Adsorption of tannin from aqueous solution by deacetylated konjac glucomannan

Journal of Hazardous Materials 178 (2010) 844–850

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Adsorption of tannin from aqueous solution by deacetylated konjac glucomannan Feng Liu, Xuegang Luo ∗ , Xiaoyan Lin Department of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, Sichuan, 621010, China

a r t i c l e

i n f o

Article history: Received 28 July 2009 Received in revised form 10 January 2010 Accepted 4 February 2010 Available online 12 February 2010 Keywords: Deacetylated konjac glucomannan Water absorbency Tannin Adsorption

a b s t r a c t Konjac glucomannan treated by alkali solution through deacetylated reaction was used as a new water insoluble adsorbent to remove tannin from aqueous solution. A comprehensive study on adsorption of tannin by deacetylated konjac glucomannan (DKGM) was conducted regarding the effects of initial pH, adsorbent dosage, contact time, temperature and initial tannin concentration. The adsorption process was much dependent on the pH and temperature and was found to follow pseudo-second-order kinetics. The optimum pH value was at pH ranging from 2 to 6. The maximum removal efficiency of tannin from aqueous solution was 90%. Increasing the adsorption temperature would result in lower adsorption capacity, suggesting that adsorption of tannin onto DKGM was exothermic in nature. The adsorption isotherms were measured at various temperatures and correlated to Freundlich isotherms. Adsorption mechanism was confirmed that the interaction of DKGM and tannin was through hydrogen bonding. It was also observed that DKGM possesses excellent reusability for tannin removal. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Tannins as one category of natural polyphenolics existed in many plants are recognized as hazardous pollutants. Hydrolysable tannin wastewaters, mainly from leather processing, plant medicine production and paper making, have caused many environmental problems [1]. In general, vegetable tannins are categorized to condensed tannins and hydrolysable tannins. Comparing toxicity of condensed tannins and hydrolysable tannins, hydrolysable tannins are more toxic than that of condensed tannins [2]. It is attributed that hydrolysable tannins are apt to decompose into gallic acid by hydrolyzation, which is easy to impair human health. Most of hydrolysable tannins are highly soluble in water and make wastewater with distinguished color. Hydrolysable tannins with average molecular weight in range from 500 to 3000 Da in general have phenolic characteristics. Large quantity of hydrolysable tannin wastewater with strong color is harmful to microorganisms, animals and humans [3–6]. Therefore, toxic hydrolysable tannins must be removed from wastewater before effluent is discharged into environment. Many methods have so far been reported for removal of tannins from wastewater, such as chemical sedimentation, biodegradation, adsorption, electrochemistry, membrane, chemical oxidation, and photo-catalytic degradation [7–9]. Among these methods, chemical sedimentation has been widely used in actual application [10]. However, in order to obtain good removal efficiency, huge amounts

∗ Corresponding author. Tel.: +86 08166089009; fax: +86 08166089009. E-mail addresses: [email protected], [email protected] (X. Luo). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.02.015

of metal agents are used to precipitate tannins, which is easy to cause secondary pollution. The method of biodegradation is suitable for treatment of tannins at low concentration. Nevertheless, growth of the microbes which are helpful to the degradation of tannins is inhibited heavily when the concentration of tannins is higher than 490 mg L−1 [11]. Adsorption was found as an effective technique to remove tannins from wastewater in recent years. Many adsorbents, such as activated carbon, organo-clays, organobentonites, and collagen fiber, have been widely studied for tannin adsorption [12–16]. Actually, a number of these adsorbents for tannin removal do not have high adsorption capacity which limits their actual application. Therefore, it is very important to develop a low-cost adsorbent with high adsorption capacity. It was found that tannins have the ability to precipitate proteins, polysaccharides, and alkaloids from the solution through hydrogen bonding and hydrophobic interaction. Konjac glucomannan (KGM) is a low-cost neutral polysaccharide derived from the tubers of Amorphophallus konjac and easily cultivate in the South East of Asia [17,18]. In our experiment, it was found that tannins have good binding ability to precipitate KGM from its sol solution. Taking consideration of this phenomenon, KGM would have good potential to adsorb tannins. The advantages of KGM used as an adsorbent to adsorb tannins from wastewater lie in its abundance, biodegradability and the relative easily chemical modification. KGM is composed of mannose and glucose in a molar ratio of 1.6:1 with a ␤-1,4-linkage and contains acetyl group per every 12 or 18 repeating units. However, KGM cannot be used directly as an adsorbent to adsorb tannins from aqueous solution, because of its high solubility in aqueous solution. High water solubility of KGM is due to acetyl substituents, which produce steric effects

