Geochemical speciation as related to the mobility of F, Mo and Se in soil leachates

doi:10.1016/S0883-2927(09)80029-2

Applied Geochemistry

Volume 8, Supplement 2, January 1993, Pages 159-163

https://doi.org/10.1016/S0883-2927(09)80029-2 Get rights and content

Abstract

Chemical speciation of soil leachates is necessary for evaluating a number of processes including solid phase controls, leaching potentials and toxicity problems. The objectives of this study were to examine chemical composition of soil leachates, potential solid phase controls of F, Mo and Se and evaluate their mobility in a calcareous soil. A typical calcareous soil was sampled at 10-cm increments from the surface to a depth of 70-cm. Soil leachates were extracted with distilled deionized H²O and subjected to speciation studies. As depth of the soil increased, concentrations of F and Mo increased from 0.77 to 6.74 mg/1 and 0.06 to 0.15 mg/1, respectively. Selenium concentration increased slightly (0.002—0.005 mg/1) but showed highest levels at intermediate soil depths. Chemical speciation indicated soil leachates were dominated by F⁻, MgMoO°₄ and MgSeO°₃ species. The F and Mo concentrations, at lower depths, indicated a close approach to saturation with respect to fluorite (CaF₂) and powellite (PbMo₄), respectively. Results obtained in this study suggest that as soil depth increases, the mobility of F, Mo and Se is increased due to the chemical form in which these elements are present in the soil leachates.

