Alumina Transformation and Copper Speciation As Revealed by EPR

May 15, 2008 - CARMEN ENID MARTÍNEZ* AND. NADIA MARTÍNEZ-VILLEGAS. Department of Crop and Soil Sciences, The Pennsylvania State. University ...
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Environ. Sci. Technol. 2008, 42, 4422–4427

Copper-Alumina-Organic Matter Mixed Systems: Alumina Transformation and Copper Speciation As Revealed by EPR Spectroscopy C A R M E N E N I D M A R T ´I N E Z * A N D N A D I A M A R T ´I N E Z - V I L L E G A S Department of Crop and Soil Sciences, The Pennsylvania State University, University Park, Pennsylvania 16802

Received December 20, 2007. Revised manuscript received March 25, 2008. Accepted March 31, 2008.

The chemical forms and solubility of Cu in alumina-organic matter systems were studied separately (Cu/Al and Cu/OM) and in mixtures (Cu/Al/OM) during long-term (up to 8 years) equilibrations at pH 6 and 7.5. The transformation of alumina was monitored by XRD, while the chemical forms of Cu were probed by EPR spectroscopy. Total dissolved Cu was determined by voltammetry. Alumina transformation to gibbsite was more rapid and complete in the Cu/Al system equilibrated at pH 7.5 than at pH 6. The presence of colloidal organic matter (Cu/Al/OM) retarded the transformation of alumina. This effect was more pronounced in the system aged at pH 7.5, likely due to the higher pH that promotes formation of Al3+-organic matter coordination complexes. As expected, the systems at pH 7.5 resulted in lower dissolved Cu concentrations than corresponding systems at pH 6. After long-term equilibrations (8 and 5 years) at pH 6 and 7.5, however, the aluminacontaining coprecipitates resulted in the lowest concentrations of Cu in solution (Cu/Al < Cu/Al/OM < Cu/OM). Analyses by EPR spectroscopy indicated that Cu forms inner-sphere complexes in all systems at both pH values. Changes in the chemical forms of coprecipitated Cu (Cu/Al and Cu/Al/OM systems) occurred with time and included Cu occupying discrete sites where Cu-O-Al bond formation was dominant followed by formation of clusters (Cu-O-Cu associations) and in some cases precipitates. The anisotropic EPR parameters of the Cu/OM systems suggested that stronger interactions exist between Cu and organic matter functional groups as compared to Cu interactions with alumina-containing coprecipitates; yet, Cu solubility was highest in the Cu/OM systems. The geochemical processes described in this investigation may be effective in forest soils and wastewater treatment plants where Al and Fe salts are used as flocculation agents and to remove metal contaminants from solution.

Introduction Oxide and organic adsorbents provide high surface areas and functional groups with a variety of binding affinities for retention of metal contaminants and nutrients in soils and natural waters. Their individual function in metal retention * Corresponding author phone: 814-863-5394; fax: 814-863-7043; e-mail: [email protected]. 4422

