1. Water Hardness
prof. V. Paulauskas

2. Soft water - is water with low mineral content
Hard water - is water that has high mineral content (in contrast with soft water)
Soft water - is water with low mineral content
prof. V. Paulauskas

3. calcium Ca2+ and magnesium Mg2+ cations iron Fe2+, aluminium Al3+ and
Hard water minerals mainly consist of:
calcium Ca2+ and
magnesium Mg2+ cations
(the two most prevalent divalent metal ions)
Other metal ions, such as
iron Fe2+,
aluminium Al3+ and
manganese Mn2+
may also be present at elevated levels in some geographical locations
prof. V. Paulauskas

4. TYPES OF HARDNESS
GENERAL HARDNESS (HG) – is total amount of divalent Me ions (or their salts) dissolved in water

5. TEMPORARY HARDNESS CaCO3(s)+ CO2(aq) + H2O ⇋ Ca2+(aq)+ 2HCO3-(aq)
Temporary hardness is caused by a combination of calcium
(magnesium) ions and bicarbonate ions - Ca(HCO3)2 in the water TH can be simply removed by boiling the water
The following is the equilibrium reaction when calcium carbonate (CaCO3) is dissolved in water:
CaCO3(s)+ CO2(aq) + H2O ⇋ Ca2+(aq)+ 2HCO3-(aq)
Upon heating, less CO2 is able to dissolve into the water. Since there is not enough CO2 around, the reaction cannot proceed from left to right, and therefore the CaCO3 will not dissolve as rapidly. Instead, the reaction is forced to the left (i.e., products to reactants) to re-establish equilibrium, and solid CaCO3 is formed. Boiling the water will remove hardness as long as the solid CaCO3 that precipitates out is removed. After cooling, if enough time passes, the water will pick up CO2 from the air and the reaction will again proceed from left to right, allowing the CaCO3 to "re-dissolve" into water.

6. PERMANENT HARDNESS
It is hardness (mineral content) that cannot be removed by boiling
It is usually caused by the presence in the water of calcium and magnesium sulfates, nitrates, also chlorides, which become more soluble as the temperature rises
Despite the name, permanent hardness can be removed using a chemical reagents (water softeners) or ion exchange column

7. GENERAL HARDNESS – is expressed in millimoles of dissolved salts of divalent Me ions (including both Ca2+ and Mg2+) per litre of water (mmol/L) - Total H
ToH (incl. both Ca2+ & Mg2+ ions) can also be expressed as parts per million (ppm) or weight/volume (mg/L) of calcium carbonate (CaCO3) in the water

8. Very soft:
0-70 ppm
0-4 dGH
Soft:
ppm
4-8 dGH
Slightly hard:
ppm
8-12 dGH
Moderately hard:
ppm
12-18 dGH
Hard:
ppm
18-30 dGH
Very hard:
>530 ppm
>30 dGH
Parts per million (ppm) - usually defined as one milligram of calcium carbonate (CaCO3) per litre of water
Degrees of General Hardness (dGH) – 1 dGH defined as 10 milligrams of calcium oxide per litre of water, which is equivalent to milligrams of calcium carbonate per litre of water, or ppm (German degrees)
Millimoles per litre (mmol/L) – 1 millimole of calcium (either Ca2+ or CaCO3) per litre of water corresponds to a hardness of ppm or dGH, since the molar mass of calcium carbonate is g/mol

9. Parts per million (ppm) - defined as one milligram of calcium carbonate (CaCO3) per litre of water
1L ≈ 1kg = 1 000g = mg = 106mg
1mg of CaCO3 / mg of water =
= 1ppm

10. Degrees of General Hardness (dGH) – 1 dGH defined as 10 milligrams of calcium oxide per litre of water
1 dGH is equivalent to milligrams of calcium carbonate per litre of water, or ppm
56 mg CaO – 100 mg CaCO3
10 mg CaO – x mg CaCO3
x = 17,85 mg CaCO3 = 17,85 ppm
1 dGH = ppm

11. Millimoles per litre (mmol/L) – 1 millimole of calcium (either Ca2+ or CaCO3) per litre of water
1 mmol/L – corresponds to a hardness of ppm or dGH (molar mass of calcium carbonate – g/mol
1mmol/L ≈ 100 mg/L CaCO3= 56 mg/L CaO
1 mmol/L = 100 ppm = 5.6 dGH

12. ORIGIN
Calcium and magnesium ions are acquired through contact with rocks and sediments in the environment
Calcium usually enters the water as either calcium carbonate (CaCO3), in the form of limestone and chalk, or calcium sulfate (CaSO4), in the form of other mineral deposits (e.g. gypsum)
The predominant source of magnesium is dolomite (CaMg(CO3)2)

13. Negative effects 2C17H35COONa + Ca2+ → (C17H35COO)2Ca + 2Na+
Hardness in water can cause water to form scales and a resistance to soap
It can also be defined as water that does not produce lather with soap solutions, but produces white precipitate (scum). For example, sodium stearate reacts with calcium:
2C17H35COONa + Ca2+ → (C17H35COO)2Ca + 2Na+
Iron, if present, is important for causing the calcification to be brownish (the color of rust) instead of white (the color of most of the other compounds)
Hard water is generally not harmful to one's health

