| The rapid development of industry has caused serious soil pollution in paddy fields.The bioavailability and toxicity of heavy metals in soil not only depend on the total content of heavy metals in the soil,but also on the form of heavy metals in the soil.Changes in the physicochemical properties of paddy soils during development may cause changes in the adsorption ratio of heavy metals in the soil active components,which in turn affect the morphology and distribution of heavy metals.Therefore,in this study,paddy soils from southern China were used as materials to analyze the differences in the distribution of heavy metals with natural soils in the same parent material.The Multi-surface Model was used to predict and compare the distribution of heavy metals in the paddy soils and its changes by using the Donnon Membrane Technique(DMT)to test and correct model predictions.The main findings are as follows:1.The collected natural soils and paddy soils all exhibited similar levels of acidity and organic matter,and the main type of clay mineral was kaolinite and hydromica.The CEC value of natural soils was higher than that of paddy soils,and the iron oxide and manganese oxide contents of the two soils were quite different.The pH values of natural soils and paddy soils were all acidic,but the pH value of paddy soils was higher than that of natural soils developed with the same parent materials.The clay content in natural soil was also higher than that in paddy soil,and the natural soil was dominated by clay while the paddy soil was dominated by loam.The CEC value of natural soil was 11.2 cmol/kg-15.7 cmol/kg while that of paddy soil was only 5.0 cmol/kg-8.6 cmol/kg.The organic matter content was not much different between natural soils and paddy soils,and their content range was 1.3%-3.6%.The amorphous iron oxide content in the soil samples was 0.47 g/kg-3.11 g/kg,and the iron oxide content in the crystalline form was 18.7 g/kg-111.6 g/kg.The content of manganese oxide was quite different,the content of manganese oxide in Liangqiao natural soil was 1.93 g/kg,the highest,and the manganese oxide content of the remaining soil samples ranged from 0.10 g/kg to 0.56 g/kg.The content of kaolinite in soil clay minerals was from 23.9% to 76.3%,and hydromica content was from 15.1% to 69.6%,1.4 nm transition mineral was from 5.2% to 30.9%.the main type of clay minerals in Liangqiao paddy soil were hydromica,and other soils samples were mainly kaolinite.2.Except for the Liangqiao paddy soil,the total amount of lead,zinc,copper,and cadmium of other soil samples exceeded the standard.The total amount and the unsteady content of the four heavy metals of the test soil layer A were higher than that of the corresponding layer B.And the content of the dissolved heavy metal was more than an order of magnitude lower than that of the corresponding unsteady heavy metal.The total contents of lead,zinc,copper,and cadmium in the tested soils were respectively 241.8 mg/kg to 1296.5 mg/kg,186.9 mg/kg to 1393.7 mg/kg,19.4 mg/kg to 610.4 mg/kg,and 0.69 mg/kg – 2.33 mg/kg,the content of unsteady heavy metals were in order of 14.8 mg/kg – 85.2 mg/kg,2.0 mg/kg – 85.2 mg/kg,1.1 mg/kg – 144.9 mg/kg and 0.09 mg/kg – 0.72 mg /kg,and the content of dissolved heavy metals were 0.02 mg/kg – 0.56 mg/kg,0.22 mg/kg – 41.66 mg/kg,0.02 mg/kg – 1.5 mg/kg,and 0.01 mg/kg – 0.25 mg/kg respectively.All the tested soils were polluted by heavy metals of different degrees.Among them,the soil of Tieshiling natural soil had the most serious pollution,and the total amount of lead,zinc,copper,and cadmium all exceeded the standard.The highest levels of unsteady lead and zinc were in Tieshiling natural soil.The higher levels of unsteady copper was only in Liangqiao natural soil which were 144.9 mg/kg and 33.2 mg/kg respectively for the layer A and layer B while other soil samples were low and the content was both less than 5.4 mg/kg.The unsteady cadmium content in the tested soil were all low.The proportion of dissolved heavy metals in unsteady heavy metals in Liangqiao paddy soil was higher than in other tested soils.Overall,the migration of zinc and cadmium in the four heavy metals was stronger and more significant while the lead and copper are less mobile.3.The results of DMT measurement showed that except for the Liangqiao paddy soil,the concentration of free heavy