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is difficult to discern because they exist mostly as oxideorganic complexes and their relative effectiveness depends on system parameters such as pH. In the acidic pH range (pH < 6.5), for example, organic materials have been shown to be more efficient adsorbents for Cu when compared with iron oxides (1), while at pH g 7 the oxide phase controlled metal solubility to the lowest level. Using single and mixed ferrihydrite-organic matter systems, Martı´nez and McBride (2) showed that during equilibration for ∼200 days at pH 5.5, the ferrihydrite-organic matter mixed system resulted in the lowest Cu solubility when compared with ferrihydrite and organic matter single systems. Whether Cu solubility was a result of bonding to the oxide or organic component, however, was not investigated. Although heavy metal solubility is initially reduced by sorption reactions, the long-term solubility may be dictated by changes occurring in the metal’s chemical form, morphological characteristics, or segregation patterns. It was demonstrated, for example, that Cu occupies isolated sites in an initially noncrystalline alumina and aging of the coprecipitate for up to 2 years resulted in alumina crystallization and migration of Cu atoms toward the surface of the primary precipitate, forming clusters and discrete CuO phases at pH 7.5 (3). Although metal (hydr)oxides may be an important sink for metal retention, their effectiveness in the presence of additional adsorbents (i.e., organic matter) has not been clearly demonstrated. Organic adsorbents contain oxygen, nitrogen, and sulfur functional groups that bind heavy metals strongly and can therefore reduce their solubility. The contribution of O- and N-containing functional groups in Cu retention, however, is hard to discern because spectroscopic techniques such as EXAFS cannot differentiate between O and N coordination environments. Studies using electron paramagnetic resonance (EPR) spectroscopy have indicated that Cu forms innersphere complexes with humic substances via coordination with oxygen-containing functional groups (4–6). Other studies have reported involvement of N ligands in Cu retention by humic substances at low Cu loadings (7–9), while still others have suggested Cu binding in organic soils and ternary complexes (Cu-organic-alumina) involves carboxyl, carbonyl, and amine functional groups (9–11). On the other hand, nitrogen involvement in Cu retention was clearly demonstrated in studies with macromolecules, where Cu-N bonding in a histidine-rich protein (12) and in a metalloenediyne (13) was reported. This study investigates the chemical forms and solubility of Cu, and the transformation of alumina in single and mixed alumina-organic matter suspensions aged for up to 8 years at pH 6 and 7.5. The efficacy of these systems (Cu/Al, Cu/ OM, Cu/Al/OM) in maintaining low Cu concentrations in solution is compared. A leaf compost was chosen as a surrogate for colloidal organic matter, and alumina was chosen as the oxide phase. Copper was studied because it interacts strongly with both alumina and organic matter and is suitable for EPR spectroscopy. EPR spectroscopy was used to monitor the chemical forms of Cu over time. X-ray diffraction (XRD) was used to identify and follow the transformation of noncrystalline alumina after its initial precipitation in single (Cu/Al) and mixed (Cu/Al/OM) systems where colloidal organic matter was present. The long periods of time used in this study provide a more realistic assessment of metal-solid phase interactions. 10.1021/es703206u CCC: $40.75

 2008 American Chemical Society

Published on Web 05/15/2008

Experimental Section Cu2+

Solid Phases. Batch-type reaction experiments with concentrations of 1500 mg of Cu2+ (kg of solid)-1 were studied. This Cu2+ level represents its concentration in the solid phase if complete coprecipitation or complexation occurred. The experiments were run for up to 8 years at room temperature (23 °C) in the presence of ambient CO2. All suspensions remained oxic throughout the duration of the experiment and had measured ORP values of ∼+400 mV (using an oxidation-reduction potential electrode). No odor and a clear supernatant were present in all suspensions throughout the duration of the experiment. Cu-Alumina Coprecipitates (Cu/Al Systems). Solutions containing 12 × 10-3 M Al3+ (as Al(NO3)3) and 3.68 × 10-5 M Cu2+ (as Cu acetate) in 1 mM KNO3 background electrolyte were titrated (25 mL min-1) with 0.1 M KOH until the pH reached 6 or 7.5. The Al concentration used was calculated to yield 0.33 g of alumina (as Al(OH)3) in 750 mL of suspension. The pH was kept constant throughout the experiment; addition of small amounts of base was necessary to keep the pH constant initially (2 weeks) but not at later times. Aliquots of the suspension were removed at time intervals (1 day, 100 days, 5 years, and 8 years) and centrifuged for 15 min at 27 000 rcf (15 000 rpm). The supernatants were filtered through a 0.2 µm membrane filter, acidified by addition of 1 M HCl (1 drop), and analyzed for total dissolved Cu by differential pulse anodic stripping voltammetry (dpasv) as described below. The Cu/Al coprecipitates (solids) collected at various time intervals were freeze dried and kept for XRD and EPR analyses as described below. Cu-Organic Matter Complexes (Cu/OM Systems). Leaf compost (predominantly sugar maple) was used as a natural model system for organic matter in our experiments. The chemical characteristics of the leaf compost are reported in Martı´nez and McBride (2) and in Table S1 of the Supporting Information. Briefly, the leaf compost has a pHwater of 7.12, 4.9 g of Al kg-1 solid, 19 mg of Cu kg-1 solid, and an organic carbon content of 37.9%. The leaf compost has an effective cation exchange capacity (CEC) of 142 cmol(+) kg-1. The pH of the leaf compost was adjusted to 5.5 by addition of 1 M HNO3. Then the leaf compost was dialyzed to remove excess salts and freeze dried prior to use. The pH of a suspension containing 3.68 × 10-5 M Cu2+ (as Cu acetate) and leaf compost (0.33 g) in 1 mM KNO3 background electrolyte was raised to pH 6 or 7.5 by addition of 0.1 M KOH (25 mL min-1) to a total volume of 750 mL. Small additions of acid were needed to keep a constant pH throughout the experiment. Aliquots of the suspension were removed, centrifuged (15 min at 27 000 rcf), and filtered (0.2 µm membrane filter) after 1 day, 90 days, and 5 years of aging. The supernatants were acidified (1 drop of 1 M HCl) and analyzed for total dissolved Cu by dpasv (described below). Dissolved organic carbon (DOC) was measured in nonacidified supernatants as described below. The Cu/OM complexes (solids) were freeze dried and kept for XRD and EPR analyses (described below). Cu-Alumina-Organic Matter Coprecipitates (Cu/Al/OM Systems). Suspensions containing 12 × 10-3 M Al3+ (as Al(NO3)3), 7.36 × 10-5 M Cu2+ (as Cu acetate), and 0.33 g of leaf compost in 1 mM KNO3 background electrolyte were titrated (25 mL min-1) with 0.1 M KOH until the pH reached 6 or 7.5 (total volume of 750 mL). The suspensions were aged for up to 5 years at constant pH. This procedure was designed to yield solid phases with a 1:1 alumina:leaf compost ratio (by weight). The suspensions were sampled after 1 day, 90 days, and 5 years of aging and Cu and DOC analyses performed as for the Cu/OM systems. The solid phases were freeze dried and characterized by XRD and EPR spectroscopy (see below). The mode of synthesis of the Cu/Al/OM systems allowed for metal (Al3+ and Cu2+) complexation with func-

