Tungsten–molybdenum fractionation in estuarine environments

Abstract

Dissolved tungsten (W) and molybdenum (Mo) concentrations were measured in surface waters and sediment pore waters of Terrebonne Bay, a shallow estuary in the Mississippi River delta, to investigate the biogeochemical processes that fractionate these Group 6 elements relative to one another during transit from weathering to sedimentary environments. Although many of the chemical properties of W and Mo are similar, the two elements behave autonomously, and the fractionation mechanisms are only partly understood. In sulfidic pore waters, dissolved Mo is depleted relative to river water-seawater mixtures, whereas dissolved W is >10-fold enriched. Reductive dissolution of poorly crystalline phases like ferrihydrite, which is a preferential host of W relative to Mo in grain coatings on river-borne particles, can explain the dissolved W enrichment. Dissolved W becomes increasingly enriched as H 2 S(aq) rises above about 60 μM due to transformation of WO 4 2to thiotungstates as well as to additional reductive dissolution of phases that host W. In contrast, as rising sulfide transforms MoO 4 2to thiomolybdates in pore waters, dissolved Mo is suppressed, probably owing to equilibration with an Fe-Mo-S phase. This putative phase appears to control the aqueous ion product, Q = [Fe 2+ ][MoS 4 2-] 0.6 [H 2 S 0 ] 0.4 /[H + ] 0.8 , at a value of 10-7.78. Concentrations of dissolved W and Mo in pore waters bear no relation to concentrations in surface waters of the same salinity. In surface waters, dissolved Mo is nearly conserved in the estuarine mixing zone. Dissolved W appears also to be conserved except for several cases where W may have been enhanced by exchange with underlying, W-rich pore waters. With increasing salinity, the molar Mo/W ratio rises from about 10 to about 1000 in surface waters whereas it is mostly <10 in underlying pore waters and in highly sulfidic pore waters is mostly near 1. Differences in two chemical properties may account for this fractionation of Mo with respect to W; Mo VI is more susceptible to reduction than W VI , but the latter is more prone to adopting octahedral coordination. We propose that the first difference facilitates Mo but not W precipitation in sulfidic sediments, whereas the second explains tungsten's proportionately greater sequestration on river-borne particles and its subsequent release to sulfidic pore waters after the particles are deposited in the delta and become subject to reductive dissolution during diagenesis. Electronic Annex Click here to download Electronic Annex: Revised Electronic Annex_KHJ.docx *Revised Manuscript (marked MS Word) Click here to download Source or Other Companion File: Revised_GRH_KHJ.docx

Ultra-trace Element Characterization of the Central Ottawa River Basin using a Rapid, Flexible, and Low-volume ICP-MS Method

“…The consistently supercrustal Mo/W ratios of the ORB waters (12.0-74.5) cover a similar range to other river waters measured to date (Mohajerin et al 2016, Bauer et al 2018. The entire Ottawa River T-C transect has supercrustal Mo/W ratios with minimal downstream variation, suggesting that the signature is inherited primarily from processes in aquatic environments nearer the source, or in soils.…”

“…However, Mo>W removal to sediment also occurs in sulfidic environments (Bauer et al 2017). Prior to the oceans, decoupling of Mo-W can be traced across estuaries where Mo behaves more conservatively than W (van der Sloot et al 1985, Bauer et al 2018 as well as into terrestrial environments as indicated by river waters with Mo/W ratios ranging from 0.3-68 (Mohajerin et al 2016, Bauer et al 2018. Terrestrial fractionation appears to begin in weathering profiles, but relative Mo-W mobility is probably controlled initially by protolith mineralogy (Greaney et al 2018) as evident by preferential W>Mo release recorded in a basaltic saprolite .…”

Rare earth element behavior during groundwater–seawater mixing along the Kona Coast of Hawaii

“…Dissolved organic carbon (DOC) concentrations were quantified by high temperature combustion using a Shimadzu TOC-500 total carbon analyzer as described previously (Burdige and Gardner, 1998). Dissolved Fe and Mn concentrations were measured by HR-ICP-MS following methods described by Roy et al (2010) and Mohajerin et al (2016).…”

Transition Metal Dichalcogenides for the Application of Pollution Reduction: A Review

“…Lastly but importantly, there is a large variety for TMDCs (about 40 kinds) and they have an abundant amount in nature or can be synthesized [9]. So far, MoS 2 , WS 2 , MoSe 2 , and ReS 2 have been naturally found [30][31][32]. Specifically, MoS 2 exists as molybdenite in nature and is the main source of molybdenum with a large amount [33].…”

“…The application of TMDCs in pollution reduction is a breakthrough as has high availability [ 30 , 31 , 32 ], high economic potential [ 280 , 281 , 282 ], and excellent efficiency [ 254 , 276 ] for the application of pollution reduction. For TMDCs availability and efficiency, we have already discussed them in Section 1 and Section 2 , Section 3 , Section 4 , Section 5 and Section 6 (depending on application) respectively.…”