Adsorption/desorption behavior of cationic and anionic dyes by biochars prepared at normal and high pyrolysis temperatures

Abstract

The purpose of this study was to determine the properties of biochar prepared at higher than normal biochar production temperature and to evaluate adsorption/desorption characteristics for three commonly used dyes of different charge and molecular nature: Methylene Blue (MB), Orange G (OG) and Congo Red (CR). The switchgrass-biochar was prepared at common pyrolysis temperature of 600 °C (SB600) and higher than normal temperature of 900 °C (SB900). The results showed that the carbon content of the two biochars was similar, but most carbon in SB900 was graphitized based on FTIR and XRD observation. Dye adsorption isotherms by SB600 and SB900 was well described Langmuir model with maximum adsorption capacity of MB, OG and CR being 37.6, 8.2 and 8.0 mg/g for SB600 and 196.1, 38.2 and 22.6 mg/g for SB900, respectively. The adsorption kinetics of dyes by both biochars were well described by pseudo-second order model. In addition, intraparticle diffusion model showed that MB adsorption was predominantly by inner pore diffusion rather than by outer boundary, while OG and CR were more adsorbed by the outer boundary. These results indicated that the strong dye adsorption by SB900 compared to SB600 was due to the significantly enhanced surface area at the high pyrolysis temperature. Additionally, the greater adsorption of cationic MB by SBs than that of anionic OG and CR was attributable to the small molecular weight, favorable electrostatic property, and strong π-π interaction of the former with biochar surface as opposed to the latter. Overall, the SB900 maintained > 85% adsorption of maximum capacity even after four adsorption-desorption consecutive cycles and could be used as an effective adsorbent for cationic dye removal from wastewater.