tional groups of organic matter and Cu2+ coprecipitation with alumina. Analyses of the Solution Phase. The total concentration of Cu in solution (acidified supernatants) was analyzed by differential pulse anodic stripping voltammetry (dpasv) using a 797 VA Computrace (Metrohm). In stripping analysis, the hanging mercury drop electrode (HMDE) was employed as the working electrode. Dissolved oxygen was removed from the samples by prepurging N2(g) through the solution for 5 min. The deposition step was carried out at -1.2 V for 2 min (stirring at 2000 rpm), followed by a 30 s equilibration period and metal stripping. Nonpurgable dissolved organic carbon (DOC) concentrations were determined by combustion to CO2 followed by IR detection using a Shimadzu TOC-5000 Total Organic Carbon Analyzer. XRD Analyses. The X-ray diffraction (XRD) patterns of freeze-dried solid phases (random orientation) were collected using a SCINTAG PAD V theta-2theta diffractometer with a liquid-nitrogen-cooled germanium solid-state detector and a Cu KR radiation source. XRD analyses were used to identify and follow the transformation of noncrystalline alumina after its initial formation in Cu/Al and Cu/Al/OM systems. Gibbsite was identified using the computer program Jade+ (version 7.1, manufactured by Materials Data, Inc., Livermore, CA). The XRD patterns of the Cu/OM systems were also collected, but as expected, only a quartz peak was present (figure not shown). EPR Spectra of Solid Phases. The EPR (electron paramagnetic resonance) spectra of the solid phases were collected using a Bruker ESP 300 E (X-band) spectrometer. The solid phases were weighed (15 mg) into quartz EPR tubes, and their spectra (10 coadded scans) were recorded at room temperature using 50 mW of power and a frequency of 9.8 GHz.

