Contrasting the geochemistry of oxic sediments across ecosystems: a synthesis

The geochemistry of oxic sediments was contrasted across a range of Canadian aquatic ecosystems; 7 freshwater lakes, (3 circumneutral and 4 acidic), 15 peatlands (5 mineral-rich, 5 moderately-poor and 5 mineral-poor), 9 wetlands (3 oligosaline, 3 mesosaline and 3 euryhaline), an estuary (deposited and suspended sediments) and an intertidal region. Sediments were characterized by a simultaneous extraction that separated sediments into easily reducible (ER) metal (oxyhydroxides of Mn and easily reducible amorphous oxyhydroxides of Fe) and reducible (R) metal (more crystalline forms of oxyhydroxides of Fe), organic matter, and, the concentrations and partitioning of Zn, Cu and Cd associated with these 3 sediment components. Ecosystems were distinct with respect to ER Fe and organic matter [canonical variate analysis (CVA)] with 53% of among system variation in geochemistry attributed to these 2 components. Sediments of peatlands and wetlands contained greater amounts of organic matter whereas sediments of lakes, intertidal and estuarine deposited and suspended sediments were characterized by greater amounts of ER Fe. A further 21% of among system variation could be ascribed to organically bound Fe that was greater in acidic lakes and mineral-rich peatlands as compared to other systems. Concentration and partitioning of Cd within sediments was regionally dependent with 41% of among system variation (CVA) attributed to differences in ER Cd and R Cd. Cadmium from peatlands and lakes located in Ontario was recovered from all 3 sediment components whereas sediment from wetlands, the estuary and the intertidal regions of British Columbia (BC) contained no organically bound Cd with amounts recovered occurring mostly as ER Cd. Lakes and peatlands located in Ontario contained 3–5-fold total Cd as compared to ecosystems located in BC. A further 21% of among ecosystem variation was attributed to Zn partitioning. Zinc in peatland and wetland sediments occurred as R Zn as compared to lake and estuarine deposited sediments where Zn was recovered both as ER and R Zn. Total Zn was also 3–5-fold greater in sediments of systems in Ontario as compared to those sampled in BC. Concentration and partitioning of Cu was similar across all systems with Cu recovered from the organic component of sediment. The geochemistry of surficial oxic sediments with respect to ER Mn, R Fe and organic matter and the geochemical associations among these sediment components is ecosystem and region dependent. For assessing impacts of metals on sediment dwelling biota the geochemical characteristics of the system under study should first be defined.     

Metal speciation and attenuation in stream waters and sediments contaminated by landfill leachate

The degree of metal contamination (Zn, Pb, Cu, Ni, Cd) has been investigated in the vicinity of an old unmonitored municipal landfill in Prague, Czech Republic, where the leachate is directly drained into a surface stream. The water chemistry was coupled with investigation of the stream sediment (aqua regia extract, sequential extraction, voltammetry of microparticles) and newly formed products (SEM/EDS, XRD). The MINTEQA2 speciation-solubility calculation showed that the metals (Zn, Pb, Cu, Ni) are mainly present as carbonate complexes in leachate-polluted surface waters. These waters were oversaturated with respect to Fe(III) oxyhydroxides, calcite (CaCO₃) and other carbonate phases. Three metal attenuation mechanisms were identified in leachate-polluted surface waters: (i) spontaneous precipitation of metal-bearing calcite exhibiting significant concentrations of trace elements (Fe, Mn, Mg, Sr, Ba, Pb, Zn, Ni); (ii) binding to Fe(III) oxyhydroxides (mainly goethite, FeOOH) (Pb, Zn, Cu, Ni); and (iii) preferential bonding to sediment organic matter (Cu). These processes act as the key scavenging mechanisms and significantly decrease the metal concentrations in leachate-polluted water within 200 m from the direct leachate outflow into the stream. Under the near-neutral conditions governing the sediment/water interface in the landfill environment, metals are strongly bound in the stream sediment and remain relatively immobile.     

