Laurie S. Balistrieri

Adsorption of selenium by amorphous iron oxyhydroxide and manganese dioxide

Abstract This work compares and models the adsorption of selenium and other anions on a neutral to alkaline surface (amorphous iron oxyhydroxide) and an acidic surface (manganese dioxide). Selenium adsorption on these oxides is examined as a function of pH, particle concentration, oxidation state, and competing anion concentration in order to assess how these factors might influence the mobility of selenium in the environment. The data indicate that 1. 1) amorphous iron oxyhydroxide has a greater affinity for selenium than manganese dioxide, 2. 2) selenite [Se(IV)] adsorption increases with decreasing pH and increasing particle concentration and is stronger than selenate [Se(VI)] adsorption on both oxides, and 3. 3) selenate does not adsorb on manganese dioxide. The relative affinity of selenate and selenite for the oxides and the lack of adsorption of selenate on a strongly acidic surface suggests that selenate forms outer-sphere complexes while selenite forms inner-sphere complexes with the surfaces. The data also indicate that the competition sequence of other anions with respect to selenite adsorption at pH 7.0 is phosphate > silicate > molybdate > fluoride > sulfate on amorphous iron oxyhydroxide and molybdate ≥ phosphate > silicate > fluoride > sulfate on manganese dioxide. The adsorption of phosphate, molybdate, and silicate on these oxides as a function of pH indicates that the competition sequences reflect the relative affinities of these anions for the surfaces. The Triple Layer surface complexation model is used to provide a quantitative description of these observations and to assess the importance of surface site heterogeneity on anion adsorption. The modeling results suggest that selenite forms binuclear, innersphere complexes with amorphous iron oxyhydroxide and monodentate, inner-sphere complexes with manganese dioxide and that selenate forms outer-sphere, monodentate complexes with amorphous iron oxyhydroxide. The heterogeneity of the oxide surface sites is reflected in decreasing equilibrium constants for selenite with increasing adsorption density and both experimental observations and modeling results suggest that manganese dioxide has fewer sites of higher energy for selenite adsorption than amorphous iron oxyhydroxide. Modeling and interpreting the adsorption of phosphate, molybdate, and silicate on the oxides are made difficult by the lack of constraint in choosing surface species and the fact that equally good fits can be obtained with different surface species. Finally, predictions of anion competition using the model results from single adsorbate systems are not very successful because the model does not account for surface site heterogeneity. Selenite adsorption data from a multi-adsorbate system could be fit if the equilibrium constant for selenite is decreased with increasing anion adsorption density.

The adsorption of Cu, Pb, Zn, and Cd on goethite from major ion seawater

Abstract The adsorption of Cu, Pb, Zn, and Cd on goethite (αFeOOH) from NaNO3 solutions and from major ion seawater was compared to assess the effect of the major ions of seawater (Na, Mg, Ca, K, Cl, and SO4) on the adsorption behavior of the metals. Magnesium and sulphate are the principal seawater ions which enhance or inhibit adsorption relative to the inert system. Their effect, as determined from the site-binding model of Davis et al. (1978), was a combination of changing the electrostatic conditions at the interface and decreasing the available binding sites. The basic differences between the experimental system of major ion seawater and natural seawater were examined. It was concluded that: 1) although the experimental metal concentrations in major ion seawater were higher than those found in natural seawater, estimates of the binding energy of Cu, Zn, and Cd with αFeOOH for natural seawater concentrations could be made from the data, 2) Cu, Pb, Zn, and Cd showed little or no competition for surface sites on goethite, and 3) the presence of carbonate, phosphate, and silicate had little or no effect on the adsorption of Zn and Cd on goethite.

