Mesoporous silica/polyacrylamide composite: Preparation by UV-graft photopolymerization, characterization and use as Hg(II) adsorbent
Graphical abstract
Introduction
In recent years, pollution by heavy metals became an important concern due to the persistence of metals in nature and their hazardous effects on health. Particularly, mercury has toxic effects on the reproduction of central nervous system, liver, and causes neurological and kidney disturbances [1]. Therefore removal of mercury ions from contaminated water is a major environmental issue to be addressed. In response to this challenge, several adsorbents were designed in view of capturing metal ions from aqueous solutions [2]; namely, activated carbon [3], natural inorganic ion-exchange materials such as zeolites [4], [5], clays [6], chitosan [7], and mesoporous materials [8]. Particularly, mesoporous silicas have emerged as interesting adsorbents for removal of Hg(II). However, due to the inertness of pristine mesoporous silica, it is imperative to modify the latter by appropriate chelatant moieties. Toward this end, mesoporous silica supports were modified by anchoring ligands [9] such as SH and NH2 [10], [11], [12] which are known to strongly adsorb mercury. Alternatively, organic polymers [13], [14], [15], [16], [17] can also be grafted to inorganic supports in order to selectively uptake heavy metal ions. Toward this end, the grafted polymers can advantageously bear carboxylic, sulphonic, amide, amino and imino groups. Particularly, polyacrylamide (PAAM) was found to be an interesting metal ions chelating polymer. For this reason, and in order to take advantage of the surface area of the inorganic support, composite materials of bentonite–polyacrylamide [18], polyacrylamide grafted attapulgite [19] and chitosan beads grafted with polyacrylamide [20] were prepared by in situ thermal radical polymerization and used as adsorbents for heavy metals ions such as Hg2+ [19], Cu2+ [18] Cd2+ [21] and Pb2+[20]. Polyacrylamide is also versatile in the sense it was found to remove different cadmium species, i.e. Cd2+ and Cd(OH)+ in acidic and alkaline media, respectively [21].
Over the recent years, UV-graft polymerization has emerged as an effective mean of preparation of nanocomposites [22] which serve as e.g. ion exchange or chelating adsorbents. The process is simple, fast and amenable to many types of monomers and using several initiating systems. For example, a hydrogen donor could be attached to the surface of the support and a photosensitizer such as benzophenone (BP) could be employed too in order to strip hydrogen from the former. This so-called hydrogen abstraction type photoinitiator (type II) is well known to efficiently photoinitiate free radical polymerization [23]. Ether [24], amine [25], alcohol [26], [27] or thiol [28] functional groups are hydrogen donors which makes them able to initiate photopolymerization. These reactive centers can be attached to materials using silane [19], ammonium [29], diazonium [26], [30], and even poly (ethylene glycol) [24].
Keeping in view the interesting properties of ordered mesoporous materials and polyacrylamide, in this work, polyacrylamide PAAM was grafted onto functionalized mesoporous silica matrix by UV-graft polymerization, a process that has not been explored so far in the literature for generating metal ion adsorbents. The synthesized PAAM amino functionalized MCM-41 composite was characterized by FTIR, TGA (thermogravimetric analysis), and N2 adsorption–desorption, XPS, SEM and XRD techniques. Hg(II) adsorption was investigated versus pH, time and temperature to define the optimal sorption conditions and determine adsorption kinetics and thermodynamic properties.
Section snippets
Materials
Cetyltrimethylammonium bromide (CTAB), tetraethylorthosilicate (TEOS 98%), mercuric chloride (HgCl2), ethanol absolute, toluene, sodium hydroxide (NaOH), hydrochloric acid (HCl), aqueous ammonia (25% NH3), (3-aminopropyl)triethoxysilane (APTES), benzophenone (BP 99%), methanol, acrylamide (AAM), nitric acid (HNO3), acetone, 1,5-diphenylthiocarbazone (dithizone) were all chemicals analytical grade and used as received.
Synthesis of MCM-41
For the synthesis of MCM-41, 2.2 g of CTAB was dissolved in 52 mL of deionized
Results and discussion
The general strategy for making MCM-41-PAAM composite is displayed in Scheme 1. It is a two-step process, starting with silanization of the mesoporous silica followed by in situ radical photopolymerization of acrylamide.
Conclusion
Polyacrylamide-grafted ordered mesoporous silica adsorbent (PAAM-NH2-MCM-41) was successfully synthesized by UV-graft polymerization and characterized using FT-IR, XPS, TGA, BET, XRD and TEM. These analytical tools permitted to monitor the changes in the composition, structure and textural properties of MCM-41 as a result of silanization followed by UV-grafting PAAM. Adsorption of Hg(II) onto PAAM-NH2-MCM-41 was investigated in aqueous media versus pH, contact time and temperature. The metal
Acknowledgements
AS wishes to thank the Tunisian Ministry of Higher Education and Scientific Research for the provision of travel grants to conduct research at ITODYS Lab (University Paris Diderot). P. Decorse is acknowledged for his technical assistance with XPS measurements. We thank S. Hamadi for her help with the TGA analyses.
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