Power benefitted bioremediation of hexavalent chromium ions in biochar blended soil microbial fuel cell

Anthropogenic activity discharges large quantities of chromium into our environment which in turn causes lethal effects. Biological remediations of chromium are progressed by soil microbial fuel cell (SMFC) whose fuel performance is made remarkable in presence of carbonaceous biochar material. In the present study, various SMFCs were constructed individually using soil admixed with prepared biochars such as Echinochloa frumentacea (EFB), Zea mays (ZMB) and Solanum melongena (SMB) as fuel. The stainless steel electrodes of 3 x 3 x 0.2 cm geometric dimensions were employed as electrodes. Various desired concentrations such as 0.1 N, 0.125 N, 0.150 N, 0.175 N and 0.2 N of synthetically prepared Cr(VI) ions from potassium dichromate stock solutions were used as catholyte. The obtained potentials were measured using multimeter and the results were compared with control SMFC incorporated with distilled water as catholyte. The addition of three prepared biochars multiplied bacterial population to a tune of 5.2 x 106 CFU/mL in EFB, 3.8 x 106 CFU/mL in ZMB and 2.7 x 106 CFU/mL in SMB which was found to be 10 times more on comparison with bare soil population (0.3 x 105 CFU/mL). EFB added to SMFC (EFS), ZMB added to SMFC (ZMS) and SMB added to SMFC (SMS) operated well in the anodic pH window of 7.04 to 8.21 and complimented bacterial population with buffering capacity under toxic Cr(VI) environment (in the order of 106). The enhanced bacterial count favoured biofilm formation at the anode biotically for electrical power production. The lower Cr(VI) concentration cathode-loaded SMFCs such as 0.125 EFS, 0.125 ZMS and 0.1 SMS delivered a static maximum potential of 774 mV for 11 days, 755 mV for 16 days and 625 mV for 11 days respectively. 0.1 EFS delivered a static potential of 748 mV for 25 long days whose pH window varied in between 8.11 and 7.54 with stable bacterial count of 106 CFU/mL. The higher Cr(VI) concentration cathode-loaded SMFCs suppressed bacterial population to an order of 104 CFU/mL and decreased the anodic pH with electrical power. The anodic electrochemical assistance of biofilm with varying Cr(VI) concentration at the cathode was accessed using parameters such as charge transfer resistance and double layer capacitance as determined by Nyquist plot of AC impedance spectroscopy. The study concluded that lower values of anodic charge transfer resistance facilitated electron transport process. Increase in charge transfer resistance values was noticed with increase of chromium concentration and biofilm thickness which retards the electrical power generation. The morphologies of biofilm were investigated using SEM with EDS tool and the study evidenced the textural structure of biofilm with involvement of chromium ions in biofilm formation. In situ studies using CV and conventional methodologies showed the reduction of chromium at the cathode. The removal efficacy was found to be 16.49%. Thus, the study concluded biochar amended soil's ability to reduce Cr(VI) ions with simultaneous power generation.