Chromate and Oxalate Adsorption on Goethite. 1. Calibration of Surface Complexation Models Karel Mesuere and Wllllam Flsh"
Department of Environmental Science and Engineering, Oregon Graduate Institute of Science & Technology, 19600 N.W. von Neumann Drive, Beaverton, Oregon 97006-1999
w Adsorption of chromate and oxalate on a-FeOOH was quantified for single-adsorbate systems as a function of pH, ionic strength, and adsorbate and adsorbent concentrations. The comprehensive data base was used to calibrate and compare two surface complexation models. Both anions exhibited Langmuir-typeadsorption with maximum adsorption densities (I'd of 2.2 (oxalate) and 2.4 pnol/m2 (chromate). Increasing ionic strength diminished the adsorption of both anions. However, the background electrolyte affected oxalate adsorption more than chromate adsorption, reflecting a higher intrinsic a f f ~ t of y chromate for the goethite surface. Measured proton/anion adsorption stoichiometry ratios ( r H )ranged from 0.3 to 1.1, increased with pH, but showed little dependence on surface concentration of adsorbates. The diffuse layer model (DLM) and triple-layer model (TLM) reproduced all major adsorption features with a single set of constants for each model, but more surface species were required for the DLM. Another difference between models arose in the computation of rH: DLM-calculatedvalues were closer to the experimental values. Comparison of DLM-based constants for goethite and those obtained elsewhere for amorphous hydrous ferric oxide indicates that the intrinsic affinity of these anions for both iron oxides is very similar. Introduction
Adsorption at solid/solution interfaces significantly alters the mobility of anions and affects the surface reactivity, dissolution rate, and aging pathway of the solid phase (1-4).The adsorption of aqueous ions onto oxide surfaces is modeled successfully by surface complexation models (SCMs) which assume that adsorption involves both a coordination reaction at specific surface sites and an electrostatic interaction between adsorbing ions and the charged surface (2,5-7). However, rigorous assessment of SCMs using all available types of data is incomplete. Evaluation of ionic strength (I) effects has been limited largely to one type of SCM, the triple-layer model (TLM) (8, 9). Data documenting the overall proton/anion adsorption stoichiometry ( r H )generally are overlooked in model evaluations, even though such data can be a powerful constraint on SCMs (5,6).Measured rH values for anion adsorption are documented in a few studies (10-12) but thus far have not been interpreted with SCMs. Moreover, modeling efforts that simultaneously consider 0013-936X/92/0926-2357$03.00/0
data for a wide range of conditions are scarce. Yet, differences among various SCMs are most likely to emerge if model calibration and comparisons are based on such comprehensive data bases. Detailed comparisons of SCMs have dealt mostly with small data sets related to surface hydrolysis and weak 1:lelectrolyte binding obtained in the absence of specifically adsorbing ions (13,14). As SCMs are incorporated into solute transport models, there is a growing need for a consistent data base of model adsorption constants (15). Especially promising is the recently developed internally consistent data base for inorganic adsorption on hydrous ferric oxide (HFO) using data from single-adsorbate/single-adsorbentsystems, as interpreted with the diffuse layer model (DLM) (5). (The DLM is also known as the two-layer model.) However, reliable prediction of adsorption in natural, multicomponent environments using single-solute adsorption constants requires that models be tested with increasingly complicated experimental systems (16-19). Unfortunately, rigorous testing with complex systems has been limited and the results have not been uniformly successful. In particular, uncertainty remains about the abilities of singlesolute constants for describing multisolute adsorption (20-22). Our objectives were to generate a consistent and comprehensive single-solute data base for the adsorption of two anionic contaminants (oxalate and chromate) on goethite (a-FeOOH) and to use these data to calibrate two SCMs (DLM and TLM) resulting in two self-consistent sets of adsorption constants. In a companion paper (23),the coadsorption of binary mixtures