Effect of pH

The adsorption process is profoundly influenced by pH by affecting the surface properties of the adsorbent and the solution chemistry of the metal at a particular pH. However, removal of Pb2+ over cellulose-based carbogel appears to be independent of the solution pH due to the electrostatic interaction between adsorbate and adsorbent having the same charge resulting in cationic exchange in the interlayers of the adsorbent (Alatalo et al. 2015). In some cases, the adsorption is feebly influenced by the pH of the solution, and highest adsorption is exhibited in acidic conditions (Li, Wang, et al. 2017; Rathore et al. 2017). At lower pH values, diethylenetriaminepentaacetic acid (DTPA) is present as H2DTPA3~, H.DTPA2-, H4DTPA~, and H6DTPA+ species on the surface of diethylenetriaminepentaacetic acid-modified magnetic graphene oxide (pHzpc=2.2) in various proportions, with the least proportion of H6DTPA+ (3.51%), and forms stable complexes with the Pb2+ ions favoring adsorption in acidic condition (Li, Wang, et al. 2017). The soft basic S2~ ligands are preferably combined with soft acidic Pb2+ ions (HSAB principle), as hard protons have lower affinity for the former in acidic conditions (pH= 1.75) (Rathore et al. 2017).

The pHzpc value of the adsorbent plays a significant role in adsorption where the adsorbent bears negative charge above it on its surface and leads to an electrostatic interaction between the surface of adsorbent and lead ions. The surface hydroxyl groups were combined with the lead ions for surface adsorption on nickel ferrite (pHzpc = 7.22) (Reddy and Lee 2013).

The maximum removal of lead ions was achieved at various pH values, e.g., 5 (Jiang and Liu 2014; Kumar et al. 2014; Jiang et al. 2012), 5.5 (Lei et al. 2015), 5.8 (Huang et al. 2011; Huang, Wu, et al. 2017), 6 (Pourbeyram 2016; Yan, Kong, et al. 2015; Hadi Najafabadi et al. 2015; Gedam and Dongre 2015; Moradi et al. 2017), 6.5 (Zhang, Yang, et al. 2017), 6.8 (Madadrang et al. 2012), 6.88 (Lv et al. 2017), and 7 (Mahmoud, Abdou, et al. 2016), under different experimental conditions. Electrostatic attraction (Huang, Wu, et al. 2017;Gedam and Dongre 2015; Moradi et al. 2017; Kumar et al.

  • 2014) , surface complex formation (Pourbeyram 2016; Huang et al. 2011), ion exchange and complex formation (Madadrang et al. 2012), dissolution-precipitation (Lei et al.
  • 2015) were held responsible for optimum adsorption at a particular pH. The hindered adsorption at lower pH values was predominantly attributed to the competition of hydronium ion with the lead ions for active sites on the adsorbent surface. In most of the cases, the decrease in adsorption capacity at pH > 6 is attributed to the prevalence of precipitation of lead ions (Pourbeyram 2016; Yan, Kong, et al. 2015). However, some authors have reported different pH values for precipitation, e.g., 6.5 (Zhang, Yang, et al. 2017), 7 (Lv et al. 2017), and 8 (Madadrang et al. 2012).

Effect of the Presence of Ions

The presence of some interfering ionic species affects the adsorption efficiency of the Pb2+ ions to some extent due to the competition for active sites on the surface of the adsorbents. The addition of cations (Zn2+ and Cd2+) and anions (NO3-, SO42-) in the solution has a negligible effect, whereas HCO,~ and HPO42" significantly decline the adsorption capacity of graphene oxide-MnFe2O4 magnetic nanohybrids for Pb2+ ions (Kumar et al. 2014).

The presence of calcium ions has a negligible effect on the adsorption capacity of chelating PE-MA-NN (porous chelating) fiber (Wang, Cheng, Yang, et al. 2013) and layered metal chalcophosphate (Rathore et al. 2017) due to the formation of a stable five-membered complex between [-NH -(CH,)2-NH2] of PE-MA-NN fiber and Pb2+ and the strong interaction between soft acidic Pb2+ and soft basic S2~ ligands of layered metal chalcophosphate (HSAB principle), respectively.

Iron oxide@diaminophenol-formaldehyde (core-shell ferromagnetic nanorod) polymer composite (Fe,O4@DAPF) (Venkateswarlu and Yoon 2015a) exhibited the highest removal efficiency for Pb2+ than other metal ions i.e. Cd2+, Zn2+, Hg2+, and As3+ . The high efficiency of Pb2+ ion can be explained by Pearson acid-base classification, which advocates the binding preference of borderline acids to borderline bases, and thus, the borderline acid i.e. Pb2+ preferentially attracted to the borderline base i.e. aminefunctional group of the DAPF resin.