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This information is key to predicting the behavior of this contaminant in environmental and engineered systems e.g. during waste water treatment processes. Mo exists in natural aqueous environments as the highly soluble tetrahedral monomeric Mo(VI) oxyanion (Mo(VI)O42-) in oxic systems above pH 6 (Xu et al., 2013; Oyerinde et al., 2008; Wichard et al., 2009; Reddy and Gloss, 1993). Heptamolybdate polyanions (Mo(VI)7O246-), consisting of MoO6 octahedra, form at lower pH (3–5) and higher total Mo concentrations (> ~10−4 M) (Davantes et al., 2017; Reddy et al., 1997).
Molybdenum (Mo) is a key trace element and a contaminant in many environments including mine tailings and acid mine drainage systems. Under oxic conditions Mo exists in a number of forms, including mono-molybdate (Mo(VI)O₄^2-) and various poly-molybdate species (e.g. Mo(VI)₇O₂₄^6-) depending on the geochemical conditions (e.g. pH). The mobility and bioavailability of Mo is often controlled by sorption to mineral surfaces, including iron (oxyhydr)oxides e.g. hematite (Fe₂O₃). This study uses adsorption isotherms, PHREEQC geochemical modeling, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and X-ray Absorption Spectroscopy (XAS) to holistically characterise the molecular scale adsorption of molybdate to hematite as a function of pH (3−12) and Mo(VI) concentration (0.01 × 10⁻⁴ - 2 × 10⁻³ M). PHREEQC and ATR-FTIR indicated both pH and Mo concentration are important variables when forming mono- vs. poly- molybdate and suggest low pH (≤ 4) and high Mo(VI) concentration (≥ 5 × 10⁻⁴ M) contribute to the formation of a poly-molybdate surface species on the hematite surface. XAS found Mo adsorbed to hematite via an inner-sphere corner-sharing bidentate binuclear complex with an octahedral mono-molybdate structure at a Mo concentration of 0.6 × 10⁻⁴ M across the pH range, and at a Mo(VI) concentration of 5 × 10⁻⁴ M and pH over 5. This is the first direct observation of octahedrally coordinated Mo(VI) adsorption species on hematite, and this information has broad implications for the mobility and transport of Mo as a contaminant in the environment.
Fate and transport of molybdenum in soils: Kinetic modeling
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A wide range of metal molybdates are quite stable both in solution and in the solid phase. Reddy and Gloss (1993) found that dissolved Mo species in alkaline soil solutions are dominated by MgMoO40, followed by CaMoO40 and MoO42 − (Table 3). MgMoO4 could also be a major Mo soluble species in seawater.
Molybdenum (Mo) is an essential trace element for human, animal and plant health. Mo deficiency in soils has frequently been reported, especially under P-deficient conditions. However, Mo is also a potentially toxic contaminant to soils and aquifers that may pose significant threat to ecological and human health. This review article illustrates briefly about Mo, its production and use, its natural occurrence, its fate and behavior in the soil-water system, and its retention mechanisms in soils. Furthermore, a range of transport models (multi-reaction model, second order model, stir-flow model, and competitive model) for describing and predicting Mo fate and transport behavior in the environment have been reviewed. It was shown that equilibrium Langmuir and Freundlich equations can be extended to simulate the pH dependent and time-dependent sorption and transport in soils. An advantage of modified Freundlich and Langmuir approaches is that they are relatively less complex and require fewer parameters. On the other hand, geochemical models, which are based on surface complexation concepts, were incorporated into kinetic models to accurately describe the reactive transport of Mo in soil components under a wide range of environmental conditions.
Human perturbation increases the fluxes of dissolved molybdenum from land to ocean — The case of the Jiulong River in China
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Pedersen (1985) reported that dissolved Mo could reach up to 0.6 μmol/L in the tailing pore waters in the Rupert and Holberg inlets, Canada. Reddy and Gloss (1995) suggested that dissolution from minerals (mainly as molybdenite) is the key process contributing dissolved Mo ions in these mine tailings. Consistently, the mining area in JR is also dominated by molybdenite (Zhang, 2010).
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Molybdenum sources and isotopic composition during early stages of pedogenesis along a basaltic climate transect
2016, Chemical Geology
Citation Excerpt :
The KCG spans a dry to intermediate precipitation gradient (660 mm to 2100 mm MAP) where soils are dominantly oxic. Under oxic conditions, Mo is expected to be mobile as a molybdate ion (MoO42 −) yielding leaching losses relative to bedrock (negative τ values) (Kabata-Pendias, 2010; Reddy and Gloss, 1993). The fact that KCG τ Mo values are positive indicates that the soils are gaining Mo from a source other than the weathering of bedrock.
Molybdenum (Mo) is an essential micronutrient and redox sensitive trace metal that has the potential to be a tracer of pedogenic processes. Globally, riverine δ⁹⁸Mo values (based on ⁹⁸Mo/⁹⁵Mo relative to NIST SRM 3134) are heavier relative to bedrock, suggesting weathering processes preferentially retain light Mo isotopes, however, the mechanisms governing this process in soils are poorly understood. We investigated soil Mo abundance and isotopic composition as a function of climate along a well-constrained climosequence on a 10 kyr lava flow in Hawaii receiving 660 to 2100 mm year^− 1 mean annual precipitation (MAP). We assessed Mo abundance and isotopic composition as a function of soil organic matter (OM) content, short-ranged-ordered (SRO) iron (Fe) (oxyhydr)oxide abundance, and Mo loss/gain. We find net accumulation of Mo across all soils (up to + 139% gain) that is positively correlated with increasing MAP. The highest Mo gains are in surface soil horizons, and are correlated with high OM content. The isotopic composition of soil Mo, deviates from underlying basaltic bedrock, which ranges from δ⁹⁸Mo values of − 0.11‰ to − 0.26‰. Soil Mo isotopic values are lightest at the dry sites (δ⁹⁸Mo = − 0.63‰) and become heavier with increasing precipitation (up to δ⁹⁸Mo = + 0.03‰). Samples with the heaviest δ⁹⁸Mo values are from horizons with the largest net gains of Mo relative to bedrock and have higher OM content. In order to further constrain Mo fluxes into and out of the soil system, we measured Mo isotope ratios in local rainwater (average δ⁹⁸Mo = + 1.11‰), groundwater (average δ⁹⁸Mo = + 0.14‰), and vegetation (δ⁹⁸Mo values between − 0.18‰ and + 0.64‰). Based on these data, we propose that Mo in these soils is substantially augmented by additions from precipitation, volcanic fog, and potentially anthropogenic inputs of Mo, and that retention of these inputs within soils is likely related to Mo–OM interactions. The large atmospheric inputs of Mo strongly modulate the δ⁹⁸Mo values of dissolved Mo fluxes from soil to values only slightly heavier than bedrock. These patterns illustrate the potential for Mo as a tracer of atmospheric inputs and pedogenic processes in soils and help elucidate the mechanisms that drive heavy δ⁹⁸Mo values in the global riverine Mo flux.
Phosphorus and micronutrient dynamics during gymnosperm and angiosperm litters decomposition in temperate cold forest from Eastern Canada
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This higher Mo retention efficiency of angiosperm litter material could result from (i) differences in the chemical composition of the two litter types such as phenolic compounds and fulvic acid (Schlesinger, 1997) and (ii) differences in litter shape (surface to volume ratio). Molybdate, the dominant species of Mo in soils, is efficiently complexed by plant derived phenolic and tannin-like compounds (Kiboku and Yoshimura, 1958; Reddy and Gloss, 1993). It has been hypothesized that the binding of Mo to phenolic compounds is a critical process supporting biological N fixation by reducing leaching of this essential trace metal from the litter (Wichard et al., 2009).
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Predicting molybdenum toxicity to higher plants: Influence of soil properties
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Soil properties such as amorphous iron (Fe) and aluminium (Al) oxides/hydroxides, texture and organic matter can also influence uptake of Mo by plants (Barrow, 1977; Gupta, 1997). In previous papers (Rooney et al., 2006, 2007; Micó et al., 2008; Li et al., 2009), we demonstrated the importance of soil properties on the toxicity and bioavailability of divalent metal cations such as Cu, Ni, and Co. However, unlike cationic metals, the availability of Mo to higher plant species generally increases with increasing soil pH (Smith et al., 1997).
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Geochemical speciation as related to the mobility of F, Mo and Se in soil leachates

Abstract

J. Colloid Inter. Sci.

Copper and molybdenum in sway-back pastures

J. agri. Sci.

Selenium adsorption by geothite

Soil Sci. Soc. Am. J.

The methods of soil analyses for soils, plants and waters

Univ. of California, Div. of Ag. Sci.

Infrared study of the adsorption of hydrocarboxylic acids on FeOOH and amorphous Fe (III) hydroxide

Colloid Polymer Sci.

Chemical equilibria of selenium in soils: a theoretical development

Soil Sci.

Element concentrations toxic to plants, animals and man

U.S. Geol. Surv. Bull. 1466.

Transformations of selenium as affected by sediment oxidation-reduction potential and pH

J. envir. Sci. Tech.

The two faces of selenium

J. Nutrit.

Fluoride adsorption by Illinois soils

J. soil Sci.

Adsorption on hydrous oxides. IV. Mechanisms of adsorption of various ions on goethite

J. soil Sci.