A series of natural composite hydrogels containing a “3-in-1” type triple adsorbent system are designed. For this purpose, Spirulina (Sp) biosorbent is immobilized on/in halloysite nanotubes in different loadings and then physically crosslinked chitosan composite hydrogels are prepared. The water absorbency and Cr (VI) adsorption capacity in neutral pH medium and wet mechanical strength as well as their morphologies are all reported as a function of Sp immobilized nanotube loadings. The use of Sp biosorbent results in composite hydrogels with high water absorbency, wet strength and thermal stability. Spirulina enlarges the metal adsorption windows efficiently and the Freundlich isotherm model can fit the fundamental metal adsorption data well. It is believed that with optimized special composite hydrogel morphologies, all positively charged receptors of the Sp and the nanotubes behave as collector domains for chromate anions.
The water pollution caused by poisonous heavy metals including Cu, Pb, Hg, Ni, Cr, As, Sn, Co, Cd and Zn (1) has been one of the biggest problems and its long-term effects and various methods for the wastewater treatment have been researched. Chromium existing as Cr (III) and Cr (VI) ions in wastewater, has attracted great attention due to its toxic effects to the environment and human health. Unlike Cr (III), hexavalent chromium ion Cr (VI) in spring and industrial waters acts as carcinogens, mutagens and teratogens in biological systems as it is a strong oxidant (2). The adsorption process has been widely studied for removal of Cr (VI) by testing different adsorbents such as fungi (3), green algae (4), peat (5), bentonite (6) and composite hydrogels (7). Recently, use of biopolymers in preparation of hydrogels has become popular due to their biocompatibility, biodegradability and nontoxicity. Chitosan, specifically, as a naturally occurring polymer, although the presence of amino and hydroxyl groups acting as potential active site for adsorption, suffers from mechanical strength and solubility problems. In order to overcome these drawbacks, new formulations including blending, crosslinking, graft polymerization and internal hydrogen bonding formations are currently being studied (8).
A number of formulations consisting of polymers and clays exist in the fields of adsorption. However, biopolymers reinforced with biocompatible nanoparticles have attracted great attention as a wastewater treatment strategy due to their nontoxicity. Halloysite nanoparticle which is a naturally occurring and biocompatible silica nanotube, has received great attention because it has a large surface area and pore volume (9). These nanotubes are composed of aluminum oxide and silicon oxide based layers and they have hollow tubes with with lengths of 500 nm to 1.2 µ and diameters smaller than 100 nm (10). The outer surface of halloysite nanotubes contains SiO4 tetrahedral layer having negative charges. The inner surface on the other hand, has octahedral (aluminol) Al-OH groups with positive charges (10).
Migroalgae, yeast, fungi and some other biomasses have also been utilized as adsorbers for heavy metal removal (11–13). Among them, Spirulina (Sp) microalgae/biosorbents has been widely used in the adsorption process (14). They have porous three-dimensional networks composed of different structures including polysaccharides, proteins and/or lipids. These polymeric structures on the cell walls of the Sp network have some functional groups such as amine, carboxylic, hydroxyl, phosphate and sulfate which provide adsorption of metal ions from different sources (13, 14). As the examples of our recent works, polyacrylamide nanocomposite hydrogels having Sp (15) and montmorillonite clay modified via Sp (Sp-MMT) (7) were reported. Particularly, in latter study, 1 wt% Sp-MMT clay containing hydrogel was found to be the best adsorber for Cr (VI) ion, which is about 312% higher than that of neat PAAm hydrogel.
In the present work, we report a design of a bio-based composite hydrogel adsorbent system with enhanced swelling, adsorption, mechanical and thermal properties by using a Sp biomass immobilized in/on natural silica nanotube. Halloysite nanotubes modified with Sp were used to prepare physically crosslinked chitosan composite hydrogels. It was expected that this “3-in-1” type triple adsorbent system would interact with much higher heavy metals via surfaces of the nanotube and Sp and pores of the chitosan hydrogel. The swelling and Cr (VI) adsorption capacity in neutral pH medium and wet mechanical strength were all discussed as a function of Sp immobilization and the nanotube content. The results were also compared with neat chitosan and the control hydrogels having as-received halloysite.
