Metastable-Equilibrium Adsorption Theory

Pan, Gang; Liss, Peter S.

doi:10.1006/jcis.1998.5397

Cited by 47 publications

(44 citation statements)

References 49 publications

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“…Following the method by Pan and Liss [33], adsorption isotherms were measured at 5, 20, and 40 • C, respectively. In brief, 30 mL of a solution containing 0.1 M NaNO 3 and a range of initial concentrations of Zn(II) (from 7.65 × 10 −6 to 3.82 × 10 −4 mol L −1 ) was mixed with 0.03 g TiO 2 in 50-mL PP centrifuge tubes.…”

Section: Adsorption-desorption Isothermsmentioning

confidence: 99%

“…Then the supernatants were filtered with 0.22-µm membrane and analyzed for Zn(II) concentration by differential pulse voltammetry (797 VA Computrace, Metrohm, Switzerland). At the Zn concentration levels tested here, the loss of Zn on the filters is negligible compared to the standard deviations of the experiment [33]. The zinc adsorption density was calculated from the difference between the initial and the final concentrations of Zn(II) in the solution phase.…”

Section: Adsorption-desorption Isothermsmentioning

confidence: 99%

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EXAFS studies on adsorption irreversibility of Zn(II) on TiO2: Temperature dependence

Pan

Zhang

et al. 2008

Journal of Colloid and Interface Science

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Adsorption irreversibility of Zn(II) on TiO 2 at various temperatures was studied using a combination of classical macroscopic methods and extended X-ray absorption fine structure (EXAFS) spectroscopy. When the temperature was increased from 5 to 40 • C, the Zn(II) adsorption capacity increased by 130%, and adsorbed Zn(II) became more reversible. The standard Gibbs free energy change ( G 0 ) of the adsorption reaction at 5, 20, and 40 • C was determined to be −19.58 ± 0.30, −22.28 ± 0.10, and −25.14 ± 0.21 kJ mol −1 , respectively. And the standard enthalpy ( H 0 ) and entropy ( S 0 ) were 24.55 ± 2.91 kJ mol −1 and 159.13 ± 0.53 J mol −1 K −1 , respectively. EXAFS spectra results showed that the hydrated Zn(II) was adsorbed through fourfold coordination with an average Zn-O bond distance of 1.98 ± 0.01 Å. Two Zn-Ti atomic distances of 3.25 ± 0.02 and 3.69 ± 0.03 Å were observed, which corresponded to an edge-sharing linkage mode (strong adsorption) and a cornersharing linkage mode (weak adsorption), respectively. As the temperature increased from 5 to 40 • C, the number of strong adsorption sites (N 1 ) remained relatively constant while the number for the weak adsorption sites (N 2 ) increased by 31%. These results indicate that the net gain in adsorption capacity and the decreased adsorption irreversibility at elevated temperatures were due to the increase in available weak adsorption sites (N 2 ) or the decrease in the ratio of N 1 /N 2 . Both the macroscopic sorption/desorption equilibrium data and the molecular level evidence of this study suggest that in a given environmental system (e.g., soils or natural waters) zinc and other similar heavy metals are likely more mobile at higher temperatures.

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Section: Adsorption-desorption Isothermsmentioning

confidence: 99%

Section: Adsorption-desorption Isothermsmentioning

confidence: 99%

EXAFS studies on adsorption irreversibility of Zn(II) on TiO2: Temperature dependence

Pan

Zhang

et al. 2008

Journal of Colloid and Interface Science

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“…Adsorption isotherm was measured by preparing a series of initial concentrations of equal amount of Cu and Cd in centrifuge tubes with each tube having the same solid suspension density (5 g l À1 ) (Pan and Liss, 1998). Adsorption suspensions were generated using 0.01 M NaNO 3 to maintain constant ionic strength.…”

Section: Adsorption Experimentsmentioning

confidence: 99%

“…the angle BAC in Fig. 5(a)) were calculated to evaluate the adsorption reversibility (Verburg and Baveye, 1994;Pan and Liss, 1998) and given in Fig. 5.…”

Section: Desorption Processesmentioning

confidence: 99%

Effect of phosphate on the adsorption of Cu and Cd on natural hematite

Zhang

Jiang

et al. 2006

Chemosphere

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Interactions between anions and cations are important to understand the chemical processes of pollutants in environment. In this study, batch experiments were carried out to investigate the simultaneous adsorption of Cu and Cd on hematite as affected by phosphate. Phosphate pretreatment suppressed the maximum adsorption of Cu and Cd on hematite and moved the adsorption pH edges to a higher pH range. Phosphate application time had a marked impact on Cu and Cd adsorption and longer contact time resulted in more reduction of Cu and Cd adsorption. Results of backtitration, Fourier transform infrared (FTIR) spectroscopy study and ionic strength effect on the adsorption revealed that Cu and Cd were adsorbed on hematite mainly through the inner-sphere complex formation mechanism and phosphate treatment reduced the inner-sphere adsorption sites, thus decreasing Cu and Cd adsorption on hematite.

