Sorption-desorption reactions

The main reasons for investigating the rates of solid phase sorption/desorption processes are to (1) determine how rapidly reactions attain equilibrium, and (2) infer information on sorption/desorption reaction mechanisms. One of the important aspects of chemical kinetics is the establishment of a rate law. By definition, a rate law is a differential equation [108] as shown in Eq. (32) ... 

Many of the early studies on kinetics of soil chemical processes were obviously concerned with diffusion-controlled exchange phenomena that had half-lives (r1/2) of 1 s or greater. However, we know that time scales for soil chemical processes range from days to years for some weathering processes, to milliseconds for degradation, sorption, and desorption of certain pesticides and organic pollutants, and to microseconds for surface-catalyzed like reactions. Examples of the latter include metal sorption-desorption reactions on oxides. 

To study rapid reactions, traditional batch and flow techniques are inadequate. However, the development of stopped flow, electric field pulse, and particularly pressure-jump relaxation techniques have made the study of rapid reactions possible (Chapter 4). German and Japanese workers have very successfully studied exchange and sorption-desorption reactions on oxides and zeolites using these techniques. In addition to being able to study rapid reaction rates, one can obtain chemical kinetics parameters. The use of these methods by soil and environmental scientists would provide much needed mechanistic information about sorption processes. 

Tien (1987) studied the kinetics of heavy metal sorption-desorption on sludge using the stirred-flow reactor method of Carski and Sparks (1985). Sorption-desorption reactions were rapid with an equilibrium reached in 30 min. The sorption-desorption reactions were reversible. The sorption rate coefficients were of the order Hg > Pb > Cd > Cu > Zn > Co > Ni, while the desorption rate coefficients were of the order Cd > Cu > Hg >... 

The results of two investigations from two diverse locations provided evidence that this distribution ratio might be due to reversible sorption-desorption reactions. Scientists at Argonne National Laboratory (ANL) equilibrated filtered Lake Michigan water with sediments that had been contaminated with Pu ten years earlier. As many as nine extractions on the same portions of sediment produced essentially identical values(15). Noshkin has observed that the concentrations of plutonium in the waters of Enewetak remain relatively constant although there is a continual replacement of water at an estimated residence time of 144 days (16). The concentration corresponds well with that predicted from the experimental distribution ratio between lagoon sediment and water. 

The first seven cycles of an in situ iron removal project in The Netherlands were simulated with the hydrogeochemical transport model PHREEQC (version 2). The concentration changes of CH4, NH/, Mn, Fe , P04 and As are discussed in detail. Arsenic shows concentration jumps in pumped groundwater which are related to oxidation/reduction and sorption/desorption reactions resulting from the water quality variations. 

Many reactions on surfaces of soils and their constituents are extremely lapid—occurring on microsecond and millisecond time scales. Examples of these include some cation and anion sorption/desorption reactions, ion-exchange processes, reactions involving hydrolysis of soil minerals, and complexation reactions. 

Rhue, R. D. and W. C. Harris. 1999. Phosphorus sorption/desorption reactions in soils and sediments. In... 

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. 

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. 

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. 

Temperature has a marked effect on the kinetics of reaction rates of solid phase sorption/desorption processes [113-116]. Arrhenius noted the following relationship between k and T (Eq. 52) ... 

The Elovich model was originally developed to describe the kinetics of heterogeneous chemisorption of gases on solid surfaces [117]. It describes a number of reaction mechanisms including bulk and surface diffusion, as well as activation and deactivation of catalytic surfaces. In solid phase chemistry, the Elovich model has been used to describe the kinetics of sorption/desorption of various chemicals on solid phases [23]. It can be expressed as [118] ... 

Wolfe (1989) suggested a model to describe abiotic reduction in sediments, where a nonreactive sorptive site and an independent reactive sorptive site are considered. The nonreactive sorptive sink is consistent with partitioning of the contaminant to the organic carbon matrix of the solids. The model is described by Fig. 13.5 where P S is the compound at the reactive sorbed site P is the compound in the aqueous phase S and S are the sediments, P S is the compound in the nonreactive sink k, k , k , and k are the sorption-desorption rate constants, and k, k, and k are the respective reaction rate constants. If the reaction constants k and k are neglected, two rate-limiting situations are observed transport to the reactive site and reduction at the reactive site. The available kinetic data, however, do not allow one to distinguish between the two mechanisms. 

Another important consideration in investigation of the reaction of sorbed pesticides is the nature of the sorption process itself. Sorption/desorption kinetics and the physicochemical characteristics of the pesticide molecules in the sediment-sorbed state can be expected to influence the kinetic observations made in experimental systems. 

Sorption/desorption processes involving the substrate and availability of active reaction sites on the iron surface... 

For sorption/desorption studies, the sorption reaction is followed by monitoring the increasing concentration of leachate with time. At an apparent equilibrium, the effluent concentration equals that of the initial sorptive solution. The desorption reaction is studied in a similar way, the reaction being followed by monitoring the decreasing concentration of the previously sorbed ion or other sorbate. In either case, the reaction is followed by determining the sorptive concentration in solution. This means that any process that effects a change in concentration will be interpreted as adsorption or desorption. 

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-... 

Radionuclide transport in natural waters is strongly dependent on sorption, desorption, dissolution, and precipitation processes. The first two sections discuss laboratory investigations of these processes. Descriptions of sorption and desorption behavior of important radionuclides under a wide range of environmental conditions are presented in the first section. Among the sorbents studied are basalt interbed solids, granites, clays, sediments, hydrous oxides, and pure minerals. Effects of redox conditions, groundwater composition and pH on sorption reactions are described. 

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