Chemical Nature of Adsorbents and Interaction with Metals and Anions
The variety of adsorbents possessing one or more unique morphological features and their characteristic properties including both physical and chemical properties that empower them to be extensively applied for the elimination of lethal organic and inorganic pollutants from water resources have been already discussed earlier. This section deals with the different types of interactions among the adsorbent and the adsorbate molecules leading to adsorption of contaminants.
Among various factors that affect the overall process of adsorption and the adsorption capacity, the presence of certain groups on the surface of the adsorbent plays a pivotal role (Cashin et al. 2018). Every adsorbent possesses its unique characteristics and functionalities such as particle size, surface area (external/internal), porosity, pore size distribution, pore volume, pore structure, bulk density including the hydrophobic and hydrophilic behavior, chemical and thermal stability, and surface morphology, in addition to attached functional groups that are quite essential for their surface chemistry and the adsorption of pollutants. In most of the cases, the interaction amid the metallic species present in the solution and the functional groups attached on adsorbents significantly improve their adsorption (Wang, Gao, et al. 2015; Bian et al. 2015). The functional groups get attached on the surface of adsorbents through the heteroatoms such as oxygen, nitrogen, sulfur, phosphorus, and halogens, which further form the basis of the classification of functional groups as oxygen-bearing functional groups, nitrogen-bearing functional groups, sulfur-bearing functional groups, etc. (Yang, Wan, et al. 2019; Cashin et al. 2018).
As a whole, the role of functional groups in the adsorption of a range of contaminants is quite complex, and the intermolecular interactions between the former and the latter are dependent upon the nature, heterogeneity, and surface chemistry of the adsorbent as well as on the ionic environment of aqueous solutions.
Adsorption Mechanisms of Metals and Anion
The metal-binding mechanism during adsorption of toxic metallic species from water resources onto various adsorbents takes place through either of the following processes: physical adsorption, ion exchange, electrostatic interaction, surface complexation, and precipitation (Figure 2.3); or sometimes several mechanisms work simultaneously in a particular aqueous environment (Xu and McKay 2017). Usually, chemisorption (i.e. precipitation, ion exchange, and surface complexation) significantly influences the metal adsorption than physisorption. All through the adsorption process, the mechanism that governs the whole process exclusively depends on the target metal (adsorbate), the adsorbent, ionic environment, and pH of the solution. The solution pH is an important factor because it affects the speciation of metal in the given aqueous medium, charge on the adsorbent surface, and complexation behavior of functional groups. The various underlying mechanisms of metal adsorption on the adsorbent surfaces in the course of the adsorption process are briefly discussed here (Yang, Wan, et al. 2019; Worch 2012a).
It is the movement of metallic ions into the pores of the adsorbent and their subsequent attachment to the surface by weak forces such as H-bonding and van der walls forces. It is strongly impacted by the surface area and pore size distribution of the adsorbents along with the nature of the target metal. Further, the heterogeneity and polarity of the adsorbent surface in addition to the presence of
Adsorption mechanisms/ adsorption interactions
Van der Waals interactions
□ Anion exchange
□ Cation exchange
□ Reversible exchange of ions
Inclusion complex Coordination complex Proton displacement Covalent binding
FIGURE 2.3 Illustration of mechanisms involved during adsorbate-adsorbent interaction in aqueous solutions.
functional groups facilitate the physical adsorption by helping in the movement of metal ions by means of electrostatic attraction and ion-dipole forces. It is most common and serves as a principal mechanism of adsorption of metal ions (Worch 2012a; Lu, Yu, et al. 2017; Moreno-Barbosa et al. 2013; Verma and Dutta 2015; Huynh et al. 2017).
The existence of oxygen-containing functional groups on the surface of the adsorbent leads to the adsorption of metal ions via an ion-exchange mechanism (Dong et al. 2018). Its efficiency depends on the divalent metallic species and protons of the oxygen-containing functional groups for which the ion size and surface chemistry of the adsorbent play a significant role. The solution pH is the dominating factor and cation-exchange capacity (CEC) is an imperative indicator of the adsorption of metallic species taking place via an ion-exchange mechanism (Liu, Zhu, et al. 2013; Kyzas et al. 2016; Hao et al. 2010).
The electrostatic interaction mechanism is operative for removal of metallic species when there is an interaction between positively charged target metals and negatively charged adsorbents (surface containing functional groups). It is a relatively weak process and is dependent on the pH of the medium and pHpzc of the adsorbent. The charged interface between the adsorbent surface and the medium is influenced by ionization of the existing functional groups on the adsorbent surface (Moreno-Barbosa et al. 2013; Afroze et al. 2016; Cui et al. 2016; Zeng et al. 2015; Yang, Tang, et al. 2014; Yuan, Zhang, et al. 2017; Cheng et al. 2012; Velazquez-Jimenez et al. 2014; Xiao et al. 2016; Lu, Yu, et al. 2017; Bayramoglu and Arica 2016).
Surface complexation leads to the formation of complexes (inner- and/or outer-sphere) during the interaction of metal ions and functional groups present on the adsorbent surface in the course of the adsorption process (Zhou et al. 2017; Kyzas et al. 2016; Shi et al. 2015; Sankararamakrishnan et al. 2014; Ma et al. 2011; Yuan, Zhang, et al. 2017; Gupta et al. 2014; Long et al. 2013; Bayramoglu and Arica 2016; Guo, Zhang, et al. 2017). The adsorption of metal onto carbon-based adsorbents is mainly governed by this mechanism (Liu, Han, et al. 2017; Li et al. 2010).
It includes the formation of a solid product in the solution during the adsorption of metal ions (Wang et al. 2018; Huynh et al. 2017). It serves as one of the major mechanisms for removal of metal ions and occasionally works cooperatively together with other adsorption mechanisms such as electrostatic interaction, ion exchange, and surface complexation (Inyang et al. 2016). The functional groups present on the adsorbent surfaces exhibit an insignificant direct effect on the precipitation of metal ions; however they promote precipitation indirectly by influencing other mechanisms of metal adsorption (Yang, Wan, et al. 2019).
Effect of Functionalization of Adsorbents on Adsorption of Metals and Anions
Adsorbent materials usually possess certain inherent limitations such as low adsorption capacity, small metal-binding capacities, prohibitive cost, small selectivity, and instability. Thus, in order to surmount the limitations, the surface of adsorbents is altered or modified to enable them to be well equipped with improved characteristics for desired applications. For this purpose, certain foreign functional groups are introduced into the adsorbent surface to enhance their performance for intended applications (Cashin et al. 2018; Wei et al. 2016; Yang, Wan, et al. 2019; McCarthy et al. 2012). An overview of the performance of a variety of adsorbents containing oxygen, nitrogen, and sulfur functional groups on the adsorption of hazardous metals and anions in terms of the contribution of specific functional groups and maximum adsorption capacities is given in this section and summarized in Table 2.1.