# Improvement of Arsenic Phytoextraction Using Indigenous Bacteria and Mobilizing Agents
## 3 Results and Discussion  
### Soil Analysis  
The main characteristics of the soil were given in [50] and the results of the arsenic extraction tests performed on the mixed soil are reported in Table 1.  
The As content of the soil was found to be 37.5 mg kg\({}^{-1}\), but the potentially bioavailable portion estimated using 0.1 M KH\({}_{2}\)PO\({}_{4}\) (which is the typical mobilizing agent of As) was equal to only 0.72 mg kg\({}^{-1}\). These results suggest the presence of particularly strong bonds which cause arsenic to be retained by soil particles and, in particular, a very strong interaction between arsenic and iron oxides/hydroxides. The values in brackets indicate the quantities of As and Fe extracted by the action of a solution of oxalate only. The data in Table 1 allowed us to confirm the synergistic action of a mixture of the two mobilizing agents, which appears to be capable of destroying a considerable amount of iron oxides/hydroxides on which As is adsorbed with very strong bonds. After evaluating different mobilizing agents of arsenic and considering that the exchange reactions with the phosphate did not produce satisfactory results, it was decided to try to destroy the iron oxides/hydroxides on which arsenic preferentially adsorbs (see [50] for more details). To this end, the use of oxalate as a mobilizing agent effectively increased the bioavailability of the pollutant. Oxalate reacts with amorphous iron oxides and, as a result, arsenic is released into the soil solution, from which it can be absorbed by plants:  
\[(\text{Fe}_{\text{x}}\text{O}_{\text{y}})\text{AsO}_{4}+\text{C}_{2}\text{O} _{4}^{2-}\rightarrow\text{Fe}_{2}(\text{C}_{2}\text{O}_{4})_{3}+\text{AsO}_{4} ^{3-}\]  
However, to try to further increase the bioavailability of arsenic (the values in brackets in the central column of Table 1 show that a maximum of 6.5% mobilization of the total was obtained), it was decided to also add a reducing agent, such as ascorbic acid, as briefly explained below. In contaminated soils, arsenic is often associated with oxides/hydroxides of various kinds and also with sulfides that may be present. Among these, iron oxides/hydroxides are the primary minerals that control the concentration of As in the different phases of the soil since, in general, they are abundant and have a high affinity for As. The dissolution of iron oxides can occur through various reactions. It has been shown that the addition of a strong acid is ineffective because the soil surface under acidic conditions becomes positively charged, thus attracting the negative charge of As oxyanions [56]. Furthermore, it has been observed that arsenic can be effectively released from the soil through the reductive dissolution of iron oxides. In natural environments, the progressive dissolution of iron (hydr)oxides is a consequence of the action of organic ligands, such as oxalate and ascorbate, which are produced by biological activity [57]. The oxalate anion (C\({}_{2}\)O\({}_{4}^{2-}\)) is an organic ligand naturally present in the soil that can form stable complexes with various metals, including iron. Under acidic conditions, oxalate can effectively adsorb onto the positively charged surfaces of iron oxides. The formation of stable complexes with iron leads to the dissolution of the iron oxides themselves. Furthermore, oxalate is a mild reducing agent which can also promote the breakdown of iron oxides by reductive dissolution. The ascorbate anion (C\({}_{6}\)H\({}_{7}\)O\({}_{6}^{-}\)) is a moderate reducing agent which significantly promotes the reductive dissolution of iron oxide [58; 59].