Diazonium-based ion-imprinted polymer/clay nanocomposite for the selective extraction of lead (II) ions in aqueous media

Highlights

• Exfoliated clay/ion imprinted polymer nanocomposites were prepared by photopolymerization.

• Montmorillonite was intercalated with diazonium salt prior to photopolymerization.

• Lead adsorption capacity of ion imprinted nanocomposites is as high as 301 mg/g.

• The clay/ion imprinted polymer nanocomposite is specific to lead.

Abstract

Ion imprinted polymer/montmorillonite clay nanocomposite (MMT/IIP) was prepared by the combination of diazonium salt chemistry and photopolymerization process, and used for the selective removal of Pb(II). Toward this end, clay was ion exchanged with dimethylaminobenzenediazonium (DMA) to facilitate incorporation of hydrogen donating sites to the aluminosilicate layers. Upon irradiation of the mixture of the intercalated clay, acrylamide, bisacrylamide and Pb(II)–dithizone complex in the presence of benzophenone in DMSO, the copolymerization initiated by the free radicals formed by the hydrogen abstraction of photoexcited benzophenone from the dimethylamino groups of the intercalated clay. The resulting ion imprinted network nanocomposite and the precursors pristine and diazonium cation intercalated clays were thoroughly characterized by detailed FT-IR, XPS, XRD and SEM. Particularly, XRD analysis brought strong supporting evidence for high exfoliation of the clay upon photopolymerization, whereas XPS permitted to demonstrate that the final nanocomposite has polyacrylamide-rich surface. The adsorption and recognition properties of MMT/IIP for Pb(II) were evaluated using atomic absorption technique. The adsorption capacity was found to be as high as 301 mg/g after about 30 min contact time, and the relative selectivity coefficients to Zn²⁺ and Fe³⁺ were 9.2 and 93, respectively. Compared with non-imprinted clay/polymer nanocomposites, the MMT/IIP exhibited much higher affinity for Pb(II) therefore stressing the importance of the artificial receptor sites within the nanocomposite. Altogether, the results are consistent with an MMT/IIP nanocomposite that is specific to lead II. This work demonstrates for the first time the potential of aryldiazonium chemistry combined with photopolymerization for the fabrication of high performance clay/ion imprinted nanocomposites for the removal of toxic heavy metals.

Introduction

One of the most serious problems affecting water is the chemical pollution by organic or inorganic compounds such as pesticides and heavy metals, respectively. Among the inorganic pollutants, lead (II) presents several health concerns [1]. At high exposure levels in blood (about 40–60 µl/dl), it attacks the brain and the central nervous system, causing coma, convulsion and even death [2]. Children who survive acute lead poisoning may suffer from mental retardation and behavioral disorders [3], [4]. Inhaled or ingested, it can lead to sub-fertility. The US Environmental Protection Agency (USEPA) has set the action level for Pb in drinking water at 15 µgl⁻¹ [5]. Several solid supports have been used for the pre-concentration of lead, such as activated carbon [6], [7], [8], [9]; and modified alumina [10], silica [11], [12] and clay [13], [14]. Also many techniques were developed for the removal of heavy metals by coagulation, chemical precipitation, solvent extraction, membrane separation, ion exchange, complexation, and adsorption. Among these methods, adsorption is preferred for its simplicity and efficiency [15], [16].

Clay minerals have good potential as adsorbents, hinders, however, by weak adsorption of heavy metal ions directly on their surface. Such adsorption occurs mainly by ion-exchange mechanism, however with low cation exchange capacity of the untreated clay which is not sufficient for large scale applications. To overcome this problem, chemical or physical modification of the clay surface with certain functional groups containing donor atoms such as oxygen, nitrogen and sulfur is necessary, yet high selectivity may not be achieved.

To address the crucial problem of selectivity, one can take advantage of the “molecular imprinting” technique. This growing strategy consists in molding the fingerprint of a template molecule in an organic polymer (or inorganic matrix), then use this fingerprint as an artificial receptor to selectively recognize the same template molecule. The same strategy is amenable to the selective recognition of metal ions. Indeed, ion imprinted polymers (IIP), an important subset of MIPs, recognize metal ions selectively. IIPs are usually designed by polymerization in the presence of a metal ion and a chelator, which is an essential building block in shaping the receptor sites of the IIP. Instead of in situ polymerization, the imprinting technique can be conducted with preformed polyelectrolytes that are employed to build, through layer-by-layer approach, multilayered films in which metal ions are complexed [17]. A number of IIPs were prepared by in situ polymerization and applied for the removal of metal ions such as Cd²⁺ [18], [19], Hg²⁺ [20], Cu²⁺ [21] and Pb²⁺ [22], [23], [24], [25], [26], [27].

