Free Ion Concentration Research Articles

Spectrophotometric measurement of freshwater pH with purified meta‐cresol purple and phenol red

Impurities in indicator salts can significantly bias spectrophotometric pH determinations. In this work, two purified sulfonephthalein indicators, meta-cresol purple (mCP) and phenol red (PR), were tested for analysis of freshwater pH on the free hydrogen ion concentration scale. These two purified indicators were characterized for the first time under low ionic strength conditions, providing their molar absorption coefficients and dissociation constants along with their temperature dependence from 8 °C to 30 °C. At 25 °C, the infinite dilution constants ( pKIo) were determined to be 8.6606 and 8.0642 for mCP and PR, respectively. The accuracy and precision of the method, evaluated with a variety of buffers with known pH, were found to be +0.0014 pH units and ±0.0022 pH units, respectively (n = 30). The pH values of different freshwater samples were also determined using both indicators. The mCP and PR results were all within ± 0.01 pH units of each other with three out of seven pH differences within ± 0.001 pH units, indicating the high consistency between these two indicator methods. The work presented here is the first parallel comparison with two purified indicators used to determine pH of the same freshwater samples.

Evaluation of free/labile concentrations of trace metals in Athabasca oil sands region streams (Alberta, Canada) using diffusive gradient in thin films and a thermodynamic equilibrium model

The Athabasca's oil sands exploitation is controversial due to its potential risks to water quality but little is known about the temporal changes in the most bioavailable fraction of metal, the free/labile species. In this study, diffusive gradient in thin films (DGT) and the Windermere Humic Aqueous Model (WHAM VII) equilibrium model were used to examine the temporal changes in free/labile metal (Cu, Ni, Zn, Pb) species in three tributaries of the north-flowing Athabasca River in the Athabasca oil sands region (AOSR). The influence of dissolved organic matter (DOM) composition (i.e. fulvic: humic ratio) on modeled Cu and Ni speciation showed a negligible effect on the labile concentration. The best agreements (92 ± 8%) between DGT-labile and WHAM calculated labile concentrations were found assuming the formation of iron oxyhydroxides (FeO(OH)). The agreement was only 70 ± 7% in the presence of inorganic colloidal aluminum oxyhydroxides (AlO(OH)) and in the absence of any inorganic colloids. Together these results suggest that a change in DOM composition had limited impacts on modeled free metal ion concentrations. Although the concentration of the main metal ligand (i.e. DOM), varied from 9 to 40 ppm, no significant temporal differences in the abundance of WHAM-modeled labile species were found, suggesting mobility and bioavailability of Cu, Ni, Pb and Zn were comparable over the 2003–2012 period.

Calculation of chloride ion concentration in expressed pore solution of cement-based materials exposed to a chloride salt solution

To investigate the relationship between the chloride ion concentration of the expressed pore solution (Ce) and the exposure solution (Ces) of cement-based materials exposed to a chloride salt solution, a calculation of the Ce, based on an electrical double layer (EDL) model was proposed. The Ce values were calculated using the proposed equations, applying the measured parameters: relative potential (δψ0), dielectric constant (ε), porosity (P), bulk density (D), volume percentage (pi) of pores with diameter di. The calculations explain why Ce does not always equal to Ces. The results indicate that the expression method does not test the concentration of free chloride ions in the bulk solution (CCl−b) or the average concentration of the transportable chloride ions (Cttr.a) in the pores. However, the ratio (Cr) of Ce to CCl−b and the relationship between Ce and Cttr.a can be calculated to more accurately measure chloride ion migration in cement-based materials.

