Nitrogen-Bearing Functional Groups

The functional groups containing nitrogen when incorporated into adsorbents increase the basic properties of the adsorbents and hence increase their uptake capacity (Yang, Wan, et al. 2019). Usually, amine groups are most commonly used for functionalization or modification of any surface where due to the availability of lone pair of electrons and readiness in the bondformation with cationic species results in enhanced adsorption. For instance, acrylamide-functionalized electrospun nano-fibrous polystyrene showed enhanced adsorption of Cd(II) and Ni(II) ions where functionalization introduced amide (-NCO) and amine (-NH-) groups onto the surface of polystyrene that lead to an increased interaction between the adsorbent and metal ions. However, the exact underlying mechanism of adsorption was unspecified (Bahramzadeh et al. 2016). Similarly, amino-functionalized metal-organic frameworks MIL-101(Cr) (Luo, Ding, et al. 2015) and polyacrylamide-functionalized Fe,O4 nanoparticles (Moradi et al. 2017) exhibited increased removal efficiency for Pb(II) ions, and the experimental findings suggested the adsorption mechanism to be governed by chemisorption.

The adsorption of V(V) ions on amine-modified poly(glycidyl methacrylate)-grafted cellulose (Anirudhan et al. 2009) and Se(IV) and Se(VI) ions on dendrimer-functionalized graphene oxide (Xiao et al. 2016) exhibited enhanced adsorption of these ions after functionalization. The underlying mechanism of adsorption was electrostatic attraction between positive adsorbent surface and negative vanadate ions at specified pH at which the tertiary amine moiety (-NH+-(CH!)2C1_) exchanged its C1‘ ions with vanadate species. For selenium oxyanions, electrostatic attraction between charged amine groups and selenite ions resulted in increased adsorption of Se(IV). The combination of electrostatic interaction between charged amine groups and selenite ions and the formation of an inner-sphere complex on the surface of GO via ligand exchanging with hydroxyl groups occurred during the adsorption of Se(VI) ions.

Ethylenediamine-modified p-zeolite (p-zeolite-EDA) involved an inner-sphere complex formation between Ni(II) ions and amine groups of P-zeolite. The amino group (-NH,) forms a protonated cationic (-NH,+) form that facilitates more availability and adsorption of nickel(II) ions in acidic solution. The formation of inner-sphere complex was explained through XPS data that evidenced that the electron density on oxygen and nitrogen atoms decreased as they easily lost electrons that migrated to nickel ions. Therefore, the complex was formed via the formation of coordination bond between oxygen and nitrogen atoms of P-zeolite-EDA and Ni(II) ions (Liu, Yuan, et al. 2017). Similarly, Hg(II) ions form coordination bonds with -NH, ligands that have one or more less electronegative donor atoms and have a strong coordination role with Hg2+ ions. This results in significant enhancement in the adsorption capacity for test metal ions. The presence of two amino groups in the structural framework further contributes to the increased adsorption. The introduction of the

-NH, groups in the adsorbent provides adsorption sites for Hg(II) ions on the surface ofMCM-41 (Zhuet al. 2012).

The adsorption of copper(II) ions on amino-functionalized magnetic nanoparticles (Hao et al. 2010) and y-aminopropyltriethoxysilane-modified chrysotile (Liu, Zhu, et al. 2013) followed the ion-exchange process. The latter one involved three mechanisms during the course of adsorption. The first one is the ion-exchange process that proceeds by exchange of protons of residual hydroxyl groups with the Cu(II) ions in solutions. Later, the amino groups attached to the surface of adsorbent using nitrogen atoms to complex Cu(II) ions. Finally, the protonated amino group adsorbed copper(II) ions by chelating or the ion-exchange process.

The amidation of graphene oxide significantly improved the adsorption of uranyl ions from aqueous solutions where ammonia was selected for modification of the -COOH groups of GO into -CONH, groups to accomplish higher specific binding with uranyl ions (Verma and Dutta 2015). The adsorption indicated a dual mechanism of physisorption and chemisorption due to the availability of heterogeneous binding sites in ammonia-modified graphene oxide (NH,-GO). Amide functionalization of graphene oxide exhibited better selectivity toward uranyl ions at experimental pH. In contrast, amino-functionalized mesoporous silica SBA-15 involves a first adsorption step followed by precipitation of uranium ions when its concentration increases in the bulk phase (Huynh et al. 2017). The same dual interaction was observed for the removal of Se(IV) and Se(VI) ions on poly(-allylamine)-modified magnetic graphene oxide (PAA-MGO) due to the heterogeneous surface of PAA-MGO. However, the mechanism was similar to that reported by Xiao et al. (2016). Both physical and chemical interactions occurred during the adsorption of selenium oxyanions where ligand exchange between -OH groups present on the surface of iron oxides and selenium oxyanions and electrostatic interaction between the amine groups and selenium oxyanions take place (Lu, Yu, et al. 2017).

