Removal of cadmium from wastewater using biofunctional reduced graphene oxide synthesized by Lysinibacillus sphaericus

Cadmium (Cd) is a toxic contaminant, which is ubiquitous in wastewater and poses a serious threat to the natural environment. In this study, biofunctional reduced graphene oxide (B-rGO) was synthesized using Lysinibacillus sphaericus and subsequently successfully used to remove 99.2% of 10.0 mg L-1 Cd(II). Extracellular polymeric substances (EPS) generated by microbes during the reduction of graphene oxide, here significantly enhanced Cd (II) adsorption. Compared to previous studies where only 34.2 and 50.9% of Cd(II) were removed using a commercial graphene oxide and green tea extract-reduced graphene oxide, respectively. The adsorption of Cd(II) by B-rGO was confirmed by advanced characterization techniques and adsorption was fitted well to the Langmuir isotherm and pseudo-second-order adsorption kinetic model. Furthermore, Raman spectroscopy showed that GO was successfully reduced to B-rGO by L. sphaericus, whereas SEM revealed a rougher B-rGO surface due to EPS binding. FTIR confirmed that the functional groups on the surface of B-rGO resulted from both rGO and EPS, which was also confirmed by XPS. A potential removal mechanism based on electrostatic attraction, ion-exchange and complexation was proposed. Finally, the application of B-rGO was practically evaluated for wastewater treatment exhibiting a 65.9% Cd(II) removal efficiency.

Automated summary

In this study, biofunctional reduced graphene oxide (B-rGO) was successfully synthesized and used to remove Cd from wastewater with an efficiency of 99.2%. The adsorption capacity of Cd was significantly enhanced by extracellular polymeric substances (EPS) generated during the reduction of graphene oxide. Advanced characterization techniques confirmed the Cd adsorption and proposed a potential removal mechanism based on electrostatic attraction, ion-exchange, and complexation.