Effects Of Humic Acids From Weathered Coal Sources On The Speciation And Bioavailability Of Cadmium In Paddy Soils

Remediation of the heavy metal contaminated farmland by humic acid is an improvement measure based on the chemical behavior of heavy metal soils.In order to analyze the effect of two humic acids on water environment,soil environment and cadmium content in rice,humic acid and fulvic acid from weathered coal mine were selected as experimental materials.Physicochemical analysis experiment,cadmium adsorption experiment,soil culture experiment and rice pot experiment were carried out to compare the effects of two humic acids on the speciation and bioavailability of cadmium in paddy soils.?1?Physicochemical analysis of humic acid andfulvic acid.Humic acid?HA?could fix or passivate soil cadmium by chelating or complexing with cadmium to form insoluble products because of its high aromatization degree,while fulvic acid?FA?could easily react with cadmium to form soluble and mobile FA-Cd because of its low aromatization.?2?Adsorption experiments of humic acid.In water environment,humic acid could adsorb Cd²⁺well and reduce the content of Cd²⁺greatly.The adsorption of humic acid on Cd²⁺could be fitted by the relevant adsorption equation,of which the Langmuir-Freundlich model was areasonably good fit.Moreover,the Cd²⁺adsorption effect of residue of humic acid after slag treatment was better.?3?Soil culture experiments with different components of humic acid.Humic acid can effectively reduce soil pH and available Cd content.In this experiment,The effect of filtrate on soil related indexes was lower than that of filter residue.After grinding and dissolving,the surface morphology of the filter residue was more conducive to its physical adsorption of Cd²⁺;the unit content of carboxyl group in the filter residue increased,which enhanced the chemical adsorption of Cd on the insoluble part of humic acid;and the unit content of phenolic hydroxyl group in the filter liquor increased,which improved the activation effect of humic acid DOM on Cd.?4?Soil culture experiment of humic acid and fulvic acid.When humic acid was added at 2.5%or higer,the passivation effect was better after 15 days;when fulvic acid was 0.5%,the activation effect was better after 1 day.In the follow-up performance,the Cd amplitude of passivation soil Cd by humic acid(0.3 mg kg^-1-4.39 mg kg^-1)was greater than that of activation soil Cd by fulvic acid(0.15 mg kg^-1-1.66mg kg^-1).?5?The effects of humic acid and fulvic acid on the distribution of cadmium in soil.Humic acid slightly reduced the available cadmium content in potted soil,while fulvic acid increased the available cadmium content in potted soil.Among them,humic acid needed 100 days to increase residual cadmium,and fulvic acid showed cadmium activation in 5 days?rice seedling stage?and 100 days?rice harvest stage?.?6?The effects of humic acid and fulvic acid on soil nutrients in rice pot experiments.Both humic acids could increase the contents of available phosphorus,available potassium and alkali-hydrolyzed nitrogen in soil.The increasing effect of fulvic acid was higher than that of humic acid,and the increasing effect of alkali-hydrolyzed nitrogen and available potassium was much higher than that of humic acid.?7?The effects of humic acid and fulvic acid on rice plants.Cadmium content of rice plants with two humic acids showed a trend in the order of root>stem>leaf>grain.Cadmium content in rice decreases because"bound"cadmium?HA-Cd?precipitation did not enter the soil aqueous solution;Low concentration of fulvic acid"bound"cadmium FA-Cd was soluble Cd,which entered into soil water solution,resulting in the reduction of Cd content in rice.High concentration of fulvic acid might change the redox environment of soil and decreased the activity of Cd.Correlation analysis showed that humic acid was more effective han fulvic acid in reducing cadmium content in rice plants.?8?Preliminary study on the mechanism of the influences of humic acid and fulvic acid on soil available cadmium content.The increase of organic matter content caused by humic acids application in soil lead to the increase of cation exchange capacity of soil,which resulted in a binding chance of Cd with two humic acids.Humic acid adsorbing cadmium existed in soil as a solid substance,and fulvic acid will as a soluble form,causing that humic acid was passivated and fulvic acid is activated.