Results and Discussion Alumina Transformation. Titration of solutions containing Al3+ and Cu2+ to pH 6 and 7.5 resulted in formation of noncrystalline alumina initially (Figure 1). Alumina transformation to gibbsite was observed after 100 days of aging the Cu/Al coprecipitates at pH 7.5. Although alumina was only partly transformed after 100 days in the Cu/Al system at pH 6, the XRD patterns of 5 and 8 years coprecipitates indicated the presence of gibbsite. Nevertheless, the XRD patterns of the Cu/Al coprecipitates aged at pH 7.5 (100 days and 5 and 8 yrs) presented lower width at half-height (w1/2) values, thus reflecting a higher degree of structural order and crystallite size (Figure 1). Increases in structural order were also observed in the Cu/Al coprecipitates at pH 6; however, the crystallite size, even after aging for 8 years, was smaller. The presence of colloidal organic matter retarded the transformation of alumina to gibbsite as evidenced by the Cu/Al/OM systems at pH 6 and 7.5 (Figure 1). In fact, the XRD patterns showed no evidence of well-crystallized gibbsite in the Cu/Al/OM systems after 5 years of aging. Colloidal organic matter seemed to have an inhibitory effect on the (trans)formation process that was more pronounced in the Cu/Al/OM system at pH 7.5 (Figure 1). This inhibitory effect was perhaps due to an increase in the extent of acid dissociation of organic matter functional groups (i.e., carboxylic, phenolic) at higher pH that promoted formation of Al3+-organic matter coordination complexes. Negatively charged functional groups present in organic matter can complex Al3+, thus hindering the precipitation process. Alternatively, organic matter functional groups can complex Al3+ and the newly formed Al-organic complexes can then serve as nucleation sites for formation of aluminum (hydr)oxide phases of nanometer size. For example, microbial polysaccharides have been shown to serve as templates for VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. X-ray diffraction (XRD) patterns for the Cu-alumina (Cu/Al) and Cu-alumina-organic matter (Cu/Al/OM) systems aged at pH 6 and 7.5 for up to 8 years. The width at half-height (w1/2) was calculated for the peak at 18.3° 2θ (d ) 0.4844 nm and hkl 002) and is shown for each plot (n.d., not determined). Ordinate scales are the same for all plots except for the 1 day samples. formation of submicrometer diameter Fe oxyhydroxides (14). Nanometer-sized aluminum (hydr)oxide phases perhaps formed but were not detected by conventional XRD analyses at initial (1 and 90 days) conditions. Using the Debye-Scherrer equation (Dhkl ) kλ/β cos θ, where D represents the size of the crystallite, the constant k is typically close to unity and ranges from 0.8 to 1.39 (0.9 used in this calculation), λ is the wavelength of the X-rays, β is the full width at half-maximum in radians, and θ is the angle of incidence), for example, we estimated the average crystallite size in the Cu/Al/OM system at pH 7.5 to be 4.24 nm after 5 years of aging. Copper in Solution and Dissolved Organic Carbon. The pH of the system affected the long-term solubility of Cu (Figure 2). As expected, corresponding systems at pH 7.5 resulted in lower concentrations of Cu in solution than at pH 6. In agreement with previous findings (3), dissolved Cu decreased in Cu/Al coprecipitates during the first 200 days of aging. However, dissolved Cu remained relatively constant for the next 8 years in the system at pH 7.5 while it decreased significantly after 8 years in the system equilibrated at pH 6 (Figure 2). The time period at which decreases in dissolved Cu occurred in the Cu/Al system (pH 7.5) coincided with the initial decrease in width at half-height (w1/2) values that reflected an increase in degree of structural order of the coprecipitate. The concentration of dissolved Cu in the Cu/ Al coprecipitates at pH 6 was lower than predicted by copper hydroxide (478 × 10-6 M) and oxide (46 × 10-6 M) phases. Copper solubility in the Cu/Al coprecipitates at pH 7.5 was close to values predicted by copper hydroxide (0.48 × 10-6 M) and oxide (0.046 × 10-6 M) phases. 4424

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FIGURE 2. Total dissolved Cu in the Cu-alumina (Cu/Al), Cualumina-organic matter (Cu/Al/OM), and Cu-organic matter (Cu/OM) systems aged at pH 6 and 7.5 for up to 8 years. The alumina-containing systems at pH 7.5 resulted in the lowest concentrations of dissolved Cu (Cu/Al < Cu/Al/OM) throughout the duration of the experiment (Figure 2). Similar long-term behavior was observed for the systems equilibrated at pH 6. The final (after 5 and 8 years) concentration of dissolved Cu was highest in the organic matter (Cu/OM) systems and lowest in the alumina-containing (Cu/Al < Cu/ Al/OM) systems at pH 6 and 7.5. Increases in total dissolved Cu in a Cu-organic matter system aged for up to ∼200 days

