Environ. Sei. Technol. 1994,28,2353-2359
Nickel Adsorption to Hydrous Ferric Oxide in the Presence of EDTA: Effects of Component Addition Sequence Amy L. Bryce,' Wllliam A. Kornlcker, and Alan W. Elrerman Environmental Systems Engineering, L. G. Rich Environmental Research Laboratory, Clemson University, Clemson, South Carolina 29634-09 19
Sue B. Clark Savannah River Ecology Laboratory, University of Georgia, P.O. Drawer E, Aiken, South Carolina 29802
Nickel, EDTA, and hydrous ferric oxide were combined in different sequences to study the effect on equilibration. In this system, the fraction of nickel adsorbed to the hydrous ferric oxide depended on the component addition sequence, but the fraction of EDTA adsorbed did not. In one sequence, nickel and EDTA were combined to preform a NiEDTA2- complex. The complex had adsorption characteristics similar to EDTA with high adsorption at pH 7. Equilibrium adsorption of the complex could be modeled using the diffuse layer model. When nickel or EDTA was individually equilibrated with HFO prior to the addition of EDTA or nickel, respectively, the system did not attain equilibrium. Rapid EDTA adsorption at pH 7, nickel adsorption was faster than the formation of the NiEDTA2- complex, but once adsorbed, nickel was slowly desorbed through the formation of solution NiEDTA2-.
Introduction Knowledge of metal speciation in soil/water systems is essential for predicting metal transport in the environment, the risk associated with bioavailability, and assessing effective remediation strategies. Processes that affect metal speciation in soil/water systems include adsorption at the surfaces of soil particles and the formation of metal complexes, Complexing agents can significantly change the solid/solution metal distribution since metal complexes have different adsorption characteristics than the uncomplexedmetal. The presence of natural organic matter, inorganic anions, and anthropogenic complexing agents have all been shown to modify surface and solution speciation (1-4). Understanding the behavior of metal complexes in natural systems is important for the development of speciation models. The ability to predict the formation of a particular metal species depends on knowledge of the probable processes in a multiphase, multicomponent system. Two-phase, multicomponent systems that include a metal, a complexing agent, and a solid phase can be difficult to describe due to the multitude of possible species (5). For example, under certain conditions metal adsorption may be favored over the formation of a metal complex, or if the complex has formed, it may adsorb to the solid phase rather than dissociate prior to adsorption. I t may be possible to distinguish important processes by allowing two compo-
* Address correspondence to this author at her present address: Savannah River Ecology Laboratory, University of Georgia, P.O. Drawer E, Aiken, SC 29802; e-mail address:
[email protected]. 0013-936X194/092&2353$04.5010
0 1994 American Chemical Society
nents to equilibrate and then monitoring reequilibration after the addition of a third component. We selected a system containing nickel, ethylenediaminetetraacetic acid (EDTA), and hydrous ferric oxide (HFO) as a model for a soil system that contains metals and organics. Iron oxides are ubiquitous soil constituents, typically found as surface coatings on soil particles, and are considered a controlling factor for subsurface metal transport (6,7). HFO, although not the predominant iron oxide in most soils, exhibits surface chemistry representative of iron oxides (6). Nickel is a suspected carcinogen found in many metal alloys and is frequently found as a contaminant near metal plating industries. EDTA is a strong complexor of most metals, and its application to metal-contaminated soils has been considered as a viable remediation technique (8). Additionally, EDTA is present in many mixed waste streams and in some cases may alternatively be considered a co-contaminant (9, 10). Finally, at a metal/ligand ratio of 1, a single Ni-EDTA species predominates over a broad pH range, facilitating the description of the solution Ni-EDTA behavior in this study. Surface complexation models (SCMs),widely accepted to describe adsorptionat the solid/solution interface, divide adsorption into chemical and electrostatic factors (11,12). Sorbates interact with discrete functional groups on the