Adsorptive removal of thallium(iii) ions from aqueous solutions using eucalyptus leaves powders

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1 Indian Journal of Chemical Technology Vol. 20, November 2013, pp Adsorptive removal of thallium(iii) ions from aqueous solutions using eucalyptus leaves powders H Dashti Khavidaki 1, M Aghaie 2, M R Shishehbore 3 & H Aghaie 1, * 1 Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran 2 Faculty of Chemistry, Islamic Azad University, North Tehran Branch, Tehran, Iran 3 Department of Chemistry, Yazd Branch, Islamic Azad University, Yazd, Iran Received 25 January 2012 ; accepted 24 May 2013 Adsorption of thallium(iii) ions from aqueous solutions onto eucalyptus leaves powders as a low cost adsorbent has been studied in batch mode. It is observed that modification of the adsorbent with basic solution (NaOH solution) is essential for achieving a significant adsorption. The alkaline modified adsorbent shows a significant amount of adsorption. However, the adsorbent treated with nitric acid shows no activity. The effects of experimental conditions such as initial solution ph, agitation speed, contact time, adsorbent amounts, initial thallium concentration and temperature are studied. The results show that adsorption is strongly ph-dependent. The maximum adsorption of thallium(iii) ion obtained at 25±1 o C is found to be 78.2%. The equilibrium data are described by the Langmuir isotherm but a slightly inferior fit is obtained using the Freundlich model. The monolayer saturation capacity is calculated to be mg g -1. The separation factor indicates that eucalyptus leaves powders are favorable for the sorption of thallium(iii). Keywords: Adsorption, Eucalyptus leaves powders, Thallium(III) ions, Langmuir isotherm, Freundlich isotherm, Separation factor Heavy metals are dangerous contaminants in environment and they must be removed effectively. Conventional methods for removing heavy metals are precipitation, solvent extraction, ion exchange, ultrafiltration, electrodialysis, reverse osmosis and chemical oxidation/reduction. However, they have several disadvantages including high cost, sensitive operating conditions and production of secondary sludge 1. Adsorption especially by the natural adsorbents is an appropriate method for removing heavy metals ions, having several advantages such as low cost, easy access to adsorbent, ease of operation and low time. Thallium is an extremely toxic heavy metal that exits in the environment commonly together with lead, zinc, iron, tellurium, alkalis, etc 2. Thallium compounds enter the environment (waters and soils) and contaminate it by the human activities, for example by exploiting and machining mineral and use of it as catalyst, dye, pigment, medicine, rodenticide and insecticide 3. Thallium is an Environmental Protection Agency (EPA) prescribed priority pollutant because its compounds are toxic in the extreme and *Corresponding author. hn_aghaie@yahoo.com they are even more severely toxic than Hg, Cd, Pb, Zn, Cu compounds in mammals 4,5. Thallium compounds accumulate in bones, renal medulla, central nervous system and throughout the body 4. They are dangerous for heart, lungs, kidneys, liver and nervous system. A trace of thallium compounds in body may create anorexia, headache, vomiting, diarrhea, temporary hair loss, and the relatively higher quantities cause blindness and death 6,7. Therefore, it is important to remove the thallium compounds and control their pollution of humans and environment. Some adsorbents have been used for removing thallium from aqueous solutions. The examples are iron powder 8, titania particle 9, silica gel 10, polyurethane foam 11, actived carbon 12, nano-al 2 O 3 2, dry biofilm biomass collected from a eutrophic lake 13, Aspergillus nigar biomass 14, nano-tio 2 15, silver nanoparticles 16, modified sugar beet pulp 17 and geological materials 18. In this study, eucalyptus powder from leaves has been used for the first time as a sorbent to remove thallium(iii) ions from aqueous solution. Eucalyptus tree is abundant and the preparation of its leaves for adsorption is easy. The prepared leaves are used as low cost sorbents.