Chitosan polymer was bought from Aldrich Chemicals (Milwaukee, WI, USA). Its deacetylation degree (DD) and viscosity average molecular weight are 75–85% and 50–190 kDa, respectively. Purified halloysite nanotube (HL) (Tabanköy-Balıkesir, Turkey) was kindly supplied from ESAN-Turkey. Acetic acid, sodium hydroxide, potassium dichromate (K2Cr2O7) and diphenyl carbazide were all bought from Aldrich Chemicals (Milwaukee, WI, USA). The Sp was supplied by Egert Natural Products Ltd. Co. (İzmir, Turkey).
2.2 Modification of halloysite with Sp
Modification of the halloysite nanotube was done by the solution mixing method, which was applied for immobilization of Sp on MMT layered silicate clay in our previous study (7). Accordingly, halloysite nanotubes and Sp (5 wt% with respect to halloysite) were dispersed separately in 300 ml and 200 ml of deionized water, respectively, at 50°C for 1 h. The biomass solution was then slowly poured into the nanotube solution. Afterwards, the solution was completed to 600 ml and vigorously stirred for 4 h at 50°C. The modified halloysite (HLSp) was obtained by centrifugation of the solution. The HLSp was dried at 50°C in a vacuum oven for 48 h (7).
2.3 Preparation of neat chitosan and composite hydrogels
The hydrogels were obtained by solution mixing and freeze-drying method as described in our previous study (10). Dispersions of 3, 5, 10 and 20 wt% HL and HLSp nanotubes were prepared in 10 ml of 1% acetic acid solution and mixed for 6 h. It was followed by addition of 0.1 g of chitosan polymer to the nanotube solutions. These mixtures was stirred for 24 h and put into cylindrical polystyrene moulds with 30 mm diameter and 10 mm height. Then, they were frozen at -20°C and lyophilized at -45°C. Subsequently, all hydrogels were neutralized with 1 m NaOH solution, and washed with deionized water. The hydrogels were then frozen and lyophilized at -45°C (10). These hydrogels were named as HLyCH and HLSpyCH where y indicates halloysite (HL) and HLSp percentages in chitosan (CH) hydrogels.
X-ray diffraction (XRD) analyses of HL and HLSp nanotubes were done by using a Rigaku D/Max 2200 Ultimat diffractometer (Rigaku, Tokyo, Japan) with CuKα radiation (λ=1.54 Å), operating at 40 kV and 40 mA with 2° min-1 scanning rate. SEM analyses were done by using ESEM-FEG/EDAX Philips XL-30 (Philips, Eindhoven, The Netherlands).
Swelling experiments were carried out at room temperature by putting the samples into deionized water at room temperature. The weight of the swollen hydrogel was measured after 24 h. The maximum swelling (S%) was determined by the equation given below:
where ms is the mass of the swollen gel and m0 is the mass of the dry gel.
The compression test was performed by doing a uniaxial compression experiment for wet hydrogels at room temperature and at a compression rate of 2 mm min-1. The measurements were conducted with a Zwick/Roell Z1.0 Universal Testing Machine (Zwick GmbH & Co. KG, Germany) equipped with a 50 N load cell until a displacement of 60% of the examined sample was reached. The results of the swelling and mechanical tests were given and discussed in the supplementary material file.
Adsorption behavior of the hydrogels was investigated by UV/VIS measurements. The dry hydrogels were put into 100 ml of solutions of different Cr (VI) concentrations (50, 100, 150 and 200 ppm), separately. The UV analyses of the solutions at a pH of 5.5–6.0 was carried out by using a UV-VIS spectrophotometer (Optizen Pop Spectrophotometer, Daejon, South Korea) and diphenyl carbazide “3500-Cr” technique at 540 nm (16).
The amount of adsorbed Cr (VI) as a function of time was found by the metal solution with concentration of 50 ppm whereas adsorption isotherms were obtained from abovementioned four different concentrations of the metal solution. Then, the metal adsorption (qe) as amount of Cr (VI) was measured with the following equation:
where Ci and Cf are the initial and final concentrations of metal ions in the solution (mg l-1) at each time intervals, V is the volume of the metal ion solution (l), W is the weight of the dry hydrogel used in the experiment (g) (7).
