International Research Journal of Environment Sciences________________________________ ISSN 2319–1414Vol. 2(6), 1-7, June (2013) Int. Res. J. Environment Sci. International Science Congress Association 1 Effect of Various Factors on the Adsorption of Methylene Blue on Silt Fractionated from Bijoypur Soil, Bangladesh Zaker Y., Hossain M.A.* and Islam T.S.A. Department of Chemistry, University of Dhaka, Dhaka-1000, BANGLADESHAvailable online at: www.isca.in Received 17th January 2013, revised 26th April 2013, accepted 30th May 2013 AbstractSilt obtained from the fractionation of Bijoypur (Netrokona, Bangladesh) soil having the particle size (53 – 140 µm) was used as an adsorbent for the removal of Methylene Blue (MB) from aqueous solution. The effect of various factors (pH, temperature and adsorbent dose) on adsorption of MB on silt fraction was investigated. The effect of pH shows that the amount adsorbed increased with the increase of pH of solution. The equilibrium adsorption isotherms were analyzed by Langmuir and Freundlich equations. Both Langmuir and Freundlich models can describe theadsorption equilibrium but the Langmuir model shows better agreement. Theamount adsorbed increased with the increase of temperature suggests the formation of dimer in the contact region. SEM micrographs and differential molar isosteric heat of adsorption (H) calculated at different surface coverage, indicate that the surface is heterogeneous having energetically different adsorption sites. Values of n calculated from Freundlich plots indicate that adsorption of MB on silt is spontaneous. At high surface coverage, the differential heat of adsorption versus surface coverage plot shows maximum value indicating the occurrence of structural rearrangement in the adsorbate. With the increase of adsorbent dose, amount adsorbed increased due to the increased surface area of adsorbent. Keywords: Methylene blue, Silt, pH, temperature, Langmuir and Freundlich isotherms, H, Adsorbent dose, Structural rearrangement. Introduction Some physico-chemical methods such as adsorption, degradation, decolorization, coagulation, reverse osmosis etc have been developed to remove dyes from wastewater1-15. Adsorption has received considerable attention for color removal from waste waters by fixed bed column process16 as it offers the most economic and effective method. Many investigators have been showing interest to study dye removal using low cost adsorbents like fly ash, coal, peat, sawdust and wood in recent years because of their local availability. Use of clay materials, rice husk, coconut coir, banana pith, wheat straw, sugarcane baggase, caladium bicolor (wild cocoyam) biomass, used tea leaves, ghassoul (natural clays from Atlas mountain of Morocco), Neem husk10, cow dung11, Sargassum muticum12, natural zeolites13 have been found highly effective, eco-friendly and cost effective. Adsorption of industrially important dyes namely Bromophenol blue, Alizarin red, MB, Ericrome black-T, Phenol red, Malachite green and Methyl violet from aqueous media on activated charcoal were studied17. Effect of initial dye concentration, agitation time, pH and temperature on adsorption of MB on ghassoul was investigated10. Adsorption isotherms and adsorption kinetics, effect of sorbent physico-chemical characteristics (pH, cation exchange capacity, ionic strength, surface area), temperature, adsorption of volatile compounds, presence of a co-solvent, association with dissolved organic matter, sorbent concentration for sorption of organic pollutants by natural sorbents (soils, sediments, clays, humic materials, and dissolved organic matters) were reviewed18. Batch adsorption experiments were carried out to investigate effect of several parameters e.g. initial dye concentration, pH, temperature and contact time for the sorption of Congo red, Malachite green and Rhodamine B dyes onto acid activated carbon19The biosorption of reactive dye from aqueous solutions using the activated sludge was studied using a batch system with respect to the initial pH, temperature, amount of adsorbent and the pre-treatment of the adsorbent. Both the Freundlich and Langmuir isotherm models could describe the adsorption equilibrium of the reactive dye onto the activated sludge with the Langmuir isotherm showing the better agreement. First- and second-order kinetic models were used to investigate the adsorption mechanism20. Out of three fractions of Bijoypur soil (sand, silt and clay), sand has the lowest adsorptive property21. Many investigators studied