Molybdenum in the Hydrosphere
Distribution of dissolved and suspended particulate molybdenum, vanadium, and tungsten in the Baltic Sea
In natural waters, dissolved oxyanions often dominate over the particle-bound element fraction. Still, the scavenging of oxyanions by suspended particles might contribute significantly to their dynamic cycling and distribution. To investigate how oxyanions are affected by manganese (Mn) redox cycling, detailed depth profiles across the pelagic redox zone at the Landsort Deep, Baltic Sea, were collected for molybdenum (Mo), vanadium (V), and tungsten (W), for both dissolved (< 0.22 mu m) and suspended particulate (> 0.22 mu m) fractions. All three oxyanions show a non-conservative behavior in the stratified Landsort Deep. Strong linear correlations with Mn in the particulate fraction in the redox zone of the Landsort Deep suggest that Mn redox cycling influences their distribution. In the dissolved fraction, Mo, V, and W exhibited rather different behavior. Molybdenum was depleted below the redox zone, while V was depleted only within the redox zone. Tungsten concentrations increased within the redox zone, being three times higher in the sulfidic zone than in the surface water. Unlike Mo, W shows no tendency for adsorption or co-precipitation under the prevailing weak sulfidic conditions in the deep water of the Landsort Deep and is, therefore, not exported to the underlying sediment. The Landsort Deep data were compared with data from the northern Baltic Sea (Bothnian Bay, Kalix River and Rane River estuaries), where particulate iron (Fe) occurs in high abundance. The particulate fractions of Mo, V, and W decreased during mixing in these estuaries. Vanadium showed the most drastic reduction, with a decrease in dissolved and particulate fractions, indicating that different processes influence the distribution of these oxyanions.
Bauer, S., Blomqvist, S., and Ingri, J.,Distribution of dissolved and suspended particulate molybdenum, vanadium, and tungsten in the Baltic Sea, Marine Chemistry, 2017, 196, 135-147.
Generating false negatives and false positives for As and Mo concentrations in groundwater due to well installation
Groundwater monitoring relies on the acquisition of 'representative' groundwater samples, which should reflect the ambient water quality at a given location. However, drilling of a monitoring well for sample acquisition has the potential to perturb groundwater conditions to a point that may prove to be detrimental to the monitoring objective. Following installation of 20 monitoring wells in close geographic proximity in central Florida, opposing concentration trends for As and Mo were observed. In the first year after well installation As and Mo concentrations increased in some wells by a factor of 2, while in others As and Mo concentrations decreased by a factor of up to 100. Given this relatively short period of time, a natural change in groundwater composition of such magnitude is not expected, leaving well installation itself as the likely cause for the observed concentration changes. Hence, initial concentrations were identified as 'false negatives' if concentrations increased with time or as 'false positives' if concentrations decreased. False negatives were observed if concentrations were already high, i.e., the As or Mo were present at the time of drilling. False positives were observed if concentrations were relatively lower, i.e., As or Mo were present at low concentrations of approximately 1 to 2mug/L before drilling, but then released from the aquifer matrix as a result of drilling. Generally, As and Mo were present in the aquifer matrix in either pyrite or organic matter, both of which are susceptible to dissolution if redox conditions change due to the addition of oxygen. Thus, introduction of an oxidant into an anoxic aquifer through use of an oxygen saturated drilling fluid served as the conceptual model for the trends where concentrations decreased with time. Mixing between drilling fluid and groundwater (i.e., dilution) was used as the conceptual model for scenarios where increasing trends were observed. Conceptual models were successfully tested through formulation and application of data-driven reactive transport models, using the USGS code MODFLOW in conjunction with the reactive multicomponent transport code PHT3D.
I. Wallis, and T. Pichler,Generating false negatives and false positives for As and Mo concentrations in groundwater due to well installation, The Science of the total environment, 2018, 631-632, 723-732.
