materials Article
Chromium(VI) Removal from Aqueous Solution by Magnetite Coated by a Polymeric Ionic Liquid-Based Adsorbent Thania Alexandra Ferreira 1 , Jose Antonio Rodriguez 1 , María Elena Paez-Hernandez 1 , Alfredo Guevara-Lara 1 , Enrique Barrado 2 and Prisciliano Hernandez 3, * 1
2 3
*
Area Academica de Quimica, Universidad Autonoma del Estado de Hidalgo, Carr. Pachuca-Tulancingo Km. 4.5, Mineral de la Reforma, Hidalgo 42184, Mexico;
[email protected] (T.A.F.);
[email protected] (J.A.R.);
[email protected] (M.E.P.-H.);
[email protected] (A.G.-L.) Departamento de Química Analítica, Facultad de Ciencias, Universidad de Valladolid, Paseo de Belén 7, Valladolid 47011, Spain;
[email protected] Área de Energías, Universidad Politécnica de Francisco I. Madero, Domicilio Conocido, Tepatepec, Hidalgo C.P. 42640, Mexico Correspondence:
[email protected]; Tel.: +52-738-7241174
Academic Editor: Eric Guibal Received: 7 April 2017; Accepted: 28 April 2017; Published: 6 May 2017
Abstract: An evaluation of the chromium(VI) adsorption capacity of four magnetite sorbents coated with a polymer phase containing polymethacrylic acid or polyallyl-3-methylimidazolium is presented. Factors that influence the chromium(VI) removal such as solution pH and contact time were investigated in batch experiments and in stirred tank reactor mode. Affinity and rate constants increased with the molar ratio of the imidazolium. The highest adsorption was obtained at pH 2.0 due to the contribution of electrostatic interactions. Keywords: chromium(VI); magnetic particles; ionic liquid; adsorption capacity
1. Introduction Chromium(VI) is a highly toxic species; it is considered on the priority list of highly toxic pollutants by the Environmental Protection Agency of the United States (EPA), which has established 50 µg/L as the maximum permitted level for chromium(VI) [1]. The main source of chromium(VI) is associated with anthropogenic activities such as electroplating, textile industries, and pigments. Depending on the pH conditions and concentration of the media, this element can be found as CrO4 2− , HCrO4− , or Cr2 O7 2− ; these species are hard oxidants, and have high solubility in water, making them a potential danger to living organisms. Chromium(VI) has negative consequences for human health. Besides causing skin irritation, chromium(VI) compounds are considered carcinogenic and mutagenic from group A according to the international agency for research on cancer [2,3]. There is a wide range of techniques for the selective removal of chromium(VI) from water, such as ultrafiltration [3], liquid–liquid extraction [4], ion exchange [5], electrochemical removal [6], and in recent years, detoxification by the presence of microorganisms [7]. Nevertheless, the most widely-used technique is adsorption because of its advantages above the other techniques: high efficiency, low cost, minimum use of organic solvents, simplicity, and reusability. Chromium(VI) adsorption has been carried out with different sorbents, including clays [8], chitosan [9], nanocomposites [10], activated carbon [11], biosorbents [12–15], and recently, magnetic particles [16]. Magnetic materials have been considered useful because they can be modified to improve selectivity and adsorption Materials 2017, 10, 502; doi:10.3390/ma10050502
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have been considered useful because they can be modified to improve selectivity and adsorption processes [16], and they can also be easily separated from the media by applying an external magnetic field, minimizing secondary [17,18]. processes [16], and they canpollution also be easily separated from the media by applying an external magnetic Sorbents basedsecondary on iron oxide particles have been used for this purpose in the past few years. In field, minimizing pollution [17,18]. all cases, the magnetic particles’ surfaces have been functional polymers orderIntoall Sorbents based on iron oxide particles have beenmodified used for with this purpose in the past fewinyears. avoid the formation of aggregates in solution, also conferring cases,air theoxidation magnetic and particles’ surfaces have been modified with functional polymers selectivity in order to and avoid stability to the magnetic particles of [19]. There areinexamples of the recovery selectivity of heavy metals, including air oxidation and the formation aggregates solution, also conferring and stability to the Cd(II), Cu(II), Ni(II), and chromium(VI) by maghemite coated with polyethylene glycol [20], magnetic particles [19]. There are examples of the recovery of heavy metals, including Cd(II), Cu(II), magnetic gelatins [18], catecholamine-coated maghemite nanoparticles [21],[20], andmagnetic polypyrrole-coated Ni(II), and chromium(VI) by maghemite