Impact and its mechanism of alkaline functional fertilizer on cadmium activity in soil.

Purpose: Irrational application of chemical fertilizers aggravated soil acidification in China, which resulted in increased cadmium (Cd) activity in soil. The aim of the study was to reduce the Cd activity in soil by applying alkaline functional fertilizer (AFF), which was prepared from NPK compound, dolomite and biochar. Methods: Incubation trials, Cd adsorption and desorption experiments, and kinetics and thermodynamics tests were applied to reveal the function and mechanism of two AFFs (DAFF and DBCAFF) in reducing Cd activity in soil. Results: The results showed that the AFF could significantly decrease Cd activity in soil by increasing the Cd adsorption capacity and reducing its desorption rate. Cd adsorption characteristics fitted the Langmuir and pseudo-second-order kinetic model, which indicated that Cd adsorption was a single-surface chemical adsorption including fast and slow processes. The thermodynamics results showed that Cd adsorption was a spontaneous endothermic reaction and was promoted by the AFF. Dolomite and biochar in the AFF could increase the Cd²⁺ adsorption sites of soil particles and the binding energy between soil and Cd²⁺, thus increasing Cd²⁺ adsorption and preventing the desorption of adsorbed Cd²⁺. The mechanism of the AFF reducing Cd activity in soil was that the AFF could promote Cd²⁺ electrostatic adsorption, ion exchange of Cd²⁺ with Ca²⁺ and Mg²⁺, formation of a complex between Cd²⁺ and oxygen-containing functional groups (OFGs) and precipitation between Cd²⁺ and OH^-, CO₃^2- and PO₄^3-. Conclusions: This study provided an effective and simple innovative method to solve soil Cd pollution by replacing conventional fertilizer with the AFF. [ABSTRACT FROM AUTHOR]