2 KHAVIDAKI et al.: ADSORPTIVE REMOVAL OF THALLIUM(III) IONS 381 Experimental Procedure Apparatus Atomic absorption spectrophotometer (AA220 Model, VARIAN Co., USA) and a ph meter (420A Model, ORION Co., USA) were used for measuring the concentration of Tl(III) ions and ph of solutions respectively. Preparation of working solutions All chemicals used in this study were of analytical grade. Thallium(III) stock solution with appropriate concentration (250 ppm) was prepared by dissolving g of its nitrate [Tl(NO 3 ) 3. 3H 2 O, Fluka] in appropriate distilled water. Preparation of adsorbents The fresh leaves of eucalyptus were fragmented, washed with distilled water and dried at room temperature for several days. Thereafter, the dried leaves were grinded by electric crusher to obtain homogenous tiny leaves. Grinded eucalyptus leaves were passed through sieves (mesh 35) to obtain eucalyptus leaves powders. Modification of adsorbent The prepared adsorbent was separately modified with 3 M nitric acid and 3 M caustic soda solutions by transferring 1 g of prepared powders into 100 ml of NaOH solution or HNO 3 solution. The mixture was shaken in a thermostatic orbit incubator shaker (Neolab, India) at 240 rpm for 60 min. Then the mixture was filtered and washed with distilled water to maintain the ph of its filtrate equal to normal (ph 7). Then the treated powders were later dried at room temperature for one day to remove moisture content completely. This experiment was repeated for many times. Adsorption experiments The adsorption experiments were conducted in 250 ml flasks containing 100 ml of Tl(III) solution of known concentration (25 ppm) prepared from the dilution of 250 ppm stock solutions. The initial ph of each flask solution was adjusted to optimum value (ph 5) with 0.1 M H 2 SO 4 or 0.1 M NaOH solution. Then, a given mass of modified eucalyptus leaves powders (0.25 g) was added to each flask solution and the resultant mixture was shaken in a thermostatic orbit incubator shaker (Neolab, India) at 240 rpm for 60 min. Upon completion, the sample was removed from the flask and filtered through a filter paper (0.2 mm pore size) to separate adsorbent particles. The filtrate was later analyzed for residual Tl(III) ions. Then, the percentage removal (% Removal) was determined as shown below: Ci C f % Removal = 100 C i (1) where C i and C f are the initial and the final concentrations of Tl(III) ions in solution phase respectively. The amount of metal ion adsorbed per unit mass of adsorbent (q e ) was calculated using the following equation: q e V = ( Ci Ce ) (2) m where C i and C e represent initial and equilibrium concentrations (mg/l) respectively; V, the volume of the solution (L); and m, the mass of the adsorbent (g). The average absolute value of relative error (AARE) was used to compare the predicted results with experimental data, as shown below: N 1 Predicted value Experimental value AARE% = 100 N i = 1 Experimental value (3) where N is the number of data points. Also, in this study the effect of various experimental parameters including modification of adsorbent (acidic and basic), solution initial ph (1-5), agitation speed ( rpm), contact time (5-60 min), amount of sorbent ( g), thallium initial concentratin (5-50 ppm) and temperature (20-45 C) on the percentage removal of Tl(III) ions was investigated. Results and Discussion Modification of adsorbent As described before, the adsorbent is separately modified with 3 M nitric acid solution and 3 M caustic soda solution and then adsorption experiments are conducted. The results show that the alkaline modified adsorbent (% removal 57.5) is superior than the acidic treated adsorbent (% removal 2.3). In the case of alkaline modification, OH - ions link to the adsorbent sites. This favors Tl(III) ions adsorption onto the surfaces of the adsorbent. The adsorbent was modified with 1, 2, 3 and 3.5 M NaOH solutions and the adsorption experiments were carried out. The modified adsorbent with