coated with polyethylene glycol gelatins [18], magnetite [19]. catecholamine-coated maghemite nanoparticles [21], and polypyrrole-coated magnetite [19]. InInaddition, thethe presence ofof functional groups such asas –OH and ononthe addition, presence functional groups such –OH and–COOH –COOH thesurface surfacecan can enhance the interaction with anions due toto electrostatic interactions. Treatment performed atat low pH enhance the interaction with anions due electrostatic interactions. Treatment performed low pH values promotes the formation of positive charges on the solid surface and favors the electrostatic values promotes the formation of positive charges on the solid surface and favors the electrostatic attraction with negatively-charged chromium(VI) attraction with negatively-charged chromium(VI)species species[18]. [18]. On the other hand, the use ofof ionic liquids (IL) in in solid phase extraction has gained interest [4].[4]. On the other hand, the use ionic liquids (IL) solid phase extraction has gained interest InIn recent years, these compounds have been physically or chemically immobilized in solids [22]. recent years, these compounds have been physically or chemically immobilized in solids [22]. Nano-silica has hexafluorophosphate forfor Pb(II) Nano-silica hasbeen beenmodified modifiedwith with1-butyl-3-methylimidazolium 1-butyl-3-methylimidazolium hexafluorophosphate Pb(II) adsorption; the synthesis of the adsorbent was based on the physical adsorption of the IL on the adsorption; the synthesis of the adsorbent was based on the physical adsorption of the IL on the surface surface of activated nano-silica by suspending the silica particles in a solution containing theInteraction IL [23]. of activated nano-silica by suspending the silica particles in a solution containing the IL [23]. Interaction between the and sorbent the analyte is attributed to physical interactions der Waals between the sorbent the and analyte is attributed to physical interactions (Van (Van der Waals forces, forces, hydrogen bonding), chemical interactions (bond formation), electrostatic interactions, the hydrogen bonding), chemical interactions (bond formation), electrostatic interactions, the formation formation of coordination complexes via theatoms, donororatoms, or ionic exchange [23,24]. Alternatively, of coordination complexes via the donor ionic exchange [23,24]. Alternatively, IL can be ILimmobilized can be immobilized using them as monomers for the preparation of polymers [25]. It has beenthe using them as monomers for the preparation of polymers [25]. It has been proved that proved that the use of IL for the adsorption of chromium(VI) enhances the desired behavior of the use of IL for the adsorption of chromium(VI) enhances the desired behavior of the sorbent, improving sorbent, improving its adsorption capacity and selectivity the ion of interest [26]. its adsorption capacity and selectivity towards the ion oftowards interest [26]. Poly(ionic liquids) (PILs) have gained considerable attention in the past few years because these Poly(ionic liquids) (PILs) have gained considerable attention in the past few years because these materials possess physical and chemical properties covering a wide range ofof applications. They are materials possess physical and chemical properties covering a wide range applications. They are considered can be be used usedas assolid solidion ionconductors, conductors, sorbents, consideredasasmultifunctional multifunctionalpolyelectrolytes polyelectrolytes that that can asas sorbents, and and in catalysis. Yuan al. described the synthesis of PIL-based core–shell nanoparticles using in catalysis. Yuan et al.etdescribed the synthesis of PIL-based core–shell nanoparticles using inorganic inorganic and cores organic coresuse for in their use in separation techniques [27],the combining unique IL and organic for their separation techniques [27], combining unique ILthe properties and properties and the small dimension of nanoparticles that amplifies the surface features, giving rise to the small dimension of nanoparticles that amplifies the surface features, giving rise to a new class of a new class of polymeric materials. PILs are obtained via radical polymerization of the some IL monomer; polymeric materials. PILs are obtained via radical polymerization of the IL monomer; examples some examples of PIL are in Figure 1 [28]. of PIL structures are structures pointed out inpointed Figure 1out [28]. Therefore, sorbents coated coated with withpolymers polymersbased basedon Therefore,this thiswork workproposes proposesthe thesynthesis synthesis of of magnetic magnetic sorbents on1-allyl-3-methylimidazolium 1-allyl-3-methylimidazoliumfor forthe theremoval removalofofchromium(VI) chromium(VI)from fromwater. water.
