Siloxane transformation reactions by means of activated carbon towards biological availability

Abstract

Siloxanes are silicon based compounds that are present in biogas. The need for new and better ways to obtain renewable energy is incrementing. The biogas produced in landfills and wastewater treatment plants can be used as a renewable energy source, however in all of the possible utilizations the siloxane removal is necessary. The costs of siloxane removal nowadays are high and not a cost effective solution has come yet. The biological removal of this compounds is a possibility currently under research, but the low solubility in water that siloxanes present is a problem. The investigation in order to make the siloxanes more bioavailable is necessary, and this study goes in that direction. A total of 4 commercial activated carbons (ACs) have been tested for the absorption and partition of the octamethylcyclotrisiloxane (D4) in batch experiments using different humidity conditions. The ACs tested present different physical and chemical properties, given by their different activation processes: ones were activated with steam and others with H3PO4. Siloxane partition into gas-water-AC phases was studied. The end goal was to prove the capacity of the ACs to catalyze the ring-opening of D4 to transform it into more soluble compounds, which may be more accessible to microorganisms to remove it biologically. All the ACs tested adsorbed the D4 completely in dry conditions. The results showed that the recovery of D4 by hexane extraction from the AC matrix was low due to the potential transformation of the D4 into other compounds either not extractable or nor detectable by the analytical methods used. When the conditions of humidity were higher, the results showed that the ACs adsorbed less D4: a little amount remained in the gas phase. A chemically AC was selected because it was the best performing AC at D4 removal and transformation. Extraction was made with a polar solvent, so the presence of silanediols could be detected. Silanediols are water-soluble by-products from siloxane hydrolisis, thus making the formation of silanediols the path to follow in future biological removal of siloxanes technologies