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INTRODUCTION
The contamination of arsenic in natural waters has been well documented in many countries including Argentina, Bangladesh, China, Chile, India, Philippines, Thailand and Taiwan (Mandal & Suzuki 2002; Sharma & Sohn 2009; Hosono et al. 2010). The primary source of arsenic in groundwater has been attributed to natural processes like geochemical reactions, volcanic emissions and weathering of arsenic rocks and minerals (Kanel et al. 2005). Another important source of arsenic is via anthropogenic activities such as ceramic and glassware production, herbicide and pesticide manufacturing, petroleum refineries, metallurgical industry and tannery operation (Altundogan et al. 2002).
Arsenic, which is a metalloid, is considered to be the 20th most abundant trace element (Mandal & Suzuki 2002; Nidheesh & Anantha Singh 2017). In general, arsenic exists in four oxidation states including +5 (arsenate), +3 (arsenite), 0 (metallic arsenic) and −3 (arsenide). Inorganic arsenic species such as arsenite [As(III)] and arsenate [As(V)] are commonly found in the aqueous environment, and are considered to be more toxic that the organic counterpart (Nidheesh & Anantha Singh 2017; Su et al. 2017). Detrimental health effects including disorder of the peripheral vascular system and central nervous system, peripheral neuropathy, conjunctivitis, gangrene, cardiovascular diseases and skin cancer have been related to long-term arsenic exposure (Bhattacharya et al. 2007). Moreover, the World Health Organization (WHO) categorized this element as a Class I human carcinogen (van Halem et al. 2009). Therefore, strict regulations have been implemented by WHO and the US Environmental Protection Agency where the current permissible limit of arsenic in drinking water is 10 μg/L (Smith et al. 2002). Meanwhile, Mexico and Taiwan have set the maximum contaminant level of arsenic for drinking water to be 25 and 10 μg/L, respectively (Chen & Chung 2006; Saldaña-Robles et al. 2017).
Adsorption is one of the most popular technologies utilized in the removal of heavy metals from waste effluents due to its simplicity in operation, low cost, safe handling, effectiveness even at a low concentration of contaminant, and applicability as a small-scale household module or in a community plant (Siddiqui & Chaudhry 2017). Adsorbents such as magnetite/non-oxidative graphene (Yoon et al. 2017), iron-coated seaweeds (Vieira et al. 2017), Fe3O4-GO-LDH (Wu et al. 2011), Fe3O4-GO (Sheng et al.