Figure 1. Chemical structures recently reported cationic poly (ionic liquids) (PILs) [28]. Figure 1. Chemical structures recently reported forfor cationic poly (ionic liquids) (PILs) [28].
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2. Results and Discussion Materials 2017, 10, 502
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2.1. Structural Characterization 2. Results and Discussion Materials 2017, 10, 502
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The synthesized sorbents were characterized by Fourier transform infrared spectroscopy (FTIR) 2.1. Structural 2. Results andCharacterization Discussion in order to evaluate the functional groups present in the solids (Figure 2). For the magnetite (Figure 2a), The synthesized sorbents were characterized by Fourier transform infrared spectroscopy (FTIR) Structural a band2.1. at 560 cm−1Characterization is characteristic for the bending vibration of the Fe–O bonds; this is also observed in in order to evaluate the functional groups present in the solids (Figure 2). For the magnetite (Figure 2a), a −1 and 1722 cm−1 correspond to the presence the modified sorbents Bands observed at 1137 synthesized sorbents were characterized by cm Fourier transform infrared (FTIR) bandThe at 560 cm−1 is(b–d). characteristic for the bending vibration of the Fe–O bonds; thisspectroscopy is also observed in −1 and −1 of C–O–C and groups in the magnetite-polymer (Figure 2b–d) due the presence of ethylene in order to C=O evaluate the functional groups present the cm solids (Figure 2). theto magnetite 2a), a the modified sorbents (b–d). Bands observed at in 1137 1722 cmFor correspond to (Figure the presence band at 560and cm−1 is(EGDMA) characteristic the bending vibration of the bonds; this is also observed in with glycolof dimethacrylate amagnetite-polymer cross-linking agent. For theFe–O spectra the magnetite coated C–O–C C=O groups in as thefor (Figure 2b–d) due to of the presence of ethylene 1coated the modified sorbents(EGDMA) (b–d). Bands observed at 1137 cm−1For and cm−1ofcorrespond presence glycol dimethacrylate as aas cross-linking agent. the1722 spectra the1635 magnetite with 1-allyl-3-methylimidazolium chloride monomer (Figure 2c,d), a band at cmto−the characteristic of −1 of C–O–C and C=O groups in the magnetite-polymer (Figure 2b–d) due to the presence of ethylene 1-allyl-3-methylimidazolium chloride monomer [25]. (Figure 2c,d), a band at 1635 cm characteristic of the C=C bond of the imidazolium ring isasobserved glycol dimethacrylate (EGDMA) as a cross-linking For the spectra of the magnetite coated with the C=C bond of the imidazolium observed agent. The morphology of the particlesring wasisstudied by[25]. scanning electron microscopy. The micrograph of 1-allyl-3-methylimidazolium chloridewas as monomer (Figure 2c,d), a bandmicroscopy. at 1635 cm−1 The characteristic of The morphology of the particles studied by scanning electron micrograph bare magnetite particles (Figure 3a) shows the formation of spherical particles with diameter around thebare C=C magnetite bond of theparticles imidazolium ring observed of (Figure 3a)is shows the[25]. formation of spherical particles with diameter 50 nm.around ForThe coated particles 3b), itbyisscanning possible to observe the formation of aggregates. of the particles(Figure was studied microscopy. Theformation micrograph 50morphology nm.magnetite For coated magnetite particles (Figure 3b), it iselectron possible to observe the of Modifying themagnetite magnetite surface with polymer gives the particles greater stability in solution of bare particles (Figure 3a)surface showscoatings the formation of spherical particles with diameter aggregates. Modifying the magnetite with polymer coatings gives the particles greater and avoids [18,19]. aroundair 50 nm. For coated magnetite particles (Figure 3b), it is possible to observe the formation of stability inoxidation solution and avoids air oxidation [18,19]. aggregates. Modifying the magnetite surface with polymer coatings gives the particles greater stability in solution and avoids air oxidation [18,19].
Figure 2. Fourier transform infrared (FTIR) spectra of the sorbents. (a) Fe3O4; (b) Fe3O4-MAA;
Figure 2. Fourier transform infrared (FTIR) spectra of the sorbents. (a) Fe3 O4 ; (b) Fe3 O4 -MAA; (c) Fe3O4-MAA-IL; (d) Fe3O4-IL. IL: ionic liquid; MAA: methacrylic acid. (c) Fe3Figure O4 -MAA-IL; (d) Fe3 O4 -IL.infrared IL: ionic(FTIR) liquid;spectra MAA:ofmethacrylic acid. 2. Fourier transform the sorbents. (a) Fe3O4; (b) Fe3O4-MAA; (c) Fe3O4-MAA-IL; (d) Fe3O4-IL. IL: ionic liquid; MAA: methacrylic acid.
