Removal of dissolved sulfides in aqueous solution by activated sludge: mechanism and characteristics

•Dissolved sulfides removal is primarily attributed to adsorption under low DO condition in activated sludge recycling process.•SEM/EDS, XPS, and FTIR results show that sulfides could be bonded with the primary amine group in the activated sludge.•Sulfides adsorption isotherm fit the Langmuir model well, and kinetic fit the pseudo-second-order model well. Activated sludge recycling has been developed as a novel technique to directly prevent volatile sulfides emission from wastewater influents. In this study, mechanisms and characteristics of dissolved sulfides removal in aqueous solution by activated sludge were investigated. When DO content in water was 0.49mg/L, 70% of removed dissolved sulfides were released back from the activated sludge by lowering pH to 1. The SEM/EDS result revealed that removed sulfur was fixed in activated sludge and the XPS result showed that fixed sulfur had an oxidation state of −2. FTIR results showed that primary amine group (R-NH2) could be one of the radical groups bonding sulfides. All these results verified that sulfides removal by activated sludge is primarily attributed to adsorption, rather than biodegradation, under low DO conditions in 40min. The equilibrium isotherm data fit the Langmuir isotherm model well. The maximum adsorption capacity (q0) ranged in 25–38mg/g at temperatures of 10–40°C. The adsorption kinetic data fit the pseudo-second-order model well. The amounts of adsorbed sulfides at equilibrium (qe) were positively proportional to temperature, initial sulfides concentration and agitation speed. These results indicate that sulfides adsorption could be a chemical sorption or ion exchange process.