Hydrogen peroxide desorption

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 most important water treatment technologies are summarized in Fig. 5-6. Depending on the source and on the water quahty, either mechanical, biological, physical, thermal, or chemical processes or their combinations may be applied. Photochemical AOPs and AOTs are subordinated to chemical processes, mainly because the current technological versions of photochemical wastewater remediation are dependent on the addition of auxihary oxidants, such as hydrogen peroxide, ozone or special catalysts such as titanium dioxide. Photochemical AOPs are attractive alternatives to non-destructive physical water treatment processes, for example adsorption, air stripping or desorption and membrane processes. The last merely transport contaminants from one phase to another, whereas the former are able to minerahze organic water contaminants (cf. Chapter 1). 

In a thermogravimetric investigation of the products of reaction between NaHCOj and HjOj, Firsova et al. [158] concluded that NajCOj.l.SHjOj decomposed exothermically, 383 to 413 K, followed by the endothermic desorption of water. Firsova and Filatov [159] reported a gravimetric study of the isothermal decomposition of sodium and potassium peroxocarbonates (MjCjOg) within the temperature interval 373 to 413 K. These compounds are prepared by the reaction of carbon dioxide with hydrogen peroxide in the presence of alkali, e.g. ... 

As in the hydrazine method, hydrogen peroxide is fed in continuously. In the steady-state cq = const and thus the oxygen formation rate is the same as its desorption rate from the liquid in the gas phase. 

The adaptability of extraction steps from commonly used SEPs to fractionating As and Se has been tested by Gruebel et al. (1988) taking standard minerals and their mixtures as an example. In particular, two steps were examined the reductive dissolution of amorphous iron oxides by hydroxylamine, and the oxidation of orgatfic matter by hydrogen peroxide and sodium hypochlorite. It has been shown that during reductive and oxidative dissolution of target elements from a certain mineral phase, readsorption on other mineral phases with subsequent desorption of elements in the next extraction step is a serious limitation on the appropriate fractionation of metalloids. 

There was a strong possibility that initiation occurred via photosensitized formation of hydrogen peroxide on the metal oxide surface followed by desorption and subsequent photolysis of hydrogen peroxide to form free radicals. Radicals formed by this means would lead to abstraction of hydrogen from the fiber surface and subsequent grafting of poly(methyl acrylate) at these sites. 

One of the main disadvantages of the Damjanovic s scheme is that it does not consider possible weak adsorptions and the reversible adsorption/desorption of hydrogen peroxide at the interface. The mechanism proposed by Wroblowa et al. [104] considers the adsorption/desorption equilibrium either for the oxygen reactant or the hydrogen peroxide intermediate. They also proposed the chemical decomposition of the intermediate (1(4), besides the electrochemical reduction to water through k3 (Scheme 2.5). 

SCHEME 2.5 Wroblowa s oxygen electroreduction mechanism with electrochemical rate constants k3 forthe direct reduction to water, k2 and k 2 for the parallel electrochemical reduction and oxidation of adsorbed hydrogen peroxide, k3 for the electrochemical decomposition of hydrogen peroxide to water, k4 for the chemical catalytic decomposition of hydrogen peroxide to oxygen, and ks and fc 5 for the parallel adsorption/desorption of the adsorbed hydrogen peroxide. The species with the super index and ° are located at the interface and in the bulk of the solution, respectively. The sub index ads refers to the adsorbed state, and the term dif symbolizes the diffusion to the bulk of the solution and vice versa. 

Even if these relationships are analyzed in detail, in some cases it will be not possible to conclude on the possibility of a direct or parallel reaction pathway because the /D//R vs. w-1/2 plot will always show a potential dependent ordinate at the origin. Thus, Wroblowa et al. [104] decided to plot the ordinate at the origin, J, as a function of the slope, S, of the NID/IR vs. oi l/ 2 curve at different /iD values. The generalization performed by Adzic and coauthors [131] based on Wroblowa s work led to k 1 0 and A > 0 (or in other words negligible desorption of hydrogen peroxide from the... 

In this scheme, Bagotskii et al. [ 106] developed a parallel reaction pathway taking into consideration fast adsorption/desorption of hydrogen peroxide and negligible oxidation of the intermediate to oxygen (Scheme 2.6). 

The polymerization process was firstly monitored by tensiometry (Figure 1). At the beginning no surface pressure was measured indicating that coniferyl alcohol is not surface active. After peroxidase and hydrogen peroxide spreading, the surface pressure increases indicating the formation of an interfacial layer. After 6-8 hours, the surface pressure levels off around 9 mN/m and then tends to decrease. This behaviour can be explained by a desorption of the molecules from the interface to the bulk or by a change of the interfacial layer structure. 

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