The Development And Application Of Surface Complexation Model Of Typical Heavy Metals On Birnessite

According to the National Survey Bulletin on Soil Pollution Status issued by the Ministry of Environmental Protection and the Ministry of Land and Resources in 2014,soil heavy metal pollution is a serious problem in China.The speciation and bioavailability of heavy metals in soils profoundly affect their ecological risks.Geochemical Speciation models,such as the multi-surface speciation model（MSM）,can be used to describe the speciation and solid/liquid distribution of trace metals in different soil environments.Manganese oxide is an important oxidizing mineral in soil,which has large specific surface area,low surface charge and strong affinity to metals,and has an important influence on the behavior of heavy metals.But the contribution of manganese oxide to metal retention was often ignored in the MSM due to the lack of surface complexation model（SCM）parameters of manganese oxide,which leads to a large deviation in the MSM prediction of Pb morphology.Therefore,in this paper,birnessite,which is widely present in the soil,wass selected as the representative of manganese oxide,and the adsorption behavior of 8 typical metals(Ca²⁺,Cd²⁺,Co²⁺,Cu²⁺,Mn²⁺,Ni²⁺,Pb²⁺,Zn²⁺)on the surface of birnessite was investigated,according to the crystal structure characteristics of birnessite,determine its surface site types,site density,specific surface area and other values,combined with spectral evidence to determine the surface reaction equation,and fit 8 kinds of divalent metal cations SCM parameters of the birnessite surface（Chapter 2）.On this basis,Pb was used as the target metal,and 11 typical soils across the country were selected to carry out Pb adsorption experiments and the common extraction methods of manganese oxide in soil were used to extract manganese oxide.Different model frameworks predict the concentration of dissolved Pb in the soil solution extracted by 0.01 mol·L^-1 Ca Cl₂ and the distribution of Pb on the active surface of the soil,evaluate the influence of different soil manganese oxide extraction methods on the model results,and select the best soil manganese oxide extraction method and the optimal MSM framework（Chapter 3）,the research results provide parameters and methods for further improving the MSM of heavy metals in the soil.The main conclusions are as follows:（1）The adsorption amount of 8 kinds of divalent metal cations(Ca²⁺,Cd²⁺,Co²⁺,Cu²⁺,Mn²⁺,Ni²⁺,Pb²⁺,Zn²⁺)on birnessite increased with increasing p H but was not influenced by the ionic strength,suggesting an inner-sphere complexation mechanism.The adsorption amount increased with the increase of the initial concentration of metal cations,and the adsorption trend is basically the same.The order of adsorption affinity of 8 metals on birnessite was Pb²⁺>Cu²⁺≈Co²⁺>Cd²⁺>Mn²⁺>Zn²⁺>Ca²⁺>Ni²⁺.（2）The birnessite is a two-dimensional lamellar structure composed of Mn O₆ octahedral sheets,and the main adsorption sites for metal are≡Mn₂O^-2/3 in the inner layer and≡Mn OH^-1/3 in the mineral edge.Combined with the results of macroscopic adsorption experiments and spectral evidence,the fitting results showed that the two-site model can better describe the adsorption behavior of various metals on the surface of birnessite:Ca²⁺/Cd²⁺/Co²⁺/Cu²⁺/Mn²⁺/Pb²⁺/Zn²⁺adsorbed on the birnessite surface to form triple-corner-sharing species（TCS）on the internal surface and double-corner-sharing species on the external surface（DCS）;Ni²⁺adsorbed on the birnessite surface to incorporat within Mn vacancies（INC）on the internal surface and form DCS on the external surface.The surface reaction of metal（Me）adsorption is as follows:TCS was formed on the internal surface(Ca²⁺/Cd²⁺/Co²⁺/Cu²⁺/Mn²⁺/Pb²⁺/Zn²⁺):3≡Mn₂O^-2/3+Me²⁺=（≡Mn₂O）₃Me⁰log K_TCSINC was formed on the internal surface(Ni²⁺):6≡Mn₂O^-2/3+Ni²⁺+2H⁺=（≡Mn₂O）₆H₂Ni⁰log K_INCDCS was formed on the external surface(Ca²⁺/Cd²⁺/Co²⁺/Cu²⁺/Mn²⁺/Ni²⁺/Pb²⁺/Zn²⁺):2≡Mn OH^-1/3+Me²⁺+H₂O=（≡Mn OH）₂Me OH^+1/3+H⁺log K_DCSA costant capacity model（CCM）and a triple layer model（TLM）were used to describe the electrostatic terms of the inner and outer surface reactions,respectively,and the corresponding model adsorption parameters were obtained by fitting.The adsorption data reported in previous literatures were used to further verify the validity of the established model and its parameters.