Particularly, IIPs were prepared as reactive and functional polymer grafts on polyethylene terephalate fibres [28], mesoporous silica (SBA-15) [29], an α-Fe₂O₃ [30] using silane chemistry [31], [32], [33]. As an alternative to these traditional coupling agents, diazonium salts have emerged as promising compounds for the modification of many types of substrates [34] since the seminal paper by Pinson and co-workers who described the mechanisms of reductive grafting of diazonium salts to electrode surface [35]. This 1992 paper has paved large avenues in materials science for development of novel processes and innovative devices [36], [37], [38], [39].

Of relevance to the actual work, aryl layers from diazonium salts permit to tether thin polymer layers to materials surfaces by either grafting to or grafting from routes [40]. Despite numerous advances in the use of diazonium salts as coupling agents for polymers to materials surfaces, only recently they were demonstrated to be excellent intercalants for layered clays [41], [42], [43]. Salmi et al. [41] were the first to modify clay by diazonium salts in view of making clay/polymer nanocomposites by radical photopolymerization. In this work, Type II initiating system consisting of hydrogen donating dimethylaminophenyl groups (DMA) attached to the clay sheets by diazonium salts intercalation and free benzophenone photosensitizer was employed. For the incorporation of hydrogen donating sites onto various surfaces to efficiently generate initiating radicals by the hydrogen abstraction of photoexcited benzophenone, several general coupling strategies were employed. Dyer et al. [44] used thiol chemistry to attach DMA groups to gold surface for the subsequent graft photoinduced polymerization of styrene and methyl methacrylate. The efficiency of this system was further exploited by Gam-Derouich et al. [45] in order to attach bioactive polymer layers on gold modified by dimethylaminophenyl groups from diazonium salts. Silane chemistry, was also demonstrated to be an efficient coupling route for the incorporation of hydrogen donating sites that was used for the modification of bentonite by the described photochemical process [46]. Provided that the silicate layers contain readily available hydroxyl functions such as in organo-modified clay, Cloisite 30B, the DMA groups can directly be incorporated by a simple esterification process [47]. Intercalation of the photoinitiator moiety through quaternized ammonium ions present in the photoinitiator structure is another alternative approach for the purpose [48]. The rationale for such an interest in these Type II initiators are suitable for radical photopolymerization of vinylic monomers and the mechanism of the process is well documented [49]. In addition, Type II initiators are interesting and very well adapted for surface-confined polymerization as they yield only one radical with initiating capability. If the hydrogen donor is attached to the material surface (e.g. through silane, thiol, diazonium or esterification), it thus restricts the polymerization process to the material under test, this contrasts with α-cleavage initiators which usually give two radicals when they dissociate, and therefore permitting to obtain simultaneously both attached polymer chains and free chains in solution. Selecting benzophenone as a hydrogen abstractor, ensures quasi no polymerization in solution when under the UV-light this photosensitizer abstracts hydrogen from the grafted DMA groups and converts to ketyl radicals.

Herein, these considerations led us to adopt the (grafted DMA)/(free benzophenone) tandem as a versatile Type II radical photopolymerization system which can be with many kinds of materials, although there are numerous ways of making clay/polymer nanocomposites for diverse purposes [50], [51], [52]. However, despite its efficiency Type II radical photopolymerization has not been employed so far to modify clays with diazonium salts in view of designing molecularly imprinted clay/polymer nanocomposites; this is what has motivated this work.

In this paper, we shall bridge the gap between pretreatment of clay by diazonium salt and in situ radical photopolymerization of acrylamide (AAm) and bisacrylamide(BAAm) for synthesizing a new ion imprinted polymer (IIP) for the selective removal of lead. Photopolymerization was conducted in the presence of Pb(II) and dithizone chelating agent in dimethyl sulfoxide (DMSO) as schematically illustrated in Fig. 1. For comparison, a non-imprinted polymer/clay nanocomposite (MMT/NIP) was prepared in the same conditions but in the absence of any Pb(II). All materials were characterized by FTIR, XPS, SEM and XRD analyses in order to account for the chemical, morphological and crystallographic structures of the nanocomposites. Lead adsorption isotherms and kinetics were determined using atomic absorption UV–vis spectrometry and both were compared to the adsorption behavior vis-à-vis zinc (II) and iron (III).