# Removal of Heavy Metal Cd Element from Paddy Soil by Geo-Electrochemical Technology
## 4 Results and Discussion  
### Effectiveness of Cd Removal from Paddy Soil  
The initial concentration of Cd in the paddy soil of the Agricultural Science Institute in Quanzhou County ranged from 0.52 to 0.64 mg/kg, with an average of 0.58 mg/kg. Figure 3 shows the total soil Cd content changes for the three experimental groups after 89 d of experimental treatment. As shown in the figure, relative to the initial soil Cd content without experimental treatment, the total Cd content in the soil of the T1, T2, and T3 experimental groups all decreased, with a significant reduction (\(p<0.05\)) in the T1 experimental group. Among the three experimental groups, the removal effectiveness of Cd in the T1 experimental group, where the GT was applied solely to the paddy soil, was the highest, reaching 36.18%. The removal effectiveness in the T2 experimental group, where the GT was applied to the paddy soil-rice system, was 10.39%. In the T3 control group, where rice was planted alone without the use of the GT, the Cd content in the soil also decreased after planting, with a reduction rate of 21.23%. In accordance with GB 15618-2018 [23] and Figure 3, in the T1 experimental group, which used the GT solely on paddy soil, the total Cd content in the soil decreased to 0.39 mg/kg after the electrokinetic remediation. That is lower than the risk screening value for agricultural soil contamination (0.4 mg/kg for paddy fields), so it met the national standard. However, the T2 experimental group, which used the GT on planted rice, and the T3 control group with planted rice without the GT had total soil Cd contents of 0.49 mg/kg and 0.46 mg/kg, respectively, after the experiment. Although the total Cd content in the soil decreased in both cases, it was still higher than the risk screening value for agricultural soil contamination. The experimental results show that the GT was effective in removing heavy metal Cd elements from paddy soil. The T1 paddy-soil-only experimental group had the best Cd-removal effect. After the remediation, the total Cd content in the paddy soil reached a safe threshold. The GT removes Cd elements from paddy soil based on the directional migration and accumulation of Cd elements toward the electrodes. It uses electrochemical effects such as electrolysis, electromigration, electro-osmosis, and electrophoresis. Then, the Cd element in the soil was collected with the use of specific adsorbent materials, thereby achieving the goal of reducing the content of the Cd element in the soil. The decrease in the soil Cd content in the T3 control group planted with rice alone was due to the absorption of Cd in the soil by rice plants. The less effective Cd removal in the T2 soil-rice system group might be related to the rice plants' absorption mechanism of Cd from the soil. According to the literature, Cd is a non-essential element of plants and enters into their roots through the absorption channels of Ca, Fe, Mn, Zn, and other essential elements [27]. For Cd  
Figure 3: Variation in total Cd content in the soil of experimental groups before and after treatment (T3 was control group). Different letters above the error lines indicate significant differences (\(p<0.05\)). The dashed line indicates the legal limit of Cd in the agricultural soil (0.4 mg/kg is derived from the Chinese national standard GB 15618-2018). contaminated soil, the toxic effects of Cd on crops can be alleviated to a certain extent by applying a certain amount of exogenous substances (such as Ca, Fe, Zn, Mn, Mg, and other elements) to the contaminated soil [28; 29; 30]. Because the above elements and Cd have similar ionic radii and chemical properties, they can share many transport proteins, ion transport channels, and binding sites in crop roots, thereby inhibiting the accumulation and transport of Cd in crops [30].