Sorbents, measuring surface area

Whereas several specific soil attributes are advocated as being responsible for DOC sorption in the mineral soil (Table V), it appears that the greater the clay or aluminum and iron oxide content of a soil, the greater its adsorptive capacity for DOC. For example, there is a positive correlation between m (the measure of the affinity of a substance for the sorbent or the partition coefficient) and soil clay content, dithionite extractable iron (Fej), and oxalate extractable aluminum (Al0) (Moore et al., 1992 Nelson et al., 1993 Kaiser and Zech, 1998). Direct measurements of the surface area of soil particles also correlate very well with DOC adsorption capacity (Nelson et al., 1993). Furthermore, Nelson et al. (1993) report that riverine DOC concentrations are negatively correlated to the clay content of watershed... 

A test matrix of about 20 different carbon samples, including commercial carbon fibers and fiber composites, graphite nanofibers, carbon nanowebs and single walled carbon nanotubes was assembled. The sorbents were chosen to represent a large variation in surface areas and micropore volumes. Both non-porous materials, such as graphites, and microporous sorbents, such as activated carbons, were selected. Characterization via N2 adsorption at 77 K was conducted on the majority of the samples for this a Quantachrome Autosorb-1 system was used. The results of the N2 and H2 physisorption measurements are shown in Table 2. In the table CNF is used to designate carbon nanofibers, ACF is used for activated carbon fibers and AC for activated carbon. 

Another issue that deserves consideration is the measurement of the surface area of a sorbent after derivatization with alkyl chains. It is generally accepted that the total surface area of a stationary phase after ligand attachment is lower than the surface area of the bare silica because bonded ligands occupy volume inside the pore [7]. 

This chapter aims to give guidelines on how to use adsorption methods for the characterization of the surface area and pore size of heterogeneous catalysts. The information derived from these measurements can range from the total and available specific surface area to the pore sizes and the strength of sorption in micropores. Note that this spans information from a macroscopic description of the pore volume/specific surface area to a detailed microscopic assessment of the environment capable of sorbing molecules. In this chapter we will, however, be confined to the interaction between sorbed molecules and solid sorbents that are based on unspecific attractive and repulsive forces (van der Waals forces, London dispersion forces). 

What are the absolute and relative abundances of important sorbent solids and what fraction of their surface areas are exposed to flowing water Any adsorption model we select that assumes a finite number of sorption sites, requires, as input, the area of a sorbing phase exposed to a given volume of water I.e.g., Cs(g/L) x 5 (m /g)] and a surface site density [ (sites/m-)] for that phase. Can we measure or estimate these values Such measurements and estimates are extremely difficult for metal adsorption by modern stream sediments, which may be mix-... 

Figure 11.1a demonstrates clearly the absence of any correlation between the sorption activity of the water-swoUen polymers and their specific surface area measured in the dry state. On the contrary, the dye uptake obviously rises with the sorbent water regain, reaching a maximum value at the highest swelling that corresponds to a 300% degree of crosshnking. The sorption capacity of Styrosorbs with a nominal 400 and... 

The spreading pressure of a sorbed gas, tt, can be calculated from the sorption isotherm, although it cannot be measured directly. Assuming the sorbent to be thermodynamically inert, and that the surface area is the same at all temperatures for different gases, then the thermodynamic functions for the sorbed phase become analogous to those of a real fluid. 

They found that the average pore size and the total pore volume are not adequate measures to predict the CO2 uptake of microporous carbon sorbents, the pore volume of micropores strongly governs the amount of adsorbed CO2 [161]. Neither high surface area CDC after chemical activation (surface area 3,101 m g ) nor high pore volume nano-TiC-CDC (Vtotai 1-61 cm g ) correspond with the highest CO2 adsorption capacity. At ambient pressure, the CO2 uptake closely follows a linear correlation with the volume of pores smaller or equal to a diameter of 1.5 mn. Pores smaller than 0.5 mn contribute to the amount of adsorbed CO2, but the best correlation is found for pore volume smaller than 0.8 mn (Fig. 2.29). The correlation between the amount of adsorbed CO2 at low partial pressures and volume of smaller pores is the basis for the well-known application of CO2 sorption as a method to calculate the pore characteristics of microporous materials. Subatmospheric pressures are of particular interest for industrial applications, where partial pressure of CO2 is below 1 bar, and here, the best prediction of the CO2 uptake capacity at 0.1 bar would be based on the volume of pores smaller or equal to a diameter of 0.5 nm (Fig. 2.29). This correlation can be used to design better CO2 sorbents and CCS devices. 

In the sorption experiments, dye solutions were added to different quantities of sorbents into glass-stoppered bottles and subsequently placed on a shaker for 24 h at 28 2°C. From the initial concentrations of sorbents (g/L) and dyes (mg/L), the amounts adsorbed in the sorbent were measured. Percent removal of dyes over synthesised zeolite as a function of contact time, shown in Figure A.4. The amounts sorbed were determined by the difference between initial and final concentrations and expressed as mg of dye/g of sorbent. Under the conditions of the experiments, all systems approached equilibrium within 15 h of contact time. The adsorption capacity of synthesised zeolite was higher due to larger pore size and surface area compared to commercial zeolite. The molecular size of methyl orange facilitates adsorption, resulting in higher adsorption capacity than methylene blue and safranine T. Reduced adsorption in synthesised zeolite (SZ) is due to the inability of the molecule to penetrate all the internal pore structures and less available surface. 

SURFACE AREA. The total or specific surface area includes the surface area of the sorbent particles plus the surface area of the pores, usually measured in m /g. Surface area is directly related to the degree of interaction between the sample and sorbent. 

Specific surface area depends on the chemical structure of the sorbent (silica, alumina, cellulose, etc.) and on the technology of its manufacturing. It can be measured and expressed numerically. 

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