Thermal, mechanical and conductive properties of imidazolium-containing thiol-ene poly(ionic liquid) networks

Abstract A series of imidazolium-based bis(trifluoromethylsulfonyl)imide [NTf2] poly(ionic liquid)s (PILs) were prepared by conducting the solventless thiol-ene ‘click’ photopolymerization of bisallylimidazolium [NTf2] and pentaerythritol tetrakis(3-mercaptopropionate) (PTMP). The thiol:ene molar ratio was varied in order to examine changes in the thermal, mechanical and conductive properties of the resulting polymer networks. The 1.0:2.0 thiol-ene PIL network exhibited the highest glass transition temperature (Tg of −5.0 °C) and storage modulus (E′ of 9.24 MPa at 100 °C) values. Temperature-dependent ionic conductivities were found to rely on both the Tg/crosslink density as well as the IL content, with the highest ionic conductivity observed for the 1.0:3.0 thiol-ene PIL network (1.42 × 10−5 S/cm at 25 °C). Application of Vogel-Fulcher-Tammann and Williams-Landel-Ferry theories revealed the impact of both Tg/crosslink density and free ion concentration on temperature-dependent conductivity data, and indicated lower free volumes and fragilities for the thiol-ene networks relative to other non-network PILs.

Effects of Chloride Ion Binding on Microstructure of Cement Pastes

AbstractThe corrosion of steel caused by chloride ions is considered to be the main durability problem of reinforced concrete structures. This paper investigates the total chloride ion content, free chloride concentration in expressed pore solution, Friedel’s salt content, and porosity of portland cement (PC) and PC/fly ash (FA) combination pastes after 7 days of water curing followed by 28 or 150 days immersion in water or 0.3, 0.7, 1.0, and 2.0 mol/L sodium chloride solutions. Results showed that the chloride ion binding capacities of the pastes increased with increase of free chloride ion concentration. The relationship between bound and free chloride ion contents could be described by Langmuir’s equation. The addition of fly ash decreased the bound chloride content and showed negative influences on Friedel’s salt formation due to low activity. Porosity of cement paste decreased as the concentration of soaking chloride ion increased, and was inversely proportional to the amount of Friedel’s salt formed.

Mixture toxicity of copper, cadmium, and zinc to barley seedlings is not explained by antioxidant and oxidative stress biomarkers.

The analysis of metal mixture toxicity to plants is complicated by mutual interactions. In the present study, mixture effects of zinc (Zn), cadmium (Cd), and copper (Cu) on barley (Hordeum vulgare L.) root elongation were analyzed using oxidative stress parameters. The hypothesis was that toxic mixture effects on plant growth are better explained by biochemical parameters than by exposure information, because the former excludes interactions among metals for root uptake. Barley seedlings were exposed for 5 d or 14 d to these metals in nutrient solutions, added in isolation and as mixtures. Root elongation in Cu+Cd mixtures was well predicted from free metal ion concentrations in solution, using concentration addition (CA) or independent action (IA) reference models. In contrast, Zn acted antagonistically when combined with Cu and/or Cd, relative to both CA and IA. This protective effect of Zn correlated with the biomarkers measured in the long-term experiment; oxidative stress (indicated by malondialdehyde level, for example) decreased after addition of Zn. In addition, it was found that some biomarkers were sensitive to both Cu and Cd dosed in isolation, but not to Cu+Cd mixtures. Overall, the exposure explained mixture effects better than most of the 16 measured biomarkers (i.e., the biochemical effects). It is concluded that these biomarkers are not robust indicators for metal mixture toxicity, potentially because different metals have different parallel modes of action on growth that are insufficiently indexed by the biomarkers. Environ Toxicol Chem 2017;36:220-230. © 2016 SETAC.

Copper affects biochemical and physiological responses of Selenastrum gracile (Reinsch).

Copper is an essential metal for several physiological and metabolic processes, but a narrow range regulate its effect in phytoplankton cells. It can affect the production of biomolecules and be toxic at concentrations slightly above those required, e.g. decreasing photosynthesis and increasing respiration. The aims of this study were to analyse the changes in growth and chlorophyll a synthesis, and in biochemistry (total carbohydrates, proteins, lipids and fatty acids) of the freshwater microalga Selenastrum gracile after exposure to copper. Exponentially growing cells were exposed to 5 concentrations of free copper ions (Cu2+) ranging from 0.7 (control) to 13×10-8 M for up to 120h. Free Cu2+ ion concentrations were calculated through the chemical equilibrium model MINEQL+. We observed that copper was responsible for a decrease in cell density and an increase in total protein and lipid production, but no effect on total carbohydrates was detected. The increase in phospholipids and sterols and a decrease in saturated fatty acids under copper exposure suggest a change in conformation of the cell membrane, by decreasing its fluidity. We suggest this serves the cell as a system to avoid the internalization of metal, thereby acting as a detoxifying mechanism.

