Assessment of Sericin Biosorbent for Selective Dye Removal*

CHEN Xinqing, LAM Koon Fung, MAK Shuk Fong, CHING Wai Kwong, NG Tsz Nok and YEUNG King Lun,2,**

1 Department of Chemical and Biomolecular Engineering, 2 Division of Environment, the Hong Kong University of Science and Technology, Hong Kong, China

Assessment of Sericin Biosorbent for Selective Dye Removal*

CHEN Xinqing1, LAM Koon Fung1, MAK Shuk Fong1, CHING Wai Kwong1, NG Tsz Nok1and YEUNG King Lun1,2,**

1Department of Chemical and Biomolecular Engineering,2Division of Environment, the Hong Kong University of Science and Technology, Hong Kong, China

The silk sericin is the main residue in silk production and it is found to be a low cost and efficient biosorbent. In this study, sericin was characterized with various techniques including SEM (scanning electron microscope), XRD, N2physisorption, FTIR (Fourier transformed infrared spectroscopy) and XPS (X-ray photoelectron spectroscopy). The nitrogen content of sericin was ca. 8.5 mmol·g?1according to elemental analysis. Dye adsorption by sericin biosorbent was investigated with the acid yellow (AY), methylene blue (MB) and copper (II)phthalocyanine-3,4′4″4′″-tetrasulfonic acid (CuPc) dyes from water. Sericin displayed large capacity for AY and CuPc adsorption with adsorption capacities of respectively 3.1 and 0.35 mmol·g?1, but it did not adsorbed methylene blue dye. This selectivity is due to the basicity of amide groups in sericin biosorbents.

dye adsorption, silk protein, biosorbent

1 INTRODUCTION

Adsorption has long been recognized as a simple and economical process for removal of water pollutants. Adsorption properties of heavy metals and persistent organic compounds by numerous materials have been reported in the past decades. Although, in general, synthetic adsorbents such as resin, zeolite and mesoporous silica have large adsorption capacity and good selectivity for various pollutants [1-4], biosorbents are considered as attractive alternatives for environmental remediation because of their low cost and availability [5]. Pollutants including dyes [6], pesticides [7] and metal ions [8-10] were successfully treated by various biosorbents derived from agricultural and industrial biomass. Biomass-derived adsorbents have been examined for dye removal by several research groups [11-19]. Chuah et al. [11] and Malik [12] reported the use of rice husk activated carbon for removal of textile dyes such as malachite green and acid yellow 36. Sugar beet pulp is shown to be effective for removing the reactive dye, Gemazol turquoise blue-G [13], while Brazil nut shell [14], wood sawdust [15], orange bagasse [16] and aerobic sludge[17] have all been shown to have varying effectiveness for removing dye pollutants from water. Rocher and coworkers [18] showed that it is possible to prepare magnetic biosorbent by embedding magnetic nanoparticles and activated carbons in alginate for removal of methyl orange and methylene blue dyes.

This work investigates the adsorption of acidic and basic dyes (acid yellow 34 and methylene blue)and large metal-organic complex dye [copper (II)phthalocyanine-3,4′4″4′″-tetrasulfonic acid] by the silk protein, sericin, a byproduct of silk manufacturing process. Emphasis is made to examine the potential of this biosorbent for the selective removal of dyes from water.

