Transcript Environmental Chemistry- Water

Environmental ChemistryWater
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
Insoluble
Ca(HCO3)2
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+ + CO32CaCO3
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
Disadvantages
• Blocks pipes leaves scale
on kettles and boilers
• Nicer Taste
• Wastes soap
• Good for brewing and
tanning
• 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 Water
Degree of Hardness
p.p.m. as CaCO3
Soft
0-75
Moderately Hard
75 – 150
Hard
150 – 300
Very hard
›300
Water Treatment
A Water Treatment
Plant
Criteria for acceptable
domestic water
• Colourless
• Odourless
• No active bacteria
present
• Fluoridated to help
teeth
Stages in Water Treatment
1.
2.
3.
4.
5.
6.
7.
Screening
Flocculation
Settlement
Filtration
Chlorination
Fluoridation
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
Experiments
1. 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 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
• 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