Adsorption And Desorption Reactions Of As And Cu With Soil Components And A Quantitative Model

In recent decades,China’s economy have been developing rapidly,and the people’s material and spiritual life is getting richer.At the same time,however,many environmental problems have occurred,among which soil heavy metal pollution is one of the environmental problems that are widely concerned around the world.Organic matter,metal oxides,and clay minerals in the soil are natural adsorbents that can effectively control the migration and transformation of heavy metals in the soil.There has been a lot of research on the adsorption equilibrium,kinetics,and complexation form of heavy metals with a certain component of the soil（e.g.,iron oxides,manganese oxides,and organic matter）.However,studies about heavy metals in multiple systems（e.g.,mixed minerals,contaminated soil particles）are scarce.This paper investigated the kinetics of adsorption and desorption of typical heavy metal（loid）s in two composite systems,and established a quantitative kinetic model to illustrate the heterogeneity of heavy metal（loid）adsorption and desorption on different adsorbents,complexation sites,and particle sizes.First,kinetics of As（V）adsorption and desorption with ferrihydrite andδ-Mn O₂ mixed minerals was studied using the stirred-flow method.A kinetics model for the mixed mineral system was developed using the component additivity approach,which incorporated the nonlinear binding of As（V）with both ferrihydrite andδ-Mn O₂ simultaneously.The kinetics model successfully predicted the kinetics of As（V）adsorption and desorption in the mixed mineral system under varying solution chemistry conditions and properly accounted for the heterogeneity of the binding sites of both ferrihydrite andδ-Mn O₂.For As（V）adsorption kinetics,the ferrihydrite bidentate non-protonated sites（Fh-bi-np）played a dominant role,followed by theδ-Mn O₂ binding sites and the ferrihydrite bidentate protonated sites（Fh-bi-p）.After the 4-h desorption,As（V）was mainly retained on ferrihydrite binding sites.Second,kinetic experiments were conducted to investigate kinetics of copper（Cu）release from different size fractions of two field-contaminated soils.The morphology and elemental distribution of different size fractions of soil particles were measured with scanning electron microscopy（SEM）and spherical aberration corrected scanning transmission electron microscopy（Cs-STEM）.A kinetics model considering both variations of soil adsorbents and particle sizes was developed.Overall,different size fractions of soil particles showed highly heterogeneous physical and chemical properties.The reactive soil adsorbents（e.g.,soil organic matter（SOM））and Cu concentrations increased with the decrease of the particle size,and the rate of Cu release from soil particles also increased with the decrease of the particle size.The kinetics model described the experimental kinetics data from different size fractions of soil particles reasonably well.More significantly,the model was able to predict Cu release from the bulk soil samples based on Cu kinetic reactions with different size fractions of soil particles.SOM appeared to be the most dominant adsorbent for Cu under our experimental conditions,and,among different SOM binding sites,bidentate sites involving carboxylic and phenolic functional groups were the most significant.In general,external environmental conditions（e.g.,p H,heavy metal concentration）and the nature of the soil（e.g.,particle size,metal oxides,organic matter）will affect the adsorption and desorption behavior of heavy metal（loid）s.The complexation heterogeneity between heavy metal（loid）s and soil active components can be further understood by mechanical model.The results of this study help to understand the migration and transformation of heavy metal（loid）s in complex environments and lay the foundation for the application of the model to actual pollution control.