A-highly-selective-sorbent-for-removal-of-Cr-VI-from-aqueous-solutions-based-on-Fe3O4-poly-methyl
A highly selective sorbent for removal of Cr(VI)from aqueous solutions based on Fe 3O 4/poly(methyl methacrylate)grafted Tragacanth gum nanocomposite:Optimization by experimental design
Susan Sadeghi ?,Fatemeh Alavi Rad,Ali Zeraatkar Moghaddam
Department of Chemistry,Faculty of Science,University of Birjand,Birjand,Iran
a b s t r a c t
a r t i c l e i n f o Article history:
Received 1March 2014
Received in revised form 25June 2014Accepted 29August 2014
Available online 6September 2014Keywords:Removal
Chromium (VI)
Modi ?ed magnetic nanoparticles Tragacanth gum Experimental design
In this work,poly(methyl methacrylate)grafted Tragacanth gum modi ?ed Fe 3O 4magnetic nanoparticles (P(MMA)-g -TG-MNs)were developed for the selective removal of Cr(VI)species from aqueous solutions in the presence of Cr(III).The sorbent was characterized by Fourier transform infrared (FTIR)spectroscopy,transmission electron microscopy (TEM),a vibrating sample magnetometer (VSM),and thermo-gravimetric analysis (TGA).A screening study on operational variables was performed using a two-level full factorial design.Based on the analysis of variance (ANOVA)with 95%con ?dence limit,the signi ?cant variables were found.The central composite design (CCD)has also been employed for statistical modeling and analysis of the effects and interactions of signi ?cant variables dealing with the Cr(VI)uptake process by the developed sorbent.The predicted optimal conditions were situated at a pH of 5.5,contact time of 3.4h,and 3.0g L ?1dose.The Langmuir,Freundlich,and Temkin isotherm models were used to describe the equilibrium sorption of Cr(VI)by the absorbent,and the Langmuir isotherm showed the best concordance as an equilibrium model.The adsorption process was followed by a pseudo-second-order kinetic model.Thermodynamic investigations showed that the biosorption process was spontaneous and exothermic.
?2014Elsevier B.V.All rights reserved.
1.Introduction
Heavy metal contamination is one of the most worldwide environ-mental problems of this century [1,2].Among the various heavy metals,chromium (Cr)is one of the most toxic contaminants generated by the electroplating,leather tanning,metal ?nishing,steel fabrication,and textile industries.Cr exists in several chemical forms displaying oxidation numbers from 0to VI,however,the higher oxidation states of Cr are of interest due to the toxic and mutagenic nature of these states.Only two species of Cr,trivalent and hexavalent Cr,are stable enough in aqueous environment.Cr(VI)is highly toxic than Cr(III)due to its high water solubility and mobility [3–5].The toxicity of Cr(VI)originates from its oxidizing property and formation of free radicals inside the cell during the reduction of Cr(VI)to Cr(III)[6,7].Therefore,the development of methods for selective removal of Cr(VI)in the pres-ence of Cr(III)is of great importance.The US Environmental Protection Agency (USEPA)has laid down the maximum contaminant level (MCL)for Cr(VI)in domestic water supplies to be 0.05mg L ?1,while total Cr is regulated to be discharged below 2mg L ?1.
For this sense,various methods have been developed to remove Cr(VI)from the industrial wastewater including chemical precipitation
[8],ion-exchange [9,10],reduction [11],electrochemical precipitation [12],solvent extraction [13],and membrane separation [14,15].These conventional chromium elimination processes are costly or ineffective in most cases,and may also lead to environmental problems from the point of view of waste https://www.wendangku.net/doc/b015918573.html,pare to the traditional methods,the adsorption method has been used as one of the most promising methods in the removal process of Cr(VI)due to high ef ?ciency,easy handling,and the effectiveness of various adsorbents.In recent years,biosorption has been focused on using readily available bioresource,low-cost,non-toxic,and effective adsorbents [16,17].
Nowadays,natural polymers such as chitosan [18],alginate [19],guar gum [20],gum Arabic [21],and cellulose [22,23]have been used in the removal of Cr(VI)from wastewaters.These biodegradable com-pounds enable the formation of a complex with various metal ions.This property can be improved by modi ?cation of these compounds with suitable functional groups through esteri ?cation [24],oxidation reactions [25],or crosslinking techniques [26].Grafting is a simple technique to introduce different functional groups onto the polymers to prevent dissolution of hydrophilic polymer chains in aquatic media and provides new binding sites.However,low surface area and draw-back in separation of natural polymers from aqueous phase limit their use in practical applications.
Magnetic separation is a promising method for solid –liquid phase separation technique,enabling the treatment of a large amount of
Materials Science and Engineering C 45(2014)136–145
?Corresponding author.Tel.:+985612502008;fax:+985612502009.E-mail address:ssadeghi@birjand.ac.ir (S.
Sadeghi).https://www.wendangku.net/doc/b015918573.html,/10.1016/j.msec.2014.08.063
0928-4931/?2014Elsevier B.V.All rights
reserved.
Contents lists available at ScienceDirect
Materials Science and Engineering C
j ou r n a l h o m e p a g e :w ww.e l s e v i e r.c om /l o c a t e /ms e c
sample within a short time.The Fe3O4magnetic nanoparticles have unique properties such low cost synthesis,low toxicity,and relatively high surface area.Therefore,it is expected that the introduction of Fe3O4into natural polymers would not only enhance the chemical and colloidal stability but also improve the separation ability of adsorbents from aqueous solution with the help of an external magnetic?eld [27–29].
