Chromium (VI) reduction in a microbial fuel cell using biogenic palladium nanoparticles and model development

Abstract

Microbial fuel cell (MFC) architectural modification is increasingly becoming an important area of research due to the need to improve energy recovery. In this study, we present a simple low-cost modification method of the anode that does not require pre-treatment-step involving hazardous chemicals to improve performance. The modification step involves deposition of granular activated carbon (GAC) which is highly conductive and provides a high specific surface area inside a carbon cloth that acts as an anode and as a supporting material. The GAC particle size of 0.6-1.1 mm led to an increase in air-cathode MFC performance due to both an increase in the available surface area of 879.5 m2 g-1 for cell attachment based on Brunauer, Emmett, and Teller (BET) results, and an increase in relevant surface for cell attachment which was rough based on the scanning electron microscope (SEM) results. This study also showed that there is an economic benefit in modifying carbon cloth with GAC.

The second part of the study explored an environmentally friendly process for the treatment of Cr(VI) with a codeposition of biologically synthesized zero-valent palladium nanoparticles on the anode electrode of a dual chambered microbial fuel cell (MFC). The MFC featured a granular activated carbon (GAC) anode modified with biogenic palladium nanoparticles (Bio-PdNPs). Temperature, pH, and initial Cr(VI) concentration were first optimized to 38 °C, pH 4, and 100 mg L-1 Cr(VI), respectively. Thereafter, the GAC average particle size was successfully optimized to 0.6-1.1 mm. The results from the study showed that GAC can be successfully modified using Bio-PdNPs to improve the performance of Cr(VI)-reducing MFC with Bio-PdNPs loading of 6 mg Bio-PdNPs g-1 GAC resulting in peak output potential difference of 393.1 mV, maximum power density of 1965.4 mW m-3, and complete removal of 100 mg L-1 Cr(VI) in 25 h.

The third part of the study was to develop a dynamic computational model for Cr(VI) reduction in MFC. The model incorporated Monod kinetics with Butler-Volmer equation. Accuracy of the parameter estimation and capacity of prediction of the model was validated with usage of two independent data sets. The results of the normalized root mean squared errors for both reduction of Cr(VI) and output voltage were less than 0.2, which indicated that the model fit for the experimental data was acceptable. The model was then used to demonstrate the effect of both the primary microbial cell and substrate concentration on Cr(VI)-reducing MFC performance. An increase in primary microbial cell and substrate concentration improved the reduction rate of Cr(VI) in the cathode chamber. Lastly, the model was used for the optimization of both concentrations. The time it takes to achieve maximum power output was minimized by using a primary microbial cell concentration of 25 mg L-1 as opposed to a value of 45 mg L-1. In addition, the substrate concentration was optimized to 60 mmol L-1 as opposed to a value of 120 mmol L-1. Overall, the model provided an initial step into determining optimal MFC operational conditions without doing much lab-work.