Ecotoxicology and Environmental Safety 122 (2015) 17–23
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Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Nanoalginate based biosorbent for the removal of lead ions from aqueous solutions: Equilibrium and kinetic studies P. Geetha a,d, M.S. Latha b,n, Saumya S Pillai c,d, Mathew Koshy d a
Department of Chemistry, D.B Pampa College, Parumala, Mannar, Kerala, India Department of Chemistry, S.N. College, Chengannur, Kerala, India c Department of Chemistry, N. S. S. Hindu College, Changanacherry, Kerala, India d Department of Chemistry, Bishop Moore College, Mavelikara, Kerala, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 8 April 2015 Received in revised form 17 June 2015 Accepted 22 June 2015
Population explosion, depletion of water resources and prolonged droughts and floods due to climatic change lead to scarcity of pure and hygienic drinking water in most of the developing countries. Recently nanomaterials attained considerable attention as biosorbent for water purification purpose. However difficulties in removing polymeric surfactants and organic solvents used for nanoproduction and instability of the generated nanoparticles limit the scope of this approach in water cleanup. Here, we describe a novel green method for synthesizing polysaccharide nanoparticles in aqueous medium using honey as the capping agent. The highly stable alginate nanoparticles, characterized by various microscopic and spectroscopic techniques, exhibited a maximum uptake capacity of 333 mg g 1of Pb(II) ions from aqueous solution. The effect of various parameters such as initial metal concentration, pH, contact time, temperature and adsorbent dose on sorption process was investigated in batch mode technique. The maximum removal percentage was 94.81 at 45 °C and at pH 4.5 in 60 min contact time. The biosorption followed Freundlich model indicating multilayer adsorption and pseudo second order kinetics. The mechanism involves both surface adsorption and pore diffusion. The positive values of ΔH°, ΔS° and the negative value of ΔG°, confirmed the endothermic nature, randomness and spontaneity of biosorption process. & 2015 Elsevier Inc. All rights reserved.
Keywords: Polysaccharide Honey Cross-linking agent Calcium alginate nanoparticles Lead removal Water purification
1. Introduction Contamination of the environment by toxic heavy metals is a worldwide problem. Lead is one of the prime toxins of the heavy metals, discharged into the environment by metal, electroplating, battery manufacturing, pigment and dye industries (Majumdar et al., 2010). Consumption of these contaminated water adversely affect liver, kidney, brain and central nervous system causing irreversible brain damage, nervous disorders and weakness of muscles (Gupta and Rastogi, 2008). Conventional methods for the removal of toxic heavy metals are economically expensive (Gupta et al., 2012a). Several techniques based on cheap waste materials have been reported recently for the removal of pollutants from wastewater (Gupta et al., 2011a, 2012b; Karthikeyan et al., 2012). Among these, biosorption is a cost effective method based on the metal/dye sequestering properties of certain natural materials such as agricultural wastes (Mittal et al., 2009a, 2009b; Gupta n
Corresponding author. E-mail address: [email protected] (M.S. Latha).
http://dx.doi.org/10.1016/j.ecoenv.2015.06.032 0147-6513/& 2015 Elsevier Inc. All rights reserved.
et al., 2010a, 2010b, 2015), fertilizer wastes (Gupta et al.,1997), industrial wastes (Jain et al., 2003) and plants such as algae (Hlaing et al., 2011). Alginates, the main constituent of the cell wall of the brown algae, are reported to have superior ability for heavy metal uptake from contaminated water (Mohan et al., 2007). Alginate is a linear copolymer composed of (1–4) linked β-D-Mannuronic acid (M) and α-L-Guluronic acid (G) residues. The carboxylate groups of the polymer provide the ability to form biodegradable gels in the presence of multivalent cations forming three dimensional net work structures called egg-box model (Lee and Mooney, 2012).Various forms of alginate such as beads, films, fibers etc are proven to be efficient biosorbents for detoxifying waste water (Torres et al., 2005; Nayak and Lahiri, 2006; Gok et al., 2013; Gomez et al., 2012; Cataldo et al., 2013, 2014). Lezehari et al. (2012) used alginate encapsulated pillared clay microbeads as a fixed bed column for the removal of a broad spectrum biocide pentachlorophenol. Recently nanoalginate is gaining momentum in detoxification of waste water due to its extraordinary tendency for rapid interaction with toxic contaminants present in wastewater (Prachi et al., 2013; Pal and Banat, 2014).
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The advancement in nanoscience and nanotechnology paved the way for the synthesis of both metal as well as polymeric nanoparticles, having myriads of applications in biology, agriculture, pharmacy, chemicals, textile industry, material industry, medicine, drug delivery, gene delivery and electronics (Kumar and Yadav, 2011; Heemasagar et al., 2014). Nanomaterials like carbon nanotubes, multi-walled carbon nanotubes, composites of nanotubes, nano form of magnetically modified orange peel etc were utilized for the removal of heavy metals and organic contaminants from wastewater (Khani et al., 2010; Gupta et al., 2011b;. Saleh and Gupta, 2012; Gupta and Nayak, 2012). Calcium alginate nanoparticles were synthesized by various groups of researchers (Rajaonarivony et al., 1993; Ahmad et al., 2006; Nesamony et al., 2012; Cheng et al., 2012; Singh et al., 2013). Since alginate is a cheap commercially available material extracted from the cell wall of the naturally occurring brown algae, use of this material in nano form for water purification is economically viable. Here we are reporting the green synthesis of calcium alginate nanoparticles (CANPs) with high surface area to volume ratio and multiple adsorption sites and their efficiency in removing lead ions from aqueous solutions.
