Effects Of Solid-liquid Interface Processes And Extractability In Different Molecular Weights Of Natural Organic Matter On Purple Soil Cd

At present,the problem of soil heavy metal pollution is becoming more and more extensive and serious in China,especially the problem of soil Cd pollution.NOM（natural organic matter）plays a special and very important role in regulating the biological activity of soil Cd due to its extensive existence and extremely high reactivity.NOM is a complex component,and its impact on the bioactivity of heavy metals in the soil is the result of the combined action of various components.It is difficult to reveal the nature of NOM’s influence on the bioactivity of heavy metals in the soil by studying only DOM components in NOM,or specific components such as HA and FA,and to show the complete picture of the continuous molecular weight distribution of NOM.In the actual environment,with different types and fertility of soil,the molecular composition of NOM is bound to be quite different.In fact,the HA and FA components in NOM have no essential difference in terms of basic molecular constituent units,and the molecular size may be the key factor affecting the activity of heavy metals.Therefore,from the perspective of the molecular weight distribution of NOM,a synchronized study was conducted on the apparent effect of NOM in regulating the activity of heavy metals and the distribution characteristics of the molecular components of NOM,to clarify the internal correlation between the two.It is expected to further deepen the understanding of the impact of NOM on heavy metals on biological and environmental behavior.Therefore,in this test,heavy metal Cd with high pollution hazard and wide pollution range was taken as the object,and the continuous classification method of tangential ultrafiltration was adopted to classify NOM into 5 components with different molecular weights（F1,<5K;F2,5-10 k;F3,10 to 30 k;F4,30 k>;F0 unrated NOM）,based on the analysis of 5 kinds of basic structural composition and properties,on the basis of NOM using total combination experimental design,the research compares the NOM components of different molecular weight in Cd adsorption-desorption of purple soil,analyzed the NOM component elements of different molecular weight,content of active functional groups and its influence on soil Cd adsorption and desorption behaviors;The effects of NOM on the solid-liquid distribution and the extractability of soil Cd under different levels of Cd pollution were discussed,revealing the correlation between the content of Cd in soil solution,pH,DOC and the relationship between the soil Cd morphology and the extractability under different levels of Cd pollution.The main research results are as follows:1.Different molecular weights:NOM element composition and structure differencesThe elemental composition and infrared spectral characteristics of NOM component with different molecular weights were basically similar,but the contents of C,H,O and other elements were significantly different.The content of C and H increased with the increase of molecular weight of NOM,while the content of O element was on the contrary.The index O/C that characterizes the structure and properties of humic acid,（N+O）/C ratio decreases with the increase of molecular weight,reflecting that the degree of oxidation of components with high molecular weight of NOM is lower and the polarity is weaker.At molecular weight>5k,the degree of condensation and aromatization of components with high molecular weight were lower.Total acid groups and phenolic hydroxyl groups decreased with the increase of molecular weight,in the same order as O/C ratio,while the content of carboxyl group changed little with the increase of molecular weight.In addition,the infrared spectrogram shows that in the binding process of NOM with Cd,alcohols,carboxylic acids,phenolic groups,polysaccharides,amino groups,aldehyde groups and associating hydroxyl groups were involved in the reaction.Although the infrared absorption spectra of NOM component with 5 different molecular weights were similar,some characteristic peak strengths of absorption were different.The results showed that F4 had high degree of aliphatic and low degree of aromatization.2.Influence of different component NOM on Cd adsorption and desorption of purple soil（1）the addition of NOM to the soil increased the apparent adsorption capacity and the adsorption strength of Cd in the soil,but the influence degree of different components and different amounts of addition was different.The adsorption capacity of soil with different addition amounts and different NOM components increased by 6.25%⁶1.22%compared with the control（original soil,CK）.With the increase of NOM dosage,the adsorption capacity of Cd on soil increased;The adsorption order of different molecular components of NOM soil on Cd is shown as follows:F1>F4>F3>F0>F2>CK.The order of holding strength is consistent with it.At the same addition amount（0.5%C）,component F1had the largest promoting effect,increasing by 37.98%compared with F0,while component F2reduced by 6.68%compared with ungraded NOM（F0）.NOM components of soil Cd adsorption-desorption vary with the size of its molecular weight doesn’t exist linear dependencies:molecular weight<5 k NOM components（F1）of soil on Cd adsorption and holding the highest intensity,and the molecular weight>10 k NOM on soil Cd adsorption quantity and holding strength increased with the increase of molecular weight increases,low molecular components may be affected mainly by soil pH and change its adsorption performance,and high molecular weight component is associated with its own adsorption ability and the molecular structure.（2）different components of NOM were first added to the solution containing Cd.After the interaction between them,the adsorption capacity of Cd in the solution was significantly enhanced.The adsorption capacity of Cd increased by 32.72%⁴8.64%compared with the control group.After the interaction between different components NOM and Cd,the adsorption amount of Cd in soil was in the following order:F4>F3>F1>F0>F2.The effect of NOM component on the adsorption of Cd on soil has no linear dependence on the change of its molecular weight.Compared with F0,the adsorption capacity of component F4 on Cd increased by 8.99%,while that of component F2 decreased by 2.68%.