surface similar to solution coordination chemistry, and the reaction is described using an intrinsic equilibrium constant,K int. At an oxide surface such as HFO,a surface functional group is typicallyrepresented as an amphoteric hydroxyl group (e.g., =FeOH, =FeO-, =FeOHZ+). The electrostatic term arises from the development of surface charge. Different versions of the model vary primarily in their description of the electrostatic term (12,13). Dzombak and Morel (14) introduced a modified version of the diffuse layer model (DLM) in conjunction with an internally consistent database for the adsorption of transition metals and inorganic anions to HFO. The DLM is one of the simplest surface complexation models available (14, 15) and is used herein. SCMs have been successfully applied to describe the adsorption of metals, inorganic anions, and organics to a variety of oxide surfaces. The adsorption of nickel to HFO has been described over a wide range of pHs, and a DLM equilibrium constant is available for these reactions (14). The adsorption of EDTA to oxide surfaces has been studied, although a DLM model constant for HFO is not readily available (16-19).In metal/ligand/sorbent systems a metal/ligand surface complex can produce significant changes from metal-only adsorption, and there are only a limited number of SCM constants available (20);however, Environ. Sci. Technol., Vol. 28, No. 13, 1994 2353
Bowers and Huang (10)have modeled the adsorption of a NiEDTAHl- complex to alumina. Experimental data for metal/ligand/sorbent systems are typically reported following a single component addition sequence (3, 22, 22). When the effect of changing the component addition sequence was tested, no significant changes in metal adsorption behavior were observed (23, 24). There is evidence, however, that the component addition sequence is an important factor in determining how metal/EDTA/sorbent systems equilibrate (8-10,2528). When EDTA is added to a system containing a metal and a sorbent, the fraction of metal adsorbed decreases after some equilibration time (8,25). However, when the metal and EDTA are combined prior to the introduction of the sorbent, metal adsorption is similar to EDTA adsorption behavior (9, 10,28). Although differences in metal adsorption behavior for different component addition sequences have been observed, the processes controlling equilibration in metal/EDTA/sorbent systems have not been thoroughly investigated. Swanson (27,28) studied a nickel/EDTA/HFO system using a variable component addition sequence and observed variable metal adsorption as a function of time and sequence. However, the EDTA concentration was maintained 10-1000 times in excess of nickel, making it difficult to distinguish the behavior of EDTA from the NiEDTA2- complex. In this study, clearly defined component addition sequences were used in a nickel/EDTA/HFO system to determine which processes were important for controlling equilibration. Component addition sequences were designed to facilitate comparison between metal adsorption, formation of the metal-EDTA complex, metal desorption, and dissociation of the metal-EDTA complex. Comparisons are made using nickel and EDTA adsorption as reference systems, respectively. In some cases, kinetic limitations prevented the establishment of equilibrium within the time scale of the experiments. Where equilibrium was achieved, however, results were described using the DLM.
Materials and Methods Materials. All chemicals were reagent-grade and were used without further purification. Carrier-free 63Ni2+and 14C-labeledethylenediaminetetraacetic acid (EDTA) were obtained from New England Nuclear and Sigma Chemicals, respectively. Iron and carrier nickel stock solutions were prepared from nitrate salts, stored in 1% “03, and standardized using atomic absorption spectroscopy (AAS). Water was distilled and deionized. All glassware was acidwashed in 10% HN03, double-rinsed, and oven-dried before use. Methods. HFO was precipitated from a solution of Fe(N0&.9H20 in 0.1 M NaN03 by the dropwise addition of 1.0 N NaOH with a continuous nitrogen sparge. Fresh precipitate was aged at pH 7.5 f 0.5 for 4 h. The amorphous character of the solid was confirmed as twoline ferrihydrite by X-ray diffraction. HFO synthesized under these conditions has been well-characterized (14), and the properties applicable to this study are given in Table 1. Aliquots of HFO were transferred to polyallomer centrifuge tubes, adjusted to a final volume of 30 mL with an ionic strength of 0.1 M NaN03, and aged an additional 6-12 h before use. On average, samples contained (9 f 0.2) X M Fe3+ as measured by AAS after HFO dissolution. 2354
