Surface Distribution Of Arsenic In The Multi-mineral Unsaturated Zone

The irrigation-induced increase in arsenic(As)content of the sediments in the unsaturated zone(UZ-sediments)has been observed in many high-As groundwater distribution areas,forming a potential human As exposure risk,and has attracted widespread attention in the academic community.Previous studies have shown that the adsorption behavior of As in UZ-sediments varied in the aspect of adsorption capacity,thermodynamics,and competitive adsorption,which may be caused by the complicated mineralogical properties of the UZ-sediment.However,there is a lack of understanding of the surface distribution behavior of As in the unsaturated zone.Considering these knowledge gaps,the present study took the Datong Basin as the study area and clarified the impact of irrigation on the distribution of As in the UZ-sediments through investigation.Based on the identification and quantification of the characteristic minerals for As adsorption in the UZ-sediments,the adsorption behavior of As in the unsaturated zone is quantitatively described through the multi-mineral surface complex model(SCM)and the reactive transport model(RTM).At last,the behavior and mechanism of As adsorption onto illite were studied.The knowledge gained in the thesis mainly includes the following points:(1)The As content in the UZ-sediments of the Datong Basin is between 4.00 and14.5 mg/kg.The content of As _Si,As _Feox2,As _Feox1,As _Car as well as the total As are mainly controlled by the provenance and weathering of the UZ-sediment hence they show a consistent spatial distribution characteristic of high north and low south.While As _Dissand As _Sorb are strongly affected by irrigation practices,causing a spatial distribution characteristic of high basin center and low piedmont.(2)The adsorption of As onto the UZ-sediment mainly controlled by the iron(Fe)oxides,carbonate minerals,phyllosilicate minerals and soil organic matters(SOM).The linear programming-based calculations show that seven minerals in the soil can adsorb As,which include Fe _Car,Fe_ox1,Fe_ox2,illite,calcite,dolomite,and SOM,of which calcite can only adsorb As(V).The average contribution of Fe-oxides,illite,carbonate minerals,and SOM to the As(III)adsorption capacity of the UZ-sediments is 73.87%,11.37%,and 8.21%,and 6.55%respectively.For the contribution of As(V)adsorption capacity,it is 74.73%,4.27%,18.76%and 2.24%respectively.The multi-mineral surface complexation model established based on the above quantitative results can well predict the adsorption behavior of As in the collected UZ-sediment samples.The prediction results also show that the contribution of non-Fe minerals is more substantial at low As concentration conditions or under strongly acidic/alkaline conditions.(3)In natural conditions,the UZ-sediments show strong retardation to As in the irrigation water.Almost all the exogenous As is adsorbed in the 0-10cm unsaturated zone after 240h of flooding,causing a 7 mg/g increase in As content of UZ-sediments.Results of the RTM show that the contribution of characteristic minerals to As adsorption in 0-10 cm unsaturated zone is carbonates>Fe-oxides>illite>SOM,and below 10 cm are illite>carbonates>Fe-oxides>SOM.(4)The adsorption of As on illite involving multiple mechanisms.The adsorption mechanism includes:(1)As-containing molecules adsorb on the basal surface((001)crystal plane)of illite through electrostatic force by using cations as bridges;(2)As-containing molecules adsorb on the edge surface(mainly(010)crystal plane)of illite by the formation of the As-O-Al bond,forming the monodentate mononuclear and bidentate mononuclear inner-sphere complexation.The former caused the ion-specific effect of the adsorption as well as the behavior of incomplete desorption,while the latter led to a decrease in the Zeta potential of illite after As adsorption.In summary,the migration and enrichment behavior of As in the unsaturated zone is largely controlled by the adsorption process,while the adsorption process is not dominated by a specific mineral(such as Fe-oxides).The quantification results show that the contribution of illite,calcite,dolomite,and SOM to As adsorption cannot be ignored,just like Fe-oxides.This suggests that these non-Fe minerals should be also considered when As-relevant studies are carried out since they have significant impacts on the environmental fate of As but are often overlooked in the previous studies.