| The existence form of heavy metals in the soil can change with soil changes in the environment. The soluble state of heavy metal concentration in animal and plant body may produce toxic in the body of plants and animals and will threat to human health through the food chain transfer into the human body, which has attracted popular attention. All kinds of salt ion and colloid exist widely in soil. Redox reaction is one of the basic soil chemical processes, thus discusses the influence degree and the process of heavy metal behavior is important to theory and practical significance.In this paper, we use two kinds of heavy metal (cadmium and zinc) and colloidal silica as the research object in the laboratory, and change the soil initial redox potentials(Eh)by adding the oxidant (ferric nitrate) and reducing agent (L-ascorbic acid) artificially. First through the static condition, studied the adsorption law of Cd and Zn in the siol in the conditions of different initial Eh of soil, ionic strength and colloidal exist or not with a single and common existence conditions.And compared the influence of different redox potentials, ionic strength, colloid and other cationic to heavy metal adsorption. Later fitting the experimental data using Origin8.0software to obtain the adsorption parameters; Then conduct the flowing experiments of Cd and Zn under the conditions of different oxidation reduction potential (respectively as509,258,-125mV), ionic strength (the concentration of NaNO3are0.002,0.01,0.03mol/L),with or without colloid by soil columns. Explore the migration laws of Cd and Zn in difference conditions. At last, simulate the breakthrough curves of Cd and Zn using HYDRUS-1D software to get the transport parameters. We can draw the following conditions:The static isothermal adsorption experiment show that, no matter whether there is colloid, the adsorption quantity of Cd and Zn in three different initial redox potential soil follow the following order:the original brown soil> reducing soil> oxidizing soil. The effect of ionic strength to the adsorption of Cd and Zn in the soil is complex, especially with colloid. Colloid can inhibit the adsorption of Cd and Zn on the soi. Under the coexist conditions the adsorption of volume of three different initial redox potential soil to Cd and Zn is more than single system.Use Langmuir and Freundlich equation to fitting the static data and the correlation coefficient R2is higher than0.95. The BTCs of heavy metal is different from Br-. The BTCs of Cd and Zn in the soil present asymmetry and obvious “bad-tail” phenomenon. This suggests that the migration of Cd and Zn in the soil have chemical non-equilibrium phenomenon.Regardless with or without colloid, the transport of Cd and Zn in different initial redox potential soil present the following order:The migration of Cd in oxidizing and reducing soil are greater than in the original brown, and performance as the flow time of Cd is ahead and flow peak of Cd increased in the oxidizing and reducing soil; In the oxidizing soil the transference of Zn is greater than the original brown soil, and in reducing the soil the transference of Zn is slightly less than in the original of the brown soil. The high and low order of pH of the liquid flow through three different initial redox potential soils is opposite to Eh.Regardless of whether there have colloid or no, the mobility of Cd in three different initial redox potentials soil is larger with the increase of ionic strength. And this relate to the competitive adsorption of Na+. Without colloid, the transference of Zn in three different initial redox potential soils becomes lager along with the increase of the ionic strength. And when there have colloid, ionic strength changes have no significant effect on the transport of Zn. This is closely related to the influence ofionic strength on colloid.When colloid exist, the out flow spikes time of Cd and Zn will be in advance, and the flow peak becomes larger, so the colloid can promote the movement of Cd and Zn, especially to Cd. The probably reason is that colloid affinity for Cd than for Zn. With the presence of colloid, the pH of flow is higher than without colloid, and Eh and Ec is lower than without colloid.Under the condition of no colloid, regardless of single system or coexistence of Cd and Zn, the mobility of Zn is larger than Cd.Under the condition of colloid presence, the mobility of Zn is larger than Cd in single system. But the migration of Zn is less than Cd in the xoexist system.Single-point adorption model can better describe the transport of Cd and Zn in soils under the influence of ionic strength and oxidation reduction potential. The performance is r2generally greater than0.75and MSE is less than0.05. The fitted value of is generally small, and mass transfer coefficient is much less than100. This indicat that there has strong heterogeneity on soil-solid interface and non-equilibruim play a leading role in the migration process of Cd and Zn, wich caused by adsorption point difference. When there is no colloid,r of fitted BTCs are greater than0.76except indibidual cases; MSE are less than0.042; fitted effect is better than with colloid. Fitting peak is generally lower than the measured values with colloid. It mainly due to the single-point adorption model does not take into the role of colloids to the migration of heavy metals. The performance is r2generally greater than0.75and MSE less than0.05. |
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