Determination of dissolved iron(III) in estuarine and coastal waters by adsorptive stripping chronopotentiometry (SCP)

An adsorptive stripping chronopotentiometric (SCP) method has been developed for quantification of dissolved iron in estuarine and coastal waters. After UV-digestion of filtered samples the Fe(III) ions in non-deoxygenated samples were complexed with solochrom violet RS (SVRS). The complexes were then accumulated by adsorption on the surface of a mercury-film electrode. The stripping step was performed by applying a constant current of −17 μA. Sensitivity and detection limit were 15 ms nmol⁻¹ L (270 ms μg⁻¹ L) and 1.5 nmol L⁻¹ (84 ng L⁻¹), respectively, for 60-s electrolysis time. Compared with the only other chronopotentiometric method available for measurement of iron in natural waters, our procedure is fifty times more sensitive in a quarter of the electrolysis time. It therefore enables detection of the concentrations currently found in estuarine and coastal waters. The method was successfully used to study the behaviour and seasonal variations of dissolved iron in the Penzé estuary, NW France.     

Determination of Fe(II), Fe(III) and Fetotal in thermal water by ion chromatography spectrophotometry (IC-Vis)

Determination of iron speciation in water is one of the major challenges in environmental analytical chemistry. Here, we present and discuss a method for sampling and analysis of dissolved Fe(II), Fe(III), and Fe_total concentrations in natural thermal water covering a wide range of temperature, pH, chemical composition, and redox conditions. Various methods were tried in the collection, preservation, and storage of natural thermal water samples for the Fe(II) and Fe(III) determinations, yet the resultant Fe speciation determined was often found to be significantly affected by the methodology applied. Due to difficulties in preserving accurate Fe speciation in natural samples for later laboratory analysis, a field-deployed on-site method using ion-chromatography and spectrophotometry was developed and tested. The IC-Vis method takes advantage of ion chromatographic separation of Fe(II) and Fe(III), followed by post-column colour reaction and spectrophotometric detection, thus allowing analysis of Fe(II) and Fe(III) in a single 15-minute run. Additionally, Fe_total can be determined after sample oxidation. The analytical detection limits are ~2 µg L^?1 (LOD) using 200–1000 µL injection volumes and depend on the blank and reagent quality. The power of this method relies on the capability to directly determine a wide range of absolute and relative concentrations of Fe(II) and Fe(III) in the field. The field-deployed IC-Vis method was applied for the determination of Fe(II) and Fe(III) concentrations in natural thermal water with discharge temperatures ranging from 12°C to 95°C, pH between 2.46 and 9.75, and Fe_total concentrations ranging from a few μg L^?c up to 8.3 mg L^?1.     

Redox behaviors of Fe(II/III) and U(IV/VI) studied in synthetic water and KURT groundwater by potentiometry and spectroscopy

The redox behaviors of Fe(II/III) and U(IV/VI) in both synthetic samples and natural groundwater were investigated with potentiometry, UV/VIS absorption spectroscopy, and time-resolved laser fluorescence spectroscopy. Total dissolved Fe(II/III) concentration along with presence of mixed redox couples of Fe²⁺/Fe³⁺ and Fe²⁺/Fe₂O₃(s) were revealed to be the major factors influencing on the redox potentials. Considerable discrepancies between redox potentials obtained with quantitative analysis and chemical speciation of Fe(II/III) and U(IV/VI) ions were identified in the KAERI Underground Research Tunnel groundwater. Chemical speciation of U(IV) in natural groundwater without considering relevant complexation reaction might cause relatively large uncertainties in redox potential calculations.     

The measurement of organically complexed Fe in natural waters using competitive ligand reverse titration

Whilst there is increasing evidence for the presence of stabilized Fe^II associated with organic matter in aquatic environments, the absence of a reliable method for determining Fe^II speciation in solution has inhibited the study of this aspect of Fe biogeochemistry. A technique is described here for the determination of Fe^II organic complexation in natural waters that is based on competitive ligand reverse titration and a model fit to experimental results, from which ligand concentration and a conditional stability constant can be obtained. Spectrophotometry was used to detect the Ferrozine (FZ) complex with reactive Fe^II, which in combination with a liquid waveguide capillary cell (LWCC) enabled high sensitivity and precision measurements of Fe^II to be made. A series of samples was collected in the Itchen River in Southampton, UK to test the method at a wide range of salinities including river water. Levels of Fe^II and total dissolved Fe were within previously reported values for this system. Fe^II was found to occur organically complexed with values for log K^′_FeIIL$\log {K^{\prime }}_{{\text{Fe}}^{\text{II}}\text{L}}$ (conditional stability constant for Fe^II-natural ligand complexes) of ≈8 at salinities between 0 and 21, whilst no measurable complexation was detected at a salinity of 31. This work demonstrates that spectrophotometry can be used in combination with ligand competition to investigate metal speciation in natural waters.     

