Adsorption and desorption of natural organic matter on iron oxide

Environ. sci. Technol. 1994, 28, 38-48

Adsorption and Desorption of Natural Organic Matter on Iron Oxide: Mechanisms and Models Baohua Gu,'vt Jurgen Schmitt,* Zhlhong Chen,s Llyuan Llang,t and John F. McCarthyt

Environmental Sciences Divlslon and Chemistry Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6038,and Department of Hydrology, Fb VI, University of Trier, 5500 Trier, Germany

+ Environmental Sciences Dividion, Oak Ridge National Laboratory. t University of Trier. 8 Chemistry Division, Oak Ridge National Laboratory.

The major mechanisms by which NOM adsorb onto mineral surfaces have been proposed to involve: (i) anion exchange (electrostatic interaction), (ii) ligand exchangesurface complexation, (iii) hydrophobic interaction, (iv) entropic effect, (v) hydrogen bonding, and (vi) cation bridging. A detailed description of these mechanisms has been discussed by Sposito (8). However, there are few direct measurements in the literature with which these mechanisms are actually evaluated. The ligand exchange mechanism between surface-coordinated OH and H20 from iron and aluminum oxides and the dissolved NOM was favored by anumber of investigators (6,9-11). Parfitt et al. (9) have shown by infrared spectroscopy that the adsorption of a fulvic acid on goethite involves the complexation between COO- of fulvic acid and OH of the goethite. Many studies have also noted that the adsorption of those organic acids is usually accompaniedby an increase in pH, indicating that the COO- groups of fulvic acid replaced the surface OH on oxides (9, 10). On the other hand, Inoue and Wada (12) indicated that soil humic substances were adsorbed on allophane through an anionexchange mechanism. Jardine et al. (13) studied the adsorption of NOM on several soils containing 1-3 % DCBextractable Fe and concluded that the predominant mechanism of NOM adsorption by the soil was physical adsorption driven by favorable entropy changes. Apparently, a discrepancy exists among investigators due to our limited understanding of the interaction mechanisms between NOM and oxide surfaces. It may also be partially explained by the different materials and experimental conditions used by the individuals and attributed to the heterogeneity and complexity of NOM and adsorbent surfaces. The physicochemical and structural features of NOM and soil minerals which lead to energetic differences give rise to different combinations of adsorption mechanisms. Desorption processes of NOM have been studied to a significantly lesser extent than adsorption, yet the former is of fundamental importance in quantitative formulation of NOM transport within the framework of mass balance. The presence of hysteretic adsorption-desorption relations has received little attention, and whether such a hysteresis is kinetically limited is largely unknown. Many mathematical formulations of the transport often considers the adsorption and desorption reaction reversible (14). Amacher et al. (15)have shown that the adsorption-desorption of several heavy metals from batch studies on several soils could not be adequately described by the use of the reversible equilibrium Langmuir or Freundlich models. On the other hand, consideration of the adsorptiondesorption hysteresis has been shown to lead to a better prediction of solute transport when using the classical convection-dispersion equation (16-18). Although many adsorption models have been developed [e.g., Langmuir and Freundlich models @)I, little consideration has been given to desorption modeling. Severalinvestigators simply

38 Envlron. Sci. Technol., Vol. 28, No. 1, 1994

0013-938X/94/0928-0038$04.50/0

The adsorption and desorption mechanisms of natural organic matter (NOM) on mineral surfaces are not completely understood because of the heterogeneity and complexity of NOM and adsorbent surfaces. This study was undertaken to elucidate the interaction mechanisms between NOM and iron oxide surfaces and to develop a predictive model for NOM adsorption and desorption. Results indicated that ligand exchange between carboxyl/ hydroxylfunctional groups of NOM and iron oxide surfaces was the dominant interaction mechanism, especiallyunder acidic or slightly acidic pH conditions. This conclusion was supported by the measurements of heat of adsorption (microcalorimetry), FTIR and I3C NMR analysis, and competitive adsorption between NOM and some specifically adsorbed anions. A modified Langmuir model was proposed in which a surface excess-dependent affinity parameter was defined to account for a decreasing adsorption affinity with surface coverage due to the heterogeneity of NOM and adsorbent surfaces. With three adjustable parameters, the model is capable of describing a variety of adsorption isotherms. A hysteresis coefficient, h, was used to describe the hysteretic effect of adsorption reactions that, at h = 0, the reaction is completely reversible, whereas at h = 1,the reaction is completely irreversible. Fitted values of h for NOM desorption on iron oxide surfaces ranged from 0.72 to 0.92, suggesting that the adsorbed NOM was very difficult to be desorbed at a given pH and ionic composition. Our results imply that a better mechanistic understanding of the interaction between NOM and oxide surfaces is needed to improve our predictive capabilities in NOM transport and cotransport of contaminants associated with NOM or iron oxides.

