## 4 Discussion
### Toluene Degradation under Different Iron Mineral Conditions
Similarly, biomass storage states can be consumed through microbial growth and decay, resulting in the depletion of the electron storage state [39]. Intermediate products, such as benzyl-succinate, benzyl-succinyl-CoA, E-phenylitaconyl-CoA, (hydroxymethylphenyl)-succinyl-CoA, benzoylsuccinyl-CoA, and succinyl-CoA, are present throughout the reaction process. The electron storage states of these intermediates can be gradually consumed over time [42; 43; 44]. Consequently, the degradation capability of intermediate states, whether in the form of an electron shuttle storage state, biomass storage state, or intermediate product storage state, can release electrons for utilization via the iron electron acceptor, leading to the reduction of trivalent iron to divalent iron. In summary, there exists an intermediate state of electron storage. Here, it is referred to as the intermediary state (electron shuttle storage state, biomass storage state, intermediate product storage state), as shown in Figure 5. The first-order degradation of toluene mentioned earlier might be toluene converting into an intermediate state. Due to the presence of this intermediate state, this reaction is more complex than a single-step reaction. Here, we discuss a relatively simpler continuous reaction process divided into two stages. Let us assume that toluene undergoes a series of processes, first generating the intermediate state B, which then further transforms into the final product C. The entire series of reactions can be represented as follows:  
\[\text{toluene}\overset{v_{1}}{\rightarrow}B\overset{v_{2}}{\rightarrow}C \tag{8}\]  
In the first stage, the conversion of toluene into an intermediate state dominates the reaction process. In the initial phase, the rate of toluene conversion is high, indicating a significant rate of intermediate state generation, while the rate of intermediate state consumption is low (\(v_{1}>v_{2}\)). Primarily, two types of reactions occur as follows: the conversion of toluene into the intermediate state and the subsequent conversion of the intermediate state into the final product. However, the conversion of toluene into the intermediate state is the predominant reaction, leading to the accumulation of the intermediate state. This indicates that this stage of the reaction is primarily driven by the conversion of toluene into the intermediate state. The initial first-order kinetics equation for toluene consumption represents the initial conversion of toluene, but the conversion of toluene is not completely mineralized, resulting in the generation of ferrous iron not following first-order kinetics. This suggests that there is still an intermediate state carrying some electrons. The ratio of toluene consumption to iron reduction is greater than 1/36. As the reaction progresses, the rate of toluene conversion gradually decreases along with the decreasing concentration of toluene. The rate of intermediate state generation diminishes, resulting in an accumulation of the intermediate state. Meanwhile, the rate of transformation of the intermediate state also increases. When the generation rate of the intermediate state equals its consumption rate (\(v_{1}=v_{2}\)), the accumulation of the intermediate state ceases, reaching its maximum value. At this point, a balance is achieved between the conversion of toluene into the intermediate state and the transformation of the intermediate state into the final product, with the ratio of toluene conversion to iron reduction equaling 1/36. The rate of toluene conversion continues to decrease, and the generation rate of the intermediate state slows down. The consumption rate of the intermediate state exceeds its generation rate (\(v_{1}<v_{2}\)), indicating that, in this stage, two reactions primarily occur: toluene conversion to an intermediate state and the subsequent transformation of the intermediate state into the final product. There is minimal generation of the intermediate state but there is significant consumption.