Philip A. Helmke

Gibbsite solubility and thermodynamic properties of hydroxy-aluminum ions in aqueous solution at 25°C

Solubility curves were determined for a synthetic gibbsite and a natural gibbsite (Minas Gerais, Brazil) from pH 4 to 9, in 0.2% gibbsite suspensions in 0.01 M NaNO3 that were buffered by low concentrations of non-complexing buffer agents. Equilibrium solubility was approached from oversaturation (in suspensions spiked with Al(NO3)3 solution), and also from undersaturation in some synthetic gibbsite suspensions. Mononuclear Al ion concentrations and pH values were periodically determined. Within 1 month or less, data from over-and undersaturated suspensions of synthetic gibbsite converged to describe an equilibrium solubility curve. A downward shift of the solubility curve, beginning at pH 6.7, indicates that a phase more stable than gibbsite controls Al solubility in alkaline systems. Extrapolation of the initial portion of the high-pH side of the synthetic gibbsite solubility curve provides the first unified equilibrium experimental model of Al ion speciation in waters from pH 4 to 9. The significant mononuclear ion species at equilibrium with gibbsite are Al3+, AlOH2+, Al(OH)+2 and Al(OH)−4, and their ion activity products are ∗K50 = 1.29 × 108, ∗Ks1 = 1.33 × 103, ∗Ks2 = 9.49 × 10−3 and ∗Ks4 = 8.94 × 10−15. The calculated standard Gibbs free energies of formation (ΔG°f) for the synthetic gibbsite and the A1OH2+, Al(OH)+2 and Al(OH)−4 ions are −276.0, −166.9, −216.5 and −313.5 kcal mol−1, respectively. These ΔG°f values are based on the recently revised ΔG°f value for Al3+ (−117.0 ± 0.3 kcal mol−1) and carry the same uncertainty. The ΔG°f of the natural gibbsite is −275.1 ± 0.4 kcal mol−, which suggests that a range of ΔG°f values can exist even for relatively simple natural minerals.

STUDIES OF THE NATURE OF CU2+ AND PB2+ BINDING SITES IN SOIL HUMIC SUBSTANCES USING X-RAY ABSORPTION SPECTROSCOPY

We have investigated the binding environments of Cu{sup 2+} and Pb{sup 2+} complexed by soil humic substances using synchrotron-based X-ray absorption spectroscopy. With the assistance of bond network analysis, analysis of X-ray absorption near edge structure (XANES) and radial structure functions derived from extended X-ray absorption fine structure (EXAFS) spectra of Cu-humate at pH 4, 5, and 6 yielded a tetragonally-distorted octahedral binding environment for Cu with 4 O atoms at an average distance of 1.94 {angstrom}, 20 atoms at an average distance of 2.02 {angstrom}, and 4 C atoms at an average distance of 3.13 {angstrom}. Analysis of Pb-humate samples at pH 4, 5, and 6 yielded 4 O atoms at average distances between 2.46 {angstrom} to 2.32 {angstrom} and 2 C atoms at an average distance of 3.26 {angstrom} for Pb. We interpret the presence of C atoms in the second atomic shell of the metal binding site as evidence that both C{sup 2+} and Pb{sup 2+} form innersphere complexes with soil humic substances. Within the pH range 4-6, there is no significant change in the structure of the binding sites for either Cu or Pb. 65 refs., 8 figs., 5 tabs.

Aqueous dissolution, solubilities and thermodynamic stabilities of common aluminosilicate clay minerals: Kaolinite and smectites

Abstract Determinations of the aqueous solubilities of kaolinite at pH 4, and of five smectite minerals in suspensions set between pH 5 and 8, were undertaken with mineral suspensions adjusted to approach equilibrium from over- and undersaturation. After 1,237 days, Dry Branch, Georgia kaolinite suspensions attained equilibrium solubility with respect to the kaolinite, for which K eq = (2.72 ± 0.35) × 10 7 . The experimentally determined Gibbs free energy of formation ( ΔG f ,298 0 ) for the kaolinite is −3,789.51 ± 6.60 kj mol −1 . Equilibrium solubilities could not be determined for the smectites because the composition of the solution phase in the smectite suspensions appeared to be controlled by the formation of gibbsite or amorphous aluminum hydroxide and not by the smectites, preventing attempts to determine valid ΔG f 0 values for these complex aluminosilicate clay minerals. Reported solubility-based ΔG f 0 determinations for smectites and other variable composition aluminosilicate clay minerals are shown to be invalid because of experimental deficiencies and of conceptual flaws arising from the nature of the minerals themselves. Because of the variable composition of smectites and similar minerals, it is concluded that reliable equilibrium solubilities and solubility-derived ΔG f 0 values can neither be rigorously determined by conventional experimental procedures, nor applied in equilibriabased models of smectite-water interactions.

