Chemosphere 141 (2015) 26–33

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Facile synthesis of monodisperse functional magnetic dialdehyde starchnano-composite and used for highly effective recovery of Hg(II)

http://dx.doi.org/10.1016/j.chemosphere.2015.06.0190045-6535/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: zhangyun@lzu.edu.cn (Y. Zhang).

Yang Wang, Yun Zhang ⇑, Chen Hou, Zhigang Qi, Xinghua He, Yanfeng LiState Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry andChemical Engineering, College of Resources and Environment, Institute of Biochemical Engineering & Environmental Technology, Lanzhou University, Lanzhou 730000, PR China

h i g h l i g h t s

� The aminothiourea functionalized magnetic dialdehyde starch was firstly synthesized.� The nano-absorbent exhibited great monodispersity and uniform particle size.� The absorbent showed the prominent adsorption capacity and strong removal ability.� The absorbent shows excellent selectively separate for Hg(II).

a r t i c l e i n f o

Article history:Received 28 September 2014Received in revised form 7 May 2015Accepted 9 June 2015

Keywords:MagneticNano-compositeDialdehyde starchAdsorptionHg(II)

a b s t r a c t

By covalently linking dialdehyde starch and amine functionalized Fe3O4 nanoparticle, and modifying withaminothiourea functional group, the novel monodisperse nano-composite has been successfully synthe-sized without any toxic crosslinking agent. The resulting nano-composite was characterized by means ofthe Fourier transform infrared spectra (FT-IR), transmission electron microscope (TEM), X-ray diffraction(XRD), elemental analysis and vibrating sample magnetometer (VSM). As the new kind of low-cost andenvironmentally friendly adsorbent with the excellent monodispersity in aqueous phase, the obtainednano-composite has shown not only the good adsorption capacity for Hg(II) on high initial concentration,but also the strong removal ability on low concentration. Moreover, the unique selectivity for Hg(II)among the mixed metal ions solution and good regeneration performance of nano-composite has alsobeen demonstrated by batch experiments.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Highly toxic Hg(II) contaminated water could cause particulardamage to aquatic life and humans, but it has been continuouslyreleased into environment with the rapid industrialization (Choet al., 2009; His et al., 2011; Xu et al., 2014). Hg(II) that accumulatedin the food chain could do extreme harm to human health. Acceptedthe lessons from the tragic example of Hg(II) contamination inMinamata (Japan), effective removal the Hg(II) from contaminatedwater has attracted the worldwide attention until now (Wanget al., 2013). Among various methods, adsorption has been devel-oped as a promising way to purify the contaminated water due toits simplicity, high efficiency and selectivity, wide-ranging availabil-ity and low cost. According to the theory of Hard–Soft-Acid–Base(HSAB), Hg(II) has a strong affinity toward ligands containing S, N

and O atoms (Ma et al., 2011). Magnetic nano-absorbents function-alized with thiol and amine derivatives, which have large surfacearea and easily separated processes, have shown the strong removalcapacity of heavy metals including Hg(II). However, the complexsynthesis processes, high preparation cost and the toxicity of rawmaterials still limit their utilization (Wu and Zhao, 2011).

Magnetic organic–inorganic nano-composite with magneticcore and polymeric shell are caused great interest compared withother nano-materials due to its good compatibility and functional-ization with other materials, and it also could prevent the agglom-eration of magnetic nanoparticles by organic shell which served assurfactant or stabilizer (Zhang et al., 2013c; Mu et al., 2013). As theburgeoning materials, varies magnetic organic–inorganicnano-composites have been reported and shown remarkablepotential in the field of heavy metal removal because their highsurface area, enhanced active sites and abundant functional groupson the surface (Zhu et al., 2010; Musico et al., 2013; Wang et al.,2011). For instance, nano-composite that used synthetic polymer,including polypyrrole, polyaniline, polyacrylonitrile, and natural

Y. Wang et al. / Chemosphere 141 (2015) 26–33 27

polymer, represented with chitosan as the organic shell, werefound to be highly efficient absorbents (Wang et al., 2012b;Monier and Abdel-Latif, 2012; Zhu et al., 2012; Liu et al., 2009).Musico et al. reported the polymer-based graphene oxidenano-composite showed improved removal of lead ion (Musicoet al., 2013). Wang et al. found that multi-cyanoguanidine modi-fied magnetic chitosan nano-composite had highly effective recov-ery for Hg(II) (Wang et al., 2013). However, the expensive, toxicmonomer and crosslinking agent still are inevitable in preparationof synthetic polymer composite and natural polymer composite,respectively. So synthesis of the economical and environmentalfriendly novel magnetic organic–inorganic nano-composite witha simple way is yet an arduous task.

