Selenite Adsorption and Desorption Characteristics in Soils: Effects of Soil Amendments and Underlying Mechanisms

Understanding the adsorption and desorption characteristics of selenium (Se) in soil is crucial for predicting the availability of exogenous Se. Biochar (BC) and sheep manure (SM) are widely used as soil amendments, but their effects on Se adsorption and desorption remain incompletely understood. This study investigated the effects of cotton straw BC and SM on the adsorption-desorption behavior and mechanisms of selenite in gray desert soil and irrigation-silted soil using batch equilibrium experiments, scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The results indicated that BC and SM addition reduced Se adsorption and increased its desorption rate in both soils, with the highest desorption rate (89%) observed in irrigation-silted soil amended with 5% BC. The Se adsorption kinetics followed a pseudo-second-order model, and the adsorption isotherms were well described by the Freundlich model. Characterization results revealed that organic compounds (e.g., aromatic compounds, carboxyl compounds, and ethers) in BC and SM influenced the adsorption process. Additionally, phosphate ions released from BC in gray desert soil and carbonate ions released from BC in irrigation-silted soil both competed with Se for adsorption sites. This study systematically elucidates the effects of BC and SM on selenite adsorption-desorption behavior in soils, providing a theoretical basis for understanding Se biogeochemical behavior and supporting the safe production of Se-enriched crops.