Glyphosate/AMPA adsorption on magnetite under different conditions: The effect of pH and electrolytes

Abstract

As a result of the frequent use, glyphosate (GLY) and its major metabolite, aminomethylphosphonic acid (AMPA), have emerged as ubiquitous environmental contaminants worldwide. Glyphosate, applied in agriculture, adsorbs on soil minerals, such as iron oxides, facilitating its fixation in soils and preventing leaching into groundwater sources. However, the adsorption capacity of various types of soils exhibits substantial variability. In this work the adsorption of GLY and AMPA on magnetite nanoparticles (MNPs) was studied under different conditions: at pH ∼ 6 and ∼ 8, and in the presence of 10 mmol/L NaCl, an indifferent electrolyte, and 10 mmol/L CaCl2, which contains a specific cation mainly present in soils. The pH-dependent zeta potential measurements revealed the difference between the effect of Na+ and Ca2+ ions on the charge state and aggregation of MNPs. The adsorption of GLY/AMPA shows significant pH dependence, with the bound amounts decreasing rapidly above pH ∼ 7 in both 10 mmol/L NaCl and CaCl2 solutions. The phosphonate adsorption capacity of MNPs, as determined from the fitting of the Langmuir isotherm equation, was approximately 0.35 mmol/g and 0,09 mmol/g at pH ∼ 6 and pH ∼ 8, respectively. The diverse bound amounts at different pHs are of environmental importance: under alkaline soil conditions, the mobilization of GLY may be more probable. GLY and AMPA bind to the surface of MNPs via complex formation at ≡Fe-OH sites, primarily involving phosphate groups, preferably at acidic pH, where the MNPs carry positive charges, and the near-surface concentration of phosphonate anions is higher due to electrostatic attraction. Furthermore, in the presence of Ca2+ ions, Ca-bridging occurs, as supported by the findings from the ATR-FTIR spectra. Additionally, it is worth noting that the complex formation between Ca2+ and phosphonates in the solution phase reduces the probability of Ca-bridge and surface complex formation, hindering the fixation of GLY/AMPA in soils.