F. Liu et al. / Journal of Hazardous Materials 178 (2010) 844–850

of acetyl groups to decrease interchain association and improve entropic penalty of chain association [19]. In contrast, deacetylation in alkali solution may make KGM water insoluble owing to increasing interchain association through hydrogen bonding. The objectives of this study were to prepare a water insoluble KGM adsorbent by modification and to evaluate its ability in adsorption tannin from aqueous solution. In order to find the optimum adsorption conditions, the influences of several operation parameters (pH, contact time, adsorbent dosage, temperature and initial concentration) on the adsorption capacity were investigated. Kinetic and equilibrium isotherm models were used to establish the rate of adsorption, adsorption capacity, and to disclose the mechanism of tannin adsorption. 2. Experimental 2.1. Materials Refined KGM flour (50–150 ␮m) with 1.85% acetyl substituted residues was purchased from Mianyang Anxian Dule Company and was used without further purification. Alcohol, sodium hydroxide and hydrolysable tannin (mean relative molecular mass of tannin: 1701.23) used in this work were all of analytical grade and obtained from Chengdu Kelong Chemicals Company. 2.2. Deacetylated reaction In order to study the effect of degree of deacetylation on water absorbency of KGM, KGM flour (40.00 g, W1 ) and varying amount of sodium hydroxide (0.000–0.690 g, W2 ) were dispersed in 240 mL alcohol/water mixture solution (100:140, V/V) in a three-necked round-bottomed flask (500 mL) equipped with a mechanical stirrer. The reaction mixture was stirred at 50 ◦ C until the reaction mixture was neutralized using phenolphthalein as an indicator. As a result, samples of deacetylated konjac glucomannan (DKGM) with varying degrees of deacetylation were obtained. Samples were filtered out and washed with distilled water, and then were dried at 60 ◦ C for 24 h. Degree of deacetylation (DD, %) was calculated by the following equation: DD(%) =

W2 × 43 × 100 40 × W1 × 0.0185

(1)

where W1 and W2 represent the weight of KGM and that of sodium hydroxide in g, respectively. Water absorbency of the sample was determined as follows: 0.10 g (m1 ) of dry sample was added into a centrifuge tube (50 mL) containing 30 mL of distilled water. The sample was allowed to swell thoroughly for 1 h, and then centrifuged at 3000 rpm for 30 min. After removing the supernatant water, the sample was weighed (m2 ). All experiments were conducted in triplicate. The criterion assigned for the relative error was 5%. The mean of the determined water absorbency was used in the discussion. Water absorbency (g g−1 ) was determined by the following equation: Water absorbency(g g−1 ) =

m2 − m1 m1

(2)

2.3. Preparation of deacetylated konjac glucomannan (DKGM) adsorbent KGM flour (40.00 g) and sodium hydroxide (0.70 g) were dispersed in 240 mL alcohol/water mixture solution (100:140, V/V) in a three-necked round-bottomed flask (500 mL) equipped with a mechanical stirrer. The reaction mixture was stirred at 50 ◦ C for 12 h. The sample of deacetylated konjac glucomannan (DKGM) with the degree of deacetylation of 100% was obtained. The sample was

