Much has been reported on the treatment of phenol containing waste, the traditional methods are found to be ineffective to remove phenol from solutions in waste water treatment for public water supplies.
The following methods are commonly used in the removal of phenol:
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(i) Centrifugal sedimentation.
(ii) Evaporation.
(iii) Oxidation with ozone.
(iv) Chlorination.
(v) Stripping and oxidation.
(vi) Solvent extraction.
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(vii) Ion exchange.
(viii) Aeration.
(ix) Biological methods.
1. Adsorption:
The adsorption process occurs at solid-solid, gas-solid, gas-liquid, liquid-liquid, liquid-solid interfaces.
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Adsorption with a solid such as carbon is dependent on the surface area of the solid. Thus carbon treatment of water involves the liquid-solid interface.
There are two methods of adsorption, physiorption and chemisorption. Both methods take place when the molecules in the liquid phase become attached to the surface of the solid as a result of the attractive forces at the surface (adsorbent) overcoming the kinetic energy of the liquid (adsorbate) molecules.
Physiorption usually involves a smaller change than does chemisorption. This type of adsorption is multilayered. Chemisorption occurs due to the reaction of adsorption molecule and the adsorbent. Physical adsorption diminishes at higher temperatures, whereas some elevation of temperature is often necessary and may be activated by heat to participate in chemisorption by a phenomenon termed as activated adsorption.
The factors affecting adsorption are:
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The physical and chemical characteristic of the adsorbent (i.e., surface area, pore size, chemical composition, etc.).
The physical and chemical characteristics of the adsorbate (i.e., molecular size, molecular polarity, chemical composition etc.).
The concentration of the adsorbate in the liquid phase (solution).
The characteristics of liquid phase. (pH, temperature).
The contact time of the system.
Adsorption with Activated Carbon:
Certain organic compounds in waste waters are resistant to biological oxidation (degradation) and many other are toxic and nuisances (odour, colour, taste) even at low concentration.
Low concentrations are not readily removed by conventional treatment methods. Activated carbon has an affinity for organics and its use for organic contaminant removal from waste is widespread.
The effectiveness of activated carbon for the removal of organic compounds from waste waters by adsorption is enhanced by its large surface area, a critical factor in the adsorption process. The surface area of activated carbon typically can range from 500 – 1400 m2/g. The chemical nature or polarity varies with the carbon type and can influence attractive forces between the molecules.
Alkaline surface are characteristic of carbon of vegetable origins and this type of surface polarity affects adsorption of dyes, colours and unsaturated organic compounds. Activated carbon surface are non- popular, making the adsorption of inorganic electrolytes difficult and the adsorption of organics are easily affected.
Manufacture of Activated Carbon:
The term activated carbon comprises a family of substances. None of the members of the family is characterised by a definite structural formula nor can be separately identified by chemical analysis. The only basis for differentiating is by adsorptive and catalytic properties.
List of source materials used for the preparation of activated carbon given below:
2. Carbonisation:
The first stage in the preparation of activated carbon involves dehydration followed by carbonisation of the raw material. This is usually conducted in the absence of air by slow heating at temperatures below 600°C. Although instances are reported in which higher temperatures have been used successfully. Sometime a dehydrating agent such as zinc, chloride, or phosphoric acid may be used. Excess water including structural water, must be driven out from the organic material.
Carbonisation converts this organic material to primary carbon which is a mixture of ash (inert inorganics) tars, amorphous carbon, crystalline carbon (elementary graphite crystallites). Non-carbon elements (H2O2) are removed in gaseous form and the freed elementary carbon atoms are graphed into crystallographic formations. During carbonisation, some decomposition products or tars will be deposited in the pores but will be removed in the activation stage.
Powdered Activated Carbon:
In waste water applications, powdered carbons are predominantly (60-75%) smaller than 325 mesh. Powdered activated carbon is usually produced by activating lump material or chip of wood charcoal or lumps of paste prepared from saw dust and subsequently grinding the activated product.
Depending upon the method of grinding the shape of the particles can differ to a certain extent and this may markedly influence the properties. Active carbon ground in ball mill has oval particles and the carbon ground in hammer mills has elongated particles.
