Free Metal Ion Concentrations Research Articles

Evaluation of thin mercury film rotating disk electrode to perform absence of gradients and Nernstian equilibrium stripping (AGNES) measurements

In the present work, the applicability of thin mercury film on a rotating disk electrode (TMF-RDE), to assess the free metal ion concentration by the absence of gradients and Nernstian equilibrium stripping (AGNES), is evaluated. The thickness of the mercury film and several AGNES parameters has been optimized. A nominal 16 nm film is chosen due to the higher signal (faradaic current) relative to the value of the noise (capacitive current). Due to the smaller volume to area ratio, the deposition time needed to reach a certain preconcentration factor (Y) is much shorter than in larger electrodes, like the HMDE. The limit of detection (3 sigma) for lead(II) is 7.4 x 10(-9)M and 7.2 x 10(-8)M for a Y of 5000 (deposition time of 150 s) and 1000 (deposition time of 100 s), respectively. A specific mathematical treatment is developed in order to subtract a corrected blank taking into account the degradation of the thin film (presumably, falling down of drops). The couple TMF-RDE/AGNES is successfully applied for speciation purposes in the systems Pb(II)-latex nanospheres and Pb(II)-IDA (iminodiacetic acid), where the stability constants calculated for both systems agree with values reported in the literature.

Metal-chelating properties of carvedilol: an antihypertensive drug with antioxidant activity

Carvedilol (CarvH) (1-[carbazolyl-(4)-oxyl]-3-[2-methoxyphenoxyethyl)-amino]-2-propanol, an antihypertensive agent with β-blocker function, has been shown to act as an antioxidant. The antioxidative properties can be correlated with metal chelating ability of the drug. Iron(III), zinc(II), and copper(II) complexes of carvedilol were synthesized and characterized with respect to their structural and spectroscopic properties. Metal interaction involves O and N donors from the aliphatic moiety of carvedilol. NMR studies allowed us to obtain the structural information on metal coordination and to suggest that the physiological concentration of carvedilol and free metal ions may be enough for a protective effect by metal chelation.

Uptake of Cd(II) and Pb(II) by microalgae in presence of colloidal organic matter from wastewater treatment plant effluents

The present study addresses the key issue of linking the chemical speciation to the uptake of priority pollutants Cd(II) and Pb(II) in the wastewater treatment plant effluents, with emphasis on the role of the colloidal organic matter (EfOM). Binding of Cd(II) and Pb(II) by EfOM was examined by an ion exchange technique and flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry in parallel to bioassays with green microalga Chlorella kesslerii in ultrafiltrate (<1 kDa) and colloidal isolates (1 kDa to 0.45 μm). The uptake of Cd by C. kesslerii was consistent with the speciation analysis and measured free metal ion concentrations, while Pb uptake was much greater than that expected from the speciation measurement. Better understanding of the differences in the effects of the EfOM on Cd(II) and Pb(II) uptake required to take into account the size dependence of metal binding by EfOM.

Incorporation of metal bioavailability into regulatory frameworks—metal exposure in water and sediment

Background, aim, and scope The cause for this position paper is the impression that risk assessors consider primarily the concentration of free metal ions dissolved in solution controlling metal bioavailabilityinaquatic systems.Aimingat a more realistic risk assessment of metals, bioavailability has to be discussed under the scope of main uptake routes of metals to organisms. Materials and methods On the basis of a review on the literature relating to bioavailability approaches, this work discusses the incorporation of metal bioavailability into the risk assessment of metals in the context of metal exposure. Results The biotic ligand model (BLM) and the concept of sulfide bound metals described by the ratio of simultaneously extracted metals and acid volatile sulfide concept (AVS) have been developed to consider the bioavailability of metals. Both approaches assume that the free ion concentration is the most relevant exposure pathway. However, apart from geochemical conditions, which control free metal concentration, bioavailability is additionally a result of contaminant/particle interaction and of organisms' activity. Asking for the relevant exposure pathways for inorganic metals to organisms, the compartments' water and sediment have been evaluated and also the importance of contaminated food. Discussion We present a conceptual model of the main processes and sources for uptake of trace metals at a biological membrane. On the basis of this model, we have to consider free metal ions, metal complexes, and particle-bound metals. The BLM approach has been proposed for use in European Union risk assessments. However, the BLM provides a means to predict ecotoxicological effect of metals in the environment, but at present assumes that total significant uptake is from the dissolved phase. It is apparent that dietary accumulation of metals is at least as important as metal uptake from the aqueous phase and in many cases dominates metal accumulation.

