Sorption-desorption kinetics, slow

Keywords. Organic pollutants, Aqueous-solid phase systems, Sorption, Desorption, Kinetics, Modeling, Transport parameters, Solid waste materials, Slow sorption/desorption, Highway materials, Remediation... 

Diazinon and Ronnel. The conclusion that neutral hydrolysis of sorbed chlorpyrifos is characterized by a first-order rate constant similar to the aqueous phase value is strengthened and made more general by the results for diazinon, 0,0-diethyl 0-(2-iso-propyl-4-methyl-6-pyrimidyl) phosphorothioate, and Ronnel, 0,0-dimethyl 0-(2,4,5-trichlorophenyl) phosphorothioate (10). The results for the pH independent hydrolysis at 35°C for these compounds in an EPA-26 sediment/water system (p=0.040) are summarized in Table IV. Because the aqueous (distilled) values of k for diazinon and Ronnel are similar in magnitude to the value for chlorpyrifos, and because these values were shown by the chlorpyrifos study to be slow compared to sorption/desorption kinetics, computer calculations of were not deemed necessary and were not made for these data. 

Similar slow sorption-desorption kinetics were observed with some other enantioselective phases in liquid chromatography (103-105). Also with highly selective crown ethers or cryptates the release of substrates from their complexes can be extremely slow (A-6). 

The main objectives of this chapter are to (1) review the different modeling techniques used for sorption/desorption processes of organic pollutants with various solid phases, (2) discuss the kinetics of such processes with some insight into the interpretation of kinetic data, (3) describe the different sorption/ desorption experimental techniques, with estimates of the transport parameters from the data of laboratory tests, (4) discuss a recently reported issue regarding slow sorption/desorption behavior of organic pollutants, and finally (5) present a case study about the environmental impact of solid waste materials/complex... 

Most researchers attribute slow kinetics to some sort of diffusion limitation (e.g., diffusion is random movement under the influence of a concentration gradient [193]), because sorbing molecules are subject to diffusive constraints throughout almost the entire sorption/desorption time course due to the porous nature of particles. Particles are porous by virtue of their aggregated nature and because the lattice of individual grains in the aggregate may be fractured. 

Generally, slow sorption or desorption has made complete remediation technology difficult. However, there have recently been legitimate questions raised by some researchers [163,187] about whether we even need to be concerned about residues that desorb so slowly and thus are apparently largely bio-unavailable. At a minimum, it is important that we understand the factors which govern slow sorption/desorption rates, their kinetics and causes at the intra-particle level of different solid phase materials (e.g., surface/subsurface and aquatic sediment particles), and how these phenomena can relate to contaminant transport, bioavailability, toxicity, remediation, and risk assessment modeling. 

Though this system is perhaps an extreme example of slow sorption kinetics, it illustrates that the assumption of rapid equilibrium between the sediment and aqueous phases is questionable. The importance of such an observation to the investigation of hydrolysis kinetics in sediment/water systems must be emphasized. Certainly, any model of hydrolysis kinetics in sediment/water systems must include explicit expressions for the kinetics of the sorption/desorption process. 

Karickhoff (1980) and Karickhoff et al. (1979) have studied sorption and desorption kinetics of hydrophobic pollutants on sediments. Sorption kinetics of pyrene, phenanthrene, and naphthalene on sediments showed an initial rapid increase in sorption with time (5-15 min) followed by a slow approach to equilibrium (Fig. 6.7). This same type of behavior was observed for pesticide sorption on soils and soil constituents and suggests rapid sorption on readily available sites followed by tortuous diffusion-controlled reactions. Karickhoff et al. (1979) modeled sorption of the hydrophobic aromatic hydrocarbons on the sediments using a two-stage kinetic process. The chemicals were fractionated into a labile state (equilibrium occurring in 1 h) and a nonlabile state. 

Selim et al. (1976b) developed a simplified two-site model to simulate sorption-desorption of reactive solutes applied to soil undergoing steady water flow. The sorption sites were assumed to support either instantaneous (equilibrium sites) or slow (kinetic sites) first-order reactions. As pore-water velocity increased, the residence time of the solute decreased and less time was allowed for kinetic sorption sites to interact (Selim et al., 1976b). The sorption-desorption process was dominated by the equilib-... 

