The Release Kinetics Of Heavy Metals From Field-contaminated Soils

The release behavior of four heavy metals?Cd,Cu,Pb,Zn?from field-contaminated soils was studied with stirred-flow system.The WHAM-based kinetic model for metal adsorption and desorption developed in this study reproduced the experimental results in stirred-flow experiments,which demonstrate the capacity of the kinetic model to quantitatively describe the metal adsorption/desorption to soils.The primary contents and results of this research are shown as follows.1.Selected properties of the soils were analyzed and four different extraction methods were used to determine the heavy metal concentration in soils.All of the five field-contaminated soils were acid soil with relative high SOC concentration and clay fraction in the soils is small.As a result of the diverse heavy metal species in soils,the heavy metal contents varied significantly with different extraction methods.The reactive metal concentrations was determined with 0.43 M HNO₃ extraction in this study,according to the WHAM modeling to0.01 M CaCl₂ extraction.2.According to the soil properties and reactive metal contents,the initial metal distribution among different soil binding sites was calculated with WHAM before stirred-flow experiments.The calculation showed SOM is the dominant adsorbent for Cu,Zn,Cd and iron?hydr?oxides can effectively compete with SOM for Pb adsorption.In SOM,bidentate sites were foremost for metal binding on account of higher site density and binding strength.3.In this research,the metal release process were studied with stirred-flow experiments and the solution pH,DOM concentration and heavy metal concentrtion in effulents were measured to explore the variational tendency of them.During the reaction process,the pH rise fast at the beginning stage and tend to stabilization in subsequent reactions.The DOM concentration rise and declined rapidly during the beginning stage and then stayed around very low concentration levels.The heavy metal concentration in effluents declined over the course of the reaction and the rate of decline varied between soils and metals.4.The WHAM-based kinetic model was used to describe the metal adsorption/desorption behavior in stirred-flow experiments and the adsorption/desorption rate coefficients of all binding sites were obtained through model mathematic fitting.The binding sites in both solid phase and DOM were considered in the kinetic model.The effect of reaction conditions on the rate of reaction was integrated into the variation of adsorption rate coefficient.According to the model fitting results,the heavy metal release was controlled by solid phase binding sites and the influence of DOM on macroscopic reaction rate is negligible.For a specific metal,the desorption rate coefficient varied by a few orders of magnitude among different SOM binding site.The desorption rate coefficient of the same binding site also changed for different heavy metals.The adsorption rate coefficient of each binding site increased with increasing pH and decreasing metal concentration on the site.For all the solid phase binding sites,the adsorption rate coefficient of Cu and Pb was larger than that of Zn and Cd,which explained the lower macroscopic release rate of Cu and Pb in the stirred-flow experiments.5.The roles of SOM,Fe/Al?hydr?oxides in controlling the metal release was analyzed according to the model calculation.It has been observed that during the reaction process,SOM is the dominant adsorbent for Cu,Zn,Cd,and iron?hydr?oxides can compete significantly with SOM for Pb binding.In SOM,the bidentate sites are most important to control the reaction rate of metal adsorption and desorption to soils.The results showed the consistence of metal adsorption/desorption kinetics with metal binding equilibrium.