Design and development of ion-selective polymer-supported reagents: The immobilization of heptamolybdate anions for the complexation of silicate through Keggin structure formation
Graphical abstract
Introduction
Silica is one of the most abundant elements on Earth. All natural water contains dissolved silica at levels of 1–30 ppm [1] with some water, such as that in New Mexico, having levels higher than 50 ppm [2]. The solubility of silica in water is 120–140 ppm [3]. Silica is a major concern for power plants, desalination units and the microelectronics industries. It deposits on the surface of boilers, cooling systems, and reverse osmosis (RO) membranes causing energy losses and increased maintenance costs. Scaling problems are obviated when the silica level is <10 ppm in RO feed water at a water recovery rate of >98% [4]. The silica concentration in water to high pressure boilers should be <8 ppm to prevent scale formation [5].
Silica is present in water in three principal forms: suspended solids (which is not problematic in water purification), colloidal silica (which can be efficiently removed by filtration and RO), and reactive silica (the major component of which is silicic acid in equilibrium with SiO2) [6]. Silicic acid is a weak acid, with pKa values of 9.5 and 13.5 [7]. It is less than 1% ionized at pH values below 7.5 and so cannot be selectively removed by ion exchange resins from natural water. It can be removed with type 1 and type 2 strong base anion exchange resins in the hydroxide form [8] but they are not selective (vide infra) and raise the pH of the effluent water (which is unacceptable in most applications).
The objective of this research was to design and prepare a reactive polymer for the selective removal of silicate from water. The main prerequisite was identifying a species that could react with silicate and be immobilized onto a polymer. Keggin anions are one class of polyanions having the general formula [XM12O40]n−, where M is either Mo or W, and X is one of a number of atoms, including Si and P [9]. The heptamolybdate ion is able to complex reactive silica to form the silicomolybdate [10] and the heptatungstate ion forms the silicotungstate [11]. Since ionically bound species retain their reactivity within a polymer support [12], crosslinked poly(vinylbenzyl chloride) beads were thus prepared, functionalized with trimethylamine, and modified with heptamolybdate and heptatungstate anions by ion exchange. The sorption affinities were then quantified and the selectivity for silicate evaluated.
Section snippets
Experimental section
Copolymer beads of vinylbenzyl chloride (VBC) and 2 wt% divinylbenzene (DVB) were prepared by suspension polymerization with 0.5% benzoyl peroxide as initiator [13]. Gel and expanded gel beads were prepared in the same manner except that toluene was added in an equal weight to the monomer when preparing the latter. The toluene was removed after polymerization by washing the beads with methanol. The beads were sieved and a particle size of 250–425 μm was used. Silica solution was prepared from a
Results
The hydrophilicity (as measured by the percent solids), nitrogen capacity, and molybdenum capacity of the trimethylammonium gel resins are given in Table 1. A comparison of the calculated and experimental nitrogen capacities shows that full functionalization has been achieved. The chloride-form resin retains a large amount of water due to the ionicity of the ligand–substrate interaction. The hydrophilicities of the heptamolybdate and heptatungstate form of the resins are significantly less
Discussion
Functionalization of the polyVBC beads with trimethylamine leads to the quaternary ammonium resin which, in the chloride form, has a low solids level (Table 1). This is due to the inherent hydrophilicity of the ionic ligand within a polymer network that is lightly crosslinked. Molybdate exists in solution as different oxomolybdenum species depending on the Mo concentration and the pH of the solution [16]. In alkaline and neutral solutions, molybdates are present as the monomeric [MoO4]2− ion.
Conclusion
Immobilization of a molybdate ion within a crosslinked polymer does not hinder its reaction with silica to form a Keggin ion despite the large size of that ion. The large size of the anion limits the overall capacity of the polymer for Si on a millimolar basis, even at full functionalization of the polymer. Access of the silica into the polymer is hindered if the matrix does not have a high water content, but increasing the resin hydrophilicity increases the apparent rate of reaction. The most
Acknowledgment
We gratefully acknowledge support from the New York State Energy Research and Development Authority, the Pall Corporation, and the Department of Energy, Office of Basic Energy Sciences through Grant No. DE-FG02-ER15287.
References (23)
Comparative chemical clarification for silica removal from RO groundwater feed
Desalin
(2003)- et al.
A critical review of fouling of reverse osmosis membranes
Desalin
(1981) - et al.
Adsorptive removal of phosphate ions from aqueous solution using synthetic zeolite
Ind Eng Chem Res
(2007) - et al.
Silica in water in relation to cooling tower operation
(1975) Silica solubility, 0-200_ and the diagenesis of siliceous sediments
J Geol
(1962)Water purification technologies and silica breakthrough
Am Lab
(2002)Silica in aqueous environments
J Non-Cryst Solids
(1996)- et al.
Silica removal using ion-exchange resins
Desalin
(2004)et al.Silica removal by ion exchange
Power
(1950) The nomenclature of polyoxometalates: how to connect a name and a structure
Chem Rev
(1998)
The preparation and properties of silicomolybdic acid III: the combination of silicate and molybdate
J Am Chem Soc
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