International Journal of PharmTech Research CODEN (USA): IJPRIF, ISSN: , ISSN(Online): Vol.10, No.

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1 International Journal of PharmTech Research CODEN (USA): IJPRIF, ISSN: , ISSN(Online): Vol.10, No.1, pp 62-76, 2017 Application of Hallosysite Nanotubes in Removal of Auramine Y and Auramine O Dyes Sanjay Desai, Astha Pandey and M.S. Dahiya Institute of Forensic Science, Gujarat Forensic Sciences University, , India Abstract : Halloysite nanotubes (HNTs) are low-cost clay minerals that have the ability to remove cationic dye from aqueous solution. Natural HNTs used as adsorbent were initially characterized by FT-IR and SEM.We have studied the effect on experimental parameters like adsorbent dose, initial ph, temperature, initial concentration and contact time were investigated by using batch adsorption technique(bat). Adsorption data were modeled using the Freundlich as well as Langmuir adsorption isotherms and first order kinetic studies. The kinetics of adsorption was observed to be first order in regard to intra-particle diffusion as the rate determining step. The values of their corresponding constants were determined from the slope and intercepts of their respective plots. Thermodynamic parameters such as free energy (ΔG ), enthalpy (ΔH ) and entropy (ΔS ) of the system were also calculated by using Langmuir constant K L. The results above indicate that HNTs had the potential to be employed as low-cost and relatively effective adsorbent for the removal of Auramine dye. Keywords : Adsorption, Halloysite Nanotubes, Auramine Y, Auramine O. 1. Introduction Many industries are facing the major problem of removal of dye waste from water. Annually tons of dyes are produced for various industrial applications and discharge of which into water has made it unportable for drinking and consumption of human beings. The discharge of this dyes in waste water is a matter of high concern for pollution to the environment and to the entire ecosystem. The two dyes (Auramine O and Auramine Y) which have been used are cationic in nature and are mainly used for dying the fabrics such as silk, wool etc where as Auramine is a cationic dye for paper, textiles, leather. They are also used as an antiseptic and as a fungicide. Auramine O is a yellow fluorescent dye and in pure form, Auramine O appears as yellow needle crystals. The solubility of it is mainly in polar solvents like water and ethanol. These dyes are difficult to biodegrade due to its complex aromatic structure. Though there are several methods like coagulation, flocculation, oxidation, membrane filtration available for purification of waste water, the adsorption method is most efficient for removal of pollutants from waste water due to its ease of operation and comparable low cost in discoloration process. Apart from the adsorbents like activated charcoal, silica, florisil, other microporous and mesoporous materials the clay minerals were reported to be unconventional adsorbents for the removal of dyes from aqueous solution due to their cheap, abundant resources and higher surface areas. Therefore the most suitable adsorbent for the removal of Auramine dyes selected was Halloysite nanotubes (HNT), an aluminosilicate mineral 1,2. Halloysite (Al 2 Si 2 O 5 (OH) 4 2H 2 O) is a double layered aluminosilicate formed by the surface weathering of aluminosilicate minerals and is composed of aluminum, silicon, oxygen and hydrogen. HNT are hollow tubes

2 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): with diameters smaller than 100 nanometers with lengths typically ranging from about 500 nanometers to over 1.2 microns. Compare to carbon nanotubes, naturally occurring halloysite nanotubes are inexpensive, nontoxic, and readily available in quantity, environmentally benign, and safe and easy to process 1,2. The objective of the study is to explore the feasibility of halloysite nanotube as an adsorbent for the removal of basic dyes like Auramine Y and Auramine O, which is basically used to stain acid-fast bacteria in sputum or in paraffin sections of infected tissue and as a component of the Truant Auramine-rhodamine stain for tubercle bacilli. The effects of adsorbent dose, initial ph, temperature, initial concentration and contact time were investigated. The equilibrium, kinetic data and thermodynamic parameters were processed to understand the adsorption mechanism of dye onto HNTs. The results indicated that natural HNT can be used as an efficient adsorbent for removal of auramine dye from waste water. Experimental Chemicals and Reagentss: Halloysite (premium grade) was obtained from New Zealand China Clays Ltd (New Zealand). Auramine Y and Auramine O were purchased from HBR chemicals Pvt Ltd, Haryana. Sample Preparation for SEM: HNT was milled and was then sieved by 75 μm mesh. It was then dispersed in water for a period of time and sprayed to dry at 200 C; the powder of halloysite nanotubes was refined. The size and morphology of halloysite nanotube were examined by scanning electron microscope as shown by in image Fig. 1. Sample Preparation for adsorption study: A stock solution (1000 mg/l) was prepared by dissolving basic dye in distilled water and further desired concentrations were obtained by diluting the stock solution with distilled water. Hydrochloric acid and sodium hydroxide solutions were prepared from analytical grade chemicals and were used for ph adjustment as and when required. Fig.1. SEM image of HNT Characterization The characterization of halloysite nanotubes can be done by various instrumental techniques but we carried out with different instruments like Scanning Electron Microscope by Carl Zeiss EVO-18, UV absorption Spectrophotometer by Shimadzu, Fourier transform infrared spectroscopy (FTIR) by Bruker FT-IR.

