Available online at www.sciencedirect.com Colloids and Surfaces A: Physicochem Eng Aspects 320 (2008) 161–168 Chitosan–magnetite nanocomposites (CMNs) as magnetic carrier particles for removal of Fe(III) from aqueous solutions Mini Namdeo ∗ , S.K Bajpai ∗ Polymer Research Laboratory, Department of Chemistry, Govt Model Science College (Autonomous), Jabalpur 482001, MP, India Received 22 September 2007; received in revised form 22 January 2008; accepted 31 January 2008 Available online 21 February 2008 Abstract This research focuses on removal of Fe(III) from aqueous solution using chitosan–magnetite nanocomposites as potential sorbent The presence of nanosized magnetic particles within the nanocomposites was confirmed by TEM and SAED analysis The particles with diameter 508 m and 84 m, follow Frendlich sorption isotherm at 30 ◦ C, and the Frendlich constants (KF , 1/n) have been found to be 5.974 mg g−1 , 2.66 and 35.98 mg g−1 , 1.385, respectively Out of various kinetic models, the experimental data for dynamic uptake of Fe(III) is best fitted on ‘pseudosecond order’ kinetic model The linear nature of plots between log (% sorption) and log (time) is indicative of intra-particle diffusion For the particles with diameters 508 m and 84 m, the value of kid was found to be 1.78 mg l−1 min−0.5 and 2.13 mg l−1 min−0.5 The sorption mean free energy from the Dubinin–Radushkevic isotherm was found to be 7.04 kJ mol−1 indicating chemical nature of sorption The increase in chitosan content in sorbent particles is found to enhance the Fe(III) uptake The various thermodynamic parameters have also been evaluated Finally, the presence of Cu2+ ions in the sorbate is found to decrease the uptake of Fe(III) © 2008 Elsevier B.V All rights reserved Keywords: Adsorption; Diffusion; TEM; Nanocomposites; FTIR Introduction Heavy metals have become an ecotoxological hazard of prime interest owing to their tendency to accumulate in living organisms At least 20 metals are classified as toxic and half of these are emitted into the environment in quantities that pose risks to human health Iron is one of the important metals that is necessary for growth and development of human being However, its overdose can lead to severe health problems such as anorexia, diarrhea, diphasic shock, metabolic acidosis and death [1] In addition, the patient may experience vascular congestion of the gastrointestinal tract, brain, spleen and thymus [2] With acute iron poisoning, much of the damage to the gastrointestinal tract and liver may result from the high-localized iron concentration and free radical production leading to heptatoxicity via lipid peroxidation and destruction of the hepatic mitochondria As a result of iron storage disease, the liver ∗ Corresponding author Tel.: +91 7612410541 E-mail addresses: mini.namdeo@gmail.com (M Namdeo), sunil.mnlbpi@gmail.com (S.K Bajpai) 0927-7757/$ – see front matter © 2008 Elsevier B.V All rights reserved doi:10.1016/j.colsurfa.2008.01.053 becomes cirrhotic Hepatoma, a primary cancer of the liver, has become the most common cause of death among patients with hemochromatosis [3] Recent past has witnessed tremendous exploitation of a variety of sorbents for the effective removal of Fe(III) from industrial effluents and ground water [4–10] Recently environmental chemists have focused their research on exploiting biopolymers as efficient tool for separation of Fe(III) like metal ions because of their easy availability, low cost, presence of a variety of functionalities, their non-toxic nature, etc [11] Among various biopolymer available, chitosan has the highest sorption capacity for several metal ions, possibly due to presence of primary amine at C-2 position of the glucosamine residues [12] This biosorbent has some drawbacks like softness, tendency to agglomerate and to form gels in aqueous solution, and finally not so readily availability of reactive binding sites [13] That is the reason that such a potential sorbent has been confined only to laboratory