André Tessier

Metal sorption to diagenetic iron and manganese oxyhydroxides and associated organic matter: Narrowing the gap between field and laboratory measurements

Diagenetic Fe and Mn oxyhydroxides were isolated in situ by vertically inserting inert collectors into the sediments of two geochemically different lakes located near Sudbury, Ontario. X-ray diffraction and electron microscopic analyses indicated that the Fe-rich material collected was predominantly ferrihydrite and poorly crystallized lepidocrocite, while the Mn-rich material was a mixture of poorly crystallized Mn oxyhydroxides. Conditional adsorption constants (KF-M and KMn-M) were calculated using the concentrations of metals (Ca, Cd, Cu, Mg, Ni, Pb, Zn) associated with the Fe- and Mn-rich material and the measured dissolved concentrations of these metals. Comparison of these in situ derived KFe-M and KMn-M values were made with: (1) the hydrolysis constants of the metals; (2) laboratory-derived intrinsic surface complexation constants obtained for adsorption of these metals on well-characterized Fe and Mn oxyhydroxides, and (3) predicted KFe-M and KMn-M values determined using the surface complexation model under the geochemical conditions observed in the lakes. Complexation of these metals with adsorbed natural organic matter was also compared to metal complexation with dissolved natural organic matter. The results are consistent with the scenario that trace metals bind directly to the OH groups of the Fe and Mn oxyhydroxides in circumneutral McFarlane Lake and to the functional groups of organic matter adsorbed on Fe oxyhydroxides in the more acidic (pH = 4.8) Clearwater Lake. Alkaline earth metals Ca and Mg bind, presumably as outer-sphere complexes, to the organic coatings. Our results provide support for the argument that laboratory-derived adsorption datasets may be useful for predicting metal adsorption in the field.

Potential artifacts in the determination of metal partitioning in sediments by a sequential extraction procedure

The partitioning of trace metals in sediments, as obtained with a sequential extraction procedure, may be affected by (i) the techniques used to preserve the sediments before analysis and (ii) the presence/absence of atmospheric oxygen during the extraction steps. No storage method tested completely preserved the initial chemical and physical characteristics of the sediments. Drying of the sediment (freeze-drying; oven-drying) should be avoided; acceptable preservation techniques include freezing or short-term wet storage (1-2/sup 0/C). Among the different metals (Cd, Co, Cr, Cu, Ni, Pb, Zn, Fe, and Mn), copper, iron, and zinc were particularly sensitive to sample pretreatment. For anoxic sediments, the maintenance of oxygen-free conditions during the extractions is of critical importance.

Interactions between arsenic and iron oxyhydroxides in lacustrine sediments

Arsenic and iron concentrations were measured in surficial sediments and in interstitial and overlying waters at 22 littoral stations of 16 lakes. The lakes were chosen to cover values of pH between 4.0 and 8.4 and various As concentrations. Depth-distributions of dissolved As and Fe concentrations suggest a close association of both elements in a dissolution-diifusion-precipitation cycle for Fe and adsorbed As. Using a simplified version of the surface complexation model, apparent adsorption constants of As onto natural Fe oxyhydroxides have been calculated from the concentrations of As and Fe determined in leachates of surficial lake sediments and the in situ measurement of dissolved As in their respective overlying waters. These calculations assume, based on thermodynamic considerations and experimental evidence, that only As(V) is associated with the natural Fe oxyhydroxides. The binding intensity values obtained from these lakes are compared to those obtained for the adsorption of As(V) onto various synthetic Fe oxyhydroxides in well-defined media. The binding constants derived from field measurements agree well with those obtained from laboratory experiments performed with amorphous Fe oxyhydroxides.

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

Abstract 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.

Geochemistry of trace metals associated with reduced sulfur in freshwater sediments