Tragacanth gum(TG)is one of the most abundant biopolymers and eco-friendly natural polysaccharides.TG is an exudate gum from shrub-by locoweeds native to arid regions of the eastern Mediterranean and southwestern Asia and desert highlands of northern and western Iran, particularly the Zagros Mountains region[30].It is an anionic carbohy-drate which has stability against heat and a wide range of pH and con-sists of major water soluble and insoluble fractions of D-galactose,D-galacturonic acid and L-rhamnose with traces of ketohexose[31].The chemical modi?cation of TG has been performed using crosslinking and grafting techniques to produce accessible binding sites in this bio-polymer and to enhance its stability and sorption capacity[32].
In this work,a new magnetic adsorbent was developed by the sur-face modi?cation of Fe3O4nanoparticles with poly(methyl methacry-late)grafted TG for selective removal of Cr(VI)in the presence of Cr(III).A possible mechanism for the adsorption of Cr(VI)by the pro-posed adsorbent has been discussed.The response surface methodology (RSM)using a full factorial design was used to explore the region of in-terest of the effective variables and?nd the optimum condition.
Up to now,no previous work on graft polymerization of magnetic TG with methyl methacrylate(Fe3O4/P(MMA)-g-TG)for removal of Cr(VI) has been reported.Therefore,the objective of the present study was to prepare a new magnetic natural adsorbent for removal of Cr(VI)and employ RSM in the optimization process of Cr(VI)by the proposed adsorbent.
2.Experimental
2.1.Chemicals
Potassium chromate(K2CrO4),methyl methacrylate(MMA),ascor-bic acid(AA),acetone,1,5-diphenylcarbazide,HNO3,and potassium persulfate(K2S2O8)were all of analytical grade and provided from Merck(Darmstadt,Germany).Tragacanth gum(TG)was given from local natural resources.Stock solution of Cr(VI)was prepared by dis-solving an appropriate amount of K2CrO4in deionized water.
2.2.Instrumentation
The developed adsorbent(Fe3O4/P(MMA)-g-TG)was characterized by transmission electron microscopy(TEM),Fourier transform infrared spectroscopy(FT-IR),thermo-gravimetric analysis(TGA),and a vibrat-ing sample magnetometer(VSM).TEM micrographs were obtained by a Carl Zeiss model Em10transmission electron microscope(Jena, Germany)operating at80kV to investigate the morphology of the nanoparticles.The FTIR spectra(400–4000cm?1)of the adsorbent dis-persed in KBr pellets were recorded by a VERTEX70FT-IR spectropho-tometer(Bruker,Munich,Germany).Thermo-gravimetric analysis (TGA)was performed on Shimadzu TGA-50(Tokyo,Japan)in N2atmo-sphere.The magnetic properties of P(MMA)-g-TG-MNPs and the naked MNPs were investigated by a Lake-Shore VSM model4700(Westerville, Ohio,USA)vibrating sample magnetometer.Deionized water was ob-tained by an AquaMax water puri?cation system(Younglin,Anyang, Korea).The pH measurements were carried out using a digital pH meter Corning125equipped with a combined glass electrode.The pH values were adjusted by the addition of1M HCl or NaOH solution. The concentration of Cr(VI)was measured using the diphenyl carbazide method by a Shimadzu2501UV–Vis.spectrophotometer(Tokyo, Japan).2.3.Synthesis of poly(methyl methacrylate)grafted TG immobilized on magnetic nanocomposite(P(MMA)-g-TG-MNPs)
Magnetic nanoparticles were prepared according to the previous work[28].P(MMA)-g-TG-MNPs was fabricated by a one step polymer-ization method.Brie?y,1.0g intact Tragacanth gum was dissolved in 250mL deionized water at70°C in a Pyrex bottle.Then,2.0g of Fe3O4 nanoparticles was added to the solution under stirring at1200rpm. Subsequently,6mL of methyl methacrylate(MMA)and1.35g of ascor-bic acid(AA)were added,and the reaction mixture was stirred for 30min.Afterwards,1.35g of K2S2O8was added and the reaction was allowed to continue for1h at room temperature.Finally,the resulting nanocomposites were separated from any homopolymer with acetone, collected by an external magnetic?eld and dried in a vacuum oven to reach a constant weight.
2.4.General procedure for removal of chromium(VI)
Adsorption experiments were performed in batch wise adsorption mode in50mL polyethylene bottles;each containing30mL of Cr(VI)so-lution by varying the initial Cr(VI)concentration from1to50mg L?1and 3.0g L?1P(MMA)-g-TG-MNPs dose at25°C were stirring for3.4h at 200rpm.After the equilibrium was reached,the adsorbent was collected under a strong external magnetic?eld and the supernatant was collected. The initial(C i)and equilibrium(C e)concentrations of Cr(VI)ions were determined by diphenylcarbazide reagent spectrophotometrically at 542nm.The uptake of Cr(VI)ions by the adsorbent was evaluated by the removal ef?ciency(R%)according to Eq.(1):
R%
eT?
C i?C e
C i
?100:e1T
To investigate the effect of the pH,10mL of10mg L?1Cr(VI)or Cr(III)with the pH ranging from2.0to8.0was mixed with10mg of the naked MNPs,or P(MMA)-g-TG-MNP adsorbents at25°C for2h. The pH values were adjusted by the addition of HNO3or NaOH solutions.
For the kinetics investigation,batch studies were conducted in
a temperature-controlled shaker using a?xed adsorbent dose of
3.0g L?1and the Cr(VI)solutions at10,20and30mg L?1concentra-tions(pH5.5).At various time intervals(0–240min),samples were collected by an external magnetic?eld and the remaining Cr(VI)were determined spectrophotometrically.
