Enhanced elemental mercury removal via chlorine-based hierarchically porous biochar with CaCO3 as template

Abstract Mercury emission from coal-fired flue gas has threatened the natural environment. It is an urgent demand for developing more effective and cheap sorbents to substitute the high-cost commercial activated carbon for removing elemental mercury. In this work, an innovative route is proposed to prepare hierarchical porous biochar via one-step co-pyrolysis of waste rice straw and polyvinyl chloride with CaCO3 as a template. The optimum material (RPC-313) delivers over 90% capture performance for Hg0 at 120 °C under 225000 h−1 of GHSV. Characterization results show that the high efficiency of this sorbent is attributed to the synergistic effect of high chlorine-containing active sites (2.02%) and the developed hierarchically pores (large specific surface area of 554.9 m2/g and pore volume of 0.6397 m3/g). It also indicates that during the preparation process, the H–Cl generated from PVC react with C C groups on the surface of bio-chars to form the C–Cl groups. The employed CaCO3 as a template creates a hierarchical structure for the sorbent. Moreover, the mercury reaction mechanism is identified as a chemisorption process which can be divided into two parts: adsorption and oxidation. An excellent mesoporous ratio (70%) accelerates mercury diffusion inside the CaCO3-activated adsorbent. And the C–Cl groups are directly beneficial for the adsorption of Hg0 according to DFT calculation. Furthermore, the C–Cl bonds and active oxygen species on the micropores surface of adsorbent act as reaction sites for oxidizing the adsorbed Hg0 into HgCl2 and HgO, respectively. The HgCl is considered as an intermediate in the reaction pathway.