Environ. Sci. Technol. 2002, 36, 328-336
Adsorption of Cu, Cd, and Ni on Goethite in the Presence of Natural Groundwater Ligands DIANE BUERGE-WEIRICH,† RENATA HARI,† HANBIN XUE,‡ P H I L I P P E B E H R A , §,| A N D L A U R A S I G G * ,† Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-8600 Duebendorf, Switzerland, Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-6047 Kastanienbaum, Switzerland, and Insitut de Me´canique des Fluides et des Solides, UMR 7507, Universite´ Louis Pasteur-CNRS, 2 Rue Boussingault, 67000 Strasbourg, France
The adsorption of copper, cadmium, and nickel on goethite was examined in natural groundwater samples from an infiltration site of the river Glatt at Glattfelden (Switzerland). Unfractionated dissolved organic matter was used at its natural concentrations. Metal concentrations were close to environmental conditions. Cu, Cd, and Ni presented the typical pH adsorption edge of cations. The major influence on metal adsorption was due to a strong organic ligand LI, which inhibited adsorption of Cu, Cd, and Ni in the alkaline pH region. Complexation of Cu, Cd, and Ni by the natural organic ligands was described with a model defining a minimum number of discrete ligands: a strong ligand LI at low concentration and additional weaker ligands with higher concentrations. The adsorption of Cu, Cd, and Ni on the goethite surface in the presence of the natural organic ligands was adequately described by considering only surface complexation and complexation in solution by organic ligands. No ternary complexes had to be invoked in the model. The major effect was complexation by the strongest ligand, whereas interactions with other cations and anions had only a minor influence. Competition reactions between Cu and Ni for complexation with the same strong ligand LI were observed.
Introduction If metals are mobilized in an aquifer, they may become bioavailable and thus toxic to some organisms or reach the deep groundwater, which is often used as drinking water. Heavy metal mobility in aquifers is linked to interactions between metals, mineral surfaces, and ligands in solution. Different effects of organic ligands on heavy metal adsorption can be expected. On one hand, organic ligands in solution can compete with the surface functional groups for complexation with heavy metals. Complexation by organic ligands * Corresponding author e-mail:
[email protected]; phone: +411 823 5494; fax: +411 823 5028. † EAWAG, Duebendorf. ‡ EAWAG, Kastanienbaum. § Universite ´ Louis Pasteur-CNRS. | Present address: Ecole Nationale Supe ´ rieure des Inge´nieurs en Arts Chimiqes et Technologiques, Laboratoire de Chimie AgroIndustrielle, UMR 1010 INRA/INP-ENSIACET, 118 Route de Narbonne, 31077 Toulouse Cedex 4, France. 328
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occurs mainly at high pH values and in the simplest case decreases the amount of metal adsorbed on surfaces. Such interactions have been observed for U(VI) (1) and Cu (2) adsorption on goethite in the presence of humic acids or Cu and Cd adsorption on kaolinite in the presence of landfill leachate organic matter (3). In this context, competition reactions between different heavy metals for complexation with organic matter have to be taken into account (4-6). On the other hand, several authors have reported enhanced metal adsorption at low pH values in the presence of organic ligands. These interactions were mostly explained by the formation of ternary surface complexes (surfaceligand-metal or surface-metal-ligand) and have been observed in different systems, such as Cu-humic acidgoethite (2), Cu-fulvic acid-kaolinite (7), U(VI)-humic acid-hematite (1), or Cu/Cd-landfill leachate organic matter-kaolinite (3). The influence of simple organic ligands on the adsorption of heavy metals on solid phases has been extensively investigated, using protocatechuic acid (8, 9), oxalate (10, 11), salicylate (8, 9, 12), or citrate (13). The effects of natural organic ligands on heavy metal adsorption have been mostly studied using humic and fulvic acids (4, 14-20). However, only few studies have considered unfractionated natural organic matter (NOM). Humic and fulvic acids represent only a part (about 50%) of the total DOC (21). Hydrophilic compounds (about 30% of the total DOC) as well as carbohydrates, amino acids, and other simple molecules (about 20% of DOC) are not taken into account when investigating only the humic and fulvic fractions (22, 23). Some of these compounds may be stronger ligands than humic and fulvic acids (24). DOC may also include xenobiotic ligands, like EDTA, NTA, or phosphonates (25-27) in anthropogenically contaminated waters. In this study, we investigated the influence of groundwater organic ligands on adsorption of Cu, Cd, and Ni on a welldefined solid phase (goethite). For this purpose, experiments were performed directly in the infiltration groundwater samples, i.e., the organic ligands were used unfractionated and at their natural concentrations. To avoid saturation of the complexation capacity of the organic ligands, metal concentrations as well were close to environmental conditions. The infiltration groundwater used came from the river Glatt in northern Switzerland, previously described in detail (28-31). Agglomerations and industrial areas within the Glatt catchment area and several sewage treatment plants provide substantial inputs of various pollutants into the river. Thus the river, and as a consequence the infiltration groundwater, can be considered to be slightly contaminated (30, 32, 33). Infiltration groundwater close to the river and deep groundwater were used in the experiments. The solid-phase goethite is used in this study to represent sorbents in natural groundwater systems. Iron(III) (hydr)oxides are often found in aquifers as coatings on other minerals, which are formed by reoxidation of Fe(II). Adsorption of Cu, Cd, and Ni is described by a model taking into account the complexation of the metal by the surface functional groups and the ligands in solution. Furthermore, competition reactions between Cu, Ni, and Cd for complexation with the organic ligands and their effects on adsorption are analyzed.
