Interaction of copper and fulvic acid at the hematite water interface

Transcription

1 Pergamon PII S (01) Geochimica et Cosmochimica Acta, Vol. 65, No. 20, pp , 2001 Copyright 2001 Elsevier Science Ltd Printed in the USA. All rights reserved /01 $ Interaction of copper and fulvic acid at the hematite water interface ISO CHRISTL and RUBEN KRETZSCHMAR* Institute of Terrestrial Ecology, Swiss Federal Institute of Technology, Grabenstrasse 3, CH-8952 Schlieren, Switzerland (Received November 16, 2000; accepted in revised form May 8, 2001) Abstract The influence of surface-bound fulvic acid on the sorption of Cu(II) to colloidal hematite particles was studied experimentally and the results were compared with model calculations based on the linear additivity assumption. In the first step, proton and Cu binding to colloidal hematite particles and to purified fulvic acid was studied by batch equilibration and ion-selective electrode titration experiments, respectively. The sorption data for these binary systems were modeled with a basic Stern surface complexation model for hematite and the NICA-Donnan model for fulvic acid. In the second step, ph-dependent sorption of Cu and fulvic acid in ternary systems containing Cu, hematite, and fulvic acid in NaNO 3 electrolyte solutions was investigated in batch sorption experiments. Sorption of fulvic acid to the hematite decreased with increasing ph (ph 3 10) and decreasing ionic strength ( M NaNO 3 ), while the presence of 22 M Cu had a small effect on fulvic acid sorption, only detectable at low ionic strength (0.01 M). Sorption of Cu to the solid phase separated by centrifugation was strongly affected by the presence of fulvic acid. Below ph 6, sorption of Cu to the solid phase increased by up to 40% compared with the pure hematite. Above ph 6, the presence of fulvic acid resulted in a decrease in Cu sorption due to increasing concentrations of dissolved metal organic complexes. At low ionic strength (0.01 M), the effects of fulvic acid on Cu sorption to the solid phase were more pronounced than at higher ionic strength (0.1 M). Comparison of the experimental data with model calculations shows that Cu sorption in ternary hematite fulvic acid systems is systematically underestimated by up to 30% using the linear additivity assumption. Therefore, specific interactions between organic matter and trace metal cations at mineral surfaces must be taken into account when applying surface complexation models to soils or sediments which contain oxides and natural organic matter. Copyright 2001 Elsevier Science Ltd 1. INTRODUCTION During the past decades, surface complexation models have been developed and tested extensively to describe the sorption of protons and metal cations to pure oxide and clay mineral surfaces (Davis and Kent, 1990). More recently, various models describing the competitive binding of protons and metal cations to purified fulvic and humic acids have been formulated, such as the nonideal competitive adsorption (NICA)- Donnan model (Kinniburgh et al., 1998) and Model VI (Tipping, 1998). In most natural environments, however, oxide mineral surfaces are coated to a large extent with adsorbed natural organic matter such as humic substances (Davis, 1982; Sposito, 1984; O Melia, 1989). Humic substances are anionic polyelectrolytes which are negatively charged and strongly bind to mineral surfaces by a variety of mechanisms, most importantly, specific adsorption by ligand exchange with protonated surface hydroxyl groups (Gu et al., 1994; Schlautman and Morgan, 1994; Murphy and Zachara, 1995). Because of their negative charge, adsorbed humic substances can profoundly alter the surface charge, zeta potential, and colloidal stability of oxide and clay mineral particles (Tipping and Higgins, 1982; Liang and Morgan, 1990; Kretzschmar et al., 1997). Therefore, the specific interactions between metal cations, organic matter, and mineral surfaces must be understood quantitatively before surface complexation models and ion binding models for humic substances can be further developed and * Author to whom correspondence should be addressed (kretzschmar@ ito.umnw.ethz.ch) applied successfully to soil or aquatic environments (Schroth and Sposito, 1998; Vermeer et al., 