Figure 3. SEM images obtained of the synthesized adsorbents. (a) Fe3O4; (b) coated Fe3O4. Figure 3. SEM images obtained of the synthesized adsorbents. (a) Fe3O4; (b) coated Fe3O4.
Figure 3. SEM images obtained of the synthesized adsorbents. (a) Fe3 O4 ; (b) coated Fe3 O4 .
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2.2. Adsorption Experiments 2.2. Adsorption Experiments 2.2.1. 2.2.1. Batch BatchStudies Studies and and Effect Effect of of the the Solution Solution pH pH The The experiments experiments to to evaluate evaluate the the equilibrium equilibrium of of adsorption adsorption were were performed performed at at pH pH values values of of 2.0 2.0 and 6.5 in order to evaluate the effect between the surface charge and the chromium(VI). Figure and 6.5 in order to evaluate the effect between the surface charge and the chromium(VI). Figure 44 shows shows the the adsorption adsorption isotherms isotherms for for the the synthesized synthesized sorbents. sorbents. The adsorption isotherms for Cr(VI) show a strong The adsorption isotherms for Cr(VI) show a strong dependence dependence on on the the pH pH value, value,and andititdecreases decreases as the pH increases as a consequence of the charge repulsion between the surface as the pH increases as a consequence of the charge repulsion between the surface of of the the solid solid 2− negatively-charged and the anionic species chromium(VI) CrO 4 .2Adsorption exhibited a dependence on − negatively-charged and the anionic species chromium(VI) CrO4 . Adsorption exhibited a dependence the electrostatic interactions. on the electrostatic interactions. ItIt was observed that that the the synthesized synthesizedsolids solidsFe Fe3O O4, Fe3O4-MAA (methacrylic acid), Fe3O4-MAA-IL, was observed 3 4 , Fe3 O4 -MAA (methacrylic acid), Fe3 O4 -MAA-IL, and and Fe Fe33O O44-IL -ILpresent presentaasignificant significantdifference differencein intheir theiradsorption adsorptioncapacity capacity(Figure (Figure4). 4). For Formagnetite, magnetite, the surface charge is neutral at pH (6.0–7.3); below this value, the surface of the magnetite the surface charge is neutral at pH (6.0–7.3); below this value, the surface of the magnetiteisispositively positively −, favoring the electrostatic attraction charged, chromium(VI) species species is is HCrO HCrO4− charged, and and the the predominant predominant chromium(VI) , favoring the electrostatic attraction 4 and also the adsorption; instead, at pH values higher than pH pzc, the magnetite surface acquires and also the adsorption; instead, at pH values higher than pHpzc , the magnetite surface acquires negative with the the predominant predominant chromium(VI) chromium(VI)species speciesCrO CrO422−−. . negative charge, charge, causing causing electrostatic electrostatic repulsions repulsions with 4 In In the the case case of of magnetite magnetite covered covered with with polymer polymer phase, phase, the the groups groups such such as as –OH –OH and and –COOH –COOH can can be be protonated protonated at at low low pH pH values, values, causing causing the the formation formation of of positive positive charges charges on on the the surface, surface, improving improving the the interaction chromium(VI)anions anionsbecause becauseofofthe thepresence presence electrostatic attraction [18]. When interaction with with chromium(VI) of of electrostatic attraction [18]. When the the polymer phase is composed of the imidazolium salt, an increase in the adsorption capacity is polymer phase is composed of the imidazolium salt, an increase in the adsorption capacity is observed. observed. It has been reported that IL-based materials show an increase in selectivity and adsorption It has been reported that IL-based materials show an increase in selectivity and adsorption capacity − of the imidazolium salt capacity due to anion exchange interactions [25], in this case, between thethe Climidazolium due to anion exchange interactions [25], in this case, between the Cl− of salt and the − and the chromium(VI) species HCrO 4 . − chromium(VI) species HCrO4 . On On the the other other hand, hand, chromium(VI) chromium(VI) can can be be reduced reduced to to Cr(III) Cr(III) in in acidic acidic solution solution in in the the presence presence of of organic matter [29]. Complexation phenomena between carbonyl groups (C=O) and Cr(III) can also organic matter [29]. Complexation phenomena between carbonyl groups (C=O) and Cr(III) can also occur, occur, as as oxygen oxygen in in this this group group is is considered considered aa strong strong Lewis Lewis base base capable capable of of complexation complexation with with metal metal cations. Then, a speciation chromium oxidation state on the solid must also be considered in cations. Then, a speciation chromium oxidation state on the solid must also be considered in order order to to propose the adsorption mechanism [30]. propose the adsorption mechanism [30]. 3.0
60.0
4.0
(a)
2.0 0.0 0.000
80.0
(d)
0.005 0.010 Ce (mmol L-1 )
40.0 (c) 20.0
qe (mmol Kg-1)
pH 2.0
qe (mmol Kg-1)
qe (mmol Kg-1)
80.0
qe (mmol Kg-1)
6.0
60.0
pH 6.5
2.0
(a)
1.0 0.0 0.000
(d) 0.005
Ce (mmol
0.010 L -1)
(c)
40.0
20.0 (b)
0.0 0.00
0.03
(b) 0.06 0.09 Ce (mmol L-1)
(A)
0.12
0.0 0.00
0.03
0.06 0.09 Ce (mmol L-1)
0.12
(B)
Figure 4. Effect of pH (A) 2.0 and (B) 6.5 on the adsorption. (a) Fe3O4; (b) Fe3O4-MAA; (c) Fe3O4-MAA-IL; Figure 4. Effect of pH (A) 2.0 and (B) 6.5 on the adsorption. (a) Fe3 O4 ; (b) Fe3 O4 -MAA; (c) Fe3 O4 -MAA-IL; (d) Fe3O4-IL. (d) Fe3 O4 -IL.
Magnetite shows a lower adsorption capacity of chromium(VI) (5.01 mmol/kg at pH 2.0)compared shows a lower adsorption capacity of chromium(VI) (5.01 mmol/kg at increase pH 2.0) to theMagnetite use of coated magnetic particles, with acrylic polymer (Fe3O4-MAA) showing a slight compared to thecapacity use of coated magnetic with polymer (Fethe -MAA) showing 3 O4imidazolium in the adsorption (6.11 mmol/kg at particles, pH 2.0). On the acrylic other hand, adding salt asa slight increase in the adsorption capacity (6.11 mmol/kg at pH 2.0). On the other hand, adding the functional monomer improves the capacity of the solid to retain the chromium(VI) anions, as shown imidazolium saltfor asFe functional monomer improves the capacity of the solid to retain the chromium(VI) in the isotherms 3O4-MAA-IL and Fe3O4-IL. The maximum adsorption capacity is 65.16 mmol/kg for Fe3O4-IL carrying out the adsorption process at pH 2.0.
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Materials as 2017, 10, 502in the isotherms for Fe3 O4 -MAA-IL and Fe3 O4 -IL. The maximum adsorption capacity 5 of 9 anions, shown is 65.16 mmol/kg for Fe3 O4 -IL carrying out the adsorption process at pH 2.0. Once the the isotherms isotherms were were obtained, obtained, Scatchard Scatchard plots plots were were used used to to calculate calculate the the values values of of affinity affinity Once constants for each solid. The values obtained for affinity constants at pH 2.0 for Fe 3O4, Fe3O4-MAA, constants for each solid. The values obtained for affinity constants at pH 2.0 for Fe3 O4 , Fe3 O4 -MAA, Fe3O O4-MAA-IL, and Fe3O4-IL were 40.7, 8.13, 5.01, and 1.41 μM, respectively. An improvement in the Fe 3 4 -MAA-IL, and Fe3 O4 -IL were 40.7, 8.13, 5.01, and 1.41 µM, respectively. An improvement in affinity of theofsolid chromium(VI) was observed by increasing the molar ratio of the imidazolium the affinity the towards solid towards chromium(VI) was observed by increasing the molar ratio of the salt in the polymer phase. The solid with a molar ratio of 4.3:2.0:1.0 (Fe 3O4:EGDMA:IL) was the one that imidazolium salt in the polymer phase. The solid with a molar ratio of 4.3:2.0:1.0 (Fe3 O4 :EGDMA:IL) presented greater adsorptiongreater capacity and the highest affinity at pH valueaffinity of 2.0. Based the results was the one that presented adsorption capacity and the highest at pHon value of 2.0. obtained, pH 2.0 was chosen to carry out kinetic studies for the modified sorbents. Based on the results obtained, pH 2.0 was chosen to carry out kinetic studies for the modified sorbents.
