Polyvinyl alcohol/silica hybrid microspheres: synthesis, characterization and potential applications as adsorbents

Industrial effluents containing large varieties of synthetic dyes from textile, leather, paper printing, food, pharmaceutical and other industries are posing a serious threat to microorganisms, aquatic life and human health. Therefore, it is of great necessity and urgency to eliminate those organic dye contaminants from polluted water. Adsorption has been considered as a simple and effective method for removal of those dyes due to its low cost, easy operation and high efficiency [1, 2]. In the past few decades, different types of adsorbents have been investigated for dye removal, among which microspheres are especially attractive in terms of their large specific surface area, good fluidity and high removal efficiency. However, most of those microspheres that have been developed so far are composed of either inorganic [3–7] (carbonaceous materials, Fe₃O₄, SiO₂, MoS₂, ZnO, et al) or polymeric materials [8, 9] (cellulose, chitosan, et al), therefore inevitably exhibiting a certain limitations when used as adsorbents for organic dyes. For instant, sol-gel derived silica xerogel has drawn extensive attention as an efficient adsorbent for dye removal owing to its chemical inertness, mesoporous nature along with functional groups [10–12]. Nevertheless, the inherent brittleness of silica xerogel makes it hard to maintain its shape and greatly limits the practical applications. It was desirable that the hybrization of silica xerogel with some polymers would improve its flexibility and shape-forming ability while maintaining their mesoporous structure [13], thus potentially generating an ideal adsorbent with improved properties and expanded application perspectives as compared to silica xerogel itself.

Much effort has been currently dedicated to combining silica with some cheap and easily available biosorbents such as chitosan [14], starch [15] and cellulose [16] for waste water treatment. Since sol-gel derived silica xerogel has been recognized as one of the most important biomaterials [17, 18], these resultant biocomposites are desirable to demonstrate a more environmental friendly adsorption process than other traditional adsorbents [19]. Polyvinyl alcohol (PVA) is one of the widely used synthetic polymers in industrial and biomedical applications [20–22]. Particularly, owing to their good properties such as hydrophilicity, biodegradability, non-toxicity, availability and shape-forming ability, the PVA-based materials have been developed as efficient adsorbents in various wastewater systems in recent years [23, 24]. Moreover, the large number of hydroxyl groups existing on the PVA chains may facilitate the PVA materials to adsorb the dye molecules via hydrogen bonding. However, to the best of our knowledge, there have been very few relevant reports on the use of the PVA/SiO₂ biohybrids as a potential adsorbent for removal of dyes from polluted water so far.

The aim of the present study was to fabricate the PVA/SiO₂ hybrid microspheres with good morphology and dispersity and then preliminarily assess their feasibility of serving as a novel bioadsorbent for waste water treatment. Firstly, the homogeneous PVA/SiO₂ hybrid sol was prepared by in situ hydrolysis of tetraethyl orthosilicate (TEOS) in the PVA aqueous solution. Subsequently, the hybrid sol was formulated into microspheres via an emulsion cross-linking process in a water-in-oil (W/O) emulsion. The influence of the SiO₂ content on the composition and morphology of the hybrid microspheres was investigated. Furthermore, rhodamine B (RhB) was chosen as a model dye to evaluate the adsorption capacity of the PVA/SiO₂ hybrid microspheres in aqueous solution. This study is expected to provide a way to extend the potential applications of the PVA/SiO₂ hybrids to the field of environmental research.

2.1. Synthesis of the PVA/SiO₂ hybrid microspheres

2.2. Morphological and compositional characterizations

2.3. Adsorption and removal of RhB by the hybrid microspheres

Rhodamine B (RhB) was selected as the model dye for the adsorption studies. A typical absorption experiment was carried out by mixing 0.15 g of the microspheres with 15 ml of RhB solution (15 mg L⁻¹) and then incubating them at 40 °C in a dark water bath for 5 h, with a shaking rate of 100 rpm. At periodic time intervals, the microspheres were centrifuged, and then 3 ml of the supernatant solutions were taken and analyzed at 554 nm by the UV/Vis spectroscopy. At the same time, an equal volume of fresh deionized water was added to maintain a constant total volume of 15 ml. The PVA-20%SiO₂ hybrid microspheres were used to investigate the effects of amount of adsorbent, adsorption time and temperature on their adsorption behaviors. The percent removal efficiency and adsorption capacity (qt, mg/g) of the RhB dye on the microspheres at any given time t were calculated by using the equations (1) and (2), respectively.

$$\begin{eqnarray} &&\% \,{\rm{Removal}}=\left({{\rm{C}}}_{0}-{{\rm{C}}}_{{\rm{t}}}\right)/{{\rm{C}}}_{0}\times 100\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{{\rm{q}}}_{{\rm{t}}}=\left({{\rm{C}}}_{0}-{{\rm{C}}}_{{\rm{t}}}\right)\times {\rm{V}}/{\rm{W}}\end{eqnarray} \tag{ 2 }$$

where C₀ (mg/L) is the initial dye concentration; C_t (mg/L) is the dye concentration at time t; V (L) is the volume of the dye solution; W (g) is the amount of the microspheres used for the adsorption of RhB. All the adsorption experiments were performed in triplicate, and the data were reported as mean ± standard deviation (SD).

The morphology of the PVA/SiO₂ hybrid microspheres with different SiO₂ contents was displayed in figure 1. The results indicated that the SiO₂ contents exerted a great influence on the dispersity and morphology of the hybrid microspheres. Among these specimens, the PVA-20%SiO₂ hybrid microspheres exhibited the most desirable morphology with the high dispersity and good spherical shape (figure 1(c)). When the SiO₂ content was less than 20%, the hybrid microspheres were observed to have a poorly spherical shape and aggregate seriously, which was very similar to the morphology of the pure PVA microspheres shown in figure 1(a). On the other hand, a higher SiO₂ content than 20% led to a decrease in the uniformity of the particle size. This was probably attributed to the change of the viscosity of the PVA/SiO₂ sol with the SiO₂ content. Moreover, a slightly coarse surface with some nano-sized pores was observed from the high-magnification SEM image of the PVA-20%SiO₂ hybrid microspheres (figure 1(f)), which would be very beneficial for their adsorption behaviors.

