A Thermodynamically Based Method to Quantify True Sorption Hysteresis

Abstract

Sorption of organic chemicals to soils and sediments often shows true hysteresis (i.e., nonsingularity of the sorption-desorption isotherm not attributable to known experimental artifacts). Since true sorption hysteresis is fundamentally important to contaminant fate, a way to quantify it is desirable. Previously proposed indices of hysteresis are empirical and usually depend on the isotherm model. True sorption hysteresis to synthetic and natural organic solids has been attributed to irreversible alteration of the solid during the sorption-desorption cycle. Given this mechanism, we propose the Thermodynamic Index of Irreversibility (TII) for quantifying hysteresis in soils where natural organic matter dominates the sorption process. The TII is based on the difference in free energy between the real desorption state and the hypothetical fully reversible state. The index is 0 for completely reversible systems and approaches 1 as the process tends toward complete irreversibility. It does not require any assumptions about the physical properties or molecular composition of the solid, and it does not depend on a specific equilibrium model. A sensitivity analysis of measurement errors provides general recommendations for the setup of sorption-desorption experiments. The TII was applied to sorption of 1,4-dichlorobenzene (DCB) to two high-organic soils, Pahokee peat (PP) and Amherst soil (AS), and a low-rank coal reference material, Beulah-Zap lignite (BZL). Common artificial causes of hysteresis were eliminated. Hysteresis was significant in the peat and the coal. The TII was clearly concentration dependent for both solids; it decreased with concentration for the peat, but increased with concentration for the coal. The TII allows quantification of hysteresis as a function of sorbate-sorbent combination, concentration, time, and other variables.

Aging effects on sorption–desorption behaviors of PAHs in different natural organic matters

“…Most prevalent theories therein include physical entrapment within micro-and nanopores, as well as irreversible pore deformation. It was declared that formation of meta-stable states of adsorbate in fixed mesopores on external surfaces or in the internal voids of soil organic matter was largely responsible for the great entrapment of PAHs in humin [7]. However, it was also suggested that pore deformation plays a major role to the observed hysteresis in kerogen compared to physical entrapment of sorbate molecules [36].…”

“…Previous studies of the effects of aging on mechanism-specific sorption and desorption have documented that aging time would lead to higher solute retention, reduced desorption, and less extractability with organic solvents that resulted from greater contaminant sequestration or binding [3][4][5]. Although some studies argued that reported sorption/desorption hysteresis phenomena may be due to experimental artifacts resulting from non-attainment of sorption equilibrium before desorption experiments, which were conceptualized as ''pseudo-hysteresis'' [6], true hysteresis phenomenon has been distinguished from artificial hysteresis by many researchers [7][8][9]. Sander et al eliminated common artificial causes by the method of quantifying hysteresis thermodynamically and found that true sorption hysteresis was significant in the peat and the coal [7], and others also confirmed irreversible sorption of neutral hydrocarbons to sediments in cyclic adsorption/ desorption studies [8,9].…”

“…Although some studies argued that reported sorption/desorption hysteresis phenomena may be due to experimental artifacts resulting from non-attainment of sorption equilibrium before desorption experiments, which were conceptualized as ''pseudo-hysteresis'' [6], true hysteresis phenomenon has been distinguished from artificial hysteresis by many researchers [7][8][9]. Sander et al eliminated common artificial causes by the method of quantifying hysteresis thermodynamically and found that true sorption hysteresis was significant in the peat and the coal [7], and others also confirmed irreversible sorption of neutral hydrocarbons to sediments in cyclic adsorption/ desorption studies [8,9]. Aging appeared to have a significant impact on sorption-desorption behavior of HOCs on NOMs.…”

Effect of temperature on the sorption and desorption of perfluorooctane sulfonate on humic acid

“…The degree of desorption hysteresis was quantified using the thermodynamic index of irreversibility (TII) proposed by Sander et al (2005). Usually, the calculated TII values lies in the range of 0 to 1, where the value 1 indicating the maximum irreversibility.…”

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

“…On the other hand, the desorption isotherms significantly deviate from the corresponding adsorption isotherms, indicating that adsorption of Zn(II) onto TiO 2 was not fully reversible. The thermodynamic index of irreversibility (TII) proposed by Sander et al [38] was used to quantify the adsorption irreversibility. The TII was defined as the ratio of the observed free energy loss to the maximum possible free energy loss due to sorption hysteresis, which was given by where C S eq is the solution concentration of the adsorption state S (C S eq , q S eq ) from which desorption is initiated; C D eq is the solution concentration of the desorption state D (C D eq , q D eq ); C γ eq is the solution concentration of hypothetical reversible desorption state γ (C γ eq , q γ eq ).…”