Abstract
Cellulose adsorbent was prepared by ATRP grafting of glycidyl methacrylate onto a cellulose backbone with subsequent functionalization with ethanediamine, and then used for the removal of Cr(VI) from aqueous solution. Batch experiments were carried out to investigate the effects of initial pH and initial Cr(VI) concentration on the adsorption performance. The optimum pH for adsorption of Cr(VI) ranged from 2 to 3, and the maximum uptake of Cr(VI) from solution was 500 mg/g at pH 3.0 and 50 °C. Langmuir and Freundlich isotherms were applied to the adsorption process, and the thermodynamic parameters were calculated. The results showed that the sorption process to be feasible, spontaneous, and endothermic. Kinetics studies revealed that the pseudo-second-order kinetic model fitted well with the experimental data and the intra-particle diffusion was not the only rate-determining step for Cr(VI)sorption onto adsorbent. The cellulose adsorbent before and after Cr(VI) adsorption were characterized using scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). Regeneration of cellulose adsorbent loaded with Cr(VI) can be achieved by treating with 2.0M NaOH.
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Thermodynamics, Kinetics, and Regeneration Studies for Adsorption of Cr(VI) from Aqueous Solutions using Modified Cellulose as Adsorbent
Chunxiang Lin, Sha Qiao, Wei Luo, Yifan Liu, Danhui Liu, Xiaojuan Li, and Minghua Liu*
Cellulose adsorbent was prepared by ATRP grafting of glycidyl methacrylate onto a cellulose backbone with subsequent functionalization with ethanediamine, and then used for the removal of Cr(VI) from aqueous solution. Batch experiments were carried out to investigate the effects of initial pH and initial Cr(VI) concentration on the adsorption performance. The optimum pH for adsorption of Cr(VI) ranged from 2 to 3, and the maximum uptake of Cr(VI) from solution was 500 mg/g at pH 3.0 and 50 °C. Langmuir and Freundlich isotherms were applied to the adsorption process, and the thermodynamic parameters were calculated. The results showed that the sorption process to be feasible, spontaneous, and endothermic. Kinetics studies revealed that the pseudo-second-order kinetic model fitted well with the experimental data and the intra-particle diffusion was not the only rate-determining step for Cr(VI)sorption onto adsorbent. The cellulose adsorbent before and after Cr(VI) adsorption were characterized using scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). Regeneration of cellulose adsorbent loaded with Cr(VI) can be achieved by treating with 2.0M NaOH.
Keywords: Cellulose adsorbent; Cr(VI); Adsorption; Kinetics; Thermodynamics
Contact information: College of Environment & Resources, Fuzhou University, Fuzhou 350108 China;
* Corresponding author: mhliu2000@fzu.edu.cn
INTRODUCTION
Chromium (VI) compounds have found widespread use in electroplating, metal finishing, chromate preparation, leather tanning, cooling towers of heavy industry, atomic power plants, mining, metallurgy operations, etc. However, they can be a major threat to public safety because of an increase in discharge into the ecosystem from waste streams, their toxic effects, and other adverse effects on receiving waters (Saha and Orvig 2010; Saha et al. 2011; Kaya et al. 2014; Liang et al. 2014; Zhang et al. 2014). According to the World Health Organization (WHO), the health effects of long term exposure to chromium (VI) levels over 0.1 ppm can cause respiratory problems, kidney problems, and liver damage (Bhaumik et al. 2014). Therefore, it is necessary that industries treat their effluents to reduce the Cr(VI) concentration in water and wastewater to acceptable levels before its transport and cycling into the natural environment.