3 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): Procedure: Adsorption experiments:- The adsorption of Auramine dye from aqueous solution was performed by using batch technique in a thermostatic shaker bath at 30 0, 40 0 and 50 0 C. After adsorption, the solution was centrifuged for 15 min at 3000 rpm and then the concentration of remaining dye was determined using spectrophotometer at λmax of 530 nm. The removal efficiency (R %), the amount of dye adsorbed at time t (Qt, mg/g) and at equilibrium (Qe, mg/g) was calculated by using the following equations respectively : R =100(Co-Ce)/Co Qt = (Co-Ct) V/m Qe = (Co-Ce) V/m were Co, Ct and Ce (mg/l) are the initial, t time and equilibrium concentrations of Auramine solution, respectively; V (L) is the volume of Auramine solution and m (g) is the weight of HNTs. Results and Discussion Characterization of HNT The characterization of halloysite nanotubes were carried out with different experimental approaches like Scanning Electron Microscopy (SEM) and FT-IR. SEM image of halloysite nanotube samples were taken with Carl Zeiss EVO-18 (Fig.1). Vibration spectra (Fig.2) for HNT were recorded from the same samples obtained after processing as describe above and it shows characteristic peak at 3623 and other peaks at 1722 and 1038 cm -1.The band at 1038 cm-1 in HNT, assigned as to be Si-O-Si plane vibrations (3). Fig.2. FT-IR spectrum of HNT Adsorption rate Effect of contact time and initial dye concentration: The effect of concentration on contact time was also studied as a function of initial dye concentration. The effect of initial dye concentration and contact time on the removal rate of dye by HNTs is shown in Figs. 3 and 4. As shown, the adsorption increases with increasing initial dye concentration. The result shows that dye uptake takes place rapidly for the first 20 min and finally attains saturation within about 30 min. The equilibrium was attained at 30 min. In order to make a comparative study for adsorption of Auramine in its

4 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): different initial concentrations, the weight of adsorbent (m = 1 g), initial ph (phi=7) and temperature (T =25 degree C) were kept the same in all adsorption experiments. The extent of removal of dye was faster in initial stages, then it showed decreasing pattern and finally it became constant showing the attainment of equilibrium. From the Fig 3, it is evident that the removal percent of Auramine increases for initial dye concentrations from 10 ppm to 15 ppm and then decreases from 15 ppm to 25 ppm. This observation is related to constant number of adsorbent sites, while the number of dye molecules increases, so to some extent with increasing the initial dye concentration the rise in amount of adsorbed dye is more than the rise in remained one and the removal percent increases. After that, rise in adsorbed amount of dye is lower than the rise in remaining dye, so removal percent of dye decreases. Figs.3 and 4 shows that the curves are single, smooth, indicating monolayer coverage of the adsorbent surface. Fig.3. Effect of initial concentration on sorption capacity of Auramine yellow on HNT Fig.4. Effect of initial concentration on sorption capacity of Auramine O on HNT Effect of adsorbent dosage: In order to make a comparative study for adsorption of Auramine in different amounts of HNT, the initial concentration of dye (C 0 = 20 ppm), phi (equal to 7) and temperature (T = 25 C) were kept the same in all adsorption experiments. It was observed that the removal percent of the dye increased with increase in adsorbent mass for Auramine yellow and Auramine O from Figs. 5 and 6. An increase in the adsorption with the adsorbent dosage can be attributed to greater surface area and the availability of more adsorption sites while the number of adsorbate molecules remains constant.

