ADVANCES IN ENVIRONMENTAL REACTION KINETICS AND THERMODYNAMICS: LONG-TERM FATE OF ANTHROPOGENIC CONTAMINANTS (Cosponsored with the Division of Geochemistry) Organized by P. Trivedi Symposia Papers Presented Before the Division of Environmental Chemistry American Chemical Society Philadelphia, PA August 22-26, 2004

METASTABLE EQUILIBRIUM ADSORPTION THEORY: A CHALLENGE AND DEVELOPMENT TO CLASSICAL THERMODYNAMIC ADSORPTION THEORIES Gang Pan, Xianliang Li, Yanwen Qin, Hao Chen, Chengxia Jia, Yuanyuan Liu, Guohua Chang and Yuhuan Yang State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China [email protected]

Introduction Over the last century, thermodynamics played a fundamental role in the understanding of adsorption, which affected many physical, chemical and environmental processes. It has been a basic concept in thermodynamic adsorption theories that adsorption density Г (mole/m2) is a state variable (a given Г corresponds to an unique value of chemical potential) so that equilibrium adsorption constants, in which Г is used, can represent the unique equilibrium characteristic of a reaction that is independent of kinetic factors under constant temperature, pressure, pH and ionic strength. The metastableequilibrium adsorption (MEA) theory1-2 proposes that Г is not a thermodynamic state variable. This hypothesis results in the re-examination of classical adsorption thermodynamics. A fundamental principle called MEA inequality is obtained when Γ is not treated as a state variable which reveals that “equilibrium” adsorption constants and adsorption isotherms may be fundamentally affected by the kinetics of adsorption1. Conventionally referred equilibrium adsorption states are generally in metastableequilibrium states, which, according to the inequality, are dependent on the irreversibility kinetics of the processes. This implies that previously measured equilibrium adsorption data may constitutionally show a lack of consistency, thereby making the use and

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comparison of the data problematic. Here, we report direct evidences obtained by EXAFS analysis (extended x-ray absorption fine structure) to prove, at molecular level, the basic hypothesis of MEA theory (i.e., Г is not a state variable) and macroscopic experimental results to confirm two effects, predicted by the MEA theory, of the adsorption reversibility and reactant concentration such as particle concentration (Cp) on the equilibrium adsorption constants/isotherms. Material and Methods Synthesis of adsorbents The synthesis of goethite, manganite (γ-MnOOH) and δ-MnO2 were described in detail in [2-4]. Macroscopic adsorption and desorption experiments Adsorption and desorption isotherms under different particle concentration conditions were measured according to [2-4]. The purpose of the experiment is to test whether adsorption isotherms are affected by adsorption reversibility. This was achieved by examining the effects of particle concentration on both adsorption hysteresis and the adsorption isotherms so that the relationships between the three (Cp, reversibility and isotherms) can be identified simultaneously. EXAFS analysis Synchrotron-based EXAFS analysis was conducted according to [3-4]. The purpose of the EXAFS measurement is to examine whether the adsorption micro-states or adsorption modes of adsorption samples with the same Г value (samples S1, S2, S3 in Fig. 1) for a given adsorption reaction under different Cp conditions/MEA states are the same or not.

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Adsorption density (mg/g)

