Dependency of migration and reduction of mixed Cr2O72−, Cu2+ and Cd2+ on electric field, ion exchange membrane and metal concentration in microbial fuel cells

Improving Scalability of copper recovery in saline microbial fuel cells with microtubular polypyrrole-based cathodic electrocatalysts

Microbial fuel cells (MFC) are emerging energy-efficient systems for copper (Cu) electrowinning from waste streams by coupling it with anodic oxidation of organics in wastewater. However, there is a lack of research examining scalable electrocatalysts for Cu electrowinning at low cathodic overpotentials in highly saline catholytes often found in e-waste leachates. The challenge of developing resilient anodic biofilms that withstand the antagonistic effects of ions migrating from catholytes in saline MFC also needs to be addressed. In this study, polypyrrole (PPy) cathodic electrocatalysts were developed and coupled with a robust halophilic anodic biofilm in MFC to improve the kinetics of Cu electrowinning from acidic chloride-based catholytes. Electrochemical characterisation of these cathodes revealed shuttling of electrons by redox-active PPy via the formation of intermediate Cu+-complexes as an energy-efficient pathway for producing metallic Cu. High power densities ranging from 0.63 ± 0.17 to 0.73 ± 0.05 W m-2 were achieved with undoped-PPy and phytic acid doped-PPy cathodes with simultaneous recovery of ∼97% Cu. These electrocatalysts also exhibited low charge transfer resistance (3-8 mΩ m2) that met the requisites for scalable cathodes in MFC. However, a decrease in the efficiency of PPy cathodes was observed over 5 d due to competing reactions at their interfaces, including re-oxidation of deposited Cu and cathodic corrosion, with further studies suggested to enhance their corrosion resistance. Nonetheless, integrating PPy electrocatalysts for Cu electrowinning in saline MFC has advanced its outlooks as an energy-efficient downstream process for urban mining of Cu from e-waste.

Bioelectrochemical remediation of Cr(VI)/Cd(II)-contaminated soil in bipolar membrane microbial fuel cells

Heavy-metal contaminated soils post great environmental and health concerns. In this study, Cr and Cd which are frequently observed in contaminated soils, were selected as representatives of hazardous heavy metals because of their different redox potentials and electric charges. Cr(VI)-, Cd(II)-, Cr(VI)/Cd(II)-contaminated soils were remediated in two-chamber air-cathode MFCs, in order to investigate the remediation of soil contaminated by single heavy metal and mixed heavy metals. Four ion exchange membranes (IEMs) were first evaluated to find out that bipolar membrane (BPM) was able to well maintain pH in both anolyte and catholyte, which was beneficial to support biological metabolism and heavy metal removal. It was also found that heavy metal ions (Cr, Cd or Cr/Cd) could migrate toward the cathode forming a concentration gradient under the weak electric field. The interaction between negatively charged Cr and positively charged Cd had no major effect to hinder each other on the migration, suggesting that the reduction reaction and electric field should be the main motivation for metal ion migration. The remediation performance of mixed heavy metal contaminated soil was superior to that of single heavy metal contaminated soil, for the possible reason of smaller internal resistance under mixed heavy metal condition.

Removal of hexavalent chromium in dual-chamber microbial fuel cells separated by different ion exchange membranes

An ion exchange membrane (IEM) is an important component in dual-chamber microbial fuel cells (MFCs) to separate cathodic chromium from anode bacteria to avoid toxicity. Common used IEMs (e.g., BPM, CEM, PEM, AEM) have different ionic transfer abilities which could influence MFC performance and chromium removal. Additionally, to distinguish chromium "removal" or "reduction" by MFCs, the chromium removal in this study was further analyzed into cathodic reduction, adsorption on the membrane and permeation through membrane to the anode chamber. It was found that BPM achieved the best performance in removing hexavalent chromium (99.4 ± 0.2 %) and balancing pH and conductivity in both chambers, followed by AEM (97.9 ± 0.8 %) and CEM (95.6 ± 0.8 %), while PEM can not well maintain pH and conductivity leading to the worst anode performance and lowest chromium removal efficiency. However, the adsorption of chromium on the AEM accounts for 91.1 ± 0.7 %, which was much higher than the other three membranes. The permeation of chromium through the membrane were all lower than 0.2% which can be ignored. SEM and EDS results showed that chromium deposits and bacteria were detected on the membrane facing cahtode and anode, respectively, indicating that membrane scaling and fouling were inevitable and happened within 24 h operation.