Simultaneous determination of free calcium, magnesium, sodium and potassium ion concentrations in simulated milk ultrafiltrate and reconstituted skim milk using the Donnan Membrane Technique

This study focused on determination of free Ca 2+, Mg 2+, Na + and K + concentrations in a series of CaCl 2 solutions, simulated milk ultrafiltrate and reconstituted skim milk using a recently developed Donnan Membrane Technique (DMT). A calcium ion selective electrode was used to compare the DMT results. The study showed that the free Ca 2+ concentrations measured by the DMT agreed well with calcium electrode data. Concentrations of free Ca 2+, Mg 2+, Na + and K + measured by the DMT agreed with concentrations predicted by existing ion speciation models. It is concluded that the DMT is suitable to measure various free metal ion concentrations simultaneously in complex milk-type systems.

METAL-INDUCED REACTIVE OXYGEN SPECIES PRODUCTION IN CHLAMYDOMONAS REINHARDTII (CHLOROPHYCEAE)(1).

Toxic effects of metals appear to be partly related to the production of reactive oxygen species (ROS), which can cause oxidative damage to cells. The ability of several redox active metals [Fe(III), Cu(II), Ag(I), Cr(III), Cr(VI)], nonredox active metals [Pb(II), Cd(II), Zn(II)], and the metalloid As(III) and As(V) to produce ROS at environmentally relevant metal concentrations was assessed. Cells of the freshwater alga Chlamydomonas reinhardtii P. A. Dang. were exposed to various metal concentrations for 2.5 h. Intracellular ROS accumulation was detected using an oxidation-sensitive reporter dye, 5-(and-6)-carboxy-2',7'-dihydrodifluorofluorescein diacetate (H2 DFFDA), and changes in the fluorescence signal were quantified by flow cytometry (FCM). In almost all cases, low concentrations of both redox and nonredox active metals enhanced intracellular ROS levels. The hierarchy of maximal ROS induction indicated by the increased number of stained cells compared to the control sample was as follows: Pb(II) > Fe(III) > Cd(II) > Ag(I) > Cu(II) > As(V) > Cr(VI) > Zn(II). As(III) and Cr(III) had no detectable effect. The effective free metal ion concentrations ranged from 10(-6) to 10(-9) M, except in the case of Fe(III), which was effective at 10(-18) M. These metal concentrations did not affect algal photosynthesis. Therefore, a slightly enhanced ROS production is a general and early response to elevated, environmentally relevant metal concentrations.

Bioenergetic effects of aqueous copper and cadmium on the grass shrimp, Palaemonetes pugio

Adult grass shrimp ( Palaemonetes pugio) were exposed to either aqueous copper (ranging from 7.54 to 41.29 μg Cu 2+/L) or cadmium (2.48– 6.55 μg Cd 2+/L) for 14 days in laboratory experiments to quantify effects on survival and bioenergetic processes, including respiration, somatic growth, energy (lipid) storage, and food consumption. The lowest observed effect concentrations for mortality were 41.29 μg Cu 2+/L or 6.55 μg Cd 2+/L, expressed as free metal ion concentrations. Both copper and cadmium caused a decrease in respiration rate at concentrations of 7.54 to 41.29 μg Cu 2+/L or 6.55 μg Cd 2+/L. Exposure to copper (≥ 27.03 μg Cu 2+/L) resulted in negative somatic growth (i.e., weight loss). Cadmium exposure (6.17 μg Cd 2+/L) caused a decrease in growth rate, relative to the control, but growth remained positive. Nonpolar lipid content and food consumption were not significantly affected by exposure to either copper or cadmium. Our results suggest that both copper and cadmium result in overall metabolic depression, decreasing energy allocation to both maintenance and production.

