# Combined Effect of Ferrous Ion and Biochar on Cadmium and Arsenic Accumulation in Rice
## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Treatment** & **BAFroot** & **BAFstraw** & **BAFhusk** & **BAFgrain** & **TFrs** & **TFsg** \\ \hline Control & 1.05 \(\pm\) 0.24 c & 0.41 \(\pm\) 0.17 b & 0.23 \(\pm\) 0.04 b & – & 0.37 \(\pm\) 0.09 a & – \\ BC & 1.09 \(\pm\) 0.09 bc & 0.20 \(\pm\) 0.11 c & 0.12 \(\pm\) 0.03 c & 0.09 \(\pm\) 0.01 b & 0.19 \(\pm\) 0.02 b & 0.47 \(\pm\) 0.05 a \\
1 BC-1 Fe(II) & 3.62 \(\pm\) 0.65 a & 0.84 \(\pm\) 0.20 a & 0.43 \(\pm\) 0.07 a & 0.17 \(\pm\) 0.03 a & 0.24 \(\pm\) 0.07 b & 0.25 \(\pm\) 0.06 b \\
3 BC-1 Fe(II) & 1.64 \(\pm\) 0.6 b & 0.29 \(\pm\) 0.14 bc & 0.11 \(\pm\) 0.03 c & 0.06 \(\pm\) 0.02 c & 0.17 \(\pm\) 0.04 b & 0.21 \(\pm\) 0.02 b \\
5 BC-1 Fe(II) & 1.54 \(\pm\) 0.30 bc & 0.26 \(\pm\) 0.12 bc & 0.11 \(\pm\) 0.04 c & 0.06 \(\pm\) 0.01 c & 0.21 \(\pm\) 0.03 b & 0.29 \(\pm\) 0.08 b \\ \hline \hline \end{tabular} – – BAF and TF value could not be calculated due to the lack of data on the control treatments. The data are mean \(\pm\) SD (\(n=3\)). Different lowercase letters indicate a significant difference at \(p<0.05\).  
\end{table} 
Table 3: Bioaccumulation factors (BF) and translocation factors (TF) of Cd. from soil [22; 30]. Fe(II) could also be oxidized to Fe(III) when exposed to an aerobic environment, and, with oxidized forms, it was possible to subsequently remove As from contaminated soil by adsorption and co-precipitation [47]. The Fe(III) ions were precipitated as Fe(III) hydroxide when BC was amended. The iron-bearing mineral was formed and bonded arsenate and arsenite, which may play a significant role in As immobilization of Fe(II) in our study [24; 25]. In addition to the liming effect, biochar may provide other benefits including adsorption of both cations and anions which may reduce nutrients' leaching owing to high surface charge density, high surface area, internal porosity, and the presence of both polar and non-polar surface sites [12; 48]. In general, the liming procedure can enhance the heavy metal barrier effect. The effect of the amendments on Cd differed from that of As, owing to the application of Fe(II), soil pH decreased by 0.7 units in 1 BC-1 Fe(II) treatment. The immobilization of heavy metals can be achieved by adsorption on the surface of biochar. It can be divided into two categories: specific and non-specific adsorption. Specific adsorption indicates that the metals were forming coordination bonds to surface in the inner layer, while nonspecific adsorption related to those metals was adsorbed by simple coulombic interaction in the diffuse electric double layer [49]. [PERSON] [50] reported that Cd\({}^{2+}\) was adsorbed by biochar occurring predominantly via two distinct cation-\(\pi\) bonding mechanisms involving Cd\({}^{2+}-\pi\) bonding with soft ligands (e.g., \(C=0\)) or electron-rich domains on aromatic structures. Such adsorption processes are probably responsible for our study. The results of this study are particularly pleasing regarding the application of this biochar to Cd and As contaminated soils, since it demonstrated that available Cd and As concentrations were decreased. However, soil pH was not distinctly changed in BC alone treatments compared to control. Thus, it can be deduced that the decrease of Cd mobility may be attributed to absorption by BC rather than pH increase. In the present study, the available As can be eliminated by Fe(II), but the DTPA-extractable Cd increased. These autologous results may owe to the variance of soil pH after addition of Fe(II) [51]. Related to the free iron oxide, the iron plaques on the outside layer of roots can accelerate the inhibition of As [22; 52]. Due to iron plaque being ubiquitous on rice roots and a barrier against As uptake, it should not be ignored while detecting the As inhibition mechanism of rice plants. The As content and BAF in root, husks, and grain increased proportionally to decreasing amounts of external Fe(II) (Figure 4, Table 4).