Contamination by Adsorption

Several other types of adsorption isotherms have been described one applicable to fractional monolayers in which there is linear variation between the amount adsorbed and log C is called Temkin adsorption. 

Adsorption is the principal source of contamination of precipitates that have large surfaces, for example, flocculated colloids (metal sulfides, silver halides, hydrous oxides). The extent of adsorption may be relatively small, as it usually is with silver halides, or severe, as it often is with hydrous oxides. 

FIGURE 9-1 Adsorption of divalent metal ions on hydrous PefUI) oxide /I, as a function of NH C1 concentration with a NH3 concentration of 0.9 M, except 0.7 M for zinc S, as a function of NH3 concentration with a NHtQ concentration of 1 M. (From Kolthoff andOverholser. ) 

Recent studies of coprecipitation with the hydrous oxides have presented further evidence for definite 1 1 compound formation between Zn or Cu and iron(III) hydroxide. X-ray information indicated a chemical compound with Cu and tin(IV) hydrous oxide, presumably with the formula Cu3SnOs H20. In each case the first step was suggested to be adsorption, followed by reaction. 

Specific reversible adsorption of up to 0.28 mole of Pb per mole of iron in colloidal hydrous iron(III) oxide has been found to be accompanied by the release of 1.6 hydrogen ions per mole of Pb adsorbed at pH 6. 

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This is a useful example to illustrate the application of charcoal (or similar substances with large surface area per gram) in the removal of contaminants by adsorption. 

In contrast to chemical processes, where the feed materials and process aids (catalysts) are in most cases well defined, clean, and consistent, the particulate solids in mechanical process technologies, especially if they are or were derived from naturally occurring raw materials (minerals, concentrates), may vary widely in composition and/ or physical characteristics. As mentioned before (Chapter 3), when agglomeration processes are involved, the surface properties of the particles (macroscopic and microscopic shape, roughness, contamination by adsorption and absorption) are of particular importance and modifications that are sometimes difficult to detect, may require a new set of operating parameters or the introduction of for example, binders, surfactants, or other additives. 

Ultra-high vacuum conditions are important for two reasons. Firstly the material to be analysed is bombarded with particles or photons, and particles (electrons or ions) leaving the surface are detected these particles suffer collisions with residual gas molecules if the pressure is >10 torr. Secondly, and more important, surfaces to be analysed can be rapidly contaminated by adsorption of residual gas molecules if the pressure is >10 torr. Hence, surface-analysis instrumentation is constructed routinely to achieve 10 ° torr (after bake-out). 

Feedstocks are natural gas, refinery fuel gas, LPG and paraffinic naphthas. After elimination of CO2, the last traces of contaminants are converted to methane (methanation) or eliminated by adsorption on molecular sieves (PSA process). 

The precipitated metallic hydroxides or hydrated oxides are gelatinous in character, and they tend to be contaminated with anions by adsorption and occlusion, and sometimes with basic salts. The values presented in Table 11.2 suggest that many separations should be possible by fractional precipitation of the hydroxides, but such separations are not always practical owing to high local concentrations of base when the solution is treated with alkali. Such unequal concentrations of base result in regions of high local pH and lead to the precipitation of more soluble hydroxides, which may be occluded in the desired precipitate. Slow, or preferably homogeneous, precipitation overcomes this difficulty, and much sharper separations may be achieved. 

Activated carbon filters are employed primarily as RW contaminant removal systems for chlorine (by chemisorption) and various organics such as trihalomethanes (THMs), petroleum products, and pesticides (by adsorption). In addition, they act as physical filters and therefore incorporate sufficient freeboard in their designs to permit periodic backwashing. 

Regardless of the location of the protein and its state, cell separation needs to be inemensive, simple, and reliable, as large amounts of fermentation-broth dilute in the desired product may be handled. The objectives are to obtain a well-clarified supernatant and solids of maximum dryness, avoiding contamination by using a contained operation. Centrifugation or crossflow filtration is t ically used for cell separation, and both unit operations can be run in a continuous-flow mode [Datar and Rosen, in Stephanopoulos (ed.), op. cit., pp. 369-503]. In recent years, e3q>anded-bea adsorption has become an alternative. It combines broth clarification and adsorption separation in a single step. 