2 EXPERIMENTAL

2.1 Silk sericin biosorbent

The silk sericin powder was supplied free-of-charge from Italy. Prior to the adsorption study, the sericin powder was dried overnight in an oven at 373 K. Repeated boiling was used to lower the solubility of sericin in powder by transforming the more soluble random coil structured protein into the less soluble β-sheets [20]. The biosorbent was examined by scanning electron microscope (JEOL 6300F). The sericin powder was attached to the specimen holder by conducting adhesive and sputter-coated with a 10 nm gold layer.Its specific surface area and pore structure were determined by N2physisorption (Coulter SA 3100),while its surface charge were measured by Zetaplus(Brookhaven Instruments Corp). The elemental and chemical compositions of the biosorbent were analyzed by various methods. The carbon, nitrogen, hydrogen and sulphur contents of the biosorbent were measured by Elementar Vario EL III. Six milligrams of sample powder were pyrolyzed at high temperature and the resulting composition of the flue gas was analyzed by a thermal conductivity detector. The surface elemental composition and their oxidation states were analyzed by Physical Electronics PHI 5000 X-ray photoelectron spectroscopy using a monochromatic Al X-ray source (1486.6 eV) at 350 W, 14 kV and 25 mA at a vacuum of 1×10?8Torr. Both regular and high resolution scans were performed and the data were plotted with respect to the binding energy. Fourier transformed infrared spectroscopy (Perkin-Elmer GX 2000) of the biosorbent was done using a Harrick praying mantis diffuse reflectance accessory. Eachspectrum was an average of 128 scans taken between 400 to 4000 cm?1at a resolution of 0.5 cm?1and optical path difference (OPD) velocity of 2 cm·s?1. The sample spectrum was plotted in transmittance for convenience.

Figure 1 Molecular structures of (a) acid yellow, (b) copper (II) Phthalocyanine-3,4′4″4′″-tetrasulfonic acid and (c) methylene blue

2.2 Dye adsorption experiments

Acid Yellow 34 (AY, 75%, Aldrich) and a metallocomplex dye, copper (II) phthalocyanine-3,4′4″4′″-tetrasulfonic acid (CuPc, 85%, Aldrich) are acidic dyes, while methylene blue (MB, 85%, Aldrich) is a basic dye. Fig. 1 shows the molecular structure of the dyes, while Table 1 lists their molecular masses, size and characteristic peaks under UV-visible spectrometer. The dyes, AY and CuPc possess the same3RSO?functional groups but CuPc is roughly four times larger than AY [Figs. 1 (a) & (b)]. Adsorption of AY and CuPc could provide insights on the accessibility of the adsorption sites in the biosorbent. MB contains amine groups and is basic [Fig. 1 (c)]. It is also slightly smaller than AY. The similarity in the size of AY and MB means that both dyes could access similar sites within the biosorbent, and their adsorption will depends mainly on the chemical interactions between the dye molecules and the surface adsorption sites.

Table 1 Physical properties of dyes used in this study

The batch adsorption experiments were performed at room temperature (298±2 K) using 0.04 g of sericin for 40 ml aqueous dye solution. The pH of the solution was adjusted by diluted hydrochloric acid (HCl,Mallinckrodt) and sodium hydroxide (NaOH, BDH)solutions. The initial and final concentrations of the dyes were analyzed by UV/visible spectrophotometer(Ultrospec 4300pro). Calibrations between 0 to 30 mg·kg?1dye were made before each set of measurements and the intensities of the absorption bands at 416 nm (AY), 630 nm (CuPc) and 665 nm (MB) display a linear correlation with their concentration. Adsorption samples were diluted and three measurements taken and averaged to obtain the equilibrium adsorption capacity [Eq. (1)].

where Co(mmol·L?1) and Ce(mmol·L?1) are the initial and final dye concentrations, respectively. V (L) is the solution volume and m (g) is the mass of adsorbent.

The batch adsorption isotherm data were modeled by Langmuir and Freundlich equations [Eqs. (2)& (3) respectively].

where Qe(mmol·g?1) is the calculated adsorption capacity. KL, bL, KFand n are the model parameters of the equations. The adsorption rates were determined by measuring the dye concentration at fixed time intervals during the adsorption. Pseudo-first-order adsorption (cf. Eq. 4) and the pseudo-second-order adsorption (cf. Eq. 5) were used to model the adsorption kinetic data [21].where Qeis the estimated steady state adsorption(mmol·g?1), Qtis the adsorption capacity at given time,t is the contact time (h), k1is the pseudo-first-order kinetic constant (h?1) and k2is the pseudo-second-order kinetic constant (mmol·g?1·h?1).

