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Page 1Selenium Sources and Treatment
Selenium Sources and Treatment
• Selenium Overview
• Selenium Characteristics
• Selenium Refinery Implications
• Selenium Treatment Techniques
• Examples
Page 2Selenium Sources and Treatment
Elemental Se is relatively non-toxic and considered an essential
trace element
Se is bioaccumulative in aquatic organisms
Narrow range between Se deficiency and toxicity
Selenium Overview
Page 3Selenium Sources and Treatment
Sources
• Drainage water, especially subsurface waters
Selenium, salinity, other trace elements
• Mine drainage
• Coal-based industries, i.e., coke plants, coal-fired power plants,
FGD wastewater
• Oil-based industries, i.e., petroleum refineries
Selenium Overview
Page 4Selenium Sources and Treatment
Selenium Characteristics
Page 5Selenium Sources and Treatment
TABLE 1
SELENIUM CHEMISTRY
Selenium
Compound
Oxidation
State
Selenide (Se-2) -2
Elemental Selenium (Se0) 0
Selenite (SeO3-2) +4
Selenate (SeO4-2) +6
Selenium Characteristics
Page 6Selenium Sources and Treatment
Selenium Characteristics
Page 7Selenium Sources and Treatment
Total Selenium
• Hydride generation atomic adsorption spectrophotometry
(HGAAS)
Quantification of Se species
• Ion chomatography w/inductively-coupled plasma
spectrophotometry/mass spectroscopy (HPLC-ICP-MS)
Selenium Characteristics Speciation and Analytical Techniques
Page 8Selenium Sources and Treatment
Selenium Characteristics Selenium Speciation
Page 9Selenium Sources and Treatment
Selenium Refinery Implications
Page 10Selenium Sources and Treatment
Selenium in wastewater is a problem for some refineries
Se treatment: not much attention until recently
Current limits vary by location and permit
Selenium Refinery Implications Background
Page 11Selenium Sources and Treatment
Carbon (84 - 87 Wt%)
Hydrogen (11 - 14 Wt%)
Sulfur (0 - 5 Wt%)
Nitrogen (0 - 0.2 Wt%)
Other Elements (0 - 0.1 Wt%)
• Oxygen
• Nickel
• Vanadium
• Selenium
• Salts
Crude Oil Composition
Page 12Selenium Sources and Treatment
Selenium enters the refineries in crude oil
Selenium is hard to remove from wastewater
Selenium removal efficiency depends on the chemical forms
(species) that are present
Selenium Refinery Implications Background
Page 13Selenium Sources and Treatment
Crude Name Se (ppb)
Crude #1 1170
Crude #2 497
Crude #3 325
Crude #4 313
Crude #5 260
Crude #6 224
Crude #7 196
Crude #8 131
Crude #9 76
Crude #10 71
Crude #11 57
Crude #12 52
Crude #13 49
Crude #14 41
Crude #15 20
Crude #16 20
Crude #17 <5
Crude #18 <5
Crude #19 260
Selenium Refinery Implications Total Selenium Results
Page 14Selenium Sources and Treatment
Selenium Refinery Implications Selenium Follows Sulfur Through Refinery
Page 15Selenium Sources and Treatment
Selenium will distribute itself into products and waste
streams similar to sulfur.
Can only leave the refinery:
• In products with Se concentrating in the heavier products rather
than the lighter, like sulfur
• In the petroleum cokes, if there is a coker present
• In the FCCU flue gas or the solids stream if there is wet gas
scrubber
• In the wastewater
There is basically no other way to escape the refinery!
Selenium Refinery Implications Where Selenium Ends Up
Page 16Selenium Sources and Treatment
Primary locations for selenium in refineries.
• Sour Water Strippers – bottoms
• Crude preparation – ex desalters
Selenium species under consideration
• Selenocyanate – (SeCN)
• Selenite – (SeO3-2)
• Selenate – (SeO4-2)
Selenium speciation is needed to optimize water treatment
Selenium Refinery ImplicationsSpeciation
Page 17Selenium Sources and Treatment
Treatment Considerations for Selenium Reduction
Page 18Selenium Sources and Treatment
A patent review will show that there have been many ideas on
how to remove selenium
ENVIRON has been actively involved in evaluating many of
these techniques
Attention will focus on the most common techniques
Each technique has limitations and the challenges from selenium
speciation
Treatment ConsiderationsMethodology
Page 19Selenium Sources and Treatment
Basically, all treatment technologies have been applied to:
• Stripped Sour Water; or,
• Total Refinery Effluent
Treatment upstream of SWS considered impractical due to
high nitrogen and sulfur concentrations.