2.5.Optimization
A full factorial design consisting of16experimental runs with6runs at the center point was used for screening and modeling of the impor-tant process variables.Four variables in the experiment process viz. sample pH(A;5–6),adsorbent dose(B;1.0–3.0mg mL?1),initial con-centration(C;10–30mg L?1),and contact time(D;2–4h)were select-ed to be analyzed,aiming to?gure out their in?uence on the removal of Cr(VI)by the adsorbent.Two other variables,i.e.temperature and stir-ring rate were kept constant at25°C and200rpm,respectively,during the experiments.A half normal plot was used for choosing the impor-tant process variables.Then,the analysis of variance(ANOVA)was per-formed to validate the model.However,the related models are somewhat limited to only two levels in these types of designs.Thus,a second-order model(response surface design)which provides more than two levels for?tting of a full quadratic model[33]is necessary to ?nd the best conditions for removal of Cr(VI).Finally,an experiment was again performed under the obtained optimal conditions to validate the de?ned model.The Design Expert Trial Version8.0software was used to develop the experimental plan for RSM.
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S.Sadeghi et al./Materials Science and Engineering C45(2014)136–145
Fig.1.(A)FTIR spectrum of (a)TG,(b)P(MMA)-g -TG,(c)MNPs,(d)P(MMA)-g -TG-MNPs,and (e)Cr(VI)loaded P(MMA)-g -TG-MNPs;TGA and DTA curves of (B)TG;(C)PMMA-g -TG;(D)MNPs;and (E)P(MMA)-g -TG-MNPs.
138S.Sadeghi et al./Materials Science and Engineering C 45(2014)136–145
3.Results and discussion
3.1.Characterization of the synthesized adsorbent
3.1.1.FTIR analysis
FTIR spectroscopy was studied to investigate structure of the pre-pared adsorbent.FTIR spectrum of intact TG shows characteristic ab-sorption bands at 3415cm ?1,at 1750and at 1640cm ?1which are related to the hydroxyl (–OH)and carbonyl(C _O)groups,respectively.The absorption peaks at 2937and 2867cm ?1were attributed to asym-metrical and symmetrical stretching vibrations of the methylene group,respectively (Fig.1A-a).The appearance of an absorption band at 1244cm ?1assigned to stretching vibrations of C –O of polyol and ether groups.Also,the absorption band around 628cm ?1belongs to a pyranose ring.Most of the characteristic absorption peaks of TG are shifted in P(MMA)-g -TG as shown in Fig.1A-b.Accordingly,absorption peaks of the methylene group are shifted to 3001and 2954cm ?1(Fig.1A-b).Furthermore,a considerable increase in intensity of C _O and methylene peaks was observed.These observations verify the for-mation of the grafted gum.
The absorption bands at 584and 3423cm ?1which are ascribed to the stretching vibration frequency of Fe –O and O –H,respectively,in Fe 3O 4magnetic nanoparticles (Fig.1A-c),could be seen in the spectrum of P(MMA)-g -TG-MNPs with a small shift to lower vibrational frequen-cy (Fig.1A-d).
The FTIR spectrum of the Cr(VI)-loaded P(MMA)-g -TG-MNP adsor-bent in Fig.1A-e showed a decrease in the intensity of O –H,C _O and C –O –C absorption peaks with a shift to lower wave numbers.Moreover,two new enlarged peaks observed at 790and 900cm ?1(Fig.1A-e)at-tributed to the Cr –O and Cr _O bonds,indicating binding of Cr(VI)to P(MMA)-g -TG-MNPs.These results indicated the interaction of Cr(VI)
with P(MMA)-g -TG-MNPs through the reaction of carboxylic acid,hydroxyl and carbonyl groups of the gum.
3.1.2.Thermal analysis
Thermo-gravimetric analysis (TGA)and differential thermal analysis (DTA)of the TG,P(MMA)-g -TG,MNPs,and P(MMA)-g -TG-MNP sam-ples were carried out in nitrogen atmosphere at the scanning rate of 10°C/min in the temperature range of 30–950°C.The results are shown in Fig.1B.A three step weight loss was observed in the TGA curve of intact TG (Fig.1B).The ?rst weight loss at about 100°C resulted from vaporization of residual adsorbed water in the sample.A two-step weight loss at 200–300°C and 400°C with the signi ?cant loss weight of 90%indicates the heterogeneous nature of the gum.
The P(MMA)-g -TG degraded showed the weight loss below 100°C similar to TG,while the sharp decline between 200and 280°C was re-lated to oxidation and degradation of the copolymer,indicating unifor-mity of the network structure (Fig.1C).
TGA curve of MNPs showed only a weight loss of 3–4wt.%below 100°C corresponding to evaporation of moisture (Fig.1D).A three step weight loss was observed in the TGA curve of P(MMA)-g -TG-MNPs (Fig.1E).The weight loss below 100°C could be attributed to the release of monolayer of water present on the surface of sample,while the weight loss at 337°C is probably due to the degradation of bonding hydroxyl groups in a multilayer structure which surrounded the nanoparticles.Final weight loss at 372°C is belonging to the bond degradation between TG and methyl methacrylate.There is no signi ?-cant weight change from 400to 1000°C,implying the presence of only iron oxide within the temperature range which is indicating that P(MMA)-g -TG bonded to the surface of Fe 3O 4nanoparticles.By calcula-tion,the magnetic content of nanocomposite was found to be about 48wt.%.
-1-0.8-0.6
-0.4-0.20
0.2
0.40
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8001000
D T A (m g /m i n )
T G A (%)
TGA
DTA
B
-3-2.5
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-1.5
-1-0.500.51
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020406080
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D T A (m g /m i n )
T G A (%)
TGA
DTA
C
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02040
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D T A (m g /m i n )
T G A (%)
TGA
DTA
D
D T A (m g /m i n )
T G A (%)
Temprature (o C)
TGA
DTA
Temprature (o C)
Temprature (o C)Temprature (o C)
Fig.1(continued ).
