Apparent Hysteresis

Apparent hysteresis occurs mainly when complete equilibrium is not reached. Diffusion into the solid matrix or into micropores of aggregates is considered a main cause of apparent hysteresis. In a transitory state, sorption occurs concurrently with desorption and the concentration of contaminant in the liquid phase is erroneously low because some fraction is associated with sorption. 

Apparent hysteresis also may be caused by other phenomena. During the consecutive extractions and dilution steps used as a common technique in desorption studies, weathering of the sorbent may occur, resulting in a possible increase of contaminant sorption and decrease in its release. Degradation of the contaminant induced by physicochemical or biological factors, or a volatilization process leading to a decreased contaminant concentration in solution, are additional factors affecting a true hysteresis result. 

Drying of the subsurface solid phase can cause an increase in the rate of desorption. If penetration of a sorbate toward inner surfaces does not reach its equilibrium by the time drying commences, a fraction of the sorbate may remain localized at more accessible outer surfaces in an amount greater than that corresponding to the equilibrium level. Under these conditions, the drying of the system may increase the rate of desorption during successive rewetting. 

The history of the surface is an additional factor affecting the release of contaminants adsorbed on solid phases into the liquid or gaseous phase. For example, the effect of drying on contaminant desorption is influenced by the time allowed for its transport into the aqueous phase. In sorbing systems, like sediments that are permanently wet, the history of the system determines the fate of sorbed molecules (Pignatello 1989). 

Release methodology may lead to incorrect desorption parameters, which in turn may be (erroneously) interpreted as hysteresis. For example, Hodges and Johnson (1987) used two experimental techniques (rapidly stirred batch and miscible 

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Hysteresis energy given from the hysteresis loop at break in carbon black-fiUed rubbers includes the apparent hysteresis energy whose fraction is roughly a half of the total one, which results from the buckling of the super-network. 

Genuine (true) and apparent hysteresis may be considered to explain contaminant release from the subsurface solid phase. Genuine hysteresis assumes that observed data are real and the equilibrium results can be explained on the basis of well-identified phenomena. Apparent hysteresis results from an experimental artifact due, for example, to a failure to reach retention or release equilibrium. 

Retention of organic contaminants on subsurface solid phase constituents in general is not completely reversible, so that release isotherms differ from retention isotherms. As a consequence, the extent of sorption depends on the nature of the sorbent. Subsurface constituents as well as the types of bonding mechanisms between contaminants and the sohd phase are factors that control the release of adsorbed organic contaminants. Saltzman et al. (1972) demonstrated the influence of soil organic matter on the extent of hysteresis. Adsorption isotherms of parathion showed hysteresis (or apparent hysteresis) in its adsorption and desorption in a water solution. In contrast, smaller differences between the two processes were observed when the soils were pretreated with hydrogen peroxide (oxidized subsamples) to reduce initial organic matter content. The parathion content of the natural... 

The electrical resistance ratio data for Chromel-P wire are shown in Fig. 2. The strained and unstrained data follow two distinctly separate curves, which form an apparent hysteresis loop . The shape of the strained curve remains basically nonlinear throughout the temperature range covered. The resistance ratio values at the extreme temperature were higher than for the corrected unstrained values. The unstrained curve, although formed by fewer data points than the strained curve, is linear between 77° and... 

This method of treating the resistance ratio data leads to the formation of the apparent hysteresis loop in Fig. 3. The data were reduced using (2), with the assumption that there would be no effect from cold-working the specimen at room temperature. How ever, the data show that the cold-working produced strains which affected the resistance data and which were partially relieved by heating to near 600°K for an hour or longer. Thus, the data for the strained specimen followed one curve with a resistance ratio of 1.0000 at 273 K after the specimen had been annealed near 600°K, the data followed a second curve, and had a resistance of 1.0118 at 273°K. 

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