Suboxic trace metal geochemistry in the eastern tropical North Pacific

Abstract We analyzed Al, Ti, Fe, Mn, Cu, Ba, Cd, U, Mo, V, and Re in water column, settling particulate, and sediment (0 to 22 cm) samples from the intense oxygen minimum zone (OMZ) of the eastern tropical North Pacific near Mazatlan, Mexico. The goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique “suboxic” geochemical signature. The water column was characterized by a Mn maximum, reaching ∼8 nmol kg−1 at 400 m. Concentrations of Cu, Ba, Cd, Mo, Re, U, and V were unaffected by the low O2 conditions and were comparable to those of the open ocean. Sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin. Al, Ti, and Fe were mostly (at least 79%) lithogenic. About 75% of the Mn was nonlithogenic. Significant amounts (at least 58%) of Cu, Ba, Cd, and Mo were nonlithogenic. Sediment geochemistry varied across the continental shelf and slope. Cadmium, U, and Re have prominent maxima centered at 310 m, with 12.3 ppm, 10.9 ppm, and 68.3 ppb, respectively, at the core top. High values of Mo (averaging 6.8 ppm) and V (averaging 90 ppm) are seen in OMZ surface sediment. Additional down-core enrichment occurs for all redox-sensitive elements in the top 10 cm. For U, Mo, V, and Re, surface sediments are a poor indicator of metal enrichment. Comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition, especially for Cd, U, and Mo. We evaluated Re/Mo and Cd/U ratios as tracers for redox environments. Rhenium and Mo concentrations and Re/Mo ratios do not lead to consistent conclusions. Concurrent enrichments of Re and Mo are an indicator of an anoxic depositional environment. In contrast, high Re/Mo ratios are an indicator of suboxic conditions. Cadmium is enriched in surface sediments, while U has considerable down-core enrichment. The concentrations of Cd and U and the Cd/U ratio do not follow patterns predicted from thermodynamics. Though the water column is suboxic, these four redox-sensitive elements indicate that the sediments are anoxic. The implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column.

Oceanic trace metal scavenging: the importance of particle concentration

Abstract Adsorption (surface complexation) has long been considered to be the dominant process involved in the oceanic scavenging of many trace metals. Much of what we know about metal removal in the ocean (i.e. rate and extent) is based on measurements of U and Th decay series isotopes. However, the scavenging equations developed from radioactive parent-daughter relationships presume no specific metal removal process and cannot be directly used to verify a particular one. In this paper we examine and compare the phenomenological model of adsorption and oceanic scavenging observations. The formalisms of surface coordination and colloid chemistry are linked to the mechanism-free observations of oceanic trace metal scavenging by the strong similarities in the description of the reaction rates and the influence of particle concentration on those rates and the equilibrium distributions. The correspondence between laboratory sorption data and field scavenging observations as well as the consistency of the hypothesis over a wide range of environmental systems successfully link oceanic trace metal scavenging with surface coordination and colloid aggregation reactions. The merging of descriptions of surface and colloid chemistry and field observations of scavenging provide a framework for interpreting field data and understanding how master variables (e.g. reaction rate, particle concentration, or particle flux) influence metal removal from the oceans.

Modeling sorption of divalent metal cations on hydrous manganese oxide using the diffuse double layer model

Manganese oxides are important scavengers of trace metals and other contaminants in the environment. The inclusion of Mn oxides in predictive models, however, has been difficult due to the lack of a comprehensive set of sorption reactions consistent with a given surface complexation model (SCM), and the discrepancies between published sorption data and predictions using the available models. The authors have compiled a set of surface complexation reactions for synthetic hydrous Mn oxide (HMO) using a two surface site model and the diffuse double layer SCM which complements databases developed for hydrous Fe (III) oxide, goethite and crystalline Al oxide. This compilation encompasses a range of data observed in the literature for the complex HMO surface and provides an error envelope for predictions not well defined by fitting parameters for single or limited data sets. Data describing surface characteristics and cation sorption were compiled from the literature for the synthetic HMO phases birnessite, vernadite and δ-MnO2. A specific surface area of 746 m2g−1 and a surface site density of 2.1 mmol g−1 were determined from crystallographic data and considered fixed parameters in the model. Potentiometric titration data sets were adjusted to a pHIEP value of 2.2. Two site types (≡XOH and ≡YOH) were used. The fraction of total sites attributed to ≡XOH (α) and pKa2 were optimized for each of 7 published potentiometric titration data sets using the computer program FITEQL3.2. pKa2 values of 2.35±0.077 (≡XOH) and 6.06±0.040 (≡YOH) were determined at the 95% confidence level. The calculated average α value was 0.64, with high and low values ranging from 1.0 to 0.24, respectively. pKa2 and α values and published cation sorption data were used subsequently to determine equilibrium surface complexation constants for Ba2+, Ca2+, Cd2+, Co2+, Cu2+, Mg2+, Mn2+, Ni2+, Pb2+, Sr2+ and Zn2+. In addition, average model parameters were used to predict additional sorption data for which complementary titration data were not available. The two-site model accounts for variability in the titration data and most metal sorption data are fit well using the pKa2 and α values reported above. A linear free energy relationship (LFER) appears to exist for some of the metals; however, redox and cation exchange reactions may limit the prediction of surface complexation constants for additional metals using the LFER.