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neutralized, filtered out and washed with distilled water, and then was dried at 60 ◦ C for 24 h. The sample was used for all adsorption study. 2.4. Bath experiment A stock tannin standard solution of 10 mmol L−1 was prepared by dissolving 17.01 g of tannin in 1 L distilled water. Initial tannin solutions at different concentrations were prepared by proper dilution from stock tannin standard solution. All adsorption experiments were conducted triple times in a 250 mL stoppered conical flask containing 50 mL of test solution. The pH of initial tannin solution was adjusted by sodium hydroxide (0.1 mol L−1 ) and hydrochloric acid (0.1 mol L−1 ). Certain amount of the adsorbent was added into the flask and the mixture in the flask was shaken for a predetermined contact time in an electrically thermostatic reciprocating shaker at 200 rpm. Sample was taken and filtered with 0.22 ␮m syringe filter, and the concentration of the filtrate was measured. The concentration of the filtrate was analyzed by Shimadzu UV-3150-VIS-NIR spectrophotometer (wavelength reproducibility: ±0.1; wavelength accuracy: ±0.3 nm) at 270 nm using calibration curve (linear equation: Y = 0.0737X, where Y is absorbency and X is tannin concentration in ␮m L−1 ; R2 = 0.999) which was established from the known concentration of standard tannin solutions (1.25 ␮m L−1 , 2.5 ␮m L−1 , 3.5 ␮m L−1 , 5 ␮m L−1 and 7.5 ␮m L−1 ) and the corresponding UV absorption and the linearity was ensured by Beer-Lambert law. The mean of the three measured residual tannin concentrations in the filtrates was used in the discussion. The criterion assigned for the relative error was 3%. The uptake capacity (qe ) and removal efficiency (E%) were calculated as follows: qe (mol g−1 ) = E(%) =

(C0 − Ce )V w

C0 − Ce × 100 C0

(3) (4)

where C0 (mmol L−1 ) and Ce (mmol L−1 ) are the initial tannin concentration and tannin concentration after adsorption, respectively. V is the volume of the solution in mL and w is the weight of the adsorbent in g. 2.5. Regenerated experiment Desorption of the adsorbed tannin from DKGM was studied by static experiment. DKGM (1.0 g) was added into 500 mL stoppered conical flask containing 250 mL of 5 mmol L−1 tannin solution and the mixture in the flask was shaken at 10 ◦ C for 6 h. The tanninloaded DKGM was filtered out and placed in the 250 mL stoppered conical flask containing 100 mL of 0.01 mol L−1 sodium hydroxide solution for desorption, and stirred continuously at 10 ◦ C for 6 h. The final tannin concentration of tannin in the filtrate and desorptive solvent was detected. The adsorption–desorption process was repeated 6 times using the same adsorbent. 2.6. Characterization of adsorbent Fourier transform infrared (FT-IR) spectra of dried samples prepared as potassium bromide discs were recorded at 400–4000 cm−1 using a Nicolet-6700 model FT-IR spectrometer. The morphologies of KGM, DKGM, tannin, and tannin-loaded DKGM were investigated by SEM (S440 Leica Cambridge Ltd.) from the appearance of the samples. All dried samples were sputter coated with gold prior to examination. UV–vis diffusion reflectance spectra of dried samples prepared as discs were recorded at 200–400 nm using Shimadzu UV-3150VIS-NIR spectrophotometer.

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Fig. 2. Effect of degree of deacetylation on water absorbency of DKGM. Fig. 1. FT-IR spectra of DKGM with different degrees of deacetylation.

3. Results and discussion 3.1. Effect of degree of deacetylation on water absorbency of KGM KGM is a water soluble non-ionic polysaccharide, which consists of ␤-1, 4-linked mannose and glucose units in a molar ratio of 1.6:1 with a low degree of acetyl groups at the side chain C–6 position. The absorption peak at 3423 cm−1 is attributed to hydroxyl groups shown in Fig. 1, which are the main functional groups in native KGM as an adsorbent for tannin adsorption. However, KGM cannot be utilized as an adsorbent for tannin removal. Native KGM has extremely high water absorbency of 100 g g−1 . In order to reduce water absorbency of KGM, it was reported that KGM was acetylated with acetic anhydride [20]. The water absorbency of KGM decreased with increasing the degree of substitution of acetyl groups. Fully acetylated KGM has water absorbency of 1 g g−1 . Due to exhaustion of many of hydroxyl groups in the modification, acetylated KGM cannot be used as an adsorbent for tannin removal. Nevertheless, it is widely accepted that the presence of acetyl substituted residues confers water absorbency on native KGM [21]. However, the relationship between water absorbency and the amount of acetyl substituted residues on the native KGM has seldom been studied. In our research, in order to study the relationship, KGM possessing 1.85% acetyl substituted residues were treated with varying amount of sodium hydroxide in alcohol/water mixture solution. Samples of DKGM with varying degrees of deacetylation were prepared. Fig. 1 shows FT-IR spectra of the DKGM samples before and after treatment using varying amount of sodium hydroxide. With increasing the dosage of sodium hydroxide in deacetylated reaction, the absorption peak at 1740 cm−1 assigned to acetyl groups was weaker and finally disappeared [22]. It indicated that deacetylated reaction easily occurred even at low concentration of sodium hydroxide. Fig. 2 shows the relationship of degree of deacetylation and water absorbency of DKGM. Water absorbency of DKGM rapidly decreased with increase of degree of deacetylation. Water absorbency of fully deacetylated KGM was about 4.8 g g−1 . The results demonstrate that acetyl substituted residues on the native KGM control its water absorbency. Thus, water insoluble KGM powder adsorbent with low water absorbency can be prepared through deacetylation of KGM using alcohol as a swelling inhibitor.