Mineralogy and geochemistry of alluvium contaminated by metal mining in the Rio Tinto area, southwest Spain

The Rio Tinto in SW Spain drains Cu and pyrite mines which have been in operation since at least the Bronze Age. Extensive metal mining, especially from 1873 to 1954, has resulted in contamination of the Rio Tinto alluvium with As, Cu, Pb, Ag and Zn. X-ray diffraction (XRD), wavelength-dispersive X-ray mapping, scanning electron microscope petrography and X-ray energy-dispersive (EDX) analysis has revealed that 4 major groups of contaminant metal and As-bearing minerals, including sulphides, Fe-As oxides, Fe oxides/hydroxides/oxyhydroxides, and Fe oxyhydroxysulphates, occur in the alluvium. Sulphide minerals, including pyrite, chalcopyrite, arsenopyrite and sphalerite, occur in alluvium near the mining areas. Iron hydroxides and oxyhydroxides such as goethite and possibly ferrihydrite occur in cements in both the mining areas and alluvium downstream, and carry minor amounts of As, Cu and Zn. Iron oxyhydroxysulphates, including jarosite, plumbojarosite and possibly schwertmannite, are the most common minerals in alluvium downstream of the mining areas, and are major hosts of Cu, Pb, Zn and of As, next to the Fe-As minerals. This work, and other field observations, suggest that (1) the extreme acidity and elevated metal concentrations of the river water will probably be maintained for some time due to oxidation of pyrite and other sulphides in the alluvium and mine-waste tips, and from formation of secondary oxide and oxyhydroxysulphates; (2) soluble Fe oxyhydroxysulphates such as copiapite, which form on the alluvium, are a temporary store of contaminant metals, but are dissolved during periods of high rainfall or flooding, releasing contaminants to the aqueous system; (3) relatively insoluble Fe oxyhydroxysulphates and hydroxides such as jarosite and goethite may be the major long-term store of alluvial contaminant metals; and (4) raising river pH will probably cause precipitation of Fe oxyhydroxides and oxides/hydroxides/oxyhydroxides and thus have a positive effect on water quality, but this action may destabilise some of these contaminant metal-bearing minerals, releasing metals back to the aqueous system.     

Distribution and partitioning of major and trace elements in pyrite-bearing sediments of a Mediterranean coastal lagoon

The formation of iron sulphide minerals exerts significant control on the behaviour of trace elements in sediments. In this study, three short sediment cores, retrieved from the remote Antinioti lagoon (N. Kerkyra Island, NW Greece), are investigated concerning the solid phase composition, distribution, and partitioning of major (Al, Fe) and trace elements (Cd, Cu, Mn, Pb, and Zn). According to ²¹⁰Pb, the sediments sampled correspond to depositions of the last 120 years. The high amounts of organic carbon (4.1–27.5%) result in the formation of Fe sulphides, predominantly pyrite, already at the surface sediment layers. Pyrite morphologies include monocrystals, polyframboids, and complex FeS–FeS₂ aggregates. According to synchrotron-generated micro X-ray fluorescence and X-ray absorption near-edge structure spectra, authigenically formed, Mn-containing, Fe(III) oxyhydroxides (goethite type) co-exist with pyrite in the sediments studied. Microscopic techniques evidence the formation of galena, sphalerite and CuS, whereas sequential extractions show that carbonates are important hosts for Mn, Cd, and Zn. However, significant percentages of non-lattice held elements are bound to Fe/Mn oxyhydroxides that resist reductive dissolution (on average 60% of Pb, 46% of Cd, 43% of Zn and 9% of Cu). The partitioning pattern changes drastically in the deeper part of the core that is influenced by freshwater inputs. In these sediments, the post-depositional pyritization mechanism, illustrated by overgrowths of Fe monosulphides on pre-existing pyrite grains, results in relatively high degree of pyritization that reaches 49% for Cd, 66% for Cu, 32% for Zn and 7% for Pb.     

