Preparation and Characterization of a Novel Spherical Cellulose Adsorbent for Reduction Adsorption of Trichloroacetic Acid①

LIN Chun-Xing TIAN Chen LIU Yi-Fn CHENG Yng-Jin LIN Zhng LIU Ming-Hu②

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Preparation and Characterization of a Novel Spherical Cellulose Adsorbent for Reduction Adsorption of Trichloroacetic Acid①

LIN Chun-XiangaTIAN ChenaLIU Yi-FanaCHENG Yang-JianaLIN ZhangaLIU Ming-Huaa②

a(350108)b(350002)

A novel spherical cellulose adsorbent has been prepared by homogeneous graft polymerization of N,N?-methylenebisacrylamide (MBA) onto cellulose in an ionic liquid, 1-N-butyl-3-methylimidazolium chloride (BMIMCl), which was then partially amine methylated through Mannich reaction to get bifunctionalized materials containing both amide and sulphinate moities. Factors affecting the attachment of functional groups were investigated. The adsorbent was characterized by Elemental Analysis (EA), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscope (SEM). Cellulose adsorbent was then tested for its potential applications in the reduction adsorption of trichloroacetic acid (TCAA) from aqueous solutions.

cellulose adsorbent, reduction adsorption, trichloroacetic acid, Mannich reaction

1 INTRODUCTION

In recent years, increasing costs and environ- mental considerations associated with the use of commercial adsorbents have led to a significant body of research work aimed at developing new low cost adsorbents derived from renewable resources. Discarded cellulosic biomass derived from forestry, agriculture, and municipal sources is potential feed stocks for the synthesis of various adsorbents. Cellulose, the main component of lignocelluloses, constitutes the most abundant and renewable polymer resource available worldwide. It is a very promising raw material available at low cost for the preparation of various functional polymers. How- ever, cellulose by itself could not be satisfactorily applied in chelating or adsorbing pollutants. Hence many attempts have been made to utilize cellulose as an adsorbent through chemical and physical modifications[1]. Recently, modified cellulose has been used to remove different types of adsorbates from water[2-10].

We herein design a novel spherical cellulose adsorbent offering the advantage of bearing two distinct functional groups, each one with its own properties, the first one being selected for its ability to reduce pollutants and the second one for its adsorption behavior towards the generated moieties. The organo-functional groups selected to achieve this goal are amide and sulphinate moieties. The effect of reaction parameters, such as dosage of reactants and catalyst, reaction temperature and reaction time is given in this report as well as the reduction adsorption of selected pollutant trichlo- roacetic acid (TCAA).

2 EXPERIMENTAL

2. 1 Materials

Cotton linter used as cellulose material was obtai- ned from commercial sources. Ionic liquid 1-N- butyl-3-methylimidazolium chloride (BMIMCl) was purchased from Henan Lihua Pharmaceutical Co., Ltd. Trichloroacetic acid (TCAA) was chroma- tographic grade, purchased from Tianjin Fuchen Chemical Reagent Factory. Thiourea dioxide (TD), formaldehyde and other reagents were of analytical grade and used as received.

2. 2 Grafting of cellulose with N,N?-methylene-bisacrylamide (MBA) in BMIMCl

In a typical reaction procedure, 0.05 g potassium persulfate used as initiator was added to 15 g cellulose/BMIMCl solutions (containing 3% of cellulose by weight) in flask. After stirring for 15 minutes, 1.5 g MBA monomer was added to the mixture and stirred for 2 h at 40～70 ℃. The polymerization reaction was operated under N2atmosphere. After the graft copolymerization reac- tion, the spherical grated cellulose (GC) copolymer was obtained from granulation using water as a coagulation bath. The grafting percentage (GP) of the copolymers was calculated by Eq. (1):

GP = (W2– W1)/W1× 100% (1)

where W1and W2are the weight of raw cellulose and grafted cellulose, respectively.

2. 3 Mannich reaction

Typical procedures are as follows: 3.2 g thiourea dioxide was first treated with 20% of H2SO4in water for minutes at 35 ℃, then an aqueous formaldehyde solution was added and stirred for 1.5 h. To the reaction mixture, 0.8 g GC was added and stirred for additional 2 h at 65 ℃. All above processes were operated under N2atmosphere. After the reaction was completed, the spherical cellulose beads were isolated and the bi-functional cellulose adsorbent was obtained. The adsorbent was vacuum dried at 60 ℃ and then the reduction-sorption per- formance towards TCAA (initial concentration 50 mg/L, pH = 7) was conducted at room temperature. The decomposition rate of TCAA () was calculated according to Eq. (2) and considered to be the index in Mannich reaction.