nin through strong intramolecular hydrogen bonding. The hydroxyl groups of DKGM and tannin can provide hydrogen bonding donators as well as hydrogen bonding acceptor. The oxygen atom of hydroxyl groups in the molecular structure of DKGM is used as the hydrogen bonding acceptor and forms hydrogen bonding with the hydroxyl hydrogen atom of tannin. However, tannin is a weak organic acid and its ionization is markedly dependent on pH. Tannin is present in a molecular form at pH below 4.5 and it ionizes with limit at the pH range of 4.5–6 [23]. With increase of pH, tannin is completely ionized at a pH value higher than 6. When the pH value is beyond 6, completely ionized tannins cannot provide hydroxyl hydrogen atom to oxygen atom of hydroxyl groups of DKGM to form hydrogen bonding. Moreover, tannin after ionization possesses negative charges in water solution and these ionized tannin molecules with the same charge sign repulse each other heavily due to the electrostatic force, but interactions between DKGM and tannins through hydrogen bonding are disrupted, therefore the comprehensive effects of the electrostatic force and disruption of H-bonding lead to a reduction in adsorption of tannins onto DKGM at higher pH value. The results suggest that DKGM has good binding capacity for uncharged tannin. 3.3. Effect of dosage To determine the removal ability of DKGM for tannin, varying amounts (0–80 g L−1 ) of DKGM were used in 50 mL tannin solution at concentration of tannin 1 or 5 mmol L−1 . The pH and temperature of tannin solutions were kept at 3.3 and 20 ◦ C, respectively. The effect of adsorbent dosage on the removal extent of tannin is shown in Fig. 4. In general, removal efficiency may increase with increas-

3.2. Effect of pH Fig. 3 shows that DKGM has significant adsorption capacity in the range of pH 2–6. Constancy of equilibrium adsorption capacity at the pH range of 2–6 is attributed that DKGM can adsorb tan-

Fig. 3. Effect of pH on adsorption of tannin onto DKGM (tannin: 5 mmol L−1 , dose: 4 g L−1 , contact time: 240 min, and temperature: 20 ◦ C).

F. Liu et al. / Journal of Hazardous Materials 178 (2010) 844–850

Fig. 4. Effect of dosage on adsorption of tannin onto DKGM (tannin: 1 and 5 mmol L−1 , pH: 3.3, contact time: 240 min, and temperature: 20 ◦ C).

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Fig. 6. Effect of temperature on adsorption of tannin onto DKGM (tannin: 5 mmol L−1 , dose: 4 g L−1 , pH: 3.3, and contact time: 240 min).

lows [24,25]:

Fig. 5. Effect of contact time on adsorption of tannin onto DKGM (tannin: 5 mmol L−1 , dose: 4 g L−1 , pH: 3.3, and temperature: 20 ◦ C).

