Vanadium for Green Energy: Increasing Demand but With Health Implications in Volcanic Terrains

Abstract The transition to a clean energy future may require a very substantial increase in resources of vanadium. This trend brings into focus the potential health issues related to vanadium in the environment. Most vanadium enters the Earth's crust through volcanic rocks; hence, vanadium levels in groundwaters in volcanic aquifers are higher than in other aquifers and can exceed local guidance limits. The biggest accumulation of volcanogenic sediment on the planet is downwind of the Andes and makes up much of Argentina. Consequently, groundwaters in Argentina have the highest vanadium contents and constitute a global vanadium anomaly. The high vanadium contents have given rise to health concerns. Vanadium could be extracted during remediation of domestic and other groundwater, and although the resultant resource is limited, it would be gained using low‐energy technology.


Groundwater in Volcanic Rocks
The high input of volcanic matter is potentially an environmental problem for Argentina. In other parts of the world, where groundwater occurs in volcanic rocks, the water contains anomalously high amounts of vanadium (Table 1; Figure 1). The water becomes enriched in vanadium by dissolution of the volcanic minerals and glass. The benchmarks against which to assess water compositions vary between countries (Table 2) but values include a health reference limit of 21 μg/L in tap water by the U.S. Environmental Protection Agency (Environmental Working Group, 2021) and lower limits for groundwater in some European countries (Länderarbeitsgemeinschaft Wasser, 2004;Smit, 2012). There is no statutory limit for the whole European Union, where vanadium values in some Italian groundwaters would probably exceed any limit due to volcanic activity (Crebelli & Leopardi, 2012 dium contents are less than 1 μg/L (MacDonald et al., 2017;Shand et al., 2007;Smit, 2012). A content of >15 μg/L vanadium in drinking water has been suggested as a potential health risk in the State of California, USA (Gerke et al., 2010).
The collated data for mean groundwater compositions in volcanic aquifers from several parts of the world show that: 1. The vanadium values are consistently higher than in nonvolcanic aquifers as represented by the British and European values. 2. In several cases, the vanadium values exceed the statutory limits set by some countries. 3. Aquifers in old volcanic rocks from the pre-Pleistocene geological record also show values higher than nonvolcanic aquifers.
In several of these regions, there is concern about the importance of groundwater vanadium for human health, including Italy (Arena et al., 2015), Germany (Härter et al., 2020), and the Canary Islands (Luengo-Oroz et al., 2014).
Given the implication of greater V contents and thus greater potential implications for health, in groundwaters in volcanic rocks, we can predict where this issue might be most acute. The extent of volcanic rocks, or sediments derived from volcanic rocks, is imposed by plate tectonics and patterns of plates on Earth over the last 100 million years. The requirements for very extensive volcanic-related aquifers are (a) long-term plate boundary subduction, causing long-term volcanic activity; (b) long length of boundary, as opposed to the short arcs that typify the west Pacific Rim; and (c) continued uplift to promote erosion of volcanics and their deposition as a sediment wedge, built up above sea level. These requirements are met most clearly in South America, where the Andes represent over 8,000 km subduction trench length, persistent volcanic activity, and uplift (Sundell et al., 2019) that have sourced sediment to the east. The Andes have been a plate margin mountain chain for tens of millions of years (Evenstar et al., 2015) and have shed enormous volumes of sediment eastward across Argentina. Importantly, Westerly winds have supplemented the eroded volcanic sediments with volcanic glass and ash (Mingari et al., 2017). Petrographic studies confirm that the sediment in Argentina and Chile contains volcanic debris derived from the Andes by mechanical erosion (Gómez et al., 2020;Horton, 2018) and volcanic glass. Magnetite grains in the sediment, in which much vanadium may be exported from the Andes, show evidence of alteration (Flint et al., 1986), which would have released the vanadium into groundwaters.
Reserves of vanadium-bearing titanomagnetite are greatest in South Africa, Russia, and China (Summerfield, 2019;Yang et al., 2021). In addition, large amounts of titanomagnetite in the Chilean Andes are mined for iron ore and contain high levels of vanadium (Broughm et al., 2017;La Cruz et al., 2020;Palma et al., 2020). Mining of iron and copper in the Andes has caused its own health concerns (Carkovic et al., 2016;Cortés et al., 2021;Reyes et al., 2020;Tapia et al., 2018). In addition to the release of toxic metals from mining spoil, the same mineralized volcanic rocks have been releasing trace elements into the environment through natural erosion over a geological timescale. The mountains are composed of magmatic rocks, which are mineralized by a range of ores including vanadium-bearing titanomagnetite. The titanomagnetite grains exhibit alteration and dispersion of the vanadium (Figure 2). Mineral alteration in the volcanic rocks and subsequently during erosion and transport could release most or all of the vanadium into groundwaters in the sediment wedge that composes Argentina.
The influence of volcanic matter on groundwater is on a larger scale in Argentina than elsewhere. There is some local concern over vanadium contents in domestic groundwater in Argentina (Espósito et al., 2011;Jaafar et al., 2018;Nicolli et al., 2012), but this has been overshadowed by concern about arsenic contamination over much of South America (Bundschuh et al., 2021;Khan et al., 2020). However, here, we bring together diverse data sets, which show that high vanadium levels occur in groundwater across Argentina and represent the largest known region of concern for vanadium toxicity.
Twenty data sets for groundwater (Table 1) represent 12 provinces along the length of Argentina (Figure 3). The mean values for V range from 6 to 1,749 μg/L. The highest individual value is 5,400 μg V/L, recorded in La Pampa Province (Smedley et al., 2002). The lowest mean value is in Salta Province in the far north near the Bolivian border (Figure 3). There is a broad distinction between values in the northern provinces (Tucumán, Salta, Córdoba, Santiago del Estero, Chaco, Santa Fe, and San Luis) and those in the south (Buenos Aires, La Pampa, Neuquén, Rio Negro, and Chubut). The weighted mean for the northern provinces is 150 μg V/L (n = 235). The weighted mean for the southern provinces is 696 μg V/L (n = 338), nearly 5 times as high. The southern provinces lie east of the major volcanoes in the Andes (Figure 3) and they would have a greater fingerprint of their output. The immediately surrounding countries of Bolivia, Uruguay, and southern Brazil also yield mildly anomalous groundwater data with mean values of 11, 40, and 22 μg V/L, respectively (Machado et al., 2020;Muñoz et al., 2013;Rezende et al., 2019). However, these mean values for vanadium in groundwater are lower than most of the mean values reported in Argentina and suggest a progressive decline with distance from the source of vanadium.

