Equilibrium sorption of heavy metals and phosphate from single- and binary-sorbate solutions on goethite

Journal of Colloid and Interface Science 275 (2004) 53–60 www.elsevier.com/locate/jcis

Equilibrium sorption of heavy metals and phosphate from singleand binary-sorbate solutions on goethite Ruey-Shin Juang ∗ and Jui-Yin Chung Department of Chemical Engineering, Yuan Ze University, Chung-Li 320, Taiwan Received 22 October 2003; accepted 7 January 2004 Available online 13 February 2004

Abstract The amounts of Cu(II), Zn(II), and phosphate sorbed from single- and binary-sorbate systems on goethite (α-FeOOH) were measured. Experiments were carried out as a function of equilibrium pH (2–7), sorbate concentration (0.21–1.57 mM), and temperature (15–35 ◦ C). The aqueous phase contained 0.1 M NaNO3 to maintain ionic strength constant. A convenient method was used to obtain sorption isotherms of single Cu(II), Zn(II), and phosphate at a fixed equilibrium pH, which could be well described by the Langmuir equation. Thermodynamic parameters for the sorption of single Cu(II) and phosphate including the free energies, isosteric enthalpies, and entropies were determined. In contrast to the single-sorbate systems, the sorption of metals was inhibited in the binary Cu(II)–Zn(II) system, whereas the sorption of both sorbates was enhanced in the binary Cu(II)–phosphate system under the conditions studied. The validity of the Langmuir competitive model for the prediction of the sorption isotherms in a binary Cu(II)–Zn(II) system was also discussed.  2004 Elsevier Inc. All rights reserved. Keywords: Sorption isotherms; Cu(II); Zn(II); Phosphate; Goethite; Langmuir equation; Thermodynamic parameters

1. Introduction Iron oxide minerals are relatively abundant in soils and sediments. The chemical nature and high specific surface area of iron oxides as discrete particles and coatings on other minerals make these oxides efficient sinks for many contaminants including cations and anions [1]. Sorption of certain ions onto the oxides in soils and sediments is significant because it affects their mobility and bioavailability. Interactions between different ions for the sorption on these oxides are of importance for the understanding of chemical processes in natural systems from an environmental perspective [2]. Goethite (α-FeOOH) is a main soil mineral in subtropical areas. The significance of goethite sorption systems is reflected by a number of experimental studies that have been conducted in the recent past. For example, phosphate sorption on goethite has been extensively studied because in most soil systems phosphate is present as a nutrient sorbed in relatively large quantities by oxides due to its high affinity for * Corresponding author.

E-mail address: [email protected] (R.-S. Juang). 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.01.035

this mineral [3–10]. An important part of most of these studies has been the modeling of sorption in order to identify many surface reactions [3,4]. Surface complexation modeling is becoming popular for the description of equilibrium sorption, because it proposes specific surface reactions that also consider complex solution chemistry. The output from such models includes equilibrium constants for the postulated surface reactions, which can be used to predict sorption under conditions different from those studied [3]. For example, phosphate sorption on goethite has been described using a surface complexation model that considers some [3,4] or, specifically, three monodentate complexes [5]. A charge distribution-multisite complexation model has been also applied for this subject [6]. However, CIR-FTIR spectroscopic studies confirmed the formation of three types of complexes, protonated and nonprotonated bridging bidentate as well as a nonprotonated monodentate, between phosphate and surface Fe3+ of goethite particles in aqueous suspensions [7]. Some discrepancies about the surface complexes formed still exist, probably due to differences in analytical tools or disagreement about peak identification. The mutual influence of different ions on sorption to a variable charged surface is caused by site competition

