Iron (Magnetite) Nanoparticle-Assisted Dark Fermentation Process for Continuous Hydrogen Production from Rice Straw Hydrolysate  
[PERSON]  
1 Department of Environmental Technology, Centro de Investigacion y Asistencia en Tecnologia y Diseno del Estado de Jalisco, A. C. Av. Normalistas 800, Colinas de la Normal, Guadalajara 44270, Jalisco, Mexico; [EMAIL_ADDRESS] (J.-P.-B. ); cristina.marinez.fraile@estudiantes.uvas (C.-M.); [EMAIL_ADDRESS] (R.M. ); iarugagheitavag  
Currently, H\({}_{2}\) is primarily produced from fossil fuels, particularly methane, and is widely used in the chemical fertilizer and petroleum industries [4]. However, using fossil fuels for H\({}_{2}\) production poses similar economic and environmental challenges as other fossil fuel-based processes. Alternatively, H\({}_{2}\) can be produced through water electrolysis, which is considered the cleanest method of production. Despite its environmental benefits, the high electricity costs associated with electrolysis--accounting for about 80% of total operating expenses--limit its practical application [5]. In contrast, biological processes for H\({}_{2}\) production from biomass (biogenic H\({}_{2}\)) offer an environmentally friendly and energy-efficient alternative [6]. Among these biological methods, dark fermentation (DF) is the most extensively researched due to its high production rates, compatibility with anaerobic digestion, and ability to process a wide range of organic waste and wastewater sources. Many types of agro-industrial residues, such as wheat straw, sugarcane bagasse, corn stover, etc., have been studied as feedstocks for H\({}_{2}\) production by DF [7]. Rice is the second most cultivated cereal crop in the world after wheat. According to the Food and Agriculture Organization of the United Nations, 787.3 million tons of rice were produced in 2021 [8], resulting in approximately 800 to 1000 million tons of rice straw (RS) as residue [9]. Only 20% of the RS produced worldwide is utilized, while the remaining 80% is discarded as waste. In many countries, the common practice for RS disposal is open burning, which contributes to air pollution [10]. RS can be a suitable feedstock for fermentative H\({}_{2}\) production due to its widespread availability and favorable quality [11]. Therefore, numerous studies have focused on the utilization of this resource for H\({}_{2}\) production. For example, [PERSON] et al. [12] investigated the effect of temperature (ranging from 37 to 55 \({}^{\circ}\)C) and total solids content (3%, 6%, and 12%) on the performance of a dark fermenter operated under a semi-continuous regime. They achieved the highest H\({}_{2}\) yield, 63.6 \(\pm\) 2.9 mL H\({}_{2}\)/g vs. added, at a total solids content of 6% and a temperature of 55 \({}^{\circ}\)C. Also, [PERSON] et al. [13] evaluated the performance of a DF process run on RS previously pretreated by NaOH/urea and electro-hydrolysis, obtaining productions of 163.0 mL H\({}_{2}\)/g RS. Currently, the use of metal nanoparticles (NPs) to enhance H\({}_{2}\) production in the DF process has become an attractive topic for numerous research groups. It is widely recognized that NPs can play a role in the DF process by influencing its mechanism in various ways, including improving electron transfer efficiency, enhancing feedstock utilization, enhancing metalloenzyme activity, and immobilizing enzymes, among others [14]. Compared with other metallic NPs, iron and nickel NPs have shown greater potential for their application in DF [14]. In particular, iron-based nanoparticles (FeNPs) play a pivotal role in enhancing the activity of metalloenzymes, including both types of hydrogenases (Fe-Fe and Fe-Ni). In addition, FeNPs show a tendency to optimize the utilization of raw materials, resulting in improved H\({}_{2}\) production [15]. Recent research efforts suggest that conductive iron oxides, such as Fe\({}_{3}\)O\({}_{4}\) (magnetite), play an important role in facilitating electron transfer between ferredoxin oxidoreductase and hydrogenases secreted by syntrophic bacteria.