Kinetics of trace element uptake by marine particles

Removal of four radiotracers (46Sc, 113Sn, 65Zn and 230Th) by natural particulates in Puget Sound seawater was carefully studied under controlled laboratory conditions. In order to analyze closely the sorption kinetics of these metals, and to determine the relative importance of different rate controlling processes, time dependent particulate and dissolved concentrations were obtained from about 1 min. to 100 days. Very fast sorption for each metal was followed by much slower and extended uptake. Overall metal sorption rates for the faster uptake reactions are proportional to their final distribution coefficient, Kd∞. Times to reach 90% of final fraction sorbed range from <1 min for Sn to about 6 days for Zn. Sorption rate variations for the different metals indicate particle residence time as a master variable in scavenging, and the rate versusKd∞ relationship allows Th to be used as an analogue for other scavenged metals. Linearizing the sorption data to an overall first order reaction model suggests that for our experiments four distinct processes exist with characteristic time periods of <1 min, ~20 min, ~4 hrs and several days. The results suggest that scavenging is affected by both surface chemical properties and biological activity. In general, stronger binding metals sorb rapidly via chemical and physical processes, while metals with weaker particle associations are taken up much more slowly and may be controlled biologically. Two uptake models and the theoretical limits of rate controlling processes such as surface reactions, diffusion and mass transfer are also discussed.

The oxidation state of manganese in marine sediments and ferromanganese nodules

Marine sediments and ferromanganese nodules from the Pacific Ocean have been analyzed for the OMn ratio of solid manganese. We tested six chemical methods and concluded that the iodometric and oxalate methods were equivalent and were the best choice in terms of accuracy and precision on natural samples. We choose the iodometric method for most of our analyses because the oxalate procedure is a method of differences. The ferromanganese nodules that we analyzed were all from MANOP site H and had MnFe ratios that ranged from 5.6 to 70. These nodules were invariably highly oxidized with OMn values ranging from 1.90 to 2.00. Our most precise analyses suggest that less than 1% of the total manganese is present as Mn(II). We also analyzed red clay and hemipelagic sediments from the eastern tropical Pacific (Baja borderland and MANOP site H) and carbonate ooze samples from the equatorial Pacific. These sediments are also highly oxidized (OMn= 1.90 to 2.00) except when Mn(II) appears in the interstitial water. As dissolved Mn(II) increases the value of the OMn ratio in the solid phase decreases. The OMn ratio decreases to values as low as 1.40. This decrease appears to be due to a decrease in oxidized manganese by reduction, however, an increase in reduced manganese in the solid sediments by adsorption or MnCO3 formation can not be ruled out in all cases.