A structural equation model of soil metal bioavailability to earthworms: confronting causal theory and observations using a laboratory exposure to field-contaminated soils

Structural equation models (SEM) are increasingly used in ecology as multivariate analysis that can represent theoretical variables and address complex sets of hypotheses. Here we demonstrate the interest of SEM in ecotoxicology, more precisely to test the three-step concept of metal bioavailability to earthworms. The SEM modeled the three-step causal chain between environmental availability, environmental bioavailability and toxicological bioavailability. In the model, each step is an unmeasured (latent) variable reflected by several observed variables. In an exposure experiment designed specifically to test this SEM for Cd, Pb and Zn, Aporrectodea caliginosa was exposed to 31 agricultural field-contaminated soils. Chemical and biological measurements used included CaC12-extractable metal concentrations in soils, free ion concentration in soil solution as predicted by a geochemical model, dissolved metal concentration as predicted by a semi-mechanistic model, internal metal concentrations in total earthworms and in subcellular fractions, and several biomarkers. The observations verified the causal definition of Cd and Pb bioavailability in the SEM, but not for Zn. Several indicators consistently reflected the hypothetical causal definition and could thus be pertinent measurements of Cd and Pb bioavailability to earthworm in field-contaminated soils. SEM highlights that the metals present in the soil solution and easily extractable are not the main source of available metals for earthworms. This study further highlights SEM as a powerful tool that can handle natural ecosystem complexity, thus participating to the paradigm change in ecotoxicology from a bottom-up to a top-down approach.

Graphene and liquid crystal mediated interactions

ABSTRACTThe two-dimensional graphene-honeycomb structure can interact with the liquid crystal’s (LC) benzene rings through π–π electron stacking. This LC–graphene interaction gives rise to a number of interesting physical and optical phenomena in the LC. In this paper, we present a combination of a review and original research of the exploration of novel themes of LC ordering at the nanoscale graphene surface and its macroscopic effects on the LC’s nematic and smectic phases. We show that monolayer graphene films impose planar alignment on the LC, creating pseudo-nematic domains (PNDs) at the surface of graphene. In a graphene-nematic suspension, these PNDs enhance the orientational order parameter, exhibiting a giant enhancement in the dielectric anisotropy of the LC. These anisotropic domains interact with the external electric field, resulting in a non-zero dielectric anisotropy in the isotropic phase as well. We also show that graphene flakes in an LC reduce the free ion concentration in the nematic media by an ion-trapping process. The reduction of mobile ions in the LC is found to have subsequent impacts on the LC’s rotational viscosity, allowing the nematic director to respond quicker on switching the electric field on and off. In a ferroelectric LC (smectic-C* phase), suspended graphene flakes enhance the spontaneous polarisation by improving the tilted smectic-C* ordering resulting from the π–π electron stacking. This effect accelerates the ferroelectric-switching phenomenon. Graphene can possess strain chirality due to a soft shear mode. This surface chirality of graphene can be transmitted into LC molecules exhibiting two types of chiral signatures in the LCs: an electroclinic effect (a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the smectic-A phase, and a macroscopic helical twist of the LC director in the nematic phase. Finally, we show that a graphene-based LC cell can be fabricated without using any aligning layers and ITO electrodes. Graphene itself can be used as the electrodes as well as the aligning layers, obtaining an electro-optic effect of the LC inside the cell.