Introduction An appreciation of the role of solid-water interfaces and surface-controlled reactions is a prerequisite for understanding the retention of natural organic matter (NOM) and the stability/transport of both organic and inorganic colloids in the subsurface soil environments. Previous studies (1-4) indicate that most particles suspended in natural aqueous systems are negatively charged due to adsorbed NOM. The adsorbed NOM can mask the physicochemical properties of the underlying solid whose behavior (e.g., electrophoretic mobility, colloidal stability, and transport) may be dominated by the adsorbed NOM. The adsorbed NOM coatings may also render hydrophilic surfaces hydrophobic and more capable of sorbing organic contaminants (5-7).

* Corresponding author.

0 1993 Amerlcan Chemical Society

considered that the hysteretic adsorption consisted of an irreversible part and a reversible part of a reaction (17, 18). Vaccari and Kaouris (17)proposed a model, which is essentially a combination of the Langmuir and Freundlich models, in which the irreversible part of the adsorption was described by the Langmuir model and the reversible part of the adsorption was modeled by the Freundlich model. However, as Di Tor0 and Horzempa (16)indicated, for many environmental modeling efforts, it may be inappropriate to model adsorption reactions as either completely reversible or completely irreversible. Therefore, the main objectives of this study were to investigate the mechanisms of NOM adsorption on iron oxide by applying such techniques as wet chemistry, microcalorimetry, FTIR, and NMR spectroscopy and to develop an adsorption-desorption model based upon our mechanistic understanding of NOM interaction with iron oxide surfaces. Adsorption-Desorption Model Description

Adsorption. The processes of NOM adsorption on iron oxide may be simplified as in eq 1,where C is the solution

0.0

-

0

NOM concentration, q is the amount of NOM adsorbed, and S = (qmm - q ) is the available surface sites on the adsorbent for NOM adsorption, where qmaxisthe maximum quantity of NOM adsorbed. Parameters kf and k b are the forward and backward rate coefficients, corresponding to the activation energies of adsorption (Ed and desorption (Eb),respectively (20,21). The net rate of adsorption can be expressed as (20, 22): aq/at = k,C(qmm - 4 ) - kbq (2) Defined according to Lindstrom et al. (21)and Haque et al. (23, 24), Ef and Eb change linearly with surface coverage to account for the heterogeneity of adsorbent surfaces and adsorbates, which ultimately influence the affinity of NOM adsorption:

+ 8q

(3)

E, = E; - 84 (4) where Efo and Ebo are the energies of adsorption and desorption at 9 = 0, and 8 is the surface stress coefficient according to Fava and Eyving (20) and Lindstrom et al. (21). By applying the rate theory (20, 217,231 12, = Ae(-Ef/Rn = ~~(-(q+&’)/Rn

I

I

4- = 0.4

(5)

where A and B are the frequency factors, R is the universal gas constant, and T is the absolute temperature. Substitution of eqs 5 and 6 into eq 2 yields:

For e uilibrium adsorption, a q h t = 0. Define K = (AI B)e-(Z-NRnand b = 8/RT, and rearrange eq 7 to yield

I

I

I

II 40

Figure 1. Simulations of Isotherms wlth the modified Langmuir model (eqs 8 and 9) by varying the two parameters, Kand b, while holding qmaxat 0.4. The concentration units are arbitrary. The model is capable of fitting a variety of isotherms such as L-type (b-e), S-type (f), H-type (a), and C-type (9) isotherms by varying the three parameters.

where

K (s)E Ke-269 (1)

kb

E, = E:

I

10 20 30 CONCENTRATIONUNITS

kr

c+s---q

I

(9)