5 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): Fig.5. Effect of on adsorbent dose on percent removal of Auramine Yellow on HNT Fig.6. Effect of on adsorbent dose on percent removal of Auramine O on HNT Effect of initial ph: ph value of dye solution plays an important role in the adsorption process. Removal rate is increased with the increase of ph. 150ml of dye solution was prepared in a conical flask with dye conc. 50mg/L and adsorbent conc. (1g/150mL) and initial ph of the conical flask is to be measured. The ph of the dye solutions was adjusted with dilute HCL (0.05N) or Na OH (0.05N) solution by using a ph meter. 150 ml of dye solution was prepared and the ph of solution is changed from 2 to 10.The flasks were put inside the incubator shaker (120 rpm fixed throughout the study) maintained at 27 C. Calibration plot of the dye after 2 hours is taken using UV spectrophotometer to measure the final concentration of dye. A graph is plotted with Qe vs. initial ph. A ph meter calibrated with 4.0 and 9.0 buffers is used. Figs. 7 and 8 indicate that maximum dye removal had occurred in basic medium. As the ph increases the sorption capacity also increases 3. The ph dependence of dye adsorption is mainly influenced by two factors: (i) distribution of dye in the solution phase, and (ii) overall charge of the adsorbent. At lower ph, the surface of the HNT is positively charged and there is no significant attraction between surface of adsorbent and the dye. At higher ph values, HNT surface will get more negatively charged, and enhances the electrostatic attraction of positively charged dye with negatively charged adsorption sites of HNT surface. This leads to the increased removal of dye.

6 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): Fig.7. Effect of ph on sorption capacity of Auramine yellow on HNT Fig.8. Effect of ph on sorption capacity of Auramine O on HNT Effect of temperature:- Temperature also has important effect on the adsorption process. In order to study the effect of temperature on adsorption of Auramine, the initial concentration of dye (C 0 =20 ppm), the weight of adsorbent (m = 1 g) and phi (equal to 7) are kept the same in all adsorption experiments. The adsorption capacity of dye onto HNTs is found to increase with increase in temperature from K to K, thereby indicating the process to be endothermic in nature. The rise of adsorption with temperature could be due to the increase of rate of diffusion of adsorbate molecules across the external boundary layer and the enlargement of the pore sizes of adsorbent particles 4. In fact, the reaction between the hydroxyl groups of HNT and cationic group of dye could be favored at high temperature.

7 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): Fig.9. Effect of temperature on sorption capacity of Auramine Yellow on HNT Fig.10. Effect of temperature on sorption capacity of Auramine O on HNT Adsorption Isotherm: Adsorption isotherm models are graphical representations which describe the interaction behavior between the adsorbent and adsorbate, and are essential for investigating the adsorption mechanism. It maps the distribution of adsorbed solute between the adsorbate and solid phases at various equilibrium concentrations. The two isotherm models which was used to analyze equilibrium data were: 1) Langmuir isotherm model 2) Freundlich isotherm model The Langmuir isotherm assumes monolayer adsorption onto a surface containing a finite number of adsorption sites of uniform strategies of adsorption with no transmigration of adsorbate in the plane of surface 5, while the Freundlich isotherm model assumes heterogeneous surface energies 6. The simplest theoretical model that can be used to describe monolayer adsorption is the Langmuir equation and is most frequently used to determine the adsorption parameters. It is based on the assumption of monolayer adsorbent on a structurally homogenous adsorbent, where all the adsorption sites are identical 7. The adsorption energy is constant and independent of surface coverage where the adsorption occurs on localized sites with no interaction between adsorbate molecules.