The presence of Ni2+, Co2+, Zn2+, and Cu2+ did not affect the metal uptake capacity of ethylenediamine-grafted MIL-101 (ED-MIL-101, chromium-based) because the -NH2 groups of ethylenediamine that coordinated with metal ions in ED-MIL-101 did not match well with other competing ions (Luo, Ding, et al. 2015), whereas Zn2+, Ni2+, Cd2+, and Cu2+ ions significantly decreased the adsorption capacity of the diethylenetriaminepentaacetic acid-modified magnetic graphene oxide composites (Li, Wang, et al. 2017).

The adsorption of Pb2+ ions on cross-linked poly(glycidyl methacrylate) microspheres functionalized with triazole-4-carboxylic acid was marginally affected by the presence of other interfering heavy metal (Mg2+, Ca2+, and Cu2+) ions. The adsorption followed Pauling’s electronegativity order, in addition to the opposite order of hydrate’s radii (Pb2+ > Cu2+ > Ca2+ > Mg2+). The cations with greater electronegativity and smaller hydrate radii tended to bind more favorably via surface complexation and electrostatic interactions (Yuan, Zhang, et al. 2017).

Zeta Potentia

The zeta potential of EDTA-graphene oxide (EDTA-GO) showed variation with the pH values as it decreased initially when the pH increased from 3 to 6, formed a plateau between 6 and 8, and then increased at pH 12. The adsorbent surface was negatively charged at all the pH ranges due to ionization of functional groups present on the surface leading to more negative charges on the surface, resulting in stronger interaction between lead ions and EDTA-GO (Madadrang et al. 2012). However, the surface of graphene oxide-zirconium phosphate (GO-Zr-P) was slightly positively charged at pH 1 (Pourbeyram 2016). The slight negative charge present on the surface slowly increased in the pH range of 2-6, and at pH >6, deprotonation of phosphate groups led to a quick rise in surface negative charge. At pH> 8, a gradual detachment of Zr-P nanoparticles from the surface of GO-Zr-P was observed.

During the adsorption of Pb2+ ions on layered metal chalcophosphate (Rathore et al. 2017), Pb2* ions occupy the Mn2+ vacancies in the layer and pull out the intercalated hydrated K+ ions from the interlayer, decreasing the inter-lamellar spacing.

This process generates the excess negative charge on surface S atoms promoting the adsorption of extra Pb2+ over the surface. The adherence of Pb2+ ions on the surface diminishes the Mn3+ concentration in Pb-MPS-1, which was evident by XPS studies. Therefore, Pb2+ ions were adsorbed both in the Mn2+ vacancies and on the surfaces. The zeta potential measurement showed that the intercalation of K+ led to increased negative charge on MPS-1, which was diminished by the sorption of Pb2+ into K-MPS-1.

Effect of Surface Modification and Material

The modification of the graphene oxide (GO) surface with EDTA groups through silanization significantly enhances the adsorption capacity of GO for heavy metal ions (Madadrang et al. 2012). Formaldehyde- and 2,3-diaminophenol-based polymer use for surface modification of ferromagnetic Fe3O4 nanorods led to the protection of Fe,O4 surfaces and enhancement of the stability as well as surface area of the composite (Venkateswarlu and Yoon 2015a). The introduction of poly (acrylamide) into reduced graphene oxide (RGO) significantly enhanced its dispersion in solution and its metal uptake capacity (Yang, Xie, et al. 2013). Similarly, functionalization of PE-MA-NN fibers synthesized with polyethylene via with hydrophilic groups (amino/amide groups) (Wang, Cheng, Yang, et al. 2013), chitosan through a-MnO2 with valine amino acid (Mallakpour and Madani 2016), and MIL-101 modification with ethylenediamine (Luo, Ding, et al. 2015) increased their adsorption capacities for lead ions.

XRD of the ED-MIL-101 and Pb/ED-MIL-101 (material after Pb2+ adsorption on ED-MIL-101) depicted the same XRD patterns, which suggested towards no change in the crystal structures of adsorbent after adsorption. However, the modification of MIL-101 with excess ED resulted in the absence of its characteristic peaks in XRD patterns that may be due to the destruction of crystalline order of framework or decomposition of crystalline MIL-101 (Luo, Ding, et al. 2015). In the XRD pattern of Fe,O4@DAPF, an additional broad peak appeared along with the characteristic peaks that were attributed to the scattering by DAPF resin. The intensities of peaks of Fe,O4@DAPF were also diminished probably by the coating of polymer resin shell on the surface of Fe,O4 nanorods (Venkateswarlu and Yoon 2015a). Upon modification of a-MnO, rods with valine, the resulting material showed well-defined diffraction lines of a-MnO2 phases with higher crystallinity and L-valine. The XRD pattern of chitosan revealed it as hydrated crystals in which chitosan chains aligned antiparallelly forming sheet-like structure stacked together by bonding with inter-molecular hydrogen leading to 3D crystals. On modification with a-MnO2-valine, the characteristic peak of chitosan disappeared due to expansion of spacing between chitosan chains in each sheet by incorporation of functional groups in the backbone (Mallakpour and Madani 2016).