The discovery of hidden arsenic species in coastal waters

The analysis of ultraviolet (UV)-irradiated and untreated seawater samples has shown that the dissolved arsenic content of marine waters cannot be completely determined by hydride generation–atomic absorption spectrophotometry without sample pretreatment. Irradiation of water samples obtained during a survey of arsenic species in coastal waters during the summer of 1988 gave large increases in the measured speciation. Average increases in the measured speciation. Average increases in total arsenic, monomethylarsenic and dimethylarsenic were 0.29 μg As dm^?3 (25%), 0.03 μg As dm^?3 (47%) and 0.12 μg As dm^?3 (79%), respectively. Overall, an average 25% increase in the concentration of dissolved arsenic was observed following irradiation. This additional arsenic may be derived from compounds related to algal arsenosugars or to their breakdown products. These do not readily yield volatile hydrides when treated with borohydride and are not therefore detected by the normal hydride generation technique. This has important repercussions as for many years this procedure, and other analytical procedures which are equally unlikely to respond to such compounds, have been accepted as giving a true representation of the dissolved arsenic speciation in estuarine and coastal waters. A gross underestimate may therefore have been made of biological involvement in arsenic cycling in the aquatic environment.     

Speciation of dissolved Fe(II) and Fe(III) in environmental water samples by micro-column packed with N-benzoyl-N-phenylhydroxylamine loaded on microcrystalline naphthalene and determination by electrothermal vaporization inductively coupled plasma-optical emission spectrometry

A novel solid phase extraction technique for the speciation of trace dissolved Fe(II) and Fe(III) in environmental water samples was developed by coupling micro-column packed with N-benzoyl-N-phenylhydroxylamine (BPHA) loaded on microcrystalline naphthalene to electrothermal vaporization inductively coupled plasma-optical emission spectrometry (ETV-ICP-OES). Various influencing factors on the separation and preconcentration of Fe(II) and Fe(III), such as the acidity of the aqueous solution, sample flow rate and volume, have been investigated systematically, and the optimized operation conditions were established. At pH 3.0 Fe(III) could be selectively retained by micro-column (20 mm × 1.4 mm, i.d.) packed with BPHA immobilized on microcrystalline naphthalene, and Fe(II) passed through the micro-column. Both Fe(II) and Fe(III) could be adsorbed by the micro-column at pH 6.5. Thus, the total Fe could be determined without the need for preoxidation of Fe(II) to Fe(III). The retained Fe(III) or the Fe(II) and Fe(III) was subsequently eluted by 0.1 ml of 1 mol l⁻¹ HCl. The adsorption capacity of the solid phase adsorption material was found to be 45.0 mg g⁻¹ for Fe(III) at pH 3.0 and 65.3 mg g⁻¹ for Fe(II) at pH 6.5, respectively. The detection limit (3σ) of 0.053 μg l⁻¹ was obtained with a practical enrichment factor of 156 at a sample volume of 17 ml. The relative standard deviations of 4.2% and 4.6% (C_Fe(III) = C_Fe(II) = 10 μg l⁻¹, n = 7) for Fe(III) and total iron were found, respectively. The method was successfully applied to the determination of trace Fe(II) and Fe(III) in environmental water samples (East Lake water, local tap water and mineral water). In order to validate the method, the developed method was applied to the determination of total iron in certified materials of NIES NO.10-b rice flour and GBW07605 tea leaves, and the results obtained were in good agreement with the certified values.     

Arsenic speciation in natural waters by cathodic stripping voltammetry

Contamination of groundwater with arsenic (As) is a major health risk through contamination of drinking and irrigation water supplies. In geochemically reducing conditions As is mostly present as As(III), its most toxic species. Various methods exist to determine As in water but these are not suitable for monitoring arsenic speciation at its original pH and without preparation. We present a method that uses cathodic stripping voltammetry (CSV) to determine reactive As(III) at a vibrating, gold, microwire electrode. The As(III) is detected after adsorptive deposition of As(OH)₃⁰, followed by a potential scan to measure the reduction current from As(III) to As(0). The method is suitable for waters of pH 7-12, has an analytical range of 1 nM to 100 μM As (0.07-7500 ppb) and a limit of detection of 0.5 nM with a 60 s deposition time. The As speciation protocol involves measuring reactive As(III) by CSV at the original pH and acidification to pH 1 to determine inorganic As(III) + As(V) by anodic stripping voltammetry (ASV) using the same electrode. Total dissolved As is determined by ASV after UV-digestion at pH 1. The method was successfully tested on various raw groundwater samples from boreholes in the UK and West Bengal.     

