Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman Research article Impacts of iron oxide and titanium dioxide nanoparticles on biogas production: Hydrogen sulfide mitigation, process stability, and prospective challenges Mohamed Farghali a,b , Fetra J. Andriamanohiarisoamanana a , Moustafa M. Ahmed b , Saber Kotb b , Takaki Yamashiro a , Masahiro Iwasaki a , Kazutaka Umetsu a,* a Graduate School of Animal and Food Hygiene, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido, 080-8555, Japan b Department of Animal Hygiene, Faculty of Veterinary Medicine, Assiut University, 71526, Egypt ARTICLE INFO Keywords: Metal oxide nanoparticles Hydrogen sulfide Biogas Methane Anaerobic digestion Manure treatment ABSTRACT Anaerobic digestion for biogas production is one of the most used technology for bioenergy. However, the adoption of nanoparticles still needs further studies. Therefore, this study was designed to examine the effect of metal oxide nanoparticles (MONPs) at four different concentrations in two different combinations, 20 (R1) and 100 (R2) mg/L for Fe 2 O 3 , 100 (R3) and 500 (R4) mg/L for TiO 2 , and a mixture of Fe 2 O 3 and TiO 2 at rates of 20, 500 (R5) and 100, and 500 (R6), on hydrogen sulfide (H 2 S) mitigation, biogas, and methane (CH 4 ) yield during the anaerobic digestion of cattle manure (CM) using an anaerobic batch system. The results showed that H 2 S production was 2.13, 2.38, 2.37, 2.51, 2.64, and 2.17 times lower than that of the control (R0), respectively, when the CM was treated by the aforementioned MONPs. Additionally, biogas and CH 4 production were 1.09 and 1.105, 1.15 and 1.191, 1.07 and 1.097, 1.17 and 1.213, 1.10 and 1.133, and 1.13 and 1.15 times higher than those of R0 when R1, R2, R3, R4, R5, and R6 were supplemented with MONPs, respectively. The highest specific production of biogas and CH 4 was 336.25 and 192.31 mL/gVS, respectively, which was achieved by R4 sup- plemented with 500 mg/L TiO 2 NPs, while the corresponding values in the case of R0 were 286.38 and 158.55 mL/gVS. 1. Introduction The production of economic and environmentally sustainable re- newable energy sources is a core area of contemporary research and industry. The anaerobic digestion (AD) of cattle manure (CM) has en- ticed considerable attention within the scientific community, because of improving in hygienic standards of digested nutrients and promoting renewable alternatives through the production of methane (CH 4 ) rich biogas (Zhang et al., 2018). Biogas usually consists of CH 4 (40–75%) and CO 2 (25–60%), with trace amounts of other impurities, including hydrogen sulfide (H 2 S) (Kadam and Panwar, 2017). The concentration of H 2 S in biogas ranges from 50 to 10,000 ppm, depending on the composition of the feed material to be digested (Muñoz et al., 2015; Pipatmanomai et al., 2009). Elevated levels of H 2 S in biogas cause several problems, for example, H 2 S significantly reduces the volume and potential uses of biogas as it is highly corrosive to the biogas purification instruments (Charles et al., 2006; Zhou et al., 2016). Ad- ditionally, it is highly poisonous and has led to the death of many people (Andriamanohiarisoamanana et al., 2015). Therefore, the pre- sence of H 2 S in biogas is one of the main obstacles to the successful implementation of AD and must be minimized to a level that can be tolerated by equipment, for example, between 200 and 500 mg/L to protect downstream biogas equipment and internal combustion engines (Lupitskyy et al., 2018). In practice, some popular biological (such as biofilters) and che- mical (such as sodium hydroxide scrubbing and iron oxide pellets) techniques have been applied to control H 2 S(Bioenergy, 1999). How- ever, post-H 2 S removal techniques are costly, require the handling of chemicals, and lack long-term stability, which greatly limits their commercialization (Blazy et al., 2014). Therefore, it is essential to re- move H 2 S during the AD process by direct mixing the feed materials to be digested with additives and then analyze the possible regulatory pathway of the entire process. In this context, it is assumed that the addition of trace minerals to feed the stream improves AD by enhancing the bacterial action that increases biogas generation. In AD, trace mi- nerals supplements serve as electron donors as they promote the total https://doi.org/10.1016/j.jenvman.2019.03.089 Received 24 January 2019; Received in revised form 6 March 2019; Accepted 18 March 2019 * Corresponding author. E-mail address: umetsu@obihiro.ac.jp (K. Umetsu). Journal of Environmental Management 240 (2019) 160–167 0301-4797/ © 2019 Elsevier Ltd. All rights reserved. T