experiments In order to overcome these limitations and other operational difficulties, we hereby propose chitosan–magnetite nanocomposite (CMN) as a novel potential sorbent for effective removal of Fe(III) from aqueous solutions This newly developed CMN has not only strong metal chelating 162 M Namdeo, S.K Bajpai / Colloids and Surfaces A: Physicochem Eng Aspects 320 (2008) 161–168 tendency due to presence of chitosan but it also possesses additional feature of nanomaterials like large surface area, dispersed nature, freedom from convectional columns of fixed packed particles and rigid membranes In addition, due to magnetic nature of CMN, it can easily be separated from the sorption system by using magnetic field of proper strength Recently, Moeser et al [14] have been able to recover 98% water-based magnetite particles coated with a bifunctional polymer using High Gradient Magnetic Separation (HGMS) technique Experimental nanocomposites(CMN) The particles were finally dried in vacuum at 70 ◦ C The jet-black appearance of freshly prepared chitosan–magnetite nanocomposites and their retention by magnet were indications of their magnetic behavior (see Fig 1) 2.3 FTIR spectral analysis The FTIR spectrum of the CMN was recorded on a FTIR spectrophotometer (Shimadzu; Model No 8400S) Samples were prepared in KBr disks (2 mg sample in 200 mg KBr) The scanning range was 400–4000 cm−1 and the resolution was cm−1 2.1 Materials Chitin was de-acetylated in 50 wt% NaOH at 90 ◦ C in nitrogen atmosphere for h [15] The final chitosan flakes were washed three times with de-ionized water (Millipore Milli-Q) and dried at 50 ◦ C in a vacuum Physical properties of chitosan flakes such as degree of de-acetylation and molar mass were determined The former was obtained to be 94 mol% following the method of Guibal et al [16] The molar mass was found to be 1.42 × 106 using the well-known Mark–Houwink equation and viscosity data of solutions containing different amounts of chitosan in 0.1 mol/l acetic acid and 0.2 mol/l NaCl [17] Other reagents such as ferrous chloride, ferric chloride and sodium hydroxide were also purchased from HiMedia and used as received The double distilled water was used throughout the investigations 2.2 Synthesis of chitosan–magnetite nanocomposites (CMNs) Chitosan–magnetite nanocomposites (CMNs) were prepared by chemical co-precipitation of Fe2+ and Fe3+ ions by NaOH in the presence of chitosan followed by treatment under hydrothermal conditions [18] To a 2% acidic solution of chitosan, iron(II) chloride and iron(III) chloride were dissolved in 1:2 molar ratio and the resulting solution was chemically precipitated at 40 ◦ C by adding 30% NaOH drop wise with constant stirring, at a controlled pH (10–10.4) The suspension was heated at 90 ◦ C for h under continuous stirring and separated by centrifuging several times in water and then in ethanol at 200 rpm The purification step was used to remove impurities from chitosan–magnetite 2.4 Transmission electron microscopy analysis (TEM) TEM and selected area electron diffraction (SAED) were obtained by employing JEM-2010 microscopes under 200 kV The sample for observation of TEM was prepared by placing three drops of nanocomposite suspension, prepared in acetone, onto a carbon-coated copper grid 2.5 Fe(III) uptake studies The sorbate stock solution was prepared by dissolving a precalculated quantity of FeCl3 in double-distilled water to give final concentration 100 mg l−1 The stock solution was diluted to obtain standard solutions with concentration in the range of 5–30 mg l−1 and their final pH was adjusted to 4.5 Fifty milliliters of FeCl3 solution of desired concentration was placed in a 125-ml Erlenmeyer flask containing 0.02 g of CMN sorbent and was agitated in thermostatic water bath at 50 rpm for h At the end of experiment the sorbent was separated by filtration and supernatant was analyzed spectrophotometrically [19] Results and discussion 3.1 Formation of chitosan–magnetite nanocomposite (CMN) Out