Adsorption of dyes on sawdust phosphate: Kinetics and equilibrium studies

Transcription

1 Indian Journal of Chemical Technology Vol. 15, March 2008, pp Adsorption of dyes on sawdust phosphate: Kinetics and equilibrium studies Aditya Prakash 1#, Sangeeta Solanki 1 & PTSRK Prasada Rao 2 1 Department of Chemistry, JNU University, Jodhpur , India 2 P B Siddhartha College of Arts & Science PG Centre,Vijayawada , India Received 23 November 2005; revised 17 September 2007 The distribution coefficients, K d, for navy brown 3REL, direct red, procion red H8B, methylene blue, metamega chrome yellow and lanasyn green 5 GL, solar violet and procion blue H5G from their aqueous solutions, at ph 3.2 and 7.5 on sawdust phosphate (SDP) have been reported. Effect of ph and added surfactant concentration on the percent removal of lanasyn green 5 GL has been studied. Percent dye removal increases with the decrease in ph. The applicability of the Freundlich and Langmuir equation has been tested for equilibrium data of which Langmuir isotherm model is found to be most appropriate. The values of Langmuir and Freundlich constants as well as the distribution constants show that SDP is a good adsorbent for all dyes, being better for basic dyes. The kinetics of adsorption of lanasyn green 5 GL on SDP was investigated at ph 3.2 and temperatures 25, 30, 35 and 40 o C. It is first order with respect to the dye and less than unity with respect to SDP. The values of k ad, k p and k f at 30 o was found to be s -1, s -1 and s -1 respectively. Keywords: Adsorption, Sawdust phosphate, Textile effluents, Adsorption isotherms Many industries in India discharge untreated or partially treated waste-water on land or in natural streams causing pollution to surface water, ground water and the soil 1-3. The colour in the water bodies even in small concentration is easily detectable and lead to inhibitory effect on the process of photosynthesis and thus affecting the aquatic ecosystem 4. The anaerobic breakdown of the dyes and incomplete bacterial degradation often produces toxic amines 5. The removal of dyes from textile effluents is one of the major environmental concern these days 6,7. Various physical and chemical methods have been employed to treat the textile effluents; these include co-agulation 8, flocculation 9, precipitation 10, adsorption 11,12, biological 13,14 treatments and membrane technology 15. However, adsorption processes have been found to be more effective for treating dye containing effluents. Adsorption by powdered activated charcoal 16 (PAC) and granular activated carbon (GAC) 17 has gained greater significance. But, in view of the high cost of activated carbon and difficulties encountered in its regeneration, search for low cost and effective adsorbents is continuing. Wood is most abundant and renewable natural source 31 available to mankind. Recently the removal of Cr(VI) from tannery effluents using SDP 32 has # For correspondence: 21, Abhay Garh, Opp. KV-1, Jodhpur , India been investigated. In the present communication, attempts have been made to obtain a phosphate derivative from saw dust, a waste material, without any pretreatment with the objective to use it as adsorbent. The results on the sorption and desorption of eight water soluble dyes used in textile processing and representing six different classes, on SDP are presented; the studies are extended to the treatment of textile effluents by SDP and the cost considerations of the process. Experimental Procedure Preparation of adsorbent Forty grams of saw dust, a waste product of saw mills engaged in cutting and chopping of sal, teak, sheesham bonsom etc. woods, procured from local industries and sieved through 40 mesh sieve, were mixed with 50 g phosphoric acid. The mixture was heated in a shallow tray in an electric oven at 60 o C for 3 h. The material was then moulded in spherical balls of diameter approx 0.5 cm. These balls were then heated at 200 o C for period 3 h in inert atmosphere of N 2. The product was then brought to room temperature and washed thoroughly with water and dried in air. Dyes Eight commercial dyes of six different classes were selected for this study and were used as such without

