## 4 Discussion  
### Toluene Degradation under Different Iron Mineral Conditions  
Under anaerobic conditions mediated by dissimilatory iron-reducing microorganisms, the degradation reaction between toluene and various iron minerals undergoes a series of slow and sequential steps. Initially, toluene enters the microbial cells from the aqueous solution, followed by a series of intracellular reactions leading to gradual degradation within the microbial cells. Ultimately, electrons are transferred from the microbial cells to the iron minerals attached to the dissimilatory iron-reducing microorganisms. Throughout this process, Fe(III) in the iron minerals undergoes reduction [33]. The degradation of toluene follows first-order kinetics, consistent with previous studies by researchers [34; 35]. During the preliminary degradation of toluene under four types of iron minerals, the first-order kinetic reaction rate constants are, respectively, as follows: amorphous iron hydroxide group, 0.128 d\({}^{-1}\); hematite group, 0.027 d\({}^{-1}\); magnetite group, 0.025 d\({}^{-1}\); and goethite group, 0.073 d\({}^{-1}\). The reaction rate constant depends on the temperature, activation energy, and concentration, and the activation energy depends on the structure (mineral type) and its interaction with the toluene, in this case. In the experiment, amorphous iron hydroxide appears as aggregates of amorphous irregularly shaped particles, while hematite exhibits long needle-like crystals. Hematite primarily exists as granular crystals, and magnetite exists in the form of inverse spinel. Amorphous iron hydroxide with a lower crystallinity and a slightly higher crystallinity of hematite demonstrate higher reaction rate constants. In contrast, hematite with a higher crystallinity exhibits lower reaction rate constants. We observed that the crystallinity of iron minerals affects their interaction with reactants such as toluene. This confirms the existence of a certain relationship between the crystallinity and reaction rate constants, although this relationship may be influenced by various factors [36]. Crystals with a higher crystallinity often have a more complete and compact crystal structure, resulting in a smaller surface area and lower exposure of active sites. In contrast, crystals with a lower crystallinity may have more grain boundaries and defects, leading to an increased surface area and more exposed active sites, thereby promoting the adsorption and conversion of reactants and increasing the reaction rate constants. Internally, crystals with a higher crystallinity typically have a more complete and ordered structure, limiting the diffusion of reactants within the crystal and affecting the rate of reaction. In contrast, crystals with a lower crystallinity may have more grain boundaries and defects, allowing reactants to diffuse more easily within the crystal, thereby increasing the reaction rate constants. Crystals with a higher crystallinity often have higher bond energy and higher activation energy, increasing the energy required for the reaction to occur and consequently reducing the reaction rate constants. In contrast, crystals with a \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Group**} & **Time Periods with Slopes** & **Time Periods with Slopes** & **Time Periods with Slopes** \\  & **Greater than 1/36** & **Approximately Equal to 1/36** & **Less than 1/36** \\ \hline Amorphous iron hydroxide Group & 0–22 & 22–28 & 28–40 \\ Hematite Group & 0–12 & 12–28 & 28–40 \\ Magnetite Group & 0–22 & 22–28 & 28–40 \\ Goethite Needle Group & 0–18 & 18–22 & 22–40 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Phases of the coupled relationship between toluene consumption and ferrous iron reduction for four Fe(III) oxides. lower crystallinity may have lower bond energy and lower activation energy, making the reaction easier to occur and thus increasing the reaction rate constants.