Elsevier

Applied Surface Science

Volume 426, 31 December 2017, Pages 29-39
Applied Surface Science

Full length article
Rapid adsorption of Pb, Cu and Cd from aqueous solutions by β-cyclodextrin polymers

https://doi.org/10.1016/j.apsusc.2017.07.103Get rights and content

Highlights

  • A high-surface-area and porous β-CD polymer adsorbent is prepared.

  • Rapid adsorption of Pb, Cu and Cd (5 min) is realized.

  • High adsorption capacities for Pb (196.4 mg/g), Cu (164.4 mg/g), Cd (136.4 mg/g) are obtained.

  • The order of removal efficiencies in multi-component adsorption is Pb>Cu>Cd.

  • H+ ions exchange and electrostatic interactions work together in the adsorption.

Abstract

Removing heavy metals from aqueous solutions has drawn more and more attentions these years because of their serious global health challenge to human society. To develop an adsorbent with low-cost and high-efficiency for removal of heavy metals (HMs), β-cyclodextrin (β-CD) polymers crosslinked with rigid aromatic groups were prepared and used for lead (Pb), copper (Cu) and cadmium (Cd) removal for the first time. The negatively charged β-CD polymers with large BET surface area were suitable to be used in HMs adsorption. The adsorption process completed in 5 min was well fit by Freundlich isotherm model and pseudo-second-order model. The intraparticle diffusion model was also appropriate to describe the adsorption of Pb, Cu and Cd on β-CD polymer. The maximum of adsorption capacities at 25 °C for Pb, Cu and Cd were 196.42, 164.43 and 136.43 mg/g when the initial concentration was 200 mg/L. The HMs adsorption process on the surface of β-CD polymer was an endothermic and spontaneous process. Both of the electrostatic interaction and distribution of Pb, Cu and Cd species influenced the adsorption process at different pH values. The order of removal efficiencies in multi-component adsorption for the three metal ions were Pb > Cu > Cd. The adsorption mechanisms were H+ ions on hydroxyl groups exchanged with heavy metal ions and electrostatic interactions. This study indicated that β-CD polymers could be developed into effective adsorbents for rapid removal of heavy metals.

Introduction

The presence of toxic heavy metals has caused a serious global health challenge to human society [1], [2]. It becomes a major water pollution problem in many countries because of the indiscriminate discharge of untreated wastewater [3]. Long-time of drinking water contaminated by heavy metals such as lead (Pb), copper (Cu), and cadmium (Cd) would result in cardiovascular, pulmonary, immunological, neurological, and endocrine disorders and cancers [4]. Thus, removing heavy metals from aqueous solutions has drawn more and more attentions these years [5]. Many stringent standards are established to control heavy metal concentrations in discharged water. Such a strict circumstance offers a huge opportunity for water treatment technologies [6]. To solve the heavy metal pollutions, it is of great significance to develop robust and effective purification processes of wastewaters contaminated by heavy metals.

So far, a mass of water treatment technologies have been developed and utilized to remove heavy metals (HMs) from water including ion exchange (IE) [7], membrane filtration [8], [9], coagulation-flocculation-sedimentation (CFS) [10] and adsorption [11], [12]. Among them, CFS is widely used owing to its simplicity and low cost. However, this method will generate a large volume of sludge with low density, which required further dewatering and disposal [13]. Membrane technology possesses high efficiency and small footprint while it also faces some limitations like high energy requirement, concentrated disposal and difficulty in maintenance [14], [15]. The disadvantage of ion exchange (IE) is the interference of competing ions and regeneration [16]. Different from those methods mentioned above, adsorption is a safe method with high effectiveness and low cost [17], [18], [19]. This method can remove soluble and insoluble pollutants effectively without generation of hazardous by-products [20], [21], [22], [23], [24], [25]. To date, a wide variety of adsorbents with specific affinity to HMs have been developed and applied into water treatment [26], [27], [28], [29], [30], [31], [32], [33]. However, there were still some problems unresolved associated with their applications, one of the most serious problems was the low adsorption rate. Most of the adsorbents needed a long time to get equilibrium, which would result in a large waste of energies. Thus, it's highly important to develop adsorbents with high adsorption rate at present.

Among the HMs adsorbents, β-cyclodextrin (β-CD) is an inexpensive, sustainably produced macrocycle of glucose, Due to the possibility in removing HMs from water through adsorption method, β-CD has drawn much attention in the present study [34], [35]. When used as an adsorbent, β-CD can encapsulate pollutants in aqueous solution to form well-defined host-guest complexes [34]. However, the application of β-CD was limited by the disadvantages of low surface areas and poor removal performance compared to conventional adsorbents like activated carbons [36], [37]. Thus, improving the surface areas and removal performance of HMs for this kind of inexpensive adsorbent is highly important for the moment.

In this study, to increase surface areas of β-CD, we crosslinked β-CD with cross-linking agent contained rigid aromatic groups like tetrafluoroterephthalonitrile. The rigid structure of tetrafluoroterephthalonitrile linked with β-CD would generate a high-surface-area and porous polymer of β-CD. Thus the adsorption capacity and adsorption rate would be largely improved. Here, Pb, Cu and Cd were selected as the representive heavy metals for the serious global health challenge to human society. The as-synthesized β-CD polymer can rapidly capture a variety of HMs with higher adsorption capacity and adsorption rate than other adsorbents. The adsorption behavior of β-CD polymer for Pb, Cu and Cd was investigated in the way of adsorption kinetics, isotherm and thermodynamic. The effects of pH and multi-component adsorption were examined. The mechanism of HMs removal was studied by FT-IR and XPS analyses to reveal the active sites in adsorption process.

Section snippets

Materials

All reagents used in our investigation are analytical grade and were purchased from Shanghai Chemical Reagents Company and used without further purification. The stock solutions of Pb, Cu and Cd were prepared with deionized water using Pb(NO3)2, Cd(NO3)2·4H2O and Cd(NO3)2.4H2O. The desired pH values of the working solutions were controlled by 0.1 mol/L HNO3/NaOH solution.

Synthesis of β-CD polymer

The synthesis process of β-CD polymer was shown in Fig. S1 (see Supplementary material). Tetrahydrofuran/dimethylformamide

Characterization of β-CD polymer

Fig. 1 showed the TEM image of the as-synthesized β-CD polymer, which demonstrated the porous structure of β-CD polymers. Fig. 2a presented the FT-IR spectra of β-CD polymer, β-CD monomer and tetrafluoroterephthalonitrile. The FT-IR spectrum of tetrafluoroterephthalonitrile showed absorbance at 2235 cm−1, which was corresponding to the nitrile stretch of tetrafluoroterephthalonitrile, the absorbance at 1670 cm−1 and 1463 cm−1 were corresponding to Csingle bondC aromatic stretches [34]. C-F stretches, which

Conclusions

In summary, mesoporous β-CD polymers crosslinked with rigid aromatic groups were prepared and used for HMs removal for the first time. The negative charge and large BET surface area made β-CD polymer a wonderful adsorbent for Pb, Cu and Cd. The adsorption process of Pb, Cu and Cd happened rapidly and got equilibrium in 5 min. The adsorption process of Pb, Cu and Cd took place on heterogeneous surfaces that varied with surface coverage. The maximum of adsorption capacities at 25 °C for Pb, Cu and

Acknowledgments

This work was supported by the National Key Scientific Program-Nanoscience and Nanotechnology (Grant No. 2011CB933700), the CASHIPS Director's Fund (Grant No. YZJJ201617), the National Natural Science Foundation of China (21177131, 61273066, 11205204, 21105001, 21077106, and 61104205).

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