Study On The Passivation Effect Of Humic Acid,Ferrihydrite And Their Coprecipitates On Cadmium In Soils

With the rapid development of industry and agriculture,heavy metal pollution in soils has become a global environmental problem.In-situ passivation remediation technology is one of the most promising remediation technologies for heavy metal contaminated farmland soil.Humic acid and ferrihydrite are widely present in the soil environment,which can form C-Fe coprecipitate with high active surface and redox potential by adsorption,coprecipitation,flocculation and chelation.C-Fe coprecipitate plays an important role in regulating the migration and transformation of heavy metal in soils.Therefore,it is of great significance to agricultural product safety and human health to explore and examine the effect of humic acid,ferrihydrite and C-Fe coprecipitate on the immobilization of heavy metal in farmland soil in China.In this paper,the farmland soil was collected from Gaowan Township,Jingyuan County,Baiyin City,Gansu Province.The soils with varying cadmium concentration were simulated by soil incubation.At the same time,ferrihydrite and C-Fe coprecipitates with different C/Fe ratios were synthesized by coprecipitation method and characterized by SEM-EDS,FTIR,XRD and N₂ adsorption-desorption experiment to examine and measure their morphology,crystallinity,functional group and specific surface area.The change of Cd speciation with time was tested using modified BCR continuous extraction method after the addition of humic acid,ferrihydrite and C-Fe coprecipitates in soil samples.The main conclusions are as follows:（1）Ferrihydrite coprecipitated humic acid by the coordination exchange of carboxyl groups on humic acid with-OH on the surface of ferrihydrite.C-Fe coprecipitates have a very similar XRD pattern to pure ferrihydrite.Humic acid addition did not change the crystal structure of 2-line ferrihydrite,but changed the surface morphology and structure properties of 2-line ferrihydrite.As the C/Fe ratio increases,the surface roughness and the pore size of particles increase,and the specific surface area and pore volume decrease.（2）When exogenous Cd enters the soil,it changes the speciation of Cd in the original soil,and increases the activity and migration of Cd in the soil.In the original soil,the Cd speciation distribution follows sequence as:weak acid soluble state>residual state>reducible state>oxidizable state,while the exogenous Cd addition to the soil,the distribution of Cd speciation follows:weak acid soluble state>reducible state>residual state>oxidizable state.When exogenous Cd content increases,the non-residual state Cd proportion increased gradually in the soil,and the residual state Cd proportion gradually decreased.（3）After the 5%humic acid and ferrihydrite were added solely to Cd-contaminated soils and reacted for 15,30 and 60 days,it promoted the conversion of weak acid-soluble Cd to the reducible and residual state in soils in all period,thus reducing the activity and migration of Cd in soils.In particular,in reaction period of30 days,the proportion of residual Cd is the biggest,thus showing the best passivation effect.By comparison,it was found that humic acid had better remediation effect on slightly Cd-contaminated soil,and the residual Cd content increased by 24.81%compared with the contrast experiment.The remediation effect of ferrihydrite on moderately and severely Cd-contaminated soils was better,and the residual Cd content increased by 23.64%and 41.69%compared with the contrast experiment,respectively.（4）After 5%C-Fe coprecipitates were applied to the soils for 15,30 and 60 days,the Cd speciation in the soils has been changed.When the passivation time reached 15and 30 days,the input of C-Fe coprecipitates converted the weak acid soluble speciation for easy plant uptake into reducible and residual speciation for difficult plant uptake.When the passivation time reached 60 days,C-Fe coprecipitates converted the weak acid soluble speciation for easy plant uptake into reducible,oxidizable and residual speciation for difficult plant uptake,,thus effectively reduced the active Cd content with high bioavailability and mobility.（5）Among C-Fe coprecipitates with C/Fe ratio of 0.5,1.0,1.5,2.0 and 2.5,C/Fe=2.0 coprecipitate had the best passivation effect on slightly and moderately Cd-contaminated soils in 15 days,while had the best passivation effect on the severely Cd-contaminated soil in 30 days,which the residual Cd content increased by30.34%,37.86%and 54.54%in the slightly,moderately and severely Cd-contaminated soil compared with the contrast experiment,respectively.（6）Compared all the passivation effect caused by humic acid,ferrihydrite and C/Fe=2.0 co-precipitate on Cd-contaminated soil with varying concentrations at the optimal passivation time.Results showed that the passivation effect for slightly Cd-contaminated soil follows sequence as:C-Fe coprecipitate（C/Fe=2.0）>humic acid>ferrihydrite,and the passivation effect for moderately and severely Cd-contaminated soil follows sequence as:C-Fe coprecipitate（C/Fe=2.0）>ferrihydrite>humic acid.