Environmental Chemistry- Water

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

Uses of Water• Drinking, Washing, flushing toilet Watering gardens etc.

• Industry uses huge amounts of water for cooling , manufacturing, generating electricity etc.

Hardness of Water• Water circulates around earth in the

water cycle

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

• 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

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

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 + H2CO3 Ca(HCO3)2 Insoluble Soluble

• 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

• Calcium hydrogencarbonate Calcium Carbonate + Carbon Dioxide + Water

Ca(HCO3)2 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

If temporary hardness is caused by Mg2+ ions

• Magnesium ions tend to form dolomite rock which contains a mixture of MgCO3 and CaCO3

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

Methods of Removing Temporary and Permanent Hardness

• Distillation• Using Washing Soda• Ion Exchange Resin

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

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+ + CO32- CaCO3

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”

• 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

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

• 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

Advantages and Disadvantages of Hard Water

Advantages• Provides calcium for

bones and teeth

• Nicer Taste

• Good for brewing and tanning

Disadvantages• Blocks pipes leaves

scale on kettles and boilers

• Wastes soap

• Produces scum

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

• 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

Hardness in WaterDegree of Hardness p.p.m. as CaCO3

Soft 0-75

Moderately Hard 75 – 150

Hard 150 – 300

Very hard ›300

Water TreatmentA Water Treatment

PlantCriteria for acceptable domestic water• Colourless• Odourless• No active bacteria

present• Fluoridated to help

teeth

Stages in Water Treatment1. Screening2. Flocculation3. Settlement4. Filtration5. Chlorination6. Fluoridation7. pH adjustment

Screening

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

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

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

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

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

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 30-50 Euro is saved in treating tooth decay!

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

Experiments1. Determine the total suspended

solids (in p.p.m.) in a sample of water by filtration

2. Determine the total dissolved solids (in p.p.m.) in a sample of water by evaporation

3. Find the pH of a sample of water

Water Pollution

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

• 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

• 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

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

• 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

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

• 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

• 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

Sample ResultsB.O.D. (mg/L) Example1-2 Clean Water20-40 Treated Sewage100 Polluted Water ( fish die)300 Raw sewage500 Brewery Effluent30,000 Pig Slurry54,000 Silage Effluent

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

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

Precautions for Collecting Water samples for B.O.D. analysis

1. Fill the bottle under the surface of the water to prevent air getting in and adding extra oxygen

2. Fill the bottle completely to ensure no air is trapped between the top of the water and the stopper of the bottle

3. Place the second bottle in the dark immediately to prevent photosynthesis

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

• 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

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

e• These metal ions can get to rivers

and lakes in discharge from industry or dumping of batteries that contain these metals

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

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

Sewage • Sewage is the used

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

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

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

• 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

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

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

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!

• 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

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

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

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

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

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

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