of various methods used to synthesize magnetite nanoparticles, the method consisting of co-precipitation of Fe(II) and Fe(III) ions, present in 1:2 molar ratio, by addition of NaOH Fig Photograph of (A) chitosan–magnetite nanocomposites and (B) their retention on the magnet surface M Namdeo, S.K Bajpai / Colloids and Surfaces A: Physicochem Eng Aspects 320 (2008) 161–168 163 Scheme has been used most frequently [20] We modified this method to prepare CMN by taking advantage of the fact that chitosan is also precipitated under alkaline conditions The preparation of CMN particles has been given in Scheme It is clear from Scheme that when NaOH is added to the acidic solution containing chitosan, Fe(II) and Fe(III) ions, magnetite nanoparticles are formed along with precipitation of chitosan Thus magnetite nanoparticles are almost entrapped uniformly within the macromolecular chitosan chains, thus resulting in the formation of chitosan–magnetite nanocomposite (CMN) particles 3.3 TEM and SAED analysis Fig depicts the TEM image of chitosan–magnetite nanocomposite particles along with SAED particles shown in inset The average diameter of particles, as determined from measurement of diameter of 30 particles, selected arbitrarily from TEM image, was found to be nearly 27 nm It is also clear that magnetite particles are completely embedded within chitosan particles due to occurring of two simultaneous processes, one involving formation of magnetite particles and the other involving precipitation of chitosan chains in alkaline medium 3.2 FTIR spectral analysis Fig 2(A–C) describes FTIR spectra of magnetite, chitosan and chitosan–magnetite nanocomposite (CMN), respectively In the spectrum (A) the presence of bands in the region 3250–3600 cm−1 and 1550–1700 cm−1 is associated with the lattice of water molecules, thus indicating the presence of water crystallization in the magnetite sample or in KBr Moreover, the characteristic peak, present at 610 cm−1 confirms the metal–oxygen stretching The presence of characteristic peak at 3594 cm−1 in the spectrum of chitosan (see Fig 2(B)) is indicative of hydroxyl groups while peaks appearing at 1650 cm−1 and 1449 cm−1 correspond to stretching vibrational frequency of amide I and amide II in chitosan molecules Finally, it can be seen in Fig 2(C) that all these characteristic peaks are also present in spectrum of chitosan–magnetite nanocomposite Fig FTIR spectrum of (A) magnetite, (B) chitosan plain and (C) chitosan magnetite 164 M Namdeo, S.K Bajpai / Colloids and Surfaces A: Physicochem Eng Aspects 320 (2008) 161–168 Fig TEM and SAED of CMN particles The headed arrow also indicates nanocomposite particles in which both magnetite and chitosan phase are clearly visible In addition, the observed agglomerations of nanocomposite particles may probably be due to the absence of stabilization agent in the reaction system as well as due to agglomerating tendency of chitosan Finally, the SAED pattern (see inset) also confirms the formation of magnetite nanoparticles 3.4 Dynamic uptake of Fe(III) The kinetics of sorption process describes the solute uptake, which, in turn governs the residence time of the sorption reaction In order to understand the kinetic aspects for Fe(III) uptake on CMN, the sorption kinetics was carried out with sorbent particles of two different sizes namely 508 m and 84 m, at room temperature for the initial Fe(III) concentration 20 mg l−1 (see Fig 4) It is clear that amount of Fe(III) sorbed increases with time, and smaller size particles exhibit higher Fe(III) uptake This may simply be attributed to the fact that particles with smaller size have larger surface area, thus providing more binding sites for iron uptake A close look at Fig reveals one more interesting fact In the first 20 (i.e phase I in the curve), the sorption process is very rapid which may be due to the simultaneous uptake of Fe(III) ions on the surface of sorbent particles However, later on, (see phase II in the curve) the uptake of iron