Bioanodes containing catalysts from onion waste and Bacillus subtilis for energy generation from pharmaceutical wastewater in a microbial fuel cell

Abstract

We propose the integration of bioanodes composed of onion waste (OW)-derived biocarbon catalysts + Bacillus subtilis (B. subtilis) for application in a microbial fuel cell (MFC). OW is activated by pyrolysis and the resulting catalysts are functionalized with methanol by intermittent microwave heating (IMH). The functionalized-activated onion waste (FAOW) catalysts are labeled as FAOW4, FAOW6, and FAOW8 after treatment at 400, 600, and 800 °C, respectively. As a comparison, an N-doped graphene catalyst (FNSG) is also studied. The substrate in the anode chamber of the MFC is highly recalcitrant pharmaceutical wastewater (PWW, pH = 9.2). The XRD results show that the FAOW catalysts have an amorphous structure, contrasting with the crystalline pattern of FNSG. It is shown that microwave heating has only a slight effect on the structural order of the FAOW series while promoting disorder at FNSG. Testing in half-cells show that the current density (j) increases at anodes in the order FAOW8 > FAOW6 > FAOW4. The j values increase even more at full bioanodes after growing a biofilm of B. subtilis on the catalyst layer of FAOW8 and FAOW6. In MFC tests, the FAOW8 + B. subtilis bioanode can produce a maximum power density (P_cell) of 30.72 mW m⁻², sustaining high performance during a 14-day long-term test. Therefore, FAOW8 and B. subtilis are proposed as a highly performing catalyst–biofilm configuration to generate energy from PWW in an MFC.