Inverse Modelling Approach to the Determination of Concentration-Dependent Transport Properties in Electrolytes: The Effect of Ion Pairing

The goal of the present investigation was to extend the recently proposed inverse modelling approach for determining concentration-dependent transport properties (salt diffusivity and Li+ cation transference number) to concentrated lithium salt solutions in solvents with relatively low permittivity. We demonstrated in our previous studies that NMR concentration profiles can be used for estimating the concentration dependent transport properties of LiTFSI solution in propylene carbonate (PC) under minimal assumption, including that of negligible ion pairing (simplified transport model) [1], which is valid for solvents with relative high permittivity [2] (εr ≈ 65 for PC). Herein we consider the case where ion paring effects cannot be neglected, due to the low permittivity of the solvent, as is the case for concentrated solutions of Li salts in mixed organic carbonate solvents (εr≈ 26 for EC:DMC 1:1). The data for the present study was obtained in the form of NMR intensity images of the 19F nuclei in the PF6 - anions of LiPF6 dissolved in a binary mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), recorded under galvanostatic conditions. The NMR images were then post-processed to obtain lithium concentration profiles. The formation of non-negligible amounts of ion pairs in the present work significantly affects the estimation of transport properties of the system. As a result, the simplified transport model produces thermodynamically inconsistent results, manifested as negative transference numbers obtained from the inverse modelling analysis (Fig. 1). To remedy this situation, we considered an extended transport model which assumes that oppositely charged ions combine to form neutral ion pairs and which tracks the concentration of neutral ion pairs in addition to the concentration of free ions. However, due to the presence of a reaction term describing the formation and disintegration of ion pairs, the resulting PDE system is extremely stiff. Using methods of asymptotic analysis, corresponding to the limit of fast reaction rates, we have reduced the modeling problem to a more simple system describing the evolution of the total lithium concentration, which is computationally tractable. As this reduced system may not satisfy all boundary conditions of the original extended system, an additional material property called boundary transference number was introduced and its optimal dependence on the total concentration was identified via inverse analysis. The unknown material properties are concentration dependent (except for the equilibrium constant K) and are reconstructed with minimal assumptions using methods of variational optimization, to minimize the least-square error between the experimentally determined and simulated concentration values. The optimization problem is solved using a gradient-based method with a careful treatment of uncertainties resulting from the presence of noise in the experimental data. A key element of our computational approach is identification of the sensitivity of the PDE system solutions to perturbations of the constitutive relations, which is achieved through a novel adjoint-based approach. As shown in Fig. 2, the reconstruction produces thermodynamically consistent results for the Li+transference number under conditions when ion pairing effects cannot be neglected.

Ion dynamics in methylcellulose–LiBOB solid polymer electrolytes

A polymer electrolyte system comprising methylcellulose (MC) as the host polymer and lithium bis(oxalato) borate (LiBOB) as the lithium ion source has been prepared via the solution cast technique. The electrolyte with the highest conductivity of 2.79 μS cm−1 has a composition of 75 wt% MC–25 wt% LiBOB. The mobile ion concentration (n) in this sample was estimated to be 5.70 × 1020 cm−3. A good correlation between ionic conductivity, dielectric constant, and free ion concentration has been observed. The ratio of mobile ion number density (n) at a particular temperature to the concentration n0 of free ions at T = ∞ (n/n0) and the power law exponents (s) exhibit opposite trends when varied with salt concentration.

Mixture toxicity and interactions of copper, nickel, cadmium, and zinc to barley at low effect levels: Something from nothing?

Metal contamination is mostly a mixture of different metals, and these multicomponent mixtures can produce significant mixture effects. The present study was set up to investigate the toxicity of multiple metal mixtures of Cu, Ni, Cd, and Zn to plants at metal doses individually causing low-level phytotoxic effects. Barley (Hordeum vulgare L.) root elongation toxicity tests were performed in resin-buffered nutrient solutions to control metal speciation. Treatments included single-metal concentrations and binary, ternary, and quaternary mixtures. Mixtures of different metals at free ion concentrations, each causing <10% inhibition of root elongation, yielded significant mixture effects, with inhibition reaching up to 50%. The independent action (IA) model predicted mixture toxicity statistically better than the concentration addition (CA) model, but some synergisms relative to the IA model were observed. These synergisms relative to IA were most pronounced in quaternary mixtures and when the dose-response curves had steep slopes. Generally, antagonistic interactions relative to the CA model were observed. Increasing solution Zn concentrations shifted metal interactions (CA based) from additive or slightly synergistic at background Zn concentrations to antagonistic at higher Zn concentrations, suggesting a protective effect of Zn. Overall, the present study shows that the CA model can be used as a conservative model to predict metal mixture toxicity to barley. Environ Toxicol Chem 2016;35:2483-2492. © 2016 SETAC.

