Advances in Chemical Engineering and Science, 2015, 5, 373-388 Published Online July 2015 in SciRes. http://www.scirp.org/journal/aces http://dx.doi.org/10.4236/aces.2015.53039 TiO Nanoparticles for Removal of Malachite 2 Green Dye from Waste Water Zeinab M. Abou-Gamra*, Mohamed A. Ahmed Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt Email: *[email protected] Received 17 June 2015; accepted 24 July 2015; published 28 July 2015 Copyright © 2015 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/ Abstract In this research, we present a simple and successful route for synthesis titania nanoparticles by controlled sol-gel progress. Chitosan as bio-template is involved in the progress of preparation to increase the surface area and manipulate defined particle and pore structure. The crystalline be- havior and the nanostructure of the prepared nanoparticles were investigated using X-ray diffrac- tion [XRD] and transmission electron microscope [TEM]. The crystalline results have pointed out the predominant existence of anatase phase that reveals the successful role of chitosan in stabi- lizing titania nanoparticles and preventing the growth of these particles into rutile phase. It is ob- vious to notice that a change in sample crystallography from anatase to completely amorphous nanoparticles upon adsorption of malachite green dye indicates a strong adsorption of this dye that destroys the crystalline feature of titania sample. TEM analysis reveals the existence of spherical nanoparticles with size about 25 nm. The adsorption isotherm indicates the adsorption capacity 6.3 mgg−1 TiO . The value of enthalpy change (∆H˚) for malachite green dye adsorption is 2 19 kJ/mol, which indicates that the removal process is endothermic. The adsorption process fol- lows pseudo-se∙cond order rate equation and the negative values of standard free energy (∆G˚) suggest that the adsorption process is spontaneous. Keywords TiO Nanoparticles, Chitosan, Malachite Green, Adsorption 2 1. Introduction The extensive uses of industrial dyes in the fields of paints, cosmetics and food industry are accompanied by various risks on human health due to the stability and toxicity of these organic materials [1]-[3]. The effective *Corresponding author. How to cite this paper: Abou-Gamra, Z.M. and Ahmed, M.A. (2015) TiO Nanoparticles for Removal of Malachite Green Dye 2 from Waste Water. Advances in Chemical Engineering and Science, 5, 373-388. http://dx.doi.org/10.4236/aces.2015.53039 Z. M. Abou-Gamra, M. A. Ahmed removal of dyes from effluent is considered one of the environmental challenges in the recent years. Malachite green (MG) (Color Index No: 42,000) is one of triphenylmethane dyes, a green crystal powder with a metallic luster, highly soluble in water and ethanol with blue-green solutions [4]. Malachite green (MG) has numerous industrial applications (dyeing of silk, leather, plastics and paper). Their appearance is harmful for humans and animals following inhalation and/or ingestion [5], produces toxicity to respiratory system and reduces fertility in humans [6]. The triphenylmethane dyes biodegradation due to presence of nitrogen in their back bone (generate carcinogenic, genotoxic, mutagenic and teratogenic problems) is a difficult task [7]. MG has high resistance to light and oxidizing agents, while its removal based on biological treatment and chemical precipitation has low efficiency [8]. It is now established that TiO is considered a promising semiconductor that is extensively in- 2 volved in removal of several toxic organic containments through both adsorption and photocatalytic process due to the stability of its chemical structure, biocompatibility, strong oxidizing power, non-toxicity and low cost of the metal precursors [9]-[14]. TiO can be only triggered by UV radiation that encompasses only about 4% - 5% 2 of natural solar radiation due to its wide band gap energy (3.2 eV). This defect limits the industrialization of this powder. Moreover, the mineralization processes through various redox reactions are encountered by the rapid recombination of the charge carriers. Another major defect in the preparation of nano titanium oxide through