8 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): Langmuir adsorption isotherm is based on the assumption that, Adsorption is a type of chemical combination in which adsorbate is adsorbed on the adsorbent surface and the adsorbed layer is unimolecular. The linear form of Langmuir equation is: Ce/Qe =Ce /Qm + 1/K L Q m where Ce (mg/l) is the concentration of dye at equilibrium(ppm), Qe(mg/g) is the amount of dye adsorbed by the HNTs at equilibrium(mg g -1 ), Qm (mg/g) is the maximum adsorption capacity corresponding to monolayer coverage, and K L (L/mg) is the Langmuir constant. When Ce/qe was plotted against Ce (Figs. 11 and 12), a straight line with slope of 1/Qm was obtained indicating that the adsorption of the adsorption of Auramine Y& Auramine O on halloysite nanotube follows the Langmuir isotherm. The Langmuir constants 'K L ' and 'Qm' were calculated from this isotherm and their values are given in Table 1. R L = 1/1+ K L C 0 Here a dimension less constant separation factor R L is defined in order to predict if the adsorption process is favorable or not. K L (L/mg) is the Langmuir constant and C 0 (mg/l) is the initial dye concentration. The R L value indicates adsorption process is irreversible when R L is 0; favorable when R L is between 0 and 1; linear when R L is 1; and unfavorable when R L is greater than 1. The correlation coefficients of the isotherms are all higher than 0.99 at the three temperatures, thereby indicating that the Langmuir isotherm fits the equilibrium data very well. The values of Qm for HNTs increases with increasing temperature, indicating the adsorption process is endothermic. The maximum monolayer adsorption capacities of HNTs vary from at 303K to at 323K. The R L values shown in Table 1 (all<1) indicate that the adsorption of basic dyes follow Langmuir isotherm. These results show that it is monolayer adsorption on the homogenous structure of HNT. Fig.11. Plot of Langmuir adsorption isotherm of Auramine Yellow on HNT

9 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): Fig.12. Plot of Langmuir adsorption isotherm of Auramine O on HNT Table 1.The values of Langmuir constant Qm and K L in addition to R L Auramine yellow Auramine O Temperature R L K L Qm R 2 R L K L Qm R The Freundlich isotherm is based on multilayer adsorption on heterogeneous surface 8. The linear form of Freundlich equation is Log Qe = log K F +1/n log Ce where Qe is the dye concentration on HNTs at equilibrium, Ce (mg/l) is the concentration of dye in solution at equilibrium, K F (mg/g (mg/l)1/n) and 1/n are Freundlich constants related to adsorption capacity and adsorption intensity or surface heterogeneity, respectively. K F can be defined as the adsorption or distribution coefficient and represents the quantity of dye adsorbed onto HNT adsorbent for a unit equilibrium concentration. The plot of log Qe versus log Ce gave straight lines. Higher value of K F indicates higher affinity for adsorbate and the values of the empirical parameter 1/n lie between 0 < 1/n < 1, indicating favorability of adsorption. Values of n>1 shows that adsorption is in favorable condition 9. The experimental data were attempted to fit into Freundlich adsorption isotherms. The correlation coefficients (R 2 > 0.95) reflects that the adsorption of these dyes agrees the Freundlich model.the values of Freundlich constants are calculated from the slope and intercept in Figs. 13 and 14 and are given in Table 2.

10 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): Fig.13. Plot of Freundlich adsorption isotherm of Auramine Yellow on HNT Fig.14. Plot of Freundlich adsorption isotherm of Auramine O on HNT Table 2.The values of Freundlich constant K F and 1/n Auramine Yellow Temperature K F 1/n R 2 30 C C C Auramine O K F 1/n R Adsorption Kinetics: The adsorption kinetic study is quite significant in wastewater treatment as it describes the solute uptake rate, which in turn controls the residence time of adsorbate uptake at the solid-solution interface. Dosage study is an important parameter because it determines the capacity of adsorbent for a given initial concentration of the dye solution. In this present investigation, the kinetics of the adsorption systems were studied by plotting the amount of dye adsorbed on the adsorbent with time for different adsorbent dosages at a constant initial