K(q)is defined as a surface excess-dependent affinity parameter. At b = 0 (which requires a constant energy of K , eq 8 becomes the well-known adsorption), Langmuir model (25). We thus refer to eq 8 as a modified Langmuir model. However, it must be pointed out that the Langmuir model was originally developed to describe the adsorption of a gas on solid surfaces. Although the model has been widely adapted to interpret reactions between solutes and solid surfaces in solution, the significance of the parameters,qmaxand K , is certainly dubious in many systems (26, 27). These parameters may be treated as empirical parameters which describe the adsorption isotherm. Similarly, our model (eq 8) may be regarded as an empirical model because a rigorous, concise description of NOM adsorption from the aqueous phase onto solid phases is difficult, consideringthe heterogeneity of solid surfaces and NOM, and the possibility of chemical adsorption and physical adsorption both occurring. An additional parameter, b, is used in our model (eq 9) whish presumably takes into account the features of changing energies of adsorption with changing surface coverage. Realistically, our model is capable of fitting a variety of adsorption isotherms including L-type, S-type, H-type, and C-type isotherms (8) as illustrated in Figure 1. Desorption. If the processes of adsorption and desorption are completely reversible, eq 8 is then used to describe both the adsorption and desorption isotherms. On the other hand, hysteresis occurs when the amount of solute remaining on the adsorbent after desorption is greater than that predicted by the adsorption isotherm (eq 8). Such a phenomenon has been observed for a number of organic contaminants and natural organic components adsorbed on soil sediments, pure clay minerals, and oxides (14,16,17,28). The term, “irreversible” (or “nonreversible”) adsorption has often been used to describe such a hysteresis (16, 17). Strictly speaking, however, completely irreversible (or nonreversible) reactions are thermodynamically impossible. For this reason, the hysteresis of an adsorption-desorption reaction is often considered as an experimental artifact due to either kinetic limitation or other factors such as chemical and biological I

Environ. Sci. Technoi.. Vol. 28, No. 1, 1994 59

activities. Realistically, however, sufficient evidence suggests that hysteresis exists even after equilibration for an extended time period (16), and it does in fact reflect a fundamental property of the adsorption since the desorption occurs at such a reduced rate as to be unobservable (e.g., within experimental error or within limited time period). With the above background and by assuming kf and k b are the same as in eq 1, eq 2 is reformulated by incorporating a correction factor, (CIC,)h, in the desorption term to account for nonequilibrium desorption hysteresis, i.e. where Ca is the equilibrium adsorbate concentration after adsorption but before commencing the desorption experiment, and h is defined as a hysteresis coefficient. Similarly to the derivation of eq 8, the nonequilibrium desorption model is given as:

It should be pointed out, however, that eqs 10 and 11 require 0 < C 5 C,, because theoretically an infinite number of desorption cycles are required for C = 0. Equation 11 satisfies conditions that, at h = 0, eq 11 becomes eq 8, indicating a completely reversible adsorption desorption process, and at h = 1, = [(K(q,qm=Ca)/(K(q,Ca + 111, Le., q becomes independent on C but dependent on the initial desorption concentration C,, indicating a completely irreversible reaction. Therefore, h may be considered as a measure of adsorption-desorption reversibility and, thus, appropriately named as a hysteresis coefficient. Materials and Methods Iron Oxide and Natural Organic Matter (NOM). Commercialiron oxide powder (100.3 % assay, J. T. Baker, Inc.) was used without further purification. Particles of iron oxide were deep-red colored and appeared to be well crystallized with an average diameter of approximately 150-300 nm as viewed by TEM. The X-ray diffraction (XRD) pattern of the iron oxide matched exactly that of hematite (a-Fez03). Its BET specific surface area was determined to be 10.1m2g',and anion exchange capacity was 38.6 f 7.4 mmol k g l at pH 6.0. The NOM sample was collected from a wetlands pond at Clemson University's Baruch Forest Science Institute in Georgetown, SC. The water chemistry of raw NOM has been provided elsewhere ( 4 ) . After being filtered through a O.l-pm Amicon hollow-fiber filter, the NOM solution was passed through a column of cation exchange resin (Na+form). No other cations were then detected by atomic absorption spectroscopy. Finally, it was stored at 4 "C. Some basic physical and chemical characteristics of NOM are listed in Table 1. In addition, FTIR and NMR spectroscopy of NOM showed that it resembled those generally shown for humic substances in aquatic systems and soil fulvic acids (29, 30). For comparison, Suwannee River fulvic acid standard obtained from the International Humic Substance Societywas also used for the experiment. Adsorption and Desorption of NOM. For NOM adsorption, the iron oxide was first suspended in purified water (Milli-Q plus system, Bedford, MA) at a pH -4 and stirred for about 1 h. Acid (HCl), base (NaOH), or 40

Envlron. Scl. Technol., Vol. 28, No. 1, 1994

Table 1. Physical and Chemical Properties of NOM elemental anal. C

0 H N S

property wt %"