of oxalate and chromate on the same sample of adsorbent is examined to determine whether modeling of such multiadsorbate systems is tractable with this set of surface complexation constants. The selected system is environmentallyrelevant and represents a larger group of adsorbents and adsorbates. Chromate is a widespread and toxic contaminant (24) representative of inorganic oxy anions of medium binding strength for metal oxides (16,17,25). Oxalate is an industrial pollutant (26)as well as a ubiquitous, naturally occurring ligand (27)with a significant adsorptive affinity and a distinct ability to dissolve metal oxides (21,28-30). Oxalate is a major cocontaminant with chromate at certain waste sites, in particular at the Hanford Nuclear Reservation (31).Goethite is an iron oxide mineral commonly found in soils and the subsurface of most climatic regions (32).It has been used as a model surface in recent kinetic
0 1992 Amerlcan Chemical Society
Envlron. Sci. Technol., Vol. 26, No. 12, 1992 2357
and in-situ spectroscopic investigations (33, 34). Materials and Methods
Synthesis and Characterization of a-FeOOH. A single batch of goethite was prepared according to Atkinson et al. (35) and used in all experiments. X-ray diffraction confirmed that the synthesized material was aFeOOH with no other detectable crystalline impurities. The specific surface area (S) determined by the N,/BET method was 66 f 3 m2/g, in good agreement with estimates from X-ray diffraction band broadening and transmission electron micrographs [60f 18and 69 f 15 m2/g (36)]. The pristine point of zero charge (PPZC) defined by the intersection of multiple titration curves at different ionic strengths was 9.3 f 0.1 (T= 25 "C). Titration data normalized to PPZC and S were in excellent agreement with published titration data for goethites with various S and PPZC values (36). Adsorption Experiments. Adsorption of oxalate and chromate was examined as a function of pH (4-11), ionic strength (I= 0.01-0.5 M), adsorbate concentration ([AIm = 0.005-5 mM), and adsorbent concentration (G = 0.18-1.8 g/L). Unless indicated otherwise, data presented pertain to I = 0.05 M and G = 1.8 g/L. All experiments were performed in a tightly capped, 200-mL Teflon cup under N2 atmosphere at 25 f 0.1 "C. Preliminary experiments showed that adsorption equilibrium was reached within 10 h for all adsorbate/adsorbent ratios (36). Adsorption pH edge experiments were set up by mixing N2-purged, ultrapure water (Nanopure, Barnstead, Boston, MA), 1M KNOBstock solution, and 0.1 M oxalate or chromate stock solution (potassium salts). After mixing and N2-purging the solution, the pH was raised to pH 10-10.5 and goethite stock suspension [12 g/L, N2-sparged for up to 8 weeks (37)] was added (total experimental suspension volume 120 mL). After 1h, the suspension was titrated with 1.0 N or 0.1 N HN03 The pH was kept constant at up to five levels for at least 10 h each. For each pH, three 1-mL aliquots were transferred to N2-purged microcentrifuge tubes and centrifuged for 10 min at 13OOO rpm and the supernatant removed for immediate, triplicate analysis. Each experiment was repeated three times so that, for each condition, at least 10 data pairs (pH, percent anion adsorbed) were recorded. Chromate isotherms were obtained similarly using 0.01 or 0.1 M K2Cr04as the titrant with constant pH 6.50. For oxalate, each isotherm data pair was obtained from a separate batch experiment at constant pH 4.00. Anion surface concentrations were calculated as the difference between total and solution concentrations. Light was excluded from the reaction vessel at all times with an aluminum foil wrapping. The proton stoichiometry ratio (rH, the number of moles of protons adsorbed per mole of anion adsorbed) was determined with a back-titration technique. A goethite suspension (1.8 g/L, I = 0.05 M) was equilibrated a t pH 4 or 6 until the pH drift was leas than 0.01 pH unit/h (85% (10-h equilibration time), iron/oxalate complexation reactions were omitted from the solution speciation calculations. Also, extensive kinetic experiments showed that ligandpromoted dissolution did not affect the overall distribution of oxalate between solution and the goethite surface under conditions of surface saturation for up to 50 h (41). Solution activity coefficientswere taken from Dzombak and Morel (5).