Granular Activated Carbon:
Granular carbons are typically larger than 40 mesh. Granular carbons may be formed by either crushing or pressing, crushed activated carbon is prepared by activating a lump material, which is than crushed and classified to desired particle size. Pressed activated carbon is formed prior to activation.
Activated Carbon Characteristics:
The properties of granular carbons which are important in performance include particle size, surface area, hardness, apparent density, pore volume etc.
Particle Size:
Activated carbons are available in diverse forms-symmetrical pellets, irregular shaped granules, powder and pre-formed shapes. The particle size of carbons must be too small because the rate of filtration is thus impaired. The rate of filtration is influenced by particle size but even more by the distribution of particle size. Head loss in the carbon contactor is an important design consideration and is affected by the carbon particle size.
Surface Area:
Adsorption is a surface phenomenon and the capacity of activated carbon to adsorb large quantities of organic molecules from its highly porous structure provides a large surface area. Carbon has been activated with a surface area yield to 2500 m2/g but 1000 m2/g is more typical.
Total surface area is normally measured by the adsorption of nitrogen gas by the Brunauer Emmet – Teller method.
Hardness:
Granular carbons should have sufficient strength to withstand the effects of handling. The resistance to abrasion is measured by comparing the screen analysis of a carbon before and after being subjected to abrasion. The conditions for conducting the abrasion are not yet standardised and vary from one supplier to another.
Apparent Density:
The apparent density of a carbon is the weight of a unit volume of it including the pores and the voids between the particles. This property is of little value in evaluation and selection of an activated carbon but it is very useful as one of the measures of successful regeneration accomplished. Apparent density of measurement is a very simple test and the density of regenerated carbon indicates the degree of regeneration accomplished.
Pore Volume:
This is a measure of the total macropore and micropore volume of the carbon which can be of some value in the selection and application of an activated carbon for a specific waste constituent relative to molecular weight.
Analytical Applications of Activated Carbon:
Activated carbon is commonly used in water, wastewater treatment, removing organics that cause odours, tastes and other detrimental effects. In addition it can be used for solvent purification or recovery of expensive materials. Koch (1918) has estimated the amount of gold in sea water. The activated carbon is used to measure the nicotinic acid content of foodstuffs, the nicotinic acid being adsorbed by carbon and subsequently eluted with a solution of sodium hydroxide in alcohol.
Tastes, colours and odours are removed from potable waters and dissolved organics such as phenols, pesticides, organic dyes, surfactants etc., are removed from industrial and municipal waste water. The removal process continues until the carbon reaches its adsorption saturation point, after which it should be regenerated. Suggested the use of carbon to determine the concentration of barbiturates, the adsorbed compound being eluted with ether.
Calls attention to precaution that should be observed when employing carbon as a clarification agent in the analysis of sugars. It has been reported that carbons with a high iodine number are usually effective for antistaining. The use of activated carbon to protect sensitive field crops against injury from a weed killer-2, 4, -D [2, 4-dichlorophenoxyacetic acid].
3. Experimental:
The low cost adsorbent groundnut husk is obtained from the nearby market. Other chemicals used in the present study namely sodium thiosulphate, phenol, potassium bromide, potassium bromate, potassium dichromate, potassium iodide, 4-amino antipyrene, starch, potassium ferricyanide, ammonium chloride, and ammonium hydroxide are of AR/CDH, BDH grade.
Preparation of Activated Carbon:
A known amount of groundnut husk is taken and it is washed with water to remove the earthy materials. Then it is kept in an oven at 120°C. It is treated with concentrated sulphuric acid and kept in an oven at 160°C for 24 hours. It is then washed repeatedly with distilled water. During the last washing two drops of BaCl2 is added to test the presence of sulphuric acid. It is dried in a hot air oven at 110°C and sieved in a 20-40 mesh size. Thus the carbon particles of uniform mesh size is prepared and activated for three hours at 200°C.
