Removal of heavy metals in aqueous solution using Antarctic krill ...

Fibers and Polymers 2013, Vol.14, No.7, 1134-1140

DOI 10.1007/s12221-013-1134-z

Removal of Heavy Metals in Aqueous Solution Using Antarctic Krill Chitosan/Hydroxyapatite Composite Hang Qi, Xiao Jiang, Dayong Zhou, Beiwei Zhu*, Lei Qin, Chun Ma, Yihang Ong2, and Yoshiyuki Murata1 School of Food Science and Technology, Engineering Research Center of Seafood of Ministry of Education, Special Sub-center of Shellfish of National Research and Development Center of Agriculture Product Processing Technology of Ministry of Agriculture, Dalian 116034, P. R. China 1 Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan 2 Post-Harvest Technology Division, Agri-food and Veterinary Authority of Singapore, Singapore 718915, Singapore (Received September 23, 2012; Revised January 3, 2013; Accepted January 4, 2013) Abstract: Discarded Antarctic krill shells can be used to prepare chitosan and adsorption materials in an attempt to save resource and protect environment. The optimal conditions of composite materials preparation were as follows: mass ratio of CS and HA was 2:1 at 40 oC, pH 10-10.5, volume ratio of chitosan and Ca(NO3)2 was 2:1 over a period of 1 day. Under these conditions, the Cr (VI) removal rate was about 75±8 %. SEM, TEM, XRD, FTIR, DT-TGA analysis indicated that CS/HA composite was made up of CS and HA and it possessed good mechanical strength and thermal stability. The ability to remove Cd (II), Cr (III) and Cu (II) in aqueous was high, and the optimal conditions were as follows: a contact time of 60 min at 35 oC and pH 6-7. The removal rates of all three heavy metals were above 90 %. Keywords: Antarctic krill shells, Chitosan, Hydroxyapatite, Preparation, Heavy metals

aqueous solution [9]. Many techniques have been employed for effluent remediation [10]. Adsorption is a promising technique for such situations, especially using low cost adsorbents viz., hydroxyapatite, clay materials, agricultural waste, etc. [11-13]. Chitosan have vast potential applications in the areas of biotechnology, biomedicine, as food ingredients [13-15], and even more applications as biopolymers reported for their high potential of adsorption of metal ions [16]. It has been known for a long time that various biocomposites which are made from the organic matrix and inorganic fraction possess good mechanical strength [17], biocompatibility and biodegradability which would make the material desirable for practical applications [17]. Chitosan blended with n-HAp have been reported to have good mechanical and chemical properties which have been recommended for the development of defluoridation technology [18]. In the present study, chitosan from Antarctic krill shells and hydroxyapatite (HA) were prepared as a composite material to adsorb heavy metal in aqueous solution. The objective of this work was to recycle and reuse the Antarctic Krill shells through the extraction of chitosan to generate a chitosan/hydroxyapatite (CS/HA) composite, determine the optimal condition of CS/HA composite preparation, characterize it and evaluate its capability in the removal of Cd (II), Cr (III) and Cu (II) from an aqueous solution.

Introduction Antarctic krill (Euphausis superb), a kind of small and shrimp-like crustacean, provide 30-90 % of the diet for marine carnivores in the Southern Ocean [1]. Commercial capture is simple because krill form high-density surface swarms. Despite their small size, krill likely has the largest biomass of any known multicellular animal species on earth [2]. The total annual capture from all fisheries has been approximately 130 million tons since 2000 (FAO, 2007). Grantham reported that krill contains 77.9-83.1 % moisture, 0.4-3.6 % lipid, 11.9-15.4 % protein, and 2 % chitin and glucide [3]. Recently, the capture amount of Antarctic krill has been increasing year by year in China. The Antarctic krill shells are discarded after extracting lipid and protein from krill as a processing waste. Preparing Chitosan and preparing adsorption materials from these discarded shells are meaningful to save resource and protect environment. Chitosan (β-(1→4)-2-amino2-deoxy-D-glucose) is a hydrophilic and cationic polymer product of chitin deacylation. Chitosan is a low-cost biopolymer, has metal sorption capability and is abundant in nature. The presence of amino groups in chitosan increases its metal adsorption capacity compared to that of chitin. It has been used in many forms: flakes [4,5], as an alumina composite [6] or molybdate impregnated chitosan beads [7]. Heavy metals are toxic pollutants released to the environment as a result of industrial, mining, and agricultural activities [8]. Heavy metal ions, such as cadmium, chromium and copper among others, are commonly detected in both natural and industrial effluents and are difficult to remove from

