colloids and surfaces a- physicochemical and engineering aspects volume issue 2014 [doi...

Accepted Manuscript

Title: Synthesis of magnetic chitosan nanoparticle and itsadsorption property for humic acid from aqueous solution

Author: Changlong Dong Wei Chen Cheng Liu Yu LiuHaicheng Liu

PII: S0927-7757(14)00108-3DOI: http://dx.doi.org/doi:10.1016/j.colsurfa.2014.01.069Reference: COLSUA 18955

To appear in: Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: 27-11-2013Revised date: 11-1-2014Accepted date: 15-1-2014

Please cite this article as: C. Dong, W. Chen, C. Liu, Y. Liu, H. Liu, Synthesis ofmagnetic chitosan nanoparticle and its adsorption property for humic acid from aqueoussolution, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014),http://dx.doi.org/10.1016/j.colsurfa.2014.01.069

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

http://dx.doi.org/doi:10.1016/j.colsurfa.2014.01.069

http://dx.doi.org/10.1016/j.colsurfa.2014.01.069

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Synthesis of magnetic chitosan nanoparticle and its adsorption property for

humic acid from aqueous solution

Changlong Dong b,*, Wei Chen a,b, Cheng Liu a,b, Yu Liub, Haicheng Liub

a Key Laboratory of Integrated Regulation and Resource Development on Shallow

Lakes, Ministry of Education, Hohai University, Nanjing 210098, PR China

b College of Environment, Hohai University, Nanjing 210098, PR China

Abstract

In this study, a novel magnetic chitosan nanoparticle (MCNP) was first prepared by

one-step in-situ co-precipitation at low temperature and normal atmosphere to remove

humic acid (HA) from aqueous solution. Modern characterization analysis showed

that MCNP was quasi-spherical in shape with size of 10 nm, and the chemical

crosslinking occurred mainly on the hydroxyl groups in chitosan. The increase in

solution pH from 4 to 10 caused an obvious decrease in equilibrium adsorption

capacity of HA from 29.3 to 7.4 mg/L due to the changes of surface charge of MCNP

and structural morphology of HA molecules. The adsorption process was

attachment-limited under low pH conditions, and both transport-limited and

attachment-limited under high pH conditions. The effect of ionic strength was

complicated at different initial HA concentrations. Low ionic strength significantly

improved adsorption performance and enhanced the interactions between adsorption

sites and HA molecules. While high ionic strength might weaken adsorption ability

because of the negative effects of screening enhanced and competitive adsorption.

Common ions existed in neutral natural water sources would not have negative effects

* Corresponding author. Address: College of Environment, Hohai University, Nanjing 210098, PR China. Tel.: +86 15195959921.E-mail address: [email protected] (C. Dong)

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on HA removal. The adsorption mechanisms during adsorption process were

electrostatic attraction and hydrogen bonding. Regeneration studies indicated that

MCNP could be recyclable for a long term. This research extended the potential

applicability of MCNP to a great extent and provided a convenient approach to

efficiently remove HA from water.

Keywords: Adsorption; Chitosan; Magnetic; Nanoparticle; Humic acid; Mechanism

1. Introduction

Humic acid (HA), together with fulvic acid, are the core of all humic substances of

natural organic matters in water resources [1]. The existence of HA with high

concentrations can lead to color, taste and odor problems [2]. Meanwhile, HA

molecules can bind heavy metals and synthetic organic chemicals firmly, and increase

the difficulty to remove these pollutants during drinking water treatment [3]. Most

seriously, HA can react with chlorine during drinking water treatment to form strongly

carcinogenic disinfection byproducts [4,5]. Therefore, the presence of HA in water

resources has been a great concern and measures should be taken to minimize the

presence of HA in drinking water treatment.

To remove HA during drinking water treatment, various processes have been

applied, including coagulation, ion-exchange, membrane technology, advanced

oxidation and adsorption [6-10]. Of them, adsorption has been found to be the most

promising method due to its simplicity of design, ease of operation and high

efficiency [11,12]. However, the structure and chemical properties of HA molecules

can undergo various changes in different water environments due to the existence of

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carboxylic and phenolic groups on HA molecules. Hence the versatility of novel

adsorbents should be considered in face of the complexity of contaminated water with

HA.

Recently, adsorption using low-cost adsorbents, which are abundant in nature or are

waste materials from other industry, has become a hot research topic [13]. Chitosan is

a cationic biopolymer obtained from deacetylation of chitin which is the second most

abundant biopolymer in nature [14]. Chitosan has been used as an excellent natural

adsorbent to remove many pollutants including dyes, heavy metals and fluoride due to

the presence of amino and hydroxyl groups [14,15]. Since the amino groups in

chitosan can be protonated, the polycationic property is expected to have a

contribution to the attraction interactions with anionic substances, such as HA.

However, raw chitosan has a tendency to agglomerate in aqueous solution and can be

dissolved in acidic media. The low specific gravity of chitosan makes itself separated

difficultly from aqueous solution and limits its use in either batch or column modes

[16]. Although lots of modified chitosan beads were synthesized to adsorb HA from

aqueous solution [1,11,15], their separation and adsorption performance can be further

improved.