2 PRAKASH et al.: ADSORPTION OF DYES ON SAWDUST PHOSPHATE 147 purification. These include navy brown 3 REL (Atul India, disperse, λ max 650 nm), direct red (Atul, direct λ max 498 nm), procion red H8B (Atul, reactive, λ max 548 nm), methylene blue (Loba, basic λ max 650 nm), metamega chrome yellow (Sandoz, metal complex, λ max 386 nm) and lansan green 5 GL (Sandoz, acid, λ max 600 nm), procion blue H5G (Atul, reactive, λ max 548 nm) and solar violet (Sandoz, acid, λ max 584 nm). The chemical structure of these dyes have been reported in colour index. The solutions of dyes were prepared a fresh, the concentration of the dye in each solution was evaluated spectrophotometrically using AIMIL MK II spectrophotometer with glass cells of one centimeter path length. All measurements were recorded at the wavelength of maximum absorbance (λ max ) corresponding to each dye. All other reagents used in this study were of AnalaR Grade and all the solutions were prepared in double distilled water. All the ph measurements were made on Naina make ph meter model 303 using combined glass electrode. Textile effluent The wastewater samples were collected from two main open drains in Jodhpur industrial area. The wastewaters were highly coloured (brownish purple), alkaline (ph ) and foul smelling. The samples were allowed to stand for 4 h, the supernatant liquid was analyzed for ph, total dissolved solids (TDS), colour (OD), conductivity, alkalinity salinity, N- nitrate, N-nitrite, chlorides, sulphates, phosphates, hardness, chemical oxygen demand (COD) and biological oxygen demand (BOD) by standard methods 33. The results are reported in Table 1. Equilibrium studies The equilibrium studies were done by agitating g of the SDP with 100 ml aqueous solution of dyes or effluent in skrew cap jars. The agitation was continued for 60 min at ambient temperature (25±0.2 o C). The initial ph of the solution was adjusted by adding requisite amount of dilute acid or alkali solution. The equilibrated solutions were centrifuged for 10 min at 10,000 RPM in T-24 model (GDR) centrifuge and analyzed for the residual dye concentration. Kinetic measurements The kinetic measurements were made in the presence of the excess of adsorbent at constant ph, adjusted by the addition of dilute solutions of NaOH or HCl. The solutions were thermally equilibrated in a constant temperature bath maintained at the desired temperature ±0.1 o C. The required volume of dye solution was then added and the time was noted. The solutions were kept at constant stirring throughout the course of reaction except when the measurements of dye concentrations were made. At known intervals of time aliquots were quickly withdrawn and transferred to the cell of the spectrophotometer and absorbance of the dye solution was recorded, at λ max of the dye. The initial dye concentration of 100 mg dm -3 was chosen to give a reasonably large initial dye concentration. Distribution coefficients (K d ) The uptake of dyes by SDP has been estimated in terms of distribution coefficients, K d. The adsorbent ( g) was stirred with 100 ml of 100 mg dm -3 aqueous dye solution at desired ph, until complete equilibrium was attained. K d values have been calculated using the formula V Kd =. m where V is the total volume of dye solution (ml), m is the weight of adsorbent in g and x is the percent amount remaining in the solution. Table 1 Characteristics of cotton, woollen and polyester processing industrial effluents Industry type Sl.No. Parameters Cotton Woolen Polyster 1 ph Total solids Total suspended solids 4 Chlorides COD BOD Colour Deep redbrown Brownish purple Light yellow 8 Conductivity Phosphates Nitrate-N Nitrite-N Traces 12 Haredness Alkalinity Oil and grease Traces (1) Average of 12 readings, throughout the year. (2) All parameters in mg dm -3.

