Johan Schijf

The influence of phosphate coprecipitation on rare earth distributions in natural waters

Abstract Although the rare earth elements (REEs) are thought to participate in essentially all of the processes that influence metal distributions in seawater, quantitative descriptions of oceanic rare earth element (REE) distributions have nearly exclusively involved removal of these elements from solution via adsorption on settling particles. Laboratory investigations involving yttrium (Y) and all REEs, except Pm, demonstrate that phosphate coprecipitation of Y and the REEs creates solution concentration patterns characteristic of input-normalized Y and REE concentrations in seawater. Removal of the REEs, and Y, from solution as phosphate coprecipitates depletes REEs in solution relative to Y, depletes solutions in Sm relative to heavier and lighter elements (relative depletions of the middle REEs), and produces a variety of near-neighbor concentration anomalies (e.g., Euue5f8Gdue5f8Tb, Dyue5f8Erue5f8Yb, and Erue5f8Ybue5f8Lu) which have been reported in high precision analyses of open ocean waters.

Comparative Scavenging of Yttrium and the Rare Earth Elements in Seawater: Competitive Influences of Solution and Surface Chemistry

Distribution coefficients were obtained for yttrium and the rare earth elements (YREEs) in aqueous solutions containing freshly precipitated hydroxides of trivalent cations (Fe3+, Al3+, Ga3+, and In3+). Observed patterns of logiKS–, where iKS = [MSi][M3+]−1[Si]−1, [MSi] is the concentration of a sorbed YREE, [M3+] is the concentration of a free hydrated YREE ion, and [Si] is the concentration of a sorptive solid substrate (Fe(III), Al, Ga, In)– exhibited similarities to patterns of YREE solution complexation constants with hydroxide (OHβ1) and fluoride (Fβ1), but also distinct differences. The logiKS pattern for YREE sorption on Al hydroxide precipitates is very similar to the pattern of YREE hydroxide stability constants (logOHβ1) in solution. Linear free-energy relationships between logiKS and logOHβ1 showed excellent correlation for YREE sorption on Al hydroxide precipitates, good correlation for YREE sorption on Ga or In hydroxide precipitates, yet poor correlation for YREE sorption on Fe(III) hydroxide precipitates. Whereas the correlation between logiKS and logFβ1 was generally poor, patterns of log(iKS/Fβ1) displayed substantially increased smoothness compared to patterns of logiKS. This indicates that the conspicuous sequence of inflections along the YREE series in the patterns of logiKS and logFβ1 is very similar, particularly for In and Fe(III) hydroxide precipitates. While the logiKS patterns obtained with Fe(III) hydroxide precipitates in this work are quite distinct from those obtained with Al, Ga, and In hydroxide precipitates, they are in good agreement with patterns of YREE sorption on ferric oxyhydroxide precipitates reported by others. Furthermore, our logiKS patterns for Fe(III) hydroxide precipitates bear a striking resemblance to predicted logiKS patterns for natural surfaces that are based on YREE solution chemistry and shale-normalized YREE concentrations in seawater. Yttrium exhibits an itinerant behavior among the REEs: sorption of Y on Fe(III) hydroxide precipitates is intermediate to that of La and Ce, while for Al hydroxide precipitates Y sorption is similar to that of Eu. This behavior of Y can be rationalized from the propensities of different YREEs for covalent vs. ionic interactions. The relatively high shale-normalized concentration of Y in seawater can be explained in terms of primarily covalent YREE interactions with scavenging particulate matter, whereby Y behaves as a light REE, and primarily ionic interactions with solution ligands, whereby Y behaves as a heavy REE.

Acantharians: a missing link in the oceanic biogeochemistry of barium

Abstract Inductively coupled plasma mass spectrometry was used to analyze the Ba and Sr concentrations of the celestite (SrSO4) skeletons and cysts of individual acantharian specimens obtained from four diverse areas of the world’s oceans. Acantharian celestite Ba/Sr mole ratios (χBa/χSr) averaged 2.6×10-3 with minimum and maximum values of 6.1×10-4 and 2.5×10-2. Celestite Ba/Sr mole ratios were compared to dissolved Ba and Sr concentration ratios ([Ba2+]T/[Sr2+]T) derived from GEOSECS stations that most closely corresponded to acantharian collection sites. Resultant Ba/Sr distribution coefficients (DBa/Sr=(χBa/χSr)/([Ba2+]T/[Sr2+]T)) in diverse areas of the world’s oceans are on the order of three or larger. These data, in conjunction with observations of acantharian mediated Sr2+ depletions in the upper ocean, indicate that acantharians play a substantial role in the global oceanic Ba budget. Observation of Ba enrichments during celestite formation is consistent with expectations based on solidsolution–aqueous-solution precipitation dynamics and the much lower solubility of BaSO4 compared to SrSO4. Furthermore, the small solubility product of RaSO4 relative to SrSO4 and BaSO4 indicates that Ra should be enriched in both celestite and barite. Consequently, acantharians may have a substantial influence on the oceanic distributions of both Ba and Ra.