In order to overcome the weakness of chitosan and improve its practicability in

drinking water treatment, chemical crosslinking, magnetic separation and

nanotechnology are selected in this study. To our best knowledge, magnetic chitosan

nanotechnology has never been applied to drinking water treatment. Magnetic

separation is a hot technology which drastically shortens separating time and

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increases processing capacity of water treatment plants. From the viewpoints of

environmental protection and high energy saving technology, it is significant to

prepare magnetic chitosan nanoparticle (MCNP) by one-step method and explore its

adsorption properties toward HA in order to expand the utilities as industrial

biomaterials. Meanwhile, it is important to prove the reusability of MCNP if they

have to be integrated into drinking water treatment as a pretreatment process.

Although MCNP with a wide size distribution was obtained [17,18], study on

preparation, characterization and adsorption properties for HA removal has not yet

been studied.

In the present study, MCNP was prepared by in-situ co-precipitation without

protection of nitrogen combined with chemical crosslinking using epichlorohydrin.

The resulting MCNP was characterized by transmission electron microscope (TEM),

scanning electron microscope (SEM), powder X-ray diffraction (XRD) and X-ray

photoelectron spectroscopy (XPS) to determine the structure of MCNP and validate

its synthetic route. The adsorption behaviors of MCNP towards HA were investigated

in batch experiments. The effects of pH and ionic strength on adsorption kinetics and

isotherms were evaluated. Influences of common ions in natural water were also

investigated. The MCNP after adsorption of HA at different solution pH values were

characterized by Fourier transform infrared spectroscopy (FTIR) and XPS to analyze

the adsorption mechanisms. The reusability of MCNP was also verified through

adsorption-regeneration cycles.

2. Experimental

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2.1. Materials

Chitosan powders 80-95% deacetylated supplied by Sinopharm Chemical Reagent

Co. Ltd., China had a molecular weight ranging from 535,000 to 620,000 g/mol. HA

was obtained from Alfa Aesar Co., and used without any further purification. All the

other chemicals used were of analytical grade. Distilled water was used throughout

the study. Stock solution of 500 mg/L was prepared by dissolving appropriate

amounts of HA in distilled water. Working solutions ranging from 3 to 60 mg/L of HA

were prepared by diluting the stock solution. Adjustment of pH was carried out using

0.1 M HCl and 0.1 M NaOH.

2.2. Preparation of MCNP

MCNP was synthesized induced by chitosan hydrogel via in-situ co-precipitation at

low temperature and normal atmosphere. Chitosan hydrogel was prepared by

dissolving 0.5 g of chitosan in 200 mL of 0.5% (v/v) acetic acid solution with

continuous stirring. 4.7 g of FeCl3·6H2O and 2.4 g of FeSO4·7H2O, which were

dissolved in 22 mL of distilled water, respectively, were added to chitosan hydrogel

by stirring at 1000 rpm for 20 min in a water bath at 40°C to form chitosan-iron ions

complex. It provided sufficient time and proper condition for chitosan to chelate iron

ions, thus protecting ferrous ions from being oxidized and replacing the protection of

nitrogen. After that, 40 mL of 28% (m/v) concentrated ammonia was added dropwise

into the reaction system. The formed magnetic Fe3O4 has a weak interaction with

amino and hydroxyl in chitosan. After 20 min, the temperature of the reaction system

was adjusted to 60°C, and 6 mL of epichlorohydrin was added to the system with

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continuous stirring at 1000 rpm for 3 h. The alkaline environment created by

concentrated ammonia in co-precipitation process makes epichlorohydrin react with

hydroxyl groups in chitosan, which avoids the consumption of amino groups. The

resulting MCNP was separated by a magnet. Finally, the obtained MCNP was washed

by 0.5% (v/v) acetic acid, distilled water and alcohol for three times, respectively, and

dried in a vacuum oven at 60°C till constant weight. The detailed synthetic route of

MCNP was shown in Fig. 1.

2.3. Adsorbent characterization

The morphology and granularity of MNCP were examined by Tecnai G20 TEM

(FEI, USA) and S4800 SEM (Hitachi, Japan). The adsorbent constituents were

identified by the combination of D8 Advance XRD (Bruker, Germany) and Ultra XPS

(Kratos Analytical Ltd, UK). The surfaces of chitosan, MCNP and HA-laden MCNP

at different solution pH values were also analyzed by XPS and Tensor 27 FTIR

(Bruker, Germany) to validate the synthetic route and adsorption mechanisms of

MCNP.

2.4. Batch adsorption experiments

Batch adsorption experiments were performed using conventional bottle-point

method at room temperature (≈25°C). 0.1 g of MCNP was dispersed into 100 mL of

HA solution contained in a 150-mL Erlenmeyer flask. The flasks were sealed and then

shaken at 150 rpm in a thermostatic orbit shaker. The effect of solution pH on

equilibrium adsorption was studied at a HA concentration of 30 mg/L in the pH range

of 4-10. For all the rest of the batch experiments, the initial pH of HA solution was

adjusted to 7.0 prior to contact with the adsorbent. NaCl was used to check the

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influence of ionic strength on adsorption isotherms at different initial HA

concentrations. To investigate the influence of coexisting common ions including Ca2+

,

Mg2+

, K+, Na

+, Cl

-, NO3

-, SO4

2- and HCO3

-, the corresponding sodium or chlorine

salts were introduced into HA solution. After the attainment of equilibrium, the

suspension was separated by a magnet. The concentration of HA remaining in

solution was measured by T6 UV/Vis spectrophotometry (Beijing Purkinje General

Instrument Co. Ltd., China) at 254 nm. The amount of adsorbed HA was calculated by

conducting mass balance between initial and final HA concentrations and is expressed

in mg of HA per g of MCNP (written as mg/g). All batch adsorption experiments were

performed in triplicate, and the average results were reported.

2.5. Regeneration

Five consecutive adsorption-regeneration cycles were performed to test the

reusability of MCNP. Regeneration of adsorbents were studied by placing 0.1 g

MCNP into 100 mL of HA solution (C0=30 mg/L) with a shaking of 150 rpm for 1 h

at 25°C and pH 7. To desorb, the HA-laden MCNP was magnetically collected,

quickly rinsed with distilled water, and stirred for 1 h in 20 mL of 1 M NaOH at 25°C.

The regenerated MCNP was then washed with distilled water until pH 7 and mixed

again with the HA solution. The amount of HA adsorption and desorption from

MCNP were calculated by the absorbance value of HA solutions at 254 nm.

3. Results and discussion

3.1. Adsorbent characterization

The TEM and SEM images of MCNP are shown in Fig. 2. TEM image in Fig. 2a

directly demonstrates that MCNP is quasi-spherical in shape with size of 10 nm. SEM

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image in Fig. 2b indicates that there is a slight aggregation of these nanoparticles due

to the existence of cross-linked chitosan on the surface of MCNP. XRD pattern of

MCNP (Fig. 3) is in good agreement with that of standard Fe3O4 (magnetite, PDF

#75-0449). The extra diffraction peak at 2θ=20.5° was introduced due to the presence

of chitosan in MCNP. The X-ray diameter of the nanoparticles deduced from the

Scherrer equation was equal to 6.4 nm, which was smaller than the size of MCNP

(around 10 nm according to TEM image) possibly due to the cross-linked chitosan

coating. Since the XRD spectra of Fe3O4 and γ-Fe2O3 are quite similar, the

composition of MCNP was further confirmed by XPS. Fig. 4 shows the XPS survey

of MCNP and the spectrum of Fe 2p (inset). It can be concluded that MCNP contained

50.2 wt.% of Fe, 31.9 wt.% of O, 14.3 wt.% of C and 3.6 wt.% of N. The XPS

spectrum of Fe 2p reveals that there was no satellite peak detected for Fe 2p3/2, and

the peak positions of Fe 2p3/2 and Fe 2p1/2 are respectively, 710.6 and 724.1 eV. These

confirm that the magnetic iron oxide existed in MCNP is Fe3O4 [19].

To validate the synthetic route of MCNP, chitosan was also characterized by

recording the photoemission bands C 1s, O1s and N 1s. Fig. 5 shows C 1s, O 1s and

N 1s XPS spectra for chitosan and MCNP. These spectra were decomposed and the

binding energy (BE) of decomposed peaks and their assignment are included in Table

1. There is C 1s from C-C, C-N, C-O-C and C=O. The bound C=O can be attributed

to acetyl groups from chitosan backbone [20]. Due to the existence of intramolecular

and intermolecular hydrogen bonding in chitosan and MCNP, O 1s and N 1s XPS

spectra show significant peaks at O(2) and N(2). Meanwhile, the weak interaction

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between chitosan and Fe3O4 is also reflected at O(2) and N(2) in MCNP. In our

system, a ΔBE of 0.5 eV is significant.

Results indicate that the atomic concentration of C-C groups increased with the

chemical crosslinking and surface changes. This increase in atomic concentration of

aliphatic carbon (C-C) can be interpreted as a result of the addition alkyl groups

induced by crosslinking reactions. There was very little change in N 1s XPS spectrum

after modification. By contrast, the O 1s bands were significantly affected by the

chemical modification. A new component appeared at the binding energy of 530.8 eV

which belongs to Fe-O groups in Fe3O4 of MCNP. The intensity of O 1s signal in

MCNP was drastically reduced due to the crosslinking reaction between epoxy groups

in epichlorohydrin and hydroxyl groups in chitosan.

3.2. HA adsorption on MCNP

3.2.1. Effect of pH

Solution pH is the most crucial and elementary parameter in drinking water

treatment. It is important to examine the effect of solution pH on adsorption

performance as many adsorbents have pH-dependent adsorption property. The

influence of solution pH (from 4 to 10) on adsorption kinetics of HA onto MCNP was

investigated and the results are presented in Fig. 6. It is obvious that the equilibrium

state of adsorption was reached much faster at higher pH. At pH 9 and 10, the

adsorption equilibrium occurred after 40 min, and the time of equilibrium was 60 min

at pH7. While the equilibrium was observed over an 80 min period at pH lower than 7.