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and/or electrostatic effects. For ions with the same charge, these two effects have a negative mutual influence on sorption [5,11–13]. For ions with opposite charge, however, the electrostatic effect has a positive mutual influence on sorption [14–20]. EXAFS data have shown that Cd(II) is innerspherically bound by bidentate double-corner-sharing sorption on the <110> surface of goethite in the presence of phosphate [18]. Macroscopic studies also indicated that the enhancement of Cd(II) sorption on goethite in the presence of phosphate is a solely electrostatic interaction and no ternary complexes are observed. On the other hand, batch sorption, ATR-FTIR, and EXAFS results indicated that Cu(II) and glyphosphate directly interact at the water–goethite interface to form two different types of ternary surface complexes at pH 4 and 9 [20]. The charge distribution-multisite complexation model has been used to describe the enhancement in sorption of Ca(II) [17] and Cd(II) [18,19] on goethite in the presence of phosphate, assuming the formation of monodentate or bidentate surface complexes with these metals, without invoking ternary surface complex formation. In principle, the parameters of such model were directly adopted from single-sorbate data sets, but the parameters of newly proposed species are assumed such that the model consistently yields good fits to the binary-sorbate data sets [21]. Unlike phosphate sorption, the sorption of metals on goethite in the absence of anionic ligands is often not described by surface complexation models because it is comparatively simple [11–13,22–26]. Batch sorption data and spectroscopic evidence confirmed that sorption of Pb(II) and Cu(II) to Fe-(hydr)oxides is dominated by the formation of inner-sphere complexes [27]. Macroscopically, inner-sphere sorption is suggested by the fact that uptake is unaffected by changes in ionic strength and occurs below pHpzc , where the mineral surface is positively charged. In general, the requirement of multiple parameters and/or computational complexity of surface complexation models may make them rather inconvenient for prediction of sorption behavior. A more empirical modeling approach involving the use of the Langmuir or similar isotherm equation is thus needed to describe the equilibrium sorption [11,12,22,23]. Because the sorption of cations and anions on iron oxide minerals such as goethite is significantly affected by solution pH [28], the aim of this work was to provide a method for accurately obtaining the sorption isotherms of Cu(II), Zn(II), and phosphate at a given “equilibrium” pH from single- and binary-sorbate systems without adding buffer solutions. An attempt was also made to correlate them with the Langmuir equation, and to obtain thermodynamic parameters for sorption. 2. Materials and methods 2.1. Synthesis and characterization of goethite All chemicals were stored in plastic (polypropylene) flasks and the experiments were carried out in plastic vessels

to avoid the possible contamination of silica. The water used throughout the experiments was always double-distillated and deionized by Milli-Q system (Millipore Co., USA), and was also purged by N2 gas to avoid CO2 dissolution. Goethite was synthesized following the procedures of Schwertmann and Cornell [1]. After slow and dropwise addition of 180 cm3 of 5 M KOH to 100 cm3 of 1 M Fe(NO3 )3 , the solution was diluted with water to 2 dm3 (pH 12). It was aged for 60 h at 60 ◦ C. During this period, the solution was agitated three times for 2 min each. After aging, a compact, yellow brown precipitate of the goethite was formed. The resulting suspension was centrifuged and filtered, and the cake was washed with water until the solution was neutral. The iron oxide was dried at 60 ◦ C for 24 h and finally stored in a desiccator. Prior to use, goethite was ground and sieved in the particle size range 0.04–0.1 mm. The crystal structure of the goethite was confirmed to be a pure product by comparing its XRD powder pattern (JOEL, JSMTS 330) with the JCPD Standards Card 29-713. The morphology was examined by means of a JOEL JSM-5600 SEM instrument, which shows agglomerates of needle-like particles. These needles appear as elongated particles, with flat surfaces and rectilinear edges [26]. The BET surface area of the goethite was found to be 84 m2 /g from N2 sorption isotherms at 77 K with a sorptiometer (Quantachrome, NOVA 2000). Based on the isotherms, the mean pore volume and pore size were found to be 0.18 cm3 /g and 2.3 nm, respectively. In addition, the apparent density of goethite was about 4.0 g/cm3 . 2.2. Zeta potential measurements Zeta potential of the goethite suspension was measured with laser Doppler electrophoresis using a Malvern Zetasizer 3000HS instrument. Goethite (0.01 g) was placed in a 100-cm3 plastic vessel, into which 50 cm3 of a CO2 -free aqueous solution was poured. The vessel was sealed and the contents were allowed to equilibrate for 24 h in a shaker at 25 ◦ C. The suspensions were then filtered through a filter paper and the final pH values were measured using a pH meter (Horiba 230FS). The aqueous phase was deionized water or 0.1 M NaNO3 , and the initial pH value was adjusted by adding an amount of CO2 -free 0.1 M HNO3 or NaOH solution. 2.3. Procedures of sorption experiments From the pH diagrams of species distribution for phosphate [29], the dominant species and corresponding pH range will be H3 PO4 (pH < 2), H2 PO− 4 (2 < pH < 6), 3− (6 < pH < 11), and PO (pH > 11). For a Cu(NO3 )2 HPO2− 4 4 containing solution, on the other hand, the ions Cu2+ are dominant at pH < 7. The amounts of some hydroxyl species + such as Cu2 (OH)2+ 2 , Cu(OH)2 , and Cu(OH) become significant under alkaline conditions. Thus, the salts Cu(NO3 )2 , Zn(NO3)2 , and NaH2 PO4 were used to prepare the target solutions of Cu(II), Zn(II), and phosphate, respectively.