The functionalization of graphene with СТАВ significantly increases the adsorption capacity of graphene for chromium(VI) ions (Wu et al. 2013). The amino group forms amide bond and, along with the -COOH and -OH groups on the surface of graphene, enhances the adsorption of Cr(VI) ions from water resources.

Ethylamine-modified montmorillonite showed an increase in the adsorption of Cs(I) ions, and the operating mechanism was ion exchange together with the complex formation via coordination of-NH, groups and surface complexation of-OH groups (Long et al. 2013). In a similar manner, the 3-(aminopropyl)triethoxysilane-functionalized coal fly ash modified with mesoporous siliceous material displayed an increased adsorption capacity of the adsorbent for Cu(II) ions by the formation of amino-copper complex through the surface amino group (Pizarro et al. 2015).

The ionizable functional groups such as -NH,, -COOH, and -OH undergo deprotonation at high pH, making the surface negatively charged, and the adsorption of Mn(II) involves chelate interactions between the adsorbent surface and Mn(II) ions (Al-Wakeel et al. 2015).

The modification of cross-linked magnetic chitosan/poly(vinyl alcohol) particles with xanthate introduces N- and S-containing functional groups on the surface of XCMP that assists in the adsorption of Cu(II) and Pb(II) ions. The mechanism of adsorption of these metal ions involves an interaction of N and S atoms of functional groups with these ions, and the metal ions are then transformed into crystals and get adsorbed on the XCMP surface (Lv et al. 2017).

Sulfur-Bearing Functional Groups

The sulfur-containing functional groups positively contribute to the adsorption of organic and inorganic contaminants from water resources on a variety of adsorbents either by increasing/decreasing specific surface area and pore volume of the adsorbents or by their extraordinary binding abilities with the soft metal ions. Thiol-functionalized ionic liquid-based mesoporous organosilica exhibited soft-soft interactions for Hg(II) and Pb(II) removal where these metal ions having low charge densities are considered as soft acids that form strong covalent bonds with sulfur-containing soft bases. The adsorption mechanism involved chemisorption where covalent bonds were formed by sharing or exchanging electrons between these metal ions and -SH groups of the adsorbent (Elhamifar et al. 2016).

The modification of chitosan beads with sodium dodecyl sulfate introduces sulfur-containing -S=O and -CSO functional groups by forming a bilayer of the surfactant on the surface of chitosan. The Cd(II) ions are adsolubilized on the surfactant bilayer formed by means of electrostatic attraction (Pal and Pal 2017).

The sulfur-functionalized ordered mesoporous carbons experienced an increase in their specific surface area (837-2865 m2/g) and pore volume (0.71-2.3 cm-’/g) after the modification and exhibited affinities for heavy metals in the following order: Hg(II)> Pb(II)> Cd(II)> Ni(II) (Saha et al. 2016). Sulfur-functionalized silica microspheres, thiol-functionalized hollow mesoporous silica microspheres, and 1,2-ethanedithiol- and vinyl-functionalized covalent organic frameworks exhibited increased Hg(II) and Hg(0) adsorption by forming a complex between the sulfur-containing functional groups (i.e. -SH, -SR) and the Hg(II) and Hg(0) metal ions, respectively (Saman et al. 2013; Zhang, Wu, et al. 2015; Sun et al. 2017).

Sulfurized activated carbon (SAC) from bagasse pith exhibited an ion-exchange mechanism for adsorption of Zn(II) ions. The surface of sulfurized activated carbon contains SAC-ОН, SAC-0 moieties, and -SO3H functional group that are collectively responsible for greater adsorption of Zn(II) ions. The adsorption mechanism involves the exchange of H+ ions on the surface of SAC with Zn2+ ions in the aqueous medium, thus forming an ion-exchange complex (Anoop Krishnan et al. 2016).

Adsorbent Containing Multiple Functional Groups

The functional groups discussed so far under different subheadings in this section revealed their significant contribution to the adsorption of a variety of metal ions by enhancing the adsorption capacity and selectivity of the adsorbents. It follows that the presence of more than one of these functional groups can also bring out certain positive changes in the adsorbents influencing the overall adsorption process. The multifunctional groups on the adsorbent surface exhibit a cooperative effect on the mechanism of adsorption. This section provides an insight into adsorbents containing more than one functional group at the surface for significant removal of metallic ions from aqueous solutions.

