Phenol removal from aqueous systems by sorption on jackwood sawdust

Transcription

1 ndian Journal of Chemical Technology Vol. 2, May 1995, pp Phenol removal from aqueous systems by sorption on jackwood sawdust V Sivanandan Achari & T S Anirudhan Depilftment of Chemistry, University of Kerala, Trivandrum Received 4 April 1994; accepted 24 July , ndia The adsorption technique using sawdust of jackwood timber has been applied for the removal of phenol from aqueous solutions. The extent of removal was dependant on concentration, ph and temperature of the solution. With an initial cqncentration of 25 mg/l at 30 C and ph 7, the removal was found to be 62.5% by using sawdust as adsorbent. The time to reach equilibrium was found to be 6 h. The higher uptake of 79.2% of the initial concentration of 25 mg/l occurs at ph 5.5 at 30 C. The applicability of Freundlich isotherm to the sawdust-phenol system was tested at 10, 20 and 30 C at ph 5.5, which can be used for the design of wastewater treatment plants. The spent adsorbent can be regenerated and reused by acid treatment. Phenol and its derivatives are toxic contaminants in aquatic environments and impart objectional taste and odour to water and is toxic to fish1.2. The permissible limit for phenol in potable water is 1,uhlL. The human consumption of phenol contaminated water can cause severe pain and ultimate damage to capillaries. The studies showed that adsorption technique is an effective method for phenol removal3 Several lowcost substances such as cellulosic materials, peanut skins, onion skins, bambara nuts, rice husks and waste tea leaves are found to be the effective adsorbents for toxic metallic ions present in sewage and waste water4-8. t has been reported that sawdust is a good adsorbent for removing heavy metals from wastewater9 Detailed studies on the sorption of metal ions such as Cd2+ and Cu2+ onto sawdust have been u}dertaken by sever~ groups of investigators However, the adsorption behaviour of phenol on sawdust has not so far been studied. The present study is intended to the use of this locally available conventional cheap material, as a phenol adsorbent. This adsorbent is a by' product of timber and plywood industries. t finds a little use either as a cheap fuel, or a packing material. The experimental material is collected from jackwood timber which is widely used for furniture and housemaking in Kerala due to the durability of its hardwood portion. The availability of sawdust is maximum in Kerala. n the study the hardwood portion of jackwood is used as an adsorbent for phenol removal from aqueous effluent simulated in the laboratory conditions. Experimental Procedure Sorbent-The sawdust of hardwood portion of jackwood was collected under hygienic,conditions from a local t4nber industry. After drying, the particles of 8{)- 230 mesh size were separated by sieving through standard test sieves. Then it was boiled in distilled water continuously for 30 min. The suspension was then left to settle to allow the supernatant to be poured off. This process was repeated several times until the coloured water soluble components were removed. Finally the washed adsorbent was dried in an oven at 80"C, allowed to cool and sieved into mesh size for subsequent use. Chemicals- All the reagentls and chemicals used were of A R Grade (BDH). The solutions were prepared in double distilled water. A stock solution of phenol was prepared by dissolving 1 g phenol in distilled water in 1000 ml capacity volumetric flask. A solution of 0.1 M NaNO) was used to maintain constant ionic strength. The ph of the medium was adjusted by using 0.1 M HN03 and 0.1 M NaOH solutions. The phenol estimation was done spectrophotometrically by the 4-amino antipyrine method using a Bausch and Lomb Spectronic 21D12. The ph of the substrate was determined using a ph meter Elico model U-120. Batch adsorption kinetics-the batch experiments were conducted using 1g of sawdust in a 250 ml capacity stoppered bottle with 100 ml of phenol solution. The adsorbate concentrations were varied in the range of mg/l. The

