This article throws light upon the three ways of treatment of industrial effluents. The ways are: 1. Primary Treatment 2. Secondary Treatment 3. Tertiary Treatment.
Way # 1. Primary Treatment:
It is of general nature and is used for removing suspended solids, odour, colour and to neutralize the high or low pH.
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It involves methods of:
(i) Screening
(ii) Neutralization
(iii) Equalization
(iv) Sedimentation
(v) Coagulation
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(vi) Precipitation
(vii) Co-precipitation etc.
Some general treatment processes are given below:
(i) Screening:
It is a process through which large materials like wooden pieces, metal pieces, paper, rags, pebbles, fibres etc. are removed. The rotary and circular filters are used now a days in modern industries to remove large materials.
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These both methods are effective and help in reducing suspended solids and BOD of the industrial effluent. The micro stainer is also used to remove five suspended particles in some treatment processes. To remove colloidal matter, ultra filters are also used although they are costly.
(ii) Neutralization:
When pH of the industrial waste is too high or too low then it should be neutralized by acid or alkali and only neutral effluent should be discharged into, the nullah or public sewer.
(a) Lime stone treatment:
For acidic effluent, lime stone should be used as it will form calcium compound [CaCl2, CaBr2, Ca(NO3)2 or CaSO4] depending upon the presence and amount of acid.
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(b) Caustic soda treatment:
Although it is costly method but it is also utilized for neutralizing the acid. Here caustic soda is added in the effluent to make the pH neutral. Only small amount of caustic soda is needed for this work.
For neutralization of alkaline effluent the following techniques are used:
(a) Carbon dioxide treatment:
If factory is producing carbon dioxide then only this method should be utilized for neutralizing the pH otherwise it would be costlier affair. Here CO2 is passed in alkaline effluent to make its pH almost 7.
(b) Sulphuric acid treatment:
This is a common method of neutralizing alkaline effluent. Here sulphuric acid is added in the effluent till pH becomes almost 7.
(c) Utilizing waste boiler – flue gas:
The stack gas which contains about 12% carbon dioxide is utilized to react alkaline effluent to make it neutral.
(iii) Equalization:
When effluent is discharged from a factory then its pH along with the quantity of suspended solids, dissolved solids etc. vary from the beginning to the last depending upon the dilution, velocity and the amount of reactants etc. Hence as the character of the effluent does not remain the same throughout hence proper treatment is not possible.
So equalization tank is necessary where effluent is kept for 10 hrs or more for the stabilization of pH and BOD. During equalization suspended solids settle down & new acid or alkaline treatment becomes economical.
The equalization tank should of sufficient size so that it may retain even the effluent of the whole day. Generally rectangular basins are selected for this purpose. It any how the arrangement for mechanical agitation is also done for some time in the tank then separation of suspended particles becomes more easier.
(iv) Sedimentation:
This treatment is only employed for the settlement of suspended particles by gravity. This technique is only used in the beginning to settle down the solid particles in a high suspension effluent.
Coagulation:
Experimental results have shown that a silt particle of size 0.05 mm requires about 11 hours to settle down through a depth of 3 m and clay particles of size 0.002 mm require about 4 days-time to settle the same height of 3 m at normal temperature of about 25°C.
As we know that water contains colloidal impurities which are even finer than 0.0001 mm and which also carry electrical charge on them. Due to electrical charges they remain in motion and never settle down.
Therefore when water is turbid due to presence of such fine size and colloidal impurities, plain sedimentation is of no use. It is also not possible to provide detention periods of longer than 4-9 hours. The coagulation becomes necessary when the turbidity is more than 40 – 55 ppm.
For dealing waters with such impurities a chemical process was evolved. This process removes all these impurities within reasonable period of 3 – 4 hours. This chemical process is called coagulation and the chemical used in the process is called coagulant.
