Applied Geochemistry (v.18, #10)

Effect of groundwater fulvic acid on the adsorption of arsenate by ferrihydrite and gibbsite by Michael A. Simeoni; Barry D. Batts; Christopher McRae (1507-1515).
Adsorption studies have been conducted at pH 4, 6 and 8 to assess the effect of fulvic acid on arsenate adsorption to ferrihydrite and gibbsite. The studies compared the adsorption of arsenate on the mineral surfaces in the absence of fulvic acid, to those cases where increasing concentrations of fulvic acid (0.3–60 μM) were added to the mineral–arsenate suspensions. Experiments where arsenate was added to mineral–fulvate suspensions were also conducted. The results suggest that arsenate adsorption on both gibbsite and ferrihydrite decreases with increasing concentrations of fulvic acid. This effect was highest at pH 4, and decreased at pH 6 and 8. Ferrous ion concentrations were very low during the ferrihydrite experiments and support the view that fulvic acid can both displace arsenate from and inhibited its adsorption to mineral surfaces. The experiments also indicated that the amount of arsenate adsorbed was lower if fulvic acid was added before rather than after arsenate. This may reflect the relative size of arsenate and fulvic acid molecules and their ability to penetrate the crystal matrices of the minerals.

A method for indirect determination of the composition of an argilleous formation interstitial water is described. It is based on the hypothesis that cation distribution on clay minerals is a fingerprint of the solution which the mineral assemblage is in equilibrium with. Precise characterisation of the retention process of all the major cations on the clay and accurate knowledge of the cation inventories of the rock is primarily required. It was shown that the studied clay acts as a multi-site ion exchanger. Three types of sites have been characterised with respect to cation exchange capacity and selectivity coefficients for each major cation. Extraction experiments have provided estimates of the total stock of Cl (free ion), Na and K of the rock but oxidation of the core sample made it impossible to get those of SO4 2−, Ca and Mg. The calculated interstitial water composition, as deduced from these experiments and the proposed model, is a slightly alkaline (pH≈8.3) Na+–HCO3 type of water with high ionic strength (I≈0.4 M).

A dissolution test with 9 natural and synthetic schwertmannite and ferrihydrite samples was performed by reaction with 0.2 M ammonium oxalate at pH 3.0 in the dark. The method was coupled with differential X-ray diffraction (DXRD) to successfully detect schwertmannite at low concentrations in oxidized mine tailings. Rapid dissolution was observed for all schwertmannites (> 94% in 60 min) and natural 2-line ferrihydrites (> 85% in 60 min); however, synthetic 2-line and 6-line ferrihydrite dissolved slower (42 and 16% after 60 min, respectively). The results showed that it was not possible to distinguish between natural schwertmannites and ferrihydrites on the basis of their dissolution kinetics. Modeling of the schwertmannite dissolution curves, examinations of mineral shape by scanning electron microscopy, and Fe/S mole ratios of the dissolved fractions indicated that two different schwertmannite particle morphologies (spherical and web-like) occurred. Collapse of spherical (sea-urchin) schwertmannite aggregates seemed to control the dissolution kinetics according to a shrinking core model. In the case of web-like schwertmannite, dissolution could be modeled with a simple first order equation, and structural SO4 2− may have affected the dissolution kinetics. No relationship was found between ferrihydrite particle shape and dissolution behavior in acid NH4-oxalate. A 1-h extraction with 0.2 M NH4-oxalate at pH 3.0 in the dark should be adequate to dissolve schwertmannite and natural 2-line ferrihydrite in most samples. In some cases, a fraction of secondary jarosite or goethite may also be dissolved, although at a slower rate. If only schwertmannite is of interest (e.g., determination by DXRD), a 15 min attack should be used to increase selectivity. A truly selective leach of schwertmannite and ferrihydrite should be based on dissolution tests, as a broad variety of dissolution kinetics can be observed in this mineral group.

