Industrial wastewater streams generally contain heavy metals such as Cr, Cu, Pb [1
], among which chromium is generally the major contaminant found in industrial wastewater, which may cause health problems and pollute natural ecosystems when at high concentrations [2
]. In aquatic ecosystems, trivalent [Cr(III)] and hexavalent [Cr(VI)] are its two major oxidation forms [3
]. Of these Cr(VI) can cause more serious injuries than Cr(III) to plants and humans, such as skin cancer, allergies and the lung cancer [4
] due to its high solubility, mobility, and mutagenicity. Several traditional treatment technologies can remove Cr(VI) efficiently, including physical, chemistry and biological methods [5
], but samples treated with all of these methods need to be verified after processing to assess whether they meet the discharge standard concentration. If the Cr(VI) emission exceeds the permitted concentration, it can cause serious environmental problems and human health disorders [6
]. Thus, it is crucial for the Cr(VI) concentration of effluents from industrial wastewater treatment plants to be strictly controlled in real time prior to its discharge to the environment. At present, several chemical analysis methods including spectrophotometry, atomic absorption spectrometry (AAS), and infrared-absorption spectrometry are applied to detect the Cr(VI) concentration in water samples [7
]. These methods display high selectivity and sensitivity, however, the existing methods have drawbacks such as a high cost or the requirement for complicated extraction, purification, and concentration procedures [8
], and they cannot provide in situ and real time values.
Recently, biosensors, which are integrated devices that combine biological materials with a transducer, have been developed for the quantitative or semi-quantitative monitoring of various analytes [9
], such as heavy metals like Cu2+
], biochemical oxygen demand (BOD) [11
] and volatile fatty acids [12
]. Previous studies have reported biosensors for Cr(VI) detection based on an amperometric enzyme biosensor utilizing cytochrome c3 or urease, relying on cell biosensors using engineered strains [13
]. Compared with traditional techniques, these biosensors exhibited simple, inexpensive and portable characteristics for practical application. However, most of the previously reported biosensors have used either purified proteins (such as enzymes, metal-binding proteins, or antibodies) or whole cells of genetically engineered microorganisms [15
]. These microbial biosensors, which usually use green fluorescent protein or other marker molecules as indicators, face the challenge of toxicity that prevents their use with samples containing high concentrations of heavy metals [16
]. Moreover, most of these biosensors cannot provide in situ and real-time Cr(VI) concentration values.
The two compartments of microbial fuel cells (MFCs), namely, the anaerobic and aerobic ones, are a promising approach that can directly utilize microorganisms as biocatalysts for transforming energy as well as to generate power [17
]. In recent years, electrical signals have frequently been applied as indicators for the monitoring of heavy metals and nitroaromatic compounds due to their highly speed, sensitivity, and quantifiability [18
]. Moreover, the potential of MFC-based biosensors for the Cr(VI) detection has been reported. A previous study used a two-chamber MFC as a device for monitoring Cr (VI) in water and highlighted the effect of the co-existing Fe(II) in the system on chromium detection as Fe(II) can reduce Cr(VI) and alter the generated voltage [20
], which is unsuitable for in situ detection of chromium. In these MFCs-based biosensors, a correlation among the Cr(VI) concentration and the voltage generation was established with the Cr(VI) acting as electron acceptor at the MFC anode [21
]. The results showed its sensitivity to higher Cr(VI) concentrations (2.5–60 mg/L), which preclude its use due to the fact the recommended maximum allowable concentration for Cr(VI) in industrial wastewater based on the Chinese National Standard is 0.5 mg/L. Furthermore, a MFC-based biosensor was developed for Cr(VI) detection using a single strain inoculated in the anode of a MFC [22
], but it is not suitable for in situ and real time measurement for the reason that medium refreshment, bacterial cultivation and aseptic techniques must be used, which must be performed in a laboratory.
The Sediment Microbial Fuel Cell (SMFC) concept is a promising alternative technology that comprises an anode and a cathode, in which the anode was embedded in sediment and the cathode is placed above the anode, where it is partly filled with water [23
]. Various complex organic compounds such as naphthalene, acenaphthene, phenanthrene [24
], and pyrene [25
] can be degraded at the anode in SMFCs while sulfate, oxygen and Cr(VI) were reduced at the cathode. Electrons produced by respiration of the microbial community colonizing the anode surface are generated firstly and then transmitted to the cathode through an external circuit [26
]. The main advantages of SMFCs over traditional batteries are that they are low cost, of simple construction and they provide long term power output [27
]. Thus, this system is applicable as an electron supply device for biosensors for long term and in situ operation. The use of SMFCs as wireless sensors for the real-time detection of environmental and ecological conditions such as temperature and the concentration of dissolved oxygen (DO) was reported [28
]. However, there are scarce reports on the development of SMFC as a biosensor for heavy metal monitoring.
In the present study, we develop an early warning system based on SMFC for long term and in situ monitoring of Cr(VI) concentrations in industrial wastewater treatment processes. A stable performance of SMFC was observed with different temperatures and pH and other co-existing ions. A relationship was obtained depending on the voltage changes in response to different Cr(VI) concentrations. Compared with colorimetric method results, the biosensor for synthesized wastewater and actual wastewater measurement displayed low deviations over 18.3 min. Thus, the system we have developed is suitable and convenient as an early warning device for in situ real time monitoring of the Cr(VI) concentrations in industrial wastewater treatment plants.