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Composition and solubility of precipitated copper(II) arsenates
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
2011 (English)In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 26, no 5, 696-704 p.Article in journal (Refereed) Published
Abstract [en]

Equilibrium reactions involving Cu(II) and As(V) have been studied with respect to formation of complexes in aqueous solutions as well as formation of solid phases. Potentiometric titrations performed at 25 °C (I = 0.1 M Na(Cl)) and at different Cu to As ratios gave no evidence for the existence of Cu(II) arsenate complexes in solution below the pH of the precipitation boundaries (pH ≈ 4), irrespective of the Cu to As ratio and pH. Mixing of solutions of Cu(II) and As(V) at different proportions and adjusting pH to values ranging from 4 to 9 resulted in precipitation of five different solid phases. The elemental composition of the solids was determined using X-ray Photoelectron Spectroscopy, and Environmental Scanning Microscopy–Field Emission Gun equipped with an energy dispersive spectroscopy detector. The average Cu/As ratio was determined by dissolving the solids. Total soluble concentrations of the components Cu(II) and As(V), as well as the basicity of the solid phases were determined by analysis of aqueous solutions. Based upon these experimental data the stoichiometric composition of the solid phases and their stability were determined. The resulting equilibrium model includes the solid phases Cu3(AsO4)2, Cu3(AsO4)(OH)3, Cu2(AsO4)(OH), Cu5Na(HAsO4)(AsO4)3 and Cu5Na2AsO4)4, where Cu5Na(HAsO4)(AsO4)3 and Cu5Na2(AsO4)4 have not been reported previously. In 0.1 M Na(Cl), Na+ was found to be a significant component in two of the solid phases. The Cu5Na2(AsO4)4 was formed in weakly alkaline conditions with pNa < 2.5. Stability constants for all solid phases have been determined. Distribution diagrams as well as predominance area (pNa–pH) diagrams are presented to illustrate stability fields of the different solid phases.

Place, publisher, year, edition, pages
Elsevier, 2011. Vol. 26, no 5, 696-704 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:umu:diva-51506DOI: 10.1016/j.apgeochem.2011.01.028OAI: oai:DiVA.org:umu-51506DiVA: diva2:482490
Available from: 2012-01-24 Created: 2012-01-24 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Modelling precipitation and surface complexation reactions in systems with Goethite, Cu(II) and Oxyanions Containing As(V) or P(V)
Open this publication in new window or tab >>Modelling precipitation and surface complexation reactions in systems with Goethite, Cu(II) and Oxyanions Containing As(V) or P(V)
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aqueous solubility of oxyanion (e.g. phosphates and arsenates), and thereby their mobility, bioavailability (phosphates) and toxicity (arsenates), in soils and sediments is dependent upon their chemical speciation. In complex, multicomponent systems, equilibrium modelling can be a useful tool to predict chemical speciation. When establishing a model, it is essential to understand the interactions between all the components not only in solution but also on mineral surfaces at a molecular level. By applying surface complexation models processes at mineral surfaces can be accounted for.

This thesis is a summary of four papers and focuses on surface complexation of the oxyanions arsenate, phosphate and monomethyl phosphate adsorbed onto the surface of goethite (α-FeOOH). Furthermore, adsorption and precipitation of copper(II) arsenates from aqueous solutions has been studied.

Solid copper(II) arsenates obtained in precipitation experiments were characterised and five different solid phases with different Cu(II) to As(V) ratio, as well as proton and Na

+content, were identified; Cu5Na(HAsO4)(AsO4)3(s), Cu5Na2(AsO4)4(s), Cu3(AsO4)2(s), Cu3(AsO4)(OH)3(s) and Cu2(AsO4)(OH)(s). The adsorption of arsenate and copper(II) to the goethite surface, could not be predicted by only applying the combined model from the two binary systems, arsenate-goethite and copper(II)-goethite. Instead, two ternary copper-arsenate-goethite surface complexes were added. In one of the surface complexes arsenate is bound to goethite surface via a copper(II) ion coordinating to surface hydroxyl groups and in the other surface complex, copper(II) is coordinating arsenate bound to the goethite surface.

Surface complexation models, in agreement with macroscopic data and detailed spectroscopic results, were designed for monomethyl phosphate, phosphate and arsenate adsorbed to goethite. The models contain monodentate inner sphere surface complexes stabilized by hydrogen bonding to neighbouring surface sites. The charge distribution of the complexes was assigned according to Pauling’s valence bond theory.

The monomethyl phosphate model consists of three singly protonated surface isomers, only differentiated by the location of the proton . In the case of phosphate and arsenate, six surface complexes, including two pair-wise surface isomers, are suggested to form; ≡FeOAsO

32.5-; (≡FeOAsO3; ≡Fe3OH)2-;(≡FeOAsO3H; ≡Fe3O)2-; (≡FeOAsO3H; ≡Fe3OH)1-; (≡FeOAsO3H2; ≡Fe3O)1- and ≡FeOAsO3H20.5-. A combination of structural information from spectroscopic measurements and quantitative data from spectroscopy, potentiometry and adsorption experiments provides a better understanding of the complexity of the coordination chemistry of particle surfaces and forms the basis for equilibrium models with high physical/chemical relevance.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet, 2012. 36 p.
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-52970 (URN)978-91-7459-381-5 (ISBN)
Public defence
2012-03-30, KBC-huset, KB3A9, Umeå universitet, Umeå, 10:00
Opponent
Supervisors
Available from: 2012-03-09 Created: 2012-03-08 Last updated: 2012-03-08Bibliographically approved

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