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In sorption experiments, an amount of goethite (0.05 g) and 50 cm3 of the CO2 -free aqueous solution were placed in a 100-cm3 plastic vessel. The vessel was sealed and was agitated for 24 h by a magnetic stirrer in a water bath, in which the temperature was controlled at 15–35 ◦ C. The aqueous solution consisted of supporting electrolyte (0.1 M NaNO3 ). Preliminary tests had shown that the sorption studied was complete after 12 h. After centrifugation, the aqueous samples were filtered through 0.45-µm PVDF filters (Whatman, USA). Then, the concentrations of metals were analyzed with an atomic absorption spectrophotometer (Varian, 220FS), and that of phosphate was analyzed by an induced coupled plasma-atomic emission spectrophotometer (Perkin–Elmer, Optima 2000DV). Each experiment was at least duplicated under identical conditions. The reproducibility of the concentration measurements was within 3%. The amount of sorbate sorbed at equilibrium q (in mmol/kg) was calculated by q = (C0 − Ce )V /W,

55

Fig. 1. Zeta potentials of the goethite suspensions under different solution conditions.

(1)

where C0 and Ce are the initial and equilibrium solution concentrations (mM), V is the volume of solution (dm3 ), and W is the weight of dry goethite used (kg). Each experiment was duplicated at least under identical conditions. The reproducibility of the measurements of q was within 4%.

3. Results and discussion 3.1. Zeta potentials of the goethite suspensions Fig. 1 shows the zeta potentials of goethite (0.2 g/l) in the suspensions of deionized water and 0.1 M NaNO3 solution. Here, deionized water means the original condition. It would contain tracer ions after pH adjustment with CO2 free 0.1 M HNO3 or NaOH solution. As expected, the zeta potential gradually decreases with increasing solution pH. The zero point of charge (pHzpc ) of pristine goethite (i.e., free of sorbates) is near 7.9, which agrees reasonably with previous results that locate in the range 7.2–9.7 [29]. With this relatively low pHzpc value, it appears that carbonate is still sorbed on the goethite surface [30], although CO2 -free aqueous solution was used in these experiments. 3.2. Sorption isotherms of single sorbates Figs. 2–4 show the effects of equilibrium pH (pHe ) on the amount of single Cu(II), Zn(II), and phosphate sorbed at different temperatures. Evidently, the sorption of heavy metals increases gradually with increasing pHe and then sharply at pHe greater than about 4. On the other hand, the sorption of phosphate anions gradually decreases with increasing pHe . Such pH trends agree with the earlier observations [8–15,21–26].

Fig. 2. Effect of equilibrium pH on the amount of single Cu(II) sorbed on goethite.