Meanwhile, an increase in solution pH from 4 to 10 caused an obvious decrease in

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equilibrium adsorption capacity of HA from 29.3 to 7.4 mg/L. These are related to the

surface charge of MCNP and the structural morphology of HA molecule at different

solution pH.

Fig.7 reveals the schematic diagram of the interactions between HA and MCNP.

The amino groups (-NH2) distributed on the surface of MCNP can react with

hydrogen ions to form protonated amino groups (–NH3+) at pH<6.4, rare amino

groups can be protonated in alkaline conditions [21]. Meanwhile, the dissociation of

H+ from carboxylic (-COOH) and phenolic (-OH) groups makes HA molecules be

electronegative. The dissociation of carboxylic groups takes place at pH>4 whereas

phenolic groups undergo dissociation at pH>8 [22]. This difference in pKa between

carboxylic and phenolic groups causes an increase in the size of HA molecules with

increased solution pH from a spherical structure to a rather linear or stretched

structure. At high solution pH, most of amino groups on the surface of MCNP

remained constant, the HA molecules were adsorbed by MCNP through hydrogen

bonding interaction with amino and hydroxyl groups. The linear HA molecules

needed more adsorption sites and the stretched configuration enhanced the

electrostatic repulsive interaction between HA molecules in the aqueous solution and

those on the surface of MCNP [23]. Hence, the adsorption sites on the surface of

MCNP were occupied quickly, and a small number of HA molecules were adsorbed.

At low solution pH, almost all of amino groups on the surface of MCNP were

protonated. The strong electrostatic attraction made a large number of negative and

spherical HA molecules adsorbed on the surface of MCNP. The spherical HA

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molecules could be adsorbed with fewer adsorption sites relative to linear

configuration. Moreover, the repulsive interaction between HA molecules was

reduced. Therefore, the adsorption sites were attached by HA molecules more

effectively, and the adsorption layer was compact. Meanwhile, the pH values of the

solutions after adsorption at different pH levels all remained almost unchanged. This

indicated that adsorption using MCNP could keep the pH level of water processing

system stable.

The rate-determining step during adsorption process can be distinctly affected by

solution pH. The adsorption of HA onto MCNP may be considered to consist of two

processes [21]: (a) the diffusion of HA from bulk solution to the surface of MCNP

through liquid film and (b) the attachment of HA to MCNP. Among these two

processes, the attachment process involves two steps suggested in Eqs. (1) and (2) (A

stands for HA). Once HA is diffused to the surface of MCNP, the step in Eq. (2) can

be a fast process if protonated amino groups are available. The protonation of -NH2 to

-NH3+, however, can be a slow process in HA attachment at higher pH values. Hence,

it can be assumed that the attachment process is controlled by Eq. (1).

(1)

(2)

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The diffusion process can be described by the Fickian diffusion model [1]:

0

2tq C S Dt

(3)

where qt is the amount of HA adsorbed per unit weight of MCNP at time t, C0 is the

initial HA concentration in the bulk solution, S is the specific surface area of MCNP

and D is the diffusion coefficient of HA in the solution. Eq. (3) indicates that under a

diffusion-controlled adsorption process, qt versus t0.5 will follow a linear relationship.

Fig. S1 shows a plot of qt versus t0.5 for the experimental results in Fig. 6 from the

beginning till almost the adsorption equilibrium. It is clearly observed that the linear

relationship of qt against t 0.5 at pH>7 is better than that at pH≤7. As shown in Fig. 7,

the linear configuration of HA molecules in alkaline condition brings stronger mass

transfer resistance in liquid film due to the larger HA molecule diameter and more

bare negative charges than those in acidic condition. Although the electrostatic

repulsion between HA molecules is existed in acidic condition, it becomes so weak

that the diffusion process is not the rate-determining step during the whole adsorption

process.

The attachment process can be described by the model proposed by Zhang [21]:

1 expt eq q k t (4)

where qe is the amount of HA adsorbed (mg/g) at adsorption equilibrium and

k′=k1[H+], k1 is the reaction constant of Eq. (1). Fig. S2 shows a plot of qt versus t for

the experimental results in Fig. 6 again. The experimental data can be adequately

fitted by the attachment model at pH>7 due to a few protonation of amino groups. By

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comparing the fitting results in Fig. S1 and Fig. S2 at pH≤7, the experiment data were

better fitted by the attachment model than the diffusion model. This may be related to

the existence of place obstruction on the HA laden surface of MCNP. Those HA

molecules that already attached to the adsorbent surface certainly have a space

squeezing effect on other coming HA molecules. This will result in an unstable

attachment for HA molecules and need more time to reach adsorption equilibrium.

Hence, the adsorption kinetics of HA adsorption onto MCNP can be considered to be

an attachment-limited process under low pH conditions, but to be a process controlled

by both transport-limited and attachment-limited mechanisms under high pH

conditions.

