Arthur W. Rose

Factors controlling the emanation of radon and thoron in soils of the eastern U.S.A.

Abstract Emanation coefficients (proportion of generated Rn atoms that escape from a solid material) have been measured for 222Rn ( t 1 2 3.8 days) and 220Rn (thoron, t 1 2 , 55 s) on 68 disaggregated soil samples from 12 soil profiles representative of the eastern U.S.A. Average values for sites are 0.20 for 222Rn and 0.16 for 220Rn. Based on distribution of 226Ra among the exchangeable, organic, Fe-oxide, sand, silt and clay fractions of the soils, a multiple regression indicates that the organic-exchangeable fraction, occurring mainly as coatings on grains, has an emanation coefficient for 222Rn of 0.46, and the silt-clay fraction has an emanation coefficient of 0.22. The latter value is verified by experiments. The organic component makes the largest single contribution to Rn in soil gas. Mineral grains have twice the 222Rn emanation as 220Rn, implying that about half the Rn atoms are emanated directly to pore space, and the remainder are freed by track-etching and diffusion over a period of days. Emanation for 220Rn appears to be dominated by Fe-oxides, the main host for pedogenic 232Th. Surface A- and E-horizons have low emanation coefficients for {Su220}Rn, probably because of a lack of Fe-oxides in these horizons.

Geology and Geochemistry of Triassic Diabase in Pennsylvania

Based on chemical composition and mineralogy, three types of Triassic diabase are recognized in Pennsylvania. The probable oldest type (Quarryville type) occurs as an olivine tholeiite dike swarm. The York Haven type is a quartz tholeiite, forming sheets, dikes, and a few flows. The youngest Rossville type is also quartz tholeiite that occurs as sheets and dikes. Within samples of the same type, chemical composition is very uniform. In content of major elements, rare earths, and Ba, the Rossville type resembles island-arc tholeiites. The York Haven type is similar to continental tholeiites. Based on calculated cooling rates and the homogeneity within each type of magma, plus paleomagnetic data, we conclude that each type was emplaced within a relatively short time period, and that all sheets, dikes, and flows of a single compositional type are essentially contemporaneous. The trend of Triassic diabase dikes in Pennsylvania parallels the trend of Precambrian and Paleozoic dikes, suggesting that trends of dikes may reflect pre-existing structural weaknesses in the basement rather than being an exact indicator of stress orientation during Triassic time. The two quartz tholeiites can be formed by crystallization of 30 to 45 percent of the olivine tholeiite magma as olivine, minor clinopyroxene, and plagioclase or spinel, accompanied by assimilation of orthopyroxene, probably from the mantle. Rare-earth and Sr-isotope data suggest that the York Haven type probably assimilated as much as 20 percent crustal material, whereas the Rossville type assimilated little or none. These phenomena of multiple-stage fractionation and reaction of the magma with mantle and crust probably apply to most magmas.

Adsorption of gold(III)-chloride and gold(I)-thiosulfate anions by goethite

Gold(III)-chloride and gold(I)-thiosulfate adsorption by goethite was investigated as a function of pH (4 to 8), Cl− concentration (0 to 0.1 M), and ionic strength (0.01 and 0.1 M). Several observations suggest that Au(III)-chloride hydrolysis species with <4 Cl ligands are preferentially adsorbed by goethite and that at low surface coverages these species are adsorbed in an inner-sphere, bidentate fashion. At pH 4.0 and in 0.01 M NaNO3, adsorption isotherms have a shape and maximum adsorption densities (210 μmol Au/g) similar to those previously observed for phosphate. In addition, excess Cl− appears in solution after adsorption, and an ionic strength increase to 0. l M NaNO3 has little effect on adsorbed amounts below 25% of maximum surface coverage. In 0.01 and 0. l M NaCl, however, adsorption increases as pH increases from 4 to 7, which is opposite to typical behavior for anion adsorption onto oxide surfaces. This “retrograde” adsorption trend is probably due to a shift in Au(III)-chloride species dominance from AuCl4− at pH 4 to preferentially adsorbed hydroxyl-substituted species such as AuCl(OH)3− at higher pH values. Maximum adsorption densities for Au(S2O3)23− at pH 4.0 in 0.01 M NaNO3 are only 35 μmol Au/g and decrease to 15 /μmol/g in 0.1 M NaNO3. Also, adsorption decreases as pH is increased from 4 to 8. This behavior suggests Au(I)-thiosulfate adsorption occurs primarily via a non-specific or outer-sphere mechanism. Moreover, the contrast in adsorption behavior of these two gold species demonstrates the importance of steric factors to adsorption processes. Gold(III)-chloride species are square planar, and the distance along an edge of this square (3.23 A) closely matches the distance between A-type hydroxyl groups on the goethite surface (3.04 A). Thus, these hydroxyl groups form an ideal “template” for bidentate coordination. Conversely, Au(S2O3)23− is a large, linear anion, and consequently it is difficult for this species to coordinate specifically with goethite surface hydroxyl groups.

