Oxygen-Bearing Functional Groups

Functional groups having oxygen atoms such as carboxyl and hydroxyl are the most widely used groups for surface modification of adsorbents. These groups significantly influence the surface properties, surface behavior, and surface reactions widely contributing to the adsorption of pollutants from water resources as discussed here in this section.

The introduction of a carboxylic functional group positively enhances Cr(VI) adsorption on activated carbon (AC), and on carbon nanotubes (CNTs), it showed a negligible effect on the removal of Cr(VI) (Ihsanullah, Al-Amer, et al. 2016). The acid treatment opened some of the blocked pores increasing the surface area of AC and forming more active adsorption sites, resulting in the enhancement of removal performance. For CNTs, although the acid treatment has introduced more active adsorption sites over the surface, the introduced and adhered negatively charged carboxylic groups lead to electrostatic repulsion between the CNT surface and chromate ions (HCrO₄~) present in the solution. Thus, acid treatment has no major part in the adsorption capacity of CNTs, and both the adsorbents (raw and modified) have comparable adsorption capacity for chromium ions.

Zhou et al. (2017) tested magnetic gelatin-modified biochar (MG-CSB) for As(V) removal and reported that functional groups having oxygen atoms significantly contributed to the increased removal of As(V). Biochar contains various oxygen-bearing groups, e.g. carboxyl, carbonyl, and hydroxyl, and bears a net negative charge due to dissociation of these groups. Under the experimental conditions, i.e. at pH 4, As(V) exists as HAsO₄²~ in the solution, the protonation and deprotonation of surface -OH groups make the iron oxide positively charged, and also some of the groups of biochar become protonated. All these cations and anions present in the solution consequently resulted in increased adsorption through electrostatic interaction and hydroxyl complexation between As(V) and MG-CSB.

The surface of nanosilica was modified with oxalic acid and tartaric acid and explored for Co(II) ions (Mahmoud, Yakout, et al. 2016). The modification increased the roughness of the surface of the adsorbent due to binding with

TABLE 2.1

Functionalized Adsorbents for the Adsorption of Metals and Anions

magnetic chitosan

Adsorbents

TABLE 2.1 (Continued)

Functionalized Adsorbents for the Adsorption of Metals and Anions

(continued)

Batch Adsorption Process of Metals and Anions

TABLE 2.1 (Continued)

Functionalized Adsorbents for the Adsorption of Metals and Anions

silica SBA-15

(continued)

Adsorbents

TABLE 2.1 (Continued)

Functionalized Adsorbents for the Adsorption of Metals and Anions

functionalized metal-organic framework

(continued)

Batch Adsorption Process of Metals and Anions

TABLE 2.1 (Continued)

Functionalized Adsorbents for the Adsorption of Metals and Anions

carboxylic acids. The roughness further increased the surface area of the sorbent due to the increased number of active sites. Further, the modification provided mechanical stability to the adsorbent to retain the spherical shape; however, a slight increase in the particle size was visualized that confirmed the complete coating of nanosilica with carboxylic acids. Metal binding to the adsorbent proceeds through a cation-exchange mechanism where the active silanol groups assisted the adsorbent in solid-phase extraction. The modification introduced additional functional groups such as -COOH, -CO, and -OH that intensely participate in metal binding along with the silanol groups that further advocated the mechanism similar to weak acid cation exchangers. In addition, a complex between Co(II) ions and the attached oxygen-donating surface functional groups also formed during the removal of target metal ions, and the selectivity of the adsorbents was governed on the basis of the soft-hard acid-base rule. Similarly, acidic modification of activated carbon prepared from potato peel was also tested for Co(II) ions where activation with H,PO₄ introduced oxygen-containing functional groups such as carboxylate, hydroxyl, phenols, and epoxy that helped in the efficient removal of metal ions (Kyzas et al. 2016). The mechanism of adsorption was explained on the basis of FTIR where the peaks corresponding to carboxylate, hydroxyl, phenol, and epoxy groups got eliminated after adsorption, showing their involvement in adsorption. Metal binding proceeds mutually through the cation-exchange, electrostatic interaction, and surface complexation mechanisms. Ramos et al. (2016) investigated carboxylate-functionalized sugarcane bagasse for adsorption of Co(II), Cu(II), and Ni(II), and FTIR spectra revealed the underlying adsorption mechanism. The FT-IR spectrum of metal ion-loaded sugarcane bagasse exhibited splitting of the band in the region of 1685 cm"¹ , which can be attributed to the stretching of C=O bond in the carboxylic acid groups. This depicted that the carboxylic groups deprotonated during the course of adsorption and involved in adsorption of metal ions. The metal ion adsorption on carboxylate-functionalized sugarcane bagasse involves an ion-exchange mechanism amid hydronium ions and metal ions with subsequent complex formation between metal ions and carboxylate groups in the order Cui-Il) > Ni(II) > Co(II). Similarly, crown ether-modified activated carbon cloth (ACC) was investigated for possible enhancement in the adsorption capacity of Cr(III), Co(II), and Ni(II) ions (Duman and Ayranci 2010). Among the monobenzo and dibenzo derivatives of crown ethers, the former are more effective in enhancing the adsorption of metal ions under study than the latter irrespective of their cavity sizes. The probable reason is their attachment onto ACC through the benzene ring that provides more space on the surface, more attachment sites, and flexible cavities for adherence of metal ions. On the contrary, both the benzene rings are involved in the attachment of dibenzo derivatives to ACC, resulting in less space and less flexible cavities, thereby leading to less enhancement in the adsorption of metal ions. The H-bonding among functional groups of ACC and metal ions along with the normal ionic interactions is involved in the adsorption where Cr(III) ions showed greater removal than Co(II) and Ni(II) ions.

