Applied Geochemistry (v.15, #2)

Effect of fulvic acids on sorption of U(VI), Zn, Yb, I and Se(IV) onto oxides of aluminum, iron and silicon by Tao Zuyi; Chu Taiwei; Du Jinzhou; Dai XiongXin; Gu Yingjie (133-139).
The sorption of Yb3+, UO2+ 2, Zn2+, I and SeO2− 3 onto Al2O3, Fe2O3 and SiO2 were determined by a batch technique in the presence and absence of fulvic acids. The effects of fulvic acid on sorption were compared. The existing general consensus, that humic substances tend to enhance metal cation sorption at low pH, reduce metal cation sorption at high pH and reduce inorganic anion sorption between pH values 3 to 10, was generally shown to be true. However, in this work many exceptions to the general consensus were found. The study indicated that the effect of humic substances on sorption of inorganic cations or anions depends not only on pH, but also on the nature of the oxide, the nature of humic substance, fractionation of the humic substance by sorption, the relative strength of complexes of both soluble and sorbed humic substances, the extent of surface coverage by humic substance, the initial concentration of humic substance and the inorganic electrolyte composition.

A natural analogue of nuclear waste glass in compacted bentonite by Gento Kamei; Yasuhisa Yusa; Takashi Arai (141-155).
A marine based argillaceous rock containing volcanic glass shards has been investigated to infer the long-term durability of vitrified nuclear waste in compacted bentonite, which is a candidate for buffer material constituting the engineered barrier system for nuclear waste disposal. Fission track ages indicate that the volcanic glass shards, andesitic scoria, have been buried in the argillaceous rock for about 1 Ma. Neither glass matrix dissolution nor precipitation on the surface was seen under an optical microscope. Little leaching of any element has been recognized by analyses using an electron microprobe analyzer. Secondary ion mass spectrometry analysis, however, indicates significant hydration which may dominantly be a permeation of molecular water.As an indicator of durability of glass against groundwater a normalized mass loss of Si (NLSi) has been evaluated for the volcanic glass based on free energy for hydration. The difference between estimated NLSi of the volcanic glass and that of a simulated waste glass is within one order so that the volcanic glass may be analogous to a waste glass with respect to durability to water. The argillaceous rock is analogous to the compacted bentonite with respect to physical properties such as dry-density, unconfined compression strength, porosity, and hydraulic conductivity. The ambient physical and chemical conditions surrounding the volcanic glass have been also investigated: temperature was in the range from 4 to 30°C due to the burial history of the volcanic glass. Over most of the past 1 Ma the volcanic glass has been in contact with groundwater originating from seawater. Thermodynamic calculations indicate (1) pH (=7.74–7.94) of the groundwater has mainly been controlled by dissolution of carbonate minerals, (2) the redox potential (Eh=−34–−73 mV) of the groundwater has dominantly been controlled by decomposition of organic materials to produce CH4(g), and (3) activity of aqueous silica of the groundwater was in equilibrium with SiO2 amorphous. Because of the equilibrium between aqueous silica and SiO2 amorphous, the volcanic glass did not dissolve during the burial.Vitrified nuclear waste sealed in compacted bentonite, therefore, will not dissolve significantly if buried in an environment as mentioned above.