The modification was also established by FT-IR spectrum where strong adsorption bands at 3386 and 3185 cm"1 corresponded to the bonding between O-H in DAPF and Fe-0 by covalent bonding (Venkateswarlu and Yoon 2015a). The FT-IR bands at 1581, 1051, and 882cm"1 were attributed to the N-H plane stretching, C-N bond stretching, and -NH2 stretching, respectively, advocating the grafting of ED on

MIL-101 (Luo, Ding, et al. 2015). A peak at 2925 and 1647cm-1 corresponding to C-H asymmetric stretching and C=O stretching vibrations, respectively, proved successful grafting of poly(acrylamide) chains on RGO (Yang, Xie, et al. 2013).

Mechanism

The adsorption mechanism of lead ions on MgO nanostructures involved solid-liquid interfacial reaction between MgO and Pb2* ions (Cao, Qu, Wei, et al. 2012) where Mg2+ ions got exchanged with Pb2* ions during adsorption and lead ions replaced the Mg2+ ions in the MgO crystal lattice. There exists a nearly linear relationship between the amount of Mg2+ ions released in the solution and the amount of Pb2+ ions adsorbed on the surface of material. The XRD pattern of material after adsorption containing diffraction peaks of MgO and PbO showed the partial chemical reaction between MgO and Pb2+ ions. The presence of FT-IR peaks of Mg(OH)2 after adsorption, but the absence of peaks of Pb(OH), after adsorption, is another indication of chemical reaction amid MgO and Pb2+ ions. Further, the SEM image revealed that the flowerlike morphology of MgO after adsorption got distorted because the formation of PbO hindered and blocked the reaction of MgO with lead ions when the first few layers of the Mg2+ got exchanged with Pb2+ ions.

The adsorption of lead ions on graphene oxide-MnFe,O4 magnetic nanohybrids occurred through the complexation mechanism - i.e. the competition between protons and metal ions gets diminished at higher pH values and -OH groups get ionized to -O-, leading to enhancement in adsorption; and cation exchange reaction where some of the GO-COO- and GO-O- groups present on the surface of graphene helped in the adsorption of Pb2+ ions (Kumar et al. 2014).

The sorption of lead ions on graphene oxide-zirconium phosphate nanocomposite involved the complexation mechanism (Pourbeyram 2016). The XPS spectrum of graphene oxide-zirconium phosphate nanocomposite after the adsorption of Pb2+ ions displayed five peaks assigned to the different forms of oxygen, viz. C-O, P-O-H, Zr-O-C, Zr-O-P, and P-O-Cd. The significant reduction in the peak intensity of P-O-H and appearance of a new peak of P-O-Cd after adsorption indicated that chemical interactions such as complexation with phosphate groups on the surface of graphene oxide-zirconium phosphate nanocomposite were responsible for the adsorption of Pb2* ions.

Desorption

The desorption of lead ions was carried out by various desorbing agents such as hydrochloric acid (Pourbeyram 2016; Kumar et al. 2014; Madadrang et al. 2012; Jiang and Liu 2014; Li. Wang, et al. 2017; Gedam and Dongre 2015; Lv et al. 2017; Bayuo et al. 2020), nitric acid (Yuan, Zhang, et al. 2017), EDTA (Luo, Ding, et al. 2015), and both hydrochloric acid and EDTA (Wang, Cheng, Yang, et al. 2013).

The surface -OH and -COOH groups protonated at lower pH values resulted in desorption (Kumar et al. 2014). The decrease in pH increased the desorption with a maximum of 90% at pH <2, and ~92% of the material was regenerated within 1 h. EDTA-graphene oxide exhibited retention of about 80% of its initial adsorption capacity on repeated use up to ten cycles (Madadrang et al. 2012). The higher concentration of H+ at lower pH changed -COO" into -COOH, which was competitive against the -COO- and Pb2+ interaction, and thus accelerated the desorption of lead ions (Jiang and Liu 2014).

The adsorption capacity and the desorption rate of lead ions remained constant after PE-MA-NN regeneration up to five consecutive cycles (Wang, Cheng, Yang, et al. 2013). The desorption of lead ions from poly(glycidyl methacrylate) microspheres functionalized with triazole-4-carboxylic acid was rapid chemical desorption, and it followed pseudo-second-order kinetics (Yuan, Zhang, et al. 2017).

 
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