The geochemical cycling of trace elements in a biogenic meromictic lake

The geochemical processes affecting the behavior and speciation of As, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, V, and Zn in Hall Lake, Washington, USA, are assessed by examining dissolved and acid soluble particulate profiles of the elements and utilizing results from thermodynamic calculations. The water column of this meromictic lake is highly stratified and contains distinctive oxic, suboxic, and anoxic layers. Changes in the redox state of the water column with depth affect the distribution of all the elements studied. Most noticeable are increases in dissolved Co, Cr, Fe, Mn, Ni, Pb, and Zn concentrations across the oxic-suboxic boundary, increases in dissolved As, Co, Cr, Fe, Mn, and V concentrations with depth in the anoxic layer, significant decreases in dissolved Cu, Ni, Pb, and Zn concentrations in the anoxic region below the sulfide maximum, and large increases in acid soluble particulate concentrations of As, Cr, Cu, Fe, Mo, Ni, Pb, V, and Zn in the anoxic zone below the sulfide maximum. Thermodynamic calculations for the anoxic region indicate that all redox sensitive elements exist in their reduced forms, the primary dissolved forms of Cu, Ni, Pb, and Zn are metal sulfide solution complexes, and solid sulfide phases of Cu, Fe, Mo, and Pb are supersaturated. Calculations using a vertical diffusion and reaction model indicate that the oxidation rate constant for Mn(II) in Hall Lake is estimated to be 0.006 d−1 and is at the lower end of the range of microbial oxidation rates observed in other natural systems. The main geochemical processes influencing the distribution and speciation of trace elements in Hall Lake appear to be transformations of dissolved elements between their oxidation states (As, Cr, Cu, Fe, Mn, V), cocycling of trace elements with Mn and Fe (As, Co, Cr, Cu, Mo, Ni, Pb, V, Zn), formation of soluble metal sulfide complexes (Co, Cu, Ni, Pb, Zn), sorption (As, Co, Cr, Ni, V), and precipitation (Cu, Fe, Mn, Mo, Pb, Zn).

The surface chemistry of δMnO2 in major ion sea water

Abstract The surface chemistry of δMnO2 in major ion seawater is defined by determining the stoichiometry of the reactions which describe the association of the surface hydroxyl groups with Na, Mg, Ca, and K ions, the intrinsic equilibrium constants which define these reactions, and the speciation of the surface at pH 8. The results indicate: 1. 1) that the surface forms monodentate complexes with Na and K ions and bidentate complexes with Mg and Ca ions; 2. 2) that pK2INT = 4.2, p ∗ K INT Na = 3.0 ± 0.3 , p ∗ K INT K = 2.0 ± 0.2 , p ∗ K INT >Mg = 3.9 ± 0.1 , and p ∗ K INT >Ca = 3.3 ; and 3. 3) ion exchange processes other than with H are important and as a result 84.8% of the surface sites of δMnO2 in major ion seawater at pH 8 are complexed by H, 8.4% by Mg, 4.6% by Ca, 1.6% by Na, and 0.6% by K. Chloride and sulphate do not adsorb on δMnO2 in the pH range of natural waters.

The surface chemistry of sediments from the Panama Basin: The influence of Mn oxides on metal adsorption☆

The solid Mn content of sediments at a site in the Panama Basin (5°21′N 81°56′W) decreases from 3.9% in the interfacial sediment to 1% at 1.5 cm and <0.2% below 5 cm. These conditions provide an opportunity to examine the influence of Mn oxides on the metal adsorption characteristics of natural marine sediments. The adsorption of 14 metals on interfacial sediment and sediment from depths of 0.5 to 3 cm and 15 to 19 cm from the Panama Basin site was studied, and distribution coefficients (KD) were determined. A comparison of the KD values for a variety of samples containing different Mn contents (i.e., Panama Basin sediments, MANOP site H interfacial sediment, red clay, and buserite) indicates that an increase in the solid Mn content enhances the ability of the particles to bind certain metals (e.g., Zn, Pb, Co, Cd, and Ba) while the binding ability for other metals (e.g., Cs, Be, Sc, Pu, Sn, and Fe) is not significantly affected. For the Panama Basin sediment, the KD values for Ni, Co, Cd, Ba, and Mn for the Mn enriched interfacial sediment are 5 to 23 times greater than the KD values for the Mn depleted deep sediment. The KD values for Cs, Be, Sc, Pu, and Fe for the two types of sediment are essentially the same. The correlation between the Mn content and the binding ability of the sediment for particular metals coincides with the Mn-metal correlations observed in bulk compositional data for ferromanganese nodules and sediments. This implies that the observed metal enrichments in nodules or hemipelagic sediments are most likely caused by preferential adsorption of the metals by Mn enriched phases.