ing adsorbent dosage, which is accompanied with more active sites available for adsorption. In each case increase in adsorbent dosage resulted in increase in percent removal of tannin. After adsorbent dosage increases to approximate 20 g L−1 , the removal efficiency increased smaller. The adsorption trend observed at high concentration (5 mmol L−1 ) was very similar to that at low concentration (1 mmol L−1 ) and at both concentrations of tannin solutions the highest dosage of DKGM corresponds to maximum removal efficiency of 90%. 3.4. Effect of contact time and adsorption kinetics Fig. 5 shows the effect of contact time on the adsorption extent of tannin on DKGM. It has been observed that adsorption yield rapidly increased to 410.3 ␮mol g−1 within 60 min at 20 ◦ C. With contact time increasing, adsorption equilibrium was established at about 240 min and fell into saturation plateau at adsorption capacity of 524.4 ␮mol g−1 . The data obtained from this experiment was further used to fit the adsorption kinetics models. The kinetics of tannin adsorption on DKGM was analyzed using pseudo-first-order Eq. (5) and pseudo-second-order Eq. (6) as fol-

ln(qe − qt ) = ln qe − k1 t

(5)

t 1 t = + qe qt k2 qe 2

(6)

where t is the contact time (min), qt and qe are the quantities of tannin absorbed at time t and at equilibrium (␮mol g−1 ), respectively, and k1 (min−1 ) and k2 (g ␮mol−1 min−1 ) are the rate constants. The kinetic parameters are listed in Table 1. The R2 value of pseudo-second-order kinetics model is higher than that of pseudofirst-order kinetics model. Moreover, the calculated adsorption capacity of 546.5 ␮mol g−1 from pseudo-second-order kinetics is very close to the experimental adsorption capacity of 524.4 ␮mol g−1 . These results suggest that the adsorption kinetics of tannin onto DKGM better fits into the pseudo-second-order kinetics model rather than pseudo-first-order kinetics model. It means that the rate adsorbed tannin onto DKGM is much dependent on the amount of tannin on the surface of DKGM and the amount of tannin adsorbed at equilibrium [26]. 3.5. Effect of temperature and adsorption thermodynamics The effect of the temperature on the adsorption capacity was studied within temperature range between 20 and 90 ◦ C. The experiments were conducted using adsorbent dosage of 4 g L−1 and at tannin concentration of 5 mmol L−1 shaken for 240 min. The adsorption capacity decreased from 524.4 to 22.2 ␮mol g−1 as temperature increased from 20 to 90 ◦ C shown in Fig. 6. A decrease in the adsorption of tannin with increase in temperature was due to the increasing Brownian movement of all molecules in the solution and breaking of existing intermolecular hydrogen bonding between tannin and DKGM which might be the reason for desorbing tannin from DKGM. In order to describe thermodynamic of tannin adsorption onto DKGM from aqueous solution, thermodynamic parameters of H and S were calculated according to the following equation

Table 1 Comparison of pseudo-first-order kinetics and pseudo-second-order kinetics constants and experimental and calculated qe values. qe (exp)

(␮mol g−1 ) 524.4

Pseudo-first-order kinetics k1

qe

(min−1 )

(cal) (␮mol g−1 )

0.01

253.0

Pseudo-second-order kinetics 2

R

k2

qe

(g ␮mol−1 min−1 ) 0.967

−5

9.52 × 10

R2

(cal) (␮mol g−1 ) 546.5

0.998

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F. Liu et al. / Journal of Hazardous Materials 178 (2010) 844–850

Fig. 7. The relationship of ln K and 1/T for tannin adsorption onto DKGM.

Fig. 8. Adsorption isotherms of tannin on DKGM (dose: 4 g L−1 , pH: 3.3, contact time: 600 min).

[27,28]: ln K =

S −H + RT R

(7)

