# Screening of Plants and Indigenous Bacteria to Improve Arsenic Phytoextraction
## 3 Results and Discussion  
### Soil Analysis  
The main characteristics of the soils obtained from the sampling procedures and the results of the arsenic extraction tests are reported in Tables 1 and 2, respectively. The concentration of As was found to vary between 24.9 and 67.2 mg kg\({}^{-1}\), while the potentially bioavailable portion estimated by using 0.05 M KH\({}_{2}\)PO\({}_{4}\) (which is the typical mobilizing agent of As) was always rather low and independent of the total amount of As present in the soil, both in the external and internal areas. These results suggest the presence of particularly strong bonds with which arsenic is retained by the surfaces of these soils and, in particular, a very strong interaction between arsenic and iron oxides/hydroxides. In fact, both the pentavalent and trivalent forms of arsenic are retained by the iron oxides through very strong bonds in which arsenate and arsenite anions compete and exchange with hydroxyl groups directly coordinated with structural Fe\({}^{3+}\) cations on the surface of the iron oxides/hydroxides. The complexes are then formed with a direct coordination of arsenic species with the iron ions of the oxides. In the case of As(V), which is the predominant form of As in soils, the arsenate anion is adsorbed by coordinating with two adjacent structural Fe\({}^{3+}\) cations forming a bidentate binuclear surface complex, characterized by a very strong bond that makes desorption difficult. Inorganic arsenic species, in particular As(V) species, form very strong bonds with iron oxides, for which in general only phosphate is highly competitive for adsorption sites, as a very similar mechanism also characterizes the adsorption of phosphate by iron oxides/hydroxides. After the addition of phosphate ions, an exchange can take place between a specifically adsorbed anion (arsenate) with another anion (phosphate). This adsorption mechanism has been used successfully in various similar applications [43; 45]. However, the ease of exchange between the two anions depends on the properties of the adsorption surface, on the specific characteristics of the soils involved, and on the time \begin{table}
\begin{tabular}{c c} \hline \hline  & **Mean** \\ \hline pH & \(8.3\pm 0.2\) \\ EC (\(\mu\)S cm\({}^{-1}\)) & \(585\pm 10\) \\ Clay (\%) & \(33.5\pm 1.8\) \\ Silt (\%) & \(18.6\pm 1.1\) \\ Sand (\%) & \(47.9\pm 0.6\) \\ CEC (Cmol\({}_{(+)}\) kg\({}^{-1}\)) & \(23.3\pm 0.7\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical properties of the individual samples collected at the site under investigation. Values are reported as mean (\(n=3\)) \(\pm\) SD. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **S2** & **S6** & **SW** & **PZ** \\ \hline Total & 40.5 \(\pm\) 2.0 & 29.3 \(\pm\) 1.7 & 24.9 \(\pm\) 1.5 & 67.2 \(\pm\) 2.1 \\ KH\({}_{2}\)PO\({}_{4}\) 0.05 M & 1.00 \(\pm\) 0.04 & 0.99 \(\pm\) 0.05 & 0.52 \(\pm\) 0.05 & 1.50 \(\pm\) 0.07 \\ KH\({}_{2}\)PO\({}_{4}\) 0.1 M & 1.30 \(\pm\) 0.07 & — & — & 2.20 \(\pm\) 0.11 \\ K\({}_{4}\)P\({}_{2}\)O\({}_{7}\) 0.1 M & 0.20 \(\pm\) 0.01 & — & — & 2.53 \(\pm\) 0.12 \\ EDTA\({}_{1\%}\) & 0.80 \(\pm\) 0.07 & — & — & 8.20 \(\pm\) 0.80 \\ K\({}_{2}\)S\({}_{2}\)O\({}_{3}\) 0.27 M & 0.10 \(\pm\) 0.01 & — & — & 0.30 \(\pm\) 0.02 \\ KHC\({}_{2}\)O\({}_{4}\) 0.2 M & 1.88 \(\pm\) 0.13 & 5.99 \(\pm\) 0.30 & 14.1 \(\pm\) 0.42 & 42.9 \(\pm\) 0.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Arsenic content (mg kg\({}^{-1}\)) in the individual samples collected at the site under investigation. Values are reported as mean (\(n=3\)) \(\pm\) SD. required to reach equilibrium, which could be very long. The process is made possible by the similar chemical characteristics of phosphate and arsenate, including the symmetry and size of the anions. However, once the arsenate has been adsorbed and depending on the type of surface complex that is formed (inner-sphere or outer-sphere), only a part can be easily desorbed and the quantitative exchange with the phosphate can become difficult or largely incomplete.