3 Results and discussion
3.1 Immobilization of Sp microalgae in/on halloysite nanotubes
Morphologies of the pure halloysite (HL) (Figure 1A,B) and Sp immobilized halloysite (HLSp) (Figure 1C,D) are all given in Figure 1. The nanotube modification was found to cause the intertubular interactions between halloysites and Sp. This leads to three dimensional orientations of halloysite through interactions of Sp with edge/face of nanotubes (17) resulting in cage-like structure (Figure 1C). The formation of this network structure is most probably due to physical interactions between HL nanotube and carboxylic, hydroxyl, phosphate and sulfate functional groups of Sp. Moreover, although pure halloysite has a tubular morphology with a diameter of ca 48 nm (Figure 1B), Sp immobilization results in enlargement of the nanotubes (Figure 1D) up to an external diameter of ca 65 nm. It is obvious that after immobilization, smooth surface of HL nanotubes was converted to a rough surface together with both expanded lumens and distortion of the tube ends (Figure 1D-solid circle). All these results may be attributed to successful immobilization of Sp both in lumens and on the nanotubes including edges by creating 3-D network structures.
In Figure 2, X-ray diffraction patterns were given for both pure halloysite (HL) and Sp immobilized halloysite (HLSp). The halloysite involves three main XRD peaks (10); the main peak at 2θ angle of 12.02° belongs to (001) reflection with a d-pacing of 0.74 nm. The existence of the (002) reflection at 24.96° indicates that HL nanotube is in the dehydrated form. Moreover, the peak at diffraction angle of 62.83° with a basal spacing of 1.48 Å is an indication of nanotube structure (18).
After modification of HL nanotubes, the peak corresponding to d001 reflection was found to shift to lower diffraction angles and showed a decreased intensity. This result confirms that the interlayer distance increases with intercalation of Sp into lumens and nanotube crystallinity decreases partially (19). It also indicates the success of immobilization of Sp into nanotubes via physical adsorption. Moreover, after immobilization process, still existing of the diffraction peak (006) at a 2θ angle of 62.79° is due to the conservation of nanotube morphology with higher diameter as it is observed in SEM images (Figure 1D).
3.2 Morphological analyses of hydrogels
Figures 3–5 show the inner structures of the chitosan hydrogels. In the HLCH composite hydrogels, on the other hand, channel-like morphology of pure chitosan (20, 21) partially turn to open cells (Figure 3). The open cell structure can be ascribed to the interaction of negatively charged nanotubes with almost positively charged chitosan molecules through the hydrogen and ionic bonding. It is also known that strong interactions between chitosan and water inhibit chitosan molecules to form aggregations in neat chitosan hydrogel (22). In terms of HLCH composites, hydrophilic HL nanotubes are believed to remove water molecules from chitosan surface and thereby leading to hydrophobic interactions of CH molecules and interchain hydrogen bonding. In other words, since nanotubes reduce the amount of intramolecular hydrogen bonding, the chitosan chains can be opened from a folded position and so leading to available intermolecular interactions between polymer molecules. This may occur through neutralization of some of the protonated amine groups of chitosan via HL nanotubes which can reduce electrostatic repulsions of cationic chitosan molecules leading to pore formation (Figure 3A,B) up to 5% loading. At higher percentages of HL nanotubes, on the other hand, the structure turns first from pore like to channel like ones in 10% loading (Figure 3C). This may be attributed to the high probability of interaction of HL nanotubes with physical junctions of chitosan molecules like hydroxyl, amine and carboxylic groups, resulting in intramolecular interactions instead of intermolecular interactions between CH molecules themselves. Higher loadings, on the other hand, destroy the sponge morphology completely by producing a heterogeneous appearance with a significant amount of closed cells (Figure 3D).
On the other hand, as compared to HLCH composites, the higher sized open cell morphologies gradually constructed up to 5% in HLSpCH hydrogels (Figure 4). This morphology is obtained most probably due to the initial cage-like structures in HLSp but further additions of HLSp nanotubes results in almost original chitosan channel structure again. This change in structural morphology after a 5% loading, can be ascribed to the interaction of polymeric chains of Sp with chitosan molecules at a high degree of HLSp nanotube loadings (Figure 4C,D) separating CH molecules from each other. This behavior is also clearly seen in their SEM images at high magnifications (Figure 5). The addition of HLSp nanotubes into chitosan biopolymer gives almost microchannel-like structures in the composite hydrogels together with some sub-networks occurring also at cell walls. In them, due to more homogeneous distribution of the organophilic HLSp nanotube, extra physical crosslinks form with Sp biomass and HL nanotubes leading to relatively bigger cells/channels. These additional fine and three dimensional network structures resulting from interaction with HLSp and CH occur mostly at the cell walls (Figure 5B,C). It is very clear that the nanotubes in this composition are very well distributed on the CH walls by creating maximum contact arrangements (Figure 5B inset). In HLSp20CH composite hydrogel (Figure 5D), these porous networks turn to mostly a double layer network like structures from Sp and CH with irregularly dispersed HL islands. This morphology at high degree of loading can be due to high number of interaction of edge/surface attached Sp with CH matrix resulting in exclusion of the free bunches of nanotube system.
