Water Hardness 2018.12.05 prof. V. Paulauskas

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 2018.12.05 prof. V. Paulauskas

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 2018.12.05 prof. V. Paulauskas

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

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.

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

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

Very soft: 0-70 ppm 0-4 dGH Soft: 70-140 ppm 4-8 dGH Slightly hard: 140-210 ppm 8-12 dGH Moderately hard: 210-320 ppm 12-18 dGH Hard: 320-530 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 17.848 milligrams of calcium carbonate per litre of water, or 17.848 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 100.09 ppm or 5.608 dGH, since the molar mass of calcium carbonate is 100.09 g/mol

Parts per million (ppm) - defined as one milligram of calcium carbonate (CaCO3) per litre of water 1L ≈ 1kg = 1 000g = 1 000 000mg = 106mg 1mg of CaCO3 /1000000mg of water = = 1ppm

Degrees of General Hardness (dGH) – 1 dGH defined as 10 milligrams of calcium oxide per litre of water 1 dGH is equivalent to 17.848 milligrams of calcium carbonate per litre of water, or 17.848 ppm 56 mg CaO – 100 mg CaCO3 10 mg CaO – x mg CaCO3 x = 17,85 mg CaCO3 = 17,85 ppm 1 dGH = 17.848 ppm

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 100.09 ppm or 5.608 dGH (molar mass of calcium carbonate –100.09 g/mol 1mmol/L ≈ 100 mg/L CaCO3= 56 mg/L CaO 1 mmol/L = 100 ppm = 5.6 dGH

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)

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

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

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

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

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

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

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

3. ION EXCHANGE COLUMN Filter filled with cationic material

3. INDUSTRIAL DEMINERALISATION UNITS Process based on ion-exchange

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

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

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

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

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

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  120000 mmol  120 mol Na2CO3 x  12720 g  12.72 kg Na2CO3

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.)