20 15

S1

S3

S2

10

Cp1-ads Cp2-ads Cp3-ads

5

Cp1-des Cp2-des Cp3-des

0

EXAFS samples

0

2

4

6

8

10

12

Ceq (mg/L) Figure 1. Adsorption and desorption isotherms of Zn on manganite (γ-MnOOH) in pH 7.5, 0.1 N NaNO3 solutions under different Cp conditions. Cp1 (0.1 g/L) < Cp2 (0.2 g/L) < Cp3 (0.4 g/L). Adsorption and desorption equilibrium time: 24 hours. Results and Discussion Effect of adsorption reversibility on adsorption isotherms and its control on the existence of particle concentration effect (Cp effect) In the adsorption of Zn on γ–MnOOH, three characters were observed (Fig. 1). A) Under each Cp condition, adsorption and desorption isotherms did not overlap each other, indicating the adsorption was irreversible. B) Under different Cp conditions, the irreversibility (angles between a solid and dotted line) increased as Cp increased. C) Adsorption isotherms (solid lines) declined as Cp increased, showing an obvious Cp effect. These observations could not be explained from classical thermodynamics where equilibrium constants have been regarded not affected by the reversibility of the process, nor the reactant concentrations. The results, however, supported the prediction of MEA inequality that equilibrium adsorption constants must be decreased as the irreversibility of the process increases. Adsorption isotherms declined as Cp increased since the increase in Cp caused an increase in adsorption irreversibility. In contrast to the system of Zn-γ-MnOOH, the adsorption of Zn on δ-MnO2 was highly reversible (adsorption and desorption isotherms coincide each other, Fig. 2). Changes in Cp did not cause discernible changes in adsorption reversibility and, as predicted from MEA theory, no Cp effect was observed in Zn-δ-MnO2 system.

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50

Γ (mg/g)

40 Cp1-ads

30

Cp2-ads Cp3-ads

20

Cp1-des Cp2-des

10

Cp3-des

0

EXAFS samples

0

5

10

15

20

25

Ceq (mg/L) Figure 2. Adsorption and desorption isotherms of Zn on δ-MnO2 in pH 5.5, 0.1 N NaNO3 solutions under different Cp conditions. Cp1 (0.1 g/L) < Cp2 (0.2 g/L) < Cp3 (0.4 g/L). Adsorption and desorption equilibrium time: 24 hours. The above-mentioned rule on the physico-chemical existence of Cp effect was also confirmed in other adsorption systems. For example, a clear Cp effect and corresponding reversibility/kinetics effect were simultaneously observed in an irreversible adsorption system of Zn-goethite, while no Cp effect and corresponding reversibility/kinetics effect were observed in a reversible adsorption system of Cdgoethite2,5. Direct evidences from molecular level to prove the basic MEA principle According to conventional thermodynamic theories, a definite Г for a given adsorbate should correspond to an unique value of chemical potential on solid surface, so that it should equilibrate to a single equilibrium concentration with equal chemical potential in solution at adsorption equilibrium, making the adsorption isotherm an unique equilibrium characteristic of the reaction. MEA theory proposed that a given Г (e.g. S1, S2, S3 in Fig. 1) could have different values of chemical potential depending on the MEA states, so that it can equilibrate with multiple equilibrium concentrations in solution, making adsorption isotherms a function of adsorption reversibility. The MEA inequality1 indicates that the more irreversible the adsorption is, the more final MEA states deviate from the ideal equilibrium state and the real equilibrium constants decrease. EXAFS analysis of samples S1, S2, S3 (Fig. 1) indicated that hydrated Zn ions (tetrahedral) adsorbed onto γ-MnOOH surfaces (octahedral MnO6 unit) in two modes: edge-linkage and corner-linkage (Fig. 3). The Zn-Mn atomic distances for the two adsorption modes were 3.07 ± 0.01 and 3.52 ± 0.02 Å, respectively3. Zn (II) adsorbed in edge-linkage has a higher surface affinity, chemical potential (µsolid) and irreversibility than that of corner-linkage3,6. The ratio between the edge-linkage (N1) and corner-

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linkage (N2) and hence the µsolid were in the order of S1 < S2 < S3 bar chart in Fig. 4), while the adsorption irreversibility was in a matching order of S1 < S2 < S3 (line chart in Fig. 4). For the reversible adsorption system of Zn-δ-MnO2, only corner-linkage mode was detected using EXAFS analysis and the Zn-Mn atomic distance was 3.52 ± 0.01 Å. These results exclusively confirmed that a given Г can indeed correspond to different MEA states with different chemical potential depending on the reversibility and that adsorption isotherms declines as the irreversibility of the process increases. By altering adsorption reversibility, Cp can fundamentally affect equilibrium adsorption constants. Hydrated Zn (II) ions

A

B

0.8

16

0.6

12

0.4

strong/weak sites % desorbed

8

0.2

4

0

0

desorption (%)

N1/N2

Figure 3. Hydrated Zn (II) ions adsorbed on γ-MnOOH surfaces as edge-linkage (A) and corner-linkage (B).