3.2.2. Effect of ionic strength

As many different kinds of salts are mixed in actual natural water source, its ionic

strength is high. To investigate the effect of ionic strength on HA adsorption capacity

and adsorption isotherms, equilibrium adsorption studies were carried out at different

ionic strengths. As shown in Fig. 8, it was found that ionic strength had a positive

effect on adsorption capacity of MCNP at HA concentrations ≥20 mg/L. The

configuration of HA molecule is affected by ionic strength. At high ionic strength, the

charge repulsion between adjacent carboxylic or phenolic groups on HA is largely

neutralized, resulting in a coiled configuration [24]. Hence, each HA molecule

occupies less surface area resulting in a higher adsorption capacity. In addition, the

DLVO theory demonstrated that the electric double layers of HA colloid particles

became compressed due to the presence of electrolytes in solution. The compression

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of the double layers results in a closer approach of HA molecules and MCNP which

increases the adsorption density. Meanwhile, it is worth noting that the enhancement

of adsorption capacity was greatly reduced when the ionic strength is 0.5 M. This may

be related to the competitive adsorption of chloride ions which are introduced by

NaCl in excess. At relatively low concentrations of HA solutions (C0=3, 10 mg/L), the

adsorption capacities increased when the ionic strength is 0.01 M and decreased with

the increasing ionic strength from 0.01 to 0.5 M. This is possible because the salt

effect on the adsorption may be classified as screening enhanced when ionic strength

is over 0.01 M [25], meaning that the electrostatic attraction between HA molecules

and surface adsorption sites is weakened resulting in a reduced adsorption capacities.

In the present study, the experiment data from all levels of ionic strength were

modeled by the Freundlich model (Eq. (5)) [26], the Langmuir model (Eq. (6)) [27]

and the Temkin model (Eq. (7)) [28] by non-linear method:

1/ne F eq k C (5)

em Le

eL

q k Cq

1 k C

(6)

ln lne eq B A B C (7)

Where Ce (mg/L) is the equilibrium concentration of remaining HA in the solution; kF

((mg/g)(L/mg)1/n) and n are Freundlich constants representing the adsorption capacity

and intensity of adsorption, respectively; qm (mg/g) is the maximum amount of HA

adsorbed at complete monolayer and kL (L/mg) is the Langmuir constant related to the

affinity of binding sites; B and A (L/mg) are the Temkin constants. Fitted equilibrium

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curves for HA adsorption are illustrated in Fig. S3. The derived unknown parameters

of each isotherm model and the correlation coefficients are reported in Table 2. The

fitting results showed that the adsorption isotherm was fitted better by Langmuir and

Temkin model than Freundlich model, indicating that the adsorption of HA onto

MCNP is a monolayer adsorption, and indeed there is interaction between HA

molecules and adsorbent surface. According to Langmuir isotherm model, the

maximum HA adsorption capacity for MCNP at 25°C and pH7 with ionic strength of

0 M was found to be 32.561 mg/g. It is evident that as ionic strength increased, qm

firstly increased, and posteriorly decreased when ionic strength is 0.5 M. The values

of kF and B represented a similar law to qm. Since the n values obtained from the

Freundlich isotherms were higher than 1.0, HA was favorably adsorbed by MCNP at

all ionic strengths studied. A one of the Temkin constants is the equilibrium

association constant related to the maximum binding energy between HA and MCNP.

It can be found that A achieved its peak value (5.873 L/mg) when ionic strength is

0.01 M, and then the value of A was on the decline. The results indicate that low ionic

strength made the interactions between HA and MCNP stronger, but high ionic

strength had a weakening effect on the interactions between HA and MCNP. Indeed,

water is considered to have very high salinity, if the concentration in NaCl equivalent

is about 0.06 mol/L [29]. At this concentration, the adsorption of HA onto MCNP

retains a good performance. The maximum adsorption capacity for HA using MCNP

is comparable with other reported adsorbents as shown in Table 3. From these data,

most of other adsorbents and modified chitosan reached maximum adsorption

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capacities with high dosage or long equilibrium time, which did not meet the

requirements of drinking water treatment. By contrast, MCNP presents excellent

adsorption efficiency and rapid separation process. Adsorption using MCNP can

significantly improve the processing capacity of water treatment plant. These all

suggest that MCNP can be an effective adsorbent for removal of HA for drinking

water treatment.