2. Materials and methods 2.1. Materials Low viscosity Sodium alginate powder (Sigma-Aldrich, London),Calcium chloride dihydrate and Lead nitrate (Merck,Darmstadt, Germany) were of analytical grade and used as such without any further purification. Natural honey used in this study was procured from Kerala Agriculture University. The pH adjustments were carried out using 0.1 M HCl and 0.1 M NaOH. 2.2. Methods 2.2.1. Synthesis of calcium alginate nanoparticles (CANPs) by ionotropic gelation method Calcium alginate nanoparticles were synthesized by the drop wise addition of calcium chloride to sodium alginate in aqueous honey medium. Continuous agitation of this homogenized mixture using a magnetic stirrer for four hours resulted in the formation of nanoparticles, which was collected by centrifugation, washed and dried under vacuum. 2.2.2. Characterization of calcium alginate nanoparticles Fourier transform infrared (FTIR) spectra of the synthesized alginate nanoparticles before and after lead biosorption were recorded between 400 and 4000 cm 1 wavelength range using Schimadzu FTIR model 1801. Surface morphology of the biosorbent and energy dispersive spectrum (EDS) was probed using a JEOL JSM-6390LA Analytical Scanning electron microscope. Transmission electron microscopy (TEM) was performed using JEOL model 1200EX instrument operated at an accelerating voltage at 80 kV. The topography of the sample was studied using Atomic Force Microscope (AFM) Nanoscope V from Veeco, USA, at a resonance frequency of 70 kHz and a spring constant of 1–5 N/m. 2.2.3. Preparation of Pb (II) solution A stock solution of 1000 mg L 1 of Pb(II) was prepared by dissolving1.598 g lead nitrate in 1000 ml deionized water. This solution was diluted as required to obtain the standard solutions containing 10–100 mg L 1 of Pb(II) ions. 2.2.4. Batch adsorption technique The effect of pH, contact time and initial concentration on the
biosorption of Pb(II) ions onto CANPs was investigated by equilibrating a definite amount of biosorbent in 100 ml Pb(II) solution with initial concentrations ranging from 10 mg L 1 to 100 mg L 1. The experiments were carried out in the pH range 3–6, by agitating the mixture for a contact time of 10–240 min. The contents of the flask were agitated well by intermittent manual shaking with the purpose of achieving a more economic process.The solution was then filtered at preset time intervals and the Pb(II) concentration was determined by AAS (Perkin-Elmer Atomic Absorption Spectrophotometer –PinAAcle900H) The percentage removal of lead ion can be calculated using the equation
=
c o −ce × 100→ co The amount of equilibrium adsorption, qe (mg g
qe =
c o −ce × V→ w
(1) 1
)
(2)
where Co and Ce are the initial and equilibrium lead ion concentrations (mg L 1), respectively, V is the volume in liter of the solution and w (g) is the mass of dry sorbent used. All the experiments were carried out in triplicate and the mean values are presented. 2.2.5. Statistical analysis The statistical analysis of the results were done using SPSS/ PC þ, version 11.5 (Bennet and Franklin, 1967).The significant differences between the variables were determined by using a one way ANOVA and Duncans multiple range tests and all the values were expressed as mean value 7SD (n¼ 3) 2.2.6. Desorption studies Regeneration studies were conducted using varying concentrations of hydrochloric acid (HCl) at room temperature. A known weight of lead adsorbed nanoparticle was equilibrated with a definite volume of HCl for 60 min and the desorbed Pb(II) ion concentration was measured using AAS.
3. Results and discussion 3.1. Characterization of the biosorbent 3.1.1. FTIR The FTIR spectrum of CANPs before and after Pb adsorption is given in Fig. 1, which confirmed the presence of both carboxyl and hydroxyl groups as binding sites in the metal adsorption process (EL –Tayieb et al., 2013).The broad peak around 3343 cm 1 is indicative of the existence of bonded hydroxyl group [υ(O–H)]. The bands at 1597 cm 1 and 1426 cm 1 are assigned to asymmetric and symmetric stretching peaks of carboxylate salt groups. The bands around 1078 cm 1 (C—O–C stretching) and 1023 cm 1 (C–O stretching) present in the IR spectrum are attributed to the saccharide structure of the biosorbent (Bansal et al., 2009). The slight shifting of the υ(O–H) band from 3343 cm 1 to 3220 cm–1 and υ (C ¼O) band from 1597 cm 1 to 1575 cm 1 after lead adsorption may be due to the participation of both hydroxyl and carboxyl functional groups on the biosorption of Pb(II) ions (Anayurt et al., 2009). 3.1.2. Characterization by microscopic techniques SEM characterization of CANPs before and after adsorption and EDS spectra of nanoalginate is shown in Fig. 2(a), (b) and (c) respectively. Before adsorption a rough surface morphology was observed, while after adsorption of lead ions on the biosorbent,
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Fig. 1. FTIR Spectrum of CANPs (a) before and (b) after lead ion adsorption.