11 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): concentration (100mg/L), at different temperatures. In all the experiments, it was observed that with increase in adsorbent loading, the fraction of dye removal increases and it was graphically plotted. Here, three kinetic models are introduced to examine the mechanism and rate-controlling step in the overall adsorption process. They are 1. Pseudo-first-order kinetic model 2. Pseudo-second-order kinetic model 3. Intra-Particle diffusion model Pseudo first order kinetic model:- The pseudo-first order kinetic model of Langergren, also known as the Langergren kinetic equation, is widely employed to understand the kinetic behavior of the system 10. It is generally expressed as follows, dq / dt = K 1 (qe - qt); where qe and qt are the sorption capacity at equilibrium and sorption capacity at time and K 1 is the rate constant of pseudo first order sorption (1/min). After integration and applying boundary condition t = 0 to t = t and q = 0 to q = qt, the integrated form becomes, log (qe-qt) = log qe-[k 2 / 2.303]t. A linear trace is expected between the two parameters log (qe-qt) and t, provided the adsorption follows first order kinetics. It is observed that the data does not fit in to first order equation. Pseudo second order kinetic model: The adsorption may also be described by pseudo second order kinetic model 11,12,14, the linearised form of which is T/Qt =1/K 2 Qe 2 + t/qe where K 2 is the rate constant of pseudo second order (g/mg min). A plot of t/qt and t should give a linear relationship if the adsorption follows second order. Qe and K 2 can be calculated from the slope and intercept of the plot. Figs. 15 and 16 show the pseudo second order plot for the adsorption of Auramine yellow and Auramine O on the HNT at various initial dye concentrations. The linear plots obtained from experimental data clearly show that the adsorption process follow pseudo second order kinetics. Fig.15. Pseudo second order plot of Auramine Yellow on HNT

12 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): Fig.16 Pseudo second order plot of auramine O on HNT Intra particle diffusion study: In the batch mode adsorption process, initial adsorption occurs on the surface of the adsorbent. In addition, there is a possibility of the dye molecules to diffuse into the interior pores of the adsorbent. Weber and Morris suggested the following kinetic model to investigate whether the adsorption is intra particle diffusion or not 13. The relationship may be given as Q t = K id T 1/2 + C Where K id is the intraparticle diffusion rate constant and is calculated by plotting Q t vs T 1/2 and the results are given in Figs. 17 and 18.The linear portion of the plot for wide range of contact time between adsorbent and adsorbate does not pass through the origin. This deviation from the origin may be due to the variation of mass transfer in the initial and final stages of adsorption. The values of K id for all concentrations studied were determined from the slopes of respective plots and the results are presented in Table 2. Fig.17. Intraparticle diffusion plot on Auramine Yellow on HNT

13 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): Fig.18. Intraparticle diffusion plot on Auramine O on HNT Adsorption Thermodynamics: The amount of auramine adsorbed at equilibrium at different temperatures for 303K, 313K and 323K was examined to obtain thermodynamic parameters. Thermodynamic parameters like ΔH and ΔS were evaluated using Van t Hoff s equation log K L = ΔS 0 /2.303R- ΔH 0 /2.303RT Where K L is the Langmuir equilibrium constant, ΔH 0 and ΔS 0 are the standard enthalpy and en-tropy changes of adsorption respectively and the values of Auramine Y and Auramine O are calculated from the slopes and intercepts of the linear plot of log K L vs 1/T shown in Fig.19 & 20 respectively. A plot of (Log K L ) versus (1/T) should produce straight line with slope equals to ΔH 0 /2.303RT and intercept equals to ΔS 0 /2.303R (Cheung et al., 2001). Figs. 19 and 20 shows linear relation between (Log K L ) and (1/T) with high correlation coefficient (R 2 > 0.9). The free energy of specific adsorption ΔG 0 (KJ.mol -1 ) is calculated using following equation ΔG 0 = -RT ln K L The thermodynamical parameters calculated are given in Table 3. Negative values of ΔG 0 indicate the feasibility of the process and spontaneous nature of the adsorption with a high performance of auramine dye for HNT. Positive value of ΔH 0 indicates the endothermic nature of the process, while positive value of ΔS 0 reflects the affinity of the adsorbents for the HNT and suggests some structural changes in adsorbate and adsorbent 15. The endothermic nature of adsorption is confirmed by the positive ΔH 0 value.

