Regional occurrence of aqueous tungsten and relations with antimony, arsenic and molybdenum concentrations (Sardinia, Italy)

Highlights

• Tungsten concentrations occurred in a range of <0.01 to 140 μg L⁻¹ in Sardinian waters.

• Maximum concentration of W occurred under slight alkaline pH and oxygenated conditions in mine waters.

• High concentrations of W were observed under alkaline pH and reducing conditions in thermal waters.

• High temperature and alkaline pH appear to enhance the W mobility in aquatic systems.

• High concentrations of W sometimes coincided with relatively high concentrations either of Sb, As or Mo.

Abstract

Tungsten (W) is rarely found in natural waters, yet it can be introduced into the food chain and cause potentially toxic effects. Uptake of W by plants and vegetables, or trace presence of W in drinking water are possible vectors for ingestion of W by humans. The latter is recognized as a possible cause of lymphatic leukemia. Increased uses of W might result in a degradation of water resources, with attendant adverse effects on biota and human health. Therefore, this study was aimed at investigating regional occurrence and speciation of W in aquatic systems in Sardinia, Italy, factors affecting W mobility and possible relations with other oxyanion-forming trace elements such as Sb, As and Mo. Although our results are specifically from Sardinia, the implications are broader and should prompt future studies in other areas with known high W concentrations.

A total of 350 sample sites are reported here, including surface waters, groundwaters, mine drainages, thermal waters and local seawater. The waters were analyzed for major and trace components, including W, Sb, As and Mo. The waters showed a variety of major chemical compositions and W concentrations. High concentrations of W were found in some mine waters and drainages from slag heaps, with W, Sb and As up to 140, 5000 and 800 μg L⁻¹, respectively. The highest concentrations of W occurred under slightly alkaline pH and oxygenated conditions, and were likely due to the dissolution of scheelite [CaWO₄] hosted in materials with which the water came into contact. High W concentrations also were observed in thermal waters, under alkaline pH and reducing conditions, and sometimes coincided with relatively high concentrations either of As or Mo.

Previous studies of W geochemistry have focused on WO₄²⁻ as the major dissolved form of W. For this study, we have augmented the thermodynamic database in PHREEQC to include possible formation of many other W-bearing complexes gleaned from the literature. The results of the speciation calculations with the newly added complexation reactions shows that the neutral species CaWO₄° and MgWO₄° are particularly dominant in most W-bearing waters and lead to undersaturation with respect to scheelite and other W-bearing minerals.

Assessing W contamination in water systems and establishing W limits in drinking water may prevent potential adverse effects of W on human and ecosystem health.

Introduction

The element tungsten (W) is a transition metal mainly hosted in the minerals scheelite [CaWO₄] and wolframite [(Fe, Mn)WO₄]. Industrial applications of W include the preparation of several alloys, filaments for incandescent and fluorescent bulbs, X-ray instruments, optical lenses, parts of cars, aircrafts, telephones, radars and weapons (Koutsospyros et al., 2006, Koutsospyros et al., 2019). The increased use of W (Clausen and Korte, 2009; Wang et al., 2016) implies its increased availability to the environment with potential negative impacts on biota (Hsu et al., 2011; Kennedy et al., 2012; Tuna et al., 2012; Leffler and Kazantzis, 2015).

The crustal abundance of W is 1.9 mg kg⁻¹ (Rudnick and Gao, 2014). Concentrations of W in some European soils vary from 0.01 to 3 mg kg⁻¹ (Reimann et al., 2015) and may reach 57 mg kg⁻¹ in W-mineralized areas (Candeias et al., 2014). Concentrations of W in uncontaminated, cold waters are usually at sub-μg L⁻¹ level (Johannesson and Tang, 2009; Johannesson et al., 2013). Average values of 0.1 μg L⁻¹ W in world rivers (Gaillardet et al., 2003), 0.085 μg L⁻¹ W in Italian bottled water (Dinelli et al., 2012), and 0.010 μg L⁻¹ W in seawater (White, 1998) have been reported. Concentrations up to 742 μg L⁻¹ W have been observed in thermal waters (Seiler et al., 2005). Concentrations of W up to 7.1 mg L⁻¹ have been reported in interstitial waters in mining wastes, likely due to the dissolution of wolframite (Petrunic and Al, 2005).

