Biogas purification by adsorption of hydrogen sulphide on NaX and Ag-exchanged NaX zeolites

Abstract

Biogas is a cheap renewable and eco-friendly alternative energy to fossil fuels. However, biogas is made up of a mixture of gases including hydrogen sulphide (H2S), a corrosive and toxic gas. The current study was carried out to purify biogas by adsorbing H2S on NaX and Ag-exchanged NaX zeolites. Pure crystalline NaX zeolites were synthesized by microwave heating technique. NaX zeolites were then modified following Ag-exchange with various aqueous solutions of AgNO3 (11.5%, 34.1%, 71.6%, 76.7%, and 78.8%) at 40 °C for 2–12 h. Compositional and structural analyses of NaX and Ag-exchanged NaX zeolites were done using powder X-ray diffraction (XRD), elemental analysis, nitrogen adsorption porosimetry, and atomic absorption spectrometry. Anaerobic digestion (AD) was carried out by feeding the digesters with a mixture of 600 mL of dairy manure and 4.0 g L−1 NaX and Ag-exchanged NaX zeolites. An almost complete exchange of Na+ cations by Ag+ cations was observed. The Ag-exchanged zeolites exhibited type I isotherm features with no modification to the shape. A well-dispersed Ag+ ion in the zeolite framework was observed. More H2S was efficiently removed in Ag-exchanged NaX zeolites digesters when compared to that of NaX zeolite. Methane yield was significantly increased from 167.31 (control) to 215.33 (78.8% Ag/NaX) mL g−1 after 48 days. Ag-exchanged NaX zeolites in the current study is a promising adsorbent for H2S removal in biogas.