Functionalization of Pine Sawdust Biochars with Mg/Al Layered Double Hydroxides to Enhance Adsorption Capacity of Synthetic Azo Dyes: Adsorption Mechanisms and Reusability

This study determined that the adsorption of azo dyes, Methyl Orange (MO) and Sunset Yellow FCF (SYF), using the pristine pine sawdust biochar (PSB) and post-modified PSB with Mg/Al layered double hydroxides (PSB-LDH) was examined to offer valuable information into the differences in their adsorption mechanisms. Although a lower specific surface area of PSB-LDH (147.2 m g) than PSB (495.7 m g), LDH were successfully functionalized on the PSB surface through co-precipitation, which was highly related to the improvements of adsorption capacity of PSB-LDH toward MO and SYF. The MO and SYF adsorption kinetics by PSB and PSB-LDH were confirmed to the pseudo-second-order and considered chemisorption. The adsorption capacity of MO and SYF adsorbed onto PSB-LDH (MO = 21.8 mg g, SYF = 23.6 mg g) were significantly higher than that of PSB (MO = 2.2 mg g, SYF = 1.6 mg g). The adsorption isotherms of MO and SYF by PSB were well fitted by Freundlich isotherm, whereas the MO and SYF via PSB-LDH were by Langmuir isotherm. Even after 3 adsorption-desorption cycles using desorbents, the PSB-LDH remained excellent reusability (reuse efficiency: >81.2%). These findings suggest that post-modification with LDH might accelerate the adsorption performance (i.e., electrostatic interaction) of azo dyes to PSB in water.