Gibberellic acid (GA3) is the most widely used plant growth regulator in the world at present, which has significant effect on the human endocrine system. After the atmospheric precipitation or intermittent irrigation, GA3 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. According to the results, the attenuation of GA3 in aqueous solution followed the pseudo-first-order kinetic model at pH=2-8 and the attenuation half-lives ranged from 16.1 to 24.6 days. Isomerized gibberellic acid (Iso-GA3) and gibberellenic acid (GEA) were the main transformation products. 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. During the interaction 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 by ~9 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.

Mechanism of Sorption and Transformation of Gibberellic acid on Ferrihydrite

ACS Fall 2021

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