Herein, our group prepared magnetic dialdehyde starchnano-composite with extraordinary monodispersity through cova-lently linking the dialdehyde starch and amine functionalizedFe3O4 nanoparticle by Schiff base. After further modified withaminothiourea functional group, the resulted absorbent had astrong affinity for Hg(II). The preparation process successfullyavoided using any toxic monomer and crosslinking agent, andaccomplished the goal that synthesis the new kind of environmen-tally acceptable and economical nano-composite with selectiveand effective removal ability for Hg(II). Dialdehyde starch, one kindof industrial starch derivatives, has been reported many times inheavy metal removal due to its good biocompatibility and easilymodified character (Ding et al., 2011; Łabanowska et al., 2012;Yin et al., 2008). But preparation of the magnetic dialdehyde starchnano-composite is rarely reported. The removal efficiency of theobtained nano-composite reached 95.7% and the adsorption capac-ity only decreased 2.6% after 5 cycles of successive adsorption–des-orption. Moreover, the composition and structure of magneticnano-composite were studied with the help of characterizationmethods. Adsorption models as well as kinetic properties ofremoval process were also clarified. In summary, with low-cost,green raw materials, and moderate synthetic processes, the novelmagnetic nano-composite, with recyclability, highly selectivity,and large adsorption capacity, could quit efficiently removeHg(II) from contaminated water.

2. Experiment

2.1. Materials

Potato starch (food-grade) was dried at 105 �C before it wasused. All other reagents were analytical grade and were used asreceived. Aqueous solutions at various concentrations were pre-pared from HgCl2, were used as sources for Hg(II), respectively.Zn(NO3)2�6H2O, Cd(NO3)2�4H2O, CuCl2�2H2O, NiNO3 were used assources for Zn(II), Cd(II), Ni(II) and Cu(II), respectively.

2.2. Preparation of dialdehyde starch

Dialdehyde starch was prepared according to the methoddescribed in an earlier report (Ding et al., 2011; Yin et al., 2008).4.0 g of potato starch suspended in 10 mL of water, sodium perio-date solutions (5.28 g) was added to the suspension as oxidant andadjusted the pH to 3.5. The mixture was stirred at 30 �C in the darkfor 4 h and filtered. The product (abbreviated to DAS) was washedwith distilled water and ethanol thoroughly and dried at 50 �C for24 h under vacuum. The percentage of dialdehyde units was 88%based on the report.

2.3. Preparation of amine-functionalized magnetite Fe3O4 nanoparticle

Typically, 1.0 g of anhydrous FeCl3 and 2.0 g of anhydroussodium acetate were added to 30 mL of ethylene glycol, after that,

10 mL of 2-aminoethanol was added to obtain a transparent solu-tion via reflux. This mixture was then transferred into aTeflon-lined autoclave and treated at 200 �C for 8 h (Wang et al.,2006; Xin et al., 2012). Amine-functionalized magnetite Fe3O4

nanoparticle (NH2–Fe3O4) was collected by magnetic decantationand washed with distilled water thoroughly. Finally, the nanopar-ticles were dried at 60 �C of 24 h under vacuum.

2.4. One-pot synthesis of aminothiourea functional magneticdialdehyde starch nano-composite

0.25 g of DAS was added to 30 mL of the above magneticnanoparticle suspension (contained 0.25 g functional Fe3O4), soni-cated the suspension for 30 min with N2 protection. The reactiontemperature was risen to 90 �C for 2 h to obtain the magneticdialdehyde starch nanoparticle (MDAS). After that, 15 mL aminoth-iourea (0.28 g) solution was added to the system directly andreacted for another 2 h. The product, aminothiourea functionalmagnetic dialdehyde starch nano-composite (AT-MDAS), waswashed with distilled water and ethanol thoroughly and dried at60 �C under the vacuum for 24 h. The synthesis routine ofnano-composite was shown in Fig. 1.

