Langmuir-type sorption behavior

A matrix of surfactant sorption experiments was conducted as a function of mixing time, initial concentration, and soil type for Tween 60, Tween 80 and Triton X-100. Although only data for Tween 80 and Appling soil are presented here, all of the soil-surfactant systems investigated ediibited Langmuir-type sorption behavior. The Langmuir equation may be written as ... 

A (rue adsorption maximum, however, is rarely observed. Precipitation reactions cau exhibit Langmuir-type behavior. If only a limited quantity of a solute that precipitates is present, a Langmuir isotherm can result as the solute increases, that is, a sorption maximum occurs. This behavior is found at low solute concentrations, where no precipitation occurs until the solute s solubility product is reached. 

Sorption and diffusion of water vapour in polymers have been studied mainly for development of water vapour barriers. Some of these studies date back to 1944 [41]. Many attempts have been made in order to describe the sorption behavior of water vapor onto solid surfaces of the membrane and its pores. As described by Vieth [41], a deviation from Henry s law was observed in 1944, in the sorption of water by hydrated cellulose membranes. It was postulated that two competing phenomena are responsible for this observation dissolution, which obeys Henry s law, and adsorption, which follows the Langmuir isotherm. With other polymer systems the ability of water molecules and/or polar groups in the polymer matrix to interact with each other has given rise to sorption isotherms which may follow Henry s law, Flory-Huggins or BET types [41]. 

The other component of molecular transport through zeoHte membranes is the equilibrium sorption strength between the molecule and zeoHte. In general, this information is reported in the form of adsorption isotherms, which are tabulated for many species and zeoHte structures [22]. For example, Langmuir isotherm commonly describes the adsorpHon behavior of molecules transporhng through MFI-type zeoHte membranes and can be represented by the foUowing equahons ... 

Consider now adsorbed molecular or ionic species that are, practically speaking, immobilized in the soil. Unless the soil is extremely acid, metals such as Cu, Cr, and Pb fall into this category. Also, certain anions such as phosphate bond so strongly on minerals that they too behave as immobile elements. The property that all of these ions have in common is that their sorption isotherms are not reversible within a time scale relevant to soil processes the adsorption (forward) isotherm is usually approximated closely by a Langmuir function of the strong-affinity type, but the desorptioii (backward) isotherm deviates markedly from the adsorption isotherm. This kind of nonequilibrium behavior, depicted in Figure 9.6, is sometimes referred to as hysteresis. Possible reasons for hysteresis in chemisorption are discussed in Chapter 4. 

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