Stable carbon isotope biogeochemistry of a shallow sand aquifer contaminated with fuel hydrocarbons by Jiasong Fang; Michael J. Barcelona; R.V. Krishnamurthy; E.A. Atekwana (157-169).
Ground-water chemistry and the stable C isotope composition (δ13CDIC) of dissolved inorganic C (DIC) were measured in a sand aquifer contaminated with JP–4 fuel hydrocarbons. Results show that ground water in the upgradient zone was characterized by DIC content of 14–20 mg C/L and δ13CDIC values of −11.3‰ to −13.0‰. The contaminant source zone was characterized by an increase in DIC content (12.5 mg C/L to 54 mg C/L), Ca, and alkalinity, with a significant depletion of 13C in δ13CDIC (−11.9‰ to −19.2‰). The source zone of the contaminant plume was also characterized by elevated levels of aromatic hydrocarbons (0 μg/L to 1490 μg/L) and microbial metabolites (aromatic acids, 0 μg/L to 2277 μg/L), non-detectable dissolved O2, NO3 and SO4. Phospholipid ester-linked fatty acid analyses suggest the presence of viable SO4-reducing bacteria in ground water at the time of sampling. The ground-water chemistry and stable C isotope composition of ground-water DIC are interpreted using a chemical reaction model involving rainwater recharge, contributions of CO2 from soil gas and biodegradation of hydrocarbons, and carbonate dissolution. The major-ion chemistry and δ13CDIC were reconciled, and the model predictions were in good agreement with field measurements. It was concluded that stable C isotope measurements, combined with other biogeochemical measures can be a useful tool to monitor the dominant terminal electron-accepting processes in contaminated aquifers and to identify mineralogical, hydrological, and microbiological factors that affect δ13C of dissolved inorganic C.

Origin and mobility of humic colloids in the Gorleben aquifer system by G Buckau; R Artinger; P Fritz; S Geyer; J I. Kim; M Wolf (171-179).
The origin and mobility of humic colloids in the Gorleben aquifer system have been examined. For this purpose, the distribution of humic colloids and relevant hydrological and geochemical parameters were examined. An investigation area was selected where sediments have been disturbed by salt dome uplift and glacial events. It is shown that, on a local scale, considerable groundwater movement and intermixing takes place from the surface down to the salt dome. Consequently no effective separation of groundwater layers occurs. Two different humic colloid sources are identified: influx from the humus horizon with recharge water and continuous in situ generation via mineralization of sedimentary organic carbon (SOC). The in situ generation leads to groundwaters with humic colloid concentrations approaching 0.4 g/L, compared to concentrations of less than 0.005 g/L in recharge waters. Young groundwaters (no 14C decay detected) between approximately 50 and 200 m depth exhibit these highly elevated humic colloid concentrations. At greater depth, salt brines are found with low humic colloid concentrations. This can be attributed to precipitation of humic acid and/or hampering of the in situ generation process due to the high salt content. There is no indication of retention or decomposition of humic colloids. The fate of in situ generated humic colloids cannot be precisely evaluated due to analytical limitations and insufficient understanding of groundwater movement.

Human and environmental contamination in the Iron Quadrangle, Brazil by Jörg Matschullat; Ricardo Perobelli Borba; Eleonora Deschamps; Bernadino Ribeiro Figueiredo; Thomas Gabrio; Michael Schwenk (181-190).
Arsenic (As) exposure is a potential health risk to local populations around Au mining areas in southeastern Brazil. In April 1998, 126 schoolchildren, aged 9.8±1.12 years in the Minas Gerais mining districts of Nova Lima and Santa Barbara, had their spontaneous urine sampled. Toxicologically low Cd (0.04–0.35 μg L−1, mean 0.13 μg L−1), partly elevated Hg (0.1–16.5 μg L−1, mean 1.1 μg L−1), and generally elevated to high As concentrations (2.2–106 μg L−1, mean 25.7 μg L−1) were found. Twenty per cent of the total sample population showed elevated As concentrations where adverse health effects cannot be excluded on a long-term basis.To assess the potential sources particularly of As, a parallel study of surface waters, sediments, soils, and tailing materials was conducted. While Cd and Hg values were low in all these media, As concentrations in water (0.4–350 μg L−1; mean 30.5 μg L−1), in soils (200–860 mg kg−1), sediments (22–3200 mg kg−1, mean 350 mg kg−1), and tailings (300–21000 mg kg−1; mean 10500 mg kg−1) reveal high concentrations which may lead to an explanation for As pathways in the investigated areas.
Keywords: Human biomonitoring; Urine; Creatinine; Freshwater; Sediments; Tailings; Arsenic; Cadmium; Mercury; Gold-mining; arsenopyrite; Iron Quadrangle; Minas Gerais; Brazil;