2.5. Characterization

The FT-IR was recorded with a Nicolet Magna-IR spectropho-tometer between 4000 and 450 cm�1 using the KBr pellettechnique. Transmission electron microscopy (TEM, FEI TecnaiG20) was obtained to elucidate the dimensions of the nanoparticle.Elemental analysis performed by a PerkinElmer 2400 CHN ana-lyzer. The concentration of ions in solution was determined byan inductively coupled plasma spectrometer (ICP/IRIS Advantage,Thermo, America). Magnetization measurements were performedon a vibrating sample magnetometry (VSM, LAKESHORE-7304,USA).

2.6. Adsorption experiment using batch methods

The adsorption of Hg(II) was studied by batch methods thatwere carried out by placing 0.01 g AT-MDAS in series of flaskscontaining Hg(II) aqueous solution (50 mL) with desired initialconcentration and pH. Thereafter, the flasks were shaken in athermostat oscillator at specific temperature with constant rate130 rpm for a given time under dark environment.

The effect of pH, contact time, initial concentration and theselective separation from multi-metal system was tested as theoperation variables on the extent of adsorption. To investigatethe effect of pH, adjusted the desired pH ranging from 2 to 7 usingHNO3 and NaOH solutions. The working concentration of Hg(II)was 30 mg L�1 for kinetic studies and 5–200 mg L�1 for isothermstudies. The selective separation of Hg(II) from mixture withNi(II), Zn(II), Cd(II), Cu(II) was carried at each ion concentration10 mg L�1. The amount of Hg(II) adsorbed on per gram of theabsorbent was calculated on the basis of following equation (Liet al., 2013; Bandaru et al., 2013).

Qe ¼ðC0 � CeÞV

Mð1Þ

Adsorption efficient ¼ C0 � Ce

C0� 100% ð2Þ

where Qe is the adsorption capacity (mg g�1); C0 and Ce are initialand equilibrium concentrations of the Hg(II) (mg L�1) in the testingsolution (mg L�1); V is the volume of the solution (L), and M is theweight of resin beads (g).

Fig. 1. Scheme for the synthesis route of AT-MDAS nano-composite.

Fig. 2. TEM images (A) of the NH2–Fe3O4 nanoparticle (a) and (b); AT-MDAS nano-composite (c) and (d); SEM images (B) of the NH2–Fe3O4 nanoparticle (a) and (c);AT-MDAS nano-composite (b) and (d).

28 Y. Wang et al. / Chemosphere 141 (2015) 26–33

3. Results and discussion

3.1. Characteristic of the magnetic absorbents

3.1.1. TEM characterizationTEM observation was undertaken to characterize the dis-

persibility and morphologies of the NH2–Fe3O4 and AT-MDASnano-composite. As shown in Fig. 2A, the NH2–Fe3O4 nanoparticleshave excellent monodispersity and uniform particle size with adiameter of about 200 nm. The little aggregation has beenobserved for AT-MDAS nano-composite as shown in Fig. 2A (c)due to the reaction between the organic DAS and the surface ofinorganic NH2–Fe3O4 nanoparticle. However, the great monodis-persity and uniform size of the nano-composites is still remained.Because of little amino content in NH2–Fe3O4 nanoparticle, only asmall amount of DAS have introduced from reaction. Therefore,the morphology of nano-composites retained the same substantialas the NH2–Fe3O4 nanoparticles as shown in Fig. 2A (d). Althoughthe content of functional DAS does not reach the high level asshown in Fig. 3d, the extraordinary adsorption ability as well ashigh selectivity for Hg(II) of the novel nano-composite has beendemonstrated due to the outstanding monodispersity and uniformmorphology.

3.1.2. SEM characterizationSEM images of the NH2–Fe3O4 and AT-MDAS nano-composite

are shown in Fig. 2B. As the same with the TEM images, the SEMimages also demonstrated the well monodispersity of both NH2–Fe3O4 (Fig. 2B (a) and (c)) and AT-MDAS nano-composite (Fig. 2B(b) and (d)). On the contrary to NH2–Fe3O4 nanoparticles inFig. 2B (c), a layer of organic ingredients could be clearly observedon the surface of AT-MDAS nano-composite in Fig. 2B (d), demon-strated the successful reaction between the functional DAS and theNH2–Fe3O4.

