What Happens to Se in the Sediment?
|In Clue #3 we saw that selenate
is changed to immobile forms that tend to remain in the sediment. In relation
to our main concern, keeping Se out of the food chain, would this be good
Unfortunately, Clue #3 doesn't explain everything. Even in environments that favor selenite and elemental Se, some selenite should change back to selenate. In other words, selenate, selenite, and elemental Se should exist in equilibrium, with some atoms continuouosly changing form. In the contaminated marsh, however, there was far less selenate than the model predicted.
Scientists used instruments like the ALS to find out more. Take a look at the following image:
The various colors indicate the locations of Se, with different colors representing different concentrations of Se. Using the color key to the right of the map (labeled "Se concentration") what can you conclude about the distribution of Se in the sample
Why do you think the Se is distributed unevenly? Does this shed any light on the problem of there being too little selenate?
Here is another SXRFM map:
Although this map is three-dimensional (3-D), the information on it is similar to the two-dimensional (2-D) SXRFM image we have already seen: it represents the spatial distribution of Se in a small area of marsh sediment. On this map the areas of high Se concentration appear as peaks as well as colors, with one large peak dominating.
There is one crucial difference between this image and the previous one: the sample used was prepared artificially, as follows:
Look at the vertical scale in the 3-D SXRFM image. It identifies an initial concentration of Se of about 70 mg/kg (70 parts per million). The concentration of Se was initially the same throughout the sample, then changed over the incubation period until a large peak of Se had accumulated near the root.
What made the Se collect in one place
How did the Se move toward the microsite
After the SXRFM image was made, the soil sample was allowed to sit a while longer, and Se continued to collect at the microsite, finally growing to a visible spot about a millimeter in diameter, the orange spot in this photo:
The 3-D SXRFM experiment shows that decomposing vegetation could lead to the uneven distribution of Se revealed in the 2D SXRFM image.
Does the microsite model suggest a reason for the unexpectedly low levels of selenate found after a week or so?
Consider some familiar examples of dissolving solids. For instance, a sugar cube takes longer to dissolve in a cup of tea than granulated sugar, and a lifesaver lasts longer if you suck on it than if you chew it. Do you know why
Now, can you relate this to the rate of dissolution of selenium at microsites
To try an activity that demonstrates diffusion, click the button: