# Enhanced Effect of Phytoextraction on Arsenic-Contaminated Soil by Microbial Reduction
## 3 Results and Discussion
### Speciation of Arsenic Uptake by _Pteris vittata_  
To further understand the process of arsenic uptake by _Pteris vittata_, speciation examination of arsenic in _Pteris vittata_ roots and fronds was performed, and the findings are presented in Figure 5 Arsenic in the roots and fronds of _Pteris vittata_ was mainly present as As(III), with a higher proportion of As(III) in the fronds (83.87-87.29%) compared to the  
Figure 4: Arsenic absorption by _Pteris vittata_. (**a**) The concentration of arsenic in roots and fronds, (**b**) the cumulative amount of arsenic absorbed by roots and fronds, (**c**) arsenic BCF and TF of _Pteris vittata_. roots (65.32-73.26%). Previous studies have proved that the valence state of arsenic did not affect the uptake rate in _Pteris vittata_. However, different valence states of arsenic were transferred at varying rates within _Pteris vittata_, with arsenic(III) being transferred from roots to fronds faster than arsenic(V). As a result, the concentration of arsenic(III) in the fronds was higher than that of arsenic(V). In the treatment group T5, where iron-sulfur-reducing bacteria and organic matter were introduced, the proportion of As(III) in both the roots and leaves of Pterocarpus indicus was significantly higher compared to the other two treatment groups. This increase can be attributed to the predominance of trivalent arsenic in anaerobic environments [35] and the metabolic activities of iron-sulfur-reducing bacteria, which enhance the solubilization of trivalent arsenic. As a result, _Pteris vittata_ is able to take up trivalent arsenic more effectively. Previous studies have indicated that _Pteris vittata_ roots absorb arsenate through the phosphate transport system, and the presence of phosphate competes with arsenate [36,37]. Phosphate inhibits arsenate uptake but not arsenite uptake and translocation, and arsenate is converted to arsenite by reductase in _Pteris vittata_ roots and translocated into the fronds. Hence, the major form of arsenic in the fronds is arsenite, accounting for over 80% of the total, and the percentage of arsenite is higher in both the roots and fronds of the Bacterial + POM group compared to the control group. The mechanism of arsenite uptake by _Pteris vittata_ differs from that of arsenate. Although the precise mechanism is not clear, several plausible explanations exist. One theory suggests that microbial-mediated oxidation of As(III) occurs on the surface of _Pteris vittata_ roots, and the oxidized As is taken up by the roots through the phosphate transport system [38]. It then interacts with substances that compete with arsenate reductase (ACR) and glutathione (GSH), resulting in enzymatic or non-enzymatic reduction to arsenite [39]. And the reduction of As(V) to As(III) is a crucial step in the detoxification of _Pteris vittata_ biomass. Another widely accepted viewpoint suggests that _Pteris vittata_ can uptake As(III) via the aquaporins (AQPs) transport system [40]. It has also been observed that the AQP inhibitor AgNO\({}_{3}\) reduces the uptake of As(III) by _Pteris vittata_ at a concentration of 0.01 mM, indicating that As(III) uptake may indeed be mediated by the AQP transport system [41], which differs from the transport system for As(V). Arsenite and phosphate do not compete during transport, making arsenite more readily translocated from roots to stems and leaves compared to arsenate. In conclusion, the phytoremediation abilities of _Pteris vittata_ in the uptake of soil arsenic can be attributed to three key roles it plays. Firstly, the roots of _Pteris vittata_ provide a stable habitat for microorganisms and secrete organic acids that serve as nutrients for these microorganisms. Secondly, _Pteris vittata_ employs distinct uptake mechanisms for both As(V) and As(III). Arsenate (As(V)) is taken up through the phosphate transport system,  
Figure 5: The speciation of arsenic in the roots and fronds of _Pteris vittata_. where it competes with phosphates for uptake.