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

摘要

Gibberellic acid (GA(3)) 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 GA(3) and minerals/soils. In this study, the GA(3) 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-GA(3) surface complexes and retention after desorption depended strongly on the pH. For pH 2.9, electrostatic interactions played an important role in GA(3) 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 GA(3) (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 GA(3) (37%). At the typical soil and groundwater pH values (4.5-9), the outer-sphere complexes predominate, where GA(3) could be out-competed by nitrate and other anions and then easily desorbed from Fh. This increases the risk of groundwater contamination. (C) 2018 Elsevier B.V. All rights reserved.