Speciation of aluminum in soils and stream waters: The importance of organic matter

doi:10.1016/j.chemgeo.2015.09.012

Chemical Geology

Volume 417, 6 December 2015, Pages 32-43

https://doi.org/10.1016/j.chemgeo.2015.09.012 Get rights and content

Highlights

•
Al(III) form mononuclear complexes with carboxylic groups in organic matter.
•
Interactions with organic matter limit the hydrolysis and polymerization of Al.
•
The stream waters contain substantial amounts of organic Al complexes.
•
The organic soils contain both organic Al complexes and solid Al phases.
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Properties and concentration of organic matter and pH control the speciation of Al.

Abstract

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.

Introduction

Aluminum (Al) is one of the most abundant elements on Earth and occurs mainly in the form of sparingly soluble silicates, oxides and hydroxides, but can also be found in complexes with organic and inorganic ligands in environmental solutions (Ščančar and Milačič, 2006). These different forms, or species, of Al have different properties with respect to mobility, bioavailability and toxicity. Some of the Al species are of environmental concern; for example elevated concentrations of soluble forms of Al have been observed to have negative effects on the growth of plants (Rout et al., 2001) and acute toxic effects on fresh water fish (Polèo et al., 1997). Al can be released from naturally occurring oxides and hydroxides into the aqueous environment as a result of both acidification and elevated pH of soils and surface waters because of its amphoteric nature (Al solubility increases under acidic (pH < 5) or alkaline (pH > 8) conditions). Furthermore, Al is also widely used in society and this adds additional anthropogenic input to the environment. For example, a common method for removing particles and colloids (including organic matter) from drinking water sources is by precipitation using Al based salts (Edzwald, 1993). Thus, the risks with high Al concentrations in the environment need to be addressed and an Environmental Quality Standard (EQS) value for Al in fresh water, drinking water, and wastewaters is greatly needed (Gardner et al., 2008).

Since speciation is a key factor influencing the adverse effects of Al, it is vital to determine its chemical forms in different natural media and under varying geochemical conditions. In this respect natural organic matter (NOM) is of importance as it is known to form strong complexes with Al (Tipping et al., 1995, Smith and Kramer, 1999, Ferro-Vázquez et al., 2014) and is abundant in most soils and surface waters. A number of different chemical equilibrium models have been developed and used to describe metal (including Al) interactions with NOM in soils and waters under varying conditions such as pH and metal concentration as well as concentration and chemical properties of the organic material (e.g., Kinniburgh et al., 1999, van Hees et al., 2001, Tipping et al., 2002, Weng et al., 2002, Sjöstedt et al., 2010, Tipping and Carter, 2011). However, these models are often based on macroscopic data alone and do not necessarily reflect the distribution of actual chemical species present in the system. This may limit the applicability of the models to capture the actual Al species of importance in environmental processes as well as to predict the effects of changing environmental conditions on metal speciation. Moreover, a proper description of Al(III)–NOM interactions is needed in order to understand the competition between Al and other metals (e.g., Pb, Cd, Cu) for binding sites in organic matter (Pinheiro et al., 2000, Chappaz and Curtis, 2013). Since most studies of Al interactions with NOM in soils and waters have utilized macroscopic techniques (e.g., Weng et al., 2002, Ferro-Vázquez et al., 2014) further research using molecular probes are motivated. Molecular-scale information on metal–NOM interactions can be obtained using different spectroscopic techniques, such as nuclear magnetic resonance (NMR), fluorescence and infrared (IR) spectroscopy (Ščančar and Milačič, 2006) as well as synchrotron-based X-ray absorption spectroscopy (XAS) (e.g., Skyllberg et al., 2006, Manceau and Matynia, 2010). XAS and IR, which are complementary techniques, have been shown to be very useful in this respect (Karlsson and Persson, 2012, Orsetti et al., 2013, Gustafsson et al., 2014, Hagvall et al., 2014) and in a few cases these techniques have been used to characterize Al in different environmental samples (e.g., Ildefonse et al., 1998, Doyle et al., 1999, Elkins and Nelson, 2002, Xu et al., 2010, Jones et al., 2011). To our knowledge, there are no previous XAS studies on Al(III)–NOM interactions but some infrared spectroscopic results from Al(III)–fulvic acid systems have been published (e.g., Patterson et al., 1992, Elkins and Nelson, 2002). These studies suggested that Al interacted with salicylic and phthalic-acid like sites of the fulvic acid. Furthermore, XAS (Hay and Myneni, 2010) and IR spectroscopy (Clausén et al., 2003, Clausén et al., 2005) studies on systems containing Al(III) and well defined organic ligands like oxalate, citrate and salicylate have demonstrated the applicability to study the interactions between these species.

