ION EXCHANGE IN WATER SOFTENING
Objective: Determine the exchange capacity of a cationic resin in water softening.
Introduction: Water softening is a process to reduce hardness in water and prevent the
build-up of lime scale and calcium deposits in pipes and equipment. Hardness is normally
measured by the amount of calcium and magnesium that is present in water and is
reported as the concentration of CaCO3. To get an idea of scale, the Saskatoon Water
Treatment and Meters Branch reports the potable water has an average hardness of 126
mg/L as CaCO3. The river water has an average hardness of 176 mg/L. 120 mg/L as
CaCO3 or greater is considered hard.
Ion exchange is an important technique to reduce hardness in water. It is the reversible
interchange of ions between a solid (ion exchange material) and a liquid. The ions in
solution become attached to the solid and the displaced ions will be forced into solution.
The process of exchange continues until both ions reach equilibrium on the surface and in
solution. This process is dynamic and can be reversible depending on the relative
concentrations of the ions in solution.
Ion exchange has been used on an industrial basis since 1910 with the introduction of
water softening. Cation exchange is widely used to soften water. The most usual ion
exchange material employed in water softening is a sulphonated styrene-based resin,
supplied by the makers in the sodium form. In the process, calcium and magnesium ions
in water are exchanged for sodium ions on the resins. Ferrous iron and other metals such
as manganese and aluminum, sometimes present in small quantities, are also exchanged
Figure 1. Ion Exchange Columns
(Picture courtesy to www.stockinterview.com/News/)
after calcium and magnesium are removed, but are unimportant in the softening process.
Removal of the hardness, or scale-forming calcium and magnesium ions, produces “soft
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water”. Softening can be carried out as a batch process by stirring a suspension of the ion
exchange resin in the water for a period until equilibrium, or an acceptable level of
hardness, is reached. However, it is more convenient to operate a continuous flow
process by passing the water downwards through a column of resin beads.
Theory: The exchange reaction for water softening with a sulphonated styrene-based
resin in the form of sodium can be described below.
2Na+R- + Ca2+(aq)↔ Ca2+R-2+ 2Na+(aq)
(1)
where R represents the resin chain and the exchange point on the beads.
The reaction takes place rapidly enough for the upper layers of the bed to approach
exhaustion before the lower layers being able to exchange ions. There is thus, a zone of
active exchange which moves down the column until the resin at all depths becomes
exhausted. The position at an intermediate stage can be illustrated as shown in Figure 2a.
Plotting the hardness readings as CaCO3 (mg/L) in the effluent against the volume of
water treated (L) generates the breakthrough curve as shown in Fig.2b. The breakthrough
point can be determined at which the concentration of CaCO3 in the effluent reaches an
acceptable level of hardness or the hardness of the feed. It is usually the limit of the
exhaustion cycle.
Figure 2a. Ion Exchange Zone
Figure 2b. Idealized Breakthrough Point
When the resin is exhausted, it can be regenerated with a copious amount of sodium salt
such as sodium chloride. The excess salt will shift the equilibrium (Eq.1) to the left and
sodium ions will replace calcium and be present on the solid.
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If the hardness is measured by CaCO3, the ion exchange capacity of the resin can be
determined as follows:
Exchange Capacity =
Removed Mass of CaCO 3 (mg)
X0.02 (meq/mg)
Vol of Wet Bed (mL)
(2)
where
πD 2
Volume of Wet Bed (mL) =
XFinal Depth of Resin Bed (cm)
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(3)
where D is the diameter of the column (cm), equal to 1.5 cm in this investigation.
The mass of CaCO3 removed from the tap water up to the breakthrough point can be
calculated. Graphically, this is given by the area in the graph of breakthrough curve
between the curve plotted and the horizontal line, representing the original hardness of
the water. The mass of CaCO3 (mg) can be converted to milliequivalent (meq) by
multiplying a factor of 0.02. (DOWEX, Ion Exchange Resins, Water Conditioning
Manual, p.74.) Knowing the wet volume of the resin bed, the exchange capacity of the
resin can be calculated as meq/mL. If other minerals such as magnesium carbonate,
calcium oxide and so on are removed, they can be converted to the equivalent
concentration as CaCO3 by certain conversion factors. The conversion factors of common
substances are given in literature (DOWEX, Ion Exchange Resins, Water Conditioning
Manual, p.75).