sol-gel route is the rapid hydrolysis of titanium alkoxide salts that lead to irregular condensation of hydroxide particles into amorphous solid [15]. This random condensation of particles is the primary cause for obtaining ir- regular particle and pore structure. It is generally accepted that the existence of active template can manipulate the particle and pore structure to facilitate the diffusion of organic containments inside the pore as well as on the oxide surface. Most of the sur- factants that are involved in the preparation of nanoparticles exhibit high degree of toxicity that affects human health and cause several environmental risks. Recently, chitosan which is available in large quantity in nature, non-toxic and biocompatible can be considered as excellent natural cationic polysaccharide biopolymer that is involved in synthesis of nanoparticles of ZnO andTiO . It is interesting to mention that chitosan molecules pos- 2 sess large number of reactive hydroxyl (-OH) and amino (-NH ) groups which can effectively co-ordinate with 2 various metal ions. It is a best issue to manipulate various oxide structures. Moreover, these natural molecules can prevent nanoparticles from agglomeration during growth, improving the adsorption capacity of the sample and can overcome the difficulty in separation and recovery of nanosized powder materials [16] [17]. Chitosan is reported as excellent adsorbent in removal of various organic dyes [18] [19]. The research work is concerned with investigation of the reactivity of TiO nanoparticles in removal of mala- 2 chite green as model pollutant dye. The crystalline feature of the synthesized sample was performed by XRD techniques. However, the nanostructure was investigated by TEM. The removal of the malachite green dye was studied over wide range of dye concentrations and different dosage of catalyst sample. Adsorption isotherms are studied using Freundlich, Langmuir, Temkin and Dubinin models to indicate the mechanism of adsorption and estimate the maximum adsorption capacity and correlation coefficients. The kinetics of adsorption process is well investigated using different models as pseudo first order, pseudo second order, Elovich, Morris and Weber. 2. Materials and Methods 2.1. Materials All chemicals including chitosan, titanium isopropoxide and malachite green dye were purchased from Sigma- Aldrich without further purification. Properties of investigated dye are listed in Table 1. Stock solution was prepared by dissolving accurately weighed sample of dye in distilled water to give a concentration of mmol∙L−1. Desirable concentrations of dye were obtained by serial dilution. 2.2. Instrumentation X-ray diffraction patterns were carried out by XRD-6100 X-ray diffractometer, with CuKα (λ = 1.5406 Å) radia- tion in the 2θ range from 5˚ to 90˚. The scanning mode is continuous with scan speed 2 deg /min, the sampling pitch 0.02 deg and the preset time 0.6 s. The nanostructure of TiO rods before and after adsorption was investigated by Transmission electron micro- 2 scope (JEOL, JEM-1200X II). The reaction was followed spectrophotometrically at λ = 617 nm for malachite green dye using thermos- max tated Evolution 300 UV-VIS spectrophotometer. 374 Z. M. Abou-Gamra, M. A. Ahmed Table 1. The molecular structure of malachite green dye. Characteristic Malachite green dye Molecular structure IUPAC name N, N, N′, N′-Tetramethyl-4,4′-diamino-triphenylcarbenium chloride Chemical formula C H NCl 23 25 2 Molecular weight (MW = 364.91 g∙mol−1) λ = 617 nm max 2.3. Methods 2.3.1. Preparation of Porous TiO Nano-Powder. 2 TiO was prepared by controlled sol-gel method as follows: About 10 ml of chitosan as bio-template solution 2 was added with constant stirring for one hour to 30 ml of titanium isopropoxide dissolved in isopropanol solu- tion. After while, few drops of water was added drop by drop until the white sol of Ti(OH) starts to be detected. 4 The sol was stirred for two hours and left for two days for condensation of sol particles into gel. The gel par- ticles were collected by filtration and washed with distilled water several times. After washing, the final product was dried at 100˚C for 24 hours. The dried powder was calcined in high temperature furnace at 500˚C for three hours to transform Ti(OH) into TiO nanoparticles. 