3 148 INDIAN J. CHEM. TECHNOL., MARCH 2008 Results and Discussion The efficiency of SDP in the removal of colour from each dye solution was determined by calculating the distribution coefficients, K d, for dyes at ph 3.2 and 7.5. Results, reported in Table 2, show that all dyes have good adsorption on SDP, and the adsorption of dyes on SDP occurs in the following order: Basic > Acid > Disperse > Direct = Reactive > Metal complex. The results also indicate, that the K d values increase with the decrease in ph of the solutions, K d values for all the eight dyes were found to be greater at ph 3.2. This may be attributed to the neutralization of surface negative charge of the SDP derivative 34. In aqueous solutions, basic dyes furnish dye-cations and get adsorbed faster due to electrostatic force of attraction, these results are in accordance with a number of studies 21,23,24,26-28,31,35-40 conducted on the adsorption of basic dyes on anionic adsorbents. The K d values for lanasyn green 5 GL on SDP also indicate that it has the potential for adsorption in the entire ph range studied, desirable from an efficient adsorbent. Adsorption isotherms The adsorption isotherms for all the eight dyes from their aqueous solutions on SDP, when the system is in equilibrium are shown in the Fig. 1. All the adsorption isotherms were found regular positive and convex to the concentration axis. The isotherms showed a sharp rise in the initial stages and then became parallel to the concentration axis, indicating that the saturation values have been reached. The experimental data was analyzed in the light of Freundlich, Langmuir and BET equations to predict the nature of adsorption. The plots of C e /x/m against C e were found linear in each case indicating a strict monolayer adsorption (Fig. 2). The values of Langmuir constants, calculated from the intercepts and slopes of the curves are reported in Table 3. The plots of log x/m against log C e were also found linear (Fig. 3). The slopes and intercepts of these curves were used to determine the values of K and 1/n, the Freundlich constants (Table 3). The values of adsorption capacities Q, as well as K were maximum for methylene blue, a basic dye, which is very much expected, as K d values for methylene blue were also found highest. Except lanasyn green 5 Gl, not much difference was found in the intensity of adsorption (1/n) for the dyes investigated. Effect of ph The effect of ph on the adsorption of lanasyn green 5 GL on SDP was investigated at ambient temperature. The percent removal of lanasyn green 5 GL was found to decrease with the increase in the ph of X/m Ce (gdm -3 ) Fig. 1 Adsorption isotherms of dyes on SDP at ph 8.0 and temperature 30ºC [Initial dye]=100 mgdm -3 ; SDP= 0.2 g Direct red, Lanasyn green 5 GL Ο Ο, Metamega chrome yellow ϑ ϑ, Solar violet ν ν, Procian blue H5G λ λ, Navy brown 3REL, Methylene blue, Procion red H8B Table 2 Distribution coefficients, K d, of dyes on sawdust phosphate (SDP) Sl. No. Dye ph K d 1 Navy brown 3 REL Direct red Procion red H8B Methylene blue Metamega chrome yellow Solar violet Procion blue H5G Lanasyn green 5 GL Initial dye concentration=5 mg dm -3 ; SDP=0.2 g; Temp 30 o C

4 [ PRAKASH et al.: ADSORPTION OF DYES ON SAWDUST PHOSPHATE 149 Ce/(X/m) 3+LOG(x/m) Ce (gdm -3 ) Fig. 2 Langmuir adsorption isotherms Direct red, Lanasyn green 5 GL, Metamega chrome yellow σ σ, Solar violet λ λ, Procian blue H5G Ο Ο, Navy brown 3REL, Methylene blue ν ν, Procion red H8B the solution (Table 4). The percent of the dye removed was higher at ph 3 as compared to percent dye removed at ph The removal of dyes from their aqueous solutions has been found to be dependant on the ph of the solution; since ph of the solution reportedly affects the surface charge of the adsorbent, solubility and the degree of ionisation of the adsorbate. The SDP and all the dyes, except basic blue, studied are negatively charged and therefore repel each other and slow down the migration of dye anions on to SDP surface. The decrease in ph helps in neutralizing the negative charge of surface and permits the movement of dye anions in the vicinity of the Vander-waal s forces. Effect of surfactants Organic surfactants of wide variety are used in the processing of textiles and they enter in the water-ways with textile effluents. These surfactants are known to be adsorbed on clays, coagulants, adsorbents etc. used in the treatment of waste-waters and have potential to alter the uptake of dye from their aqueous solutions. 3+LOG Ce Fig. 3 Freaundlich linear plots Direct red, Lanasyn green 5 GL Ο Ο, Metamega chrome yellow ϑ ϑ, Solar violet ν ν, Procian blue H5G λ λ, Navy brown 3REL σ σ, Methylene blue, Procion red H8B In the present study, effect of the nature and concentration of added surfactants on the adsorption of lanasyn green 5 GL from its aqueous solution on SDP was investigated at room temperature. The results in Table 5 show that the surfactants have little effect on the adsorption behaviour of SDP. The percent removal of lanasyn green 5 GL increased with the increase in concentration of added cationic surfactants and decreased with increasing anionic surfactant concentration, whereas non-ionic surfactants had no effect. It appears that the cationic surfactant molecules neutralize some negatively charged sites of the adsorbent resulting in the improved migration of dye anions onto the SDP surface. The reduced percent removal of the dye on SDP in presence of anionic surfactant may be explained on the concept of competitive behaviour of dye and surfactant anions for the positive sites of the SDP. Kinetics of adsorption The kinetics of adsorption of lanasyn green 5 G L from its aqueous solutions by SDP at 25, 30, 35 and 40 o C was investigated. The first order rate constants, k ad was calculated, using Lagergren s 41 equation, at different initial dye concentration and with different amount of SDP at different temperatures.

