Composite synthesized from nano layered double hydroxides (or oxides) and spherical biochar: Application for removing Congo red and methylene blue dyes from water

Abstract

In this study, a new composite (LDH–SB) was synthesized from layer double hydroxides (LDH) and spherical biochar (SB) through hydrothermal carbonization. LDH–SB was then calcinated at 500 ^◦C to generate another composite (LDO–SB). Under such a temperature, LDH in the surface of LDH–SB was converted to layer double oxides (LDO). Their properties were thoroughly investigated. SEM images confirmed that the nano particles (LDH or LDO) were successfully loaded in the surface of SB. XRD data validated the existence of LDH or LDO in the structure of the composites. The BET surface area of LDH–SB và LDO–SB was 81.7 m²/g and 192 m²/g. The pHPZC of LDH–SB and LDO–SB was 8.49 and 10.23. Leaching test demonstrated that two composites exhibited a strong chemical stability within solution pH from 2.0 to 10. The results of pH-dependent adsorption indicated that two composites well adsorbed methylene blue (MB) dye under basic condition (pH 10) and Congo red (CR) dye under acid (pH 3.0). Adsorption kinetics, isotherms, and thermodynamics were conducted under batch experiments. The maximum adsorption capacity of the MB⁺ cations and CR^- anions by LDH–SB (92.2 and 266 mg/g) and LDO–SB (64.4 and 795 mg/g, respectively) was calculated from the Langmuir model. LDO–SB exhibited an excellent adsorption capacity towards CR dye in water compared to LDH–SB.