Nica-Donnan Model Prediction Of Metal Speciation In Soil Solutions: Integrating DOM Reactivity And Al/Fe Competitive Effect

Shaoguan is a non-ferrous metal town in Guangdong Province.It is also an important ecological function area and rice production area in the Northern Guangdong.In recent years,The mining and smelting of local industry and many metal mines have resulted in the enrichment of heavy metals,the increased of soil acidification,the increased of effectiveness,migration and biological toxicity of heavy metal ions.It poses a serious threat to the health of residents in the vicinity of the mining area.It is particularly necessary to comprehensively carry out investigation of heavy metal pollution in soils in the region,and to clarify the key chemical morphological features and effectiveness of heavy metals in soil.This study selected of the samples were collected from the villages surrounding three typical industrial and mining areas,i.e.Fankou Lead-Zinc Mine,Dabaoshan Polymetallic Mine and Qujiang Power Plant.Contaminated paddy soil as the research object.The Donnan Membrane Techique?DMT?was also analyzed for the key chemical forms and activities of heavy metal free ions in the soil.Comprehensive consideration of dissolved organic matter?DOM?,iron-aluminum oxide and iron-aluminum ion competition affect the possible distribution of heavy metal ion species.A multi-surface model was used to simulate the distribution and intrinsic mechanism of heavy metal speciation,providing a theoretical basis for the accurate prediction and assessment of heavy metals in the environment.The main research results are as follows:?1?The contents of heavy metals Cd and Pb in the paddy soils around the three typical industrial and mining areas in Shaoguan were 0.38³.58 mg·kg^-1(average value was 2.04mg·kg^-1),and 69.64¹408.06 mg·kg^-1(average value was 126.31 mg·kg^-1);among them,the content of Cd was excessively exceeded,the rate of exceeding the standard was 100%,and the over-standard rate of lead was 30%.?2?The content of dissolved Cd in soil ranged from 1.42 to 38.81?g?L^-1(average of13.11?g?L^-1),which accounted for 1.20%to 39.38%of the total soil content of Cd?average of 11.46%?.And mainly in free form,accounting for 66.57%to 100.00%of the dissolved content of Cd?average of 94.88%?.The dissolved content of Pb in the soil ranged from 0.19to 2120.11?g?L^-1(average of 108.57?g?L^-1),which accounted for 0.00%to 1.51%of the total soil content of Pb?average of 0.21%?.This indicates that Pb is harder to migrate than Cd in surface soil.Pb in the soil solution is mainly free ion,accounting for 5.28%to 100.00%of the dissolved content of Pb?average of 76.61%?.?3?The three components of Fulvic acid,FA,Humic acid,HA,Hydrophilic Acids,Hy,and Hydrophobic Neutral Organic Matter?HON?were obtained by batch separation of soil dissolved organic matter?DOM?.The recovery rate was 86.00%to 125.50%?average of103.50%?.The DOM is mainly composed of Hy and FA,with the average of 71.23%and24.10%of the total DOM.The UV-Visible Spectroscopy?UV-VIS?absorption spectrum of DOM shows that the absorption peak height is related to the content of DOM in soil solution.The UV-VIS absorption spectra characteristic values of SUVA₂₅₄,SUVA₂₆₀,SUVA₂₈₀ and E₂/E₃ can characterize the aromaticity of DOM,the size of hydrophobic components,and the apparent molecular weight of paddy soils and rice grains around the three industrial and mining areas.The three Dimensional Excitation-Emission?3D-EEM?Spectroscopy absorption spectra of DOM of characteristic values of BIX and HIX can be used to characterize the humic component origin,the relative contribution rate of autogenous sources,and the degree of humification in the DOM soil around the three industrial and mining areas.The characteristics of humic substances in the DOM of rice soil solution around the three industrial and mining areas are more important and strong.However,the newly self-generated source components are weak and have important terrestrial source contributions.?4?The DOM activity was corrected by the DOM default activity?82.50%?,FA measurements,SUVA₂₅₄,SUVA₂₆₀,SUVA₂₈₀,E₂/E₃,Slope_280³50,FI,BIX,HIX,and other spectral parameters.Multi-surface model calculation results show:after using SUVA₂₅₄,SUVA₂₆₀ and HIX to correct DOM activity,the model fits well.Among them,after using HIX corrected DOM activity,the modeled values of free ions'contents of Cd,Pb were in good agreement with the measured values(RMSE_Cd=0.16 log units;RMSE_Pb=0.74 log units).After adjusting the DOM activity with HIX parameters,assuming that ions of Fe,Al are controlled by ferrihydrite and amorphous aluminum hydroxide in the soil solution.The free ions activities of Cd,Pb obtained the best fitting results(RMSE_Cd=0.12 log units;RMSE_Pb=0.33 log units).This indicates that the competitive adsorption of iron and aluminum,with the adsorption of oxides and hydroxides have an important effect on the activity of Cd and Pb ions in soil solution.