Orientation construction and comparison of mercury removal sites by physical and chemical adsorption on cotton stalk biochar

The capture of elemental mercury (Hg 0 ) is the crucial step for synergetic control of mercury emission via dust removal equipment. Micropore structure, oxygen-containing groups and adding active elements are three key factors in the biochars activation for mercury capture. Previous studies usually focused on single modification factor. This study aims to evaluate and contrast the influence of the three key factors on the mercury capture of biochars. Physical activation (microwave and steam), H 2 O 2 modification and NH 4 Cl impregnation were used to directionally develop pore structure, form new oxygen-containing groups and introduce halogen active sites. The testing results showed that physical activation causes distinct increase in the V micro ratio by 420 % yet just modest increase in the highest mercury removal efficiency ( η ) of C6W by only 43.0 %. The 30 % H 2 O 2 modification and 5 % NH 4 Cl impregnation distinctly increased η by 82.6 % and 116.2 %, respectively. Moreover, synergetic activation of H 2 O 2 modification with physical activation and that of NH 4 Cl impregnation with physical activation further increased η by 159.5 % and 180.6 %, respectively. The pore structure development was not the key factor in mercury removal, yet it improved the chemical modification effectiveness. Physicochemical characterization methods, molecular adsorption kinetic models and mercury desorption energy calculation were used to analyze the mercury capture active sites and investigate the mercury adsorption mechanism. The H 2 O 2 modification and NH 4 Cl impregnation formed chemisorption sites of C-O, C=O and C-Cl groups. The mercury desorption energy increased from 32.2-33.0 kJ∙mol −1 of physisorption to 60.2–70.7 kJ∙mol −1 of chemisorption. This paper provides guides for future preparation of highly effective mercury removal sorbents.