Aging influence and long-term stable mechanism of biofuel ash immobilizing cadmium in alkaline soil combining artificial aging simulation

The potential harm of Cd in alkaline soil are weaker than that in acidic soil generally, but once contaminated, remediation of alkaline soil is more difficult to realize by the way of increasing pH which is usually used in acidic soil. Biofuel ash (BFA) from biomass combustion in the biomass power plant can effectively immobilize Cd in alkaline soil, and BFA is aging simultaneously. Natural aging in soil and three methods of artificial aging of alkaline oxidation, freeze-thaw cycle and alkaline freeze-thaw cycle which were intended to simulate the key factors of alkaline and freeze-thaw cycles of soil environment were applied. Traditional characterization and XAFS, adsorption experiment, precipitation experiment and in-situ immobilization experiment in soil by DGT were conducted to research the influence of aging on the physicochemical properties and immobilization performance and to reveal the long-term stable immobilization mechanism of BFA for Cd. After natural aging, the anorthite crystal strengthened, specific surface area reduced, silico-aluminate rearranged, aluminum was substituted by silicon, the ionic bonding weakened and hydroxyl reduced, the changes above which were the same as artificial aging simulation partly inhibited Cd immobilization, and the change in pH was the external manifestation. Aging changed immobilization performance of BFA including adsorption and precipitation by affecting its pH. And the pH was positively correlated with the immobilization capacity. With BFA aging, its immobilization rate reduced, but the effect of immobilization promoted by 51.9 % after BFA application. Cd was found to undergo obligate adsorption, this behavior was attributed to coprecipitation with Ca on the BFA surface, which played an important role in long-term stable immobilization. The research was beneficial to the application of BFA in alkaline seasonal frozen soil contaminated by Cd.