Experimental Methods Materials Antarctic Krill (Euphausis superb) was obtained from Liaoning Province Dalian City Ocean fisheries Company (Dalian, China) in Antarctic (60o.00~60o.20S/ 45o.30~46o.30W).

*Corresponding author: [email protected] 1134

Removal of Heavy Metals Using CS/HA Composite

All other reagents were of analytical grade. Preparation of Chitin The Antarctic krill shells were obtained after the lipid and protein were extracted, washed with water, dried at 80 oC and preserved at −30 oC. 50 g dried shells underwent a series of alkaline (NaOH) and acid (HCl) treatments, and washing with deionised water and adjustment of neutral pH between each treatment. After the final washing and pH adjustment, shells mass was dried at 105 oC for 4 h, pink chitin was obtained. Preparation of Chitosan 10 g chitin was mixed with 1 l 42-45 % NaOH, continuously stirred at 95 oC for 8 h. The mixture was cooled, washed with tap water and adjusted to pH 7.0. The neutralized mixture was further washed with deionized water for 3 times and finally dried at 80 oC to obtain refined chitosan (the degree of deacetylation was 79.3 %). Preparation of Hydroxyapatite Hydroxyapatite (HA) was synthesized by the reaction of calcium nitrate and ammonium dihydrogen phosphate at a stoichiometric Ca/P ratio of 1.67, with the pH value being maintained above 10 during mixing by adding ammonia solution. The resulting precipitate was rinsed with deionised water until neutral and then dried at 80 oC to moisture content of less than 1 % (w/w). Preparation of Chitosan/Hydroxyapatite Composite The corresponding chitosan/Hydroxyapatite (CS/HA) composite was prepared by the precipitation method. Ammonium dihydrogen phosphate (NH4H2PO4) and calcium nitrate (Ca(NO3)2) was dissolved in deionized water separately based on the a stoichiometric Ca/P ratio of 1.67. 3 % (w/v) CS solution was made up by dissolving CS powder in 2 % acetic acid. The CS solution was mixed with Ca(NO3)2 solution. The NH4H2PO4 solution was added to CS-Ca(NO3)2 solution at the rate of 4 ml/min in the volume ratio of 3:2. pH was maintained above 10 using ammonia solution during the entire 2 h addition process with continuous stirring. The solution was centrifuged at 9500 g for 10 min. The precipitate was neutralized by rinsing with deionised water. The CS/HA composite was obtained finally by drying the neutralized precipitate [19]. Analysis The CS/HA composite was characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and differential thermal-thermogravimetry analysis (DT-TGA). The morphology of the composite was studied with scanning electron microscope (SEM) with S-

