James H. Weber

Review of possible paths for abiotic methylation of mercury(II) in the aquatic environment

Abstract High concentrations of methylmercury in freshwater and marine fish lends an urgency to the problem of how inorganic mercury, the predominant pollutant, is converted to methylmercury in the aquatic environment. Numerous experiments by microbiologists suggest that biotic sediments are major sites for methylation of inorganic mercury and that sulfate-reducing bacteria contribute considerably to methylmercury production. However, the sufficient but low sulfate concentration needed for sulfate-reducing bacteria to produce methylmercury makes it difficult to explain its ubiquitous occurrence in coastal and deep sea marine organisms and sediments. In addition, the high concentration of mercury(II) added to samples allows only mercury-resistant species, which do not predominate in the aquatic environment, to survive. Finally, sterilization methods change chemistry as well as biology, and conclusions based on experiments comparing active and sterile sediments are not definitive. For these reasons there is a strong possibility that abiotic methylation of mercury(II) contributes to the formation of methylmercury. This review emphasizes potential abiotic methylation of mercury(II) by methylcobalamin, methyltin compounds, and humic matter. Although methylcobalamin readily methylates mercury(II) in non-environmental matrices, it is least likely of the three methyl donors because it is not abundant in the aquatic environment. In contrast methyltin compounds (mono-, di-, and trimethyltin) occur in water, biota, and sediments of the marine environment. All three methylate mercury(II) in seawater and may be involved in abiotic methylation. Humic matter is the most promising environmental methylating agent for several reasons. Its concentration in waters and sediments is high. It is associated with the solubility and thus mobility of mercury in freshwaters and marine waters, probably by complexation. In addition, several model studies have shown that humic matter effectively methylates mercury(II) forming methylmercury.

Fulvic acid: modifier of metal-ion chemistry

Fulvic acid, which is derived from the decay of plants and animals, is being studied for its role in the transport and toxicity of metal ions in soil and water. It is discussed in relation to the origin of humic substances and its interactions with metal ions. Techniques for investigating complexes of fulvic acid and metal ions are presented. They are separation and nonseparation analyses which are applied to speciation problems. The applicability, advantages, and disadvantages of both methods are presented. Separation of free and complexed metal ions can be done by chromatography, or with membranes that exclude the metal-ion complexes. Chromatographic techniques include liquid chromatography by size exclusion. Nonseparation techniques include voltametry and potentiometry, as well as fluorescence. A comparison of methods for calculation of the conditional stability constant K for complexes containing fulvic acid and copper (II) or cadmium (II) is presented.

Sorption of trace metals by humic materials in soils and natural waters

Trace metal sorption by coagulated humic acid and peat materials can be described macroscopically either as a complexation or a cation exchange phenomenon. Model equilibrium constants or selectivity coefficients for trace metal sorption by H‐form humic materials are special cases of two general models which do not depend on the sorption mechanism. Sorption mechanisms can be elucidated by spectroscopic techniques, both optical and magnetic resonance, but no information about mechanisms can be inferred from equilbrium constants.

The isolation and characterization of fulvic acid and humic acid from river water

Abstract Many rivers in New Hampshire are rich in dark brown organic matter from natural sources. Because the interactions between metal ions like Fe3+ and the organic fulvic and humic acids makes water treatment difficult, we are studying the nature of the organic acids. Fulvic acid was isolated from the B2 horizon of a Podzol soil obtained at Conway, NH, and fulvic and humic acids were isolated from the Oyster River (Lee, NH) and Jewel Pond (Stratham, NH). The method of isolating the organic matter from water involves new techniques. The oxygen-containg functional groups, carbon, hydrogen, and nitrogen were analyzed for all samples. The fulvic acid and humic acid samples isolated from water are different from each other, but similar to analogous soil samples.

Copper(II) complexing capacities of natural waters by fluorescence quenching.

The natural fluorescence of humic substances in both fresh waters and marine samples is quenched upon complexation to Cu2+ ion. Binding curves obtained from titration data are mathematically modeled and complexing capacities ( CL) determined by computer curve fitting. The results indicate that the fluorescence technique directly measures organic matter complexation only, and CL values are unaffected by hydrolysis. Filtered water samples are analyzed at their natural pH under N2 but are otherwise unaltered. Rayleigh scattering, measured along with fluorescence, signals precipitate formation and was used to indicate a suitable stopping point for titrations. Scattering and fluorescence trends were similar for all samples except an estuarine sample. A multiple correlation study of several commonly measured water characteristics and the titration parameters showed a statistically significant trend between UV absorbance and CL values.