Treatment ConsiderationsMethodology
Page 20Selenium Sources and Treatment
Bioremediation: Aerobic and Anoxic
Chemical Oxidation-Iron Treatment
• Chemical Reduction/Oxidation/Precipitation
• Sodium Metabisulfite w/ Cu
• H2O2, O3, Air
• Iron: Ferric, Ferrous
• ZVI (Zero Valence Iron)
Reverse Osmosis
• Not feasible due to high reject load and need for disposal
Methodologies ENVIRON has
investigated
Page 21Selenium Sources and Treatment
Ion Exchange
• Not feasible due to salt and organic competition for sites.
Granular Activated Carbon
• Total removals = 31- 91% removal
• Carbon consumption high due to other organics present
Wetlands
• Total removals = 43-90% removal
Methodologies ENVIRON has
investigated
Page 22Selenium Sources and Treatment
Attention will focus on the most common techniques
Each technique has limitations and the challenges from
selenium speciation
Treatment Techniques
Page 23Selenium Sources and Treatment
Selenium Speciation
Selenate
Selenite
Selenocyanate
Page 24Selenium Sources and Treatment
Selenate (SeO4-2) – more soluble and not readily removed,
must be reduced
Selenite (SeO3-2) – more insoluble, can be removed by
adsorption on ferrihydrite
Selenocyanate (SeCN) – most reduced form, can be precipitated
with Cu salts or oxidized to selenite
Selenium Species Characteristics
Page 25Selenium Sources and Treatment
Iron Co-Precipitation
ZVI (Zero Valence Iron)
Bioremediation
Treatment TechniquesPredominant Techniques to focus on
Page 26Selenium Sources and Treatment
Iron Co-Precipitation
Page 27Selenium Sources and Treatment
Iron Co-Precipitation
Page 28Selenium Sources and Treatment
Selenite removed by adsorption on ferrihydrite solid
Selenate is more soluble and not readily removed. Must be
reduced. Can be done by the addition of Fe2+. Fe2+ to Fe3+
and followed by adsorption on ferrihydrite floc
Selenocyanate (SeCN) must first be oxidized to selenite and
then adsorbed by ferrihydrite solids
Iron Co-Precipitation
Page 29Selenium Sources and Treatment
Iron dosages in the range of 50-200 mg/L
Sulfate hinders removal of selenate.
Negative selenite ion is adsorbed on positive iron floc
(ferrihydrite).
Increasing Fe improved Se removal, but with diminishing return
Optimum pH range for Se reduction is 5.5-6.5
Can achieve treated concentrations of Se in range of 20 to 50
ppb
Iron Co-Precipitation
Page 30Selenium Sources and Treatment
ENVIRON has successfully employed a catalyst (Cu) to
enhance reduction and achieve a lower range of effluent
concentrations
Oxidation/reduction reaction using copper catalyst a
reducing agent and aeration
Achieved Se <30 ppb in steel coke wastewater application
Iron Co-Precipitation
Page 31Selenium Sources and Treatment
Zero Valence Iron
Page 32Selenium Sources and Treatment
ZVI – Fe0, elemental iron
Produces electrons as iron oxidizes:
• Fe0 = Fe+2 + 2e- = Fe+3 + 3e-
Treats metals by two techniques
• Galvanic displacement
Cu+2 + Fe0 = Cu0 + Fe+2
• Co-precipitation
Zero Valence Iron (ZVI)
Page 33Selenium Sources and Treatment
Challenges include passavation or iron oxides (Fe2O3) forms
and prevents electron transfer
Results in poor selenium treatment results
Zero Valence Iron (ZVI)
Page 34Selenium Sources and Treatment
Hybrid approach uses ZVI + ferrous salts and other reagents to
avoid passavation
Keeps the iron sites available
Hybrid - Zero Valence Iron (hZVI)
Page 35Selenium Sources and Treatment
ZVI most prevalent in mining. Much of original research
came from mining
• Results shows that ZVI can achieve effluent Se = 5-15 ppb.
