Mid Sweden University

An investigation combining IR spectroscopy, potentiometric titrations, and adsorption experiments was carried out to study pyromellitate (1,2,4,5-benzenetetracarboxylate) sorption at the goethite (α-FeOOH)/water interface. The IR spectra show evidence of outer-sphere complexation throughout the pH range from 3 to 9. Below pH 6 additional IR spectroscopic features appear, which are tentatively assigned to inner-sphere complexes. A normalized IR peak area plot for each peak indicative of inner- and of outer-sphere complexes as a function of pH provided a semiquantitative surface speciation scheme. This scheme was successfully reproduced using surface complexation theory with a multisite complexation model calibrated on potentiometric titration and on adsorption data. The surface speciation was described with a binuclear outer-sphere complex on the {110} plane of goethite and a mononuclear inner-sphere complex on the {001} plane. Furthermore, as the IR spectra also indicated partial protonation of pyromellitate complexes at low pH, a partially protonated outer-sphere species on the {110} plane was included in the model.

Concentrations of Al3+ were calculated in soil solutions from concentrations of the monomeric ‘reactive Al’ species ([Al3+] + [Al(OH)2+] + [Al(OH)2+] + [AlF2+]) obtained using a recently reported flow injection analysis (FIA) chelating resin technique. Soil solution samples came from 7 sites encompassing a range of New Zealand soils (Brown, Gley, Pallic, Podzol, and Recent Soils) and vegetation types (pasture, shrub lands, and indigenous and exotic forest). Previously published data from a further 7 sites, obtained using a rapid (7 s) FIA technique, were transformed to give compatible results. The resultant data (n = 85) covered the pH range 2.7–7.6, and showed a single curvilinear relationship for log [Al3+] v. soil solution pH, regardless of vegetation or soil type. At pH >5.6, the data had a slope of –2.98 and fell between the amorphous Al(OH)3 and gibbsite solubility lines. At pH <5.0, the data had a slope of –0.46; further, the soil solutions were under-saturated with respect to both minerals. These results are interpreted as indicating control of Al solubility by Al(OH)3(s) (at pH >5.6) and soil organic matter (at pH <5.0), respectively. This interpretation is supported by data from a pH-dependent Al–fulvic acid binding curve, for which calculated values of [Al3+] follow the same curvilinear relationship determined from the soil solution samples.

Kinetic and thermodynamic factors associated with the use of pyrocatechol violet (PCV) for the determination of total reactive Al or `free' Al[Al3++Al(OH)2++Al(OH)+2] have been investigated. The rate of reaction of Al with PCV (in MES buffer, pH 6.2) was strongly influenced by the presence of competing ligands. The rate of formation of Al(PCV)2 on the addition of Al3+ to a PCV–competing ligand mixture was: oxalate≈F−≈malonate>salicylate>>no competing ligand>citrate. A similar increase in the reaction rate relative to standards (i.e. no competing ligand) was observed for Al pre-equilibrated in humic waters and soil solution (at concentrations above or below the Al-complexation capacity). The discrepancy in reaction rates may be ascribed to the inhibition through pH-induced hydrolysis of Al3+ in the absence of ligands (i.e. in standards) or to acceleration in the presence of naturally occurring ligands. It has serious implications for the use of kinetic-based FIA protocols for the determination of Al fractions or total Al in natural waters. Specifically, the (usually) slower reaction for Al3+standards implies that measurements on systems containing organic ligands may overestimate the concentration of `free' or total Al. Quantitative studies on the thermodynamics of the citrate–Al3+–PCV system established that the attainment of equilibrium in the pH range 5.0 to 6.6 required ≈300 min. Thus, determination of total Al by FIA in systems containing this or closely related ligands is not feasible.