The distribution of elements between co-existing phases in some marine ferromanganese-oxide deposits

Analyses of ferromanganese oxides from the Indian and Atlantic Oceans for the elements Mn, Fe, Co, Ni, Cu, Zn, Pb, Ca, AI, Ti, Cr and Cd have helped to elucidate some of the controls on their geochemistry. In most samples virtually all of the Mn and much of the Fe are present as acid-reducible phyllomanganates and Fe oxyhydroxides respectively. By contrast, in samples in which goethite was identified, much of the Fe and significant amounts of the Mn. are not acid-reducible. The partition patterns of the minor elements reflect to varying extents the mineralogy of the hydrous Mn and Fe oxide phases. In δ-Mn-O₂-rich samples the ratio of adsorbed to crystallographically-bound Ni. Cu and Zn, is higher than in todorokite-rich samples, but in each case these metals are virtually entirely phyllomanganate-associated. In goethite-rich samples, however, significant amounts of Ni. Cu and Zn may be associated with the goethite itself rather than with phyllomanganate minerals. Cobalt shows very close association with the phyllomanganates irrespective of the specific mineralogy, but Pb behaves in a way which cannot yet be fully characterised. The non-reducible fractions of the samples contain most of the Ca, Al. Ti and Cr. Some Ca however is also present in the phyllomanganates.     

Mine drainage from the weathering of sulfide minerals and magnetite

Pyrite and pyrrhotite are the principal minerals that generate acid drainage in mine wastes. Low-pH conditions derived from Fe-sulfide oxidation result in the mobilization of contaminant metals (such as Zn, Cd, Ni and Cr) and metalloids (such as As) which are of environmental concern. This paper uses data from detailed mineralogical and geochemical studies conducted at two Canadian tailings impoundments to examine the mineralogical changes that pyrite, pyrrhotite, sphalerite and magnetite undergo during and after sulfide oxidation, and the subsequent release and attenuation of associated trace elements. The stability of sphalerite in tailings impoundments generally is greater than that of pyrrhotite, but less than pyrite. Dissolved Ni and Co derived from Fe sulfides, and to a lesser extent, dissolved Zn and Cd from sphalerite, are commonly attenuated by early-formed Fe oxyhydroxides. As oxidation progresses, a recycling occurs due to continued leaching from low-pH pore waters and because the crystallinity of Fe oxyhydroxides gradually increases which decreases their sorptive capacity. Unlike many other elements, such as Cu, Pb and Cr, which form secondary minerals or remain incorporated into mature Fe oxyhydroxides, Zn and Ni become mobile. Magnetite, which is a potential source of Cr, is relatively stable except under extremely low-pH conditions. A conceptual model for the sequence of events that typically occurs in an oxidizing tailings impoundment is developed outlining the progressive oxidation of a unit of mine waste containing a mixed assemblage of pyrrhotite and pyrite.     

Partitioning of copper and zinc in the sediments and porewaters of a high-elevation alkaline lake,east-central Arizona,U.S.A.

In Pacheta Lake, a high-elevation alkaline lake proximal to the smelting region of southern Arizona-New Mexico, concentrations of transition metal ions in pore waters and co-existing sediments were compared. Copper, Fe, Mn and Zn have been partitioned among operationally defined sediment solid phases (exchangeable sites, organic complexes, amorphous oxides, crystalline oxides, sulfides and residual silicates) and their concentrations in interstitial waters were measured. Concentrations are reported as a function of depth in the sediment column. The diagenetic environment is described and cycling mechanisms postulated for the above metals.Selective, sequential extraction of metals from lake sediments showed different binding mechanisms for Cu and Zn, the former most strongly associated with organic complexes, and the latter with iron oxyhydroxides. This difference has strong implications for selective metal remobilization under variable environmental conditions, both naturally and anthropogenically induced. Copper and zinc in porewaters were estimated to diffuse to overlying waters at 12.8 and 21.9 μg/cm²/a, respectively. These fluxes are large enough to account for observed concentrations of Cu and Zn in overlying waters. No sediment metal contamination was directly attributable to smelting activity. However, this study does document a flux from sediments, which have accumulated Cu and Zn, to overlying waters no longer receiving trace metal deposition from now inactive smelters.     