3.1.3. FTIR characterizationFig. 3A shows the FTIR spectra of NH2–Fe3O4, DAS and AT-MDAS

nano-composite. The peaks in NH2–Fe3O4 spectra (a) at 1622,3417 cm�1 indicates that the particle contains amount of aminegroup (Guo et al., 2009). The peak at 580 cm�1 is related to thevibration of FeAO functional group, which is in agreement withthe characteristic peak for Fe3O4 (Koehler et al., 2009; Zhanget al., 2013a). The spectrum of DAS (b) is just like the referencereported, the characteristic bands at 1735 cm�1 relates to thevibration of C@O (Ding et al., 2011; Yin et al., 2008). Compare withthe NH2–Fe3O4 and DAS, the spectrum of AT-MDAS (c) shows much

4000 3500 3000 2500 2000 1500 1000 500

c

3417

b

1735

Wavenumber(cm-1)

Tran

smitt

ance

%

2046

1614

a1622

(A)

20 25 30 35 40 45 50 55 60 65 702-Theta

Inte

nsity

a

b

(B)

-10000 -5000 0 5000 10000-100

-80

-60

-40

-20

0

20

40

60

80

100

mag

netiz

atio

n (e

mg/

g)

magnetic field (Oe)

a

b

a

b

(C)

100 200 300 400 500 60060

70

80

90

100

W (%

)

Temperature (oC)

AT-MDAS

22.5%

NH2-Fe3O4

(D)

Fig. 3. FT-IR spectrum (A) of NH2–Fe3O4 (a); MDAS (b) and AT-MDAS (c); XRD patterns (B) of the NH2–Fe3O4 nanoparticle (a) and AT-MDAS nano-composite (b); VSMmagnetization curves (C) of the NH2–Fe3O4 nanoparticle (a) and AT-MDAS nano-composite (b); and the thermal behaviors (D) of NH2–Fe3O4 and the AT-MDAS nano-composite.

Y. Wang et al. / Chemosphere 141 (2015) 26–33 29

differences indicates the functionalization of aminothiourea group.The vibration of C@O disappeared and the band of amine group in1614 cm�1 increased, the new bind of aminothiourea group in2046 cm�1 formed. These observations reveal that the aminoth-iourea functionalized magnetic dialdehyde starch nano-composite has been successfully synthesized.

Fig. 3B shows the XRD patterns of the NH2–Fe3O4 nanoparticleand AT-MDAS nano-composite. Both of the samples are observedthe six characteristic diffraction peaks of Fe3O4 at 2h = 30.1�,35.5�, 43.3�, 53.4�, 57.2�, and 62.5� marked by their indices(220), (311), (400), (422), (511) and (440). The results indicatethat the functional DAS has successful introduced to the surfaceof the NH2–Fe3O4 nanoparticle and AT-MDAS nano-composite keepintrinsic phase of magnetite.

3.1.4. Magnetization characterizationFig. 3C shows the hysteresis loops of NH2–Fe3O4 and the

AT-MDAS nano-composite which is characterized the magneticmeasurements of each product. The saturation magnetization ofNH2–Fe3O4 is 79.13 emu g�1 and the AT-MDAS is 59.6 emu g�1.Although the magnetic intensity decreased after the reactionbetween functional DAS and NH2–Fe3O4, the AT-MDASnano-biosorbent could be separated from treated water vary con-veniently by using magnetic field (Zhang et al., 2011, 2013b).

3.1.5. TGA characterizationThe thermal behavior of NH2–Fe3O4 and the AT-MDAS

nano-composite are further investigated by TGA. As shown inFig. 3D, the NH2–Fe3O4 nanoparticle almost has no mass lossaround the tested temperature. As for AT-MDAS nano-composite,

the weight loss over the temperature range of 200–460 �C isattributed to the functional DAS, and 22.5% of weight loss provedthat are 22.5% of functional DAS in AT-MDAS nano-composite(Liu et al., 2012).

