Environmental Engineering Reference
In-Depth Information
A disk shape (diameter 40 mm and height 20 mm) agarose solid (2 wt%) containing
excess water was prepared by dissolving agarose in water on a hot plate at about 90 , and
then by cooling the hot solution up to room temperature (25 ) in a glass cell (figure 14).
5mm
5mm
20mm
20mm
40mm
40mm
Figure 14. The cell and the size of the solid to investigate diffusion of thionine dye in the solid
containing excess water [6].
The 2 wt% agarose solid was tight and elastic, the hardness being almost the same as a
brick cheese and one third of a conventional rubber eraser. An air was injected from a syringe
to make a round air bubble (diameter 5.0 mm) in the center of the solid, and the bubble was
filled with an aqueous solution of 5.0 mmoldm -3 thionine (3) by using a syringe. Three-
dimensional diffusion of the thionine was observed from the top of the cell, and the diffusion
distance was plotted against time at 25 (figure 15). As a reference, the same thionine
solution was injected by a syringe in the middle of the same shape liquid water (as in
figure14) in the absence of any stirring and flow, and the diffusion of the thionine was
observed.
In the liquid water the injected thionine became homogeneous in the water within 40 min,
while in the agarose solid its diffusion was remarkably slow as shown in the figure15.
The diffusion length of a substance (d cm) in a gas or liquid phase in the absence of any
other driving forces such as convection and flow of the liquid or gas is represented by the
equation (1), where D is the diffusion coefficient (cm 2 s -1 ) of the substance and t is time (s).
d= (6 D t) 1/2
(1)
The D values of thionine in a 2 wt% agarose solid and in an aqueous solution were
determined by cyclic voltammograms (as for the method see the reference [3]) to be 4.98×10 -
6 cm 2 s -1 (in the 2 wt% agarose solid) and 2.21×10 -6 cm 2 s -1 (in aqueous solution). The
diffusion coefficient in the solid is even larger than that in water showing that self-diffusion
of thionine should take place in the agarose solid the same as or even quicker than in a liquid
water if only self-diffusion contributes to the diffusion. However, as it was mentioned, the
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