• Results a function of media mass and HRT
• Residence time in hours, working towards minutes
• Nitrate interfere
ZVI – Results
Page 36Selenium Sources and Treatment
ZVI in FGD wastewater
• Results shows that ZVI can achieve effluent Se <10 ppb
• Residence time is in terms of hours
• Many interferences
ZVI – Results
Page 37Selenium Sources and Treatment
ZVI in Refinery wastewater
• Selenocyanate is removed from SWS w/o the need for
peroxidation to selenite/selenate.
<2 ppb seen w/ no preoxidaiton.
• Amines reduce removal rate.
• Total Se <10 ppb can be achieved, but highly variable.
• Currently HRT is 2-4 hours. Staged approach used to reduce
HRT.
ZVI – Results
Page 38Selenium Sources and Treatment
Aerobic Activated Sludge
Anoxic Fixed Film
Bioremediation
Page 39Selenium Sources and Treatment
Aerobic biological treatment appears to be an assimilative
reduction process
Has shown to oxidize selenocyanate to selenite
Some refineries have shown 20-95% removal of Se through
biological systems
Soluble forms of Se observed in bio-effluent believed to be
soluble organic Se compounds or colloidal selenite.
Achieves 50-100 ppb total Se in effluent
Activated Sludge
Page 40Selenium Sources and Treatment
Selenite/Selenate are microbially reduced to elemental Se under
reducing conditions in presence of an organic food source
Relies on selenium-reducing cultures and controlled
environmental conditions to reduce selenium to Se0
In carbon deficient waters, a supplemental COD source is
required
Can achieve treated concentrations of Se in range of 5 to 10 ppb
Anoxic Fixed Film
Page 41Selenium Sources and Treatment
Biological Treatment
Organic Carbon
CO2 + Biomass
O2 NO3 SeO4 SO4 CO2
H2O N2 Se H2S CH4
Electron Transfer
Bacterial Cell
Decreasing Energy Yield
Electron Donor Electron Acceptor
Page 42Selenium Sources and Treatment
Biological Selenium Reduction
+200 mV -500 mV
O2 H2O NO3 N2 SeO4 Se SO4 HS- CO2 CH4
Page 43Selenium Sources and Treatment
Anoxic System Design
Page 44Selenium Sources and Treatment
1
16
31
46
61
76
91
106
121
136
151
166
0
500
1000
1500
2000
2500
[Se]
ug
/L
Days
Selenium Removal
Reactor 1
Reactor 2
Reactor 3
Influent
Reactor
Start Up
11 8 hr5.5
EPA Pilot Test(Mine Waste Technology Program)
Page 45Selenium Sources and Treatment
Selenium Precipitation
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Most Effective Technologies
Page 53Selenium Sources and Treatment
Iron Treatment
• Achieves Se <30 ppb
• Can pretreat if Se >500 ppb
• Can polish if Se <100 ppb
• Ferrous compounds
• Slightly acidic conditions
• Co-precipitation of Se with ferric hydroxide
• Catalyst enhances
• Produces sludge
Treatment ConsiderationsSelected Technologies
Zero Valence Iron
• Achieves Se <10 ppb
• Can pretreat if Se >500 ppb
• Can polish if Se <100 ppb
• Slightly acidic conditions (mine)
• Some products require chemical addition to prevent passavation
• High HRT
• Produces sludge
Page 54Selenium Sources and Treatment
Aerobic Bioreactor
• Achieves 50 ppb
• Combined with Chemical
Treatment for high strength Se
wastewaters (>500 ppb)
• Can combine chemical
treatment within bioreactor
• Se is oxidized in aeration basin
and part is trapped in biofloc.
Treatment ConsiderationsSelected Technologies
Anoxic Bioreactor
• Achieves Se <5 ppb
• Can follow chemical
pretreatment or aerobic
bioreactor
• Once established produces consistent results
• SeCN requires oxidation
• Produces sludge