| Groundwater Cr(Ⅵ)exceeding the standard is one of the main types of groundwater pollution in our country at present,which has a serious impact on human health and safety in living and producing areas relying on groundwater.The groundwater Cr(Ⅵ)is mainly caused by the discharge of Cr(Ⅵ)wastewater from surface industrial production or the accumulation of waste residues through leaching migration.The adsorption,transformation and desorption of Cr(Ⅵ)on the ground soil will affect the difficulty of Cr(Ⅵ)entering the groundwater.A large number of studies have shown that soil physicochemical properties and environmental factors affect the adsorption,conversion and desorption properties of Cr(Ⅵ).Therefore,the migration performance of Cr(Ⅵ)in different types of soil is also different.The study on the adsorption performance of Cr(Ⅵ)in soil in different areas is of great significance for identifying the potential impact of local surface pollutants containing Cr(Ⅵ)on groundwater Cr(Ⅵ).In this paper,the static adsorption and desorption properties of Cr(Ⅵ)on four typical farmland soils in Shaanxi Province,namely yellow spongy soil(HM),brown soil(HT),paddy soil(SD)and eolus soil(FS),are studied to determine the kinetic and thermodynamic characteristics of Cr(Ⅵ)on different soils.To understand the effects of initial concentration,p H,time,temperature,ionic strength and other factors on soil adsorption and desorption of Cr(Ⅵ).The purpose is to clarify the adsorption and desorption performance of Cr(Ⅵ)in four soils,and to analyze the potential sources of Cr(Ⅵ)in groundwater in different regions.The following results were obtained:(1)The adsorption of Cr(Ⅵ)on 4 kinds of soils can reach adsorption equilibrium at 8 h.Although the adsorption kinetics of Cr(Ⅵ)on 4 kinds of soils can be fitted by pseudo-first-order and pseudo-second-order kinetic models,there are some differences between the fitting results and the actual results.Both Langmuir model and Freundlich model can be used for the thermodynamic characteristics of Cr(Ⅵ)adsorption in the four soils,but the Langmuir model has a better phase relationship.In the four soils,the adsorption capacity of Cr(Ⅵ)on HT and SD is higher than that on HM and FS,and the maximum adsorption capacity of Cr(Ⅵ)on SD is up to36.26 mg/kg,while the maximum adsorption capacity of HM is only 4.202 mg/kg.(2)With the increase of p H,the adsorption capacity of Cr(Ⅵ)increased,and the p H was positively correlated with the adsorption of Cr(Ⅵ).When the temperature increases,the adsorption performance of Cr(Ⅵ)on the four soils also increases,and there is a positive correlation between temperature and soil adsorption of Cr(Ⅵ).After soil organic matter is removed,the adsorption capacity of soil for Cr(Ⅵ)is significantly decreased,indicating that organic matter is the main factor for soil adsorption of Cr(Ⅵ).By increasing the content of soil organic matter,the adsorption of Cr(Ⅵ)in soil can be increased,and the pollution of ground Cr(Ⅵ)on groundwater can be alleviated.Free iron oxide is also the key component of soil adsorption of Cr(Ⅵ),and its content directly affects the adsorption of Cr(Ⅵ)by soil.After the removal of free iron oxide,the adsorption performance of Cr(Ⅵ)by four soils is significantly decreased.(3)The desorption kinetics study showed that the desorption of Cr(Ⅵ)in 4 soils also reached equilibrium at 8 h.The desorption kinetic characteristics of Cr(Ⅵ)in four soils were consistent with double constant,parabola and pseudo-first-order model.In contrast to the adsorption properties,the desorption capacity of Cr(Ⅵ)in the four soils was compared,and the desorption capacity of Cr(Ⅵ)in HT and SD was lower than that in HM and FS.Compared with the soil treated with Cr(Ⅵ)solution with a concentration of 10 mg/L,the desorption rates of HM and FS after the soil treated with Cr(Ⅵ)solution with a concentration of 100 mg/L are1.2%-2.1% higher than those of HT and SD.(4)With the increase of Cr(Ⅵ)concentration,temperature and ionic strength,the Cr(Ⅵ)desorption rate of the four soils increases correspondingly,but these influencing factors will not change the order of Cr(Ⅵ)desorption rate of the four soils.Under the influence of different factors,the desorption rates of the four soils are still high in HM and FS,while low in HT and SD;Different from other factors,with the increase of p H,the desorption rate of Cr(Ⅵ)in 4 kinds of soil has decreased,mainly due to the increase of p H leading to the increase of soil surface electronegativity,Cr(Ⅵ)is easier to form precipitation on the soil surface,thus promoting the adsorption of Cr(Ⅵ)in soil and reducing the desorption rate of Cr(Ⅵ).In summary,the four typical soil types in Shaanxi Province,yellow spongy soil(HM),brown soil(HT),paddy soil(SD)and sandy soil(FS),all have adsorption and desorption properties for Cr(Ⅵ),in which HT and SD have larger adsorption capacity and adsorption capacity for Cr(Ⅵ),but smaller desorption capacity and desorption capacity for Cr(ⅵ).The results show that Cr(Ⅵ)pollutants are more likely to be adsorbed by soil when entering the brown soil and paddy soil,and are not easy to migrate into groundwater,while the yellow spongy soil and wind-sandy soil are on the contrary. |
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