1999; Floroiu et al., 2001). Based on the current state of knowledge, adsorbed humic substances are expected to increase metal sorption at low ph, where humic substances are strongly sorbed to the mineral surfaces and metal sorption to the mineral surfaces is low (Murphy and Zachara, 1995). However, it has not yet been resolved whether adsorbed humic substances simply contribute additional binding sites without affecting metal sorption to the mineral surface itself, according to the linear additivity model proposed by Zachara et al. (1994), or whether the interactions between metal cations and adsorbed organic matter at the mineral surface are more complex, leading to nonadditive behavior (Robertson, 1996; Vermeer et al., 1999). Some studies suggest that humic substances are fractionated upon adsorption to mineral surfaces (Davis and Gloor, 1981; Wang et al., 1997; Vermeer and Koopal, 1998) and that metal cations (Cu, Cd, Ni) may have a higher binding affinity to surface-bound humic substances than to dissolved humic substances (Davis, 1984; Laxen, 1985). If this is the case, a linear additivity model may underestimate metal cation sorption in ternary systems containing minerals and organic matter. In other studies, however, electrostatic interactions between mineral surfaces and adsorbed humic substances have been proposed to affect metal binding to both the mineral surface and adsorbed humic substances (Vermeer et al., 1999). Robertson (1996) and Vermeer et al. (1999) reported that under acidic conditions metal cation sorption in ternary systems was much lower than expected from a linear additivity model. In contrast, Zachara et al. (1994) did not find any specific interactions between the sorption of cobalt

2 3436 I. Christl and R. Kretzschmar Table 1. Selected characteristics of the soil fulvic acid (FA) isolated from an organic horizon of a humic Gleysol at Unterrickenzopfen (northern Switzerland). Sample M w a (kd) Elemental composition (g kg 1 ) C H N S O b Carboxylic carbon c (mol kg 1 ) Phenolic carbon c (mol kg 1 ) FA a Average apparent molecular weight determined by size exclusion chromatography. b Calculated as difference to 100%. c Calculated from 13 C-NMR spectra and carbon contents. and sorption of humic acid to different mineral sorbents. The effect of humic acid on cobalt binding was merely additive, that is, the sorption of cobalt to the solid phase could be explained by cobalt binding to the mineral surface and adsorbed humic acid contributing additional high-affinity sites without specific interactions between mineral surface and adsorbed humic acid. The objective of this study was to investigate the interactions of adsorbed fulvic acid and Cu(II) at the hematite ( -Fe 2 O 3 ) surface (ternary systems) as a function of ph and ionic strength. In two previous papers, we reported on the binding of Cu to purified soil fulvic acid and to colloidal hematite particles (binary systems). Copper binding in binary systems was quantitatively described using the NICA-Donnan model for fulvic acid (Christl and Kretzschmar, 2001) and a 2-pK basic Stern surface complexation model for hematite (Christl and Kretzschmar, 1999). Based on the model parameters obtained from these binary systems, we now compare experimental results for Cu sorption in hematite fulvic acid systems (ternary system) with model calculations based on the linear additivity assumption. The purpose of this comparison is to investigate the magnitude and direction of deviations from the linear additivity model, which can provide valuable insights about the interactions between fulvic acid and Cu at the hematite water interface. 