3.1.6. Adsorption studyBecause the excellent monodispersity extremely increases the

specific surface area and decreases mass transfer resistance, andwith successful modifying with plenty of aminothiourea functionalgroups, the AT-MDAS nano-composite could effectively removeHg(II) in water samples (Liu et al., 2013). As the functional groupscontain S and N have exceptional selectively binding affinitytoward Hg(II), metal ions co-adsorption of the AT-MDASnano-composite was investigated (Wei et al., 2011). The experi-ment was carried out by placing 0.01 g AT-MDAS in flak containing50 mL mixed solution contain Zn(II), Cd(II), Mg(II), Ni(II) and Hg(II)with 10 mg L�1 of the initial concentration for each metal ion. Theresults show in Fig. 4A demonstrates that the nano-composite hasextraordinary selectivity for Hg(II) among the mixture. Theremoval ratio of Hg(II) reached 96% also indicates strong adsorp-tion ability that the AT-MDAS has.

3.2. The effect of pH on Hg(II) adsorption

As one of the critical parameters in the adsorption, pH of themetal ion solution could affect the surface charge of the adsorptionand the metal ion chemistry (Wang et al., 2012a). For that, we com-pared the adsorption processes between NH2–Fe3O4 and AT-MDASnano-composite when the pH ranged 2–7 and the results areshown in Fig. 4B. Contrast with NH2–Fe3O4, the adsorption capacity

Fig. 4. Selective study (A) for the AT-MDAS nano-composite; effect of initial pH (B); shacking time (C) on Hg(II) adsorption by 0.01 g absorbent with initial concentration of30 mg L�1 at 30 �C; and effect of initial concentration and temperature (D) on Hg(II) adsorption.

30 Y. Wang et al. / Chemosphere 141 (2015) 26–33

of AT-MDAS for Hg(II) enhances conspicuously. At low pH, theadsorption surface will be completely covered with hydroniumions which compete strongly with metal ions for adsorption sites(Laus et al., 2010; Yuan et al., 2013). Therefore, the maximumadsorption of AT-MDAS is observed at pH of 6–7. Owning to thepowerful chelating ability of resulted nano-composite to Hg(II),the adsorption amount is basically maintained at the optimumlevel with the pH range of 3–7. Consequently, the rest of adsorp-tion experiments were done at natural pH of Hg(II) solution.

3.3. The effect of time on Hg(II) adsorption

The effect of contact time on the adsorption capacity of theAT-MDAS nano-composite for Hg(II) is shown in Fig. 4C. Theadsorption rate increased rapid within few minutes, and theamount of Hg(II) adsorbed onto AT-MDAS increased with increas-ing of contacting time until approaching adsorption equilibrium.Because the excellent monodispersity of the nano-biosorbent sig-nificantly increases the specific surface area, the AT-MDAS couldreach the saturated adsorption in a short time. Furthermore, thehigher initial concentration provides higher drive force andenhances adsorption ability is also observed form the results(Yang et al., 2011).

3.4. Effect of the initial concentration and temperature

The effect of initial concentration on the adsorption of Hg(II)onto AT-MDAS nano-composite is also investigated and the results

are shown in Fig. 4D. It is observed that the adsorption capacityincreased almost linear with increasing initial concentration untilthe equilibrium. When the initial concentration of Hg(II) increasedmore than 150 mg L�1, the maximum adsorption capacity reachedsaturation and the amount is 318.87, 310, and 306.78 mg g�1, atthe temperature of 289, 303, and 313 K, respectively. These resultsindicate that the temperature plays an important role in theadsorption of Hg(II), the adsorption capacity essentially increasedwith temperature rising, suggesting an endothermal process.Thanks to the strong chelating force between the functional group,aminothiourea, in nano-composite with the Hg(II), the AT-MDASexhibits excellent removal ability at low initial concentration asshown in Fig. 5A. All of the removal ratio for Hg(II) is higher than90% with the initial concentration ranged from 5 to 30 mg g�1.Therefore, the AT-MDAS nano-composite not only has a largeadsorption capacity for Hg(II), but also the powerful removalability.