2.2.2. Adsorption Adsorption Kinetics: Kinetics: Stirred Stirred Tank Tank Experiments Experiments 2.2.2. The chromium(VI) chromium(VI) adsorption adsorption with with respect respect to to contact contact time time was was evaluated evaluated at at pH pH 2.0. 2.0. The The results results The are presented presented in in Figure Figure 5A. 5A. The The adsorption adsorption of of chromium(VI) chromium(VI) increases increases with with contact contact time, time, achieving achieving are valuesof of at at least least 70% 70% in in the the first first 120 120 min min with with the the solids solids containing containing IL IL in in the the polymer polymer phase. phase. Removal Removal values efficiency decreases as follows: Fe 3 O 4 -IL > Fe 3 O 4 -MAA-IL > Fe 3 O 4 -MAA. The highest chromium(VI) efficiency decreases as follows: Fe3 O4 -IL > Fe3 O4 3 O4 -MAA. The uptake was was 90.94% 90.94% with with respect respectto tothe theinitial initialCr(VI) Cr(VI)concentration concentrationemployed. employed. uptake Adsorption kinetics was evaluated using pseudo-first-order kinetic model, and results results have have aa Adsorption kinetics was evaluated using pseudo-first-order good linear linear correlation. correlation. The value of the the rate rate constant constant (k) was was calculated calculated from from the the slope slope of of the the linear linear good plot of ofln(q ln(qee − − qqtt))versus correlation plot versus time time (t), (t), as as shown shown in in Equation (5). Adsorption rate constants and correlation coefficient for given in Table 1. In1.allIn cases, resultsresults had a good correlation adjusting coefficient foreach eachsolid solidare are given in Table all cases, had linear a good linear correlation to a pseudo-first-order process. According the results Figure 5Binand in Table 1, the adjusting to a pseudo-first-order process. to According to presented the resultsinpresented Figure 5B and in adsorption rate increases the ILwith content decreases over timeover duetime to the of sites Table 1, the adsorption ratewith increases the ILand content and decreases duesaturation to the saturation available for interaction or ion exchange. Rate constants other chromium(VI) sorbents reported are of sites available for interaction or ion exchange. Rateofconstants of other chromium(VI) sorbents summarized in Table 1. The synthesized solids in thissolids work in have rate constants. reported are summarized in Table 1. The synthesized thishigher work have higher rate constants. A
60.0 50.0 qe (mmol Kg-1)
(c) 40.0 30.0
(b)
20.0 10.0 0.0
(a) 0
rate of adsorption (mmol L-1 min -1)
0.004
70.0
B
(c) 0.003 (b) 0.002
0.001 (a) 0.000
20
40
60
80
100
0
120
20
time (min)
40
60
80
100
120
time (min)
Figure 5. 5. Adsorption Adsorption kinetics: kinetics: (A) Adsorption capacity with respect to contact contact time time and and (B) (B) Rate Rate of of Figure adsorption with with respect respectto tocontact contacttime time(pH (pH2.0); 2.0);(a) (a)Fe Fe33O O44-MAA; (b) Fe33O adsorption O44-MAA-IL; -MAA-IL;(c) (c)Fe Fe33OO4-IL. -IL. 4 Table 1. 1. Kinetic Kinetic data data obtained obtained from from stirred stirredtank tankexperiments experimentsat atpH pH2.0. 2.0. Table Sorbent Sorbent Fe3O4 Fe Fe3O 4-MAA 3 O4 4 -MAA FeFe 3O34O -MAA-IL Fe3Fe O34O -MAA-IL 4-IL Fe3 O4 -IL Activated carbon derived from Activated carbon derived from acrylonitrile–divinylbenzene copolymer acrylonitrile–divinylbenzene copolymer Acinetobacter Acinetobacterjunii juniibiomass biomass
(×10−3) Rate Constant min−1 Rate Constant min−1 (×10−3 ) 6.56 ± 0.75 6.56±± 0.75 25.40 5.50 25.40± ± 5.50 25.30 3.20 25.30± ± 3.20 27.80 6.10 27.80 ± 6.10 5.99 5.99 18.00 18.00
R2 2 R 0.98 0.98 0.93 0.93 0.97 0.97 0.94 0.94 0.8369 0.8369
Reference Reference - work This This work - [11] [11]
0.991 0.991
[12] [12]
According to the results presented in Figure 5b and in Table 1, the adsorption rate increased with the IL content, and decreased over time due to the saturation of sites available for interaction or ion exchange.
