Biochar modified by amine improving its adsorption of SO2

Abstract: To study the effect of amine impregnation modification on biochar structure and its adsorption of SO2, corncob was used as raw materials to prepare biochars under N2 at 850 ℃, followed by activation stage in gaseous CO2 at the same temperature. The derived activated biochars (CC850) were then impregnated with MDEA-Methonal solutions to obtain modified biochars. The physicochemical properties, the structure, and the SO2 adsorption properties related to surface microtographs of activated biochars and modified chars were investigated. The production and activation of biomass chars were carried out in a self-designed vertical furnace reaction system which includes gas generating zone and modification reaction zone. The amine modified biochars were obtained by impregnating activated chars (CC850) in MDEA-Methonal solutions with different concentrations (from 1% to 10%), and labeled as CC850-X% (X presented the mass concentration of MDEA in the mixed solutions). To determine the specific surface area and micropore characteristics of biochars before and after modification, nitrogen adsorption/desorption isotherms were performed at 77 K with a Micromeritics ASAP2020 automatic adsorption instrument. And the elemental compositions of biochars were determined by the ultimate analysis. The results showed that the specific surface area and micropore volume of CC850 were high up to 756.25 m2/g and 0.2971 mL/g, respectively. However, with the increase of impregnated concentration, the pore structure parameters of biochars decreased rapidly to a degree that can not be detected. While the nitrogen content of biochars gradually increased from 0.61% to 6.91% which indicated the successful introduction of N onto the surface of the biochars. The SO2 adsorption properties of CC850 and its modified chars (CC850-X%) showed a breakthrough curve and adsorption capacity. The results showed that the saturated adsorption time as well as the saturated adsorption capacity of biochars firstly decreased from 20.02 min, and 57.78 mg/g (CC850) to 13.35 min, and 38.53 mg/g (CC850-4%), and then gradually increased up to 54.13 min, and 156.22 mg/g (CC850-10%), respectively. Fourier transform infrared spectrum analyzer (VERTEX70, Bruker) was used to analyze the variation of chemical properties and surface functional groups of activated biochars and modified biochars before and after desulfuration. The Fourier transform infrared spectra of the corresponding biochars linked with ultimate analysis and SO2 adsorption capacity suggested that the amount of nitrogen functional groups such like -NH and C-N introduced onto the surface of biochars increased with the impregnated concentration, but the adsorpiton capacities of biochars presented a trend of first increase and then decrease. This phenomenon matched well with the results of the pore structure development trend of biochars, which may be accounted by the main adsorption mechanism transition from physical adsorption to chemical adsorption, and the chemical adsorption of modified chars was enhanced with the increasing impregnated concentration in the experimental range. The surface microstructure of CC850 and CC850-10% before and after desulfuration were presented by SEM micrographs (Sirion 200, FEI), and the surface elemental components of biochars were analyzed by EDX analyzer (GENESIS, EDAX Inc) in the specific areas. The CC850 showed an existence of a regular surface with abundant pores while the CC850-10% had a rough surface with low amount of pores. However, both of these two biochars presented effective adsorption abilities to a certain extent.