The adsorption of Pb(II) and Zn(II) ions on activated carbons of watermelon (GACW) and walnut shell (GACN) activated by phosphoric acid followed physisorption due to the heterogeneous distribution of pores on GACW which could avoid pore blocking and electrostatic interaction between the negatively charged GACW surface and positively charged test metal ions (Moreno-Barbosa et al. 2013).

Eucalyptus sheathiana bark was modified by NaOH for adsorption of Zn(-II) metal ions where electrostatic interaction between zinc cations and negatively charged surface increased the adsorption performance (Afroze et al. 2016). Further, cations with surface hydroxyl groups formed a surface complex with subsequent formation of other complexes after the adsorption of metal ions. A comparable remark was for the result exhibited by the removal of Ge(IV) ions on catechol-functionalized nanosilica, where the adsorption mechanism also involved electrostatic interaction and the complex formation amid germanium ions and hydroxyl groups of catechol-functionalized nanosilica (Cui et al. 2016) and catechol-functionalized activated carbon (Marco-Lozar et al. 2007). The surface complexation mechanism of adsorption was adopted by carboxyl-functionalized magnetite nanoparticles (CMNPs) for removal of Pb(II), Cd(II), and Cu(II) where negatively charged carboxylate ions capture the metal ions by chelating and forming complexes with them (Shi et al. 2015). The electrostatic interaction between Cd(II) ions and carboxyl groups containing polyacrylic acid-modified magnetic mesoporous carbon (Zeng et al. 2015) and a-ketoglutaric acid-modified magnetic chitosan (Yang, Tang, et al. 2014) is found responsible for the acceleration in the monolayer adsorption of cadmium ions.

The adsorption of As(III) and As(V) on zerovalent iron nanoparticles immobilized on oxidized multiwalled carbon nanotubes (MWCNTs) using EDTA as chelating agent (ZCNT) involved solution-oxidation-adsorption and surface complexation as the underlying mechanisms. Due to aerial oxidation, ZVI generates Fenton’s reagent that oxidizes As(III) to As(V) and hydrated ferric oxides are formed on the surface with which As(V) forms complex. EDTA plays a bifunctional role as a chelating agent for arsenic and helps in the preservation of zerovalent state of iron. ZVI and EDTA conjointly aid in the enhancement of adsorption capacity (Sankararamakrishnan et al. 2014).

