Study On The Effect Of Acid Rain On Passivation And Remediation Of Cd Contaminated Soil

The cadmium（Cd）pollution in soil in our country is serious,scientifically and effectively reducing the content and activity of cadmium in soil is of great significance for soil environmental risk management and control,ensuring the quality of agricultural products and human health.The chemical passivation remediation technology has short cycle,low cost,simple operation and high remediation efficiency.It has been widely used in the remediation of soil contaminated by cadmium.Since Cd remains in the soil after restoration,the mobility and bioavailability of Cd may be improved under the effects of long-term rain,sunlight,and microorganisms.Therefore,it is of great theoretical and practical significance to evaluate the long-term effectiveness of the chemical passivation repair technology and to develop a more stable repair reagent based on the Cd morphological change mechanism.In this thesis,phosphate remediation reagents,which are widely used in engineering,were used to passivate and remediate simulated cadmium-contaminated soil.The effect and mechanism of acid rain on the form of Cd in the remediation soil were studied,and the combined remediation effect of phosphate+MgO on cadmium contaminated soil and the effect of acid rain were also studied,the main conclusions are as follows:（1）Phosphate is effective in remediating cadmium contaminated soil,the combined repair effect of phosphate+MgO is better than that of single phosphate repair.Under the same application rate(calculated as PO4^3-),after using dipotassium hydrogen phosphate,calcium dihydrogen phosphate and diammonium hydrogen phosphate,the available Cd content in the soil was reduced by 33.58%,34.87%and33.24%,respectively.The biological effectiveness of Cd was reduced by 29.27%,27.32%,and 33.33%,respectively,and the plant availability was reduced by 17.54%,13.36%,and 19.08%,respectively;under the same application rate(calculated as PO4^3-),the addition of MgO（based on soil quality 5%）,the available Cd content in the soil was reduced by 49.80%,49.05%,and 47.20%,the Cd bioavailability was reduced by 37.21%,35.14%,and 51.98%,and the plant availability was reduced by31.21%,29.83%,and 38.94%,respectively.The effect is significantly better than single phosphate repair.（2）BCR continuous extraction experiment results show that compared with the blank,after repairing dipotassium hydrogen phosphate,calcium dihydrogen phosphate and diammonium hydrogen phosphate,the weak acid extraction state Cd decreased by15.9%,20.1%,and 3.5%,and the reducible state increased respectively.The residual state increased by 5.3%,7.4%,and 4.9%,and the residual state increased by 12.4%,12.3%,and 8.7%,respectively.Phosphate remediation of cadmium contaminated soil mainly converts the more active weak acid-extracted Cd into a more stable reducible state.Compared with the blank,after MgO combined with dipotassium hydrogen phosphate,calcium dihydrogen phosphate and diammonium hydrogen phosphate,respectively,the weak acid-extracted Cd decreased by 33.8%,29.1%and 27.7%,and the reducible state increased by 9.0%,7.4%,3.3%,the oxidizable state increased by4.4%,5.2%,and 8.4%,and the residue state increased by 20.4%,16.5%,and 16.0%.After MgO combined with phosphate remediation of cadmium contaminated soil,the weak acid with higher activity.The extracted state of Cd is mainly transformed into a residue state,partly reducible state and oxidizable state.（3）Under the action of acid rain,the repair effect of a single phosphate becomes worse with the increase of the simulated years（acid rain）.Under the effect of long-term acid rain（simulating 35 years）,the soil available Cd,Cd bioavailability,and Cd plant availability of the samples with the best initial effect of the restoration of ammonium dihydrogen phosphate increased by 29.00%,44.09%and 50.38%respectively.As high as 63.88%,52.38%and 69.45%,the main reason is that a large amount of residual Cd is transformed into the most active weak acid extraction state again under the action of H⁺erosion,which increases the environmental risk.MgO can significantly increase soil pH.Although the combined remediation effect of phosphate+MgO will become worse with the increase of the simulation period（acid rain）,the remediation effect will be maintained at the end of the acid rain experiment simulation period（35 years）under the same conditions.At a good level:soil available Cd,Cd bioavailability,and Cd plant availability are only 34.52%,31.30%and 49.03%.In addition,the effects of single phosphate repair and phosphate+MgO combined repair become worse as the pH value of acid rain decreases.（4）There is a significant positive correlation between the content of available Cd in the soil and the bioavailability of Cd and plant availability（P<0.01）;the pH value of soil is significantly related to the content of available Cd,Cd bioavailability and plant availability.There is a negative correlation（P<0.05）,and the content of soil available phosphorus is only negatively correlated,indicating that acid rain aging changes the soil pH value than the content of available phosphorus is the main factor affecting the activity of Cd;acid rain aging leads to a large amount of oxidizable.The Cd in the state and the residue state is converted into the weak acid extraction state and the reducible Cd.The strong pH buffering capacity of MgO and its adsorption to Cd significantly inhibit the increase in the reducible Cd content,so that the repair effect is maintained at a good level.