Quantitative adsorption and local structures of gallium(III) at the water-alpha-FeOOH interface

The adsorption of Ga(III) at the water-alpha-FeOOH (goethite) interface has been investigated by means of quantitative adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, and surface complexation modeling. Under the conditions studied, pH range 3-11 and surface coverages of 0.9-3.2 micromol/m2, Ga(III) was found to adsorb strongly to alpha-FeOOH, and the surface species were more resistant toward hydrolysis and formation of soluble Ga(OH)4- than either solid gallium hydroxides or soluble polynuclear complexes. The EXAFS measurements revealed the presence of octahedral Ga(III) complexes at the water-alpha-FeOOH interface, with practically no structural variations as a function of pH or total gallium concentration. Analysis of the first coordination shell required an anharmonic model indicating a distorted geometry of the GaO6 octahedra, with mean Ga-O distances at 1.96-1.98 angstroms. A method based on the continuous Cauchy wavelet transforms (CCWT) was used to identify backscattering atoms in the higher coordination shells. This analysis indicated predominately Fe backscattering, and the quantitative data fitting resulted in three Ga-Fe paths at 3.05, 3.2, and 3.55 angstroms, which correspond to two edge-sharing and one corner-sharing linkage, respectively. The collective results from EXAFS spectroscopy showed that Ga(III) adsorbs to Fe equivalent sites at the surface alpha-FeOOH as an extension of the rows of Fe octahedra in the bulk structure. This interpretation was further corroborated by a Ga-Fe-Fe multiple scattering path at 6.13 angstroms. The quantitative adsorption and proton data were modeled using a surface complexation formalism based on a 1 pK(a) constant capacitance model. In agreement with the EXAFS results, the model obtained included one predominating surface complex with the stoichiometry [triple bond]FeOGa(OH)2(-0.5) and the stability constant log beta(intr.) = -2.55 +/- 0.04 ([triple bond]FeOH(-0.5) + Ga3+ + 2H2O <--> [triple bond]FeOGa(OH)2(-0.5) + 3H+).     

Surface complexes of monomethyl phosphate stabilized by hydrogen bonding on goethite (α-FeOOH) nanoparticles

Typically, a significant fraction of phosphorus in soils is composed of organic phosphates, and this fraction thus plays an important role in the global phosphorus cycle. Here we have studied adsorption of monomethyl phosphate (MMP) to goethite (α-FeOOH) as a model system in order to better understand the mechanisms behind adsorption of organic phosphates to soil minerals, and how adsorption affects the stability of these molecules. The adsorption reactions and stability of MMP on goethite were studied at room temperature as a function of pH, time and total concentration of MMP by means of quantitative batch experiments, potentiometry and infrared spectroscopy. MMP was found to be stable at the water-goethite interface within the pH region 3-9 and over extended periods of time, as well as in solution. The infrared spectra indicated that MMP formed three predominating pH-dependent surface complexes on goethite, and that these interacted monodentately with surface Fe. The complexes differed in hydrogen bonding interactions via the auxiliary oxygens of the phosphate group. The presented surface complexation model was based on the collective spectroscopic and macroscopic results, using the Basic Stern approach to describe the interfacial region. The model consisted of three monodentate inner sphere surface complexes where the MMP complexes were stabilized by hydrogen bonding to a neighboring surface site. The three complexes, which had equal proton content and thus could be defined as surface isomers, were distinguished by the distribution of charge over the 0-plane and β-plane. In the high pH-range, MMP acted as a hydrogen bond acceptor whereas it was a hydrogen bond donor at low pH.     

Surface complexation of mellitic acid to goethite: an attenuated total reflection Fourier transform infrared study

The nature of the interaction between mellitic acid (benzene hexacarboxylic acid) and the common soil mineral goethite (alpha-FeOOH) has been investigated as a function of pH and ionic strength by use of attenuated total reflection Fourier transform infrared spectroscopy. Molecular orbital calculations of the theoretical vibrational frequencies of the mellitate ion (L6-) and dihydrogen mellitate (H2L4-) have allowed the measured absorption frequencies to be accurately assigned. At pH values above 6, adsorption involves outer-sphere complexation of the deprotonated L6- ion. At lower pH values, there is evidence of a second outer-sphere surface complex involving a partially protonated species, although the extent of protonation of the surface species is significantly less than that found for the solution species at the same pH. While there is no evidence of inner-sphere complexation, increasing the ionic strength to 2.0 M does not displace the adsorbed species but does increase the fraction present on the surface as the fully deprotonated L6-. The small effect of ionic strength suggests that the adsorptive interaction, although outer-sphere in character, is still relatively strong, which indicates that hydrogen bonds may play a significant role. Hydrogen bonding may also help to account for the observed outer-sphere complexation at pH values above the pHiep of goethite.