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Global Biogeochemical Cycle of Vanadium

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Global Biogeochemical Cycle of Vanadium Global biogeochemical cycle of vanadium William H. Schlesingera,1, Emily M. Kleina, and Avner Vengosha aEarth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC 27708 Contributed by William H. Schlesinger, November 9, 2017 (sent for review September 1, 2017; reviewed by Robert A. Duce, Andrew J. Friedland, and James N. Galloway) Synthesizing published data, we provide a quantitative summary problematic environmental contaminant, but high concentrations of the global biogeochemical cycle of vanadium (V), including both of V can be toxic to humans and other organisms (18, 19). human-derived and natural fluxes. Through mining of V ores Reflecting a new level of concern, the State of California has re- (130 × 109 g V/y) and extraction and combustion of fossil fuels cently imposed a new standard (15 μg/L) for V in drinking water (600 × 109 g V/y), humans are the predominant force in the geo- (https://oehha.ca.gov/water/notification-level/proposed-notifica- chemical cycle of V at Earth’s surface. Human emissions of V to the tion-level-vanadium). In some regions, the release of V to the atmosphere are now likely to exceed background emissions by as atmosphere and its deposition in natural ecosystems have de- much as a factor of 1.7, and, presumably, we have altered the clined in recent decades due to changes in fuel use and industrial deposition of V from the atmosphere by a similar amount. Exces- practices (20, 21). sive V in air and water has potential, but poorly documented, Vanadium’s primary commercial use is in the manufacture consequences for human health. Much of the atmospheric flux of steel alloys to increase hardness and corrosion resistance. probably derives from emissions from the combustion of fossil An emerging application is the development of V redox bat- fuels, but the magnitude of this flux depends on the type of fuel, teries as an alternative for lithium-ion batteries for storage of with relatively low emissions from coal and higher contributions renewable energy. Worldwide, about 82 × 109 gofVare from heavy crude oils, tar sands bitumen, and petroleum coke. produced from virgin ore annually, and, at current rates of Increasing interest in petroleum derived from unconventional de- consumption, reserves (∼15 × 1012 g)areexpectedtomeet posits is likely to lead to greater emissions of V to the atmosphere demand into the next century (22). In the United States, in the near future. Our analysis further suggests that the flux of V recycling of products and fly ash provides as much as 40% of in rivers has been incremented by about 15% from human activ- the V in commercial use, including exports (23). ities. Overall, the budget of dissolved V in the oceans is remark- Huang et al. (13) provide a comprehensive review of the oxi- ably well balanced—with about 40 × 109 g V/y to 50 × 109 g V/y dation and reduction processes that transform V in Earth’s inputs and outputs, and a mean residence time for dissolved V in surface environments, and Hope (24) quantifies the movement seawater of about 130,000 y with respect to inputs from rivers. of V on the present-day and future Earth, based on its mobili- zation by human activities. In this paper, we provide an updated, vanadium | petroleum | geochemical cycle | aerosols | rock weathering quantitative summary of the global biogeochemical cycle of V, including both human and natural fluxes. Using published data, anadium (V) occurs in a wide range of earth materials and is we compile and calculate global-scale estimates of the flux of V Va relatively abundant trace metal, with an average concen- in nature, especially as modified by recent changes in the com- tration in the upper continental crust (97 mg/kg) more than bustion of coal and petroleum products and byproducts. Due to double those of nickel (Ni) and copper (Cu) (1). In modern limited data, many of these estimates must be presented without society, the majority of V is used to improve the strength and boundaries of uncertainty, but we are encouraged by the general corrosion resistance of steel; it is also of increasing strategic internal consistency that emerges when the independent flux and technological interest as a specialty metal in electronics estimates produce a roughly balanced global budget. and batteries. V is an essential trace element in prokaryotic Mobilization of V from Earth’s Crust biochemistry, where it is found as an alternative to molybde- ’ num in the molecular structure of nitrogenase, the enzyme of N Natural Sources. Natural V mobilization from Earth s crust occurs fixation (2–4). It also appears in the structure of enzymes in the dominantly