3 RESULTS AND DISCUSSION

3.1 Biosorbent characterization

The silk sericin powder received from the supplier was recovered from the silk degumming process water by filtration. Although the original powder is slightly soluble in water, repeated boiling in water can render it insoluble. The SEM image in Fig. 2 (a) shows the irregular, elongated shape of the dried sericin powder. The particle size ranges from less than a micron to tens of microns. Based on the nitrogen physisorption measurement, the specific surface area of the silk sericin is only 1.5 m2·g?1and can be considered to be non-porous.

The chemistry of sericin was examined by FTIR.Fig. 2 (b) plots the infrared spectrum of the silk protein.The sericin powder displays prominent peaks at 3339,2940, 1676 and 1568 cm?1corresponding to amide A,B, I and II as reported by Gulrajani and coworkers [22].The peaks at the vicinity of 2989 and 3298 cm?1are mainly from the N H stretching vibration belonging to amide A and B. The peak at 1676 cm?1in the spectrum is assigned to amide I peak and is believed to originate from the stretching vibrations of C O and C N groups. The peak found between 1530-1570 cm?1was reported to belong to amide II and arises from the in-plane N H bending while the peaks at 1071 (C OH), 1240 (N H), 1398 (O H), 1568 cm?1(N H) had been assigned to amides III and V.

Figure 2 Characterization of sericin by (a) scanning electron microscopy, (b) Fourier-Transform Infra-Red and (c) X-ray photoelectron spectroscopy

Table 2 Bulk and surface composition of silk sericin

Figure 3 (a) AY isotherm at pH 2.5; (b) CuPc isotherm at pH 2.5; (c) MB isotherm at pH 2.5; (d) pH effect on AY adsorption; (e) pH effect on CuPc adsorption; (f) pH effect on MB adsorption Langmuir model; Freundlich model

The bulk and surface elemental compositions of the silk sericin were determined by elemental analyzer and X-ray photoelectron spectroscopy and the results are presented in Fig. 2 (c) and Table 2. It can be seen from the C1 peak of sericin that the biosorbent contains carboxyl (287.8 eV), carbonyl (286.6 eV), alkyl(285 eV) and carbon (282.6 eV) [23]. Sericin consists of three polypeptides with molecular masses of 400000, 250000 and 150000 [24] and composed of 18 different amino acids including serine (30%-40%),aspartic acid (10%-20%) and glutamic acid (3%-15%)rich in hydroxyl groups [25]. Wu and coworkers [26]reported that sericin recovered from silk wastewater contains 92% protein, 1% sugar and 4% ash with a total nitrogen content of 14.6%. Similarly, Vaithanomsat and Kitpreechavanich [27] also reported the nitrogen content of sericin to be within 12%-14%. The sericin biosorbent prepared in this study has total nitrogen and sulphur contents of 11.9% and 5.0%, respectively.Nitrogen contents are comparable to literature values while most of the sulphur content is present in bulk phase instead of the surface by comparing the bulk and surface composition.

