Mechanism identification of temperature influence on mercury adsorption capacity of different halides modified bio-chars

•The temperature rising improved mercury adsorption capacity of NH4Br and NH4I modified bio-chars.•The high temperature inhabited mercury adsorption capacity of NH4Cl modified bio-char.•The disparate effect of high testing temperature resulted from enthalpy change difference of chemical reaction.•The halid mercury on modified bi-chars shows high desorption energy.•The C-X groups were main adsorption site for elemental mercury.

Bio-chars as mercury sorbent were derived from cotton straw via pyrolysis carbonization, microwave/steam activation and ammonium halides impregnation. Mercury adsorption capacity of various impregnated bio-chars was tested at different temperatures. Rising temperature improved the adsorption capacity of bio-chars modified with NH4Br and NH4I yet inhabited that of bio-char modified with NH4Cl. Kinetic model analysis indicated that the changes of mercury adsorption capacity were related to the difference of enthalpy change for various bio-chars. In temperature programmed desorption technic (TPDT) analysis, high desorption energy (80.9–82.1 kJ mol−1) proved that mercury adsorption mainly depended on chemisorption for halide modified bio-chars. Modification effect of ammonium halides on mercury adsorption capacity followed the order: NH4I (7643.1 μg g−1) > NH4Br (842.0 μg g−1) > NH4Cl (365.3 μg g−1). X-ray photoelectron spectroscopy (XPS) analysis proved that C-X groups served as mercury adsorption sites and determined the mercury removal capacity of bio-chars. This study explained the mechanism of temperature effects and halides modification as well as proposed useful suggestion on mercury sorbent preparation and application.

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