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JSM-6460 LV model (Electronics Co., Ltd., Japan). The size and shape of CS/HA composite were measured by TEM JEM 2EX model (JEOL, Japan). XRD was used to determine the crystalline phases present in composite, XRD spectra of CS/HA composite were identified with D/max-3B model (Rigaku Co., Ltd, Japan). FTIR spectra of the samples as solid by diluting in KBr pellets were recorded with Frontier model (Perkin-Elmer, US). The results of FTIR spectrometer were used to confirm the functional groups present. The mass changes of composite were determined with DT-TGA (TA, US). Sorption Experiments The sorption experiments were performed by batch equilibration method. The solution of potassium bichromate (K2Cr2O7) (10 mg/l) was prepared at pH 4.0 condition, while the solution of copper chloride (10 mg/l), chromic chloride (10 mg/l) and cadmium chloride (10 mg/l) were individually diluted in double distilled water. The batched adsorption experiments were carried out in duplicates by mixing 0.2 g of HA/CS composite with 50 ml of metal salts solution. A thermostated shaker rotating operating at a speed of 200 rpm were used to ensure thorough mixing of K2Cr2O7 solution (10 min) and the mixed metal chloride solutions (60 min). The solutions were then centrifuged at 9500 g for 10 min and the supernatant was collected for determination of sorption capability. Determination of Cr (VI) Content One milliliter sample solution was mixed with 0.5 ml sulphuric acid, 0.5 ml phosphoric acid and 2 ml diphenylcarbazide, and make up to 50 ml with double distilled water. The solution was incubated at room temperature for 10 min, the absorbance was measured at 540 nm using UV Spectrophotometer. Potassium dichromate standard solution was used to establish the standard curve. Determination of Metal Content The obtained samples were heated in a 60 oC water bath for 20 min. The samples were centrifuged to separate the deposits formed from the supernatent. The supernatant obtained from the samples were made up to 10 ml with double distilled water. The metal content of the final solution was determined by ICP-OES (PerkinElmer Optima 7000 DV, US). Calibration was carried out using different standard solutions of Cd, Cr and Cu. Statistical Analysis Data were subjected to Analysis of variance (anova) and the differences between means were evaluated by leastsignificant difference Test. The SPSS statistic program (Version 13.0) (SPSS Inc., 2001) was used for data analysis.

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Results and Discussion

this study exhibited very high viscosity (>1000 centipoises) mainly because Antarctic krill were small and the shells were thin making Antarctic krill shells easy to deal with acid and alkaline. The low degradation level of the main chain of chitosan also contributed to high viscosity of chitosan from Antarctic krill shells.

Properties of Chitosan from Antarctic Krill Shells As shown in Table 1, chitosan from Antarctic krill shells was light pink and schistose. It contained 0.74 % moisture and 0.74 % ash, the degree of deacetylation was 79.32 %, pH was 7.2-7.5, and apparent viscosity was 1240 centipoises. The properties of obtained chitosan from Antarctic krill shells analysed were lower than those published in food standard, but higher than industrial standard. Chitosan from Antarctic krill shells derived using the method presented in

Effect of Mass Ratio, Temperature, pH, Volume Ratio and Time on Cr (VI) Removal Rate with CS/HA Composite The effects of the mass ratio of CS/HA, temperature, pH, the volume ratio of chitosan and Ca(NO3)2 and time were

Table 1. Properties of chitosan from Antarctic krill shells Color

Appearance

Sample Light pink Schistose Industry grade White or light Schistose or yellow powder Food grade

Moisture (%) 0.74 ≤12.0 ≤10.0

Degree of deacetylation (%) 79.32 1000 means super high viscosity

Figure 1. Effect of mass ratio, temperature, pH, volume ratio and time on Cr (VI) removal rate with CS/HA composite.

Yeild (%) 21.22



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Figure 3. TEM photographs of (a) HA and (b) CS/HA composite.

Figure 2. SEM micrograph of HA, CS and CS/HA composite; (a) HA×200, (b) HA×1000, (c) CS×100, (d) CS/HA composite ×200, (e) CS/HA composite ×1500, and (f) CS/HA composite ×4000.

investigated using Cr (VI) removal rate as an index. As shown in Figure 1(a)-(e), under the optimal conditions where mass ratio of CS and HA was 2:1, temperature of 40 oC, pH 10-10.5, the volume ratio of chitosan and Ca(NO3)2 was 2:1 and time of 1 day, the Cr (VI) removal rate was about 75±8 %. The analysis of the CS/HA composite characteristics prepared under the optimal conditions are as follows.