Results and Discussion Effect of pH. Adsorption of oxalate and chromate on a-FeOOH a t low solution concentrations increased from 0 to 100% with decreasing pH, but the pH edge for chromate was markedly steeper than for oxalate (Figure 1A and B). Fractional adsorption of both anions became significant (greater than a few percent) between pH 9 and 10, reflecting the high PPZC of the adsorbent. The absence of significant adsorption above the PPZC emphasizes that a favorable electrostatic environment is necessary for adsorption of these anions. Maximum adsorption of diprotic acids on metal oxides typically occurs at pH is: pKd (42). Because chromate is a weaker acid than oxalate (log = 4.27 and log Kcr,2= 6.51; Table IV), chromate adsorption reached a maximum at a higher pH (Figure 1A and B). For the pH range studied here (pH 4-11), three surface species were required for the DLM to obtain a good model 2960 Envlron. Scl. Technol., Vol. 26, No. 12, 1992
fit at relatively low surface coverage of anions (100% solute adsorption in Figure 1A and B corresponds to 1520% of rmJ. If only two surface species were selected (=SHAo and =SA-, reactions 5 and 6 in Table 11),the DLM underpredided chromate adsorption at high pH (Figure 1B). The more negative surface species =SOHA2- (reaction 7 in Table 11) was necessary to account for chromate adsorption at pH >8.5 (Figure 1B and D). For oxalate, adsorption a t pH >7 could only be accounted for if the species =SOHA2-was replaced by the even more negative surface species =SOA3- (reaction 8 in Table 11; Figure 1A and C). In fact, it is the drastic change in the proton dependence of reaction 8 compared to that of reaction 6 that allowed the prediction of the gradual slope of the oxalate pH edge. We acknowledge that highly charged, mononuclear, monodentate representations such as 4OA" may not be chemically realistic. An equally good fit may be possible by hypothesizing more complex (e.g., bidentate or binuclear) surface species that exhibit the necessary pH dependence with more realistic coordination chemistry. However, as stated above, our intent here in model evaluation was to follow common practice and employ only surface complexes with 1:l stoichiometries. With the TLM, adequate simulations of adsorption data could be obtained using only one or two surface reactions. Oxalate adsorption was best modeled with the outer-sphere surface species =SOH2+-C2042and =SOH2+-HC2O4-(reactions 9 and 10 in Table 11). These reactions (analogous to reactions 5 and 6 for DLM Table II) are commonly used in TLM modeling of anion adsorption on metal oxides (7, 9, 17), and their choice is supported by recent kinetic studies (43,44). For chromate, reaction 9 in Table I1 was irrelevant in explainingthe data and was omitted from the TLM. The reactions discussed so far for the data presented in Figure 1described all data seta very well, but the optimal log K values fitted to these reactions varied over 1 log unit (Table 111). Typically, the DLM represented the data
-
I
11 :I:
pH = 6.50
cccc0 1.80 g/L 4*00 (EXP)
2
,
-
1
c
I
I
I
DLM
I
I
I
I
+HA’
=SOH’+
b
4 I
II
10
-10
-a
-6
-2
-4
L 0 G(0x)sor. ( m o W
)
-10
i I
I
-a
I
I -6
LOG(WS0L
I
I
I
-4
I
-2
( m o w )
Flgure 2. Constant pH adsorption isotherms for oxalate (A) and chromate (8)on a-FeOOH (1.8 g/L; Z = 0.05 M). SolM lines are MM fits obtained using model constants listed in Table 11. The logarithms of the calculated concentrations of surface species are shown In C (oxalate) and D (chromate).