FIGURE 3. Electron paramagnetic resonance (EPR) spectra of Cu coprecipitates with alumina (Cu/Al) formed by titration to pH 6 and 7.5 and aged for up to 8 years. All spectra collected with the gain set to 1 × 104. The EPR parameters of the rigid-limit (paramagnetic) Cu are indicated by g| (2.36) and g⊥ (2.07) and the free electron resonance position by ge (2.0023). at pH 5.5 were observed and attributed to Cu complexation with dissolved organics (2). In that study, a ferrihydriteorganic matter system aged for ∼200 days at pH 5.5 resulted in the lowest concentration of dissolved Cu when compared to ferrihydrite and organic matter single systems. Although the shorter term (up to 200 days) solubility results of this work (Figure 2) agree with those reported for the ferrihydrite-organic matter systems (2), we now find that after longer equilibration times (5 and 8 years) the Cu/Al systems resulted in the lowest concentration of Cu in solution. Similar long-term behavior might be expected in systems where other oxide phases (i.e., ferrihydrite) are present. No definite trend was observed between dissolved organic carbon (DOC) and aging time; yet, lower concentrations of DOC were released to the solution phase from the Cu/Al/ OM systems than from the Cu/OM systems at pH 6 and 7.5 (Figure S1, Supporting Information). It is possible that Al3+ complexation suppressed the decomposition or hydrolysis of organic matter in the Cu/Al/OM systems or that Al3+ served as a flocculant, thus preventing oxidation and/or physical disruption of the organic material. Dissolved organics might also sorb onto the surface of alumina, thus resulting in a decrease in the concentration of DOC. Furthermore, the Cu/ OM system at pH 7.5 released the most DOC, possibly due to increased organic matter dissolution at higher pH. No clear trends were observed between total dissolved Cu and DOC (data not shown). Solid-Phase Speciation of Cu As Revealed by EPR Spectroscopy. Electron paramagnetic resonance (EPR) spectroscopy was used to determine the chemical forms and monitor the dynamic behavior of Cu after long-term reaction in alumina (up to 8 years), organic matter (up to 5 years), and alumina-organic matter (up to 5 years) mixed systems. Cu/Al Coprecipitates. The EPR spectra of Cu coprecipitates with alumina aged for up to 8 years at pH 6 and 7.5 are presented in Figure 3. The rigid-limit (anisotropic) Cu-EPR spectra prevailed in all coprecipitates and indicated (on the time scale of the instrument) reduced movement (“tumbling”) of the Cu2+ ion due to inner-sphere bonding. The rigid-limit Cu-EPR spectra indicate Cu-O-Al bond formation and are attributed to Cu occupying magnetically isolated (well-dispersed) sites within the alumina and/or at the