The sorption isotherms of single sorbates at a fixed pHe can be obtained as follows. Selecting a suitable pHe from Figs. 2–4 (e.g., 5.0 for metals and 3.5 for phosphate), we find the values of q by interpolation at different initial sorbate concentrations (C0 ), and then the corresponding Ce values from Eq. (1). The main reason why these pHe values are

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Fig. 3. Effect of equilibrium pH on the amount of single Zn(II) sorbed on goethite and the sorption isotherm.

selected is to neglect the possible sorption of carbonate from the present CO2 -free aqueous solutions on goethite [31]. The results are shown in Figs. 3,5, and 6. Judging from the shapes of sorption isotherms, it is expected that the common two-parameter Langmuir equation can be applied. qmonKCe q= , 1 + KCe

(2)

where qmon refers to the amount of sorption corresponding to monolayer coverage (i.e., site density) and K is the Langmuir constant. A linear plot of (Ce /q) vs Ce would yield the values of qmon and K. The fit is good under the concentration ranges studied (correlation coefficient, r 2 > 0.994). Tables 1 and 2 list the results of K and the capacity per unit area of goethite qmon /A for sorption of metals and phosphate. Evidently, Cu(II) has a much higher sorption capacity than Zn(II) [13,23,24], but has a comparable affinity to goethite when the equilibrium concentrations (Ce ) are normalized to the saturated concentrations. For Cu(II) sorption, the qmon /A value slightly increases with increasing temperature at a given pHe . It increases from 0.41 to 0.95 µmol/m2 at 25 ◦ C, e.g., when pHe increases from 4.5 to 5.0. They are comparable to those obtained earlier in similar metal–goethite systems [11–13,22,26]. However, Roddy et al. [25] found different isotherms of Cu(II) sorption on goethite at pH 5.5 at low and high Cu(II) concentrations. They explained this by competitive sorption of different species Cu(OH)+ and

Fig. 4. Effect of equilibrium pH on the amount of single phosphate sorbed on goethite. Table 1 The Langmuir parameters obtained for the sorption of single metals on goethitea Metal

Cu(II)

Zn(II)

T

pHe 4.5

pHe 5.0

(K)

K (1/mM)

qmon /A (µmol/m2 )

K (1/mM)

qmon /A (µmol/m2 )

288 298 308 308

3.7 4.9 7.2

0.39 0.41 0.45

2.8 3.9 4.9 18.2

0.88 0.95 0.98 0.30

a The BET surface area of goethite, A = 84 m2 /g.

Cu2 (OH)2+ 2 for surface sites on goethite. When the concentration increases, the proportion of the latter species increases, leading to enhanced sorption. On the other hand, the qmon /A value in phosphate sorption decreases from 2.29 to 1.93 µmol/m2 at 25 ◦ C when pHe increases from 2.5 to 4.5 (Table 2). In the pH range studied, these values are less than those reported by Li and Stanforth [10] who investigated the sorption of single phosphate onto goethite with a BET surface area of 55.7 m2 /g. They indicated that the reaction between phosphate and goethite involves sorption and surface precipitation, and the transi-

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57

Fig. 5. Sorption isotherms of single Cu(II) on goethite at different equilibrium pH values.

Fig. 6. Sorption isotherms of single phosphate on goethite at different equilibrium pH values.

Table 2 The Langmuir parameters obtained for the sorption of single phosphate on goethitea

layer caused by the change in ionic strength in the solution phase should be caused by specific interactions. Thus, the role of specific sorption is likely predominant in the sorption of phosphate and this could be the reason why the results support a simple Langmuir isotherm. Although Eq. (2) is applicable to the present sorption systems under the conditions studied, the good description of the isotherms does not mean that the sorption occurs on one single type of site [32]. The fact that K changes with pH (Tables 1 and 2) supports that different kinds of sorption sites (with different interaction energies) are present on a goethite surface [33].