Stability constants for mono- and dioxalato-complexes of Y and the REE, potentially important species in groundwaters and surface freshwaters

We present the first measured set of stability constants for mono- and dioxalato-complexes of yttrium and all rare earths except Pm (Y+REE), Oxβn = [MOxn3−2n] [M3+]−1 [Ox2−]−n(where [ ] ≡ concentrations, M ≡ Y+REE, and Ox2− ≡ C2O42−). Aqueous solutions of Y+REE were titrated with oxalic acid in the presence of a cation-exchange resin, and Y+REE concentrations in the solution phase were measured by ICP-MS. This method allows investigation of all Y+REE simultaneously under identical conditions and is thus very sensitive to subtle inter-element variations in log Oxβn. Experiments were performed at a single ionic strength (I = 0.05 M), but at two values of pH. Patterns of log Oxβ1 and log Oxβ2, determined from our experiments, are similar in shape and reminiscent of those for carbonato-complexes. The average ratio of stepwise stability constants K2/K1 = Oxβ2/(Oxβ1)2 is 0.05 ± 0.02 for Y+REE excluding La and Ce. n nLiterature values of Oxβ1(Eu) for 0.03 mol/L ≤ I ≤ 1 mol/L were used to derive the relation log Oxβ1(Eu) = log Oxβ10(Eu) − 6.132√I/(1 + 1.47√I) + 0.902I, where log Oxβ10(Eu) is the stability constant at infinite dilution. Applying this relation to all Y+REE, the following values of log Oxβ10 (at zero ionic strength) were found: 6.66 (Y), 5.87 (La), 5.97 (Ce), 6.25 (Pr), 6.31 (Nd), 6.43 (Sm), 6.52 (Eu), 6.53 (Gd), 6.63 (Tb), 6.74 (Dy), 6.77 (Ho), 6.83 (Er), 6.89 (Tm), 6.95 (Yb), 6.96 (Lu). These values, which are based on direct measurements for each individual Y+REE, agree quite well with published extrapolations that are mostly based on linear free-energy relationships. n nTotal oxalate concentrations of 10−5−10−3 M have been reported for soil solutions. Free oxalate ions persist at much lower pH than free carbonate ions and a simple speciation model demonstrates that oxalato-complexes can dominate Y+REE speciation in mildly acidic groundwaters of low-to-moderate alkalinity.

Determination of stability constants for the mono- and difluoro-complexes of Y and the REE, using a cation-exchange resin and ICP-MS

Abstract Stability constants of the form F β n =[MF n 3− n ][M 3+ ] −1 [F − ] − n (where brackets denote the concentrations of complexes MF n 3− n , free metal ions M 3+ and free fluoride F − ) were determined for the mono- and difluoro-complexes ( n =1, 2) of yttrium and the rare earth elements (Y+REE). A new method was developed, using equilibration with Bio-Rad AG 50W-X2 cation-exchange resin and analysis of the solution phase using inductively-coupled plasma mass spectrometry (ICP-MS). This method enabled us to measure all stability constants under identical conditions and is, therefore, especially sensitive to subtle inter-element variations in F β 1 and F β 2 . The pattern of logxa0 F β 1 differs distinctly from the majority of those in the literature and contains more structure. The set of F β 1 values that is commonly used for modeling REE speciation in natural waters appears to be in error for REE heavier than Eu. The pattern of logxa0 F β 2 is similar in shape to that of logxa0 F β 1 , but substantially deviates for Ce. This is reflected in the pattern of stepwise stability constant ratios across the Y+REE series: the average value of K 2 / K 1 = F β 2 /( F β 1 ) 2 is 0.07±0.03 for Y+REE excluding Ce, whereas for Ce it is 0.49. While anomalous behaviour of Ce has been reported for many environments and is known to be caused by its unique redox chemistry, only Ce(III) should be present under the conditions of these experiments.

Late Precambrian Balkan-Carpathian Ophiolite - a slice of the Pan-African ocean crust? Geochemical and tectonic insights from the Tcherni Vrah and Deli Jovan massifs, Bulgaria and Serbia

