Envlron. Scl. Technol. 1992,26,2365-2370
Chromate and Oxalate Adsorption on Goethite. 2. Surface Complexatlon Modeling of Competitive Adsorption Karel Mesuere and Wllllam Flsh
Department of Environmental Science and Engineerlng, Oregon Graduate Institute of Science & Technology, 19600 N.W. von Neumann Drive, Beaverton, Oregon 97006-1999 Adsorption of binary mixtures of chromate and oxalate onto a-FeOOH was quantified as a function of pH for a wide range of adsorbate concentrations. Oxalate diminished the adsorption of chromate most effectively at low pH and when adsorbate concentrations were near surface-saturation levels. Chromate significantly inhibited oxalate adsorption over the entire pH range, reflecting the higher affinity of chromate for the a-FeOOH surface. The results indicate the adsorption of organic acids can enhance the mobility of chromate in acidic environments, while competitive adsorption of inorganic oxy anions may sharply diminish organic acid adsorption, thereby exerting an important control on mineral weathering rates. The ability of two surface complexation models to quantitatively predict the binary-solute data was evaluated using model constants that successfully described goethite surface hydrolysis and single-solute adsorption. Diffuse layer and triple-layer model simulations were highly similar and quantitatively accounted for binary-solute adsorption as a function of pH when surface concentrations of both coadsorbates were high. However, these models significantly underpredicted adsorption of minor species. Model predictions were particularly poor when the concentration of only one adsorbate was very low. Introduction Anion partitioning onto minerals, soils, and subsurface materials is often limited by the competitive adsorption of anionic cosolutes (1-8),while cation adsorption has little Anionic adsorbates direct influence on anions (1,9,10). directly compete for available binding sites and indirectly interact through alteration of the electrostatic charge at the solid surface. Both interactions are influenced by solution pH and by the relative concentrations and intrinsic binding affinities of the adsorbates. Competitive adsorption interaction between anions with similar adsorptive affinities can be very pronounced over a wide range of pH and relative concentrations (7,11, 12). In contrast, anions with intermediate to weak binding strength (Mood2-,CrOd2-,organic acids, SO:-, SeO:-) significantly reduce the adsorption of strongly binding Se032-,SiO2-, only if the weaker anions (Po43-, anions are in large excess and if solution pH is low (2, 13-1 9). Chromate is readily adsorbed by a variety of soil minerals (1,6,20-23), but coadsorption of sulfate, carbonate species, and other inorganic oxy anions can distinctly reduce chromate adsorption and enhance the mobility of chromate (1,6,24-26). Low molecular weight organic acids are common anions in natural environments (27,28)and are major cocontaminants of chromate at certain hazardous waste sites (29). Their adsorption interactions with chromate have not been documented in detail. Among organic acids, oxalic acid has an intermediate binding strength (3,17-19,30-32). Oxalate in large excess can effectively reduce phosphate adsorption (17-19,30,32-35), but an excess of sulfate represses oxalate adsorption (3). Solution concentrations of oxalate may be limited by precipitation with Ca2+(36)and biodegradation (37,38), 0013-936X/92/0926-2365$03.00/0
but recent investigations have revealed unexpectedly high levels of oxalic acid (up to 1 mM) and other carboxylic acids in a variety of soils and sediments (38,391.Therefore, organic acids may contribute to the mobility of inorganic contaminants such as chromate. Surface complexation models (SCMs) have been applied successfully to single-solute adsorption data (23,26,40). However, the ultimate goal of such modeling is to accurately predict adsorption in multicomponent, natural aqueous environments,using uniquely defined adsorption constants. Examination of binary-adsorbate mixtures is an important step toward this goal. Binary-adsorbate behavior should be predictable with models that combine information obtained from single-adsorbate experiments, provided all chemical components and reactions in the multiple-solute systems are defined, i.e., all adsorbing species or adsorptive solids also appear in the single-adsorbate systems. Application of SCMs to multiple-solute systems has produced some promising results (1,6,lo), but often has been quantitatively unsuccessful (2,3,14, 41,42).Limited evidence suggests the accuracy of model predictions of multiple-solute adsorption depends on adsorption density (1)and also may be affected by surface heterogeneity (11, 14,41). In addition, the accuracy of predictions is impaired by unreliable model parameters not experimentally verified or else optimized to very limited single-solute adsorption data. Finally, predicting adsorption of anion-cation and multiple-cation mixtures can be complicated by ill-defined ternary surface complexes or solid phases that do not exist in single-solute systems (9,42-45). The work discussed here extends the single-solute adsorption studies of oxalate and chromate on goethite (aFeOOH) presented in the first paper in this series (46)to binary-solute systems. Specifically, the objectives were to determine the extent of