NC0004979_Interaction of Selenium_20040609

imap://[e-mail] t,et:143...
Subject: Interaction of Selenium w/ Ferric or Ferrous
From: John J Kim <[e-mail]>
Date: Wed, 9 Jun 2004 10:21:03 -0400
To: [e-mail]
CC: Donald L Scruggs <[e-mail]>, Robbin M Jolly <[e-mail]>,
Ronald E Lewis <relewisl @duke-energy.com>
Thanks for coming to Allen Steam Station. Sorry for not being able to be
with you at the meeting.
We have been using ferric ions ( in sulfate form) to reduce the selenium
in our ash basin discharge. The exact chemistry of removal mechanism(s) is
still not well defined. Because of that, the term, "coprecipitation" is
used loosely to describe the mechanism. Ferric compounds such as ferric
sulfate and ferric chloride reacts with water to produce hydrated ferric
oxide species ( Fe3+ + nH2O = Fe00H.nH2O), which are referred to as "floc"
in water treatment. As this solid floc starts to form and grow in size,
it provides the large active sites for selenium to interact with. The
removal mechanism can be both chemical reactions (iron -selenium complexes)
and surface phenomenon (adsorption). The fact that the selenium removal
efficiency is strongly pH dependent suggests the important role of the
surface charge of these ferrihydrite solids. Good selenium reduction can
be achieved under a slightly acidic condition (pH 3.5 - 5.5), The removal
efficiency drops off considerably at pH above 6. At a high pH, selenium
in solid particles leaches out. This is why we have high selenium levels
in our flyash sluice water when we burn high alkaline coal.
Selenium is present in natural waters as anions (oxy - or oxo compounds)
and is soluble at any pH. As such, it can not be removed with pH
adjustment alone (as the case of many toxic metal ions). Typically,
selenite(Se032-) and selenate Se042-) are the forms of selenium present in
our flyash sluice water. Selenite ions have a high affinity to the solid
surface such as ferrihydrite, while selenate ions exhibits a higher
affinity to anion ion exchange resins. So, it is advantageous to convert
selenate to selenite for the iron coprecipitation process.
Ferrous and ferric compounds are used in water treatment to remove the
suspended solids. However, in our effort, we will be using ferrous ions
to help convert selenate to selenite, or even to selenium element to some
extent. We will be conducting a series of testing to find out the
effectiveness of ferrous ions relative to ferric ions for removing selenium
from the sluice water. Below are the equations describing the various
reactions;
Coagulation: Fe2+ + 2H2O = Fe(OH)2 (s)+ 2H+ --- Eq. 1
Selenate Reduction to Selenite: Fe2+ + Se042- + H2O = Fe3+ + Se03
2- + 2H+ + 02 -- Eq 2
Selenate Reduction to Elemental Selenium: Fe2+ + Se042- + 2 H2O = Se0
+ Fe00H (s) + 3H+ + 202 --- Eq 3
Factors such as oxygen in water, reduction reaction time, temperature,
coagulation time will affect the conversion.
I hope this answers your question. Please, feel free to call me
(704-756-0559) if you have additional questions or need clarification.
1 of 2 6/9/2004 10:31 AM

imap://[e-mail].143...
2 of 2 6/9/2004 10:31 AM

Duke Power Company
In -Progress Report:
Emergency Response to
Elevated Ash Basin Selenium
Presented by: Robbin Jolly
Allen Steam Station
Belmont, North Carolina

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Ash Basin Se Control Chart
Jan. 2002 - July 2003
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Sequence Number
�- SPC Average LCL CL UCL Compliance Value
30 32

Ash Basin pH vs Selenium
Selenium (ppb) pH
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19 -Apr 29 -Apr 9 -May 19 -May 29 -May 8 -Jun 18 -Jun 28 -Jun 8 -Jul
Date
■ Selenium (ppb) ♦ pH
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5.0
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Recovery Plan
-Stop suspected coal source.
-The released selenium in the basin would
have to be removed chemically:
coagulate/floc/filter
- Treatment would have to be in a controlled
environment.
- Treatment would have to go to the tower
for blending

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Initial Selenium Concentration 43 ppb
Initial Solution pH 9.23 -- - --
Filtered after 5 min agitation upon addition of 15 ppm of FeCl3 61-12O (-20% as Fe)

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Treatment Lines
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Allen Ash Basin Selenium Results
60 °
55
50
45
40
35
30
25
20
15
10
90 /°
85%
80%
75%
70%
65%
60%
55%
50%
45%
40%

Fl