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“…However, few studies have correlated adsorption mechanisms in aqueous solutions with degradation behavior occurring under the same conditions. Recently, a metastable-equilibrium adsorption (MEA) theory has been developed by Pan et al [7,8], which points out that the adsorption density  (mol/m 2 ) has been incorrectly used in the past as a thermodynamic state variable in the theoretical foundations of classical surface thermodynamics. Once this deficiency is removed from classical surface thermodynamics, a new principle, called MEA inequality, is obtained, based on strict theoretical deductions from first principles.…”

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confidence: 99%

Photocatalytic degradation pathways and adsorption modes of H-acid in TiO2 suspensions

2012

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Effect of adsorption mode on photodegradation of H-acid in TiO 2 suspension was studied using DFT calculation, UV-Vis spectroscopy, FTIR, and ionic chromatography. At pH 2.5, H-acid was adsorbed on TiO 2 surfaces by one dissociated sulfonic group. The adsorbed sulfonic group was attacked by surface ·OH, resulting in the production of SO 4 2− and the cleavage of the naphthalene ring. At pH 5.0, H-acid was adsorbed on TiO 2 surfaces by two sulfonic groups. The two adsorbed sulfonic groups were simultaneously attacked by surface ·OH, leading to a faster initial production of SO 4 2− and initial degradation rate of H-acid than those under pH 2.5. Microscopic adsorption structures may be more important than adsorption amount in controlling the photodegradation pathways of organic pollutants. The heterogeneous photocatalytic degradation of organic compounds on TiO 2 surfaces has been extensively investigated with a view to mineralizing environmentally harmful organic compounds. The adsorption of organic molecules is critical in determining photocatalytic degradation pathways; these pathways are directly related to the equilibrium and kinetic properties of photocatalytic degradation reactions. The process is initiated through band-to-band excitation of the TiO 2 particles by ultraviolet (UV) radiation to generate free ·OH radicals derived from valence band hole oxidation of terminal OH − groups and hydration water on the particle surfaces. These radicals have been assumed to be the dominant oxidizing agents, with a preference for attacking organic compounds adsorbed on or near the TiO 2 surfaces for quick quenching [1]. Since surface adsorption is required for interactions between the organic molecules and the photo-excited electrons or holes, differences in adsorption modes should affect photocatalytic degradation pathways. Many authors have investigated the adsorption of organic compounds on TiO 2 surfaces [2−6]. However, few studies have correlated adsorption mechanisms in aqueous solutions with degradation behavior occurring under the same conditions. Recently, a metastable-equilibrium adsorption (MEA) theory has been developed by Pan et al. [7,8], which points out that the adsorption density  (mol/m 2 ) has been incorrectly used in the past as a thermodynamic state variable in the theoretical foundations of classical surface thermodynamics. Once this deficiency is removed from classical surface thermodynamics, a new principle, called MEA inequality, is obtained, based on strict theoretical deductions from first principles. The MEA inequality indicates that real "equilibrium" adsorption constants decrease as the actual MEA states deviate from the ideal equilibrium state. Equilibrium adsorption constants or adsorption isotherms, when expressed in terms of , are fundamentally influenced by kinetic factors such as reaction rate and the reversibility of the adsorption process [9−12]. This implies that the equilibrium constants of heterogeneous catalytic reactions can be fundamentally influenced by MEA states. This wo...

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Metastable-Equilibrium Adsorption Theory

Cited by 47 publications

References 49 publications

EXAFS studies on adsorption irreversibility of Zn(II) on TiO2: Temperature dependence

EXAFS studies on adsorption irreversibility of Zn(II) on TiO2: Temperature dependence

Effect of phosphate on the adsorption of Cu and Cd on natural hematite

Photocatalytic degradation pathways and adsorption modes of H-acid in TiO2 suspensions

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