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

Abstract

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 degrees 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 degrees 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 (Fe-0). 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.