(v) Principle of Coagulation:
The principle of coagulation can be explained from the following two aspects:
1. Roc formation, and
2. Electrical charges.
1. Floe formation:
When coagulant is added to the water and thoroughly mixed, it produces a thick insoluble gelatinous precipitate. This precipitate is called floe. The floe has the property of arresting the suspended impurities in water during its downward settlement towards the bottom of the tank.
The gelatinous precipitate has therefore the property of removing fine and colloidal particles quickly. The coagulation process also removes colour and test in general.
2. Electric charges:
The flock ions are electrically charged (positive) while all the colloidal particles have negative charge. Therefore floes attract the colloidal particles and cause their removal easily by settlement at bottom of the vessel in which it is used.
Coagulants:
The chemicals given below can be used as coagulants either alone or in combination.
a. Sodium aluminate.
b. Sodium aluminate + aluminium sulphate.
c. Aluminium sulphate.
d. Sodium aluminate + Ferric chloride.
e. Aluminium chloride (but used under exceptional circumstances only).
f. Aluminium sulphate + caustic soda.
g. Ferric chloride alone.
h. Aluminium sulphate + hydra ted lime.
i. Polyelectrolytes.
j. Ferrous sulphate.
k. Copper sulphate.
l. Sodium aluminate + magnesium chloride.
m. Copper sulphate+ hydrated lime.
n. Ferric sulphate.
o. Aluminium sulphate + sodium carbonate.
p. Ferric sulphate + hydrated lime.
q. Ferrous sulphate + hydrated lime.
r. Ferrous sulphate + chlorine.
s. Potassium permanganate + Ferrous sulphate.
t. Magnesium carbonate.
In water treatment plants following are the usual coagulants most commonly used:
i. Ferrous sulphate and lime.
ii. Magnesium carbonate.
iii. Polyelectrolytes.
iv. Aluminium sulphate.
v. Sodium aluminate.
vi. Chlorinated copperas.
Characteristics of some of these six coagulants are given below:
i. Ferrous sulphate and lime:
When lime and Ferrous sulphate are added to water, the ferrous hydroxide is formed in the form of floes.
FeSO4+ 7H2O + Ca(OH)2 → Fe(OH)2 + CaSO4 + 7H2O
4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3
Ferric hydroxide Fe(OH)3 is a floe which settles down and removes all the colloidal impurities in water. Ferrous sulphate is most effective in pH range of 8.5 and above.
ii. Magnesium carbonate:
(i) It removes turbidity and colour.
(ii) Floes formed are heavier than formed by other processes and thus detention period is very much reduced.
(iii) It removes iron and manganese completely.
(iv) It is possible to recycle and reuse the coagulant by passing the sludge through water containing carbon dioxide as follows:
Mg(OH)2 + CO2 → MgCO3 + H2O
iii. Polyelectrolytes:
Polyelectrolytes are special types of polymers. They may be anionic, cationic and non-ionic, depending upon the charge they carry. Out of these only cationic polyelectrolytes can be used independently, while other types can be used only along with other conventional coagulants.
Separam, wisprofloc, Mogul etc. are some of its patented forms available in foreign countries. In India, extract from seeds of a plant named Nirmali can act as polyelectrolyte. The plant is found in Andhra, Madhya Pradesh, West Bengal and Orissa.
iv. Aluminium sulphate:
It is also known as simply alum or filter Alum. Its chemical composition is Al2(SO4)3.18H2O. It reacts with alkaline water to form aluminium hydroxide (floe), calcium sulphate and carbon dioxide. The following are the chemical reactions which alum performs with various types of alkaline substances.
(i) Al2(SO4)3. 18H2O + 3Na2CO3 → 2Al(OH)3 + 3Na2SO4 + 18H2O + 3CO2
(ii) AL2(SO4)3. 18H2O + 3Ca(HCO3)2 → 2Al(OH)3 + 2CaSO4 + 18H2O + 6CO2
(iii) AL2(SO4)3. 18H2O + 3Ca(OH)2 → 2Al(OH)3 + 3CaSO4 + 18H2O
Alum is in most common use due to following reasons:
(i) In addition to turbidity it also reduces taste and odour.