A long list of methods have been applied to reduce and recover the toxic heavy metals from final effluent (Peng et al. 2004; Golder et al. 2007; Bhuvaneshwari and Elango 2009; Liu et al. 2009; Saha and Orvig 2010; Saha et al. 2011; Xing and Beauchemin 2011; Xie et al. 2011; Wang et al. 2013; Wu et al. 2013; Song et al. 2014). The methods described by these authors include chemical precipitation, oxidation, coagulation, ion exchange, reverse osmosis, solvent extraction, flocculation, membrane separation, filtration, evaporation, electrolysis, and adsorption. Among all the treatment processes mentioned, adsorption using the chelating properties of adsorbents is one of the effective and widely considered techniques for the removal of toxic heavy metals from wastewaters. Various adsorbents have been developed for removing heavy metal ions from aqueous solutions, such as clay (Zhao et al.2013), ion exchange resins (Kusku et al. 2014), activated carbon (Bayrak et al. 2013), carbon nano-tubes (Kumar et al. 2013), and metal oxides (Xu et al.2014). In particular, cellulose, the main constituent of plants, is the most abundant biomass in the world. It is renewable, cheap, biodegradable, and can be chemically modified for enhancing its effectiveness by introduction of new functional groups (O’Connell et al. 2006; Liu and Sun 2008). There is intensive research nowadays for the application of cellulosic material for hazardous contaminant adsorption from aqueous wastes.
In this work, an adsorbent was prepared by ATRP grafting of glycidyl methacrylate (GMA) onto a cellulose backbone with subsequent functionalization with ethanediamine. The use of the ATRP technique instead of conventional redox grafting methods can provide good control of the graft length and composition, and the subsequent functionalization with ethanediamine can produce amine functional groups, NRH. The latter can be protonated to form NRH2+ in aqueous solution, thus having the ability to adsorb the anionic pollutants through electrostatic interaction. The obtained adsorbent was then used to remove Cr(VI) from aqueous solution. Batch adsorption experiments were conducted to evaluate the Cr(VI) adsorption behavior including kinetics, equilibrium, and regeneration studies. The effects of adsorbent amount, pH, contact time, and initial Cr(VI) concentration on the adsorption capacity were investigated. The rate kinetics and equilibrium parameters were determined. Adsorption isotherm models and thermodynamic parameters were also investigated to know the adsorption behavior.
EXPERIMENTAL
Materials
Cotton linter was used as the cellulose material. The ionic liquid (1-buty1-3-methylimidazolium chloride, BMIMCl, mp.73 oC) was obtained from Henan Lihua Pharmaceutical Co., Ltd. Copper-(I) bromide (CuBr) was treated according to Matyjaszewski et al. (1999). CuBr was stirred in glacial acetic acid overnight, filtered, and washed with absolute ethanol three times. The compound was dried under vacuum at 60 °C overnight. Chloroacetyl chloride was purchased from Enterprise Group Chemical reagent Co., Ltd. Glycidyl methacrylate (GMA), 2,2’-dipyridyl,ethanol absolute and other reagents were all analytical grade and used as received. Potassium dichromate (K2Cr2O7) was used as a source of Cr(VI). All aqueous solutions and standards were prepared using deionized water.
Preparation of Cellulose Adsorbent
Graft copolymerization
Cellulose was first dissolved in BMIMCl completely, and then 3 folds amount of chloroacetyl chloride was added with a pipet into the cellulose solution. The mixture was stirred in water bath at 40 oC under N2 atmosphere for 2 h. After the end of the reaction, the temperature was reduced, and the catalyst Bpy and Cu(I)Br was introduced. Then nitrogen-degassed GMA was added with a separatory funnel. The molar ratio of the reactant was as follows: [Cellulose ester]:[CuBr]:[Bpy]:[GMA]=1:1:1:100. The polymerization was allowed to react for a 4 h period at 20 oC or 30 oC, and then it was stopped by exposing the mixture to air. Finally, the obtained product was precipitated into de-ionized water, washed by ethanol absolute, repeatedly Soxhlet-extracted to remove homopolymer using acetone solution, and then freeze-dried. The monomer conversion was calculated from the weight balance.
Ring-opening reaction
The obtained grafted cellulose (5g) was dispersed in 200 mL ethanediamine (EDA) solution containing small amount of sodium carbonate. The mixture was reacted at 50 to 90 oC for 2 h. The amine-functionalized adsorbent was filtered, washed with distilled water and acetone, and then dried in vacuum at 60 oC.
Batch Experiments
The point of zero charge of the adsorbent was determined by the following procedure (Lataye et al. 2006): A series of 50 mL conical flasks were prepared with a cork stopper, each containing 20 mL of 0.01 M KNO3 solution. The initial pH (pH0) in each flask was adjusted between 3 and 11 by adding either 0.1 M KOH or 0.1 N HNO3. The adsorbent (0.2 g) was added to each flask, and the flasks were securely capped immediately. The suspensions were then shaken and allowed to equilibrate for 24 h. The difference between the initial and final pH (pHf) values (ΔpH = pH0 – pHf) was plotted against the pH0. The point of intersection of the resulting curve with abscissa, at which aΔpH value of zero gave the pHpzc.
