What Happened to the Se in the Marsh?



  When scientists concluded that the wildlife casualties and deaths were the result of Se poisoning, the flow of waste water into the wetlands was stopped. However, water-soluble forms of Se are easily cycled through the environment. Plants take up Se through their roots, insects eat plants, and birds eat insects. Fish and their predators would also be exposed to Se. As long as there were substantial quantities of Se dissolved in the water, it would be difficult to prevent Se from entering the food chain.

The solubility of Se varies, depending on its form. Selenium has three common forms:

To summarize:

  • Selenate has a charge of (-2), with an Se ion of charge +6.
  • Selenite has a charge of (-2), with an Se ion of charge +4.
  • Elemental Se has a charge of 0.
Based on the preceding information, what form do you think most of the dissolved Se was in--

Scientists found that Se remained in solution for only about a week. Not long after flow into the marsh was stopped, there was very little dissolved Se remaining in the ponded water. A small amount was released as a gas, dimethyl selenite, which disperses harmlessly in the atmosphere. But most of the Se didn't go far. It changed forms, becoming selenite and elemental Se.

To see how elements change forms by gaining and losing electrons try the following experiment:

CLUE #3: When Se first entered the marsh, it was dissolved in the water as selenate. In this form it is very mobile and enters the food chain through plants and insects. The shortage of oxygen in the marsh creates an environment in which elements like Se can act as electron acceptors for microorganisms involved in decomposing organic material. While a small amount of the selenate (with a +6 Se ion) is given off as dimethyl selenite gas, most becomes selenite (with a +4 Se ion) or elemental Se (0 charge). These insoluble, immobile forms of Se collect in the sediment at the bottom of the pool and are not taken up by plants or easily diffused into the groundwater.

Wetlands Introduction

Clue #1

Clue #2

Clue #4

Putting the Pieces Together


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