Study on the Effects of the Pyrolysis Atmosphere on the Elemental Mercury Adsorption Characteristics and Mechanism of Biomass Char

Biochars prepared in three pyrolysis atmospheres of N 2, O 2, and CO 2 were investigated. The pyrolysis characteristics, pore structures, and functional groups of the biochars were characterized by thermogravimetric analysis (TGA), specific surface area and porosity analyses, and Fourier transform infrared (FTIR) spectroscopy. The adsorption mechanism was further explored by the temperature-programmed desorption (TPD) technique in combination with the adsorption kinetics. The results showed that the Hg 0 adsorption capacities of the biochars produced in an O 2 pyrolysis atmosphere were worse than those of the biochars obtained in a N 2 atmosphere, whereas CO 2 significantly promoted the adsorption capacity. The biochar prepared in 20% CO 2 had the highest adsorption capacity. Three pyrolysis reaction pathways were possible in the O 2 atmosphere. In addition, a critical concentration existed between O 2 concentrations of 5 and 7%. Above 750 °C, CO 2 could react directly with the biochar through the Boudouard reaction. Mercury adsorbed to the surface of the biochar in various ways in the mercury adsorption process and thus was attached to the surface of the biochar in several forms. The products of chemical adsorption were Hg-OM and HgO, and a “secondary adsorption layer” was proposed to form in the mechanism. In the process of mercury adsorption by the biochar, a small part of the Hg 0 chemically adsorbed to the active sites through their functional groups, which were mainly carbonyl and carboxyl groups. At the same time, the formation of a monolayer or multilayer “secondary adsorption layer” occurred, and the remaining Hg 0 could further adsorb to this “secondary adsorption layer”.