Sustainable synthesis of graphene sand composite from waste cooking oil for dye removal

Waste cooking oil (WCO) appears to be a potential carbonaceous source for synthesizing graphene sand composite (GSC) adsorbent in removing pollutants. This study presents a green synthesis method of GSC using WCO as a sustainable carbon source for the synthesis of GSC through the thermal graphitization method. Characterization analysis conducted on GSCWCO verified the successful coating of WCO onto the sand surface and conversion to graphene, which possessed distinct functional groups and features of graphene materials. GSCWCO adsorbent effectiveness in removing Congo Red dye through batch adsorption was studied under the influence of different initial concentrations (20 to 100 mg/L), and the optimum pH (pH 2 to 10), contact time (5 to 240 min), and temperature (25 to 45 °C) were investigated. The GSCWCO showed removal rates of 91.5% achieved at an initial dye concentration of 20 mg L-1, 1.0 g of adsorbent dosage, a temperature of 25 °C, and 150 min of contact time. The GSCWCO exhibited a maximum capacity of 5.52 mg g-1, was well-fitted to the Freundlich isotherm model with an R2 value of 0.989 and had an adsorption mechanism that followed the pseudo-second-order kinetic model. Negative values of enthalpy (ΔH) and Gibbs free energy (ΔG) revealed that CR adsorption onto GSCWCO was a spontaneous and exothermic process. The presence of functional groups on the surface of GSCWCO with such interactions (π-π attractive forces, hydrophobic forces, and hydrogen bonding) was responsible for the anionic dye removal. Regeneration of GSCWCO adsorbent declined after four cycles, possibly due to the chemisorption of dyes with GSC that resulted in inefficient adsorption. Being a waste-to-wealth product, GSCWCO possessed great potential to be used for water treatment and simultaneously benefited the environment through the effort to reduce the excessive discharge of WCO.