surface. Slow reactions between Cu and noncrystalline alumina resulted in increased intensity of the rigid-limit CuEPR signal after 100 days of aging. The EPR parameters of the rigid-limit Cu were estimated to be g| ) 2.36 and g⊥ ) 2.07 for coprecipitates formed at pH 6 and 7.5. The EPR parameters are similar to those reported for Cu adsorption to noncrystalline alumina over the pH range 4-8 (g| ) 2.344; g⊥ ) 2.069) (9) and for Cu coprecipitation with alumina (g| ) 2.35; g⊥ ) 2.07) (3). Broadening of the EPR signal occurred after aging the coprecipitates for 5 and 8 years; however, the hyperfine lines of the g| component were still present (Figure 3). The appearance of an underlying broad signal suggests clustering of Cu atoms, leading to shorter Cu-Cu distances and magnetic interactions (4) either at the surface or within the primary precipitate. Broadening could also indicate the presence of Cu complexes with a range of g values and thus different chemical environments. The presence of an underlying broad signal together with the parallel hyperfine components suggests a composite spectrum of more than one Cu species: Cu at magnetically isolated sites and Cu forming clusters or discrete solid phases. The simultaneous occurrence of clusters (Cu-O-Cu associations) and adsorption at isolated sites (Cu-O-Al bond formation) have been shown for Cu in alumina-gibbsite (3) and boehmite (15, 16) systems. Thus, the Cu-EPR spectra indicated that changes in the chemical forms of coprecipitated Cu occurred with time that included Cu-O-Al bond formation between Cu and alumina followed by formation of clusters and perhaps precipitates. Cu/OM Complexes. The rigid-limit Cu-EPR spectra were also present in the Cu/OM systems, indicating formation of inner-sphere complexes. The hyperfine lines were present and had estimated values of g| ) 2.31 and g⊥ ) 2.06 at both pH 6 and 7.5 (Figure 4). Slow reactions between Cu and colloidal organic matter increased the intensity of the rigidlimit Cu-EPR signal with increasing aging time. The EPR parameters are consistent with those reported for Cu2+ complexation with humic acid extracted from an acid surface soil (4) and humic substances (HS) isolated from swamp water at a HS:Cu molar ratio of 0.6 (5). Moreover, a resonance near g ) 2.14 appeared in the Cu/OM system at pH 6. This resonance represents the motional averaging of the g| and VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Electron paramagnetic resonance (EPR) spectra of Cu in Cu-organic matter (Cu/OM) complexes and Cu-alumina-organic matter (Cu/Al/OM) mixed systems formed by titration to pH 6 and 7.5 and aged for up to 5 years. Gain: 1 × 104 for all pH 6 systems; 1 × 105 for all Cu/Al/OM at pH 7.5; and 6.3 × 104 (1 day), 6.3 × 103 (90 days), and 8.0 × 102 (5 years) for Cu/OM systems at pH 7.5. The EPR parameters of the rigid-limit (paramagnetic) Cu are indicated by g| (2.36 in Cu/Al/OM and 2.31 in Cu/OM systems) and g⊥ (2.07 in Cu/Al/OM and 2.06 in Cu/OM systems), and the free electron resonance position by ge (2.0023). g⊥ values (gaverage ) 1/3g| + 2/3g⊥) by dynamic Jahn-Teller distortion and indicates Cu2+ coordination to organic matter functional groups (4). The resonance at g ) 2.14 differs from the g ) 2.184 isotropic Cu2+ line characteristic of aqueous Cu(H2O)62+ and reported for Cu-exchanged montmorillonite at pH 4 (17). The Cu/OM (pH 7.5) and Cu/Al/OM (pH 6) systems showed the g ) 2.14 resonance at initial conditions (1 day), also indicating Cu2+ coordination to organic matter and/or OH- ligands in alumina (Figure 4). Furthermore, the initial (1 day) EPR spectra of the Cu/OM systems at pH 6 and 7.5 showed three resonances at g values lower than 2.00 (Figure 4). These resonances were also present, to a lesser extent, in the Cu/Al/OM system at pH 6. These three resonances at g < 2.00 revealed the presence of natural Mn2+ (hyperfine splitting) in the leaf compost and were initially superimposed on the Cu2+ signal. The Mn2+ g value is centered at 2.00, very close to the free electron resonance position, and reveals six resonances. The Mn2+ resonances disappeared after aging of the Cu/OM systems, presumably the result of increased sorption of Cu by organic matter. Alternatively, their disappearance may be due to Mn2+ oxidation. Due to the heterogeneity of functional groups present in organic matter, it is impossible to unequivocally assign a specific type of chemical bond between Cu and organic matter in the Cu/OM systems using EPR spectroscopy. The 4426

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g| (2.31) component of the Cu/OM systems is higher than those reported for Cu-N ligation in macromolecules and therefore indicates a weaker type of interaction. For example, recent EPR studies reported Cu-N ligation with estimated g values of 2.285 in a histidine-rich protein (12) and 2.27 in a metalloenediyne (13). Cooper bonding to a large synthetic metalloprotein, however, revealed coordination to four oxygen donor atoms where g| values of 2.385 and 2.32 suggested the existence of two different Cu(II) complexes (18). Studies of Cu interactions with environmental samples are more challenging, but some have reported involvement of N ligands in Cu retention by humic substances at low Cu loadings (6–8), while others have suggested Cu binding in organic soils and ternary complexes involves carboxyl, carbonyl, and amine functional groups (9–11). Using the Peisach and Blumberg plots (19), which relate EPR parameters to the number and nature of coordinating (organic) ligand atoms based on the formal charge of the complex (charge/ ligand model), we can deduce information about the type and number of ligands in our system. The EPR parameters of the Cu/OM systems lie in the region characteristic for coordination to four oxygens with the number of axial ligands not being defined. Similar coordination environments (Cu(H2O)2O4) and g| values (2.31) have been reported for Cu-humic substance complexes (5).