Laboratory study of calcite–gypsum sludge–water interactions in a flooded tailings impoundment at the Kristineberg Zn–Cu mine,northern Sweden

Due to liming of acid mine drainage, a calcite–gypsum sludge with high concentrations of Zn (24,400 ± 6900 μg g⁻¹), Cu (2840 ± 680 μg g⁻¹) and Cd (59 ± 20 μg g⁻¹) has formed in a flooded tailings impoundment at the Kristineberg mine site. The potential metal release from the sludge during resuspension events and in a long-term perspective was investigated by performing a shake flask test and sequential extraction of the sludge. The sequentially extracted carbonate and oxide fractions together contained ⩾97% of the total amount of Cd, Co, Cu, Ni, Pb and Zn in the sludge. The association of these metals with carbonates and oxides appears to result from sorption and/or coprecipitation reactions at the surfaces of calcite and Fe, Al and Mn oxyhydroxides forming in the impoundment. If stream water is diverted into the flooded impoundment, dissolution of calcite, gypsum and presumably also Al oxyhydroxides can be expected during resuspension events. In the shake flask test (performed at a pH of 7–9), remobilisation of Zn, Cu, Cd and Co from the sludge resulted in dissolved concentrations of these metals that were significantly lower than those predicted to result from dissolution of the carbonate fraction of the sludge. This may suggest that cationic Zn, Cu, Cd and Co remobilised from dissolving calcite, gypsum and Al oxyhydroxides were readsorbed onto Fe oxyhydroxides remaining stable under oxic conditions. In a long-term perspective (≳10² a), ⩾97% of the Cd, Co, Cu, Ni, Pb and Zn content of the sludge potentially is available for release by dissolution of calcite and reductive dissolution of Fe oxyhydroxides if the sludge is subject to a soil environment with lower dissolved Ca concentrations, pH and redox than in the impoundment.     

Remobilization of transition metals in surficial pelagic sediments from the eastern Pacific

The top thirty centimeters of sediment at two sites in the eastern equatorial Pacific contain evidence of post-depositional remobilization of Mn, Fe, Co, Ni, Cu, and Zn. Remobilization takes place as Mn and Fe oxyhydroxides are released to the pore water during the microbially-mediated decomposition of organic matter. Precipitation of the dissolved metals in near-surface more oxic strata controls the solid-phase distribution of Mn, Fe, and Zn. The solid-phase redistribution of Co and Ni requires only suitable material for readsorption. Comparison of pore water fluxes with solid-phase metal distributions in the solid sediment indicates no loss of dissolved metal to the overlying water column at the present time. Loss of Mn during the Quaternary is indicated by the composition of the sediments, however. Leaching experiments suggest that portions of the mobile Mn, Fe, Co, Ni, and Cu are fixed by incorporation in authigenic smectite in the surficial sediments.     

Trace metals in oxic lake sediments: possible adsorption onto iron oxyhydroxides

Apparent overall equilibrium constants for the adsorption of Cd, Cu, Ni, Pb and Zn onto natural iron oxyhydroxides have been calculated from the partitioning of these trace metals in oxic lake sediments and the in situ measurement of trace metal concentrations in the associated pore waters. Such values obtained from lakes of various pH located on the Precambrian Shield, in the area of Sudbury, Ontario, are compared with equilibrium constants obtained for the adsorption of the trace metals onto iron oxyhydroxides in well-defined media.The field data are consistent with laboratory experiments reported in the literature and with theory. Both the influence of pH upon adsorption and the binding strength sequence observed for the field data agree with theory. At high sediment pH values, the partitioning of Cd, Ni and Zn between the pore waters and the natural iron oxyhydroxides is similar to those reported in the literature for the adsorption of these metals at low surface coverage onto amorphous iron oxyhydroxides in a NaNO₃ medium; deviation from this simple model is however observed for Cu and Pb, presumably due to the competitive action of dissolved ligands. At low sediment pH values, the adsorption is much higher than predicted by the simple model and can be explained by the formation of ternary complexes with the iron oxyhydroxide surface.     