Naturally Occurring versus Anthropogenic Sources of Elevated Molybdenum in Groundwater: Evidence for Geogenic Contamination from Southeast Wisconsin, United States
Molybdenum (Mo) is an essential trace nutrient but has negative health effects at high concentrations. Groundwater typically has low Mo (<2 mu g/L), and elevated levels are associated with anthropogenic contamination, although geogenic sources have also been reported. Coal combustion residues (CCRs) are enriched in Mo, and thus present a potential anthropogenic contamination source. Here, we use diagnostic geochemical tracers combined with groundwater residence time indicators to investigate the sources of Mo in drinking-water wells from shallow aquifers in a region of widespread CCR disposal in southeastern Wisconsin. Samples from drinking-water wells were collected in areas near and away from known CCR disposal sites, and analyzed for Mo and inorganic geochemistry indicators, including boron and strontium isotope ratios, along with groundwater tritium-helium and radiogenic He-4 ingrowth age-dating techniques. Mo concentrations ranged from <1 to 149 mu g/L.Concentrations exceeding the U.S. Environmental Protection Agency health advisory of 40 mu g/L were found in deeper, older groundwater (mean residence time >300 y). The delta B-11 = 22.9 +/- 3.5 parts per thousand) and Sr (Sr-87/Sr-86 = 0.70923 +/- 0.00024) isotope ratios were not consistent with the expected isotope fingerprints of CCRs, but rather mimic the compositions of local lithologies. The isotope signatures combined with mean groundwater residence times of more than 300 years for groundwater with high Mo concentrations support a geogenic source of Mo to the groundwater, rather than CCR-induced contamination. This study demonstrates the utility of a multi-isotope approach to distinguish between fossil fuel-related and natural sources of groundwater contamination.
Harkness, J. S., Darrah, T. H., Moore, M. T., Whyte, C. J., Mathewson, P. D., Cook, T., and Vengosh, A.,Naturally Occurring versus Anthropogenic Sources of Elevated Molybdenum in Groundwater: Evidence for Geogenic Contamination from Southeast Wisconsin, United States, Environmental Science & Technology, 2017, 51, 12190-12199.
Sulfur and molybdenum fractionation in marine and riverine alluvium paddy soils
Intermittently submergence and drainage status of paddy fields can cause alterations in morphological and chemical characteristics of soils. We conducted a sequential fractionation study to provide an insight into solubility of Sulfur (S) and Molybdenum (Mo) in flooded alluvial paddy soils. The samples (0-15 and 15-30 cm) were taken from marine and riverine alluvial soils in Kedah and Kelantan areas, respectively, and were sequentially extracted with NaHCO3, NaOH, HCl, and HClO4-HNO3. Total S in upper and lower layers of Kedah and Kelantan ranged between 273 and 1121 mg kg-1, and 177 to 1509 mg kg-1, respectively. In upper layers and subsoil of Kedah, average total Mo were 0.34 and 0.27 mg kg-1, respectively. Average total Mo in Kelantan were 0.25 mg kg-1 (surface layer) and 0.28 mg kg-1 (subsoil). Cation exchange capacity (CEC) was positively correlated with plant available amounts of Mo in upper layers of Kedah area. Also, total and medium-term plant-available S was correlated with total carbon (C) at lower layers of Kelantan soil series. But in surface layers of Kelantan soil series, CEC was strongly correlated with total and medium-term plant-available S. Our results indicates that the influence of flooding conditions on soil S and Mo contents in paddy fields may cause long-term changes in S and Mo chemical reactivities.
Zakikhani, H., Yusop, M. K., Hanafi, M. M., Othman, R., and Soltangheisi, A.,Sulfur and molybdenum fractionation in marine and riverine alluvium paddy soils, Chemical Speciation and Bioavailability, 2016, 28, 170-181.