4 2 2.3.2. Adsorption Studies The equilibrium isotherm of a specific adsorbent represents its adsorptive characteristics and is very important to the design of adsorption processes. Experiments for the estimation of the adsorption isotherms of (MG) dye onto TiO nanoparticles were performed by adding fixed amounts of TiO powder to a series of Erlenmeyer flasks, 2 2 each containing dye solutions of concentrations (3.6 - 22 mg/L). The vessels were then agitated using shaker for 30 min. at room temperature (30˚C) to attain equilibrium. Then, 5 ml of suspension was withdrawn and adsor- bent was removed by centrifugation for 5 minutes at 1800 rpm. Supernant concentration was determined spec- trophotometrically. The amount of dye adsorbed onto TiO nanoparticles was calculated based on the following 2 mass balance equation: V(C −C ) q = o e e m where q is the adsorption capacity (mg dye adsorbed onto the mass unit of TiO , mg/g), V is the volume of the e 2 dye solution (L), C and C (mg/L) are initial and equilibrium dye concentrations, and m (g) is the mass of dry o e TiO added. The equilibrium relationship between the quantity of adsorbate per unit of adsorbent (q) and its 2 e equilibrium solution concentration (C) at a constant-temperature is known as the adsorption isotherm. Several e isotherm models have been developed for evaluating the equilibrium adsorption of compounds from solutions such as Langmuir, Freundlich, Helsey, Dubinin-Radushkevich, Temkin, etc. Moreover, the kinetics of dye ad- sorption is investigated using pseudo-first order, pseudo-second order, Elovich and Weber-Morris models. 3. Results and Discussion 3.1. XRD XRD analysis was performed to assess the nature and size of titania crystalline phases. Figure 1 illustrate the diffraction pattern of titania prepared in presence and absence of chitosan to explore the influence of the bio-template on titania crystalline features. On examining Figure 1, one can observe several titania crystalline 375 Z. M. Abou-Gamra, M. A. Ahmed 10 20 30 40 50 60 A TiO with chitosan 2 A A A A A TiO without chitosan y 2 nsitu.) R ea. nt( R I R A A A 10 20 30 40 50 60 2θ Figure 1. XDR pattern of TiO in presence and absent chitosan. 2 376 Z. M. Abou-Gamra, M. A. Ahmed peaks for sample prepared in absence of chitosan at 2θ = 25.3, 36.9, 37.7, 38.5, 48, 51.9, 53.8, 55.1, 62.6, 68.7 and 75 [JCPDS No. 71-1167 were a = 3.786Å and c = 9.507Å] revealing the existence of anatase phase and oth- er peaks at 2θ = 27.4˚, 39˚, 41˚, and 44˚ [JCPDS No. 88-1175, a = 0.4.5 Å and c = 2.940 Å] referred to rutile phase. It is obvious to notice several diffraction peaks referred only to anatase phase in the sample prepared in presence of chitosan revealing the successful role of chitosan in preventing the particles agglomeration and inhi- biting the rutile phase transformation. Also Figure 1 shows a change in sample crystallography from anatase to completely amorphous nanoparticles upon adsorption of malachite dye indicates a strong adsorption of this dye that destroys the crystalline feature of titania sample. 3.2. TEM TEM is considered a powerful tool in determining the nanostructure of the prepared sample. It is clear that tita- nia nanoparticles exist in spherical structure with size about 25 nm, Figure 2(a). It is interesting to notice from Figure 2(b) that the malachite dye is strongly attached to titania nanoparticles as various rods linked to titania nanoparticles. 3.3. Dye Adsorption Analysis Malachite green dye was taken as pollutants models to investigate the adsorption capacity of TiO nanoparticles. 2 3.3.1. Effect Contact Time Equilibrium time is one of the important parameters to design a low cost adsorbent for removal of organic wastes. The adsorption of malachite green dye onto TiO was studied as a function of contact time to determine 2 the necessary adsorption equilibrium time. The results reveal that about 85% of malachite green dye was ad- sorbed in 30 minutes, Figure 3. 