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S.Sadeghi et al./Materials Science and Engineering C 45(2014)136–145
On the basis of the results obtained from TGA studies,it was found that the TG undergoes a crosslinking reaction so that P(MMA)-g-TG and P(MMA)-g-TG-MNPs have more regular structures compared to that of MNPs with less hydrophilicity than TG.
3.1.3.Magnetic properties
The magnetic properties of the synthesized nanoparticles were also studied by a VSM.The magnetization curves of the MNPs and P(MMA)-g-TG-MNPs were obtained at room temperature.The variation in mag-netic moments of the nanoparticles as a function of the magnetic?eld in the range of?20,000to20,000Oe was investigated which is shown in Fig.S1.The hysteresis loops were recorded at room temperature (Fig.S1)and the coercivities and reduced remanences(M H=0/M sat)of these samples were also measured.Saturation magnetization for MNPs and P(MMA)-g-TG-MNPs was about64emu/g and41emu/g,respective-ly.The coercivity for MNPs and P(MMA)-g-TG-MNPs was about12and 4Oe,respectively.The reduction of measured saturation magnetization and the ferromagnetic feature of P(MMA)-g-TG-MNPs probably ex-plained by the larger particle diameter and some aggregation or nonmag-netic layering on MNPs(Fig.S1).Although immobilization of P(MMA)-g-TG on the MNPs reduced the magnetic property of MNPs,it still could be separated from aqueous solution by using an external magnet?eld.
3.1.
4.Transmission electron microscopy(TEM)
The morphology of PMMA and PMMA-CH particles was character-ized by using TEM.Fig.S2shows the typical TEM images of the Fe3O4 nanoparticles(Fig.S2-a)and P(MMA)-g-TG-MNPs(Fig.S2b).It is ob-served that after grafting polymerization,the dispersion of MNPs im-proved greatly.The polydispersity in the system was low because of the narrow size distribution of the particles in the range of10–22nm.
3.2.Effect of pH on the selective removal of Cr(VI)
As it is shown in Fig.2A,the maximum removal ef?ciency of Cr(VI) was observed at a pH of2for both of the naked MNPs,and P(MMA)-g-TG-MNPs.Above this pH,the amount of adsorbed Cr(VI)sharply de-creased to about zero in the case of naked MNPs,but the value of remov-al ef?ciency was decreased to about50%in the case of P(MMA)-g-TG-
MNPs.It is obvious that the pH affects not only the surface charge of the adsorbent,but also the species forms of Cr.Below pH6,the adsor-bent surface charge is positive due to protonation of oxygen atoms of the naked MNPs or those of the ester group of P(MMA)presented in the P(MMA)-g-TG-MNPs,which were bene?cial for the adsorption of the predominate forms of Cr species,i.e.CrO42?,and HCrO4?forms (Fig.S3a).In addition,hydrogen bond breaking in polymeric chains leads to an increase in the thickness of the polymeric layer(Fig.S3b). So,at low pH values,trapping of Cr(VI)in the polymeric layer is an alter-native mechanism for description of Cr(VI)adsorption by P(MMA)-g-TG-MNPs.As the pH increased,the positive surface charge of the adsor-bent decreased,leading to negligible electrostatic interactions of Cr(VI) with the adsorbent.Nevertheless,appreciable removal ef?ciency was also observed,suggesting that the electrostatic attractions are not the only driving force in the Cr(VI)adsorption,and other mechanisms such as complex reaction may also contribute in the adsorption process (Fig.S3a).Although,with increasing pH,the polymeric layer thickness decreased,trapping of Cr(VI)in polymeric layer is still involved in the adsorption mechanism with little effect.As a result,the ability of the P(MMA)-g-TG-MNPs for adsorption of Cr(VI)within the studied wide pH range,comparing with the naked MNPs,is obvious.It is clear that Cr(III)could not be adsorbed by P(MMA)-g-TG-MNPs at lower pH values than6,because of repulsion between the same charges of the adsorbate and the adsorbent.At high pH values,the surface charge of P(MMA)-g-TG-MNPs becomes negative,resulting in an increase of Cr(III)adsorption. Thus,the pH range of2–6was studied for selective removal of Cr(VI)in the presence of Cr(III),because of minimum removal of Cr(III)in this pH range(Fig.2B).Among the studied adsorbents,the role of P(MMA)-g-TG-MNPs for selective removal of Cr(VI)species was obvious at higher pH above5.The experiments were not conducted at initial pH values above6,because precipitation of Cr(OH)3is likely occur.Moreover, Cr(VI)was partially reduced to Cr(III)species by the reductive hydroxyl groups on the adsorbent.Finally,the pH range of5.0–6.0was selected.
3.3.Optimization of adsorption
3.3.1.Screening
In this study,a full factorial design was used for screening and deter-mination of important variables affecting the selective removal of Cr(VI).Table1shows the main variables,their symbols and levels.The levels of the variables were chosen according to preliminary experi-ments.According to the preliminary experiments,pH(A),adsorbent dose(B),initial concentration of Cr(VI)solution(C)and contact time (D)were selected as variables.The experiments which were designed at two levels of variables were conducted in a randomized manner. The design matrix with the corresponding response is given in Table S1shows the experimental conditions with response values obtained from the experiments.Considering three factors,eight factorial,six axial and six center points,this design involves20experiments which were performed in a random order.