better than the TLM. For most data sets, V y values for DLM optimizations were 1 order of magnitude smaller than for TLM optimizations (Table IV). We note however that such may result in part from the larger number of anion adsorption reactions considered for DLM modeling. Further discussion of modeling focuses primarily on DLM results, but significant differences between DLM and TLM data are highlighted. Effect of Adsorbate Concentration. Isotherms shown in Figure 2A and B indicate that adsorption of oxalate at pH 4 and chromate at pH 6.5 was Langmuirian, with adsorption proportional to solution concentrations at low surface coverage and a gradual decline in fractional adsorption as surface saturation was approached. In the region of proportional (“linear”)adsorption, the relative distribution of the model surface species was constant (Figure 2C and D). As the surface approached saturation, the relative importance of each surface species depended on the adsorbate concentration (Figure 2C and D). Near surface saturation, the concentration of charged surface species diminished and the uncharged species =SHAo became dominant, both for oxalate at pH 4 and chromate at pH 6.5. This model behavior is consistent with the absence of electrophoretic mobility observed for a-FeOOH with high oxalate adsorption density at pH 4 (29) as well as the reduced electrophoretic mobility with increased adsorbate concentrations reported for other anions on goethite (45). Evidence of a saturable surface also was detected in a series of pH edges obtained a t incrementally increasing adsorbate concentrations (Figures 3A and 4A). The lowest total anion concentration shown ([AITOT= 0.01 mM) corresponded to the upper limit of proportional adsorption. Below this limit the fraction of [AIToTadsorbed was a function only of pH so that pH edges obtained for [AITOT < 0.01 mM (data not shown) could not be distinguished from the pH edge for [A]TOT= 0.01 mM. For [A]TOT> 0.01 mM, the pH edges shifted downward because the fraction of [ A ] m adsorbed was inversely related to [AITOT.
Flgure 3. Fractional adsorption of oxalate as a function of pH for varying levels of total oxalate concentrations (A), ionlc strengths (B), and goethite concentrations (C). SolM lines are DLM fits obtained using model constants listed in Table 11.
Environ. Sci. Technol., Vol. 26, No. 12, 1992 2381
1
100
9
CHROMATE (mM): 0 0 0 0 0 0.01
2 80 Y
2
2
?1
.6
71 7
Ns= 1.5 sites/nmz
N s = 2 . 3 sites/nm2
60 40
2
3
20
7$;,/'I 1 {'I 1 I
0
I
I
I
,
2 e.
IONIC STRENGTH: 00000 0.01 M 00000 0.05 AAAAA
4
5
6
PH
@a
4
5
6
8
7
9
1
0
1
"
PH Figure 4. Fractional adsorption of chromate as a function of pH for varying levels of total chromate concentratlons (A) and ionic strengths (B). Solid lines are DLM fits obtained using model constants listed in Table 11.
The highest [AITOTin the pH edge experiments (0.8 mM) corresponded to near surface-saturation levels in the isotherms. Overall, the selected DLM surface reactions (with a unique set of mass law constants; Table 11) accounted well for oxalate and chromate concentration effects over the entire pH range (Figures 3A and 4A). Effect of Adsorbent Concentration. Increasing the adsorbate/adsorbent ratio ( [AITOT/G)by reducing the adsorbent concentration (G) for a given [ A ] ~ lowered T the fractional adsorption (Figure 3C), very much like the effect of increasing [A]* for a constant G discussed before. The constant-pH isotherms obtained at different adsorbent concentrations (Figure 2B) are indistinguishable because the data were normalized with respect to G (and 8).The effect caused by changing the adsorbate/adsorbent ratio on adsorption is well-documented and, on some occasions, has been interpreted as evidence of heterogeneous sites on the a-FeOOH surface. In particular, it has been argued that the constancy of the distribution coefficient [KD= ([A]SURF/ [A]SOL) / GI with increasing adsorbate surface density is a prerequisite for a homogeneous surface (46). However, such a simple partition coefficient (KD) fails to account for electrostatic effects. As the surface concentration of adsorbates increases, changes in surface charge may become important and induce nonlinear adsorption well before saturation is reached (Figure 2A and B). The extent of such nonproportional adsorption is affected by pH as well as the affinity of the adsorbate for the surface. Its occurrence, by itself, does not imply a heterogeneous surface. The capability of the DLM, with homogeneous surface sites (i.e., with a single surface component), to account for the effects of changing adsorbate and adsorbent concentrations underscores this point. A means to assess site heterogeneity is to document competitive, multiligand adsorption interactions over a wide range of adsorbate concentrations, as discussed in the second paper in this series (23). Effect of Ionic Strength. Raising the background electrolyte concentration over almost 2 orders of magnitude 2362
Environ. Scl. Technoi., Vol. 26, No. 12, 1992
I
I
I
I
I
I
N s = 1 . 5 sites/nm2 3
DLM
I
4
a. 0
0.5
I
7
8
N s = 2 . 3 sites/nm2 9 3
4
5
6
7
0
7
PH
Flgwe 5. Measured proton stoichiometry ratios obtained at pH 4 and 6, for total adsorbate concentrations of 0.05 (0)and 0.8 mM (A). DLMcalculated r Hvalues as a function of pH are indicated by dashed (0.05 mM) and solid (0.8 mM) lines for N , = 1.5 sites/nm2 (model constants were taken from Table 111) and N , = 2.3 sites/nm2 (model constants from ref 36).
