Thin disk flow method

Adsorption kinetics of Zn on the untreated soil and the soils with the removal of organic matter and Fe oxides has been studied in detail by Hinz and Selim (1999) using a thin disk flow method. They reported that Zn sorption was highly concentration-dependent. When Zn concentration was... 

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

Fig. 2-6. Thin-disk flow method experimental setup. Background solution and. solute arc pumped from the reservoir through the thin disk and are collected as aliquot.s by the friicli< ti collector.

The experimental setup for this method is shown in Fig. 2-7. It is identical to the setup for the thin-disk method except that the stirred-flow reactor is used in place of the thin disk. Flow through the reactor is maintained ul a constant velocity by a pump, and a fraction collector is again used to collect reactor effluent. 

These relative surface excess parameters can be determined experimentally using different methods /)(w) can be determined by radiotracer studies, o-measurements, electroanalytical techniques (twin-electrode thin-layer, flow-through thin layer, rotating ring-disk experiments) etc., whereas q can be determined by charging curves, capacitance measurements etc. Isotherm conversion q-Fiiyf) is obtained by the corresponding Maxwell relations ... 

This method is also referred to as the miscible-displacement or continuous-flow method. In this method a thin disk of dispersed solid phase is deposited on a porous membrane and placed in a holder. A pump is used to maintain a constant flow velocity of solution through the thin disk and a fraction collector is used to collect effluent aliquots. A diagram of the basic experimental setup is shown in Fig. 2-6. A thin disk is used in an attempt to minimize diffusion resistances in the solid phase. Disk thickness, disk hydraulic conductivity, and membrane permeability determine the range of flow velocities that are achievable. Dispersion of the solid phase is necessary so that the transit time for a solute molecule is the same at all points in the disk. However, the presence of varying particle sizes and hence pore sizes may produce nonuniform solute transit times (Skopp and McCallister, 1986). This is more likely to occur with whole soils than with clay-sized particles of soil constituents. Typically, 1- or 2-g samples are used in kinetic studies on soils with the thin disk method, but disk thicknesses have not been measured. In their study of the kinetics of phosphate and silicate retention by goethite, Miller et al. (1989) estimated the thickness of the goethite disk to be 80 /xm. 

Stirred-flow reactors retain all the advantages of flow methods in general and eliminate all the problems associated with the thin-disk method. They also retain many advantages of the batch method. Reaction products desorbed into solution are continually removed. Film or particle diffusion is reduced or eliminated by mixing within the reactor. Direct control with feedback of reactor conditions is possible. Although stirred-flow reactors normally have much smaller volumes than conventional batch reactors, it is possible to construct a pH-stat stirred-flow reactor by inserting a microcombination pH electrode and microburette tip into the reactor. The O-rings can provide leak-proof seals where the electrode and burette tip enter the reactor. The electrode and burette tip are connected to an autotitrator as in the conventional batch reactor. 

As with the batch reactor method, radiotracers are an excellent means of following the extent of kinetic reactions in flow methods. Flow methods using radiotracers are identical to those where other analytical methods are used to determine the solute of interest except that a radiolabeled solute is used. Radiotracers have been used numerous times in column transport studies but apparently have not as yet been used with thin-disk and stirred-flow methods. 

It is clear that most of the limitations with flow methods apply to the thin-disk method. Hybrid methods such as the stirred-flow and fluidized bed reactor combine the best features of batch and flow methods and eliminate or control many of the limitations of each. Future progress in the study of reaction kinetics in soils and soil constituents will most likely come from the use of hybrid batch-flow methods and from the use of relaxation methods where rapid chemical reactions can be studied. 

Equally disturbing is that sorption parameters (i.e., Langmuir sorption parameters) obtained from batch and thin-disk methods are different (Miller cl al., 1989). This was attributed to the fact that batch systems are closed whereas flow systems arc open so that the competitive antecedent solute species is removed, ll is apparent that much more work needs to be done on... 

The rate functions in Table 2-2 can be applied to batch kinetic data directly because no transport (flow) processes occur. In flow methods the appropriate transport equation must be coupled to the kinetic rate function to achieve a correct solution. Kinetic-rate functions cannot be applied directly. For the thin-disk method the transport equation (Skopp and McCallister, 1986) is... 

The dynamic method for surface force measurement [14,155,228] consists in deducing the dependence of the surface force n on film thickness h from film thinning experiments. If a liquid film (viscosity rj and radius r) is squeezed by a pressure difference Ap between two circular flat solid disks, then the Reynolds flow is applicable to film thinning (Section 3.2). 

To facilitate the measurement of the bulk conductivity, the toner powders were compressed into thin cylindrical disks under a hydrostatic pressure of 100,000 psi. Under these pressures the toner flowed readily and formed a uniform pellet in which the toner particle surfaces were in more intimate contact than in a loose powder. Gold electrodes were evaporated onto the flat faces and the current as a function of the applied electric field measured. This measurement method of the conductivity has certain inherent disadvantages, however, this measurement was intended to be used simply to contrast the kinetic conductivity values reported in Section IV, not to extract absolute values of the material parameters. Nonetheless, several samples of various thicknesses were prepared and measured for each type of toner and the results were found to be quite reproducible. 

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