3.2.3. Effect of common ions

There are all kinds of ions existed in natural water sources. Previous reports suggest

that oxyanions and divalent cations present in natural water systems could

significantly influence the adsorption of organic matter [34,35]. Fig. 9 shows the

effect of added Ca2+

, Mg2+

, K+, Na

+, Cl

-, NO3

-, SO4

2- and HCO3

-on the adsorption of

HA onto MCNP. From the adsorption experiment results, it was observed that all of

the ions except HCO3-

showed positive effects on removal of HA. The cations Ca2+

and Mg2+

greatly improved the adsorption ability of MCNP towards HA. This may be

due to the salting-out effect produced by Ca2+

and Mg2+

. The addition of high valent

cations would make the solubility of HA reduced. The precipitated HA molecules are

easier to be adsorbed on the surface of MCNP. The competitive adsorption of NO3-

and SO42-

seemed much less crucial. It proves that the chitosan coated adsorbent

shows a better affinity to organic matter. The anion HCO3-caused a slight decrease in

HA adsorption. This may be because of the change in solution pH. The pH value of

the HA solution was 8.63 for HCO3-. It caused a decline of the number of protonated

amino groups, resulting in a slight decrease of adsorption capacity. Hence, it can be

concluded that the common ions existed in neutral natural water sources may not have

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negative effects on HA removal in most cases.

3.3. Adsorption mechanisms

3.3.1. FTIR spectra

Infrared spectroscopy is a very useful tool for determination of molecular structure

because direct information about the presence of functional groups is easily provided.

The FTIR spectra of HA and MCNP before and after reaction with HA at different

solution pH are shown in Fig. 10. The characteristic peaks in the spectrum of HA (Fig.

10a) can be assigned as follows: 3418 cm-1 (O-H stretching of carboxyl and phenol

groups), 1582 cm-1 (C-C stretching of aromatic rings and to C=O stretching of

conjugated carbonyl groups), 1376 cm-1 (the stretching of carboxylate) [25]. As for

MCNP (Fig. 10b), the broad band at 3384 cm-1 may be due to the overlapping of -OH

and -NH2 stretching, which is consistent with the peak at 1591, 1071 and 1017 cm-1

assigned to N-H deformation, C3-OH and C6-OH stretching, respectively. The peak

located at 1631 cm-1 is the characteristic of amide I band, which is the character peak

of chitosan in MCNP. The peak at 1372 cm-1 can be attributed to the -CH3 symmetric

deformation.

As shown in Fig. 10, the peak at the wavenumber of 3384 cm-1, corresponding to

the overlapping of -OH and -NH2 stretching in MCNP, decreased to 3375 cm-1 after

adsorption at pH 10, but remained unchanged after adsorption at pH 7, and increased

to 3409 cm-1 after adsorption at pH 5. The decreased wavenumber can be attributed to

the formation of hydrogen bonding between -OH, -NH2 groups on MCNP and

carboxyl, phenol groups on HA molecules. The unchanged and increased

wavenumber may be caused by the loaded HA molecules on the surface of MCNP

after adsorption, which brought a large number of carboxyl and phenol groups,

resulting in offsetting the impact of hydrogen bonding. The reason why the

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wavenumber of -OH and -NH2 stretching after adsorption at pH 10 was not increased

is that the number of adsorbed HA was too small to bring this peak to high

wavenumber. After adsorption at different solution pH, the peak at 1376 cm-1

appeared which belongs to the carboxylates in HA molecules. Another significant

change in the FTIR spectra is found that the adsorb band at 1591 cm-1, assigned to

N-H deformation, disappeared at pH 5 after adsorption. This can be attributed to the

formation of the coordinated -NH3+ and A

- complexes, which greatly reduced the

vibration frequency of the -NH2 groups. The peak of N-H deformation was slightly

shifted to lower wavenumber after adsorption at pH 7 and remained unchanged after

adsorption at pH 10, indicating that much more coordinated -NH3+ and A

- complexes

were formed at pH 5 than at pH 7, and again at pH 7 than at pH 10. Meanwhile, the

C3-OH and C6-OH stretching became weaker and shifted to lower wavenumber in

higher solution pH after HA adsorption. This indicates that hydrogen bonding

dominates the adsorption interactions in alkaline conditions. The peak of amide I band

was also appeared in the FTIR spectra of MCNP after reaction with HA at different

pH, showing that the MCNP is chemically stable in both acidic and basic solutions.

The FTIR results clearly explain the experimentally observed adsorption phenomenon

of HA on MCNP at different solution pH values, adsorption decreased with increasing

solution pH.

3.3.2. XPS spectra

Due to its surface sensitivity and chemical specificity, XPS is also used to analysis

the surfaces of materials. To investigate the adsorption mechanisms of HA on MCNP,

XPS studies of MCNP after HA adsorption at different solution pH values were

conducted. It appears that the different forms of amino and hydroxyl groups on

MCNP played a very important role in the adsorption performance. The N 1s and O 1s

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XPS spectra were therefore obtained to provide evidences and to explain the observed

phenomenon. Fig. 11 shows the N 1s and O 1s XPS spectra for MCNP with HA

adsorption obtained from experiments conducted at solution pH 5, 7 and 10,

respectively. The relative peak ratios of amino and hydroxyl groups have been

calculated from the areas under each peak in Fig.11 and the results are given in Table

4.

With HA adsorbed on MCNP, the N 1s XPS spectra in Figs. 11b and 11c show a

new component of N(3) peak at 400.8 eV. This peak can be attributed to the formation

of -NH3+··A

- complexes, in which a lone electrons in the nitrogen atom was donated

to the shared bond between the N and A-, and, as a consequence, the electron cloud

density of the nitrogen atom was reduced, resulting in a higher BE peak observed [25].

