Chpt.19: Environmental Chemistry - Water

Water is essential for life:

- 80% earths surface covered with water - makes up 2/3 of body weight- can live without food for several weeks but can only survive without water for few days

We take water for granted!!!! When we turn on the tap we expect to have a plentiful supply of clean safe drinking water that we use for cleaning, washing etc. We forget that as a country with piped water system we are in the minority!!!!!

In this chapter we will study and gain an appreciation for the importance of water in our lives. We will investigate:

- water as an important solvent- how substances dissolved in water affect its properties- how water is treated in water treatment plants- water pollution- analyse the composition of water- how sewage is treated

Hardness In Water

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

• A grey precipitate (scum) is formed instead

• Hardness caused by dissolved calcium and magnesium salts:

- calcium sulphate - CaSO4, magnesium sulphate - MgSO4

- calcium chloride - CaCl2, magnesium chloride - MgCl2

- calcium hydrogencarbonate - Ca(HCO3)2, magnesium - Mg(HCO3)2

• One of the most common substances in soap is sodium stearate, C17H35COONa. When soap is added to Hard Water the stearate ion (active part of soap) reacts with the calcium or magnesium ions in the water to form the grey insoluble compound calcium stearate (scum)

Calcium ion + Stearate ion → Calcium Stearate (scum)

Ca2+ + 2C17H35COO- → (C17H35COO)2Ca↓

• Lather is only formed once all the Ca2+ /Mg2+ ions have been precipitated in the scum – great deal of soap wasted in reaching this stage

• Modern detergents (washing powders, washing up liquids, shampoos etc.) not affected by hard water as they DO NOT contain soap – made from crude oil

Types of Hardness:

• Permanent and Temporary

• Expressed as ppm

Temporary Hardness:

Temporary hardness is hardness in water that can be removed by boiling.

Caused by:- calcium hydrogencarbonate – Ca(HCO3)2

- magnesium hydrogencarbonate – Mg(HCO3)2

Limestone (calcium carbonate) is insoluble in water thus calcium ions are not immediately available to cause hardness

However, limestone does react with carbonic acid:

Rainwater + CO2 → Carbonic Acid H2O + CO2 → H2CO3

This acidic solution of carbonic acid reacts with the limestone in the soil to form calcium hydrogencrbonate:

Limestone + Carbonic Acid → Calcium Hydrogencarbonate

CaCO3 + H2CO3 Ca(HCO3)2

INSOLUBLE SOLUBLE

The Ca2+ ions cause hardness while the HCO3- ions have

no effect. However, if water containing these two ions is heated the water is softened as a chemical reaction occurs which removes the Ca2+ ions from the water:

Calcium → Calcium Carbonate + CO2 + H2O Hydrogncarbonate

Ca(HCO3)2 → CaCO3↓ + CO2 + H2O Soluble Insoluble

HEAT

An undesirable side effect of this is that the insoluble calcium carbonate precipitate can form a fur/scale that builds up on kettles, boilers, hot water pipes etc. The build up of this limescale can cause problems such as wasting heat, or even explosions due to pipes being completely clogged!!!

*Note: Mg2+ ions often come from dolomite rock, which contains a mixture of MgCO3 and CaCO3 (MgCO3.CaCO3)

*Demonstration: Test on scale deposits in a kettle

Permanent Hardness:

Permanent hardness is hardness in water that can only be removed by methods other than boiling i.e. ion exchange, distillation

Caused by:

- sulphates of calcium (gypsum rock) and magnesium

- chlorides of calcium and magnesium

Methods of removing hardness (permanent & temporary) from water:

a) Distillation

b) Washing Soda

c) Ion Exchange Resin/Deionisation

a) Distillation:

- involves boiling the water and then cooling the vapour

- ALL dissolved and suspended solids and dissolved liquids are removed from the water

- very pure water

- not used on a large scale to soften water due to the expense involved in boiling the water

Distillation

b) Washing Soda (Hydrated Sodium Carbonate – Na2CO3.10H2O)Before the development of modern detergents clothes were washed with soap and crystals of this compound were added to the water to soften the water!!!

- carbonate ions in the washing soda react with Ca2+ ions in the water and removes them as insoluble calcium carbonate:

Ca2+ + CO32- → CaCO3↓

- Na+ ions (sodium carbonate) remain dissolved in water

- bath salts – crystals of washing soda coloured with perfume added

c) Ion Exchange Resin/ Deionisation (Swap Shop): Easiest way to remove hardness from water

Modern ion exchange resins are man-made materials which exchange or ‘swap’ ions that cause hardness (Ca2+, Mg2+) with ions that do not cause hardness (Na+).