where K is the thermodynamic equilibrium constant, H is enthalpy for adsorption, S is entropy for the adsorption, R is the ideal gas constant and T is the absolute temperature in Kelvin. Thermodynamic equilibrium constant (K) was obtained from the equation of K = (C0 − Ce )/Ce , where C0 in mmol L−1 is the initial tannin concentration and Ce in mmol L−1 is the equilibrium tannin concentration after adsorption. According to Eq. (7), it is shown in Fig. 7 that the parameters of H and S can be calculated from the slope and the intercept of the plot of ln K versus 1/T, respectively. The H parameter was found to be −35.3 kJ mol−1 . The negative H indicated the exothermic nature of tannin adsorption onto DKGM. It was reported that energy of adsorption from hydrogen bond forces was about 2–40 kJ mol−1 [29]. The value (−35.3 kJ mol−1 ) of H suggested that hydrogen bonding interaction between tannin and DKGM played an important role in adsorption process. The S parameter was calculated as −119.5 J mol−1 K−1 . The negative sign of S revealed the randomness decreasing at the solid–solution interface during the adsorption of tannin onto DKGM. 3.6. Adsorption isotherms The adsorption isotherms of tannin on DKGM in water and ethanol at different temperatures are shown in Fig. 8. As the adsorption temperature was lower, the equilibrium adsorption capacity (qe , ␮mol g−1 ) increased significantly with increase of tannin equilibrium concentration in aqueous solution. With the increase of temperature, the equilibrium adsorption capacity declined in general. In order to confirm the adsorption mechanism of tannin onto DKGM through hydrogen bonding, ethanol was chosen to dissolve tannin to form tannin solution. In the medium of ethanol, DKGM cannot adsorb tannin at any tannin concentration range shown in Fig. 8. The adsorption process of tannin on DKGM was heavily interfered by ethanol. It can be explained that the hydrogen bonding

interactions between DKGM and ethanol are stronger than hydrogen bonding interactions between DKGM and tannin. The reaction sites of hydroxyl groups presented in DKGM are surrounded by ethanol molecules through hydrogen bonding, which leads to nonadsorptive process between DKGM and tannin. The adsorption equilibrium data presented in Fig. 8 were applied to Langmuir Eq. (8) and Freundlich isotherms Eq. (9). The Langmuir isotherm is based on the monolayer adsorption. Freundlich isotherm is assumed to heterogeneous adsorption with uniform energy. Their expressions are given by the following equations [30,31]: Ce Ce 1 = + qe qmax qmax b log qe = log KF +

(8)

1 log Ce n

(9)

where qe (␮mol g−1 ) and Ce (mmol L−1 ) are the amount of adsorbed tannin per unit weight of adsorbent and tannin concentration in solution at equilibrium, respectively. b (L ␮mol−1 ) is Langmuir constant relating the free energy of adsorption. qmax is the monolayer uptake capacity of the adsorbent. b and qmax were calculated from slope and intercept of the linear plot of Ce /qe and Ce , respectively. KF (␮mol g−1 ) and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. KF and 1/n were determined from the intercept and slope of linear plot of log qe and log Ce , respectively. The isotherm constants are presented in Table 2. The results showed that the equilibrium data well fitted Freundlich isotherm model at the temperature range from 10 to 50 ◦ C. Compared with Freundlich isotherm model, the Langmuir isotherm model only well describes the adsorption behavior of tannin onto DKGM at the lower temperature. It was also found that equilibrium adsorption capacity increased with increase of tannin concentration and hardly reached a maximum point. It indicates that the interaction between tannin and DKGM is multiple-layer adsorption process rather than monolayer adsorption process.

Table 2 Langmuir and Freundlich isotherms constants. Temperature ◦

Langmuir constant

T ( C)

b (L ␮mol

10 30 50

0.77 0.18 0.04

−1

)

Freundlich constant qmax (␮mol g 787.40 1129.48 1331.58

−1

)

2

R

n

KF

R2

0.975 0.949 0.351

2.21 1.34 1.10

319.67 169.49 51.05

0.994 0.989 0.977

F. Liu et al. / Journal of Hazardous Materials 178 (2010) 844–850

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3.8. Characterization of adsorbent before and after adsorption

Fig. 9. Adsorption–desorption cycle of DKGM.

3.7. Desorption and regeneration studies Regeneration of the spent adsorbent for repeated reuse is very important in industrial treatment. It was found that the adsorption behavior of tannin onto DKGM was much dependent on pH value. When pH value is beyond 6, tannin acid is easily ionized and neutralized in alkali condition. At high pH value, the hydroxyl bonding interactions between tannin and DKGM are weaker or disrupted. Therefore, desorption of the adsorbed tannin from DKGM can take place in alkali solution. Comparing the adsorption data and desorption data, the desorption ratio was calculated as 99% in 0.01 mol L−1 sodium hydroxide solution. The adsorption–desorption cycle of DKGM was shown in Fig. 9. It was obvious that DKGM could be utilized repeatedly and the adsorption capacity was slightly declined.