competitive adsorption between oxalate and chromate on a-FeOOH as a function of pH, adsorption density, and relative adsorbate concentrations and to ascertain the accuracy of two well-calibrated SCMS, the diffuse layer model (DLM, &o known as the two-layer model) and the triple-layer model (TLM), in predicting adsorption in binary-solute systems for the wide range of conditions presented by the data. The models have been previously optimized to numerous single-solute adsorption data (461,as well as to acid-base titrations of the adsorbent in the absence of oxalate and chromate (23).Because data acquisition was systematic and consistent, variability in the experimental methods was greatly reduced. However, because this study focused on anion adsorption it was important to consider possible interferences such as reduction-oxidation and precipitation-diisolution reactions. No significant redox coupling between Cr(V1) and oxalate was expected at pH 14 (the lowest pH in the experiments) because undissociated oxalic acid (H2C204)is a necessary reactant (47-49). The absence of Cr(V1) reduction was confiied in our laboratory by the constancy of the Cr(VI) absorbance (0.1567 f 0.0005 AU) at X = 350 nm of a pH 3 solution of chromate and excess oxalate monitored for 24 h (46).Fe(I1) rapidly reduces chromate
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Environ. Sci. Technol., Vol. 26, No. 12, 1992 2965
over a wide pH range (50,51). Because oxalate participates in rapid photoreductive dissolution of iron oxides, light was excluded from the reaction vessel for all experiments. Under some experimental conditions (pH 4; [OX]TOT > 0.5 mM), nonreductive oxalate-promoted dissolution of a-FeOOH did occur (52). Only a few data were obtained under these conditions, and dissolution was always less than 0.5% of total iron. Dissolution was slow relative to adsorption and did not alter adsorbate partitioning (52). The effect of chromate adsorption on oxalate-promoted dissolution was studied in detail and is presented elsewhere (52). Because titrations were always performed from high to low pH, the possibility of reprecipitation of the dissolved iron was eliminated.
Materials and Methods The synthetic goethite used for all experiments was taken from the same sample described in the first paper in this series (23). This goethite sample has a specific surface area (S) of 66 f 3 m2/g (N2/BET) and a pristine point of zero charge (PPZC) of 9.3 f 0.1 (2' = 25 "C). Details of the adsorbent synthesis and characterization are described elsewhere (46). Adsorption as a function of pH was measured in goethite suspensions (1.8 g/L) at constant ionic strength (I= 0.05 M KN03)and temperature (25 f 0.1 "C)in tightly capped, 200-mL Teflon cups under N2 atmosphere. Nz-purged, ultrapure water (Nanopure, Barnstead, Boston, MA) was mixed with appropriate amounts of 1 M KN03 stock solution and 0.1 M K2C204and KZCrO4stock solutions. The pH was adjusted with KOH to a value beyond the region of anion adsorption (pH 10-10.5); an aliquot of goethite stock suspension [12 g/L, Nppurged for up to 8 weeks (5311 was added and the pH adjusted incrementally downward with "0% At regular intervals, the pH was fixed for at least 12 h after which samples were taken. Preliminary kinetic experiments at pH 4,6, and 8 and for a wide range of relative adsorbate concentrations ([OX]TOT/ [Cr]TOT = 0.02-20) indicated that adsorption equilibrium was reached within 12 h (46). All samples were immediately centrifuged and analyzed. Further details of the experimental procedure and analysis are described in ref 23. Total adsorbate concentrations ([AITOT)of oxalate and chromate ranged between 0.01 and 0.8 mM. For [AlTWI 0.01 mM, adsorption was proportional to [AIToT, while for [AITOT 1 0.8 mM, surface saturation was reached (23). Data were modeled with the DLM and TLM by assuming homogeneous sites and using the model parameters specified in the first paper in this series without further optimization (23). Only outer-sphere surface complexes were used for TLM modeling. For each model, the site density (N,) was set equal to the values that were optimal for modeling surface hydrolysis data and exceeded the maximum adsorption density (r-) of the adsorbates used (oxalate and chromate): 1.5 sites/nm2 for the DLM and 5.0 sites/nm2 for the TLM (23). As in the companion paper (23), B = Ns/r'mm was used as the mass balance coefficient for the component =SOHoin all anion adsorption reactions. Results and Discussion One hundred percent adsorption of 0.01 and 0.05 mM chromate or oxalate (Figure 1A and B) corresponded to approximately 5% and 15% of the maximum adsorption density of each anion [I',,(Ox) = 2.2 pmol/m2, and rm,(Cr) = 2.4 pmol/m2 (23)]. At these low surface densities, chromate adsorption was unaffected by coadsorption of equimolar concentrations of oxalate and it suppressed 2388
Environ. Scl. Technol., Vol. 26, No. 12, 1992
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PH Flgure 1. Binary-solute adsorption of equlmolar concentrations IO.01 (A) and 0.05 mM (B)] of oxalate (0)and chromate (W) on a-FeOOH as a funof pH (G= 1.8 g/L; I = 0.05 M). Open symbols present data collected in singlesolute systems under otherwlse Identical condltions (23).DLM slmulations are shown wtth solM (slngle-solute) and dashed (binary-solute) lines. Model constants were taken from ref 23.
