Adsorption behavior and mechanism of glufosinate onto goethite

- Glufosinate is bonded onto goethite through an inner-sphere surface complexation.
- The adsorption is pH-dependent and ionic strength-independent.
- Cu2 + can enhance the binding by formation of goethite-Cu-GLU ternary complex.
- SO42 −, PO43 − and high level of FA can inhibit the adsorption by competitive effect.
- ATR-FTIR and DFT calculation were used to identify the surface configuration.

The adsorption of glufosinate (GLU), a widely used herbicide similar to glyphosate (GLY), onto goethite was investigated as a function of the pH, ionic strength, background cations and anions, heavy metal ions and fulvic acids (FAs) by using batch adsorption experiments. In situ ATR-FTIR spectroscopy and density functional theory (DFT) calculations were carried out to characterize the molecular interactions between GLU and goethite surfaces. The macroscopic results indicated that an increasing pH exerted an adverse effect on GLU adsorption because of the electrostatic repulsion, and the adsorption was not sensitive to ionic strengths or background cation types, indicating that an inner-sphere surface complex was involved. GLU adsorption can be considerably depressed by PO43 −, SO42 −, and a high level of FA because of the competitive effect, while being enhanced by Cu2 + with a maximum adsorption at approximately pH 5 because of the metal ion bridging effect. Other examined divalent metal cations (Cd2 +, Zn2 +, and Pb2 +) showed almost no effect on GLU adsorption, indicating weak interaction between them. ATR-FTIR spectra and the DFT calculations further proved that GLU was bonded to goethite surfaces through the formation of a monodentate mononuclear inner-sphere complex between the phosphinic moiety and surface Fe(III) centers under an acidic condition. The results showed that GLU had a similar adsorption mechanism to that of GLY onto goethite, but with a lower adsorption affinity, possibly exerting higher mobility and risk in soils.

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