Stabilization Characteristics Of Cadmium In Three Typical Agricultural Soils

Soil cadmium(Cd)pollution,leading to a series of environmental problems,has attracted widespread attention in recent years.After a series of process of adsorption and desorption,migration and transformation,the availability of Cd decreases with time going on,and changes little finally.Many biotic factors(species,growth stages,biological adaptability,different evaluation objects,etc.)and abiotic factors(aging effects,soil properties,environmental conditions,etc.)affect the stability of Cd in soils.Following incubation experiment,soil column simulation experiment and field plot experiment,the process of adsorption,migration and transformation,and stabilization of Cd in three typical agricultural soils in Sichuan province was investigated in this study.The main results are presented as follows.(1)According to the adsorption-desorption experiment,the Cd adsorption of the three typical soils increased rapidly and then increased slowly with the increasing Cd concentrations.The Fluvo aquic soil showed the greatest adsorption ability for Cd,followed by the purple soil and yellow soil.The Cd adsorption of Fluvo aquic soil,Purpish soil and Yellow soil was up to 1415.18 mg kg-1,1202.09 mg kg-1 and 1013.34 mg kg-1 at the treatment of 100 mg kg-1 Cd.The adsorption characteristics of Cd in soils could be well described by the Freundlich model among the four adsorption equilibrium models,with R2?0.86(P<0.05).Besides,the Cd desorption increased with the increasing Cd concentrations.The desorption rates of Cd were 9.6%-20.3%,13.6%-25.5%and 13.6%-35.2%for Fluvo aquic soil,Purplish soil and Yellow soil.(2)In the soil column stimulation experiment,the Cd concentrations in leachate of the three typical soils were less than 0.3 ?g L-1 and the ? level of the sewage quality standards in China(1?gL-1),demonstrating that the Cd did not penetrate the soil column.Cd in the purple soil and yellow soil migrated more and more along their profiles with increasing leaching times or leaching amounts.The Cd in the three typical soils were enriched in 0-2 cm,0-5 cm and 0-20 cm of the Fluvo aquic soil,Purplish soil and Yellow soil,respectively.The Yellow soil presented the greatest migration ability,followed by the Purplish soil and Fluvo aquic soil.Thus,greater migration was observed along with greater adsorption of Cd in soils.The dominant Cd fraction in the Purplish soil and Yellow soil was exchangeable fraction,accounting for 45%and 50%of the total Cd in soils.There is little change among all Cd fractions in the purple soil and yellow soil over time.For the Fluvo aquic soil,the dominant fractions were exchangeable and Fe/Mn oxide bound fractions.The exchangeable fraction was transformed to Fe/Mn oxide bound fraction gradually with time going on,and the Fe/Mn oxide bound fraction accounted more than 41%after 30 days.(3)In the incubation experiments,the available Cd decreased rapidly in the first 15d,after which the available Cd decreased slowly.The Cd concentrations in soils changed little after 30 d when the addition of exogenous Cd was less than 2 mg kg-1.When the exogenous Cd was more than 5 mg kg-1,the soil Cd showed no changes after 60 d,and the available Cd in the three soils accounted 52.6%-66.7%of the added amount.According to the Cd fraction analysis,the exogenous Cd showed higher availability than the Cd in the soil.When the addition of Cd was not more than 2 mg kg-1,the major Cd fractions in soils were exchangeable,Fe/Mn oxide bound and residue fractions,which accounted for 14.9%-25.9%,29.6%-38.5%and 19.56%-36.62%of the added amount.When the addition of Cd was more than 5 mg kg-1,the exchangeable and the Fe/Mn oxide fractions accounted for 25.4%-36.1%and 34.1%-40.7%and dominated the added amount.Cd in soils gradually transformed from higher availability to lower availability over time.(4)In the field experiment,the available Cd in the Fluvo aquic soil presented similar changes with that in the laboratory,and accounted for 40%-45%of the added amount after 60-90 d.The exchangeable and carbonate bound fractions in soils accounted more and more,and the other three fractions accounted less and less with increasing addition of Cd.Cd in soils gradually transformed from higher availability to lower availability over time.Greater inhibition for the growth of Chinese cabbage was observed with increasing addition of Cd.After the stabilization for 210 d,there is still obvious Cd toxicity to the growth of Chinese cabbage.Compared to the soils without the plantation of Chinese cabbage,the available Cd in soils enhanced with the plantation of Chinese cabbage.The available Cd in soils decreased firstly and then increased when the addition of Cd was less than 2 mg kg-1.Meanwhile,the available Cd in soils with the plantation of Chinese cabbage was higher than that in soils without the plantation of Chinese cabbage,and showed little change with the addition of 2 mg kg-1 Cd.