3.3 Metal adsorption behavior of the hydrogels
Metal adsorption capacities of hydrogels were investigated by examining the uptake of Cr (VI) ion and using UV/Vis measurements. Figure 6 shows the adsorption test results. In aqueous solutions, Cr (VI) anion is present in different forms of chromate anions. Depending on pH of the solution, Cr (VI) can be in different forms such as H2CrO4, HCrO4-, CrO42- and Cr2O72- (2). It is mostly in HCrO4- form when the pH is between 2 and 6. When the solution has a pH value which is higher than 6, CrO42- and Cr2O72- anions are dominant. The aqueous solution has only CrO42- anion at pH >7.5 (23). On the other hand, for pH below 6.2, chitosan molecules have positive charge due to protonated amine groups (-NH2), resulting in interaction between NH3+ functional groups and chromate anions (24). Moreover, the aluminols (AlOH) on the inner/lumen surfaces of the nanotubes are subjected to different surface ionization reactions in the presence of water leading to AlOH2+ and AlO- ions depending on pH (25). Under the metal adsorption conditions (pH 5.5–6.0) in our study, both types of ions can be present in lumens of HL nanotube (26). Thereby, the electrostatic interaction may exist between the inner aluminol surface of halloysite nanotubes and the adsorbed chromate anions.
As seen in Figure 6, neat CH hydrogel adsorbed 11.14 mg of Cr (VI) ion per gram of hydrogel. HL nanotube loaded composite hydrogels, have an average adsorption of 12.88 mg g-1 in the 50 ppm metal ion solution. The lowest metal adsorption capacity of HL20CH hydrogel, compared to all the hydrogels, can be ascribed to both its closed cells containing morphology (Figure 5D) and the presence of high amount of negatively charged surface blocking active protonated amine groups of CH molecules.
For the HLSpCH composite hydrogels, on the other hand, the average adsorption value increased up to 16.04 mg g-1. The highest value obtained by the HLSp20CH hydrogel (18.28 mg g-1) may be ascribed to interaction of chromate ions with hydroxyl, carboxylic, phosphate, amine, amide, and sulfate groups of Sp (7). This improvement can also be due to protonated amine groups of Sps protein molecules under the metal adsorption conditions (pH 5.5–6.0) in our study. Also, in terms of increased amount of HLSp, there occurs an aforementioned “double layer network” like structures from Sp and CH with irregularly dispersed HL islands (Figure 7A). This morphology results from most probably due to high interaction of edge/surface attached Sp with CH matrix at high degree of loading. The high magnification SEM image of this special morphology (Figure 7B) was found to have many bunches of nanotubes excluded from the surface whose lumens/inner surfaces act as positively charged receptors and extra collector domains for chromate anions. In other words, existence of Sp enlarges the adsorption windows up to 18 mg g-1.
Adsorption isotherms of the hydrogels were also examined via the Freundlich adsorption model, which is basically used for determination of adsorption on heterogeneous surface (27). For the adsorption experiment, chromium (VI) ion containing solutions having four different concentrations (50–200 mg l-1) were used; adsorption capacity qe (mg g-1) and equilibrium Cr (VI) concentration, Ce (mg l-1) were determined. As it can be seen from Figure 8 that as initial metal ion concentration increases, Cr (VI) adsorption value increases. Neat CH hydrogel showed an adsorption value of 11.14 mg g-1 at the lowest concentration (50 mg l-1) and it increased up to 39.21 mg g-1 in the case of the solution concentration of 200 mg l-1. This behavior was also observed via all the composite hydrogels that their metal adsorption values were found to increase with increase in the initial metal ion concentration (Figure 8). HLSpCH hydrogels exhibited higher adsorption capacities for each metal ion concentration than neat CH and HLCH hydrogels. The HLSp20CH hydrogel showed the maximum adsorption values of 18.28 and 66.89 mg Cr (VI) per g of hydrogel at the lowest and the highest metal ion concentrations, respectively. Moreover, HLSp20CH composite hydrogel was found to have an adsorption capacity which is 70% higher than those of neat CH and HL20CH hydrogels at 200 mg l-1 metal ion concentration.
