| Due to the influence of human activities such as mining and smelting activities,sewage irrigation,and the application of excessive fertilizers and other agricultural chemicals,the problems of heavy metal(loid)pollution and soil acidification in farmland soil have become increasingly prominent.Acidic rainfall,changes in soil redox state caused by frequent flooding and drainage during rice growth,and changes in organic matter after returning straw to the field strongly affect the occurrence,migration and bioavailability of heavy metals in soil.Soil colloid,especially water dispersible colloid(WDC),a very important pollutant carrier due to its small particle size and small interception effect of porous media pores,play important roles in metal release,migration and bioavailibility.In this paper,the simulated long-term acid rain leaching test,the microcosm culture test and the simulated dissolved organic matter-iron(DOM-Fe)colloid formation test were conducted,and the asymmetric flow field flow fractionatorultraviolet-visible spectrophotometer-inductively coupled plasma mass spectrometer(AF4-UVICP-MS),ultrafiltration,three-dimensional fluorescence and other technical means were employed.This study was dedicated to investigate the main factors which influence the release of WDC-metals from polluted soils,to reveal the migration and transformation mechanism of heavy metals in polluted soil from colloidal and nano scale,and to provide scientific insights for the environmental pollution of heavy metals in agricultural land and the protection of land resources.The main findings are as follows:(1)By simulating long-term acid rain leaching of four typical heavy metal complex polluted soils,the changing trends of heavy metals and WDC in soil extracts were investigated.The results showed that the concentrations of arsenic(As)and(Cd)in the four soil leaches all exceeded the limits of national Class III quality standard for groundwater,and iron(Fe),manganese(Mn),and lead(Pb)in part of leaches exceeded the standards,indicating that heavy metals in the tested soils may have the risk of polluting groundwater under the action of longterm acid rain.Moreover,it was found that with the increase of acid rain extraction times,the dissolution of copper(Cu)and Cd decreased,but the proportion of colloidal Cd and Cu increased;the dissolution of Pb in neutral soil increased,and most of Pb was in colloidal state;and the particle size of WDC became smaller,and Pb and Fe were transformed into small particles.The results indicated high migration risk of Pb,Cu and Cd with the colloids influenced by acid rainfall.(2)The typical heavy metal-contaminated paddy soil was selected,and the dynamic relationship among soil solution chemical properties,DOM,soil WDC,and metal release was analyzed through microcosmic experiments combined with three-dimensional fluorescence spectroscopy.The results showed that the addition of straw increased the release of Fe,Mn,As,Cd and other metals during the anaerobic period,and also affected the WDC and DOM composition.The soil WDC particles were mainly distributed in three size ranges: 0.3-3 k Da were inorganic clay minerals and iron-manganese minerals,3-40 k Da were mainly organic matter and iron minerals,130 k Da-450 nm were organic matter-inorganic composite colloids.The contents of colloidal As and Cu in the reduced state were significantly higher than those in the aerobic state,while the contents of colloidal Cd and Pb in the oxidized state were significantly higher than those in the reduced state.The effect of organic materials addition on the polluted elements was related to the types of elements.On the other hand,the content of protein-like(C3)in soil DOM decreased overall during the cultivation period(p<0.05);the content of humus-like substances increased in the anaerobic period and decreased in the aerobic period.Structural equation model analysis showed that the change of Eh was significantly negatively correlated with the dissolution of Mn and As(path coefficients-0.662 and-0.528,p<0.01),and at the same time,Eh also affected soil humic acid C2(path coefficient-0.816,p<0.01)controlled As release(path coefficient 0.244,p<0.05).Therefore,the change of Eh caused by the alternation of dry and wet paddy soil and the measures of returning straw to the field can not only directly affect the dissolution of heavy metals,but also further control the release of heavy metals through the content and composition of WDC and DOM,which was helpful for further clarify the mechanism of metal release in farmland soil.(3)The effects of p H and C/Fe molar ratio on the formation and particle size of dissolved organic matter-iron(DOM-Fe)colloids,as well as the adsorption and stability of colloids to Cd and As,were studied by simulating the formation of DOM-Fe colloids.The study found that organic matter content(C/Fe molar ratio),p H,and heavy metal types all significantly affected the co-migration behavior of heavy metals and colloids.Cd was more likely to co-migrate in the form of colloid,while As was released in the form of true solution,which further verified the importance of the WDC composed of organic matter and iron mineral for the release and migration of heavy metals.In conclusion,water dispersible colloids played important role in the release and migration of heavy metals in soil,and the distribution characteristics of different elements in WDC are quite different.Soil properties,precipitation,Eh,exogenous organic matter and other factors affected the particle size and composition of WDC and the distribution of heavy metals on colloids,and further affected the release,migration and availability of heavy metals in soil. |
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