Figure 4. XRD patterns of (a) HA and (b) CS/HA composite.

composite. It was observed that there were aggregation in HA and CS/HA composite powder, but this phenomenon was not severe, indicating a uniform dispersion of HA and CS. HA powder was observed to be elongated crystal and the particle size was 10×30 nm. In comparison to HA, CS/HA composite was thin and needle-like with particle size of 20×80 nm. CS could have possibly accelerated HA crystal nuclear growth; henceforth increasing the particle size of HA in the presence of CS. This result supports the findings of SEM.

Scanning Electron Microscopy (SEM) Analysis Figure 2 shows SEM micrographs of the surface porosity of the materials. Figure 2(a) and 2(b) showed that HA was made up of aggregated fine particles. Figure 2(c) shows that the chitosan possessed sheet-like structure in comparison to the fine aggregation of particles exhibited by CS/HA composite in Figure 2(b); The homogenous CS/HA composite possessed larger surface area to volume ratio as compared to chitosan alone. These findings correspond with Ito et al.’s results [20]. Irregular and porous surfaces could be observed through these micrographs (Figure 2(c), Figure 2(d) and Figure 2(f)). Based on this observation, it was concluded that the material presented an adequate morphology for metal adsorption.

X-ray Diffractometry (XRD) Analysis The XRD technique which produces information about the crystalline structure of the material was used for characterizing CS/HA composite. Figure 4 shows the obvious diffraction peak feature of HA and it is consistent with standard figure (PDF Card No. 90-432). CS/HA composite showed similar feature diffraction peak, but the intensity of peak was lower, probably due to the amount of CS in the composite. The crystalline structure of HA was mainly retained in the CS/HA composite. The diffraction peak feature of HA and CS/HA composite is consistent with Chen et al.’s result [17].

Transmission Electron Microscopy (TEM) Analysis Figure 3 shows the TEM photographs of HA and CS/HA

Fourier Transform Infrared Spectroscopy (FTIR) Analysis The FTIR spectrum was carried out as a qualitative

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Figure 5. FTIR spectra of (a) HA, (b) CS and (c) CS/HA composite.

analysis to determine the main functional groups present in HA, CS and the CS/HA composite (Figure 5). In the FTIR spectra of HA, it was observed that the bands at 1094, 1026, and 962 cm-1 were the characteristic bands of phosphate stretching vibration, while the bands at 602 and 563 cm-1 were due to phosphate bending vibration. In the FTIR spectra of CS, the adsorption peaks at 1646 cm-1 were attributable to the C=O bonds, the peaks observed at 3433 and 1640 cm-1 were caused by the N-H bonds, and the adsorption peaks from 1030 to 1150 cm-1 were due to C-O. In the FTIR spectra of CS/HA composite, the adsorption peaks feature of HA and CS indicated that the chemical composition of CS and HA did not change after integration. However, the strength of feature peaks of CS and HA, such as 1026 cm-1, 962 cm-1, and 563 cm-1, shifted to a lower wave number at different levels. This might be due to hydrogen bonding interaction occurring between OH− in HA and -NH2 in CS or the chelation between -NH2 and Ca2+ or HA micro particles entering the molecular chain of CS to break the hydrogen bonding interaction between chains [21]. Differential Thermal-thermogravimetry Analysis (DTTGA) Analysis The thermal analysis of the sample showed that there was a loss of mass from temperatures below 500 oC. Figure 6 shows the thermogravimetric profile obtained for HA, CS and CS/HA composite. The mass of HA remain unchanged between 20 to 800 oC (Figure 6(a)). The CS mass dramatically decreased from 90 to 45 % when the temperature was between 300 to 400 oC (Figure 6(a)). From 400 to 800 oC, the mass loss of CS was 24 % and final mass was 21 % of the original. Figure 6(c) shows three stages of mass loss, which are represented by regions I, II and III. The first stage extended from the beginning of the analysis until 230 oC, the mass loss was about 5 %. During this stage, water loss corresponded to the loss of adsorbed water and water of

