Mechanism of birnessite-promoted oxidative dissolution of antimony trioxide

The most important naturally occurring forms of antimony (Sb) are the ore mineral stibnite (Sb2S3) and its principal weathering product antimony trioxide (Sb203). Moreover, most Sb is released into the environment as Sb,03 from manufacturing, formulations, processing, and the use and disposal of Sb products. In this study, bimessite (h-MnO,) was employed to promote Sb203 dissolution. The Sb3O3 dissolution rate accelerated from 2 % to >99% after 9 days of reaction, and more than 98 % of the dissolved Shill was oxidised into Sbv in the presence of birnessite. The birnessitepromoted oxidative dissolution mechanism of Sb703 was studied through experiments on the effects of the reaction time and pH. The release rate of Sb in the aqueous phase at the initial stage (335.4 hmol L 1 day 1, pH 6.5) was much higher than that at the late stage (13.28 hmol L day 1, p116.5), and when the pit increased from 4.7 to 8.0, the Sb release rate decreased from 351.2 mot L-1 day -1 to 257.7 j.imol L-1 day -1. The dissolution promotion effects of birnessite on Sb203 were the most evident under acidic and neutral conditions (the percentages of dissolved Sb203 under acidic and neutral conditions were 98.3 % and 100.0 % after 9 days of reaction). Not all of the produced Sb'' was released in the reaction solution because 50 % of it was adsorbed by bimessite, and the amount of Sb adsorbed increased with increasing pH. Therefore, alkaline conditions are shown to reduce the release of Sbv by inhibiting Sb203 dissolution (86.7%) and enhancing Sbv adsorption. These results could help clarify the geochemical cycle and fate of Sb in the environment.