Study On The Passivation Effect Of Purple Rock Collapse Product On Cadmium Polluted Soil

At present,with the increasing frequency activities of metal smelting,sewage irrigation,mineral resources development and so on,a variety of heavy metals enter the farmland soil,causing heavy metal pollution in farmland.Among which,the problem of cadmium pollution is most prominent.The in-situ passivation remediation technology has become one of the major means of remediation of heavy metal pollution in recent years,because of it has some unique characteristics,like simplicity,speed,low cost,and high efficiency.Purple rock in Sichuan is recognized by large amount,easy disintegration,alkalinity,and high content of mineral elements.It is a new passivation agent for cadmium pollution restoration.Before,it has been found that the passivation of cadmium-contaminated soils by purple rock minerals is mainly affected by soil p H,CEC,and soil cation availability,and the specific interactions are not yet known.Therefore,I select Sichuan purple rock as passivation material,and carrie out the purple rock disintegration test and purple rock disintegration product cadmium contaminated soil passivation test,and then study the effect of the disintegration product of purple rock on the cadmium-contaminated paddy soil.The main analysis results are as follows:（1）The purple rock exhibited a large amount of cations under the condition of water-heat oscillation at room temperature（25°C,180 rpm）.Ca²⁺,Fe³⁺,K⁺,and N^a+were the major cations.And with the extension of the extraction time,its release gradually increases.After shaking for 4 hours,the release of Ca²⁺,Fe³⁺,K⁺,and N^a+reached 108.63mg/kg～249.81 mg/kg,14.64 mg/kg～17.25 mg/kg,41.38 mg/kg～52.13 mg/kg,and 18.58mg,respectively./kg～35.27mg/kg.The purple rocks from Suining show greater release of Ca²⁺,Fe³⁺,and Na⁺than Nanchong Purple Rock and Dazhou Purple Rock;The purple rock from Nanchong has greater K⁺release than Suining Purple Rock and Dazhou Purple Rock.Purple rock cation release is also affected by the disintegration temperature and the frequency of oscillation.From 15°C to 45°C,the release of Ca²⁺,Fe³⁺,K⁺,and Na⁺from the three purple rocks was larger,with the largest increase in the Suining Formation.From100 rpm to 250 rpm,the increase of Ca²⁺,Fe³⁺,K⁺,and Na+emission from the three purple rocks was larger,with the largest increase in Fe³⁺and K⁺in the Suining Formation.（2）In the cadmium-contaminated paddy soil（all Cd content 3.32mg/kg）,the addition of purple rock disintegration products can reduce the available cadmium content of the soil,and the difference between the particle sizes is small.In the soil culture period（0-60d）,the treatment of purple rock raw mineral powder,disintegration residue and mixture can reduce the effectiveness of soil Cd,and the drop rate ranged from 33.99%to47.06%.The treatment of purple rock disintegration solution can significantly reduce soil Cd effectiveness by 7.33%～23.21%.With the increase of time,the reduction effect gradually weakens.The purple rock raw mineral powder treatment,the purple rock disintegration residue treatment and the purple rock disintegration mixture treatment can significantly reduce the exchangeable cadmium in the soil.During the 0-60d culture cycle,it showed a tendency of firstly reducing and then stabilizing.The reduction of the exchangeable state in the purple rock disintegration solution treatment reached a significant level within 0-15 days of culture,and there was no significant difference between the late test（30-60 days）.By correlation analysis,the main source of available cadmium in the treatment of the original minerals,disintegrated residues and disintegration mixture of purple rocks was exchangeable cadmium;In the purple rock disintegration solution treatment,the main sources of available cadmium are exchangeable and iron-manganese oxide bound cadmium.（3）In the cadmium-contaminated paddy soil（full Cd content of 3.32mg/kg）,adding purple rock and purple rock disintegration products can significantly increase soil p H and CEC.In the 0-60 day culture period,the treatment of purple rock raw mineral powder,disintegrated minerals,and disintegration mixture showed a trend of increasing first and then stabilizing.From 30 to 60 days,the relationship between soil p H and CEC size for each treatment was as follows:purple rock disintegration residue>purple rock original mineral powder>purple rock disintegration solution.During the culture period,each treatment increased the content of available calcium and copper and decreased the content of available iron and manganese.（4）The correlation analysis of soil p H,CEC,available cations and cadmium forms showed that:In the treatment of the purple rock raw mineral powder,disintegration residue and disintegration mixture,the exchangeable cadmium was extremely negatively correlated with p H,CEC,available calcium,zinc and copper,and it is significantly positively correlated with available iron and manganese.In the purple rock disintegration solution treatment,the exchangeable cadmium was extremely negatively correlated with p H,available iron and manganese,and it is significantly positively correlated with available calcium,zinc,and copper.In general,in the treatment of disintegration solution of purple rock,the soil p H,effective iron and manganese content were increased,and the content of available calcium,zinc and copper in the soil was reduced.Therefore,the soil exchangeable Cd content in the soil was reduced,and the effective Cd content in the soil decreased,with a range of 7.33%to 23.21%,reaching a significant level.The treatment of purple rock raw mineral powder,disintegration residue and disintegration mixture increased soil p H,CEC,effective calcium,zinc and copper content,reduced the content of available iron and manganese,and significantly reduced the exchangeable cadmium content in soil.The drop reached 33.99%to 47.06%.