Determination of speciation and bioavailability of Cd in soil solution using a modified soil column Donnan membrane technique

The concentration of free metal ion (Mz+) has a strong impact on bioavailability and toxicity. In this study, Cd2+ concentration in soil solution was measured by a modified soil column-Donnan membrane technique (S-DMT). Compared with the previously reported DMT, the equilibrium time was relatively short (8 hours) and the multiple metals in the solution can be determined using ICP-AES. This technique was successfully applied in a pot experiment to evaluate Cd speciation and availability to rice. The measured Cd2+ concentration in the soil solutions agreed with the model calculation and correlated very well with the rice growth and the Cd uptake in rice.

Impact of pH on CdII partitioning between alginate gel and aqueous media

Environmental context. Biogels, such as those in cell walls or biofilm matrices, generally comprise negative structural charge which leads to accumulation of positively charged species, e.g. metal ions. The magnitude of the effective charge, and hence the local chemical speciation within the gel phase, is pH dependent. In situ speciation measurements in biogels, such as the model alginate studied in this work, offer a better estimate of bioavailable concentrations than does analysis of the surrounding aqueous medium. Abstract. Many microorganisms exist in a biogel-mediated micro-environment such as a cell wall or a biofilm, in which local concentrations of ionic nutrients and pollutants differ from those in the surrounding bulk medium. The local concentration is the relevant parameter for considerations of bioavailability. These modified concentrations arise as a consequence of the negative charges within biogels which may induce a Donnan potential inside the biogel phase. For metals, the net effect on the speciation within the biogel, relative to the bulk medium, is an enhancement of the concentration of free cations. Since the structural charge in the biogel arises from protolytic functional groups, the Donnan potential is pH dependent. Here we apply in situ voltammetry to measure the free metal ion concentration inside alginate gel as a function of pH. In the pH range 3 to 7, the speciation of CdII within this model biogel can be explained by specific binding to carboxylic functional groups and electrostatic binding resulting from the Donnan potential.

Chemodynamics of heavy metals in long-term contaminated soils: Metal speciation in soil solution

The concentration and speciation of heavy metals in soil solution isolated from long-term contaminated soils were investigated. The soil solution was extracted at 70% maximum water holding capacity (MWHC) after equilibration for 24 h. The free metal concentrations (Cd2+, CU2+, Pb2+, and Zn2+) in soil solution were determined using the Donnan membrane technique (DMT). Initially the DMT was validated using artificial solutions where the percentage of free metal ions were significantly correlated with the percentages predicted using MINTEQA2. However, there was a significant difference between the absolute free ion concentrations predicted by MINTEQA2 and the values determined by the DMT. This was due to the significant metal adsorption onto the cation exchange membrane used in the DMT with 20%, 28%, 44%, and 8% mass loss of the initial total concentration of Cd, Cu, Pb, and Zn in solution, respectively. This could result in a significant error in the determination of free metal ions when using DMT if no allowance for membrane cation adsorption was made. Relative to the total soluble metal concentrations the amounts of free Cd2+ (3%-52%) and Zn2+ (11%-72%) in soil solutions were generally higher than those of Cu2+ (0.2%-30%) and Pb2+ (0.6%-10%). Among the key soil solution properties, dissolved heavy metal concentrations were the most significant factor governing free metal ion concentrations. Soil solution pH showed only a weak relationship with free metal ion partitioning coefficients (K(p)) and dissolved organic carbon did not show any significant influence on K(p).

Conditional Affinity Spectra of Pb2+−Humic Acid Complexation from Data Obtained with AGNES

The new electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) has been applied to follow Pb2+ complexation to Purified Aldrich Humic Acid. A refined methodology of AGNES, allowing considerably larger gains, reached free metal ion concentrations down to subnanomolar values in a reasonable deposition time due to the lability and mobility of these complexes. Further insights into the meaning of the binding data, fitted to a NICA (Non Ideal Competitive Adsorption) isotherm, can be obtained with the concept of conditional affinity spectrum (CAS). For this purpose, we present the analytical expression for the CAS of NICA isotherm and show the CAS distributions for the Pb binding at fixed pH. Results reveal that the underlying spectra of each elementary distribution of the bimodal NICA evolve with pH yielding different overlapping and nonsymmetrical distributions. A non-negligible occupation of phenolic and carboxylic sites by Pb2+ takes place in the range of 4 < pH < 9.

