Adsorption properties of amidoxime resins for separation of metal ions from aqueous systems

doi:10.1016/j.reactfunctpolym.2008.09.011

Reactive and Functional Polymers

Volume 68, Issue 12, December 2008, Pages 1665-1670

https://doi.org/10.1016/j.reactfunctpolym.2008.09.011 Get rights and content

Abstract

In this study, the acrylonitrile (AN)–divinylbenzene (DVB)–methylacrylate (MA) resin was synthesized via suspension polymerization in the presence of toluene as diluent, and benzoylperoxide (BPO) as initiator. The effects of MA as a hydrophilic agent and toluene as a diluent on the anion and cation exchange capacity of the synthesized amidoxime resins were investigated before and after alkaline treatment. The results showed that the anion exchange capacity decreased with an increase in the amount of MA while alkaline treatment had no significant effect. Also, the cation exchange capacity increased with an increase in the amount of hydrophilic agent and reached a maximum point. The sorption equilibrium was achieved relatively fast within 40 min, and the resin exhibited affinity towards lead (II), copper (II) and in particular U(VI), and the order of adsorption affinity were ${UO}_{2}^{2 +}$ > Pb²⁺ ≫ Co²⁺ > Cu²⁺ ≫ Cd²⁺. The adsorption of uranium was directly depended up on the pH value. Furthermore, the macroreticular chelating resin, containing amidoxime group had higher adsorption of uranium (VI) than other metal ions studied. Finally, the alkaline treatment enhanced the potential for much faster adsorption and the highly porous chelating resin provided a more favorable pore structure for the rapid rate of diffusion of metal ions.

Introduction

There is a continuous need for new separation techniques which selectively extract metal ions from dilute waste waters and industrial process streams. Concern over environmental issues and possible re-cycle of extracted metals have enhanced the search for new processes. Ion exchange has been widely studied for the recovery of metal ions from diluted streams [1], [2], [3], [4], [5]. Commercially available ion exchange resins show high performances but generally poor selectivity towards different metal ions [6]. A high selectivity can be observed in some cases [7], but kinetics is slow due to the hydrophobic character of the polymeric backbone [8]. An appealing alternative method is the use of chelating agents grafted on a hydrophilic support in a solid–liquid extraction process.

Many complexing agents which form negatively charged complexes have been employed for the separation of metal ions on anion exchange resins. The separations are based on the different stabilities of the complexes, and their different affinities for the resins [9], [10], [11], [12], [13]. It is worth noting that, chelating groups have been introduced into resins via synthetic techniques or by simple loading, and the modified resins have been used for the selective recovery or preconcentration of metal ions [14], [15], [16], [17].

The process using adsorbents is thought to be the most effective method for recovering heavy metals because of its high selectivity, the ease of handling and the environmental issues. Adsorbents containing both amidoxime and hydrophilic groups have a much higher uptake potential than those containing amidoxime. This is due to less hydrophilic nature of the amidoxime group [18], [19].

Varraest et al. [20] demonstrated that Inuline modified with an amidoxime groups formed stable complexes with Cu(II). Colella et al. [21] also demonstrated that poly (acrylamidoxime) can be successfully used for the preconcentration of trace metals from aqueous solutions. Hence, in this work, a new polymeric adsorbent having both hydrophilic and amidoxime groups were synthesized via suspension polymerization and the effects of different parameters such as hydrophilic and diluent agents investigated. Also the adsorption potential of the resin for some metal ions, especially those which have impact on the environment, from aqueous media was considered.

Section snippets

Experimental

All the reagent and chemicals used were of analytical grade obtained from Merck.

Results and discussion

The result on amidoxime polymer air dried is shown in Table 1. The pore structure of amidoxime polymer was estimated from the mercury intrusion measurement on the polymer sample swollen in water and air dried. The pore structure of amidoxime polymer depends on the quantity of toluene. As the quantity of toluene increases, the average pore radius, and the pore volume increase and hence the polymer exhibits a higher porosity.

An Infrared spectrum of the amidoximated resin is shown in Fig. 1. The

Conclusion

The effect of hydrophilic agent, diluent and alkaline treatment on the exchange capacity of AN–DVB–MA showed that the anion exchange capacity decreased with an increase in the amount of hydrophilic agent, while alkaline treatment had no significant effect. It is worth nothing that in the amidoxime resins, the anion exchange capacity determines the content of amidoxime groups. Furthermore, the cation exchange capacity increased with an increase in the amount of hydrophilic agent to reach a

