The effect of humic acid on mercury solubility and complexation

Utilization of mercury in gold mining in the tropics has contributed to large inputs of the metal into the aquatic environment. Although in this activity mercury is utilized in its elemental state, which is relatively immobile and inert, transformations to methylmercury occurring in natural systems lead to mercury contamination of aquatic organisms and the food chain. We investigated the effectiveness and mechanisms involved in the solubilization of mercury in the presence of humic acid, which is an important component of dark river waters in the tropics. Results showed that the solubility of elemental mercury was enhanced due to the presence of humic acid through a solubilization–complexation mechanism, which was attributed to the presence of acid sites on the humic acid molecule, mainly the carboxyl group. Calcium ions in solution prevent humic‐acid‐induced elemental mercury solubility. Although it was demonstrated that the mercury complex formed is more mobile in the presence of humic acid, preliminary results indicated that this mercury complex seem absorbed by fish. Copyright © 2000 John Wiley & Sons, Ltd.     

The pzc of mercury in the presence of humic acids and their complexes with aluminium

Variation of the lifetime of a mercury drop with potential was used to determine the pzc of mercury in the presence of soil humic acids and their aluminium complexes. In all cases there was an overall net shift in the pzc in the cathodic direction. Variation in the extent of the shift with pH and concentration indicated greater adsorption of negatively charged species. The shift was smaller in the presence of aluminium humates, probably due to a decrease in the negative charge of the humic acid molecules after complexing. The use of electroanalytical techniques for metal speciation studies in soils and natural water, if humic materials are known to be present, might therefore be limited.     

Effect of surface-active compounds on voltammetric stripping analysis at the mercury film electrode

The effects of various organic compounds on the differential-pulse anodic-stripping voltammetric response at the in-situ plated mercury film electrode are explored. These effects vary from metal to metal and from one organic compound to another. The most pronounced effects are observed in measurements of copper. The main effect of the organic compound is to depress the peak current rather than change the peak shape or potential. The differences between the organic interferences observed at the mercury film electrode and those reported at the hanging mercury drop electrode are explained by the different morphology and geometry of the two electrodes. The implications of these interferences for the reliability and feasibility of stripping measurements in natural waters are discussed. Gelatin, camphor, humic acid, starch, agar, sodium dodecyl sulphate and albumin were used as representative organic compounds, and cadmium, lead, and copper as test metal ions.     

On-chip generated mercury microelectrode for heavy metal ion detection

In this paper, the on-chip generated mercury microelectrode for heavy metal ion detection has been presented. A mercury droplet (approximately 150 microm in diameter) is on-chip generated to form a microelectrode through the mercury microfluidics control. The mercury microelectrode is used to electrochemically detect the heavy metal ions. The sample solutions with different concentration of heavy metal ions (Pb(2+) and Cd(2+)) have been successfully detected on the mercury droplet microelectrode using the square wave stripping voltammetry.     

Fourier transform ion cyclotron resonance mass spectral characterization of metal‐humic binding

The interaction between metals and naturally occurring humic substances and the thereby induced issues of bioavailability and hydrogeochemical turnover of metal ions in natural waters have been the subject of intense study for decades. Traditional bulk techniques to investigate metal‐humic binding (e.g. potentiometry and inductively coupled plasma mass spectrometry (ICP‐MS)) can provide quantitative results for the relative abundance and distribution of metal species in humic samples and/or overall binding constants. The shortcoming of these bulk techniques is the absence of structural detail. Ultra‐high‐resolution mass spectrometry, currently the only technique demonstrated to resolve individual humic ions, is not generally employed to provide the missing qualitative information primarily because the identification of metal complexes within the already complex mixtures of humic substances is non‐trivial and time‐consuming to the extent of eliminating any possibility for real‐time manipulation of chelated analytes. Here, it is demonstrated that with tailored selection of the metal ion, it is possible to visually identify large numbers of metal‐humic complexes (～500 for Be²⁺, ～1100 for Mn²⁺, and ～1500 for Cr³⁺) in real‐time as the spectra are being acquired. Metal ions are chosen so that they form primarily even‐m/z complexes with humic ions. These even‐m/z complexes stand out in the spectrum and can readily be characterized based on molecular formulae, which here revealed for example that Suwannee River fulvic acid (SRFA) complexes encompassed primarily highly oxygenated fulvic acids of relatively low double‐bond equivalence. Facile, real‐time identification of even‐m/z metal‐humic complexes additionally allows for the specific selection of metal‐humic complexes for MSⁿ analysis and in‐trap ion‐neutral reactions enabling investigation of metal‐humic complex structure. MS/MS data were collected to demonstrate the potential of the technique as well as highlight some of the remaining challenges. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of organic colloids on ASV signals of Cd, Pb and Cu