Molybdenum speciation and burial pathway in weakly sulfidic environments: Insights from XAFS
Sedimentary molybdenum (Mo) accumulation is a robust proxy for sulfidic conditions in both modern and ancient aquatic systems and has been used to infer changing marine redox chemistry throughout Earth's history. Accurate interpretation of any proxy requires a comprehensive understanding of its biogeochemical cycling, but knowledge gaps remain concerning the geochemical mechanism(s) leading to Mo burial in anoxic sediments. Better characterization of Mo speciation should provide mechanistic insight into sedimentary Mo accumulation, and therefore in this study we investigate Mo speciation from both modern (Castle Lake, USA) and ancient (Doushantuo Formation, China) environments using X-ray Absorption Near Edge Structure (XANES) spectroscopy. By utilizing a series of laboratory-synthesized oxythiomolybdate complexes-many containing organic ligands-we expand the number of available standards to encompass a greater range of known Mo chemistry and test the linkage between Mo and total organic carbon (TOC). In weakly euxinic systems ([H2S(aq)] < 11 μM), or where sulfide is restricted to pore waters, natural samples are best represented by a linear combination of MoO3, MoOxS4-x 2- (intermediate thiomolybdates), and [MoOx(cat)(4-x)2- (cat = catechol, x = 2 or 3). These results suggest a revised model for how Mo accumulates in weakly sulfidic sediments, including a previously unrecognized role for organic matter in early sequestration of Mo and a de-emphasized importance for MoS42- (tetrathiomolybdate). (C) 2017 Elsevier Ltd. All rights reserved.
Wagner, M., Chappaz, A., and Lyons, T. W.,Molybdenum speciation and burial pathway in weakly sulfidic environments: Insights from XAFS, Geochimica Et Cosmochimica Acta, 2017, 206, 18-29.
RIVERS AND ESTUARIESImpact of water-particle interactions on molybdenum budget in humid tropical rivers and estuaries: insights from Nethravati, Gurupur and Mandovi river systems
The study presents the seasonal and inter-annual monitoring of molybdenum (Mo) distribution and variability in humid tropical riverine and estuarine systems (Nethravati, Gurupur and Mandovi estuaries) of west coast of India. The study investigates the geochemical behaviour of Mo in the river and estuaries, and their ultimate fluxes into the ocean. The riverine flux of dissolved Mo (DMo) to the Nethravati, Gurupur and Mandovi estuaries are 1800 mol yr(-1) (4.88 mol day(-1)), 195 mol yr(-1) (0.53 mol day(-1)) and 10.5 x 10(3) mol yr(-1) (28 mol day(-1)) respectively, and the riverine particulate Mo (PMo) flux to Nethravati estuary is 10.8 x 10(3) mol yr(-1). The DMo in river (similar to 30 to 40%) is scavenged onto particles under oxidized acidic river water conditions and subsequently released in the estuary, impacting the solute budget of Mo to the sea. In the estuaries, under low salinity conditions, DMo is sequestered onto particles during pre-monsoonal season. The DMo sequestration in the estuary is estimated to be similar to 2 mol day(-1) in the Nethravati estuary and similar to 1.9 mol day(-1) in the Mandovi estuary. During this season sequestration in the estuary is higher than the riverine supply, indicating the sequestration of both marine and river borne DMo. However, the mechanisms involved in the removal process are different in these estuaries viz. oxidative adsorption process in the Nethravati-Gurupur estuary and microbial utilization in the Mandovi estuary. The lower salinity region during monsoon and post-monsoon season shows slight excess of DMo, river borne particulate Mo could release up to 3 to 4 nmol L-1 by desorption under alkaline higher ionic strength conditions. At higher salinity (>20 psu) in both the estuaries and in all the seasons, DMo gain is systematic (similar to 1 to 37 nmol L-1). Mo release from river borne particles could contribute only up to 3 to 4 nmol L-1, which is not sufficient to balance the observed Mo excess. On the other hand, the reductive Mo remobilization from bottom sediments (Mo = 4 mg kg(-1)) during sediment diagenesis and subsequent tidal activity, release up to 28 nmol L-1 of DMo to the estuarine water. Mo release to water column is supported by the gradual enrichment of DMo with depth in the estuary. Therefore, diagenetic release of DMo forms the potential source of DMo excess in the estuary. (C) 2016 Elsevier B.V. All rights reserved.
Gurumurthy, G. P., Tripti, M., Riotte, J., Prakyath, R., and Balakrishna, K.,Impact of water-particle interactions on molybdenum budget in humid tropical rivers and estuaries: insights from Nethravati, Gurupur and Mandovi river systems, Chemical Geology, 2017, 450, 44-58.
