Wastewater depollution of textile dyes and antibiotics using unmodified and copper oxide/zinc oxide nanofunctionalised graphene oxide materials†

Abstract

Textile dyes and pharmaceutical discharges are playing a major role as wastewater contaminants, with serious impacts on both the environment and individuals' health and wellbeing. In this article, graphene oxide was produced by oxidising graphite following a modified Hummers method, which then formed a platform to make two composite materials, copper oxide-reduced graphene oxide (CuO-rGO) and zinc oxide-reduced graphene oxide (ZnO-rGO). These materials' adsorption capacity for textile dyes rhodamine-6G (R-6G) and malachite green (MG), and antibiotic pharmaceuticals amoxicillin (AMOX) and tetracycline (TC) was analysed using a UV-visible spectroscopic method. GO showed an adsorption capacity of 625 mg g-1 and 813 mg g-1 for removing R-6G and MG dyes at pH 7 and 333 K, with a 99% regeneration efficacy and <90% reuse after 5 recycle experiments. CuO-rGO provided the highest antibiotic removal of 405 mg g-1 and 552 mg g-1 for AMOX and TC, at pH 7 and 333 K, with an 80% regeneration efficacy and 82% reuse after 5 recycle experiments. The adsorption process of R-6G, MG, AMOX and TC on GO, CuO-rGO and ZnO-rGO materials confirmed the Langmuir adsorption isotherm and pseudo-second order kinetic equations, suggesting chemisorption adsorption processes. A thermodynamic analysis indicated that each adsorption process is spontaneous. The R-6G and MG adsorption on GO, CuO-rGO and ZnO-rGO was endothermic and the AMOX and TC antibiotic adsorption on CuO-rGO and ZnO-rGO was exothermic, while the AMOX and TC adsorption on GO was endothermic as identified by the thermodynamic analysis. Chemical changes in the nanomaterials were observed after adsorption of dye and antibiotics using an attenuated total reflectance Fourier transform mid-infrared (ATR-FTIR) spectroscopic method. The hydroxyl and carbonyl functional groups were typical of GO material characterisation, which efficiently removed the cationic dye molecules. The reduced-GO materials efficiently removed the anionic antibiotic molecules from water. The plausible adsorption mechanisms are electrostatic interactions and p–p interactions between the dye and antibiotic molecules and the adsorbent materials. The study demonstrates that unmodified GO can be used as an efficient adsorbent for removing cationic contaminants and the reduced-GO materials effectively remove anionic contaminant molecules from wastewater systems.