Different Initial Redox Potential Of Soil Heavy Metals Cd / Zn / Cu Migration Experiments And Numerical Simulation

Soil chemical environment is one of the important environmental factors in a human life, which affects plant growth and development, to a large extent restricts the quality of food. However, Soil chemical environment is vulnerable to heavy metal contamination of materials. The heavy metals which are introduced into the soils either accumulate in the surface layer or transport into the groundwater. Redox reaction is a basic soil chemical and biochemical processes, which changs the valence state of ions in soil affected by the presence of inorganic and organic forms, migration and transformation. Redox potential plays an important role on various chemical, biological processes of the soil. Therefore the study of transport processes is of utmost importance for the evaluation of environmental risks.First of all, the present study evaluates the adsorption and competitive adsorption of three heavy metals (Cd, Zn, Cu) in five different initial redox potential of soils by means of sorption isotherms, evaluates the capacity of these soils to sorb these metals. And then, we are to evaluate the transport and competitive transport of the metals (Cd, Zn, Cu) in these soils based on miscible displacement experiments and HYDRUS-1D package is used to determine model parameters and predict the outflow dynamics. The conclusions have been drawn as followed:In the static adsorption isotherm experiment, five different initial redox potential of soils have significant differences of adsorption capacity of a single heavy metal, reduced soil> brown> oxidized soil. The adsorption amounts of heavy metals in soils are increased with its increasing equilibrium concentration. The adsorption amounts of two ions in the competitive experiment are less than that in single system. Coexistence of three heavy metal ions, the sequence of the competitiveness of three heavy metals is Cu2+>Cd2+>Zn2+. With its increasing equilibrium concentration, pH is decreased, Eh is increased and Ec is increased. In this paper, the Freundlich equation is more accurately than the Langmuir equqtion to describe adsorption behavior of three heavy metals.Compared with the BTCs of Br", the five different initial redox potential of soils can obviously influence the shape and peak values of the BTCs of heavy metals. The BTCs of heavy metals from the transport experiment are exceedingly asymmetric.In the five different initial redox potential of soils, compared with the BTCs of heavy metals, it was found that the initial redox potential were positively correlated with the transport of heavy metals. The change of the leachate pH is degressive, then leveling off. The change of the leachate Eh is increased, then leveling off.In the competitive experiment of Cd and Zn, the resident concentration of heavy metals was higher than that in the single system. The total adsorption of heavy metals (the amount of the Cd and Zn competitive adsorption) than a single condition in the adsorption is significantly lower. The change of the leachate pH in the competitive experiment is lower than single ion and the Eh is the higher, the Ec is the lower.Compared with the competitive experiment of Cd and Cu, Zn and Cu, in the single-ion experiments, it was found that the flow time of copper is higher than that of cadmium/zinc, the peak value of copper is higher than that of cadmium/zinc in the oxidized soil, the peak value of the reduced soil is lower than the oxidized soil. In the competitive experiment, the resident concentration of heavy metals was higher than that in the single system. The change of the leachate pH in the competitive experiment is lower than single ion and the Eh is the higher, the Ec is the lower.In the competitive experiment, the sequence of the capacity of the three heavy metals adsorption by the soils was Zn>Cd>Cu. It can be explained by the hydrolysis capacity, atomic weight, the hydrolysis constant and coefficient of soft.The BTCs of Br- can all be fitted by One-site sorption model with high goodness of fit(r2>0.98, MSE<0.006). When we use the retardation factor to fit the BTCs, we find the retardation factors from Freundlich are suitable in most cases, which can understand the initial redox potential of heavy metals in soils of different adsorption characteristics.The BTCs of heavy metals in soil column can all be fitted by One-site sorption model with high goodness of fit (r2>0.814, MSE<0.028). Through these model parameters acquired by the One-site sorption model, we can predict the BTCs of heavy metals at different depth of soil columns. As the depth of soil column increased from 5cm to 15cm, the hysterestic effect of response of relative concentration of heavy metals to the pore volumes will be more and more obvious.