Peter J. Swedlund

Adsorption and polymerisation of silicic acid on ferrihydrite, and its effect on arsenic adsorption

The interaction between silicic acid (H4SiO4) and ferrihydrite involves both adsorption and polymerisation, and has a significant effect on the adsorption of anionic species such as the oxyanions of arsenic. Using the diffuse layer model (DLM), the adsorption constants for H4SiO4, As(V), and As(III) adsorption onto ferrihydrite have been determined from experimental adsorption data. Silicic acid could be considered to adsorb as a monomer when the total Si to Fe mole ratio (Si(T)/Fe) was <0.1, and the inhibitory effect of H4SiO4 on As(III) and As(V) adsorption could be accurately modelled using the DLM. At higher Si(T)/Fe ratios, H4SiO4 polymerisation on the ferrihydrite surface appeared to become increasingly important. However, even when Si(T)/Fe=1.8 and there would have been significant H4SiO4 polymerisation, the DLM continued to predict almost all of the observed effect of H4SiO4 on As(III) and As(V) adsorption while taking only H4SiO4 adsorption into account. This suggests that H4SiO4 adsorption inhibits As adsorption to a greater degree than H4SiO4 polymerisation.

Cu and Zn ternary surface complex formation with SO4 on ferrihydrite and schwertmannite

The use of adsorption data from single sorbate systems to model metal adsorption in SO4-rich waters, such as acid mine drainage, can lead to inaccurate predictions of metal speciation. The adsorption of Cu and Zn on ferrihydrite, for example, is enhanced at low pH values in the presence of SO4. This effect can only be accurately modeled using the diffuse layer model and surface complexation theory if ternary surface complexes, ≡FeOHCuSO4 or ≡FeOHZnSO4, are taken into consideration. Intrinsic adsorption constants for the formation of these ternary complexes on ferrihydrite have been derived from experimental data. When included in the model, Cu and Zn adsorption in the presence of SO4 is accurately predicted for a wide range of metal, ferrihydrite and SO4 concentrations. Adsorption of Cu and Zn onto the SO4-rich Fe oxyhydroxide, schwertmannite, could also be accurately predicted and is indistinguishable from adsorption onto ferrihydrite in the presence of high solution SO4 concentrations (e.g. 0.01 mol kg−1 SO4).

The effect of SO4 on the ferrihydrite adsorption of Co, Pb and Cd: ternary complexes and site heterogeneity

Abstract Ferrihydrite adsorption of Co, Pb and Cd is enhanced in the presence of sulfate. This effect, which has been observed previously for Cu and Zn, is not predicted by the diffuse layer model using adsorption constants derived from single sorbate systems. However, by including ternary surface complexes with the stoichiometry ≡FeOHMeSO 4 , where ≡Fe is an Fe on the ferrihydrite surface and Me is Co, Pb, or Cd, the effect of SO 4 on cation adsorption is accurately predicted for the range of cation, ferrihydrite and SO 4 concentrations studied. Surface site heterogeneity for Co, Pb and Cd adsorption in the absence of SO 4 also was considered. While Co adsorption could be described with a 2-site model (with site densities of 5×10 −3 and 2×10 −1 mol mol Fe −1 ) Pb adsorption required a third site with a higher affinity and a site density of 3.5×10 −4 mol molFe −1 . Cadmium adsorption was reasonably well described with a 2-site model but showed some evidence for a third higher affinity site. Recommended values of adsorption constants for use in SO 4 -rich environments, such as acid mine drainage, are presented.

Insights into H4SiO4 surface chemistry on ferrihydrite suspensions from ATR-IR, Diffuse Layer Modeling and the adsorption enhancing effects of carbonate

Silicic acid (H(4)SiO(4)) adsorbs at the ferrihydrite-water interface as monomeric or oligomeric surface silicate complexes.ATR-IR spectra were used to determine the proportions of monomeric and oligomeric surface silicate as a function of pH and Si surface concentrations (Γ(Si)) for H(4)SiO(4) in ferrihydrite suspensions.At each pH the proportion of adsorbed silicate present as monomers decreased as Γ(Si) increased while at a given Γ(Si) the proportion of adsorbed silicate present as monomers was higher at higher pH. ATR-IR spectra for ferrihydrite suspensions in combination with the adsorption isotherm data were used to calibrate the Diffuse Layer Model (DLM) to describe H(4)SiO(4) adsorption as monomers and oligomers on ferrihydrite surface sites (≡FeOH). Using a set of reactions that were consistent with the ATR-IR spectra the DLM could accurately describe the H(4)SiO(4) adsorption isotherms, the distribution of surface monomeric and oligomeric silicates, and the decrease in surface potential with Γ(Si).The reactions included the formation of monomeric complexes (≡FeH((3-n))SiO(4)((-n))) and trimeric silicate complexes formed between two surface sites(≡Fe(2)H((6-n))Si(3)O(10)((-n))). This oligomer stoichiometry is consistent with previous studies suggesting the surface silicate oligomer is formed by a solution H(4)SiO(4) bridging two adjacent adsorbed monomers to form a linear trimer. This study also showed that carbonate can enhance H(4)SiO(4) adsorption between pH 9 and 11. The data were consistent with formation of an outer-sphere complex between a solution H(3)SiO(4)(-) and a protonated adsorbed carbonate species which is analogous to the mechanism by which carbonate enhances the goethite adsorption of sulfate.

