Exothermic adsorption mechanism

Prasad (1994, 139) found that As(V) sorption attained equilibrium in contact with hematite in about 35 minutes. Using an initial As(V) concentration of 1.3 x 10-4 mol L-1 (9.7 mg As/1) at 20 °C, maximum As(V) removal with hematite occurred at a pH of 4.2 (Prasad, 1994, 141). At this pH, 200-/im hematite removed 81 % of the As(V), whereas hematite with a mean particle size of only 100 pm removed 97 % (Prasad, 1994, 139-140). The percentage As(V) removal by hematite decreased with increasing temperatures of 20-40 °C, which is consistent with an exothermic adsorption mechanism (Prasad, 1994, 141). 

Adsorption is an exothermic process. The adsorption enthalpy, decreases as the load of adsorbed molecules increases. In activated carbon adsorption systems for solvent recovery, the liberated adsorption enthalpy normally amounts to 1.5 times the evaporation enthalpy at the standard working capacities which can result in a 20 K or more temperature increase. In the process, exothermic adsorption mechanisms may coincide with endothermic desorption mechanisms. ... 

The corresponding temperature dependence of the absolute CO2 capacities on K-promoted HTlc obtained from these results at 980 torr is also shown in Figure 1. This capacity initially decreased with increasing temperature, reached a plateau at around 300 to 400 °C, and then decreased again with further increases in the temperature. This behavior was indicative of an exothermic adsorption mechanism because of the increasing CO2 capacity with decreasing temperature ... 

The surface proton adsorption which occurs after Step 2, however, complicates the determination of the heat content change resulting from anion adsorption. In order to make this correction, the heat associated with proton adsorption must be determined from the previous potentiometric-calorimetric titrations. Proton adsorption on goethite is exothermic, and Figure 1 provides an average value of -29.6 kj/mol near pH 4. This value, when multiplied by the moles of protons required to return to pH 4 after anion adsorption, allows correction for the heat associated with proton adsorption. This correction, however, is based on the assumption that the proposed two-step anion adsorption mechanism described above represents the only surface reactions which occur during anion adsorption. As such, the results obtained by this procedure are model dependent and are best used for comparative purposes. 

A lowering of the interaction temperature increased the adsorption tendency of most of the soils studied (Table 3). Such exothermicity can be reconciled with a physical adsorption mechanism [ 16 ]. The retention mechanism appeared to be slightly endothermic on GI, a soil characterized by a very high sand content, a low pH, and a very low organic matter content. It seemed possible that, once... 

Anatase is another interesting phase of Ti02- The natural crystallographic faces are the (101) and (100) orientations. The acid-base adsorption mechanism is very similar to the one of the metal/oxide already discussed. On the (101) surface, molecular adsorption is exothermic (16.6 kcal/mol for full coverage) [99]. In contrast, water dissociates on the (001) plane, which is more reactive [100]. It is worth noticing that the surface morphology has a very important effect on the hydroxyl coverage [100]. 

For metal desorption from the biomass certain dilute solutions of mineral acids like hydrochloric acid, sulfuric acid, acetic acid and nitric acid were used [219, 76]. Batch system was carried out to study the desorption of the adsorbed Hg (II) from the biosorbent - immobilized and heat inactivated Trametes versicolor and Pleurotus sajur-caju [8]. Hg (II) ions adsorbed onto the biosorbents were eluted with 10 mmol dm HCl and the results showed that more than 97% of the adsorbed Hg (II) ions were desorbed from the biosorbents. In order to evaluate the feasibility of applying the prepared biosorbents in the heavy metals removal processes, the metal desorption efficiency from loaded biosorbents, and the reusability of the biosorbent in repeated adsorption-desorption operations were determined. The charged species exhibited desorption-resistance fraction whereas the desorption of the neutral form was completely reversible. The difference in sorption and desorption between the neutral and charged species is attributed to the fact that the anionic species sorbs by a more specific exothermic adsorption reaction whereas the neutral form partition by the hydrophobic binding to the soil [206]. Desorption of soil-associated metal ions and possible mechanisms have received considerable attention in literature [148],... 

These mechanisms are characterized by the relative magnitudes of the heats of reaction, solution, or adsorption (see Adsorption, separation). AH useflil drying mechanisms are exothermic. Phosphoms pentoxide is a Class 1 drying agent that reacts with water to form a polyphosphoric acid (2) ... 