（3）For the studied metal cations(except Ni²⁺and Ca²⁺),there was an linear free-energy relationship（LFER）between log K_TCS and hydrated ionic radius（HIR）and electronegativity（EN）.log K_TCS was negatively correlated with HIR and EN（r=-0.891,p=0.017）and positively correlated（r=0.909,p=0.012）.In addition,when ionic radius（IR）,HIR,HFE and EN were included in the regression analysis,the regression equations of log K_DCS and log K_TCScan be better obtained as follows:log K_DCS=275.47–188.43*IR–473.95*HIR+0.02*HFR–15.86*EN（R²=0.79）log K_TCS=464.27–188.19*IR–847.95*HIR+0.02*HFR–30.66*EN（R²=1.00）These two equations can be used to estimate the complex constants of other divalent metal cations with similar surface morphology on the surface of birnessite.（4）The results of different extraction methods showed that the content of soil manganese oxides extracted by Suda method was significantly higher than that by Chao method.Among the 11 kinds of soils,the content of manganese oxides in Kunming soil was the highest,which was 1.92±0.13 g·kg^-1（Chao method）and 2.86±0.20 g·kg^-1（Suda method）.The content of manganese oxides in Yingtan soil was the lowest,which was 31.10±1.02 mg·kg^-1（Chao method）and 0.19±0.01 g·kg^-1（Suda method）.The contents of manganese oxides in the other nine soils were similar in the range of 0.20～0.70 g·kg^-1（Chao method）and 0.38～0.97 g·kg^-1（Suda method）.（5）The prediction effects of Pb dissolution under different model frameworks were compared.When the traditional MSM was used,that was,without considering the manganese oxide（NS1）,the prediction results of the model overestimate the dissolution of Pb from some acidic soils and underestimate the dissolution of Pb from some alkaline soils.When manganese oxide was added to MSM,but only the adsorption of Pb²⁺（YS1）was considered,the adsorption of Pb was greatly overestimated.When the adsorption of competitive ions Ca²⁺,Cu²⁺,Zn²⁺,Ni²⁺and Mn²⁺（YS2）were further considered,the prediction result of the model was significantly improved.When only Mn²⁺was considered as the competitive ion in MSM（YS3）,the prediction effect was similar to that of YS2.This indicated that the influence of competing ions must be considered when adding manganese oxide to MSM,and Mn²⁺was the most important competing ion of Pb²⁺in manganese oxide in the soil studied in this study.（6）According to the speciation distribution predicted by the model,in the NS1 model framework without considering manganese oxide,soil organic matter in acidic soil was the main adsorption surface for Pb,and iron oxide in alkaline soil was the main adsorption surface for Pb.When manganese oxide was added to MSM,but only the adsorption of Pb²⁺（YS1）is considered,more than 80%of Pb was adsorbed by manganese oxide,which was also the reason why YS1 overestimates Pb adsorption.After considering multiple competing ions（YS2）,organic matter and manganese oxide in acidic soil were the main adsorption surfaces for Pb,while iron oxide in alkaline soil was still the main adsorption surfaces for Pb.This indicated that the contribution of manganese oxide to Pb adsorption was mainly shown in acidic conditions and soils with high manganese content,such as the contribution of more than 50%in the soil of Kunming,while the contribution of manganese oxide was not significant under alkaline conditions because the content of iron oxide was generally much higher than that of manganese oxide.The improved MSM can help people to understand the speciation distribution of Pb in soil deeply,and realize the quantitative prediction of the distribution of Pb in soil solid/liquid phase.In conclusion,based on the SCM of typical metals adsorbed on the surface of birnessite,this study incorporated the surface of manganese oxide into MSM,which can better predict the concentration of dissolved Pb in soil with different physicochemical properties and the morphology distribution of Pb on each active surface of soil,and can provide a scientific basis for studying the migration behavior of Pb in soil.