reduced the pHW(pH correspondingto 50% adsorption) by 1 pH unit for oxalate and 0.5 pH unit for chromate (Figures 3B and 4B). Ionic strength effects are almost undetectable for strongly adsorbing anions such as phosphate, selenite, and molybdate (9, 47), whereas I has a pronounced effect on weakly adsorbed anions such as sulfate and selenate (9). Effects of I on oxalate and chromate (Figures 3B and 4B) are intermediate (17). Increased ionic strength reduces anion adsorption by diminishing the solution activity of adsorbing species and by decreasing the positive potential in the plane of adsorption at pH < PPZC (5). This potential determinesthe degree of electrostatic attraction between surface groups and adsorbates but has less effect on adsorption as the intrinsic affinity of the adsorbate for the surface increases. Ionic strength effects therefore are largely a measure of the relative contribution of Coulombic forces to the overall change in free energy associated with the adsorption process. Such effects can be taken into account by both the DLM and TLM. The DLM simulated well the effect of ionic strength on chromate adsorption (Figure 4B). For oxalate, DLM simulations were accurate for I < 0.1 M, but at higher ionic strength, the DLM overpredicted the effects on oxalate adsorption (Figure 3B). We conclude that for anions with intrinsic binding affinities a t least as strong as that of chromate the DLM effectively models ionic strength effects over a wide range of background electrolyte concentrations. For anions with weaker intrinsic affinities, such as oxalrite, the DLM is accurate up to moderate ionic strengths; at high background electrolyte the DLM apparently fails to accurately represent either the Coulombic forces or the competition from background anions. Measured Proton Stoichiometry. Data documenting the overall proton stoichiometry for anion adsorption onto iron oxides are scarce, but nearly all indicate a coadsorption of protons (FH > 0) (10-12).The value of rH may be affected by both pH and adsorption density r (10-12). Measured rH values for two concentrations of oxalate and chromate at pH 4 and pH 6 (Figure 5A and C) fell within the range of published values for anions on iron oxides
OXALATE Ns=2.3 sites/nmz
OXALATE N,=5.0 sites/nmz
16
f 12 h
28
u
4 0
4 CHROMATE
'1
I ,6
0.01 3
5
I 6
PH
1
7
I E
0
CHROMATE
2
4
6
8
1 0 1 2 0
8
4
PKCT
12
16
logKso1
Figure 7. Correlation of DLh4 surface complexah constants (log K2 and log K3;Table 11) for oxalate (0)and chromate (0)with pKe2(A) and log K values for corresponding solution complexatbnreactkns (B). sdid lhes are lhear free energy relatbnshlps derived for anbn eaptkn on hydrous ferric oxlde (5). The solution complexation reactbns are as folkws: Fe0I-f' 00:- H+ FeCrO,' H20, log K2 = 10.2; H20, log K 1 = 11.7; FeOH2+ FeOH2+ OX2- 2H+ = FeHOX" OX2- H+ = FeOX' H20, log K 2 = 11.5.
+
1,
I
I
93
4
5
I 6
I
,
1
7
8
9
PH
Figure 6. Measured proton stolchiometty ratios obtained at pH 4 and 6, for total adsorbate concentrations of 0.05 (0)and 0.8 mM (A). TLMcalculated r H values as a function of pH are Indicated by dashed (0.05 mM) and solld (0.8 mM) lines for N, = 5.0 sltes/nm2 (model constants were taken from Table 11) and N, = 2.3 sltes/nm2 (model constants from ref 36).