The contribution of linear,quadratic and interaction terms to the re-gression polynomials was calculated using an analysis of variance (ANOVA).A quadratic model correlating removal ef?ciency and interac-tive factors based on a CCD model was evolved by Design Expert soft-ware.The model F value for the response was45.73,indicating the R
e
m
o
v
a
l
(
%
)
pH
20
40
60
80
100
120
012345678 R
e
m
o
v
a
l
(
%
)
pH
Fig.2.Removal ef?ciency of(A)Cr(VI)at different pH values using MNPs,and P(MMA)-g-TG-MNPs as adsorbents,(B)Cr(VI)and Cr(III)at different pHs using P(MMA)-g-TG-MNPs as an adsorbent.
140S.Sadeghi et al./Materials Science and Engineering C45(2014)136–145
signi?cance of the model.According to ANOVA,it is observed that the B, C,D,B2and D2of the model terms were signi?cant for these values of “Prob N F”greater than0.05,while other terms such as C2,CD,BD and BC were not signi?cant(Table3).
The adaption of?t of the proposed second polynomial model was speci?ed by the coef?cient of determination in backward modes to eliminate the not signi?cant factors(R2,adjusted R2,predicted R2and adequate precision).R2and adjusted R2for the selected model equaled to0.9808and0.9595,respectively.The low difference between adjusted R2versus R2value represents the goodness of data?t.The predicted R2 obtained was0.8122which is acceptable.Herein,the signal to noise ratio estimated by the“Adeq.precision”parameter was equal to 20.095.Ratios greater than4indicate adequate model discrimination.
The removal ef?ciency,as the response,for Cr(VI)species was math-ematically?tted by means of response surface regression and postulated a quadratic model,including interaction terms according to coded Eq.(2):
R%
eT2?t6440:71t2838:95B?2027:76C
t567:63D?522:36B2–356:22BC?471:30D2:e2T
The magnitude of the coef?cients in Eq.(2)exhibits the impact of the effect,while the sign determines the nature of in?uence of factors.
The normal probability plot of the residuals and the plot of the resid-uals versus the predicted response were used for the investigation of model adequacy and also veri?cation of the obtained results(Fig.S5a). It is apparent that the residuals commonly fall on a straight line in a nor-mal probability plot,supporting?tness of the least-square?t.The plot of the residuals versus the predicted values(Fig.S5b)revealed that there is no assignable pattern and anomalous structure.These?gures proved that experiments were performed randomly and errors of response were distributed normally.Consequently,the obtained model for the re-sponse has required adequacy.
3.3.3.Effect of experimental variables on removal of Cr(VI)
The response surface plots illustrate combined effects of the signi?-cant variables of contact time,adsorbent dose,and Cr(VI)initial concen-tration on removal ef?ciency of Cr(VI)in which one of the variables was kept constant at the middle value.Fig.3A shows that although both con-tact time and adsorbent dose variables have positive effects on removal of Cr(VI),the variation of adsorbent dose has a higher in?uence on the removal of Cr(VI)than the variation of contact time.This result is con-sistent with the F-value in Table2.Also,there is no signi?cant interac-tion between these variables.The best removal ef?ciency was obtained at contact time over3.4h.The adsorbent dose is one of the most important variables in the adsorption process.Fig.3B shows the shape of the response surface plot representing the variation of adsor-bent dose according to the variation of the initial Cr(VI)concentration. The removal ef?ciency seems to be ef?cient when adsorbent dose was greater than3.0g L?1and initial concentration near30mg L?1,approximately.The high adsorbent dose and low initial Cr(VI)concen-tration were favored for maximal removal ef?ciency of Cr(VI)species due to more available sorption active sites at high adsorbent dose.In ad-dition,the interaction between initial Cr(VI)concentration and adsor-bent dose was not signi?cant.Fig.3C shows that the removal ef?ciency was increased by decreasing the initial Cr(VI)concentration, while it was increased slightly as contact time increased.This may be due to the fact that all the active adsorption sites have been utilized at higher Cr(VI)concentrations at a certain time.There was no interaction between contact time and initial Cr(VI)concentration.A satisfactory re-sponse was found when contact time was3.4h;adsorbent dose was 3g L?1,and initial Cr(VI)concentration was less than20mg L?1at pH5.5.The optimum predicted response with95%con?dence level was94.0%for Cr(VI)removal.The veri?cation of the predicted result was accomplished by performing experiments using the optimized con-ditions.The experimental removal ef?ciency of Cr(VI)was found to be 97.8%that was in close agreement with the CCD model predictions.
3.4.Adsorbent capacity
In order to determine the adsorption capacity of P(MMA)-g-TG-MNPs,30mg portions of the adsorbent were added to10mL solutions containing various initial concentrations of Cr(VI)and the mixture was shaken for3.4h.The Cr(VI)adsorption capacity was calculated using the following equation:
q?
C o?C e
w
?Ve3T
where q is the capacity of adsorbent(mg g?1),C o and C e are the Cr(VI) initial and equilibrium concentrations(mg L?1),respectively,V is the vol-ume of the Cr(VI)solutions(L)and w is the weight of adsorbent(g).The results showed that the adsorption capacity increased with an increase in initial Cr(VI)concentration from5to30mg L?1that provides maximum mass transfer from the aqueous phase to solid adsorbent.
By plotting q versus C e,the maximum adsorption capacity of P(MMA)-g-TG-MNPs for Cr(VI)was obtained to be7.64mg g?1.The experimental adsorption capacity was compared with the adsorption capacity predicted by the three adsorption isotherms of Langmuir, Freundlich,and Temkin.
