Effects Of Humic Acids Fractions With Different Molecular Weight On Speciation Of Bioavailability Of Lead

Lead?Pb?is a heavy metal element in agricultural soils of China,which has a wide range of pollution and a serious degree of pollution.Humus is the main component of soil organic matter.Fulvic acids?FA?and Humic acids?HA?are the main active components of humus.They are commonly referred to as humic acids?HAs?,which have a large specific surface area and a variety of functional groups,and can react with various organic-inorganic pollutants,thus changing the form and biological activity of heavy metal pollutants in soil.It is generally believed that HA has the characteristics of higher degree of polymerization,larger molecular weight and more complex structure compared with FA.Therefore,HA often forms complex with soil inorganic colloid components in the soil,which can absorb and fix heavy metals in the soil with high strength,thus reducing its bioavailability and producing passivation effect on heavy metals,while FA with low molecular weight helps to dissolve heavy metals in the soil,thus providing heavy metals mobility and bioavailability of metals.The influence of organic materials on the activity of heavy metals is the result of the interaction of different components.In fact,there are no essential differences between the active components?HA and FA?in the basic molecular components of HAs,and the molecular weight may be the key to the difference in the reactivity of HAs from different sources and the activity of heavy metals.In this study,four groups of continuous components?F1,<5K;F2,5-10K,F3,10-30k and F4,>30K?with different molecular weight were divided into soil HAs.Earthworms were used as model organisms to study the effects of different molecular weight components on the bioavailability and bioavailability of Pb in soil.Combined with batch isotherm adsorption tests,chemical single extractions and continuous extraction morphological classification,the non availability of HAs was explained the mechanism of the same components affecting the biological activity of Pb provides a scientific basis for further understanding the environmental behavior of HAs and the remediation of Pb pollution in soil by using organic materials containing has.The main results are as follows:?1?After adding different molecular weight HAs components,the adsorption capacity of soil to Pb changes,and its effect is related to the molecular weight of HAs components.Compared with the control,F1 component promotes the adsorption of Pb in soil,and F3 and F4 component weaken the adsorption capacity of Pb in soil.The quasi second-order kinetic model can well fit and describe the adsorption kinetic process.Low molecular weight components?F1,F2?can promote the adsorption of heavy metals in soil,and high molecular weight components?F3,F4?can promote the desorption of heavy metals in soil.?2?There is no significant difference in the effect of different molecular weight components of HAs on the transformation of Pb form in soil.Pb in soil mainly exists in the form of reducible state?S2?,which accounts for 81.21%of the total,indicating that the activity of Pb in soil is low.The proportion of S3 and S4 is relatively low,and there is little change in the whole culture process.?3?The influence of HAs on the extractability of soil Pb is closely related to the molecular weight of its components.The components with molecular weight<10K?F1,F2?can promote the activation of soil Pb,while the components with molecular weight>10K?F3,F4?can passivate soil Pb.The extractability?effectiveness?of exogenous Pb decreases sharply after entering the soil,and tends to be stable after 2 months of aging.After the treatment with low molecular weight components F1 and F2,the extractability of soil Pb is significantly higher than that of the control,in which the effective Pb content of ammonium acetate in F1 is 50.9mg/kg,higher than 40.4mg/kg of the control,and the extractable Pb after the transformation of high molecular weight components is lower than that of the control,The content of effective Pb in ammonium acetate decreases by 17.6mg/kg after transformation equilibrium.The extraction rate of heavy metals by three extractants is EDTA>DTPA>HCl.?4?There are differences in bioavailability between the two methods of In vitro?PBET,SBET?:BA_SBET<BA_PBET.Compared with the control,the components with molecular weight less than 10k improves the bioavailability of Pb,while the components with molecular weight higher than 10k decreases the bioavailability of Pb in soil,and the effect is stronger with the increase of molecular weight of HAs.There is no significant relationship between the bioavailability of Pb in soil and the amount of Pb adsorbs by soil.?5?The addition of F1 enhances the inhibition of Pb on earthworm growth.The addition of F3 and F4 significantly reduces the mortality of earthworm?P<0.05?,and with the increase of the molecular weight of HAs,the growth inhibition rate of earthworm showes a gradual increase trend.The content of Pb in earthworm decreases with the addition of different molecular components of HAs.The accumulation of Pb in soil by earthworm is positively correlated with the concentration of Pb.?6?When the components with molecular weight less than 10k?F1,F2?are applied to the soil,the activities of two enzymes in earthworm are significantly enhanced,while the components with molecular weight higher than 10k?F3 and F4?decreases the activities of POD and CAT in earthworm,and with the increase of the molecular weight of HAs,the overall activities of the two enzymes decreased.The results showes that the low molecular weight component promotes the activity of Pb in soil and increases the oxidative stress in earthworm,while the high molecular weight component has the opposite effect.