The effect of organic colloids, such as humic and fulvic acids, on the ASV signals of Cd, Pb and Cu in acetate-buffer media has been investigated. Owing to their ability to adsorb metal ions, or form complexes (some sparingly soluble), the presence of the organic acids caused the magnitude of the signal peaks to decrease, the effect increasing with increase in organic acid: metal-ion ratio, and pH (from 5 to 8). Analysis before and after nitration through a 0.45-mum membrane indicated that most of the Cd retained was labile; in the Pb and Cu system less than half of the sorbed metal ion was labile, though a further fraction was displaced in the presence of Chelex 100 resin. The magnitude of the interference effect varied with the molarity of the medium and the presence of diverse anions (e.g. chloride, bromide) in the system. Cu displayed the most anomalous behaviour, with the stripping peak tending to broaden and split, particularly in the presence of humic acids. This behaviour was enhanced when the contact time with the preformed Hg film before the deposition/stripping cycle was increased above 5-10 min. This effect has been attributed to reduction (and subsequent oxidation) of Cu-humate species adsorbed on the mercury film.     

Effect of model organic compounds on square-wave voltammetric stripping analysis at the Nafion/chelating agent mercury film electrodes

The effect of various organic compounds on the Nafion/chelating agent mercury film electrodes (NCAMFEs) in square-wave anodic-stripping voltammetry (SWASV) is explored. Two chelating agents used to prepare the NCAMFEs are dimethylglyoxime and 2,2'-bipyridyl. Triton X-100, sodium dodecyl sulfate, albumin, gelatin, starch, camphor, and humic acid are used as model organic compounds, while cadmium, lead, and copper are used as test metal ions. The NCAMFEs are considerably more resistant to organic interferences than the Nafion-coated mercury film electrode. The implications of these interferences for the reliability and feasibility of stripping measurements using the NCAMFEs in real samples are discussed. Results presented for untreated urine and natural water samples demonstrate the analytical utility of the NCAMFEs in SWASV.     

Analysis of sorption and bioavailability of different species of mercury on model soil components using XAS techniques and sensor bacteria

The present work studies the adsorption behaviour of mercury species on different soil components (montmorillonite, kaolinite and humic acid) spiked with CH₃HgCl and CH₃HgOH at different pH values, by using XAS techniques and bacterial mercury sensors in order to evaluate the availability of methyl mercury on soil components. The study details and discusses different aspects of the adsorption process, including sample preparation (with analysis of adsorbed methyl mercury by ICP-OES), the various adsorption conditions, and the characterization of spiked samples by XAS techniques performed at two synchrotron facilities (ESRF in Grenoble, France and HASYLAB in Hamburg, Germany), as well as bioavailability studies using mercury-specific sensor bacteria. Results show that XAS is a valuable qualitative technique that can be used to identify the bonding character of the Hg in mercury environment. The amount of methyl in mercury adsorbed to montmorillonite was pH-dependent while for all soil components studied, the bond character was not affected by pH. On the other hand, clays exhibited more ionic bonding character than humic acids did with methyl mercury. This interaction has a higher covalent character and so it is more stable for CH₃HgOH than for CH₃HgCl, due to the higher reactivity of the hydroxyl group arising from the possible formation of hydrogen bonds.The bioavailability of methyl mercury adsorbed to montmorillonite, kaolinite and humic acids was measured using recombinant luminescent sensor bacterium Escherichia coli MC1061 (pmerBR_BSluc). In case of contact exposure (suspension assays), the results showed that the bioavailability was higher than it was for exposure to particle-free extracts prepared from these suspensions. The highest bioavailability of methyl mercury was found in suspensions of montmorillonite (about 50% of the total amount), while the bioavailabilities of kaolinite and humic acids were five times lower (about 10%). The behaviour of methyl mercury in the presence of montmorillonite could be explained by the more ionic bonding character of this system, in contrast to the more covalent bonding character observed for humic acids. Thus, XAS techniques seem to provide promising tools for investigating the mechanisms behind the observed bioavailabilities of metals in various environmental matrices, an important topic in environmental toxicology.     

Treatment of humic acids with ferric, aluminum, and chromium ions in water

The interaction of ferric, aluminum, and chromium ions with humic acids at different pH of mixture has been investigated. The concentration of metal ions, bound with humic acid is proportional to the concentration of metal ion in water solution at pH 3 at low metal ion concentrations. The ratio of the number of ferric ions, bound with humic acid, to the number of acidic groups in humic acid is more than 1.5 at high ferric ion concentration in solution. That is bound with the formation of dimers of ferric ions and their treatment with acidic groups of humic acid. The radius of particles formed in the reaction of humic acid with ferric ion does not depend on the pH and decreases in the reaction with chromium and aluminum ions at pH higher than 4. An increase of metal ion concentration leads to the increase of the particle radius at low metal ion concentration reaching the constant value at high metal ion concentration and pH 6.1.     