Ion exchange units used to soften water contain cation (positive ion) exchange resin

- Ion exchange involves the water being passed through a cation exchange resin.

- The resins used are complex sodium compounds and may be represented as RNa.

- The Ca2+ and Mg2+ ions in the hard water swap places with the Na+ ions in the resin:

Learn:

2RNa(s) + Ca2+(aq) → R2Ca(s) + 2Na+

(aq)

- 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

Ion Exchange Resin

c) Deionisation

In some cases it is necessary to remove ALL the ions from the water i.e. deionised water. In this case:

- the resin used is a mixture of a cation (+ive) exchanger, replaces metal ions in water with hydrogen ions, H+, and an anion (-ive) exchanger, which replaces anions in water with hydroxide ions, OH- - MIXED BED RESIN

Removal of Na+ and Cl- ions in water using a deioniser:*Learn:

- Cation exchange resin, RH, replaces Na+ ions with H+ ions:

RH + Na+ → RNa + H+

- Anion exchange resin, ROH, replaces Cl- with OH- ions:

ROH + Cl- → RCl + OH-

- H+ and OH- ions then combine to form water:H+ + OH- → H2O

Deionisation

Deionised Vs. Distilled(Higher Level)

Deionised

• Easily and cheaply produced

• Can contain dissolved gases as well as non-ionic material (organic) dissolved in the water

Distilled

• Expensive to produce

• Purest form of water as all dissolved and suspended solids as well as dissolved gases have been removed

Advantages & Disadvantages of Hard Water

Advantages DisadvantagesProvides calcium for teeth and bones

Blocks pipes leaves scale on kettles and boilers

Nicer taste Wastes SoapGood for brewing and tanning

Produces scum

Student Questions:- Bk pg’s 288-289 Questions 19.1-19.2- Workbook pg. 48 Question W19.1

Mandatory Experiment: To determine the total hardness in a water sample (Higher Level)Must understand

- theory behind this experiment- theory of experiment - associated calculation- how to determine the concentration of temporary and permanent hardness

Student Questions:- Bk pg. 289 Questions 19.3 and 19.4, 19.5 (Tricky)- Workbook pg’s 48-49 Questions W19.2-W19.5

Water Treatment

Water supplied to houses around the country must fulfil certain criteria. It must be attractive looking, odourless, safe to drink and fluoridated. In order to ensure that the quality of domestic water supplies is of a certain standard it is necessary to carry out certain procedures on the water before going to houses. Water treatment plants have been set up around the country to ensure this standard is reached.

Stages of Water Treatment:

1. Screening

2. Flocculation

3. Settlement (Sedimentation)

4. Filtration

5. Chlorination

6. Fluoridation

7. pH adjustment

1. Screening:Water is first passed through a wire mesh to remove any floating debris – twigs, plastic bags etc.

2. Flocculation:Flocculation is the coming together of small `suspended solids in water to form larger

particles (flocs) This is done by adding certain chemicals to the water – flocculating agent Al2(SO4)3

A flocculant is a chemical added to water to coagulate suspended particlesThe addition of Al2(SO4)3 causes the suspended solids to stick together to form larger particles, which are allowed to settle out in the

settlement stage.

Flocculation

3. Settlement (Sedimentation):The flocculated water is pumped into the bottom

of large settlement tanks and rises up slowly (allows maximum settlement to take place at bottom of tank) to the surface where clear water is collected in channels

The suspended particles settle to the bottom

Approximately 90% of particles are removed in settlement stage

Settlement

4. Filtration:The water, from top of settlement tanks, is

allowed to fall through beds of graded sand and gravel

These filter beds remove any remaining suspended solids

The sand acts just like a sieve or filter paper in removing the suspended solids

The water coming out of filter beds is now clear but not yet fit for human consumption

Filtration

5. Chlorination:Chlorine is added to water in order to sterilise it

i.e. kill any harmful micro-organisms

Chlorine may be added as elemental chlorine or more commonly in the form of sodium hypochlorite (NaOCl) (active disinfecting agent chloric(I) acid)

Chlorine requirements in drinking water 0.2-0.5ppmChlorine requirements in swimming pool water 1-

5ppm

Amount of chlorine added must be carefully controlled need enough to give continued protection until use but also not too much as gives water unpleasant taste and smell.

