Measuring the saturation limit of low-volatility organic compounds in soils: implications for estimates of dermal absorption.

Estimating dermal absorption from contaminated soils typically requires extrapolations from measurements obtained on soils artificially contaminated at much larger concentrations. Such extrapolations should be constrained by the fact that maximum absorption will occur for the largest possible concentration of chemical on the soil without neat chemical being present; i.e., at the soil saturation limit (S(soil)). Saturation limits of two low-volatility model compounds (4-cyanophenol and methyl paraben) were determined on the 38-63µm sieve fraction of four soils with different fractions of organic carbon (f(oc)=0.015-0.45) and specific surface areas (σ(soil)=4-34m(2) g(-1)) using two methods: equilibrium uptake into silicone rubber membranes and differential scanning calorimetry. Except for Pahokee peat, which had the largest f(oc), a model assuming contributions from both surface adsorption and organic carbon absorption provided excellent predictions of S(soil). In all soils, the surface saturation concentration of both chemicals was estimated at 2.2mg m(-2). The saturation concentration of 4-cyanophenol in the soil organic carbon was 1.7-fold higher than methyl paraben, which is consistent with the estimated solubility limits of these two chemicals in octanol.

Isotopic variability of mercury in ore, mine-waste calcine, and leachates of mine-waste calcine from areas mined for mercury.

The isotopic composition of mercury (Hg) was determined in cinnabar ore, mine-waste calcine (retorted ore), and leachates obtained from water leaching experiments of calcine from two large Hg mining districts in the U.S. This study is the first to report significant mass-dependent Hg isotopic fractionation between cinnabar ore and resultant calcine. Data indicate that delta202Hg values relative to NIST 3133 of calcine (up to 1.52 per thousand) in the Terlingua district, Texas, are as much as 3.24 per thousand heavier than cinnabar (-1.72 per thousand) prior to retorting. In addition, delta202Hg values obtained from leachates of Terlingua district calcines are isotopically similar to, or as much as 1.17 per thousand heavier than associated calcines, most likely due to leaching of soluble, byproduct Hg compounds formed during ore retorting that are a minor component in the calcines. As a result of the large fractionation found between cinnabar and calcine, and because calcine is the dominant source of Hg contamination from the mines studied, delta202Hg values of calcine may be more environmentally important in these mined areas than the primary cinnabar ore. Measurement of the Hg isotopic composition of calcine is necessary when using Hg isotopes for tracing Hg sources from areas mined for Hg, especially mine water runoff.

Enhanced solubilization of a metal-organic contaminant mixture (Pb, Sr, Zn, and perchloroethylene) by cyclodextrin.

Prior work has suggested that (carboxymethyl)-beta-cyclodextrin (CMCD) is capable of simultaneously enhancing the solubility of organics and metals, but sparse experimental data and no theoretical models have been published on this process. Preciously, a geochemical model for metal complexation by CMCD was formulated using PHREEQC on the basis of conditional stability constants measured in experiments using single-metal salts. In this study, the model is expanded to simultaneous metal and organic (perchloroethylene, PCE) complexation by CMCD. Experiments to verify the application of the formulation to mixed-waste systems were performed using solutions containing multiple metal ions (Pb, Sr, and Zn) and in a separate experiment introducing PCE with multiple metal ions. These experimental results show simultaneous solubility enhancement of metals and PCE. For solutions up to about 50 g/L CMCD, the model accurately predicted the simultaneous solubility enhancement for PCE, Pb, and Zn, while the difference between the measured and predicted Sr concentrations was accurate to within 15%. At CMCD concentrations greater than 50 g/L, the observed metal solubilities were greater than predicted (10% for Pb and Zn), probably due to the difficulty in accurately representing the activity and the effect on the ionic strength of functional groups on large organic molecules at higher concentrations.

Electron transfer capacities and reaction kinetics of peat dissolved organic matter.

Information about electron-transfer reactions of dissolved organic matter (DOM) is lacking. We determined electron acceptor and donor capacities (EAC and EDC) of a peat humic acid and an untreated peat DOM by electrochemical reduction and reduction with metallic Zn and H2S (EAC), and by oxidation with complexed ferric iron (EDC) at pH 6.5. DOC concentrations (10-100 mg L(-1)) and pH values (4.5-8) were varied in selected experiments. EAC reached up to 6.2 mequiv x (g C)(-1) and EDC reached up to 1.52 mequiv-(g C)(-1). EDC decreased with pH and conversion of chelated to colloidal iron, and the electron-transfer capacity (ETC) was controlled by the redox potential Eh of the reactant (ETC = 1.016x Eh - 0.138; R(2) = 0.87; p = 0.05). The kinetics could be adequately described by pseudo first-order rate laws, one or two DOM pools, and time constants ranging from 2.1 x 10(-3) d-1 to 1.9 x 10(-2) d(-1) for the fast pool. Reactions were completed after 24-160 h depending on the redox couple applied. The results indicate that DOM may act as a redox buffer over electrochemical potentials ranging from -0.9 to +1.0 V.

Application of saccharose as copper(II) ligand for electroless copper plating solutions.

Saccharose, forming sufficiently stable complexes with copper(II) ions in alkaline solutions, was found to be a suitable ligand for copper(II) chelating in alkaline (pH>12) electroless copper deposition solutions. Reduction of copper(II)-saccharose complexes by hydrated formaldehyde was investigated and the copper deposits formed were characterized. The thickness of the compact copper coatings obtained under optimal operating conditions in 1h reaches ca. 2 microm at ambient temperature. The plating solutions were stable and no signs of Cu(II) reduction in the bulk solution were observed. Results were compared with those systems operating with other copper(II) ligands.

