Co-adsorption of cadmium(II) and glyphosate at the water-manganite (γ-MnOOH) interface

The co-adsorption of Cd(II) and glyphosate (N-(phosphonomethyl)glycine, PMG) at the manganite (γ-MnOOH) surface has been studied in the pH range 6-10 at 25 °C and with 0.1 M Na(Cl) as ionic medium. Batch adsorption experiments, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy were used for the quantitative analysis and the determination of the molecular structure of the surface complexes. The adsorption of Cd(II) and PMG in the ternary Cd(II)-PMG-manganite system was compared with the adsorption in the binary Cd(II)-manganite and PMG-manganite systems. The formation of three inner sphere surface complexes was observed, a ternary Cd(II)-PMG-manganite complex, a binary Cd(II)-manganite complex and a binary PMG-manganite complex. The surface concentration of the ternary complex and the Cd(II)-manganite complex was more or less constant throughout the pH range studied. However, the surface concentration of the binary PMG-manganite complex decreased with increasing pH. The major part of the binary PMG-surface complex was protonated. The ternary surface complex displayed a type B structure (Cd(II)-PMG-manganite). The average Cd-Mn distance obtained from EXAFS (3.26 Å) indicates that the binary and ternary Cd(II)-surface complexes are formed by edge-sharing of Mn and Cd octahedra on the (010) plane of the manganite crystals.