Chemical heterogeneity effects

Filella, M., Buffle, J. and van Leeuwen, H. P. (1990). Effect of physico-chemical heterogeneity of natural complexants Part I. Voltammetry of labile metal-fulvic complexes, Anal. Chim. Acta, 232, 209-223. 

Through such chemisorption studies, the values of >, have been determined not only by geometric accessibility, but also by the chemical heterogeneity of the surface. This can result in abnormal values of D, and demonstrates the scale effect on the kinetics and selectivity of catalytic reactions. For such studies, Farin and Anvir [213] derived the equations that can be applied for characterization of supported catalysts ... 

If the biological effect of a chemical is related to its dose, there must be a measurable range between concentrations that produce no effect and those that produce the maximum effect. The observation of an effect, whether beneficial or harmful, is complicated by the fact that apparently homogeneous systems are, in fact, heterogeneous. Even an inbred species will exhibit marked differences among individuals in response to chemicals. An effect produced in one individual will not necessarily be repeated in another one. Therefore, any meaningful estimation of the toxic potency of a compound will involve statistical methods of evaluation. 

In chemical heterogeneous catalysis, it is common to use highly porous catalysts that come in particles of millimeter to centimeter size to increase the effective catalyst surface. In practical electrocatalysis, in particular applying electrocatalysis in fuel cells, it is also usual to use highly porous— although accounting for the low diffusion coefficients in liquid electrolytes compared to gases, 10 5 cm2/sec vs 1 cm2/sec, much smaller—catalyst particles. 

Biofilms enhance bacteria-DOM interactions by several means. Their spatial and chemical heterogeneity provides additional sorption sites for DOM compared with clean surfaces. Their loose architecture with interstitial voids and channels increases diffusivity and to some extent allows convective flow within biofilm structures. Because bacteria metabolize organic matter sorbed to the biofilm, a diffusion flux from the free water to the biofilm is maintained. Large proportions of organic matter sorbed to the biofilm are not instantly turned over but remain in the biofilm as a reservoir, which buffers direct effects of DOM depletion in the water column. 

The adsorption-desorption reaction in Eq. 4.3 has been applied to soils in an average sense in a spirit very similar to that of the complexation reactions for humic substances, discussed in Section 2.3.11 Although no assumption of uniformity is made, the use of Eq. 4.3 to describe adsorption or desorption processes in chemically heterogeneous porous media such as soils does entail the hypothesis that effective or average equilibrium (or rate) constants provide a useful representation of a system that in reality exhibits a broad spectrum of surface reactivity. This hypothesis will be an adequate approximation so long as this spectrum is unimodal and not too broad. If the spectrum of reactivity is instead multimodal, discrete sets of average equilibrium or rate constants—each connected with its own version of Eq. 4.3—must be invoked and if the spectrum is very broad, the sets of these parameters will blend into a continuum (cf. the affinity spectrum in Eq. 2.38). 

Strictly speaking, Balke s system combined SEC in the first dimension with a mixed mode separation in the second dimension. Since SEC separates with respect to hydrodynamic volume and not molar mass, the copolymers under investigation could not be quantified with respect to molar mass distribution (see discussion in Sect. 3). The effect of the SEC separation was simply to obtain fractions with narrower molar mass distribution as compared to the total sample. Considering this fact, it is clear that for chemically heterogeneous copolymers no quantitative data can be obtained from the first dimension. Only the second dimension, separating with respect to chemical composition, can provide quantitative information on the chemical composition distribution. Accordingly, a coupled information on both MMD and CCD was not available for this system. 

Figure 1.29. (a) Successive configurations of a meniscus on a vertical solid surface consisting of two macroscopic phases when the liquid volume is increased, (b) Effect of a small chemical heterogeneity... 

O. Smidsrpd, B. Larsen, A. J. Pemas, and A. Haug, The effect of alkali treatment on the chemical heterogeneity and physical properties of some carrageenans, Acta Chem. Scand., 21 (1967) 2585-2598. 

Gerke H. H., Molson J. W., and Frind E. O. (1998) Modelling the effect of chemical heterogeneity on acidification and solute leaching in overburden mine spoils. J. Hydrol. 209, 166-185. 

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