Mechanism of Sorption and Transformation of Gibberellic Acid on Ferrihydrite

Author:Zhang Li

Supervisor:liu fei chen liang


Degree Year:2019





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-OH21/2+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.