Effect of high hydrostatic pressure on the supramolecular structure of corn starch with different amylose contents

Corn starches with amylose contents ranging from 0 to 80% were suspended in 60 wt% water or ethanol and subjected to high hydrostatic pressure (HHP) up to 600 MPa. The impact of HHP treatment on the granule morphology, lamellae structures, and crystalline characteristics were examined with a combination of SAXS, WAXS and optical microscopy. All starch dispersed in water showed a decrease in area of the lamellar peak in the SAXS data at q∼0.6 nm(-1). The lamellae thickness (d) increased for pressurized waxy, normal, and Gelose80 corn starches, suggesting water is forced into starch lamellae during HHP. However, for Gelose50 corn starch, the d remained constant over the whole pressure range and light microscopy showed no obvious granule swelling. WAXS studies demonstrated that HHP partially converted A-type starches (waxy and normal corn) to starches with a faint B-type pattern while starches with a B+V-type pattern (Gelose50 and Gelose80), were not affected by HHP. All corn starches suspended in ethanol showed no detectable changes in either granule morphology, or the fractal, the lamellae, and the crystalline structures.

Calcium Phosphates in Ca2+-Fortified Milk: Phase Identification and Quantification by Raman Spectroscopy

Calcium phosphate nanoclusters (CPNs) are important for the structure, function, and nutrient density of many dairy products. Phosphorylated amino acids in caseins stabilize calcium phosphate as nanoclusters which are amorphous to X-ray diffraction and exist within casein micelles, and these CPNs play a key role in micelle stability. Addition of calcium to milk results in further calcium phosphate removal from the serum, and there is uncertainty about the nature of the material formed and its stability. In this work we investigate both the solution and colloidal phases in CaCl2 enriched bovine milk to identify, quantify, and determine the solubility of the calcium phosphate material formed in response to calcium addition to milk. The P-O stretching bands are quite distinct in the Raman spectra of the main synthetic calcium phosphate mineral phases, including the amorphous calcium phosphate phase. In response to adding between 5 and 40 mM CaCl2 to milk, the serum phosphate concentration decreased asymptotically from 7.5 ± 0.2 to 0.54 ± 0.05 mM. Using Raman spectroscopy with a combination of internal and external standards, it was possible to show that the calcium phosphate material formed after Ca(2+) addition to milk was the same as amorphous calcium phosphate nanoclusters present in the absence of added calcium. The use of an internal standard allowed a quantitative analysis of the spectra which demonstrated that the amorphous calcium phosphate formed accounted for all of the calcium and phosphate that was removed from solution in response to calcium addition.

X-ray diffraction investigation of amorphous calcium phosphate and hydroxyapatite under ultra-high hydrostatic pressure

The changes in the crystal structures of synthetically prepared amorphous calcium phosphate (ACP) and hydroxyapatite (HAP) in water (1:1 mass ratio) were studied by synchrotron X-ray diffraction (XRD) under ultra-high hydrostatic pressures as high as 2.34 GPa for ACP and 4 GPa for HAP. At ambient pressure, the XRD patterns of the ACP and HAP samples in capillary tubes and their environmental scanning electron micrographs indicated amorphous and crystalline characteristics for ACP and HAP, respectively. At pressures greater than 0.25 GPa, an additional broad peak was observed in the XRD pattern of the ACP phase, indicating a partial phase transition from an amorphous phase to a new high-pressure amorphous phase. The peak areas and positions of the ACP phase, as obtained through fitting of the experimental data, indicated that the ACP exhibited increased pseudo-crystalline behavior at pressures greater than 0.96 GPa. Conversely, no structural changes were observed for the HAP phase up to the highest applied pressure of 4 GPa. For HAP, a unit-cell reduction during compression was evidenced by a reduction in both refined lattice parameters a and c. Both ACP and HAP reverted to their original structures when the pressure was fully released to ambient pressure.

Retrogradation of Maize Starch after High Hydrostatic Pressure Gelation: Effect of Amylose Content and Depressurization Rate

High hydrostatic pressure (HHP) has been employed to gelatinize or physically modify starch dispersions. In this study, waxy maize starch, normal maize starch, and two high amylose content starch were processed by a HHP of the order of 600 MPa, at 25°C for 15min. The effect of HHP processing on the crystallization of maize starches with various amylose content during storage at 4°C was investigated. Crystallization kinetics of HHP treated starch gels were investigated using rheology and FTIR. The effect of crystallization on the mechanical properties of starch gel network were evaluated in terms of dynamic complex modulus (G*). The crystallization induced increase of short-range helices structures were investigated using FTIR. The pressure releasing rate does not affect the starch retrogradation behaviour. The rate and extent of retrogradation depends on the amylose content of amylose starch. The least retrogradation was observed in HHP treated waxy maize starch. The rate of retrogradation is higher for HHP treated high amylose maize starch than that of normal maize starch. A linear relationship between the extent of retrogradation (phase distribution) measured by FTIR and G* is proposed.

Modeling Cd2 + sorption onto ferrihydrite in the presence of phthalic acid

Cadmium sorption by ferrihydrite in the presence of phthalic acid was examined over a range of pH (4.0 approximately 8.5) conditions and sorbate/sorbent ratios. The presence of phthalic acid enhanced Cd(2+) sorption by forming ternary complexes on ferrihydrite surface, especially at low pH, but for high pH and high total organic ligand/Fe(mmol/mol) ratios, it decreased Cd(2+) sorption onto ferrihydrite by forming soluble complexes with the phthalate in solution. In binary systems, Cd(2+) and phthalic acid sorption by ferrihydrite was well reproduced using the diffuse layer model with sorption constants derived from the experimental data. Prediction using the optimised binary sorption constants for Cd(2+) sorption onto ferrihydrite in the presence of phthalic was poor and achieving a good fit required the inclusion of two additional ternary complexes.