This phenomenon was not found in Fig. 11a due to the basic condition which makes

amino groups disable to be protonated. In the case of pH 5, the peak area percentage

of N(1) and N(3) is found to be 21.7% and 30.5%. While in the case of pH 7, the peak

area percentage of N(1) and N(3) is found to be 29.8% and 24.3%. The lower pH

condition results in a weaker N(1) and a stronger N(3) of MCNP after HA adsorption.

Moreover, as the adsorption capacity increasing with the decreasing solution pH, the

peak area percentage of N(2) increased, indicating that the hydrogen bonding is also

existed between HA molecules and amino groups on MCNP in both acidic and basic

conditions. From Figs. 11d, 11e and 11f, it is clear that the ratio of O(1)/Ot decreased

and that of O(2)/Ot increased with the increase of solution pH values. This indicates

that hydroxyl groups on MCNP have a good affinity to HA molecules through

hydrogen bonding, especially in the basic condition. Therefore, it can be concluded

that the electrostatic attraction and hydrogen bonding are the main adsorption

mechanisms in acidic solutions, and the HA adsorption in alkaline solutions is mainly

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through forming hydrogen bonding with the neutral amino groups and hydroxyl

groups on the surface of MCNP.

3.4. Regeneration

Good desorption performance of an adsorbent is important in its potential practical

applications. From the results in Fig. 6, it has been found that MCNP did not adsorb

HA significantly at pH>9, suggesting that the adsorbed HA on MCNP may be

possibly desorbed in a basic solution with a high solution pH value. The desorption

reaction taking place in the basic desorption solutions can be given as

(8)

As shown in Eq. (8), the protonation effect of amino groups was significantly

weakened and more neutral amino groups were generated due to the deprotonation of

-NH3+ groups in an extreme alkaline solution. Hence, the electrostatic attraction

between HA molecules and -NH3+ groups was non-existent. The structure of desorbed

HA molecules became stretched so that it was difficult for HA molecules to attach to

the surface of MCNP. Meanwhile, the solubility of HA molecules was highly

improved in basic solutions. All of these indicate that an extreme alkaline solution is

suitable for the desorption of MCNP. Satisfactory regeneration results were achieved

as demonstrated in Fig. 12. It was observed that after the first desorption step for 1 h,

the amount of desorbed HA was approximately 95.4%. The incomplete desorption of

HA on MCNP may due to the dual process of physisorption and chemisorption [36]. It

was worth noting that the amount of desorption was slightly decreasing with the

increasing contact time during every single desorption process. This may be because

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the mass driving force that resulted from the already desorbed HA will make small

amounts of desorbed HA adsorbed onto MCNP again. The following adsorption steps

revealed a similar dynamical shape of the first adsorption step, and maintained more

than 90% of adsorption capacities against the first adsorption step. This suggests that

MCNP can be reused almost without any significant loss in the adsorption

performance. Therefore, MCNP is a promising adsorbent for removing HA from

aqueous solution due to its potential practical applications in drinking water treatment.

4. Conclusions

The adsorbent studied in this paper is based on chitosan, a low-cost, biocompatible

and biodegradable natural biopolymer. The novel magnetic chitosan nanoparticle

(MCNP) was prepared induced by chitosan hydrogel via in-situ co-precipitation at

low temperature and normal atmosphere without nitrogen protection to remove humic

acid (HA) from aqueous solutions. The resulting MCNP is quasi-spherical in shape

with size of 10 nm. The magnetic iron oxide existed in MCNP has been identified as

Fe3O4. Chemical crosslinking reaction was achieved between epoxy groups in

epichlorohydrin and hydroxyl groups in chitosan, which avoided the consumption of

active amino groups. The increase in solution pH from 4 to 10 caused an obvious

decrease in HA adsorption capacity from 29.3 to 7.4 mg/L. The adsorption kinetics of

HA adsorption onto MCNP can be considered to be an attachment-limited process

under low pH conditions, but to be a process controlled by both transport-limited and

attachment-limited mechanisms under high pH conditions. Low ionic strength made

the interactions between HA and MCNP stronger, but high ionic strength had a

weakening effect on the adsorption interactions due to the introduction of competitive

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adsorption and screening enhanced. The common ions existed in natural water sources

would not have obvious negative effects on HA removal. The chitosan coated MCNP

showed a better affinity to organic matters. The electrostatic attraction and hydrogen

bonding are the main adsorption mechanisms in acidic solutions, and the HA

adsorption in alkaline solutions is mainly through forming hydrogen bonding with the

neutral amino groups and hydroxyl groups on the surface of MCNP. Regeneration

studies indicate that MCNP can be recyclable for a long term. This fundamental

research should be helpful for the future development of sustainable magnetic

adsorbent materials in drinking water treatment plants.

Acknowledgements

This work was supported by the National Natural Science Foundation of China

(51178159), and Key Laboratory of Integrated Regulation and Resource Development

on Shallow Lakes, Ministry of Education (Hohai University) China.

