Adsorb Of Lead By Different Physical Compositions Of Paddy Soil And Its Response To Changes In Organic Matter

Soil heavy metal pollution has been one of the most widespread environmental problem in the world for decades.Soil is the major carrier of heavy metal pollutants.The soil absorbs and fixes the incoming heavy metals in various ways and forms.Lead is a non-degradable environmental pollutant in soil.Most of the lead entering the soil is distributed on the soil surface which has important impacts to surface biological and ecological health.The composition and properties of soil aggregates of different particle sizes express strong heterogeneity in the soil,affecting the adsorption and fixation mechanism of soil heavy metal pollutants on its surface.As a result,different soil physical components show enormous differences in the amount,manner and molecular morphology of heavy metals.The mineralogical composition of different soils will be inherited from the parent material.The huge difference caused by the different mineral composition in the parent rock.It will affect the composition properties of the soil and it always influent the adsorption capacity and form of heavy metals.Soil organic matter is an important component in soil.It can determine soil properties.Due to its special physical and chemical properties,soil organic matter can adsorb and fix heavy metals,reducing bioavailability and environmental mobility of heavy metals.Therefore,studying the composition of organic matter in the physical components and the adsorption form and characteristics of different parent soils can help us understand the pollution distribution of lead in soil by organic matter and reveal the mechanism of lead adsorption in soil.This has important significance.In this study,we select two types of paddy soils that developed from granite?S_G?and basalt?S_B?as soil parent materials in Hainan.Samples were prepared through soil incubation tests such as adding organic fertilizer,remove soil organic matter by using hydrogen peroxide,and soil simulated lead pollution.Each soil sample was divided into five physical components with different particle sizes?2-0.25 mm,0.25-0.05 mm,0.05-0.01 mm,0.01-0.001 mm,<0.001mm?by wet sieving.Investigate the composition and properties,lead adsorption morphology,lead adsorption-desorption characteristics of each physical component.And find out the effects of different particle size physical components on the adsorption of lead and organic matter in paddy soil.The conclusion obtained are as follows:1.Comparing the soils of two different parent material soils,the soil organic matter?SOM?,CEC and free iron oxide of the whole soil of basalt soil?S_B?were higher than those of granite soil?S_G?.The soil texture of S_B is more glutinous than S_G,and the proportion of the small particle size physical components is higher,and the pH is closer to neutral.There is a big difference about the mineral structure of the two soils.The existence of pyroxene in basalt is an important reason that the content of iron,magnesium and other metal elements in basalt paddy soil?S_B?are higher than those in granite paddy soil?S_G?.This shows that the parent material has a huge influence on the composition and properties of the soil.2.As the particle size decreasing of the two soils,the content of primary minerals decreased in the physical components,and proportion of clay minerals increased.Their organic matter content,CEC and free iron oxide content all increases.The content decreased,and the content of elements such as Fe,Mg,Ca,Al,N,and P increases with decreasing particle size.3.Comparing the soils that come from two different parent materials,the total amount of lead adsorption by basalt paddy soil is higher than that of granite paddy soil because of the differences in their composition properties.The content of reducible and residual lead in the SB is higher than that of S_G.It indicates that Pb adsorption form of S_B is more stable than that of S_G.Comparing the molecular forms of lead adsorption between the two soils,the combined forms of lead silicate and lead phosphate were significantly different.4.With the decrease of particle size,the content of lead in each form and total adsorption of the two soils both increased significantly.The proportion of reducible lead increased most significantly.It explains that the adsorption amount and stability of lead with small particle size physical components are higher than those with large particle size.As the particle size decreases,the content of primary minerals decreases in each component,the number of clay minerals increases and the content of metal elements increases such as calcium,iron,aluminum,magnesium,which will increase the binding sites of each soil samples for heavy metal adsorption.It will increase the adsorption capacity of Pb,and make the adsorption form become more stable.The main form of lead in large particle size components is lead silicate and lead phosphate,while in small particle size components,it mainly exists in the form of Pb-metal-silicate combination.5.The addition of organic matter promoted the formation of large aggregates in the soil.The addition of organic fertilizer increased the content of soil organic matter,free Fe₂O₃,and CEC in each physical component,and the increase was more obvious in large particle size physical components,indicating that the organic matter was mainly preserved in large particle size physical components.After fertilizing,a large amount of substances that can fix heavy metals remain in the large particle size components.As the amount of adsorption increases,the amount of desorption also increases.It shows that the large particle size components in the fertilizer have poor adsorption stability for lead,and the components with small particle size have more stable adsorption of lead.6.After the organic matter was removed,the q_m?maximum adsorption amount of lead?of the two soils and their physical components of each particle size were significantly higher than those of the original soil.The reason is that the content of iron oxide and CEC in the soil are increased.It brings a large number of oxygen-containing functional groups,and a reduced specific surface area also is an important factor.Many factors contribute to this result.After removing the organic matter,the desorption of<0.001mm physical components has't significant effect,which indicates that the physical composition of<0.001mm is more stable for the combination of lead.7.The isothermal adsorption test found that the curve increased almost linearly in the low equilibrium concentration range(1-5mg·L^-1),and tended to be flat at high equilibrium concentrations.The adsorption characteristics of lead with soil and its physical components are more in line with the Langmuir model representing a single molecular layer.The physical components of the original soil,the organic matter removal group and the organic matter addition group,and the smaller the particle size,the better the fit.All the qm?the maximum adsorption amount of lead?has increased.Moreover,the maximum adsorption amount of lead from the physical components of the basalt paddy soil was higher than that of the granite paddy soil.8.The kinetic adsorption test found that with the change of time,the adsorption amount reached more than 80%of the total adsorption amount in the first 10 minutes,and then the adsorption rate decreased and the adsorption tended to stabilize.It is found through the fitting of the kinetic equation that the adsorption characteristics of lead are more in line with the quasi-second-order kinetic model,indicating that chemical adsorption is a step to control its adsorption rate.As the particle size decreases,its initial rate gradually increases.This phenomenon is caused by non-single factors,which results from the combined effect of different compositions and properties of various physical components.9.pH has a great influence on the desorption amount and desorption morphology of lead.Among them,the ion-exchanged lead is the main chemical form of desorption.As the pH increases,the maximum desorption amount of each physical component increases.Both the physical adsorption and ion-exchange lead contents and their proportions decreased,while the complex and hydrogen-bonded lead contents and their proportions increased,indicating that an increase in pH is beneficial to the chemical form of lead in soil steady direction shift.