Adsorption-based removal of organic dyes via a sustainably produced few-layered graphene: A potential method for industrial wastewater treatment

Abstract

This study discusses adsorption-based removal of six chemically distinctive organic dyes using graphene produced via a chamber explosion method. The method of graphene synthesis is sustainable as it produces zero waste, requires low energy and is scalable. The dye removal was investigated via UV–visible spectroscopy and proved to be efficient, with near-complete removal of all dyes within 10 min of adsorption by graphene. Specifically, 99 %, 100 %, 98 %, 95 %, 99 % and 100 % of removal was observed for Congo red, crystal violet, methylene blue, malachite green, indigo carmine and Rhodamine B, respectively, within 10 min. The physicochemical properties of the graphene were thoroughly characterised and it was shown to consist of few-layered forms, with high surface area despite being non-porous, and it comprised of almost 99 % carbon. The dye adsorption behaviour fitted pseudo second-order kinetics with Congo red showing the highest rate of 0.627 g/mg.min. Adsorption capacity of graphene was investigated via isothermal analysis using Congo red and indicated an estimated qmax of 44.24 mg/g. Congo red-adsorbed graphene was regenerated by calcination, and showed a retention of 99.5 % carbon, evidencing the robustness of the graphene. Subsequent reuse of calcined graphene for further dye adsorption indicated 97 % and 100 % dye removal within 15 and 60 min, respectively, indicating potential for recyclability. There appeared to be a direct relationship between fast dye adsorption and the number of rotatable bonds within each dye that led to improved π-π stacking and electrostatic attractions. This sustainably-produced graphene could be a good contender for specialised applications in water purification against conjugated dyes or related organic contaminants, due to its fast, irreversible adsorption capability and recyclability.