(ii) It produces clear water. In other words, it is very efficient type of coagulant
(iii) Floes formed by it are more stable and heavy than that formed by other coagulants.
(iv) It is not harmful to health.
v. Sodium aluminate:
Chemical equation may be as given below:
Na2Al2O4 + CaSO4 → CaAl2O4 + Na2SO4
Na2Al2O4 + CaCl2 → CaAl2O4 + 2NaCl
Na2Al2O4 + Ca(HCO3)2 → CaAl2O4 + Na2CO3 + CO2 + H2O
The effective range of pH for this coagulant is 6.0 to 8.5.
vi. Chlorinated copperas:
The combination of Ferric sulphate Fe2(SO4)3 and Ferric chloride FeCl3 is called chlorinated copperas. When solution of Ferrous sulphate is mixed with chlorine, both Ferric sulphate and Ferric chlorides are produced. About 1 kg chlorine reacts with 7.3 kg of ferrous sulphate.
6FeSO4. 7H2O + 3CL2 → 2Fe2(SO4)3 + 2FeCl3 + 42H2O
Ferric sulphate and Ferric chloride each is an effective floe and their combination is also very effective.
It is found that Ferric chloride and Ferric sulphate can be used independently with lime to act as coagulants. In this case following are the chemical reactions.
Fe2(SO4)3 + 3Ca(OH)2 → 3CaSO4 + 2Fe(OH)3
2FeCl3 + 3Ca(OH)2 → 3CaCl2 + 2Fe(OH)3
Ferric chloride is effective in pH range of 3.50 to 6.50 or above 8.50 but Feme sulphate is effective in pH range of 4 to 7 and above 9.
Dry Fed Devices:
There are a number of devices which may be used for dry feeding. But two most commonly used devices are shown in Figs. 1 and 2. Dry powder of coagulant is filled in the conical hopper. The hoppers are fitted with agitating plates which prevent the chemical from being stabilized. At the bottom of the hopper a revolving helical screw or the toothed wheel is fixed.
The rotation of the helical screw or the toothed wheel is regulated through a venturi device in the raw water pipe. When more discharge is passed through the venturi device, the rotation of the screw or toothed wheel gets increased and more coagulant is thrown in the water.
Mixing Channels:
In this method, the mixture of raw water and coagulant is made to pass the channel in which fluming is done. After fluming vertical baffles are also fixed on both the sides of the channel. The complete arrangement has been shown in Fig. 3.
Way # 2. Secondary Treatment:
Generally secondary treatment involves biological processes and bacteria which are utilized for the oxidation. The organic substances present in the effluent serve as food for the bacteria and thus within a short time they make the effluent free from organic matter by consuming it.
The main biological treatments are given below:
i. Trickling filter
ii. Anaerobic digestion
iii. Oxidation ditch
iv. Aerated lagoon
v. Activated sludge process
vi. Oxidation pond
i. Trickling Filter:
Generally circular trickling filters are used but hexagon form can also be utilized. The bed broken stones serves as a trickling filter. When effluent after primary treatment is sprinkled over a bed of broken stones, then bacterial slimes (aerobic organisms) are generated on the stones and oxidize the effluent. Thus colloidal organic matter is consumed by them.
Now a days instead of stones, plastic and PVC circular pieces are used being lighter and economical. The depth of bed is generally taken about 2.5 to 3 metre depending upon the size of stone, temperature of effluent and retention time. As the function of organisms is directly proportional to temperature hence temp, is very important factor in this treatment. A low temperature decreases the efficiency of the filter.
According to bio-chemist the bacteria includes species of genera, pseudomonas, Alcaligens, Micrococcus, Flavo bacterium and Entero bactericeae. It has been observed that during the treatment Fungi and algae are also present in trickling filters. Thus it is a mixture of various organisms which is responsible for the oxidation of organic matter present in the effluent.
ii. Anaerobic Digestion:
This treatment is in fact slow oxidative digestion process carried out in absence of air in a closed container where ammonia and methane are released as the end products of the reaction. A very important point to be taken under consideration is that pH must be maintained almost 7 during the oxidation process.
