Mechanism Of Sorption And Transformation Of Gibberellic Acid On Ferrihydrite

Gibberellic acid?GA₃?is the most widely used plant growth regulator in the world at present.Because its molecular structure is similar to steroids,GA₃ as an endocrine disrupter could affect the human endocrine system.GA₃ is typically applied by foliar spraying,some of which would enter the shallow soil of the vadose zone.After the atmospheric precipitation or intermittent irrigation,it would migrate to the deep soil and groundwater,threatening the groundwater quality.However,few studies have focused on the fate of GA₃ in vadose zone and aquifer media.Ferrihydrite?Fh?widely occurs in soil and groundwater,and always plays a substantial role in the migration and transformation of contaminants in vadose zone and aquifer.In this study,the interaction process and mechanism of sorption and transformation of GA₃ on Fh were investigated.The main conclusions obtained are as follows:?1?The results demonstrated special sorption characteristic of GA₃ on Fh with high sorption capacity?65.01 mg/g?,but goethite,hematite,montmorillonite and kaolinite all hardly absorbed GA₃.The sorption of GA₃ on Fh showed a strong dependence on pH.For 2.9<pH?8.0,the surface hydration-shared ion pair,the solvent-surface hydration-separated ion pair and the outer-sphere complex which interacted electrostatically with Fh predominated.These three outer-sphere complexes are partly reversible,with a high total desorption percentage?69–80%?.For 2.4?pH?2.9,the monodentate complex was dominant and showed more retention after desorption than the outer-sphere complexes with a low total desorption percentage?37%?.?2?The attenuation of GA₃ in aqueous solution followed the pseudo-first-order kinetic model at pH=2–8 and the attenuation half-lifes ranged from 16.1 to 24.6 days.Isomerized gibberellic acid?Iso-GA₃?and gibberellenic acid?GEA?were the main transformation products.Groundwater from China University of Geosciences?Beijing?matrix has no effect on GA₃ attenuation process.After reaching the attenuation equilibrium at pH=8,the mass balance of GA₃ and its products was 76.2%,including Iso-GA₃?58%?,GEA?7.9%?,GA₃?7.3%?and hydroxy gibberellic acid?OH-GA₃??3%?.Toxicological predictions showed that the toxicity of GA₃'s products was stronger than that of GA₃.?3?During the interaction kinetics between GA₃ and Fh,sorption was dominant in the first 4 h,while transformation was dominant after 4 h.The outer-sphere complexes formed by complexation of GA₃ with?Fe-OH₂^1/2+on Fh were the precursors of transformation products.Fh mainly catalyzed the transformation of GA₃ into Iso-GA₃,followed by Iso-GA₃ transforming into OH-GA₃.As a result,the transformation rate of GA₃ on Fh increased by⁹ times compared to that in aqueous solution?pH=4.9?.Based on the measured concentration data of GA₃ and its products in solid and liquid phases,the conceptual model and mathematical model of multi-step reaction kinetics were established.Therefore,the sorption and transformation process of GA₃ on Fh were described in detail.