Mechanochemical bromination of unburned carbon in fly ash and its mercury removal mechanism: DFT study

The mechanochemical (MC) brominated fly ash is a cost-effective mercury removal adsorbent, in which unburned carbon (UBC) plays an important role. The MC bromination mechanism of UBC and its mercury removal mechanism were completely studied through the density functional theory (DFT) method. Various defects on zigzag and armchair edge models were constructed at the micro-scale to simulate the MC effect on UBC at the macro-scale. The results reveal that the intact surface of zigzag and armchair can be constructed into abundant defective structures by MC action. Compared with the complete surface, bromine is more favorable to bind on the defective surface, resulting in more and stronger C-Br covalent bonds and more active sites. These defective structures also have a promoting effect on mercury adsorption. For the bromine-embedded structure, although the appropriate defective structure accounts for less, it not only can promote the adsorption and oxidation of mercury by improving adsorption ability or decreasing the oxidation energy barrier but is also easier to generate. Due to defect types formed by MC interaction on the UBC surface are much more diverse and complex, this study provides the theoretical basis for further research.