smoother morphology was observed. The EDS spectrum shows the presence of calcium ions which indicates the formation of crosslinks between polymeric chains by the electrostatic interaction of carboxylate ions in the polysaccharide and divalent calcium ions. The TEM and AFM images are shown in Fig. 2(d) and (e) respectively. The TEM image shows well-defined spherical alginate nanoparticles of 50 nm size. No polymeric chains or
expanded structures were seen showing that low concentration of metal salt solution was sufficient enough to cause effective compaction of polymeric chains to form stable nanoparticles. The nano-scale dimension of the prepared alginate particles was also confirmed by AFM images. Aggregation of nanoparticles was observed in the AFM image due to the non-homogeneity of the prepared solution.
Fig. 2. SEM Images of nanoalginate (a) before (b) after lead ion adsorption (c) EDS (d) TEM and (e) AFM images of nanoalginate.
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Fig. 3. The effect of (a) pH, (b) Concentration, (c) Contact time and (d) Amount of biosorbent on the removal of Pb(II) ions onto CANPs.
3.2. Investigation of adsorption parameters 3.2.1. Effect of pH The initial pH of the solution has been reported as one of the most important parameters in the biosorption of metal ions (Saeed et al., 2002). The pH determines the net charge on the biosorbent which invariably determines whether the metal ions can bind or not. Fig. 3(a) shows the effect of pH on the removal of Pb(II) ions from an initial concentration of 10 mg L 1 in 60 min contact time. The percentage removal increases gradually from pH 3, reaches a maximum at 4.5 and thereafter decreases. Under high acidic conditions the biosorbent surface becomes more positively charged and the attraction between adsorbent and the metal cation reduces (Beuno et al., 2008). As the pH increase, the carboxyl and hydroxyl active sites of the biosorbent get deprotonated which enhance metal binding capacity. After complete deprotonation further increase in pH reduce the metal adsorption due to the formation of Pb(II)hydroxides in solution which compete with Pb (II) ions for the adsorption sites (Amboga et al., 2014; Tsai and Chen, 2010) 3.2.2. Effect of initial metal ion concentration and contact time There is a decrease in the removal percentage from 91.06 to 50% and increase in metal uptake capacity from 37.02 to 248.76 mg g 1 with increase in initial Pb(II) concentration from 10 to 100 mg L 1 at pH 4.5 which is shown in Fig. 3(b). At low concentrations the adsorbent sites take up the available metal ions more quickly. With the same amount of adsorbent at higher metal ion concentration, more metal ions will compete with the fewer
number of available biosorbent sites, which show a decrease in the percentage removal (Das and Mondal, 2011; Horsefall and Spiff, 2005).The metal ions diffuse to the adsorbent surface by intraparticle diffusion and highly hydrolyzed ions will diffuse slowly (Ashraf et al., 2011). Hence at higher concentrations there will be an increase in the amount of unadsorbed ions in the solution. Similar results have been obtained for the adsorption of cadmium ions using Hypnea valentiae biomass (Aravindan et al., 2009).The effect of contact time for Pb(II) biosorption on CANPs is shown in Fig. 3(c). As the contact time is increased to 60 min, the adsorption of Pb(II) ion also increases, but after 60 min, there is a very small change in the removal efficiency of Pb(II) ions up to 240 min. Hence the equilibrium contact time is considered as 60 min for all further experiments. 3.2.3. Effect of adsorbent dosage Adsorbent dose has a strong effect on the adsorption of the metal ions at a particular initial concentration. Fig. 3(d) shows that an increase in biosorbent amount from 0.02 to 0.08 g L 1 resulted in an increase of percentage removal from 91.06 to 94.81% (uptake capacity 37.02–11.45 mg g 1). This is due to the increase in the number of active sorption sites at the adsorbent surface. 3.3. Isotherm analysis The equilibrium data of nanoalginate for Pb(II) adsorption was investigated using Langmuir and Freundlich isotherm models which are shown in Fig. 4(a) and (b) respectively. The regression coefficients and the isotherm parameters are presented in Table 1.
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Fig. 4. (a) Langmuir (b) Freundlich adsorption isotherm models (c) pseudo-first order and (d) pseudo-second order kinetic models. Table 1 Isotherm constants and regression data for biosorption of Pb(II) on CANPs. Sl No.
Adsorption Isotherms
1.
Langmuir model
2.
Freundlich model
Qm (mg g 1) b (L mg 1) kf (L g 1) nf
Isotherm parameters
R2
333 0.0833 38.28 2.083
0.970
separation and favorable multilayer adsorption of metal ions from aqueous medium (Xu et al., 2012; Subhashini et al., 2013; Sharma et al., 2009). The smaller value of 1/n obtained (0.480) indicates better adsorption and formation of relatively stronger bond between CANPs and Pb(II) ions.