Chlorination

6. Fluoridation:Involves adding small quantities (1ppm) of

fluorine compounds to water

Fluoridation of water is carried out because it has been shown that the presence of fluoride ions in

water can help prevent tooth decay – strengthens enamel of teeth

Sources of fluoride ions:- sodium fluoride, NaF- hexafluorosilicic acid, H2SiF6

Fluoridation

7. pH Adjustment:It may be necessary to adjust the pH of water

before it leaves treatment plant

Optimum pH of water for distribution – pH 7-9

If water supply slightly acidic (due to original source or chemicals added) can lead to corrosion of pipes so calcium hydroxide (lime) added to raise pH above 7

If water supply slightly basic (due to added softeners-sodium carbonate) sulphuric acid added to lower pH.

pH Adjustment

Mandatory Experiment: To determine

(a) the total suspended solids (p.p.m) of a sample of water by filtration

(b) the total dissolved solids (p.p.m) of a sample of water by evaporation

(c) the pH of a sample of water

Must understand:- theory of experiment- associated calculations

Student Questions:

Book – pg’s 289-230 Questions 19.6Workbook – pg. 49 Question W19.6

Water Pollution

Dissolved Oxygen:

Dissolved oxygen in rivers, lakes and the sea is vital for the survival of fish and other forms of life.

Although oxygen (non-polar) is only slightly soluble in water aquatic life cannot exist without it.

The solubility of oxygen in water is dependent on the temperature of the water – the solubility of gases in water decreases with increase in temperature - when water is heated bubbles of air are seen to come out of solution.

Another factor affecting the amount of dissolved oxygen in water is the discharge of organic waste (domestic sewage, animal slurry, silage effluent etc.) into waterways.

When organic waste is discharged into the water, the bacteria and other organisms which are naturally present in the water are provided with nutrients.

These nutrients cause the organisms to multiply into large numbers. As the organisms break down the waste (aerobic respiration) into compounds like CO2 and water, they use up the oxygen dissolved in the water:

Organic Matter + Oxygen CO2 + H2O

If a considerable amount of organic waste is present, it is possible that the dissolved oxygen level in the water will be reduced so much that fish life in the water will start to decline e.g. trout and salmon require 5 p.p.m of dissolved oxygen in order to survive.

If the dissolved oxygen level drops to low levels such as zero concentration, anaerobic bacteria will take over and start producing H2S gas leaving the river with a foul smell!!!!

Such POLLUTION results in the death of vast amounts of fish and other aquatic life!!!!!

Biochemical Oxygen Demand (B.O.D.) (Higher Level):

The Biochemical Oxygen Demand (B.O.D.) test was developed to determine the level of pollution in our waterways:

Biochemical Oxygen Demand is defined as:

- the amount of dissolved oxygen- consumed by biological action- when a sample of water is kept at 20oC- in the dark- for five days

*Note: B.O.D. is measured in mg/L of oxygen

This process involves collecting two water samples from the same area. The dissolved oxygen in bottle A is measured immediately using a titration called The Winkler Method (or using a dissolved oxygen meter). Bottle B is incubated in the dark at 20oC for five days:

- kept in the dark to prevent the production of oxygen by photosynthesis that might be carried out by any plant life present.

- amount of oxygen dissolved in water depends on temperature, experiment is carried out at a fixed

temperature to allow the valid comparison of the BOD values of different water samples.

- the Winkler method of measuring dissolved oxygen is carried out on the sample of water in bottle B at the end of the 5 day period. The difference between the dissolved oxygen level for bottle A and the dissolved oxygen level for bottle B is the B.O.D. This B.O.D. value represents the amount of oxygen required by bacteria and other micro-organisms to break down organic material over the five day test period.

- if the sample is very polluted, (effluents with BOD greater than 9mg/L) it must be diluted by a fixed amount with well-oxygenated water. This ensures that the dissolved oxygen present does NOT run out before the end of the 5 day test period and that a measureable amount of oxygen (at least 2 p.p.m) will be left after the five day period. When the B.O.D. calculation is carried out, the result is multiplied by the dilution factor to get the true B.O.D. value.

- The higher the B.O.D. value the more polluted the water. A high B.O.D. value means that there is a large amount of organic waste in the water and as this organic waste is acted on by bacteria, the level of oxygen decreases.