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In fact, with a facile amidoximation functionalization, PAN can carry numerous amidoxime groups and become an excellent adsorbent for strong affinity of heavy metal ions (Nilchi et al., 2008). In recent years, amidoxime-PAN in form of porous PAN fiber textile has been reported (Nilchi et al., 2008; Saeed et al., 2008). Mccomb et al. modified a commercial PAN textile and the obtained amidoxime-PAN cloth could be used in a device for the in-situ concentrating of trace metals in water (Mccomb and Gesser, 2015).
Porous modified polyacrylonitrile (PAN) with an ultrahigh percentage of amidoxime groups (UAPAN) was synthesized for the first time and used to adsorb antimonite (Sb(III)) and antimonate (Sb(V)) from aqueous solution. Fourier transform infrared (FT-IR), Zeta potential, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) were adopted to characterize UAPAN and explore adsorption mechanism. Moreover, batch experiments were performed to investigate the influence of various adsorption parameters, including initial pH, contact time, temperature, coexisting ions and reusability on adsorption capacities. Results showed that the maximum adsorption capacities for Sb(III) and Sb(V) were 125.4 and 177.3 mg g⁻¹, respectively, which were much higher than those of other adsorbents reported in literature. The adsorption thermodynamics was evaluated, indicating the spontaneous and endothermic adsorption. The adsorption isotherm was suitable to be modeled by Langmuir isotherm (R² > 0.96). Results of FT-IR, Zeta potential and XPS indicated that adsorption was involved with electric charge attraction and ligand exchange. DFT further explained that better adsorption of Sb(V) on UAPAN than that of Sb(III) was caused by the higher adsorption energy, more favorable bond lengths and atom charge density. Accordingly, UAPAN is expected to be a compelling candidate for antimony decontamination from aqueous environment.
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Amidoximes are polar and ionizable at both low and high pH (pKa1 ∼4.5 and pKa2 ∼13.3): at pH < 4, they are cationic; at pH 5–13, they are primarily charge-neutral; and at pH > 13, they are anionic (Figure 3A).49,50 Such functionality grants expansive opportunities for cation exchange membrane development at high pH. Prior to our use of them as pore-lining chemical functionality in microporous polymer membranes for electrochemical devices, amidoxime-based polymeric materials have been previously implemented in gas separations10 and in extracting lanthanides and actinides from processed ores, where their stability at pH > 13 is a strict requirement.8,9,51 To address the structural rigidity of the polymer backbone, we were drawn to an architectural platform based on ladder polymers, where conformational and configurational entropy are low and intrinsic microporosity can be high (10%–30%), relative to other classes of polymers (e.g., cellulosics, polysulfones, polyamides, polyimides, polyolefins, etc.).
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This gives an order of U > Pb ≫ Cd, Cu using commercial poly(amidoxime) chelating polymer. Moreover, the result matches well with those using other adsorbents Nilchi et al., reported the order of U > Pb ≫ Cu ≫ Cd using acrylonitrile (AN)–divinylbenzene (DVB)–methylacrylate (MA) adsorbent [40], Kavakli et al., reported U ≫ Cu > Pb using a chelating polymer containing double amidoxime groups [41]. According the HSAB theory, NH and NOH of P(AO)-g-CTS/BT act as hard base, uranyl ions act as hard acid so the adsorbent exhibits good adsorption capacity towards uranyl ions.
A novel amidoxime functionalized adsorbent, poly(amidoxime)-grafted-chitosan/bentonite composite [P(AO)-g-CTS/BT] was prepared by in situ intercalative polymerization of acrylonitrile (AN) and 3-hexenedinitrile (3-HDN) onto chitosan/bentonite composite using ethylene glycol dimethacrylate (EGDMA) as cross linking agent and potassium peroxy disulphate (K₂S₂O₈) as free radical initiator. The adsorbent was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), BET surface area analyser and X-ray photoelectron spectroscopy (XPS). Nitrile groups from two monomers converted to amidoxime groups and therefore, increases the adsorption efficiency of uranium(VI) [U(VI)] from seawater. The optimum pH for U(VI) adsorption was found to be 8.0. The adsorbent dosage of 2.0 g/L was sufficient for the complete removal of U(VI) from seawater. The kinetic data fitted well with pseudo-second-order kinetic model which assumes the presence of chemisorption. The equilibrium attained within 60 min and well agreement of equilibrium data with Langmuir adsorption model confirms monolayer coverage of U(VI) onto P(AO)-g-CTS/BT. The maximum adsorption capacity was found to be 49.09 mg/g. Spent adsorbent was effectively regenerated using 0.1 N HCl. Six cycles of adsorption-desorption experiments were conducted to study the practical applicability and repeated use of the adsorbent. The feasibility of the adsorbent was also tested using natural seawater. The results show that P(AO)-g-CTS/BT is a promising adsorbent for the removal of U(VI) from seawater.

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Adsorption properties of amidoxime resins for separation of metal ions from aqueous systems

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Talanta

Coord. Chem. Rev.

Inorg. Chim. Acta

Eur. Polym. J.

J. Radiat. Phys. Chem.

React. Funct. Polym.

J. Alloy. Comp.

Carbohyd. Polym.

Environmental Chemistry

Ion Exchange, Science and Technology

J. Radioanal. Nucl. Chem.

Rad. Phys. Chem.

Sep. Sci. Technol.