1. Environmental Chemistry- Water

2. Importance of Water
Water is the most important and abundant chemical on Earth
It makes up 80% of the Earth’s surface
2/3 of our body weight

3. Uses of Water Drinking, Washing, flushing toilet Watering gardens etc.
Industry uses huge amounts of water for cooling , manufacturing, generating electricity etc.

4. Hardness of Water
Water circulates around earth in the water cycle

5. Hardness of water
As water is such a good solvent it is very rare to get pure water, even rainwater contains dissolved gases
In some parts of Ireland water is hard (ie. It will not easily form a lather with soap but will form a grey precipitate referred to as scum)
Hard water contains definite quantities of Ca2+ or Mg2+ ions

6. Hard water is water that will not easily form a lather with soap
Hard water is water that will not easily form a lather with soap. Hardness in water is caused by the presence of Ca2+ or Mg2+ ions

7. Reactions to learn! Soap contains sodium stearate (C17H35COONa)
Calcium or magnesium ions react with stearate ions to form a grey insoluble compound called calcium stearate (scum) which floats on water
Ca2+ + C17H35COO (C17H35COO)2Ca

8. Temporary Hardness
Temporary hardness can be removed by boiling the water
Temporary hardness arises when slightly acidic rainwater (carbonic acid) falls on limestone and reacts with it to form calcium hydrogen carbonate
Limestone + Carbonic acid Calcium Hydrogen carbonate
CaCO3 + H2CO Ca(HCO3)2
Insoluble Soluble

9. The Ca2+ ions cause the hardness the HCO3- ions have no effect on hardness
If water containing these two ions is heated a chemical reaction occurs which removes the Ca2+ ions from the water thus softening the water

10. Calcium hydrogencarbonate Calcium Carbonate + Carbon Dioxide + Water
Ca(HCO3) CaCO3 + CO2 + H2O
soluble insoluble
The calcium ions that were dissolved are now precipitated as insoluble calcium carbonate (limestone), this is often referred to as fur or scale and builds up on kettles and water pipes

12. If temporary hardness is caused by Mg2+ ions
Magnesium ions tend to form dolomite rock which contains a mixture of MgCO3 and CaCO3

13. Permanent water Hardness
Permanent Hardness is usually caused by the presence of Calcium Sulphate CaSO4 or Magnesium Sulphate MgSO4
This hardness cannot be removed by boiling

14. Methods of Removing Temporary and Permanent Hardness
Distillation
Using Washing Soda
Ion Exchange Resin

15. Distillation This involves boiling the water and cooling the vapour
All dissolved solids + liquids are removed this way
However it is not practical to use this method on a large scale

16. Using Washing Soda
Washing Soda softens water as the carbonate ions in the washing soda react with the calcium ions in the water and removes them as insoluble calcium carbonate
Ca2+ + CO CaCO3

17. Ion Exchange Resin
These are man made materials which “Swap” ions that cause hardness in water with ions that don’t cause hardness in water
As the ions involved
are positive ions they
may often be called
“Cation exchange resins”

18. Frequently the Ca2+ ions in the water are replaced with Na+ ions
The calcium ions remain behind in the resin
Eventually the resin loses all of its Na+ ions and it needs to be regenerated by passing a concentrated solution of sodium chloride through it

19. Deionised Water
For laboratory and medical purposes it may be necessary to remove all ions from water
This is achieved by passing water through a deioniser which contains a mixture of cation and anion exchange resins

20. The cation exchange removes all the positive ions and the anion exchange resin removes all the negative ions
Positive ions are replaced with H+ ions and negative ions with OH- ions which recombine to form water

22. Advantages and Disadvantages of Hard Water
Provides calcium for bones and teeth
Nicer Taste
Good for brewing and tanning
Blocks pipes leaves scale on kettles and boilers
Wastes soap
Produces scum

23. Determining the Hardness of a Water Sample
Total hardness = Magnesium + Calcium ions present
Total water hardness is expressed in terms of mg per litre of CaCO3 (p.p.m. CaCO3)
The concentration of Calcium and Magnesium ions present in water is determined using a chemical called ethylenediaminetetraacetic acid or EDTA for short!
The EDTA forms a complex with Ca2+ and Mg2+ ions and reacts in the ration 1:1 with these ions

24. In the presence of Eriochrome Black T indicator calcium and magnesium ions have a wine red colour
If all the ions have reacted with EDTA the colour changes to blue
By measuring how much EDTA is needed to react with all the ions in a sample of water the hardness of the water may be determined

25. Hardness in Water Degree of Hardness p.p.m. as CaCO3 Soft 0-75
Moderately Hard
75 – 150
Hard
150 – 300
Very hard
›300

26. A Water Treatment Plant
Criteria for acceptable domestic water
Colourless
Odourless
No active bacteria present
Fluoridated to help teeth

27. Stages in Water Treatment
Screening
Flocculation
Settlement
Filtration
Chlorination
Fluoridation
pH adjustment

28. Screening
Involves passing the water through a wire mesh to remove nay large floating debris eg twigs, plastic bags etc.