Adsorptive behavior of butyltin compounds under simulated estuarine conditions

Abstract A 2 3 + 1 factorial design was used to study adsorption of BuSnCl 3 , Bu 2 SnCl 2 , and Bu 3 SnCl under simulated estuarine conditions. The variables included artificial seawater and its dilutions (salinity 5–35 g kg −1 ), pH (6.2–8.2), and hydrous iron oxide concentration (10–1000 mg l −1 ). Fulvic acid concentration was constant at 10 mg l −1 , and initial concentration of butyltin compounds was 10 ng ml −1 (asSn). Adsorption of butyltin compounds varied from 72 to 100% for BuSnCl 3 , 0 to 56% for Bu 2 SnCl 2 , and 57 to 95% for Bu 3 SnCl. At the 95% confidence level all three variables were significant for BuSnCl 3 adsorption. pH was significant for Bu 2 SnCl 2 adsorption, and pH and salinity were significant for Bu 3 SnCl adsorption. Discussion includes the importance of, and reasons for, differing adsorptive behavior of the three butyltin compounds.

An EPR study of the reduction of vanadium(V) to vanadium(IV) by fulvic acid

Abstract The reduction potential of a soil-derived fulvic acid (SFA) sample is 0.5 V (vs. NHE). The SFA reduction potentials increase with decreasing pH which means that H+ is consumed when SFA acts as an oxidizing agent. We used this SFA as a reducing agent toward vanadium(V) in an electron paramagnetic resonance (EPR) study. At pH 2 a 10-fold SFA molar excess reduces 50% of a vanadium(V) to a vanadium(IV) species (VO2+) within 15 min, but no additional reduction occurs within 4 h. The semiquinone radical in SFA is not involved in the reduction process because its EPR signal intensity does not decrease and because the amount of radical present could account for only 10% of the observed reduction. In a similar experiment at pH 6, only 4.5% soluble VO2+ is measured by EPR. Most of the VO2+ formed in the experiment forms EPR-inactive (VOOH)22+ and solid VO(OH)2. At pH 2.3 a solution of SFA and vanadium(V) in a 10 1 molar ratio has a reduction potential of 0.57 V (vs. NHE) in agreement with the reduction potential of SFA alone. EPR measurements show that the reduced vanadium product, VO2+, is partially complexed to SFA at pH 2, but predominantly complexed at pH 4 and 6.

Determination of mercury(II), monomethylmercury cation, dimethylmercury and diethylmercury by hydride generation, cryogenic trapping and atomic absorption spectrometric detection

Abstract Mercury(II), monomethylmercury cation (MeHg), dimethylmercury (Me 2 Hg) and diethylmercury (Et 2 Hg) were simultaneously determined from aqueous samples by hydride generation volatilization, trapping and separation on a chromatographic column, and detection by atomic absorption spectrophotometry in a heated quartz furnace. Simplex optimization showed us the most effective choice of hydride formation and purge conditions. Absolute detection limits are 50 pg for the three organomercury compounds and 110 pg for Hg(II). Calibration curves are linear from 0.05 to 5 ng, and the reproducibility range for 1 Hg of the four analytes is 3–7%. Large differences among measured MeHg concentrations in a recent interlaboratory study using different measurement methods confirm that a new, completely independent method for determining mercury compounds in environmental samples is important. The method was applied to estuarine samples of the marsh grass Spartina alterniflora and eelgrass ( Zostera marina L.) and found Hg(II), MeHg (eelgrass only) and Me 2 Hg. Our difficulties in determining Me 2 Hg suggest that it is more common in the aquatic environment than commonly believed.

Lead(II) complexation by fulvic acid: how it differs from fulvic acid complexation of copper(II) and cadmium(II).

Abstract Pb2+, like Cu2+, forms strong complexes with fulvic acids (Cd2+-fulvate complexes are much weaker), but Pb-fulvate precipitates at a much lower mole ratio of metal ion to fulvic acid than either Cu-fulvate or Cd-fulvate does. Physical association of Pb2+ with Pb-fulvate solids as well as complexation by sites still available in the precipitates probably causes the increased removal of free Pb2+ from solution after precipitation begins.

Corrected fluorescence spectra of fulvic acids isolated from soil and water.

The fluorescence of humic matter is a ubiquitous phenomenon that occurs for isolated soil and aquatic matter and for natural water samples. This property is used to compare humic substances, but uncorrected emission spectra can be especially misleading for spectra taken on different instruments. This paper details the corrections of emission fluorescence spectra of well-characterized fulvic acids isolated from soil and a fresh-water river. The corrections significantly modify the uncorrected spectra. This effect demonstrates the need for emission spectra corrections before comparing the fluorescence properties of diverse humic matter samples.