The Langmuir isotherm is based on monolayer adsorption on the ac-tive sites of the adsorbent,while the heat of adsorption is constant for all sites[34],while the Freundlich isotherm is an empirical model which is used to describe multilayer adsorption on heterogeneous surfaces[35]. The linear forms of the Langmuir and Freundlich isotherms are given by Eqs.(4)and(5),respectively:
1
q e
?1
q m
t1
q mábáC e
e4Tlogq e?logk ft
1
n
logC ee5T
where q e(mg g?1)is the equilibrium value for removal of Cr(VI)per unit weight of adsorbent,b(mL mg?1)is the Langmuir constant related to the energy of adsorption,C e(mg L?1)is the equilibrium concentration of Cr(VI)in solution,and K f and n are the Freundlich isotherm constants which are related to the adsorption capacity and intensity of the adsor-bent,respectively.The adsorption condition is favorable for n N1values.
The Temkin isotherm describes the behavior of the adsorption system on heterogeneous surfaces and it is expressed by the following linear equation[36]:
q e?
RT
b
lnk Tt
RT
b
lnC ee6T
Table1
Factors,factor notation,and their levels in FFD and CCD.
Factor Notation Levels
?αa?10+1+α
FFD
Initial pH A–5 5.56–Adsorbent dose(mg mL?1)B–123–Initial Cr(VI)concentration(mg L?1)C–102030–Contact time(h)D–234–
CCD
Adsorbent dose(mg mL?1)B0.32123 3.68 Initial Cr(VI)concentration(mg L?1)C 3.1810203036.82 Contact time(min)D 1.32234 4.68
a For this rotatable CCD,axial distance is1.68.141
S.Sadeghi et al./Materials Science and Engineering C45(2014)136–145
where R is the gas constant (8.341J mol ?1K ?1),T is the absolute tem-perature (K),K T is the equilibrium binding constant (L g ?1),and b T is a constant related to the heat of adsorption (J mol ?1).
The estimated parameters of the models with corresponding corre-lation coef ?cient R 2are summarized in Table 3.The result of comparison of experimental and calculated adsorption capacity was in high accor-dance with the Langmuir isotherm model.Besides,the high value of the Langmuir constant (b)in Table 3re ?ects a high af ?nity between the adsorbate and adsorbent which is indicative of the chemisorption process.Furthermore,a dimensionless separation factor,R L ,obtained
from the Langmuir model describes the tendency of the adsorption pro-cess and can be calculated from the following equation:R L ?1tbC 0eT
?1
e7T
where C i is the initial concentration of Cr(VI)in aqueous solution.For the favorable Langmuir isotherm,the values of R L should be between 0and 1.Greater af ?nity between adsorbent and adsorbate is inferred when R L is smaller.The R L value of 0.03for the P(MMA)-g -TG-MNPs signi ?es favorable adsorption of Cr(VI).Further,the linear plot for
the
Fig.3.Response surface plots,and interactions of (A)contact time and adsorbent dose,(B)adsorbent dose and initial Cr (VI)concentration,and (C)contact time with initial Cr(VI)concentration.
Table 2
Analysis of variance (ANOVA)for CCD.Source Sum of squares Degree of freedom Mean square F value a p-Value b Prob N F Model 1.78E+089 1.98E+0745.73b 0.0001Signi ?cant c B 1.10E+081 1.10E+08254.55b 0.0001Signi ?cant C 5.58E+071 5.58E+07128.97b 0.0001Signi ?cant D 4.40E+061 4.40E+0610.180.0128Signi ?cant BC 1.02E+061 1.02E+06 2.350.164Not signi ?cant BD 2.48E+051 2.48E+060.57
0.4724Not signi ?cant CD 1163.5511163.5 2.69E ?070.9599Not signi ?cant B 2 3.93E+061 3.93E+069.090.0167Signi ?cant C 299,298.38199,298.380.230.4646Not signi ?cant D 2
3.20E+061 3.20E+067.40.0263Signi ?cant Residual 3.46E+068
4.32E+05Lack of ?t 3.17E+065 6.34E+05 6.56
0.0762
Not signi ?cant
Pure error 2.90E+05396,629.28
Cor.total d
1.86E+08
19
a Test for comparing model variance with residual (error)variance.
b Probability of ?nding the observed F-value if the null hypothesis is true.
c Signi ?cant at P b 0.05.
d
Totals of all information corrected for the mean.
142S.Sadeghi et al./Materials Science and Engineering C 45(2014)136–145
Temkin adsorption isotherm with high correlation coef ?cient of ?tting data (0.946)supports that the adsorption of Cr(VI)onto P(MMA)-g -TG-MNPs is a chemisorption process (Table 3).3.5.The effect of temperature
The effect of temperature on removal ef ?ciency of Cr(VI)by P(MMA)-g -TG-MNPs was studied in the temperature range of 298–318K with constant initial Cr (VI)concentrations of 10,20,and 30mg/L and shown in Fig.S6A.The removal ef ?ciency of P(MMA)-g -TG-MNPs was found to decrease with increasing temperature from 298K to 318at different concentrations,indicating that the adsorption of Cr (VI)onto P(MMA)-g -TG-MNPs was more favorable at lower tem-peratures.Based on these results,it implies that the adsorption process was controlled by an exothermic process.
The experimental data obtained at different temperatures can be used in calculating the thermodynamic parameters (Fig.S6B).The values of standard enthalpy ΔH 0(kJ mol ?1)and entropy of adsorption ΔS 0(kJ mol ?1K ?1)were calculated from the slope and intercept of a Van't Hoff plot (ln K 0versus 1/T),respectively,according to the follow-ing equations:ΔG o
??RT lnK 0
e8T
lnK 0?