T

pHe 2.5

(K) 288 298 308

pHe 3.5

pHe 4.5

K qmon /A K qmon /A K qmon /A (1/mM) (µmol/m2 ) (1/mM) (µmol/m2 ) (1/mM) (µmol/m2 ) 52 62 71

1.93 2.29 2.38

28 47 68

1.75 2.12 2.17

63 116 165

1.53 1.93 1.95

a A = 84 m2 /g.

tion point from sorption to surface precipitation occurs at a lower surface coverage when the pH increases. For example, the transition point is about 4 µmol/m2 at pH 4.42. In both sorption of single Cu(II) and phosphate, it is also found that at a given pHe the sorption capacity increases with increasing temperature. This is probably related to changes in the water properties such as hydrogen bonding and hydration cell of the ions, as discussed in the next section about the positive enthalpy of sorption. It is noted that the concentration dependence of sorption should reflect a specific sorption in the presence of supporting electrolyte [27]. Thus, a change in the double

3.3. Effect of temperature on sorption Because K is essentially an equilibrium constant, the isosteric enthalpy of sorption (H ) can be obtained from the van’t Hoff equation [9,22,25]: d(ln K)/d(1/T ) = −H /R.

(3)

Fig. 7 shows the linear relationships (correlation coefficient, r 2 > 0.986). Other thermodynamic parameters of the sorption including free energy and entropy are also calculated by [22,34,35]

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Fig. 7. van’t Hoff plot for sorption of single Cu(II) and phosphate on goethite at different equilibrium pH values.

Table 3 Thermodynamic parameters for the sorption of single Cu(II) on goethite pHe 4.5 5.0

H

G (kJ/mol)

S (J/(mol K))

(kJ/mol)

288 K

298 K

308 K

(average)

24.5 20.7

−3.1 −2.4

−3.9 −3.4

−5.1 −4.1

95.7 80.4

Table 4 Thermodynamic parameters for the sorption of single phosphate on goethite pHe 2.5 3.5 4.5

H

G (kJ/mol)

S (J/(mol K))

(kJ/mol)

288 K

298 K

308 K

(average)

12.1 32.2 33.9

−8.2 −9.0 −9.8

−10.2 −10.6 −12.1

−10.5 −10.7 −12.9

73.0 144.8 163.9

G = −RT ln K,

(4)

S = (H − G)/T .

(5)

It is noted that the Langmuir constant K used in Eq. (4) is in 1/M. The calculated results are listed in Tables 3 and 4. The small but negative G values obtained in the sorption of both single Cu(II) and phosphate indicate the sorption to be spontaneous. Also, the positive S value may be due to some structural changes in both sorbates and goethite during the sorption process [34]. In fact, the positive H and S values have been reported previously for the sorption of metals on iron oxide minerals, such as Co(II)/goethite and Cd(II)/goethite [22], and Cd(II)/hematite [36]. Angove et al. [22] have actually stated that the Langmuir parameters yield

Fig. 8. Effect of equilibrium pH on the amount of binary Cu(II) and Zn(II) sorbed on goethite and the sorption isotherms.

positive entropies, while some surface complexation parameters generate negative sorption entropies. The results of positive H values for phosphate sorption (12–34 kJ/mol) are inconsistent with those (−25 kJ/mol or less, depending on surface coverage) obtained by titration calorimetry [9]. It was also postulated in that paper that free energy has a large favorable entropic component with enthalpic factors of more minor importance in anion sorption. One possible explanation of our results is that the overall process is virtually a combination of proton desorption and phosphate sorption, because the enthalpy change for proton sorption on goethite was found to be −30 to −39 kJ/mol [30,36]. The sorption of phosphate or other strongly bound oxyanions such as arsenate generally not only makes the surface of goethite more negative but also shifts the pHzpc to more acidic values. The shape of zeta potential versus pH profile remains relatively constant [7,10]. 3.4. Equilibrium sorption of binary sorbates Fig. 8 compares the effect of equilibrium pH on amount of Cu(II) and Zn(II) sorbed in single and binary sorbate systems at a fixed total metal concentration (upper part). It is found that Cu(II) still has a higher sorption than Zn(II), even when Zn(II) is in a large excess to Cu(II) in solution. An attempt was made to predict the sorption isotherms in binary sorbate systems based on the K and qmon values that are directly adopted from single sorbate systems. A simple “one-site” Langmuir competitive model is thus tried, which

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however, at higher sulfate concentrations (> 0.25 mM), the ≡FeOHCuSO4 complex is dominant at pH < 5. Such mutually enhanced sorption cannot be described by the modification of combined forms of simple Langmuir and other empirical equations because the interactions among metals, anionic ligands, and goethite are strong and complicated [38]. A surface complexation modeling approach would be more powerful for this subject.

