# Enhanced Effect of Phytoextraction on Arsenic-Contaminated Soil by Microbial Reduction
## 3 Results and Discussion
### Function of Iron-Sulfur-Reducing Microbial  
The effect of iron-sulfur-reducing bacteria on the solubilization and differentiation of arsenic in soil was investigated under anaerobic incubation conditions, and the results are shown in Figure 2. As depicted in Figure 2a, the pH value of the leachate solution was hardly altered after the addition of iron-sulfur-reducing bacteria, similar to the control group. However, when plant organic matter was introduced, the pH value of the soil leachate increased, eventually reaching 7.39, with a difference of 0.31 compared to the initial period. This increase can be attributed to the anaerobic bacterial community dominated by Desulfovibrio, which becomes metabolically active in the presence of an organic carbon source. However, when plant organic matter was added, Desulfovibrio was able to oxidize organic acids produced by plant fermentation and transfer electrons to the sulfate in the soil, thus increasing alkalinity [28]. This reaction can be represented by the following reaction Equation (1):  
\[\mathrm{C_{2}H_{4}O_{2}+SO_{4}^{2-}\to 2 HCO_{3}^{-}+H_{2}S} \tag{1}\]  
The concentration of dissolved As produced in the Bacterial + POM group was significantly higher than that in the other culture groups, as shown in Figure 2b. The concentration of As increased rapidly in the pre-culture period; by the end of the culture period, the concentration of As in the leachate had reached 4.76 mg/L, which was higher than that in the control group (3.24 mg/L). The above results suggested that the metabolism of iron-sulfur-reducing flora dominated by Desulfovibrio can promote the dissolution and release of arsenic from the soil under the supply of a sufficient organic carbon source. The first reasonable explanation is that HCO\({}_{3}\)- produced by the respiration of Desulfovibrio competed with arsenate and caused the desorption of arsenic. On the other hand, HS-produced by the reduction of sulfate (via Desulfovibrio) created crucial conditions for the reduction of Fe(III), which could enhance the mobility of As by the reductive dissolution of iron oxides [29; 30; 31]. More importantly, the reduction products of sulfate could stimulate the reduction of As(V) to As(III) [32]. The reaction process is represented by the following Equations (2) and (3):  
\[\mathrm{HS^{-}+2 FeOOH+5H^{+}\to 2 Fe^{2+}+S^{0}+H_{2}O} \tag{2}\]  
\[\mathrm{HS^{-}+HAsO_{4}^{2-}+H^{+}\to S^{0}+HAsO_{3}^{2-}+H_{2}O} \tag{3}\]  
Figure 1: Relative abundance histogram of artificially enriched iron-sulfur-reducing microbial at the genus level. Furthermore, it has been demonstrated that organic acids could promote the reductive solubilization of iron oxides, leading to the release of As bound to the iron oxides. The reaction process in the case of lactate can be expressed by the following Equation (4):  
\[\text{C}_{2}\text{H}_{4}\text{O}_{2}+8\text{FeOOH}\rightarrow 8\text{Fe}^{2+}+2\text{HCO}_{3}^{-}+12\text{H}_{2}\text{O} \tag{4}\]  
In the presence of a specific dissolved organic matter, in addition to the reductive solubilization of As-containing iron oxides, As(V) could be reduced to As(III) by Desulfovibrio through an allochthonous reduction process. For instance, when lactate serves as the electron donor, the plausible direct reduction process of As(V) is described in Equation (5):  
\[2\text{H}\text{As}\text{O}_{4}^{2-}+\text{CH}_{3}\text{CHOH}\text{CO}^{-}+ \text{H}^{+}\rightarrow 2\text{H}_{2}\text{As}\text{O}_{3}^{-}+\text{HCO}_{3}^{-}+ \text{CH}_{3}\text{CO}^{-} \tag{5}\]  
In conclusion, iron-sulfur-reducing bacteria play a crucial role in facilitating the release and reduction of arsenic in contaminated soil. Firstly, they indirectly promote this process by controlling the reduction of iron oxides and sulfates present in the soil. Secondly, these bacteria directly contribute to the reduction of arsenic by transferring electrons to arsenic through the oxidation of organic matter as electron donors.