2. MATERIALS AND METHODS 2.1. Fulvic Acid Fulvic acid was extracted from a well-humified organic horizon (H) of a Humic Gleysol at Unterrickenzopfen (northern Switzerland) following a standard procedure recommended by the International Humic Substances Society (Swift, 1996). After separating the fulvic acid from the humic acid fraction, the fulvic acid was further purified using a DAX-8 resin (Supelco) column technique (Thurman and Malcolm, 1981) and then transformed into the protonated form by passing it through a column filled with proton-saturated cation exchange resin (Amberlite IR-120, Fluka). The purified fulvic acid was stored until use at 4 C in the dark as aqueous stock solution, which had a total organic carbon concentration of 4.52 g/l. Some important properties of the fulvic acid are summarized in Table 1. Additional details on the isolation and purification procedures and the characterization using a combination of spectroscopic and analytical techniques are reported in Christl et al. (2000) Colloidal Hematite Particles Submicrometer sized, uniform hematite particles of spheroidal shape were synthesized by aging a concentrated Fe(III) hydroxide gel (Sugimoto and Sakata, 1992). To remove excess salts, the particles were washed twice with deionized water by centrifugation and then dialyzed against deionized water for 12 d. The resulting stock suspension (64.2 g/l hematite) was stored in a polyethylene bottle in the dark at 4 C. Particle characterization by X-ray diffraction and transmission electron microscopy (TEM) revealed that the particles were well-crystallized hematite particles of spheroidal shape exhibiting an average particle diameter of nm (Christl and Kretzschmar, 1999). The specific surface area measured by the N 2 -BET method was m 2 /g. The point of zero charge of the hematite derived from acid base titrations at different ionic strengths was at ph 9.5 (Christl and Kretzschmar, 1999) Acid Base Titrations All titration experiments reported in this paper were conducted in a thermostated room at 25 1 C using a computer-controlled titration set-up (Kinniburgh et al., 1995). For acid base titrations, four burettes (Dosimat 605, Metrohm), a glass electrode ( , Metrohm), and an Ag/AgCl reference electrode ( , Metrohm, Herisau, Switzerland) were connected to a personal computer by a Microlink MF18 interface (Biodata, Manchester, UK). Suspensions containing either hematite ( 18 g/l) or fulvic acid ( 2 g/l FA) in NaNO 3 electrolyte were titrated from ph 3.5 to 10 with CO 2 -free, 0.05 M NaOH and backtitrated with 0.05 M HNO 3. Ionic strength was kept constant within each forward and backward titration cycle by appropriate additions of DI-H 2 O or 2 M NaNO 3 solution at each titration step. After completion of a titration cycle, the ionic strength was adjusted to the next higher value by addition of 2 M NaNO 3. Additional details on the titration procedures and data evaluation are given in Christl and Kretzschmar (1999) for hematite and Christl and Kretzschmar (2001) for fulvic acid, respectively Metal Sorption Experiments in Binary Systems The ph-dependent sorption of Cu to hematite was studied in batch equilibration experiments. Suspensions containing 2 g/l hematite and 4to100 mol/l Cu(NO 3 ) 2 were prepared in 0.01, 0.03, or 0.1 M NaNO 3 background electrolyte solutions. Suspension ph was adjusted to values between 3 and 9 using CO 2 -free NaOH and HNO 3, respectively. After equilibration for 21 h under N 2 atmosphere at 25 C, all samples were centrifuged for 20 min at 6000 g. The ph values of the supernatants were measured with a combined glass electrode ( , Metrohm, Herisau, Switzerland). Supernatants were filtered through 0.1- m membrane filters (NC10, Schleicher & Schuell, Dassel, Germany), acidified by adding a drop of distilled HNO 3, and analyzed for dissolved Cu by flame atomic absorption spectroscopy. Sorbed amounts were calculated from the difference between total and dissolved Cu concentrations. Cu-binding to fulvic acid was investigated by potentiometric titration experiments using the same titration set-up as for acid base titrations, but with an additional ion-selective electrode for Cu 2 (Orion 9429, Thermo Orion, Beverly, MA). Fulvic acid solutions ( 1 g/l FA) were adjusted to ph 4, 6, or 8 and then titrated with Cu(NO 3 ) 2 solutions, while keeping the ph constant at ph 4, 6, or 8 by NaOH or HNO 3 addition. The activity of free Cu 2 in solution was monitored using the ion-selective electrode. All titration experiments were performed at a constant ionic strength of 0.1 M in NaNO 3 electrolyte. To exclude carbonate, all solutions were prepared under N 2 atmosphere using carbonate-free water and the titration vessel was kept under N 2 atmosphere throughout the experiment. Further details on electrode calibration and data evaluation can be found in Christl et al. (2001).