In this treatment bacteria desulfo vibrio acts in reducing sulphates to sulphides and H2S is liberated. Besides this, bacteria belong to genera, methanosarcina, methano bacterium and methanococcus act to produce methane.
The bacteria flavobacterium alcaligenes, pseudomonas, aerobacter and escherichia act to produce acids. This treatment is generally used in paper mill, dairy, slaughter house and other factories which produce soluble organics.
iii. Oxidation Ditch:
It is a treatment generally carried out in beet sugar manufacturing plants, dairy, slaughter house and other factories to reduce BOD to 85 to 95%.
The oxidation ditch is an improvement over activated sludge process in shape and the aerator. The aerator is cage rotor placed across the channel.
Oxidation Ditch Process:
Oxidation ditch is a long continuous channel (lined with butyl rubber or plastic) generally oval in shape and about 1 to 2 metre in depth. The effluent after primary treatment is passed into oxidation ditch and kept here for long time as it is a slow process. High efficiency can be obtained by recycling about 90% sludge in such a fashion that overall sludge production is lowered down.
iv. Aerated Lagoons:
These are big cement tanks having a depth of 4-6 metres. These tanks or lagoons are used for the oxidation of dissolved organics. When the waste of the factory is passed in these lagoons and aerated mechanically then after 3- 4 days (depending upon the quantity and temperature) flocculent sludge is formed which is responsible for the oxidation of soluble organic material present in the effluent.
If proper precautions are taken then 85-90% BOD is reduced with aerated lagoons. The only drawback of this method is that it requires a long space for tanks.
v. Activated Sludge Process:
It is an important biological oxidation method for the removal of suspended and colloidal solids and also reduces BOD of the effluent. In this method, effluent is continuously subjected to biological degradation carried out by microbial floe suspended in the reaction tank into which oxygen is introduced by mechanical methods.
The effluent which comes after reaction tank is allowed to settle down and a portion of sludge is recycled to the tank itself for microbial population.
The process diagram is given below:
The important factors which determine the efficiency of the activated sludge are as follows:
(i) pH value
(ii) Temperature
(iii) Volume of tank
(iv) Nature of organic matter
(v) Velocity
(vi) Oxidation – reduction potential of sludge.
The microbial mass consists of bacteria, fungi, protozoa, nematodes and rotifers, which are formed in activated sludge process and are the active agents of activated sludge. The micro-organisms are supplied by adding essential nutrients mainly nitrogen & phosphorous. Generally Nitrogen & Phosphorous are supplied by addition of urea and mono ammonium or di-ammonium hydrogen phosphate.
The Nitrogen & Phosphorous requirement are about 10% & 2% per day. The actual amount can be calculated on the basis of the quantity of the sludge. The researches have shown that if trace amount of iron, cobalt & molybdenum are added then the efficiency of the oxidation increases.
The optimum pH for activated sludge should be between 6.5 – 9.0 as below 6.5 fungi will complete with bacteria and above 9.0 rate of metabolism will be reduced. For maintaining this range of pH a buffer can also be added (Bicarbonate can be added to maintain pH).
In this technique as pH is a very important factor hence continuous monitoring of pH, is necessary during the treatment. Besides this high temperature is also needed as it enhances metabolic activity due to which oxygen is consumed in a quicker time & thus anaerobic conditions are attained immediately.
Some chemists have suggested the following modifications in this process:
(i) Contact stabilization
(ii) High rate aerobic treatment
(iii) Tapered aeration
(iv) Extended aeration
(v) Tapered aeration.
Out of these aeration techniques, extended aeration is preferred where only aeration period is extended (from 10-12 hrs) to 30 – 45 hrs. If the effluent is passed at several places of the aeration tank then it is called ‘Stepped Aeration’ while reduction of oxygen supply continuously in the tank is called ‘Tapered aeration’.
