Kristin Vala Ragnarsdottir

The dissolution of apatite in the presence of aqueous metal cations at pH 2–7

Apatite dissolution was studied at 25°C in a series of batch experiments carried out within the pH range of 2–7 with or without the presence of aqueous Pb2+ or Cd2+. The synthetic, microcrystalline hydroxylapatite used in the majority of the experiments was found to have a significantly higher solubility than natural fluorapatite, but a lower dissolution rate. The dissolution rates of both phases increased with decreasing pH. When Pb2+ was present in solution in contact with synthetic hydroxylapatite its concentration decreased over a time interval ranging from several days to several weeks, to a steady state minimum. The rate of Pb2+ loss from solution was sensitive to acidity, and progressed faster at lower pH, but maximum loss was independent of pH. Calcium release to solution matched aqueous lead loss on a mole for mole basis. By the end of each experiment mass calculations suggest that all apatite had been consumed regardless of reaction rate and pH. The solid residue was newly crystallised Pb–hydroxylapatite. This reaction was also observed in situ using Atomic Force Microscopy (AFM) and was found to take place epitaxially onto apatite surfaces. The concentration of aqueous Cd2+ in solution was also reduced in the presence of hydroxylapatite. Cadmium losses were, however, substantially lower. Unlike Pb2+, the maximum amount of Cd2+ lost from solution was a function of pH, and was higher as solution composition approached neutral pH. Cadmium was present in the solid residue at the end of these experiments, probably as a Ca–Cd phosphate solid solution. This work suggests that the interaction between apatite and metals in solution is controlled by apatite dissolution and results in the precipitation of new metal phosphates. The new phosphates nucleate heterogeneously onto the hydroxylapatite surfaces, which acts as a catalyst for the reaction.

The mechanism of cadmium surface complexation on iron oxyhydroxide minerals

Abstract Many sediment and soil systems have become significantly contaminated with cadmium, and earth scientists are now required to make increasingly accurate predictions of the risks that this contamination poses. This necessitates an improved understanding of the processes that control the mobility and bioavailability of cadmium in the environment. With this in mind, we have studied the composition and structure of aqueous cadmium sorption complexes on the iron oxyhydroxide minerals goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH), and schwertmannite (Fe8O8(OH)6SO4) using extended X-ray adsorption fine structure spectroscopy. The results show that adsorption to all of the studied minerals occurs via inner sphere adsorption over a wide range of pH and cadmium concentrations. The bonding mechanism varies between minerals and appears to be governed by the availability of different types of adsorption site at the mineral surface. The geometry and relative stability of cadmium adsorption complexes on the goethite surface was predicted with ab initio quantum mechanical modelling. The modelling results, used in combination with the extended X-ray adsorption fine structure data, allow an unambiguous determination of the mechanism by which cadmium bonds to goethite. Cadmium adsorbs to goethite by the formation of bidentate surface complexes at corner sharing sites on the predominant (110) crystallographic surface. There is no evidence for significant cadmium adsorption to goethite at the supposedly more reactive edge sharing sites. This is probably because the edge sharing sites are only available on the (021) crystallographic surface, which comprises just ∼2% of the total mineral surface area. Conversely, cadmium adsorption on lepidocrocite occurs predominately by the formation of surface complexes at bi- and/or tridentate edge sharing sites. We explain the difference in extended X-ray adsorption fine structure results for cadmium adsorption on goethite and lepidocrocite by the greater availability of reactive edge sharing sites on lepidocrocite than on goethite. The structures of cadmium adsorption complexes on goethite and lepidocrocite appear to be unaffected by changes in pH and surface loading. There is no support for cadmium sorption to any of the studied minerals via the formation of an ordered precipitate, even at high pH and high cadmium concentration. Cadmium adsorption on akaganeite and schwertmannite also occurs via inner sphere bonding, but the mechanism(s) by which this occurs remains ambiguous.

The boron isotope systematics of Icelandic geothermal waters: 1. Meteoric water charged systems

We have measured the boron isotope composition and boron and chloride concentrations of 27 Icelandic geothermal fluids from both high- and low-temperature systems. The δ11B values range from −6.7‰ in the Krafla system, to +25.0‰ in a warm spring from the Southern Lowlands. In addition, we have also determined the δ11B values of basaltic glass from Nesjavellir (−5.3 ± 1.4‰) and travertine from Snaefellsnes (−22 ± 0.5‰). The B isotope and Cl/B systematics of the high-temperature systems are dominated by the composition of the local basalts. The lower temperature systems show evidence for mixing with B and Cl of a marine origin, together with some uptake of B into secondary mineral phases. The data from the Snaefellsnes geothermal system indicate that the fluids have undergone interaction with basalts that have undergone significant low-temperature alteration by seawater.

