Mercury sorption study of halides modified bio-chars derived from cotton straw

• The C-X (Cl, Br and I) groups sever as chemisorption sites for elemental mercury.
• The different ratio of C-X groups explains the result that halide modified effect follows the order: I > Br > Cl.
• The formation of C-X groups depended on the reducibility and size of halide ions.

The mechanism of three halides (chloride, bromide and iodide) modification and reason for their modified effect difference was investigated in this study. Bio-chars derived from cotton straw were further activated and modified with microwave/steam and halides impregnation. Modified bio-chars serving as mercury sorbent was measured via BET, SEM and XPS analyses. The chemisorption site was identified as crucial factor for mercury adsorption performance via pore structure, SEM analysis and adsorption testing study. Based on XPS analysis of bio-char prior to and after adsorption testing, carbon–halide functional groups (C-X) were dominant chemisorption sites oxidizing Hg0 into mercury halide (HgCl2, HgBr2 and HgI2). The amount of C-X groups possibly decided halide modification effect order: I− > Br− > Cl−. The formation of C-X groups depended on the reducibility and size of halide ions. In order to verify the adsorption mechanism, the molecular sieve (MCM-22) as non-carbon carrier was used to compare with bio-char. Results suggested that the lack of C-X groups decreased mercury removal performance in experimental condition. This paper firstly explained the reason for modified difference of three halides in term of chemisorption site formation mechanism.

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