Retention hysteresis

Several other explanations have been put advanced to explain retention hysteresis, including (1) surface precipitation of metallic cations whose hydroxides, phosphates, or carbonates are sparingly soluble (2) chemical reactions with solid surfaces, including organic surfaces, which form complexes with metallic cations and (3) incorporation into the subsurface organic matter through chemical reactions and biochemical transformation. For the case described by Fig. 5.9 or explanations (1) and (2), the contaminant release always exhibits a hysteresis... 

The effect of these factors on the adsorption isotherm may be elucidated by reference to specific examples. In the case of the isotherm of Fig. 5.17(a), the nonporous silica had not been re-heated after preparation, but had been exposed to near-saturated water vapour to ensure complete hydroxylation. The isotherm is of Type II and is completely reversible. On the sample outgassed at 1000°C (Fig. 5.17(h)) the isotherm is quite different the adsorption branch is very close to Type III, and there is extrensive hysteresis extending over the whole isotherm, with considerable retention of adsorbate on outgassing at 25°C at the end of the run. 

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

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 effect of pore impregnation with nonpolar material was studied by treating samples with dichlorodimethylsilane (DCDMS). In each case a decrease in hysteresis area, compared to the untreated material, was observed after coating samples with DCDMS. The increases in the extrusion contact angle, with DCDMS compared to untreated sample, resulted in decreases in W q. In some cases impregnation with DCDMS led to greater mercury retention or an increase in IF, over the untreated material. However, this was always accompanied by a larger decrease in and thus a decrease in the pore potential. 

Furthermore, the history of a hysteresis loop plays an important role in the determination of lifetime and reliability of ferroelectric capacitors, especially for applications in ferroelectric memories. Three main effects are characterized in particular as changes in the hysteresis loop under various conditions, which are described later in this chapter as fatigue, retention, and imprint with the corresponding ways to measure these effects. 

The pulses have trapezoid shape or triangular shape with similar rise times of pulses to measure a closed hysteresis loop. The retention is measured by pulses instead of a standard hysteresis loop since the excitation has to be modified to get the unknown initial polarization state, but compare it to a reference value, e.g. polarization with 1 second delay. In principle this could also be measured using the hysteresis measurement as described above, and monitoring the polarization during the prepolarization pulse (pulse no. 1 in Figure 3.6). The... 

The adsorbates and adsorbents in this study were chosen to reflect a range of different types of molecular interactions as well as to observe whether the retention volume data trends could be generalized. In addition, solutes were picked which would rapidly equilibrate with the chosen adsorbents (no hysteresis) and whose distribution coefficients could be measured conveniently over as wide a pressure range as possible. As shown in Table I, the adsorbents corresponded to two distinctly different chemical types as classified by the criterion of Kiselev (40). The alumina represented an adsorbent capable of specific interactions with sorbates having localized peripheral... 

Methylcellulose (Dow Chemical Co., 1990) is a water-soluble, thermostable ether that gels reversibly upon heating at 40-50°C. Sucrose lowers Tgel- The gels are prone to hysteresis (Grover, 1993). This biopolymer is an excellent foam stabilizer, as demonstrated in Fig. 1 in Chapter 2 by the visible retention of air bubbles at the solid-liquid interface. 

The water retention of cellulose fibers at a given relative humidity varies depending on whether the equilibration has taken place by desorption or adsorption. (hysteresis). The water uptake also continuously decreases after repeated drying and moistening of the fibers. Additional factors influencing the ability of pulp fibers to swell are their chemical composition, such as their hemicellulose and lignin content. 

The desorption isotherms are reversible, except for sample PC76, which presents a smalt hysteresis loop. This is indicative of a slight development of mesoporosity. On the other hand, only the original sample (i.e., PC) presents gas retention at low pressures. 

Fig. 6-7. Adsorption and desorption isotherms illustrating hysteresis behavior of Cu retention in McLaren soil.

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