Now a days a modified activated sludge process called ‘High-rate aerobic treatment’ is used. In this technique, aeration period is extended from 30 to 70 hrs, settling and returning of settled sludge to the aeration tank. Here a big tank is taken so that complete aeration period may be given to the effluent
vi. Oxidation Pond:
It is a pond where oxidation takes place with the help of bacteria- pseudomonas, alcaligenes and flavo bacterium and algae. In fact the presence of bacteria and algae both help in the oxidation of organic load of effluent.
The dimensions of the pond depend upon the quantity of the effluent releases per day. The depth of the pond should be between 1.5 metre – 2 metre otherwise it would become breeding place for mosquitoes & midges. Preference has been given for the depth of 2 metre as in this case pond becomes anaerobic rather than aerobic.
The bottom of the pond and all areas should be made of concrete or brick to avoid seepage of sludge etc. through the pores. Polythene sheeting should be spread in the lower portion to avoid any chance of seepage through any side. The tank should be made in such a way that at the entrance of the placement it should be more deeper than at the outlet to create anaerobic zone for solid deposition & digestion.
The mechanism of the pond reaction is as follows:
When the effluent is added in the pond the solids slowly & slowly settle in the bottom and this solid layer acts as anaerobic phase and anaerobes act to convert organic matter into ammonia, methane and carbon dioxide.
In the top layer of the pond the aerobic oxidation takes place releasing the products water and carbon dioxide. The facultative zone is near anaerobic phase hence the presence of excess algae in the pond will enhance the rate of oxidation of the organic matter.
In fact the oxygen needed for the metabolism is given by the algae in the pond and which in turn absorbs carbon dioxide released by bacteria from photosynthesis. Thus both bacteria and algae help in the oxidation of the organic matter of the pond and hence their presence in good amount is necessary for the effective purification of the effluent.
As we know that for photosynthesis of algae the sun light is necessary hence precautions should be taken in such a way that maximum light penetration should occur in the pond.
The following factors are important for the effective anaerobic and aerobic oxidation of the pond:
(i) The pond should be open and larger and depth of pond should be 2 metre.
(ii) Sun Light penetration in the pond for photo synthesis of algae.
(iii) Wind action for mixing.
(iv) Nutrient level should be neither too high & nor too low as in case of high euglena and chlorella dominate and for low nutrient level filamentous algae such as ulothrix, vaucheria and spirogyra will be generated.
(v) The pH of the effluent should be high as it will lead to precipitation of die heavy metals as hydroxides which settle as sludge.
(vi) For disinfection of waste material, it should be taken into another pond or disinfectant should be added separately. Disinfectant should not be added in the oxidation pond otherwise the whole mechanism will be stopped.
Way # 3. Tertiary Treatment:
This type of treatment is needed for the effluent for the removal of bacteria and dissolved inorganic matter (metals, metal oxides, metal Carbonates, metal sulphates etc.). The bacteria of fecal origin are removed by keeping the effluent in maturation ponds for a definite period.
If any how there are still bacteria in the effluent then bleaching powder is added to kill them or a higher dose of chlorine gas is passed for a definite interval of time say 1 to 1.5 hrs.
According to latest researches conducted by V. P. Kudesia and Co-Workers if a mixture of chlorine and bromine solution is passed in the effluent then it kills all type of bacteria within 30 minutes while if chlorine of same concentration takes 1 hour.
This reduction of time is due to the fact that a mixture of 4 species of chlorine (Cl2, HOCI, CI3– and Cl+) and 4 species of bromine (Br2, HOBr, Br3 and Br+) when combine lead to formation of several new species along with more formation of molecular chlorine and molecular bromine & which cause the rupture of the bacterial cell (See Water Pollution by V. P. Kudesia, Pragati Prakashan, Meerut).
