Covalent Attachment of Quaternary Ammonium Group to Geopolymer Surface With Silane Coupling Agent to Enable Simultaneous Adsorption of Cations and Anions

Abstract

Geopolymers are amorphous aluminosilicates that are actively studied as adsorbents for wastewater treatment. However, their use is limited to separation of cations due to their negatively charged surface. In this study, conventional metakaolin geopolymer (Si:Al:Na = 3:1:1) was modified with a silane coupling agent, N-[3-(trimethoxysilyl)propyl]-N,N,N-trimethylammonium chloride in acidified methanol at pH 3.5, to covalently attach a positively charged quaternary ammonium group to the surface which acts as an anion exchange site. The successful attachment of the silane coupling agent was confirmed with X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS), and scanning electron microscopy (SEM). The modified geopolymer was capable for the simultaneous adsorption of ammonium (NH₄⁺) and nitrate (NO₃⁻) with adsorption capacities of 3.29 and 0.15 mmol/g (or 59.4 and 9.3 mg/g), respectively, according to the Langmuir-Freundlich isotherm (R² ≥ 0.98 and R² ≥ 0.94, respectively). Under same conditions, the non-modified geopolymer adsorbed 0.74 mmol/g of NH₄⁺ and showed negligible NO₃⁻ adsorption. Thus, this study demonstrates a proof of concept that silanization enables the development of dual cation-anion geopolymer ion exchangers.