Geochemistry of radium in soils of the Eastern United States

Abstract The abundance and chemical/mineralogical form of 226Ra, 238U and 232Th were determined on samples of soil and associated vegetation at 12 sites in the eastern United States. Progressive, selective chemical extraction plus size fractionation determined the abundance and radiometric equilibrium condition of these nuclides in 6 operationally defined soil fractions: exchangeable cations, organic matter, “free” Fe-oxides, sand, silt, and clay. In soils, profile-averaged 226Ra/238U activity ratios (AR) are within 10% of unity for most sites, implying little fractionation of U and Ra when the entire soil profile is considered. However, 226Ra greatly exceeds 238U activity in most surface soil (AR up to 1.8, av 1.22), in vegetation (AR up to 65, av. 2.8), in the exchangeable+organic fraction (AR up to 30, av. 13), in some soil Fe oxides (AR up to 3.5, av. 0.83) and in the C horizons of deeply weathered soils (AR up to 1.5). A major factor in Ra behavior is uptake by vegetation, which concentrates Ra>U and moves Ra from deeper soil to surface soil. Vegetation is capable of creating the observed Ra excess in typical surface soil horizons (AR up to 1.8, av. 1.22) in about 1000 a. Of the total Ra in an average A horizon, 42% occurs as exchangeable ions and in organic matter, but only 6–8% of the parent U and Th occur in these soil forms. In contrast, U is slightly enriched relative to Ra in Fe-oxides of A horizons, implying rapid chemical partition of vegetation-cycled U and Ra. In deeper horizons, transfer by vegetation and/or direct chemical partitioning of Ra into organic and exchangeable forms provides a source for unsupported 226Ra in Ra-rich organic matter, and leaves all soil minerals Ra-poor (AR=0.73). Organic matter evidently has a strong affinity for Ra. The phenomena discussed above are relevant to evaluation of indoor Rn hazard, and behavior of Ra at sites affected by radioactive waste disposal, phosphate tailings, Ra-rich brine, and uraniferous fertilizer.

Temporal variability of radon concentration in the interstitial gas of soils in Pennsylvania

In order to determine the extent and causes of radon variability in soil gas, repeated measurements of 222Rn, 220Rn, and soil properties including moisture, temperature, permeability, and diffusivity have been made at five sites in central Pennsylvania. Concentrations of 222Rn and 220Rn to a depth of 2 m varied temporally in an annual, approximately sinusoidal pattern having an amplitude of twofold to tenfold at all five sites. The existence of the annual pattern is independent of monitoring method, soil drainage, or bedrock parent material. Radon variability is caused by changes in soil moisture content and distribution; the action of soil water on Rn is classified into two distinct regimes: (1) At low and moderate moisture contents, diffusion occurs dominantly in air-filled pores, and Rn is distributed between air and water approximately at equilibrium. Under these conditions Rn in soil air at depth increases with increasing moisture. (2) At high moisture contents diffusion occurs largely in water-filled pores, and air/water equilibrium exists only near interfaces. Under these conditions, Rn in soil air can be low. Since Rn partitioning between gas and water is temperature sensitive and because soil moisture and temperature change in annual cycles, much of the variability in 222Rn concentration occurs in annual cycles. As a result, knowledge of regional and temporal soil moisture and temperature patterns allows estimates of 222Rn concentration in soil gas. In areas where the soil substrate has a significantly lower Ra concentration, emanation coefficient, or dry bulk density than the soil, the substrate can act as a Rn sink with respect to the soil. Under extreme conditions the [Rn] profile in the soil may actually display a concentration gradient toward the rock. The process of horizon formation during soil genesis significantly affects the vertical distribution of Ra, emanation coefficient and porosity, and therefore Rn production in soil.