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According to the results presented in Figure 5b and in Table 1, the adsorption rate increased with the IL content, and decreased over time due to the saturation of sites available for interaction or ion exchange. Rate constants of other chromium(VI) sorbents reported are summarized in Table 1. These studies indicate that chromium(VI) adsorption obeys a pseudo-first-order kinetic model; however, the synthesized solids in this work have higher rate constants. 3. Materials and Methods 3.1. Materials All solutions were prepared with deionized water (Millipore system) with a resistance of 18.2 MΩ cm or greater. All chemicals used were reagent grade. Potassium dichromate (K2 Cr2 O7 ) was purchased from Sigma Aldrich (St. Louis, MO, USA), and a stock solution of 500 mg/L of chromium(VI) was prepared. Chromium(VI) solutions were prepared from dilutions from the stock solution. 1,5-Diphenylcarbazide, sodium persulfate (Na2 S2 O8 ), ethylene glycol dimethacrylate (EGDMA), methacrylic acid (MAA), 1-allyl-3-methylimidazolium chloride (IL), iron (II) sulfate heptahydrate (FeSO4 ·7H2 O), sodium hydroxide, sulfuric acid, and methanol were also purchased from Sigma Aldrich. 3.2. Synthesis and Characterization of Polymer-Coated Fe3 O4 Particles Precipitation method was employed for the preparation of Fe3 O4 particles; 12.96 mmol (3.6 g) of FeSO4 ·7H2 O were dissolved in 100 mL of deionized water, and NaOH (6 M) was added until pH 10.0 ± 0.2 and dark green color were obtained. The suspension was stirred at 300 rpm, aerated, and heated at 100 ◦ C during 45 min, keeping pH value at 10.0 ± 0.2. Magnetic particles were obtained according to the reaction represented in Equation (1) [20]. Fe2+ + 2 OH− → Fe(OH)2 ↓ 3 Fe(OH)2 + 0.5 O2 → Fe(OH)2 + 2 FeOOH + H2 O
(1)
Fe(OH)2 + 2 FeOOH → Fe3 O4 + 2 H2 O The resulting suspension with a black precipitate was separated using a magnet to retain the magnetic particles, and the supernatant was decanted. Magnetite was washed with deionized water (3 × 10 mL) followed by cold ethanol (2 × 10 mL). Magnetite was dispersed in methanol (15 mL), and it was transferred into a ball flask containing methacrylic acid (MAA), IL monomer, and EGDMA. Fe3 O4 (4.3 mmol) and EGDMA (4 mmol) were kept constant while varying the concentration of MAA (0–2 mmol) and IL (0–2 mmol). The mixture was stirred for 15 min. Then, 0.5 mmol of solid Na2 S2 O8 (0.12 g) was added as radical initiator, and a reflux system was mounted. The temperature was ramped from room temperature to 60 ◦ C over the first 2 h, and maintained for 2 h [31]. The obtained solid was washed with deionized water, and left in the oven at 60 ◦ C for 8 h to dry. The dried particles were kept in a desiccator prior to use. The resulting sorbents are composed as follows, considering the molar ratio mentioned above. Fe3 O4 , Fe3 O4 -MAA, Fe3 O4 -MAA-IL, Fe3 O4 -IL (Table 2). Table 2. Molar ratio for the synthesized sorbents (mmol); EGDMA: ethylene glycol dimethacrylate. Sorbent
Fe3 O4
EGDMA
MAA
IL
Fe3 O4 Fe3 O4 -MAA Fe3 O4 -MAA-IL Fe3 O4 -IL
4.3 4.3 4.3 4.3
4.0 4.0 4.0
2.0 0.0
0.0 2.0
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Once the sorbents were synthesized, they were characterized by Fourier transform infrared spectroscopy (FT-IR) in a Perkin-Elmer Frontier spectrometer (Waltham, MA, USA) between 4000 and 400 cm−1 in order to identify the functional groups in the structure. Micrographs of the sorbents were taken using scanning electron microscopy (FEI Model Quanta 200 F, Amsterdam, The Netherlands). 