0.998
3.4. Adsorption kinetics
Out of the two isotherm models studied, the data fitted well with Freundlich model with a high value for correlation coefficient R2 (0.998), compared to Langmuir model (R2 ¼0.970).The maximum adsorption capacity obtained by Langmuir model is 333 mg g 1 and the value of the Langmuir constant b is 0.0833 L mg 1. The best fit of equilibrium data with Freundlich model suggests that heterogeneous biosorption plays a major role in removing Pb (II) ions from aqueous solution. The Kf value indicates the adsorption capacity of the adsorbent and the value of 1/n below one or n (dimensionless constant) greater than one indicates easy
The plots of pseudo-first order and pseudo-second order kinetic models are shown in Fig. 4(c) and (d).The values of the rate constants along with the corresponding correlation coefficients are presented in Table 2. Although the plot showed linearity, the calculated qe values for varying concentration of Pb(II) ions was not in agreement with the experimental qe values which suggested that biosorption of Pb(II) on nanoalginate is not pseudofirst order in nature. The higher values of R2 (0.999) for all the varying initial concentrations of Pb(II) ions indicate the better fit of pseudo-second order kinetic model. The decrease in the rate of biosorption with
Table 2 Regression coefficients and equilibrium parameters of kinetic study Pseudo-first order kinetics
Pseudo- second order kinetics
K1 (min 1)
R2
qe (mg g
0.0529 0.0461 0.0415 0.0484
0.952 0.999 0.843 0.987
11.64 24.83 69.82 161.43
1
)
qe (exp) value
K2
R2
qe (mg g
0.0119 0.0047 0.0014 0.0004
0.999 0.999 0.999 0.999
38.46 76.92 166.7 333
1
)
qe (mg g 37.02 72.66 155.66 248.76
1
)
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increasing initial metal concentration is evident from the decreasing values of k2 with increase in Pb(II) concentration. So also the value of qe obtained from the slope of the plot for varying concentrations of Pb(II) seems to be in close agreement with the calculated qe values. 3.5. Intraparticle diffusion mechanism The mechanism of lead biosorption was studied by the intra particle diffusion model by plotting the amount of the metal adsorbed at time t (qt) and the square root of time (t1/2) which is shown in Fig. 5(a) and the data in Table 3(included in the supplementary file). Since the linear plots at various concentrations do not pass through the origin, the rate controlling step is not governed by intraparticle diffusion alone. The initial curved portion of the plot indicates the boundary layer effect while the second linear portion indicates intraparticle diffusion. The higher values of the intercept C (boundary layer effect) indicate greater contribution of surface adsorption in the rate determining step (Arivoli et al., 2009). Hence both intraparticle diffusion and surface adsorption play an important role in the biosorption mechanism of Pb(II) ions by nanoalginate.
obeyed pseudo-second order kinetics. The evaluation of the thermodynamic parameters indicated the spontaneity and endothermic nature of the biosorption process. The positive values of ΔH°, ΔS° and also the increase in the positive value of Ea with rise in temperature, all confirmed the endothermic nature of the sorption process. The positive value of ΔS° revealed the increase in randomness, the sorption stability as well as the irreversibility of the biosorption process while the negative values of ΔG° indicated the spontaneity of adsorption process. The decrease in the negative value of ΔGo with increasing temperature showed the more favorable adsorption of Pb(II) ions on CANPs at higher temperatures.Thus Calcium alginate nanoparticles can be considered as an efficient biosorbent for removing lead ions from aqueous solutions.
Acknowledgments Geetha P is thankful to the University Grants Commission, SWRO for providing financial support for the conduct of the research work under the Faculty Improvement Programme during XIth plan period.
3.6. Thermodynamic studies Appendix A. Supplementary material The thermodynamic parameters for the biosorption of Pb(II) ions onto CANPs at various temperatures were computed by plotting ln kc vs. 1/T for 10 mg L 1 initial Pb(II) ion concentration [Fig. 5(b)] and the values are listed in the Table 4 (included in the supplementary file). ΔH° and ΔS° have been computed from the slope and the intercept of the plot and the Gibbs free energy change ΔG° was also calculated. The positive value of ΔH° confirmed the endothermic nature of the biosorption process. The positive value of ΔS° showed the increase in entropy, sorption stability and irreversible nature of the adsorption process. The negative value of ΔG° confirmed the feasibility of the sorption process. 3.7. Desorption studies The regeneration studies conducted using four different concentrations of hydrochloric acid (0.1, 0.01, 0.001 and 0.0001 M) showed a maximum desorption ability of 97.6% with 0.1 M HCl. At lower pH the greater number of H þ ions present will compete with metal ions for the same binding sites, resulting in greater desorption of Pb(II) ions and the data is presented in Table 5 (included in the supplementary file). Even after four cycles of adsorption–desorption process, the biosorbent was found to be highly stable and hence it can be reused. 3.8. Comparison with commercial and low cost biosorbents The maximum biosorption capacities of various sorbents including alginate nanoparticles for Pb(II) ions are summarized in Table 6 (included in the supplementary file). This comparison showed that CANPs has higher biosorption capacities than some of the commercially available and low cost biosorbents.
4. Conclusion In the present study, the maximum lead removal capacity of Calcium alginate nanoparticles was found to be 94.81% at 45 °C and at pH 4.5 in 60 min contact time. The best fit of the equilibrium data with Freundlich adsorption isotherm model confirmed the heterogeneity of the adsorption sites.The biosorption process
Supplementary data associated with this article can be found in the online version at http://doi:10.1016/j.ecoenv.2015.06.032.