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PH Flgure 2. Singlasolute adsorption of oxalate and chromate on aF e w as a function of pH (0= 1.8 g/L; I = 0.05 M),illustrating the shifts In pH edges that result from doubllng the solute concentratlon. No experimental data were obtained for total chromate concentratlons of 0.1 mM (only model slmulatlon shown). Constants for DLM Slmulatlons were taken from ref 23.
oxalate adsorption only to a very small extent (Figure 1A and B). The pH edges in single-solute experiments shifted to a lower pH when the total adsorbate concentrations were doubled from 0.01 to 0.02 mM or from 0.05 to 0.1 mM (Figure 2A and B). This shift indicated that incremental adsorption was decreasingly favorable due to mass action and electrostatic effects and was no longer a constant
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PH Figure 3. Binary-solute adsorption of equimolar concentrations [0.2 (A) and 0.8 mM (B)] of oxalate (0)and chromate on a-FeOOH as a function of pH (0 = 1.8 g/L; I = 0.05 M). Open symbols present data collected In singlasolute systems under otherwise Identical conditions (23). DLM simulations are shown with solid (slngle-solute) and dashed (blnary-solute) lines. Model constants were taken from ref 23.
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proportion of the total added anion. Yet, when the total concentration of anions in the binary-solute systems was doubled from 0.01 to 0.02 mM or from 0.05 to 0.1 mM by the addition of the second anion, there was no comparable shift in the pH edge of either anion. The absence of mass action and electrostatic surface interactions between oxalate and chromate in binary systems suggests that anion adsorption at low surface density occurred at ion-specific sites on different crystal planes, as discussed later. Similar results were obtained for chromate and sulfate adsorption on Na-kaolinite (20). DLM simulations of adsorption at 0.01 and 0.05 mM total chromate in the presence of equimolar oxalate were nearly indistinguishable from simulations for chromate adsorption in the absence of oxalate, in good agreement with experimental data (Figure 1A and B). However, adsorption of the more weakly binding oxalate was clearly underpredicted by the DLM in binary-solute systems (Figure 1A and B). The model underprediction was most pronounced for adsorption from equimolar concentrations of 0.05 mM oxalate and chromate between pH 6 and 8 (Figure 1B). The inadequacy of the model simulation in this case exceeded the uncertainty expected based on the 99% confidence intervals for the intrinsic equilibrium constants used for modeling [f0.1 log unit for most log K values (2311.The suppression of oxalate adsorption predicted by the model was largely due to electrostatic effects because the dominant model surface species under these conditions were negatively charged [=SOAS for oxalate and =SA- for chromate (23)], while adsorption sites were still in excess. In contrast, at pH 4, where the dominant model surface species for both oxalate and chromate were uncharged (=SAH0)(23),DLM-predicted differences between oxalate adsorption in single- and binary-solute systems became comparatively smaller (Figures 1A and B). Competitive adsorption effects became apparent when both adsorbates were present in concentrations resulting in intermediate to high adsorption densities. For a total
Flgurr 4. Blnary-solute adsorptlon of oxalate (0)and chromate (m)
on a-FeOOH as a function of pH (0= 1.8 g/L; I = 0.05 M). The r a h [OX]TOT/[Cr]TOT equal 4 (A) and 16 (B). Open symbols present data
collected in singlesolute systems under otherwise Identical conditlons (23). DLM simulations are shown with solid (single-solute) and dashed (blnary-solute) lines. Model constants were taken from ref 23.
concentration of chromate or oxalate of 0.2 mM, surface densities at pH 4 were between 60% and 70% of rmax. When both oxalate and chromate were present at 0.2 mM each, chromate adsorption was unaffected at pH >7, but exhibited a small amount of competitive inhibition at more acidic pH (Figure 3A). Oxalate adsorption in the presence of chromate was reduced by approximately 50% a c r m the entire pH range of adsorption (Figure 3A). In binary-solute systems with 0.8 mM equimolar adsorbate concentrations, oxalate adsorption was reduced by more than 75% compared to single-solute systems, but chromate adsorption was still only slightly inhibited, and only for pH 8.5 adsorption of the major adsorbate (oxalate, [OX]TOT = 0.8 mM) was