Standardisation of Phenol:
Phenol is standardised by bromination of phenol. First the aliquot is .taken in a iodine flask and the Winkler’s solution [KBr + KBrO3] is added and the flask is shaken. Then concentrated hydrochloric acid is added and allowed to stand for few minutes. Potassium iodine solution is added and titrated against sodium thiosulphate using starch as indicator. Sodium thiosulphate is standardised by using standard potassium dichromate.
Calibration Curve:
Calibration curve is drawn by following procedure adopted in the literature. Phenol reacts with 4-amino antipyrene to give a red coloured complex whose concentration is estimated in a UV Spectrophotometer (Systronics) at 510 nm.
With the help of these data calibration graph is drawn (Fig. 4.26). The readings are given in Table 4.17 and it is used in the subsequent analysis.
Optimisation of Carbon Dosage:
10 ppm of phenol solution is prepared from stock solution. It is taken in conical flasks and the carbon that has been prepared from groundnut husk is added in the range of 0.2g, 0.4 g, 0.6 g, 0.8 g, 1 g, 1.2 g, 1.4 g, 1.6 g, 1.8 g, 2 g, 2.2 g, 2.4 g, in each of the flasks and it is kept
in the shaker for 5 hours. Later it is taken and filtered. To the filtrate, buffer solution, 4-amino antipyrene reagent and potassium ferricyanide solution are added.
Then the absorbance values are measured as per the standard procedure. From the absorbance value the amount of phenol adsorbed can be calculated. A graph is plotted between the carbon dosage in gm and percentage of phenol adsorbed. The data is given in Table 4.18 and represented in Fig. 4.27. From the graph of optimum dosage of carbon is found to be 2 gm.
Optimisation of Time:
The amount of phenol adsorbed by the activated carbon is found to vary with time. A stock solution containing 10 ppm of phenol is prepared. Adsorption of phenol from this solution is studied with 2 gms of carbon containing in different conical flasks.
The solution is stirred in the mechanical shaker to attain adsorption equilibrium and the filtrate was analysed with Spectrophotometer at 510 nm. A plot of phenol adsorption versus time is drawn. The results are depicted in Fig. 4.28 and in the Table 4.19. It is observed that an effective removal of phenol required 4 hours of shaking.
Optimisation of pH:
20, 40, 60, 80 ppm solutions were prepared from the stock solution. Then the solution is taken in a beaker; the pH of the solution is adjusted to the desired value by a digital pH meter. It is kept in the mechanical shaker for 4 hours and then it is filtered. The filtrate is treated with the buffer solution, 4-amino antipyrene and potassium ferricyanide solutions. Later it is analysed in a Spectrophotometer at a wavelength of 510 nm. The absorbance values are measured. The procedure was repeated for various pH values of the aliquot sample. The adsorption was found to be maximum at pH5 (Table 4.20).
Fruendlich Adsorption Isotherm:
In this, adsorption was studied for various concentrations (20 ppm to 100 ppm). This experiment was carried out to study the influence of phenol concentration on carbon. The high concentration solutions were diluted to a level to obtain the absorption values limited to standard values.
4. Effect of Ions over Fruendlich Adsorption Isotherm:
The effect of CI–, SO42–, HCO3–, CO32– over Freundlich adsorption isotherm were studied.
Column Studied:
To remove the impurities from large volumes of waste water, continuous flow adsorbers are preferred over batch adsorbers. The desalter waste and the hot well sample from the refinery waste was taken and the phenol present in the respective waste was removed by adsorption with the activated carbon by column studies. The process was carried out in steady state fixed bed flow adsorbers because this type of operation is relatively simple and it also functions as a filter unit.
Hence the solutions (20-100 ppm) was taken in a 250 ml conical flask and kept in the shaker for 4 hours. Then it is taken out and filtered. To the filtrate, the reagent was added and absorbance value was measured as per the standard procedure. This procedure was repeated for various concentrations of the aliquot sample. The adsorption was found to increasing with increasing concentration of phenol. The results obtained is given in Table 4.21. A straight line is obtained.
Experimental Set Up:
The column is a glass tube with 1 cm thickness and 10 cm in length. It contains glass wool at the bottom to ensure even distribution of flow and for supporting the carbon material. 5g of the granular solid activated carbon made from groundnut husk was weighed and packed uniformly into the column.