Biogeochemistry of manganese oxide coatings on pebble surfaces in the Kikukawa River System, Shizuoka, Japan by Yukinori Tani; Naoyuki Miyata; Keisuke Iwahori; Mitsuyuki Soma; Shin-ichi Tokuda; Haruhiko Seyama; Benny K.G Theng (1541-1554).
The biogeochemistry of Mn-oxide coatings formed over submerged pebble surfaces on the streambed of the Kikukawa river system has been investigated. Located in central Shizuoka, Japan, this system drains strongly acidified soils under tea plantations. Besides containing high amounts of Mn (up to 450 μg/cm2), the coatings are capable of scavenging and accumulating other elements including Ba, Zn, Ni, Co, W, Mo and Sb. When suspensions of the coating material were incubated with 0.2 mM Mn2+, the Mn(II) ion was microbially transformed into Mn-oxides. When the same suspensions were spread on agar plates containing acetate, yeast-extract, and 1.0 mM Mn2+ (AY agar medium) both Mn-oxidizing bacteria and fungi appeared, indicating the existence of a diversity of Mn-oxidizing microorganisms in the system. Plate counts using two agar culture media with varied nutrient levels indicated that the ability of these microorganisms to oxidize Mn(II) was strongly dependent on nutrient supply. The relatively nutrient-poor AY agar medium was more conducive to microbial growth than the K1 agar medium with a higher organic nutrient content. Concentrations of Mn dissolved in the stream waters did not correlate well with the amounts of solid Mn on submerged pebbles. Thus, factors other than dissolved Mn concentration (e.g., organic nutrient supply and pH) determined the ability of microorganisms to oxidize Mn in the streambeds. A survey of dissolved Mn in streams and water draining tea plantations combined with chemical analysis of Mn in the underlying soils indicate that the soils have been strongly acidified through excessive applications of N-fertilizers. As a result, Mn was leached from the soil column into the Kikukawa river system. Biogenic Mn-oxide coatings on streambeds can therefore serve as an indicator of soil acidification and metal leaching from soils of the corresponding watershed.

In the context of the proposed low- and intermediate-level radioactive waste repository at Wellenberg (Switzerland), calculations simulating the interaction between hyperalkaline solutions and a fractured marl, at 25 °C, have been performed. The aim of these calculations is to evaluate the possible effects of mineral dissolution and precipitation on porosity and permeability changes in such a fractured marl, and their impact on repository performance. Solute transport and chemical reaction are considered in both a high-permeability zone (fracture), where advection is important, and the wall rock, where diffusion is the dominant transport mechanism. The mineral reactions are promoted by the interaction between hyperalkaline solutions derived from the degradation of cement (a major component of the engineered barrier system in the repository) and the host rock. Both diffusive/dispersive and advective solute transport are taken into account in the calculations. Mineral reactions are described by kinetic rate laws. The fluid flow system under consideration is a two-dimensional porous medium (marl, 1% porosity), with a high-permeability zone simulating a fracture (10% porosity) crossing the domain. The dimensions of the domain are 6 m per 1 m, and the fracture width is 10 cm. The fluid flow field is updated during the course of the simulations. Permeabilities are updated according to Kozeny's equation. The composition of the solutions entering the domain is derived from modeling studies of the degradation of cement under the conditions at the proposed underground repository at Wellenberg. Two different cases have been considered in the calculations. These 2 cases are representative of 2 different stages in the process of degradation of cement (pH 13.5 and pH 12.5). In both cases, the flow velocity in the fracture diminishes with time, due to a decrease in porosity. This decrease in porosity is caused by the precipitation of calcite (replacement of dolomite by calcite) and other secondary minerals (brucite, sepiolite, analcime, natrolite, tobermorite). However, the decrease in porosity and flow velocity is much more pronounced in the lower pH case. The extent of the zone of mineral alteration along the fracture is also much more limited in the lower pH case. The reduction of porosity in the fractures would be highly beneficial for repository performance, since it would mean that the solutions coming from the repository and potentially carrying radionuclides in solution would have to flow through low-conductive rock before they would be able to get to higher-conductive features. The biggest uncertainty in the reaction rates used in the calculations arises from the surface areas of the primary minerals. Additional calculations making use of smaller surface areas have also been performed. The results show that the smaller surface areas (and therefore smaller reaction rates) result in a smaller reactivity of the system and smaller porosity changes.