The Saguenay Fjord sediments are characterised by high trace metal, organic matter and Fe monosulfide (AVS) concentrations. The presence of important maritime activities, dredging operations around harbour installations, and natural slumpling events contribute to the resuspension of the Fjord sediments, their exposure to an oxidising environment, and the remobilization of a variety of contaminants to the water column.In order to simulate these processes, sediments collected from various depths in a box core recovered on the landward slope of the fjord's interior basin were resuspended in aerated seawater in the laboratory at room temperature. The slurries were sampled at various time intervals over a period of nearly 2000 h and the dissolved Fe, Mn and As concentrations were determined. Results of this study indicate that the amount of Fe and As released to the solution is strongly correlated to the acid volatile sulfide (AVS) content of the resuspended sediments. The release of Mn to the water phase, however, appeared to be mainly controlled by the dissolution of a solid phase, other than the Fe monosulfides but soluble in 1 N HCl, to which Mn (II) is associated. This phase could be a Mn carbonate or a mixed Mn–Ca carbonate.The dissolved metal concentrations measured at any time during the resuspension experiments reflect the reactivity of the carrier phases and the competing kinetics of the release and removal mechanisms. Upon their release to solution, Fe, Mn and As were scavenged by the newly precipitated Fe and Mn oxyhydroxides. The differential rate of formation of these phases and removal of associated metals in solution reflects their respective oxidation kinetics. The residence time of As in solution was very similar to that of Mn, implying that it was more efficiently removed with the slowly precipitated Mn oxyhydroxides. This observation is consistent with the faster oxidation kinetics of As (III) to As (V) in the presence of Mn oxyhydroxides. Nevertheless, results of As speciation analyses also suggest that a portion of the solubilized As was adsorbed and possibly oxidised by the rapidly precipitated Fe oxyhydroxides.

Fluid chemistry and the hydraulic regime in a marl formation of the Swiss Alps were studied by a number of techniques. Fluid inclusions record the conditions of maximum burial and regional low-temperature metamorphism, whereas fluid samples and hydraulic tests derived from deep boreholes reflect present-day, near-surface conditions. The characterization of the different types of fluids places constraints on the geochemical and hydraulic evolution of low-permeability argillaceous rocks during uplift and exhumation.Fluid inclusions were studied by microthermometry and sampled directly by decrepitation techniques. They contain a two-phase system consisting of an aqueous fluid and a coexisting CH4-rich gas (T=190–250°C, P lith≈2500 bar). Bulk and isotopic compositions of aqueous fluid inclusions are consistent with a mixture of connate seawater and water derived from the dehydration of clay minerals. Methane was generated in situ by thermal cracking of kerogen. Textural evidence and stable isotopic signatures of carbonates in veins and in the rock matrix indicate local buffering of fluid compositions and very low water/rock ratios. Free fluids residing in the present-day fracture and matrix porosity consist of CH4-saturated Na–Cl groundwater with minute amounts of free CH4 gas which occurs in druses. Their chemical and isotopic compositions are very similar to those of the fluid inclusions, suggesting a common origin. Post-metamorphic admixtures of externally derived waters cannot be identified, and it is suggested that present-day Na–Cl groundwaters that occur in the central parts of the marl have resided in the formation since the time of metamorphism some 20 Ma b.p. The only major change in the fluid composition has been the outgassing of CH4 from the formation, most probably by diffusion.The hydraulic regime during metamorphism was characterized by localized fluid underpressures in open veins because widely scattered, sub-hydrostatic pressures were often identified in fluid inclusions. The central part of the argillaceous rock body, approximately coinciding with the region where Na–Cl groundwaters occur, has sub-hydrostatic pressures today, as indicated by hydraulic tests in deep boreholes.Both the closed-system behavior derived from the chemical and isotopic characteristics of the fluids and the (recurrent or continuous) existence of hydraulic underpressures suggest very low permeabilities of argillaceous rocks during metamorphism and throughout subsequent uplift and exhumation. All fluids present in the central parts of the formation are either connate or produced in situ. Even though major events of brittle faulting and unloading due to uplift occurred since the peak of metamorphism, fluid flow through the formation has been negligible.