（3）NOM can change the adsorption property of soil to Cd by affecting soil properties or changing the existing form of Cd in solution.The effect of different molecular weight components is significantly different.Different component NOM was added to the soil,the soil with component F1had the strongest adsorption capacity for Cd,while the soil with component F2 had the weakest adsorption capacity for Cd.Under the condition of 0.5%C,the adsorption capacity of F1 soil increased by 61.25%compared with CK,and that of F2 soil increased by 10.58%compared with CK.Under the condition of 0.5%C,after the interaction of different component NOM with Cd,the adsorption capacity of Cd on the soil significantly changed.The soil showed the highest adsorption capacity for Cd after the reaction with component F4,and the adsorption capacity increased by 48.64%compared with CK.However,after the action of component F2,the adsorption amount of Cd in soil was the lowest,only 32.72%higher than that of CK.3.Influence of different component NOM on the solid-liquid distribution of soil Cd（1）in the fixed soil,with the pollution level of Cd at 5mg/kg and 10mg/kg respectively,different molecular weight components of NOM were added to the soil,which significantly changed the distribution of Cd in the solid-liquid phase of the soil.There was a significant negative correlation between the concentration of Cd in the liquid phase of soil（0.01 mol·l-1kno3 solution,shaken overnight,centrifugated for 10 min at 10000 r·min-1）and the molecular weight of NOM component.The higher the molecular weight of NOM component,the lower the concentration of Cd in the soil solution.With the same amount of NOM,the concentration of Cd in the soil solution was in the following order:F1>F0>F2>F3>F4.The adsorption and fixation effect of high molecular weight NOM component on Cd in soil was higher than that of low molecular weight component.The higher the amount of NOM in the soil,the higher the concentration of Cd in the soil solution,and the weaker the soil’s fixation and retention effect on Cd.Soil of different components,（2）add NOM solid-phase Cd to the solution of the release ability and the consistency between the dissolution performance of NOM,namely the Cd concentration in the soil solution（x）（y）and DOC concentration exists positive correlation,the regression equation is:y=0.00715+0.0012 x（r=0.392,N=96）,show that the more NOM component dissolution ability to release the Cd concentration of soil solution is higher,especially when exogenous Cd pollution levels are relatively low（5 mg/kg）,the trend is more obvious;There was a significant negative correlation between the concentration of Cd（y）in the soil liquid phase and the pH（x）of the soil liquid system（r=0.396,N=96）.That is,the higher the pH of the system,the lower the ability of Cd to migrate to the soil solution.（3）NOM all components promote the transformation of acid-soluble Cd and reducible Cd into residual Cd in the soil,and the promotion ability is related to the molecular weight of NOM component.The contents of acid-soluble Cd and reduced Cd in the NOM soil of each component are positively correlated with the molecular weight of NOM,while the contents of residual Cd are negatively correlated with the molecular weight of NOM.（4）the concentration of Cd in the soil liquid phase is restricted by the morphology of solid phase Cd.The concentration of Cd in the soil solution is positively correlated with the content of acid-soluble Cd and reduced Cd,negatively correlated with the content of residual Cd,but not significantly correlated with the content of oxidizable Cd.4.Influence of different molecular weight component NOM on the extractability Cd of purple soil CH₃COONH₄（1）in a fixed level of Cd pollution in soil were 5 mg/kg and 10 mg/kg,under the condition of different molecular weight NOM components added to the soil,NOM to join in the soil of components,all reduced can extract sexual content of Cd in soil（CH3COONH₄ as the extraction agent）,the treatment can extract sexual Cd content in soil with than control（0.825 mg/L）was reduced by 14.0%⁷9.0%,and different dosage of components and the NOM and different level of Cd contaminated soil extract Cd content is different.When the pollution level of Cd was low（Cd2+=5mg/kg）and the amount of NOM was low（0.1%,0.2%）,the extractable Cd content of different components of NOM soil was as follows:F1>F4>F0>F3>F2.The extractable Cd content was not significantly correlated with the molecular weight of NOM.When the pollution level of Cd was high（Cd2+=10 mg/kg）and the amount of NOM was low（0.1%,0.2%）,the extractable Cd content in the soil of NOM of different components was as follows:F4>F3>F0>F2>F1.Its extractability:the content of Cd is proportional to the molecular weight of NOM.The amount of NOM was higher（0.5%）.At different levels of Cd pollution,the extractable Cd content in the soil of each component was in the same order:F1>F2>F0>F3>F4.Its extractability:the content of Cd is inversely proportional to the molecular weight of NOM.（2）with the addition of NOM,the extractable Cd content of the soil generally decreased first and then increased with the extension of the culture time.It reached the minimum value about 30 days after the culture,then began to rise,and finally stabilized after 60 days.In addition,the extractable Cd content of each treatment was lower than that of the control.（3）correlation analysis showed that the content of extractable Cd in soil was positively correlated with the content of acid soluble Cd and the content of reduced Cd in soil,but the correlation with the content of acid soluble Cd in soil was stronger than that of reduced Cd.Soil extractable Cd content was negatively correlated with residual Cd content,while soil extractable Cd content was not significantly correlated with oxidizable Cd content.