The flow rate of 1 ml/minute was adjusted by adding distilled water. Both the desalter waste and hot well sample was taken and adsorption was conducted in stages of half-an-hour. The results obtained are given in Table 4.22 and Table 4.23.
Continuous Adsorption:
Continuous adsorption of phenol was carried out with desalter waste and hot well sample by packing the column with 5g of activated carbon and it was found that there was 93% adsorption of phenol in the case of desalter waste and there was 94.1% adsorption of phenol in the case of hot well sample.
Activated carbon is prepared from groundnut husk. It was tried in place of commercially available granular activated carbon.
Effect of Carbon Dosage:
It was found that the adsorption of phenol increases with increase in the carbon dosage. With 2 gm of activated carbon there was 83% phenol adsorption. So as we increase the carbon dosage, 100% phenol adsorption can be achieved. A plot of % adsorption of phenol versus weight of activated carbon was drawn and it was found from the Fig. 4.28 that 2g of activated carbon was fixed as optimum carbon dosage.
Time Dependence on Adsorption of Phenol:
The amount of phenol adsorbed by carbon was found to vary with different time intervals. A plot of phenol adsorption versus time was drawn. The percentage removal of phenol and the time of shaking are given in Table 4.21. It was observed that an effective removal of phenol required a period of 4 hours shaking with 2g of activated carbon.
Effect of pH:
The hydrogen ion concentration (pH) of the solution also affects the adsorption of phenol. A wide range of pH is encountered in the industrial effluents. The effect of pH on phenol adsorption by the activated carbon was studied. The studies carried out at pH 3, 4, 5, 6 and 7. The parameters K and 1/n were calculated for these pH values. It was found that the phenol adsorption was more effective at pH5 (Table 4.22).
Adsorption Isotherm:
Adsorption isotherm tests are one of the important tests in selecting a particular type of carbon for water, waste water and industrial treatment systems. The Fruendlich adsorption isotherm is usually used to quantify the equilibrium relationship between the amount of impurity adsorbed on the carbon and the impurity concentration remaining in the solution.
Mathematically, x/m = Kcl/n
where x = Amount of impurity adsorbed.
m = Weight of carbon.
c = Equilibrium concentration of impurity in solution.
K, n = Fruendlich parameters.
These parameters represent the adsorption capacity and intensity of adsorption respectively. The experimental data obtained for the system by adsorption of phenol on activated carbon obey the well-known Fruendlich adsorption isotherm.
The Fruendlich equation can also be given in the form –
log x/m = log K + 1/n log c
Where the values given in Table 4.21, a plot of log x/m versus log c is drawn which gives Fruendlich adsorption isotherm. From the graph the intercept K and slop 1/n are calculated.
The parameters 1/n and K are of definite importance in the characterisation of the capacity of removal of phenol by activated carbon. From the inclined straight line obtained, it is evident that the adsorption increases with increase in concentration.
Effect of Ions on Adsorption Isotherm:
The effect of ions such as Cl–, SO42–, CO32–, HCO3– on the Fruendlich adsorption was studied. A plot of log x/m versus log c was drawn. Values of K and 1/n were calculated. The presence of ions like Cl–, SO42–, CO32–, HCO3– in the phenolic waste do not affect considerably the adsorption of phenol on activated carbon.
Column Studies:
The hot well sample and the desalter waste from the refinery waste was treated in a column of carbon. It is found that there was an effective removal of 93% of phenol in the case of desalter waste and 98% of phenol in the case of hot well sample.
Activated carbon adsorption is one of the latest techniques in waste water treatment using which the stringent discharge quality standards can be made. Investigations have been undertaken to determine whether cheaply, commercially available materials held promise in the treatment of the waste water.
One such material groundnut husk was selected and its feasibility was studied. Parameters such as carbon dosage, time, pH were optimised with regard to phenol adsorption using groundnut husk activated carbon. The Fruendlich adsorption isotherm was studied. The desalter waste and hot well sample from the refinery waste was effectively treated with the activated carbon for the removal of phenolic waste.