Using biofilms for monitoring metal contamination in lotic ecosystems: The protective effects of hardness and pH on metal bioaccumulation.

Biofilms can make good bioindicators and biomarkers, offering a convenient tool to monitor metal contamination in streams that results from mine tailing sites. Biofilm metal content (Cu, Zn, Cd, Pb) as well as diatom diversity and the presence of teratologies (diatom abnormalities) were determined for biofilms from rivers with a variety of physicochemical properties across a metal contamination gradient. The results of metal accumulation were highly consistent from year to year, with significant relationships between calculated free metal ion concentrations and biofilm metal contents for samples from different rivers. This indicates the "universal nature" of the metal accumulation process in biofilms. The authors observed that protons and major cations protected against metal accumulation. A very low number of diatom taxa were found at the most contaminated sites, and the highest proportions of deformities were observed at these sites. However, it was difficult to distinguish the effect of metal contamination from the effect of other parameters, especially pH. The results suggest that the development of biofilm-based proxies for metal bioavailability is useful and that incorporation of the effects of hardness and pH in this metal contamination monitoring tool is important. Environ Toxicol Chem 2016;35:1489-1501. © 2015 SETAC.

AGNES at vibrated gold microwire electrode for the direct quantification of free copper concentrations

The free metal ion concentration and the dynamic features of the metal species are recognized as key to predict metal bioavailability and toxicity to aquatic organisms. Quantification of the former is, however, still challenging. In this paper, it is shown for the first time that the concentration of free copper (Cu2+) can be quantified by applying AGNES (Absence of Gradients and Nernstian equilibrium stripping) at a solid gold electrode. It was found that: i) the amount of deposited Cu follows a Nernstian relationship with the applied deposition potential, and ii) the stripping signal is linearly related with the free metal ion concentration. The performance of AGNES at the vibrating gold microwire electrode (VGME) was assessed for two labile systems: Cu-malonic acid and Cu-iminodiacetic acid at ionic strength 0.01 M and a range of pH values from 4.0 to 6.0. The free Cu concentrations and conditional stability constants obtained by AGNES were in good agreement with stripping scanned voltammetry and thermodynamic theoretical predictions obtained by Visual MinteQ. This work highlights the suitability of gold electrodes for the quantification of free metal ion concentrations by AGNES. It also strongly suggests that other solid electrodes may be well appropriate for such task. This new application of AGNES is a first step towards a range of applications for a number of metals in speciation, toxicological and environmental studies for the direct determination of the key parameter that is the free metal ion concentration.

Statistical evaluation of biogeochemical variables affecting spatiotemporal distributions of multiple free metal ion concentrations in an urban estuary

Free metal ion concentrations have been recognized as a better indicator of metal bioavailability in aquatic environments than total dissolved metal concentrations. However, our understanding of the determinants of free ion concentrations, especially in a metal mixture, is limited, due to underexplored techniques for measuring multiple free metal ions simultaneously. In this work, we performed statistical analyses on a large dataset containing repeated measurements of free ion concentrations of Cu, Zn, Pb, Ni, and Cd, the most commonly measured metals in seawater, at five inshore locations in Boston Harbor, previously collected using an in-situ equilibrium-based multi-metal free ion sampler, the ‘Gellyfish’. We examined correlations among these five metals by season, and evaluated effects of 10 biogeochemical variables on free ion concentrations over time and location through multivariate regressions. We also explored potential clustering among the five metals through a principal component analysis. We found significant correlations among metals, with varying patterns over season. Our regression results suggest that instead of dissolved metals, pH, salinity, temperature and rainfall were the most significant determinants of free metal ion concentrations. For example, a one-unit decrease in pH was associated with a 2.2 (Cd) to 99 (Cu) times increase in free ion concentrations. This work is among the first to reveal key contributors to spatiotemporal variations in free ion concentrations, and demonstrated the usefulness of the Gellyfish sampler in routine sampling of free ions within metal mixtures and in generating data for statistical analyses.