3 382 INDIAN J. CHEM. TECHNOL., NOVEMBER M NaOH solution shows 60.8 % removal and is found to be the most suitable. The adsorbents modified with 2, 3 and 3.5 M NaoH show 56.5, 57.7, 59.5% removal. Effect of initial ph on adsorption capacity Initial ph of the test solutions was varied in the range of 1-5. At relatively higher ph values, Tl(III) ions could be precipitated as Tl(OH) 3 (K sp = ) 19. The results are shown in Fig. 1 a. The optimum ph is found to be 5. At low ph range (1-3), H + ions that are adsorbed on the surfaces of the adsorbent oppose Tl(III) ions while OH - ions favor it. Effect of agitation speed Adsorption tests were carried out using different agitation speeds (60, 120, 180, 240 and 300 rpm). Fig. 1b shows a plot of the adsorption (% adsorbed) of Tl(III) ions on the alkaline modified eucalyptus leaves powders against agitation speeds. The maximum adsorption is obtained at 120 rpm. This can be explained by the fact that the film boundary layer thickness surrounding the particles is reduced and the external film transfer coefficient is increased with increasing agitation speed. Therefore, adsorption increases. Effect of contact time Adsorption of Tl(III) ions onto the alkaline modified eucalyptus leaves powders depends on different contact times (Fig. 1c), when other operating conditions are kept constant. The optimum contact time is found to be 20 min. Decrease in adsorption at relatively higher contact time may be because of desorption of thallium ions. Effect of sorbent dosage The dosage of the sorbent in the test solutions is varied from 0.1 g to 0.5 g while the other conditions are maintained constant (Fig. 1d). It is clear that the dosage of 0.4 g is the optimum value. Effect of temperature Adsorption is affected by the adsorption temperature (Fig. 1e). The optimum temperature range is C. It is observed that the studied adsorption may be exothermic. Effect of thallium initial concentrations Thallium initial concentration is another parameter affecting the adsorption of thallium ions. The measured percentage removal of Tl(III) ions on the alkaline modified eucalyptus leaves powders is determined for different initial concentrations of 5-50 ppm Tl(III) ions. The other conditions are kept constant. The results are presented in Fig. 1f. The suitable concentration range of Tl(III) ions is 10 mg/l upto 20 mg/l. Adsorption isotherms The experimental results are analyzed by Langmuir and Freundlich models. The respective linear equations are given below: Langmuir equation: 1 = 1 + ( 1 ) 1 (4) qe qm qmkl Ce 1 Freundlich equation: log qe = log K F + logc (5) e n where C e (mg L -1 ) and q e (mg g -1 ) are the liquid phase and solid phase concentrations of sorbate at equilibrium respectively; K L (L mg -1 ), the Langmuir isotherm constant; q m (mg g -1 ), the maximum sorption capacity of Langmuir model; K F, the Freundlich constant (mg 1-(1/n) L 1/n g -1 ), and n, the heterogeneity factor. At first, we analyzed the adsorption data at different initial concentrations of Tl(III) ions according to the linear form of the Langmuir isotherm [Eq. (4)]. The plot of 1/q e against 1/C e shows a straight line with a slope of 1/K L q m and intercept of 1/q m. It is found that the Langmuir isotherm is linear over the suitable concentration range and the data are correctly fitted the Langmuir equation. The monolayer saturation capacity (q m ) is found to be mg g -1. We then examined our data according to the Freundlich isotherm [Eq. (5)]. The plot of log q e versus log C e also shows a straight line with slope 1/n and intercept log K F. This plot is also found to be linear. On comparing langmuir and Freundlich isotherms, It is indicated that Langmuir model yields a slightly better fit than Freundlich model, because the value of its regression coefficient (R 2 ) is higher (0.9989) and AARE value is lower (1.55%) than the values obtained from the Freundlich isotherm (R and AARE 7.67% ). The value of Freundich exponent (n, 1.05) in the range 1-10 shows the favorable sorption. The essential characteristics of the Langmuir isotherm can be defined by a dimensionless constant separation factor (R L ), that is obtained by the following equation 20 : R L = K L C i (6)

