## 4 Discussion
### Toluene Degradation under Different Iron Mineral Conditions
It should be noted that the relationship between the crystallinity and reaction rate constants is influenced by specific reaction systems, reaction conditions, and crystal structures, and is not an absolute rule. The order of rate constants is amorphous iron hydroxide > goethite > hematite > magnetite. In this study, we focused on exploring the effect of the reactant structure on reaction rate constants, particularly for iron minerals with different crystallinities, providing valuable insights into the reaction kinetics between iron minerals and organic pollutants. During the degradation process of toluene, the theoretical amount of iron reduction is much higher than the actual reduction amount. This suggests that the complete oxidation of toluene and the reduction of iron do not occur synchronously. This discrepancy may be due to the degradation process of toluene not being a one-step mineralization, but rather involving intermediate states, leading to the incomplete reduction of ferrous iron and poor model fitting [28; 37]. The Conceptual Model of the Interrelationship between Toluene Degradation and Iron Reduction Processes  
Observation reveals a deviation between the experimental curve and the theoretical straight line. Initially, the slope of the experimental curve exceeds that of the theoretical curve. As the reaction progresses, the slope gradually decreases, eventually becoming less than that of the theoretical curve. This suggests the occurrence of electron-delayed ferrous iron reduction during the reaction. After analysis, three potential reasons have been identified. Firstly, during the extracellular anaerobic respiration of microorganisms with solid electron acceptors, electrons are typically transported by electron shuttles, leading to the storage of some electrons on these shuttles. This storage mechanism may cause a deviation in the stoichiometric ratio between toluene and iron during the reaction process [38]. Secondly, the synthetic metabolism and assimilation by microorganisms may result in the partial assimilation of toluene as a carbon source, promoting microbial biomass growth. With an increase in the microbial population, the amount of assimilated toluene also increases, leading to the accumulation of un-reduced toluene inside microbial cells. This accumulation prevents the detection of toluene, resulting in delayed electron transfer, which can be released upon microbial decay, achieving delayed electron release [39]. Thirdly, toluene may be completely degraded into multiple intermediate products within a short period of time. These intermediate products may accumulate during the degradation pathway, causing delayed electron transfer. For example, under the catalysis of benzoyl-CoA synthase, fumarate is added to the methyl group of toluene to form benzoyl succinate. Subsequently, a series of modified \(\beta\)-oxidation reactions occur after the addition reaction, converting phenylsuccinic acid into benzoyl-CoA, which is an intermediate product of the anaerobic degradation of aromatic compounds. Then, benzoyl-CoA is reduced by benzoyl-CoA reductase to form non-aromatic products, with the final products being CO\({}_{2}\) and H\({}_{2}\)O [40]. Only one suspected intermediate product was found in the gas chromatography analysis. It is speculated that many intermediates have their specificity, and a specific treatment is required for each intermediate product to achieve the identification of the product. The existence cycle of each intermediate is short, and the identification and characterization of intermediate products can be specialized in the later stage. Under the influence of microorganisms, all intermediate states can undergo mineralization. For instance, electron shuttles can transfer all their electrons to an electron acceptor, transitioning from a reduced state to an oxidized state, thereby achieving the reconversion of the electron shuttle storage state [41].