Batch adsorption studies were performed at room temperature by shaking 100 mg of cellulose adsorbent with 100 mL of Cr(VI) solutions in 250 mL. Then, the mixtures were stirred by a water bath shaker at 200 rpm for a predetermined period. Afterwards, the resultant solution was filtered out, and the filtrate was analyzed for its Cr(VI) concentration by a spectrophotometric method using a UV VIS spectrophotometer (T6, China PERSEE Co. Ltd.). The parameters affecting the adsorption process, i.e., pH, agitation time, initial Cr(VI) concentration, and adsorption temperature were studied. The pH of the aqueous solution was adjusted using 0.01N HCl and 0.01N NaOH solutions. All experiments were replicated thrice for all the adsorbents and results were averaged. The amount of Cr(VI) adsorbed by the cellulose adsorbent(q) and the removal percentage (R%) of Cr(VI) were calculated for each run by following expressions,
(1)
(2)
where q is the adsorption capacity (mg/g), Ci, C0, and Ce are the initial, outlet, and equilibrium concentrations of Cr(VI) (mg/L), V is the volume of Cr(VI) solution (mL), and W is the total amount of cellulose adsorbent (g).
Adsorption isotherm studies were carried out with five different initial concentrations of Cr(VI) from 100 mg/L to 600 mg/L at pH 3, contact time 3 h, and adsorbent dosage level of 1.0 g/L.
Kinetic studies of adsorption were also carried out at concentration of 100 mg/L at different temperature wherein the extent of adsorption was analyzed at regular time intervals.
Regeneration
Regeneration of adsorbent as well as recovery of adsorbate material is quite important, and the desorption and regeneration experiments were conducted with different types of desorbing agents through a batch technique. The adsorbent loaded with Cr(VI) was placed in the desorption medium and was stirred at 200 rpm at room temperature for 3 h. The elution ratio was calculated from the amount of Cr(VI) adsorbed on the adsorbent and the amount of Cr(VI) desorbed. The adsorption-desorption cycle was repeated several times to determine the reusability of the adsorbent.
Characterization
The Fourier transform infrared (FTIR) spectra of cellulose adsorbent before and after sorption were recorded with a AVAT-AR360 FT-IR spectrometer (Nicolet, USA) in the range of 4000 to 400 cm-1. X-ray photoelectron spectroscopy (XPS) analyses of the sorbents before and after sorption were conducted on a ESCALAB 250 spectrometer (Thermo Scientific, USA). Scanning electron microscopy (SEM) images of the sorbents before and after adsorption were obtained with a XL30 ESEM-TMP environmental scanning electron microscope (Philips-FEI, Holland). The X-ray diffraction (XRD) pattern of adsorbent before and after adsorption were performed on an X/Pert Pro MPD X-ray diffractometer (Philips, Holland). The energy-dispersive X-ray spectroscopy (EDX) analysis of cellulose adsorbent before and after adsorption were analyzed with a Nova NanoSEM 230 field-emission microscope (FEI, Hillsboro, US).
RESULTS AND DISCUSSION
Preparation and Characterization of the Adsorbent
Scheme 1 represents the functional reaction involved in the adsorbent preparation. There was remarkable weight gain of cellulose after graft copolymerization of GMA, and the monomer conversion was found to be 49.8%. The epoxy group content in the graft copolymer was determined to be 2.739 mmol/g by the pyridine-HCl titration method (Anirudhan and Senan 2011). After ring-open reacting with EDA, the epoxy group on the grafted cellulose was 100% converted to an amino group, which was confirmed by a nearly zero value of epoxy group content of cellulose adsorbent.