4 KHAVIDAKI et al.: ADSORPTIVE REMOVAL OF THALLIUM(III) IONS 383 Fig. 1 Effect of process parameters on % removal of Tl (III) ions

5 384 INDIAN J. CHEM. TECHNOL., NOVEMBER 2013 Fig. 2 Separation factor for the adsorption of Tl(III) ions onto the alkaline modified eucalyptus leaves powder in terms of initial concentration of Tl(III) ions where K L is the Langmuir constant; and C i, the initial concentration of the sorbate in solution. The separation factor (R L ) illustrates the shape of the isotherm and the nature of the adsorption process, as given below: R L value : Nature of process R L >1 : Unfavorable R L =1 : Linear 0<R L <1 : Favorable R L =0 : Irreversible The calculated R L values against initial thallium(iii) concentration are shown in Fig. 2. It is found that the adsorption is more favorable at higher concentrations. Moreover, the value of R L in the range of 0-1 at all initial thallium(iii) concentrations confirms the favorable adsorption of Tl(III) ions. Conclusion The study shows that the alkaline modified eucalyptus leaves powder can be used as a adsorbent for the removal of Tl(III) ions from aqueous media. It is observed that the modification of adsorbent with the alkaline solution (NaOH solution) is essential for obtaining a significant adsorption. The alkaline modified adsorbent show the significant adsorption capacity but the adsorbent treated with nitric acid does not exhibit any such activity. The experimental data show that the adsorption amount is dependent on operating conditions such as initial solution ph, agitation speed, contact time, amount of adsorbent, initial thallium concentrations and temperature. The optimum conditions are ph 5, agitation speed 120 rpm, contact time 20 min, amount of adsorbent 0.4 g, initial thallium concentrations mg L -1, and temperature C. Under the conditions, the maximum adsorption amount of thallium(iii) ion on eucalyptus leaves powder obtained is 78.2%. A comparison of Langmuir and Freundlich isotherm plots for the fitting experimental data indicate that Langmuir model yields a slightly better fit than Freundlich model. The monolayer saturation capacity (q m ) is calculated to be mg g -1. The R L values signify that the alkaline modified eucalyptus leaves powder is suitable for the adsorption of thallium(iii). References 1 Yao Z Y, Qi J H & Wang L H, J Hazard Mater, 174 (2010) Zhang L, Huang T, Zhang M, Guo X & Yuan Z, J Hazard Mater, 157 (2008) Lan C H & Lin T S, Ecotox Environ Safe, 61 (2005) Zitko V, Sci Total Environ, 4 (1975) Cheam V, Water Qual Res J Can, 36 (2001) John Peter A L & Viraraghavan T, Environ Int, 31 (2005) Zhang Z, Zhang B, Long J, Zhang X & Chen G, Sci China Ser D, 41 (1998) Kikuchi E, Itoh K, Fujishima A, Yonezawa T & Kimura T, Chem Lett, 19 (1990) Kajitvichyanukul P, Chenthamarakshan C R, Rajeshwar K & Qasim S R, Adsorp Sci Technol, 21 (2003) Akl M A A, Kenawy I M M & Lasheen R R, Microchem J, 78 (2004) Cao X A, Chen Y H & Zhang Q, Fuel Chem Technol, 19 (2000) Hanafi A, J At Mol Sci, 1 (2010) Yin Z, Zhang D & Pan X, Int J Environ Pollut, 37 (2009) John Peter A L & Viraraghavan T, Bioresour Technol, 99 (2008) Zhang L, Huang T, Guo X & Liu X, Chem Res Chinese, 26 (2010) Campbell F W, Zhou Y G & Compton R G, New J Chem, 34 (2010) Zolgharnein J, Asanjarani N & Shariatmanesh T, Toxicol Environ Chem, 93 (2011) Liu J, Lippold H, Wang J, Lippmann-Pipke J & Chen Y, Chemosphere, 82 (2011) Villaverde M S & Verstraeten S V, Arch Biochem Biophys, 417 (2003) Hall K R, Eagleton L C, Acrivos A & Vermeulen T, Ind Eng Chem Fund, 5 (1966) 212.