(-0.1 < rH < 1.2). For both oxalate and chromate the measured rHrose significantly as pH increased from 4 to 6, in agreement with data for phosphate adsorption on HFO (IO). Reliable estimates at pH >7 could not be obtained because of the small extent of adsorption. A m had less effect on rHthan did pH, with only a minor increase in rHas r increased from less than 20% of rmax ([A]ToT = 0.05 mM) to near surface saturation ( [ A l m = 0.8 mM). The measured rHreflects charge compensation at the oxide surface through a redistribution of surface species upon adsorption of ionic solutes (48). Hence, the measured rH can be compared to model results only through an analysis that takes species redistribution into account. The model rHis derived from the model-calculated total proton surface concentration (TOTHSURF) a t a given pH in the presence and absence of the adsorbing ion. For example, for chromate adsorption on a-FeOOH, the DLM TOTH~URF equations are TOTHsmF = [=SOH2+] - [=SO-] (1) TOTH*sURF = [=SOH2+]*- [=SO-]* +2[S3AHo]
+ [=SA-] (2)
where the starred terms represent concentrations of surface species for systems with chromate. For a given pH, the rHvalue is then calculated by subtracting eq 1from eq 2 and dividing the excess surface proton concentration (mol/L) by the total concentration of chromate surface species (mol/L). The DLM-generated rHvalues (Figure 5A and C) increased with pH and [AITOT,in reasonable agreement with experimental trends. However, in absolute terms, the agreement was less satisfactory, especially for chromate. The calculated trend with pH arises largely because the difference of [SOH2+]*and [SOH2+]becomes increasingly negative as pH drops below 5.5 and becomes positive above this pH. The larger rH values calculated for chromate and the occurrence of a maximum at higher pH (compared to oxalate) were due to the higher affinity of chromate for goethite. The trend with pH became more pronound when N , was increased to 2.3 sites/nm2 (Figure 5B and D) and agreed better with experimental rH values.
+ +
+
+
+
+
+
+
However, with this higher site density the DLM was less accurate in reproducing surface hydrolysis (36).The TLM with outer-sphere complexes poorly represented the experimental trends and erroneously predicted a decrease in rH with pH for oxalate (Figure 6A and C). Over the range of 2.3-5 sites/nm2,N , did not significantly affect rH in the TLM (Figure 6B and D). Extrapolation of DLM Constants. The intrinsic DLM surface complexation constants obtained here for oxalate and chromate adsorption on a-FeOOH conformed well to a h e a r free energy relationship (LFER) established by Dzombak and Morel (5) for adsorption of divalent anions on amorphous HFO (Figure 7A). This LFER relates the logarithms of surface complexation constants to the pK, of the adsorbing anions. Because we used the same parameter fitting methodology as Dzombak and Morel, a direct comparison can be made. [We used N , = 1.5 sites/nm2 for a-FeOOH whereas Dzombak and Morel (5) used N , = 2.3 sites/nm2 for HFO, but the values of log K2 and log K3 obtained here for chromate were virtually identical (fO.l log unit) for either value of N,. For oxalate, log K2was approximately 0.5 log unit lower when the lower site density was used (36).] The agreement between goethite constants and the LFER for HFO suggests that the affinity of divalent anions for amorphous HFO is similar to that for the highly crystalline goethite. This is supported by chromate adsorption data for different iron oxides (49). Another LFER relates analogous log K values for surface and solution reactions (5). The results for chromate on goethite were also in excellent agreement with this LFER established for HFO (Figure 7B). For oxalate, adsorption constants for the protonated and unprotonated sites (log Kl and log K2in Table 11) deviated significantly from the LFER. Note however that two of the four HFO constants used to establish this LFER showed comparable deviations from the LFER regression line (5). For chromate, no data are available for a solution reaction analogous to surface reaction 1 (Table 11). Conclusions Binding of oxalate and chromate onto a-FeOOH was proportional to solution concentrations at low adsorption densities (
