Functionalization of pine sawdust biochars with Mg/Al layered double hydroxides to enhance adsorption capacity of synthetic azo dyes: Adsorption mechanisms and reusability

Abstract

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-LDHMgAl) was examined to offer valuable information into the differences in their adsorption mechanisms. Although a lower specific surface area of PSB-LDHMgAl (147.2 m2 g−1) than PSB (495.7 m2 g−1), LDHMgAl were successfully functionalized on the PSB surface through co-precipitation, which was highly related to the improvements of adsorption capacity of PSB-LDHMgAl toward MO and SYF. The MO and SYF adsorption kinetics by PSB and PSB-LDHMgAl were confirmed to the pseudo-second-order and considered chemisorption. The adsorption capacity of MO and SYF adsorbed onto PSB-LDHMgAl (MO = 21.8 mg g−1, SYF = 23.6 mg g−1) were significantly higher than that of PSB (MO = 2.2 mg g−1, SYF = 1.6 mg g−1). The adsorption isotherms of MO and SYF by PSB were well fitted by Freundlich isotherm, whereas the MO and SYF via PSB-LDHMgAl were by Langmuir isotherm. Even after 3 adsorption-desorption cycles using desorbents, the PSB-LDHMgAl remained excellent reusability (reuse efficiency: >81.2%). These findings suggest that post-modification with LDHMgAl might accelerate the adsorption performance (i.e., electrostatic interaction) of azo dyes to PSB in water.