3.5. Desorption and reusability

In order to reduce the cost of removal process for potentialpractical application, examining the desorption of the Hg(II) fromAT-MDAS and regeneration the absorbent is necessary. Thesaturated EDTA has been chosen as the eluent in the study. Afterdesorption, the AT-MDAS was treated with deionized water towash away the residual EDTA solution and explored for Hg(II)adsorption in the succeeding cycles. As demonstrated in Fig. 5B,the adsorption capacity of the AT-MDAS nano-composite for

Fig. 5. The removal ratio of the AT-MDAS nano-composite (A) at low initialconcentration and adsorption capacity of the AT-MDAS nano-composite afterrepeated regeneration (B).

Y. Wang et al. / Chemosphere 141 (2015) 26–33 31

Hg(II) only decreased 2.6% after 5 successive adsorption–desorp-tion cycles. Consequently, the AT-MDAS nano-composite with thegood regeneration performance and high stability could be effec-tively and economically used for the selectively treatment ofHg(II) contaminated wastewater (Kawalec et al., 2013).

3.6. Adsorption kinetics and isotherms

In order to evaluate the kinetic mechanism that controls theadsorption process, the pseudo-first-order, pseudo-second-order

Table 1Parameters of kinetics model for adsorption of Hg(II) on AT-MDAS nano-composite with dHg(II) on AT-MDAS nano-composite (B).

Initial conc. (mg L�1) Qexp (mg g�1) Pseudo-first-order

k1 � 102 (min�1) Qe (mg g�1) Ra

(A)20 175.75 7.43 168.34 0.827230 234 4.55 269.77 0.86540 256 1.75 240.21 0.9191

Temperature (K) Langmuir parameters

Qmax (mg g�1) b (L mg�1)

(B)298 312.5 0.159303 322.58 0.156313 323.64 0.179

and intraparticle diffusion kinetic models were employed to inter-pret the experimental data (Wang and Chen, 2014).

The pseudo-first-order (3), pseudo-second-order (4) and intra-particle diffusion (5) kinetic model are respectively represented as:

logðQ e � Q tÞ ¼ log Q e �k1t

2:303ð3Þ

tQ t¼ 1

k2Q 2e

þ tQ e

ð4Þ

Q t ¼ kit1=2 þ C ð5Þ

where k1, k2 and ki are pseudo-first-order rate constant (min�1),pseudo-second-order rate constant (g mg�1 min�1) and intra-particle diffusion rate constant (mg g�1 min�1/2) of adsorption,respectively. Qe and Qt are the adsorption capacity (mg g�1) at equi-librium time and at time t (min), respectively. C (mg g�1) is a con-stant of intra-particle diffusion model.

The kinetic parameters for adsorption of Hg(II) with differentinitial concentrations by AT-MDAS nano-composite are given inTable 1A. There is no doubt that the pseudo-second-order modelobviously fits for the experimental kinetic data because the corre-lation coefficient (R) is more higher and the calculated Qe by thepseudo-second-order model is also more in agreement with theexperimental Qexp than the Lagergren kinetics in all three initialconcentrations. Because the pseudo-second-order model isassumed that the rate determining step may be chemisorptionwhich involves valence forces through sharing or exchangingelectrons sorbent and sorbate and the adsorption capacity is pro-portional to the number of active sites occupied on the adsorbentsurface, which suggested that the adsorption of Hg(II) onAT-MDAS is mainly the chemical reactive adsorption.

As fundamental study in describing the interactive behaviorbetween the adsorption and absorbent, the equilibrium adsorptionisotherm is important in the design of adsorption systems. For theadsorption isotherm studies, we selected the data in Fig. 8 andtreated them with Langmuir and Freundlich equations, respec-tively (Liu and Lee, 2014).

Ce

Qe¼ 1

bQmaxþ Ce

Q maxð6Þ

ln Q e ¼ ln KF þ1n

� �ln Ce ð7Þ

where Ce is the equilibrium concentration of metal ions in solution(mg L�1), and Qe is the equilibrium adsorption capacity (mg g�1),Qmax (mg g�1) and b (L mg�1) are the Langmuir constant whichare related to the adsorption capacity and energy of adsorption,

ifferent initial concentration (A) and parameters of isotherm model for adsorption of

Pseudo-second-order Intraparticle diffusion

k2 � 103 (g g�1 min�1) Qe (mg g�1) Ra ki (mg g�1 min�1/2) Ra

0.5659 188.67 0.9906 10.67 0.78010.2488 270.27 0.9842 17.6 0.96930.3146 277.77 0.991 16.72 0.9722

Freundlich parameters

Ra KF (mg g�1) 1/n Ra

0.9905 71.087 0.332 0.90950.9912 68.033 0.351 0.87490.9916 74.619 0.339 0.9073

Table 2Maximum adsorption capacities of adsorption of Hg(II) onto various adsorbents (A) and Langmuir adsorption isotherm at different temperatures (B).