29. Flocculation
Causing smaller particles in water to come together to form larger particles or “Flocs”
This is achieved using flocculating agents such as Aluminium sulphate
The larger particles settle out in the next stage

30. Settelement Water is passed into settlement tanks
It is allowed to sink then rise very slowly leaving particles at the bottom
90% of particles are removed at this stage

31. Filtration
Water from the top of settlement tanks passes through large beds of sand
Sand acts like a sieve or filter paper and removes suspended solids
The sand is cleaned regularly

32. Chlorination
The water that comes out after filtration is not fit to drink as harmful micro-organisms may still be present
Chlorine is added to kill micro-organisms
Only small amounts of chlorine are added as it gives the water an unpleasant taste and smell if too much is present
Water needs to be monitored for bacteria control

33. Fuoridation
Fluorine compounds such as Sodium flouride nad hexafluorosilicic acid are added to water to help reduce dental decay
Estimated for every 1Euro spent on fluoridation Euro is saved in treating tooth decay!

34. pH Adjustment
If water is slightly acidic it may damage pipes so Calcium hydroxide may be added to raise the pH to 7
If the water is very hard it may be softened by adding sodium carbonate
Optimum pH of water for distribution to homes is 7-9

35. Experiments
Determine the total suspended solids (in p.p.m.) in a sample of water by filtration
Determine the total dissolved solids (in p.p.m.) in a sample of water by evaporation
Find the pH of a sample of water

36. Water Pollution

37. Dissolved Oxygen
Dissolved oxygen in rivers, lakes and the sea is vital for fish and other forms of life
O2 is non polar and not very soluble in water
Its solubility depends on the temperature of the water

38. Organic wastes such as sewage, slurry, effluent from farms, effluents from food processing factories, milk industrial waste etc. Can leak into rivers, lakes etc.
These provide nutrients to bacteria and other organisms naturally present in water and allows them to multiply to large numbers

39. These large numbers produce more carbon dioxide and use up available oxygen meaning fish life in the water may be reduced or killed off completely
If it gets very bad the dissolved oxygen level may reach 0 and only anaerobic bacteria will survive meaning the water can become foul smelling due to the production of Hydrogen Sulphide gas

40. Methods used by Chemists to indicate levels of pollution in water
A test to measure the demand water has for dissolved oxygen was introduced in the 1900’s and is called the Biological Oxygen Demand or B.O.D. Of the water

41. The Biological Oxygen Demand is defined as
The amount of dissolved oxygen consumed by biological action when a sample of water is kept at 20⁰C in the dark for 5 days

42. Measuring the B.O.D. Of Water
Completely fill two bottles to be tested with water
Measure the dissolved oxygen of one of the samples with a dissolved oxygen meter or by a titration called the Winkler Method
Incubate the second bottle at 20⁰C in darkness for 5 days

43. The water is kept in darkness to prevent photosynthesis from taking place as this would increase the amount of dissolved oxygen
The temperature must be kept fixed so that a fair comparison is made also the amount of dissolved oxygen is dependent on temperature

44. After 5 days measure the dissolved oxygen in the second bottle
The B.O.D. Is the difference in the two dissolved oxygen levels as this is the amount of oxygen that has been used up in the test
B.O.D. Is measured in mg/L of oxygen
The higher the B.O.D level the more polluted the water

45. Sample Results B.O.D. (mg/L) Example 1-2 Clean Water 20-40
Treated Sewage
100
Polluted Water ( fish die)
300
Raw sewage
500
Brewery Effluent
30,000
Pig Slurry
54,000
Silage Effluent

46. The more organic waste in water the more bacteria present to feed on it this decreases the available oxygen

47. Effluents Effluent means waste water or sewage water
It is possible to measure the B.O.D. of effluents
Oxygen has low solubility in water (9.2mg/L at 20⁰C) so effluents with B.O.D. Higher than 9mg/L must be diluted with Fresh distilled water and well shaken to ensure there will be a measurable amount of oxygen after 5 days
NB Example in book p 275

48. Precautions for Collecting Water samples for B.O.D. analysis
Fill the bottle under the surface of the water to prevent air getting in and adding extra oxygen
Fill the bottle completely to ensure no air is trapped between the top of the water and the stopper of the bottle
Place the second bottle in the dark immediately to prevent photosynthesis