The limited number of XAS studies on Al in environmental samples is likely due to the technical difficulties caused by the low K-edge energy of Al (1.5596 keV) that results in strong scattering and absorption from air and light elements present in the samples. Special conditions such as helium atmosphere or vacuum are necessary, and typically only highly concentrated samples can be analyzed. This limits the possibilities to analyze the extended X-ray absorption fine structure (EXAFS) region of the XAS spectrum and as a result most studies on Al systems focus on the X-ray absorption near edge structure (XANES) region. Gallium (Ga; K-edge at 10.367 keV) on the other hand is readily accessible to both EXAFS and XANES and has been shown to be a suitable analog for Al due to the similar coordination chemistries in association with organic ligands and NOM (Clausén et al., 2003, Clausén et al., 2005, Hagvall et al., 2014). Recent EXAFS results (Hagvall et al., 2014) showed that Ga(III) formed mononuclear chelate complexes with NOM that suppressed the hydrolysis and polymerization of Ga(III), and these results were also in general agreement with previous studies on Fe(III)–NOM interactions (Karlsson and Persson, 2010, Karlsson and Persson, 2012).

The present study was conducted to investigate the distribution of Al(III) between NOM complexes and inorganic forms in different environmental samples such as isolated aquatic NOM, organic soils and stream waters, and thereby to assess the influence of NOM on Al(III) speciation. The two main objectives of the study were to determine the extent of Al(III)–NOM complexation and to identify the functional groups participating in the Al(III)–NOM interactions. To accomplish this we have used the complementary XAS and IR spectroscopy techniques to probe the local Al(III) structures and changes of the functional group chemistry of NOM, respectively. The results have been compared to the previously studied Fe(III)– and Ga(III)–NOM systems.

Section snippets

Samples and sample preparation

The natural organic matters used in this study (Suwannee River Natural Organic Matter (SRN, 1R101N) and Suwannee River Fulvic Acid (SRFA, 1S101F)) were purchased from the International Humic Substance Society (IHSS) (for materials descriptions see the Supplementary data, Table S1). Both materials are collected from the Suwannee River, which is classified as a blackwater river, having a dissolved organic carbon (DOC) concentration of 25–75 mg/L which lowers the pH below 4.0 (Averett et al., 1994).

IR spectroscopy

The IR and potentiometric titration data showed that the Al(III)–SRN and Al(III)–SRFA systems behaved similarly in the studied pH range. We will focus on the results for the Al–SRN system while the Al–SRFA results will be presented in the Supplementary data (Figs. S1, S2 and S3). The IR spectra of SRN in the absence and presence of Al as a function of pH (Fig. 1; for the full spectra series see Fig. S3, Supplementary data) were dominated by features mainly originating from carboxylic functional

Complexation of Al(III) by NOM

Our IR spectroscopic results showed that Al(III) mainly interacted with carboxylate groups of the SRN and SRFA under the current experimental conditions. This was revealed in the IR spectra as a loss of carbonyl intensity relative to the asymmetric carboxyl band and a concomitant shift of the latter in the pH range 3 to 6 upon addition of Al to NOM (Fig. 1 and Figs. S1 and S2, Supplementary data). Thus, Al was able to outcompete some of the carboxylic protons and coordinate directly to the

Conclusions and environmental implications

The collective spectroscopic results of the present study showed that Al(III)–NOM complexes were similar to those of Ga(III)– and Fe(III)–NOM (Karlsson and Persson, 2012, Hagvall et al., 2014), and that organic Al complexes were favored in the slightly acidic pH range between pH 3 and 6 and by decreasing Al/R–COOH ratios. These similarities support the conclusion that also Al forms mononuclear chelate complexes with carboxylic groups in NOM, and that these complexes are sufficiently stable to

Acknowledgments

We acknowledge the French synchrotron SOLEIL for provision of synchrotron radiation facilities (proposal 20110841), and we greatly appreciate the help and advice from Delphine Vantelon during data collection at the LUCIA beamline. The IR spectroscopy measurements were conducted at the Vibrational Spectroscopy Platform at Umeå University and Janice Kenney and Andras Gorzsas are greatly acknowledged for their contribution in the development of the new IR titration setup. Ulf Skyllberg is

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Speciation of aluminum in soils and stream waters: The importance of organic matter

Highlights

Abstract

Introduction

Section snippets

Samples and sample preparation

IR spectroscopy

Complexation of Al(III) by NOM

Conclusions and environmental implications

Acknowledgments

Geochim. Cosmochim. Acta

J. Mol. Struct.

J. Inorg. Biochem.

Coord. Chem. Rev.

Geoderma

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Chem. Geol.

Colloids Surf. A: Physicochem. Eng. Aspects

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Sci. Total Environ.

Geochim. Cosmochim. Acta

Environ. Pollut.

Geochim. Cosmochim. Acta

Water Res.

Environ. Int.

Chem. Geol.

Sci. Total Environ.

Geochim. Cosmochim. Acta

Coord. Chem. Rev.

Sci. Total Environ.

Water Res.

Geochim. Cosmochim. Acta

Humic substances in the Suwannee River, Georgia: interactions, properties, and proposed structures

The Hydrolysis of Cations