Adsorbent within particles

The adsorption of organics from the liquid to a solid phase is generally assumed to occur in three stages [50]. The brst is the movement of the contaminant (adsorbate or solute) through a blm surface surrounding the solid phase (adsorbant). The second is the diffusion of the adsorbate within the pores of the activated carbon. The bnal stage is the sorption of the material onto the surface of the sorbing medium. The overall rate of adsorption is controlled by the rate of diffusion of the solute molecules within the capillary pores of the carbon particles [27]. 

The apparent rates of adsorption into adsorbent particles usually involve the resistances for mass transfer of adsorbate across the fluid film around adsorbent particles and through the pores within particles. Adsorption perse at adsorption sites occurs very rapidly, and is not the rate-controlling step in most cases. 

The evidence for the reduction of Cr(VI) to Cr(III) is only indirect, because Cr(III) is not detected in solution. Cr(III) has a strong tendency to adsorb to particle surfaces and to precipitate as insoluble (hydr)oxide. Thus, Cr(III) produced within the water column by reduction is expected to bind to particles and to be found in the particulate phase. No evidence for release of Cr from sediments was found. Cr(III) is expected to be retained very strongly in sediments, so the release of Cr(III) under anoxic conditions is unlikely. Under oxic conditions the oxidation of Cr(III) by Mn oxides, for example, and release of Cr(VI) from the sediments is plausible such a mechanism in sediment pore waters is indicated in ref. 84. 

Recent chemical experiments with transactinides have been carried out by application of refined methods. Fast transport is achieved by thermalizing the products of nuclear reactions recoiling out of the target in helium gas loaded with aerosol particles (e.g. KCl, M0O3, carbon clusters) of 10 to 200 nm on which the reaction products are adsorbed. Within about 2 to 5 s the aerosols are transported with the gas through capillary tubes over distances of several tens of metres with yields of about 50%. 

Trace metals in seawater can exist in a variety of physical and chemical forms. The simplest physical distinction is particulate versus dissolved forms. This is somewhat of an operational definition with 0.4 p.m or 0.2 p.m pore size filters generally providing this separation. Particulate forms include those metals adsorbed onto particle surfaces, incorporated within particles of biogenic origin and incorporated in the matrix of... 

Absorption is distinguished from adsorption by defining it as the filling in of the intergranule void between and within particles. It is thus a filling of three-dimensional space in or between adsorbent particles, whereas adsorption is purely a surface phenomenon. 

The two important properties of an activated carbon that affect its use as an adsorbent are particle size and stirface area. Adsorption rates increase as particle sizes decrease, while the total adsorptive capaciQr of a carbon, depends on the on its total sur ce area. The size of the carbon does not have a great effect on total surface area, since most of the sur ce area lies within the pores of a carbon particle. Thus, equal weights of the same carbon would have essentially the same capaci in the granular form as in the powder form. 

Equation 4.5 represents the surface excess of substance i relative to an aqueous solution that contains kilograms of water plus substance i at molality m,-. This surface excess is assigned to a surface at which there is no net accumulation of water. If water in the interstitial space is not adsorbed (in the sense defined in Chap. 2), then this surface can be taken as congruent with the geometric boundaries of the adsorbing soil particles. If some of the interstitial water is adsorbed, say, within the region bounded by a surface 1.0 nm from the boundary of a soil particle, then the surface of zero net accumulation of water could differ slightly from the soil particle surface. 

The bi-dispersed solid dealt with in Section 14,6 is zeolite type solid where the micropores within the grain are of molecular dimension, and hence they only accommodate one type of adsorbate within the micropore, namely the adsorbed species. In this section we shall consider another type of bi-dispersed solids where the micropores are large enough to accommodate adsorbate in both forms free form as well as adsorbed form. For such cases, the mass balance equations inside the particle are ... 

Multilayers of cationic PDADM AC (200-350 kg/mol) and anionic PSS (70 kg/mol) were deposited onto silica (particles of 70-100 nm in size) colloids (Rodriguez et al. 2000). Adsorbed water peaks in PSS and PDADMAC were observed at 5h=3.66 and 4.29 ppm, respectively (Figure 5.20). The water peak in the PSS/PDADMAC complex shifted to 3.85 ppm. For PEM on silica, two sharp peaks were observed 4.9 (protons associated with the silica surface) and 3.85 ppm (water adsorbed within the polymer film). 

Migration of adsorbate within the pores of the adsorbent by intra-particle diffusion (pore diffusion)... 

Both active carbons differ in the development of porous structure within particles, as well as in the contribution of various kinds of pores into their total porosity. AG carbon is a typical polydisperse adsorbent, of a comparable contribution of all kinds of pores. On the contrary, RN active carbon is a microporous adsorbent with dominant microporous structure and lower content of macro- and mesopores. 

The absorbance of the blue phosphorus complex reaches a maximum in a few minutes and stays almost constant for many hours. However, the absorbances should be read within 10-30 min, using a wavelength of 880 nm. This determination, on an unfiltered sample, gives the amount of dissolved inorganic phosphate ions in true solution and probably also includes a small fraction of those ions that are adsorbed onto particles and subsequently dissolved by the acid in the mixed reagent. The latter fi-action is not included when a filtered sample is analysed. 

An adsorbing (virtual) particle is created whose position and orientation is selected at random within prescribed limits defining the simulation domain. 

An adsorbing (virtual) particle is generated at random within the simulation area AS with periodic boundary conditions on its perimeter if it does not touch any of sites, the particle is rejected and another virtual particle is generated (the number of attempts N t is increased by one). 

Fig. 6. Concentration profiles through an idealized biporous adsorbent particle showing some of the possible regimes. (1) + (a) rapid mass transfer, equihbrium throughout particle (1) + (b) micropore diffusion control with no significant macropore or external resistance (1) + (c) controlling resistance at the surface of the microparticles (2) + (a) macropore diffusion control with some external resistance and no resistance within the microparticle (2) + (b) all three resistances (micropore, macropore, and film) significant (2) + (c) diffusional resistance within the macroparticle and resistance at the surface of the...

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