Once the exchange capacity of the resin bed is determined, it can be used to design a
column packed with the resins for water softening at large scale. The required resin
volume can be determined in the following equation:
Resin Volume (L) =
Feed Hardness (meq/L)X Throughput (L) −3
X10 (L/mL)
Exchange Capacity (meq/mL)
(4)
where the throughput refers to the volume treated per exhausted cycle of resin.
Apparatus: Water softening in this investigation is carried by the Armfield Ion
Exchange Apparatus W9. The sketch is shown below in Figure 3. The system consists of
a column packed with a sulphonated styrene-based resin, a pump to supply liquids to the
column, four tanks to store solutions of HCl, NaCl, test and deionized water and a sump
tank. A rotameter is used to measure the flowrate of the feed. A conductivity meter is
used to monitor the concentration of sodium in the effluent. An anion exchange column
was set up for the experiment of demineralization but not used in this investigation.
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A Mettler Toledo DL28 automatic titrator with an attached DP5 Phototrode is used for
determination of CaCO3 concentration in the effluent. For this purpose, 100 ml plastic
sample cups are provided.
Figure 3. – Diagram of Ion Exchange Apparatus
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Materials and Methods:
The hardness of the test water passing through the ion exchange column is determined by
titrating the effluent of the W9 Ion Exchange apparatus with a complexometric reaction.
(Appendix A).
The concentration of sodium ions [Na+] in solution is measured by the conductivity of the
collected effluent.
The required chemicals are:
Ammonia buffer ~pH10
Calcium chloride dihydrate
Sodium chloride
pH 4, 7, & 10 buffer solutions
Disodium ethylenediamine-tetraacetic acid dihydrate (EDTA) solution 0.01 M
Calmagite 1% solution – indicator
Procedure:
A.
B.
Adding Cation Resin to the Column
1.
Drain the column by first placing a waste vessel at valve 10, then opening
valves 6 and 10
2.
Remove the cation column by undoing the plastic holders on each end of
the column. The column pulls out towards you and has no catches.
3.
Fill the cation column to ~300 mm of cationic resin (golden-brown
colored granules).
4.
Replace the column and close valves 6 and 10
Regenerate the Cationic Resin
This may already be done for you, check with your TA
Regeneration is required at the beginning of the experiment to ensure that the
cation column has the requisite amount of Na+ ions. You are assuming that the
column is depleted.
For apparatus configuration see Data Sheet I, Figure 1c.
1.
Select Tank B, open valves 2 and 12 turn on the main switch.
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C.
2.
Add 30 g of salt to a beaker. Add RO water.
3.
Set the flow meter to 20 -50 mL/min and add the salt to the column.
Continue until the conductivity reaches 1.1 x 10-2 Siemens for three
minutes or all the salt has been placed in the column. This means that the
column has reached the saturation point and has an excess of Na+ ions.
3.
Select Tank “D” and flush the column for 5 minutes at 70 ml/min.
4.
Close all valves afterwards.
Fluidizing the Resin Bed
The Resin is pre-regenerated with excess sodium. Backwashing ensures that the
remaining regeneration solution and debris from the last experimental run are
washed out of the column. Plus it expands the resin beads so that no air pockets
remain in the resin bed.
For apparatus configuration see Data Sheet I (Page 9 of this manual), Figure 1b.
1.
Make sure all valves are closed.
2.
Open valves 3 and 6.
3.
Select Tank D and turn on the pump, then backwash the cation column at
a flow rate of 50-70 ml/min for 5 minutes.
Large air bubbles can be gotten rid of by closing valve 6 and opening 12 until
the water has reached the top of the column. Then close 12 and re-open 6.
Repeat if necessary.
D.
4.
When the air bubbles have been eliminated and the resin has settled, turn
off valves 3 and 6.
5.
Measure the final depth of the resin.
Softening of Water Sample
For apparatus configuration see Data Sheet I, Figure 1d.
1.
Select Tank C containing the test water.
2.
Open valves 2 and 10.
3.
The TA will provide the appropriate flow rate
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E.
4.
Collect the initial 2 samples at 300-400 ml intervals, the remainder at 100400 ml intervals.
5.
Determine the hardness of each sample as per Appendix “A” and continue
testing each sample until hardness rises above 100 mg/L as CaCO3.
Shutdown
1.
Select Tank “D” and flush the column at max flow for 5 minutes
2.
Turn off the pump, open valve 10 and completely drain the resin column.
3.
Rinse the beakers and equipment with RO water.