Results of FT-IR analysis of the cellulose, grafted cellulose, and cellulose adsorbent are shown in Fig. 1. Compared to the cellulose spectrum (Fig. 1a), the appearance at 1728 cm-1, 906 cm-1, and 848 cm-1 of grafted cellulose (Fig. 1b) were related to the GMA structure, indicating the successful ATRP grafting of GMA onto cellulose. After the ring-opening reaction, the characteristic peaks of epoxy group disappeared (Fig. 1c), while a broad band ranging from about 3100 to 3500 cm-1 corresponds to the combination of the stretching vibration of both OH and NH groups. In addition, a new signal appearing at 1571 cm-1 could be assigned to the deformation vibration of the NH group (Lin et al. 2013).
Scheme 1. Synthesis route of cellulose adsorbent
Fig. 1. FTIR spectra of cellulose (a), cellulose-g-GMA (b), and cellulose adsorbent (c)
Effect of pH
The variation of (pH0 – pHf) with initial pH0 is shown in Fig. 2. The value of pHpzc was found to be 7.92.
Solution pH plays an important role during the adsorption process, for the variation of pH can affect the surface charge of the cellulose adsorbent, the degree of ionization, and speciation of adsorbate (Elliott and Huang 1981; Bajpai et al. 2004). In this study, the adsorption of Cr(VI) as a function of pH was conducted in the initial pH range of 1.0 to 8.0, and results are shown in Fig. 3. It can be seen that the adsorption performance of the cellulose adsorbent was highly pH dependent. Cr(VI) removal efficiency was found to decrease with an increase in the initial pH. The optimum pH range for Cr(VI) removal was found to be between 2 and 3, and a maximum 99.6% of removal efficiency was obtained. The results were consistent with the findings reported previously by Saha et al. (2013a,b) and Saha and Saha (2014), where the maximum adsorption of Cr(VI) by chattim tree saw dust, mosambi peel, and mango leaves was found in each case at pH=2. At a lower pH (pH≤3) value, the dominant form of Cr(VI) is HCrO4–, while the surface of adsorbent is charged positively (pHpzc=7.92). The stable forms of chromium such as H2CrO4 and CrO3 exist as polynuclear species at high Cr(VI) concentration (Bayat 2002), and hence the low pH value results in a higher percentage removal of Cr(VI). The decrease in adsorption of Cr(VI) by increasing the value of pH may be due to the dual competition between both the anions (CrO4 2- and OH–) to be adsorbed on the surface of the adsorbent of which OH– predominates.
Previous work reported that when Cr(VI) comes in contact with organic substances or reducing agents, Cr(VI) is easily or spontaneously reduced to Cr(III) in an acidic medium due to its high redox potential value (Park et al. 2004a,b; 2005a,b,c). In this work, the mechanism of Cr(VI) removal by cellulose adsorbent is an electrostatic attraction process in which Cr(VI) are present in the form of anions, such as HCrO4– and Cr2O7–, while the surface of cellulose adsorbent has positive charges. The adsorption characteristics of Cr(VI) on the cellulose adsorbent were determined to be different from those observed on biomass or biomass-based activated carbons. The different adsorption behaviors were caused by the different surface properties of the materials. In our paper, the reaction sites for Cr(VI) are -NRH groups at the chain end, which should not be treated as biomaterials or organic substances to follow a biosorption process. This idea is also supported by studies from other laboratories on similar types of materials (Baes et al. 1997; Garg et al. 2004; Wartelle and Marshall 2005).
Basically, the authors have studied the oxidation states in the adsorption studies and found that the reduction of Cr(VI) to Cr(III) is very low. Therefore, in the present work the dominant mechanism is modeled as adsorption of Cr(VI) species. Since the main goal was the removal of total chromium from solution, much emphasis was focused on total removal, particularly on hexavalent chromium. In fact, Cr(VI) adsorption on adsorbent is not completely “adsorption” but “adsorption + reduction”. Therefore, further work will be carried out to check the reduction of Cr(VI).
Fig. 2. Variation of (pH0 – pHf) with initial pH0
Fig. 3. Effect of pH on the Cr(VI) removal
Effect of Cr(VI) Initial Concentration
The influence of initial concentration of Cr(VI) on adsorption was studied by varying it from 30 to 500 mg·L-1 at an initial pH value of 3 while maintaining an adsorbent amount of 1.0 g·L-1 and a contact time of 3 h duration. The obtained result is presented in Fig. 4. It is shown that with an increase in the initial concentration from 30 to 500 mg·L-1, the removal rate decreased from 99.6% to 62.4% and the adsorption capacity increased from 30.0 to 311.8 mg·g-1. The decrease in the removal ratio of Cr(VI) with increasing initial concentration may be due to the fact that the adsorbent has a limited number of active sites, which would have become saturated above a certain concentration. On the other hand, the increase in adsorption capacity with an increase in initial Cr(VI) concentration may attributed to increased rate of mass transfer due to increased concentration of driving force (Juang et al. 1997; Dönmeza and Aksu 2002; Garg et al. 2008).
