Design and development of ion-selective polymer-supported reagents: The immobilization of heptamolybdate anions for the complexation of silicate through Keggin structure formation

Abstract

A silicate-selective polymer-supported reagent has been developed that utilizes the reactivity of silicate to generate polyanionic Keggin structures. Heptamolybdate and heptatungstate anions are immobilized onto trimethylammonium ligands bound to microporous poly(vinylbenzyl chloride) beads. The heptamolybdate complexes >90% of the silicate from a 20 ppm solution at pH 7; the heptatungstate has a lower affinity, complexing 40% of the silicate. Complexation by the heptamolybdate remains high throughout the pH range 3.8–10.7. Sorption is unaffected by the presence of chloride, sulfate, and nitrate ions. The apparent rate of reaction is maximized by immobilizing the ligand on an expanded gel support: whereas the microporous polymer requires 24 h to complex all of the silicate from a 100 ppm solution, the expanded gel attains that level in 4 h. The rate-limiting step is thus identified as accessibility of the silicate to the heptamolybdate rather than the rate of reaction to form the silicomolybdate. FTIR spectra confirm silicomolybdate formation: The heptamolybdate polymer has four characteristic bands at 715, 845, 912 and 945 cm⁻¹ and these bands get weaker as silicate reacts with the heptamolybdate; at complete reaction, the band at 845 cm⁻¹ disappears. The spectrum of the silica-saturated polymer has strong bands at 900–904 cm⁻¹ and 790–800 cm⁻¹, consistent with the spectrum of tert-butylammonium silicododecamolybdate.

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 pK_a 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.