Determination of \u3csub\u3eSO\u3c/sub\u3e\u3csub\u3e\u3csub\u3e4\u3c/sub\u3e\u3c/sub\u3eβ\u3csub\u3e1\u3c/sub\u3e for Yttrium and the Rare Earth Elements at I = 0.66 M and T = 25°C—implications for YREE Solution Speciation in Sulfate-rich Waters

We present a complete set of stability constants (SO4β1) for the monosulfato-complexes of yttrium and the rare earth elements (YREE), except Pm, at I = 0.66 m and t = 25°C, where SO4β1 = [MSO4+] × [M3+]−1[SO42−]−1 (M ≡ YREE and brackets indicate free ion concentrations on the molal scale). Stability constants were determined by investigating the solubility of BaSO4 in concentrated aqueous solutions of MCl3. This is the first complete set to be published in more than 30 years. The resulting SO4β1 pattern is very similar in shape to one reported by de Carvalho and Choppin (1967a) (I = 2 mol/L; t = 25°C) that has been largely ignored. Stability constants vary little between La and Sm, but display a weak maximum at Eu. Between Eu and Lu, SO4β1 decreases by 0.2 log units, substantially exceeding the ±0.02 log unit average analytical precision. The stability constant for Y is approximately equal to that for Er. Our SO4β1 pattern is consequently distinctly different from the consensus pattern, based on a single data set from 1954, which is essentially flat, with a range of only 0.07 log units between the lowest and highest SO4β1 values within the lanthanide series (excluding Y). Values of SO4β1 obtained in this work, in conjunction with the ion-pairing model of Millero and Schreiber (1982), allow prediction of SO4β1 between 0 and 1 m ionic strength. These results are used to assess both the absolute and relative extent of YREE sulfate complexation in acidic, sulfate-rich waters

Determination of the Side-Reaction Coefficient of Desferrioxamine B in Trace-Metal-Free Seawater

Funding for Open Access provided by the UMD Libraries Open Access Publishing Fund.Electrochemical techniques like adsorptive cathodic stripping voltammetry with competitive ligand equilibration (ACSV-CLE) can determine total concentrations of marine organic ligands and their conditional binding constants for specific metals, but cannot identify them. Individual organic ligands, isolated from microbial cultures or biosynthesized through genomics, can be structurally characterized via NMR and tandem MS analysis, but this is tedious and time-consuming. A complementary approach is to compare known properties of natural ligands, particularly their conditional binding constants, with those of model organic ligands, measured under suitable conditions. Such comparisons cannot be meaningfully interpreted unless the side-reaction coefficient (SRC) of the model ligand in seawater is thoroughly evaluated. We conducted series of potentiometric titrations, in non-coordinating medium at seawater ionic strength (0.7 M NaClO4) over a range of metal:ligand molar ratios, to study complexation of the siderophore desferrioxamine B (DFOB) with Mg and Ca, for which it has the highest affinity among the major seasalt cations. From similar titrations of acetohydroxamic acid in the absence and presence of methanesulfonate (mesylate), it was determined that Mg and Ca binding to this common DFOB counter-ion is not strong enough to interfere with the DFOB titrations. Stability constants were measured for all DFOB complexes with Mg and Ca including, for the first time, the bidentate complexes. No evidence was found for Mg and Ca coordination with the DFOB terminal amine. From the improved DFOB speciation, we calculated five SRCs for each of the five (de)protonated forms of DFOB in trace-metal-free seawater, yet we also present a more convenient definition of a single SRC that allows adjustment of all DFOB stability constants to seawater conditions, no matter which of these forms is selected as the “component” (reference species). An example of Cd speciation in seawater containing DFOB illustrates the non-trivial use of different SRCs for polyprotic, polydentate organic ligands

Effect of Mg and Ca on the Stability of the MRI Contrast Agent Gd–DTPA in Seawater

