Changes of distribution and chemical speciation of metals in hexavalent chromium loaded algal-bacterial aerobic granular sludge before and after hydrothermal treatment

Abstract

Algal-bacterial aerobic granular sludge (AGS) was applied for hexavalent chromium (Cr(VI)) biosorption from wastewater and the dynamic distribution and mobility of different metals in granules were systematically examined before and after hydrothermal treatment. The loaded Cr on algal-bacterial AGS was found to mainly localize in microbial cells and mineral particles; little Cr was detected in extracellular polymeric substances (EPS) after 6 h contact, which increased to 5.1% after 24 h biosorption. Along with Cr localization, 9.3–22.8% of Mg and 11.5–26.4% of Ca in algal-bacterial AGS were released from loosely bound EPS, then replenished to maintain their stable proportion in EPS, probably contributing to granular stability. In addition, chemical fractionation showed that the Cr mobility in algal-bacterial AGS, indicated by a low mobility factor of 4.7%, decreased to 1.4% with the co-existence of mineral salts and acetate during biosorption, which was further declined to 0 after hydrothermal treatment.