Deionization utilizing reduced graphene oxide-titanium dioxide nanotubes composite for the removal of Pb2+ and Cu2+

Abstract

Apart from organic and salt pollutants, bodies of water where wastewater is discharged contain heavy metals such as copper and lead. These heavy metals have great health and environmental impacts due to their toxicity even at low concentration, making heavy metal removal in wastewater effluent highly significant. As an alternative to other chemical engineering technologies for heavy metal treatment, capacitive deionization (CDI) was investigated in this study to improve its current copper and lead low electrosorptive performance. To this end, a composite of synthesized reduced graphene oxide (rGO) with titanate nanotubes (TNT) at 3:1 mass ratio was used for CDI application as electrochemical tests indicate that the material is capable of behaving as an electrical double layer (EDL) behavior, a property desired for capacitive deionization. The performance of the composite in removing copper and lead was tested via CDI. Both Cu2+ and Pb2+, at 80 ppm concentrations, were subjected to 2-h electrosorption runs. The maximum specific electrosorptive capacities achieved for Cu2+ and Pb2+ are 3.99 mmol (253.25 mg) and 1.17 mmol (241.65 mg) of heavy metal per gram of rGO-TNT composite, respectively. These electrosorption capacities achieved have been found to surpass the performance of currently used materials in capacitive deionization for copper and lead removal. Such electrosorptive performance of the composite in CDI could be attributed to its high BET surface area of 511.226 m2/g, of which 99.83% is mesoporous. However, its discharge during desorption at zero voltage after 20 min was only 5.28% and 3.63% for Cu2+ and Pb2+, respectively. These electrosorption and desorption behavior revealed that both heavy metal ions, especially copper, have high affinity to the electrode due to the presence of electron-rich functional groups, sp and sp2 hybridized carbon in the composite. The strong EDL behavior and highly conductive mesoporous molecules of rGO-TNT composite make it a suitable CDI electrode for copper and lead removal and, potentially, to other heavy metals.