Ion Removal
(Sep 5th, 2016)
by Dr. Arun Kumar ([email protected])
Objective: To learn about methods for removing
ions from water
September 11, 2016
Arun Kumar
([email protected])
1
Surface Water Treatment
Conventional Surface Water Treatment
• Screening (remove relatively large floating and
suspended debris)
• Rapid-mix (mixing water with chemicals that
encourage suspended solids to coagulate into
larger particles that will settle easily)
• Flocculation (gently mixing water and
coagulant allowing the formation of large
particles of floc)
Conventional Surface Water Treatment
• Sedimentation (flow is slowed enough so that
gravity will cause flocs to settle)
• Sludge processing (mixture of solids and
liquids collected from settling tank are
dewatered and disposed of)
• Disinfection (ensure that water is free of
harmful pathogens)
• Distribution system protection (residual
disinfection)
Ion types
• Types:
– Cations: calcium, magnesium, sodium etc.
– Anions: sulfate ions; chromate ions; nitrate, etc.
Hardness
• Total Hardness
– Technically - the sum of all polyvalent cations
– Practically - the amount of calcium and
magnesium ions (the predominant minerals
in natural waters)
– It is divided into carbonate and noncarbonate
hardness.
Description
Soft
Moderately hard
Hard
Very hard
Hardness range
(mg/L as CaCO3)
0 - 75
75 - 100
100 - 300
> 300
Hardness
Expressed as mg/L as CaCO3
Alkalinity
Alkalinity is often used describe a
water’s ability to resist pH changes
upon the addition of an acid. It
could also be called acidneutralizing capacity.
Expressed as mg/L as CaCO3
Alkalinity (mol/L) = [HCO3-] + 2 [CO32-] + [OH-] – [H+]
Alkalinity
Ion Removal
• Removal depends on concentration and their
valence states
• Methods:
– chemical precipitation; softening; ion exchange;
adsorption
(1)Ion Exchange Process
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Ion exchange method
• Ion of interest is retained on material surface
and exchangeable ions are released from
materials.
• Example: arsenic removal by resin where
arsenate ions replace hydroxyl ions from resin
surface by surface complexation formation
methods
Cation Exchange Process
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Anion Exchange Process
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Zeolite for Hardness Removal
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Zeolite
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Ion Exchanger
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/FreshWater/hardness.html
Ion Exchanger
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Problem: IITD GW to Drinking Water
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Example: Breakthrough curve
• After how many bed volumes of water, different
ions are breaking through the resin column?
• Breakthrough
concentration
limit
for
arsenic=0.010 mg/L;
• Breakthrough concentration limit for sulfate
ions=0.10 mg/L;
• Breakthrough concentration limit for nitrate
ions=0.10 mg/L;
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Follow different curve and discuss what
they show
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(2)Softening
• This process is used to remove bivalent and
multivalent cations from water (hardness; for ex:
Ca2+;
• Cations with carbonate and bicarbonate ions
give non-permanent hardness (also known as
carbonate hardness). It is a unstable hardness
and can easily be removed using boiling (a
simplest method).
• Cations with sulfate, chloride, and nitrate give
non-carbonate hardness and permanent
hardness. This cannot be easily removing using
boiling.
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Conventional
drinking water
treatment
Water
softening
Precipitative Softening
• Add quick lime (CaO) or hydrated lime [Ca(OH)2] in
hard water, raise pH higher than 10, convert
soluble bicarbonate ions (HCO3-) to insoluble
carbonate (CO3=).
• Form calcium carbonate (CaCO3) and magnesium
hydroxide [Mg(OH)2] precipitates.
• Add soda ash (Na2CO3) if insufficient natural
bicarbonate alkalinity.
• Re-carbonated with CO2, covert carbonate
particles into soluble bicarbonates, prevent filter
clogging from fine particles that do not settle.
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Example
• A sample of water
concentrations of ions
Ca2+
Mg2+
Na+
K+
HCO3SO42Cl-
has
the
following
40 mg/L
10 mg/L
11.8 mg/L
7.0 mg/L
110 mg/L
67.2 mg/L
11 mg/L
Calculate Total Hardness, total alkalinity?
Ion
Conc.
M.W.
(mg/L) (mg/mmol)
40
Ca2+
10
Mg2+
Na+ 11.8
7
K+
HCO3- 110
SO42- 67.2
Cl-
11
n
Eq. Wt. Conc.
Conc.
(mg/meq) (meq/L) mg/L as
CaCO3
20.0
12.2
23.0
39.1
61.0
40.1
24.3
23
39.1
61
2
2
1
1
1
96.1
2
48.1
35.5
1
35.5
Sample Calculation:
Equivalent weight of Ca2+
mg