3.2 Dye adsorption on biosorbent

Acid yellow 34 (AY) is roughly quarter of the size of CuPc and has a single sulfonate group compared to four in CuPc, while MB is comparable in size to AY but differs in its chemistry. The adsorptions of the dyes on sericin are shown in Fig. 3. Silk sericin adsorbs 0.96 mmol·g?1(i.e. 400 mg·g?1) of AY and 0.18 mmol·g?1(i.e., 180 mg·g?1) of CuPc separately, but not MB at pH 2.5 and room temperature. The AY and CuPc adsorption occurs at pH below sericin’s point-of-zero charge (i.e., pH 3.5, Fig. 4) during which the amines become protonated. CuPc being larger than AY can only access sites located near the surface or in large pores. It is therefore not surprising that the adsorption capacity of sericin for CuPc [Fig. 3 (b)] is significantly lower than AY [Fig. 3 (a)]. The dyes were adsorbed by electrostatic interaction between the negatively charged sulfonate group (i.e.,3SO?) of the dye molecule and the positive protonated amino groups on sericin. Figs. 3 (d) and 3 (e) show that the adsorption capacities of the dyes are higher at lower pH, i.e. higher hydrogen ion concentration with adsorption capacity for AY reaching 3.1 mmol·g?1(i.e.,1280 mg·g?1) at pH 1.5. The Zeta-potential plot in Fig. 4 for sericin at various pHs shows its surface is positively charged below pH 3.5 and negatively charged above this pH due to deprotonated amino and carboxylgroups. According to Fig. 4, the surface charge of sericin at pH 1.5 is higher than that at pH 2.5, which could explained the higher adsorption capacity of sericin for the acid dyes at low pH.

Table 3 Dye adsorption on various adsorbents listed in literature

Figure 4 Zeta-potential of sericin against pH

Methylene blue does not adsorb on sericin as shown by Figs. 3 (c) and 3 (f). This is likely due to the repulsion between the positively-charged surface of the biosorbent and the dye molecules at low pH. Thus,this study shows that sericin is selective for acid dye at low pH similar to modified mesoporous silica adsorbent [28]. Table 3 lists the adsorption capacities of acid dyes and methylene blue on various adsorbents.The sericin has a relatively high adsorption capacities for acidic dyes compared to other adsorbents, while it does not adsorb any MB. This adsorption selectivity of sericin is unique compared to most biosorbents.

Langmuir and Freundlich models were used to fit the adsorption data and Table 4 shows that the Langmuir adsorption isotherm has better fit compared to Freundlich for the adsorption of AY and CuPc on sericin.

The batch adsorption kinetics of both dyes on sericin were measured with initial AY and CuPc concentration of 2 mmol·L?1and L/S (liquid per solid ratio) of 1000 ml·g?1at pH 1.5. Figs. 5 (a) and 5 (b) plots the adsorp-tion with time and it can be seen that the adsorption rate of CuPc is faster than AY in spite of less CuPc being adsorbed on the biosorbent. This could be due to the stronger electrostatic charge of CuPc than AY and CuPc being mainly adsorbed on the surface. The correlation coefficient (R2) for the pseudo-second-order model has higher value (＞0.99) from Table 5 suggesting that the dye adsorption on sericin is likely to be chemisorptive.

Table 4 Langmuir and Freundlich model parameters of dye on silk sericin

Figure 5 Adsorption kinetics (Pseudo-second order models are plotted on the graphs) of (a) AY and (b) CuPc on sericin (Experimental conditions: 0.1 g sericin in 100 mldye solution at pH 1.5)

Table 5 Kinetic model parameters of dyes on sericin

4 CONCLUSIONS

Silk sericin derived from waste biomass is low cost and effective for removal of acidic dyes from water. Also, the strong similarity in structure and chemistry of AY and CuPc similar to many active pharmaceuticals suggests that sericin could have potential application for treatment of these pollutants as well. Sericin is complex biosorbent rich in amide groups that could be further altered to achieve different adsorption behaviour and selectivity for targeted remediation of polluted water. Indeed, it has been demonstrated that sericin biosorbent could selectively adsorb precious metals (i.e., gold, palladium) from solutions containing other metals [23]. Gold, palladium and platinum find uses in catalysis [37-40] and membranes [41-44] and improper disposal could lead to pollution by these heavy metals. We also believe that lesson learned from sericin biosorbents could be translated to synthetic adsorbents to achieve high adsorption capacity and selectivity [45-47].

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2012-03-15, accepted 2012-04-04.

* Supported by the Hong Kong Research Grant Council (605009), the Hong Kong Innovation Technology Fund (ITS/108/09FP)and the Environment and Conservation Fund (ECWW11EG02).

** To whom correspondence should be addressed. E-mail: kekyeung@ust.hk