In the literature (7, 11), the Cr (VI) uptake capacity by some adsorbents expressed as mg Cr (VI)/g adsorbent are given as follows: Aeromonas Caviae (Bacterial biomass: 284.4), Lab cultivated yeast (fungal biosorbent: 32.6), Penicillium Purpurogenum (fungal biosorbent: 36.5), Sp (algal biomass: 48 mg) and montmorillonite (Smectite clay: 3.6). Pristine halloysite as silica nanotube, on the other hand, has very low adsorption capacity to hexavalent chromium (2). As a conclusive remark, our result particularly indicated that while 1 g of Sp by itself can remove only about 48 mg Cr (VI), in the presence of supportive hydrogel including HL, this removal increases up to about 6700 mg Cr (VI).
It is also worth to say that after metal adsorption process, easy removal of biosorbent from the solution as a bulk without needing conventional centrifuging process will be an additional advantage of the system.
The equilibrium Cr (VI) ion concentration (Ce, mg l-1) was also found and log qe-log Ce plots were given in Figure 9. The adsorption capacity (k), adsorption intensity (n) and correlation coefficient (r2) as the adsorpsion constants (7), were calculated by using the the qe and Ce values and tabulated in Table 1. As compared to neat CH hydrogel, all the HLSpCH and HLCH composite hydrogels exhibited higher k values. The highest k value (0.61) was obtained with HLSp20CH hydrogel as the best adsorbent system for Cr (VI) based on the Freundlich adsorption model. This result was also confirmed with metal adsorption behavior of the same hydrogel versus time data (Figure 6). On the other hand, HL loaded composite hydrogels showed increased k values up to 5% loading which are still higher than HLSp20CH, then decreases at higher loadings. The lowest k value may be attributed to its abovementioned closed cells containing morphology (Figure 3D) and is in accordance with its adsorption capacity versus time data (Figure 6). According to this result, it can be stated that the combination of Sp biomass, halloysite biocompatible nanotubes and CH biopolymer gives as a new biotechnological approach for adsorption. Moreover, at optimum compositions, this “3-in-1” type adsorbent system can result in effective removal of metals without contamination of water. The production of HLSpCH hydrogels having high adsorption values is accomplished most probably due to existence of immobilized Sp biomass on/in HL nanotubes. The maximized interaction of polysaccharide chains of Sp with CH polymer matrix produces a co-network or a “double layer network” which results in a large surface area. Thereby all the functional groups of Sp biomass and the nanotubes become highly available and thus adsorbs the chromate anions much more in comparison with HL nanotube loaded hydrogels or neat CH hydrogel.
We report a novel “3-in-1” type triple composite adsorbent having Sp microalgae immobilized in/on natural silica nanotube. The morphological and X-ray diffraction studies confirmed the success of immobilization of Sp biosorbent into/onto the nanotubes by resulting in cage-like structures without disturbing the typical tubular structure of nanotubes. It is worth to say that the immobilization of Sp increases water absorption of neat chitosan due to special double network layered open cell morphologies. While the pure chitosan can swell only 2156%, for both HL and Sp loaded hydrogels, this value reached up to 3249%. Two-fold and four-fold increase in toughness and compression modulus values of pure chitosan hydrogel, respectively, were found to be achievable with HLSp loading. The physical crosslinks between Sp-CH double networks as well as silica nanotubes particularly at cell walls seems to be contributing factor in efficient stress/energy dissipation. Fundamental metal adsorption studies showed that the existence of Sp enlarged the metal adsorption windows efficiently up to 18 mg Cr (VI) per gram hydrogel. At the highest metal ion concentration (200 mg l-1), 20% HLSp containing hydrogel showed 70% higher adsorption capacity than pure CH hydrogel and 20% HL containing hydrogel.
It has been believed that with the abovementioned optimized special morphologies, all functional moieties of the Sp biomass and the nanotubes attract the Cr (VI) species from the solution much more deeply as compared to the use of HL nanotube loaded hydrogels or neat CH hydrogel. This new, “3-in-1” type triple adsorbent system seems to provide a novel route for the preparation of natural, simple but high strength as well as very efficient composite hydrogel adsorbers with low cost.
Supports given by Scientific Research Projects Coordination Departments of Yalova University (project no. 2014/BAP/087) and Boğaziçi University are gratefully acknowledged.
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