becomes very slow thus indicating intra-particle diffusion of sorbate Similar results have also been reported elsewhere [20] Fig The kinetic data for Fe(III) uptake by CMN of different particle sizes Fig Pseudo-second order kinetic model for Fe(III) uptake by CMN of different sizes Using the Fe(III) uptake kinetic data at room temperature (from Fig 4) we fitted following kinetic models [21–23]: • • • • pseudo-first order Lagergren model, pseudo-second order model, simple Elovich model and power function model The estimated models and related kinetic parameters are listed in Table Based on linear regression values, it can be very well seen from Table that kinetics of Fe(III) uptake onto CMN can be best described by the ‘pseudo-second order kinetic model’ as shown in Fig 3.5 Intra-particle diffusion The bi-phasic nature of iron uptake profiles, as observed in Fig 4, has already indicated towards intra-particle diffusion of Fe(III) into sorbent particles Moreover, diffusion is an endothermic process, the rate of sorption will increase with an increased solution temperature when pore diffusion is the rate-limiting step [23] So, in the present study also we may expect the Fe(III) uptake process to be pore-diffusion controlled To further confirm this, graphs were plotted between log (% sorption) and log (time) which yielded straight lines (see Fig 6) thus confirming the occurrence of intra-particle diffusion [24] The rate constants of intra-particle (kid ) were calculated from the slopes of linear Fig Plot between log (% sorption) and log (time) showing intra-particle diffusion M Namdeo, S.K Bajpai / Colloids and Surfaces A: Physicochem Eng Aspects 320 (2008) 161–168 165 Table Parameters of various kinetics models fitted to experimental data S no Kinetic model Equationa Parameters with particle size q Pseudo-first order Lagergren ln − Pseudo-second order t q Simple Elovich q = a + 2.303 b log t Power function log q = log a + b log t a = qe k2 qe2 + = −K1 t qe t 508 m 84 m R2 R2 = 0.9463 K1 = 0.033 R2 = 0.9943 k = 0.0031 R2 = 0.9595 a = 24.006 b = 7.685 R2 = 0.9606 a = 31.024 b = 0.1381 = 0.8513 K1 = 0.0259 R2 = 0.9925 k = 0.0035 R2 = 0.915 a = 25.68 b = 6.371 R2 = 0.9351 a = 29.95 b = 0.1307 All notations carry their usual meaning portion of the plots of amount sorbed q (mg g−1 ) versus square root of time at room temperature for two particle sizes (see Fig 7) using the Weber and Morris equation: q = kid t 0.5 According to Weber and Morris, initial curve portion represents boundary layer adsorption while slope of final linear portion is measure of intra-particle pore diffusion coefficients The values of kid at room temperature for 508 m and 84 m sized particles were found to be 1.78 mg l−1 min−0.5 and 2.13 mg l−1 min−0.5 , respectively 3.6 Sorption isotherms In a two-component system (sorbent and solution), at equilibrium there is always a defined distribution of the solute between the liquid and the solid phase, which can generally be expressed by one or more isotherms [25] In the present study, the equilibrium sorption of Fe(III) onto CMN was studied in concentration range 5–30 mg−1 l at room temperature and the experimental data was fitted in the re-arranged Frendlich isotherm: respectively The linear nature of plots between log qe and log Ce at room temperature (Fig 8) indicates the applicability of Frendlich sorption isotherm The values of KF and 1/n for room temperature were also calculated for both the particle sizes using the least square method through a regression analysis and are presented in Table along with the experimental values It can be seen that the two values are in quite close agreement with each other To classify the physical or chemical sorption, the data was applied to the Dubinin–Radushkevich (D–R) isotherm model The D–R equation is Cad = Cm exp(−Bε2 ) where Cad is the amount of Fe(III) sorbed on CMN, Cm is the maximum Fe(III) uptake, B is a constant with a dimension of energy, and Polyanyi potential ε is given as ε = RT ln + Ce where