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Fig. 1. Synthetic route of MCNP.

Fig. 2. (a) TEM and (b) SEM images of MCNP.

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Fig. 3. XRD pattern of MCNP.

Fig. 4. XPS survey of MCNP and the spectrum of Fe 2p (inset).

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Fig. 5. C 1s, O 1s and N 1s XPS spectra for chitosan and MCNP.

Fig. 6. Effect of solution pH on the HA adsorption kinetics (initial HA concentration

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30 mg/L; temperature 25°C).

Fig. 7. Schematic diagram of the interactions between HA and MCNP at (a) high and (b) low solution pH.

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Fig. 8. Effect of ionic strength on the HA adsorption capacity at different initial HA concentrations (solution pH 7; temperature 25°C; contact time 60 min).

Fig. 9. Effect of other common ions on removal of HA (ion concentration 0.01 M; initial HA concentration 30 mg/L; solution pH 7; temperature 25°C; contact time 60 min).

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Fig. 10. FTIR spectra of (a) HA, (b) MCNP and MCNP after reaction with the HA at pH (c) 10, (d) 7 and (e) 5.

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Fig. 11. N 1s and O 1s spectra of MCNP with HA adsorbed from solutions at different pH values.

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Fig. 12. Adsorption and desorption of HA on MCNP.

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Table 1Assignments of main spectral bands based on their binding energies (BE) and atomic concentration (AC) for chitosan and MCNP.

Chitosan MCNPElement

BE (eV) AC (%) BE (eV) AC (%)Assignments

C(1) 285.1 4.3 285.3 5.8 C-CC(2) 286.6 27.8 286.3 19.7 C-N, C-O-CC(3) 288.1 4.0 288.1 2.0 C=O

Total C 36.1 27.5O(1) 532.5 30.8 532.0 12.6 -OHO(2) 533.2 24.9 533.7 15.3 -OH--O, -OH--N, -OH--Fe3O4

O(3) 530.8 18.0 Fe-OTotal O 55.7 45.9

N(1) 398.0 7.1 398.0 5.0 -NH2

N(2) 400.3 1.1 399.9 0.9 -NH2--O, -NH2--Fe3O4

Total N 8.2 5.9

Fe710.6, 724.1

20.7 Fe-O

Total Fe 20.7

Table 2Isotherm constants for HA onto MCNP.

Freundlich Langmuir TemkinIonic strength

n kF R2 qm kL R2 A B R2

0 M NaCl 2.913 10.087 0.932 32.561 0.336 0.997 4.802 6.180 0.9940.01 M NaCl 2.341 15.145 0.969 51.937 0.373 0.992 5.873 9.803 0.9890.1 M NaCl 1.813 15.459 0.920 70.141 0.271 0.968 2.621 15.721 0.9940.5 M NaCl 1.993 9.481 0.936 51.745 0.168 0.984 1.716 11.332 0.999

Table 3The comparison of maximum HA adsorption capacities of various adsorbents on the basis of Langmuir isotherm model at 25°C and pH7 with ionic strength of 0 M.

Adsorbent Dosage (g/L) Equilibrium time (h) qm (mg/g) ReferenceAminopropyl functionalized rice husk ash 5.0 0.5 8.2 [4]Acid-activated Greek bentonite 12.0 5 10.752 [10]Fly ash 0.5 100 10.7 [30]palygorskite 5.0 2 17.0 [31]Activated carbon (rice husk) 3.3 2.5 45.4 [32]Chitosan hydrogel beads 33.3 24 0.6 [33]Chitosan/zeolite composites 1.0 24 74.1 [2]MCNP 1.0 1 32.561 This study

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Table 4Surface composition of the different types of amino and hydroxyl groups on MCNP with HA adsorption at different solution pH values.

Contents of different types of amino and hydroxyl groups (%)N(1)/Nt N(2)/Nt N(3)/Nt O(1)/Ot O(2)/Ot O(3)/Ot

pH 10 71.4 28.6 22.1 41.2 36.7pH 7 29.8 45.9 24.3 23.7 39.7 36.6pH 5 21.7 47.8 30.5 42.1 37.7 20.2

Note. N(1) for -NH2; N(2) for -NH2--A-, -NH2--Fe3O4 or -NH2--O; N(3) for -NH3

+··A

-;

O(1) for -OH; O(2) for -OH--A-, -OH--O, -OH--N or -OH--Fe3O4; O(3) for Fe-O; Nt

denotes summation of peak areas of N(1), N(2) and N(3); Ot denotes summation of peak areas of O(1), O(2) and O(3), based on the analysis of the XPS results.

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Highlights

Magnetic chitosan nanoparticle (MCNP) was synthesized at mild conditions.

MCNP has pH and ionic strength dependent adsorption property toward humic acid.

The adsorption process is controlled by transport and attachment mechanisms.

The adsorption mechanism involves electrostatic attraction and hydrogen bonding.

MCNP presents a good desorption performance.

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*Graphical Abstract (for review)

colloids and surfaces a- physicochemical and engineering aspects volume issue 2014 [doi...

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