## 3 Results
### Effect of Different DO Concentrations on Cr Content in the L. hexandra CW-MFC System
#### 3.4.2 Chemical Speciation Analysis of Cr in \(L\). hexandra  
All forms of Cr content reached the maximum residual state (10,273.44 mg\(\cdot\)kg\({}^{-1}\)), HCl extraction state (2626.25 mg\(\cdot\)kg\({}^{-1}\)), H\({}_{2}\)O extraction state (1577.75 mg\(\cdot\)kg\({}^{-1}\)), ethanol extraction state (1344.31 mg\(\cdot\)kg\({}^{-1}\)), HAc extraction state (1771.19 mg\(\cdot\)kg\({}^{-1}\)), and NaCl extraction state (1296.69 mg\(\cdot\)kg\({}^{-1}\)) at 4.5 mg\(\cdot\)L\({}^{-1}\) (Figure 5). The proportion of the residual state increased to the maximum, accounting for 55% of the total. This indicated that the increased DO concentration contributed to the transformation of Cr to a stable state. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**DO Concentration** & \multicolumn{2}{c}{**Total Chromium (mg\(\cdot\)kg\({}^{-1}\))**} & \multicolumn{2}{c}{**Cr (VI) (mg\(\cdot\)kg\({}^{-1}\))**} & \multicolumn{2}{c}{**Cr (III) (mg\(\cdot\)kg\({}^{-1}\))**} \\ \cline{2-9}
**(mg\(\cdot\)L\({}^{-1}\))** & **Anode** & **Cathode** & **Anode** & **Cathode** & **Anode** & **Cathode** \\ \hline
3.0 & 7988.54 & 8386.46 & 572.50 & 448.33 & 7416.04 & 7938.13 \\
3.5 & 8295.83 & 8517.71 & 487.50 & 416.25 & 7808.33 & 8101.46 \\
4.0 & 9045.83 & 9525.00 & 453.75 & 246.25 & 8592.08 & 9278.75 \\
4.5 & 14,945.83 & 16,350.00 & 357.92 & 204.17 & 14,587.92 & 16,145.83 \\
5.0 & 10,132.29 & 12,778.13 & 422.08 & 241.67 & 9710.21 & 12,536.46 \\ \hline \end{tabular}
\end{table} 
Table 1: Cr content in the substrate at different DO concentrations. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**DO Concentration** & \multicolumn{2}{c}{**Total Chromium (mg\(\cdot\)kg\({}^{-1}\))**} & \multicolumn{2}{c}{**Cr (VI) (mg\(\cdot\)kg\({}^{-1}\))**} & \multicolumn{2}{c}{**Cr (III) (mg\(\cdot\)kg\({}^{-1}\))**} \\ \cline{2-9}
**(mg\(\cdot\)L\({}^{-1}\))** & **Root** & **Stem** & **Leaf** & **Root** & **Stem** & **Leaf** & **Root** & **Stem** & **Leaf** \\ \hline
3.0 & 8428.13 & 1428.13 & 1191.67 & 1927.08 & 635.42 & 483.33 & 6501.04 & 792.71 & 708.33 \\
3.5 & 9913.54 & 1851.04 & 1365.63 & 1616.67 & 495.83 & 472.92 & 8296.88 & 1355.21 & 892.71 \\
4.0 & 13,070.83 & 2402.08 & 1913.54 & 1104.17 & 481.25 & 450.00 & 11,966.67 & 1920.83 & 1463.54 \\
4.5 & 18,869.79 & 3642.71 & 3070.83 & 641.67 & 418.75 & 385.42 & 18,228.13 & 3223.96 & 2685.42 \\
5.0 & 14,313.54 & 2588.54 & 2187.50 & 1081.25 & 441.67 & 414.58 & 13,232.29 & 2146.88 & 1772.92 \\ \hline \end{tabular}
\end{table} 
Table 2: Cr content in \(L\). hexandra at different DO concentrations. Figure 4: The content and proportion of Cr following chemical extraction from the _L. hexandra_ CW-MFC system under different DO concentrations: (**a**) Cr content of the anode; (**b**) Cr proportion of the anode; (**c**) Cr content of the cathode; and (**d**) Cr proportion of the cathode. Figure 5: The content and proportion of chemical-extracted Cr in the roots of _L. hexandra_ under different DO concentrations: (**a**) Cr content in _L. hexandra_ roots; and (**b**) Cr proportion in _L. hexandra_ roots. The content of chemically extracted Cr in the stems of _L. hexandra_ reached the maximum residual state (2024.06 mg\(\cdot\)kg\({}^{-1}\)), HCl extraction state (480.00 mg\(\cdot\)kg\({}^{-1}\)), H\({}_{2}\)O extraction state (350.69 mg\(\cdot\)kg\({}^{-1}\)), ethanol extraction state (220.13 mg\(\cdot\)kg\({}^{-1}\)), HAc extraction state (311.25 mg\(\cdot\)kg\({}^{-1}\)), and NaCl extraction state (324.25 mg\(\cdot\)kg\({}^{-1}\)) at 4.5 mg\(\cdot\)L\({}^{-1}\) (Figure 6). Chromium mainly existed as the residual form, the oxalate form, and as other bound chromium in the stems of _L. hexandra_. The chemical form of Cr in _L. hexandra_ leaves is mainly in the residual state. The content of residual Cr increased, then decreased as the DO concentrations were raised. A maximum of 1303.75 mg\(\cdot\)kg\({}^{-1}\) was observed at a DO concentration of 4.5 mg\(\cdot\)L\({}^{-1}\); this accounted for 43% of the total Cr amount (Figure 7).