# Enhanced Effect of Phytoextraction on Arsenic-Contaminated Soil by Microbial Reduction
## 3 Results and Discussion
### Changes in Soil Arsenic Content and Fractionation  
To better evaluate the effectiveness of phytoremediation, changes in soil arsenic content were assessed. After _Pteris vittata_ was growing for 4 months, significant changes in soil arsenic content were observed, as depicted in Figure 6a. The soil arsenic content decreased in all three treatment groups with _Pteris vittata_, with reductions of 11.95%, 14.86% and 19.62% in each group, respectively. The treatment group with the addition of iron-sulfur-reducing bacteria and organic fertilizer (T5) demonstrated the highest reduction in arsenic content. This indicates that the inclusion of iron-sulfur-reducing bacteria and organic fertilizer may enhance the uptake of arsenic by _Pteris vittata_, with organic fertilizers playing a particularly significant role. Figure 6: Total arsenic (**a**) and available arsenic (**b**) content of soil in each treatment group. As illustrated in Figure 6b, in the control groups (T2, T4) without the planting of _Pteris vittata_, the soil arsenic content remained relatively stable, but there was a notable increase in available arsenic content. This can be attributed to the enhanced bacterial metabolic activity resulting from the presence of sufficient organic matter as a substrate. This, in turn, promoted the dissolution of acicular iron ore and hematite in the soil, leading to the release of arsenic and an increase in available arsenic content. The reduction rates of available arsenic in the three experimental groups after fern remediation were 85.88%, 89.41% and 98.36%, respectively, with the highest reduction rate observed in the bacteria + POM group. This suggests a synergistic relationship between the bacterial-induced transformation of arsenic species and the accumulation of arsenic by plants. Furthermore, the absorption of arsenic by _Pteris vittata_ remained within its tolerance range, preventing arsenic leakage. The BCR sequential extraction method was utilized to analyze the fractionation of arsenic in the soil following four months of remediation; the results are shown in Figure 7. The primary fraction of arsenic in the raw soil was the residual fraction (56.37%), followed by the reducible fraction (20.34%), the oxidizable fraction (13.44%) and the acid-soluble fraction (6.75%). Among these fractions, the acid-soluble fraction was identified as the most mobile and bioavailable form of arsenic. And the transport and transformation of arsenic in the reducible fraction were strongly influenced by chemical or microbially mediated redox processes of iron oxides in the natural environment [42]. Iron (hydro)oxides in soils, especially those with low crystallinity (hydrous iron ore, needle iron ore, etc. ), provide the main adsorption sites for arsenic in soils [43]. And the reductive dissolution of iron (hydro) oxides is the main cause of arsenic release from soils or subsurface environments [44]. Consequently, in treatments T3, T4 and T5 inoculated with reducing bacteria, a significant decrease was observed in the reducible fraction of arsenic, indicating that the inoculation of reducing bacteria facilitated the reductive transformation of Fe(hydro) oxides in the soil, leading to the release of reducible arsenic. The reduction in the reducible fraction was more pronounced in treatment group T5, which can be attributed to the fact that the small molecular organic acids in the plant organic matter (POM) also contributed to the solubilization of iron oxides. A decrease in the reducible fraction of arsenic was also observed in the T1 and T3 treatment groups without POM application, as _Pteris vittata_ itself secretes small amounts of organic acids from its roots during growth. The dissolution of iron (hydro) oxides resulted in a reduction in the sorption sites for arsenic, which was then released into the soil pore water as a free fraction and subsequently absorbed by the roots of _Pteris vittata_.