Speciation of Al is determined by a 1.3-s reaction with 8-quinolinol (oxine)-derivatised Fractogel in a 22 μl column reactor in a flow injection (FI) manifold. Al (pre)concentrated on the column from a 650 μl sample is selectively eluted with 250 μl of 0.02 M NaOH and detected spectrophotometrically as the Al-chrome azurol S (CAS) complex at pH 5.0. This Al (referred to as ‘free Al’) comprises Al3+ + A1(OH)2+ + Al in very labile complexes. Tests with synthetic solutions established that Al is not significantly sequestered from the citrate, oxalate and malonate complexes. Al-hydroxo polymers [Al13(OH)327+] are quantitatively retained by the column but are not desorbed by 0.02 M NaOH in the time frame of the FI method; therefore they do not contribute to the analytical signal. However, they can be quantified after stopped-flow elution with 0.2 M NaOH. The AlF2+ and AlF2+ complexes are retained and eluted quantitatively and therefore contribute to the measurement of ‘free Al’. The method has been applied to humic waters and soil solutions and the results for ‘free’ Al3+ compared with those obtained by the 7-s CAS method. The method has a 2σ detection limit of 70 nM, a linear working range of 0.3–16 μM and relative standard deviations of 7% and 1% at 0.5 and 16 μM, respectively.

Complexation of o-phthalate (1,2-benzenedicarboxylate) and competitive complexation of phosphate and phthalate at the goethite-water interface have been studied in 0.1 M Na (NO 3) media at 298.2 K within the range 3.0 < -log [H +] < 8.5. Equilibrium measurements were performed as potentiometric titrations supplemented with spectrophotometric phosphate and phthalate analyses.

The binary and ternary chemical subsystems H +-goethite and H +-goethite-H 2PO 4−have been investigated earlier and described according to the constant capacitance model. The adsorption of phthalate showed a strong ionic strength dependence which indicated that phthalate is adsorbed as outer-sphere complexes. The experimental data in the subsystem H +-goethite-phthalate were evaluated on the basis of an extended constant capacitance model with the aid of the computer program FITEQL, version 2.0. One plane for inner sphere complexation and one plane for outer-sphere complexation, each with an associated constant capacitance, were included in the extended constant capacitance model. Surface complexation of phthalate is described by two outer-sphere complexes, ≡FeOH 2+L 2− and ≡FeOH 2+ HL −.

In the experiments with simultaneous complexation of phosphate and phthalate, the complexation of phosphate was not influenced by the presence of phthalate. On the other hand, the complexation of phthalate was decreased even at low phosphate concentrations. The equilibrium models determined for the subsystems were used to predict the adsorption of phosphate and phthalate in the quaternary system. It was found that these predictions were in good agreement with experimental titration and phosphate/ phthalate adsorption data.

Diffuse reflectance IR-spectra were recorded to obtain structural information of the phthalate complexes. The spectroscopic data did not contradict the outer-sphere model. However, because of the complexity of the phthalate molecule conclusive structural assignment could not be made.

The surface speciation of orthophosphate ions on goethite has been studied as a function of pH, time, total phosphate concentration, and ionic medium by means of diffuse reflectance FTIR spectroscopy. The samples were prepared in accordance with a distribution diagram of surface species as calculated from thermodynamic data. In agreement with the thermodynamic model three dominating surface complexes could be distinguished with IR spectroscopy, and the relative distribution of the species was shown to be primarily a function of pH. The IR characteristics of the individual surface complexes were indicative of molecular symmetries of the PO4unit of C3v, C2v, and C3v, respectively. This was concluded to be incompatible with the bidentate, bridging structural model previously suggested. Instead, the IR data are in good agreement with a monodentate coordination of phosphate to the surfaces, where the three surface complexes only differ in the degree of protonation. A comparison between the adsorption behavior of phosphate on goethite and hematite was also made. Here the importance of the aqueous stability of the adsorbent on the adsorption mechanism was shown.

Phosphate complexation at the goethite-water interface has been studied in 0.1 M Na(NO3) medium at 298.2 K in the range 3.0 <—log[H+] < 9.0. Equilibrium measurements were performed as potentiometric titrations supplemented with spectrophotometric phosphate analyses. The experimental data were evaluated on the basis of the constant capacitance model and with the aid of the computer program FITEQL, version 2.0.

The acid/base properties of the goethite-water interface have been investigated earlier and are described by two intrinsic equilibrium constants logβs 1,1,0(int) = 7.47 and logβs-1,1,0(int) = −9.51 and with a specific capacitance of 1.28 F m−2. The binding between phosphate and goethite was found to be strong within the investigated -log[H+] range, and the amount of adsorbed phosphate is slowly decreasing with increasing—log[H+] value. The model describing the phosphate complexation is represented by the following equilibria and intrinsic constants:

Further support for the proposed model is given by zeta-potential measurements.