Studies of the nature of binding sites of first row transition elements bound to aquatic and soil humic substances using x-ray absorption spectroscopy

The coordination environments of first row transition elements (Co, Ni, Cu, and Zn) complexed by aquatic and soil humic substances were studied using X-ray absorption spectroscopy (XAS). With the assistance of bond network analysis, analysis of X-ray absorption near edge structure (XANES) spectra and radial structure functions (RSF) derived from extended X-ray absorption fine structure (EXAFS) spectra of Co, Ni, Cu, and Zn in humic substances indicate an octahedral binding environment for Co, Ni, and Zn and a tetragonally-distorted octahedral binding environment for Cu. Analysis of EXAFS spectra of each element provides detailed information on the internuclear distances, bond angles, and Debye-Waller factors within the first two atomic shells surrounding Co, Ni, Cu, and Zn bound to humic substances. We interpret the presence of C atoms in the second atomic shell of the metal binding site as further evidence that Co, Ni, Cu, and Zn form innersphere complexes with humic substances. The number and type of ligands involved in the binding are different for different elements. 71 refs., 10 figs., 5 tabs.

The pH dependence of free ionic activities and total dissolved concentrations of copper and cadmium in soil solution

Abstract Concentrations of total dissolved Cu and Cd and activities of their free ions in soil solution are suggested to be influenced by soil pH. The objective of this research was to study the relationships between free ionic activities and total dissolved concentrations of Cu and Cd and soil pH. Soil samples from Elkhorn, WI, USA (Typic Argiudoll), were used as a model. Free ions Cu 2+ and Cd 2+ were determined in the soil saturation extracts by using Donnan analysis with graphite furnace AAS. The values of soil pH beyond their indigenous values were obtained by additions of dilute nitric acid to the soil-water mixtures. The results showed that logarithmic concentrations of total dissolved Cu and Cd showed curve-linear relationships with soil pH. Free ionic Cu 2+ and Cd 2+ activities were negatively correlated with soil pH. The slopes of log (Cu 2+ ) vs. pH plots were − 0.10 and − 0.30 and those of log (Cd 2+ ) vs. pH plots were − 0.60 and − 0.70 for the control soil and the sewage-sludge-treated soil, respectively.