through mechanical and chemical rock weathering, marine algae responsible for the formation of bromoform (5) with lesser contributions from other pathways such as volcanic and methyl bromide (6), which contributes to the depletion of emissions and aeolian processes. For mechanical weathering, × 9 stratospheric ozone. Whether V is an essential trace element in Hope (24) estimates that 2,300 10 g V/y is transported to the higher plants and animals is unknown (7), with some evidence sea in suspended sediments. We suggest a somewhat (30%) favoring an essential role in at least some higher organisms (e.g., refs. 8 and 9). Higher plants accumulate about 1 mg/kg Significance in their tissues (10). The mean concentration of V in the continental crust is about Human emissions of vanadium to atmosphere exceed natural 9 97 mg/kg (1, 11), and ∼20 × 10 g V/y enters biogeochemical sources by a factor of 1.7 and are destined to rise dramatically cycles at Earth’s surface through chemical weathering (12). V, as we switch to the use of heavy oils, tar sands, and bitumen as which can exist in three common oxidation states, is largely combustion sources. Breathing vanadium-rich aerosols has − found as the vanadate ion (H2VO4 ) in natural oxidized waters unknown but potentially adverse health impacts. The human of near-neutral pH. The dissolved concentration in river water is impacts on the global vanadium cycle parallel impacts on the about 0.7 μg/L, less than half of the concentration of ∼1.8 μg/L in global cycles for Pb and Hg. seawater (13). V is widely recognized as a trace contaminant in coal and oil Author contributions: W.H.S., E.M.K., and A.V. designed research, performed research, and is released to the atmosphere through particle emissions as- analyzed data, and wrote the paper. sociated with their combustion (14–17). While V has not received Reviewers: R.A.D., Texas A&M University, retired; A.J.F., Dartmouth College; and J.N.G., as much attention as other anthropogenically mobilized elements, University of Virginia. data presented here show that the human perturbation of V The authors declare no conflict of interest. cycling through the atmosphere may exceed that of mercury Published under the PNAS license. (Hg) and a variety of other metals (Table 1). V is not viewed as a 1To whom correspondence should be addressed. Email: [email protected]. E11092–E11100 | PNAS | Published online December 11, 2017 www.pnas.org/cgi/doi/10.1073/pnas.1715500114 Downloaded by guest on September 26, 2021 Table 1. Movement of certain crustal elements through the atmosphere PNAS PLUS Volatilization Continental Sea Volcanic Industrial Fossil fuel Ratio anthropogenic: Element dust spray emissions Biomass burning Natural Human induced particles combustion natural V 155 0.52 7 5 100 to 287 0.59 to 1.71 Hg 0.12 0.009 0.5 0.6 3.7 4.1 3.4 1.52 Pb 32 5 4.1 38 32 85 1.48 Cu 50 14 9 27 43 4 0.47 Zn 100 51 10 147 88 5 0.30 Ag 2.3 0.01 0.01 1.2 0.44 0.05 0.14 Fe 55,000 200 8,800 830 641 4,200 0.07 Al 96,000 810 4,500 2,125 397 5,900 0.06 From Rauch and Pacyna (88), except for V (this paper) and Hg (107, 108). All data in 109 grams per year. lower value, 1,625 × 109 g V/y, derived from the transport of total lated on fly ash and other coal combustion residues (CCRs), with suspended sediment, 12.6 × 1015 g/y (25), and the mean con- little or no V loss due to volatilization (e.g., ref. 44). The V centration of 129 mg V/kg in sediments (26). composition in CCRs reflects the V content of the parent coal. With respect to chemical weathering, Shiller and Mao (12) The ratio between the volume of CCRs and the volume of uti- observed that world rivers are characterized by relatively con- lized coal is about 0.10 (45). Combined with efficient uptake of V − stant dissolved V/Si ratio (∼66 × 10 6 mol V/mol Si), suggesting in CCRs, this suggests an approximate 10-fold V enrichment in that dissolved V and Si are dominantly derived from chemical the residual ash compared with coal, averaging ∼250 mg/kg weathering of silicate rocks (27). In a detailed study of global based on the global average V in coal. This estimate is within the weathering, Gaillardet et al. (28) concluded that virtually all range of values (150 mg/kg to 700 mg/kg) reported for V in dissolved silica in rivers derives from silicate weathering. Fol- CCRs (e.g., refs. 37 and 46). lowing the approach of Shiller and Mao (12), we calculate the V In recent decades, installation of high-efficiency cold-side 9 flux from chemical weathering as ∼21 × 10 g/y, using their V/Si electrostatic precipitators, fabric filters, and wet flue gas desul- correlation and a recent estimate of global dissolved silicate flux phurization systems has reduced particulate matter (PM) emis- 12 (380 × 10 g/y; ref.
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