Iron oxides adsorbed

Conventional adsorbents such as alumina, iron oxide, manganesia, titania, and ferric phosphate have been studied extensively to remove As from water (Mohan et ah, 2007). Absorption is a physical-chemical process by which the adsorbates (ions of targeted solutes) are adsorbed to the surface of an adsorbent. Cupric oxide and iron oxide adsorbers for instance have been investigated... 

The jS-D-galactosidase in homogenates of E. coli has been purified by affinity chromatography on an iron oxide adsorbent containing attached residues of sebacic acid. Related studies on the purification and specificity of E. coli -D-galactosidase have been reported. ... 

Arsenic(V) removal with a Ce(IV)-doped iron oxide adsorbent. Chemosphere, 51, 945-52. 

The examples in the preceding section, of the flotation of lead and copper ores by xanthates, was one in which chemical forces predominated in the adsorption of the collector. Flotation processes have been applied to a number of other minerals that are either ionic in type, such as potassium chloride, or are insoluble oxides such as quartz and iron oxide, or ink pigments [needed to be removed in waste paper processing [92]]. In the case of quartz, surfactants such as alkyl amines are used, and the situation is complicated by micelle formation (see next section), which can also occur in the adsorbed layer [93, 94]. 

As a possible method of concentrating trace amounts of bioactive organic compounds occurring in the hydrosphere, adsorption properties of various compounds have been explored by employing hydrous metal oxides as the adsorbents. To date, a family of organophosphoms compounds and carbonic acids were adsorbed onto hydrous iron oxide, along with the adsoi ption of monosaccharides onto hydrous zirconium oxide. 

Bound in minerals such as uraninite, pitchblende, coffinite, etc. fixed as a replacement ion for Y, Ce, Zr, Th, Ca, and Ba in other, particularly accessory minerals and adsorbed as ion on clay minerals, hydrous iron oxides, etc. 

Mossbauer spectroscopy can be used for in situ study of electrodes containing nuclei capable of resonance absorption of y radiation for practical systems, primarily the 57Fe isotope is used (passivation layers on iron electrodes, adsorbed iron complexes, etc.). It yields valuable information on the electron density on the iron atom, on the composition and symmetry of the coordination sphere around the iron atom and on its oxidation state. 

The effects of the removal of organic matter and iron oxides on Zn adsorption on soils are also influenced by Zn concentration. At low concentrations (5-10 mg L initial concentration), both treated soils (removed organic matter and iron oxides) behaved similarly. At high Zn concentration, however, treated soils behaved differently. When the initial Zn concentration was between 5 and 10 mg kg-1, adsorption of Zn by soils without organic matter and without both organic matter and iron oxides were 2-2.5 times that of the untreated soil. With an increase in initial Zn concentration, the soil without both iron oxides and organic matter adsorbed more Zn than the soil without organic matter. This indicates that the available sites for Zn decrease with increases in the initial Zn concentration. 

In order to understand the general behavior of the tested materials, scoping tests were conducted in 25 mg/L gold(III)-chloride solutions at 25°C and 80°C with different initial pH values, namely 1.5, 4 and 8. After the start of the experiment the pH was not further adjusted, i.e., it could change freely. It remained similar to the initial pH in all cases. From these tests it was found that, in the case of the iron-oxide based adsorbents, dissolution of the solid took place at pHaluminum oxides and titanium oxide was observed at this pH. At pH>2 all adsorbents were found to be stable and did not dissolve during the experiment. 

Dissolution of gelatinous hydrous iron oxide, which adsorbs and occludes metals... 

In coastal environment, detrital and authigenic Fe and Mn oxides, which accumulate in oxic surface sediments, play a pivotal role in determining the geochemical behaviour of arsenic (Mucci et al., 2000) and selenium (Belzile et al., 2000). Arsenic and selenium differ in their affinities for metal oxide surfaces. Although both adsorb onto iron oxides, arsenate (As(V)) adsorbs more strongly than arsenite (As(lll)), and selenite (Se(IV)) adsorbs more strongly than selenate (Se(VI)) (Belzile et al., 2000). 

Contaminants in soils and sediments can be adsorbed on to inorganic minerals such as clays and metal oxides, notably hydrated iron oxide, FeOOH and manganese dioxide, Mn02, or adsorbed on to organic matter such as humic... 

In Table II, it can be seen that decomposition at low temperatures left a CO/Fe ratio on the surface of between 0.8 and 1.3. A reasonable explanation for the high CO to iron ratios on the surfaces, given the fact that Mossbauer spectroscopy studies show that a large fraction of the iron is oxidized, is that iron subcarbonyl species are stable on the surface at 383 K. This is true since CO does not adsorb strongly on iron oxides (35). 

Dissolved iron(III) is (i) an intermediate of the oxidative hydrolysis of Fe(II), and (ii) results from the thermal non-reductive dissolution of iron(III)(hydr)oxides, a reaction that is catalyzed by iron(II) as discussed in Chapter 9. Hence, iron(II) formation in the photic zone may occur as an autocatalytic process (see Chapter 10.4). This is also true for the oxidation of iron(II). As has been discussed in Chapter 9.4, the oxidation of iron(II) by oxygen is greatly enhanced if the ferrous iron is adsorbed at a mineral (or biological) surface. Since mineral surfaces are formed via the oxidative hydrolysis of Fe(II), this reaction proceeds as an autocatalytic process (Sung and Morgan, 1980). Both the rate of photochemical iron(II) formation and the rate of oxidation of iron(II) are strongly pH-dependent the latter increases with... 

The primary anion studied in both the titration calorimetry7 and CIR-FTIR experiments reported here was the salicylate (2-hydroxybenzoate) ion (SAL). Acidity constants for salicylic acid are pK = 3.0 and pK = 13 (12), and the aqueous solubility of salicylic acid is 2.4 g/L, while that of NaSAL is 975 g/L. SAL has been shown to adsorb on both iron oxides (13) and aluminum oxides (14). Several other anions were also studied, and results for these anions are given as needed to illuminate certain features of the salicylate-goethite adsorption process. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

Iron oxide is one of the oldest media used for H2S removal from process gas. Its use is limited by the necessity of periodically replacing the solid adsorbent (3), as the adsorbent cannot be regenerated completely and gradually loses its effectiveness. Hot-gas purification processes using iron oxide include the Appleby-Frodingham Process and the METC Fixed-Bed Process (1). In these processes the iron oxide removes the H2S at elevated temperatures (700-1000°F) and is also regenerated at these temperatures. None of these or other hot-gas purification processes is commercial today. 

Hydrated iron oxides can adsorb heavy metals. These adsorption properties arise from the presence of structural hydroxyl groups on their surface, which exhibit amphoterism (56) ... 

Figure 6. XPS C Is (left) and 0 Is (right) spectra of the interfacial substrate surface of epoxy-amine coating on bare steel following cathodic polarization testing (solid curves). Also shown is an approximate curve resolution for each spectrum components of the C Is spectrum are, from left, alkane, ether, and carbonate principal components of the 0 Is spectrum are, from left, iron oxide, carbonate, ether, ad adsorbed water (29.) or sodium Auger ( ). Reproduced with permission from Ref. 29. Copyright 1981, Pergamon Press.

The photocatalytic activity of iron oxides with semiconductor properties is of two kinds. The holes at the solid surface can oxidize either adsorbed or solution species (see Chap. 11), or both holes and electrons may induce dissolution of the solid phase (see Chap. 12). 

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