An experimental and modeling study of carbon nanomaterial membranes, bacterial growth, and their interactions towards Pb(II) removal from wastewater

Abstract

Pb(II) removal is imperative due to its inherent toxicity at low levels and its tendency to accumulate in ecosystems. Conductive carbonaceous nanomaterials (CCNs), such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs), have recently gained the interest of researchers due to their superior properties and ease of functionalization. The aim of this study is to utilize CCNs for Pb(II) removal within membrane technology and bioremediation strategies. Membranes have shown promise in their treatment abilities, producing excellent effluent quality while reducing plant footprints. The integration of CNTs within membrane technology provides an opportunity to couple its removal capacity with Pb(II) removal that exhibits regeneration capabilities. However, membrane fouling can be problematic for membrane longevity and regeneration. CNTs have also shown to be capable of mitigating fouling via electrostatic repulsion and pollutant degradation. However, little work has been conducted on its fabrication. In this work, CNTs were incorporated with poly(vinyl) alcohol (PVA) in thin film composites, where the effects of PVA chain length and degree of crosslinking were investigated. It was found that a pseudo-optimal coating can be obtained using 31-50kDa PVA with 10% crosslinking. This combination lead to a highly permeable, hydrophilic surface with good electrical conductivity that exhibited a molecular weight cut off of 2000kDa. Biosorption has shown promise in Pb(II) removal in the lab scale but its large-scale use is hindered from rapid saturation of binding sites and low regeneration abilities. Exoelectrogens were proposed as reactive biosorbents to couple biosorption with bioreduction in an attached growth configuration. CCNs were investigated as bacterial scaffolds, where their efficacy and Pb(II) dosage concentration was studied. It was found that CNFs were superior in removing Pb(II), exhibiting Pb(II) concentrations ≤0.10 ppm where removal increased when Pb(II) dosage increased from 0.5 to 5ppm. SEM-EDX analysis provided evidence that bioreduction dominated Pb(II) removal. A long-term study was further conducted using CNFs, revealing its robustness in long term removal over suspended growth reactors with a sustained removal of ≈ 80%. A numerical model was further proposed which exhibited a goodness of fit with an R-squared of 0.92. This model confirmed that bioreduction dominated Pb(II) removal and revealed biofilm thickness and Monod kinetics to be the main influential parameters on Pb(II) removal.