## 3 Results
### The Relationship between Toluene and Ferrous Products under Different Iron Mineral Conditions  
In order to investigate the proportional relationship between the consumption of toluene and the generation of ferrous iron during the reaction process, we measured the toluene consumption (vertical axis) and ferrous iron generation (horizontal axis) at specific time points (3 rd, 5 th, 7 th, 12 th, 18 th, 22 nd, 28 th, and 40 th days) in the biodegradation of toluene. These findings are visually depicted in the two-dimensional graph shown in Figure 4.  
Figure 3: A comparison between the theoretical Fe(II) concentration curves and the actual Fe(II) concentration curves. (**a**–**d**), respectively, represent the four iron minerals, namely, amorphous iron hydroxide, hematite, magnetite, and goethite, respectively (initial Fe(III) concentration of 3.6 mmol/L) as electron acceptors. The initial concentration of toluene (C\({}_{0}\)) was 0.1 mmol/L. The theoretical Fe(II) concentrations (C\({}_{\text{FAT}}\), C\({}_{\text{FHT}}\), C\({}_{\text{FMT}}\), and C\({}_{\text{RGT}}\)) correspond to the mineralization of toluene to Fe(II) at time t for amorphous iron hydroxide, hematite, magnetite, and goethite, respectively.  
According to the graph, the vertical axis, from bottom to top, represents the increasing amount of toluene consumption, while the horizontal axis, from left to right, represents the gradually increasing ferrous iron production, reflecting the progression of the reaction over time.  
Hematite group: At the beginning of the reaction, the toluene consumption increases slowly from 0 mmol/L, and the ferrous iron production also starts at 0 mmol/L and increases gradually. If the initial toluene consumption and ferrous iron production follow the theoretical ratio of 1/36, the curve would tend toward the direction of the theoretical curve. However, as observed in the graph, from the first point to the second point, the curve deviates from the theoretical curve toward the \(Y\)-axis, indicating higher toluene consumption and relatively insufficient ferrous iron production. From the first point to the sixth point, the slope of the curve decreases, gradually approaching the slope of the theoretical curve, indicating a gradual reduction in toluene consumption and a relative increase in ferrous iron production from 0 to 22 days, reaching a stage where toluene and iron approach the theoretical 1/36 ratio. From the sixth point to the seventh point, a temporary equilibrium is reached, indicating that at some point between 22 and 28 days,  
Figure 4: The mutual relationship between the degradation of toluene and iron reduction is depicted in the figure, where (**a–d**), respectively, represent the four iron minerals, namely, amorphous iron hydroxide, hematite, magnetite, and goethite, with an initial Fe(III) concentration of 3.6 mmol/L. The figure illustrates the real-time consumption of toluene (initial concentration of 0.1 mmol/L) and the real-time generation of ferrous ions under the influence of iron-reducing bacteria. For comparison, a reference line y = 1/36x is plotted, representing the theoretical ratio of 1/36 between toluene consumption and ferrous ion generation. This line serves as a reference, indicating that, in the theoretical reaction, the complete oxidation of 1 mmol of toluene would theoretically yield 36 mmol of ferrous ions. The direction of the arrow indicates the direction of the reaction.  
toluene consumption and iron production are in a theoretical 1/36 ratio. From the seventh point to the eighth point, the slope of the curve is less than the slope of the theoretical curve, indicating that from 28 to 40 days, the ratio of toluene to iron is less than the theoretical 1/36 ratio. Similar analyses were conducted for the fitting relationship between toluene consumption and ferrous iron production in the hematite, magnetite, and goethite groups, as shown in Table 4.