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 or bad

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:

This is an SXRFM map of a sample of sediment taken from the marsh. It is a map of a very small area of marsh sediment. Look at the scale on the x and y axes. How large is the area represented

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:

  1. Soil from an uncontaminated marsh pond was carefully prepared to be of uniform consistency, with about 1% organic matter.
  2. Several samples of this soil were made up and wetted with water containing selenate.
  3. Some samples were then embedded with sections of root from a common wetland plant.
  4. The samples were covered to prevent oxygen from entering and allowed to sit for 6 to 17 days.
  5. After this incubation period they were mapped using the SXRFM.
Do you see any resemblances between this experiment and the activity for Clue #3? Can you guess what this experiment is trying to determine

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:

CLUE #4: The disappearance of selenate from the water can be explained by the action of microorganisms, which reduces the selenate to the insoluble forms selenite and elemental Se. Study of actual sediment suggests that these insoluble forms of Se accumulate in microsites rather than as a uniform layer on the sediment. SXRFM mapping identifies decaying vegetation as a possible cause of these microsites. Since the microsites present less surface area to the water than a uniform layer of Se, conversion back to selenate is slow, accounting for the low levels of selenate in the pond water.

Wetlands Introduction

Clue #1

Clue #2

Clue #3

Putting the Pieces Together


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