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Contents lists available at ScienceDirect
Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Nanoalginate based biosorbent for the removal of lead ions from aqueous solutions: Equilibrium and kinetic studies P. Geetha a,d, M.S. Latha b,n, Saumya S Pillai c,d, Mathew Koshy d a
Department of Chemistry, D.B Pampa College, Parumala, Mannar, Kerala, India Department of Chemistry, S.N. College, Chengannur, Kerala, India c Department of Chemistry, N. S. S. Hindu College, Changanacherry, Kerala, India d Department of Chemistry, Bishop Moore College, Mavelikara, Kerala, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 8 April 2015 Received in revised form 17 June 2015 Accepted 22 June 2015
Population explosion, depletion of water resources and prolonged droughts and floods due to climatic change lead to scarcity of pure and hygienic drinking water in most of the developing countries. Recently nanomaterials attained considerable attention as biosorbent for water purification purpose. However difficulties in removing polymeric surfactants and organic solvents used for nanoproduction and instability of the generated nanoparticles limit the scope of this approach in water cleanup. Here, we describe a novel green method for synthesizing polysaccharide nanoparticles in aqueous medium using honey as the capping agent. The highly stable alginate nanoparticles, characterized by various microscopic and spectroscopic techniques, exhibited a maximum uptake capacity of 333 mg g 1of Pb(II) ions from aqueous solution. The effect of various parameters such as initial metal concentration, pH, contact time, temperature and adsorbent dose on sorption process was investigated in batch mode technique. The maximum removal percentage was 94.81 at 45 °C and at pH 4.5 in 60 min contact time. The biosorption followed Freundlich model indicating multilayer adsorption and pseudo second order kinetics. The mechanism involves both surface adsorption and pore diffusion. The positive values of ΔH°, ΔS° and the negative value of ΔG°, confirmed the endothermic nature, randomness and spontaneity of biosorption process. & 2015 Elsevier Inc. All rights reserved.
Keywords: Polysaccharide Honey Cross-linking agent Calcium alginate nanoparticles Lead removal Water purification
1. Introduction Contamination of the environment by toxic heavy metals is a worldwide problem. Lead is one of the prime toxins of the heavy metals, discharged into the environment by metal, electroplating, battery manufacturing, pigment and dye industries (Majumdar et al., 2010). Consumption of these contaminated water adversely affect liver, kidney, brain and central nervous system causing irreversible brain damage, nervous disorders and weakness of muscles (Gupta and Rastogi, 2008). Conventional methods for the removal of toxic heavy metals are economically expensive (Gupta et al., 2012a). Several techniques based on cheap waste materials have been reported recently for the removal of pollutants from wastewater (Gupta et al., 2011a, 2012b; Karthikeyan et al., 2012). Among these, biosorption is a cost effective method based on the metal/dye sequestering properties of certain natural materials such as agricultural wastes (Mittal et al., 2009a, 2009b; Gupta n
Corresponding author. E-mail address: [email protected] (M.S. Latha).
http://dx.doi.org/10.1016/j.ecoenv.2015.06.032 0147-6513/& 2015 Elsevier Inc. All rights reserved.
et al., 2010a, 2010b, 2015), fertilizer wastes (Gupta et al.,1997), industrial wastes (Jain et al., 2003) and plants such as algae (Hlaing et al., 2011). Alginates, the main constituent of the cell wall of the brown algae, are reported to have superior ability for heavy metal uptake from contaminated water (Mohan et al., 2007). Alginate is a linear copolymer composed of (1–4) linked β-D-Mannuronic acid (M) and α-L-Guluronic acid (G) residues. The carboxylate groups of the polymer provide the ability to form biodegradable gels in the presence of multivalent cations forming three dimensional net work structures called egg-box model (Lee and Mooney, 2012).Various forms of alginate such as beads, films, fibers etc are proven to be efficient biosorbents for detoxifying waste water (Torres et al., 2005; Nayak and Lahiri, 2006; Gok et al., 2013; Gomez et al., 2012; Cataldo et al., 2013, 2014). Lezehari et al. (2012) used alginate encapsulated pillared clay microbeads as a fixed bed column for the removal of a broad spectrum biocide pentachlorophenol. Recently nanoalginate is gaining momentum in detoxification of waste water due to its extraordinary tendency for rapid interaction with toxic contaminants present in wastewater (Prachi et al., 2013; Pal and Banat, 2014).
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The advancement in nanoscience and nanotechnology paved the way for the synthesis of both metal as well as polymeric nanoparticles, having myriads of applications in biology, agriculture, pharmacy, chemicals, textile industry, material industry, medicine, drug delivery, gene delivery and electronics (Kumar and Yadav, 2011; Heemasagar et al., 2014). Nanomaterials like carbon nanotubes, multi-walled carbon nanotubes, composites of nanotubes, nano form of magnetically modified orange peel etc were utilized for the removal of heavy metals and organic contaminants from wastewater (Khani et al., 2010; Gupta et al., 2011b;. Saleh and Gupta, 2012; Gupta and Nayak, 2012). Calcium alginate nanoparticles were synthesized by various groups of researchers (Rajaonarivony et al., 1993; Ahmad et al., 2006; Nesamony et al., 2012; Cheng et al., 2012; Singh et al., 2013). Since alginate is a cheap commercially available material extracted from the cell wall of the naturally occurring brown algae, use of this material in nano form for water purification is economically viable. Here we are reporting the green synthesis of calcium alginate nanoparticles (CANPs) with high surface area to volume ratio and multiple adsorption sites and their efficiency in removing lead ions from aqueous solutions.