3.3.2. Effect of TiO Dose 2 The amount of solid catalyst is important parameter for detection of adsorption capacity of solid in removing organic wastes. A rapid uptake of pollutants and establishment of equilibrium in a short period signify the effi- ciency of the solid in removal of various organic pollutants. The effect of TiO dose on adsorption of malachite 2 green dye was investigated in range of 0.1 - 0.4 g/100 ml at fixed amount of dye and contact time = 30 min. The removal of dye increases with increase catalyst amount, Figure 4. (a) (b) Figure 2. TEM micrographs of TiO nanoparticles. 2 377 Z. M. Abou-Gamra, M. A. Ahmed 90 80 70 % 60 al 50 v o m 40 e r 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 time / min Figure 3. Effect of contact time on adsorption of malachite green onto TiO surface. [MG] = 2.5 × 10−5 mol∙dm−3, 2 0.1 g of TiO/100 ml, Temp. = 30˚C. 2 Figure 4. Effect of TiO dose on adsorption of malachite green onto TiO surface. [MG] = 5 × 10−5 mol∙dm−3, 2 2 Temp. = 30˚C. 3.3.3. Effect of Dye Concentration The initial concentration of adsorbate also plays an important role as a given mass of the adsorbent can adsorb only a fixed amount of the solute. Batch adsorption experiments were carried out by shaking the adsorbent with an aqueous solution of the dye of desired concentration in corning glass bottles at 30˚C. The adsorption capacity of malachite green dye increased from 1.01 to 17.5 mg∙g−1 upon increase in dye concentration as indicated in Figure 5. Several authors reported that dye removal by TiO increased with increases dye concentration up to 2 optimum value. 378 Z. M. Abou-Gamra, M. A. Ahmed 20 18 16 14 12 1 − g g∙ 10 m / 8 e q 6 4 2 0 0 1 2 3 4 5 6 7 [M.G]×105/mol∙dm−3 Figure 5. Effect of initial dye concentrations on adsorption of malachite green onto TiO surface. TiO 2 2 dose = 0.1 g/100 ml, Temp. = 30˚C. 3.4. Adsorption Isotherm Models The equilibrium data of malachite analyzed by fitting them into Langmuir, Freundlich and Temkin equation to find out the suitable model that may be used for design consideration. Table 2 summarizes the constants and coefficients of different models. 3.4.1. Langmuir Isotherm The Langmuir isotherm assumes the absence of any interactions between adsorbate molecules and the adsorp- tion process is account for monolayer formation. The linear form of the Langmuir isotherm, assuming monolay- er adsorption on a homogeneous adsorbent surface, is expressed as follows [20]: C 1 C e = + e q bq q e max max where the q (mg∙g−1) is the maximum adsorption capacity of the adsorbent corresponding to monolayer for- max mation and illustrates the maximum value of q that can be attained as C is increased. The b parameter is a e e coefficient related to the energy of adsorption and increases with increasing strength of the adsorption bond. Values of q and b are determined from the linear regression plot of (C/q) versus C, Figure 6(a). Linear plot max e e e in negative direction indicates that Langmuir model fails to explain the process of adsorption and absence of formation of monolayer. 3.4.2. Freundlich Isotherm It is well established that the Freundlich isotherm is often applied to heterogeneous solid catalyst. The Freun- dlich equilibrium isotherm equation is an empirical relation involved for the description of multilayer adsorption with interaction between adsorbed molecules. The Freundlich equation [21] is expressed as follows in its linear form: 1 logq =logK + logC e f n e where, K represents the capacity of the adsorbent for the adsorbate, and 1/n shows adsorption intensity of dye f on solid which is a function of the strength of adsorption. A linear regression plot of log q versus log C, Figure e e 6(b) gives the K and n values. The model is applicable to the adsorption on heterogeneous surfaces by a uniform f energy distribution and reversible adsorption. Linear plot with high regression factor indicating the suc- 379 Z. M. Abou-Gamra, M. A. Ahmed (a) (b) (c) (d) Figure 6. Isotherm plots for adsorption of malachite green on TiO surface. (a) Langmuir isotherm, (b) Freundlich isotherm, 2 (c) Temkin isotherm, (d) Dubinin isotherm. Table 2. Adsorption isotherms parameters of malachite green on TiO . 