Several researchers have confirmed that biodegradation can be limited by the slow desorption of organic compounds [22-25]. Though significant research has been conducted to study the sorption and desorption kinetics of organic compounds and their bioavailability, few studies have focused on the bioavailability of contaminants in soils containing only the desorption resistant fraction and how the degradation rates compare to those for freshly contaminated soils. 

Desorption kinetic studies were conducted with freshly contaminated soils and aged soils (i.e., soils that were allowed to have extended contact time of 3 months and 5 months during the sorption step). Three levels of contamination were used. To illustrate here, soil desorption kinetics data for 1,3-DCB with silty soil from the PPI site was plotted with the fraction of the contaminant released as a function of time in Fig. 5. Again, the results for other chemicals are not discussed here as the findings are very similar. As shown in Fig. 5, a substantial portion of the contaminant is released within the first 20-30 h, followed by a very slow release over a very long period. This slow release was observed over the entire duration of the experiment (100-450 h). Approximately 60% of 1,3-DCB was desorbed in the first 24 h for freshly contaminated soils. 

Wall effects, or the adherence of material to the bare silica capillary wall, has been a difficult problem since the early days of HPCE, particularly for large molecules such as proteins. Small molecules can have, at most, one point of attachment to the wall and the kinetics of ad-sorption/desorption are rapid. Large molecules can have multiple points of attachment resulting in slow kinetics. Several solutions have been proposed, including the use of (a) extreme-pH buffers, (b) high-concentration buffers, (c) amine modifiers, (d) dynamically coated capillaries, and (e) treated or functionalized capillaries. 

A particular emphasis has been placed on the detachment rate of colloids from the mineral surface. Colloid sorption is irreversible (or at least shows very slow desorption kinetics regardless of solution composition either in electrolyte solution or in the presence of a carrier colloid such as silica or humic acids (Table ID) moreover, desorption tests up to three months have not shown any colloid detachment. No marked influence of temperature on the release of retained ceria colloids has been observed between 20 and 90°C. 

It must be noted that a retardation coefficient is strictly applicable only when a linear relationship exists between Cs and Caq and the partitioning equilibrium is rapidly established. In some nonlinear cases, or when the kinetics of sorption and desorption are slow, mixing is increased as chemicals are released from aquifer solids after some time has passed, resulting in a smearing or tailing of a pollutant peak (Fig. 3-28). 

Because many organic chemicals are nonionic and have low water solubilities, they will exist primarily in the sorbed state in soil- and sediment-water systems. The sorption of nonionic chemical occurs through hydrophobic sorption or partitioning to the organic matter associated with the soil or sediment (Karickhoff, 1980 Chiou et al., 1983). Furthermore, because desorption kinetics may be slow relative to hydrolysis kinetics, to accurately predict the fate of hydrolyzable chemicals in soil-and sediment-water systems an understanding of hydrolysis kinetics in the sorbed... 

An intriguing application of these Pd nanoparticles in basic research concerns the question of the solubility of H2 in such materials relative to bulk palladium [47]. Hydrogen concentration-pressure isotherms of surfactant-stabilized palladium clusters and polymer-embedded palladium clusters with diameters of 2, 3 and 5 nm were measured with the gas sorption method at room temperature. The results show that, compared to bulk palladium, the hydrogen solubility in the a-phase of the clusters is enhanced fivefold to tenfold, and the miscibility gap is narrowed. Both results can be explained by assuming that hydrogen occupies the subsurface sites of the palladium clusters. The Pd-H isotherms of all clusters show the existence of hysteresis, even though the formation of misfit dislocations is unfavorable in small clusters. Compared to surfactant-stabilized clusters, the polymer-embedded clusters show slow absorption and desorption kinetics. Moreover, evidence for a cubic-to-icosahedral transition of quasi-free Pd-H clusters by the hydrogen content was reported [47c]. 

Peak diffuseness may be a result of the kinetics of the sorption-desorption process (i.e., slow mass transfer or exchange at sorbent surfaces). Peak diffusion in this case is usually nonsymmetric because the rates of sorption and desorption are not the same. Band spreading due to the final rate of mass exchange is closely related to the diffusion phenomenon. Physical adsorption, for all practical purposes, is instantaneous. The overall process of sorption, however, consists of several parts (a) the movement of sorbate molecules toward the sorbent surface, resulting fi om intergrain diffusion (outer diffusion), (b) movement of sorbate molecules to the inside of pores (i.e., internal diffusion of the sorbate molecules in the pores and surface diffusion in the pores), and (c) the sorption process in general. 

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