3 Copper sorption in fulvic acid hematite suspensions Metal Sorption Experiments in Ternary Systems The ph and ionic strength dependent sorption of Cu to hematite in the presence of fulvic acid was studied with batch equilibration experiments. Hematite suspensions were prepared in 0.01, 0.03, and 0.1 M NaNO 3 electrolyte solutions and the ph was adjusted to values between ph 3 and 9 by addition of HNO 3 or carbonate-free NaOH. Then, fulvic acid and Cu(NO 3 ) 2 solutions were concurrently added to give final concentrations of 38 mg/l fulvic acid and 22 mol/l Cu, respectively. The final hematite concentration of the suspensions was 2 g/l. For each ph series, a series of blank samples without fulvic acid was prepared for comparison. All samples were equilibrated for 21 h at 25 C under N 2 atmosphere and then centrifuged for 60 min at 6000 g. Supernatants were analyzed for ph, total organic carbon (TOC), and dissolved Cu concentrations. For TOC measurements, subsamples of the supernatants were acidified by HCl addition and analyzed on a TOC analyzer (TOC-5000, Shimadzu, Kyoto, Japan). For Cu analysis, subsamples of the supernatants were acidified by adding a drop of distilled HNO 3 and analyzed by flame atomic absorption spectroscopy. To assess the effect of Cu on fulvic acid adsorption, similar experiments were conducted in the absence of Cu under otherwise identical conditions Data Analysis The proton and Cu binding data for colloidal hematite and purified soil fulvic acid were used to calibrate appropriate sorption models for these binary systems. For hematite, a 2-pK basic Stern surface complexation model allowing for outer-sphere surface complexes of Na and NO - 3 (ion-pair formation) was used in this study. Sorption of Cu was modeled with an inner-sphere, outer-sphere, and a singly hydrolyzed outer-sphere complex. The surface site density of the hematite was fixed to 7 sites/nm 2, which gave the best prediction of competitive sorption experiments for Cu and Pb for the same hematite (Christl and Kretzschmar, 1999). The proton and Cu binding to fulvic acid was modeled using the NICA-Donnan model (Kinniburgh et al., 1998). The model parameters, which were obtained from a combination of data fitting and chemical characterization by 13 C-NMR spectroscopy, are reported elsewhere (Christl and Kretzschmar, 2001; Christl et al., 2001). The model parameters obtained from the best-fit descriptions of the binary systems were used for model calculations of Cu sorption in the ternary system containing hematite and fulvic acid as sorbent phases. The calculations were based on the linear additivity assumption, that is, specific interactions between Cu and fulvic acid at the hematite surface are neglected. These model calculations were then compared to experimental results to obtain information about the direction and magnitude of possible deviations from the additivity model. As pointed out earlier, such comparisons have been conducted previously, but with variable results. Calculations of Cu sorption in the ternary system were carried out with the chemical speciation program ECOSAT 4.4 (Keizer and van Riemsdjik, 1998). Formation of inorganic aqueous complexes and possible precipitation of metal (hydr-)oxide phases was included into the calculations using stability constants given by Baes and Mesmer (1976) and Smith and Martell (1976). 3. RESULTS AND DISCUSSION 3.1. Cu Sorption to Hematite and Fulvic Acid The results of acid base titration and batch Cu sorption experiments with pure hematite are shown in Figure 1 (symbols), along with the best-fit model description using the basic Stern surface complexation model (solid lines). The acid base titration data at different ionic strengths exhibited a common intersection point near ph 9.5 (Fig. 1a), which corresponds to the point of zero charge (PZC) of the hematite surface. Cu sorption followed a typical adsorption edge behavior, that is, a sharp increase in metal cation sorption with increasing ph, until 100% of the total Cu was sorbed (Fig. 1b). The maximum surface coverages in these experiments ranged between 0.07 Fig. 1. Experimental data (symbols) and respective surface complexation model fits (lines) for proton and Cu sorption by colloidal hematite particles as a function of ph, total metal concentration, or ionic strength (NaNO 3 electrolyte). (a) Acid base titrations of pure hematite at four different ionic strengths. (b) Cu sorption to hematite (2 g/l) in batch experiments with three different total Cu concentrations and 0.1 M NaNO 3 electrolyte concentration, and (c) Cu sorption to hematite (2 g/l) in batch experiments with 20 M total Cu and varying ionic strength (NaNO 3 electrolyte).