R is the gas constant in kJ mol−1 K−1 , T is the absolute temperature and Ce is the equilibrium concentration of Fe(III) in solution The obvious linearized form of D–R isotherm is log Ce n where Ce and qe are the equilibrium Fe(III) concentration in solution (mg l−1 ) and in the solid phase (mg g−1 ), respectively, and KF and 1/n are Frendlich constants related to the maximum sorption capacity (mg g−1 ) and energy of sorption (l mg−1 ), when ln Cad was plotted against ε2 , a straight line was observed (see Fig 9) The computed values of B, as determined from the slope of linear plot, was 10.1 × 10−3 kJ mol−1 From the Fig q (mg g−1 ) vs t0.5 plot for the evaluation of kid Fig Frendlich isotherms for Fe(III) uptake by CMN of different particle sizes log qe = log KF + ln Cad = ln Cm − Bε2 166 M Namdeo, S.K Bajpai / Colloids and Surfaces A: Physicochem Eng Aspects 320 (2008) 161–168 Table Values of Frendlich constants KF and 1/n for the iron(III) uptake by CMN Particle diameter (m) Graphical values KF 508 84 Regression values (mg g−1 ) 1/n 5.974 35.98 (l mg−1 ) 2.663 1.385 KF (mg g−1 ) 1/n (l mg−1 ) 5.546 33.57 2.734 1.470 Fig 10 Plot between ln kc and 1/T for the evaluation of H◦ and S◦ Fig Dubinin–Radushkevich isotherm for evaluation of B which is the free energy transfer of one mole of solute from infinity to the surface of chitosan–magnetite nanocomposites The numerical value of E was found to be 7.04 kJ mol−1 which lies approximately in the expected range of 8–16 kJ mol−1 thus indicating chemisorption or ion exchange [26] and 69.08 kJ mol−1 , respectively The values of kc , G◦ , H◦ and S◦ have been tabulated in Table It is clear that G◦ is negative and decreases with temperature, thus indicating that better separation is obtained at higher temperature It also suggests that the sorption process is spontaneous in nature This simply attributes to greater chances of co-ordination which results in the formation of higher linkages The positive value of H◦ confirms the endothermic nature of the sorption process The positive value of S◦ also indicates the increased randomness during sorption process 3.7 Evaluation of thermodynamic parameters 3.8 Effect of adsorbent dose Thermodynamic parameters were calculated from the variation of the thermodynamic distribution coefficient, kc with change in temperature The standard free energy change, G◦ , was calculated using the expression: The sorbate solution of definite concentration 50 mg l−1 was agitated with different quantities of sorbent, in the range 10–60 mg per 50 ml and the solutions were analyzed for percent Fe(III) uptake and x/m (i.e iron sorbed in mg for g of sorbent) The results, as shown in Fig 11, reveal that % sorption and x/m values show opposite trends The observed increase in % sorption with sorbent dose can be simply attributed to the fact that increase in amount of sorbent result in increase in surface-area thus providing more and more binding sites for Fe(III) uptake This finally results in enhancement in percent uptake However, the decrease in the value of x/m with sorbent dose may be explained as follows: it is clear from Fig 11 that with increase in sorbent dose from 10 mg to 60 mg, the % uptake increases from 83 to 94 It means that for sixfold increase in the calculated value of B, the mean sorption energy E was computed as E= √ −2B G◦ = −RT ln kc The value of H◦ and S◦ were calculated from the Van’t Hoff equation: ln kc = H ◦ S ◦ − R R T The plot of ln kc versus 1/T was found to be linear (see Fig 10) The values of S◦ and H◦ were calculated from the intercept and slope of linear plot and were found to be 243.35 J K−1 mol−1 Table The values of kc , G◦ , H◦ and S◦ Temperature (◦ K) Kc × 102 G◦ kJ mol−1 H◦ kJ mol−1 S◦ J K−1 mol−1 283 293 303 78.10 244.5 675.0 −0.581 −2.177 −4.809 69.08 243.35 M Namdeo, S.K Bajpai / Colloids and Surfaces A: Physicochem Eng Aspects 320 (2008) 161–168 167 Fig 13 Effect of chitosan content in feed mixture on Fe(III) uptake by resulting CMN Fig 11 Effect of sorbent