Relation between colloid composition and the environment of their formation: application to the El Berrocal site (Spain)

In the framework of nuclear waste topics, the role of colloids is more and more emphasized in relation to the transport of radioelements in natural groundwaters. We attempt here to check if the nature and physico-chemical characteristics of the colloids from the granitic site of El Berrocal are closely related to the bedrock composition.The colloids are studied in three types of groundwaters (sulfate and carbonate waters in different redox conditions) as well as colloidal suspensions extracted from clayey materials present in the fractures. The particle size distributions are performed by photon correlation spectroscopy. Their chemical composition is obtained by X-ray microanalysis after transversal ultrafiltration.The main results deal with the chemical composition of colloids. They are mainly composed of silica, aluminosilicate phases, and Fe, Ti, Al oxyhydroxides. In the case of the carbonate and oxidized hydrofacies, traces of Mg, Cu, Zn, Ba, S as well as carbonate phases (Zn, Ca) occur. In the particular case of the sulfate hydrofacies, traces of S, Mg, Cu are identified. Finally in the less oxidized and carbonate groundwater, a lot of phases consist of sulphur with Cu, Zn, Fe as well as of carbonate with Cu, Zn, Fe, Ca and Mg. For the clayey suspensions, colloids are mainly composed of illite and silica.In conclusion, the contribution of the bedrock as well as the influence of the physico-chemical conditions are discussed in order to decipher the role of neoformation and dispersion processes in the colloid formation.     

Trace metal occurrence in a mineralised and a non-mineralised spodosol in northern Sweden

In order to determine the metal-bearing phases with special emphasis on Cu, a sequential extraction has been carried out on seven soil samples from a sulphide-bearing spodosol profile in Liikavaara Östra, close to the Aitik Copper Mine in northern Sweden. A reference spodosol profile with very low abundances of sulphides located far from anthropogenic emissions was also studied. Five fractions were selected for the extraction: (I) CH₃COONa-extractable (exchangeable/adsorbed/carbonate); (II) Na₄P₂O₇-extractable (labile organics); (III) 0.25 M NH₂OH·HCl-extractable (amorphous Fe oxyhydroxides/Mn oxides); (IV) 1 M NH₂OH·HCl-extractable (crystalline Fe oxides); and (V) KClO₃/HCl-extractable (organics and sulphides). The distribution of trace elements (Co, Cr, Cu, Ni, Pb and Zn) in the profile in Liikavaara Östra is different from that in the reference profile. Possible explanations for these differences are (i) the presence of sulphides in the soil, (ii) atmospheric deposition of dust derived from mining activities at the Aitik Copper Mine, and (iii) mineralogical heterogeneities inherited from the deposition of the till. There is no straightforward correlation between the amount of the extracted phases and the metal extractability in the soils. This fact indicates that other factors are important for the retention of trace metals as well. The data presented in this study suggest that Co, Cr and Ni, to a fairly large extent, are associated with the organic matter in the B-horizon in both profiles, while in the C-horizon in Liikavaara Östra, sulphides are probably the more important carriers of these elements. For Co and Ni, Fe oxyhydroxides seem to be important. Most of the Cr occurred in the residual remaining after the leaching procedure. Copper and Zn seem to be associated with the organic matter to some extent in the B-horizon. The concentration of Cu in the C-horizon in Liikavaara Östra is high (2310 ppm), but only a very small fraction is likely to be hosted by sulphides. It is concluded that the major part of Cu in the C-horizon and a prominent fraction in the B-horizon in Liikavaara Östra are associated with some secondary phase that is extractable during extractions III and IV. Possible candidates for this phase are goethite and inclusions of native Cu in weathered biotite.     

Seasonal variations in the composition of mine drainage-contaminated groundwater in Dalarna, Sweden

Groundwater down-gradient from a mine rock dump in Dalarna, Sweden was sampled from the onset of snowmelt runoff (April) until October in order to investigate seasonal variations in groundwater composition. The results demonstrate that considerable variation in solute concentration (Al, Cu, Fe, SO₄²⁻, Zn) and acidity occurs in groundwater; the greatest change in solute concentrations occurs during the melting of the snow cover, when sulfide oxidation products are flushed from the rock dump. During this period, groundwater flow is concentrated near the soil surface with an estimated velocity of 1 m/day. Groundwater acidity varied by a factor of four closest to the rock dump during the sampling period, but these variations were attenuated with distance from the rock dump. Over a distance of 145 m, groundwater pH increases from 2.5 to 4.0 and acidity decreases from 3–13 to 0.8–1.1 meq/L, which is the combined effect of ferric iron precipitation and aluminosilicate weathering. As a result of flushing from the upper soil horizons, peaks in total organic carbon and ammonium concentrations in groundwater are observed at the end of snowmelt. In soils impacted by acidic surface runoff, the sequential extraction of C horizon soils indicates the accumulation of Cu in well-crystallized iron oxyhydroxides in the upper C horizon, while Cu, Fe, Ni and Zn accumulate in a well-crystallized iron oxyhydroxide hardpan that has formed 2.5m below the ground surface. Surface complexation modeling demonstrates that SO₄²⁻ and Cu adsorb to the abundant iron oxyhydroxides at pH < 4, while Zn adsorption in this pH range is minimal.     