B.O.D (mg/L) Source of sample1-2 Clean Water20-40 Treated Sewage100 Polluted Water (fish

die)

30,000 Pig slurry 54,000 Silage Effluent

Most fish kills in Ireland are caused by slurry and silage effluent into rivers and lakes

Example 1:

To find the B.O.D. of a sample of polluted river water, 25cm3 of the water was diluted to one litre with well-oxygenated pure water. Two bottles, A and B, were filled with the diluted water and their dissolved oxygen concentrations were determined. The analysis was carried out immediately for bottle A and five days later for bottle B. The results obtained were 12.8 p.p.m and 8.2 p.p.m respectively.

i) Why dilute polluted river water? What was advantage in using well oxygenated water for this purpose?

ii) Under what conditions should bottle B have been kept for the five days before it was analysed? In the case of one of these conditions explain why it is necessary.

iii) What was the B.O.D. of the polluted river water?

Example 1 Solution:

Example 2:

Why is it necessary to analyse the sample of water in bottle A immediately???

Example 2 Solution:

• So that oxygen content does not increase due to photosynthesis

• So that the oxygen content does not decrease due to respiration

• So that the oxygen content does not increase/decrease due to activity of organisms

Example 3:A sample of polluted water was analysed, in order to determine the amount of dissolved oxygen in the water, using the Winkler method. The main reactions may be represented as follows:

4Mn(OH)2 + O2 → 2Mn2O3 + 4H2O

Mn2O3 + 2I- + 6H+ → 2Mn2+ + I2 + 3H2O

A 50cm3 sample of the water was diluted to one litre with pure well-oxygenated water. 300cm3 of this diluted solution were analysed by the Winkler method and 14.7cm3 of 0.02M sodium thiosulphate were required. A second 300cm3 sample was stored in the dark at 20OC for five days and then analysed for dissolved oxygen. The liberated iodine required 4.8cm3 of 0.02M sodium thiosulphate. Calculate the amount of dissolved oxygen in each sample of water. Express your answers in p.p.m. Hence find the B.O.D. of the water.

Example 3 Solution:

Student Question:Please complete W19.8 pg 50 in workbook

Try the following:Book – pg 290 No’s 19.7 – 19.9Workbook – pg. 49 W19.7

Mandatory Experiment: To measure the amount of dissolved oxygen in a sample of water by means of a redox titration (Winkler Method)

Must understand:

- reaction equations- theory of experiment- associated calculations (calculating amount of dissolved oxygen in water in p.p.m.

Eutrophication:

Another process which results in the reduction of the amount of dissolved oxygen in water is EUTROPHICATION.

Defnition:

Eutrophication is the enrichment of water with nutrients, which leads to excessive growth of algae

This is caused when water is overloaded with plant nutrients, in particular nitrate ions (NO3

-) and phosphate ions (PO4

3-). This is caused by untreated or partially treated sewage or by run off from farmland of slurry or fertilisers.

As the nitrate and phosphate levels rise, many plants and floating algae undergo population explosions.

The algae are short lived.

As the algae decay, micro-organisms use up much of the dissolved oxygen in the water, leading to the death of many forms of animal life.

‘Algal Bloom’ covers much of the surface of the water with a green

scum

Higher Level

Eutrophication may occur artificially or naturally

Natural Eutrophication:- occurs mainly in lakes- when sediments build up in a lake leads to a gradual increase in nitrogen and phosphorous levels.

Artificial Eutrophication:- caused by a sudden increase in nutrients - source of these nutrients – artificial fertilisers

being washed into rivers and lakes or by domestic sewage/waste from farming entering waterways

There is some concern about the levels of nitrate ions in water. It is thought that high

levels of nitrates may cause stomach cancer and also death in babies.

Heavy Metal Pollution (Higher Level):

Water pollution is also caused by the release of toxic metal ions into the water e.g. lead ions (Pb2+) , mercury ions (Hg2+) and cadmium ions (Cd2+)

• These elements considered cumulative poisons i.e. frequent exposure causes a build up in the body, with consequent serious health damage.

• Quantities of these elements can be found in rivers and lakes as a result of the discharge of industrial effluents or the dumping of batteries that contain these metals.

• Consequently they can enter drinking water!!!!!

• Associated problems:- lead in drinking water - old houses

containing lead plumbing

- mercury poisoning – metallic mercury is dangerous when inhaled but less

dangerous when swallowed as most of it passes out of body within a few days. However, it is a build up of mercury salts which can pose serious health risks! – Minamata Bay 1950’s – birth defects and

death

• Before the effluent is run into a waterway, the metal ions are removed by means of precipitation – lead ions are reacted with dilute HCl:

Pb 2+ + 2Cl- → PbCl2↓

EU and Water Quality

Water quality is controlled by EU legislation. Heavy metal limits have to be set because of the toxic effects of metals like mercury, cadmium and lead. Limits on phosphates and nitrates help to reduce the occurrence of eutrophication in waterways. Limits are also set for chemical species dissolved in drinking water.

