Environ. Sci. Technol. 1998, 32, 743-748
Solubility of Cd2+, Cu2+, Pb2+, and Zn2+ in Aged Coprecipitates with Amorphous Iron Hydroxides C A R M E N E N I D M A R T IÄ N E Z * A N D MURRAY B. MCBRIDE Department of Soil, Crop, and Atmospheric Sciences, Cornell University, Ithaca, New York 14853
Coprecipitates of heavy metals in iron oxides are possible long-term sinks that could limit metal mobility and toxicity in soils. To test the efficacy of this process to maintain low metal solubilities, rapid and slow simultaneous titrations of Cd2+, Cu2+, Pb2+, and Zn2+ were conducted in the presence of Fe3+ by titration with KOH to pH 6 in order to form coprecipitates having concentrations of 100 mg kg-1 Cd2+, 1500 mg kg-1 Cu2+, 500 or 1500 mg kg-1 Pb2+, and 3000 mg kg-1 Zn2+. The final soluble concentrations of Cd2+ (0.28 and 0.16 µM), Zn2+ (8.4 µM), Cu2+ (0.60 µM), and Pb2+ (99%) under the experimental conditions used.
Results Rate of Titration and Aging of the Coprecipitate. The simultaneous titration of Cd2+, Cu2+, Pb2+, Zn2+, and Fe3+ to pH 6 removed variable fractions of the heavy metals with iron hydroxide (Figures 1-3). At pH 6, copper(2+) and lead(2+) hydroxides could have formed but cadmium(2+) or zinc(2+) hydroxide formation could not have occurred during the coprecipitation. Yet, the rapid addition of base can result in localized high alkalinity that may promote metal hydrolysis and polymerization, especially if a solid phase is already present in the system (e.g., iron (hydr)oxide). Both the rapid and slow titrations to pH 6 resulted in incomplete Fe3+ hydrolysis, as indicated by an unstable pH condition. The rapid titration, however, resulted in a more pronounced decrease in suspension pH with aging of the coprecipitate. The pH of the suspensions decreased to about pH 5.0 upon aging of the precipitate after initially being raised to 6.0 as shown in Figures 1-3. However, after about 100 days, the pH rebounded to about 5.5. This pH reversal may be the result of slow system equilibration with atmospheric CO2, which should control pH at about this value. An inverse relationship existed between soluble Cu2+ and suspension pH. The slow titration resulted in a rebound in soluble Cu2+ from the lowest value immediately following coprecipitate formation to about 6% of total Cu2+ added (0.60 µM, pCu ) 6.22) after 200 days (Figure 1). The Cu2+ concentration in solution after rapid titration increased from about 3% of total Cu2+ added at time zero (pH 6) to about 20% after 40 days, apparently due to desorption induced by the pH decrease. Subsequently, soluble Cu2+ again decreased, to about 6% of total Cu2+ added, after 200 days (0.60 mM, pCu ) 6.22) in response to an upward drift of pH. Thus, Cu2+ solubility appears to be responding more strongly to pH changes than to any structural changes in the coprecipitate induced by aging. Although a similar final “steady state” is reached independent of the initial rate of titration, the rapid coprecipitation results in greater soluble Cu2+ during the first 100 days of aging (Figure 1). Rapid titration creates very alkaline conditions locally, leading to localized hydrolysis of trace metal cations as well as Fe3+. Metastable metal hydroxy polymers and precipitates can form rapidly under these conditions, redissolving slowly with reduction of suspension pH (18). During titration, Cu2+ hydrolysis may lead to (a) precipitation as a separate Cu(OH)2 phase, (b)
FIGURE 1. Soluble Cu2+ and pH of suspension as a function of aging of the coprecipitate. The insert shows soluble Cu2+ as a function of suspension pH. Open circles represent soluble Cu2+ after the rapid (6.24 mL min-1) titration, while open squares represent soluble Cu2+ after the slow (0.03 mL min-1) titration. Closed circles and squares show changes in suspension pH after the rapid and slow titration, respectively.