3.3. Adsorption Experiments 3.3.1. Batch Studies Batch studies were performed by mixing the synthesized sorbents (8.0 mg) with 10 mL of chromium(VI) solutions (0–20 mg/L). The contact time was 30 min in a multi-wrist shaker (model 3589). Different factors, such as solution pH and contact time were evaluated. Chromium(VI) adsorption was first studied at two pH values (2.0 and 6.5) to investigate the dependence on solution pH. Sulfuric acid 0.01 M and sodium hydroxide 0.01 M were used for pH adjustment. Once the contact time was completed, the magnetic sorbent was recovered by an external magnet, and the supernatant was decanted. Adsorption capacity values were calculated from change in the concentration of the chromium(VI) in the solutions employed using the diphenylcarbazide method measuring at 540 nm in a HACH spectrophotometer (DR-2700, Dusseldorf, Germany). To describe the equilibrium of adsorption, the data was fitted to an adsorption isotherm by plotting the remaining concentration of chromium(VI) with respect to the adsorbed chromium(VI), which is calculated according to Equation (2): (C0 − Ce )V qe = (2) w where qe is the adsorbed chromium (mmol/kg), C0 and Ce are initial and final concentrations, respectively (mmol/L), V is the volume of the solution (L), and w is the sorbent mass (kg). Affinity constant values were calculated using the Scatchard method by plotting qe /Ce versus Ce (where qe is expressed in terms of mol/kg and Ce in terms of mol/L) [32]. 3.3.2. Semi-Continuous System Adsorption kinetic studies were carried out in a semi-continuous system implemented to calculate the saturation rate of the synthesized sorbents. One-hundred milliliters of 2.0 mg/L chromium solution were mixed with the different sorbents individually (80.0 ± 0.3 mg). Volumes of 2.0 mL were taken every 10 min for chromium(VI) measurement. The experiments were performed in a stirred tank mode using a stir-pak laboratory stirrer from Cole-Parmer with a helix stirrer from multi-craft. The velocity for a first-order kinetic model for the adsorption obeys Equation (3) [33]: dCe = kCe dt
(3)
Lagergren proposed an adaptation of the equation starting from the concentration of adsorbed chromium(VI); Equation (4) is the velocity equation for a pseudo-first-order reaction (Equation (4)), where the velocity of the adsorption process depends on the velocity constant (k), the maximum adsorbed concentration of chromium(VI) (qe ), and the adsorption at time t (qt ) with the units described above. dqt = k [qe − qt ] dt
(4)
Equation (4) was integrated with respect to the initial and final conditions, and Equation (5) was obtained where t is the time when the sample was taken. ln(qe − qt ) = ln qe − kt
(5)
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By plotting ln(qe − qt ) versus t from the pseudo-first-order equations for each solid, it is possible to calculate the velocity constant (k) for the adsorption and obtain the velocity equation. 4. Conclusions Magnetic sorbents with potential use for chromium(VI) removal were synthesized and evaluated. Adsorption exhibited a clear dependence on the pH of the chromium solution. Highest adsorption capacity was obtained in acidic solutions (pH 2.0), and a speciation of chromium oxidation state is required to identify the adsorption mechanism. Fe3 O4 -IL was the solid that had the highest affinity and the best adsorption capacity. The rate constants for the adsorption process fit to a pseudo-first-order equation, and the value of the constant increased by increasing the IL molar ratio. The use of the ionic liquid-modified magnetic particles for chromium(VI) removal is feasible, economically attractive, and environmentally-friendly by diminishing secondary pollution because of their easy separation from the medium. Acknowledgments: The authors wish to thank PRODEP (Project RedNIQAE-2015) and Junta de Castilla y Leon, (project VA171U14) for the financial support. Author Contributions: Thania Alexandra Ferreira and Jose Antonio Rodriguez performed the experiments; María Elena Paez-Hernandez and Alfredo Guevara-Lara analyzed the adsorption data; Enrique Barrado and Prisciliano Hernandez performed instrumental characterization; the paper was written under supervision Jose Antonio Rodriguez and Prisciliano Hernandez; Thania Alexandra Ferreira is responsible for the writing of the work. Conflicts of Interest: The authors declare no conflict of interest.
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