Removal of dyes using low cost adsorbents

Transcription

1 Indian Journal of Chemical Technology Vol. 15, March 2008, pp Removal of dyes using low cost adsorbents V K Verma* & A K Mishra Chemical Engineering Department, H.B. Technological Institute, Kanpur , India Received 8 June 2007; revised 20 September 2007 The low cost adsorbents namely Rice Husk Carbon (RHC), Wheat Straw Carbon (WSC) and Saw Dust Carbon (SDC) have been tested for the effectiveness in decolourisation of wastewater containing a mixture of dyes. Three dyes namely crystal violet, direct orange and magenta have been used for imparting colour in the representative samples of wastewater. Effect of various parameters such as agitation time, ph, temperature and adsorbent dosage has been investigated in the study. The adsorption of dyes are best described by pseudo first order mechanism. The rate constants of adsorption (K ad ) for the three dyes have been determined for the three adsorbents separately which are found to be , and min 1 for crystal violet, , and min 1 for direct orange and , and min 1 for magenta when treated with RHC, WSC and SDC respectively. Keywords: Adsorbents, Dyes, Decolourisation, Rice husk carbon, Wheat straw carbon, Saw dust carbon The discharge of large quantity of highly coloured wastewater from the textile industries poses serious environmental problems. Untreated effluents from textile industries and other industries such as leather, paper, plastic, cosmetic and food also contribute to the pollution load. The colouration of water due to the presence of dyes may have an inhibitory effect on the process of photosynthesis and thus may affect the aquatic ecosystem. The wastewater from dyeing industries are released into nearby land or rivers without any treatment because the conventional treatment methods are not cost-effective in the Indian conditions. On the other hand, the low cost technologies don t allow a wishful colour removal and have certain disadvantage. Thus, the removal of dyes from coloured effluents, particularly textile industries, is one of the major environmental problems. Various technologies have been employed in the past for the removal of dyes from wastewater. But adsorption process has been found to be more effective method for treating dye-containing wastewater Recently a number of low cost adsorbents like pearl millet husk 11, coconut shell 12, almond shells 13, pine bark 14 and wool waste 15, tropical gross 16, saw dust 17, rice husk 18, Azadirachta indica leaves 19 and jack fruit peel 20 have been used for removal of dyes form wastewater. In the present study the adsorption of three dyes namely crystal violet, direct orange and magenta have been investigated using Rice Husk Carbon (RHC), Wheat Straw Carbon (WSC) and Saw Dust Carbon (SDC) as the adsorbents. Experimental Procedure The dyes crystal violet, direct orange and magenta were obtained from Thomas Baker Chemicals Ltd. Mumbai, India. The stock solutions of these dyes were prepared in distilled water. The adsorbents were prepared by heating rice husk, saw dust and wheat straw using muffle furnace. The masses obtained from the furnace were treated with concentrated sulphuric acid and washed with water and finally dried in sunlight to remove excess acid and water. Batch experiments Dye adsorption experiments were carried out by taking 50 ml stock solution of dye (10 mg/l) and treated with a dose of 1.0 g of adsorbent. The variables studied were agitation time, ph, temperature and adsorbent dose. After desired time of treatment samples were filtered to remove adsorbents. The progress of adsorption was estimated by using spectrophotometer (spectromic-20, Bausch & Lomb) at 500 nm. The ph was measured by digital ph meter (NIG 333). Results and Discussion Effect of agitation time The samples of three dyes were taken in separate flasks and treated with 1.0 g of different adsorbents. The variation in per cent removal of dyes with the agitation time for the three adsorbents has been shown in Fig. 1(a-c). It is evident from Fig. 1a, that RHC treatment resulted in 70% removal of crystal violet in first 15 min, which increased upto 82.5% in 60 min. It

2 VERMA & MISHRA: REMOVAL OF DYES USING LOW COST ADSORBENTS 141 also shows that the percent removal of direct orange and magenta are 47 and 54% in 15 min which increased to 77 and 84% in 45 min respectively for both the dyes indicating optimum time of 45 min for maximum adsorption. Figure 1b shows that the crystal violet was removed to the extent of 62.5% in 15 min which increased upto 70% in 60 min when treated with WSC. The equilibrium was obtained in 45 min for this dye. The maximum removal of dye was observed to be 70%. This figure also shows the effect of WSC on direct orange and magenta, where the removal increased from 47 to 85% and 67 to 94.5% respectively. When the dyes were treated with SDC, the removal was found to be 75% in case of crystal violet, 54% in case of direct range and 50% in case of magenta in first 15 min which increased to 95, 85 and 73% respectively in 60 min (Fig. 1c). Fig. 1 Effect of time for the removal of dyes by (a) RHC, (b) WSC and (c) SDC Effect of ph The aqueous solution of the dyes having concentration 10 mg/l were treated by 1.0 g of different adsorbents for half an hour under varying ph. The ph was varied from 2 to 8 with the help of 0.1 N HCl and 0.1 N NaOH solution. The results shown in Fig. 2(a-c). Figure 2a reveals that when the dyes were treated with RHC, 80% removal was obtained at ph 2, while at ph 8 it was found to be 85% in case of crystal violet dye. The equilibrium was reached at ph 6 in the acidic medium. In case of direct orange, the removal was 62%, which increased upto 85% as, the ph increased from 2 to 8. However in case of magenta, the removal was observed to be 45 to 73% for the same conditions. The effect ph for WSC is shown in Fig. 2b, which indicates that the removal of crystal violet was 72.5 to 87.5%, direct orange was 69.2 to 92.4% and magenta was 50 to 84% when ph varied from 2 to 8. Similarly all the three dyes were also treated with SDC at ph 2 to 8. The removals were found to be 85 to 90% in case of crystal violet, 69.2 to 85.0% in case of direct orange and 67 to 94.5% in case of magenta (Fig. 2c). From the above results it is evident that the removal of dyes are more at higher ph because the surface of activated carbons are negatively charged 21, the decrease in adsorption capacity in the low ph region would be expected as the acidic medium would lead to an increase in hydrogen ion concentration which would then neutralize the negatively charged carbon surface thereby decreasing the adsorption of the positively charged cation because of a reduction in the force of attraction between adsorbent and adsorbate.

