Gibberellic acid surface complexation on ferrihydrite at different pH values: Outer-sphere complexes versus inner-sphere complexes

Abstract

Gibberellic acid (GA₃) is a widely used plant growth regulator and environmental toxin especially in China, but no study has focused on the mechanism of the interactions between GA₃ and minerals/soils. In this study, the GA₃ surface complexation mechanism on ferrihydrite (Fh) was investigated by combining sorption-desorption batch experiments with Fourier transform infrared (FTIR) spectroscopy and moving-window two-dimensional (MW2D) correlation spectroscopy. The results showed that the Fh-GA₃ surface complexes and retention after desorption depended strongly on the pH. For pH > 2.9, electrostatic interactions played an important role in GA₃ sorption on Fh in two ways. One was directly forming an outer-sphere complex by electrostatic attraction to a minor extent. The other was acting as a driving force to facilitate the formations of surface hydration-shared ion pair (mainly formed at pH < 5.7) and solvent-surface hydration-separated ion pair (mainly formed at pH > 5.7). Those three outer-sphere complexes were partially reversible according to the high total desorption percentage of GA₃ (69-80%). For pH ≤ 2.9, the generated monodentate complex was observed and increased with decreasing pH, which showed more retention on Fh after desorption than the outer-sphere complexes according to the lower total desorption percentage of GA₃ (37%). At the typical soil and groundwater pH values (4.5-9), the outer-sphere complexes predominate, where GA₃ could be out-competed by nitrate and other anions and then easily desorbed from Fh. This increases the risk of groundwater contamination.

Keywords: Ferrihydrite; Gibberellic acid; Retention after desorption; Surface complexation; pH.