Fig. 4. Effect of Cr(VI) initial concentration on the adsorption capacity and Cr(Ⅵ) removal rate
Adsorption Isotherms
The adsorption equilibrium data for the Cr(VI) removal were obtained using Cr(VI) initial concentration between 100 mg/L and 500 mg/L at an adsorbent dosage level of 1.0 g/L. Two of the most commonly used isotherm models, Langmuir and Freundlich models, were adopted to analyze the isotherm data. The mathematical expressions are given by Eqs. 3 and 4, respectively, as follows,
(3)
(4)
where qe is the amount of chromium (VI) adsorbed at equilibrium (mg·g-1), Q0 is the Langmuir maximum adsorption capacity (mg·g-1), Ce is the equilibrium concentration of the Cr(VI) (mg·L-1), and b (L ·mg-1) is the Langmuir constant. K and n are Freundlich’s constants related to adsorption capacity and the heterogeneity factor, respectively.
The Langmuir model (Eq. 3) is an ideal model for homogeneous monolayer adsorption. The model assumes uniform energies of adsorption on to the surface and no transmigration of adsorbate in the plane of the surface. Meanwhile, the Freundlich model (Eq. 4) is an empirical equation in nature, assuming that the sorption process takes place on heterogeneous surfaces and sorption capacity is associated with the concentration of adsorbate in the liquid phase at equilibrium.
The Langmuir and Freundlich isotherm for adsorption of Cr(VI) on cellulose adsorbent at different temperatures are illustrated in Figs. 5 and 6. The calculated parameters for isotherm models are presented in Table 1. It can be seen that the maximum adsorption capacity of Cr(VI) calculated from Langmuir model was 500 mg/g at 50℃, higher than that of cellulose-g-acrylonitrile graft copolymers (371.96 mg/g) ( Hajeeth et al. 2014), mosambi peel (250 mg/g) (Saha et al. 2013a), chattim tree saw dust (333.33 mg/g) (Saha et al, 2013b), mango leaves (250.23 mg/g) ( Saha and Saha 2014), cellulose-g-styrene copolymer (123.4 mg/g) (Zhang et al. 2012), and other biosorbent, such as gelatin (200 mg/g) (Mahmoud and Mohamed, 2014a), Gelatin-Grafted-Yeast (240 mg/g) (Mahmoud and Mohame, 2014b), and novel silica gel (100 mg/g) (Abdel-Fattah and Mahmoud 2011).
Table 1. Calculated Parameters and the Regression Equations for the Langmuir Model
The Langmuir model was found to describe the adsorption more successfully than the Freundlich model for all the temperatures studied, according to the coefficients of determination, R2. The value of the Langmuir parameter b, which is related to the binding energy, was found to increase with increase in temperature, indicating the contribution of stronger binding sites at higher temperature conditions (Deniz et al. 2006).
Fig. 5. Langmuir isotherms for Cr (VI) adsorption on cellulose adsorbent
The magnitude of the Freundlich parameter n gives an indication of feasibility of the adsorption (Ncibi et al. 2006). The 1/n value was in the range 0.1 to 0.5, which indicates that the adsorption process is feasible. For both the isotherm models, the adsorption capacity also increased as temperature was increased. This suggest the endothermic nature of the adsorption process.
Fig. 6. Freundlich isotherms for Cr (VI) adsorption on cellulose adsorbent
To confirm the favorability of Cr(VI) adsorption onto cellulose adsorbent, the separation factor (RL) was calculated by the following equation,
(5)
where RL is a dimensionless separation factor indicating the shape of the isotherm, b is the Langmuir constant, and C0 is the initial Cr(VI) concentration. If RL = 0, RL > 1, and 0 < RL < 1, it represents that the isotherm is irreversible, unfavorable, and favorable, respectively (Mohan et al. 2005). The values of RL at different temperature were determined and listed in Table 2. It can be seen that all RL values were in the range 0 to 1, indicating good uptake of Cr(VI) toward the adsorbent.