49. Eutrophication
Overloading water with plant nutrients ( Nitrate and Phosphate ions) will also reduce the amount of dissolved oxygen
These nutrients cause plants and floating algae to undergo population explosions
When these die and decay they
put a high oxygen demand
on the water and algal bloom
covers much of the water with
a green scum

50. Eutrophication is the enrichment of water with nutrients which leads to the excessive growth of alagae
Natural Eutrophication = gradual increase in nitrogen and phosporous levels in sediments in lakes
Artificial Eutrophication = sudden increase in nutrients caused by artificial fertilisers or domestic sewage and waste getting into water
Concern that nitrate ions in water may cause stomach cancer and death in babies

51. Toxic Metal Ions in water
Lead ions ( Pb2+ ), Mercury ions ( Hg2+ ), and Cadmium ions (Cd2+ ) are commonly referred to as Heavy Metals due to their high relative atomic masses
These are classified as Cumulative poisons as their concentrations tend to build up in the body upon continuous exposure

52. e
These metal ions can get to rivers and lakes in discharge from industry or dumping of batteries that contain these metals

53. Mercury Poisoning
Metallic Mercury is dangerous inhaled into lungs, if ingested it can pass out of body in a few days
High levels of mercury slats cause damage to kidneys and intestines
Minamata Bay in Japan in 1950’s was contaminated with industrial waste
Mercury made its way into fish which was consumed by local population causing birth defects and death

55. Removal of Heavy Metal ions
Mercury and other heavy metal ions are recovered from industrial waste before it is discharged by Precipitation
Eg. Lead ions are reacted with HCl and the lead is precipitated as lead chloride
Pb2+ + 2Cl PbCl2

56. Sewage
Sewage is the used water from homes, factories, businesses etc. That is discharged into sewers that take it to a sewage treatment plant

57. Sewage
Sewage may consist of organic waste, inorganic waste and gases like hydrogen sulphide and methane
If sewage was dumped straight in rivers and lakes it would have a huge demand on oxygen and the water would become vey polluted
Sewage needs to be treated to reduce its B.O.D. Before it is released into rivers and lakes

58. Primary Treatment
A mechanical process involving screening and settlement
Screening is the removal of solids such as twigs, leaves and plastic bags that could damage equipment later on it is done by passing the sewage through metal bars

59. Settlement takes place when the sewage flows into settlement tanks and the suspended solids are allowed to settle to the bottom over a period of a few hours this is also know as Sedimentation
The solids that settle out are removed periodically to land or sea
The liquid goes on for secondary treatment

60. Secondary Treatment
This is a biological process where the suspended and dissolved organic matter is reduced and harmful bacteria are broken down
The Activated Sludge Process is used to achieve this

61. Activated Sludge Process
This involves pumping the sewage into large aeration tanks where it provides nutrient for lots of microorganisms (called activated sludge)
These micro organisms digest the sewage in the same way they would in a river
However the mixture is constantly churned to mix it with air and keep the oxygen levels high in some tank compressed air is constantly bubble through

62. Activated sludge process contd.
From the aeration tank the sewage passes into a settling tank
Some sludge is removed and some is recycled back to the aeration tank to come in contact with fresh sewage
The sludge removed can be used as fertiliser as it has lots of nutrients
It may also be stored in a sealed container where anaerobic bacteria work on it and convert it to methane which can be used as a fuel!

64. After secondary treatment 95% of the B. O. D
After secondary treatment 95% of the B.O.D. Of the original sewage is removed and the effluent may be released into a nearby waterway
If compounds of phosphorous and nitrogen are present they are removed in tertiary treatment

65. Tertiary Treatment Phosphates come from household detergents
Nitrates come from organic materials in sewage
Phosphates are removed by precipitation ( adding aluminium sulphate or IronIIIChloride) Lime may also be used

66. Tertiary Treatment
Nitrogen may be present in the form of ammonia, nitrite compounds, nitrate compounds or organic compounds containing nitrogen
Removal of these is expensive and difficult
Tertiary treatment plants are found in Mullingar and Killarney

67. No Confusion!!
Remember water treatment is to treat water to make it fit for domestic use, Sewage treatment treats sewage to prepare it for release into waterways

68. Instrumental Methods of Water Analysis
pH Meter – Water from limestone regions tends to be basic with a pH of 7.8, Water from sandstone regions tends to be acidic with pH ‘s as low as 5.8
The pH is measured with a
ph meter and adjusted to
be in the range 7-9

69. Atomic Absorption Spectrometry
This can be used to detect the presence of certain elements since each element has its own atomic absorption spectrum
It can also be used to detect the concentration of heavy metals in water

70. Colorimetry
In colorimetry light is passed through a sample of water and the absorption of the sample is measured
This is then compared to samples of known concentration