scholarly journals Competing Sorption of Se(IV) and Se(VI) on Schwertmannite

Minerals ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 764
Author(s):  
Bouchra Marouane ◽  
Ning Chen ◽  
Martin Obst ◽  
Stefan Peiffer

Schwertmannite (SHM) is a naturally occurring mineral that has been shown to effectively scavenge oxyanions from contaminated water. In this study, Fourier-transform infrared spectroscopy and X-ray absorption spectroscopy techniques in combination with wet-chemical techniques were used to study the competitive sorption of Se(IV) and Se(VI) at pH 3. The experiments were conducted with three types of schwertmannite obtained from oxidative synthesis, biogenic synthesis and high-pressure compaction at different initial Se concentrations and mixing ratios for 48 h and 56 days, respectively. A threshold value for the uptake mechanisms was identified, which reflects the amount of easily exchangeable sulphate (~0.5 mmol/g). At adsorbate concentrations below this threshold, an inner-sphere corner-sharing bidentate binuclear complex forms upon exchange with sulphate. At higher concentrations, both oxyanions become bound to SHM through co-occurrence of mainly inner-sphere and partly outer-sphere corner-sharing bidentate binuclear complexes with Fe(III) containing surface sites. Single species experiments clearly indicate a higher affinity of SHM for Se(IV). However, in mixed species experiments, competitive sorption occurs with equal or even preferential uptake of Se(VI) at concentrations much lower than the threshold value, presumably due to geometrical similarity between selenate and sulphate, and increasing preference for Se(IV) at high Se concentrations.

2006 ◽  
Vol 985 ◽  
Author(s):  
James L Jerden ◽  
A Jeremy Kropf

AbstractBatch adsorption experiments in which neptunium bearing solutions were reacted with goethite (alpha-FeOOH) have been performed to study uptake mechanisms in sodium chloride and calcium-bearing sodium silicate solutions. This paper presents results identifying and quantifying the mechanisms by which neptunium is adsorbed as a function of pH and reaction time (aging). Also presented are results from tests in which neptunium is reacted with goethite in the presence of other cations (uranyl and calcium) that may compete with neptunium for sorption sites. The desorption of neptunium from goethite has been studied by resuspending the neptunium-loaded goethite samples in solutions containing no neptunium. Selected reacted sorbent samples were analyzed by x-ray absorption spectroscopy (XAS) to determine the oxidation state and molecular speciation of the adsorbed neptunium. Results have been used to establish the pH adsorption edge of neptunium on goethite in sodium chloride and calcium-bearing sodium silicate solutions. The results indicate that neptunium uptake on goethite reaches 95% at a pH of approximately 7 and begins to decrease at pH values greater than 8.5. Distribution coefficients for neptunium sorption range from less than 1000 (moles/kg)sorbed / (moles/kg)solution at pH less than 5.0 to greater than 10,000 (moles/kg)sorbed / (moles/kg)solution at pH greater than 7.0. Distribution coefficients as high as 100,000 (moles/kg)sorbed / (moles/kg)solution were recorded for the tests done in calcite equilibrated sodium silicate solutions. XAS results show that neptunium complexes with the goethite surface mainly as Np(V) (although Np(IV) is prevalent in some of the longer-duration sorption tests). The neptunium adsorbed to goethite shows Np-O bond length of approximately 1.8 angstroms which is representative of the Np-O axial bond in the neptunyl(V) complex. This neptunyl(V) ion is coordinated to 5 or 6 equatorial oxygens with Np-O bond lengths of 2.45 angstroms. The absence of a clearly recognizable Np-Fe interaction for the sodium chloride sorption tests suggests that neptunium in these solutions adsorbs as an outer-sphere complex. XAS results from the calcium-bearing sodium silicate sorption tests show evidence for a neptunyl(V) inner-sphere surface complex with a Np-Fe interaction at 3.5 angstroms. Desorption tests indicate that samples in which neptunium is bound as inner-sphere complexes show significant sorption hysteresis relative to samples in which neptunium is bound largely as outer-sphere complexes.