Environ. Sci. Tschnol., Vol. 28, No. 13, 1994
Table 1. Properties Assigned to Hydrous Ferric Oxide, Taken from Dzombak and Morel (14)
surface area (m2/g) site densities
600
=Fe*OH, 0.005 mol siteimol of Fe3+ =FewOH, 0.2 mol siteimol of Fe3+
In all experiments, 0.1 N NaOH or “ 0 3 was added to adjust the pH before the addition of the adsorbates. Adsorbate concentrations (Ni2+or EDTA) were 1 X 10-5 M unless noted otherwise. Radiolabeled Ni2+ and/or EDTA were present as tracers and represented less than 1% of the total adsorbate concentration (carrier and tracer). The total volume change during pH adjustment and adsorbate addition was less than 3 % ;therefore,volume corrections were not necessary. To check for loss of adsorbates to the container walls, blanks containing no adsorbent were prepared a t each pH. Bright (29) demonstrated that loss to centrifuge tube surfaces was negligible a t all pH values used within the time required to complete the experiments. Samples were equilibrated a t room temperature in a tumbler apparatus for the desired amount of time, followed by centrifugation a t 15000 rpm for 15 min for solid/liquid separation. The fraction of adsorbate remaining in solution was determined by analyzing an aliquot of the supernatant using a Beckman 3801 liquid scintillation counter. The decay for 63Ni2+(0.07 MeV) and 14C (0.2 MeV) is sufficiently separated in energies to allow simultaneous determination in each sample. All samples were counted until the error associated with the measurement was less than 2% or a maximum of 20 min. The fraction of nickel and/or EDTA adsorbed was determined by difference. The pH of the remaining supernatant in contact with the HFO was determined using a Ross-15 combination electrode with an estimated uncertainty of jz0.05 pH unit. All experiments contained at least two sets of replicate samples. Experiments were conducted to determine the time required for equilibration in the single adsorbate systems (nickel/HFO or EDTA/HFO) as well as the equilibrium fraction of nickel or EDTA sorbed a t any given pH. Single adsorbate experiments are referred to as “binary” experiments. No change in the distribution of nickel or EDTA between the solution and the solid phase was observed after 6 h. Nickel/HFO samples were equilibrated for 12 h, and EDTA/HFO samples were equilibrated for 24 h. Ternary systems involving all three components (nickel/ EDTA/HFO) were studied using well-defined component addition sequences. In all cases, two components were allowed to equilibrate a t pH 7 prior to the addition of the third component. The sorption of a Ni-EDTA complex to HFO was studied by contacting a 0.003 M solution of 1:l Ni2+-EDTA for 24 h. These experiments are referred to as “nickel/EDTA preequilibrated” studies. Aliquots of this solution were then added to centrifuge tubes containing HFO and adjusted to the desired pH to yield total M NiEDTA2-. concentrations of 1 X Other component addition sequences involved equilibrating nickel or EDTA with HFO and then observing the effect of adding the second adsorbate to the equilibrated binary system. These ternary experiments are designated as either “metal-first” in which the nickel was preequilibrated with HFO prior the addition of EDTA or as “ligandfirst” in which EDTA was first equilibrated with HFO
Table 2. Reaction Stoichiometries and Equilibrium Constants at Zero Ionic Strength as Used Within MINTEQA2 Solution Reactionsa log K eq no.
EDTA’ + H+ HEDTA” EDTA’ + 2H+ H2EDTAZEDTA’ + 3H+ I)H3EDTA1EDTA’ + 4H+ Q H4EDTA Ni2+ + EDTA’ NiEDTA2Ni2+ + EDTA’ + H+ Q NiHEDTAl-
11.05
(1)
Q
17.82 20.94 22.66 20.38 24.02
(2)
Q
(3) (4) (5)
Q
(6) Surface Reactionsb
=FeOH + H+ =FeOHz2+ =FeOH Q =FeO- + H+ =FesOH + NiZ+ Q =FenONi+ H+ =FeWOH + EDTA’ + 2H+ Q FeWEDTAH2-+ HzO =FeWOH + Ni2+ EDTA’ + H+ Q =FeWEDTANi-
log Kint
eq no.
Q
+
+
+ HzO
Smith and Martell (40).* Equations 7 and 8 are from Dzombak and Morel (14);eqs 9-11 are from this work. IQnt= KapP exp(-AZF$/R!l‘‘) where R is the ideal gas constant, F is Faraday’s constant, $ is the potential at the surface, T is temperature, and A2 is the change in charge on the surface (14).