Abstract The Balkan-Carpathian ophiolite (BCO), which outcrops in Bulgaria, Serbia and Romania, is a Late Precambrian (563xa0Ma) mafic/ultramafic complex unique in that it has not been strongly deformed or metamorphosed, as have most other basement sequences in Alpine Europe. Samples collected for study from the Tcherni Vrah and Deli Jovan segments of BCO include cumulate dunites, troctolites, wehrlites and plagioclase wehrlites; olivine and amphibole-bearing gabbros; anorthosites; diabases and microgabbros; and basalts representing massive flows, dikes, and pillow lavas, as well as hyaloclastites and umbers (preserved sedimentary cover). Relict Ol, Cpx and Hbl in cumulate peridotites indicate original orthocumulate textures. Plagioclase in troctolites and anorthosites range from An60 to An70. Cumulate gabbro textures range from ophitic to poikilitic, with an inferred crystallization order of Ol-(Plag+Cpx)-Hbl. The extrusive rocks exhibit poikilitic, ophitic and intersertal textures, with Cpx and/or Plag (Oligoclase-Andesine) phenocrysts. The major opaques are Ti-Magnetite and Ilmenite. The metamorphic paragenesis in the mafic samples is Chl-Trem-Ep, whereas the ultramafic rocks show variable degrees of serpentinization, with lizardite and antigorite as dominant phases. Our samples are compositionally and geochemically similar to modern oceanic crust. Major element, trace element and rare earth element (REE) signatures in BCO basalts are comparable to those of MORB. In terms of basalt and dike composition, the BCO is a ‘high-Ti’ or ‘oceanic’ ophiolite, based on the classification scheme of Serri [Earth Planet. Sci. Lett. 52 (1981) 203]. Our petrologic and geochemical results, combined with the tectonic position of the BCO massifs (overlain by and in contact with Late Cambrian island arc and back-arc sequences), suggest that the BCO may have formed in a mid-ocean ridge setting. If the BCO records the existence of a Precambrian ocean basin, then there may be a relationship between the BCO and the Pan-African ophiolites from the Arabian–Nubian Shield. We suggest that the BCO is the missing link between the Pan-African and the Avalonian–Cadomian peripheral orogens of Murphy and Nance [Geology 19 (1995) 469].

Mg/Ca ratios in stressed foraminifera, Amphistegina gibbosa, from the Florida Keys

Abstract Since 1991, significant proportions of Amphistegina populations in the Florida Keys and elsewhere have exhibited stress symptoms that include loss of symbiont color (‘bleaching’), anomalous shell breakage and reproductive damage. Previous studies of other taxa have reported elevated Mg/Ca ratios in tests from pollution-stressed foraminiferal populations. The purpose of this study was to test the hypothesis that anomalous shell breakage in stressed Amphistegina gibbosa is the result of loss of control of calcification, resulting in elevated concentrations of Mg that weaken the crystal structure of the test. Analysis of Mg and Ca concentrations in A. gibbosa tests, using an Inductively-Coupled Plasma Mass Spectrometer, revealed normal Mg/Ca (2–5xa0mol%) in all specimens analyzed, including normal specimens collected in 1982 (prior to the onset of the stress event), and both normal and broken specimens collected quarterly from afflicted populations in 1996. Analysis of specimens from the high Mg calcite taxon, Archaias angulatus , revealed Mg/Ca of 10–14xa0mol%. This study, which presents an ICP-MS procedure that can be used to assess Mg/Ca in individual foraminifera, does not support the hypothesis that shell breakage in stressed Amphistegina results from disruption of calcification at the ionic level.

Investigation of the ionic strength dependence of Ulva lactuca acid functional group pKas by manual alkalimetric titrations.

We performed a series of manual alkalimetric titrations in NaCl solutions (0.01-5.0 M) at T = 25 degrees C on both fresh and dehydrated samples of the marine chlorophyte Ulva lactuca (sea lettuce), a strong metal accumulator holding considerable promise in biosorbent and biomonitor applications. Functional groups were characterized in terms of their number, site densities, and acid dissociation constants (pK(a)s). FITEQL4.0 modeling shows that, at any ionic strength, titration curves for dehydrated biomass in the pH range 2-10 are adequately described by three functional groups with remarkably uniform site densities of about 5 x 10(-4) mol/g. Lower site densities for fresh U. lactuca are consistent with approximately 87% water content. The pK(a)s display pronounced ionic strength dependent behavior obeying an extended Debye-Huckel relation. Extrapolation to I = 0 yields values of 4.26 +/- 0.04, 6.44 +/- 0.02, and 9.56 +/- 0.04. This information by itself is insufficient to unambiguously identify the groups. Similar site densities suggest that all three are linked to major molecular building blocks of the cell material, pointing to carboxylic acids, phosphate esters, and amines as likely candidates. Highly acidic sulfate esters, not detected in our titrations, may also play a role in trace metal adsorption on U. lactuca.

Comparative Coprecipitation of Phosphate and Arsenate with Yttrium and the Rare Earths: The Influence of Solution Complexation

AbstractCoprecipitation of yttrium (Y) and rare earth elements (REEs) with phosphate and arsenate removes these elements from solution in variable proportions. During both phosphate and arsenate Coprecipitation, middle REEs (Sm and Eu) are progressively depleted in solution relative to heavier and lighter elements. Solution complexation by oxalate (Ox2-) influences Y and REE removal patterns by strongly enhancing the retention of Y and the heaviest REEs in solution. The extent of this enhancement is well described by a quantitative account of the comparative solution complexation of Y and REEs as M(Ox)+ and M(Ox)n