4 3438 I. Christl and R. Kretzschmar and 1.7 mol/m 2, based on a specific surface area of the hematite particles of 28.3 m 2 /g (Christl and Kretzschmar, 1999). The weak ionic strength dependence of Cu sorption is also correctly described by the model, as shown in Figure 1c. According to our model calculations and X-ray absorption spectroscopy results for these systems (not shown), Cu was predominantly sorbed as inner-sphere complex at the hematite surface, which is in agreement with the small ionic strength dependence. Overall, the basic Stern surface complexation model yielded an excellent description of the entire data set for proton and Cu sorption to hematite in NaNO 3 background electrolyte solutions. Additional details on the model formulation and resulting model parameters are discussed in Christl and Kretzschmar (1999). In the context of the present paper, mainly the goodness of model fits for the binary systems will be important. The binding of protons and Cu to fulvic acid (symbols) and the corresponding best-fit description with the NICA-Donnan model (solid lines) are presented in Figure 2. The fulvic acid exhibited typical charging behavior as a function of ph and ionic strength, as has been observed in many other studies on fulvic and humic acid (Milne et al., 2001). Binding of Cu to fulvic acid increased with increasing ph and increasing free Cu 2 concentration. The NICA-Donnan model provided a good description of the entire data set, although slight deviations remain at the highest Cu 2 concentrations used. In the concentration range relevant for the ternary experiments presented in this paper ( 22 M), the model accurately described the experimental data (see inset Fig. 2b). The charging behavior and Cu binding to fulvic acid and resulting NICA-Donnan model parameters were discussed in detail by Christl and Kretzschmar (2001) and Christl et al. (2001). Again, in the context of discussing the ternary systems, mainly the goodness of model fits is important Binding of Fulvic Acid to Hematite The influence of ph and ionic strength on the sorption of fulvic acid (38 mg/l) to hematite (2 g/l) is shown in Figure 3a. The lines are polynomial fits to the experimental data. At ph 4.5, 85% of the fulvic acid was adsorbed to the hematite and the effect of ionic strength on fulvic acid adsorption was small. At higher ph values, fulvic acid adsorption decreased with increasing ph and the effect of ionic strength on fulvic acid sorption became more pronounced. At ph values above the point of zero charge of the hematite (ph 9.5), still a substantial portion of the fulvic acid was adsorbed. Increasing ionic strength resulted in increased fulvic acid adsorption. The adsorption behavior observed for fulvic acid on hematite as a function of ph and ionic strength is in good agreement with previously reported results for similar systems. For example, similar ph dependence of natural organic matter adsorption was observed for goethite, hematite, alumina, and kaolinite (Tipping, 1981; Davis, 1982; Day et al., 1994; Gu et al., 1994; Schlautman and Morgan, 1994; Kretzschmar et al., 1997; Lenhart and Honeyman, 1999). Schlautman and Morgan (1994) studied the adsorption of Suwannee River fulvic and humic acid to alumina particles and discussed the possible adsorption mechanisms involved. At ph 4, ionic strength had little effect on humic and fulvic acid adsorption and ligand exchange was Fig. 2. Experimental data (symbols) and respective NICA-Donnan model fits (lines) for proton and Cu binding by a purified soil fulvic acid as a function of ph and/or total metal concentration. (a) Acid base titrations of fulvic acid at four different ionic strengths, and (b) Cu binding isotherms to fulvic acid as a function of free Cu concentration and ph at 0.1 M ionic strength (NaNO 3 electrolyte). The inset is an enlargement of the relevant region for the sorption experiments in the ternary systems. postulated as the predominant adsorption mechanism. At ph 7 and ph 10, increasing NaCl concentrations resulted in higher adsorption densities of humic acid, which was explained by increased hydrophobic bonding due to screening of charge and increased adsorption by Na cation-bridging at ph values above the point of zero charge of alumina. In addition, reduced electrostatic repulsion between dissolved and adsorbed fulvic acid molecules would favor higher adsorption at higher ionic strength, due to screening of charge. The adsorption of fulvic acid to hematite in the ternary systems containing 22 M Cu is shown in Figure 3b. The lines are identical to the ones shown in Figure 3a and only serve for better comparison. The presence of Cu had little effect on the sorption of fulvic acid to hematite at 0.03 and 0.1 M NaNO 3 electrolyte concentrations. At the lowest ionic strength (in 0.01 M NaNO 3 ), however, the presence of Cu resulted in a slight