For the removal of inorganic substances, the following methods have been recommended:
i. Reverse osmosis
ii. Chemical Precipitation
iii. Evaporation
iv. Dialysis
v. Removal by algae
vi. Ion Exchange
vii. Activated carbon
viii. Exchange resins
ix. Bark of Babul or other plants for the removal of metals.
i. Reverse Osmosis:
The passage of solvent from a solution of low concentration to high concentration through semi permeable membrane is called osmosis. The reverse of this process is called reverse osmosis.
Here effluent containing dissolved solids is passed through semipermeable membrane at the pressure excess of osmotic pressure of feed waste (about 45 – 50 atm) the water of the effluent passes through the semi permeable membrane leaving the dissolved solids on the surface.
Thus through this technique the solid inorganic matter is separated from the water and the material can be recovered by using proper techniques. Although this method is costly as collulose acetate or polyamide hydrazide are used as semipermeable membranes but it is an effective method & yields immediate results.
ii. Chemical Precipitation:
It is a chemical technique by which metals are removed by precipitating them either as hydroxides at high pH or as sulphates etc. For example, chromate from electroplating industry can easily be removed by reaction with ferrous sulphate and then precipitating it with lime.
From Barium chloride factory, the excess of Barium is precipitated as Barium sulphate by the addition of Dil. Sulphuric acid. The nickel can be precipitated with the help of dimethyl glyoxime and mercury with the help of potassium iodide as mercuric iodide. Thus by use of suitable reagents, metals can be precipitated & thus can be separated & reutilized.
iii. Evaporation:
This method is generally employed when waste solid/solids are reused in the industry. It is a method used for recovery of radioactive substances. Here the effluent is boiled and after the evaporation of the water, the concentrated solution is left out in the vessel which is again used in the recycle process of the industry.
iv. Dialysis:
The process of separating crystalloid from colloid by diffusion or filtration through a membrane is called dialysis.
The dialyzer consists of a shallow cylinder open at both ends, over one end of which a membrane is tied. The effluent which is to be dialyzed is placed in this cylinder which is then suspended in a large dish containing water.
The flow of water is continuous. Dialysis is a slow process and takes several hours for the complete removal of the substances. Take for example the effluent containing hydrochloric acid, and sodium silicate then resulting sodium chloride and hydrochloric acid diffuse away, leasing only colloidal silicic acid in the membrane.
v. Removal by Algae:
As we know that algae require metals – cobalt, copper, zinc, manganese, iron, molybdenum etc. in trace amounts and potassium, calcium, magnesium, phosphorus, nitrogen and sulphur for their growth hence presence of algae in the effluent will reduce the above contents from the effluent It is important to note that algae must be removed prior to its reuse otherwise it would create problems.
vi. Activated Carbon:
As activated carbon has high adsorption power hence it is utilized for the removal of pesticides such as DDT, hexachlorobenzene, Dieldrin, Heptachlor, Lindane, Aldrin, Chlordane, Toxophene, Methoxychlor, Heptachlor epioxide & others. The simple mechanism involves in this technique is based on the phenomenon of adsorption.
As adsorption is a surface phenomenon hence the pesticides or other substances get adsorb on the surface of the activated carbon & can easily be removed & reutilized. The advantage of this technique is that adsorbed substances can be recovered in the same form in which they were present.
vii. Resins:
Generally, ion exchanger resins are insoluble in water and in organic solvents, and they contain active or counter ions that will exchange reversibly with other ions, in a surrounding solution without any appreciable physical change occurring in the material.
The ion exchanger is of complex nature and is polymeric. In a cation exchanger the active ions are cations while in an anion exchanger, active ions are anions. The structure of cross-linked sulphonated polystyrene is shown on next page.
It is said that when resin is brought in contact with water, some water penetrates into the resin, and the hydrogen atoms of the sulphonic acid ionize. They may then be replaced by an equivalent quantity of another cation.
(a) Cation Exchange Resins:
They have a high molecular weight, having cross-linked polymer containing sulphonic, carboxylic, phenolic etc. groups as an integral part of the resin and an equivalent amount of cations.