The geochemical behavior of W is poorly known although it bears similarities to that of molybdenum (Mo), which lies immediately above W in the periodic table of elements (Mohajerin et al., 2016). However, the mobility of W is considerably lower than that of Mo in weathering and transport processes (Gaillardet et al., 2003). Differences in chemical properties may account for fractionation of Mo with respect to W in estuarine environments. In sulfidic pore waters, dissolved Mo is depleted relative to river water–seawater mixtures, whereas dissolved W is enriched by >10-fold; reductive dissolution of poorly crystalline phases like ferrihydrite, which is a preferential host of W relative to Mo, can explain the dissolved W enrichment (Mohajerin et al., 2016).

Experimental results carried out at 300–600 °C and 0.5–2 kbar showed that the solubility of scheelite as a simple oxyacid species [H₂WO_4(aqueous) under acid pH] can theoretically attain W values between 10 and 100 ppm under geologically reasonable conditions, and complexation by additional ligands is not required for W transport by hydrothermal solutions (Wood, 1992). The solubility of W minerals may be further increased by ion pairing of alkali metal cations with tungstate or chloride complexation of Fe, Mn (wolframite), or Ca (scheelite) (Wood, 1992).

In oxic environments, aqueous W is expected to occur as WO₄²⁻ under the pH conditions observed in most waters. Several studies demonstrate that the ion WO₄²⁻ is mobile under oxic conditions and high pH in natural waters (e.g., Seiler et al., 2005; Bednar et al., 2009; Johannesson and Tang, 2009). Data and geochemical modeling demonstrated that WO₄²⁻ can be mobile in anoxic groundwater too (e.g., Johannesson et al., 2013). Experimental studies indicated a maximum leaching of W from soils under neutral to alkaline pH conditions (Bednar et al., 2009). The WO₄²⁻ species is strongly sorbed to Fe(III) oxides/oxyhydroxides and other sorbents under low to circumneutral pH, and desorbs from mineral surface sites at higher pH (Johannesson et al., 2013). The presence of natural organic matter, such as humic acids, may decrease the WO₄²⁻ adsorption rate on ferrihydrite, thus favoring its mobilization (Du et al., 2020). The formation of thiotungstate ions and polymerization processes may allow the persistence of W in solution under a large range of pH and temperature (Strigul, 2010; Mohajerin et al., 2014). The formation of thiotungstate species may also be important in some anoxic waters (Johannesson et al., 2013). Nearly conservative behavior of W in alkaline groundwater from a shallow, unconsolidated and fractured tuffaceous aquifer in southern Nevada has been reported (Johannesson and Tang, 2009).

Information on the effects of W on biota is scarce, however, a few studies have indicated that W can have toxic effects on human health. In particular, the ingestion of W has been suggested as a possible cause of lymphatic leukemia (Witten et al., 2012; Koutsospyros et al., 2019), attracting special attention of the US Environmental Protection Agency in 2011 (Lemus and Venezia, 2015). Kennedy et al. (2012) found significant bioaccumulation of W in certain varieties of cabbages and snails, with toxic or lethal effects observed at the highest concentration levels. The World Health Organization has not set a guide value for W in drinking water (WHO, 2011). Likewise, W in drinking water is not regulated in Italy, nor the W concentrations in soils and areas to be reclaimed (GURI, 2006).

Despite the efforts undertaken by some researchers, further studies are required to fully understand the geochemical behavior of W in natural environments (Koutsospyros et al., 2019). This study was aimed at assessing the occurrence of W in aqueous solutions at a regional level in Sardinia (Italy). An area of particular interest included in the study is the Su Suergiu abandoned mine (Gerrei district), which exploited antimonite [Sb₂S₃] and scheelite ore hosted in carbonate and quartz gangue. The deposit was the largest in Italy and it was mined until the 1960s. Mining and metallurgical processing residues were dumped near the mine and have caused measurable Sb and As contamination in the aquatic system (Cidu et al., 2014, Cidu et al., 2018), but W contamination has not been assessed so far. Specific objectives of this study were to: i) evaluate the regional occurrence of aqueous W and assess the W contamination at Su Suergiu; ii) compare the W occurrence with other oxyanion-forming trace elements such as Sb, As and Mo; iii) perform speciation-solubility calculations for W; iv) estimate factors that control the transport and fate of W in aquatic systems.