A kinetic study of nickel complexation in model systems by adsorptive cathodic stripping voltammetry

Nickel speciation in the presence of different sources and fractions of dissolved organic matter

This study evaluated nickel (Ni) speciation in the presence of different fractions (humic acid (HA), fulvic acid (FA)) and sources (natural sediment, Suwannee River, peat moss) of dissolved organic matter (DOM) at Ni concentrations toxicologically relevant to the freshwater amphipod, Hyalella azteca. The free Ni ion, Ni(2+), was measured in reconstituted water (with or without DOM) using a miniaturized ion-exchange technique (IET). Ni speciation from these experiments was compared to calculated results obtained from equilibrium modelling (WHAM, Model VI). While it is known that Ni will complex with DOM, it was found that under acutely toxic Ni exposure concentrations ([Ni(Total)]=5mg/L, or 85.1 microM) representative surface-water DOC concentrations ( approximately 10mg/L) played little or no role in Ni speciation. Conversely, at sublethal Ni exposure concentrations ([Ni(Total)]=0.2 and 0.5 microg/L, or 3.4 and 8.51 microM, respectively) DOM significantly affected Ni speciation with [Ni(2+)] decreasing with increasing concentration of DOM. It was found that for similar concentrations of DOC (same fraction, different sources), the measured Ni(2+) concentrations were reduced (relative to the control), but similar to one another. Conversely, at similar DOC concentrations, the HA fraction reduced Ni(2+) levels to a greater extent than the associated FA fraction. Overall, this study provides proof of principle that Suwannee River and peat humic substances are suitable analogues for natural sediment pore-water DOM when evaluating Ni bioavailability in freshwater.

Speciation of nickel in surface waters measured with the Donnan membrane technique

The evaluation of the ecotoxicological risk of nickel (Ni) in surface water is hampered by a lack of speciation data. Six surface waters were sampled and speciation of Ni(II) was measured by the Donnan membrane technique (DMT) combined with radiochemical determination of 63Ni. The free Ni2+ ion fraction in the dissolved (<0.45 microm) phase was determined at background Ni concentration ((4-8) x 10(-8) M) and at concentrations in the range of toxicity thresholds for the Ni sensitive species Cerodaphnia dubia (5 x 10(-8) to 2 x 10(-6) M). The free ion fraction ranged from 4 to 45% at background Ni and increased with increasing Ni concentration and water hardness and with decreasing pH. The equilibration time after addition of Ni2+ (3h-7d) did not significantly change the measured free ion fraction. Predictions of the Humic-Ion Binding Model WHAM (Windermere Humic Aqueous Model) VI overestimated the observed free Ni2+ fraction (median>two-fold), even when assuming that all dissolved organic matter (DOM) was present as fulvic acid (FA). The impact of several model parameters affecting the prediction of Ni speciation were evaluated, including the solubility product of Fe(OH)3, which affects the Fe competition for complexation by DOM. The best fit (R2=0.88) was obtained by increasing only the distribution term DeltaLK2, which modifies the binding strength of multi-dentate sites, to accommodate the observed dependence of free ion fraction on Ni concentration.

Chemical speciation of Co, Ni, Cu, and Zn in mine effluents and effects of dilution of the effluent on release of the above metals from their metal–dissolved organic carbon (DOC) complexes

The paper explores the lability of DOC complexes of Co, Ni, Cu, and Zn in the mining effluent, and investigates the effects of dilution of the effluent on the release of metals from the metal-DOC complexes. This study was done using competing ligand exchange method in conjunction with adsorptive cathodic stripping voltammetry on effluent samples from Copper Cliff Mine, Sudbury, Canada, using two samples of the effluent: one, undiluted (100%) effluent, and the other, diluted (45%) effluent. The dilution was done with tap water in order to determine the effects of dilution on the metal complexes in the effluents when they were discharged into receiving streams of freshwaters. The dilution of the effluent had a small effect on release of Cu from the Cu-DOC complexes, but had a much greater effect on the release of Zn from the Zn-DOC complexes. The release of Ni and Co from their DOC complexes decreased drastically on dilution of the effluent. The much greater release of Cu from the Cu-DOC complexes compared to the release of Ni, Co, and Zn from their DOC complexes in both the undiluted and the diluted effluent was probably due to the higher absolute concentration of Cu and the higher [Cu]/[DOC] ratio. The drastic decrease in the release of Ni and Co from the Ni- and Co-DOC complexes in the diluted (45%) effluent compared with the undiluted (100%) effluent probably resulted from strengthening of the metal-DOC bond due to the reduced screening of charges by a smaller concentration of Ca2+ in the diluted (45%) effluent. This work also shows that change in the ionic strength produces conformational changes in and in aggregation and precipitation of the DOC and also changes in electrostatic interactions between the metal cations and the humate polyanions.