Gaseous diffusion and permeability in four soil profiles in central Pennsylvania

Bulk diffusion and gaseous permeability coefficients were measured in situ in most morphologic horizons of four soil profiles in central Pennsylvania. Such data are rare in the literature. From the eluvial to the illuvial horizons of individual soil profiles, bulk diffusion coefficients generally decrease by nearly an order of magnitude, and gaseous permeability coefficients decrease by about two orders of magnitude. In all four profiles, the diffusion and permeability coefficients are higher in the upper, coarsely textured horizons than the lower horizons and, at corresponding depths, in more coarsely textured than finely textured pedons. The accuracy of the diffusion coefficients is confirmed by the similarity of an observed radon-222 (Rn) concentration profile to that estimated using the measured diffusion coefficients and a two-dimensional finite difference model. Several published methods of estimating bulk diffusion coefficient from air-filled porosity are statistically compared with the data following log transformation. Although all the methods tested were highly correlated to the in situ data, the estimates of Millington (1959, Science, 130:100–102) and Sallam et al. (1984, Soil Science Society of America Journal, 48:3–6) produced values most similar to those measured. Logarithmically transformed values of the bulk diffusion and permeability coefficients are highly correlated with each other for both our data and previously published data. This relationship holds true for eight different soils measured by separate researchers using different methodologies and appears to be generally applicable. Since the permeability values encompass a larger relative range than the diffusion values, measured gaseous permeability coefficients can be used to estimate bulk diffusion coefficients. This empirical approach to estimating diffusion coefficients is useful in that, in addition to air-filled porosity, permeability reflects the continuity and tortuosity of the air-filled pore system. Since the continuity and tortuosity of air-filled pores affect the diffusion coefficient, and neither of these properties are directly reflected in estimates based on air-filled porosity, estimation of diffusion coefficient using permeability accounts for important soil properties not directly accounted for with methods based on air-filled porosity alone.

Interactions of gold(III) chloride and elemental gold with peat-derived humic substances

Abstract Peat-derived humic and fulvic acids, isolated using widely accepted techniques, were investigated for their ability to reduce gold (III) chloride and dissolve elemental gold. Au(III)-chloride (1–20 mg 1 −1 ) was reduced to elemental colloidal Au by both humic and fulvic acids (1–20 mg 1 −1 ) under all conditions studied. The growth rate and mean size of the colloidal Au formed was monitored using UV-visible spectroscopy. Larger colloids were formed in the presence of humic (∼60-nm mean diameter) than fulvic acid (∼20-nm mean diameter). Otherwise, colloidal growth rates were similar for humic and fulvic acids. Colloidal Au formation was completed in ∼8 days at pH 4 and slowed to > 14 days at pH > 7. Cupric ion (2 · 10 −5 -2 · 10 −3 M) accelerated colloidal Au development from 2 to 20 times while equal concentrations of Ca 2+ had no effect. About 10 mg 1 −1 humic and fulvic acids were necessary to completely reduce 10 mg 1 −1 Au (III)-chloride which is equivalent to a reduction capacity of ∼15 meq e − /g humic substance. IR spectroscopy and acid titration results suggest that humic substance oxidation included the addition of phenolic, alcohol and ketone groups which helps to account for the appreciable reduction capacity observed. Ten mg 1 −1 solutions of these same humic substances did not dissolve elemental Au to levels > 1 μ g 1 −1 over periods up to 150 days. Consequently, it is concluded that humic and fulvic acids function primarily as reductants of oxidized Au species rather than as dissolution and complexation agents for elemental Au. This implies that other components of natural organic matter are primarily responsible for the dissolution and complexation of elemental Au in surficial environments.