Dissolution kinetics of heulandite at pH 2–12 and 25°C

Abstract The dissolution rate of heulandite depends strongly on pH. Based on silica release, the logarithm of the steady-state dissolution rate at pH 2 is −13.1 mol cm−2 s−1. The logarithm of the rate decreases to −15.8 mol cm−2 s−1 at pH 7.2 and increases again to − 14.6 mol cm−2s−1 at pH 12.2. At low pH, Al is released preferentially to silica; but at intermediate and high pH, the release of silica appears to be congruent relative to Al. The change in dissolution rate with pH indicates that at low pH, the dissolution mechanism is controlled by the detachment of a positively charged Al species, > A1-OH2+. Below pH 5, however, a silica-rich surface layer is formed requiring diffusion through the layer. At intermediate and high pH, it is likely that the dissolution rate is controlled by the detachment of a negatively charged silica species, > Si −O−. The reaction order of the hydrogen ion under low pH conditions is 0.7, and the reaction order of the OH− ion is 0.3 at high pH. The measured dissolution rates indicate that a 1 mm heulandite crystal would dissolve in 300,000 yrs if the solution composition is maintained undersaturated.

Aqueous speciation of yttrium at temperatures from 25 to 340°C at Psat: an in situ EXAFS study

Abstract The solvation environment of aqueous yttrium in chloride bearing solutions was characterised by EXAFS spectroscopy at temperatures from 25 to 340°C. Four solution compositions were considered containing 0.1 M YCl 3 , 0.1 M YCl 3 +0.23 M HCl+1.0 M NaCl, 0.1 M YCl 3 +0.23 M HCl+2.0 M NaCl, and 0.05 M Y(NO 3 ) 3 , respectively. Yttrium was found to be surrounded by an inner coordination shell of H 2 O, the coordination number decreasing from 9–10 to ∼8 with increasing temperature from 25 to 340°C. The Y–O interatomic distance is constant 2.37±0.02 A at temperatures up to 340°C. Yttrium chloride inner sphere complexing is negligible up to 340°C for all solutions, including that containing 2.5 M chloride. This observation implies that chloride inner sphere complexes play an insignificant role in the solubility and transport of yttrium, and by analogy the heavy rare-earth elements in sub critical crustal fluids. Data analysis of EXAFS spectra of the 0.1 M YCl 3 solution at temperatures in excess of 250°C indicate the presence of yttrium atoms located at ∼3.6 A and ∼4.9 A from the central yttrium atom. This latter observation is consistent with yttrium polyatomic species formation, or polymerisation.

Removal of Uranium(VI), Lead(II) at the Surface of TiO2 Nanotubes Studied by X-Ray Photoelectron Spectroscopy

A thin film of well-ordered anatase TiO2 nanotubes prepared by anodic oxidation of titanium metal were synthesised and used as adsorbent medium for the purification of water from aqueous uranium and lead. The amount of subtracted metal ions was quantified by using X-ray photoelectron spectroscopy at the surface of the reacted TiO2 surface. Batch experiments for the sorption of U and Pb at the surface of the titania substrate were carried out in separated solution equilibrated with air of uranyl acetate and lead nitrate, in the pH range 3–9. For uranium, the experiments were also repeated in anoxic (N2) atmosphere. The amount of metal ions adsorbed onto the titania medium was quantified by measurements of the surface coverage expressed in atomic percent, by recording high-resolution XPS spectra in the Ti2p, U4f and Pb4f photoelectron regions. Adsorption of the uranyl species in air atmosphere as a function of pH showed an adsorption edge near pH 4 with a maximum at pH 7. At higher pH the presence of very stable uranyl–carbonate complexes prevented any further adsorption. Further adsorption increased until pH 8.5 was obtained when the uranyl solution was purged from dissolved CO2. Lead ion showed a sorption edge at pH 6, with a maximum uptake at pH 8. The results showed that the uptake of uranium and lead on the selected titania medium is remarkably sensitive to the solution pH. This study demonstrates the reliability of this type of material for treating water polluted with heavy metals as well as leachates from radioactive nuclear wastes.

An EXAFS spectroscopic study of aqueous antimony(III)-chloride complexation at temperatures from 25 to 250°C

Abstract The X-ray adsorption fine structure (EXAFS) spectroscopy of antimony(III)-chloride solutions were obtained at temperatures from 25 to 250°C at pressures corresponding to the liquid–vapor equilibrium curve for H2O. Two solution compositions were considered: solution A consisted of 0.042 M SbCl3+2.9 M HCl and solution B consisted of 0.1 M SbCl3+2.29 M HCl. Interpretation of resulting spectra indicates the presence of aqueous Sb–Cl inner sphere complexes at all investigated temperatures. The average number of chlorine ions in these complexes increases with increasing temperature over the range 25 to 250°C from ∼3.0 to ∼3.4 and from ∼2.6 to ∼2.9 for solutions A and B, respectively. These results also indicate an increasing average number of chloride ions per complex with increasing aqueous chloride concentration. The Sb–Cl interatomic distances for the two solutions are approximately equal and decrease from 2.42 to 2.38 A with increasing temperature over this range. This latter observation is consistent with theoretical models of aqueous complexation that predict decreasing aqueous species electrostatic radii with increasing temperature.