Porewater concentration profiles were determined for Fe, trace elements (As, Cd, Co, Cu, Mn, Ni, Pb, Zn), sulfide, SO4 and pH in two Canadian Shield lakes (Chevreuil and Clearwater). Profiles of pyrite, sedimentary trace elements associated with pyrite and AVS were also obtained at the same sites. Thermodynamic calculations are used, for the anoxic porewaters where sulfide was measured, to characterize diagenetic processes involving sulfide and trace elements and to illustrate the importance of sulfide, and possibly polysulfides and thiols, in binding trace elements. The ion activity products (IAP) of Fe sulfide agree with the solubility products (Ks) of greigite or mackinawite. For Co, Ni and Zn, IAP values are close to the KS values of their sulfide precipitates; for Cu and Pb, IAP/Ks indicate large oversaturations, which can be explained by the presence of other ligands (not measured) such as polysulfides (Cu) and thiols (Pb). Cobalt, Cu, Ni and Zn porewater profiles generally display a decrease in concentration with increasing ΣH2S, as expected for transition metals, whereas Cd, Pb and Zn show an increase (mobilisation). The results suggest that removal of trace elements from anoxic porewaters occurs by coprecipitation (As and Mn) with FeS(s) and/or adsorption (As and Mn) on FeS(s), and by formation of discrete solid sulfides (Cd, Cu, Ni, Pb, Zn and Co). Reactive Fe is extensively sulfidized (51–65%) in both lakes, mostly as pyrite, but also as AVS. Similarities between As, Co, Cu and Ni to Fe ratios in pyrite and their corresponding mean diffusive flux ratios suggest that pyrite is an important sink at depth for these trace elements. High molar ratios of trace elements to Fe in pyrite from Clearwater Lake correspond chronologically to the onset of smelting activities. AVS can be an important reservoir of reactive As, Cd and Ni and, to a lesser extent, of Co, Cu and Pb. Overall, the trace elements most extensively sulfidized were Ni, Cd and As (maximum of 100%, 81% and 49% of the reactive fraction, respectively), whereas Co, Cu, Mn, Pb and Zn were only moderately sulfidized (11–16%).

Characteristics of lacustrine diagenetic iron oxyhydroxides

Abstract Diagenetic iron oxyhydroxides formed by the oxidation of iron (II) were allowed to deposit onto inert collectors vertically inserted into the sediments of eight lakes chosen to represent a range of lake pH values. Particles comprising the iron-rich deposits have been characterized at macroscopic (composition; X-ray diffraction) and microscopic (transmission electron microscopy; energy dispersive spectroscopy; electron diffraction) levels. The only crystalline forms of Fe(III) oxyhydroxides identified were poorly ordered ferrihydrite and lepidocrocite. Morphologies of lepidocrocite (lath-type) and presumably of ferrihydrite (spherical and ellipsoidal particles) are similar to those reported previously as being formed in the water column of seasonally anoxic lakes. Deposition of iron oxyhydroxides in the lake sediments also appears to occur on bacterial cells and on their exopolymers. Silicon, sulfate, chloride, phosphate, manganese, calcium, and aluminum represented minor components of the diagenetic Fe(III) oxyhydroxides, whereas organic carbon was a more abundant component. Calculations with a diffuse layer version of the surface complexation model using the ambient dissolved concentrations, and comparison of the predicted and measured compositions of the iron-rich particles suggest that sulfate, phosphate, and probably calcium were sorbed on the iron oxyhydroxides, whereas silicon was probably more firmly bound at the surface.

Sediment porewater sampling for metal analysis: A comparison of techniques

Abstract The concentrations of Ca, Mg, Fe, Mn, Cr, Co, Ni, Cu, Zn, Cd and organic carbon were measured in a sediment porewater sampled by two different techniques: in situ dialysis, and centrifugation followed by filtration. Sediment centrifugation at 5,000 rpm followed by filtration (0.45 μm membrane) was equivalent to dialysis for Co, Ni, Cr, Fe, Mn but gave higher and more variable concentrations for Cu, Zn and organic carbon. Concentrations comparable to those obtained by dialysis were found when centrifugation speed was increased to 11,000 rpm and when 0.2 or 0.03 μm membranes were used to filter the supernatant. This procedure was also found equivalent to dialysis for Cr, Co, Ni, Cd and organic carbon. A reduction of the dialysis membrane pore size to 0.002 μm did not produce any apparent change in the porewater composition; however, a further reduction to 0.001 μm indicated either incomplete equilibration after two weeks, or exclusion of medium sized metal-organic complexes. Because of its inherent simplicity, in situ dialysis appears particularly well adapted to the study of trace constituents in sediment porewaters under field conditions.