Table 2. Values of RL at Different Temperature
Thermodynamics for Adsorption
Thermodynamic parameters such as free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) are crucial for determining the spontaneity of an adsorption process. These parameters, which are estimated to evaluate the feasibility and endothermic nature of the adsorption process, can be calculated from the following equations,
(6)
(7)
where R is the ideal gas constant 8.314 (J mol−1 K−1) and T is temperature (K).
The values of ΔG can be calculated according to the values of b (presented in Table 3), while ΔH and ΔS are obtained from the slope and intercept by plotting ln b versus 1/T. The obtained values of thermodynamic parameters are listed in Table 3. The increasingly negative values of ΔG with temperatures indicates the spontaneity and feasibility of the sorption of Cr(VI) on the cellulose adsorbent.
Table 3. Results of Calculated Thermodynamic Parameters
The positive value of ΔH indicated the endothermic nature of adsorption process, and the positive values of ΔS at different temperatures indicate the increase in the randomness in the (solid + liquid) interface. The parameters are all supporting each other and confirming the positive influence of temperature for Cr(VI) adsorption on the adsorbent .
Kinetics Study
The effect of adsorption time on Cr(VI) removal was investigated to determine an optimum contact time for the adsorption of Cr(VI) on cellulose adsorbent (Fig. 7). As can be seen from the figure, there was a rapid uptake kinetics within 30 min, with adsorption equilibrium attained within 3 h. The pseudo-first-order kinetic model (Eq. 8) and pseudo-second-order kinetic model (Eq. 9) were applied to studied the sorption kinetics of Cr(VI) onto cellulose adsorbent.
In Eqs. 8 and 9, qe and qt are the amounts of Cr(VI) adsorbed (mol/g) at equilibrium and at time t (min), respectively, and k1 (min-1) and k2 (g·mg-1·min-1) are the rate constants of the pseudo-first-order kinetic model and the pseudo-second-order kinetic model, respectively.
The results are presented in Table 4 and in Figs. 7 and Fig. 8.
Table 4. Pseudo-First-Order Dynamic Equations and Parameters
Fig. 7. Pseudo-first-order curve of Cr(VI)
Fig. 8. Pseudo-second-order curve of Cr(Ⅵ)
From the results, it could be found that the pseudo-second-order model provided a better relationship for kinetics of adsorption process, because the values of correlation coefficients for the pseudo-second-order model of all examined data were found to be very high (R2≥0.99), indicating a better fit with the experimental data compared to the pseudo-first-order kinetic model.
Intra-particle diffusion model was also applied (Eq. 10) to study the effect of intra-particle diffusion resistance on sorption. Figure 9 illustrates the plots of qt vs. t1/2. It shows that there is a two-stage linear region over the entire time range; one was attributed to the membrane diffusion, while the other was attributed to the intra-particle diffusion.
Fig. 9. Diffusion curve
The linear portions of the curves do not pass through the origin. This implies that the intra-particle diffusion was not the only phenomenon to govern the adsorption process. Both the membrane diffusion as well as intra-particle diffusion contribute to the rate-determining step. The slope of the initial linear portion of the curve was calculated to derive the intra-particle rate constant KT, and the results are presented in Table 5.
In Eq. 10, KT is the intra-particle diffusion coefficient (mmol·g-1·min-1/2).
Table 5. Intra-particle Diffusion for Adsorption of Cr(VI) by Cellulose Adsorbents at Different Temperatures
Fig. 10. ESEM micrograph of cellulose adsorbent before (A and B) and after (C and D) adsorption
Characterization of Adsorbent
ESEM images of cellulose adsorbent before and after adsorption are shown in Fig. 10. It can be observed that the surface of the adsorbent after adsorption changed, and the visible pores of the adsorbent had disappeared or had been filled up.
EDX analysis was then carried out on the cellulose adsorbent pre-adsorption and the Cr(VI) loaded adsorbent samples post adsorption. The EDX spectra obtained are shown in Fig. 11. These spectra indicate the presence of the adsorbed Cr(VI) on the cellulose adsorbent material as evidenced by the Cr peak at 5 to 6 keV. Coupled with the results from the adsorption studies, the EDX plots provide clear evidence for the physical presence of the adsorbed metal on the cellulose adsorbent.