ΔS 0R ?ΔH 0
RT
e9T
K 0?
q e C e
e10T
where K 0is the equilibrium constant obtained from the adsorption iso-therm at different temperatures.Thermodynamic parameters for the removal of Cr(VI)by the proposed adsorbent are summarized in Table 3.The negative standard Gibbs free energy ΔG 0(kJ mol ?1)values at all investigated temperatures indicated that the adsorption process was feasible and spontaneous.The obtained negative ΔH 0value showed that the Cr(VI)adsorption process by P(MMA)-g -TG-MNPs has an
exothermic nature.Also,the negative value of ΔS 0indicated greater order of reaction during adsorption of Cr(VI)onto P(MMA)-g -TG-MNPs [37,38].3.6.Kinetic study
In order to understand the behavior of the adsorbent and to examine the controlling mechanism of the adsorption process,the experimental capacities obtained at various times was used (Fig.4A).In this study,kinetic aspects of the adsorption process in a solid –liquid system were considered based on three kinetic models:pseudo-?rst-order,pseudo-second-order,and intraparticle diffusion models [39].
The pseudo-?rst-order rate expression of Lagergren based on the adsorption capacity of solid sorbent is generally expressed as follows:log q e ?q t eT?logq e ?
k 1;ads t
2:303
:e11T
Ho and McKay's pseudo-second order kinetic model can be expressed as Eq.(12):t q t ?1k 2;ads áq 2e tt q e e12T
which
h ?k 2;ads áq 2
e
e13T
where q e ,and q t are the amounts of Cr(VI)ions adsorbed on the surface of P(MMA)-g -TG-MNPs at equilibrium (mg g ?1)and at any time t(mg g ?1),respectively,k 1and k 2are the rate constant of pseudo-?rst order (min ?1)and pseudo-second order (g mg ?1min ?1)adsorption,respectively,and h is the initial adsorption rate (mg g ?1min ?1).
The equation constants can be determined from the linear plots of log (q e ?q t )versus t and t/q t versus t (Fig.4B,C)which are listed in Table 3.The best ?t between the two kinetic models is assessed by the linear coef ?cient of determination (R 2)and calculated adsorption ca-pacity.It can be seen that the correlation coef ?cient of the pseudo-second-order kinetic model is much better than that of the pseudo-?rst-order kinetic model.Also,according to this model,equilibrium
Table 3
Equilibrium isotherm,thermodynamic,and kinetic parameters for the removal of Cr(VI)by P(MMA)-g -TG-MNPs.Isotherm models Langmuir
Freundlich Temkin
q max (mg g ?1
)=7.81K F (mg g ?1
)=4.4b T (KJ mol ?1)=1839.33b =1.83;R L =0.03n =3.3K T (L g ?1)=40.93R 2=0.936
R 2=0.957
R 2=0.946
Thermodynamic parameters Temperature (K)
Parameters ΔG o (KJ mol ?1)
ΔS o (J mol ?1K ?1)ΔH o (KJ mol ?1)298?15.367?319.92
?97.5
303?3.521308?2.478313?1.603318
?1.095
Kinetic models C 0(mg L ?1)
Pseudo-?rst-order kinetics Pseudo-second-order kinetics Intraparticle diffusion q e (calc.)(mg g ?1)
K 1(min ?1)R 2K 2(g mg ?1min ?1)h (mg g ?1min ?1)R 2q e (calc.)(mg g ?1)K id (mg g ?1min ?0.5)C (cm 2s ?1)R 210 2.950.01150.9630.02080.23760.994 3.380.135 1.4230.96920 5.230.01150.9810.00450.20050.964 6.540.3800.5810.98030
7.91
0.0161
0.974
0.0007
0.7547
0.930
8.41
0.506
0.071
0.991
q e (exp;mg g ?1):3.23(at 10mg L ?1);6.31(at 20mg L ?1);7.84(30mg L ?1).
143
S.Sadeghi et al./Materials Science and Engineering C 45(2014)136–145
capacity of adsorbent is more close to experimental amount of capacity.Therefore,it may be concluded that Cr(VI)adsorption onto the pro-duced adsorbents obeyed the pseudo-second-order kinetic model.
When the solid –liquid adsorption process is studied,the transfer of adsorbate from aqueous solution to the sorbent particles is generally characterized by either ?lm or intraparticle diffusion or both.In this study,the adsorption data was also analyzed in terms of an intra-particle diffusion mechanism.The intraparticle diffusion model was used to examine the adsorption mechanism according to the Eq.(14):q t ?K id t
0:5
tC e14T
where K id (mg g ?1min ?0.5)is the rate constant for intra-particle diffu-sion adsorption and C is the intercept.The values of rate constant can be calculated by plotting q t versus t 0.5(Fig.4D).The value of the intercept C provides information related to the thickness of the boundary layer.The mechanism of adsorption based on the intraparticle diffusion model is generally considered to be in three steps,i.e.adsorption of the adsorbate on the external surface of the adsorbent (boundary layer diffusion),intraparticle diffusion,and ?nal equilibrium step.One or any combina-tion of these steps can be the rate-controlling mechanism.The rate of the ?rst step depends on the binding process (physical or chemical)and the sharpness of this portion is attributed to the high diffusion of adsorbate through the solution to the external surface of adsorbent.This step is often assumed to be extremely rapid [34,40].
