Sorption desorption

A wide variety of guest molecules may be trapped by the Wemer-type crystalline host lattice, ranging, eg, from noble gases to condensed aromatic hydrocarbons. These clathrates may be formed from solution or by sorption. Kinetics of sorption—desorption have been studied (83). 

Desorption is the reverse of the sorption process. If the pesticide is removed from solution that is in equdibrium with the sorbed pesticide, pesticide desorbs from the sod surface to reestabUsh the initial equdibrium. Desorption replenishes pesticide in the sod solution as it dissipates by degradation or transport processes. Sorption/desorption therefore is the process that controls the overall fate of a pesticide in the environment. It accomplishes this by controlling the amount of pesticide in solution at any one time that is avadable for plant uptake, degradation or decomposition, volatilization, and leaching. A number of reviews are avadable that describe in detad the sorption process (31—33) desorption, however, has been much less studied. 

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

Wolfsberg, K. Aguilar, R.D. Bayhurst, B.P. Daniels, W.R. DeVilliers, S.J. Erdal, B.R. Lawrence, F.O. Maestas, S. Mitchell, A.J. Oliver, P.Q. Raybold, N.A. Rundberg, R.S. Thompson, J.L. Vine, E.N. "Sorption-Desorption Studies on Tuff. III. A Continuation of Studies with Samples from Jackass Flats and Yucca Mountain, Nevada", Report LA-8747-MS, Los Alamos National Laboratory, 1981. 

Kishk FM, Abu-Sharar TM, Bakry NM, et al. 1979b. Sorption-desorption characteristics of methyl parathion by clays. Arch Environ Contam Toxicol 8 637-645. 

The influence of sorption/desorption on biodegradation, which has been discussed in a wider context in Chapter 4. 

An important consideration prior to sample collection is transportation and storage. Samples should be treated so as to retain the integrity of the sample from the moment of collection to the time of analysis. The physico-chemical characteristics of a sediment sample change during drying, with effects on the sorption-desorption behavior of chemicals. 

Ivanovich M, Harmon RS (eds) Clarendon Press, Oxford, p 34-61 Giammar DE, Hering JG (2001) Time scales for sorption-desorption and surface precipitation of uranyl on goethite. Environ Sci Technol 35 3332-3337... 

Quigley MS, Honeyman BD, Santschi PH (1996) Thorium sorption in the marine enviromnent equilibrium partitioning at the Hematite/water interface, sorption/desorption kinetics and particle tracing. Aquat Geochem 1 277-301... 

Chromatography is essentially a physical method of separation in trtiich the components to be separated are distributed between two phases one of which is stationary (stationeury phase) while the other (the mobile phase) percolates through it in a definite direction. The chroaatographic process occurs as a result of repeated sorption/desorption acts during the movement of the sample components along the stationary bed, and the separation is due to differences in the distribution constants of the Individual sample components. 

A procedure for characterizing the rates of the volume change of gels has not been uniformly adopted. Often, the kinetics are simply presented as empirical sorption/desorption curves without quantitative analysis. In other cases, only the time required for a sample of given dimensions to reach a certain percentage of equilibrium is cited. One means of reducing sorption/desorption curves to empirical parameters is to fit the first 60% of the sorption curve to the empirical expression [119,141]... 

WASP/TOXIWASP/WASTOX. The Water Quality Analysis Simulation Program (WASP, 3)is a generalized finite-difference code designed to accept user-specified kinetic models as subroutines. It can be applied to one, two, and three-dimensional descriptions of water bodies, and process models can be structured to include linear and non-linear kinetics. Two versions of WASP designed specifically for synthetic organic chemicals exist at this time. TOXIWASP (54) was developed at the Athens Environmental Research Laboratory of U.S. E.P.A. WASTOX (55) was developed at HydroQual, with participation from the group responsible for WASP. Both codes include process models for hydrolysis, biolysis, oxidations, volatilization, and photolysis. Both treat sorption/desorption as local equilibria. These codes allow the user to specify either constant or time-variable transport and reaction processes. 

Sorption and desorption are usually modeled as one fully reversible process, although hystersis is sometimes observed. Four types of equations are commonly used to describe sorption/desorption processes Langmuir, Freundlich, overall and ion or cation exchange. The Langmuir isotherm model was developed for single layer adsorption and is based on the assumption that maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface, that the energy of adsorption is constant, and that there is no transmigration of adsorbate on the surface phase. 

The tables were designed to encompass processes included in most models of the various media of concern. Although selected processes are not rigorously defined for each media (e.g., sorption/desorption in air refers to toxicant-particulate interactions), the goal was to provide a concise ranking table for each level of analysis. 

In all of the workshops, but especially in the FAT and Exposure Assessment workshops, the need for better understanding and model representation of soil systems, including both unsaturated and saturated zones, was evident. This included the entire range of processes shown in Table II, i.e., transport, chemical and biological transformations, and intermedia transfers by sorption/desorption and volatilization. In fact, the Exposure Assessment workshop (Level II) listed biological degradation processes as a major research priority for both soil and water systems, since current understanding in both systems must be improved for site-specific assessments. 

In the first stage, unfixed dyes, salt and alkali present in the liquor phase must be removed and this is best done by replacing this liquor with fresh water. Sorption, desorption and diffusion processes play only subordinate roles in this stage, the key factors being liquor flow, mechanical action and liquor exchange. The dilution laws are generally applicable. 

An increase of the pH in the aqueous medium, and capture of SO42- by the mineral surface, could lead to the liberation of Pi. In addition, there seem to be selfregulation mechanisms for the Pi sorption-desorption process, depending on the S042 concentration at the interface. Processes such as enrichment and liberation... 

Hinz C., Selim H.M. Kinetics of Zn sorption-desorption using a thin disk flow method. Soil Sci 1999 164 92-100. 

Graphite finds wide range of applications in the electrodes for certain types of rechargeable batteries and supercapacitors, in electro-sorption/desorption electrodes, as anodes in a number of processes of... 

Pigna M, Krishnamurti GSR, Violante A (2006) Kinetics of arsenate sorption-desorption from metal oxides Effect of residence time. Soil Sci Soc Am J 70 2017-2027... 

In order to test the reversibility of metal-bacteria interactions, Fowle and Fein (2000) compared the extent of desorption estimated from surface complexation modeling with that obtained from sorption-desorption experiments. Using B. subtilis these workers found that both sorption and desorption of Cd occurred rapidly, and the desorption kinetics were independent of sorption contact time. Steady-state conditions were attained within 2 h for all sorption reactions, and within 1 h for all desorption reactions. The extent of sorption or desorption remained constant for at least 24 h and up to 80 h for Cd. The observed extent of desorption in the experimental systems was in accordance with the amount estimated from a surface complexation model based on independently conducted adsorption experiments. 

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