Water Pollution

Water pollution refers to the alteration in the physical, chemical and biological

characterization of water which will cause harmful effects on human, and

aquatic biota and disturb the normal use of water for irrigation, agriculture,

public water supply and industrial use.

Types of water pollution: -

1. Physical pollution of water – It is related to changes in colour, odour, taste,

density, turbidity and thermal pollution.

i) Colour- Colour affects the quality of light that penetrates to a given

depth, and inhibits plant and animal metabolism. Colours are generally

due to organic dye and inorganic complexes. Trade effluents, and

tannary waste, are responsible for deep green or blue colouration of

water which makes it unsuitable for various purposes.

ii) Turbidity- Turbidity arises from colloidal matter, finely suspended

particles and soil erosion. It is due to sewage and industrial effluents.

These are mainly from anthropogenic activities caused by human

beings. Non-turbid polluted water bodies may be contaminated with

acids and alkali, and thus it is also unsuitable for industrial and

domestic uses.
 iii) Taste- Change in taste is generally due to industrial effluents and the

main reason for taste change is Fe, Mn, phenols, free chlorine, oils,

hydrocarbons, chlorophenols, petroleum products, synthetic

detergents, pesticides, decomposed organic matter, algae, fungi,

bacteria, and pathogens. They impart a peculiar taste in water bodies.

Water purification techniques like chlorination, fluorination, chemical

precipitation, and activated carbon are used for purification of taste.

iv) Odour- Change in odour is caused due to chemical as well as

biological agents:-

Chemical agents include H2S, free chlorine, NH3, phenols, alcohols, esters,

and hydrocarbons.

Biological agents include algae, fungi, and microorganisms.

Untreated sewage if discharged into water bodies, causes foul odour.

Anaerobic algae cause grassy odour. Rivularia causes muddy odour.

Protozoa causes a fishy odour. Organic amines also cause fishy odour.

Phosphorous compounds cause a vermin smell. Earthy odours are due to

human beings.

Odours can be eliminated by chlorine, activated carbon and aeration.

 v) Thermal Pollution- Thermal pollution arises mainly by the

discharge of unutilized heat produced in various thermal power plants, it

reduces the amount of dissolved oxygen due to which organic matter

degrades faster.

2. Chemical pollution of water:- In this category, it causes changes in

acidity, alkalinity, pH, dissolved oxygen and other gases in water. It can be

caused by both organic and inorganic pollutants.

Organic Pollutant:

i) Biodegradable pollutants: These pollutants can be degraded with

time. These includes proteins, domestic sewage, waste from cannery

and slaughterhouses, fats from sewage soap production, food

processing and wool processing, carbohydrates, sugar, starch, from

textile mills, paper mills, sewage, polymers, resins, coal, oil and other

organic substances which are found in domestic and industrial waste.

ii) Non- biodegradable pollutants: These pollutants persist in the

aquatic system for a long period. Pesticides, Herbicides, fungicides, and

insecticides, due to agricultural run-off or from rain that can be washed

into nearby water bodies.

3. Biological Pollution of water:- It is caused by excretory products of warm-

blooded mammals, birds, bacteria, viruses, algae, protozoans, rotifers, etc.

Infection of the intestinal tract, dysentery, cholera, typhoid, polio,

gastroenteritis, infectious hepatitis.

4. Physiological pollution of water:- This is caused by several chemical

agents such as Cl2, F2, SO2, H2S, ketone, phenols, amines, mercaptans, and

hydroxyl benzene. They affect the central nervous system.

Sources Of Water Pollution: -

1. Sewage and domestic waste- Sewage contains minerals and organic matter,

human excreta, soap, detergent, garden waste, and metals. Sewage contains

a trace quantity of toxic metals such as Cu, Cr, Zn, Mn, Pb, and Ni. It also

contains decomposable organic matter and it exerts O2 demand. Organic

matter includes fatty acids, esters, amino acids, amides, amino sugars and

proteinaceous amine.

2. Industrial Effluents- In this category, we have toxic chemicals, hazardous

compounds, phenols, aldehydes, ketones, amines, cyanides, metallic waste,

acids, alkalis, oil, grease, suspended solid dyes, non-biodegradable matter,

radioactive waste and thermal pollutants.

3. Agricultural Discharge- Plant nutrients, pesticides, insecticides,

herbicides, farm waste, manure slurry sediments, plants and animals debris

and drainage from soil erosion.

4. Fertilizers- (main components of fertilizers are N, K, and P)

Fertilizers increase the total crop yield but at the expense of protein loss. The

balance of amino acids within the protein is disrupted, super-phosphates lead

to Fe, Cu, and Zn deficiency in plants, and potash treatment decreases

valuable nutrients like ascorbic acid and carotene in fruits and vegetables.

Clear water bodies are known as oligotrophic. When clear water bodies are

converted into marshy land by excessive algal bloom which is promoted due

to N, P, and K fertilizers and are washed into water bodies, this phenomenon

is known as eutrophication.

5. Detergents- Detergents contain surface active agents and contribute

phosphates of sodium silicates, sodium sulphates, and amides. They are

responsible for foaming. We have surfactants like alkyl benzyl sulphonates

(ABS) that persist for a longer time so they have been replaced by linear

alkyl benzyl sulphonates (LABS) which can be degraded.

6. Toxic Metals- Heavy metals such as Hg, Cd, Pb, As, Co, Mn, Fe, and Cr are

very harmful to aquatic ecosystems. They are added from industrial

processes, domestic sewage discharge, land runoff and fossil fuel burning.

