| Arsenic（As）is a highly toxic and carcinogenic element,which is extremely harmful to the ecological environment and human health.Geogenic high-arsenic groundwater is widely distributed in arid regions of northern China,which is widely used for agricultural irrigation.In northern China,due to the fact that there is little precipitation,large evaporation,low content of organic matter,high alkalinity and secondary salinization in the soil,the migration and transformation of As in the soil may be different from other regions.In this study,the cropland soil in Kuitun Reclamation Area,which was knows as the first endemic arsenism area in Chinese mainland,was taken as the research object.In view of the environmental background of strong evaporation in arid region and the characteristics of saline-alkali cropland soil,the direct positioning of As was realized through kinetic and thermodynamic experiments of adsorption and desorption,the simulation experiments of inserting ion exchange membrane into soil under different conditions（strong evaporation,p H and salinity）,as well as measuring by energy spectrometer.In addition,the vertical and level of migration processes of As in cropland saline-alkali soil were analyzed,and the characteristics of adsorption,desorption,migration and transformation of As in saline-alkali soil were studied.The morphological changes and influencing factors of As in saline-alkali soil were expounded,which provided theoretical and practical guidance for comprehensively revealing the migration and transformation process and mechanism of As in saline-alkali soil in arid areas,correctly preventing and controlling soil As pollution,with the main results as follows:（1）At 35℃,the adsorption capacity of 10 mg/L As（Ⅴ）in Kuitun cropland soil was 137.75 mg/kg,and the adsorption rate was 68.88%,both of which were higher than that at 15℃and 25℃.The desorption amount was 21.51 mg/kg,and the desorption rate was 15.64%,which were lower than those at 15℃and25℃.The the process of adsorption and desorption of As（Ⅴ）in Kuitun cropland soil is able to be fitted by pseudo-second-order kinetic and intraparticle diffusion model.The adsorption and desorption of 10 mg/L As（Ⅴ）in soil were divided into rapid membrane diffusion stage（5～60 min）,intraparticle diffusion stage（60～480 min）and adsorption equilibrium stage（>480 min）.In the process of adsorption of As（Ⅴ）by soil,monolayer and multilayer adsorption occurred simultaneously,in which the monolayer adsorption was dominant,and the adsorption was easy to proceed（Freundlich equation parameter n>2）.Physical and chemical adsorption were involved in the process,which was more influenced by chemical adsorption.The adsorption and desorption of As（Ⅴ）in the soil was mainly complexed with five functional groups including-OH,N-H,C=O,C-H and-SH,and As（Ⅴ）was adsorbed on organic matter,various metal oxides/hydroxides and clay minerals containing elements such as C,Si,K,Ca,Na,Mg,Al and Fe.（2）After 10 times of irrigation with 10 mg/L As（V）solution under different treatments,the migration of As in the vertical direction mainly occurred within 10 cm in 21 days,and the horizontal of migration mainly occurred within 5 cm.Under the condition of strong evaporation,the vertical and horizontal migration of As in soil were less than that in indoor.Compared with P1（p H=7.5）and P2（p H=8.5）treatment groups,the vertical migration distance of As in P3（p H=9.5）treatment group increased by 5 cm.Compared with T1（TDS=0.5 g/L）and T2（TDS=1.5 g/L）treatment groups,T3（TDS=4.5 g/L）treatment group moved 1 cm more horizontally and 6 cm more vertically.With the decrease of Eh as well as the increase of p H and salinity,As in soil solution was easier to migrate.（3）In the soil environment with 75%field water capacity,As（Ⅴ）in the soil solution of outdoor and indoor treatment groups did not transform into As（Ⅲ）.Under different treatments,the sequence of As in Kuitun cropland soil was O-As>Ca-As>Al-As>Fe-As>A-As>H2O-As.There was no significant difference in As speciation content in soil under different evaporation conditions.The available As and insoluble As in soil of P3（p H=9.5）treatment group were significantly higher than those of P1（p H=7.5）and P2（p H=8.5）treatment groups.The available As and insoluble As in T3（TDS=4.5 g/L）treatment group were significantly higher than T1（TDS=0.5 g/L）and T2（TDS=1.5 g/L）treatment groups.The p H was the most important factor affecting the variation of soil available As after irrigation under different conditions,with its contribution rate reaching 66.3%. |
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