Kipton J. Powell

Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg2+– Cl–, OH–, CO32–, SO42–, and PO43– aqueous systems (IUPAC Technical Report)

This document presents a critical evaluation of the equilibrium constants and reaction enthalpies for the complex formation reactions between aqueous Hg(II) and the common environmental inorganic ligands Cl–, OH–, CO32–, SO42–, and PO43–. The analysis used data from the IUPAC Stability Constants database, SC-Database, focusing particularly on values for 25 °C and perchlorate media. Specific ion interaction theory (SIT) was applied to reliable data available for the ionic strength range Ic < 3.0 mol dm–3. Recommended values of log10βp,q,r° and the associated reaction enthalpies, ∆rHm°, valid at Im = 0 mol kg–1 and 25 °C, were obtained by weighted linear regression using the SIT equations. Also reported are the equations and specific ion interaction coefficients required to calculate log10βp,q,r° values at higher ionic strengths and other temperatures. A similar analysis is reported for the reactions of H+ with CO32– and PO43–. Diagrams are presented to show the calculated distribution of Hg(II) amongst these inorganic ligands in model natural waters. Under typical environmental conditions, Hg(II) speciation is dominated by the formation of HgCl2(aq), Hg(OH)Cl(aq), and Hg(OH)2(aq).

Chemical speciation of environmentally significant metals with inorganic ligands Part 2: The Cu2+-OH-, Cl-, CO32-, SO42-, and PO43- systems (IUPAC Technical Report)

The numerical modeling of CdII speciation amongst the environmental inorganic ligands Cl–, OH–, CO32–, SO42–, and PO43– requires reliable values for the relevant stability (formation) constants. This paper compiles and provides a critical review of these constants and related thermodynamic data. It recommends values of log10βp,q,r° valid at Im = 0 mol kg–1 and 25 °C (298.15 K), along with the equations and empirical reaction ion interaction coefficients, ∆ε , required to calculate log10βp,q,r values at higher ionic strengths using the Brønsted–Guggenheim–Scatchard specific ion interaction theory (SIT). Values for the corresponding reaction enthalpies, ∆rH, are reported where available. Unfortunately, with the exception of the CdII-chlorido system and (at low ionic strengths) the CdII-sulfato system, the equilibrium reactions for the title systems are relatively poorly characterized. In weakly acidic fresh water systems (–log10 {[H+]/c°} < 6), in the absence of organic ligands (e.g., humic substances), CdII speciation is dominated by Cd2+(aq), with CdSO4(aq) as a minor species. In this respect, CdII is similar to CuII [2007PBa] and PbII [2009PBa]. However, in weakly alkaline fresh water solutions, 7.5 < –log10 {[H+]/c°} < 8.6, the speciation of CdII is still dominated by Cd2+(aq), whereas for CuII [2007PBa] and PbII [2009PBa] the carbonato- species MCO3(aq) dominates. In weakly acidic saline systems (–log10 {[H+]/cϒ} < 6; –log10 {[Cl–]/c°} < 2.0) the speciation is dominated by CdCln(2–n)+ complexes, (n = 1–3), with Cd2+(aq) as a minor species. This is qualitatively similar to the situation for CuII and PbII. However, in weakly alkaline saline solutions, including seawater, the chlorido- complexes still dominate the speciation of CdII because of the relatively low stability of CdCO3(aq). In contrast, the speciation of CuII [2007PBa] and PbII [2009PBa] in seawater is dominated by the respective species MCO3(aq). There is scope for additional high-quality measurements in the Cd2+ + H+ + CO32– system as the large uncertainties in the stability constants for the Cd2+-carbonato complexes significantly affect the speciation calculations.

Development and application of the diffusive gradients in thin films technique for the measurement of total dissolved inorganic arsenic in waters.

The diffusive gradients in thin films (DGT) technique, utilizing an iron-hydroxide adsorbent, has been investigated for the in situ accumulation of total dissolved inorganic As in natural waters. Diffusion coefficients of the inorganic As(V) and As(III) species in the polyacrylamide gel were measured using a diffusion cell and DGT devices and a variety of factors that may affect the adsorption of the As species to the iron-hydroxide adsorbent, or the diffusion of the individual As species, were investigated. Under conditions commonly encountered in environmental samples, solution pH and the presence of anions, cations, fulvic acid, Fe(III)-fulvic acid complexes and colloidal iron-hydroxide were demonstrated not to affect uptake of dissolved As. To evaluate DGT as a method for accumulation and pre-concentration of total dissolved inorganic As in natural waters, DGT was applied to two well waters and a river water that was spiked with As. For each sample, the concentration obtained with use of DGT followed by measurement by hydride generation atomic absorption spectrometry with a Pd modifier (HG-AAS) was compared with the concentration of As measured directly by HG-AAS. The results confirmed that DGT is a reliable method for pre-concentration of total dissolved As.