Cadmium – 5 mg/L

Mercury – 1 mg/L

Lead – 10 mg/L

Nitrates – 50 mg/L

Sewage Treatment

For many years, domestic sewage running untreated into rivers, lakes and the sea has been one of the most serious causes of pollution in Ireland. In general a small quantity of a pollutant would not pose a problem as it would be degraded quickly, posing no threat to the environment. However, large quantities of sewage cannot be broken down in a reasonable period of time and so constitutes serious pollution.

Sewage may consist of organic waste, inorganic waste and various gases. If this were simply dumped into a local river or lake there would be such a high demand on the dissolved oxygen that the water would become very polluted!!!

The essential purpose of a sewage treatment plant is to reduce the B.O.D. of the sewage before it is discharged into the local river or lake.

There are three recognised stages in sewage treatment:

- Primary Treatment

- Secondary Treatment

- Tertiary Treatment

Primary Treatment (Mechanical Process):

• Involves screening and settling

• Screening: sewage passes through steel bars that remove large floating solids and physically break up sewage. Sewage is then slowly passed through grit channels in which pebbles, grit etc. settle and are removed periodically.

• Sewage flows into bottom of primary settling tanks where it remains for a few hours.

• Settlement (Sedimentation): suspended solids settle to the bottom of the tank and form a sludge. Scraper at bottom of tank removes sludge periodically.

• Liquid on top (supernatant liquid) flows away to secondary treatment.

• Removes approx 1/3 B.O.D. in sewage

Definition:Primary treatment involves screening and settlement, and is a physical process.

Secondary Treatment (Biological Oxidation of Sewage):

• involves biological process which reduces the levels of suspended and dissolved organic materials – ACTIVATED SLUDGE PROCESS

• consists of an aeration tank followed by a settling tank.

• Aeration Tank: The sewage is fed continuously into an aerated tank that is kept oxygenated by mechanical agitators. Micro-organisms present in the tank oxidise most of the organic matter (activated sludge)

*Note: function of aeration tank is to breakdown organic matter without letting the concentration

of dissolved oxygen in the liquid drop too low.

• After the biological stage the sewage flows into a settling tank where it undergoes further settlement.

• Settling: some of the sludge (rich in nutrients) is removed and used as fertiliser or further converted to methane. Some of the sludge is recycled back into the aeration tank to come in contact with fresh sewage.

• Thus the micro-organisms are continually decomposing the organic waste in the sewage into compounds like carbon dioxide, nitrates etc.

• After secondary treatment about 95% of the B.O.D. of the original sewage is removed.

• Effluent may now be discharged into waterways, however, still contains compounds of phosphorous and nitrogen – moves to tertiary treatment.

Definition:Secondary treatment is a biological process involving the oxidation by micro-organisms of the nutrients present in the sewage.

Secondary Treatment

Tertiary Treatment:

• Involves chemical and biological processes

• Involves the removal of phosphates (household detergents) and nitrates (organic materials) from the effluent resulting from secondary treatment

• Remember nitrates and phosphates cause pollution by eutrophication

• Phosphates removed by precipitation. Reacted with a compound such as aluminium sulphate producing an insoluble salt – aluminium phosphate. Other additives used are iron(III) chloride and lime. Insoluble phosphate compounds allowed to settle before effluent discharge.

• Nitrates present in the form of ammonia, nitrite compounds, nitrate compounds or organic compounds containing nitrogen.

• Nitrates removed by biological denitrification.Bacteria is used to reduce nitrates to

nitrogen gas. The removal of all nitrogen compounds can be very difficult and quite expensive.

Effluent may now be discharged into waterways!!!

Definition:Tertiary treatment involves the removal of phosphates and nitrates from the effluent

Instrumental Methods of

Water Analysis

A) pH meter

B) Atomic Absorption Spectrometry

C) Colorimetry

*Notes on separate handout*

Mandatory Experiment: To estimate the concentration of free chlorine in swimming pool water or bleach using a comparator

Must understand:

- theory of experiment

Student Questions:

Book – pg.290 No’s 19.11-19.12Workbook – pg.49 W19.9