineering, Modelling, Electronics and Systems (DIMES), University of Calabria, 87036 Rende, Italy; rosy.paletta@unical.it 2 Department of Mechanical, Energy and Management Engineering (DIMEG), University of Calabria, 87036 Rende, Italy; sebastiano.candamano@unical.it (S.C.); pierpaolo.filippelli@unical.it (P.F.) * Correspondence: catialopresto@gmail.com or catiagiovanna.lopresto@unical.it Abstract: The anaerobic digestion (AD) of biomass is a green technology with known environmental benefits for biogas generation. The biogas yield from existing substrates and the biodegradability of biomasses can be improved by conventional or novel enhancement techniques, such as the addition of iron-based nanoparticles (NPs). In this study, the effect of different concentrations of Fe 2 O 3 -based NPs on the AD of brown macroalga Sargassum spp. has been investigated by 30 days trials. The effect of NPs was evaluated at different concentrations. The control sample yielded a value of 80.25 ± 3.21 NmL CH4 /g VS . When 5 mg/g substrate and 10 mg/g substrate of Fe 2 O 3 NPs were added to the control sample, the yield increased by 24.07% and 26.97%, respectively. Instead, when 50 mg/g substrate of Fe 2 O 3 NPs was added to the control sample, a negative effect was observed, and the biomethane yield decreased by 38.97%. Therefore, low concentrations of Fe 2 O 3 NPs favor the AD process, whereas high concentrations have an inhibitory effect. Direct interspecies electron transfer (DIET) via Fe 2 O 3 NPs and their insolubility play an important role in facilitating the methanogenesis process during AD. Keywords: Fe 2 O 3 ; nanoparticles; anaerobic digestion; Sargassum spp.; macroalgae 1. Introduction Anaerobic digestion (AD) of wastes is a promising green approach to valorize various waste streams and generate renewable bioenergy. Indeed, organic wastes are the most profitable source of renewable energy, and the production of biogas by AD seems to be the closest to commercial-scale exploitation [1]. Nanotechnology is an emerging technology to improve AD performance. Nano-sized particles (1–100 nm) have excellent physicochemical properties, such as high activity, high reactive surface area, chemical stability, high specificity for improving performance, and ability to stimulate microbial growth in the AD process. The addition of nanoparticles (NPs) affects the microbial community [2] and, in suitable concentrations, increases the degradation of biomass through direct or indirect interspecies electron transfer [3,4], thus enhancing biogas production [5–7]. Among NPs, iron-based NPs (Fe-NPs) seem to be the most promising nanomaterials for enhancing biogas production, improving biodigester process stability, achieving better substrate treatment, and increasing pathogen reduction [8–10]. Fe-NPs include zero-valent iron (ZVI) with paramagnetic properties and iron oxide NPs (IONPs) with ferromagnetic properties. Recently, their influence on the fundamental mechanisms of the anaerobic digestion process and on the fertility of effluents have been extensively discussed [11,12]. Among IONPs, magnetite (Fe 3 O 4 ) NPs have been widely applied in recent years for their magnetic properties, non-toxicity, high coercivity, biocompatibility, and ability to improve electron transport efficiency, increasing the activity of enzymes during methanogenesis, providing nutrients to microorganisms, reducing the inhibiting effect of sulphate-reducing Processes 2023, 11, 1016. https://doi.org/10.3390/pr11041016 https://www.mdpi.com/journal/processes