# Improvement of Arsenic Phytoextraction Using Indigenous Bacteria and Mobilizing Agents
## 3 Results and Discussion
### Soil Analysis
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **As** & **Fe** \\ \hline Total & 37.5 \(\pm\) 1.9 & 17102 \(\pm\) 210 \\
0.1 M KH\({}_{2}\)PO\({}_{4}\) & 0.72 \(\pm\) 0.06 & 1.32 \(\pm\) 0.07 \\
0.01 M C\({}_{6}\)H\({}_{8}\)O\({}_{6}\) & 0.05 \(\pm\) 0.01 & 16.7 \(\pm\) 0.8 \\
0.05 M KHC\({}_{2}\)O\({}_{4}\) + 0.01 M C\({}_{6}\)H\({}_{8}\)O\({}_{6}\) & (0.77 \(\pm\) 0.05) 0.87 \(\pm\) 0.05 & (0.15 \(\pm\) 0.01) 154 \(\pm\) 8 \\
0.1 M KHC\({}_{2}\)O\({}_{4}\) + 0.01 M C\({}_{6}\)H\({}_{8}\)O\({}_{6}\) & (1.35 \(\pm\) 0.08) 1.5 \(\pm\) 0.1 & (2.97 \(\pm\) 0.15) 2284 \(\pm\) 91 \\
0.2 M KHC\({}_{2}\)O\({}_{4}\) + 0.01 M C\({}_{6}\)H\({}_{8}\)O\({}_{6}\) & (2.45 \(\pm\) 0.12) 3.6 \(\pm\) 0.1 & (369 \(\pm\) 17) 4711 \(\pm\) 141 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Potentially bioavailable arsenic and iron contents (mg kg\({}^{-1}\)) in the mixed soil collected at the site under investigation. Values are reported as means (\(n\) = 3) \(\pm\) SDs. The combination of chelating and reducing agents can therefore promote the dissolution of iron oxides, causing the release of arsenic associated with them through synergistic action. The use of organic reagents, such as oxalate and ascorbate, is to be considered as a practice with high environmental compatibility, since the two substances are biodegradable and have very low toxicities; moreover, their addition does not affect the stability of soil structures. The extraction of iron with oxalate and ascorbate is influenced by the properties of soil and depends on pH. In general, it is observed that:  
* The solubilization of As and Fe with oxalate is more effective at a pH of about 2;
* At pH 7, the quantity of solubilized As tends to decrease;
* At a very basic pH, unlike Fe, solubilized As tends to increase because a direct ion exchange mechanism can occur between the oxyanions of As and the hydroxyl groups of the iron oxy-hydroxides. When soil is treated with a mixture of ascorbate and oxalate, the iron (Fe\({}^{2+}\)) that becomes available through the reductive action of the ascorbate can form complexes with the oxalate (for example, Fe(C\({}_{2}\)O\({}_{4}\)) and [Fe(C\({}_{2}\)O\({}_{4}\))\({}_{2}\)]\({}^{2-}\)) as a function of the molar ratio between species. When the Ox/Fe ratio is close to 1, the formation of the Fe(C\({}_{2}\)O\({}_{4}\)) complex is dominant, while higher Ox/Fe ratios favor the formation of [Fe(C\({}_{2}\)O\({}_{4}\))\({}_{2}\)]\({}^{2-}\). Fe(C\({}_{2}\)O\({}_{4}\)) tends to precipitate due to low solubility (the solubility of the dihydrated salt Fe(C\({}_{2}\)O\({}_{4}\)) \(\times\) 2H\({}_{2}\)O is only 0.97 g L\({}^{-1}\) at 25 \({}^{\circ}\)C [60]). The use of a mixture of oxalate and ascorbate also tends to increase arsenic extraction, indicating a synergistic action resulting from the combination of the two reagents. Therefore, the presence of a sufficient amount of binder (oxalate) is important to favor the extraction of arsenic. In summary, oxalate extracts iron from the soil mainly through reactions in which it acts as a binder, while ascorbate acts as a reducing agent. Following the dissolution of iron oxides/hydroxides, the previously adsorbed As is released into the soil solution. The extraction of As through the combination of chelating and reducing agents is therefore influenced both by the reducing power of the reducing agent and by the stability of the complex that the chelating agent forms with Fe. A strong reducing agent is theoretically capable of extracting As from the soil with all types of chelating agents, while a mild reducing agent requires the use of a strong chelating agent. Based on these considerations, arsenic and iron extractability tests were also carried out on the mixed soil using different oxalate-ascorbate combinations before starting the microcosm tests.