FIGURE 2. Soluble Zn2+ and pH of suspension as a function of aging of the coprecipitate. Open circles represent soluble Zn2+ after the rapid (6.24 mL min-1) titration, while open squares represent soluble Zn2+ after the slow (0.03 mL min-1) titration. Closed circles and squares show changes in suspension pH after the rapid and slow titration, respectively. polymerization and adsorption onto the surface of ferrihydrite, or (c) polymerization and occlusion within the ferrihydrite structure. In the first and second case, if the pH subsequently decreases, Cu2+ hydrolysis products become less stable and can depolymerize or disassociate, consuming H+ or releasing OH- to the system. This would allow the Cu2+ ion either to readsorb onto the surface or be incorporated into the ferrihydrite structure. If the hydrolysis
product is occluded within the ferrihydrite structure, rearrangement of the solid structure could release OH- into solution and maintain a low solubility. Unlike Cu2+, soluble Cd2+ in the suspensions was virtually unaffected by the pH changes induced by aging (Figure 3), possibly because the threshold pH value for Cd2+ precipitation as hydroxides is above 6. Even 200 days after the initial coprecipitation (rapid titration), about 72% of the total Cd2+ VOL. 32, NO. 6, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Soluble Cd2+ and pH of suspension as a function of aging of the coprecipitate. Open circles represent soluble Cd2+ after the rapid (6.24 mL min-1) titration, while open squares represent soluble Cd2+ after the slow (0.03 mL min-1) titration. Closed circles and squares show changes in suspension pH after the rapid and slow titration, respectively. added remained in solution (0.28 µM, pCd ) 6.55). The slow titration resulted in about 41% of the total Cd2+ added remaining in solution (0.16 µM, pCd ) 6.79) after 200 days of aging (Figure 3). This decreased solubility occurred after an approximately constant value of 0.30 µM was maintained during the first 100 days of aging. A study by Ainsworth et al. (19) suggested Cd2+ incorporation into the recrystallizing HFO structure after long reaction times and transformation to goethite. In their study, Cd2+ was added to freshly prepared HFO and the pH was maintained at about 7. Despite having an ionic radius (0.097 nm) incompatible with structural incorporation, Cd2+ displayed increased desorption hysteresis with aging at pH values below 6.5. Yet another study found that Cd2+ coprecipitated with iron oxide partitioned into goethite at pH 11 but became more extractable over time (14). The experimental conditions of the latter study were favorable to retention by various mechanisms, and Cd2+ may have been located within the oxide structure, may have been adsorbed onto the ferrihydrite surface, or may have formed a separate (hydr)oxide phase. Our results show a slight increase in soluble Zn2+ between 50 and 75 days after the initial coprecipitation (rapid titration), but about 42% of the total added Zn2+ (8.4 µM, pZn ) 5.07) remained in solution after 200 days, independent of the rate of titration (Figure 2). Although the overall relationship of soluble Zn2+ to pH is weak (see Figure 5), the rapid titration clearly produced a reciprocal temporal relationship (mirror image) between soluble Zn2+ and pH (Figure 2), comparable to that seen for Cu2+ (Figure 1). This illustrates a transitory dependence of Zn2+ and Cu2+ solubility on pH, which was not observed with Cd2+. Lead was the only metal sufficiently removed from solution by coprecipitation that it could not be detected by dpasv, and therefore the Pb2+ coprecipitate had a solubility below 5 nM, the detection limit of the dpasv method used in this study. Coprecipitation, Surface Complexation, and Pure Mineral Phases. The solubility of Cu2+ in the coprecipitates formed is pH controlled. This is evident after pCu is calculated and plotted against pH (Figure 4). Despite the 746
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FIGURE 4. Cu2+ solubility (pCu) as calculated from the data obtained after rapid (open circle) and slow (open square) titration and pH of the suspension. The complete line represents the pCu as calculated from the surface complexation constants compiled by Dzombak and Morel (21). Dashed lines denote the pCu of different Cu compounds according to Lindsay (20). observed fluctuations in pH, the solubility of Cd2+ and Zn2+ (pCd and pZn) is a constant value during aging of the coprecipitates (Figures 5 and 6). Thus, there is no evidence that solubility values change systematically with aging of the coprecipitates or with changes in pH. Solubility lines of pure mineral phases (Ksp) and surface complexation constants for high-affinity binding sites on hydrous ferric oxide (K1int) are plotted in Figures 4-6 for comparison with soluble Cd2+, Cu2+, and Zn2+ obtained from this coprecipitation study. The solubility lines for various
constants are derived from adsorption experiments with approximately 0.001-0.01 mol of M2+/mol of Fe, with calculations fixed to 0.005 mol of M2+/mol of Fe. By comparison, loadings in our study are 0.004 mol of Zn2+/mol of Fe, 7.9 × 10-5 mol of Cd2+/mol of Fe, 0.0002 or 0.0006 mol of Pb2+/mol of Fe, and 0.002 mol of Cu2+/mol of Fe, adding to a total of 0.0065-0.0069 mol of M2+/mol of Fe. The intrinsic surface complexation constants for HFO, K1int, are based on the reaction:
FesOH0 + M2+ S FesOM+ + H+
FIGURE 5. Zn2+ solubility (pZn) as calculated from the data obtained after rapid (open circle) and slow (open square) titration and pH of the suspension. The complete line represents the pZn as calculated from the surface complexation constants compiled by Dzombak and Morel (21). Dashed lines denote the pZn of different Zn compounds according to Lindsay (20).
