Sorption specificity and desorption hysteresis of gibberellic acid on ferrihydrite compared to goethite, hematite, montmorillonite, and kaolinite

Formation of ferrihydrite induced by low pe+pH in paddy soil reduces Cd uptake by rice: Evidence from Cd isotope fractionation

Ferrihydrite (Fh) is an important iron mineral in paddy soil and is prone to phase transition during dynamic redox condition, which affects Cd distribution and induces Cd isotope fractionation across soil to rice. Here, we conducted rice culture experiments with or not Fh application under different irrigation regimes to study the relationship between Fe species and Cd availability, as well as the isotope ratio of Cd in different Cd pools in paddy soil-rice system. Fh addition under continuous flooding (FL) with the decrease of pe + pH from 9.36 to 3.44 promoted the formation of amorphous Fe oxides as increased by 120.1% and facilitated Cd immobilization along with the increase of Fe/Mn oxides bound Cd by 25.3%, compared with continuous drying (DY) treatment. The isotopically heavy Cd were preferentially enriched from soil to extractable Cd (Δ^114/110Cd_{extractable Cd-soil} = 0.39-0.62‰) and from soil to grain (Δ^114/110Cd_grain-soil = 0.40-0.66‰) particularly at low pe + pH and with Fh addition, while light Cd were enriched in Fe/Mn oxides (Δ^114/110Cd_{Fe/Mn oxides bound Cd-extractable Cd} = -0.65 ∼ -0.14‰). Besides, the expression of transporters involved in Cd transport in rice like OsNRAMP1, OsNRAMP1, OsHMA3, OsHMA2 and OsLCT1 were suppressed under low pe + pH condition. These findings indicated that low pe + pH facilitated Cd stabilization by the existence of more amorphous iron oxides, which induced the enrichment of heavy Cd isotope in liquid phase and light in Fe/Mn (oxy)hydroxides, respectively.

Transformation kinetics and mechanism of gibberellic acid with ferrihydrite: Building a novel adsorption-transformation multi-step kinetic model

Gibberellic acid (GA₃), a widely used phytohormone, is easily transformed into more toxic products. The soil and groundwater environment are an important sink for GA₃, but its transformation catalyzed by soil minerals has not been studied. In this study, the transformation kinetics and mechanism of GA₃ with ferrihydrite (Fh) were examined through kinetic batch experiments, microscopic-spectroscopic investigation and mathematical modeling. The results showed that rapid adsorption of GA₃ on Fh occurred in the first 4 h, followed by a catalytic pseudo-first-order transformation of the parent compound and products generation (4 h-30 d). Fh predominantly enhanced the transformation of GA₃ into Iso-GA₃ which was further hydrolyzed into OH-GA₃, in which adsorption was a prerequisite for transformation. The catalytic transformation likely resulted from the surface hydroxy of Fh, which not only stabilized the transformation intermediates by forming surface complexes with the carboxyl group of GA₃ and its products, but also served as a powerful nucleophile to attack the γ-lactone of GA₃ and Iso-GA₃. Based on the catalytic isomerization and hydrolysis mechanism of GA₃ with Fh, a novel adsorption-transformation multi-step kinetic conceptual model and mathematical model were developed. This model fitted the measured data well (R² > 0.97) and the fitted parameters suggested that the transformation rate constants of the transformation of GA₃ into Iso-GA₃ and the transformation of Iso-GA₃ into OH-GA₃ were facilitated with Fh by ∼26 and ∼9 times, respectively. The multi-step kinetic model has great potential in simulating GA₃ fate in soil and groundwater to assess its environmental health risk.

Role of Pond Sediments for Trapping Pesticides in an Agricultural Catchment (Auradé, SW France): Distribution and Controlling Factors

In agricultural areas, ponds are suitable wetland environments to dissipate and reduce the occurrence of pesticides in aquatic environments. However, their impact at a catchment scale is still poorly understood. This study aims to determine how these organic contaminants were trapped in a pond located in an agricultural critical zone from SW France (Auradé catchment). The spatial distribution of pesticide concentrations and their different controlling factors were investigated in waters and sediments collected during two distinct seasons. The results highlighted (i) the link between the presence of the molecules and the agricultural practices upstream, (ii) the influence of hydrological/seasonal conditions, especially on hydrophobic molecule accumulation such as tebuconazole, (iii) the key role of clay content in sediments on the control of moderately hydrophilic pesticides (metolachlor and boscalid), but also the unexpected role of coarse particles for boscalid; and (iv) the influence of sediment depth on pesticide storage. Nevertheless, other physico-chemical parameters, such as mineralogical composition of sediment, needed to be considered to explain the pesticide patterns. This study brings a new hypothesis to be investigated in the future about pesticide behaviour in such pond environments.

Different binding characteristics of ciprofloxacin to iron mineral surfaces: Thermodynamic evidence and site energy distribution analysis

Iron minerals in soil play an important role in controlling the migration of fluoroquinolones. In this study, batch experiments were carried out to investigate interactions in ciprofloxacin (CIP) adsorption to goethite, hematite, and magnetite at pH 6.0. Thermodynamics and the site energy distribution theory (SEDT) were adopted to clarify the complexation types. Using the adsorption results, pH-dependent interactions were qualitatively elucidated. The thermodynamic data revealed the difference in adsorption mechanisms. With increasing sorbate loading, CIP adsorption to hematite and magnetite was endothermic, and both enthalpy change and entropy change decreased; however, CIP sorption to goethite showed opposite characteristics. The higher adsorption capacity and affinity of CIP to hematite and magnetite than those to goethite were caused by their higher site energy of the highest occurring frequency (E₀ ^* ) and the temperature-dependent average site energy, respectively. The E₀ ^* on the surface of goethite was about 17-19 kJ mol^-1 , where E₀ ^* values of hematite and magnetite were 20-26 kJ mol^-1 . When temperature increased from 289.15 to 308.15 K, the high- and low-energy site densities for three iron minerals changed by -32 to 167% and by -36 to 223%, respectively. The different thermodynamic and SEDT results indicated that CIP adsorption mechanisms to goethite and hematite/magnetite were mainly outer- and inner-sphere complexation, respectively. The findings of this study reveal the adsorption mechanisms and are helpful in evaluating the transport of antibiotics in soils containing typical iron minerals.

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

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.