Humic acids coagulation: influence of divalent cations by Nathalie A. Wall; Gregory R. Choppin (1573-1582).
The effects of the ionic strength (maintained by LiCl, NaCl or KCl) and Ca2+ and Mg2+ concentration on the coagulation of purified humic acids (HA) was studied. Solutions of known ionic strengths, pcH, Ca2+ and Mg2+ concentrations were prepared with HA and filtered to obtain the fraction with a size smaller than 100 kD. After a 50 day storage, samples of these solutions were filtered again (100 kD) and the total organic C (TOC) of the filtered solutions measured. The HA coagulation increased with salt concentration, with the cationic charge, and for cations of the same charge, with the cationic charge density. The coagulation decreased for pcH values of 4 to 7–8 in the absence of and presence of Mg2+ and Ca2+. In the absence of the divalent cations, the coagulation has a constant value for pcH>8, but, in the presence of Mg2+ and Ca2+, increases at pcH values greater than 9. The coagulation of humic materials occurs whether the samples are exposed to light or kept in the dark, although the coagulation kinetics are slower for the samples kept in the dark. The size distribution of size-fractionated humic solutions changes over time to a size distribution similar to that of the original humic solution before it was size-fractionated. The results are explained by the DLVO theory.

The solid state partitioning of contaminant metals and As in river channel sediments of the mining affected Tisa drainage basin, northwestern Romania and eastern Hungary by Graham Bird; Paul A. Brewer; Mark G. Macklin; Dan Balteanu; Basarab Driga; Mihaela Serban; Sorin Zaharia (1583-1595).
The solid state partitioning of metals and As in river channel sediments has been investigated along an 828 km study reach comprising the rivers Lapuş, Someş and Tisa, using a 4-stage sequential extraction procedure (SEP) that identified 4 chemical phases: (1) exchangeable, (2) Fe and Mn oxides, (3) organic matter/sulphides and (4) residual. Peak total concentrations of As (240 mg kg−1), Cd (95 mg kg−1), Cu (24,500 mg kg−1) and Pb (21,100 mg kg−1) were found in the River Lapuş immediately downstream of Băiuţ base-metal mine, with Zn concentrations (23,000 mg kg−1) peaking 2 km further downstream. Contaminant metals and As within samples close to Băiuţ mine were largely bound to sulphides (>92%). Ten km downstream from Băiuţ mine, however, Cd and Zn were found to be environmentally significant due to their strong association with the exchangeable phase (73 and 47%, respectively). Lead was found to be predominantly associated with Fe and Mn oxides (71%), but concentrations in the residual phase were relatively high in the upper Tisa. Copper was more evenly distributed between the 4 phases than the other 4 elements analysed. Elevated metal and As concentrations were found in the rivers Lapuş and Someş downstream of polluted tributaries, but sites on the River Tisa had total metal concentrations below Dutch guideline values. The implications of the river sediment metal partitioning results on the environment are considered in light of Cd and Zn ‘exchangeable hotspots’ at sites on the Lapuş and Someş Rivers.

Laboratory investigation of silica removal from geothermal brines to control silica scaling and produce usable silicates by Darrell L. Gallup; Frankie Sugiaman; Vilma Capuno; Alain Manceau (1597-1612).
Laboratory and field jar test experiments show that desilication of geothermal brines may be achieved by treating the brine with various metal cations at elevated pH to precipitate metal silicates. Below ∼90 °C and at elevated pH, metal treatment of synthetic and field brines precipitated only amorphous or poorly crystalline metal-rich silicates of little commercial value. Treatment of brine with alkaline-earth metals below ∼90 °C, except Mg, tends to co-precipitate metal carbonates. Laboratory reactions conducted at ∼130 °C demonstrated that certain metal ions may react with silica in brine to precipitate crystalline compounds of commercial value. For example, kerolite clay was precipitated upon treating synthetic and field brines with Mg at 130 °C, while under similar conditions, sodalite and Zeolite P were precipitated upon treatment with Al. Treatment of brines with transition and heavy metals only precipitated amorphous silicates at ∼130 °C, except in the case of addition of a combination of Mg and Fe, which precipitated a smectite clay. Desilication of brine and precipitation of minerals may add value to geothermal power projects by simultaneously controlling scale deposition and producing salable products. By decreasing dissolved silica concentrations in brines, undersaturation of amorphous silica is achieved. Controlled precipitation of silica from brine in crystallizers, tanks or ponds can mitigate silica scaling potential in brine-handling equipment.

