With the development of factories, environmental problems are getting serious, including potentially toxic metal pollution, which has been a threat to the ecology, environment and human health due to the characterizations of high toxicity and bioaccumulation. Among them, Cd (II) is a typical pollutant, and it sources from electroplating, paints, electronics and so on (Mohan et al., 2014). It is well known that Cd (II) is environmentally persistent and not degraded in water, and what’s more, Cd (II) can be harmful to the human body and life (Kołodyńska et al., 2017; Zhou et al., 2018). Some common ways, such as ion exchange, sorption and ultrafiltration are used to remove heavy metal from wastewater (Luo et al., 2014; Romero-Dondiz et al., 2016; Salima et al., 2013; Wang et al., 2018b). But these methods still face challenges due to the restrictions including implementation process and expenditure (Ding et al., 2014). Among the different methods, adsorption run costs are lower and its processes studied to remove heavy metal are simpler relatively (Hernández-Montoya et al., 2013; Shaban et al., 2018). And studying a high adsorption capacity and low-cost sorption material to repair the wastewater polluted by Cd (II) was important and meaningful.
Biochar is known as a fantastic sorbent to adsorb heavy metals (Lu et al., 2017; Lu et al., 2015b; Nyamunda et al., 2019; Paranavithana et al., 2016). Research shows that the pore structure and surface properties of biochar can affect the removal efficiency of heavy metals (Huff & Lee, 2016; Wang et al., 2015; Xiao et al., 2018). With the larger superficial area and further surface function groups, biochar will obtain more interaction sites to remove heavy metals (Ahmad et al., 2018; Beesley & Marmiroli, 2011; Xian et al., 2018). But it is not easy to obtain biochar with both abundant void structures and surface functional groups using common preparation methods (Gronnow et al., 2013). Normally, functional groups decrease and surface area increases with increasing pyrolysis temperature (Jing et al., 2018; Tomczyk et al., 2020). Because these contradictions limit the use of biochar as an effective adsorbent, biochar needs to be modified to improve its performances (Hu et al., 2015; Jin et al., 2018; Lu et al., 2015a).
Novel biochar modification methods are used to boost biochar adsorption capacity to pollutants from wastewater. The methodologies of surface modification are concerned widely due to its simple modification procedure and sorption effectiveness. Biochar-based materials can be obtained by impregnating or covering biochar surface with compounds (Michalekova-Richveisova et al., 2017), carbonaceous structures such as sulfapyridine, amino, naphthenic acids and so on (Frankel et al., 2016; Inyang et al., 2015; Yang & Jiang, 2014). The adsorption capacity of biochar can be improved by increasing the number of adsorption sites, oxygen-containing functional groups or other specific adsorption structures on the surface of biochar by methods of surface modification, oxidant or chemical synthesis (Bashir et al., 2020; Gu et al., 2020; Yazovskikh et al., 2020; Zhang et al., 2020).
Chitosan is a low-cost material, which is prepared by hydrolysis of chitin with sodium hydroxide, and the amide functional group was converted into amino functional groups. Because amino polysaccharide is renewable and biodegradable worldwide, chitosan is relatively cheap. What’s more, chitosan have been used to remove heavy metal ions and have fantastic adsorption ability to heavy metals from soil and water as an adsorbent (Gerente et al., 2007; Pontoni & Fabbricino, 2012). In addition, Chitosan contains a large number of amino functional groups, which have strong binding ability to different heavy metals. Therefore, chitosan can be introduced into the surface of adsorption materials to improve adsorbents’ adsorption ability to heavy metals (Bhatnagar & Sillanpaa, 2009; Gerente et al., 2007; Pontoni & Fabbricino, 2012). There is a hypothesis that chitosan accumulates easily, which can become more disperse by combinating chitosan with biochar. The combination is very advantageous, both materials are cheap and environmentally friendly. What's more, chitosan modified biochar will combine the larger surface area and porous network structure of biochar as well as the higher adsorption capacity of chitosan, so the chitosan modified biochar obtained will have a stronger adsorption capacity for heavy metals. However, no previous studies explored in detail the adsorption characteristics, influencing factors and potential mechanisms of Cd (II) on kiwi biochar modified with chitosan.
In this research, a novel chitosan-modified kiwi branches biochar (CHKB) was synthesized and used as an adsorbent for the removal of Cd (II). Batch sorption experiments were used to examine the sorption behaviors of Cd (II) on the CHKB under various conditions. The objectives of this study were to: (a) synthesize and characterize an effective CHKB adsorbent that can be used for Cd (II) removal; (b) determine the adsorption mechanisms of the CHKB to Cd (II); (c) investigate the kinetics and equilibrium isotherms of Cd (II) adsorption on CHKB; and (d) identify the effect of solution pH, dosage and regeneration cycles of the Cd (II) adsorption on the CHKB.