2 138 NDAN J. CHEM. TECHNOL., MAY ,0 80 ionic PH-7 0 stren9h.0.01 M Time, min Fig. i-effect of initial concentration and contact time on phenol adsorption whole study was done at ph below 7, because at higher ph ranges the texture of the adsorbent is changed. The ph and ionic strength were kept constant. Bottles containing 100 ml of phenol solution of different concentrations, ph and ionic strength were shaken with 1 g of sawdust at a uniform speed in a temperature controlled shaking machine. At pre-determined time intervals, the contents were centrifuged and the remaining concentration of phenol in the supernatant was analysed spectrophotometrically. The bottles containing solutions of different phenol concentrations and ph were shaken for 6 h to ensure the complete saturation. The amount of phenol adsorbed was determined as the difference between the amount of phenol initially added and that left after adsorption. The effect of ph of the medium on the removal process was indicated at different ph values. Batch experiments were performed to study the effect of temperature (10, 20 and 30 C) on adsorption of phenol by sawdust. Batch desorption and regeneration studies~ After the attainment of equilibrium the overlying solution was carefully de~ted and filtered without the loss of adsorbent. The desorption experiments were carried out at the same conditions of ph and ionic strength used for adsorption processes, but without phenol. To regenerate, the spent adsorbent was treated with 0.1 M HCl, washed and dried in a hot air oven at a temperature of 80 C. The spent sawdust did not show much reduction in its adsorption capacity, and would be resued a number of times. Results and Discussion Effect of concentration and contact time-fig. 1 shows the effect of phenol concentration on the extent of adsorption as a function of time for the initial phenol concentrations of 25, 50, 100 and 250 mg/l. A constant ph of 7 and ionic strength of 0.01 M NaN03 was maintained throughout the experiment. t is evident from Fig. 1 that the adsorption of phenol on sawdust increases with increase of contact time up to 6 h, afterwards no significant change is found. t is noted that the period of equilibration is independent of concentration and hence this period was chosen for further equilibrium batch adsorption studies. The removal was very rapid within initial one hour duration. The maximum removal was found for the lowest concentration of 25 mgl. At equilibrium condition of 6 h, 1.56 mg/g dry weight was adsorbed which constituted 62.5% of the initial total concentration. Out of 62.5% of the total adsorbed amount, 78.2% was removed within 1 h duration. The removal of phenol increased with the increase in solution concentration, but the percentage of adsorption decreased. The removal of phenol by sawdust 'decreased from 62.5 to 35.3% when the phenol concentration was varied from 25 to 250 mgl at ph 7 at 30 C. The examination of plotted curves also reveals that at a fixed adsorbent dose, the amount adsorbed increases with the concentrations of solution. n the case of lower concentrations, the ratio of the initial number of moles of phenol to the available surface area is low and subsequently the fractional adsorption becomes independent of initial concentrations. However, at higher concentrations the available sites of adsorption become fewer and hence the percentage removal of phenol depend upon the initial concentration. Adsorption dynamics-the rate of phenol adsorption was studied and rate constant K r of the process was determined using Lagergren rate13 equation: log(qe - q)=log qe -(Krt/2.303).., (1) The straight time plot of log( qe - q) versus t for phenol adsorption on sawdust. at 30 C (Fig. 2) indicate the applicability of Lagergren equation and explains that the process follows first order kinetics. The value of rate constant Kr are 2.~8 x 10-2, 2.39 X 10-2, 2.36 X 10-2 and 2.45 x 10-2 min-1 for an initial concentration of 25, 50, 100 and 250 mgl, respectively, at 30 C. The plot is linear for a wide range of concentrations and contact period. The values of K r at different initial concentrations clearly indicate that this parameter is H' '1111 ', i;1 ", '<, ii 1,1 ' 1"1',

3 ACHAR & ANRUDHAN: PHENOL REMOVAL FROM AQUEOUS SYSTEMS ph ionic., st~ c:r J 0.0 ]' mgll 5().0 mgll " l9l ~Omg/L.m:. 210.,fO o 30 0_ 0, ,8 H) -0-8 o Time, min 5() 6"0 70 Fig. 4-Equilibrium isotherm 25 Fig. 2-Lagergren c Q(L ionic atrwlglh 0.01 M /' '\. ;3 / C?r 56./'D.~ 10 8 ~ 4 ;2 ~ ct -&- Adaorpion4 o-plon 1"5 Fig. 3-Effect plot for phenol adsorption of ph of the solution totally independent of initial concentration and hence phenol adsorption onto sawdust follows first order rate kinetics. Effect of ph-the removal of phenol from wastewater by adsorption is highlydependant on ph of the solution, which affects the surfaee charge of the adsorbent and tlegree of ionisation and speciation of phenol. Fig. 3 shows the extent of phenol adsorption as a function of ph for an initial concentration of 25 mgl. The uptake is small in the low ph ranges and gradually increases up to ph 5.5, where maximum removal 79.2% occurs. At this optimum ph, 1.98 mglg of the initially present 25 mgl is adsorbed. The lower ph results in the protonation of phenol molecules as well as the adsorbent surface, which leads to the 5 extensive repulsion of protonated phenol. This results in the net reduction of phenol adsorption. With the increase of ph from 5 to 7 molecular form of phenol persist in the medium and surface protonation will be minimum, leading to the enhancement of phenol adsorption. Since the required ph of the adsorption system was adjusted using 0.1 N HN03, the formation of nitrophenop4 in stable molecular form and its high adsorption compared with phenol onto the surface of the adsorbent cannot be neglected. t has been reported that the relative affinities of adsorbent for a series of -CH3, - OCH3, - Cl or - N02 group into the orthol-, meta - or /para - position results in an increased adsorption compared with phenop5. After the adsorption; the desorption was done using the same background solution, keeping constant ph. Samples which adsorbed maximum phenol desorbed minimum amount, while the extent of desorption was maximum at lower ph ranges. The desorption behaviour of phenol is also shown in Fig. 3. Variation of adsorption with temperature-studies on the variation of adsorption with tempeatur-e at 10, 20 and 30 C at 6 h showed that the process was almost exothermic. The adsorption isotherm of phenol on sawdust at these temperatures are given in Fig. 4. The isotherms are concave to the Y-axis upto 17.5 mglg at 10 C, mglg at 20 C and 12.5 mglg at 30 C. The shape of the isotherms gives an indication whether the adsorption is favourable or unfavourable16 The shape of the curve obtained in the present investi-