40.1

 M.W.  
mmol
=
=
 n   2 meq
mmol


  20 mg 
 = 

  meq 


Ion
Conc.
M.W.
(mg/L) (mg/mmol)
40
Ca2+
10
Mg2+
Na+ 11.8
7
K+
HCO3- 110
SO42- 67.2
Cl-
11
n
Eq. Wt. Conc.
Conc.
(mg/meq) (meq/L) mg/L as
CaCO3
20.0
1.995
12.2
0.823
23.0
0.510
39.1
0.179
61.0
1.800
40.1
24.3
23
39.1
61
2
2
1
1
1
96.1
2
48.1
1.400
35.5
1
35.5
0.031
Sample Calculation:
mg 


mg 
 40.0

 conc. in

meq
L




L  = 1.995 meq
Conc. of Ca2 + 
=
=
L
mg   20.05 mg 
 L  
 eq. weight in
 

meq 
meq  

Ion
Conc.
M.W.
(mg/L) (mg/mmol)
40
Ca2+
10
Mg2+
11.8
Na+
7
K+
HCO3- 110
SO42- 67.2
Cl-
Sample
Calculation:
11
n
Eq. Wt. Conc.
Conc.
(mg/meq) (meq/L) mg/L as
CaCO3
20.0
1.995
99.8
12.2
0.823
41.2
23.0
0.510
25.7
39.1
0.179
9.0
61.0
1.800
90.2
40.1
24.3
23
39.1
61
2
2
1
1
1
96.1
2
48.1
1.400
69.9
35.5
1
35.5
0.031
15.5
 mg 
 mg

 meq 
Ca2 + 
as CaCO3  = Ca2 + 

 × eq. wt. of CaCO3 
L
L
meq






 mg 
mg
 mg

 meq 
Ca2 + 
as CaCO3  = 1.995 
 = 99.8
 × 50 
L
 L

 L 
 meq 
I) Total hardness ( sum of the polyvalent cations)
[Ca2+] + [Mg2+] = 99.8 + 41.2 =141 mg/L as CaCO3
II) Carbonate hardness (hardness associated with CO32- or
HCO3-)
= 90 mg/L as CaCO3
III) Non-carbonate hardness (the total hardness minus
the carbonate hardness)
= 141 - 90 = 51 mg/L as CaCO3
IV) Total alkalinity ( sum of concentrations of bicarbonate ions,
sulfate and chloride ions
=90.2+69.9+15.5 =175.6 mg/L as CaCO3
(3) Adsorption Method
• Ion of interest (i.e., adsorbate)is retained on
material surface
(i.e., adsorbent) through
surface complex formation; water becomes
cleaner after this process
• Adsorbents are regenerated after some
treatment of water (when concentration of ions
exceed some pre-decided limit on ion
concentration in water
• Adsorbents are disposed on hazardous landfill
(if removal of toxic ions) after passing it
through the USEPA toxicity characteristics
leaching procedure (TCLP) test
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Activated Carbon Adsorption
• Contaminant reduction in AC filters takes place by
two processes:
– physical removal of contaminant particles,
blocking any that are too large to pass through
the pores (filters with smaller pores are more
effective),
– and a process called adsorption by which a
variety of dissolved contaminants are attracted to
and held (adsorbed) on the surface of the carbon
particles. The characteristics of the carbon
material (particle and pore size, surface area,
surface chemistry, density, and hardness)
influence the efficiency of adsorption.
• Organic contaminants, unwanted coloring,
and taste-and-odor-causing compounds
can stick to the surface of granular or
powder activated carbon and are thus
removed from the drinking water.
http://www.cyber-nook.com/water/Solutions.html#carbon