Transcript Monitoring and Assessing Water Quality

Iron
WQT 134
Aquatic Chemistry II
Standard Methods 20th ed
#3500 Iron (#110)
Applied Water and Spent water Manual
Chapter 38
Lecture 6
Week 6 Objectives
Reading assignment:
American Public Health Association (APHA), American Water Works Association
(AWWA) & Water Environment Federation (WEF). 1999. Standard Methods for
the Examination of Water and Wastewater, 20th edition
Jackson. 1993. Applied Water and Spentwater Manual.
1. Understand the role and function of
Iron in water treatment.
2. Understand how to measure Iron (SM
#3500, #110)
3. Comprehend iron concentrations in
nature.
Iron #3500, UNKNOWN
SAMPLE
Total Iron: The amount of ferrous (Fe+2) and ferric iron (Fe+3)
in a sample. Determined using a colorimeter.
Measure 50 ml of your unknown sample into a
50 ml erlenmeyer flask and BOIL IT!
Add 2 ml concentrated HCl (use pipette)
Add 2 ml hydroxylamine hydrochloride
solution
Cool to room temperature and pour 38
ml into 50 ml volumetric flask
Add 10 ml ammonium acetate buffer
Add 2 ml phenanthroline solution
Dilute to mark with water
Digestion
Process
Color
Development
Iron #3500, Standard CURVE
To make your Fe Std Curve:
Measure X ml of 10 mg/L Fe in 50 ml volumetric flask
Add 10 ml ammonium acetate
buffer
Add 2 ml phenanthroline solution
Dilute to 50 ml mark with Boiled DI water
Digestion
Process
Color
Development
Bring to
Volume
Iron #3500, #110
Spectrophotometer: Instrument that measures the intensity
of the light (Fe 510nm) entering a sample and the light exiting
a sample and compares the two intensities. Information about
the two intensities can be expressed as transmittance (the
ratio of the intensity of the exiting light to the entering light)
or percent transmittance (%T) or absorbance.
A=εlc
Figure 3. Total Fe analysis at 510 nm
Iron #3500, #110
Beer’s Law : (physical chemistry) The law which states that
the absorption of light by a solution changes exponentially
with the concentration, all else remaining the same.
“As concentration goes up so does absorbance”
Iron #3500, #110
Beer’s Law :
A=εlc
The quantity ε is the molar absorptivity; in older literature it is sometimes called the
extinction coefficient. The molar absorptivity varies with the wavelength of light used in the
measurement. The absorption spectrum is sometimes displayed in the form ε vs λ rather
than A vs λ.
•If T = 30%, then 30% of the photons passing through
the sample reach the detector and the other 70% are
absorbed by the analyte.
•The absorbance is a slightly less intuitive quantity. If
A = 0, then no photons are absorbed. If A = 1.00, then
90% of the photons are absorbed; only 10% reach the
detector. If A = 2.00, then 99% of the photons are
absorbed; only 1% reach the detector
Factoids
Iron #3500, #110
• Iron (Fe) is the first element in Group VIII of the periodic
table
• atomic number of 26, atomic weight of 55.85
• Common valences of 2 and 3; solubility of ferrous ion
(Fe2+) is controlled by the carbonate concentration.
• Average Fe in the earth’s crust is 6.22%
• Soils Fe ranges from 0.5 to 4.3%
• Streams averages ~0.7 mg/L
• Groundwater averages 0.1 to 10 mg/L. Iron
• Minerals hematite, magnetite, taconite, and pyrite.
Iron #3500, #110
Factoids
• Ferrous ion (Fe2+) is soluble form in nature
• On exposure to air or addition of oxidants, ferrous iron is
oxidized to the ferric state (Fe3+) and may hydrolyze to
form red, insoluble hydrated ferric oxide
Pipe Corrosion/Fe staining/Fe in GW
~20,000 gpd
Iron #3500, #110
flow direction
Day 1
Day 2
Day 4
Day 16
Iron #3500, #110
goethite
low
BC Column
Final Pore Water
magnetite
1
medium
Fe(II)aq
siderite
2
inhibitors
green rust
Zachara et al., (2001) proposed that dissolved
Fe(II) concentrations control secondary solid
phase products of Fe oxide reduction.
Flow Direction
high
3
4
5
dark
orange
6
7
0
5 10 15 20 25
-1
Iron #3500, #110
Iron #3500, #110
What is Iron?
• Iron occurs in the minerals hematite(Fe2O3),
magnetite (Fe3O4), ferrihydrite (FeOOH),
aresenopyrite (FeAsS), siderite (FeCO3),
aluminosilicates, and pyrite (FeS). It is widely used
in steel, alloys, as well as for environmental
remediation of nitrates and arsenic.
Why do we care?
•Iron in residual waters after treatment can cause
laundry and fixture staining, corrosion, rust, odor
and aesthetic taste problems and iron bacterial
blooms in drinking water.
Iron #3500, #110
How is it done?
1. Iron in your unknown sample is brought into solution,
reduced to the ferrous state by boiling with acid and
hydroxylamine, and treated with 1,10-phenanthroline at pH
3.2 to 3.3. Three molecules of phenanthroline chelate each
atom of ferrous iron to form an orange-red complex.