3 142 INDIAN J. CHEM. TECHNOL., MARCH 2008 Effect of temperature To study the effect of temperature, the experiments were carried out for the three adsorbents at temperature varying from 20 to 80 C. It was observed that RHC removed 80 to 87.5% of crystal violet dye. However, in case of direct orange and magenta, the removal of dyes was 69.2 to 85% and 50 to 84% respectively. The results have been shown in Fig. 3a. Similarly the stock solution of dyes treated with WSC for the removal of dyes at the same temperature the removal was 65% which increased upto 75% in case of crystal violet, 38 to 77% in case of direct orange and 54 to 94.5% in case of magenta (Fig. 3b). When the stock solution was treated with SDC the removal of crystal violet was found to be 87.5% at 20 C which increased upto 95% at 80 C, removal of direct orange was found to be 62 to 92.4% and removal of magenta was found to be 73 to 89% when temperature was increased from 20 to 80 C (Fig. 3c). All the experiments were carried out for 30 min. The equilibrium was obtained at 60 C for all the dyes when treated with RSC and WSC. Effect of adsorbent dose The effect of adsorbent dose was also tested for the removal of all the three dyes from aqueous solution. The experiments were carried out by mixing 50 ml aqueous solution of dyes (10 mg/l) and adsorbent dose was varied form 0.5 to 2.0 g. The removal of crystal violet ranged from 70 to 77.5% when treated with different doses of RHC. The removal of direct orange was found to be from 54 to 69.1% and the removal of magenta from 67 to 89% (Fig. 4a). The removal of crystal violet was found to be 70 to 82.5%, direct orange 62 to 85% and magenta 50 to 83.4% when treated with different doses of WSC (Fig. 4b). When treated with different doses of SDC, the removal was found to be significant for all the three dyes. It was 87.5 to 95% in case of crystal violet, 54 to 85% in case of direct orange and 67.5 to 94.5% in case of magenta (Fig. 4c). The increase in the removal of dyes with increased adsorbent doses was due to the introduction of more binding sites for adsorption. Kinetic model of adsorption In order to determine the controlling mechanism of adsorption process such as mass transfer and chemical reaction pseudo first order kinetic model 18 is used to test the experimental data. A simple kinetics of adsorption is given by a Lagergren rate equation 22. Fig. 2 Effect of ph on the removal of dyes by (a) RHC, (b) WSC and (c) SDC dqt dt = k q q ) (1) ad ( e t

4 VERMA & MISHRA: REMOVAL OF DYES USING LOW COST ADSORBENTS 143 Fig. 3 Effect of temperature on the removal of dyes by (a) RHC, (b) WSC and (c) SDC Fig. 4 Effect of adsorbent dose on the removal of dyes by (a) RHC, (b) WSC and (c) SDC

5 144 INDIAN J. CHEM. TECHNOL., MARCH 2008 Table 1 Adsorption rate constant (k ad min 1 ) for different dyes with different absorbents Dye Adsorbent RHC WSC SDC Crystal violet Direct orange Magenta where k ad is the rate constant of pseudo first order adsorption, q e denotes the amount of dye at equilibrium and q t is the amount of dye absorbed at time t. Applying conditions q t = 0 at t = 0 q t = q t at t = t log( q e kad. t qt ) = log qe (2) Based on experimental results, linear plots of log (q e q t ) versus t suggest the applicability of Lagergren equation [Fig. 5(a-c)]. The rate constants were calculated from the slopes and values are presented in Table 1. The effect of dye concentration on rate constants (k ad ) helps to describe the mechanism of removal of colour 23. Conclusion The adsorption of crystal violet, direct orange and magenta on the different absorbents namely, RHC, WSC and SDC have been tested by pseudo first order rate equation. The rate constant for crystal violet, direct orange and magenta vary from to , to and to min 1 while using RHC, WSC and SDC respectively. The linear plots of log(q e q t ) versus t suggested the applicability of the Lagergren first order rate equation. Acknowledgement The authors are thankful to the Director, Harcourt Butler Technological Institute (HBTI), Kanpur for providing necessary research facilities. Fig. 5 Test of pseudo first order equation for adsorption of (a) crystal violet, (b) direct orange and (c) magenta, on different adsorbents References 1 Khan A Tabrez, Singh Ved Vati & Kumar D, J Sci Ind Res, 63 (2004) McMullan G, Mechan C, Conneely A, Kirby N, Robinson T, Nigam P, Banat I M, Merchant R & Smyth W F, Appl Microbiol Biotechnol, 56 (2001) Nigam P, Armour G, Banat I M, Singh D & Merchant R, Bioresour Technol, 72 (2000) 219.

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