The species of Silene compacta Fischer as indicator of zinc,iron and copper mineralization

The possibility of using the species Silene compacta Fischer (Caryophyllaceae) as a plant indicator for Zn, Fe and Cu is discussed in this paper. Rock and plant samples were collected from an area near Essimi, north-eastern Greece, which is known for its Cu–Pb–Zn mineralization and analyzed for their content in 14 elements (Fe, Mn, Ag, As, Cd, Co, Cu, Mo, Ni, Pb, Sb, Sn, Zn and V). The data illustrate positive linear correlation between the concentrations of Zn, Fe and Cu in plant ash and rock samples. In particular, these correlations are 0.99 for Zn, 0.97 for Fe and 0.97 for Cu and are significant at the 99.9% confidence level. Therefore, S. compacta could be used as a plant indicator for Zn, Fe and Cu mineralization.     

Geochemistry and specific features of manganese ore formation in sediments of oceanic bioproductive zones

Processes of authigenic manganese ore formation in sediments of the northern equatorial Pacific are considered on the basis of study of the surface layer (<2 mm) of ferromanganese nodule and four micronodule size fractions from the associated surface sediment (0–7 cm). Inhomogeneity of the nodule composition is shown. The Mn/Fe ratio is maximal in samples taken from the lateral sectors of nodule at the water-sediment interface. Compositional differences of nodules are related to the preferential accumulation of microelements in iron oxyhydroxides (P, Sr, Pb, U, Bi, Th, Y, and REE), manganese hydroxides (Co, Ni, Cu, Zn, Cd, Mo, Tl, W), and lithogenous component trapped during nodule growth (Ga, Rb, Ba, and Cs). The Ce accumulation in the REE composition is maximal in the upper and lower parts of the nodule characterized by the minimal Mn/Fe values. The compositional comparison of manganese micronodules and surface layers of the nodule demonstrated that the micronodule material was subjected to a more intense reworking during the diagenesis of sediments. The micronodules are characterized by higher Mn/Fe and P/Fe ratios but lower Ni/Cu and Co/Ni ratios. The micronodules and nodules do not differ in terms of contents of Ce and Th that are least mobile elements during the diagenesis of elements. Differences in the chemical composition of micronodules and nodules are related not only to the additional input of Mn in the process of diagenesis, but also to the transformation of iron oxyhydroxides after the removal of Mn from the close association with Fe formed in the suspended matter at the stage of sedimentation.     

Comparison of Cu,Zn and Fe bioleaching from Cu-metallurgical slags in the presence of Pseudomonas fluorescens and Acidithiobacillus thiooxidans