A novel sequential injection--Cold vapour atomic absorption spectrometric system for rapid and reliable determination of mercury

A simple and rapid method is described for reliable determination of ultra-trace concentrations of mercury based on a novel combination of a sequential injection (SI) system with cold vapour atomic absorption spectrometry (CVAAS). The SI-CVAAS method achieved a very high sampling frequency of 180h(-1), a detection limit of 0.2-0.3ngmL(-1), a dynamic range of 0-60ngmL(-1) and requires only 0.2-0.6mL of sample. The presence of common metal ions, such as Cu(2+), Pb(2+), Cd(2+), Zn(2+), Fe(3+), Co(2+), Ni(2+) and Mn(2+), did not interfere with the measurement of mercury by this method. Also the interference from organic matter, such as humic substances, was easily reduced or eliminated by appropriate sample dilution. The method was successfully applied to the determination of mercury in lake sediment samples and a river sediment reference material. An average percentage recovery of 101.2% was achieved by the SI-CVAAS method for mercury in the reference material with a R.S.D. of 1.8%.     

An Electrochemical Flow Cell For The Detection Of Metal Complexes

Abstract

An electrochemical flow cell has been developed using reticulated vitreous carbon (RVC) coated with a thin film of mercury. The flow cell and electrode components are inexpensive and easily constructed. Reproducible results have been obtained for concentrations as low as 100 ppb. Peak currents were dependent on flow rate, electrode size, and mercury deposition time. Oxygen interference was reduced by nitrogen purging to enable analysis of sub-ppm metal concentrations. The cell was tested as a detector for metal complexes eluated from a modified gel permeation chromatographic column.     

Mercury,a Structural Building Block and Source of Localized Reactivity in Extended Metal–Metal Bonded Systems

Transition metal–mercury complexes were among the first compounds of study for the concept of direct metal–metal bonding which was established more than three decades ago. Since then, a large number of such systems have been synthesized and studied. The fact that mercury is readily attached to a large variety of main group or transition metals has stimulated its use as a general building block in the systematic synthesis of mixed-metal clusters. The past decade has witnessed a rapid expansion of bimetallic cluster chemistry in which species containing mercury have played a prominent role, and which has led to the discovery of many unprecedented cluster structures and reactions. In particular, the ability of mercury to form multicenter metal–metal bonds with polynuclear cluster fragments has substantially extended its coordination chemistry which was thus far dominated by simple linear structural arrangements. Although certain structural motifs are found to be common to many of the transition metal–mercury clusters investigated to date and thus enable a relatively systematic synthetic approach, the multitude of surprising discoveries has kept the interest in the chemistry of the element itself alive. The recent discovery of the redox and photochemical reactivity of some of these systems has opened up an exciting and promising area of cluster research. Its significance for the synthetic methodology lies in the fact that the increasing redox activity of molecular carbonyl clusters on going to higher nuclearities appears to set a limit on the size of metal frameworks attainable by the standard preparative methods. On the other hand, their potential use as photochromes or redox mediaters in coupled electron-transfer reactions provides an additional stimulus for future studies in this field.     

Kinetic aspects of the reduction of mercury ions by humic substances

A combination of a computer controlled titration device with a cold vapour atomic absorption spectrometer was set up to examine the kinetics of the reaction between humic substances and Hg(II). Subsequent injections of Hg(II) into an excess of humic acid produce data about short and long term changes in the reaction velocity of humic substances. This experimental design is suitable for the examination of the effects of pH, reaction time, light and interfering substances like chloride on the formation of elemental mercury. The obtained data agree with data from the literature. With the accumulation of the produced elemental mercury on a gold net it is possible to enhance the sensitivity of the method. This enables examinations at environmental concentrations.     

Synthesis and properties of a chelating resin containing triazolethiol groups

A macroreticular poly(acrylic acid)-based resin with triazolethiol as the functional group has been synthesized. The stability of the resin in acidic media and the behaviour in sorption and desorption of various metal ions have been investigated and compared with those of the acylthiosemicarbazide resin which is an intermediate in synthesis of the triazolethiol resin. Both resins show high affinity for copper(II) silver, cadmium and mercury(II), and high selectivity for silver and mercury(II) at low pH (1–2), and even at pH 7 if EDTA is present. The triazolethiol resin sorbs metal ions faster than the acylthiosemicarbazide resin does and sorbs mercury(II) from high concentrations of acids and neutral salt solutions. This resin has been applied to the concentration of silver and mercury(II) from sea-water samples by column operation.     

