Effect of soil particle size on the kinetics of Cu release from field-contaminated soils: Experiments and a quantitative model

Predicting the kinetics of heavy metal release from the field-contaminated soils requires the consideration of the chemical and physical heterogeneity of soil composition, such as multiple soil adsorbents, heterogeneous binding sites, and varying soil particle sizes. In this study, we conducted kinetic experiments 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. Our results suggested that a unified modeling framework, which specifically considers the chemical heterogeneity of soil binding sites and variations of particle sizes, is promising for predicting kinetic reactions of metals with heterogeneous soil particles in the field.