## 4 Discussion
### Effect of Cathode DO Concentration on the Migration and Distribution of Cr  
Substrate adsorption, plant absorption, and degradation (photodegradation, hydrolysis, and biodegradation) are considered the main pathways for Cr removal in CW-MFC [40]. Increasing the DO concentration can improve the absorption of Cr by the substrate and _L. hexandra_, and promote the conversion of Cr (VI) to Cr (III). In this study, when the DO concentration was 4.5 mg-L\({}^{-1}\), the Cr (III) concentration in the substrate was the highest (Table 1), because the increase in the DO concentration could improve the activity of cathode micro-organisms (such as electrochemically active bacteria, EAB) and produce more bioelectricity [41], thus, promoting the cathodic electrochemical reduction of Cr (VI) to Cr (III) and total Cr removal. The substrate contains 59.5% of the total Cr in the wetland [42,43]. This indicates that the substrate type and chemical composition may be a key factor in improving the metal-removal efficiency of the wetland [44]. The accumulation of Cr (III) in _L. hexandra_ reached a maximum value at 4.5 mg-L\({}^{-1}\) (Table 2), indicating that the increase in the O\({}_{2}\) content improved the enrichment ability and adsorption capacity of Cr in _L. hexandra_[45], and the Cr content in the plant roots was greater than that in the stems and leaves [46], indicating that the roots played an important role in absorbing heavy metal ions. On the one hand, the increase in the DO concentration promoted the conversion of Cr (VI) to Cr (III), thereby reducing Cr toxicity, which was more conducive to plant growth  
Figure 9: Relative abundance map of the _L. hexandra_ CW-MFC microbial community composition at the genus level. and Cr absorption [47]. On the other hand, the promotion effect of plants was mainly due to its indirect effect. The increase in the cathodic DO concentration improves the microbial environment of the plant rhizosphere, enhances biodegradation [48], and accelerates the metabolism of EAB [49; 50]. Therefore, an appropriate increase in the DO concentration can improve the absorption of heavy metal Cr by the substrate and _L. hexandra_, and promote the conversion of Cr (VI) to Cr (III). The Effect of Cathode DO Concentration on the Valence Change and Morphological Characteristics of Cr  
The level of chemically extracted Cr was higher in the cathode than that in the anode (Figure 4). This may be because the reduction of Cr (VI) mainly occurred in the cathode [47], and the increase of the DO concentration increased the electron acceptor of the cathode. The oxidation potential of Cr (VI) (1.33 V) is higher than that of O\({}_{2}\) (1.23 V) [51]. Therefore, Cr (VI) is a better oxidant than O\({}_{2}\). The Cr (VI) electron acceptor in the cathode is first reduced when the cathode O\({}_{2}\) content increased. This proves that the increased DO concentration promotes Cr (VI) reduction. However, further increases in the DO concentration results in the O\({}_{2}\) cathode penetrating into the anode area and destroying the anaerobic environment of the anode; this is not conducive to the electricity generation and pollutant degradation of the system [37; 52]. The reducing ability of Cr (VI) is reduced when there is insufficient electron transfer to the cathode. The forms of Cr in the cathode and anode are mainly residual and oxidizable; this may be one of the reasons why most of the Cr remains in the substrate [43]. Increasing the DO concentration results in Cr mainly existing in the bound form with pectin, protein, heavy metal phosphate, and oxalate in _L. hexandra_ (Figures 5-7). These forms reduce the toxicity and mobility of Cr [53; 54; 55]. Plant cell walls (including intercellular spaces and vacuoles) mainly contain Cr in bound forms to pectinate, protein, heavy metal phosphate, and oxalate; this reduces the levels of free Cr, thereby reducing cellular toxicity [56; 57].