## 1 Introduction  
Organic contaminants, particularly benzene compounds, are widespread in the groundwater of sites contaminated with substances such as petroleum, posing significant adverse effects on both the environment and the economy [1; 2; 3; 4; 5; 6]. Due to the prevalent anaerobic conditions in the groundwater of polluted sites [7; 8], anaerobic biodegradation has become a crucial approach for addressing organic pollution in site remediation. Organic contaminants undergo biological degradation through processes such as nitrate reduction, iron reduction, manganese reduction, sulfate reduction, and methane production [9; 10; 11]. Iron, as one of the most abundant elements in the Earth's crust, constitutes approximately 6% of its surface. It is one of the most abundant electron acceptors, and the dissimilatory iron reduction process stands out as a primary mechanism in anaerobic biodegradation [12]. Research on the iron reduction of organic compounds, especially aromatic hydrocarbons, is particularly crucial [13]. Over the past three decades, researchers have extensively studied dissimilarity iron-reducing bacteria and the reduction of iron-containing minerals. These minerals, including ferrihydrite, goethite, amorphous iron minerals, magnetite, iron-bearing clay minerals, jarosite, and schwertmannite, among others [14; 15; 16; 17; 18; 19; 20; 21], serve as electron acceptors in dissimulatory iron reduction reactions, while certain compounds serve as electron donors in these reactions. \[\mathrm{CH_{3}COOH+8 Fe^{3+}+4H_{2}O\rightarrow}\mathrm{8 Fe^{2+}+2 HCO_{3}-+10H^{ +}\ \Delta H=651.98\ kJ/mol} \tag{1}\]  
\[\mathrm{C_{6}H_{6}+30 Fe^{3+}+18H_{2}O\rightarrow}\mathrm{30 Fe^{2+}+6 HCO_{3}-+36H^{ +}\ \Delta H=-457.92\ kJ/mol} \tag{2}\]  
\[\mathrm{C_{7}H_{8}+36 Fe^{3+}+21H_{2}O\rightarrow}\mathrm{36 Fe^{2+}+7 HCO_{3}-+3 4H^{+}\ \Delta H=-550.39\ kJ/mol} \tag{3}\]  
\[\mathrm{C_{6}H_{5}OH+28 Fe^{3+}+17H_{2}O\rightarrow}\mathrm{28 Fe^{2+}+6 HCO_{3} -+34H^{+}\ \Delta H=-468.99\ kJ/mol} \tag{4}\]  
\[\mathrm{C_{8}H_{10}+42 Fe^{3+}+24H_{2}O\rightarrow}\mathrm{42 Fe^{2+}+8 HCO_{3} -+50H^{+}\ \Delta H=-637.7\ kJ/mol} \tag{5}\]  
In actual pure cultivation experiments, iron-reducing bacteria utilize trivalent iron compounds as electron acceptors for the biological degradation of compounds such as acetate, benzene, toluene, phenol, ethylbenzene, and xylene isomers [22; 23]. In these reactions, the electron donor-substrate and the electron acceptor-trivalent iron react in a specific ratio. However, the actual ratio often deviates from the theoretical ratio. Taking the extensively studied organic pollutant, toluene, as an example, the theoretical molar ratio of toluene oxidation to the generated reduced iron in the reaction is 1/36. However, previous studies [24; 25; 26; 27] have found that the ratio of these two components often exceeds the theoretical value of 1/36. The following two perspectives explain this phenomenon: firstly, it is suggested that some organic carbon may be bound to bacterial cells, ultimately promoting biomass growth [28]. On the other hand, it is possible that toluene has undergone partial loss over time. Whether it is due to the loss of synthesized biomass or the adsorption and volatilization loss of toluene over time, these processes are functions of time. However, as of now, no coupled analysis has been conducted regarding the temporal evolution between toluene oxidation and iron reduction. In order to delve into the stoichiometric relationship of the microbial degradation reaction of toluene with iron under pure cultivation conditions, we designed and conducted an experiment. Four prevalent and extensively studied iron oxides commonly found in geological formations--amorphous iron hydroxide, hematite, magnetite, and goethite--were selected as electron acceptors. Toluene served as the electron donor in this experiment.