# Combined Effect of Ferrous Ion and Biochar on Cadmium and Arsenic Accumulation in Rice
## 4 Discussion  
The yield of rice is associated with the tiller number of plants. More rice plant tillers mean a higher yield of rice [32]. As is well known, Fe as an essential mineral elements for the chloroplast synthesis and enzymes involved in the biosynthesis express has a very important role in graminaceous plants [33], and thus promotes plant growth [24; 34]. The iron may generate the formation of iron plaque on the surface of root, promoting the growth and bifurcation of root [23]. Iron plaque both increases the uptake of nutrients and promotes the growth of rice; these results can be found in field applications [35]. In addition, extra addition of SO\({}_{4}\)\({}^{2+}\) into the heavy metal contaminated soil could increase the plant height and the soil biomass [36]. In soil, pH is a key factor which influence the availability of heavy metals [37]. As is easily transferred in soils with a pH between 3 and 8 [38]. On the contrary, in a low pH condition, the availability and mobility of Cd are increasing. In previous study, the lined biochar had been proved to be effective in eliminating soil contamination by increasing the soil pH [39; 40; 41]. In our study, BC has a high pH value (pH 9.4) which could induce a liming effect on the mobility of Cd. Furthermore, with the pH increasing, the number of positively charged sites on soil minerals decreases, lowering the sorption capacity of negatively charged oxy-anions of As [42]. However, in our study, the pH slightly increased with biochar addition, and the changes in soil pH were brought about by the Fe(II) application. The pH value was significantly decreased in 1 BC-1 Fe(II) treatment owing that Fe(II) accounted for a significant portion of amendment. A similar result was reported by [PERSON] [43], who found that addition of Fe(II) could obviously decrease the soil pH value. It is well known that the Fe-hydroxides may result in a lower pH. It can be ascribed to the hydrolysis and precipitation of Fe(III)-hydroxides, after the oxidation of Fe(II) to Fe(III) in the air. With a relatively rapid kinetics, Fe(II) can be oxidized to the trivalent state [44]; this process produces protons and thus may lower the pH [45]. Heavy metal's toxicity in the soil and its potential risk of entering human body was evaluated by its bioavailability. The mobility and bioavailability of metals can be eliminated dramatically by applying soil amendments in the multiple contaminated soils [11; 46]. The mechanisms of stabilizing the As in soil can be concluded in two aspects: the generation of amorphous or crystalline Fe(III) arsenate compounds and the other one is absorption on the layer of iron (oxy)hydroxides, including zero talent iron, iron hydroxides, and oxides, which have the ability to remove both As(III) and As(v) \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Treatment** & **BAFroot** & **BAFstraw** & **BAFhusk (\(\times 10^{-2}\))** & **BAFgrain (\(\times 10^{-2}\))** & **TFrs** & **TFrg** \\ \hline Control & 2.69 \(\pm\) 0.29 a & 0.12 \(\pm\) 0.03 a & 0.64 \(\pm\) 0.21 a & – & 0.04 \(\pm\) 0.01 b & – \\ BC & 2.86 \(\pm\) 0.44 a & 0.11 \(\pm\) 0.04 a & 0.66 \(\pm\) 0.15 ab & 0.90 \(\pm\) 0.30 a & 0.04 \(\pm\) 0.01 b & 0.13 \(\pm\) 0.04 a \\
1 BC-1 Fe(II) & 0.83 \(\pm\) 0.10 b & 0.04 \(\pm\) 0.01 b & 0.38 \(\pm\) 0.10 b & 0.04 \(\pm\) 0.02 c & 0.04 \(\pm\) 0.01 b & 0.02 \(\pm\) 0.01 b \\
3 BC-1 Fe(II) & 0.86 \(\pm\) 0.29 b & 0.04 \(\pm\) 0.01 b & 0.52 \(\pm\) 0.17 ab & 0.20 \(\pm\) 0.10 b & 0.04 \(\pm\) 0.01 b & 0.03 \(\pm\) 0.01 b \\
5 BC-1 Fe(II) & 1.46 \(\pm\) 0.44 c & 0.08 \(\pm\) 0.03 c & 0.60 \(\pm\) 0.14 ab & 0.20 \(\pm\) 0.01 b & 0.06 \(\pm\) 0.01 a & 0.03 \(\pm\) 0.02 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 4: Bioaccumulation factors (BF) and translocation factors (TF) of As.