Enhanced energy recovery in a microbial fuel cell and improved catalytic chromium (VI) reduction using biogenic palladium nanoparticles

Abstract

Palladium (Pd) is a cheap and effective electrocatalyst that is capable of replacing platinum (Pt) in various applications. However, the problem in using chemically synthesized Pd nanoparticles (Chem-PdNPs) is that they are mostly fabricated using toxic chemicals under severe conditions. In this study, we present a more environmentally friendly process in the fabrication of biogenic Pd nanoparticles (Bio-PdNPs) using Citrobacter sp. isolated from wastewater sludge. Successful fabrication of Bio-PdNPs was achieved under anaerobic conditions at pH 6 and a temperature of 30 °C using sodium formate (HCOONa) as an electron donor. Citrobacter sp. showed biosorption capabilities with no enzymatic contribution to palladium (II) [Pd(II)] uptake during absence of HCOONa in both live and dead cells. Citrobacter sp. live cells also displayed high enzymatic contribution to the removal of Pd(II) by biological reduction. This was confirmed by scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) and X-ray diffraction (XRD) characterization, which revealed the presence of Bio-PdNPs deposited on the bacterial cells. The Bio-PdNPs successfully enhanced the anode performance of the Microbial Fuel Cell (MFC). The MFC with the highest Bio-PdNPs loading (4 mg Bio-PdNP cm-2) achieved a maximum power density of 539.3 mW m-3 (4.01 mW m-2) and peak voltage of 328.4 mV.

The discharge of hexavalent chromium [Cr(VI)] from several anthropogenic activities which are responsible for the production of Cr(VI) leads to environmental pollution and concerns over plant growth inhibition and carcinogenesis in animal life-forms. In this study, we explored a simple yet cost effective method for the catalytic reduction of Cr(VI) using chemically and biologically synthesized Pd nanoparticles. The Bio-PdNPs were fabricated at a wide range of Pd(II) concentrations within 24 h, pH of 6 and a lower temperature of 30 °C as compared to Chem-PdNPs which were fabricated at the same pH but at a different temperature of 70 °C. In addition, the presence of elemental Pd was confirmed by SEM, EDS and XRD. In this study, it was shown that the Bio-PdNPs have the capability of improving the catalytic reduction of Cr(VI) due to them being smaller in size and also being highly dispersed as compared to Chem-PdNPs. Furthermore, although both the synthesis methods used in this study required less chemical agents which are not severely harmful and are cost effective, the Bio-PdNPs fabricated using the catalyst concentration of 1.1 g Bio-PdNPs L-1 resulted in a faster removal rate where 0.962 mmol L-1 of Cr(VI) was removed within 2 h.

In order to model the kinetics of the catalytic Cr(VI) reduction, the Langmuir–Hinshelwood mechanism was successfully used. The Langmuir–Hinshelwood mechanism is used to describe the bimolecular reactions on the catalysts surfaces and considers a case where two reactants get adsorbed on the surface of the catalyst and then react together afterwards.