Effective adsorptive removal of dyes and heavy metal using graphene oxide based Pre-treated with NaOH / H2SO4 rubber seed shells synthetic graphite Precursor: Equilibrium Isotherm, kinetics and thermodynamic studies

Abstract

Graphene oxide (GO) has excellent adsorption capacity as a promising adsorbent due to its large surface area consisting of large number of oxygen-functional groups. Nonetheless, the properties of GO are heavily influenced by the graphite sources and the type of pretreatment. This study aims to investigate the effect of pre-treatment of graphite sourced from agricultural wastes on the properties of the GO for the adsorption of dyes and heavy metal ion. Two sources of graphite (pre-treated with NaOH and H2SO4) were synthesised using biomass wastes (rubber seed shells) and were utlilized to produce high-efficiency GO for the adsorption of Methylene blue (MB), Congo red (CR) and Cadmium (II) from aqueous solutions. The effect of pre-treatment on the GO properties, behavior and performance was evaluated through batch adsorption studies which were conducted under operational condition of activation temperature (25–55 ○C), contact time (2–20 min), dosage (0.02–0.1 g), pH (2–12) using impregnation ratio of 1:4. The morphological analysis revealed that GO pretreated with NaOH (GO-N) graphite source exhibited more porous surface area compared to GO pretreated with H2SO4 (GO-H) graphite source, the surface properties was attributed to higher oxygen ratio as indicated in the EDX mapping and as demonstrated in the BET analysis. In general, both GO demonstrated excellent adsorption capacity due to their single-layered planes with noticeable functional groups which included hydroxyl, alkoxy, and carboxyl groups along with the π − π interactions and hydrogen bonds. Maximum monolayer adsorption capacities for the effective removal of CR, MB and Cd (II) was achieved as 299.38, 311.54, and 107.65 mg g−1 respectively onto GO-N compared to GO-H as illustrated by the Langmuir isotherms. Furthermore, the results showed that the experimental data were well-fitted to the pseudo-second-order model for both CR and MB including Cd (II) with GO-N with higher correlation coeficient R2 (R2 > 0.9) recorded. Thermodynamic study signified that the enthalpy (ΔH) and Gibbs free energy (ΔG) values of the adsorption process for both pollutants were negative indicating that adsorption of CR, MB and Cd were spontaneous and exothermic in nature which suggests that the adsorption of dyes and heavy metal from an aqueous solution was attributed to the presence of active sites and functional groups on the GO surface. However, the study revealed that GO–N proved to be a better adsorbent compared to GO-H under same operational condition and can be considered as a promising and cost-effective adsorbent for the removal of dyes and metal ions from industrial wastewater.