# Removal of Heavy Metal Cd Element from Paddy Soil by Geo-Electrochemical Technology
## 4 Results and Discussion
### Variation in Cd Content in Various Parts of Rice  
For the T2 and T3 experimental groups, after 89 d of experimental treatment, the changes in Cd content in various tissues and organs of the rice in both groups were recorded. Figure 4 shows a reduction in Cd content in various tissues and organs of rice in the T3 control group, which did not use the GT, and in the T2 planted-rice experimental group, which used the GT. In both groups, the Cd content in rice plants from high to low was in the order of rice roots > rice stems > rice leaves > rice grains (brown rice). That was basically consistent with the research of [PERSON] et al. [31], [PERSON] et al. [32], and others, i.e., the Cd content in organs storing nutrients (rice grains) is lower than that in organs with vigorous metabolisms (rice roots, rice stems, and rice leaves). According to Figure 4, relative to the T3 control group without the technology, the Cd content in various parts of rice in the T2 experimental group decreased significantly, especially in the roots and stems of rice, where there was a significant difference (\(p\) < 0.05) compared with the T3 control group. Relative to the T3 control group, the Cd content in the roots, stems, leaves, and grains of the T2 experimental group decreased by 59.78%, 73.95%, 72.76%, and 64.56%, respectively. That shows that the GT effectively reduced the Cd element content in the tissues and organs of the rice plants. The Cd content in brown rice samples from the T2 and T3 experimental groups was evaluated in accordance with the \"National Standard for Food Safety: Maximum Residue Limits for Pollutants in Food\" (GB 2762-2022) [33] to determine whether it exceeded the limited standard of 0.2 mg/kg. It  
Figure 4: Variations in Cd contents in various tissues and organs of rice. The T3 group was the control group, and the T2 group used the geo-electrochemical technology. Different letters above the error lines indicate significant differences (\(p\) < 0.05). The dashed line indicates the legal limit of Cd in brown rice (0.2 mg/kg is derived from the Chinese national standard GB 2762-2022). was found that neither group's brown rice exceeded the limit of heavy metal Cd, indicating good food safety. That shows that although the initial total soil Cd content of the T2 and T3 experimental groups exceeded the risk screening value for agricultural soil contamination, indicating that there might be ecological and environmental risks in that soil, brown rice had no above-standard Cd content after actual rice planting. In addition to the GT's reduction of the Cd content of the entire paddy soil-rice system, it also shows that the Cd content of rice grains did not have a strong positive correlation with soil Cd content. That is consistent with the views of [PERSON] et al. [34], [PERSON] et al. [35], and others; i.e., at the regional scale, the correlation between rice Cd content and soil Cd content is usually low, with a correlation coefficient R\({}^{2}\) of typically less than 0.2. The Cd content of rice is related to the Cd content in the soil but also to rice variety, soil pH, soil organic matter content, and soil moisture content. [PERSON] et al. [36] found that low-accumulation rice varieties can still guarantee the safety of rice consumption when grown in paddy fields where the Cd content exceeds the risk screening value for agricultural soil contamination. Through research, [PERSON] et al. [37] found that the soil pH value has an important effect on the Cd content of rice. As the soil pH value rises from 5.5 to 7.5, the safe critical value of soil Cd content to ensure that the Cd content in rice does not exceed the standard rises from 0.27 mg/kg to 0.8 mg/kg. [PERSON] et al. [38] studied the effect of organic fertilizer on the Cd content in double-season rice. They found that the organic carbon components in organic fertilizer can reduce the availability of Cd in the soil and ultimately reduce the Cd content in rice. [PERSON] et al.