Modelling precipitation and surface complexation reactions in systems with Goethite, Cu(II) and Oxyanions Containing As(V) or P(V)

University dissertation from Umeå : Umeå Universitet

Abstract: 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; ≡FeOAsO32.5-; (≡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.

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