Acid production from sulfide minerals using hydrogen peroxide weathering by Stuart R Jennings; Douglas J Dollhopf; William P Inskeep (235-243).
Sulfide mineral weathering is a major source of acid generation in mining environments. Oxidation and hydrolysis reactions in soil and geologic material under earth surface conditions causes weathering of reduced sulfide minerals resulting in liberation of weathering products including acid. Pyrite and marcasite are minerals common in mine environments that cause acid generation. Many other sulfide minerals are present in mining environments which may or may not form acid upon weathering. Characterization of complex mineral assemblages containing S compounds is therefore critically important to pre-mine planning and postmine waste characterization. Despite the importance of mineral weathering behavior, little is known about the acid generation characteristics of common sulfide and sulfate minerals. To assess the response of common sulfide and sulfate minerals to oxidizing conditions, 13 minerals were subjected to treatment with 10% H2O2. The resulting leachate was analyzed for pH, electrical conductivity, S and titratable acidity. The sulfide minerals arsenopyrite, pyrite, chalcopyrite, pyrrhotite, marcasite and sphalerite demonstrated significantly elevated levels of titratable acidity and are acid generating in contrast to galena, chalcocite and all the sulfates. The sulfate minerals barite, anhydrite, gypsum, anglesite and jarosite were included in experimentation and were found not to form acid under strongly oxidizing conditions. Remediation strategies for disturbed lands containing reduced S minerals must therefore consider not only the total quantity of sulfide minerals present, but the specific mineralogy of the S compounds.

Early weathering of palladium gold under lateritic conditions, Maquiné Mine, Minas Gerais, Brazil by C.A.C Varajão; F Colin; P Vieillard; A.J Melfi; D Nahon (245-263).
Since the 80's, studies have shown that Au is mobile in supergene lateritic surficial conditions. They are based either on petrological, thermodynamic studies, or experimental works. In contrast, few studies have been done on the mobility of the Pt group elements (PGE). Moreover, at the present time, no study has addressed the differential mobility of Au, Ag and Pd from natural alloys in the supergene environment. The aim of this study is to understand the supergene behavior, in lateritic conditions, of Au–Ag–Pd alloys of the Au ore locally called Jacutinga at the Maquiné Mine, Iron Quadrangle, Minas Gerais state, Brazil.The field work shows that the host rock is a “Lake Superior type” banded iron formation (BIF) and that the Au mineralization originates from sulfide-barren hydrothermal processes. Primary Ag–Pd-bearing Au has developed as xenomorphous particles between hematite and quartz grains. The petrological study indicates that the most weathered primary Au particles with rounded shapes and pitted surfaces were found, under the duricrust, within the upper friable saprolite. This layer, however is not the most weathered part of the lateritic mantle, but it is where the quartz dissolution resulting porosity is the most developed. The distribution of Au contents in the weathered rocks are controlled by the initial hydrothermal primary pattern. No physical dispersion has been found. Most of the particles are residual and very weakly weathered. This characterizes early stages of Au particle weathering in agreement with the relatively low weathering gradient of the host itabiritic formations that leads essentially to the development of isostructural saprolite lateritic mantle. Limited dissolution of primary Au particles issued from the friable saprolite induces Pd–Ag depleted rims compared to primary Au particle Pd–Ag contents.In addition, limited very short distance in situ dissolution/reprecipitation processes have been found at depth within the primary mineralization, as illustrated by tiny supergene, almost pure, Au particles. The supergene mobility order Pd>Ag>Au as reflecting early weathering stages of Au–Ag–Pd alloys under lateritic conditions is proposed.