For instance, acrylamide- and hydroxyl-functionalized metal-organic framework exhibits increased removal of Hg(II) ions where hydroxyl and acrylamide groups play a pivotal role. On functionalization, both these groups are introduced into the pore wall of MOF and provide strong sites for coordination and chemisorption of Hg(II) ions. The experimental findings suggested that the adsorption involved both physisorption and chemisorption mechanisms. However, the formation of Hg-0 from both these groups advocated chemisorption as the dominating mechanism (Luo, Chen, et al. 2015).

For the removal of U(VI) ions, silica particles were modified with amidoxime (AMD) and carboxyl (CA) groups where amine and hydroxyl groups on AMD ligand act as chelation sites and on CA ligand act as ion-exchange groups for U(VI) ions (Bayramoglu and Arica 2016). The presence of amino, imino, hydroxyl, and carboxyl groups on the surface of adsorbent significantly increases the removal of test metal ions by the process of electrostatic interaction. The complexation and electrostatic interactions between AMD ligands and U(VI) at pH 5 and the deprotonation of negatively charged carboxyl groups for ion-exchange reaction with U(VI) ions after pH 3.0 are the main reasons for increased adsorption. Moreover, the affinity of uranyl ions for nitrogen and hydroxyl groups is higher than that of the carboxyl-containing counterparts.

The introduction of three carboxylic groups and amine groups into the cornstalk through the functionalization with nitrilotriacetic acid resulted in excellent adsorption capacity for Cd(II) and Pb(II) ions (Huang, Yang, et al. 2015). The mechanism involved surface chelation between carboxyl and amine groups present on the surface and Cd(II) and Pb(II) ions, and ion exchange between Na+ on the surface and Cd2+ and Pb2+ ions in the solution. It was found that some of the metal ions undergo coordination with two carboxyl groups and one amine group and the other metal ions were adsorbed by ion exchange process through carboxyl groups.

During the activation of carbon prepared from Phragmites australis with urea phosphate, a number of oxygen-containing and nitrogen-containing functional groups are introduced into the prepared activated carbon (Guo, Zhang, et al. 2017). It also increases the porous structure of the carbon. The adsorption involved various mechanisms, namely microporous filtration due to the small size of Cd(II) ions, ion exchange between the protons present in the functional groups attached on the adsorbent surface and Cd(II) ions in the solutions, electrostatic attraction between the deprotonated phenolic and carboxyl groups and Cd(II) species, and surface complexation. The surface complexation occurred via covalent bonding with Cd(II) ions and O atoms in deprotonated carboxyl, ketone, ether phenol, and ester groups, and N atoms in C-NH, and O=C-NH groups and P atoms in pyrophosphate. These atoms donated their electrons to Cd(II) ions to form coordination complexes.

Chitosan modification of vermiculite introduced amine- and hydroxyl-containing functional groups that facilitate the adsorption of As(III) ions. The probable mechanism of adsorption involved chelation or complex formation between the As(III) and NH, groups of chitosan or ion exchange between H+ ions of attached NH, groups and As(III) ions. Further, the oxalic groups adhered on the surface of vermiculite act as a bridge between the adsorbent surface and chitosan chains that aid in the enhancement of adsorption capacity. The mean adsorption energy indicated ion exchange as the dominant mechanism of removal together with the adsorption of As(III) ions into the pores present on surface or among the inner layers of adsorbent (A. Saleh et al. 2016).

Due to the introduction of multiple thio-triazole units as binding sites in the structural framework, lignin-based adsorbents exhibited an enhanced adsorption capacity for Cd(II) ions. The adsorption involved chemical interactions where the Pearson hard-soft acid-base (HSAB) principle governed the overall adsorption mechanism. Cd(II) ions acted as Lewis soft acid that reacts readily with imino and thioether groups attached to the surface considered as Lewis soft bases to form covalent bonds, and thus, the adsorbent showed promising adsorption capacity (Jin et al. 2017).

The functionalization or modification of adsorbents not only contributes to the adsorption of metallic species but also influences the removal of certain anions from aqueous solutions. For instance, chitosan microspheres were modified by formaldehyde, PEG2ooo, and epichlorohydrin (ECH). This results in the introduction of amino and hydroxyl groups onto the surface of the adsorbent. At lower pH (i.e. 5), protonation of amino groups of chitosan occurred that enhanced the electrostatic attraction of nitrate and phosphate anions and resulted in the increased adsorption of these anions (Zhao and Feng 2016).

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