Evaluation of the impact of organic matter composition on metal speciation in calcareous soil solution: Comparison of Model VI and NICA-Donnan

To assess the influence of the composition of dissolved organic matter (DOM) on metal speciation, two models have been used: Model VI and NICA-Donnan. The speciation of Cu, Ni, Pb and Zn in the soil solution of 36 samples characterized by alkaline pH and different contamination levels has been determined. Four assumptions about DOM composition have been tested: (i) 100% of humic substances (HS) are fulvic acids (FA), (ii) 100% are humic acids (HA), (iii) 35% are HA and 65% FA and (iv) 84% are HA and 16% FA. The results obtained with the two distinct models are of the same order of magnitude; however, few differences have been highlighted regarding the description of the non-specific interactions, the Zn–FA interactions and the lowest free ion concentrations. The main result of this study is that the free ion concentrations calculated with the two models may differ by one order of magnitude depending on whether HA and/or FA are used to represent DOM. This work demonstrates the impact of the assumptions made about the DOM nature for metal speciation modelling.

Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) for the determination of [Zn2+] in estuarine waters

Zinc (Zn) has been classified as a “Specific Pollutant” under Annex VIII of the EU Water Framework Directive by two thirds of the EU member states. As a result, the UK Environmental Quality Standard (EQS) for Transitional and Coastal (TrAC) Waters has been reduced from 612 nM to 121 nM total dissolved Zn. It is widely accepted that the free metal ion ([Zn2+]) is the most bioavailable fraction, but there are few techniques available to determine its concentration in these waters. In this work, Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) has been applied, for the first time, to determine [Zn2+] in estuarine waters. The AGNES method had a mean RSD of ±18%, a (deposition time dependent) limit of detection of 0.73 nM and a [Zn2+] recovery of 112 ± 19% from a certified reference material (BCR-505; Estuarine Water). AGNES results for 13 estuarine samples (salinity 0.1–31.9) compared well (P = 0.02) with Competitive Ligand Exchange Cathodic Stripping Voltammetry (CLE-AdCSV) except for one sample. AGNES requires minimal sample manipulation, is unaffected by adsorption of interfering species at the electrode surface and allows direct determination of free zinc ion concentrations. Therefore AGNES results can be used in conjunction with ecotoxicological studies and speciation modelling to set and test compliance with water quality standards.

Assessment of H+ in complex aqueous solutions approaching seawater

pH measurements are used to assess both the free acidity of aqueous solutions and the concentrations, mi, of chemical species i related to hydrogen ion, H+, through chemical equilibrium, provided that measurements in reference solutions with well-known compositions are also carried out. Seawater is a highly complex electrolytic matrix of variable composition, rich in NaCl, where the determination of pH=−lg aH+ (aH+=mH+γH+ is the activity of the specified ion, H+ with the concentration mH+ and activity coefficient γH+≠1) is an issue of controversy.Electroneutrality prevents measurement of individual ion activity coefficients, which are affected and depend on the other ions in presence, namely the counter ions. Nevertheless, mean activity coefficients, γ±=γ+γ−, in this case γ±=γH+γCl−, can be experimentally assessed from the application of the Nernst equation to Harned cell (Pt|H2 vs. Ag|AgCl electrodes, without transference) in solutions of known mH+ and mCl−. In this work measurements of electrical potential, E/V, have first been taken in strategically planned solutions, ranging from 0.01molkg−1 HCl+(NaCl) to 0.01molkg−1 HCl+(NaCl+KCl+CaCl2+MgCl2), of ionic strength, I≈0.67molkg−1 at 15°C, 25°C and 35°C with experimental pressure corrected to reference pressure of 1atm. Mean activity coefficients were experimentally assessed with associated uncertainty. Further addition of sodium sulfate, Na2SO4, targeting artificial seawater (ASW), originates acid–base equilibria that affect the free hydrogen ion concentration, mH+. The changes introduced by the addition of sulfate in the measured Harned cell potential have been used to assess the free proton concentration and, based on the obtained results, to propose for the first time a procedure aiming to calculate values of bisulfate association constants with the corresponding uncertainty at 25°C for both studied media. Further, the effective ionic strength of artificial seawater (ASW) was calculated and compared with the value obtained using the salinity based equation largely conveyed in the literature.

