Fabrication of carbon-slag composite via a pyrolytic platform and its environmental application for arsenic removal as a case study

For the valorization of biomass and steel slag, co-pyrolysis of rice straw and steel slag was carried out as a case study. To achieve the more sustainable pyrolytic platform, carbon dioxide (CO₂) was employed as reactive medium. Therefore, this study laid great emphasis on chasing the mechanistic roles of CO₂ in the thermolysis of the mixture (rice straw + steel slag) at the fundamental level. This study experimentally validated that CO₂ reacted with thermally induced hydrocarbon species from rice straw via the gas phase reactions. Such reactions resulted in CO formation at temperatures ≥ 480 ˚C, of which the reaction kinetics was catalytically accelerated when steel slag was co-pyrolyzed with rice straw. Note that steel slag contained the significant amount of metals and alkaline compounds. However, co-pyrolysis of rice straw and acid-washed slag revealed that the enhanced reaction kinetics resulting in CO formation at temperatures ≥ 480 ˚C was imparted from alkaline compounds such as CaCO₃. Also, this study showed that CO₂ effectively suppressed dehydrogenation during the thermolysis of rice straw. Such mechanistic roles of CO₂ played a pivotal role to shift carbon distribution from pyrolytic oil to pyrolytic gas. The different thermal degradation routes triggered by CO₂ led to the morphologic change to carbon-slag composite. In detail, the surface area of carbon-slag composite was enlarged in the CO₂ atmosphere. To impart the desirable functionality, the surplus amount of CO formed from CO₂ was re-used to transform iron oxides in the composite into zero-valent iron (Fe⁰). Porosity and zero-valent iron in carbon-slag composite increased the As(V) sorptive capability, of which the removal efficiency reached up to 99.3 % at pH 6.9. Copyright © 2019 Elsevier B.V. All rights reserved.