metal ions in layers A and B of other soil showed little difference.The classical parameters of the Multi-surface Model predicted the free zinc,copper and cadmium better than that of lead,and the optimized parameters obtained better forecast results.The DMT technology could separate and determine free heavy metal.And free lead level was from 0.01 μg/kg to 204.67 μg/kg,zinc was from 2.16 μg/kg to 1935.3 μg/kg,and copper was from 0.01 μg/kg to 876.5 μg/kg,finally the free state of cadmium was 0.12 μg/kg – 101.58 μg/kg.The Multi-surface Model predicted free zinc,copper and cadmium well in the soil,but the prediction of lead is slightly biased.The model was optimized,to optimize the parameters of the Multi-surface Model for adsorption of lead could improve the accuracy of prediction.The results showed that the prediction accuracy of lead ions had improved,but it still need further optimization.Then the model system had been added the manganese oxide parameters to take into account the adsorption of lead on manganese oxide,and the results showed that the prediction accuracy was further improved,so the model after optimization was better for prediction of free heavy metals in soil.4.The Multi-surface Model prediction results showed that the main active component of lead adsorption was organic matter and manganese oxide.The main active component of zinc adsorption was crystalline iron oxide,followed by organic matter.The main active components of copper and cadmium adsorption in soil were all organic matter.The parameter-optimized Multi-surface Model predicted the distribution of lead species in the soil solid phase active components.It is found that the rank of the sorption forms in LQPA,TSNA,TSNB,TSPA was Organic matter bound state > Manganese oxide bound state > Amorphous iron oxide bound state > Clay mineral bound state > Crystalline iron oxide bound state,but in other soil was Manganese oxide bound state > Organic matter bound state > Amorphous iron oxide bound state > Clay mineral bound state > Crystalline iron oxide bound state.The main active component of zinc adsorption in soil of Liangqiao was crystalline iron oxide,followed by organic matter,while the main active component of zinc adsorption in Tieshiling soil was organic matter,followed by crystalline iron oxide and clay minerals.In addition to Liangqiao paddy soil,the binding state of copper with organic matter in all other soil samples exceeded 90%,followed by the binding state of crystalline iron oxides,and the order of adsorption capacity of the active components for cadmium in Liangqiao soil was Organic matter > Clay mineral > Crystalline iron oxide,while the Tieshiling soil was Organic matter > Crystalline iron oxide > Clay mineral.5.In the paddy soil,the Eh value gradually decreased and the pH value gradually increased and then reached a stable level with the flooding time.The contents of Fe,Mn,DOC and heavy metal ions in the soil solution first increased and then decreased,and the distribution on the surface of the active component of heavy metals changed more obviously during the initial flooding period and the drainage period.The results of submerged paddy soil experiment showed that the Eh value of bottom layer soil and upper layer soil decreased with prolonged flooding time,reached a minimum around in 13 days and then remained stable.The water layer basically maintained oxidation state,thus the change of Eh was small.The pH of all the three layers of soil solution increased first with the flooding time and then remained stable after reaching 8.The concentration of Fe,Mn and DOC in the soil solution first increased and then decreased with time.The dissolved heavy metals and free heavy metals decreased with time and reached to a minimum around in 13 days,and then remained stable.When the soil environment changed from oxidation state to reduction state,the relative proportion of lead absorbed by manganese oxide and the proportion of zinc,copper,and cadmium adsorbed by organic matter both decreased first and then increased.And after 13 days,the species distribution did not change substantially.When the soil environment changed from reduction state to the oxidation state,the relative proportion of the active component which adsorbed heavy metal most will undergo strong and rapid changes. |
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