Cu/Al/OM Coprecipitates. The rigid-limit Cu-EPR spectra prevailed in the Cu/Al/OM systems aged for up to 5 years at pH 6 and 7.5 (Figure 4). As with the Cu/Al coprecipitates, the EPR parameters of the rigid-limit Cu were estimated to be g| ) 2.36 and g⊥ ) 2.07, suggesting formation of Cu-O-Al bonds. Opposite to what was observed in the Cu/OM complexes, the rigid-limit Cu-EPR signal intensity for the Cu/Al/OM systems decreased with time. A reduction in signal intensity has been attributed to exchange coupling interactions with neighboring paramagnetic ions (Cu) or spin-spin relaxation as Cu ions cluster (16). Thus, Cu-O-Cu associations occurred even though alumina transformation to gibbsite was far from complete (Figure 1) and a substantial decrease in surface area of the primary precipitate (alumina) is not expected. A potential explanation is that smaller (nanometer size) alumina colloids formed in the presence of the organic matrix (as suggested by the XRD patterns presented in Figure 1), thus providing smaller volumes for coprecipitation of Cu. This scenario can result in higher Cu: Al ratios within the alumina and promote Cu-Cu interactions. Another potential explanation is that Al3+ formed complexes with functional groups of organic matter that resulted in a reduction of the number of organic matter retention sites otherwise available for complexation of Cu2+. The later scenario could also result in increased Cu-Cu interactions and decreased intensity of the rigid-limit CuEPR signal. Furthermore, a broad EPR signal appeared after 5 years in the Cu/Al/OM system formed at pH 7.5, while signal broadening was not apparent in the pH 6 Cu/Al/OM system. Signal broadening, together with a reduction in signal intensity and weak parallel hyperfine components, suggests copper (hydr)oxide phases formed after aging the Cu/Al/ OM system at pH 7.5 for 5 years. System Comparison: Coprecipitation vs Complexation. The EPR parameters of the rigid-limit Cu were estimated to be g| ) 2.36 and g⊥ ) 2.07 in alumina-containing systems (Cu/Al and Cu/Al/OM) and g| ) 2.31 and g⊥ ) 2.06 in the organic matter (Cu/OM) systems equilibrated at pH 6 and 7.5 and aged for up to 8 years. The fact that the Cu-EPR parameters are identical in alumina-containing systems and differ from the organic matter systems provide evidence for the predominance of Cu coprecipitation with alumina in the mixed systems under the experimental conditions used. It also suggests similar ligation environments in alumina-containing systems. Since g| reflects the strength of Cu-solid interactions, it is clear from the decrease in g| that stronger bonds are formed between Cu and organic matter functional groups in the Cu/OM systems. Although the EPR data indicate stronger electron donation by ligand groups in the organic matter than by OH groups of alumina, the g| value is not low enough to indicate participation of N functional groups of organic matter. It seems that when specific adsorption reactions are the main retention mechanism for Cu, organic soil constituents (both soil organic matter and dissolved organic matter) control Cu partitioning and solution speciation (20). On the other hand, when the potential for coprecipitation reactions exists, as in the mixed Cu-alumina-organic matter systems of this investigation, Cu will tend to form a coprecipitate with the metal oxide phase. The geochemical processes reported herein may occur in forest soils and wastewater treatment plants where Al and Fe salts are used as flocculation agents and to remove metal contaminants from solution.

Acknowledgments This research was funded by the NRI-USDA Competitive Grants Program (Award no. 2003-35107-13650). We thank Drs. John H. Golbeck and Rama Balasubramanian in the Department of Biochemistry and Molecular Biology at The Pennsylvania State University for use of the EPR spectrometer.

Supporting Information Available Chemical composition of the leaf compost (Table S1), and concentration of dissolved organic carbon in the Cu/Al/OM and Cu/OM systems as a function of time (Figure S1). This material is available free of charge via the Internet at http:// pubs.acs.org.

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