5 150 INDIAN J. CHEM. TECHNOL., MARCH 2008 Table 3 Freundlich and Langmuer constants for adsorption of dyes on saw dust phosphate (SDP) at ph 8.0 and temperature 30 o C Sl. No 10 3 [C e ] (g) 10 3 (g) SDP m (g) x/m 10-3 (g/g) C e /(x/m) Langmuir Constants Freundlich Constants [Lanasyn green 5GL] = 0.05 g dm S = S = I = I = r = r = Q º = K = b = R L = /n = [Metamega chrome yellow] = 0.05 g dm S = S = I= I = r = r = Q º = K = b = R L = /n = [Direct red] = 0.05 g dm S = S = I = I = r = r = Q º = K = b = R L = /n = [Procion red H8B] = 0.05 g dm S = S = I = I = r = r = Q º = K = b = R L = /n = [Solar violet] = 0.05 g dm S = S = I = I = r = r = Q º =0.021 K = b= /n = R L = [Procion blue H5G] = 0.05 g dm S = S = I = 3.79 I = r = r = Q º =0.011 K = b = R L = /n = [Navy brown 3REL] = 0.05 g dm S = S = I = I = r = r = Q º = K = b= /n = R L = [Methylene blue] = 0.05 g dm S = S = I = I = r = 0.99 r = Q º = K = b= /n = R L = 0.022

6 PRAKASH et al.: ADSORPTION OF DYES ON SAWDUST PHOSPHATE 151 Table 4 Percent removal of lanasyn green 5G L by SDP at different ph Sl. No. ph % Dye removed [Lanasyn green 5G L] i = 100 mg dm -3, Temp = 30 o C, SDP = 0.2 g Table 5 Percent removal of lanasyn green 5 G L by SDP in presence of surfactants Absorbent (g) Surfactant (g dm -3 ) Arkoline SPW (Anionic) kad. t log( qe qt) = log qe Arkoline N45 (Non-ionic) Arkoline HCS (Cationic) Percentage dye removed [lanasyn green 5GL] i =100 mg dm -3, ph=3.2, Temperature=30 o C where q e and q t are the amount of dye adsorbed at equilibrium and the lapse of time t. The plot of log (q e q t ) versus time was found linear, indicating that the above equation is valid for this system (Fig. 4). The rate of adsorption of lanasyn green on SDP with respect to the dye was found unity while with respect to the SDP, the order was found to be less than one. In all the cases, it was found that the rate of the dye uptake, k ad, increased with the mass of adsorbent. The dependence of rate on the adsorbent mass was found to be less than unity. The mechanism of the dye uptake by the adsorbent appears to involve four steps (i) migration of the molecules from the bulk of solution to the surface of the adsorbent (ii) diffusion of dye molecules through the boundary layer to the surface of the adsorbent (iii) adsorption of dye molecules at the sites on the adsorbent (iv) intraparticle diffusion in the interior of the adsorbent. In the batch reactor with rapid mixing, there is also a possibility that transport of adsorbate ions from Fig. 4 First order plots for adsorption of lanasyn green 5 GL on SDP at 30ºC [lanasyn green 5 GL]= 100 mgdm -3 ; SDP= 200 mg; volume= 0.1 dm 3 solution into the pores of the adsorbent may be the rate controlling step as suggested by Weber and Morris 42 and Poots et al. 31. This was tested by plotting the amount of dye adsorbed against the square root of the time, according to equation 45. Q t /Q o = k p t 0.5 where k p is the pore diffusion constant (Fig. 5). The upper part of curve in each case was found linear and the lower portion curved, similar plots have been obtained by Gupta et al. 21 in the adsorption of chrome dye on fly ash and coal and by Mckay et al. 43 in the removal of colour from effluents by silica. These authors have attributed the initial curve portion to the boundary layer effect as suggested by Crank 44 and the upper linear portion to the intra-particle diffusion effects, the values of k p was calculated from the slope of the linear portion of these curves and is reported in Table 6. The plots of C t /C o against t 0.5, for different masses of the adsorbent, were also found linear and were used to calculate intra particle diffusion constant. These values have also been reported in Table 6. The product of mass transfer co-efficient and surface area of particle for the adsorption of dyes on SDP were also calculated using the method and equation 46 described as,

