Siloxane adsorption by porous silica synthesized from residual sand of wastewater treatment

Abstract

Wastewater from municipal sewage may be an interesting source of biogas, in which case siloxanes are commonly present above the spec limit to be used in power engines. On the other hand, residual sand from Wastewater Treatment (WWT) Plants is also rich in inorganic matter, which can be a suitable feedstock for the synthesis of porous inorganic adsorbents. This work describes the synthesis of an alternative adsorbent for siloxane adsorption using residual sand from a WWT plant. The ultimate goal was to find a substitute for commercial activated carbons or silicas currently employed in siloxane removal from biogas. The residual sand was subject to thermal and acid treatment, which successfully increased its silicon content while developing adequate internal porosity. The textural characteristics and the chemical composition of the new adsorbent were similar to those found in commercial silica gels. Liquid phase adsorption isotherms and uptake rate curves were measured for siloxanes Octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). The siloxane uptakes and adsorption kinetics were comparable and even superior to those found in commercial silicas, especially at low adsorbate concentrations. These results demonstrate the potential of residual sand from WWT as a suitable raw material to produce a good adsorbent for the removal of cyclic siloxanes from biogas.