= (C1– C2)/C1× 100% (2)

where C1is the initial concentration of TCAA (mg/L), and C2is the final or equilibrium con- centration of TCAA (mg/L).

The adsorption capacity of cellulose adsorbent towards TCAA or reduction products (i.e. mono- chloroacetic acid (MCAA) and dichloroacetic acid (DCAA)) was obtained by the following equation:

Qe= (M2– M1)/M1× 100% (mg/g) (3)

where M1and M2are the weight of adsorbent before and after adsorption (g). Concentrations of TCAA, DCAA, MCAA and other ions in solutions were determined using an ion chromatography (881 Compact IC pro, Switzerland).

2. 4 Instrumentation-material characterization

The surface morphology of cellulose, GC and cellulose adsorbent were observed with a XL30 SEM-TMP Environmental Scanning Electron Microscope (Philips-FEI, Holland). IR spectra were recorded on a Nicolet iS10Fourier Transform Infrared Spectrometer (Thermo Fisher Scientific, USA) using KBr pellets. The contents of C, H, N and S in adsorbent were determined by a Vario MICRO Elemental Analyzer (Elementa, Germany).

3 RESULTS AND DISCUSSION

3. 1 Graft copolymerization of cellulose with MBA

In previous work, dissolution and graft copoly- merization of cellulose with vinyl monomers in BMIMCl were reported[12-16]. The high concentra- tion of chloride and its activity in ILs were con- sidered to play an important role in cellulose dissolution, by breaking the extensive hydrogen- bonding network present in cellulose. A completely homogeneous system was achieved when the reaction proceeded at 50 ℃. The extent of grafting was evaluated in terms of grafting percentage (GP), which is the ratio of the amount of introduced side chains to that of the main chain. The GP of MBA onto cellulose was listed in Table 1. As summarized in Table 1, the GP of the copolymer depended on the mass ratio of the MBA/cellulose, reaction time, and reaction temperature used. Appropriately, increasing the mass ratio of MBA/cellulose and extending the reaction time may raise GP. The highest GP achieved was 38.1%. The reaction could be also accelerated through raising the temperature which might not exceed 70 ℃because higher temperature may lead to uncontrolled side reaction (self-crosslinking of the MBA).

Table 1. Conditions and Results of the Homogeneous Grafting of Cellulose in BMIMCl

3. 2 Mannich reaction of grafted cellulose (GC)

Mannich reaction is one of the most important carbon-carbon bond formation reactions in organic synthesis[17]and an atom-economy reaction. It is a multi-component condensation between a non- enolizable aldehyde (commonly formaldehyde), an amine, and an enolizable carbonyl compound[18]. The products of Mannich reaction are mainly b-amino carbonyl compounds and 1,2-amino alcohol derivatives[19], while other products may also formed with different dosage and adding sequence of materials.

Previous work reported that Mannich reaction occurs between a carbon with high electron density and an immonium ion formed from formaldehyde and an amine[20]. Therefore, an aminomethyl group in the Mannich reaction can be introduced into thiourea dioxide. Then the electrophilic addition reaction occurs between the aminomethyl group in thiourea dioxide and the amide group in GC. Thus, the bi-functional spherical cellulose adsorbent with amide and sulphinate group was obtained via Mannich reaction, showing as Scheme 1.

Scheme 1. Mannich reaction route of the grafted cellulose

In Scheme 1, TD and formaldehyde were condensed firstly by catalyzing with sulfate acid, and then electrophilic addition occurred with GC to form bi-functional cellulose adsorbent. The Man- nich reaction was evaluated in terms of reduc- tion-removal of TCAA (η) in aqueous. The removal ratio of TCAA depended on the Mannich reaction parameters, and the results are shown in Table 2. It can be seen that increasing the ratio of TD/for- maldehyde and the amount of sulfate acid may raise the removal percentage of TCAA. The highest performance of the adsorbent was obtained under the conditions when the molar ratio of TD/for- maldehyde was 2:1, sulfate acid dosage 2 g, condensation reaction temperature (T1) 35 ℃ and addition reaction temperature (T2) 50 ℃. Higher reaction temperature might lead to the decomposition and oxidation of the reactant and products, causing the decrease in the removal percentage of TCAA[21].

Table 2. Conditions and Results of the Mannich Reaction

3. 3 Characterization of the adsorbent

The formation of grafted cellulose and cellulose adsorbent were confirmed by elemental analyses, FT-IR spectroscopy, and SEM. The elemental analy- ses of the grafted cellulose sample (Table 3) indicated that 4.1 wt-% N was detected. This suggested that successful substitution of the OH group by MBA (GP = 38.1%) occurred in the first step. Elemental analyses of cellulose adsorbent have also demonstrated successful loading of the sulphi- nate groups onto the grafted cellulose because the S content averages 5.3 wt-%.

