The thermodynamics of metal-complex and ion-pair formation

“…These findings also corroborate that W(VI) adsorption is suppressed in the presence of HA. The entropy changes (ΔS) can further provide information on the coordination environments of the adsorbed W oxyanions, i.e., inner-sphere complex formation commonly generates a positive ΔS while outer-sphere complex formation produces a negative ΔS because the former displaces solvating water molecules from the adsorption coordination sites, whereas the latter does not (Nancollas, 1970;Gorman-Lewis et al, 2006;Du et al, 2017). Our ΔS data (12.96-15.03 J/mol/K) therefore provide clear evidences for the formation of inner-sphere type of W species on Fh and Fh-HA surfaces.…”

Section: Thermodynamic Adsorption Characteristicsmentioning

confidence: 99%

Natural organic matter decreases uptake of W(VI), and reduces W(VI) to W(V), during adsorption to ferrihydrite

et al. 2020

Chemical Geology

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Tungsten is both naturally occurring and an anthropogenically released contaminant metal in soils, sediments and water systems that typically exits as the soluble tungstate oxyanions, W(VI)O4 2− . Tungsten mobility and fate are strongly dependent on the adsorption of tungstate to mineral surfaces. However, environmental mineral surfaces are commonly coated with natural organic matter (NOM), and the role of this coating in the tungsten adsorption process, and thus in controlling tungsten reactivity and transport, is unclear. This study investigates W(VI) adsorption to ferrihydrite (Fh), a ubiquitous iron (hydr)oxide in soils and sediments, both in the absence and presence of humic acid (HA), a widely occurring type of NOM, using batch experiments coupled with spectroscopic and thermodynamic techniques. Kinetic results indicate that access to the adsorption sites for W(VI) on the organomineral surfaces is limited when Fh is coprecipitated with HA. Commensurate with this observation, batch experiments show that HA decreases W(VI) adsorption to Fh over a wide pH range (4-11), and this inhibitory effect is more pronounced at higher HA concentration. X-ray photoelectron spectroscopy (XPS) measurements demonstrate the formation of inner-sphere type W complexes on both the Fh and HA fraction of the Fh-HA binary composite. In particular, ~40% of the adsorbed W(VI) species is reduced to W(V) in the presence of HA. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) results show the presence of poly tungstate species on Fh, particularly at lower pH and in the presence of HA. Isothermal titration calorimetry shows that W(VI) adsorption to Fh is an exothermic process both in the presence and absence of HA, and that process is accompanied by a positive entropy. The findings of this work suggest that NOM not only mobilizes tungstate but also reduces tungstate from W(VI) to W(V) at environmental iron (hydr)oxide-water interfaces, which is of significance for evaluating the migration and bioavailability of tungsten in both natural and contaminated environments.

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“…In spite of thermodynamic studies on several metal dicarboxylate complexes (9,10) no systematic investigations have been carried out on comparable complexes originating from the association of the cupric ion with the dicarboxylic acids reported here.…”

Section: Introductionmentioning

confidence: 88%