2. Materials and methods 2.1. Materials Low viscosity Sodium alginate powder (Sigma-Aldrich, London),Calcium chloride dihydrate and Lead nitrate (Merck,Darmstadt, Germany) were of analytical grade and used as such without any further purification. Natural honey used in this study was procured from Kerala Agriculture University. The pH adjustments were carried out using 0.1 M HCl and 0.1 M NaOH. 2.2. Methods 2.2.1. Synthesis of calcium alginate nanoparticles (CANPs) by ionotropic gelation method Calcium alginate nanoparticles were synthesized by the drop wise addition of calcium chloride to sodium alginate in aqueous honey medium. Continuous agitation of this homogenized mixture using a magnetic stirrer for four hours resulted in the formation of nanoparticles, which was collected by centrifugation, washed and dried under vacuum. 2.2.2. Characterization of calcium alginate nanoparticles Fourier transform infrared (FTIR) spectra of the synthesized alginate nanoparticles before and after lead biosorption were recorded between 400 and 4000 cm 1 wavelength range using Schimadzu FTIR model 1801. Surface morphology of the biosorbent and energy dispersive spectrum (EDS) was probed using a JEOL JSM-6390LA Analytical Scanning electron microscope. Transmission electron microscopy (TEM) was performed using JEOL model 1200EX instrument operated at an accelerating voltage at 80 kV. The topography of the sample was studied using Atomic Force Microscope (AFM) Nanoscope V from Veeco, USA, at a resonance frequency of 70 kHz and a spring constant of 1–5 N/m. 2.2.3. Preparation of Pb (II) solution A stock solution of 1000 mg L 1 of Pb(II) was prepared by dissolving1.598 g lead nitrate in 1000 ml deionized water. This solution was diluted as required to obtain the standard solutions containing 10–100 mg L 1 of Pb(II) ions. 2.2.4. Batch adsorption technique The effect of pH, contact time and initial concentration on the
biosorption of Pb(II) ions onto CANPs was investigated by equilibrating a definite amount of biosorbent in 100 ml Pb(II) solution with initial concentrations ranging from 10 mg L 1 to 100 mg L 1. The experiments were carried out in the pH range 3–6, by agitating the mixture for a contact time of 10–240 min. The contents of the flask were agitated well by intermittent manual shaking with the purpose of achieving a more economic process.The solution was then filtered at preset time intervals and the Pb(II) concentration was determined by AAS (Perkin-Elmer Atomic Absorption Spectrophotometer –PinAAcle900H) The percentage removal of lead ion can be calculated using the equation
=
c o −ce × 100→ co The amount of equilibrium adsorption, qe (mg g
qe =
c o −ce × V→ w
(1) 1
)
(2)
where Co and Ce are the initial and equilibrium lead ion concentrations (mg L 1), respectively, V is the volume in liter of the solution and w (g) is the mass of dry sorbent used. All the experiments were carried out in triplicate and the mean values are presented. 2.2.5. Statistical analysis The statistical analysis of the results were done using SPSS/ PC þ, version 11.5 (Bennet and Franklin, 1967).The significant differences between the variables were determined by using a one way ANOVA and Duncans multiple range tests and all the values were expressed as mean value 7SD (n¼ 3) 2.2.6. Desorption studies Regeneration studies were conducted using varying concentrations of hydrochloric acid (HCl) at room temperature. A known weight of lead adsorbed nanoparticle was equilibrated with a definite volume of HCl for 60 min and the desorbed Pb(II) ion concentration was measured using AAS.
3. Results and discussion 3.1. Characterization of the biosorbent 3.1.1. FTIR The FTIR spectrum of CANPs before and after Pb adsorption is given in Fig. 1, which confirmed the presence of both carboxyl and hydroxyl groups as binding sites in the metal adsorption process (EL –Tayieb et al., 2013).The broad peak around 3343 cm 1 is indicative of the existence of bonded hydroxyl group [υ(O–H)]. The bands at 1597 cm 1 and 1426 cm 1 are assigned to asymmetric and symmetric stretching peaks of carboxylate salt groups. The bands around 1078 cm 1 (C—O–C stretching) and 1023 cm 1 (C–O stretching) present in the IR spectrum are attributed to the saccharide structure of the biosorbent (Bansal et al., 2009). The slight shifting of the υ(O–H) band from 3343 cm 1 to 3220 cm–1 and υ (C ¼O) band from 1597 cm 1 to 1575 cm 1 after lead adsorption may be due to the participation of both hydroxyl and carboxyl functional groups on the biosorption of Pb(II) ions (Anayurt et al., 2009). 3.1.2. Characterization by microscopic techniques SEM characterization of CANPs before and after adsorption and EDS spectra of nanoalginate is shown in Fig. 2(a), (b) and (c) respectively. Before adsorption a rough surface morphology was observed, while after adsorption of lead ions on the biosorbent,
P. Geetha et al. / Ecotoxicology and Environmental Safety 122 (2015) 17–23
19
Fig. 1. FTIR Spectrum of CANPs (a) before and (b) after lead ion adsorption.