prior to the addition of nickel. In both cases, a series of vials containing the single adsorbate (nickel or EDTA) and HFO were equilibrated at pH 7. Immediately before introduction of the second adsorbate, three samples were removed for analysis to establish representative conditions of all samples. In the remaining samples, the pH was adjusted, the second adsorbate was added, and changes in nickel and EDTA adsorption were monitored with time. Modeling. The equilibrium model MINTEQA2 was used for all speciation calculations (30). The reaction stoichiometries and equilibrium constants used are given in Table 2. Intrinsic adsorption constants are consistent with the diffuse layer model (DLM). The DLM incorporates two site types: strong (=FesOH) and weak (=FewOH), where strong sites represent a small fraction of the available sites and weak sites represent the remainder of sites (Table 1).Cations are expected to bind preferentially to strong sites at low sorbate/sorbent ratios, whereas anion adsorption is limited to weak sites (14). Under the conditions used herein, cations and anions are not expected to compete for surface sites. Equilibrium DLM adsorption constants were calculated using the nonlinear least squares optimization program FITEQL (14,31). Experimental data were input into the program as the total and/or free solution concentrations. Reactions that described the system were included from Table 2, in addition to the equation for which the adsorption constant was calculated. Ionic strength adjustments were made using the Davies equation.
Results and Discussion
Binary System, Nickel/HFO. Experimental data for nickel adsorption to HFO as a function of pH are shown in Figure 1. The fraetion of nickel adsorbed increased sharply from near 0% at pH less than 5.5 to approximately 100$7, at pH values 2 7 . The shape of the adsorption curve and the position of the adsorption edge are consistent with data previously reported by Dzombak and Morel (14).Due to the low ratio of metal to sorbent, nickel adsorption was limited to strong sites for modeling purposes. The simplest model reaction providing a reasonable fit of the data is obtained using eq 9 (Table 2) in which Ni2+ displaces hydrogen at a strong surface site. Modeling results using eq 9 are shown in Figure 1by the solid line. While more
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complex speciation models involving two or more Ni surface species, Le., =FeaONi+, =FeOsNi(OH)o, etc., also provided similar representation of the experimental data, the calculated equilibrium constant log K int = 0.87 at I = 0 using the single =FesONi+ species agreed well with data previously modeled by Dzombak and Morel (14). When the pH of a system containing sorbed nickel (i.e., pH 16.5) is decreased below the adsorption edge to induce metal desorption, the fraction sorbed should be very similar to the adsorption curve if the processes are reversible. However, secondary processes such as diffusion of nickel into the HFO solid (32, 33) or structural changes in the HFO due to aging (34) may result in hysteresis in the adsorption curve when the pH is decreased. To check for reversibility in this study, nickel desorption experiments were conducted. Vials containing a binary system of nickel and HFO were first equilibrated at pH 7.5 for 12 h such that >99% of the nickel was adsorbed (Figure 2). The pH of the vials was then systematically adjusted to provide a range of hydrogen ion concentrations. The kinetics of reequilibration were monitored by measuring the pH and fraction of nickel adsorbed between 10 and 96 h. In less than 10 h, the resulting adsorption curve was very similar to the curve predicted by DLM calculations, with the exception of pH values less than 5.5. Below pH 5.5, 15% of the Ni2+ remained adsorbed, whereas the model predicted 100% desorption. While the reason for this is Environ. Scl. Technol., Vol. 28, No. 13, 1994
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unclear, nickel adsorption was considered reversible in the region where this study was conducted, i.e., pH >5.5. Binary System, EDTA/HFO. The adsorption of EDTA to HFO was studied from pH 4 to pH 10 (Figure 3). In this case, the fraction of EDTA adsorbed was approximately 100% at lower pH values and decreased significantly between pH 6 and pH 7 to near 0 % . A t pH 1 7 , little or no adsorption occurred, and HEDTA3- was expected to be the predominant solution EDTA species. Although the extent of EDTA protonation varies significantly with pH, the changes in the fraction of EDTA adsorbed correlated to the predicted changes for one solution species, H2EDTA2- (dashed line, Figure 3). Several different surface species have been suggested for EDTA, where EDTA can coordinate one or more surface sites (19). However, modeling results were again comparable in all cases, regardless of whether one or more EDTA carboxyls were allowed to interact with the surface. Consequently, the simplest model was where one EDTA carboxyl group displaced a single hydroxyl group on a weak surface site, as described by eq 10 (Table 2). The estimated log K int for the =FeWEDTAH2-is 18.9 at zero ionic strength. The model results are represented by the solid line in Figure 3. Ternary System, Nickel and EDTA Preequilibrated. In these experiments, nickel and EDTA were preequilibrated a t pH 7, which favored the formation of a 1:l NiEDTA2- complex. The adsorption of nickel and EDTA to HFO as a function of pH is shown in Figure 4. The fraction of nickel and EDTA adsorbed after contact 2356
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Flgure 4. Percent nickel and EDTA adsorbed, premixed addition sequence: M total nickel, M total EDTA, 0.009 M Fe3+,0.1 M NaN03.Times are from contact with HFO. Solid symbols represent nickel; open symbols represent EDTA. Solid line represents model fit for surface species =FeWEDTANii-; log Kin‘ = 28.76 at zero ionic strength. Dashed line represents the fraction of nickel and EDTA as solution NiEDTA2-
with HFO for 2,24, and 49 h was near 100% below pH 6 and dropped sharply to approximately 0 % adsorbed when the pH was increased to 7 or greater for both adsorbates. The shape and position of the adsorption curve is very similar to the adsorption curve observed in the binary EDTAiHFO system (Figure 3). Furthermore, the fraction of nickel adsorbed (filled symbols, Figure 4)at a given pH was equivalent to the fraction of EDTA adsorbed. This suggests that the NiEDTA2- complex did not dissociate prior to the adsorption of the complex and that EDTA controls adsorption. This type of interaction has been termed a “ligand-like ternary surface” complex (5) and has been observed in other studies where a metal and EDTA were preequilibrated (9, I O ) . The absence of change in the fraction of adsorbed species within the experimental time frame indicates that equilibrium is achieved within 2 h. At all pH values, speciation calculations predict that the concentrations of free Ni2+ and uncomplexed EDTA are very low M). Because such small concentrations are difficult to model in FITEQL (35), the system was defined in terms of the NiEDTA2complex. In the absence of the sorbent, this species predominates a t pH 1 4 . Between pH 4 and pH 8, t,he six-coordinate species NiEDTA2- can also exist as the sixcoordinate Ni(H20)EDTA” with one unprotonated EDTA carboxyl not coordinated with the nickel (36). The uncoordinated carboxyl group was selected to interact with the surface due to the similarity between the adsorption curve in the binary EDTA/HFO system and the ternary nickel/EDTA/HFO system. Adsorption of the complex was restricted to weak surface sites to remain consistent with DLM theory for anion adsorption. The data were fit using the following reaction: =Fe”OH
+ NiEDTA2- + H+ ++ = Fe”EDTANi-
+ H,O
(I)
with log K int = 8.38 a t I = 0. To obtain the reaction stoichiometry required by MINTEQA2, this was combined with eq 5 in Table 2 to yield eq 11in Table 2. The model results are shown by the solid line in Figure 4. The dashed line in Figure 4 indicates the corresponding solution fraction of nickel and EDTA as the NiEDTA2- complex. The surface species proposed in eq 1differs slightly from Girvin et al. (9)and Bowers and Huang ( I O ) ,who proposed
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Figure 5. Percent (A) EDTA and (B) nickeladsorbed: Metal-first addition M total nickel equilibrated with 0.009M Fe3+in 0.1 M sequence. M total EDTA. Measurements NaN03at pH 7 prior to the additionof at 2,26, and 50 h indicate time elapsed since EDTA addition. Solid lines represent model results of binary adsorption for (A) EDTAIHFO and (B) nickei/HFO.
Figure 6. Percent (A) EDTA and (B) nickeladsorbed for the ligand-first addition sequence. M total EDTA equilibrated with 0.009 M Fe3+ In 0.1 M NaN03 at pH 7 (99% of nickel was adsorbed after the equilibration period (not shown). Figure 5 illustrates the effects of changes in the pH and the introduction of EDTA on the equilibrated nickel/HFO system. The fraction of EDTA adsorbed as a function of pH (Figure 5A) is comparable to EDTA adsorption to HFO in the binarysystem. For convenience, the solid line in Figure 5A represents the model results of the binary EDTA/HFO system previously presented (Figure 3). Adsorption was essentially complete within 2 h and exhibited no significant changes with time. Nickel adsorption in the metal-first addition sequence was significantly different from the preequilibrated system (Figure 5B). Two hours after the addition of EDTA, the fraction of adsorbed nickel at pH