Geogenic As and Mo groundwater contamination caused by an abundance of domestic supply wells
Lacking a connection to a municipal water supply, each household in the municipality of Lithia, approximately 30 km southeast of Tampa, Florida (USA), is responsible for its own supply of drinking water, causing a high-density of private domestic supply wells (DSW) in this area. There, a multilayered aquifer system exists, which can be subdivided into three distinct hydro stratigraphic units, which are, from the top down: the Surficial Aquifer System (SAS), the Intermediate Aquifer System (IAS), and the Upper Floridan Aquifer System (UFA). Despite the relatively small area, the geochemical and hydro geological setting in Lithia is complex, consisting of: i) extensive cyclical pumping in a municipal well field to the west, ii) large seasonal changes in hydraulic head, ii) multiple aquifers with different hydraulic heads, and iv) a large density of domestic supply wells (DSW). Within the zone of highest As concentrations, there are approximately 100 wells in an area of 2.5 km x 1.5 km. Most of these wells have large open screened intervals, often open to all three aquifers, allowing the downward flow of oxygenated and upward flow of anoxic groundwater. A survey of groundwater quality found that As and Mo concentrations in the DSW were up to 371 mu g/L and 4740 mu g/L, respectively. To obtain information about the individual aquifers, 5 well clusters with 4 monitoring intervals (approximately 50 m, 65 m, 80 m and 95 m below surface) and 8 push core wells (approximately 9 m below surface) were installed and sampled. In those wells, As and Mo were only elevated in a permeable layer within the IAS at a depth of 50 m. Values were up to 195 mu g/L for As and up to 5050 mu g/L. for Mo. Using the tritium-helium (H-3 -He-3) method, the ages of those samples high in As and Mo were determined to be 40, 30 and 30 years, respectively, while all other samples had ages older than 50 years. This indicated that mixing between young and old groundwater could be responsible for the high As and Mo concentrations. A good negative correlation for the whole data set was also observed between the concentration H-3 and delta O-18 values, which together with hydrogeological modeling confirmed that the increased permeability created by the high density of DSW resulted in flow paths that permitted the perpetual mixing of shallow and deep groundwater. The release of the As and Mo appeared to be a consequence of changes to the physicochemical conditions in the aquifer, either via the introduction of oxygen-rich fluids into the IAS or the mixing of different fluids in the IAS or the introduction of oxygen-depleted fluids into the IAS. While the mobilization of geogenic trace metals is often associated with pumping-induced hydraulic gradient changes, we found that a certain density of multi-aquifer wells can be sufficient to alter hydrologic flow paths and induce the mobilization of geogenic trace metals even in the absence of significant pumping. In Lithia, the DSW effectively increased the local scale permeability of the aquifer, causing the mixing of oxygen-rich surface and deeper anoxic groundwater across a confining unit. Because the alteration to the hydrologic flow paths was a consequence of changes to the physical structure of the aquifer system rather than due to pumping, the alteration is not easily reversible, thus significantly complicating site remediation. Our results provide a cautionary warning of the risks of undue private DSW development in rapidly growing communities. (C) 2016 Elsevier Ltd. All rights reserved.
Pichler, T., Renshaw, C. E., and Sueltenfuss, J.,Geogenic As and Mo groundwater contamination caused by an abundance of domestic supply wells, Applied Geochemistry, 2017, 77, 68-79.
Molybdenum in natural waters: A review of occurence, distributions and controls
Molybdenum is an essential trace element for human, animal and plant health and has played an important part in the evolution of life on earth. Nonetheless, exposure to the element can be harmful and although the evidence for symptoms in humans is sparse, it has been linked with a number of health conditions in animal models.
Molybdenum is present in trace quantities (1-10 mg/kg) in most rocks and soils and at concentrations less than, and often orders of magnitude less than, 10 µg/L in most fresh-waters. It is the most abundant transition metal in open seawater (10 µg Mo/L) owing to the dominance, and low chemical reactivity, of the molybdate ion (MoO42-).
