Over the last decades, the effects of industrialization, urbanization, and population growth have led to the occurrence and magnification of severe pollutants in the environment, including synthetic organic compounds (El Morabet, 2018; Kodavanti et al., 2014). Two common organic pollutants in industrial and domestic effluents are catechol (CAT) (1, 2-dihydroxybenzene) and hydroquinone (HQ) (1, 4-dihydroxybenzene). CAT is used as a reagent for photography, fur dye development, an antioxidant in manufacturing rubber, plastic production, and in the pharmaceutical industry (Amin et al., 2014; Schweigert et al., 2001b). HQ is used in varnishes, oils, and hair dyes (U.S. Food and Drug Administration, 2009), in industry as an agent in photography, stabilizer in paints, and antioxidant in rubber (El Morabet, 2018), and in pharmaceutical and cosmetics as skin brightening products, skin lighteners, and topical treatment for skin disorders (DeCaprio, 1999; Odumosu and Ekwe, 2010).
Industrial wastewater containing phenolic compounds without proper treatment has a severe effect on aquatic life, plants, animals, and humans (Milligan and Häggblom, 1998). Phenol and its derivatives are toxic and carcinogenic and can persist for many years in the environment due to their resistance to biological degradation (Deisinger et al., 1996; Zheng et al., 2013). Several studies have reported the toxicity of CAT for a variety of aquatic organisms (Anku et al., 2017; Duan et al., 2018; Elmenaouar et al., 2017; Neilson et al., 1991). In humans, CAT can irritate skin, eyes, and the upper respiratory tract as well as cause DNA damage provoking mutagenesis and carcinogenesis (Subramanyam and Mishra, 2013). HQ exposure can result in eye pigmentation, corneal effects, and impaired vision (DeCaprio, 1999; Subramanyam and Mishra, 2013). Exposure to phenols can result in severe toxic effects in humans and animals (DeCaprio, 1999; Schweigert et al., 2001a), bacteria (Subramanyam and Mishra, 2013), and aquatic organisms (Enguita and Leitão, 2013; Saha et al., 1999; U.S. Department of Health and Human Services, 2008). Due to their toxicity and impact, the United States Environmental Protection Agency (USEPA) has classified phenols as priority pollutants (Anku et al., 2017).
Treatment approaches for removing organic compounds from wastewater include physical, chemical, and biological processes; however, adsorption process has been found to be economical and effective method of removing organic compounds (Karpińska and Kotowska, 2019; Sophia and Lima, 2018). Adsorption mechanisms were studied to facilitate phenolic removal from polluted water using various materials such as metal oxides, activated carbon, waste materials, biochar, and other oxides (Abugazleh et al., 2020; García-Araya et al., 2003; Sophia and Lima, 2018; Yang et al., 2018).
The importance of metal oxides emerged from their physical, chemical, magnetic, and optical properties (Alias et al., 2020; Lewandowski et al., 2015), and although they are small in size, they exhibit a relatively large and unique surface structure. These properties result in high surface reactivity, leading some metal oxides such as titanium dioxide (TiO2) and iron (III) oxide (Fe2O3) to be used in industrial and biomedical applications (Lewandowski et al., 2015; Nagpal and Kakkar, 2019). TiO2 is currently used in electronics, personal care products, paints, coatings, solar cells, and photocatalysis, and has been reported in environmental remediation to effectively remove phenols from contaminated water (Bahri et al., 2011; Rasalingam et al., 2014; Vasudevan and Stone, 1996). Fe2O3 is abundant, low cost, environmentally friendly (MacHala et al., 2011; Wu et al., 2014), and has been used in adsorbents for contaminated water remediation (Dave and Chopda, 2014). Attributes of Fe2O3 that enable its effective separation of adsorbents include its particle size, magnetic and polymorphism properties (Wu et al., 2014). This has led to studies investigating Fe2O3 as a potential adsorbent and detoxifying agent for heavy metals and organic compounds (Anku et al., 2017; Sophia and Lima, 2018; Wu et al., 2014).
The toxicity of phenols and their derivatives has been reported to cause substantial damage to aquatic organisms (Bährs et al., 2013; Enguita and Leitão, 2013; Schweigert et al., 2001; Shadnia and Wright, 2008). The USEPA recognizes the standard freshwater test organisms, Ceriodaphnia dubia and Pimephales promelas, for short-term chronic toxicity testing (U.S. Environmental Protection Agency, 2002). These 7-d tests are utilized to examine Whole Effluent Toxicity (WET) testing to fulfill requirements for National Pollution Discharge Elimination System, pesticide and industrial chemical registration, and ambient toxicity in surface waters (U.S. Environmental Protection Agency, 2002). They are used in ecotoxicological studies due to their sensitivity to a wide range of pollutants in relevant aqueous ecosystems (Blaise and Férard, 2005). C. dubia is recommended as an ideal toxicity test organism due to its sensitivity and rapid generation time (Pakrashi et al., 2013; Versteeg et al., 1997). C. dubia is recommended as a bioindicator for environmental risk of many toxic materials in freshwater ecosystems (Brayner et al., 2006; Pakrashi et al., 2013). P. promelas (fathead minnows) are small omnivorous fish with a relatively short life span and the ability to survive a wide range of aquatic conditions (Geiger et al., 1986; Watanabe et al., 2007). P. promelas are used in many environmental studies to predict toxic effects on resident fishes and their ecosystems (Ankley and Villeneuve, 2006; Babich and Borenfreund, 1987).
In this study, the chronic toxicity induced by CAT and HQ to C. dubia and P. promelas was investigated before and after adsorption with metal oxides (TiO2 and Fe2O3). Toxicological data in aquatic invertebrates to CAT and HQ is limited with high variability (Bährs et al., 2013; Duan et al., 2018; Warnecke et al., 2014); thus, the lack of information warrants the need to determine the toxicity to standard aquatic test organisms. Additionally, the efficacy of using TiO2 and Fe2O3 to reduce the toxicity of these phenolic compounds is reported.