No abstract

As a component of herbicides, the fate of glyphosate (PMG) in the environment is of significant interest. The nature of PMG adsorption on mineral surfaces plays a significant role in the degradation of PMG. The adsorption of PMG on goethite (alpha-FeOOH) has been studied as a function of pH and PMG concentration. Adsorption was investigated with batch experiments, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS). The N 1s line in XPS spectra showed deprotonation of the amine group of PMG (NH2+) with increasing pH. IR analyses showed no evidence for the interaction of PMG's carboxylate group with the goethite surface, while the phosphonate group formed inner-sphere complexes. There is evidence for intramolecular hydrogen bonding between NH2+ and both the carboxylate and the phosphonate groups at low pH. Intramolecular hydrogen bonding is lost when the amine group is deprotonated, and the trend in intramolecular hydrogen bonding between NH2+ and phosphonate shows that PMG adsorbs via predominantly monodentate complexation. A minor quantity of bidentate complexes is thought to form both at near-neutral pH and when the surface concentration of PMG is low. While the phosphonate group of PMG binds directly, the carboxylate group remains relatively "free" from complexation with goethite, leaving it subject to degradation and/or complexation with metal ions present in the environment.

No abstract

No abstract

No abstract

No abstract

Synthetic gibbsite was prepared and thoroughly characterized using X-ray diffraction, high-resolution transmission electron microscopy, atomic force microscopy, and Fourier transform infrared techniques. The protonation and charging of the particle surface(s) in 20 and 100 mM (Na)Cl ionic media were investigated using potentiometric titrations and zeta potential measurements. These measurements indicated a pHpznpc ) 9.0 ( 0.2 and pHIEP ) 10.0 ( 0.1. Furthermore, it was concluded that the concentration of singly coordinated sites (tAlOH) located at the edges of the particles is too low (less than 7% of the total number of crystallographic sites) to explain the proton uptake. Therefore, the doubly coordinated sites (tAl2OH) on the basal planes must be proton reactive. Both proton data and zeta potentials could be described using a three-plane model where adsorbed Na + at the basal planes was placed in the 1-plane and all other adsorbed medium ions were placed in the 2-plane. The following results were obtained:The protonation constant for the singly coordinated sites (reaction 3) was set equal to pHIEP. The values of reactions 4 and 5 were taken from the literature. The effect of different reaction times on proton uptake and zeta potential is highlighted. It is clear that increased "equilibrium times" will result in an increased proton uptake, whereas the zeta potential remains constant. This is also reflected in the divergent models presented in the literature with respect to the protonation and charging properties of gibbsite particles.

Complexation of o-phthalate (1,2-benzenedicarboxylate) at the water−boehmite (γ-AlOOH) interface has been studied in 0.1 N NaCl media at 298.2 K within the range 4.0 < pH < 10.0. Equilibrium measurements were performed as potentiometric titrations and adsorption measurements. Attenuated total reflectance IR spectra were recorded to obtain structural information of the phthalate surface complexes. Additional adsorption and IR spectroscopic measurements were performed as a function of ionic strength. Infrared data strongly indicated the formation of two complexes at the surface, one outer-sphere and one inner-sphere complex with a chelating bidentate structure. The relative concentrations of these complexes were shown to vary with ionic strength and pH. The potentiometric and adsorption data were modeled using the extended constant capacitance approach to describe the electrostatics at the water−boehmite interface. Furthermore, constraints from the infrared spectroscopic results were used in the model. The surface complexation model which provided the best fit to all experimental data included the following equilibria:  ⋮AlOH + HL- ↔ ⋮AlOH2 +L2- (outer sphere); ⋮Al(OH)2 + H+ + HL- → ⋮AlL + 2H2O (inner sphere).