smoother morphology was observed. The EDS spectrum shows the presence of calcium ions which indicates the formation of crosslinks between polymeric chains by the electrostatic interaction of carboxylate ions in the polysaccharide and divalent calcium ions. The TEM and AFM images are shown in Fig. 2(d) and (e) respectively. The TEM image shows well-defined spherical alginate nanoparticles of 50 nm size. No polymeric chains or
expanded structures were seen showing that low concentration of metal salt solution was sufficient enough to cause effective compaction of polymeric chains to form stable nanoparticles. The nano-scale dimension of the prepared alginate particles was also confirmed by AFM images. Aggregation of nanoparticles was observed in the AFM image due to the non-homogeneity of the prepared solution.
Fig. 2. SEM Images of nanoalginate (a) before (b) after lead ion adsorption (c) EDS (d) TEM and (e) AFM images of nanoalginate.
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Fig. 3. The effect of (a) pH, (b) Concentration, (c) Contact time and (d) Amount of biosorbent on the removal of Pb(II) ions onto CANPs.
3.2. Investigation of adsorption parameters 3.2.1. Effect of pH The initial pH of the solution has been reported as one of the most important parameters in the biosorption of metal ions (Saeed et al., 2002). The pH determines the net charge on the biosorbent which invariably determines whether the metal ions can bind or not. Fig. 3(a) shows the effect of pH on the removal of Pb(II) ions from an initial concentration of 10 mg L 1 in 60 min contact time. The percentage removal increases gradually from pH 3, reaches a maximum at 4.5 and thereafter decreases. Under high acidic conditions the biosorbent surface becomes more positively charged and the attraction between adsorbent and the metal cation reduces (Beuno et al., 2008). As the pH increase, the carboxyl and hydroxyl active sites of the biosorbent get deprotonated which enhance metal binding capacity. After complete deprotonation further increase in pH reduce the metal adsorption due to the formation of Pb(II)hydroxides in solution which compete with Pb (II) ions for the adsorption sites (Amboga et al., 2014; Tsai and Chen, 2010) 3.2.2. Effect of initial metal ion concentration and contact time There is a decrease in the removal percentage from 91.06 to 50% and increase in metal uptake capacity from 37.02 to 248.76 mg g 1 with increase in initial Pb(II) concentration from 10 to 100 mg L 1 at pH 4.5 which is shown in Fig. 3(b). At low concentrations the adsorbent sites take up the available metal ions more quickly. With the same amount of adsorbent at higher metal ion concentration, more metal ions will compete with the fewer
number of available biosorbent sites, which show a decrease in the percentage removal (Das and Mondal, 2011; Horsefall and Spiff, 2005).The metal ions diffuse to the adsorbent surface by intraparticle diffusion and highly hydrolyzed ions will diffuse slowly (Ashraf et al., 2011). Hence at higher concentrations there will be an increase in the amount of unadsorbed ions in the solution. Similar results have been obtained for the adsorption of cadmium ions using Hypnea valentiae biomass (Aravindan et al., 2009).The effect of contact time for Pb(II) biosorption on CANPs is shown in Fig. 3(c). As the contact time is increased to 60 min, the adsorption of Pb(II) ion also increases, but after 60 min, there is a very small change in the removal efficiency of Pb(II) ions up to 240 min. Hence the equilibrium contact time is considered as 60 min for all further experiments. 3.2.3. Effect of adsorbent dosage Adsorbent dose has a strong effect on the adsorption of the metal ions at a particular initial concentration. Fig. 3(d) shows that an increase in biosorbent amount from 0.02 to 0.08 g L 1 resulted in an increase of percentage removal from 91.06 to 94.81% (uptake capacity 37.02–11.45 mg g 1). This is due to the increase in the number of active sorption sites at the adsorbent surface. 3.3. Isotherm analysis The equilibrium data of nanoalginate for Pb(II) adsorption was investigated using Langmuir and Freundlich isotherm models which are shown in Fig. 4(a) and (b) respectively. The regression coefficients and the isotherm parameters are presented in Table 1.
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Fig. 4. (a) Langmuir (b) Freundlich adsorption isotherm models (c) pseudo-first order and (d) pseudo-second order kinetic models. Table 1 Isotherm constants and regression data for biosorption of Pb(II) on CANPs. Sl No.
Adsorption Isotherms
1.
Langmuir model
2.
Freundlich model
Qm (mg g 1) b (L mg 1) kf (L g 1) nf
Isotherm parameters
R2
333 0.0833 38.28 2.083
0.970
separation and favorable multilayer adsorption of metal ions from aqueous medium (Xu et al., 2012; Subhashini et al., 2013; Sharma et al., 2009). The smaller value of 1/n obtained (0.480) indicates better adsorption and formation of relatively stronger bond between CANPs and Pb(II) ions.