KAOLINS: SOURCES OF DIFFERENCES IN CATION-EXCHANGE CAPACITIES AND CESIUM RETENTION

Seven kaolins from Georgia (southeastern U.S.A.), ranging from high to low commercial grade, were characterized by X-ray powder diffraction and chemical techniques to establish that the variation in quality was caused by impurities. The Ca and Cs cation-exchange capacities (CEC) varied from 2.67 to 8.17 and from 3.29 to 8.77 meq/100g, respectively. Selective dissolution and correlation analyses strongly indicated that expandable 2:1 minerals, particularly smectite (1.2-5.9%), were responsible for most of the observed variations in Ca CEC (r = 0.85*). The external surface CEC of kaolinite ranged from 0 to 1 meq/ 100 g. The positive significant correlation (r = 0.90**) between the Ca CEC and the K-mica content (03.9%) suggested that Ca CEC may be related to the degree of mica weathering through an expandable mineral stage.The Cs-retention capacity (0.19–1.14 meq/100 g) was closely related to Cs-measured vermiculite content (r = 0.80*), and this content plus specific surface (R = 0.93**) or mica content (R = 0.86*). The Cs retention appeared to be primarily related to the presence of interlayer wedges at mica/vermiculite XY interfaces.РезюмеСемь каолинов из Джорджии (юго-восток США), представленные в диапазоне от высоко- до низкокачественных коммерческих марок, были охарактеризованы порошковым методом рентгеноструктурного анализа и химическими анализами, чтобы доказать что изменения качества вызываются примесями. Катионно-обменные способности (КОС) Са и Cs изменялись от 2,67 до 8,17 и от 3,29 до 8,77 М.ЭК./100 г соответственно.Селективные анализы растворения и корреляции убедительно доказали, что расширяющиеся минералы 2:1, особенно смектит (1,22–5,9%), обусловили большинство замеченных изменений КОС Са (г = 0,85*). Наружная поверхностная КОС каолинита колеблется от 0 до 1 М.ЭК./ЮО г. Существенная положительная корреляция (г = 0,90**) между КОС Са и содержанием К-слюды (02–3,9%) указывает на то, что КОС Са может быть связана со степенью выветривания слюды в течение фазы расширения минерала.Способность удержания Cs (0,192–1,14 М.ЭК./ЮО г) тесно связана с измеренным по Cs содержанием вермикулита (г = 0,80*) и этим содержанием плюс удельной поверхностью (R = 0,93**) или содержанием слюды (R = 0,86*). Удержание Cs, по-видимому, связано в основном с присутствием межслойных клиньев в промежутках XY слюда/вермикулит. [N.R.]ResümeeSieben Kaoline aus Georgia (südöstliche USA), die von hoch- bis niedrig-qualitativ reichen, wurden mittels Röntgenpulverdiffraktometrie und chemischen Methoden charakterisiert, um festzustellen, daß die Qualitätsunterschiede durch Verunreinigungen hervorgerufen werden. Die Ca- und Cs-Kationenaustauschkapazitäten, (CEC), variierten von 2,67 bis 8,17 bzw. von 3,29 bis 8,77 mÄq/100 g. Selektive Auflösungs- und Korrelationsanalysen zeigen sehr stark an, daß quellfähige 2:1 Minerale, vor allem Smektit (1,2–5,9%), für die meisten der beobachteten Variationen in der Ca CEC (r = 0,85*) verantwortlich sind. Die CEC der äußeren Oberfläche von Kaolinit reicht von 0–1 mÅq/100 g. Die positive beachtliche Korrelation (r = 0,90**) zwischen der Ca CEC und dem Gehalt an K-Glimmer (0–3,9%) deutet daraufhin, daß die Ca CEC im Zusammenhang stehen könnte mit dem Ausmaß, in dem die Glimmerverwitterung durch ein Stadium eines quellfähigen Minerales geht.Die Cs-Retentionskapazität (0,19–0,14 mÄq/100 g) stand in engem Zusammenhang mit dem Cs-gemes- senen Vermiculitgehalt (r = 0,80*) und mit diesem Gehalt plus spezifischer Oberfläche (R = 0,93**) bzw. Glimmergehalt (R = 0,86*). Die Cs-Retention scheint vor allem mit der Anwesenheit von Zwischenschichtkeilen an den Glimmer/Vermikulit XY Grenzflächen im Zusammenhang zu stehen. [U.W.]RésuméSept kaolins de Géorgie (Sud Est des E.U.) s’étageant de grade commercial haut à bas ont été caractérisés par diffraction poudrée aux rayons-X et par des techniques chimiques pour établir que la variation en quantité était causée par des impuretés. Les capacités d’échange de cations Ca et Cs (CEC) ont varié de 2,67 à 8,17 et de 3,29 à 8,77 meq/100 g, respectivement. Les analyses de dissolution sélective et de corrélation ont intensément indiqué que les minéraux expansibles 2:1, la smectite en particulier, (1,25,9%), étaient responsables pour la plupart des variations observées dans Ca CEC (r = 0,85*). La surface externe CEC de la kaolinite s’étageait de 0 a 1 meq/100 g. La corrélation positive significative (r = 0,90**) entre Ca CEC et le contenu en mica-K (0–3,9%) suggère que Ca CEC peut être apparenté au degré d’altération du mica à travers un stage minéral expansible. La capacité de retention-Cs (0,19–1,14 meq/100 g) était apparentée de près au contenu en vermiculite mesuré Cs (r = 0,80*), et ce contenu plus la surface spécifique(R = 0,93**) ou le contenu en mica (R = 0,86*). Larétention de Cs semblaitètre principalement apparentée à la présence de parties interfeuillets aux interfaces mica/vermiculite XY. [D.J.]

Robust geochemical indices for redox and weathering on a granitic laterite landscape in central Uganda