Perfluorosulfonated ionomer-modified diffusive gradients in thin films: tool for inorganic arsenic speciation analysis.

A new concept in speciation analysis based on the diffusive gradients in thin films (DGT) technique is described. By use of two sets of DGT devices, one set with perfluorosulfonated ionomer (Nafion) diffusive membranes and the other with polyacrylamide, anionic and uncharged analytes can be fractionated on the basis of charge. The dual device method is applied to speciation analysis of dissolved inorganic arsenic species. Over the environmentally significant pH range, inorganic As(III) exists as neutral H(3)AsO(3), whereas As(V) is present as anionic H(2)AsO(4)(-) and HAsO(4)(2-). The measured diffusion coefficient of As(III) through the negatively charged Nafion membrane is significantly larger than that of the As(V) species, whereas diffusion rates are similar through polyacrylamide diffusive gels. Hence, after simultaneously deploying DGT devices with and without Nafion membranes, measurement of the amount of accumulated As in each type of device enables the concentration of both oxidation states to be determined.

The aluminium(III)-4-nitrocatechol system: potentiometry, voltammetry and application to the determination of reactive Al(III)

Abstract Potentiometric and voltammetric investigations of the Al(III)-4-nitrocatechol system indicate that 4-nitrocatechol (4ncat) is a suitable ligand for amperometric determination of Al(III) at environmental concentrations. The potentiometric study of solution equilibria was undertaken over the pH range 3–11 and [4ncat]: [Al(III)] ratios from 1 to 4. Complexes [Al(4ncat)]+, [Al(4ncat)2]−, [Al(4ncat)3]3− and [Al(4ncat)2OH]2− were characterised; evidence for [Al(4ncat)OH]0 was equivocal. The voltammetric responses of 4ncat and its Al(III) complexes were examined by cyclic voltammetry and hydrodynamic voltammetry at the rotating disk electrode. The potential for oxidation of free 4ncat decreases as the solution pH increases from ca. 4 to 9, and the potential for oxidation of coordinated 4ncat decreases as the number of coordinated ligands increases. At pH ca. 9, with a suitable excess of ligand, formation of [Al(4ncat)3]3− is almost quantitative, and close to 300 mV separates the oxidation of free and coordinated ligand. These conditions were chosen for amperometric determination of Al(III) in a flow injection analysis (FIA) system. Targeting of the reactive Al(III) fraction and removal of interferents were achieved online using an oxine-derivatised microcolumn. The detection limit for reactive Al(III) was 0.08 μM (2σ for a 1 μM Al3+ standard). The method was applied to the determination of reactive Al(III) in soil solutions and measurement of the Al(III) complexation capacity of a soil-derived fulvic acid.

Flow injection determination of Al3+ and Al13O4(OH)24(H2O)127+ species using a 1.3-s reaction with 8-quinolinol-derivatised Fractogel

Speciation of Al is determined by a 1.3-s reaction with 8-quinolinol (oxine)-derivatised Fractogel in a 22 μl column reactor in a flow injection (FI) manifold. Al (pre)concentrated on the column from a 650 μl sample is selectively eluted with 250 μl of 0.02 M NaOH and detected spectrophotometrically as the Al-chrome azurol S (CAS) complex at pH 5.0. This Al (referred to as ‘free Al’) comprises Al3+ + A1(OH)2+ + Al in very labile complexes. Tests with synthetic solutions established that Al is not significantly sequestered from the citrate, oxalate and malonate complexes. Al-hydroxo polymers [Al13(OH)327+] are quantitatively retained by the column but are not desorbed by 0.02 M NaOH in the time frame of the FI method; therefore they do not contribute to the analytical signal. However, they can be quantified after stopped-flow elution with 0.2 M NaOH. The AlF2+ and AlF2+ complexes are retained and eluted quantitatively and therefore contribute to the measurement of ‘free Al’. The method has been applied to humic waters and soil solutions and the results for ‘free’ Al3+ compared with those obtained by the 7-s CAS method. The method has a 2σ detection limit of 70 nM, a linear working range of 0.3–16 μM and relative standard deviations of 7% and 1% at 0.5 and 16 μM, respectively.

