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Speciation of aluminum in soils and stream waters: The importance of organic matter
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
2015 (English)In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 417, 32-43 p.Article in journal (Refereed) Published
Abstract [en]

Aluminum (Al) is the most common metal in the Earth's crust, and exists mainly in the form of silicates, oxides and hydroxides, or in complexes with natural organic matter (NOM) or inorganic ligands. Since speciation is a key factor for understanding the environmental impact of Al, it is important to determine the chemical forms of Al that predominate in different natural media and under varying geochemical conditions. This study investigated how complexation with NOM influenced the speciation of Al(III) in different environmental samples (isolated aquatic NOM, organic soils and stream waters) using X-ray absorption spectroscopy (XAS) and Infrared (IR) spectroscopy. The combined spectroscopic results showed that Al(III) formed mononuclear complexes with carboxylic functional groups in NOM that were sufficiently stable to suppress the hydrolysis and polymerization of Al(III). These organic Al complexes were favored at slightly acidic pH values between pH 3 and 6 and at decreasing Al concentrations. The XAS results of the organic soils and the stream water samples indicated a variation in the speciation from a predominance of organically complexed Al in the stream waters to a mixture of Al-NOM complexes and precipitated Al phases (Al hydroxides and/or Al silicates) in the organic soils. Although the presented XAS results were limited by relatively low signal-to-noise caused by the low K-edge energy of Al, the combined spectroscopic results provided new and useful information about Al in different environmental samples and showed that NOM and pH are important parameters controlling the speciation of Al.

Place, publisher, year, edition, pages
2015. Vol. 417, 32-43 p.
Keyword [en]
Aluminum, Organic matter, Soil, Stream water, XAS, IR spectroscopy
National Category
Geochemistry
Identifiers
URN: urn:nbn:se:umu:diva-102843DOI: 10.1016/j.chemgeo.2015.09.012ISI: 000365098800004OAI: oai:DiVA.org:umu-102843DiVA: diva2:810537
Note

Originally included in thesis in submitted form.

Available from: 2015-05-07 Created: 2015-05-07 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Characterization of aluminum in environmental systems using X-ray absorption and vibrational spectroscopy: The importance of organic matter
Open this publication in new window or tab >>Characterization of aluminum in environmental systems using X-ray absorption and vibrational spectroscopy: The importance of organic matter
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The fate and behavior of many metals in the environment are highly dependent on interactions with natural organic matter (NOM), which is abundant in most soils and surface waters. The complexation with NOM can influence the speciation of the metals by affecting their hydrolysis and solubility. This in turn will also have an effect on the mobility and potential toxicity of the metals. For aluminum (Al) these interactions are of high environmental importance since Al have been shown to have negative effects on plant growth, water living organisms, and fish.

This thesis will focus on the interactions between Al(III) and NOM in different environments and under varying geochemical conditions. To study this, infrared (IR) spectroscopy and X-ray absorption spectroscopy (XAS) have primarily been used. Due to the difficulties in analyzing Al using XAS, gallium(III), shown to be a suitable analogue for Al(III), was used as a probe to get complementary information from the Ga(III)-NOM system. The combined results from these studies showed that Ga(III) and Al(III) formed strong chelate complexes with carboxylic groups in NOM and that these complexes were strong enough to suppress the hydrolysis and polymerization of the metals. Furthermore, Al in organic soil and stream water samples was also studied using XAS and the results showed a variation in the speciation from a predominance of organically complexed Al(III) in the stream waters to a mixture of Al(III)-NOM complexes and precipitated Al phases (Al-hydroxides and/or Al-silicates) in the organic soils. To further study mineral-NOM interactions the effects of NOM on the dissolution of gibbsite (g-Al(OH)3(s); a common mineral in the environment) were investigated. The results showed that NOM can promote mineral dissolution and presence of inner-sphere Al(III)-NOM species on the gibbsite surface, detected by IR spectroscopy, could indicate a ligand induced dissolution. To further investigate the structure of the complex formed at the surface of the mineral, an EXAFS study was conducted on the ternary Ga(III)-NOM-gibbsite system. The results indicated either formation of inner-sphere complexes with Ga(III) acting like a bridge between NOM and the gibbsite surface, or the presence of two separate species; Ga(III)-NOM complexes in solution and a precipitated Ga(OH)3(s) phase.

As a sidetrack to the Al(III)-NOM studies, a new way of characterizing NOM was developed using simultaneous infrared and potentiometric titrations, multivariate data analysis, and chemical equilibrium modeling. An acid/base model for a fulvic acid was constructed, based on spectroscopic information about functional groups and their pKa values, and indicated that the fulvic acid is to be regarded as a tetra carboxylic acid consisting of at least four fractions of carboxylic acids. This demonstrates new possibilities to study the acid/base and metal complexing properties of NOM, in which the presence of carboxylic acid groups predominate, and to design equilibrium models more reliable than presented before.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2015. 76 p.
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-102847 (URN)978-91-7601-277-2 (ISBN)
Public defence
2015-06-04, Lilla Hörsalen (KB3A9), KBC, Umeå Universitet, Umeå, 14:01 (English)
Opponent
Supervisors
Available from: 2015-05-12 Created: 2015-05-07 Last updated: 2015-05-12Bibliographically approved

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