FIGURE 6. Cd2+ solubility (pCd) as calculated from the data obtained after rapid (open circle) and slow (open square) titration and pH of the suspension. The complete line represents the pCd as calculated from the surface complexation constants compiled by Dzombak and Morel (21). Dashed lines denote the pCd of different Cd compounds according to Lindsay (20). pure minerals were obtained from data compiled by Lindsay (20). Solubility lines for metal carbonates were calculated assuming a constant CO2(g) pressure (0.0003 atm). Solubility lines of “soil-Zn” and “soil-Cu” are based on studies in which (Zn or Cu)EDTA and (Zn or Cu)DTPA were reacted with soils of different pH and the total Zn or Cu remaining in solution was measured after 30 days. These measurements provide an estimation of the equilibrium constant for the soil-Zn or soil-Cu reactions and provide a useful reference solubility for Zn and Cu in soils (20). The intrinsic oxide surface complexation constants were obtained from data collected by Dzombak and Morel (21). The surface complexation
where, FesOH0 is a high affinity binding site at the oxide surface and M2+ represents a divalent cation. The log K1int is 0.47 for Cd2+, 0.99 for Zn2+, 2.89 for Cu2+, and 4.65 for Pb2+, predicting solubility to follow the order Cd2+ > Zn2+ > Cu2+ > Pb2+. In deriving this equation, activities of the free (FesOH0) and metal complexed (FesOM+) surface sites are assumed constant. The nature of the surface complexation reaction (involves one H+) results in solubility lines with a slope of one. The solubility values for Cu2+ reveal that copper hydroxide, carbonate, and oxide have higher solubility than the coprecipitates. The solubility line of Cu2+ from soils (soil-Cu) and cupric ferrite (assuming Fe3+ activity is controlled by Fe(OH)3(amorp)) reported by Lindsay (20) are close to the limits of the oxide surface complexation constant at pH values of 5.5-6.5, but coprecipitated Cu2+ is more soluble (Figure 4). Franklinite (assuming Fe3+ activity is controlled by Fe(OH)3(amorp)) has the lowest solubility of all Zn2+ species reported in Figure 5, with the pure zinc(2+) hydroxide, carbonate, and oxide the highest, and Lindsay’s solubility line of Zn2+ from soils (soil-Zn) results in about the same solubility as Zn2+ in the iron oxide coprecipitates at pH values of about 5.5 (Figure 5). The solubility of Cu2+ and Zn2+ (expressed as pCu and pZn), calculated from the intrinsic surface complexation constants for HFO (K1int) predict a lower solubility than found for the amorphous coprecipitates of this study (Figures 4 and 5). This difference is larger for Cu2+ than for Zn2+, possibly related to the different tendencies of these metals to hydrolyze (18). Cadmium solubility was too low in the coprecipitates to be controlled by pure cadmium oxide, hydroxide, or carbonate compounds. Coprecipitation with iron (hydr)oxide resulted in a higher pCd (lower solubility) than adsorption (Figure 6). However, this comparison of pCd from coprecipitation and adsorption may not be valid, since the Cd/Fe ratio used to obtain the surface complexation constant (0.005) is 2 orders of magnitude higher than the Cd/Fe ratio used in this coprecipitation study (about 0.00008). On the basis of the detection limit of the dpasv method used in this study (5 nM), soluble Pb2+ is not controlled by its hydroxide or carbonate forms, indicating that reaction with the oxide controls Pb2+ activity.