Metal leaching from metallurgical wastes (slags) by means of environmentally friendly approaches is promising for practical applications. The goal of this study was to compare the feasibility of metal bioleaching from Cu slags by means of Pseudomonas fluorescens and Acidithiobacillus thiooxidans. Two size particles (<0.3 mm and 1–2 mm) of two types of Cu slags (massive crystalline slag and granulated amorphous slag) were used to study metal (Cu, Zn and Fe) bioleaching. The 40-days bioleaching experiments with P. fluorescens began at circumneutral pH (7.0), whereas the experiments with A. thiooxidans were started under acidic (pH 2.5) conditions. The results demonstrated that A. thiooxidans catalyzes metal leaching from both slag types investigated. After 21 days of incubation, optimal leaching was achieved and up to 79% Cu, 76% Zn and 45% Fe could be extracted from crystalline slag under conditions of 1 wt.% pulp density and particle size <0.3 mm. The optimal efficiency achieved with amorphous slag was 81% Cu, 79% Zn and 22% Fe when 1% pulp density and 1–2 mm particle size were used. The use of P. fluorescens resulted in poor leaching efficiencies as compared to the performance of A. thiooxidans, presumably due to the higher pH conditions maintained during the P. fluorescens incubations. The maximum metal leaching efficiencies with P. fluorescens were achieved at 1% pulp density and particle size <0.3 mm and did not exceed 10% Cu, 4% Zn, 0.3% Fe for crystalline slag and 4% Cu, 3% Zn, 0.7% Fe for amorphous slag. Both slags exhibited a good potential for bioleaching with A. thiooxidans, however; further optimization of the process parameters (e.g. pulp density, particle size and pH) is needed to improve the efficiency.     

Fractionation of Cu and Zn isotopes during adsorption onto amorphous Fe(III) oxyhydroxide: Experimental mixing of acid rock drainage and ambient river water

Fractionation of Cu and Zn isotopes during adsorption onto amorphous ferric oxyhydroxide is examined in experimental mixtures of metal-rich acid rock drainage and relatively pure river water and during batch adsorption experiments using synthetic ferrihydrite. A diverse set of Cu- and Zn-bearing solutions was examined, including natural waters, complex synthetic acid rock drainage, and simple NaNO₃ electrolyte. Metal adsorption data are combined with isotopic measurements of dissolved Cu (⁶⁵Cu/⁶³Cu) and Zn (⁶⁶Zn/⁶⁴Zn) in each of the experiments. Fractionation of Cu and Zn isotopes occurs during adsorption of the metal onto amorphous ferric oxyhydroxide. The adsorption data are modeled successfully using the diffuse double layer model in PHREEQC. The isotopic data are best described by a closed system, equilibrium exchange model. The fractionation factors (α_soln-solid) are 0.99927 ± 0.00008 for Cu and 0.99948 ± 0.00004 for Zn or, alternately, the separation factors (Δ_soln-solid) are −0.73 ± 0.08‰ for Cu and −0.52 ± 0.04‰ for Zn. These factors indicate that the heavier isotope preferentially adsorbs onto the oxyhydroxide surface, which is consistent with shorter metal-oxygen bonds and lower coordination number for the metal at the surface relative to the aqueous ion. Fractionation of Cu isotopes also is greater than that for Zn isotopes. Limited isotopic data for adsorption of Cu, Fe(II), and Zn onto amorphous ferric oxyhydroxide suggest that isotopic fractionation is related to the intrinsic equilibrium constants that define aqueous metal interactions with oxyhydroxide surface sites. Greater isotopic fractionation occurs with stronger metal binding by the oxyhydroxide with Cu > Zn > Fe(II).     

Bacterially-mediated authigenesis of clays in phosphate stromatolites

Authigenic clays in close textural relation to carbonate fluorapatite within finely laminated phosphate stromatolites of Upper Jurassic age have been studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and analytical electron microscopy (AEM). Stromatolite laminae consist of hexagonal prisms of francolite (sizes ranging between 0·1 and 1 μm) that are surrounded by poorly crystalline smectite and amorphous Fe–Si–Al oxyhydroxides. Microanalyses show that smectite is Fe rich, with highly variable composition, particularly regarding Fe and Si contents. Smectite has significant beidellitic, montmorillonitic and non-tronitic substitutions. Although the lack of fringe contrast in some areas adjacent to the smectite packets with 1·0–1·3 nm spacing is due to differences in orientation of layers, textural and analytical data clearly indicate the presence of Fe–Si–Al amorphous phases intimately intergrown with smectite. The occurrence of poorly crystalline smectite and associated amorphous phases within microbially precipitated stromatolite laminae, both as envelopes around, and as pore-fillings between extremely small calcium phosphate crystals, demonstrates authigenic smectite growth from a precursor Fe–Si–Al amorphous material. This material is formed in close association with a phosphate-rich precursor. The textural and structural relations, the preservation of chemical precursors of glauconite such as nontronitic montmorillonite, and the presence of Fe–Si–Al amorphous mineral phases, imply crystallization of the observed crystalline phases from synsedimentary (bacterially precipitated) amorphous precursors during early diagenesis in postoxic environments. Carbonate fluorapatite was the first phase to crystallize from the primary gel; smectite and associated amorphous Fe–Si–Al oxyhydroxides were the residual material of the crystallization process. The slow rate of transformation (at low temperatures) from Fe–Si–Al-rich gels to smectite, explains the textural relations between the poorly crystalline phases and the phosphate crystals, as well as the preservation of amorphous substances in relation to clays. Authigenic smectite represents the first step in glauconitization.     