Selective separation and determination of dissolved chromium species in natural waters by atomic absorption spectrometry

A procedure for determining the concentrations of dissolved chromium species in natural waters is described. Chromium(III) and chromium(VI), separated by co-precipitation with hydrated iron(III) oxide, and total dissolved chromium are determined separately by conversion to chromium(VI), extraction with APDC into MIBK and determination by a.a.s. The detection limit is 40 ng l^?1 Cr. The dissolved chromium not amenable to separation and direct extraction is calculated by difference. In the waters investigated, total concentrations were relatively high (1–5 μg l^?1) with Cr(VI) the predominant species in all areas sampled with one exception, where organically bound chromium was the major species.     

2,2′:6′,2″-Terpyridine[hydroxypropyl-β-cyclodextrin] 1:3 complex used as chelating agent for the determination of iron with a sensitive, selective and fast liquid chromatographic method

In the present work, 2,2′:6′,2″-Terpyridine (terpy), a substance with very poor aqueous solubility, was dissolved in water, after formation of its inclusion complex with hydroxypropyl-β-cyclodextrin (HPβCD), in a 1:3 stoichiometry. The obtained [terpy:(HPβCD)₃] supramolecule, with enhanced aqueous solubility, enables its usage as a reagent at RP-LC methods. It was found that, terpy after inclusion complexation retains unaffected the ability of binding to Fe²⁺. It was also observed that, the stable, reddish-purple [Fe(terpy)]²⁺ complex was formed quantitatively in a wide pH range (2-9). Subsequently, iron as active substance or impurity in a drug product, can be determined through UV-vis measurements of [Fe(terpy)₂]²⁺. Speed, sensitivity and selectivity are the most important features of the isocratic RP-LC method, developed to determine iron in pharmaceutical formulations. The duration of the chromatographic separation was less than 4.0 min. The method was linear, precise and accurate from 0.17 to 2.2 mg l⁻¹ of iron and the detection limit was found to be 5 μg l⁻¹. The absorbance at 318 and 552 nm allowed the quantitation of Fe (II) and Fe (III) after reduction, as well as of total Fe (II + III). Moreover, there were no interferences from Fe³⁺, Ni²⁺, Co²⁺ or Cu²⁺.     

Catalytic flow-injection determination of reactive iron in non-acidified water samples.

A flow-injection determination method was proposed for the speciation of reactive species of iron in non-acidified water samples. Iron was determined by the spectrophotometric monitoring of the iron-catalyzed oxidation of o-phenylenediamine with hydrogen peroxide. The reactive iron was characterized as Fe3+ and Fe(III)Li(3-in), where i = 1 or 2 for a unidentate ligand and i = 1 for a bidentate ligand (L(n-)) from the chemical equilibria of hydroxo, oxalato and fluoro Fe(III) complexes. A useful parameter was also proposed to check the reliability of the determination when suffering the adsorption of iron on the inner wall of a PTFE tube of a flow-injection system. Under the optimized FIA condition and by measuring the peak height, a linear calibration curve of iron was obtained up to 0.1 mg L(-1) with a detection limit of 0.1 microg L(-1). The proposed method was successfully applied to the speciation of reactive and unreactive iron in tap- and river-water samples.     

The use of Nafion-coated thin mercury film electrodes for the determination of the dissolved copper speciation in estuarine water

Differential pulse anodic stripping voltammetry (DPASV) using a Nafion-coated thin mercury film electrode (NCTMFE) was implemented to determine the dissolved copper speciation in saline estuarine waters containing high concentrations of dissolved organic matter (DOM). The study used model ligands and estuarine water from San Francisco Bay, California, USA to demonstrate that the NCTMFE is more effective at distinguishing between electrochemically inert and labile copper species when compared to the conventional thin mercury film electrode (TMFE). Copper titration results verify that the NCTMFE better deals with high concentrations of DOM by creating a size-exclusion barrier that prevents DOM from interacting with the mercury electrode when performing copper speciation measurements. Pseudovoltammograms were used to illustrate that copper complexes found in natural waters were more apt to be electrochemically inert at the NCTMFE relative to the TMFE when subjected to high negative overpotentials. Copper speciation results using the NCTMFE from samples collected in San Francisco Bay estimated that >99.9% of all copper was bound to strong copper-binding ligands. These L₁-class ligands exceeded the concentration of total dissolved copper in all samples tested and control the equilibrium of ambient [Cu²⁺] in the San Francisco Bay estuary.     