Gadolinium diethylenetriaminepentaacetic acid (Gd–DTPA) is widely applied as a contrast enhancer in medical MRI. As Gd–DTPA is only minimally captured in wastewater treatment plants (WTPs) or degraded by UV light and other oxidative processes, concentrations in rivers have increased globally by orders of magnitude following its introduction in 1987. The complex also seems impervious to estuarine scavenging and is beginning to emerge in coastal waters, yet it is unknown how its stability is changed by competition for the DTPA ligand from major seawater cations. We performed potentiometric titrations at seawater ionic strength (0.7 M NaClO4) to determine dissociation constants of the five DTPA carboxylic acid groups, as well as stability constants of Mg, Ca, and Gd complexes with the fully deprotonated and single-protonated ligand. These are in general agreement with literature values at low ionic strength and confirm that complexes with Ca are more stable than with Mg. A new finding, that the DTPA complexes of Mg and Ca appear to be hydrolyzed at elevated pH, implies that their coordination in these chelates is less than hexadentate, enabling additional competition with Gd from dinuclear Mg and Ca species. Side-reaction coefficients for trace-metal-free seawater, calculated from our results, suggest that the higher abundance of Mg and Ca may significantly destabilize Gd–DTPA in coastal waters, causing dissociation and release of as much as 15% of the organically complexed Gd from the ligand. This effect could magnify the particle-reactivity and bioavailability of anthropogenic Gd in sensitive estuarine habitats, indicating an urgent need to further study the fate of this contaminant in marine environments

Validation and application of a new microwave-digestion/ICP-MS method for the analysis of trace metals in tree increment cores

A method is presented for the routine analysis of eight trace metals (As, Cd, Co, Cr, Cu, Ni, Pb, V) in wood samples of 100–200 mg. The organic matrix is broken down by microwave digestion and the resulting solutions are analyzed by ICP-MS, after appropriate dilution, against an external multi-element calibration line. Detection limits are ∼200 ng metal per g of dry wood for Cr and Cu, and as low as 10–60 ng g−1 for the other six metals. Frequent analysis of the Certified Reference Material NJV 94-5 (Wood Fuel) and a series of spike recovery experiments gave chemical yields ranging from 90% for Cr to 103% for V, but only 80% for As. Precision is generally 2–7%, yet better for Cd and Cu (∼1%) and somewhat worse for As (∼20%). This new method is suitable for the analysis of tree increment cores cut into sections representing 1–5 years of growth and can be readily modified to include additional metals. As a practical application, increment cores of eastern cottonwood (Populus deltoides) from Ohio (USA) were analyzed for the period 1970–2009. Pairwise comparisons of cores taken from the same tree, or from adjacent trees at the same location, illustrate some acknowledged limitations of their use as a temporal record of trace metal loading in soils

Determination of Stability Constants for the Mono- and Difluoro-complexes of Y and the REE, Using a Cation-exchange Resin and ICP-MS

Stability constants of the form Fβn=[MFn3−n][M3+]−1[F−]−n (where brackets denote the concentrations of complexes MFn3−n, free metal ions M3+ 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 Fβ1 and Fβ2. The pattern of log Fβ1 differs distinctly from the majority of those in the literature and contains more structure. The set of Fβ1 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 log Fβ2 is similar in shape to that of log Fβ1, 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 K2/K1=Fβ2/(Fβ1)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

Rare earth element exchange through the Bosporus: The Black Sea as a net source of REEs to the Mediterranean Sea

The Bosporus is the only source of seawater to the Black Sea and helps to maintain the basin-wide salinity gradient that caused the Black Sea to become the largest permanently anoxic basin in the world, some 3000 years ago. Concentrations of dissolved rare earth elements (REEs) in each of the three layers of water that make up the Bosporus inflow/outflow system, substituted into a simple hydrographic model that evaluates the entrainment of outflowing Black Sea water in the inflowing Mediterranean Sea water, suggest that the Black Sea acts as a net source of REEs to the Mediterranean Sea. This holds true for Ce, which shows a considerable range of concentrations in the outflowing Black Sea water, even if the lower end of that range is taken to represent the Ce concentration of the Black Sea endmember.