Seasonal variations of Zn, Cu, As and Mo in arsenic-rich stream at the Mokrsko gold deposit, Czech Republic

Monthly sampling of slightly alkaline arsenic-rich stream in the Mokrsko gold deposit revealed seasonal variations in dissolved Zn, Cu, As and Mo. Concentrations of trace metal cations (Zn, Cu) increased as much as 330 and 178%, respectively, from minimum mean values at autumn to maximum mean values at spring. In contrast, concentrations of trace element oxyanions (As, Mo) revealed opposite seasonal pattern with increase to 189% (As) and 123% (Mo) during summer–autumn, indicating that in-stream biogeochemical process(es) played the main role in controlling the seasonal variations of these trace elements. The trace elements were mainly scavenged by low crystalline Mn oxyhydroxide and Fe oxyhydroxide (ferrihydrite). Results are consistent with sorption and coprecipitation processes controlling seasonal variations of dissolved Zn and Cu, while As and Mo dynamics appear linked to Mn redox reactions. The sorption processes and Mn redox processes are attributed to the changes of pH and oxic/anoxic conditions on the surface of oxyhydroxides, respectively, which are themselves controlled by the balance between photosynthesis and respiration. Under the geochemical conditions of the stream, inferred Mn redox reactions can only be explained by microbial activity.     

Effect of adsorbed metal ions on the transport of Zn- and Ni-EDTA complexes in a sand and gravel aquifer

Adsorption, complexation, and dissolution reactions strongly influenced the transport of metal ions complexed with ethylenediaminetetraacetic acid (EDTA) in a predominantly quartz-sand aquifer during two tracer tests conducted under mildly reducing conditions at pH 5.8 to 6.1. In tracer test M89, EDTA complexes of zinc (Zn) and nickel (Ni), along with excess free EDTA, were injected such that the lower portion of the tracer cloud traveled through a region with adsorbed manganese (Mn) and the upper portion of the tracer cloud traveled through a region with adsorbed Zn. In tracer test S89, Ni- and Zn-EDTA complexes, along with excess EDTA complexed with calcium (Ca), were injected into a region with adsorbed Mn. The only discernable chemical reaction between Ni-EDTA and the sediments was a small degree of reversible adsorption leading to minor retardation. In the absence of adsorbed Zn, the injected Zn was displaced from EDTA complexes by iron(III) [Fe(III)] dissolved from the sediments. Displacement of Zn by Fe(III) on EDTA became increasingly thermodynamically favorable with decreasing total EDTA concentration. The reaction was slow compared to the time-scale of transport. Free EDTA rapidly dissolved aluminum (Al) from the sediments, which was subsequently displaced slowly by Fe. In the portion of tracer cloud M89 that traveled through the region contaminated with adsorbed Zn, little displacement of Zn complexed with EDTA was observed, and Al was rapidly displaced from EDTA by Zn desorbed from the sediments, in agreement with equilibrium calculations. In tracer test S89, desorption of Mn dominated over the more thermodynamically favorable dissolution of Al oxyhydroxides. Comparison with results from M89 suggests that dissolution of Al oxyhydroxides in coatings on these sediment grains by Ca-EDTA was rate-limited whereas that by free EDTA reached equilibrium on the time-scale of transport. Rates of desorption are much faster than rates of dissolution of Fe oxyhydroxides from sediment-grain surfaces and, therefore, adsorbed metal ions can strongly influence the speciation of ligands like EDTA in soils and sediments, especially over small temporal and spatial scales.