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

Heavy metal pollution has become a major environmental problem in the world.The heavy metal elements in the soil environment migrate,transform and accumulate in animals and plants,and enter the human body with the food chain,thus threatening human health.Cadmium?Cd?is a heavy metal element with the widest pollution range and the most serious pollution degree in agricultural soil in China.Humus is the main organic colloid in soil,and also the main component of organic materials.Its active components mainly include fulvic acid?FA?and humic acid?HA?,both of which are commonly referred to as humic acid?HAs?which has a large specific surface area and contains a variety of functional groups,so has a high reactivity,can react with a variety of organic-inorganic pollutants,thus changing the form and activity of pollutants in the soil.In the soil environment,the effect of HAs on the biological activity of heavy metals,including Cd,is the result of the comprehensive action of various components.The different content and proportion of different molecular weight components lead to different apparent effects.In fact,there is no essential difference between the active components?HA and FA?in the basic molecular components of HAs,and the molecular size may be the key to the differences in the reactivity of HAs from different sources and the activity of heavy metals.In this study,batch isotherm adsorption test,chemical continuous extraction morphology classification method and in vitro experiment were used to study the apparent effect of HAs on the activity of heavy metals.Combined with the correlation analysis and multiple regression analysis of the existing form of Cd and its bioavailability,Cd accumulation in earthworms,the mechanism of different components of HAs on the biological activity of Cd was explained.It can provide scientific basis for further understanding of environmental behavior of HAs and remediation of heavy metal pollution in soil by using organic materials containing HAs.The main results are as follows:?1?HAs can change the adsorption capacity of Cd on soil,and its action size and direction are related to the molecular weight and addition amount of HAs components.The fitting results of Langmuir equation shows that when the amount of HAs is 0.2%C,0.5%C and 1%C,the components of F1?<5K?promote the adsorption of Cd,while the components of F3?10-30k?and F4?>30K?with higher molecular weight weaken the adsorption of Cd.?2?The effect of HAs on the transformation of Cd form in soil is affected by the molecular weight of its components,but the effect of application amount is less.The addition amount of HAs is within the test range,F1?<5K?and F2?5-10K?components promoted the increase of Cd proportion of S1?weak acid extraction state?.When the addition amount was 0.2%C,0.5%C and 1%C,the Cd proportion of S1?weak acid extraction state?increased by 6.44%,17.36%and 8.12%respectively,while the Cd proportion of F3?10-30k?and F4?>30K?components decreased with the increase of molecular weight,and both of them were smaller than the control.High molecular weight components?F3,F4?promoted the transformation of Cd from available to difficult to use.When the added amount was 0.5%C,the ratio of Cd in S4?residual?was significantly higher than that in CK?P<0.05?under the influence of F3?10-30k?and F4?>30K?,8.09 and 8.33 times higher than that in CK,respectively.?3?Different molecular weight components of HAs had different effects on the bioavailability of Cd in soil.Molecular weight 10k Da was the critical threshold of Cd activation and deactivation.The bioavailability of Cd was determined by PBET,SBRC and IVG.The results showed that F1?<5K?and F2?5-10K?components increased the bioavailability of Cd.The BA_Cd of PBET was 70%,61.82%and 23.64%higher than CK.Under the influence of F3?10-30k?and F4?>30K?,BA_Cd decreased with the increase of the molecular weight of HAs,and were smaller than CK.There were some differences in the results of different methods.The bioavailable gastric and intestinal stages of CD showed that BA_PBET<BA_SBRC<BA_IVG.?4?Through the animal simulation test,it is shown that the effect of different molecular weight components of HAs on Cd enrichment by earthworm has obvious molecular scale effect.Low molecular weight components?F1,F2?inhibited the growth of earthworms and promoted the accumulation of Cd in soil.When the concentration of Cd pollution was 10mg/kg,the content of Cd in earthworm was3.88mg/kg,11.17%higher than CK.The addition of F3?10-30k?and F4?>30K?decreased the toxicity of Cd on earthworms,and significantly reduced the mortality of earthworms?P<0.05?.With the increase of molecular weight of HAs,the inhibition rate of HAs on earthworms showed a gradual increase trend.?5?HAs affects the biological activity of Cd by affecting the adsorption of Cd on soil,the distribution of Cd form in soil,the bioavailability of Cd and the concentration in animals.The bioavailability of Cd and the concentration of Cd in earthworm were correlated with the adsorption of Cd by soil under different molecular weight components of HAs.S1?weak acid extract?Cd in soil has a positive contribution to the bioavailability of Cd and the concentration of Cd in earthworm,and there is a moderate correlation between the bioavailability of Cd and the concentration of Cd in earthworm.Therefore,in the discussion of the influence of different molecular weight components of HAs on the biological activity of heavy metals,we should combine the bioaccumulation ability with bioavailability study and other chemical indicators for comprehensive evaluation.