Dermally adhered soil: 1. Amount and particle-size distribution.

The risk associated with the dermal absorption of chemicals from contaminated soil is, in part, a function of particle size distribution, as determined by either dry or wet sieving techniques. For the soils tested, the adhered soil fractions were shown to be independent of organic matter content and soil origin. Soil moisture content becomes a factor only for very moist soils. Results show that the adhered fractions of dry or moderately moist soils with wide distributions of particle sizes generally consist of particles of diameters <63 microm. Consequently, dermal absorption experiments using larger size fractions may be of limited relevance to actual situations of soil exposure.

Dermally adhered soil: 2. Reconstruction of dry-sieve particle-size distributions from wet-sieve data.

In the evaluation of soil particle-size effects on environmental processes, particle-size distributions are measured by either wet or dry sieving. Commonly, size distributions determined by wet and dry sieving differ because some particles disaggregate in water. Whereas the dry-sieve distributions are most relevant to the study of soil adherence to skin, soil can be recovered from skin only by washing with the potential for disaggregation whether or not it is subsequently wet or dry sieved. Thus, the possibility exists that wet-sieving measurements of the particle sizes that adhered to the skin could be skewed toward the smaller fractions. This paper provides a method by which dry-sieve particle-size distributions can be reconstructed from wet-sieve particle-size distributions for the same soil. The approach combines mass balances with a series of experiments in which wet sieving was applied to dry-sieve fractions from the original soil. Unless the soil moisture content is high (i.e., greater than or equal to the water content after equilibration with water-saturated air), only the soil particles of diameters less than about 63 microm adhere to the skin. Because of this, the adhering particle-size distribution calculated using the reconstruction method was not significantly different from the wet-sieving determinations.

Evidence for the aquatic binding of arsenate by natural organic matter-suspended Fe(III).

Dialysis experiments with arsenate and three different NOM samples amended with Fe(lll) showed evidence confirming the formation of aquatic arsenate-Fe(Ill)-NOM associations. A linear relationship was observed between the amount of complexed arsenate and the Fe(lll) content of the NOM. The dialysis results were consistent with complex formation through ferric iron cations acting as bridges between the negatively charged arsenate and NOM functional groups and/or a more colloidal association, in which the arsenate is bound by suspended Fe(lll)-NOM colloids. Sequential filtration experiments confirmed that a significant proportion of the iron present at all Fe/C ratios used in the dialysis experiments was colloidal in nature. These colloids may include larger NOM species that are coagulated by the presence of chelated Fe(lll) and/or NOM-stabilized ferric (oxy)hydroxide colloids, and thus, the solution-phase arsenate-Fe(Ill)-NOM associations are at least partially colloidal in nature.

Stability of low levels of perchlorate in drinking water and natural water samples.

Perchlorate ion (ClO4-) is an environmental contaminant of growing concern due to its potential human health effects, impact on aquatic and land animals, and widespread occurrence throughout the United States. The determination of perchlorate cannot normally be carried out in the field. As such, water samples for perchlorate analysis are often shipped to a central laboratory, where they may be stored for a significant period before analysis. The stability of perchlorate ion in various types of commonly encountered water samples has not been generally examined-the effect of such storage is thus not known. In the present study, the long-term stability of perchlorate ion in deionized water, tap water, ground water, and surface water was examined. Sample sets containing approximately 1000, 100, 1.0, and 0.5 microg l(-1) perchlorate ion in deionized water and also in local tap water were formulated. These samples were analyzed by ion chromatography for perchlorate ion concentration against freshly prepared standards every 24h for the first 7 days, biweekly for the next 4 weeks, and periodically after that for a total of 400 or 610 days for the two lowest concentrations and a total of 428 or 638 days for the high concentrations. Ground and surface water samples containing perchlorate were collected, held and analyzed for perchlorate concentration periodically over at least 360 days. All samples except for the surface water samples were found to be stable for the duration of the study, allowing for holding times of at least 300 days for ground water samples and at least 90 days for surface water samples.

Natural organic matter affects arsenic speciation and sorption onto hematite.

Arsenic mobility in natural environments is controlled primarily by sorption onto metal oxide surfaces, and the extent of this sorption may be influenced strongly by the presence of other dissolved substances that interact with surfaces or with arsenic itself. Natural organic matter (NOM), a prevalent constituent of natural waters, is highly reactive toward both metals and surfaces and is thus a clear candidate to influence arsenic mobility. The objectives of this study were therefore to reveal the influences of diverse NOM samples on the sorption of arsenic onto hematite, a model metal oxide, as well as to reveal influences of arsenic on the sorption of NOM, using conditions and concentrations relevant to natural freshwater environments. Of the six NOM samples tested, four formed aqueous complexes with arsenate and arsenite. The extent of complexation varied with the NOM origin and, in particular, increased with the cationic metal (primarily Fe) content of the NOM sample. In addition, every NOM sample showed active redox behavior toward arsenic species, indicating that NOM may greatly influence redox as well as complexation speciation of arsenic in freshwater environments. When NOM and As were incubated together with hematite, NOM dramatically delayed the attainment of sorption equilibrium and diminished the extent of sorption of both arsenate and arsenite. Consistent with this result, when NOM and As were introduced sequentially, all NOM samples displaced sorbed arsenate and arsenite from hematite surfaces, and arsenic species similarly displaced sorbed NOM from hematite in significant quantities. Competition between NOM and As for sorption thus appears to be a potentially important process in natural waters, suggesting that NOM may play a greater role in arsenic mobility than previously recognized. In addition, in all sorption experiments, arsenite was consistently desorbed or prevented from sorbing to a greater extent than arsenate, indicating that interactions with NOM may also partially explain the generally greater mobility of arsenite in natural environments.