A combination of adsorption mechanisms were reported for the removal of Co(II), Cs(I), and Sr(II) on phosphate-modified montmorillonite (PMM) (Ma et al. 2011). Surface modification by phosphate increases the surface area and pore volume of the adsorbent due to the formation of inner-sphere complex with sorbed phosphate ions and Al-O-P-OH surface precipitates. Among the target metal ions, Cs(I) ions were preferentially adsorbed due to stronger electrostatic attraction. In addition, the phosphate modification provided extra sorption sites for Cs(I) ions. Further, high selectivity of Cs⁺ as compared to K⁺ resulted in its exchange with K⁺ into the interlayer of PMM, by the ion-exchange method. Co(II) ions formed COOH⁺ that interacts with negatively charged adsorbent surface, thus assisting in adsorption. In addition, the hydroxyl groups of phyllosilicate sheets bound to Co(II). Adsorption was found to be strongly pH-dependent, which advocates surface complexation as the main mechanism of Co(II) adherence onto PMM. The adsorption of Sr(II) ions involved ion exchange (pH < 8) and surface complexation (pH >8) as the operating mechanisms.

Similarly, multiple mechanisms such as electrostatic interactions, ion exchange, and complexation were accounted for the adsorption of Pb(II) ions on triazole-4-carboxylic acid-functionalized poly(glycidyl methacrylate) microspheres (Yuan, Zhang, et al. 2017). The terminal carboxylic groups are deprotonated to form negatively charged COO^- that binds lead ions via electrostatic interactions and ion-exchange methods, and imine (-NH-) and tertiary amino groups (-N=) of triazole moieties donate their lone pair of electrons to empty atomic orbital of lead ions forming bidentate and tetradentate triazole-Pb(II) chelates or complexes. The OH - groups also interact with Pb(II) ions through chelation or complexation.

The activated carbon prepared from scrap tires was activated by H₂O₂ and tested for adsorption of Ni(II) ions (Gupta et al. 2014). The functional groups such as hydroxyl and carbonyl on the surface of adsorbent undergo protonation at lower pH values and deprotonation at higher pH values. At pHpzc, electrostatic repulsion between positively charged species at the adsorbent surface and nickel ions lowers the adsorption, whereas at pH > pH_pzc, the adsorbent surface bears negatively charged species that undergo electrostatic attraction with nickel ions. This interaction results in the formation of complexes, thereby enhancing the adsorption process. Thus, surface complexation and ion-exchange mechanisms govern the adsorption of Ni(II) ions on activated carbon prepared from scrap tires, whereas on ZnCl₂ activated carbon from Glycyrrhiza glabra residue, Ni(II) and Pb(II) ions are adsorbed via electrostatic attractions.

For the removal of V(V) ions, ZnCl₂ activated coir pith carbon is used where the surface functional groups introduced by activation actively participate in the sorption. The reactive OH groups at the adsorbent surface enhance the removal performance via the ion-exchange mechanism (Namasivayam and Sangeetha 2006).

H₂O₂-modified attapulgite is tested for Sr(II) ions where modification with H₂O₂ brings about morphological changes in the adsorbent. It exfoliates the surface and forms more of the silicate layers that increase the surface area from 96.34 to 174.14 m²/g with smaller pore size and larger pore volume. This leads to increased adsorption sites of clay particles resulting in the enhancement of the adsorption abilities of the modified attapulgite for Sr(II) ions. The same effect was reported for adsorption of Sr(II) ions on KOH-modified activated carbon from textile sewage sludges where treatment with KOH significantly increased the surface area from 69.13 to 135m²/g. Although the structure of activated carbon was less complex than that of the precursor material, the remaining functional groups contributed positively to the adsorption by reducing the equilibrium time to 5 min (Kagan and Kiitahyah 2012). During Sr(II) removal with Fe,O₄ particles modified sawdust, the functionalization of the adsorbent assisted in its easy separation as well as in the enhancement of adsorption performance by electrostatic interactions (Cheng et al. 2012).

Surface modification can also influence the adsorption of various anions such as F" ions from aqueous solutions. The potential of zirconium-carbon hybrid sorbent was explored for F~ ions (Velazquez-Jimenez et al. 2014). The adsorption mechanism involved three main steps, where first of all, Zr(IV) species get adsorbed on the surface of activated carbon by COOH groups via electrostatic interactions to form C-O-Zr bonds. The next reaction involved the formation of zirconium oxalate complex by interaction of Zr(IV) with the -OH groups of oxalic acid. It regulates the nucleation and limits the growth of ZrO₂ particles that positively contribute to increased fluoride adsorption. The final step involved the fluoride attack on Zr-oxalate complexes and the displacement of hydroxyl groups by fluoride ions and the adsorption of latter on the ZrOx-AC surface.