5 Copper sorption in fulvic acid hematite suspensions 3439 Fig. 3. Fulvic acid binding to hematite as a function of ph and NaNO 3 concentration (a) in the absence of Cu and (b) in the presence of 22 M Cu. Symbols represent measured data; lines represent polynomial fits of fulvic acid adsorption in the absence of Cu. Total concentrations of fulvic acid and hematite were 38 mg/l and 2 g/l, respectively. increase in fulvic acid adsorption, which was particularly obvious at high ph. The effect of Cu on fulvic acid adsorption may be explained by a reduction of fulvic acid negative charge as a result of Cu binding and, possibly, enhanced formation of cation-bridging complexes at the hematite surface above the point of zero charge Influence of Fulvic Acid on Cu Sorption Fig. 4. Cu sorption in ternary systems containing 2 g/l hematite and 38 mg/l fulvic acid in NaNO 3 background electrolyte solutions. The total Cu concentration (Cu T ) was 22 M. (a) Cu sorption to the solid phase (i.e., hematite plus surface-bound fulvic acid) in the ternary system is shown by closed symbols. The thick solid line represents a model calculation based on the linear additivity assumption. The dashed lines show the contribution of Cu sorbed to the hematite surface itself and Cu sorbed to surface-bound fulvic acid, respectively. For comparison, Cu sorption to pure hematite is shown by open symbols along with the corresponding surface complexation model fit (thin solid line). (b) Cu sorption to the solid phase in ternary systems at three different ionic strengths and corresponding model calculations based on the additivity assumption. The sorption of Cu in a ternary system containing 2 g/l hematite and 38 mg/l fulvic acid at 0.1 M ionic strength is presented in Figure 4a. For comparison, Cu sorption to pure hematite under otherwise identical conditions is also presented (open symbols) along with the surface complexation model fit (thin solid line). The closed symbols represent the amounts of Cu sorbed to the entire solid phase separated by centrifugation, that is, hematite particles plus surface-bound fulvic acid. The sorption of Cu was strongly affected by the presence of fulvic acid. Below ph 5.8, Cu sorption to the solid phase was increased when compared to the pure hematite suspensions. Above ph 5.8, Cu sorption was reduced by 5% compared to the pure hematite system, which is due to reduced fulvic acid adsorption at higher ph (Fig. 3b) and formation of dissolved Cu fulvic acid complexes. Additional ternary sorption experiments at different ionic strengths are shown in Figure 4b, demonstrating the influence of ionic strength on Cu sorption to hematite fulvic acid complexes (Fig. 4b). At all ionic strengths, Cu sorption was enhanced below ph 5.8 and reduced by approximately 5 to 15% at higher ph values, when compared to the pure hematite (Fig. 1c). In the presence of fulvic acid, ionic strength had a somewhat different effect on Cu sorption than for the pure hematite system. At ph values below 4.7, a decrease in ionic strength from 0.1 to 0.01 M resulted in an increase in Cu sorption to the solid phase by 10%. This observation is consistent with the effect of ionic strength on Cu binding to humic substances and

6 3440 I. Christl and R. Kretzschmar the fact that more than 70% of the fulvic acid was adsorbed at these ph levels (Fig. 3b). Above ph 4.7, Cu sorption decreased with decreasing ionic strength. This effect is in good agreement with the observed ionic strength effects on fulvic acid adsorption (Fig. 3b) and the increasing Cu binding by fulvic acid with increasing ph (Fig. 2b). All observed trends in Cu sorption in ternary hematite fulvic acid systems are in qualitative agreement with previous reports on metal sorption in other mixed mineral organic matter systems (Davis, 1984; Zachara et al., 1994; Murphy and Zachara, 1995; Ali and Dzombak, 1996; Schroth and Sposito, 1998; Benyahya and Garnier, 1999; Floroiu et al., 2001). In all these studies, metal cation sorption was enhanced at low ph values, where the organic molecules adsorbed most strongly to the oxide mineral