Selective extraction, separation and speciation of iron in different samples using 4-acetyl-5-methyl-1-phenyl-1H-pyrazole-3-carboxylic acid

A method for speciation, preconcentration and separation of Fe(II) and Fe(III) in different matrices was developed using solvent extraction and flame atomic absorption spectrometry. 4-Acetyl-5-methyl-1-phenyl-1H-pyrazole-3-carboxylic acid (AMPC) was used as a new complexing reagent for Fe(III). The Fe(III)-AMPC complex was extracted into methyl isobutyl ketone (MIBK) phase in the pH range 1.0-2.5, and Fe(II) ion remained in aqueous phase at all pH. The chemical composition of the Fe(III)-AMPC complex was determined by the Job's method. The optimum conditions for quantitative recovery of Fe(III) were determined as pH 1.5, shaking time of 2 min, 1.64 × 10⁻⁴ mol L⁻¹ AMPC reagent and 10 mL of MIBK. Furthermore, the influences of diverse metal ions were investigated. The level of Fe(II) was calculated by difference of total iron and Fe(III) concentrations. The detection limit based on the 3σ criterion was found to be 0.24 μg L⁻¹ for Fe(III). The recoveries were higher than 95% and relative standard deviation was less than 2.1% (N = 8). The validation of the procedure was performed by the analysis of two certified standard reference materials. The presented method was applied to the determination of Fe(II) and Fe(III) in tap water, lake water, river water, sea water, fruit juice, cola, and molasses samples with satisfactory results.     

Speciated isotope dilution analysis of Cr(III) and Cr(VI) in water by ICP-DRC-MS

An isotope dilution method has been developed for the speciation analysis of chromium in natural waters which accounts for species interconversions without the requirement of a separation instrument connected to the mass spectrometer. The method involves (i) in-situ spiking of the sample with isotopically enriched chromium species; (ii) separation of chromium species by precipitation with iron hydroxide; (iii) careful measurement of isotope ratios using an inductively coupled plasma mass spectrometer (ICP-MS) with a dynamic reaction cell (DRC) to remove isobaric polyatomic interferences. The method detection limits are 0.4 μg L⁻¹ for Cr(III) and 0.04 μg L⁻¹ for Cr(VI). The method is demonstrated for the speciation of Cr(III) and Cr(VI) in local nullah and synthetically spiked water samples. The percentage of conversion from Cr(III) to Cr(VI) increased from 5.9% to 9.3% with increase of the concentration of Cr(VI) and Cr(III) from 1 to 100 μg L⁻¹, while the reverse conversion from Cr(VI) to Cr(III) was observed within a range between 0.9% and 1.9%. The equilibrium constant for the conversion was found to be independent of the initial concentrations of Cr(III) and Cr(VI) and in the range of 1.0 (at pH 3) to 1.8 (at pH 10). The precision of the method is better than that of the DPC method for Cr(VI) analysis, with the added bonuses of freedom from interferences and simultaneous Cr(III) determination.     

Photoreduction of Terrigenous Fe‐Humic Substances Leads to Bioavailable Iron in Oceans

Humic substances (HS) are important iron chelators responsible for the transport of iron from freshwater systems to the open sea, where iron is essential for marine organisms. Evidence suggests that iron complexed to HS comprises the bulk of the iron ligand pool in near‐coastal waters and shelf seas. River‐derived HS have been investigated to study their transport to, and dwell in oceanic waters. A library of iron model compounds and river‐derived Fe‐HS samples were probed in a combined X‐ray absorption spectroscopy (XAS) and valence‐to‐core X‐ray emission spectroscopy (VtC‐XES) study at the Fe K‐edge. The analyses performed revealed that iron complexation in HS samples is only dependent on oxygen‐containing HS functional groups, such as carboxyl and phenol. The photoreduction mechanism of Fe^III‐HS in oceanic conditions into bioavailable aquatic Fe^II forms, highlights the importance of river‐derived HS as an iron source for marine organisms. Consequently, such mechanisms are a vital component of the upper‐ocean iron biogeochemistry cycle.     

