Studies On Interface Electrochemistry Properties And Heavy Metal Desorption Behavior Of Meadow Brown Soil In Northeast China

Beacause of longtime development of extensive heavy industry in old industrial base in northeast china,heavy metal pollution in this place has posed potential threat to entironment and food security.To know and understand the function mechanism of heavy metal pollution of the soil in Northeast China,provide theoretical evidences for restoration and remediation of heavy metal pollution of soil there.The electrochemistry properties and adsorption-desorption behavior on clay-water interface has important effect on the chemiscal speciation,transformation and fate of heavy metals in soil.And thus has always been the main issue of soil chemistry and enviromental chemistry.In this work we studied the electrochemistry porperties of water-clay interface,the desportion behavior and chemical speciation of heavy metals on meadow brown soil of Northeast China,and investigated effects of some environmental factors which may enfluence the desorption behavior of heavy metals.A better understanding of function mechanism of heavy metal pollution was expected.The research work I performed and the the achievements are as follows:1.The point of zero net charge(PZNC)for the meadow brown soil in Northeast China with structural negative charges was measured with two methods: potentiometric titration and mass titration.The result is for meadow brown soil possessing structural negative charges,the PZNC independent of electrolyte concentration also exists.And the values of pH_PZNCobtained by the two methods are 2.77 and 5.6 respectively.It was found that the values of pH_PZNCdetermined by potentiometric titration were more precise than that determined by mass titration.The extroplating method was used to obtain the total density of surface proton-active sites (Ns),which is 0.667 mmol·g^-1.According to the value of pH_PZNC,Ns and structural negative charge density(δ_st)measured,the intrinsic surface reaction equilibrium constants,pK of 1-pK model,pK_a1^intand pK_a2^intof 2-pK model,were directly calculated to be 3.52,2.50,4.54 respectively for the meadow brown soil in northeast of China. 2.Speciation of Cu,Zn,Cd,Cr,Pb,Ni,As in the meadow brown soil sample were studied by Tessier sequential extraction method and BCR method respectively. The speciation of these metals is:Cu:residual＞associated with organic matter＞associated with Fe-Mn Oxides＞associated with carbonates＞exchangableZn:residual＞associated with Fe-Mn Oxides＞associated with organic matter＞exchangable＞associated with carbonatesCr:residual＞associated with Fe-Mn Oxides＞associated with organic matter＞exchangable,associated with carbonatesCd:exchangable＞residual＞associated with carbonates＞associated with Fe-Mn Oxides＞associated with organic matterNi,Pb:residual＞associated with Fe-Mn Oxides＞exchangable＞associated with organic matter＞associated with carbonatesAs:residual＞associated with Fe-Mn Oxides＞associated with organic matter＞associated with carbonates＞exchangableThe speciation of heavy metals determined with BRC method accords with the that measured with the Tessier method.It was found the Tessier method was suitable for the sample studied.3.Part of the sample was freezed at -70℃for one month,and another part was freezed at -20℃for three months.The speciation for these two samples was determined using the Tessier method.And it turned out that the extent to which the speciation was affected after freezing differ from one metal to another.Compared to the original sample,the variation was not great.4.The desorption of Cu,Ni,Pb,Zn,Cd,As,Cr from soil when suspended in the 0.02mmol/LNaNO₃ solution was observed.The amount desorbed at equilibrium was related to the pH,solid/liquid ratio,the concentration and valence of the electrolyte.Na₂EDTA promoted the desorption of every metal studied.5.0.02mol/LNaNO₃ electrolyte is not efficient on promoting the desorption of metals.The ratio of Pb desorbed decreased a little at first and then slowly went up. When It come to As,the desorption ratio go through a rapid increase,followed by a decreasing and a increasing consequently,and finally achieved a stability.The desorption of Zn went down fast at first,after a certain time of retaining,it finally went up and achieved a stability.The sample freezed at -70℃for one month was also tested for the desorption rate of Pb,As,Zn in NaNO₃ electrolyte.The ratio of As and Zn desorbed decreased a little,while a little increase was found for that of Pb.6.0.1mol/L Na₂EDTA solution was found to promote the desorption of all the metal studied,excerpt for Cd with a solid/liquid ratio of 0.05.the desorption rate was found increased rapidly,and the rate slows down.7.After extraction of NaNO₃ electrolyte,speciaton of metals on the sample was determined by Tessier method.The result was the speciation was similar to the original soil,which indicated that there ware resorption and redistribution processes happened during the extraction.