surface and metal cation sorption to the mineral surface itself was low. At high ph values, reductions in metal cation sorption were often observed and explained by increasing concentrations of dissolved metal organic complexes with increasing ph, competing for metal sorption to the mineral surface. The magnitude of these effects of course depends on the sorbent phases and metal cations used as well as the concentrations of all involved components. Although these general trends can be explained qualitatively, they are far from understood at the quantitative or even molecular level. Some studies on metal cation sorption in the presence of organic matter and minerals involved no attempt of modeling the ternary systems (Liu and Gonzalez, 1999; Floroiu et al., 2001). In other studies, the results from ternary systems were fitted (not predicted) by invoking additional ternary surface complexes (Davis, 1984; Ali and Dzombak, 1996; Schroth and Sposito, 1998; Benyahya and Garnier, 1999). However, the nature of such ternary complexes remains speculative without spectroscopic evidence, particularly for natural organic matter such as humic substances. Comparisons of experimental sorption data with model calculations based on the linear additivity hypotheses have led to contrasting conclusions. For example, Zachara et al. (1994) reported that Co sorption at low metal concentrations was predicted rather well with an additive model for systems containing humic acid and gibbsite, goethite, or kaolinite. Dalang et al. (1984) found that fulvic acid adsorbed to kaolinite did not significantly affect Cu sorption to the clay surface and had the same complexing properties with respect to Cu as dissolved fulvic acid, which would support the linear additivity hypothesis. However, Davis (1984) and Laxen (1985) proposed that Cu forms stronger complexes with surface-bound than with dissolved natural organic matter. Furthermore, Tipping et al. (1983) have speculated that mineral-bound humic substances can electrostatically enhance metal ion binding to hydroxylated sites on oxide surfaces. Both of these factors would tend to increase Cu sorption in excess of model predictions assuming linear additivity. In contrast to these earlier findings, however, two recent studies on the sorption of Cu to goethite (Roberston, 1996) and of Cd to hematite (Vermeer et al., 1999) in the presence of humic acid found that total metal sorption was much lower than predicted with an additive model, at least under acidic conditions. Robertson (1996) reported that Cu sorption was up to an order of magnitude lower than predicted. Vermeer et al. (1999) explained these results by electrostatic interactions between the hematite surface and the humic acid molecules Comparison with Additivity Model Calculations To obtain insights about the possible interactions between Cu and fulvic acid at the hematite surface, the experimental results were compared with model calculations based on the linear additivity assumption and the model parameters obtained from the binary systems (Figs. 1 and 2). Let us first examine the experiment shown in Figure 4a. The solid line represents the predicted sum of Cu adsorbed to hematite and Cu adsorbed to surface-bound fulvic acid molecules. Even though the additivity model correctly predicts the general trends, it consistently underestimates Cu sorption to the solid phase. Greatest deviations from additive behavior are observed near ph 4.5, where observed Cu sorption was roughly 30% larger than expected from the additivity model. In ph ranges where the sorption of Cu to the solid phase is clearly dominated either by binding to adsorbed fulvic acid (ph 4) or by complexation at the hematite surface (ph 7), the model predictions are much more accurate. The dashed lines in Figure 4 show the calculated distribution of sorbed Cu to surface-bound fulvic acid and the hematite surface, respectively. These calculations demonstrate, that at ph 5.5 most of the sorbed Cu is complexed with surfacebound fulvic acid. At higher ph values, sorption of Cu to the hematite surface becomes stronger and outcompetes complexation by fulvic acid. Thus, at high ph most of the sorbed Cu is sorbed to the hematite surface itself. Let us now examine the ionic strength dependence of Cu sorption in the ternary system (Fig. 4b). At all ionic strengths, the additivity model underpredicted Cu sorption in the range from ph 4 to 7. However, the reversal of the ionic strength dependence of Cu