# Practical Considerations for Continuous Monitoring of Hexavalent Chromium in Wastewater Using a Microbial Fuel Cell Biosensor: Biosensor Fabrication, Sample Pretreatment, and Bacterial Community Analysis

## TL;DR

A new biosensor using a microbial fuel cell was developed to continuously monitor hexavalent chromium in wastewater with high accuracy and stability.

## Contribution

A novel microbial fuel cell biosensor with a genetically engineered E. coli strain for real-time Cr(VI) detection in wastewater.

## Key findings

- The biosensor showed excellent linearity (R2 ≥ 0.999) across a wide Cr(VI) concentration range (0.015–200 mg/L).
- The engineered E. coli strain retained functionality after 450 days of storage at −20 °C.
- The biosensor achieved high accuracy in wastewater monitoring with minimal deviation from standard methods.

## Abstract

Hexavalent chromium (Cr(VI)) is a high-priority environmental pollutant due to its strong oxidizing properties, which cause DNA damage and other severe health effects. Conventional detection methods are often costly and lack real-time monitoring capabilities, creating a strong demand for cost-effective, real-time biosensors that meet industrial requirements. In this study, we developed a novel biosensor for continuous Cr(VI) monitoring using a single-chamber microbial fuel cell (MFC). The biological element is an engineered Escherichia coli strain (ChrA-ChrB-E. coli), constructed by introducing Cr(VI)-resistant (ChrA) and Cr(VI)-reducing (ChrB) genes. The presence of Cr(VI) affects bacterial metabolism and electron transfer within the MFC, generating a measurable signal proportional to the contaminant’s concentration. The biosensor demonstrated robust performance and characteristics. The recombinant strain retained functional activity after 450 days of storage at −20 °C. The system exhibited high sensitivity and excellent linearity (R2 ≥ 0.999) across a broad Cr(VI) concentration range of 0.015–200 mg/L. During continuous monitoring of chrome tanning and electroplating wastewater, measurements deviated by less than 2.33% from the standard diphenylcarbazide (DPC) method; electroplating deviation was further reduced to −0.69% with EDTA pretreatment. In fishery water, the deviation was higher (−7.12%) due to dissolved oxygen (DO) interference but was reduced to −0.75% after mechanical stirring to remove DO. The biofilm bacterial community remained highly stable over six months in both wastewater types, with the inoculated ChrA-ChrB-E. coli strain maintaining dominance (>99.6%). These results substantiate the feasibility of using this biosensor for continuous, online, real-time detection of Cr(VI) in actual wastewater environments.