48.3 42.1 3.3 1.6 3.1

acidityb HbA-NOMc HbN-NOMC H1-NOMC

12 mmol g-1 C 42% 18% 40 %

Corrected for ash content. Acidity was determined by potentiometric titration upto pH 11 under Ar purging. HI-NOM, HbANOM, and HbN-NOM represent hydrophilic, hydrophobicacid, and hydrophobic neutral fractions of NOM fractionated on XAD-8, respectively ( 4 ) .

inorganic salts (NaC1, NazSOr, and Na3P04) were then added to achieve the desired ionic composition and pH (in 100mL). While stirring, 1mL of the iron oxide suspension (1% 1 was pipeted into 4 mL of a series of NOM solutions with desired ionic composition, pH, and C concentration in 10-mL Teflon centrifuge tubes. The suspensions were then shaken for 18 f 2 h a t room temperature. Preliminary results indicated that NOM adsorption on iron oxide was very fast and plateaued within minutes. The suspensions were then centrifuged at 15 000 rpm for 15min, which was found to be sufficient for separation of the adsorbent from adsorbate. At the most extreme cases (e.g., at a low pH with no NOM or at a high pH with high NOM concentrations), the amount of iron oxide remaining in the supernatant was 0.97qmW) This may also offer an explanation for the uncertainty (or high standard deviations) of b and K values estimated from the NOM adsorption isotherms (Table 2). An inaccurate determination of low C concentration may result in large deviations

I t is interesting to note that the calculated AGad,O values (correspondingto K values in Table 2) for NOMadsorption are about -35 kJ mol-' C (Table 2), which is in a fairly good agreement with the measured adsorption enthalpy (-J -46 kJ mol-l C) although the two experiments were performed at different pH values and by different techniques. These observations again support our model assumption that the activation energies of adsorption and desorption change with surface coverage (eqs 3 and 4). A positive b value (Table 2) indicated that a high-affinitytype interaction occurred first between NOM and iron oxide surfaces and that adsorption progressed with decreasing affinity. Therefore, parameter b is regarded as a measure of the heterogeneity of a system, which leads to energetic differences in adsorption. Finally, a hysteresis coefficient, h, was used to describe the relative reversibility of NOM adsorption and desorption (Table 3). The advantages of using the desorption model (eq 11) are that only one parameter, h, is needed for model calculations. All other parameters, K , b, and qmaX,are the same as those for the adsorption (eq 8). In addition, the model can be used quantitatively to describe the hysteresis. Data presented in Table 3 in general fitted those desorption data quite well (Figure 3) and indicated very strong hysteretic desorption of NOM on iron oxide. It should be pointed out, however, that an inaccurate determination of low C concentrations after a series of desorption cycles contributed largely to the deviations of estimated h values (Table 3). Further studies are certainly needed, and studies by using 14C-labeledhumic substances would be an advantage for desorption studies. General Conclusions

The general conclusion of this study is that NOM adsorption on iron oxide surfaces is largely due to the ligand exchange mechanism, at least at pH below the ZPC of iron oxide, which may be verified by a variety of direct or indirect experimental techniques. Although the NOM sample used for the present study may not be representative of all aquatic humic substances, general rules may apply since these humic substances are known to be abundant in carboxyland hydroxylfunctionalgroups. This is further supported by the observation that Suwannee River fulvic acid exhibited similar adsorption behavior as did NOM. By recognizing the heterogeneity of both NOM and adsorbent surfaceswhich lead to energetic differences of the interactions, a modified Langmuir model (which considers the dependence of adsorption affinity on surface coverage) may be used to describe different types of adsorption isotherms. Our results also suggest that strong hysteresis (h= 0.72-0.92) in NOM adsorption-desorption should be considered for a better modeling of NOM transport, and a failure to consider the hysteresis may result in a large discrepancy between the calculated and observed effluent concentration distributions. Environ. Scl. Technol., Vol. 28, No. 1, 1984 45

Acknowledgments

We thank Dr. P. F. Low at Purdue University for the use of his microcalorimeterand Ms. H. Lin for her technical assistance. We also thank G. Sposito, P. M. Jardine, R. Cook,C. Francis, and three anonymous reviewersfor their helpful comments. This research was supported by the Subsurface Science Program, Environmental Sciences Division, Office of Health and Environmental Research, US. Department of Energy, under Contract DE-ACOB 840R21400 with Martin Marietta Energy Systems, Inc. This is Publication No. 4148of the Environmental Sciences Division, Oak Ridge National Laboratory. Literature Cited

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Abstract published in Advance ACS Abstracts, November 1, 1993.