(b) Anion Exchange Resins:
They are also polymers containing amine or, quaternary ammonium groups as integral parts of the polymer lattice and an equivalent amount of anions such as chloride, hydroxyl or sulphate ions.
Characteristics of a Useful Resin are:
1. The resin must be sufficiently cross-linked to have only a negligible solubility.
2. The resin must contain a sufficient number of accessible ionic exchange groups and must be chemically stable.
3. The resin must be sufficiently hydrophobic to permit diffusion of ions through the structure at a finite and unstable rate.
4. The Swollen resin must be denser than Water.
Action of ion-exchange Resins:
It is clear that cation exchange resins contain free exchangeable cations.
(Res. A–)B++ C+(soln.) <=> (Res. A–) C++ B+(soln.)
where (Res. A–)B+ represents the cation-exchange resin in which resin is the basic polymer of the resin, A– is the anion attached to the polymeric frame work and B+ is the active or mobile cation.
Hence a sulphonated polystyrene resin in the hydrogen form may be written as (Res. SO3–)H+. Similarly an anion exchange resin may be written as (Res. NMe3+) Cl–
If we observe that the equilibrium is completely displaced from left to right, the ions C3+ is completely fixed on the cation exchanger.
If the solution contains several ions (C+, D+ and E+) the exchanger may show different affinities for them thus making separation possible. We here consider the displacement of sodium ions in a sulphonate resin by calcium ions.
2(Res. SO3–) Na+ + Ca++ (soln.) ↔ (Res. SO3–)2 Ca++ + 2Na+ (soln.)
Since the reaction is reversible, hence the resin may be recovered by passing a solution containing sodium ions through the product, the calcium ions are thus removed from the resin.
(a) Cation Exchange Resins:
Consist of small, simple cations and can be exchanged by large, high molecular weight anions.
Resin – H + NaCl → Resin – NaHCl
(b) Action Exchange Resins:
Consist of small, simple anions which can be exchanged by large, high molecular weight cations.
Resin – OH + NaCl → Resin – CI + NaOH
Both natural and synthetic materials are available as ion exchange resins.
The factors determining the distribution of ions between an ion- exchange resin and a solution include.
(1) Nature of Exchanging Ions:
(a) The extent of exchange increases with increasing valency of the exchanging ions (At low concentration and at ordinary temperatures), e.g.,
Na+ < Ca++ < Al+++ < Th++++
(b) The extent of exchange increases with decrease in size of the hydrated cation (under similar conditions and constant valence).
Li+ < H+ < Na+ < NH4+ < K+ < Rb+ < Cs+
(c) When a cation in solution is being exchanged by an ion of different valency the relative affinity of the higher valent ion increases in direct proportion to the dilution.
(d) With strongly basic anion exchange resins univalent anions appear to behave similarly to univalent cations.
(2) Nature of Ion-Exchange Resin:
The absorption of ion will depend upon the nature of the functional groups in the resin.
The result obtained in ion-exchange separations can be affected by:
i. The solvent or eluant,
ii. Temperature,
iii. Particle size,
iv. Nature of the ion-exchange resins,
v. The hydrogen ion concentration,
vi. The length of the column, and
vii. The rate of flow of eluant
The important properties which notice the behaviour of a resins are:
a. Number of functional groups.
b. Nature of functional groups.
c. Degree of cross linking.
d. Size of particles.
e. Strength of functional groups.
Theoretical Principles:
Ion Exchange Equilibria:
The net result of an ion exchange reaction may be expressed as a replacement of equivalent quantities of like charged ions.
The value of the equilibrium constant or selectivity coefficient K remains constant if activities of various species are used – otherwise K will change due to change in activity coefficients.
The experimental results have given useful rules:
i. The greater the charge on the ion, the greater the affinity for the resin.
ii. The selectivity coefficients are nearly one as the cross linking is decreased.
iii. The affinity of high molecular weight organic ions and some anionic complexes of metal ions are generally high.
iv. The smaller the (hydrated) ion the greater the affinity for the resin.
v. The observed order of affinity for groups of ions is
The above order exhibits the following changes:
(a) Change in pH
(b) Change in relative concentrations
(c) Change in nature of resin,
(d) Ionic strength and
(e) Complex formation.