Antimony transport in hydrothermal solutions: an EXAFS study of antimony(V) complexation in alkaline sulfide and sulfide–chloride brines at temperatures from 25°C to 300°C at Psat

Existing electrochemical data suggest that under alkaline conditions, Sb(V) sulfide complexes may be stable under conditions as reducing as those found in hydrothermal ore solutions. To assess the nature of Sb(V) complexes in such solutions, X-ray absorption fine structure spectra (EXAFS) of antimony(V) solutions were obtained at temperatures from 25°C to 300°C at pressures corresponding to the liquid–vapour equilibrium curve for H2O. Three solution compositions were considered: Solution A consisted of 0.1 m Sb+1.15 m Na2S; solution B had the composition of 0.05 m Sb+0.2 m NaHS+0.06 m NaOH; and solution C consisted of 0.05 m Sb+0.2 m NaHS+0.06 m NaOH+1 m NaCl. At temperatures <150°C, the inner coordination shell of aqueous Sb(V) contains four sulfur atoms corresponding to the Sb(HS)4+ complex. Above 150°C, SH− ligands are replaced by OH− to form mixed-ligand Sb complexes. Antimony(V) atoms are found in a second coordination shell at temperatures greater than 250°C, indicating the existence of aqueous polyatomic clusters. No antimony(V) chloride complexation was found in the presence of HS−.

Speciation of tin (Sn2+ and Sn4+) in aqueous Cl solutions from 25°C to 350°C: an in situ EXAFS study

Abstract X-ray absorption fine structure spectroscopic (EXAFS) measurements were made of tin chloride solutions at temperatures from 25°C to 350°C. In Cl concentrations of 0.5 to 2.5 M, aqueous tin(II) forms SnCl n 2− n complexes with n increasing from 3.4 at 25°C to 3.9 at 250°C, indicating the presence of SnCl 3 − and SnCl 4 2− . Between 250°C and 350°C, SnCl 4 2− is the dominant species. In oxidized Cl brines, aqueous tin(IV) occurs as the SnCl 6 2− complex from 25°C to 250°C. Neither the SnCl 4 2− nor the SnCl 6 2− complexes have been previously observed. The existence of these complexes, however, implies that SnO 2 will be much more soluble than predicted by previous geochemical models. In particular, oxidized acidic brines will be capable of transporting tin as tin(IV) complexes.

Assessing Long Term Sustainability of Global Supply of Natural Resources and Materials

The human population has grown exponentially over the past century and is expected to increase to nine or ten billion by the year 2050 (Evans, 1998). This growth has been accompanied by an increasing rate of consumption of natural resources (Brown & Kane, 1994, Brown, 2009a,b). On several key resources, the use of materials and energy has increased faster than the population growth alone. At present, humans are challenging planetary boundaries and capacities (Humphreys et al., 2003, Rockstrom et al., 2009). For many fossil resources (energy, most metals and key elements), the rate of extraction is now so high that it can only with difficulty be further increased (Hubbert, 1956, Pogue & Hill, 1956, Ehrlich et al., 1992, Smil, 2001, 2002, Fillipelli, 2002, 2008, Greene et al., 2003, Arleklett, 2003, 2005, Hirsch et al., 2005, Gordon et al., 2006, Heinberg, 2007, Zittel & Schindler, 2007, Roskill Information Services, 2007a,b,c,d, 2008, 2009a,b, 2010a,b,c, 2011, Strahan, 2007, 2008, Ragnarsdottir et al., 2011, Sverdrup & Ragnarsdottir, 2011). In many cases, known resources are dwindling, because prospecting cannot find more. There have been several earlier warnings about the prospect of upcoming future material scarcity (Forrester, 1971, Meadows et al., 1972, 1992, 2004, Graedel & Allenby, 1995), though these have been seen as “interesting”, but have generally been shrugged off as academic studies. In the years after world war II, there has been a redefinition of success and wealth to imply increased consumption and material through-put (Friedman, 1962, Friedman & Friedman, 1980, Jackson, 2009). This success, reported as gross national product (GDP), has been adopted by most leaders of the world as a generic measure of success (growth), leading to enormous flows of materials, and as a result, waste. Fossil fuels are arguably the most essential modern commodity that may become scarce during the coming decades (Hubbert, 1966, 1972, 1982, Hirsh, 1992, Graedel et al., 1995, 2002, 2004), but rare minerals and metals, used, for example, in mobile phones, are also not in unlimited supply (Cohen, 2007, Ragnarsdottir, 2008). New technologies, such as transistors, pin-head capacitors, compound semiconductors, flat-screen liquid-crystal displays, light emitting diodes, electric car batteries, miniature magnets and thin-film solar cells therefore need to be developed according to the long-term availability of their key material ingredients.