Complexation by natural heterogeneous compounds: Site occupation distribution functions, a normalized description of metal complexation

Abstract This paper presents a new conceptual approach to interpreting titration curves of metal complexation by physically and chemically heterogeneous natural complexants such as humic acids, clays, complete soils, or sediments. The physico-chemical and analytical difficulties encountered with such systems are reviewed by comparison with a system containing only a few simple ligands, followed by discussion of the new approach on the same basis. It is shown that interpretation of heterogeneous complexant properties necessitates a preliminary transformation of experimental raw data into a function sufficiently normalized so as to allow comparison of results obtained under different conditions. A normalized function called a Site Occupation Distribution Function (SODF) and its potential usefulness is described here. The SODF is a readily computable function which relates the complexation buffer intensity of the system to the differential free energy of the complexation sites present. Its major interest is that it enables one to obtain both a rigorous mathematical description of the complexant properties (even when highly heterogeneous) at the macroscopic level and, in certain cases, an estimation of the molecular-scale behavior of particular site types. The relationship of the SODF to other distribution functions proposed in the literature is discussed and applications are exemplified using simulated and real natural systems. In particular, its utility is discussed in detail for 1. (1) discriminating between different site types (major, minor, dominant, background), 2. (2) evaluating the degree of heterogeneity of an unknown complexant system, 3. (3) estimating the nature and true thermodynamic constants of complexes, and 4. (4) yielding a rigorous definition of “complexation capacity.”

Particulate trace metal speciation in stream sediments and relationships with grain size: Implications for geochemical exploration

Sediment samples were collected from streambeds in an undisturbed watershed in eastern Quebec (Gaspe Peninsula). Two sampling sites were located on a stream draining an area of known mineralization (Cu, Pb, Zn) and two on a control stream. The sediment samples were separated into 8 distinct size classes in the 850 μm to <1 μm size range by wet sieving, gravity sedimentation or centrifugation. Each sediment subsample was then subjected to a sequential extraction procedure designed to partition the particulate heavy metals into five fractions: (1) exchangeable; (2) specifically adsorbed or bound to carbonates; (3) bound to Fe-Mn oxides; (4) bound to organic matter; (5) residual. The following metals were analyzed in each extract: Cu, Pb, Zn; Fe, Mn. Comparison of samples from the mineralized area with control samples revealed the expected increase in total concentrations for Cu, Pb and Zn. Non-detrital metals were mainly associated with Fe oxides (specifically adsorbed; occluded) and with organic matter or resistant sulfides. For a given sample, variation of trace metal levels in fractions 2 and 3 with grain size reflected changes in the available quantities of the inorganic scavenging phase (FeOx/MnOx); normalization with respect to Fe and Mn content in fraction 3 greatly reduced the apparent dependency on grain size. The results of this study suggest that a single reducing extraction (NH2OH.HCl) could be used advantageously to detect anomalies in routine geochemical surveys. A second leaching step with acidified H2O2 could also be included, as the trace metal concentrations in fraction 4, normalized with respect to organic carbon content, also showed high {anomaly/background} ratios.

Partitioning of zinc between the water column and the oxic sediments in lakes

Zinc concentrations were measured in oxic sediments and in the associated interstitial and overlying waters (vertical profiles) at 40 littoral stations of various lakes; the stations were chosen to represent a large range of lake pH (pH 4–8.4) and of zinc concentrations in the sediments and the overlying waters. Large concentration gradients of dissolved zinc (leading presumably to large downward fluxes) were found at the sediment-water interface of acid lakes (pH < 6). Concentrations of dissolved zinc [Zn]in the overlying waters were found to be highly undersaturated with respect to Zn(OH)2 (s) and ZnC03 (s), and in general with respect to ZnSiO3 (s), ruling out solubility equilibrium with pure solid phases as a mechanism of control of zinc concentrations in the overlying waters. The partitioning of zinc between the overlying water and the surficial sediments was described by sorption processes, using either the distribution coefficient model or a simplified version of the surface complexation model. In relation with this latter model, binding intensity values, ka, for the sorption of zinc to natural iron oxyhydroxides have been estimated using the field data, I.E., the partitioning of Zn in oxic lake sediments and their dissolved concentrations in the associated overlying waters. The following empirical relationship between ka and lake pH has been found: log ka = 1.21 pH - 2.83 (r2 = 0.89). The pH dependence of KA is in agreement with recent theory on the adsorption of trace metals at oxyhydroxide surfaces; however, the binding intensity values derived from field measurements differ markedly in some cases from those of the equilibrium constants obtained for zinc in laboratory experiments. As several observations suggest that Fe-bound zinc is mobile, the distribution of labile zinc between the water column and the sedimentary iron oxyhydroxides has been calculated as a function of pH for typical shallow lakes with the log KA-pH empirical relationship. This model predicts that most of the mobile Zn should be present in the water column at pH < 5 and in the surficial sediments above pH 7. The pH interval 5.5–6.5 (into which fall a large number of poorly buffered lakes receiving acidic deposition) defines a region where changing pH in the water column should have the most pronounced effect on the partitioning of Zn between the water column and the sediment.