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

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 Hg-0 at 120 degrees 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 m(2)/g and pore volume of 0.6397 m(3)/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 Hg-0 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 Hg-0 into HgCl2 and HgO, respectively. The HgCl is considered as an intermediate in the reaction pathway.

Automated summary

Mercury emission from coal-fired flue gas poses a threat to the environment, prompting the development of more effective and affordable sorbents as alternatives to commercial activated carbon for mercury removal. In this study, a hierarchical porous biochar was prepared through co-pyrolysis of waste rice straw and polyvinyl chloride with CaCO3, exhibiting over 90% capture performance for Hg-0. The high efficiency of the sorbent can be attributed to the synergistic effect of high chlorine-containing active sites and developed hierarchically pores.