Continuous Affinity Spectra

FIGURE 11.6 Sorption isotherms of dodecylpiridinium on a soil material (EPA-12). (a) Points experimental data line fitted isotherms (almost indistinguishable) with discrete and continuous affinity spectra (b) discrete affinity spectrum, found using regularization for a small number of sites (c) continuous affinity spectrum, after regularizing for smoothness note that the smaller peaks are magnified by a factor of 30. (Reprinted with permission from Cernik, M. et al.. Environ. Sci. TechnoL, 29,2,413-425. Copyright 1995 American Chemical Society.)... 

For a heterogeneous population of sites with a discrete affinity distribution, the overall degree of protonation, 0, is the weighted sum of the degrees of protonation of the different categories of sites for a continuous affinity spectrum, the summation is replaced by an integral... 

A variety of alternative models more realistic than the Langmuir isotherm may be used to fit the isotherm data the most accurate but most demanding (in terms of amount and accuracy of data required) is the affinity spectrum approach, which yields a continuous spectrum of number of binding sites vs. [27]. The simplest approach, however, is to model the isotherm as resulting from just two classes of binding site ... 

The affinity spectra or constant distribution is a matter that has attracted several researchers, especially in the case of HS (Koopal and Vos 1993 Borkovec and Koper 1994a Manunza et al. 1995 Borkovec et al. 1996 Lin and Rayson 1998 Avena, Koopal, and van Riemsdijk 1999 Lin, Drake, and Rayson 2002 Garces, Mas, and Puy 2004 Orsetti, Andrade, and Molina 2009 David et al. 2010). The proton affinity spectrum is treated separately from the spectra of other ions. The reason is that, as H" " binding is almost always present, for all other ions (either metal cations or anions), there is always competition with protons thus, competitive adsorption must be considered. Here, discrete and continuous affinity spectra are discussed, both for noncompetitive and for competitive cases, and in Section 11.3, methods to extract them from experimental data are presented. 

Humic materials fractionated on the basis of hydrophobicity and proton affinity continue to exhibit two fluorophores as discussed in the section "Exciation-Emission Spectra. Strong evidence to establish the existence of at least two chromophores is seen in the phase-resolved spectra. These spectra are shown in Figures 4 a-f. They consist of the phase-resolved emission spectrum of each of the two fluorophores plotted separately and the normal emission spectrum of the humic fraction. If the nulling out of one fluorophore is exact then the sum of the two separate phase resolved spectra should be additive to equal the normal spectrum. In these figures the normal emission spectrum was measured separately from the two phase resolved emision spectra. The phase resolved spectra were then superimposed onto the scan of the normal emission spectrum. 

Figure 9. Continued, (c) Ultraviolet—visible spectrum of the supernatant after affinity precipitation of avidin from an avidin—myoglobin solution (both 10" M in 0.2 M ammonium carbonate buffer at pH 8.9) by a DMPE-B-C12E8 solution. The supernatant was sampled after precipitation for two hours and centrifugation at 5000 rpm for twenty minutes, (d) Ultraviolet—visible spectrum of avidin—DMPE-B precipitate described in text and caption 9(c) after resuspension in 10" M C12E8 solution in 0.2 M ammonium carbonate buffer at pH 8.9.

NO— NO dimer species with a triplet state (referred to as the (NO)2 bi-radical or radical pair in the following). The ESR spectrum of (NO)2 bi-radical shows the forbidden transition. Arris = 2, at ca. 170 mT g 4), when the corresponding allowed transitions, Ams = 1, are observed for the same sample at ca. 340 mT ig 2). These verify the presence of the triplet electronic state. Thus the ESR study suggested that the zeolite can stabilize the (NO)2 dimer as the triplet rather than the usual singlet state, indicating a great affinity of the NO molecule for the zeolites. The (N0)2 bi-radical may play an important role as an intermediate species in the decomposition of NO [29], ESR studies have accordingly continued to be of interest in recent decades. 

---