Data Analysis:
a) Plot the hardness as CaCO3 concentration (mg/L) against effluent volume (L). Identify
the breakthrough point. Determine the total amount of CaCO3 (mg) removed by the
column up to the breakthrough point.
b) Calculate the exchange capacity (meq/mL).
c) Design a column (area and height) to reduce the hardness of 10,000 L of water to 100
mg/L of CaCO3. The initial hardness of water is the same as your experimental. Keep the
height-to-diameter ratio of the wet resin bed the same as that of the resin column used in
this experiment. Provide brief discussion on your design results including the feasibility
of using one column and one exhausted cycle to complete the task.
Hint: you need to first determine the required resin volume for this project. A safety
factor should be applied to the exchange capacity figure to compensate for non-ideal
operating conditions and resin aging on a working plant. Typical safety margin is 5% for
cation resins. Column sizing should be adjusted to allow for resin expansion if
backwashing is performed (80–100% of the settled resin bed height) and resin swelling
during service, approximately 5-8% for strong acid cation resin.
Suggestion: Determine the hardness of the untreated test water at the beginning of the
experiment. (5 ml sample)
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Data Sheet I – Configuration of Ion Exchange Apparatus
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Appendix “A”
Complexometric Titration for the Determination of Water Hardness
Titration Procedure
Water Hardness Titration
Disodium Ethylenediamine-tetraacetic acid dihydrate (EDTA or Na2H2Y∙2H2O)
forms a chelated soluble complex when added to a solution of certain metal
cations. If a small amount of dye (Calmagite) is added to a solution containing
calcium and magnesium ions at a pH of 10 ± 1, the solution becomes wine red
due to the MgIn- formation. If EDTA is added as a titrant, the calcium and
magnesium will become complexed.
The calcium will be complexed out first as it has a larger formation constant with
EDTA than magnesium. When all of the calcium ions have been complexed with
EDTA, the trace amount of magnesium ions in the buffer will react. Once the
trace amount of magnesium is complexed, the solution will change to a blue. The
addition of the trace amount of magnesium is required for the complexometric
titration and eliminates the need for a blank correction titration.
pH is very important in this experiment as having a higher value than 10.5 will
precipitate out CaCO3 or Mg(OH)2 immediately. However, even at a pH of 10
Ca2+ will precipitate out eventually. Thus, a maximum of 5 minutes from the
addition of the buffer solution should be observed, to prevent interference from
Calcium (III) hydroxide precipitation.
Mettler Toledo DL28 with Phototrode DP5
A Mettler Toledo DL28 auto-titrator is used for the titrations. The main switch is
in the back. The Phototrode DP5 allows for an automatic titration based on a
colorimetric endpoint.
Procedure:
1.
Take 10-25 mL of collected sample then add calgomite and buffer..
2.
Press F3 to reset the display, if needed. Press “100”, “OK”
3.
The end point is dark blue (24 mV) or 50 mL max EDTA
4.
Dispose of all solutions in the waste container provided.
Calculation of Hardness: Ideally, 1 mL EDTA used is equivalent to 1 mg CaCO3 titrated.
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Volume of EDTA Added (mL) x Correction Factor x 1000 mL/L
= mg/L CaCO 3 as hardness
Volume of Water Sample (mL)
The correction factor will be provided by the Lab Demonstrator.
Example 1
If 9.5 mL of EDTA is added to 25 mL of sample and there is a correction factor of 0.909:
9.5 mL x 0.909 x 1000 = 345.4 mg/L CaCO3 as hardness
25 mL
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References:
Armfield Instruction Manual. 2000. Ion Exchange Apparatus W9. Issue 14. WO014461.
Dowex, 2007. “Water Condition Manual – A Practical Handbook for Engineers and
Chemists”. Dow Liquid Separations. http://www.reskem.com/pages/resinpdfs.php.
Bailey, S. J., et al. 2003. “Standard Test Method for Hardness in Water, D1126-02”.
Annual Book of ASTM Standards, Vol 11.01. ASTM International, West
Conshohocken, PA. Pg 98-101.
Frason, M. 2005. “2340C EDTA Titrimetric Method.” Standard Methods for the
Examination of Water and Wastewater. 21st Ed. American Public Health
Association, Washington D.C. Pg 2-37 – 2-39.
Harris, D. 2003. “EDTA Determination of Total Water Hardness.” Quantitative
Chemical Analysis. 6th Ed. W.H. Freeman, New York. Pg. 259-267, 272-277.