Abstract Most geochemical studies of laterite formation have been based on detailed examinations of a small number of hopefully “representative” profiles. Due to spatial variability in bedrock composition and geomorphic processes that sort materials by size, density, and other properties, experimental designs developed for the geochemical analysis of single soil profiles are of limited use in landscape investigations. In this study of a laterite landscape on a granitic rock in Uganda, we used element concentrations from neutron activation analysis (NAA) to develop a suite of robust geochemical indices for landscape-scale analysis of weathering and oxidation-reduction (redox) processes in laterite formation. To capture K-feldspar weathering the K/Hf ratio served as a good proxy for feldspar/zircon mineral ratios while K/Na ratios indexed feldspar concentration to quartz. Using principal components analysis, we derived an index of Rare-Earth Elements (REE) leaching (relative to Th), and an index of Heavy to Light REE (HREE/LREE) fractionation. We used the Fe/Sc ratio to ascertain the absolute (redox) vs. relative (weathering) Fe accumulation in laterite formation—with absolute Fe accumulation also associated with lower Fe/As ratios. Mn-Ce-Ba nodules and concretions were identified by characteristically high Ce values—with individual Ce values as high as 5000 μg g −1 . The relatively large amounts of K-feldspar (K avg is 4000 μg g −1 ) and low degree of HREE/LREE fractionation suggest that this landscape is weathered less than generally assumed. The Fe- and Mn-derived indices highlight the important role of oxidation-reduction processes in iron accumulation. Overall, our findings suggest that the current ferricrete-mantled hilltops were formerly located on a lower, poorly drained part of the landscape.

Determination of mononuclear dissolved aluminum in near-neutral waters

Abstract Concentrations of mononuclear dissolved Al are determined in samples of natural waters and of solutions separated from experimental suspensions of clay minerals, by solvent extraction and spectrophotometry of aluminum 8-hydroxyquinoline. Samples are treated with a solution of 8-hydroxyquinoline buffered at pH 5.0 by sodium acetate-acetic acid. The resulting Al-hydroxyquinoline complex is extracted immediately into 2.00 ml toluene, to prevent inclusion of secondary mononuclear Al released from poly-nuclear Al(OH)x ions or aluminous solids. After centrifugation-separation of the toluene and aqueous phases, the overlying toluene layer is transferred to a spectrophotometer equipped with a miniature fixed quartz cell. A detection limit of 0.2 μg Al l−1 can be attained with 100-ml samples, and at 1.0 μg Al l−1, the analytical uncertainty is ± 10% or less. Preliminary extractions with chloroform solutions of diethylammonium diethyldithiocarbamate eliminate interferences due to dissolved V, Fe, Ni, Cu, Ga, Mo and U. Extracting the Al-hydroxyquinoline complex at pH 8.3 eliminates any interference due to F. The procedure was used to measure the solubility of gibbsite over the pH range 5–8.

Rare-earth elements, Co, Sc and Hf in the Steens Mountain basalts

Abstract The concentrations of the rare-earth elements (REE) in 52 of 70 consecutive lava flows from Steens Mountain, Oregon, are reported. The concentrations of Co, Sc and Hf were measured in 17 of the flows. Logarithmic partitioning theory is used to correlate the concentrations of the trace elements with the major element and mineralogical compositions of the samples. Production of the basalts by partial melting requires a parent material that has concentrations of the REE that are several times those of chondrites. Those samples enriched in alumina have positive Eu anomalies compared to chondrites and those depleted in alumina have negative Eu anomalies. Production of these samples by partial melting is unlikely because the Eu anomalies would require that plagioclase was stable under the conditions of melting. All of the samples can be related to a single parent magma by fractional crystallization processes. The concentrations of the trace elements are controlled by the addition or removal of plagioclase and the removal of clinopyroxene. Samples with high concentrations of alumina are interpreted as plagioclase cumulates while those with low concentrations of alumina are residual liquids produced by crystallization of plagioclase. During fractional crystallization, the concentrations of the light REE are increased selectively in the residual liquid. The material that must be crystallized and removed during fractional crystallization has characteristics that are similar to those reported for gabbros.

Major and trace elements in igneous rocks from apollo 15

The concentrations of major and trace elements have been determined in igneous rocks from Apollo 15. All materials analyzed have typical depletions of Eu except for minerals separated from sample 15085. Four samples have concentrations of trace elements that are similar to those of KREEP. The samples of mare basalt from Apollo 15 have higher concentrations of FeO, MgO, Mn, and Cr and lower concentrations of CaO, Na2O, K2O, and rare-earth elements (REE) as compared to the samples of mare basalt from Apollos 11, 12, and 14. The samples can be divided into two groups on the basis of their normative compositions. One group is quartz normative and has low concentrations of FeO while the other is olivine normative and has high concentrations of FeO. The trace element data indicate that the samples of olivine normative basalt could be from different portions of a single lava flow. At least two and possible three parent magmas can be identified from the samples of the quartz normative group on the basis of their concentration ratios of Sm to Eu. Within each group, the compositions of the samples appear to be related by crystallization of olivine or pyroxene. Significant variations of the ratio of concentration of Sm to Eu cannot be produced without plagioclase-liquid equilibrium. The source material of mare basalt may be depleted in Eu. Alternatively, the magmas may have assimilated a small volume of material similar to KREEP.