S1 S2 S3 Adsorption samples

Figure 4. Bar chart: the ratio between edge-linkage (N1) and corner-linkage (N2) in samples S1, S2, S3 (see Fig. 1). Line chart: percentage of Zn desorbed (24 h) for S1, S2, S3. Recent development of MEA theory and its applications to environmental problems: 1. Theoretical. Using quantum chemical ab initio calculation method, we have successfully calculated the micro-structure/geometry and electric charge density of different zinc species in solution (Zn(H2O)n2+, Zn(OH)(H2O)n+, Zn(OH)2(H2O)n0, Zn(OH)3(H2O)n-, Zn(OH)4(H2O)n2-) and their adsorption onto γ-MnOOH (clusters) as different MEA states. Most of the calculated data are difficult to be obtained experimentally, but a few of them that can be measured by EXAFS agreed well with the experiment. 1285

2.

3.

Novel experimental technique. We have invented a “circulative temperature scanning adsorption-desorption” device (patented) that can quantitatively measure the irreversibility kinetics of adsorption reactions. Information obtained by this technique together with XAFS analysis may be a breakthrough for the development of MEA theory using nonlinear non-equilibrium thermodynamics. Environmental applications. A new phosphorus adsorption model in natural waters/soils/sediments was developed based on the MEA inequality, which can be used to describe crossover-type adsorption isotherms in environment7. A quantitative assessment of the phosphorus flux of Saharan dust storms and Nile River/Aswan Dam to the East Mediterranean Sea indicated that the new model may reduce some 70 % of the uncertainty of the current model prediction, which may be a significant contribution to the global change modeling.

References 1. Pan, G. and Liss, P. S. (1998). Metastable Equilibrium Adsorption Theory I. Theoretical, J. Colloid Interface Sci., 201, 71-76. 2. Pan, G. and Liss P. S. (1998). Metastable Equilibrium Adsorption Theory II. Experimental, J. Colloid Interface Sci., 201, 77-85. 3. Pan, G., Qin, Y., Li, X., Hu, T., Wu, Z. and Xie, Y. (2004). EXAFS studies on adsorption-desorption reversibility at manganese oxides-water interfaces I. Irreversible adsorption of zinc on manganite (γ-MnOOH), J. Colloid Interface Sci., 271, 28-34. 4. Li, X., Pan, G., Qin, Y., Hu, T., Wu, Z. and Xie, Y. (2004). EXAFS studies on adsorption-desorption reversibility at manganese oxides–water interfaces II. Reversible adsorption of zinc on δ-MnO2. J. Colloid Interface Sci., 271, 35-40. 5. Pan, G., Liss, P. S. and Krom, M. D. (1999). Particle concentration effect and adsorption reversibility, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 151, 127-133. 6. Trivedi, P., Dyer, J., Sparks, D. L. and Pandya, K. (2004). Mechanistic and thermodynamic interpretations of zinc sorption onto ferrihydrite, J. Colloid Interface Sci., 270, 77-85. 7. Pan, G., Krom, M.D. and Herut, B. (2002). Adsorption-desorption of phosphate onto airborne dust and riverborne particulates in East Mediterranean seawater, Environ. Sci. & Technol., 36, 3519-3524.

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1281 ADVANCES IN ENVIRONMENTAL REACTION ...

Trivedi, P., Dyer, J., Sparks, D. L. and Pandya, K. (2004). Mechanistic and thermodynamic interpretations of zinc sorption onto ferrihydrite, J. Colloid Interface. Sci., 270, 77-85. 7. Pan, G., Krom, M.D. and Herut, B. (2002). Adsorption-desorption of phosphate onto airborne dust and riverborne particulates in East Mediterranean ...

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