2009 ◽  
Vol 74 (10) ◽  
pp. 1543-1557 ◽  
Author(s):  
Herman P. Van Leeuwen ◽  
Raewyn M. Town

The degree of (de)protonation of aqueous metal species has significant consequences for the kinetics of complex formation/dissociation. All protonated forms of both the ligand and the hydrated central metal ion contribute to the rate of complex formation to an extent weighted by the pertaining outer-sphere stabilities. Likewise, the lifetime of the uncomplexed metal is determined by all the various protonated ligand species. Therefore, the interfacial reaction layer thickness, μ, and the ensuing kinetic flux, Jkin, are more involved than in the conventional case. All inner-sphere complexes contribute to the overall rate of dissociation, as weighted by their respective rate constants for dissociation, kd. The presence of inner-sphere deprotonated H2O, or of outer-sphere protonated ligand, generally has a great impact on kd of the inner-sphere complex. Consequently, the overall flux can be dominated by a species that is a minor component of the bulk speciation. The concepts are shown to provide a good description of experimental stripping chronopotentiometric data for several protonated metal–ligand systems.


RSC Advances ◽  
2021 ◽  
Vol 11 (35) ◽  
pp. 21359-21366
Author(s):  
Debabrata Chatterjee ◽  
Marta Chrzanowska ◽  
Anna Katafias ◽  
Maria Oszajca ◽  
Rudi van Eldik

[RuII(edta)(L)]2–, where edta4– =ethylenediaminetetraacetate; L = pyrazine (pz) and H2O, can reduce molecular oxygen sequentially to hydrogen peroxide and further to water by involving both outer-sphere and inner-sphere electron transfer processes.


2021 ◽  
Vol 520 ◽  
pp. 120315
Author(s):  
Svetlana E. Korolenko ◽  
Aleksey S. Kubasov ◽  
Lyudmila V. Goeva ◽  
Varvara V. Avdeeva ◽  
Elena A. Malinina ◽  
...  

2020 ◽  
Vol 2020 (1) ◽  
Author(s):  
Xiangjun Dai ◽  
Suli Wang ◽  
Weizhi Xiong ◽  
Ni Li

Abstract We propose and study a stochastic delay single-species population system in polluted environment with psychological effect and pulse toxicant input. We establish sufficient conditions for the extinction, nonpersistence in the mean, weak persistence, and strong persistence of the single-species population and obtain the threshold value between extinction and weak persistence. Finally, we confirm the efficiency of the main results by numerical simulations.


2017 ◽  
Vol 18 (2) ◽  
pp. 737-744 ◽  
Author(s):  
Efthimia Kaprara ◽  
Fani Pinakidou ◽  
Eleni C. Paloura ◽  
Anastasios I. Zouboulis ◽  
Manassis Mitrakas

Abstract In this study, the implementation of an iron oxy-hydroxide (FeOOH) as a surface catalyst for Cr(VI) reduction by inorganic sulfur reductants (ISRs) was investigated. Batch Cr(VI) removal tests, performed to evaluate and compare the efficiency of ISRs in the presence of FeOOH, qualified Na2S2O4 as the optimum for drinking water treatment. Application of Na2S2O4 in continuous flow rapid small scale column tests, using a FeOOH adsorbent at pH 7 ± 0.1 and artificial (resembling natural) water matrix, verified the high potential for Cr(VI) removal at sub-ppb level. Indeed, a 15 mg S/L Na2S2O4 dose diminished an initial Cr(VI) concentration of 100 μg/L below the method's detection limit of 1.4 μg/L at least for 105 bed volumes. X-ray absorption fine structure spectroscopy revealed that Cr(VI) forms outer sphere complexes, while Cr(III) is involved in 2E, 2C and 1 V geometries with the surface Fe-oxyhydroxyl groups. It can, therefore, be concluded that FeOOH attracts Cr(VI) to its surface via physisorption, offering a solid surface that promotes the transfer of electrons through bridging ions. Thus, when Na2S2O4 is added in the system, Cr(VI) is reduced to Cr(III), which is subsequently chemisorbed onto the FeOOH surface.


2018 ◽  
Vol 57 (4) ◽  
pp. 2085-2095 ◽  
Author(s):  
Samira M. Abozeid ◽  
Eric M. Snyder ◽  
Timothy Y. Tittiris ◽  
Charles M. Steuerwald ◽  
Alexander Y. Nazarenko ◽  
...  
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