0.998
3.4. Adsorption kinetics
Out of the two isotherm models studied, the data fitted well with Freundlich model with a high value for correlation coefficient R2 (0.998), compared to Langmuir model (R2 ¼0.970).The maximum adsorption capacity obtained by Langmuir model is 333 mg g 1 and the value of the Langmuir constant b is 0.0833 L mg 1. The best fit of equilibrium data with Freundlich model suggests that heterogeneous biosorption plays a major role in removing Pb (II) ions from aqueous solution. The Kf value indicates the adsorption capacity of the adsorbent and the value of 1/n below one or n (dimensionless constant) greater than one indicates easy
The plots of pseudo-first order and pseudo-second order kinetic models are shown in Fig. 4(c) and (d).The values of the rate constants along with the corresponding correlation coefficients are presented in Table 2. Although the plot showed linearity, the calculated qe values for varying concentration of Pb(II) ions was not in agreement with the experimental qe values which suggested that biosorption of Pb(II) on nanoalginate is not pseudofirst order in nature. The higher values of R2 (0.999) for all the varying initial concentrations of Pb(II) ions indicate the better fit of pseudo-second order kinetic model. The decrease in the rate of biosorption with
Table 2 Regression coefficients and equilibrium parameters of kinetic study Pseudo-first order kinetics
Pseudo- second order kinetics
K1 (min 1)
R2
qe (mg g
0.0529 0.0461 0.0415 0.0484
0.952 0.999 0.843 0.987
11.64 24.83 69.82 161.43
1
)
qe (exp) value
K2
R2
qe (mg g
0.0119 0.0047 0.0014 0.0004
0.999 0.999 0.999 0.999
38.46 76.92 166.7 333
1
)
qe (mg g 37.02 72.66 155.66 248.76
1
)
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increasing initial metal concentration is evident from the decreasing values of k2 with increase in Pb(II) concentration. So also the value of qe obtained from the slope of the plot for varying concentrations of Pb(II) seems to be in close agreement with the calculated qe values. 3.5. Intraparticle diffusion mechanism The mechanism of lead biosorption was studied by the intra particle diffusion model by plotting the amount of the metal adsorbed at time t (qt) and the square root of time (t1/2) which is shown in Fig. 5(a) and the data in Table 3(included in the supplementary file). Since the linear plots at various concentrations do not pass through the origin, the rate controlling step is not governed by intraparticle diffusion alone. The initial curved portion of the plot indicates the boundary layer effect while the second linear portion indicates intraparticle diffusion. The higher values of the intercept C (boundary layer effect) indicate greater contribution of surface adsorption in the rate determining step (Arivoli et al., 2009). Hence both intraparticle diffusion and surface adsorption play an important role in the biosorption mechanism of Pb(II) ions by nanoalginate.
obeyed pseudo-second order kinetics. The evaluation of the thermodynamic parameters indicated the spontaneity and endothermic nature of the biosorption process. The positive values of ΔH°, ΔS° and also the increase in the positive value of Ea with rise in temperature, all confirmed the endothermic nature of the sorption process. The positive value of ΔS° revealed the increase in randomness, the sorption stability as well as the irreversibility of the biosorption process while the negative values of ΔG° indicated the spontaneity of adsorption process. The decrease in the negative value of ΔGo with increasing temperature showed the more favorable adsorption of Pb(II) ions on CANPs at higher temperatures.Thus Calcium alginate nanoparticles can be considered as an efficient biosorbent for removing lead ions from aqueous solutions.
Acknowledgments Geetha P is thankful to the University Grants Commission, SWRO for providing financial support for the conduct of the research work under the Faculty Improvement Programme during XIth plan period.
3.6. Thermodynamic studies Appendix A. Supplementary material The thermodynamic parameters for the biosorption of Pb(II) ions onto CANPs at various temperatures were computed by plotting ln kc vs. 1/T for 10 mg L 1 initial Pb(II) ion concentration [Fig. 5(b)] and the values are listed in the Table 4 (included in the supplementary file). ΔH° and ΔS° have been computed from the slope and the intercept of the plot and the Gibbs free energy change ΔG° was also calculated. The positive value of ΔH° confirmed the endothermic nature of the biosorption process. The positive value of ΔS° showed the increase in entropy, sorption stability and irreversible nature of the adsorption process. The negative value of ΔG° confirmed the feasibility of the sorption process. 3.7. Desorption studies The regeneration studies conducted using four different concentrations of hydrochloric acid (0.1, 0.01, 0.001 and 0.0001 M) showed a maximum desorption ability of 97.6% with 0.1 M HCl. At lower pH the greater number of H þ ions present will compete with metal ions for the same binding sites, resulting in greater desorption of Pb(II) ions and the data is presented in Table 5 (included in the supplementary file). Even after four cycles of adsorption–desorption process, the biosorbent was found to be highly stable and hence it can be reused. 3.8. Comparison with commercial and low cost biosorbents The maximum biosorption capacities of various sorbents including alginate nanoparticles for Pb(II) ions are summarized in Table 6 (included in the supplementary file). This comparison showed that CANPs has higher biosorption capacities than some of the commercially available and low cost biosorbents.
4. Conclusion In the present study, the maximum lead removal capacity of Calcium alginate nanoparticles was found to be 94.81% at 45 °C and at pH 4.5 in 60 min contact time. The best fit of the equilibrium data with Freundlich adsorption isotherm model confirmed the heterogeneity of the adsorption sites.The biosorption process
Supplementary data associated with this article can be found in the online version at http://doi:10.1016/j.ecoenv.2015.06.032.
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