Photochemistry of organic iron(III) complexing ligands in oceanic systems

Iron is a limiting nutrient for primary production in marine systems, and photochemical processes play a significant role in the upper ocean biogeochemical cycling of this key element. In recent years, progress has been made toward understanding the role of biologically produced organic ligands in controlling the speciation and photochemical redox cycling of iron in ocean surface waters. Most (>99%) of the dissolved iron in seawater is now known to be associated with strong organic ligands. New data concerning the structure and photochemical reactivity of strong Fe(III) binding ligands (siderophores) produced by pelagic marine bacteria suggest that direct photolysis via ligand-to-metal charge transfer reactions may be an important mechanism for the production of reduced, biologically available iron (Fe[II]) in surface waters. Questions remain, however, about the importance of these processes relative to secondary photochemical reactions with photochemically produced radical species, such as superoxide (O2-). The mechanism of superoxide-mediated reduction of Fe(III) in the presence of strong Fe(III) organic ligands is also open to debate. This review highlights recent findings, including both model ligand studies and experimentallobservational studies of the natural seawater ligand pool.     

Determination of chromium(III) and total chromium in marine waters

The development of an analytical technique is described which may be used to determine chromium, chromium(III) and chromium(VI) in estuarine and coastal waters. The method is based on selective micro-solvent extraction with subsequent GFAAS. The technique has been applied in a major North Sea estuary. The results obtained confirm that thermodynamic factors alone cannot be relied upon to describe the form of chromium in estuaries. Kinetic factors appear to have a strong influence over speciation and lead to the persistence of Cr(III) species in environments where Cr(VI) would be expected to be present.     

Highly mobile iron pool from a dissolution-readsorption process

Oxalate (C2O4 2-) acts synergistically on the dissolution of goethite (alpha-FeOOH) in the presence of siderophores that are secreted by plants and microorganisms to sequester iron. We report here the first in situ molecular-scale observations of synergistic ligand-promoted dissolution processes. We show that there are conditions under which oxalate promotes goethite dissolution, but dissolved Fe(III) concentrations do not increase because Fe(III)-oxalate complexes readsorb to the mineral surface. We demonstrate that these readsorbed Fe(III)-oxalate complexes are highly mobile, extremely reactive in the presence of uncomplexed siderophores, and responsible for the synergistic effects on the dissolution of goethite.     

Determination of chromium(III) and total chromium in marine waters

The development of an analytical technique is described which may be used to determine chromium, chromium(III) and chromium(VI) in estuarine and coastal waters. The method is based on selective micro-solvent extraction with subsequent GFAAS. The technique has been applied in a major North Sea estuary. The results obtained confirm that thermodynamic factors alone cannot be relied upon to describe the form of chromium in estuaries. Kinetic factors appear to have a strong influence over speciation and lead to the persistence of Cr(III) species in environments where Cr(VI) would be expected to be present.     

Sensitive determination of iron(III) by gold electrode modified with 2-mercaptosuccinic acid self-assembled monolayer

Preparation and application of gold 2-mercaptosuccinic acid self-assembled monolayer (Au-MSA SAM) electrode for determination of iron(III) in the presence of iron(II) is described by cyclic voltammetry, electrochemical impedance spectroscopy, and Osteryoung square wave voltammetry. The square wave voltammograms showed a sharp peak around positive potentials +0.250 V that was used for construction of the calibration curve. Parameters influencing the method were optimized. A linear range calibration curve from 1.0 × 10⁻¹⁰ to 6.0 × 10⁻⁹ M iron(III) with a detection limit of 3.0 × 10⁻¹¹ M and relative standard deviation (R.S.D.) of 6.5% for n = 8 at 1.0 × 10⁻⁹ M iron(III) was observed in the best conditions. Possible interferences from the coexisting ions were also investigated. The results demonstrated that sensor could be used for determination of iron(III) in the presence of various ions. The validity of the method and applicability of the sensor were successfully tested by determining of iron(III) in natural waters (tap and mineral waters) and in a pharmaceutical sample (Venofer^® ampoule) without interference from sample matrix. The experimental data are presented and discussed from which the new sensor is characterized.     

The determination of dissolved chromium(III) and chromium(VI) and particulate chromium in waters at μg l-1 levels by thin-film x-ray fluorescence spectrometry

A method is described for the determination of particulate chromium and dissolved chromium(III) and (VI) in water at μg l^-1 levels. Particulate material is collected by filtration of the water sample through a membrane filter (0.4-μm pore-size). Chromium(III) and chromium(VI) are then coprecipitated, separately and in that order, with iron(III) hydroxide (at pH 8.5) and a cobalt—pyrrolidinedithiocarbamate carrier complex (at pH 4.0). Both precipitates are collected as thin films on membrane filters and, with the particulate material, analysed directly for chromium by x-ray fluorescence spectrometry. Detection limits, for a 100-ml water sample and counting times of 100 s, are 0.1 μg Cr l^-1. The method is unaffected by sea salt and is applicable, without modifications, to river and estuarine waters.