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INTRODUCTION
Currently, increasing amounts of industrial and agricultural effluents are discharged into the environment, resulting in more inorganic and organic pollution of soil and water. Most organic pollutants are biodegradable, but inorganic pollutants, especial heavy metals, are non-biodegradable and can be accumulated through the human and animal food chains. This accumulation of heavy metals is a serious threat to human health, aquatic organisms and ecological safety over a long period (Dong et al. 2014). Nickel (Ni) mainly derives from steel factories, batteries manufacturing, and metal and alloy production (Gautam et al. 2015). It is well known that as a micronutrient, Ni is part of the synthesis of vitamin B12, and an activator in some enzyme systems. However, high dosages can cause serious diseases such as hepatitis, nephritic syndrome, anaemia, and diarrhoea (Kasprzak et al. 2003). It is therefore necessary to reduce ionic Ni to permissible limits before its discharge into the natural environment.
The United States Environmental Protection Agency (EPA) provides current maximum allowable contaminant levels for many heavy metals. To meet the EPA standards, various removal techniques such as ion exchange, precipitation, sorption, and electrocoagulation are used in sewage treatment (Inyang et al. 2015). Among these techniques, the sorption approach has been extensively adopted due to its ease of operation, low cost, and high efficiency. However, different adsorbents exhibit different sorption abilities. For instance, the sorption behaviours of Ni on a series of adsorbents, including activated carbon (Kadirvelu et al. 2001), clays (Gupta & Bhattacharyya 2006), Na-attapulgite (Fan et al. 2009), carbon nanotubes (Chen et al. 2009), mordenite (Yang et al. 2011), magnetic nanoparticles (Panneerselvam et al. 2011), graphene oxides (Chen et al. 2016a), and montmorillonite (Chen et al. 2016b) showed a large variation. Not only that, many feedstock materials in nature have the potential to be cost-efficient adsorbents. Thus, potential adsorbents deserve further exploration.
Biochar can be produced from agricultural residues, the majority of which are waste or by-products from the harvesting and processing of crops (Inyang et al. 2015). Thus, there are dual benefits for environmental protection to utilize agricultural wastes as adsorbents for the removal of heavy metals. Furthermore, many low-cost adsorbents have been used to remove contaminants from water, such as tea factory waste (Malkoc & Nuhoglu 2005), diatomite (Sheng