The Remediation Effects And Mechanism Of Natural Composite Clay On Cd Contaminated Paddy-wheat Rotation Farmland Soil

Rice and wheat are the largest crops in China and both of them have a strong ability to uptake and accumulate Cd from soils.The quality safety of agricultural products has attracted increasing attention in recent decades.Cd pollution in the soil of some paddy-wheat rotation farmland in Southwest China seriously threatens food safety.Many studies on the safe utilization of Cd polluted cultivated land in a paddy field in South China,nevertheless,few studies focus on the safe utilization technology of Cd pollution in paddy-wheat rotation farmland.The immobilization effect and influence mechanism for heavy metals in primary soil have been studied a lot,but the sorption effect mechanism of Cd from different granular soils was rarely reported.This study selected the Cd-contaminated paddy-wheat rotation farmland in Chengdu Plain as the research object,and natural composite clay with low economic cost as the amendment,the remediation effect of composite clay on rice-wheat rotation farmland was identified through field experiments.On this basis,the micro-remediation mechanism of Cd pollution by composite was investigated via Cd sorption on different granular soils and typical soil colloidal components with surface charge under composite clay treatment.The study was supported by the project"Research on the conditioning and remediation of farmland soil in heavy metal polluted areas of Tuojiang River Basin".The main conclusions are as follows:(1)The field experiments showed that the composite clay had a significant remediation effect on paddy-wheat rotation farmland.The concentration of Cd in wheat and rice grain after adding 2‰～8‰composite clay into the soil decreased from 37.5%to 58.3%and from 38.7%to 58.1%,respectively.The inhibitory effect of composite clay on Cd accumulation in crops mainly depended on regulating Cd bioavailability content in the soil.The application of composite clay could lift the soil p H,enhance Cd sorption,and promote toxic-carbonate precipitation.In paddy-wheat rotation,the soil extractable Cd in the composite clay treatment group was significantly less than that in the control group(DTPA-Cd and HCl-Cd declined by 16.2%～49.5%and 23.8%～79.6%,respectively)and remained stable during the total planting seasons.Meanwhile,the maximal sorption capacity(Q_m)of soil treated with composite clay for Cd was 14.6%～56.2%higher than the control group,implicating the sorption capacity of soil for Cd was significantly enhanced.(2)Particle sizes of arable soil significantly influenced the sorption capacity for Cd.The clay could fix Cd in each particle size,whereas the mechanism varied from particle size.For the soil below 0.002 mm,the clay mainly improved the sorption performance by enhancing the non-specific adsorption;For 0.02-0.002 mm and 0.2-0.02 mm soils,it promoted specific sorption while inhibiting non-specific sorption.Since specific sorption was greater than non-specific sorption,the overall sorption capacity of the soil increased.Flooding treatment was conducted in a dry and wet environment to simulate paddy-wheat rotation.The flooded soil had a stronger sorption capacity than the original soil.However,the flooding treatment affected the specific and non-specific sorption of Cd.As the size was less than 0.002 mm,flooding treatment weakened the non-specific sorption and strengthened the specific adsorption of Cd.As soil size in the range of 0.02 to 0.002 mm,flooding treatment enhanced non-specific and specific sorption.Yet,the non-specific sorption capacity was still lower than the original soil.As soil size from 0.2 to 0.02 mm,flooding treatment further reduced the nonspecific sorption of Cd on the amended soil.It strengthened the specific sorption of Cd on the composite clay-treated soil.(3)The clay affects the Cd sorption properties of different surface-charged colloidal components.The soil organic matter plays an important role in Cd sorption.The total Cd sorption in the original soil decreases by 20%-40%by removing organic matter afterward,mainly attributed to the decline of soil-specific sorption that corresponds to organic matter.The clay had a good immobilization effect on the no organic soil.Its sorption capacity was 19.9%-43.9%higher than that of the control soil without organic matter.Birnessite and goethite had a strong sorption capacity for Cd.The Cd sorption by birnessite was specific.At the same time,goethite was mainly specific adsorption with a small amount of non-specific adsorption.The addition of clay enhanced the sorption capacity of sodomite and goethite,and this sorption enhancement was characterized by specific adsorption enhancement.Illite could adsorb Cd,and its sorption capacity of Cd was affected by temperature.Clay would inhibit the sorption of Cd by illite,and the reduction of sorption capacity was corresponding to the simultaneous reduction of specific sorption and non-specific sorption.Kaolinite had no obvious sorption capacity for Cd,and neither did the addition of composite clay.(4)The Freundlich model could fit the Cd sorption on the soil with each particle size before and after immobilization(flooding)better than the Langmuir model.For organic matter-removed soil,the Freundlich and Langmuir models can better fit Cd sorption on goethite and illite.The fitting parameters of the Freundlich model revealed that the Cd sorption on each particle size of soil after clay treatment was primarily heterogeneous multi-layer sorption.Meanwhile,the clay had impact on the thermodynamic properties of each particle size of soil but did not change the trend of its sorption performance.Langmuir fitting showed that the maximum sorption capacity of organic matter removed soil and goethite increased after immobilization,in contrast,the maximum sorption capacity of illite decreased.In addition,the sorption processes of different particle size soils,organic matter removed soils,goethite,and illite before and after immobilization treatment had differentΔG~θvalues.All negative values,ΔH~θ,andΔS~θ.The sorption process was an endothermic spontaneous process,and the temperature rise was conducive to the sorption.