The origin of speciation: Trace metal kinetics over natural water/sediment interfaces and the consequences for bioaccumulation

The speciation of heavy metals was measured over a variety of natural and undisturbed water/sediment interfaces. Simultaneously, two benthic species (oligochaete Limnodrilus spp. and the midge Chironomus riparius) were exposed to these sediments. Under occurring redox conditions, free ion activities of trace metals Cd, Cu, Ni, Pb, and Zn were measured with a chelating exchange technique, while geochemical conditions (i.e., redox) remained in tact. Free ion activities were compared with total dissolved concentrations in pore waters and surface waters in order to relate speciation to bioaccumulation. Limnodrilus spp. and C. riparius have accumulation patterns that could be linked to time-dependent exposure concentrations, expressed as chemical speciation, in the surface water and the sediment's pore water. Concentrations of free metal ions in the overlying surface water, rather than in sediment pore water, proved to be the best predictor for uptake. For the first time, measurements are obtained from sediments without disturbing physical–chemical conditions and thus bioavailability, a major restriction of other studies so far.

Relationship between Metal Speciation in Soil Solution and Metal Adsorption at the Root Surface of Ryegrass

The total metal content of the soil or total metal concentration in the soil solution is not always a good indicator for metal availability to plants. Therefore, several speciation techniques have been developed that measure a defined fraction of the total metal concentration in the soil solution. In this study the Donnan Membrane Technique (DMT) was used to measure free metal ion concentrations in CaCl(2) extractions (to mimic the soil solution, and to work under standardized conditions) of 10 different soils, whereas diffusive gradients in thin-films (DGT) and scanning chronopotentiometry (SCP) were used to measure the sum of free and labile metal concentrations in the CaCl(2) extracts. The DGT device was also exposed directly to the (wetted) soil (soil-DGT). The metal concentrations measured with the speciation techniques are related to the metal adsorption at the root surface of ryegrass (Lolium perenne L.), to be able to subsequently predict metal uptake. In most cases the metal adsorption related pH-dependently to the metal concentrations measured by DMT, SCP, and DGT in the CaCl(2) extract. However, the relationship between metal adsorption at the root surface and the metal concentrations measured by the soil-DGT was not-or only slightly-pH dependent. The correlations between metal adsorption at the root surface and metal speciation detected by different speciation techniques allow discussion about rate limiting steps in biouptake and the contribution of metal complexes to metal bioavailability.

Effects of rice straw on the speciation of cadmium (Cd) and copper (Cu) in soils

Four soils were collected from different sites of China in Lechang (LC, Guangdong province), Changsha (CS, Hunan province), Jiaxing (JX, Zhejiang province) and Hangzhou (HZ, Zhejiang province), and were spiked with Cu (50 mg kg − 1 ) and Cd (5 mg kg − 1 ). The effects of rice straw addition (6%) on the chemical distribution of both metal ions were studied by measuring the soluble metal ion concentrations and the free metal ion concentrations (using the Donnan membrane technique) after 1-, 3- and 6-month incubation. Results show that the addition of rice straw increased soil pH by about 0.4 pH unit on average, and increased DOC significantly in soil LC, CS and JX, but not in soil HZ. With the addition of rice straw, total soluble Cu concentration increased from 0.82 µmol l − 1 (0.26–2.4 µmol l − 1 ) to 1.22 µmol l − 1 (0.70–3.60 µmol l − 1 ), whereas total soluble Cd concentration decreased from 20 nmol l − 1 (2–70 nmol l − 1 ) to 15 nmol l − 1 (2–56 nmol l − 1 ). When rice straw was added, both free Cu 2+ and Cd 2+ concentrations decreased, for free Cu 2+ concentration from 217 nmol l − 1 (31 to 369 nmol l − 1 ) to 124 nmol l − 1 (22 to 263 nmol l − 1 ) and for free Cd 2+ concentration from 16 nmol l − 1 (1–55 nmol l − 1 ) to 12 nmol l − 1 (1–43 nmol l − 1 ). With the increase of incubation time, free Cu 2+ concentration tended to increase but free Cd 2+ concentration decreased. Speciation model calculations show that compared to the binding capacity of soil organic matter, the capacity of rice straw is much less important. The decrease of free metal ion concentration upon rice straw addition can be attributed mainly to the increased pH. The higher DOC content in the rice straw treatment could be the reason for higher soluble Cu concentration when rice straw was added. Adsorption to DOC is much less important for Cd than for Cu. Calculation also shows that adsorption to clay minerals plays a more important role for Cd than for Cu, which may explain the stronger ageing effects on Cd distribution.

