Characteristics and Mechanisms of Cadmium Adsorption Kinetics in Arable Soil Under the Influence of Electric Field

Objective This study investigated the adsorption kinetic characteristics of heavy metal cadmium (Cd) adsorption by soils from different types of cultivated land based on soil electrochemical theory.

Method Typical arable topsoil samples from permanently charged soils (Black soil, Yellow brown soil, Anthropogenic alluvial soil) and variably charged soils (Latosolic red soil, Red soil) were collected as the research object. Ion adsorption kinetic experiments were conducted using miscible displacement technology. The adsorption experimental data were fitted using an adsorption kinetic model that takes into account the role of the electric field, to characterize the number of adsorption reaction levels, the equilibrium adsorption amount and other kinetic parameters of Cd^{2 +} , and to calculate the ion- soil surface interaction energy and diffusion activation energy.

Result Except for the lowest ion concentration (2.67 × 10⁻² mmol L⁻¹), Cd^{2 +} showed zero-order and first-order adsorption kinetic on the black soil surface, the rest of the treatments showed first-order adsorption kinetic of Cd^{2 +} on the surfaces of the five soils. The equilibrium adsorption of Cd^{2 +} on the surfaces of the five soils followed the order of Black soil > Yellow brown soil > Anthropogenic alluvial soil > Latosolic red soil > Red soil, and the trends of the ion-soil surface interaction energies roughly followed the same order, while the changes in the diffusion activation energies showed the opposite order. The adsorption kinetics of Cd²⁺ on the surfaces of five different agricultural soils exhibit differences. The adsorption behavior of Cd^{2 +} in constant charge soils was jointly influenced by clay particles, organic carbon and calcium carbonate content, while pH dominates the adsorption behaviour of Cd^{2 +} by the variable charge soils. The further findings showed that the surface charge number and the surface electric field intensity of the permanently/variably charged soils were important factors influencing their adsorption of Cd^{2 +} . As the surface charge number and the surface electric field intensity of the soils were elevated, the interaction between Cd^{2 +} and the soil surface was intensified and the required diffusion activation energy was lowered, leading to an increase in Cd^{2 +} adsorption.

Conclusion The results elucidate the adsorption mechanism of Cd²⁺ on charged soil surfaces from an electrochemical perspective, and may provide new insights into the modelling and prediction of the environmental behaviour of heavy metals in agricultural soils.