Rare earth elements and Y (REY) have been analysed in 49 groundwaters from localities in the areas of Lake Kinneret and the Jordan and Arava Valleys. These waters originate from various aquifers and the REY abundances are expected to be controlled by the aquifer rocks. The REY pattern allow one to distinguish interaction of waters with basalts, basalt-limestone interaction zones (silicified limestones), limestones from the Judea and Avdat Groups (Upper Cretaceous and Eocene, respectively), and sandstones of the Lower Cretaceous Kurnub Group. Groundwater from the Quaternary alluvial fill (Dead Sea Group) are either controlled by Judea Group limestone or Kurnub Group sandstone. The REY patterns show characteristic features for each group. In hydrogeological systems, the rocks of natural replenishment areas are usually not the same as rock units from which the waters are collected. This becomes evident by comparing the lithostratigraphic groups from which the waters were collected and the hydrochemical grouping according to REY patterns with their characteristic trends and anomalies. In many cases, there is a correspondence between the lithostratigraphic and the hydrochemical grouping; in other cases, the 2 groupings disagree. This disagreement proves inter-aquifer flow of groundwater. In some cases, the geologically derived aquifers rocks of origin, differ from those indicated by REY patterns. Thus, applying the REY grouping, new fundamental information for hydrological models can be given and sources of salinisation can be elucidated.

Soil organic C (SOC) concentrations in topsoil samples taken at 678 sites in the grassland of Ireland were investigated using statistics and geostatistics. SOC concentrations (Walkley–Black method) follow a lognormal distribution, with a median and geometric mean of 5.0%, and an arithmetic mean of 5.3%. Statistically significant (P<0.01) positive correlation between SOC and silt-plus-clay, and negative correlation between SOC and sand were observed, with lower correlation (P=0.17) between SOC and pH. Lower SOC concentrations were associated with higher percentages of land in tillage. In order to obtain a robust measurement of spatial structure, spatial outliers were detected, and subsequently eliminated, using the local Moran's I index. The spatial distribution of SOC concentrations based on kriging interpolation showed coherent spatial patterns, with the highest values in the western coastal area, and relatively low values in the inland and southeastern coastal areas; soils at higher elevation were also found to contain higher SOC concentrations. These patterns are consistent with the distribution of rainfall within the country.

Compound-specific H isotope analysis has been used to monitor bioremediation of petroleum hydrocarbons. However, the success of this approach requires a full evaluation of the isotopic effects resulting from evaporation, because light petroleum hydrocarbons undergo both biodegradation and evaporation under natural conditions. The authors determined the H isotope fractionation of common volatile petroleum hydrocarbons, including the C10–C14 n-alkanes, MTBE (tert-butyl methyl ether), and BTEX (benzene, toluene, ethylbenzene, p-xylene and o-xylene) during progressive vaporization under simulated experimental conditions. A decrease in δD values for n-alkanes of up to 33.3‰ and up to 44.5‰ for BTEX compounds when 99% of these substances had evaporated was observed. The results also show that H isotope fractionation increases with n-alkane chain length. Such fractionation patterns are interpreted in terms of competition between the decreased intermolecular binding energy in D-enriched species, and the isotope effect due to the mass difference. In contrast to hydrocarbons, methanol and ethanol show H isotopic enrichment during vaporization, indicating that H-bonding, when present in organic molecules, plays a controlling role on the vapor pressure of different isotope species.

The through- and out-diffusion of HTO, 36Cl and 125I in Opalinus Clay, an argillaceous rock from the northern part of Switzerland, was studied under different confining pressures between 4 and 15 MPa. The direction of diffusion and the confining pressure were perpendicular to the bedding. Confining pressure had only a small effect on diffusion. An increase in pressure from 4 to 15 MPa resulted in a decrease of the effective diffusion coefficient of ∼20%. Diffusion accessible porosities were not measurably affected. The values of the effective diffusion coefficients, D e, ranged between (5.6±0.4)×10−12 and (6.7±0.4)×10−12 m2 s−1 for HTO, (7.1±0.5)×10−13 and (9.1±0.6)×10−13 m2 s−1 for 36Cl and (4.5±0.3)×10−13 and (6.6±0.4)×10−13 m2 s−1 for 125I. The rock capacity factors, α, measured were circa 0.14 for HTO, 0.040 for 36Cl and 0.080 for 125I. Because of anion exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than the uncharged HTO. Unlike 36Cl, 125I sorbs weakly on Opalinus Clay resulting in a larger rock capacity factor. The sorption coefficient, K d, for 125I is of the order of 1–2×10−5 m3 kg−1. The effective diffusion coefficient for HTO is in good agreement with values measured in other sedimentary rocks and can be related to the porosity using Archie's Law with exponent m=2.5.