A voltammetric study of the aluminium complexes of catechol and 1,2-dihydroxy-anthraquinone-3-sulfonic acid

Abstract The oxidative electrochemistry of Al-catechol (catechol1,2-benzenediol) and Al-DASA (DASA1,2-dihydroxyanthraquinone-3-sulfonic acid) complexes in aqueous solution has been studied by cyclic voltammetry and steady-state voltammetry at the rotating disk electrode. Speciation calculations enabled the nature and concentration of complexes to be determined, hence the electrochemical response(s) could be identified with particular species. The oxidation potentials for the complexes do not depend on AlIII concentration. For the Al-catechol system, complexes [Al(cat)3]3-, [Al(cat)2]- and [Al(cat)2 OH]2- are electroactive, each with Epa0.27 V versus SCE (irreversible two-electron oxidation; v100 mV s-1). The primary anodic processes for [A1(DASA)]0, [Al (DASA)2]3- and [A1(DASA)3]6- are irreversible two-electron oxidations of one coordinated ligand at Epa0.93, 0.92 and 0.80 V versus SCE, respectively (v50 mV s-1). The initial (reduced) forms of the catechol and DASA complexes are inert on the experimental timescale. After oxidation the ligand dissociates from the metal centre and, when the experimental timescale is sufficiently long, further oxidation of the resultant lower stoichiometry complex is observed for [A1(DASA)2]3-. The implications of the results for the indirect electrochemical determination of aluminium are discussed.

Lability of metal ion-fulvic acid complexes as probed by FIA and DGT: a comparative study

Two kinetic-based analytical techniques with very different time constants (flow injection analysis, FIA and diffusive gradients in thin films, DGT) have been compared in a study of metal ion lability in the systems Al 3+ -FA and Cu 2+ -FA (fulvic acid, FA). Flow injection analysis used an in-line micro-column of chelating ion exchanger to capture the labile metal fraction during the short contact time, 1-3 s, with the injected sample. The moderately labile and inert metal fractions were rejected by the ion exchanger but the former gave a colorimetric reaction down-line. The labile fraction was quantified by subsequent elution of captured metal ions and down-line analysis. The chelating resins used were oxine-derivatised fractogel (for Al 3+ ) and oxine-derivatised controlled-pore glass (for Cu 2+ ). Diffusive gradients in thin films utilised cross-linked polyacrylamide films as the diffusive barrier and Chelex-100 to capture the diffusing metal ions that are labile on the experiment time-scale (min). Diffusion coefficients, D, for metal-FA systems at different metal:FA ratios were measured independently in a diffusion cell. They indicated that for both the labile (Cu 2+ -FA) and slowly labile (Al 3+ -FA) systems the metal ion diffuses at a similar velocity to the FA, even when the total metal ion concentration exceeds the capacity of FA to bind metal ions in non-labile forms (FA complexation capacity, CC). Complexation capacity was used as a basis for comparison of the two techniques. It was observed that for the less labile Al 3+ -FA system, the DGT-labile Al 3+ equated to the sum of the labile + moderately labile fractions determined by FIA. For the Cu 2+ -FA system the CC determined by DGT was smaller than that determined by FIA (significant at IS.D.). Further, the results indicated that D values measured for labile metal-FA complexes in a diffusion cell may not be appropriate for interpretation of diffusion processes that occur in the DGT experiment.

Speciation determination of aluminium (Al3+) by flow injection amperometry via the 1,2-dihydroxyanthraquinone-3-sulfonic acid complex

Two flow injection methods for the determination of ‘free Al’ (Al3+ and Al(OH)2+) in natural samples have been developed and compared. Both employed amperometric detection by measuring the decrease (in the presence of Al) in the oxidation current for DASA (1,2-dihydroxyanthraquinone-3-sulfonic acid) at pH 5.0 at a glassy carbon electrode. One method sought to achieve selective determination of free Al by use of a short reaction time (10 s or 3 s) and a low concentration of DASA (3 μM at the detector). The other effected an initial fractionation of Al on a column (22 μl) of oxine(8-quinolinol)-derivatised Fractogel (< 2 s contact time) followed by elution of Al with NaOH solution. The latter protocol achieved better speciation determination and countered the interference of Fe(III) and Ca2+. The 3σ detection limits for the two methods were 75 and 120 nM, respectively. The procedures were applied to the determination of free Al in synthetic Al-ligand solutions and of the Al complexation capacity of a soil fulvic acid and humic water.

Mechanistic studies on the trapping and desorption of volatile hydrides and mercury for their determination by electrothermal vaporization-inductively-coupled plasma mass spectrometry

Abstract Metallic coatings in the pyrolytic graphite furnace have been used for the pre-concentration of Hg (using electrochemically reduced Au) and AsH 3 (using both thermally and electrochemically reduced Ir and Pd/Ir) prior to electrothermal volatilization. The analyte trapping efficiency during accumulation and the analyte and modifier release processes during volatilization were monitored in real time. This was achieved by using the fast response capability of inductively-coupled plasma-mass spectrometry to determine simultaneously both the analyte and modifier elements as a function of time. The temperature dependence of the analyte trapping process points to contrasting mechanisms for Hg adsorption on Au (reversible, physical adsorption/amalgamation) and for AsH 3 adsorption on Pd, Ir or Pd/Ir (physical adsorption followed by an irreversible hydrogen abstraction reaction at active sites). Results also indicate that only minor volatilization of modifier occurs at the temperature required for volatilization of analyte from the furnace.