Ion Exchange Complexing Resins as Sensors for the Determination of Free Metal Ion Concentration at a Low Level

Two complexing resins containing iminodiacetic and carboxylic groups have been examined as sensors for the determination of free metal ion concentration at low level (RS method). The free metal ion concentration is proportional to the concentration of metal ion adsorbed in the resin phase. Free metal ion concentrations as low as 10−18 mol · l−1 were determined for copper (II), and 10−14 mol · l−1 for lead (II) in solutions containing known ligands. The concentrations obtained by RS method were in agreement with those calculated from the literature constants. The method was applied to the free copper (II) determination in natural waters of high and low salinity.

The use of differential pulse anodic stripping voltammetry and diffusive gradient in thin films for heavy metals speciation in soil solution

Abstract In the soil solutions obtained in situ with suction cups from soils (Cambisol and Fluvisol) of pot experiment with Salix smithiana Smith, Lolium perenne L. and Thlaspi caerulescens J. &amp;amp; C. Presl heavy metals species (Cd, Pb and Cu) were assayed by differential pulse anodic stripping voltammetry and diffusive gradient in thin films. Prediction of accumulation performed best at free metal ion concentrations in unchanged pH (in 10−3 mol L−1 NaClO4 base electrolyte). The speciation provided by differential pulse anodic stripping voltammetry according to pH can provide a detailed description of the soil solution matrix. The concentration of free metals in unchanged pH represents a small part of the total content and varied from 0.04 to 0.75% with two exceptions found for accumulating plants (the content of Cd2+ in the soil solution from T. caerulescens was about 6% and the content of Cu2+ in the soil solution from S. smithiana was about 30%). The available concentration as determined by diffusive gradient in thin films was not in correlation with the heavy metals concentration in plant biomass.

Determination of Free Nickel Ion Concentrations Using the Ion Exchange Technique: Application to Aqueous Mining and Municipal Effluents

Free metal ion concentration is generally considered a useful indicator of a metal’s bioavailability and ecotoxicity to aquatic biota. This article reports the speciation of nickel in mining and municipal effluents from Sudbury (Ontario, Canada), and also in model solutions containing a fully characterized laurentian fulvic acid (LFA) at environmentally relevant concentrations. A column ion exchange technique (IET) using a cation exchange resin (Dowex 50W-X8) was applied to determine free nickel ion concentrations. In model solutions, reasonable correlation was found between the predictions of an equilibrium-based computer speciation code, Windermere Humic Aqueous Model (WHAM) VI, and the results obtained by IET at low nickel-to-fulvic acid ratios. However at higher mole ratios, the WHAM VI predicted higher free nickel ion concentrations than IET. Only three out of six effluent samples showed reasonable agreement between the IET and the WHAM VI results, indicating the need for further development of IET for application to effluent waters.