Kinetic aspects of the reduction of mercury ions by humic substances

A combination of a computer controlled titration device with a cold vapour atomic absorption spectrometer was set up to examine the kinetics of the reaction between humic substances and Hg(II). Subsequent injections of Hg(II) into an excess of humic acid produce data about short and long term changes in the reaction velocity of humic substances. This experimental design is suitable for the examination of the effects of pH, reaction time, light and interfering substances like chloride on the formation of elemental mercury. The obtained data agree with data from the literature. With the accumulation of the produced elemental mercury on a gold net it is possible to enhance the sensitivity of the method. This enables examinations at environmental concentrations.     

Characterization of Mercury – Humic Acids Interaction by Potentiometric Titration with a Modified Carbon Paste Mercury Sensor

Stability constant for mercury binding by commercial and natural humic acids (HA) were determined using a new potentiometric mercury(II) sensor based on dithiosalicylic acid modified carbon paste electrode. The sensor present a high selective and sensitive response to mercury(II) ions, and a low detection limit of 1.8×10^?8 M. The potentiometric titrations curves of humic acids against mercury(II) ions were modeled. For 1.00×10^?7 to 3.00×10^?4 M mercury(II) ion concentration levels the results are consistent with the presence of two different binding sites in the humic acid macromolecule. The strongest binding sites (log K₁ ranging from 10.1 to 6.8) are probably due to interaction with carboxylic acid and amine groups in the molecule, whereas weakest binding sites (log K₂ ranging from 8.8 to 4.5) can be associated to phenolic groups.     

Accurate determination of element species by on-line coupling of chromatographic systems with ICP-MS using isotope dilution technique

The instrumental design for coupling different liquid chromatographic systems such as ion, reversed phase, and size exclusion chromatography as well as capillary gas chromatography, with ICP-MS for the determination of element species is described. For accurate analyses obtaining ‘real time’ concentrations of chromatographic peaks, the isotope dilution mass spectrometric (IDMS) technique is applied. Two different spiking modes are possible, one using species-specific and another one using species-unspecific spike solutions of isotope-enriched labelled compounds. The species-specific mode is only possible for element species well defined in their structure and composition, for example iodate or selenite, whereas the species-unspecific mode must be applied in all cases where the structure and composition of the species is unknown, for example, for metal complexes with humic substances. For accurate determinations by the isotope dilution technique the mass discrimination effect must also be taken into account. Iodate, iodide and organoiodine species, including those of humic substances, have been analysed in mineral, drinking and environmental water samples by coupling different liquid chromatographic methods with ICP-IDMS. Heavy metal complexes with humic substances in water samples of different origin have been characterized by size exclusion/ICP-IDMS. The possibilities of determining different environmental selenium species are discussed and the results for the analysis of selenite and selenate, which has been carried out by GC/ICP-IDMS after converting these species into a volatile piazselenol compound, are presented.     

Heavy metal content of stormwater runoff in St. Petersburg

A procedure has been developed for the prediction of heavy metal concentrations in storm rainfall in St. Petersburg on the assumption of equal rates of metal influx with precipitations to the Baltic Sea and St. Petersburg territory. The concentrations of mercury, copper, and zinc in stormwater runoff from different administrative districts of the city have been calculated, and the predicted values have been compared with those determined experimentally.     

Fractionation analysis of trace metals in humic substances of soils irrigated with wastewater in Central Mexico by particle induced X-ray emission

The Mezquital Valley in Central Mexico has received wastewater from Mexico City for nearly 100 years. Wastewater brings in organic matter and nutrients but also trace metals. Humic substances, the main components of organic soil matter, are responsible for retaining and regulating the mobility of trace metals in soils. In this study, humic substances were extracted from the soil and separated into distinct fractions (humic acids, fulvic acids and humins). The particle induced X-ray emission (PIXE) technique was applied to determine the metal content in bulk soil as well as in humic acids and fulvic acids not soluble in H₃PO₄. In order to assess whether the long-term input of organic matter and metals modifies the metal association with these humic substances, parcels irrigated for three time periods (5, 47 and 89 years) were selected for this study. It was observed that metals such as Zn and Cu are mainly associated with the humic acids. Fulvic acids retain mainly Cr while Pb is distributed among humic and fulvic acids. It was also observed that in general, metal retention by humic substances increases with irrigation time. Depth also affects metal association with the humic substances.     

STUDY OF THE INFLUENCE OF HUMIC SUBSTANCES ON THE SORPTION OF RARE EARTH ELEMENTS AT THE KAOLINITE AND BENTONITE SURFACES

The isotherms for sorption of fulvic acid from weathered coal,and the distribution ratios,Rd,of ^169Yb at various concentration of fulvic acid on Na-bentonite and Na-Kaolinite have been measured.Meantime,the dependences of Rd of ^169 Yb on pH values at a constant initial concentration of FA were obtained.It was not found that the parallel relationship between the high affinity of humic substance to the clay and the high uptake of metal which has strong ability to complex with this humic substance.