2. The colored solution obeys Beer’s law; its intensity is
independent of pH from 3 to 9. Thus, sample is run at 510
nm on a colorimeter
3. A set of standards is run along with unknown sample and a
blank to determine Fe concentration.
4. Plot absorbance or percent transmission (on the vertical or
y-axis) vs. iron concentration (on the x or horizontal axis)
in mg/l as a linear graph.
5. Determine unknown Fe concentration
Iron #3500, #110
What are advantages of the procedure?
•Accuracy of dissolved or total concentrations
of iron as low as 10 μg/L can be determined
with a spectrophotometer using cells with a 5
cm or longer light path.
•No pretreatment necessary for well or
potable water samples
Iron #3500, #110
What are disadvantages of the procedure?
•Preliminary treatment is a must for waste or organic rich
stream water
•Cyanide, nitrite, and phosphates (polyphosphates more so
than orthophosphate), chromium, zinc in concentrations
exceeding 10 times that of iron, cobalt and copper in excess
of 5 mg/L, and nickel in excess of 2 mg/L.
•Bismuth, cadmium, mercury, molybdate, and silver
precipitate phenanthroline.
•Color or organic matter may necessitate digestion before
use of the extraction procedure.
Iron #3500, #110
What are typical values in nature?
•The United Nations Food and Agriculture
Organization recommended level for irrigation
waters is 5 mg/L.
•The U.S. EPA secondary drinking H2O MCL is
0.3 mg/L.
Iron #3500, #110
What are the units and conversions?
•Fe in mg/l
Calculations and Formulas?
mg/L as Fe = mg/l as read from standard curve
mg/L as Fe = (curve value)(dilution factor)
Iron #3500, #110
Example Problem?
If 25 ml of sample has been diluted to 100 ml, and then 50 ml
of this solution was used for analysis, the dilution factor to
multiply times the curve value (which is in terms of
concentration- would be 4.0) Suppose the diluted sample
produced an absorbance equivalent to 0.145 mg/L then the
actual concentration would be:
mg/l as Fe = (0.145)(4)= 0.580 mg/l as Fe
Iron #3500, #110
Tips and Suggestions?
1. If It says Fume Hood….. Keep it in the Fume Hood!!
2. 36 N Sulfuric acid will burn on contact, wear gloves!
3. Let spectrophotometer warm up a bit to 510 nm
4. Run all samples in succession
5. Take your time and try to learn/play with making a
graph in Excel; its a great tool to have.
6. Wipe the Cuvette when using the
Spectrophotometer!
7. Check for Spec drift by reanalyzing your blank and
readjusting.
Water containing high iron is
objectionable in a public water
supply because:
no
.
...
ss
n
iro
d.
as
co
nt
en
ir o
n
th
ca
u
se
st
a
ill
w
c.
ex
ce
ce
ex
b.
a.
ex
ce
ss
ss
ir o
n
iro
n
w
ill
sc
a.
..
.
a. excess iron will scale pipes
b. excess iron will stain plumbing
fixtures
c. excess iron causes “baby blue”
syndrome
d. iron content has no effect on
the water supply
..
25% 25% 25% 25%
Red water may be caused by iron
concentrations above:
m
g/
L
0.
3
d.
c.
0.
1
m
g/
L
m
g/
L
0.
03
b.
01
0.
a.
a. 0.01 mg/L
b. 0.03 mg/L
c. 0.1 mg/L
d. 0.3 mg/L
m
g/
L
25% 25% 25% 25%
Dissolved iron in excessive
amounts results in consumer
complaints about:
di
ty
tu
rb
i
sm
el
l
d.
co
b.
c.
rr
os
es
s
rd
n
ha
a.
a. hardness
b. corrosion
c. smell
d. turbidity
io
n
25% 25% 25% 25%
Which of the following
chemicals will most likely keep
iron in suspension?
te
d.
po
c.
rm
pe
ta
s
po
si
um
ly
p
a.
..
ha
ho
sp
b.
a.
ch
lo
rin
e
a. chlorine
b. lime
c. polyphosphate
d. potassium permanganate
lim
e
25% 25% 25% 25%
Before iron, manganese, and
hydrogen sulfide can be
removed by filtration they
must first be converted to:
...
gh
th
ro
u
or
fr
e
es
as
G
A
n
od
th
ro
u
G
as
es
e
so
lu
bl
e
gh
pr
ec
...
...
Insoluble precipitates through oxidation
Gases through flash mixing
An odor free state through settling
Gases through adequate detention time
In
a.
b.
c.
d.
s.
..
25% 25% 25% 25%
Unlike most surface waters,
ground waters may need to
have these materials removed
n.
..
fte
ni
So
n
an
d
ng
co
ag
e
pp
e
...
m
an
Iro
pe
r
op
C
Iro
n
an
d
an
d
m
an
ga
...
1. Iron and manganese
2. Copper and manganese
3. Iron and copper
4. Softening agents
...
25% 25% 25% 25%
Iron and manganese may be
removed from source waters
by oxidation. The following
chemicals are often used to
precipitate iron. Check all
that apply
1
on
ly
an
d
2
ro
x
i..
.
lu
m
hy
d
A
di
um
So
m
...
pe
r
hl
or
in
e
si
um
C
ta
s
Po
1. Chlorine
2. Potassium permanganate
3. Alum
4. Sodium hydroxide
5. 1 and 2 only
20% 20% 20% 20% 20%