the adsorptive property of clay22; but the adsorptive property of silt has not been thoroughly investigated. The present investigation aims at the detailed study of equilibrium adsorption of dye on silt. The present study was carried out to investigate the effect of different factors e.g. pH, temperature and adsorbent dose on the adsorption of MB on silt. The objective was to study whether the adsorption process can be described by Langmuir and Freundlich models, to determine thermodynamic parameters and the change of surface morphology by SEM. International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 2(6), 1-7, June (2013) Int. Res. J. Environment Sci. International Science Congress Association 2 Material and MethodsSilt was fractionated from Bijoypur (Netrokona) soil. In our present investigation, titrometric method23 was used for fractionation of Bijoypur soil into sand, silt and clay on the basis of particle size. Fractionated silt was characterized by LIBS, FT-IR, SEM, XRD and pHzpc24. The adsorption experiment was carried out at pH 7.0 with MB of concentration 5.0×10-5 M at 30°C. 0.1 g of silt was taken in each of 6 bottles containing 40 mL MB solution. The bottles were shaken in a thermostatic mechanical shaker (SWB-20, HAAKE, Fisons Ltd, Germany). After a definite interval of time each bottle was withdrawn from the shaker. The supernatant of the bottle was transferred and centrifuged repeatedly until a clear liquid was obtained. The absorbance of the clear solution was measured spectrophotometrically at max 663.0 nm. The adsorption experiments were carried out to investigate the effect of pH on adsorption at solution pH 3.0, 7.0 and 11.0 keeping other parameters constant. In all cases, the pH of the solution was adjusted by using acid or alkali without affecting the volume of the solution. To investigate the effect of temperature on adsorption, adsorption experiments were carried out with different initial concentrations of MB from 3.0 to 30.0 mg/L at pH 7.0. The experiments and the analysis were performed as described earlier. The equilibrium concentration and amount adsorbed for different initial concentrations were calculated at different temperatures and adsorption isotherms were constructed. To determine the effect of adsorbent dose on adsorption isotherms, adsorption experiments were performed at pH 7.0 and temperature 30ºC using 0.1, 0.2 and 0.3 g of silt following the same procedure as mentioned earlier. A small portion of fractionated silt were separately taken in a SEM sample holder and made it platinum coated using a Pt-coated auto system (JFC-1600, JEOL, Japan). Platinum coated silt was placed in the SEM sample chamber and SEM picture was taken at 20 kV with 2,000 and 30,000 magnification. Again this procedure was carried out for the sample collected after adsorption of MB on silt with 30,000 and 60,000 magnifications. Results and DiscussionCharacterization of Adsorbent: Silt was characterized by LIBS, FT-IR, SEM, XRD and pHzpc and results have been discussed in the earlier study24. Estimation of Equilibrium: Equilibrium time for adsorption of MB on silt water interface was estimated as 90 minutes beyond which no change of amount adsorbed was found. Effect of pH on Adsorption: Figure-2 shows that the adsorption of MB on silt increases with the increase of pH indicating favorable adsorption at alkaline medium which can be explained from the pHzpc value of silt. The pHzpc value of silt is 6.39±0.0224. When the pH of solution is higher than the pHzpc, the surface becomes negatively charged. MB is a cationic dye. Due to electrostatic force of attraction molecules of MB are adsorbed on the surface of silt. Similar trends were observed in the literature25. t (min.) 020406080100120140 q (mg/g) 0.00.20.40.60.81.01.21.41.6 Figure-1 Equilibrium time of the adsorption of MB on silt at pH 7.0 (min.) 020406080100 (mg/g) 01234 pH 3.0 pH 7.0 pH 11.0 Figure-2 Change of surface coverage of silt at different pHs of MB solution Adsorption Isotherms at Different Temperatures: Adsorption isotherms of MB on silt were determined at different temperatures as shown in figure-3. In all cases, the nature of isotherms is almost same suggesting the possible formation of bi-layers. Figure-3 shows the amount of MB adsorbed on silt increases with increasing concentration non-uniformly and amount adsorbed increases with increasing temperature indicating endothermic process. Similar trend was observed during the investigation of the adsorption of Brilliant Red on activated