The 2011 WHO Guidelines for Drinking-Water Quality (fourth edition) advised a health-based value of 70 µg/L. for Mo but this is no longer promulgated as a formal guideline value as WHO consider such concentrations to be rarely found in drinking water. This is indeed usually the case, but there are instances where currently-used drinking waters do exceed 70 µg Mo/L. We therefore recommend more routine measurement of Mo in water, at least on a reconnaissance scale, in order to improve knowledge on occurrence in water used for potable supply. Where multi-element analytical procedures are already used (e.g. ICP-MS), the marginal cost of adding Mo to the list of elements to be analysed should not be great.
We have reviewed nine areas in the world where high concentrations of Mo in freshwater, and in some cases drinking water, have been found: Argentina, Jordan, Qatar, Ethiopia, UK, USA (three) and Chile. These represent a range of geochemical environments. A common theme of the high-Mo occurrences is (i) oxic, alkaline conditions where, as for seawater, the Mo occurs as the stable molybdate ion; groundwater in oxic, alkaline conditions within volcanogenic sediments can have exceptionally high Mo concentrations (up to hundreds of µg/L) where felsic volcanic ash is present; (ii) anoxic, non-sulphidic waters where Mo can be released to solution by reductive dissolution of Mn and Fe oxides or by release from degradation of organic matter, notably within high-Mo organic-rich muds, black shales or oil shales; or (iii) surface waters or groundwater impacted by metal sulphide mining and/or mineralisation, in particular occurrences of porphyry deposits. Under such conditions, Mo concentrations can reach several tens to several hundreds of µg/L and while not all are otherwise suitable for drinking water, some are.
Much of the basic geochemistry of Mo in oxic natural environments is now quite well understood. Critically, its behaviour is redox-sensitive like its near neighbours in the Periodic Table, W and V. At the near-neutral pH values characteristic of most natural waters, Mo is rather weakly sorbed and formation of Mo minerals is either not indicated or is extremely slow. Molybdenum becomes less mobile when converted to thiomolybdates under the strongly reducing conditions found in some present-day ocean basins (e.g. the Black Sea), fjords, stratified lakes and confined aquifers. This leads to concentrations of around 100 mg Mo/kg or more in black shales and other organic-rich mudstones. However, despite the many studies of these water bodies and the importance of Mo as a palaeoredox indicator, the mechanism of the highly-efficient and diagnostic scavenging of Mo in euxinic (H2S-rich) waters remains uncertain. Possibilities include the formation of an as yet unidentified Mo-Fe-S mineral or solid solution, or the scavenging by some pre-existing solid such as a sulphide or oxide mineral, or organic matter. The possible role of dispersed and reduced natural organic matter has become more prominent in recent years but this has proven difficult to quantify and the mechanism of binding is poorly understood. Molybdenum isotope studies now play an important role in constraining reaction pathways. At a more fundamental level, there is a lack of up-to-date thermodynamic and kinetic data for many of the reactions of importance for Mo in the natural environment and this limits the ability of current geochemical models to predict its fate and transport. This is particularly true for the strongly reducing conditions where Mo partitions to the solid phase, leading to the formation of the Mo-rich shales, Even the existence of reduced aqueous Mo species (e.g. in the Mo(V) and Mo(111) oxidation states) in natural waters is uncertain. These uncertainties will only be resolved with focused laboratory experiments using the benefits of modern instrumentation, combined where necessary with supporting molecular dynamics calculations. The mobility of Mo in aqueous systems has to date received far more attention in the marine than the freshwater setting, The value of Mo speciation as an indicator of redox conditions and of stable-isotopic variations as a tracer, can have more value in the arena of environment and health, and studies of the element's mobility in aqueous systems can be useful for themes varying from radioactive waste disposal, sustainability of unconventional hydrocarbon exploitation and wider surficial pollution. (C) 2017 BGS, A component Institute of NERC. Published by Elsevier Ltd.
Smedley, P. L., and Kinniburgh, D. G.,Molybdenum in natural waters: A review of occurence, distributions and controls, Applied Geochemistry, 2017, 84, 387-432.