7 152 INDIAN J. CHEM. TECHNOL., MARCH 2008 Table 6 Rate of adsorption constants, intra-particle diffusion coefficients and mass transfer coefficients of lanasyn green 5 GL on SDP Temperature ( C) Amount of adsorbent (mg) 10 4 k ad s k p s k f S S s k p s [lanasyn green 5GL] = 100 mg dm -3, ph = 3.2,Volume = 0.1 dm -3 where k f is the mass transfer constant and S s is the particle area. The values of k f.s s are also reported in Table 6. The values of k f.s s and k ad.k p were found to increase with the temperature, which can be explained on the basis of widening of the pore size, increase in the kinetic energy of the dye molecules and increase in the ionization of dye etc. C t /C o Time (min) Fig. 5 C t /C o plots for intraparticle diffusion constant at 30ºC [lanasyn green 5 GL] = 100 mgdm -3 ; SDP = 100 mg Ο Ο, ϑ ϑ, 150 mg λ λ 200 mg and 250 mg, volume = 0.1 dm 3. ds/dt = k f.s s (Q t Q o ), as Q t approaches Q o and t o one gets Qt d / dt k = 0 = kf. Sst. Qo Studies with textile effluents The applicability SDP in the treatment of textile effluents was evaluated by studying the adsorption of colour of the textile effluents by SDP in the ph range of The percent decrease in OD was taken as a measure of reduction of the total dye concentration in the effluents and percent decrease in COD as the measure of total organic carbon. The results in Table 7 show that the percent decrease in OD was found to increase with decrease in ph and for the same amount of adsorbent, percent colour removed was maximum at ph 3. The results indicate that, at lower concentrations of colour in the effluents, the percent decrease in OD is proportional to the amount of adsorbent added whereas at high concentrations of colour, the percent decrease in OD tends to reach a limiting value. Further addition of adsorbent does not decrease the colour proportionately, so is the case with COD. The analysis of the treated effluents given in Table 8 shows that the optimum dose of SDP is different for different effluents depending upon the nature and concentration of pollutants in the effluents. The desired doses of SDP to remove 80 percent colour from effluents of woolen, polyester and cotton fibre/fabric dyeing industries were found to be 500, 1500 and 2000 mg dm -3 respectively. The lower dose of SDP for woolen mill effluents is due to the increased adsorptivity of basic raw material, the saw dust 47, for cationic dyes. On the basis of optimum

8 PRAKASH et al.: ADSORPTION OF DYES ON SAWDUST PHOSPHATE 153 Table 7 Decrease in OD and COD of effluents by SDP Sl.No. Source of Dose Percent decrease in effluent (mg dm -3 ) OD* COD** > >10 1 Cotton processing industries Wool processing industries Polyester processing industries *Average of six observations. **Average of three observations. Table 8 Characteristics of effluents after treatment with SDP* Sl.No. Parameter Cotton a Wool b Polyster c 1 ph <7 < Colour Nil Nil Colourless 3 TDS TSS Nil Nil Nil 5 COD Nil 6 BOD Nil 7 Sulphides Nil 8 Sulphates Chlorides Phosphates Nitrates Nil 12 Nitrites Traces Traces Alkalinity Nil Nil Nil 14 Hardness < Oil and grease Traces <10 Traces *Values are in mg dm -3 except ph and conductivity. **Average of 12 samples. # Optimum dose: a-2000 mg dm -3, b-50 mg dm -3, c-1500 mg dm -3 dose of SDP required to treat 100M 3 (1 lakh litre) for the cotton, woolen, polyester processing industrial effluent, the approximate cost of effluent treatment was computed considering only the cost of material used in the treatment process. For treating 100M 3 cotton, woolen and polyester effluent, the optimum dose of SDP was found to be 200, 30 and 30 kg with cost in rupees 2500, 380 and 380 respectively. Regeneration studies The preliminary studies have shown that the SDP can be regenerated by heating it at 300 o C and can be used again, the dyes and other organic material adsorbed onto the SDP are decomposed to nitrogen, water and CO 2 leaving behind carbon only; no appreciable change in the properties SDP was found after the regeneration. Further the use of SDP has no sludge disposal problem like treatment with lime and alum/feso 4 etc. mixtures. Conclusion The SDP acts similar to activated carbon with modified surface. The treatment of saw dust with H 3 PO 4, imparts it the ability to remove dye colour from the industrial effluent. SDP is the only adsorbent amongst the various saw dust modifications which is able to remove the colour from the effluents of cotton, woolen and polyester processing textile industries in the entire ph range having maximum efficiency at ph 3-4. The SDP may prove a useful and cost effective adsorbent for textile effluents. Acknowledgements The authors are grateful to the Head, Department of Chemistry, JNV University, Jodhpur for providing research facilities and Dr Raj N Mehrotra for his valuable help. One of the authors (PTSRK Prasada Rao) acknowledges the encouragement given by the Siddhartha Academy.

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