Study of the Effect of Adsorption Temperature on Elemental Mercury Removal Performance of Iron-Based Modified Biochar

An iron-based modified biochar was prepared by the coprecipitation method, including single iron-based modified biochar with FeCl3 and iron-based biochar doped with Cu and Mn from CuSO4 and KMnO4. Combined with the physicochemical properties of sorbents, the effect of flue gas temperature on the mercury adsorption performance of the sorbents was explored within the temperature range of 50–350 °C. The adsorption mechanism was further explored by adsorption kinetics, thermodynamics, and temperature-programmed desorption (TPD) analyses. The results showed that the mercury adsorption capacity of unmodified biochar decreased monotonously with temperature, but the adsorption capacities of Fe/BC, FeCu/BC, and FeMn/BC all enhanced first and then weakened, with 200 °C being the optimum adsorption temperature. FeCu/BC had the strongest adsorption capacity, while Fe/BC had the worst one. C═O and COOH, metal oxides and ions, lattice oxygen, chemisorbed oxygen, and Cl ions were all active sites for oxidative adsorption of Hg0, among which Fe2O3 and CuO or CuFe2O4 exerted a synergistic effect. Excessive temperature caused destruction to the porous structure and deactivation of the active centers, leading to the reduction of physisorption and chemisorption. Mercury adsorption on sorbents needs more activated energy due to the enhancement of chemisorption, and the moderate increase of temperature contributed to the adsorption properties of the sorbents. The Hg0 adsorption on modified biochar was spontaneous and endothermic, and the complexity degree of the adsorption process was promoted. TPD results showed that at 200 °C the removal of Hg0 by modified biochar was mainly strong chemisorption and that HgO was mainly formed.