Binding of Pb(II) in the system humic acid/goethite at acidic pH
Introduction
Natural colloids such as humic acids (HA) and iron oxides are an important part of soils as well as other organic matter, aluminium and other metal oxides, and clays (Sparks, 1995, Jones and Bryan, 1998, Helmke, 1999). They have a central role in defining the fate of foreign species, such as pollutants, contributing in great measure to contaminant fixation. Metallic cations, such as Pb (II), are commonly found in soils as a result of human activity, being sorbed (or binded) to both kind of materials under mechanisms which are still under study.
A number of studies have evaluated the adsorption of metal cations onto oxide particles (Zachara and Westall, 1999, Kovačević et al., 2000, Hiemstra and Van Riemsdijk, 2002, Bradl, 2004) and the binding to humic substances (Tipping and Hurley, 1992, Benedetti et al., 1995, Kinniburgh et al., 1996, Kinniburgh et al., 1999, Dupuy and Douay, 2001, Alvarez-Puebla et al., 2004a, Alvarez-Puebla et al., 2004b, Alvarez-Puebla et al., 2005, Ghabbour et al., 2005), as well as other substrates individually. Some authors reported the binding of some cations to several soil components (Wu et al., 2003) but separately.
However, the above mentioned components are present together in natural soils, and their mutual interactions can affect significantly the metal binding capacity of the soil. There are several studies on that interaction; particularly for humic substances–iron oxides showing the formation of complexes (Varadachari et al., 2000, Saito et al., 2004, Fu and Quan, 2005). It can be thus expected that the sorption by a whole soil will not follow the so called additivity rule, that is the total amount sorbed will in general differ form that calculated by the simple sum of the amounts sorbed by the components individually. In spite of this, there are very few reports of the metal cation binding to several soil components simultaneously. It can be mentioned the work of Murphy et al. (1999) studying the sorption of Th(IV) and U(VI) to hematite in presence of natural organic matter; the research of Vermeer et al. (1999), one of the first to report deviations from the additivity rule, in the case of Cd(II) sorption to the Aldrich humic acid–hematite system, finding positive and negative deviations as a function pf pH. Lenhart and Honeyman (1999) studied the system hematite–humic acid–U(VI) and found that the additivity rule underestimated the amount adsorbed; they proposed the presence of oxide–uranyl–humic ternary complexes (Type A complexes) to explain the discrepancy. Alcacio et al. (2001) conducted a molecular scale investigation of Cu(II) bonding to goethite–humic acid complexes with EXAFS and XANES measurements; they reported the presence of two types of ternary complexes: Type A complexes with the Cu(II) cations bridging the oxide and the HA, and Type B complexes with the cations bonded to the HA, in turn bound of the oxide surface. Arias et al. (2002) measured the competitive adsorption equilibrium isotherms of Cu(II) and Cd(II) on kaolin in presence and absence of humic acids. Humic acids were found to enhance the metal adsorption capacity of mineral surfaces. This enhancement was also observed in the competitive adsorption of copper and cadmium on kaolin and kaolin-humic complex. The Freundlich isotherm equation was found to provide an excellent fit to the experimental data. More recently, Saito et al. (2005) studied the same system from a macroscopic point of view and found positive deviations of the additivity rule; they also determined the effect of Cu(II) on humic adsorption to goethite, finding increased or decreased adsorption depending on the experimental conditions. These authors reported some discrepancies with the conclusions of Alcacio et al. (2001), attributing the effect to proton displacement (and consequently a less positive surface charge) upon HA adsorption onto the oxide; they also proposed the formation of both Type A and B complexes. On the other hand, Weng et al. (2005) studied the system goethite–fulvic acid–Ca(II), finding positive and negative deviations form the additivity rule, depending on pH; they also found that the presence of Ca(II) increased fulvic acid adsorption to hematite, and attributed the deviations mainly to electrostatic effects. From the theoretical point of view, there are a number of models describing with relatively good agreement the interaction of metal cations with humic substances, such as NICA-Donnan (Benedetti et al., 1995, Kinniburgh et al., 1996), WHAM model V–VI (Tipping and Hurley, 1992, Smith et al., 2004) or the Stockholm humic model (Gustafsson, 2001); on the other hand, metal cation–oxide particle interaction are successfully interpreted by several models, specially the CD-MUSIC model (Hiemstra and Van Riemsdijk, 1996, Bradl, 2004). However, to date there is no theoretical model describing the binding of metal cations to several substrates simultaneously. The outcome of these studies is that the simultaneous binding of metal cations to soil components is a complex issue and is the next step in this field of research.
In this paper, the investigation of Pb(II) binding to goethite–HA systems is reported. The main objective is to compare the sorption of Pb(II) to iron oxide–HA systems with the sorption to these substrates separately with the substrates essentially in solid state, and to gain insight into the molecular aspects of this sorption phenomenon. Goethite was chosen because it is an iron oxide representative and commonly found in soils. Because HAs have composition and properties dependent on their origin, two samples are studied: a commercial Fluka humic acid, and a sample extracted from soil of the Buenos Aires province, Argentina. The range of Pb(II) concentrations studied is typical of moderately polluted soils. The binding isotherms are determined analytically by atomic absorption spectrometry (AAS), and the Pb(II)-substrates interaction is probed by IR spectroscopy.
Section snippets
Goethite
Synthetic goethite was provided by the Surface Science group of INQUIMAE. It was prepared following the technique described by Schwertmann and Cornell (1991), with a OH/Fe ratio of 1.5. Its specific surface area, determined by the BET technique, was of 89 m2 g−1 and its mean particle size, measured with a Brookhaven Instruments Corp. 90 plus particle size analyzer, was 120 nm.
Humic acids
Two different samples of humic acids were used: one of them was Fluka humic acid (FHA), obtained from natural coal and used
Binding isotherms
The UV–vis measurements of the HA concentration remaining in solution after centrifugation revealed that, in the absence of goethite, 14% of FHA and 18% of VHA remained in solution. In the presence of goethite, the amount of both HAs remaining was not more than 1%. These results are independent, within experimental error, of the amount of Pb(II) present. Because the AAS technique measures the metal concentration present in solution irrespective of its speciation, a correction was performed in
Acknowledgements
The authors gratefully acknowledge financial support from the Universidad de Buenos Aires (UBACYT 2004–2007 X105), the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET, PIP 02287) and the Agencia Nacional de Promoción Científica y Tecnológica (grant No. 06-12467). The authors wish to thank Dr. Jorge Stripeikis for his kind help with the AAS measurements as well as Dr. Roberto Candal for kindly supplying the goethite samples used in this work.
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2019, Environment InternationalCitation Excerpt :For example, the adsorption of Cu on hematite increased by approximately 30% in the presence of FA (Christl and Kretzschmar, 2001). Similar increments were also observed for Cd onto hematite-HA composites at pH > 6 (Vermeer et al., 1999), Pb onto goethite-humic and γ-Al2O3/FA composites (Wu et al., 2003; Orsetti et al., 2006; Xiong et al., 2015), Cu onto goethite-HA complex and HA-coated gibbsite (Saito et al., 2005; Antelo et al., 2009). The increased adsorptions were ascribed to the decreased surface potential of Fe (hydro)oxides induced by humic substances (HS), and changed affinity of HS for heavy metals or the formation of ternary composites (Fig. 3).