4. Conclusions

Fig. 9. Effect of equilibrium pH on the amount of binary Cu(II) and phosphate sorbed on goethite.

is given by [37] qi =

qmonKi Ce,i
. 1 + nj=1 Kj Ce,j

(6)

It is evident that the prediction of Cu(II) sorption is acceptable, as shown in lower part of Fig. 8. But, the sorption of Zn(II) is overestimated. That is, Zn(II) sorption is significantly depressed in the presence of Cu(II), the more strongly bound metal. This agrees with the previous results that site competition and/or electrostatic effects have a negative influence on sorption for ions with the same charge [5,11–13]. Other modified forms of the Langmuir competitive model may improve the fitness, such as considering different types of surface sites (e.g., monodentate or bidentate) of sorbents available for sorption of different sorbates [11,12]. Fig. 9 shows the effect of equilibrium pH on sorption of Cu(II) and phosphate in single and binary systems at a fixed total sorbate concentration. Evidently, Cu(II) sorption is enhanced in the presence of phosphate and phosphate sorption is enhanced in the presence of Cu(II). These results are generally consistent with those reported previously in similar metal–phosphate–goethite systems, such as Zn(II) [14,15], Pb(II) [16], Ca(II) [17], and Cd(II) [18,19]. The explanations for enhanced effect of anions on metal sorption include (i) the formation of ternary complexes, and (ii) reduction of surface potential by anion sorption, making the surface more attractive to metal sorption, and the formation of metal– ligand precipitates [3,4]. For example, the increased sorption of Cu(II) in the presence of sulfate can be modeled considering the formation of ternary complex ≡FeOHCuSO4 [21]. In the sulfate-free systems, ≡FeOCu+ is the dominant species;

This work indicated that the sorption isotherms (q vs Ce ) of single metals and phosphate on goethite at a fixed equilibrium pH (pHe ) could be obtained from a common pH profile of the amount of sorption (q vs pHe ), without the help of buffer agents. The isotherms were well described by the Langmuir equation. Increasing pHe led to an increased metal sorption and a decreased phosphate sorption. At 25 ◦ C, the sorption capacity of Cu(II) increased from 0.41 to 0.95 µmol/m2 when pHe was increased from 4.5 to 5.0, but that of phosphate decreased from 2.29 to 1.93 µmol/m2 as pHe was raised from 2.5 to 4.5. The Langmuir constants yielded negative G and positive S values in sorption of single Cu(II) and phosphate, indicating the spontaneous sorption and some structural changes in sorbates and goethite during the process. The positive H value for phosphate sorption obtained (12–34 kJ/mol) inferred that the overall process was a combination of proton desorption and phosphate sorption. The sorption of Cu(II), the more strongly bound metal, could be acceptably predicted by the one-site Langmuir competitive model in a binary Cu(II)–Zn(II) solution, but that of Zn(II) was far overestimated. On the other hand, the enhanced sorption of Cu(II) and phosphate in binary sorbate solutions was not described by such simple or any modified models because the interactions among Cu(II), phosphate, and goethite were complicated. A surface complexation modeling approach is needed for this subject.

Acknowledgment Support for this work by the National Science Council, ROC, under Grant NSC 90-2214-E-155-001 is gratefully appreciated.

Appendix A. Nomenclature A C0 Ce G

BET surface area of goethite (m2 /g) initial sorbate concentration in the aqueous phase (mM) equilibrium sorbate concentration in the aqueous phase (mM) free energy of sorption defined in Eq. (4) (kJ/mol)

60

H K q qmon S T V W

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isosteric enthalpy of sorption defined in Eq. (3) (kJ/mol) Langmuir constant defined in Eq. (2) (1/mM) amount of sorption at equilibrium (mmol/kg) amount of sorption corresponding to monolayer coverage (mmol/kg) entropy of sorption defined in Eq. (5) (J/(mol K)) temperature (K) volume of the solution (dm3 ) amount of dry sorbent used (kg)

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