Discussion Among the mechanisms responsible for the removal of heavy metals from solution, thermodynamic principles predict that coprecipitation (“true” solid solution formation) will often result in the lowest metal solubility. Since Kolthoff and Moskovitz (22) and Swallow et al. (23) found a very slight to no difference in percent of Cu2+ sorbed after coprecipitation or adsorption onto freshly prepared HFO, true solid solution formation was not indicated. Kolthoff and Moskovitz (22) explained this behavior as a result of the amorphous character of the primary precipitate. Swallow et al. (23) attribute the results to micropore diffusion into a “swollen ion-exchange resin” like structure of a freshly prepared HFO so that adsorption is not restricted to external surface sites. However, Ainsworth et al. (19) ruled out micropore diffusion as the VOL. 32, NO. 6, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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interpretation for the hysteresis in desorption found with Co2+, Cd2+, and Pb2+ in their system. Zinc coprecipitation and adsorption on HFO were equivalent in their efficiency of metal removal from solution at pH 5-6 (15). The presence of Ni2+ during Zn2+ coprecipitation had no effect on its removal from solution (24). Recent spectroscopic and microscopic evidence suggest that the formation of multinuclear and mixed-metal phases at mineral surfaces is common and that adsorption at discrete sites is probably less prevalent than previously thought (3, 4, 6, 7). Consequently, metal solubilities calculated from surface complexation models may underestimate solubilities in real systems. The data presented herein indicate that discrete metal adsorption and the formation of a true solid solution may not happen under the experimental conditions used. Polymerization of Cu2+ hydroxy species, and to a lesser extent Zn2+, and segregation/adsorption within/on the iron (hydr)oxide may explain this phenomenon. A pure precipitated phase is not formed either, so that metal solubility falls between that of discrete-site adsorption and precipitation. An adsorbed metal hydroxy polymer may have characteristics leading to solubility higher than that for the metal adsorbed at isolated sites. However, spectroscopic analyses will be required to verify the suggested forms of sorbed metals based on this study. The low tendency of Cd2+ to hydrolyze increases the possibility of its retention by high affinity binding sites on or in the iron (hydr)oxide. Furthermore, when very small mole fractions of metals are retained, there is less chance of severe structural distortion and instability induced in the ferrihydrite structure upon incorporation (2, 19). Direct evidence (EXAFS) indicates that Cd2+ substitutes for Fe3+ within the R-FeOOH lattice upon coprecipitation (2). In addition, a metal adsorption/desorption study (19) suggests that Cd2+ may diffuse into structural positions of iron oxides after contact with the freshly produced HFO for several weeks. Even so, our results indicate a low efficiency of incorporation, with Cd2+ solubility remaining at a large fraction of that prior to coprecipitation. Solution chemistry was affected by the rate of titration (Figures 1-3), although pH changes seemed to control the overall Cu2+ solubility behavior. This can be understood if the distribution and form of metals in the solid phase differ with rate of precipitation. One would expect that hydrolysis of solutions with low M2+/Fe ratios would promote the formation of a true solid solution, but the intermediate solubilities resulting from this coprecipitation study suggest adsorption and/or segregation on/within the ferrihydrite structure. Of these metals, Cu2+ has an ionic radius closest to that of Fe3+, so it could readily be incorporated into the structure, but spectroscopic methods will be required to investigate this assumption. Earlier work using ESR to investigate the position of Cu2+ with aluminum hydroxide coprecipitates gave indications that most of the coprecipitated Cu2+ was at or very near the particle surfaces (13). If there were any effects of aging of the coprecipitate or slow structural incorporation of metals on metal solubility, they were obscured in this study by spontaneous pH changes. Titrations of Fe3+ salts to obtain ferrihydrite followed by maintenance at a constant pH during aging are more likely to reveal effects of recrystallization of this mineral on trace metal solubility. Significance to the Soil System. Work on the retention of trace metals by iron oxides has, for the most part, been reported as percent of metal retained versus pH. This results in little attention given to the actual concentrations remaining in solution, which can still be high enough to be toxic to biological systems. The present report uses a model system to provide values for soluble metals under realistic conditions of pH, potential metal loadings on the oxide fraction through 748
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the application of sewage sludge, and long-term reaction times. Furthermore, results from this study show that metals coprecipitated with iron (hydr)oxides have solubilities of free Cd2+ (pCd ) 6.55-6.79), Cu2+ (pCu ) 6.22), and Zn2+ (pZn ) 5.07) that in hydroponic experiments (25, 26) have been shown to be toxic to sensitive crops such as alfalfa (threshold toxicity limit of pCd ) 8.65-8.75, pZn ) 5.0-5.4) and wheat (threshold toxicity limit of pCu ) 8.2). It is recognized, however, that this comparison is not totally valid without making corrections for ionic strength, solid concentration effects (solid:solution ratio), and ion competition (e.g., Ca). Although iron (hydr)oxides may be an important sink for metal retention (especially Pb) in the long term and have been used in attempts to remediate metal-contaminated soils, they may still prove relatively ineffective in lowering activities of some metals such as Cd and Cu at U.S. EPA permitted heavy metal loadings.
Acknowledgments This research was supported by the USDA-NRI Competitive Grants Program, Award 95-37107-1620.
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Received for review March 21, 1997. Revised manuscript received December 10, 1997. Accepted January 2, 1998. ES970262+