Figure 14.1 presents the main components of a typical in situ SVE system.910 Vertical extraction wells are installed inside the contaminated zone at appropriate distances from one another. The SVE wells are typically constructed of PVC pipe, with a screened interval, which is placed within the contaminated zone. The wells are connected to blowers or vacuum pumps, which induce a continuous airflow through the pores of the unsaturated zone. The soil surface is sometimes covered with an impermeable seal, made from high density polyethylene (HDPE) or bentonite clay for example, to prevent the vertical influx of air from the surface, which might cause short-circuiting problems, and promote horizontal gas flow through the contaminated area. The airstream, which contains the contaminant vapors, passes initially through an air-water separation unit to remove the entrained moisture and is then directed to the gas treatment unit, where the contaminants are thermally destroyed or removed by adsorption. 

Pure Ti02 was recently reported to be active in the disinfection of water contaminated by spores of the type Fusarium solani [142], Bacillus anthracis [143], or Cryptosporidium parvum oocysts [144], or when supported as nanocomposites on zeolite H(i for E. coli deactivation [145], and it found applications in water treatment as a replacement for chlorine. Ag-Ti02 immobilized systems were used for inactivation of bacteria, coupling the visible light response of the system and the strong bactericidal effect of Ag [146]. Silver was deposited on hydroxyapatite to form nanocomposites with a high capacity for bacterial adsorption and inactivation [147], or used for airborne bacterial remediation in indoor air [148],... 

Mesoporous materials (SBA-15 and Al-SBA-15 with various Si/Al ratios) were synthesized and investigated in relation to their capacity to be used as adsorbents for depollution of the contaminated air or wastewater. The compositional and the structural properties were determined by XRD, N2 isotherms, NMR, chemical analysis and XPS. The acidity and adsorption properties of the solids were checked by adsorption microcalorimetry using various basic or polluting molecules in gas phase. 

Laird DA, Barriuso E, Dowdy RH, Koskinen WC (1992) Adsorption of atrazine on smectites. Soil Sci Soc Am J 56 62-67 LeBaron PC, Wang Z, Pinnavaia TJ (1999) Polymer-layered silicate nanocomposites an overview. Appl Clay Sci 15 11-29 Lee J-F, Crum JR, Boyd SA (1989) Enhanced retention of organic contaminants by soil exchanged with organic cations. Environ Sci Technol 23 1365-1372 Lee J-F, Mortland MM, Boyd SA, Chiou CT (1989a) Shape-selective adsorption of aromatic molecules from water by tetramethylammonium-smectite. J Chem Soc Faraday Trans I 8 2953-2962... 

Because carbon has a natural affinity for adsorption of heavy hydrocarbon species and polar molecules, CMS membranes need to be used at a sufficiently high temperature to eliminate contribution/interference of the adsorption. In contrast, strong adsorption of heavier molecules may be used to separate those species by adsorption as discussed earlier by the SSF mechanism (Rao and Sircar, 1993b). The SSF carbon membranes typically have pore dimensions much greater than those needed for CMS membranes since the separation is based on the adsorbed species effectively blocking permeation of other components (Fuertes, 2000). Carbon membranes are resistant to contaminants such as H2S and are thermally stable and can be used at higher temperatures compared to the polymeric membranes. For the synthesis gas environment, the hydrothermal stability of carbon in the presence of steam will be a concern limiting its operation temperature. 

All but the most heavily contaminated fresh and marine waters contain total PAH concentrations in the part-per-trillion or low part-per-billion range (Table 25.3) (Neff 1982b). A large proportion of the PAH content in water is probably adsorbed onto suspended solids (Harrison et al. 1975). In Lake Michigan, concentrations of total PAHs in the surface microlayer varied from 0.15 to 0.45 pg/L, representing on a relative scale, 106 times the concentration in air, suggesting that aerosols are a major source of these compounds and that the microlayer is a repository until the PAHs are removed by adsorption and sedimentation (Strand and Andren 1980). 

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