Biofuel Ash Aging in Acidic Environment and Its Influence on Cd Immobilization

Abstract

Biofuel ash (BFA), which is the ash generated by biomass combustion in a biomass power plant, can be prepared as a heavy metal immobilizer and have a good immobilization effect on Cd in the soil environment of southern China, but the long-term effects of BFA on Cd immobilization remained unclear. Therefore, research about BFA aging and its influence on Cd immobilization was conducted in the paper. BFA was naturally aged into BFA-Natural aging (BFA-N) in the soil environment of southern China, and to simulate BFA-N, BFA was also artificially acid aged into BFA-Acid aging (BFA-A). The result indicated that BFA-A could partially simulate BFA-N in physicochemical properties. The Cd adsorption capacity of BFA reduced after natural aging and the decrease was more obvious in BFA-A according to Qm in Langmuir equation and qe from the pseudo-second-order kinetic model. The adsorption processes of BFA before and after aging were mainly controlled by chemical action rather than physical transport. The immobilization of Cd included adsorption and precipitation, and adsorption was the dominant factor; the precipitation proportion was only 12.3%, 18.8%, and 1.7% of BFA, BFA-N, and BFA-A, respectively. Compared with BFA, both BFA-N and BFA-A showed Ca loss, and BFA-A was more obvious than BFA-N. Ca content level was consistent with Cd adsorption level among BFA, BFA-N, and BFA-A. It could be inferred that the main immobilization mechanism of Cd by BFA before and after aging was consistent and closely related to Ca. However, the adsorption mechanism of electrostatic interaction, ion exchange, and hydroxyl complexation changed to varying degrees in BFA-N and BFA-A.