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Selenium Sources and Treatment


• Selenium Overview

• Selenium Characteristics

• Selenium Refinery Implications

• Selenium Treatment Techniques

• Examples


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


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


Selenium Characteristics


TABLE 1

SELENIUM CHEMISTRY

Selenium

Compound

Oxidation

State

Selenide (Se-2) -2

Elemental Selenium (Se0) 0

Selenite (SeO3-2) +4

Selenate (SeO4-2) +6



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


Selenium Characteristics Selenium Speciation


Selenium Refinery Implications


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


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


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


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


Selenium Refinery Implications Selenium Follows Sulfur Through Refinery


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


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


Treatment Considerations for Selenium Reduction


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


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


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


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


Attention will focus on the most common techniques

Each technique has limitations and the challenges from

selenium speciation

Treatment Techniques


Selenium Speciation

Selenate

Selenite

Selenocyanate


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


Iron Co-Precipitation

ZVI (Zero Valence Iron)

Bioremediation

Treatment TechniquesPredominant Techniques to focus on


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 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



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



Zero Valence Iron


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)


Challenges include passavation or iron oxides (Fe2O3) forms

and prevents electron transfer

Results in poor selenium treatment results

Zero Valence Iron (ZVI)


Hybrid approach uses ZVI + ferrous salts and other reagents to

avoid passavation

Keeps the iron sites available

Hybrid - Zero Valence Iron (hZVI)


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


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


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


Aerobic Activated Sludge

Anoxic Fixed Film

Bioremediation


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


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


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


Biological Selenium Reduction

+200 mV -500 mV

O2 H2O NO3 N2 SeO4 Se SO4 HS- CO2 CH4


Anoxic System Design


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)


Selenium Precipitation


Most Effective Technologies


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


• 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


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


• Can follow chemical

pretreatment or aerobic

bioreactor

• Once established produces consistent results

• SeCN requires oxidation

• Produces sludge

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