The permeation liquid membrane as a sensor for free nickel in aqueous samples

Polymer inclusion membrane to access Zn speciation: Comparison with root uptake

Metal speciation studies can be performed with a new technique based on a functionalized membrane. The estimation of not only the total amount of metal, but also the metal available to living organisms is very important. In this context, we have investigated the use of a polymer inclusion membrane (PIM) in a new tool for the determination of free metal ion concentration. In order to check the usefulness of PIM devices in metal speciation studies and metal availability to potato plants (Solanum tuberosum), Zn has been chosen as a case study. The PIM designed for Zn transport uses polyvinyl chloride (PVC) as polymer and di-(2-ethylhexyl) phosphoric acid (D2EHPA) as carrier, with 0.01M nitric acid in the receiving solution. The stability of the PIM has been demonstrated and good linearity of PIM-device fluxes (J_PIM) with free metal concentration was observed for total metal concentrations ranging from 3μM up to 70μM. The presence of different ligands, such as ethylenediaminetetraacetic acid (EDTA), humic acid (HA) and citrate, greatly influences the measured J_PIM because the formation of metal complexes in the donor phase decreases the free Zn concentration in the sample. Good correlation has been found when comparing PIM fluxes and metal accumulation in potato plants roots in the presence of EDTA. But, the root uptake did not change when adding citrate and HA to the hydroponic medium, so the uptake does not always follows the Free Ion Activity Model (FIAM). These ligands might induce physiological changes in the roots and enhance metal uptake.

Passive sampling methods for contaminated sediments: state of the science for metals

"Dissolved" concentrations of contaminants in sediment porewater (Cfree ) provide a more relevant exposure metric for risk assessment than do total concentrations. Passive sampling methods (PSMs) for estimating Cfree offer the potential for cost-efficient and accurate in situ characterization of Cfree for inorganic sediment contaminants. In contrast to the PSMs validated and applied for organic contaminants, the various passive sampling devices developed for metals, metalloids, and some nonmetals (collectively termed "metals") have been exploited to a limited extent, despite recognized advantages that include low detection limits, detection of time-averaged trends, high spatial resolution, information about dissolved metal speciation, and the ability to capture episodic events and cyclic changes that may be missed by occasional grab sampling. We summarize the PSM approaches for assessing metal toxicity to, and bioaccumulation by, sediment-dwelling biota, including the recognized advantages and limitations of each approach, the need for standardization, and further work needed to facilitate broader acceptance and application of PSM-derived information by decision makers.

Nickel (II) Preconcentration and Speciation Analysis During Transport from Aqueous Solutions Using a Hollow-fiber Permeation Liquid Membrane (HFPLM) Device

Nickel (II) preconcentration and speciation analysis using a hollow fiber supported liquid membrane (HFSLM) device was studied. A counterflow of protons coupled to complexation with formate provided the driving force of the process, while Kelex 100 was employed as carrier. The influence of variables related to module configuration (acceptor pH and carrier concentration) and to the sample properties (donor pH) on the preconcentration factor, E, was simultaneously studied and optimized using a 3 factor Doehlert matrix response surface methodology. The effect of metal concentration was studied as well. Preconcentration factors as high as 4240 were observed depending on the values of the different variables. The effects of the presence of inorganic anions (NO₂⁻, SO₄²⁻, Cl⁻, NO₃⁻, CO₃²⁻, CN⁻) and dissolved organic matter (DOM) in the form of humic acids were additionally considered in order to carry out a speciation analysis study. Nickel preconcentration was observed to be independent of both effects, except when cyanide was present in the donor phase. A characterization of the transport regime was performed through the analysis of the dependence of E on the temperature. E increases with the increase in temperature according to the equation E(K) = −8617.3 + 30.5T with an activation energy of 56.7 kJ mol⁻¹ suggesting a kinetic-controlled regime. Sample depletion ranged from 12 to 1.2% depending on the volume of the donor phase (100 to 1000 mL, respectively).

Trace metal speciation and bioavailability in surface waters of the Black Sea coastal area evaluated by HF-PLM and DGT

Trace metal speciation in seawater from the Bulgarian Black Sea coast was studied in situ by hollow fiber permeation liquid membrane (HF-PLM) and by diffusion gradients in thin-film gels (DGT). The concentrations of Cd, Cu, Ni, and Pb determined by HF-PLM were lower than those measured by DGT, in agreement with their analytical windows, e.g., free metal ions provided by the HF-PLM and dynamic (mobile and labile) species by the DGT. The obtained suite of data was further used to evaluate the bioavailability of these metals to the microorganisms, which was then compared with experimental results of metal uptake to green microalga Chlorella salina. Uptake fluxes of the Cd, Cu, Ni, and Pb to C. salina, were predicted from the measured HF-PLM concentrations and laboratory experimentation in artificial seawater, in agreement with theoretical considerations. The HF-PLM and DGT appear to be promising analytical techniques for speciation and bioavailability studies in complex environmental media and allow improved understanding of the role of different chemical species in metal bioavailability (and impact) in seawaters.

Ni uptake by a green alga. 1. Validation of equilibrium models for complexation effects

Short-term (< 60 min) internalization fluxes (J(int)) by the green alga Chlamydomonas reinhardtii were measured in the presence of a variety of ligands forming hydrophilic (citric, diglycolic, or nitrilotriacetic acids), amphiphilic (Suwannee River fulvic and humic acids), and hydrophobic (oxine, diethyldithiocarbamate) Ni complexes. Free nickel concentrations, ranging from 10(-10) to 10(-3) M, were evaluated by thermodynamic modeling and by a resin exchange technique. Ni internalization appeared to occur via a single transport site that was characterized by a conditional stability constant (pH 6.0, I = 10(-2) M) of 10(5.1) M(-1). In the presence of humic substances, Ni uptake could be quantitatively predicted on the basis of Ni2+ concentrations for two algae: C. reinhardtii and Chlorella kesslerii. In the presence of the hydrophobic ligands, an increase in permeability was observed due to the formation of neutral, lipophilic complexes.