To identify the possible functional groups on cellulose adsorbent involved in the binding of Cr(VI), FTIR spectra were obtained before and after metal adsorption. If a ligand coordinates to a metal, the energy of the ligand material will most likely be perturbed leading ultimately to subtle shifts in the absorption peaks in the FTIR spectra. These FTIR absorption bands are usually shifted to lower or higher frequencies. Figure 12 shows the FTIR spectra of cellulose adsorbent before and after Cr(VI) adsorption. There was a small shift of the ring vibration peak at 1573 cm-1 in cellulose adsorbent to 1657 cm-1 for Cr(VI)-loaded adsorbent.
Fig. 11. EDX spectra of cellulose adsorbent pre- adsorption (a) and post adsorption (b)
Fig. 12. The FT-IR spectrum of cellulose adsorbent (a) and Cr(VI) loaded adsorbent (b and c)
Fig. 13. XPS spectra for (a) cellulose adsorbent and (b) Cr(VI) loaded cellulose adsorbent
XPS spectra are also widely used to distinguish different forms of the same element and to identify the existence of a particular element in a material (Dambies et al. 2001). The surface structure of cellulose adsorbent before and after Cr(VI) adsorption was investigated by XPS spectra (Fig. 13). The results showed that the Cr peak is observed at 570 to 590eV after Cr(VI) adsorption, indicating the existence of Cr atoms on the surface of adsorbent.
Regeneration of Cellulose Adsorbent
From an economical point of view, the regeneration of cellulose adsorbent was investigated by conducting regeneration study with different types of desorbing agents (Table 6) using batch experiments. It could be seen that the NaOH solutions was found to be the most effective desorbing agent.
Table 6. Effect of Different Desorbing Agents on the Desorption Percentage
The relationship between the desorption percentage and the concentration of NaOH was studied and results are shown in Fig. 14. The percentage of desorption increased with the increase in NaOH concentration and was 98.0% when 2.0 M of NaOH was used as a desorption agent.
Figure 15 shows the ESEM images of the cellulose adsorbent after regeneration. The results show that the micro-pores on the surface of the adsorbent appeared again after regeneration with NaOH solutions.
Fig. 14. Effect of sodium hydroxide aqua concentration on the desorption
Fig. 15. ESEM images of cellulose adsorbent after regeneration
The reusability of adsorbent was studied by repeating the adsorption-desorption cycles, and the results are shown in Table 7.
Table 7. Relationship between Adsorption Capacity & Desorption Percentage and Adsorption-Desorption Cycles
The data presented in the table show that the adsorbent could be reused five times at least without noticeable change in adsorption capacity during the repeated adsorption-desorption operations. The above results indicated that the cellulose adsorbent had excellent ability of reuse in Cr(VI) adsorption by using 2.0M NaOH desorbing agent.
CONCLUSIONS
A modified cellulose adsorbent was synthesized and then used for the removal of Cr(VI) from aqueous solutions. Batch experiments were performed to study the effects of adsorption conditions on adsorptive performance. Equilibrium data could be well modeled by the Langmuir isotherm, and a kinetic study showed that the adsorption process followed the pseudo-second-order kinetics model. Thermodynamic parameters were calculated, and the results showed that the adsorption process is spontaneous and endothermic in nature. Characterization with ESEM, EDX, FTIR, and XPS confirmed the binding of Cr (VI) onto the cellulose adsorbent. The Cr(VI)-loaded cellulose adsorbent can be regenerated by treating with a 2.0 M NaOH solution and reused at least five times without noticeable change in adsorption capacity.
ACKNOWLEDGMENTS
The authors are grateful for the support of the Science and Technology Project of Fujian Province Educational Department (JK2013004, JA12040), Science & Technology Development Fund of Fuzhou University (2012-XY-10,2014-XQ-11), and the National Science Foundation of Fujian Province (41372346).
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Article submitted: July 24, 2014; Peer review completed: September 14, 2014; Revised version received: September 24, 2014; Second revision received: September 25, 2014; Accepted: September 26, 2014; Published: October 6, 2014.