The geochemistry of cadmium in some sedimentary rocks

Abstract The cadmium contents of 11 shales, 11 sandstones, 7 limestones, 14 metamorphic rocks and 28 stream sediments, all from Pennsylvania, were determined by flameless atomic absorption spectrometry. On the basis of these cadmium values and those in the literature, cadmium is found to be enriched in dark shales and soils, depleted in red shales, sandstones and limestones, and about the same in stream sediments as in most igneous and metamorphic rocks and the crust. In stream sediments, cadmium correlates most significantly with zinc, followed by carbon, weight loss on ignition, cobalt, readily extractable lead and manganese. Examination of the correlation plots and of an Eh-pH diagram indicates that the primary cause of cadmium enrichment in the sedimentary environment is the adsorption and/or complexation of cadmium with organic matter followed by the accumulation of organic debris in a reducing depositional environment.

Gold distribution and mobility in the surficial environment, Carajas region, Brazil

Abstract Increasing evidence indicates gold is mobile in the surficial environment. In arid regions this is attributable to complexing by chloride and near oxidizing sulfides to thiosulfate or related ligands. In this exploratory study, the extent and mechanism of gold mobility in a deeply weathered tropical rainforest environment has been investigated by analysis of waters, vegetation and stream sediment at the Salobo Cu-Au and Bahia Au-Cu deposits in the Carajas Mineral Province, Para Province, Brazil. Waters from drill holes, an adit and a stream draining the Salobo deposit contain 11 to 73 ng/L dissolved Au, compared to background levels of 2 to 3 ng/L. Elevated concentrations of dissolved Au are tentatively attributed to complexing by thiosulfate generated by accelerated weathering of sulfides beneath the steep slopes at the deposit. At the Bahia deposit, which occurs beneath an ancient deeply weathered plateau surface, the highest level of dissolved Au in small streams draining the deposit is 3 ng/L. Vegetation (multiple species) over ore at both deposits contains elevated concentrations of gold (65–400 ng/g of ash). The data suggest that vegetation may be a useful medium for gold exploration, but that in view of the results at Bahia, waters are of questionable value in deeply weathered low-relief tropical rainforest areas. The mobilization of gold by vegetation on ancient surfaces over 106–107 year time periods appears adequate to explain near-surface enrichment of Au and its lateral dispersion, thereby contributing to lateritic gold deposits.

The geochemistry of mercury in sedimentary rocks and soils in Pennsylvania

Abstract The mercury contents of 11 sandstone, 11 shale and 6 limestone samples from Pennsylvania average 7, 23 and 9 ppb Hg, respectively, which is lower than the values for sedimentary rocks reported in the literature. The differences may arise because many of the reported high values are from regions characterized by more mineralization and volcanism than is present in central Pennsylvania. The lowest values found for shale and sandstone in Pennsylvania (0.4 and 0.5 ppb Hg, respectively) are lower by an order of magnitude than the lowest previously reported values. The mercury content of sedimentary rocks varies markedly due to the effects of volcanism, organic material and sulfur in reducing environments, iron and manganese oxides in oxidizing environments, diagenesis, hydrothermal processes, and the thermal history of the rock. Soils in Pennsylvania have much greater amounts of mercury than their parent rocks even after taking into account possible residual concentration, suggesting that mercury is added to the soils from an outside source. Rain is the major source of mercury absorbed by the soil. A portion of the absorbed mercury returns to the atmosphere, establishing a rain-soil-atmosphere mercury cycle. The general enrichment of mercury in soils and sediments compared to rocks is supported by the observation that the mercury content of rain is greater than freshwater. The much higher values of mercury in unconsolidated sediments compared to sedimentary rocks suggest that mercury is lost during diagenesis. Mans contribution of mercury to the surface environment is nearly equal to the natural contribution. Industrial loss contributes more than 65 per cent of mans total, and the contribution of fossil fuel consumption is small, although it may be locally important. The implications of mercury loss and absorption by soils may be an important factor in concentrating mercury in crops and other living matter, especially near industrialized areas.