Table 3. Results of the Elemental Analyses

The presence and integrity of organo-functional groups were further checked by FTIR (Fig. 1). The spectrum of the grafted cellulose was characterized by N–H and C–N vibrations of amide chains at 3150, 1571 and 1334 cm-1, and a vibration corres- ponding to the C=O group at 1657 cm-1. The sulphinate moieties on the cellulose adsorbent can be identified via the vibration of band at 1090 cm-1while the peak at 1189 cm-1is corresponding to the oxidation product of part sulphinate group.

The SEM micrograph (Fig. 2a) of cellulose illus- trates a relatively smooth, continuous structure. After the graft copolymerization reaction and Man- nich reaction, rough morphology of the surface of functionalized microparticles is clearly observed (Fig. 2b and 2c). It may arise because of the polar difference between cellulose and the interruption of intermolecular hydrogen bonds and crystalline regions in cellulose. Moreover, the micro sphere structure of different sizes and irregular shapes can be observed on the surface of the adsorbent (Fig. 2c), which might be ascribed to that the sulphinate groups (-SO-OH) of thiourea dioxide attached to the surface of grafted cellulose bead with an amorphous globular shape.

Fig. 1. IR spectra of cellulose (a), GC (b) and cellulose adsorbent (c)

Fig. 2. SEM images of cellulose (a), grafted cellulose (b) and cellulose adsorbent (c) at 5000× magnification

3. 4 Performance of the spherical cellulose adsorbent

The prepared bi-functional spherical cellulose adsorbent was then used to test the reduction- adsorption ability towards TCAA in aqueous solution, with the results summarized in Table 4. The data showed that the bi-functional cellulose adsorbent led to significant TCAA reduction, espe- cially between pH 6 and pH 10. One already knows that thiourea dioxide has been described as a reducing agent[22, 23], and it can also be applicable for TCAA reduction. The reduction of TCAA was possibly stepwise, following the pathway illustrated in Eqs. (4)～(6). Moreover, the reduction activities decreased in the order of TCAA, DCAA and MCAA[24].

This equation indicated that TCAA reduction was favored at higher pH values (pH = 6～10). This explained the values of degradation rate of TCAA listed in Table 3, indicating dramatic decrease in reduction percentage below pH 4, because the medium was not basic enough for sulphinate groups to reduce TCAA species.

On the other hand, the rest of TCAA and the generated species (i.e. MCAA and DCAA) were immobilized on the adsorbent, which was also ascertained by elemental analysis (Table 5). The appearance of Cl element and the increase of C and H elements confirmed that chloroacetic acids were adsorbed onto the adsorbent. The adsorption process then proceeded due to the electrostatic interaction between amide groups and chloroacetic acids. However, the adsorption capacity of spherical cellulose adsorbent was decreased with increasing the pH value, as shown in Table 4. The reason might be ascribed to that more amide groups were pro- tonated to form groups -NH3+, thereby increasing the electrostatic attractions between chloroacetic acids and adsorption sites, causing the observed increase in adsorption capacity

Table 4. Degradation Rate of TCAA and Adsorption Capacity of Adsorbent for the Generated Moieties*

*Adsorption conditions: adsorbent dosage, 1 g/L; initial concentration of TCAA, 50 mg/L; solution pH, 7; contact time, 3 h; adsorption temperature, 30℃

Table 5. Elemental Analyses of the Adsorbent before and after Adsorption

4 CONCLUSION

A novel spherical cellulose adsorbent with reduc- tion and adsorption capacity was successfully prepared by Mannich reaction after homogeneous graft copolymerization with MBA in BMIMCl. The degradation rate of TCAA could reach 97.3% when the adsorbent was prepared under optimum reaction conditions. The EA, IR spectrum and the SEM images of the adsorbent confirmed the existence of functional group on cellulose adsorbent. The functionalized cellulose behaved as good reduction sorbent for TCAA, thus showing their potential for other oxidative pollutants removal.

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21 October 2013;

22 July 2014

the Science and Technology Project of Fujian Province Educational Department (JK2013004, JA12040), Science & Technology Development Fund of Fuzhou University (2012-XY-10, 2014-XQ-11) and the National Natural Science Foundation of China (41372346)

. Born in 1970, Doctor, majoring in environmental materials. E-mail: mhliu2000@fzu.edu.cn