Absorbent Adsorption capacity (mg g�1)

(A)Magnetic chitosan–thioglyceraldehyde (Monier, 2012) 98 ± 2Magnetic chitosan–phenylthiourea (Monier and Abdel-Latif, 2012) 135 ± 3Magnetic silica nanocomposites (Song et al., 2011a) 19.79Polyrhodanine-encapsulated magnetic nanoparticles (Song et al., 2011b) 179Multi-cyanoguanidine modified magnetic chitosan (Wang et al., 2013) 285Thiourea modified magnetic chitosan (Donia et al., 2008) 560AT-MDAS (this work) 310

298 K 303 K 313 K

b Concentration (mg L�1) RL b Concentration (mg L�1) RL b Concentration (mg L�1) RL

(B)0.159 15 0.295 0.156 15 0.299 0.179 15 0.271

20 0.239 20 0.242 20 0.21830 0.173 30 0.176 30 0.15640 0.135 40 0.138 40 0.12250 0.112 50 0.113 50 0.10160 0.0948 60 0.0965 60 0.085180 0.0722 80 0.0741 80 0.0652

100 0.0591 100 0.0602 100 0.0529120 0.0498 120 0.0507 120 0.0444150 0.0402 150 0.0409 150 0.0359200 0.0304 200 0.0311 200 0.0271

32 Y. Wang et al. / Chemosphere 141 (2015) 26–33

respectively. KF is the Freundlich constant related to adsorptioncapacity.

Table 1B shows the model parameters and correlation coeffi-cients (R) obtained by using Langmuir and Freundlich models. Itcould see that the monolayer Langmuir model is describe theexperiment data more precisely than the Freundlich model at stud-ied temperature because that the correlation coefficients (R) ofLangmuir model is better, and the theoretical Qmax value calculatedby Langmuir model is also very closely to experimentally observedcapacity. The maximum adsorption capacity of Hg(II) adsorbedonto AT-MDAS were calculated to be 312.5, 322.58 and323.64 mg g�1 at 298, 303 and 313 K, respectively. The Langmuirisotherm is based on three assumptions that sorption is limitedto monolayer coverage; all surface sites are alike and can onlyaccommodate on adsorb atom; the ability of a molecule to beadsorbed on a given site is independent of its neighboring sitesoccupancy. Table 2B compares the maximum adsorption capacityof the AT-MDAS for Hg(II) with other magnetic adsorbentsreported in the literatures. To determine if the adsorption processis favorable, a dimensionless constant separation factor, ‘RL’, couldbe used to classify isotherm for the Langmuir type adsorption pro-cess, which is defined as below.

RL ¼1

ð1þ bC0Þð8Þ

The RL values are found in the range of 0.0271–0.299 forAT-MDAS at 25, 30 and 40 �C, respectively, showing favorableadsorption.

4. Conclusions

In summary, aminothiourea functional magnetic DASnano-composite has been prepared through the covalently linkingDAS and NH2–Fe3O4 and functionalized with aminothiourea group.With the excellent monodispersibility in aqueous phase, theobtained nano-composite showed not only the good adsorptioncapacity for Hg(II), but also the strong removal ability. With conve-nient magnetic operation and synthesized by common industrial-ized raw materials and mature synthetic routes, as well as theextraordinary removal ability and high selectivity, the AT-MDAS

nano-composite has a great potential to be used as environmentalfriendly and economical absorbent to treatment the Hg(II) contam-inated wastewater.

Acknowledgments

The authors gratefully acknowledge financial supports from theNational Natural Science Foundation of China (No. 21304040),Natural Science Foundation of Gansu Province (1308RJYA027)and Chinese Postdoctoral Funds (2013M532090). This paper isdedicated to memory of Prof. Yanfeng Li, who passed awayrecently.

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