Cadmium Pollution Control In Paddy Field By Composite Passivation Materials

Passivation technology is a widely used method in the remediation of heavy metal contaminated paddy fields.Based on the research and development of new composite passivation materials,this study analyzed the effects of composite passivation materials on paddy soil and rice through indoor research and field experiments.The research results have important practical reference value for farmland soil remediation in China.（1）Orthogonal test was used to screen out the raw material ratio（kaolin:apatite:red mud=1:0.2:0.2）with the best effect on soil effective cadmium passivation.and the composite passivation materials processed with this ratio for follow-up research.The content of heavy metals in the composite passivation materials were all lower than the corresponding national standard limits.The maximum adsorption capacity was47.82 mg/g at 25℃ and the initial cadmium ion concentration of 500 mg/L.The adsorption of Cd²⁺was mainly monolayer adsorption.（2）Field experiments were carried out with the addition of composite passivation materials at a mass ratio of 0.025%to 0.4%（750-12000kg/ha）.On the 10th,90th and 210th days,the pH change rates were0.40%～23.80%,0.83%～20.12%,and-2.63%～14.14%respectively.There was a significant positive correlation between the amount of composite passivation materials and soil pH（P<0.01）.At the same time,the correlation between composite passivation materials and soil pH decreased with the passage of time.（3）In the field experiment,the soil available cadmium content decreased by 9.4%～51.36%（the 10th day）after the composite passivation materials with different addition ratios were applied to the soil.The content of available cadmium in soil decreased with the addition of composite passivation materials.There was no significant difference in the content of available cadmium in soil at different sampling times（p>0.05）,and the passivation effect of the composite passivation material on available cadmium in soil was relatively stable.The gradient addition of composite passivation materials had no significant effect on the contents of soil organic matter,alkali hydrolyzable nitrogen and available potassium at different sampling times（p>0.05）.There was a significant positive correlation between the amount of composite passivation materials and the content of soil available phosphorus in different periods.（4）The addition of composite passivation materials had a significant positive correlation with the yield of early rice and late rice（P<0.01）,but with the passage of time,the effect of adding composite passivation materials on rice yield gradually weakened.The content of cadmium in brown rice decreased after adding composite passivation materials.Both early and late rice reached the maximum cadmium reduction rate of 75.48%and 70.75%in brown rice at 0.4%（12000 kg/ha）.There was a very significant negative correlation between the content of cadmium in brown rice and the amount of composite passivation materials（p<0.01）.During the two seasons of rice planting,the passivation effect of cadmium on brown rice was relatively stable.（5）After the compound passivation material was applied,the cadmium content in the roots,stems and leaves of early rice and late rice decreased significantly.The order of cadmium content in each part of early rice and late rice plants from large to small was root>stem>leaves>brown rice.The root bioconcentration factor of early rice and late rice decreased with the increase of the content of composite passivation materials.The plant height of early rice and late rice was not significantly affected by the gradient addition of composite passivation materials,but the biomass of early rice was significantly increased after the gradient application of composite passivation materials.（6）Based on the soil pollution and rice varieties selected in this study,0.2%treatment（6000 kg/ha）can achieve the best balance between treatment effect and treatment cost,the reduction rate of soil available cadmium reaches a high level of 44.12%,and the cadmium content of brown rice in early and late rice meets the requirements of national standards.When using this addition amount,the comprehensive cost per hectare of application is about 10800 yuan,and the net income per hectare of paddy field after restoration is 35325.3 yuan,which has a good cost performance ratio.