2 Frendlich model K 1.4 f n 0.67 R2 0.971 Temkin model α (L∙g−1) 1.45 β 4.99 R2 0.988 Dubinin model B (×107 mol2∙J−2) 6 Q (mol∙g−1) 11.72 m E (J∙mol−1) 912.87 R2 0.9581 cessful model in explaining the adsorption model. Table 2 summarizes Freundlich constants. The results of Freundlich and Langmuir models suggest that adsorption of malachite green dye is accompanied by multilayer formation. 3.4.3. Temkin Isotherm The Temkin model takes into the account adsorbing species–adsorbent interactions. This isotherm proposed that 380 Z. M. Abou-Gamra, M. A. Ahmed the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent–ad- sorbate interactions and the adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The linear Temkin [22] equation is q =βln +βlnC e α e α is the equilibrium constant corresponding to the maximum binding energy/L⋅g−1. β=RT b T is the absolute temperature in Kelvin. R is the universal gas constant 8.314 J∙mol−1∙K−1. b is the Temkin constant related to heat sorption/J∙mg−1. α and β are calculated from the slope and intercept of q versus lnC, Figure 6(c). The Temkin equation better e e holds for the prediction of gas phase equilibria rather than liquid phase. The liquid phase is a more complex phenomenon since the adsorbed molecules do not necessarily organized in a tightly packed structure with iden- tical orientation. Linear plot and high regression value suggest the successful model in explaining the adsorption mechanism. Table 2 summarizes Temkin constants. The adsorption energy obtained from Temkin plot 503.988 J∙mg−1 which indicates that the adsorption process is endothermic and a strong interaction between TiO and 2 malachite green dye. 3.4.4. The Dubinin Radushkevich Isotherm This model is involved to estimate the porosity, free energy and the characteristics of adsorbents [23]. The iso- therm assumes the surface heterogeneity and the variation of adsorption potential during sorption process. The model has commonly been applied in the following linear Equation: lnq =lnQ −B€2 e m Polanyi potential, €, can be calculated according the following equation €=RTln(1+1C ) e where B is a constant related to the adsorption energy, Q the theoretical saturation capacity. The slope of the plot m of lnq versus €2 gives B (mol2∙J−2) and the intercept yields the adsorption capacity, Q (mg∙g−1) as shown in e m Figure 6(d). Table 2 summarizes Dubinin constants. The mean free energy of adsorption (E) which is energy require to transfer one mole of the dye from infinity in solution to the surface of the solid can be calculated from the B value using the following relation E =1 2B The value of energy is about 912.87 J/mole revealing physisorption of malachite on titania nanoparticles. 3.5. Adsorption Kinetics Adsorption of organic dyes on metal oxide surface are influenced by three mass transfer process which are ex- ternal diffusion of dye molecules from liquid phase to the solid surface, actual adsorption and intraparticle diffu- sion of dye molecules in the interior of pores. The adsorption process is usually very fast rather than internal and external diffusion. It is well known that the adsorption equilibrium is reached within several minutes. However, the long adsorption equilibrium time suggests that the internal diffusion controls the reaction rate. Kinetics is key factor for adsorption investigation because it can predict the rate at which a pollutant is re- moved from aqueous solution and provides valuable data for understanding the mechanism of adsorption process. Several models are available to investigate the adsorption mechanism and description based on experi- mental data such as pseudo-first order, pseudo-second order, intramolecular diffusion and Elovich models. The pseudo-first order adsorption rate [24] and pseudo-second order adsorption rate [25] have the following linear forms ln(q −q )=lnq −kt e t e 1 where k (min−1) is pseudo first order rate constant, q (mg∙g−1) is the amount of dye adsorbed on surface at equi- 1 e librium, q (mg∙g−1) is the amount of dye adsorbed on surface at time t (min). The adsorption rate constant, k t 1 and q were calculated from the plot of log(q − q) vs t, Figure 7(a), and are listed in Table 3. e e t 381 Z. M. Abou-Gamra, M. A. Ahmed (a) (b) (c) 382