Use of a non-thermal plasma technique to increase the number of chlorine active sites on biochar for improved mercury removal

Abstract

Biochar, known as a byproduct of biomass pyrolysis, was prepared from rice straw (R6), tobacco straw (T6), corn straw (C6), wheat straw (W6), millet straw (M6), and black bean straw (B6) in high purity nitrogen at 600°C. Chlorine (Cl) non-thermal plasma was used to increase Cl active sites on biochar to promote the mercury removal efficiency. The physio-chemical properties of biochar were characterized by proximate analysis, ultimate analysis, BET, SEM, TGA, FTIR, and XPS. Modification by chlorine plasma increased the Hg0 removal efficiency of the biochar from around 8.0% to 80.0%. The Hg0 adsorption capacity of T6 was 36 times higher after Cl2 plasma modification. Plasma caused the biochar surface to become porous and promoted the thermal stability of the biochar. Sulfur (S) content remained in the range of 0.5–0.7%, elemental/organic sulfur and sulfide were converted to sulfate during plasma treatment. The relative intensity of the oxygen functional groups (CO, CO and C(O)OC) were enhanced, while the content of oxygen (O) in biochar decreased. The main reason for the improved mercury removal efficiency by modified biochars was attributed to the increased number of CCl groups on the surface of the biochars induced by Cl2 plasma. The CCl groups functioned as activated sites and promoted the Hg0 removal efficiency.