Mitigation
There are strategies available to mitigate high vanadium contents in groundwaters, intended primarily to cope with contamination from mining and other short-term commercial activities. The methods include adsorption onto iron oxides, activated carbon, liquid membranes, and combinations of these materials (e.g., Kamal et al., 2017;Leiviskä, 2021;Sirviö et al., 2016;Sharififard et al., 2016). The emphasis has been in the removal of a toxic element, but there is a move toward sustainability using innovative extraction of V from wastewater and V-rich solutions (Petranikova et al., 2020). The mean V content of sea water is much lower than in the groundwaters discussed, commonly cited as 0.3 μg/L and up to ∼2 μg/L. The extraction of vanadium from seawater is possible (Ivanov et al., 2017;Suzuki et al., 2000) but not economically feasible. Extraction from groundwater would be more economic if the large scale was not essential. Groundwater with a V content of 100 μg V/L in 10% aquifer porosity would contain 10 4 kg V. Given the rate at which groundwaters move in Argentina, estimated as 0.01-0.42 m/day and more specifically in one study as 0.07 m/day (Cabrera et al., 2010(Cabrera et al., , 2017Maldonado et al., 2016), groundwaters with 100 μg V/L in 10% aquifer porosity would be replenished within 40 years. Less conservative values of 20% porosity and 200 μg V/L would see replenishment of all the groundwater V within 10 years. The quantities of V that could be obtained from groundwater sources may be limited but the technology has the advantage of low temperature and low energy processing. Incidentally, the mass of V in a large ore body of 10 8 kg vanadium in sandstone (Kelley et al., 2017) would be sourced in less than a million years in the 1 km 3 of groundwater.
Most of the data are from groundwaters in relatively young (<5 Ma) volcanic rocks. An example that includes older (>100 Ma) volcanic rocks, in Scotland, includes groundwaters with several times the V contents for the region (MacDonald et al., 2017). However, the contents are modest compared with the values in young volcanic rocks. This implies that volcanic rocks release much of their mobile V when they are young, probably from reactive volcanic glass and unstable magmatic minerals. A further implication is that if large volumes of volcanogenic sediment can be identified in the geological record, they could have hosted V-rich groundwaters and even sediment-hosted V mineralization.

Conclusions
This review of V contents in groundwaters in volcanic terranes confirms previous implications that they are higher than in nonvolcanic terranes. In particular.
1. In several aquifers in volcanic rocks, the mean vanadium values exceed the statutory limits of some countries. 2. The anomalously large volume of volcanogenic sediment contributed from the long-term erosion of the Andes is reflected in the very high groundwater V levels in Argentina. 3. The high V contents in groundwaters in young volcanic rocks suggest that the V is liberated early in the rock history.
Faced with a possibly very big increase in the demand for vanadium to support battery manufacture, new and more environmentally acceptable technologies, and new sources of vanadium, may be required. A new landscape for the processing of vanadium must take into account the potential implications for human health. The data suggest that V-rich groundwaters may incidentally make a modest contribution to resources of the element.

Conflict of Interest
The authors declare no conflicts of interest relevant to this study.

Data Availability Statement
All data are reviewed from published literature and included in this paper.

Acknowledgments
Research was supported by the Natural Environment Research Council (grant NE/T003677/1). J. Johnston and J. Bowie provided skilled technical support. The manuscript benefitted from a constructive review.