sorption at around ph 5 was correcty predicted. At ph 5, Cu sorption is predicted to increase with increasing ionic strength due to the increase in fulvic acid sorption to hematite (Fig. 3b). At ph 5, Cu sorption is predicted to decrease with increasing ionic strength, due to decreased Cu binding by fulvic acid at higher ionic strength. Compared with the large deviations from the additivity model reported by Robertson (1996) for Cu sorption in a humic acid goethite system, the deviations observed in our study were much smaller (less than 30%). Further, the deviations observed in this study pointed in the opposite direction at low ph than reported by Robertson (1996) and Vermeer et al. (1999) and are more consistent with earlier reports postulating synergistic interactions between humic substances and metal cations on overall metal sorption (Tipping et al., 1983; Davis, 1984; Laxen, 1985). To find a reasonable explanation for the observed nonadditive effects, let us reconsider the assumptions of the linear additivity approach. In this approach, electrostatic interactions at the mineral water interface are neglected. However, electrostatic interactions between the adsorbed fulvic acid and the hematite surface may play an important role for the fate of Cu in the ternary system. The negative electrostatic field of the adsorbed fulvic acid will interact with the electrostatic field of the hematite surface (Vermeer and Koopal, 1999). At ph values below the PZC of the hematite particles, the positive electrostatic field of the hematite surface may be lowered by the negatively charged adsorbed fulvic acid. Concurrently, the negative electrostatic field of the adsorbed fulvic acid will be

7 Copper sorption in fulvic acid hematite suspensions 3441 affected by the positively charged hematite surface. Due to this mutual interaction, additional positively charged ions such as protons and metal cations may be bound to the hematite surface, while on the other hand, less cations may be bound to the adsorbed fulvic acid (Vermeer and Koopal, 1999, Vermeer et al. 1999). Thus, Vermeer et al. (1999) concluded that depending on the binding affinity of the hematite surface and the fulvic acid for the metal cation and the respective change of the electrostatic fields, the mutual electrostatic interaction between adsorbed polyelectrolyte and hematite surface may either lead to higher or lower metal sorption compared to model calculations based on the linear additivity approach. They predicted that the linear additivity model would tend to overestimate metal cation sorption at low ph and underestimate metal cation sorption at high ph. This was consistent with their experimental data for Cd sorption to hematite, but has not been conclusively shown for other metal cations. An alternative explanation for the enhanced Cu sorption in the presence of fulvic acid compared to the additivity model could be a greater affinity of Cu fulvic acid complexes for the hematite surface compared to free Cu cations. As mentioned earlier, several researchers have invoked such ternary surface complexes to fit experimental data for mineral organic matter systems. Because the concentration of binding sites on fulvic acid is roughly an order of magnitude larger than the Cu concentration used in our experiments, a greater affinity of Cu fulvic acid complexes for the hematite surface may only be evident in the Cu sorption data, and less so in the fulvic acid adsorption data. Although this would be consistent with the results shown in Figure 3, further experimental evidence is needed to test this hypothesis. A further assumption of the linear additivity approach is that fulvic acid is not fractionated with respect to its Cu binding affinity upon adsorption. However, Davis and Gloor (1981) found that dissolved organic matter from a lake was fractionated with respect to its size when adsorbed to colloidal alumina. Similary, Wang et al. (1997) reported that a fulvic acid was fractionated during adsorption to goethite and that the more acidic and aromatic compounds were preferentially adsorbed. Christl et al. (2001) studied Cu binding by two size-fractions of a soil humic acid. The smaller size-fraction was higher in carboxylic and phenolic functional groups and aromatic carbon and exhibited by 10 to 20% higher Cu binding at given free Cu concentration than the larger size-fraction, which contained more aliphatic carbon backbones. 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