charcoal over the temperature range from 5ºC to 55ºC26. But both exothermic and endothermic effect of temperature on the adsorption of methylene blue onto clay was International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 2(6), 1-7, June (2013) Int. Res. J. Environment Sci. International Science Congress Association 3 also observed25. Exothermic effect was observed over the range of 20ºC to 40ºC. But above 40ºC, endothermic effect was found. High temperature favors the diffusion of dye molecules in the internal porous structure of surface. Figures-4 and 5 show the application of Langmuir and Freundlich models to the adsorption equilibrium separately. (mg/L) 0510152025 (mg/g) 0.00.51.01.52.02.53.03.5 30 40 50 Figure-3 Adsorption isotherms of MB adsorption onto silt at different temperatures (mg/L) 0510152025 e (g/L) 1012 30 40 50 Figure-4 Langmuir plots for MB adsorption onto silt at different temperatures From Langmuir plots (igure-4) amount adsorbed for monolayer formation (), Langmuir adsorption-desorption equilibrium constant () and regression constant () were determined and values are shown in able-1. From Freundlich plots (Figure-5), Freundlich adsorption-desorption equilibrium constant () and regression constant () and were estimated and are shown in able-2. Values of as shown in able-1 and able-2 suggest that the equilibrium data fit better in Langmuir isotherm than Freundlich isotherm over the temperature range of 30 to 50ºC. loge (log{mg/L}) -0.6-0.4-0.20.00.20.40.60.81.01.21.41.6 log e (log{mg/g}) -0.10.00.10.20.30.40.5 30 40C 50 Figure-5 Freundlich plots for MB adsorption onto silt at different temperatures Table-1 Langmuir parameters for the adsorption of MB on silt (K) m (mg/g) l (L/mg) 2 303 2.505 0.287 0.989 313 2.737 0.439 0.995 323 3.120 0.641 0.991 Table-2 Freundlich parameters for the adsorption of MB on silt T (K) n 2 K f (mg/g) 303 3.554 0.987 0.930 313 3.865 0.952 1.181 323 4.329 0.961 1.525 Adsorption Isobar: The isobar in figure-6 shows positive temperature coefficient. This may be due to the activated adsorption. An increase in temperature leads to an increase in kinetic energy of adsorbate molecules. This favors the molecules to move from single adsorption site to the cavities and pores to form higher order aggregates depending on concentration and other interactions like adsorbed- adsorbed MB molecules, adsorbed- unadsorbed molecules and adsorbed molecule-surface etc. The formation of dimer of MB is also very likely. The adsorption of MB from aqueous solution onto kaolinite and four soil samples to determine the effect of MB dimerization on the measured surface area have been reported27. The visible spectra of adsorbed molecules indicated that MB was present at the surface as a mixture of monomeric and dimeric species. The results also suggest that dimmers are formed in the contact region between two aggregating particles. Adsorption Isoster: From figure-3, adsorption isosters were drawn as shown in igure-7. Differential molar isosteric heats of adsorption () were estimated at different values of amount of MB adsorbed by silt as shown in table-3. In figure-8, heat of adsorption is plotted as a function of MB adsorbed. International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 2(6), 1-7, June (2013) Int. Res. J. Environment Sci. International Science Congress Association 4 T (K) 300305310315320325 q (mg/g) 2.02.22.42.62.83.0 Figure-6 Adsorption isobar of MB on silt 1/ T (1/K) 0.00300.00310.00320.00330.00340.0035 ln (ln {mg/L}) -3-2-1 2.062 mg/g 1.885 mg/g 1.750 mg/g 1.552 mg/g 1.344 mg/g 0.750 mg/g 1.000 mg/g 1.115 mg/g Figure-7 Adsorption isosters of MB on silt for different initial concentrations Table-3 Heat of adsorption of MB on silt for different amount adsorbed (mg/g) (mg/L) ln(ln{mg/L}) (K) 1/ × 10 (K) Differential heat of adsorption, (kJ/mol) 0.750 0.935 -0.067 303 3.300 88.627 0.575 -0.553 3133.194 0.105 -2.254 3233.096 1.000 1.720 0.542 303 3.300 85.302 0.990 -0.010 3133.194 0.210 -1.561 3233.096 1.115 2.290 0.829 303 3.300 88.295 1.250 0.223 3133.194 0.260 -1.347 3233.096 1.344 3.855 1.349 303 3.300 81.353 1.980 0.683 3133.194 0.520 -0.654 3233.096 1.552 7.810 2.055 303 3.300 79.715 4.425 1.487 3133.194 1.095 0.091 3233.096 1.750 9.530 2.254 303 3.300 63.768 5.935 1.781 313 3.194 1.980 0.683 323 3.096 1.885 10.935 2.392 303 3.300 33.256 6.510 1.873 313 3.194 4.845 1.578 323 3.096 2.062 14.010 2.639 303 3.300 39.683 7.135 1.965 313 3.194 5.310 1.669 323 3.096 References 1. 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