Comparison of AGNES (absence of gradients and Nernstian equilibrium stripping) and SSCP (scanned stripping chronopotentiometry) for trace metal speciation analysis

The free metal ion concentrations obtained by SSCP (stripping chronopotentiometry at scanned deposition potential) and by AGNES (absence of gradients and Nernstian equilibrium stripping) techniques have been compared and the usefulness of the combination of both techniques in the same electrochemical cell for trace metal speciation analysis is assessed. The free metal ion concentrations and the stability constants obtained for lead(II) and cadmium(II) complexation by pyridinedicarboxylic acid, by 40 nm radius carboxylated latex nanospheres and by a humic acid extracted from an ombrotrophic peat bog were determined. Whenever possible, the free metal ion concentrations were compared with the theoretical predictions of the code MEDUSA and with the free metal ion concentrations estimated from ion selective electrodes (ISE). SSCP values were in agreement with the ones obtained by AGNES, and both of them agreed reasonably with the ISE values and the theoretical predictions. For the lead(II)-humic acid, it was not possible to obtain the stability constants by SSCP due to the heterogeneity effect. However, using AGNES it is possible to obtain, for these heterogeneous systems, the free bulk metal concentration, which allows us to retrieve the stability constant at bulk conditions.

Assessing toxicity of mining effluents: equilibrium- and kinetics-based metal speciation and algal bioassay

Environmental context. The release of mining effluents exposes natural waters to excess metals and thereby threatens both human and environmental health. The present study explores the toxicity of aqueous mining effluents collected from a mining area in Sudbury (Ontario, Canada), using two different methods for determination of metal speciation, and an algal toxicity study. The results show reasonable correlation between metal speciation and the observed toxicity and suggest the importance of taking into account other factors related to water quality criteria such as nutrient concentrations, diluent water and presence of other toxic metals that can greatly influence the toxicological result. Abstract. The present study explores the toxicity of aqueous mining and municipal effluents from the Sudbury area (Canada) using equilibrium- and kinetics-based estimates of metal speciation and chronic toxicity studies using algae (Pseudokirchneriella subcapitata). Free metal ion concentration was determined by the Ion Exchange Technique (IET) and a computer speciation code, Windermere Humic Aqueous Model (WHAM) VI. Labile metal concentration was determined using the Competing Ligand Exchange Method. In general, no correlation was found between the observed IC25 (concentration of test substance that inhibits growth of organism by 25%) and the [Ni]labile, [Ni2+]IET or [Ni2+]WHAM, probably because of contributions by other metals such as Cu and Zn being also significant. Reasonable correlation (r2 = 0.7575) was found when the observed toxicity was compared with the sum of free metal ions of Cu, Ni, and Zn predicted by WHAM. The results of the present study reveal the importance of taking into account other factors related to water quality criteria such as nutrient concentrations, diluent water, and the presence of other toxic metals, which greatly influence the toxicological result in complex, multi-metal contaminated waters.

Influences of metal‐ligand complexes on the cadmium and zinc biokinetics in the marine bacterium, Bacillus firmus

Uptake kinetics of cadmium and zinc in gram-positive bacteria, Bacillus firmus, isolated from Hong Kong sediments were examined in the present study. The metal uptake by the bacteria was measured at different ambient free metal ion concentrations (10(-12)-10(-6) M Cd(2+) and 10(-10)-10(-6) M Zn(2+)) by adding different concentrations of total dissolved Cd and Zn and hydrophilic organic ligands (ethylenedinitrilotetraacetic acid, nitrilotriacetic acid, and citrate). Our data suggest that Cd and Zn uptake by B. firmus is best predicted by Cd(2+) and Zn(2+) activities. Free metal ions were complexed with the active sites on the bacterial surface, and an equilibrium between the free metal ion and surface-metal complex was reached quickly. After binding, the metal ions were then biologically transported into the bacteria. In addition, with the presence of lipophilic organic ligands (diethyldithiocarbamate and oxine), the lipophilic metal complex was internalized rapidly into B. firmus by passive diffusion through the bacterial plasma membrane. The uptake of the lipophilic metal complex could not be predicted by the free ion activity model because the mass transport through plasma membrane was the most important metal uptake pathway. Furthermore, the efflux of Cd and Zn by B. firmus was determined in the present study. The calculated efflux rate constants of Cd and Zn were (5.55 +/- 1.96) x 10(-4)/min and (3.75 +/- 1.04) x 10(-4)/min, respectively. The present study helps us to understand the process of bioaccumulation of metals in marine bacteria, which remains a poorly studied area.