Figure 6. TG scans for (a) HA, (b) CS and (c) CS/HA composite.

crystallization. The second stage which ranged from 230 to 500 oC, the mass loss was about 35 %. This corresponded to organic matter decomposition as well as the volatile substance released from inorganic compound decomposition. This phenomenon was similar to DT-TGA analysis of grape bagasse [22]. Finally, region III at temperature above 600 oC, which represents the ash residues have almost no mass loss detected. This result indicates that CS was not simply adhered on HA surface, but complexly joined together. Effect of Contact Time, pH and Temperature on Heavy Metal Removal Rate with CS/HA Composite The CS/HA composite effectiveness in removal of heavy



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The effect of the pH on rate of removal of Cd (II), Cr (III) and Cu (II) from the aqueous solution was found to be low in the present study. The CS/HA composite was determined at different pH levels and the results are shown in Figure 7(c). It was apparent from Figure 7(c) that the pH had low influence on the removal ability of heavy metal using CS/ HA composite; The removal rate was between 97 % and 99.9 %. At pH 11, the removal rates of three ions were 99.6 % (Cd (II)), 99.9 % (Cr (III)), and 99.9 % (Cu (II)) respectively. That might be due to precipitation of heavy metal hydroxide produced under alkaline condition [9]. In the acidic and neutral medium, the removal of Cd (II), Cr (III) and Cu (II) was governed only by ionic exchanges, the CS/HA composite was effective to remove heavy metal. Taking into consideration of the above factors, the optimal pH of the medium should beat pH 6-7 which was similar to the Fe (III) adsorption optimal pH of 6-8 with nanohydroxyapatite chitin/chitosan hybrid biocomposites [19].

Conclusion

Figure 7. Effect of (a) contact time, (b) temperature, (c) pH and temperature on heavy metal removal rate with CS/HA composite.

The Cr (VI) removal rate of CS prepared from Antarctic krill shells and HA composite under optimal conditions was about 75±8 %. SEM, TEM, XRD, FTIR, DT-TGA analysis indicated that composite was made up of CS and HA, it possessed a better mechanical strength and relatively thermal stability in comparison to industrial standards. The capacity of removal Cd (II), Cr (III) and Cu (II) in aqueous was high and all of removal rates of three metals were above 90 %. The ability of removing heavy metals in marine bioactive materials using this CS/HA composite material with Antarctic krill shells will be conducted in the next study to further utilize the Antarctic krill resource.

Acknowledgements metal was determined by varying the contact time within the range of 5-60 min As shown in Figure 7(a), the removal rate increases as the contact time increases with the highest removal rate of Cd (II), Cr (III) and Cu (II) reaching 97.2 %, 95.3 % and 94.3 % at 60 min respectively. Figure 7(b) shows the effect of temperature on the removal rate of heavy metal. At 5 oC the removal rate of was highest for Cd (II) followed by Cu (II) and Cr (III) at 96.4 %, 93.9 % and 86.51 %, respectively. However, as the temperature increases to 35 oC, the removal rate reached maximum for all heavy metal at 97.6 % for Cd (II), 97.8 % for Cr (III), and 97.0 % for Cu (II) respectively. This result indicated that the removal rates of Cd (II) and Cu (II) were independent with temperature change, but the removal rate of Cr (III) from the aqueous solution was highly dependent on the temperature as it might alter the surface charge on the CS/HA composite. This result was not consistent with the report that heavy metal adsorption materials are generally highly dependent on temperature [9].

This work was supported by National Projects of the 863 Plan (No. 2011AA090801) and Program for Liaoning Excellent Talents in University (LJQ2011056).

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