Co-adsorption Mechanism Of Glyphosate And Cd(Ⅱ) On Three Soil Minerals

Heavy metals in excess of the standards and organic pollutants residues, such as pesticide residues, are dominant in agricultural production and agricultural product safety, in recent years. It has becomes a focal point. Much studies focus on the migration, transformation and fate of heavy metals or organic pollutants in soil, but the mechanism of adsorption and migration of pollutants is ignored in multiple pollutant coexistence system, especially heavy metal and organic pollution coexistence system. The interactions among these pollutants can affect their migration, transformation, biological availability and biological toxicity. Heavy metal Cadmium(Cd) and organic phosphorus pesticide compound in soil are very common in China. In this study, we investigated the co-adsorption of Cd(Ⅱ) and glyphosate at the goethite, Montmorillonite and Kaolinite surface. Their co-adsorption mechanisms were studied using Fourier Transform Infrared Spectroscopy(FTIR), Isothermal Titration Calorimetry(ITC) and X-ray Photoelectron Spectroscopy(XPS). The main results were summarized as the following:1. The adsorption of Cd(Ⅱ) in minerals-Cd(Ⅱ) binary system and minerals-Cd(Ⅱ)-glyphosate ternary system were affected by pH. In mineral-Cd(Ⅱ) binary systems, the adsorption of Cd(Ⅱ) increases with increasing pH. In goethite-Cd(Ⅱ)-glyphosate and kaolinite-Cd(Ⅱ)-glyphosate ternary system, the adsorption of Cd(Ⅱ) increases with increasing pH. In montmorillonite-Cd(Ⅱ)-glyphosate ternary system, the adsorption of Cd(Ⅱ) increases with increasing pH from 3 to 6 and then decreases.2. The FTIR spectrum shows that there is a complexation between glyphosate and Cd(Ⅱ) according formation of P-O-Cd, and it indicates the formation of P-O-Fe, P-O-H, C-O-Fe between phosphate and carboxyl group with goethite, forming ternary surface complex. The strength of the chemical bond decreased with increasing p H. The XPS result indicated goethite can adsorb glyphosate and glyphosate-Cd(Ⅱ) in the meantime. The negative charge on the goethite surface increases gradually in the pH range 3 to 4, and increased immediately in the pH range 4 to 8, after adding glyphosate. The co-adsorption of Cd(Ⅱ) and glyphosate on goethite leads to decreased Cd(Ⅱ) adsorption at pH 3-4 while an increase in adsorption is observed at pH 4-8, compared to the binary goethite-Cd(Ⅱ) system.3. The co-adsorption of Cd(Ⅱ) and glyphosate on montmorillonite did not affect the Cd(Ⅱ) adsorption at pH 3-6, while a decrease in adsorption is observed at pH 6-8, compared to the binary montmorillonite-Cd(Ⅱ) system. The co-adsorption of Cd(Ⅱ) and glyphosate on kaolinite leads to decreased Cd(Ⅱ) adsorption at pH 3-5 while a increase in adsorption is observed at pH 5-8, compared to the binary kaolinite-Cd(Ⅱ) system. The negative charge on the montmorillonite and kaolinite surface increases gradually after adsorbing glyphosate. In addition, no new absorption peaks were observed in the FTIR of montmorillonite or kaolinite-Cd(Ⅱ) system.4. Cd(Ⅱ) or glyphosate adsorption and co-adsorption on the three minerals were exothermic reaction. The released heat of co-adsorption roughly equal with the sum of liberated heat of Cd(Ⅱ) and glyphosate adsorption, and the heat of glyphosate adsorption is considerably higher than that of Cd(Ⅱ) adsorption. It indicated that the emitted heat of co-adsorption is mainly from the reaction heat of glyphosate adsorption.