Transformation kinetics and mechanism of gibberellic acid with ferrihydrite: Building a novel adsorption-transformation multi-step kinetic model

Gibberellic acid (GA₃), a widely used phytohormone, is easily transformed into more toxic products. The soil and groundwater environment are an important sink for GA₃, but its transformation catalyzed by soil minerals has not been studied. In this study, the transformation kinetics and mechanism of GA₃ with ferrihydrite (Fh) were examined through kinetic batch experiments, microscopic-spectroscopic investigation and mathematical modeling. The results showed that rapid adsorption of GA₃ on Fh occurred in the first 4 h, followed by a catalytic pseudo-first-order transformation of the parent compound and products generation (4 h–30 d). Fh predominantly enhanced the transformation of GA₃ into Iso-GA₃ which was further hydrolyzed into OH-GA₃, in which adsorption was a prerequisite for transformation. The catalytic transformation likely resulted from the surface hydroxy of Fh, which not only stabilized the transformation intermediates by forming surface complexes with the carboxyl group of GA₃ and its products, but also served as a powerful nucleophile to attack the γ-lactone of GA₃ and Iso-GA₃. Based on the catalytic isomerization and hydrolysis mechanism of GA₃ with Fh, a novel adsorption-transformation multi-step kinetic conceptual model and mathematical model were developed. This model fitted the measured data well (R² > 0.97) and the fitted parameters suggested that the transformation rate constants of the transformation of GA₃ into Iso-GA₃ and the transformation of Iso-GA₃ into OH-GA₃ were facilitated with Fh by ∼26 and ∼9 times, respectively. The multi-step kinetic model has great potential in simulating GA₃ fate in soil and groundwater to assess its environmental health risk.