Electron Transfer Budgets and Kinetics of Abiotic Oxidation and Incorporation of Aqueous Sulfide by Dissolved Organic Matter

The reactivity of natural dissolved organic matter toward sulfide and has not been well studied with regard to electron transfer, product formation, and kinetics. We thus investigated the abiotic transformation of sulfide upon reaction with reduced and nonreduced Sigma-Aldrich humic acid (HA), at pH 6 under anoxic conditions. Sulfide reacted with nonreduced HA at conditional rate constants of 0.227–0.325 h^–1. The main transformation products were elemental S (S⁰) and thiosulfate (S₂O₃^2–), yielding electron accepting capacities of 2.82–1.75 μmol e^– (mg C)⁻¹. Native iron contents in the HA could account for only 6–9% of this electron transfer. About 22–37% of S reacted with the HA to form organic S (S_org). Formation of S_org was observed and no inorganic transformation products occurred for reduced HA. X-ray absorption near edge structure spectroscopy supported S_org to be mainly zerovalent, such as thiols, organic di- and polysulfides, or heterocycles. In conclusion, our results demonstrate that HA can abiotically reoxidize sulfide in anoxic environments at rates competitive to sulfide oxidation by molecular oxygen or iron oxides

Structural Incorporation of As⁵⁺ into Hematite

Hematite (α-Fe₂O₃) is one of the most common iron oxides and a sink for the toxic metalloid arsenic. Arsenic can be immobilized by adsorption to the hematite surface; however, the incorporation of As in hematite was never seriously considered. In our study we present evidence that, besides adsorption, the incorporation of As into the hematite crystals can be of great relevance for As immobilization. With the coupling of nanoresolution techniques and X-ray absorption spectroscopy the presence of As (up to 1.9 wt %) within the hematite crystals could be demonstrated. The incorporated As⁵⁺ displays a short-range order similar to angelellite-like clusters, epitaxially intergrown with hematite. Angelellite (Fe₄As₂O₁₁), a triclinic iron arsenate with structural relations to hematite, can epitaxially intergrow along the (210) plane with the (0001) plane of hematite. This structural composite of hematite and angelellite-like clusters represents a new immobilization mechanism and potentially long-lasting storage facility for As⁵⁺ by iron oxides

Thallium Speciation and Extractability in a Thallium- and Arsenic-Rich Soil Developed from Mineralized Carbonate Rock

We investigated the speciation and extractability of Tl in soil developed from mineralized carbonate rock. Total Tl concentrations in topsoil (0–20 cm) of 100–1000 mg/kg are observed in the most affected area, subsoil concentrations of up to 6000 mg/kg Tl in soil horizons containing weathered ore fragments. Using synchrotron-based microfocused X-ray fluorescence spectrometry (μ-XRF) and X-ray absorption spectroscopy (μ-XAS) at the Tl L₃-edge, partly Tl­(I)-substituted jarosite and avicennite (Tl₂O₃) were identified as Tl-bearing secondary minerals formed by the weathering of a Tl–As–Fe-sulfide mineralization hosted in the carbonate rock from which the soil developed. Further evidence was found for the sequestration of Tl­(III) into Mn-oxides and the uptake of Tl­(I) by illite. Quantification of the fractions of Tl­(III), Tl­(I)-jarosite and Tl­(I)-illite in bulk samples based on XAS indicated that Tl­(I) uptake by illite was the dominant retention mechanism in topsoil materials. Oxidative Tl­(III)­uptake into Mn-oxides was less relevant, probably because the Tl loadings of the soil exceeded the capacity of this uptake mechanism. The concentrations of Tl in 10 mM CaCl₂-extracts increased with increasing soil Tl contents and decreasing soil pH, but did not exhibit drastic variations as a function of Tl speciation. With respect to Tl in contaminated soils, this study provides first direct spectroscopic evidence for Tl­(I) uptake by illite and indicates the need for further studies on the sorption of Tl to clay minerals and Mn-oxides and its impact on Tl solubility in soils

Uranium Redox Transformations after U(VI) Coprecipitation with Magnetite Nanoparticles

Uranium redox states and speciation in magnetite nanoparticles coprecipitated with U­(VI) for uranium loadings varying from 1000 to 10 000 ppm are investigated by X-ray absorption spectroscopy (XAS). It is demonstrated that the U M₄ high energy resolution X-ray absorption near edge structure (HR-XANES) method is capable to clearly characterize U­(IV), U­(V), and U­(VI) existing simultaneously in the same sample. The contributions of the three different uranium redox states are quantified with the iterative transformation factor analysis (ITFA) method. U L₃ XAS and transmission electron microscopy (TEM) reveal that initially sorbed U­(VI) species recrystallize to nonstoichiometric UO_2+<i>x</i> nanoparticles within 147 days when stored under anoxic conditions. These U­(IV) species oxidize again when exposed to air. U M₄ HR-XANES data demonstrate strong contribution of U­(V) at day 10 and that U­(V) remains stable over 142 days under ambient conditions as shown for magnetite nanoparticles containing 1000 ppm U. U L₃ XAS indicates that this U­(V) species is protected from oxidation likely incorporated into octahedral magnetite sites. XAS results are supported by density functional theory (DFT) calculations. Further characterization of the samples include powder X-ray diffraction (pXRD), scanning electron microscopy (SEM) and Fe 2p X-ray photoelectron spectroscopy (XPS)

Labile or Stable: Can Homoleptic and Heteroleptic PyrPHOS–Copper Complexes Be Processed from Solution?

Luminescent Cu­(I) complexes are interesting candidates as dopants in organic light-emitting diodes (OLEDs). However, open questions remain regarding the stability of such complexes in solution and therefore their suitability for solution processing. Since the emission behavior of Cu­(I) emitters often drastically differs between bulk and thin film samples, it cannot be excluded that changes such as partial decomposition or formation of alternative emitting compounds upon processing are responsible. In this study, we present three particularly interesting candidates of the recently established copper–halide–(diphenylphosphino)­pyridine derivatives (PyrPHOS) family that do not show such changes. We compare single crystals, amorphous bulk samples, and neat thin films in order to verify whether the material remains stable upon processing. Solid-state nuclear magnetic resonance (MAS ³¹P NMR) was used to investigate the electronic environment of the phosphorus atoms, and X-ray absorption spectroscopy at the Cu K edge provides insight into the local electronic and geometrical environment of the copper­(I) metal centers of the samples. Our results suggest thatunlike other copper­(I) complexesthe copper–halide–PyrPHOS clusters are significantly more stable upon processing and retain their initial structure upon quick precipitation as well as thin film processing