Rice husk waste-derived super-biochar with the max surface area and Philic-CO2 textural Structure: Boosting effect and mechanism of post-desilication

A super-carbon material as adsorbents of CO2 were prepared with rice husk waste by a new two-step process, pyrolysis-activation and HF post-desilication process. This paper was focused on investigating the boosting effect and mechanism of post-desilication. It was found that the function of the post-desilication was considerably different from that of the reported desilication. It could not only effectively remove Si element from rice husk biochar (RHC) but also significantly improve its performance, producing a super-biocarbon with a well-developed pore structure, rich C-F groups, and a 4230-m2/g surface area, which is the max surface area of known biocarbon. It could adsorb 341.5 mg g−1 CO2, a max CO2 adsorption capacity of reported biocarbon, and its CO2/N2 and CO2/H2O adsorption selectivity index reached as high as 22.2 and 2.1, respectively. The large surface area was mainly originated from the expansion effect of SiF4 gas from the post-desilication, generating numerous ultra-microspores, while these C-F groups created by the co-reactions of the post-desilication. The ultra-high adsorption capacity was mainly from the fill-adsorption driven by the size effect of these ultra-micropores and the strong CO2-affinity of C-F groups, and the adsorption selectivity was confirmed to depend on water-resistant and CO2-philic characteristics of its C-F groups by theoretical calculations and experimental observations. These findings presented not only an advanced CO2-capture material but also a facile way to tailor textural structure of other biochars.