14. Negative effects
Hard water causes scaling (limescale), which is the left-over mineral deposits that are formed after hard water had evaporated
Scale can clog pipes, ruin water heaters, coat the insides of tea pots, decrease life of toilet flushing units and washing machines
Similarly, insoluble salt residues will remain in hair after shampooing, clothes after washing, food products after cooking
In industrial settings, water hardness must be constantly monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment that comes in contact with water

15. Negative effects
Very soft water can corrode the metal pipes in which it is carried and as a result the water may contain elevated levels of cadmium, copper, lead and zinc

16. Softening
It is often considered desirable to soften hard water – removal of divalent metal ions
A water softener works on the principle of cation exchange in which ions of the hardness minerals (mainly calcium and magnesium) are exchanged for sodium or potassium ions, effectively reducing GH to tolerable levels
Hardness can be removed using a water softener (chemical reagent) or ion exchange column
In drinking water, the recommended limits for total hardness expressed as the sum of the calcium and magnesium ion concentrations is – 2-4 mmol/L

17. Softening: 1. Thermal method
Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling t
Ca(HCO3)2  CaCO3 + CO2 + H2O

18. 2. Chemical precipitation
Softening
2. Chemical precipitation
MgCl2 + Na2CO3  MgCO3 + 2NaCl
3CaCl2 + 2Na3PO4  Ca3(PO4)2 + 6NaCl
CaSO4 + 2NaOH  Ca(OH)2 + Na2SO4
Ca(HCO3)2 + Ca(OH)2  2CaCO3 + 2H2O
Divalent Me ions forms solid precipitates – are removed from water

19. Na2R (cat.) + CaSO4  CaR (cat.) + Na2SO4
Softening:
3. Ion exchange
Natural or synthetic zeolites (alumosilicates) used as cationites
Na2R (cat.) + CaSO4  CaR (cat.) + Na2SO4
Na2R + Ca2+  CaR + 2Na+
This process is called ion exchange
Large-scale softening is caried out with:
zeolites (Na2Al2Si2O8 . xH2O) or
ion exchange resins

20. 3. ION EXCHANGE COLUMN
Filter filled with cationic material

21. 3. INDUSTRIAL DEMINERALISATION UNITS
Process based on
ion-exchange

22. 3. WATER DEMINERALISATION
Cationite H+
Anionite
OH-
All ions are removed from water
Product: deionised water

23. Softening – ion exchange
Many zeolite minerals occur in nature,
but specialized ones are often made artificially

24. Softening – ion exchange
When sodium zeolite has a low concentration of sodium ions left, it is exhausted, and can no longer soften water
The resin is recharged by flushing (often back-flushing) with saltwater
The resulting saltwater and mineral ion solution is then rinsed away, and the resin is ready to start the process all over again. This cycle can be repeated many times

25. 1 Problem: 0. 2 g Ca(NO3)2, 0. 15 g Ca(HCO3)2 and 0
1 Problem: 0.2 g Ca(NO3)2, 0.15 g Ca(HCO3)2 and 0.05 g Mg(HCO3)2 are dissolved in 0.8 L of drinking water. Calculate: general, temporary and permanent water hardness.
X1  1.52 mmol/L
X2  1.16 mmol/L
X3  0.43 mmol/L
HP  x1  1.52 mmol/L
HT  x2 + x3  1.59 mmol/L
HG  HP + HT  3.11 mmol/L

26. 2 Problem: General water hardness - 4 mmol/L
2 Problem: General water hardness - 4 mmol/L. Calculate the amount of FeCl2 which is dissolved in 15 m3 of this water
1mM (iron chloride)  127 mg/mmol
x  7620 g  7.62 kg FeCl2

27. 3 Problem: Calculate the amount of reagent (sodium carbonate) needed to soften 20 cubic meters of water used in a thermoelectric power-station as a cooling agent (H = 6 mmol/L)?
1 M (sodium carbonate)  106 g/mol
x  mmol  120 mol Na2CO3
x  g  kg Na2CO3

28. Cycle of Carbon Dioxide
Carbon dioxide reacts with water to form carbonic acid (1) which at ordinary environmental pH exists mostly as bicarbonate ion (2). Microscopic marine organisms take this up as carbonate (4) to form calcite skeletons which, over millions of years, have built up extensive limestone deposits. Groundwaters, made slightly acidic by CO2 (both that absorbed from the air and from the respiration of soil bacteria) dissolve the limestone (3), thereby acquiring calcium and bicarbonate ions and becoming "hard". If the HCO3– concentration is sufficiently great, the combination of processes (2) and (4) causes calcium carbonate ("lime scale") to precipitate out on surfaces such as the insides of pipes. (Calcium bicarbonate itself does not form a solid, but always precipitates as CaCO3.)