###### Abstract  
Cr (VI) is hazardous to humans and our environment. _Lersia hexandra_ Swartz (_L. hexandra_) is the first wet chromium hyperaccumulator found in China. This study constructed the _L. hexandra_ constructed wetland-microbial fuel cell (CW-MFC) system to treat Cr (VI) wastewater. It also determined the effects of different dissolved oxygen (DO) concentrations on power generation, pollutant removal, and Cr (VI) reduction. Cathode aeration promoted the voltage output and pollutant removal of the _L. hexandra_ CW-MFC when the DO concentration was 4.5 mg\(\cdot\)L\({}^{-1}\): the highest voltage was 520 mV, the chemical oxygen demand (COD) removal rate was 93.73%, and the Cr (VI) removal rate was 97.77%. Moreover, the increase in the DO concentration improved the absorption of heavy metal Cr by the substrate and _L. hexandra_, and promoted the transformation from Cr (VI) to Cr (III). Chromium mostly exists as a residue with low toxicity and low mobility in _L. hexandra_ and the substrate. This proves that the increased DO concentration promotes the redox reaction in the system and plants, reducing Cr (VI) to Cr (III). At the same time, the key micro-organism _Grobacter_ that enhances the performance of the system and Cr (VI) reduction was found. The research results can provide a reference for the subsequent CW-MFC treatment of actual Cr-containing wastewater. Leverisia hexandra Swartz; constructed wetland-microbial fuel cell; dissolved oxygen (DO); Cr (VI) +
Footnote †: journal: Medical  
and _L. hexandra_ CW detoxifies Cr (VI) by reducing Cr (VI) to Cr (III) [14]. Some MFCs can remove Cr (VI) from water by generating large amounts of electrons [15; 16]. The CW and MFC systems may be coupled because the redox gradient of the natural layer of the CW system is highly consistent with the MFC operating conditions [17]. In recent years, the coupled constructed wetland-microbial fuel cell (CW-MFC) system has received extensive attention [18; 19; 20]. For example, the optimal removal rate of Cr (VI) by CW-MFC was approximately 91% [21]. However, current research has only aimed to improve the treatment capacity of Cr (VI) [20], and the removal process of Cr (VI) in water by the CW-MFC system is unclear. Oxygen as an electron acceptor is considered to be a key parameter for optimizing CW-MFC [22; 23]. When the system cathode is in an aerobic environment, aromatic amine mineralization [24], nitrification [25; 26], and anode unused organic matter [27] will be further degraded under the action of micro-organisms, which is conducive to improving the removal efficiency of pollutants in the system [26; 28]. Intermittent aeration (IA) can promote the wastewater treatment and power generation of the _Canna indica_ L. CW-MFC system [22]. The overall performance of CW-MFC is improved by adding aeration devices or cultivating plants in the cathode area [29]. The aeration process consumes more organic pollutants by inhibiting the activity of anaerobic heterotrophic micro-organisms [30]. Therefore, the cathode dissolved oxygen (DO) concentration plays an important role in improving the system performance. Cr (VI) is reduced to Cr (III) after receiving electrons from the anode at the cathode as an electron acceptor [31]. At the same time, the cathode O\({}_{2}\) acts as another electron acceptor to combine with electrons to form water and another reducing compound [23]. In this process, both Cr (VI) and O\({}_{2}\) act as electron acceptors to consume electrons from the anode and promote the degradation of anode organic matter. The introduction of MFC provides an effective way for electron conduction to the cathode [22]. Therefore, based on the above analysis, we speculate that there may be a competitive relationship between O\({}_{2}\) and Cr (VI) in the CW-MFC system, which affects the valence state conversion of Cr (VI) to Cr (III). Nevertheless, the role of O\({}_{2}\) in the reduction process of Cr (VI) needs to be further explored and clarified. The first wet chromium hyperaccumulator discovered in China was _L.