Experimental Section Chemicals. Metal solutions were prepared from the metal stock standard from Baker Instra Analyzed (Cu, Cd, Ni, and Mg: 1 mg/L; Ca: 10 mg/L). Following chemicals were from 10.1021/es010892i CCC: $22.00
2002 American Chemical Society Published on Web 01/04/2002
TABLE 1. Mean Dissolved (6. However, at pH values above 7, Cu adsorption was somewhat underestimated by the model. A possible explanation could be that the adsorption of Cu was enhanced by reactions not yet considered in the calculations, e.g., influence of anions contained in the groundwater matrix. Phosphate is a possible candidate to influence metal adsorption, as this anion adsorbs most probably as an inner-sphere complex (50, 57, 58). As a consequence, the surface charge will be lowered and thus Cu adsorption on the surface may be increased due to electrostatic attraction. According to our model calculations, phosphate adsorption can indeed increase the adsorption of Cu at pH values above 6 (Figure 1a). However, at pH values below 5, electrostatic interactions between phosphate and Cu could not represent this enhanced adsorption. Thus, the elevated adsorption of Cu at low pH values was most probably due to some specific interactions not yet considered in the model. Possibly Cu-phosphate complexes are very stable because of the Jahn-Teller distortion of the Cu ion (59) and can thus adsorb on goethite to form ternary complexes of type B. The model combining the complexation of Cd and Ni by the organic ligands with the adsorption of Cd and Ni was able to adequately represent the metal pH adsorption edge in the presence of the natural organic ligands (Figure 1b,c). By combining the model for Cu complexation with the model for Cu adsorption on goethite, Cu adsorption in the presence of the deep groundwater organic ligands could be sufficiently well-described (Figure 1d). Similarly as observed in the infiltration groundwater, interactions with phosphate could as well explain the slight underestimation of Cu adsorption at pH values above 7 as well as the enhanced Cu adsorption in the acidic pH range. Competition Reactions between Cu and Ni for Complexation with Organic Ligands. Cu adsorption on goethite in the presence of natural organic ligands was slightly enhanced by addition of Ni (system Cu-Ni-DOC-together: Figure 7). The Cu adsorption isotherm and its intercept with the x-axis were shifted to the left, which is a
hint that less Cu was complexed in solution by strong ligands due to competition with Ni. Ni adsorption as well was enhanced by the addition of Cu (data not shown) (34): the fraction of Ni adsorbed in the presence of increasing Cu concentrations was about 40%, whereas in the absence of Cu it amounted to less than 20%. This means that less Ni was complexed in solution in the presence of Cu, which corroborates the fact that Cu and Ni are competing for complexation with the same strong specific ligand in solution. Recording the Ni isotherm at pH 7.35 in the presence of a constant Cu concentration ([Ni]tot ) 3 × 10-8 M, [Cu]tot ) 1.2 × 10-7 M) gave similar results. The Ni adsorption isotherm and its intercept with the x-axis were shifted clearly to the left in the presence of Cu (Figure 3), i.e., less Ni was complexed in solution by strong ligands due to competition reactions with Cu. As described above, Cu adsorption increased from 44% adsorbed Cu in the absence of Ni to around 55% in the presence of Ni. Competition experiments were also performed in the system Cu-Ni-first. In this case, for which Ni and Cu were adsorbed on goethite prior to the addition of natural organic ligands, no effect on the adsorption of Cu was observed in the presence of Ni (solid triangles, Figure 7). After addition of the ligands, metal desorption reactions occurred, in which Cu is expected to react much faster than Ni (60). Similarly as described above for the systems Cu-DOCtogether and Ni-DOC-together, the slope of the adsorption isotherm decreased as compared to the reference adsorption isotherm. However, no clear difference in the slope was observed in the systems Cu-Ni-DOC-together and CuDOC-together. This indicated competition reactions of Cu or Ni between the surface and the ligand but no competition between Cu and Ni for complexation with the weaker ligands, which are present in excess of the total metal concentrations. So again, the competition reactions point out the importance of the strong ligand LI. Since Cd adsorption on goethite was not influenced by Cu or Ni, whatever the pH (results not shown) (34), Cd may be bound to other ligands than Cu or Ni.