14 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): Fig.19. Plot of Log K L against 1/T for the adsorption of Auramine Y onto HNT Fig.20. Plot of Log K L against 1/T for the adsorption of Auramine O onto HNT Table 3.Thermodynamical parameters for adsorption of Auramine yellow and Auramine O Dyes ΔG 0 (KJmol -1 ) 303K 313K 323K ΔS 0 (KJmol -1 K -1 ) ΔH 0 (KJmol -1 ) 303K 313K 323K R 2 Auramine yellow Auramine O Conclusion: The halloysite nanotube can be efficiently utilized as an adsorbent for the removal of Auramine Yellow and Auramine O from aqueous solutions. The amount of Auramine Y& O adsorbed increased with the increase in the initial concentration of dye, ph and temperature. The isotherm adsorption data fit well with Langmuir model indicating monolayer adsorption and maximum adsorption capacity increased from to (mg/g) with increasing temperature from 30 0 to 50 0 C. By calculating the thermodynamic parameters like ΔG 0, ΔH 0 and ΔS 0, the variation of temperatures show the spontaneity of the system. The surface

15 Astha Pandey et al /International Journal of PharmTech Research, 2017,10(1): morphology studies reports that the excess amount of dye contents was adsorbed on the surface which show changes on the active sites of the adsorbent. Acknowledgement: The Authors are thankful the Director IFS for providing the infrastructural and instrumental facility for the research work. References: 1. Mall, I.D., Srivastava, V.C., and Agarwal, N.K. (2007). Adsorptive removal of Auramine-O: Kinetic and equilibrium study. Journal of Hazardous Materials., Vol. 143, pp Rawtani, D., and Agrawal, Y.K. (2012). Multifarious Applications of Halloysite Nano Tubes: A Review. Rev.Adv.Mater.Sci., Vol. 30, pp Luo, P., Zhao, Y., Zhang, B., Liu, J., Yang, Y., Liu, J. (2010). Study on the adsorption of Neutral Red from aqueous solution onto halloysite nanotubes. WATER RESEARCH., Vol. 44, pp Wang, S., Zhu, Z.H. (2007). "Effects of acidic treatment of activated carbons on dye adsorption. Dyes Pigm., Vol. 75, pp Fytianos, K., Voudrias, E., Kokkalis, E. (2000). Sorption-desorption behavior of 2,4-dichlorophenol by marine sediments. Chemosphere., Vol. 40, No. 1, pp Singh, S.R., Singh, A.P. (2012). Removal of methylene blue dye from aqueous solutions using carbon adsorbent prepared from sugarcane bagasse. IJESR., Vol. 2, No. 4, pp Langmuir, I. (1918). The adsorption of gases on plane surface of glass, mica and platinum. J. Am. Chem. Soc., Vol. 40, pp Freundlich, H.M.F. (1906). Uber die adsorption in losungen Z. Phys. Chem., Vol. 57, pp Hameed, B.H., Mahmoud, D.K., Ahmad, A.L. (2008). Equilibrium modeling and kinetic studies on the adsorption of basic dye by a low-cost adsorbent: Coconut (Cocos nucifera) bunch waste. Journal of Hazardous Materials., Vol. 158, pp Annadurai, G., Juang, S.R., and Lee, J.D. (2002). Use of Cellulose-Based Wastes for Adsorption of Dyes from Aqueous Solutions. Journal of Hazardous Materials., Vol. 92, No. 3, pp Ho, Y.S., and McKay, G. (1999). Pseudo-Second Order Model for Sorption Processes. Process Biochemistry., Vol. 34, No. 5, pp [46]Elmorsi, T.M. (2011). Equilibrium Isotherms and Kinetic Studies of Removal of Methylene Blue Dye by Adsorption onto Miswak Leaves as a Natural Adsorbent. Journal of Environmental Protection., Vol. 2, pp Weber, W.J., Morris, J.C. (1963). Preliminary appraisal of advanced waste treatment process. Proc. lnt. Conf., Advances in Water Poll Res., Vol. 2, pp Cheung, C.W., Porter, J.F., McKay, G. (2001). "Sorption kinetic analysis for the removal of cadmium ions from effluents using bone char." Water Res., Vol. 35, pp Hong, S., Wen, C., He, J., Gan, F., Ho, Y.S. (2009). Adsorption thermodynamics of Methylene Blue onto bentonite. Journal of Hazardous Materials., Vol. 167, pp *****