If the surface is nearly covered (0A 1) the reaction will be first-order in the gas phase reactant and zero-order in the adsorbed reactant. On the other hand, if the surface is sparsely covered (0A KAPA) the reaction will be first-order in each species or second-order overall. Since adsorption is virtually always exothermic, the first condition will correspond to low temperature and the second condition to high temperatures. This mechanism thus offers a ready explanation of a transition from first-to second-order reaction with increasing temperature. 

A solid surface interacts with its surrounding molecules (in the gas or liquid phase) in varying degrees. For example, if a solid is immersed in a liquid, the interaction between the two bodies will be of interest. The interaction of a substance with a solid surface can be studied by measuring the heat of adsorption (besides other methods). The information one needs is whether the process is exothermic (heat is produced) or endothermic (heat is absorbed). This leads to the understanding of the mechanism of adsorption and helps in the application and design of the system. Calorimetric measurements have provided much useful information. When a solid is immersed in a liquid (Figure 5.10), in most cases there is a liberation of heat ... 

The interaction of N2 with transition metals is quite complex. The dissociation is generally very exothermic, with many molecular adsorption wells, both oriented normal and parallel to the surface and at different sites on the surface existing prior to dissociation. Most of these, however, are only metastable. Both vertically adsorbed (y+) and parallel adsorption states (y) have been observed in vibrational spectroscopy for N2 adsorbed on W(100), and the parallel states are the ones known to ultimately dissociate [335]. The dissociation of N2 on W(100) has been well studied by molecular beam techniques [336-339] and these studies exemplify the complexity of the interaction. S(Et. 0n Ts) for this system [339] in Figure 3.36 (a) is interpreted as evidence for two distinct dissociation mechanisms a precursor-mediated one at low E and Ts and a direct activated process at higher These results are similar to those of Figure 3.35 for 02/ Pt(lll), except that there is no Ts... 

The effect of temperature on sorption equilibrium is a direct indication of the strength of the sorption process. The weaker the interaction between sorbent and sorbate, the less the effect of temperature (Hamaker and Thompson, 1972). While temperature can influence sorption, the strength and direction of the effect depends on the properties of the sorbent and sorbate and on the sorption mechanism. Adsorption processes are generally exothermic, so the higher the temperature, the less the adsorption (Hamaker and Thompson, 1972). Hydrophobic sorption, however, has been shown to be relatively independent of temperature (Chiou et al., 1979). 

For uptake of solute from solution by porous solids the rate will be endothermic rather than exothermic if intraparticle transport is the rate-limiting mechanism. Because diffusion is an endothermic process while adsorption is exothermic, rate of uptake of solute by porous solids will often increase with increasing temperature while for the same system the equilibrium position of adsorption or adsorption capacity will decrease with increasing temperature. 

An alternative mechanism to account for the formation of alkanes is that the alkene which is released from the catalytic site is immediately readsorbed unless the vacated site is filled by some other donor molecule. The process may be likened to the cage effect in the formation of radical pairs in solution. Adsorption of the alkene is exothermic and, accordingly, is likely to have a low energy barrier. The lower selectivity of Os, Ir, and Pt has been attributed to the readsorption of alkene. ... 

As we have seen previously, the separation mechanism in pervaporation is explained by an adsorption-diffusion process. In this way, the selective adsorption of the components in the zeolite will be responsible for the selectivity in the separation. Adsorption is an exothermic nonactivated process. In general, the isotherm of adsorption on zeohtes follows a single site Langmuir-type isotherm [74]. 

Thus, the process is seen as physical adsorption with subsequent substitutive desorption. The characteristic time of the reaction could be roughly evaluated from the ratio of areas of the two peaks as 45 15 s ( rr). It indicates an activation energy of the reaction of 80 kJ mol-1 or so. The enthalpy of the gaseous chlorination of W02C12 is -40 kJ mol-1 and an estimate for the adsorption enthalpy of W02C12 is about -lOOkJ mol-1 these three values are consistent with the proposed mechanism. The reason why the chlorination does not proceed in gas, though, is its quite exothermic may be the short time, not more than 0.1 seconds. 

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