dose on percent Fe(III) uptake 3.10 Effect of chitosan content in CMN sorbent dose, the percent sorption increases marginally by 1.13fold only This suggests that value of ‘x’ does not increase in the same proportion as that of adsorbent dose ‘m’ increases The overall effect of this marginal increase in ‘x’ value is that x/m decreased gradually 3.9 Effect of sorbate concentration on Fe(III) uptake The amount of Fe(III) sorbed per gram of sorbent (i.e x/m) was found to vary in interesting way with initial concentrations of sorbate solution in the range 20–360 mg l−1 The results as depicted in Fig 12, clearly indicate that x/m increases with initial Fe(III) concentrations up to 160 mg l−1 and then it shows decreasing trend with further increase in sorbate concentrations The initial increase in x/m may be attributed to the fact that with increase in Fe(III) concentration, more and more metal ion sorbed into the CMN particles However, beyond 160 mg l−1 concentration of Fe(III) the decrease in x/m may be due to dissolution of chitosan from CMN particles as the pH of the sorbate solution was found to be in the range 2–3 Since chitosan is soluble in acidic pH, its dissolution from the CMN particles resulted in decrease in Fe(III) uptake The loss in weight of CMN particles due to chitosan dissolution was also confirm in a separate experiment by adding a known quantity of CMN in aqueous solution of pH 3.0 and determining its dry weight after h There was observed an appreciable loss in the weight of CMN particles, thus supporting our arguments Fig 12 Effect of percent removal of Fe(III) by CMN The variation in concentration of chitosan in nanocomposites is expected to cause a change in sorption behavior of Fe(III) To investigate this, chitosan–magnetite nanocomposites with different contents of chitosan were synthesized by varying the amount of chitosan in the feed mixture in the range 0.5–3.0% The results, as shown in Fig 13 indicate that as the amount of chitosan is increased in nanocomposite, Fe(III) also increases This may be explained on the basis of the fact that the higher concentration of chitosan provides more binding sites for sorption and therefore the uptake of Fe(III) increases It is also worth mentioning that as the chitosan concentration is increased beyond 2.5%, nearly all active sites are saturated and so Fe(III)-uptake attains optimum value 3.11 Effect of presence of co-ions It is almost unlikely that only a single type of metal ion (i.e Fe(III)) is present in the industrial effluents or domestic water For example, there are some regions in India where the ground water contains other metal ion along with iron So it may be interesting to see the effect of presence of co-ions on the Fe(III) uptake by chitosan–magnetite nanocomposites (CMNs) To investigate this, to Fe(III) solution of initial concentration 40 mg l−1 , varying amounts of copper sulphate were added to yield copper concentrations in the range 2–10 mg l−1 and sorption studies were carried out with same amount of 508 m sized sorbent particles The results, as depicted in Fig 14 clearly indicate that there is drastic decrease in uptake of Fe(III) due to presence of Cu2+ ions in the sorption system This may sim- Fig 14 Effect of presence of Cu2+ ions on Fe(III) uptake 168 M Namdeo, S.K Bajpai / Colloids and Surfaces A: Physicochem Eng Aspects 320 (2008) 161–168 ply be explained on the basis of the fact that Cu(II) ions also compete with Fe(III) during the sorption process Being transition metal, Cu(II) has strong tendency to bind with N atoms, present in chitosan chains of CMN As a result the uptake of Fe(III) decreases with increasing concentrations of Cu2+ ions in the sorbate solution 3.12 Desorption studies Known quantities of chitosan–magnetite nanocomposites (CMNs), loaded with Fe(III), were put in distilled water as well as in KCl solutions of different concentrations, in the range 0.01–0.06 M, for a period of 24 h However, the percent desorption was found to be almost negligible (