Chemical speciation of sulfur and metals in biogas reactors – Implications for cobalt and nickel bio-uptake processes

This article deals with the interrelationship between overall chemical speciation of S, Fe, Co, and Ni in relation to metals bio-uptake processes in continuous stirred tank biogas reactors (CSTBR). To address this topic, laboratory CSTBRs digesting sulfur(S)-rich stillage, as well as full-scale CSTBRs treating sewage sludge and various combinations of organic wastes, termed co-digestion, were targeted. Sulfur speciation was evaluated using acid volatile sulfide extraction and X-ray absorption spectroscopy. Metal speciation was evaluated by chemical fractionation, kinetic and thermodynamic analyses. Relative Fe to S content is identified as a critical factor for chemical speciation and bio-uptake of metals. In reactors treating sewage sludge, quantity of Fe exceeds that of S, inducing Fe-dominated conditions, while sulfide dominates in laboratory and co-digestion reactors due to an excess of S over Fe. Under sulfide-dominated conditions, metals availability for microorganisms is restricted due to formation of metal-sulfide precipitates. However, aqueous concentrations of different Co and Ni species were shown to be sufficient to support metal acquisition by microorganisms under sulfidic conditions. Concentrations of free metal ions and labile metal complexes in aqueous phase, which directly participate in bio-uptake processes, are higher under Fe-dominated conditions. This in turn enhances metal adsorption on cell surfaces and bio-uptake rates.

Determination of the free-ion concentration of rare earth elements by an ion-exchange technique: implementation, evaluation and limits

Environmental context The lanthanides are a group of heavy elements (from lanthanum to lutetium) increasingly used in many electronic consumer products and little is known about their environmental mobility and toxicity. In natural systems, these elements will bind to natural organic matter but metal toxicity is usually defined by the free metal ion concentration. Here, we propose a method based on sample equilibration with an ion-exchange resin to measure the free lanthanide ion concentration in the presence of natural organic matter. Abstract An ion-exchange technique that employs a polystyrene sulphonate ion-exchange resin was developed for determining environmentally relevant free-ion concentrations of Ce, Eu, La and Nd. Owing to the high affinity of rare earth elements (REE) for the selected resin, this method requires the addition of an inert salt to increase the concentration of the counter-ions (i.e. cations that are exchanged with REE bound to the resin). The use of a batch equilibration approach to calibrate the resin allowed the implementation of the ion-exchange technique at reasonably low ionic strength (I = 0.1M). Several ligands were used to test the selectivity of the method, which proved to be highly selective for the free metal ion in presence of the tested cationic and anionic complexes (REE–nitrate, REE–malic acid and REE–nitrilotriacetic acid systems) and operational for very low proportions of REE3+, owing to the strong REE–resin interactions. The ion-exchange technique was also implemented to determine [Eu]inorg in the presence of natural humic matter (Suwannee River Humic Acid) and the results were compared with those obtained using equilibrium dialysis and those calculated with chemical equilibrium models. At pH 4.00, the measured [Eu]inorg values were in fairly good agreement with those predicted with the Windermere Humic Aqueous Model and Stockholm Humic Model, whereas the Non-Ideal Competitive Absorption model appeared to underestimate the [Eu]inorg. However, the inorganic europium concentrations were strongly underestimated (4 &lt; [Eu]inorg, IET/[Eu]inorg, calc &lt; 18) with the three prediction models at higher pH (5.3 and 6.2).