Fig. 10 shows the scanning electron micrographs of KGM (×500), DKGM (×500), tannin (×1000) and tannin-loaded DKGM (×500). The scanning electron micrographs of pure KGM and DKGM samples are similar with the size of 50–150 ␮m, and show its irregular granular structure. It indicates that deacetylated reaction has no effect on the surface structure of KGM. The morphologies of KGM and DKGM have both become shrunken and wrinkled made by drying. The micrograph of dried tannin-loaded DKGM shows a difference in morphology from DKGM. After the tannin adsorbing on DKGM, wrinkled appearance of DKGM was disappeared. The main reason was that large amount of tannin was adsorbed on DKGM and free volume of DKGM in the molecular structure was occupied and stuffed by tannin. It also suggests that DKGM has certain swelling degree in aqueous solution and provides enough free volume to accommodate large quantity of tannin. To further elucidate the mechanism of tannin adsorption on DKGM, UV–vis diffusion reflectance spectra were conducted on tannin before and after adsorption on DKGM. As shown in Fig. 11, tannin gives an adsorption band at 256 nm. After tannin adsorption on DKGM, the adsorption band near 265 nm represents a red shift from the 256 nm adsorption band for tannin. Such a red shift may be due to interaction between DKGM and tannin. It can be explained that –OH groups on tannin have good electron affinity to make tannin as the electrons’ acceptor because of the delocalization of the lone pairs of p-electrons from oxygen atoms. The O atoms on DKGM have lone pairs of electrons in hydroxyl groups. When DKGM adsorbent is immersed into tannin solution, the O atoms on DKGM naturally donate their electrons to tannin unsaturated structure to form n → ␲ charge transfer complexes and adsorb tannin on its backbone, which lead to a red shift in UV–vis diffusion reflectance spectra. This n → ␲ complex can be also used to explain the interactions between absorbent and tannin.

Fig. 10. SEM of KGM, tannin, DKGM and tannin-loaded DKGM.

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Fig. 11. UV–vis diffusion reflectance spectra of tannin before and after adsorption on DKGM.

4. Conclusion Water insoluble deacetylated konjac glucomannan was prepared by a simple method. The adsorption characteristics of DKGM were studied based on tannin solution. The results showed that DKGM can be utilized as a low-cost and readily available biosorbent for removal of tannin from aqueous solutions. The adsorption process depends on the pH and temperature. The biosorbent can be easily regenerated and reused by the pH adjustment. Nonadsorptive phenomenon at the high pH value or in the medium of ethanol, the value of enthalpy (H) and UV–vis diffusion reflectance spectra of tannin before and after adsorption on DKGM indicate that hydrogen bonding interactions play an important role in adsorption process. Acknowledgement This work was supported by National Key Technology R&D Program of China (no. 2007BAB18B08). References [1] W. Edwards, R. Bownes, W.D. Leukes, E.P. Jacobs, R. Sanderson, P.D. Rose, S.G. Burton, A capillary membrane bioreactor using immobilized polyphenol oxidase for the removal of phenols from industrial effluents, Enzyme Microb. Technol. 24 (1999) 209–217. [2] X. Liao, Z. Lu, B. Shi, Selective adsorption of tannins onto hide collagen fibres, Sci. China Ser. B: Chem. 46 (2003) 495–504. [3] A. Gnanamani, G. Sekaran, M. Babu, Removal of tannin from cross-linked and open chain polymeric tannin substrates using heme peroxidases of Phanerochaete chrysosporium, Bioproc. Biosys. Eng. 24 (2001) 211–217. [4] V.P. Vinod, T.S. Anirudhan, Sorption of tannic acid on zirconium pillared clay, J. Chem. Technol. Biotechnol. 77 (2001) 92–101. [5] T.S. Anirudhan, M. Ramachandran, Adsorptive removal of tannin from aqueous solutions by cationic surfactant-modified bentonite clay, J. Colloid Interface Sci. 299 (2006) 116–124.

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