The plate theory of partition of chromatography can be applied directly to anion exchange column.
The distribution ratio d can be defined as:
D = quantity of sample in resin of a given plate/quantity of sample in interstitial solution of same phase. Suppose a sample contains cations A+ and B+ which are to be separated by this technique. The sample is introduced into the column, it is retained at the top of column by exchange of cations
In all equations (7) to (9), we have neglected the activity coefficients. The cations A+ and B+ can be eluted from the column if they are replaced by another cation contained in the eluent. Suppose there is a high concentration of [H+] ions, so the increase of [H+] will also increase [A+] and [B+].
Experimental Technique:
The apparatus consists of a burette provided with a glass wool plug at the lower end. The resin particles should be packed uniformly in the column so that solutions may pass through the column in a uniform manner. It is necessary that resin bed should be free from air bubbles so that there is no channelling.
Generally ion-exchange resins contain ionic impurities and water soluble intermediates. They must be washed out before use. This washing is done by passing 2M-HCL and 2M – NaOH alternatively, with distilled water rinsing’s in between, and then washing with water until the effluent is neutral and salt free.
Both acidic cationic exchangers and basic cationic exchangers are used in this chromatography. (The acidic cationic exchangers are those having -OH, -SO2OH or –CH2SO2OH groups in the molecule while basic anionic exchangers are styrene quaternary ammonium compounds).
The effluent containing the mixture of salts is first passed through the column. A cation exchanger replaces all cations present in solution with H+ ions (or other). The least adsorbed ions come out first and the most strongly remain near the top.
An eluting solution containing a strongly absorbable cation e.g. H+ ion in HCL is now passed through the column which gradually displaces other cations. The eluates are collected in separate containers and then purity is detected by the suitable techniques.
The presence of toxic heavy metals ions in industrial waste waters has become a matter of concern in recent years. The toxic heavy metal ions which pose potential danger from the industrial waste waters include lead, mercury, copper, cadmium, zinc and nickel.
Precipitation, ion-exchange, evaporation, reverse osmosis, electro-dialysis, adsorption by activated carbon and clays, solvent extraction and cementation are some of the techniques that have been used for removing heavy metal ions from industrial waste waters (1). Ion exchange methods can be efficient but synthetic ion-exchange resins are relatively expensive.
Many of the methods which are used at present to scavenge the heavy metal ions from industrial waste waters are either uneconomical or are unable to meet effectively the stringent water quality limits for waste effluents, usually much less than 1 ppm. for each heavy metal.
Interest has arisen recently in the investigation of some unconventional methods and materials for scavenging heavy metal ions from industrial waste waters. Insoluble starch xanthenes have been found to be very useful for removing heavy metal ions from solutions.
Agricultural waste materials such as waste wool, Peanutskin, Walnut expeller metal, modified cotton and modified barks have been investigated by various workers to study their effectiveness in binding heavy metal ions. Many of these agricultural by-products are widely available and are of little or no economic value and some of them in fact present a disposal problem.
The use of coastal red wood bark, water hycnith, black and red oak, western hemlock, pine and sweet gum bark for scavenging toxic heavy metal ions has been reported by several workers.
The efficiency of commonly available Indian barks such as Babul (Accacia, Arabica) for the retrieval of toxic heavy metal ions has been evaluated by Kudesia in 1978. It has been observed that the bark of Babul absorbs about 99% heavy metals from the effluent.
Thus it can be used as an effective tool for the removal of metals from the industrial effluents. Moreover as the raw material is very cheap hence the technique can be used without any investment.
Further studies with other plants are going on will be helpful in preventing environmental pollution and giving a clue for the recovery of heavy metals thus saving crores of rupees wasted in purchasing these metals every year by industrialists.