## 4 Discussion
### Toluene Degradation under Different Iron Mineral Conditions
The intermediates accumulated in the early stage  
Figure 5: Schematic chemical proposal for the implied mechanism during the toluene degradation. of this phase begin to gradually deplete. Overall, this stage signifies a predominance of the transformation of the intermediate state. Consequently, in the overall reaction, the electrons obtained from ferrous iron reduction exceed those released from toluene consumption during this period. In other words, the lagged electrons at this point contribute to the reduction of ferrous iron. The ratio of toluene conversion to iron reduction is less than 1/36. It can be inferred that under microbial mediation, the degradation reaction of toluene using trivalent iron minerals as electron acceptors is likely to proceed in stages rather than in a single step; initially, toluene is transformed into certain intermediate states, and eventually transformed into final products. Therefore, through analysis, we have developed this conceptual model, as shown in Figure 6.  
By integrating experimental data into the model, we can ascertain the predominant stages of various iron minerals. The slope of the regression line, depicted in Figure 4, with the ferrous iron concentration on the x-axis and toluene consumption on the y-axis (representing the ratio of toluene consumption to iron reduction, hereinafter referred to as \"the slope\"), effectively delineates the phases of different iron minerals within the model. Those with slopes greater than 1/36 denote the dominant phase of toluene transitioning to an intermediate state, while those with slopes less than 1/36 signify the dominant phase of intermediate state mineralization. The phases of the coupled relationship between toluene consumption and ferrous iron reduction for four Fe(III) Oxides represents the division of stages, as illustrated in Table 4.  
In the process of toluene transformation to an intermediate-state-dominated phase, the slopes are largest for magnetite and amorphous iron hydroxide, followed by goethite, and then red hematite. A larger slope indicates a faster oxidation rate of toluene relative to the iron reduction rate, with a rapid accumulation of the intermediate state. Active oxidized  
Figure 6: Conceptual model of the interrelationship between toluene degradation and iron reduction processes, where the red curve represents the reaction progress line, and dashed lines indicate reactions that have not occurred. The \(\Circle\) points denote the points with a slope of 1/36, with the left side representing the transformation of toluene to intermediate-state-dominated phase and the right side representing the mineralization of the intermediate-state-dominated phase. The solid black line, y = 1/36x, represents the reaction progress line according to the theoretical equation. iron acts as a catalyst, promoting the rapid degradation of toluene. The catalytic effects are in the following order: magnetite > amorphous iron hydroxide > goethite > red hematite. Through analyzing the duration of toluene transformation to the intermediate-state-dominated phase, the process is longest in amorphous iron hydroxide and magnetite, followed by goethite, and then by red hematite. This suggests that different activities of oxidized iron lead to different accumulation times of the intermediate state. The order of accumulation times is as follows: amorphous iron hydroxide and magnetite > goethite > red hematite. Arranging based on the slope of the line connecting the endpoint and starting point, the order is amorphous iron hydroxide > magnetite > goethite > red hematite. The slope of the line represents the final ratio of toluene consumption to ferrous iron reduction. This also suggests that higher activity of oxidized iron leads to a lower ratio of toluene consumption to ferrous iron reduction, which might be due to the high activity of oxidized iron affecting the iron reduction in the intermediate state of electron storage, resulting in the accumulation of electrons.