The multi-linearity of the q t vs.t 0.5plot for Cr(VI)removal by provid-ed adsorbent (Fig.4C)indicated that adsorption took place in three steps.As can be seen from this ?gure,the slope of the ?rst portion was higher than the second portion,which implied that diffusion of Cr(VI)in bulk phase to the exterior surface of adsorbent is faster than intra-particle diffusion of Cr(VI)into the P(MMA)-g -TG-MNPs in the ad-sorption process.Because,the plot of q t versus t 0.5does not pass through the origin,it can be concluded that intraparticle diffusion is not the only
rate-limiting step for the whole reaction and the boundary layer diffu-sion controlled the adsorption to some https://www.wendangku.net/doc/b015918573.html,paring the slope of the second linear portions of different initial concentrations of Cr(VI)in the time interval of 20–120min showed the enhancement in diffu-sion of Cr(VI)into the adsorbent with increasing of initial concentration.The calculated intraparticle diffusion coef ?cient,K id value and intercept are listed in Table 3.The low value of intercept veri ?ed that surface ad-sorption had low contribution in the rate determinant step.3.7.Effect of coexisting metal ions
Effects of alkali,alkaline earth and some transition metals which are the commonly coexisting ions with Cr(VI)in natural waters and indus-trial ef ?uents on the removal ef ?ciency of Cr(VI)were investigated under optimum conditions.This study was performed with varying ini-tial concentration of coexisting ions (0–1000mg L ?1)by keeping the constant concentration of Cr(VI)at 20mg L ?1.The results are shown in Table 4.It can be seen that there is no interference for removal of Cr(VI)in the presence of more than 1000fold of the studied ions includ-ing Cr(III)ions.3.8.Application
In order to further evaluate the effects of different matrices on the removal ef ?ciency of Cr(VI),the electroplating waste water and well water (Industrial town area,Birjand,Iran)spiked with various amount of Cr(VI)were used in the removal experiment.The electroplating wastewater and well water contained 10mg L ?1and 100μg L ?1Cr(VI),respectively.The pH of waters was adjusted to 5.5before pro-ceeding with the standard addition method.The obtained results in Table 5revealed that the removal ef ?ciencies up to about 20μg mL ?1Cr(VI)were over 89.0%which indicated that the coexisting ions had
q t
t(min)
L o g (q e -q t )
Time(min)
t /q t
t (min)
t 0.5 (min 0.5)
q t
Fig.4.(A)Effect of concentration on the sorption kinetics of Cr(VI)by P(MMA)-g -TG-MNPs,and the plots of (B)pseudo-?rst order,(C)pseudo-second order,and (D)intra-particle dif-fusion kinetic models of Cr(VI)adsorption onto P(MMA)-g -TG-MNPs at different initial concentrations of the adsorbate.
144S.Sadeghi et al./Materials Science and Engineering C 45(2014)136–145
little impact on the removal ef?ciency of Cr(VI)and this adsorbent can be used to purify these waters containing toxic Cr(VI).
Although the sorption capacity was lower compared to some adsor-bents[17,20],it is higher or comparable than the other adsorbents[41–44].Furthermore,the sorption of Cr(VI)by the P(MMA)-g-TG-MNPs was possible at a wide pH range(2–6),which could facilitate the appli-cation of the adsorbent for wastewater treatment.Moreover,high selec-tivity of the prepared adsorbent to Cr(VI)in the presence of Cr(III),ease of separation from the bulk solution,lower adsorbent dosage(3.0g/L), and high percent removal(N95%),make this adsorbent more ef?cient and useful compared to most of other adsorbents[20,41–43].
4.Conclusion
A new adsorbent produced from MNPs and grafted TG for the selec-
tive removal of Cr(VI)from aqueous solutions was introduced.Optimi-zation of the experimental conditions to maximize the removal ef?ciency of Cr(VI)was achieved using the statistical experimental de-sign methodology.The CCD shows that the initial concentration (C)has a negative effect on the removal ef?ciency(response)of Cr(VI),whereas adsorbent dose(B)and contact time(D)have positive effects on the response.The proposed quadratic model obtained by CCD agreed well with the experimental data with correlation coef?cients (R2)of0.9808.The Cr(VI)adsorption behavior on the P(MMA)-g-TG-MNPs sorbent was?tted with the Langmuir isotherm model,indicating that homogeneous adsorption occurred.Kinetic study veri?ed that the adsorption of Cr(VI)by the prepared adsorbent obeyed the pseudo-second-order kinetic model.The proposed method has been successful-ly employed for the determination of Cr(VI)species in the presence of Cr(III).The coexisting ions such as Cr(III)ions have no obvious effects on Cr(VI)adsorption,suggesting that the prepared adsorbent can be used as highly ef?cient and economically viable adsorbent for selective Cr(VI)removal.Furthermore,the adsorbent can be utilized over a wide pH range which is applicable for waste waters with various pH values. The developed method is characterized with simplicity,selectivity,safe-ty,low-cost and easy operation,because of the distinct and advanta-geous features of MNPs.
Acknowledgment
Authors are grateful to the Research Council of Birjand University for funding this work.
Appendix A.Supplementary data
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Table4
Effect of diverse ions on removal ef?ciency of Cr(VI)by P(MMA)-g-TG-MNPs.
Interfering ion Removal
ef?ciency(%)Amount of reduction in removal ef?ciency of Cr(VI)
None98.5–NaCl94 4.5 CaCl293.6 4.9 NaNO394.9 3.6 KCl94.54 Cu(NO3)2980.5 Cr(NO3)3.9H2O91.7 6.8 Mg(NO3)2980.5 Na2SO494.54 Na(CH3COO)98.40.1 Zn(CH3COO)297.80.7 MnCl297.90.6 NiCl298.20.3 NaHCO394.4 4.1 Cd(CH3COO)2980.5Table5
Investigation of Cr(VI)removal in real samples by P(MMA)-g-TG-MNPs.
Real samples Amount of added Cr(VI)
μg mL?1
Removal ef?ciency of
Cr(VI)
Underground water097.8±1.4
1093.6±2.2
2083.0±2.5
3076.4±1.1
4066.9±3.5
5059.8±2.5 Electroplating water096.2±1.2
592.8±0.7
1092.6±0.7
1591.0±0.6
2089.0±0.9
145
S.Sadeghi et al./Materials Science and Engineering C45(2014)136–145