Discussion Ligand Effect on Cu, Cd, and Ni Adsorption. The most important finding in this study is the presence of the strong ligand LI for Cu, Cd, and Ni, which could only be distinguished because experiments were performed at such low total metal concentrations and because unfractionated organic material was used. It inhibited Cu adsorption on goethite at Cu concentrations below 5 × 10-8 M in the infiltration groundwater, respectively, at 1 × 10-8 M in the deep groundwater; Ni adsorption at concentrations below 5 × 10-8 M; and Cd adsorption at concentrations below 5 × 10-10 M. Hence, this strong ligand was found as well in the infiltration as in the deep groundwater, which is consistent with previous works done on the same site (26). The nature and origin of these strong ligands are yet unknown. They may either be of natural origin (61-66) or be anthropogenic ligands, e.g., EDTA or NTA (nitrilotriacetate). Natural strong ligands may be derived from biogenic material and include amino and/or thiol functional groups. EDTA is known to be a strong complexant that could remobilize adsorbed heavy metals on the infiltration path. It has been shown that EDTA is not a very effective ligand for Cu as compared to the natural ligands in this groundwater (33), as the concentration of Cu complexed by EDTA was always less than 0.1% of total Cu in the groundwater samples. However, in our system where we have added Cu, it is difficult to distinguish between the effect of the natural and the anthropogenic ligands. Biodegradation of NTA is very fast, and it has been shown on the same sampling site that this substance is almost completely degraded on the infiltration path from the river to the groundwater (67, 68).
The less specific ligands at higher concentrations probably correspond to humic or fulvic acids. Their complexation properties for Cu and Cd appear to be described by the parameters of the WHAM model (54). Consequences for the Natural System. The presence of these strong ligands, even if present only at low concentrations is of environmental significance. At ambient metal concentrations of the Glatt groundwater, the main Cu, Cd, and Ni species are most probably metal organic complexes. Hence these metals will be mobilized in this aquifer system. Additionally to the strength of the ligand, the ratio of metal to ligand is an important factor to describe metal mobilization. It could be shown that mobilization of heavy metals is not only possible in waters rich in humic substances (15, 18, 70) but also in waters with low DOC content. Surprising is the fact that no ternary surface complexes with the natural organic ligands had to be invoked to explain the results. As goethite is a very strong adsorbent, metals are under these conditions preferentially adsorbed by the free goethite functional groups rather than complexed by the organic ligands. The strong ligands LI are probably maintained in solution due to complexation with the metals. Finally, competitive interactions between heavy metals may also play an important role in metal transport. A heavy metal present at high concentrations may induce immobilization of another heavy metal.
Acknowledgments We thank David Kistler for help with ICP-MS measurements and sampling, Rene´ Schoenenberger for EDTA measurements and help with sampling, and Bernhard Wehrli for discussions. BASF goethite was provided through the GDR Practis (CNRS, CEA, EdF, Andra).
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Received for review April 23, 2001. Revised manuscript received September 5, 2001. Accepted October 1, 2001. ES010892I