7. Silt- These are soil particles which are the most damaging pollutants in hill-

streams. They create high turbidity in water and hinder the free movement

of aquatic organisms and, growth and productivity of fishes. They may also

lead to flood as the deposition of slits increases the water level.

8. Thermal Pollutants- These are due to atomic, nuclear and thermal power

plant waste, and discharge of unutilized heat in water bodies.

9. Radioactive Pollutants- These include fission and fusion products. Radio-

nuclei are added from nuclear power plants, nuclear reactors, nuclear tests,

nuclear installations or medical waste. They have a very long lifetime, so

they cause various diseases like genetic aberration, cancer etc.

Chemical Speciation:

The term chemical speciation means the identification of inorganic,

organic or organometallic species of an element/chemical present in the

environment. It is important to identify such species. The biological

activity e.g., the toxicity of an element may vary widely depending on the

species. The physical properties (volatility, solubility, etc.) and chemical

behaviour differ among the different species of an element. They affect the

mobility or transportation of the element concerned in the environment and

its toxicity to man and other organisms. These species can be identified and

quantified by various analytical methods.

Lead (Pb):

The possible species of Pb in water may be classified according to size.

The soluble portion passing through a 0.45μm membrane filtered may be

associated with colloidal particulate material.

In freshwater at pH 6, Pb occurs in the form of inorganic non-colloidal,

non-labile species and the composition is Pb2(OH)2CO3. At the higher pH

values of most surface water, Pb is more likely to be associated with

colloidal matter. In seawater, Pb is generally present as colloidal inorganic

matter. The species which have been identified as Pb in aqueous media are

Pb(CH3)4 and Pb(CH3)3+, Pb(CH3)22+, PbSO4, Pb3O4, PbO, and PbS. They

reach water bodies through street dents. The other species of lead are

(CH3)3C2H5Pb, (CH3)2(C2H5)2Pb, (CH3)(C2H5)Pb, (C2H5)4Pb.

Pb(CH3)4 → (CH3)3Pb+ → (CH3)2Pb2+ → CH3Pb3+

Air borne Pb compounds are found in both gaseous and particulate

matter. X-ray powder diffraction confirms the presence of PbSO4,

(NH4)2SO4, Pb3O4, PbO, PbSO4, etc. in street dust and reported to be

PbSO4(NH4)2SO2, PbBrCl22NH4Cl, PbBrCl(NH4)2BrCl etc.

Fig. Pathways of Pb in the environment

As mentioned above the atmosphere over Los Angeles alone has input of

about 20 tonnes of Pb daily. Greenland ice and Southern California basin

sediment have shown appreciable Pb content. The organo-lead compounds

are more toxic than inorganic lead. Commercially, five tetraalkyl lead

(TAL) compounds are important: tetramethyl lead (TML),

tetramethylethyl lead (TMEL), dimethyldiethyl lead (DMDEL),

methyltriethyl lead (MTEL) and tetraethyl lead (TEL). Most organic Pb

enters the atmosphere during manufacture, transfer of leaded gasoline and

use in vehicles.

Mercury (Hg):
Interest in chemical speciation virtually originated from the incident of

Mercury poisoning, starting with the Minamata episode (1953). Extensive

research has been carried out since the late 1950s on the aquatic chemistry

of Hg, its biotransformation, toxicity to biota and monitoring and analysis

techniques. It has been established that there is a remarkable difference in

toxicity between methyl mercury compound (CH3Hg)+, (CH3)2Hg and

other species, viz. Hg0, Hg2+, etc. These methyl mercury compounds are

villains in all fatal incidents due to Hg. Hence, the estimation of methyl

mercury compounds, particularly in biological material, is of considerable

importance. The major analytical approach in speciation has been to

distinguish between. Hg0, Hg2+, CH3Hg and (CH3)2Hg. A summary of the

environmental chemistry of Hg is given below-

Organic compounds (alkyl) of Hg have attained notoriety for their toxic

effect. Their presence in waterbodies requires constant monitoring. The

permissible limit of Hg in drinking water is 2 ppb. The best and most

convenient method is the Flameless Atomic Absorption Method, while the

dithizone method may also be used as it is fairly selective for Hg.

Arsenic (As):

Methyl arsenic compounds are found in several water bodies. Air contains

Arsenic mostly as an inorganic species. Some trimethyl arsenic in vapour

form can also be found and some methyl arsenic compounds as

particulates. Arsenic analysis is generally carried out by the silver

diethyldithiocarbonate spectrophotometrically.

Arsenic speciation in air samples is carried out using a glass wool filter to

trap the particulates, followed by silvered glass beads to trap volatile

arsines. The arsines are removed by dil. NaOH wash and analysed as

above, after NaBH4 reduction. Ambient air appears to contain As mostly

as the inorganic species, some (CH3)3 As in vapour form, and some methyl

arsenic compounds as particulates.

In water, speciation of arsenic (As) has been conducted by iodide (I-)

reduction. As(V) compound converted to iodides in the presence of I-,

which is then allowed to react with diethylammonium diethyl

dithiocarbamate to form arsenic complexes of diethyl dithiocarbamate.

 As[(C2H5)2N-CS2]3

CH3-As[(C2H5)2N-CS2]

(CH3)2-As[(C2H5)2N-CS2]

These species can be identified by gas chromatography. Organometallic

and organometalloidal species are identified by liquid chromatography.

Fig: Environmental Chemistry of Arsenic