4 ) ),)_,, 11,(lillie,d, i' i i );j11 ;lll i il _'jj~lllllll '.'." '" ',_ 140 NDAN J. CHEM. TECHNOL., MAY , E 1'4 n K o.~ T;C ru !l7 ")( E. Cl' o -1; C>O 0-5 log Ce Fig. 5-Freundlich isotherm plot x/m= Ce.389 at 30 C xlm= Ce.369 at 20 C xlm= Ce.329 at looc... (3).., (4)... (5) From the above equations for an equilibrium concentration of 1 mglml each gram of sawdust can remove mg of phenol at 30 C, which is increased up to 1.37 mg at lo C. Regeneration-1 g of spent adsorbent after adsorption at ph 5.5 was washed with distilled water to remove any adhering phenol molecules and was shaken with 100 ml of 0.1 N HC for regeneration. The desorption and regeneration was complete within 30 min duration. Of the adsorbed quantity of 1.95 mglg of phenol from the initially present 25 mgl, 1.91 mg was desorbed, constituting a net regeneration of 98% of the total adsorbed in a single step. t was then washed with distilled water dried in a hot air oven and was reused in subsequent operations. gation is favourable for adsorption. These isotherms suggest phenol adsorptions is more effective at higher concentrations. The percent of adsorption decreased with increase of temperature from 10 to 30 C and hence a lower temperature is more favourable for the adsorption of phenol on the new adsorbent. The least adsorption at 30 C is partly due to the weakening of the attractive forces between the phenol and sawdust and partly due to the enhancement of thermal energies of the adsorbate, thus making the attractive force between phenol and sawdust insufficient to retain the adsorbed molecules at the binding sites. Adsorption isotherm-the adsorption behaviour of phenol on sawdust at au temperatures was in close agreement with the Freundlich equation. The constants K F and n were obtained from the linear regression analysis of 10gxlm=logKF+1/nlogCe... (2) The intercept log KF is roughly an indicator of sorption capacity and the slope n is adsorption intensity (Fig. 5). The correlation coefficient obtained for the Freundlich adsorption isotherm was found to be 0.90, 0.87 and 0.89 at 10, 20 and 30 C. To test the significance of the correlation coefficient students t-test was applied. The test statistics value of t was calculated and found to be 4.60, 4.06 and 4.48 and the critical value from the table for (n- 2) degree of freed0m was 4.03 at 1% level of significance. The equilibrium sorption capacity of the adsorbent can be determined from the isothermal data. Conclusions On the basis of the present study it may be cdncluded that the hardwood portion of jackwood sawdust is found to be very useful in the removal of phenol from wastewater. The most important parameter of the process is the solution ph. The maximum accumulation is noted within 6 h, by removing 79.2% of the initially present 25 mgll at ph 5.5 at 30 C. The optimum ph of the sorption is 5.5, though efficient sorption is obtained over a range of ph 5-7. The simple and semi-empirical Freundlich adsorption isotherm constants can be evaluated experimentally for phenol at different temperatures. The sorption process is temperature dependant and exothermic in nature. Acknowledgement The authors are thankful to Head, Department of Chemistry, University of Kerala for providing the laboratory facilities. Nomenclature Ce = phenol concentration in water at equilibrium mg/ml Kr = Lagergren rate constant, min-) KF = Freundlich isotherm constant, m = mass of sawdust adsorbent n = Freundlich isotherm constant qe = mass of phenol adsorbed at equilibrium q = mass of phenol adsorbed at time t t = time,min T = temperature, C X = mass of phenol adsorbed, mg References 1 Sax N, Dangerous properties of industrial materials, 2nd ed (Reinhold, New York), ~ r f f ~ 'l'r'

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