Cascade ultrafiltration and competing ligand exchange for kinetic speciation of aluminium, iron, and nickel in fresh water

Kinetic speciation of nickel, aluminium, and iron in fresh water has been investigated by cascade ultrafiltration followed by competing ligand exchange of the ultrafiltered fractions. Graphite furnace atomic absorption spectrometry was used to measure the kinetics of metal complex dissociation. Dissolved metal species were fractionated by cascade ultrafiltration. Metal speciation in each ultrafiltered fraction was then characterized as free metal ions, "labile" metal complexes (with dissociation rate constants >/=10(-3) s(-1)), "slowly labile" metal complexes (with dissociation rate constants >10(-6) s(-1)), and "inert" metal complexes (with dissociation rate constants <10(-6) s(-1)). The experimental results were compared with the predictions of a computer-based equilibrium speciation model, the Windermere humic aqueous model (WHAM) V. Cascade ultrafiltration coupled with kinetic speciation of the metal species in each molecular weight cut-off (MWCO) fraction provided a more comprehensive picture and insight into the physical and the chemical characteristics of the metal species than either ultrafiltration or measurement of dissociation kinetics alone.

Influence of dissolved organic matter on nickel bioavailability and toxicity to Hyalella azteca in water-only exposures

Dissolved organic matter (DOM) is known to reduce the bioavailability of metals in aquatic systems. This study evaluated the effects of DOM from various sources (e.g., Little Bear Lake sediment, Suwannee River, peat moss) and various DOM fractions (humic acids, HA; fulvic acids, FA) on the bioavailability of nickel (Ni) to Hyalella azteca, a common freshwater benthic invertebrate. In particular, this study was conducted to evaluate the effect of surficial sediment DOM on Ni bioavailability. Short-term (48 h) acute toxicity tests with H. azteca conducted in synthetic water demonstrated that the aqueous Ni concentrations required for lethality were greater than what could be significantly complexed by environmentally relevant concentrations of dissolved organic carbon (DOC: 0.6-30.4 mg/L). At Ni concentrations sublethal to H. azteca (500 microg/L), the bioavailability of Ni was significantly reduced in the presence of representative surface water DOC concentrations regardless of DOC source or fraction. DOC fraction (i.e., FA and HA) differentially affected Ni speciation, but had little or no effect on Ni accumulation by H. azteca. Tissue Ni was found to be strongly dependent upon the Ni(2+) concentration in the exposure solutions and the Ni:DOC ratio. Overall, the concentration of DOC played a greater role than either DOC source or fraction in determining Ni speciation and hence bioavailability and toxicity to H. azteca.

Separation of low-molecular mass organic acid–metal complexes by high-performance liquid chromatography

The solution speciation of metals is a critical parameter controlling the bioavailability, solution-solid phase distribution and transport of metals in soils. The natural metal-complexing ligands that exist in soil solution include inorganic anions, inorganic colloids, organic humic substances, amino acids (notably phytosiderophores and bacterial siderophores) and low-molecular mass organic acids. The latter two groups are of particular significance in the soil surrounding plant roots (the rhizosphere). A number of analytical methodologies, encompassing computational, spectroscopic, physico-chemical and separation techniques, have been applied to the measurement of the solution speciation of metals in the environment. However, perhaps with the exception of the determination of the free metal cation, the majority of these techniques rarely provide species specific information. High-performance liquid chromatography (HPLC) coupled to a sensitive detection system, such as inductively coupled plasma mass spectrometry (ICP-MS) or electrospray ionisation mass spectrometry (ESI-MS), offers the possibility of separating and detecting metal-organic acid complexes at the very low concentrations normally found in the soil environment. This review, therefore, critically examines the literature reporting the HPLC separation of metal-organic acid complexes with reference to thermodynamic equilibrium and kinetic considerations. The limitations of HPLC techniques (and the use of thermodynamic equilibrium calculations to validate analytical results) are discussed and the metal complex characteristics necessary for chromatographic separation are described.

Kinetic speciation of Co(II), Ni(II), Cu(II), and Zn(II) in model solutions and freshwaters: lability and the d electron configuration

The kinetic speciation of Co(II), Ni(II), Cu(II), and Zn(II) in model solutions of a well-characterized fulvic acid (Laurentian fulvic acid), freshwater samples from the Rideau River (Ottawa, Ontario), and freshwater samples from the Sudbury (Ontario) area were investigated by the competing ligand exchange method using Chelex 100 as the competing ligand and by inductively coupled plasma-mass spectrometry to measure the dissociation kinetics. The metal species were quantitatively characterized by the rate coefficient for the first-order dissociation of metal complex to free metal ion. This technique can be applied to almost all elements and represents an important advance in our ability to investigate the kinetic availability of metal species in the freshwater environment. The order of the lability of the metal complexes, Co(II) > Ni(II) > Cu(II) < Zn(II), follows the reverse order of the ligand field stabilization energy with the exception of Cu(II); the behavior of Cu(II) is also due to the Jahn-Teller effect, which shortens the equatorial bonds and lengthens the axial bonds of a tetragonally distorted Cu(II)-L6 complex. This study has demonstrated a relationship between the lability of metal-DOM complexes of the 3d transition metals in freshwaters and their d electron configuration. This is the first time that the importance of the d electron configuration on the lability of metal complexes in the freshwater environment has been demonstrated. The slow complexation kinetics of both Ni(II) and Cu(II) suggestthatthe usual equilibrium assumption for freshwaters may be invalid.