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The particle size distributions of the PVA/SiO₂ hybrid microspheres were determined by randomly measuring 300 particles in the SEM images, while that of the pure PVA microspheres was not measured due to the serious aggregation among the particles. The statistical data was plotted as a distribution histogram of the particle diameter of the microspheres with respect to the percentage of the corresponding particle size. As displayed in figure 2, all the four samples exhibited a narrow particle size distribution mainly ranging between 1 and 30 μm. Especially, the PVA-20%SiO₂ hybrid microspheres presented the most uniform distribution pattern. The calculated average particle sizes of the PVA/SiO₂ hybrid microspheres with the SiO₂ content of 10%, 20%, 30% and 40% were approximately 10.3 ± 3.0, 13.3 ± 2.3, 12.0 ± 3.4 and 11.1 ± 4.9 μm, respectively. By combination of the SEM results shown in figure 1, it can be revealed that the change of the SiO₂ content exerted an acceptable impact on the average particle size of the PVA/SiO₂ hybrid microspheres, but had an unneglectable effect on their particle size distribution. It was observed in our experiments that the viscosity of the PVA/SiO₂ hybrid sol significantly increased with the increase of the SiO₂ contents. Especially for the PVA-40%SiO₂ hybrid sol, it was liable to gel after being incubated at 60 °C for a period of time. When the hybrid sol with a high viscosity was added to the oil phase, it tended to be difficult to uniformly disperse under the same conditions, thus creating a relatively wide particle size distribution.

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The FT-IR spectra of the pure PVA and PVA/SiO₂ hybrid microspheres were presented in figure 3(a). As compared with the spectrum of the pure PVA microspheres, the PVA/SiO₂ hybrid microspheres experienced an obvious change in the absorption pattern of the O–H groups in the range of 3000–3800 cm⁻¹, which was mainly attributed to the involvement of the silanol groups (Si–OH). The appearance of the absorption peaks of the Si–O–Si groups at 1063, 796 and 465 cm⁻¹ indicated the formation of amorphous silica in the hybrid microspheres [26]. Moreover, the absorption intensity of the three peaks was gradually enhanced with the increase of the SiO₂ content. In addition, the amorphous structure of silica was further confirmed by the XRD results. As shown in figure 3(b), the PVA/SiO₂ hybrid microspheres only exhibited a characteristic peak of the PVA molecules at about 20°, whereas no obvious diffraction peak was assigned to the SiO₂ phase, regardless of the SiO₂ contents.

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The TG measurement was carried out to investigate the thermal stability of the PVA/SiO₂ hybrid microspheres. As shown in figure 4, the small weight loss below 300 °C was mainly due to the water removal from the microspheres. The larger one occurring in the temperature range of 300 °C–600 °C was probably associated with the decomposition of PVA as well as the progressive poly-condensation and dehydration of silica xerogel [27], The SiO₂ content in the PVA/SiO₂ hybrid microspheres can be determined based on the percentage of the remnant. After deducting the residual amount of the pure PVA microspheres (∼1.9 wt%), the actual SiO₂ contents of the PVA-10%SiO₂, PVA-20%SiO₂, PVA-30%SiO₂ and PVA-40%SiO₂ hybrid microspheres were measured to be about 4.2, 16.2, 26.6 and 32.8 wt%, respectively.

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A preliminary study of the PVA/SiO₂ biohybrid microspheres as a potential adsorbent for removal of the RhB dye was performed. The adsorption behaviors of the dyes by the PVA/SiO₂ hybrid microspheres were monitored by the UV/Vis spectroscopy. In order to determine the optimal adsorption conditions, the effects of the adsorbent dosage, contact time and temperature on the removal efficiency of the RhB dye on the PVA-20%SiO₂ hybrid microspheres were firstly investigated, and the results are shown in figure 5. It was indicated in figure 5(a) that, with the increase of the adsorbent amount, the removal efficiency of RhB increased rapidly and then decreased slightly probably arising from the stack of the extra adsorbents. The maximum removal efficiency (∼88.4%) of RhB was obtained when 0.15 g of the microspheres were used. Thus, this amount of adsorbent was used for further studies. When the contact time was varied from 10 to 300 min, the removal efficiency of RhB increased gradually with time, while the increment in contact time after 300 min led to a little increase in the percentage of the adsorbed RhB (figure 5(b)). Therefore, a contact time of 300 min was chosen as the optimal value. In addition, the percentage of the adsorbed RhB increased gradually with temperature until 40 °C, after which it decreased notably with a further increase in temperature probably due to the accelerated escape of the adsorbed dye from the surface of the microspheres to the solution at higher temperature [28].

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Three kinetic models of pseudo-first-order, pseudo-second-order and intraparticle diffusion adsorption models were used to investigate the adsorption kinetic process of RhB on the PVA-20%SiO₂ hybrid microspheres, and their mathematical representations were given below:

$$\begin{eqnarray*}&&\mathrm{ln}\left({{\rm{q}}}_{{\rm{e}}}-{{\rm{q}}}_{{\rm{t}}}\right)=\,\mathrm{ln}\,{{\rm{q}}}_{{\rm{e}}}-{{\rm{k}}}_{1}{\rm{t}}\,\left(\mathrm{pseudo} \mbox{-} \mathrm{first} \mbox{-} \mathrm{order}\right)\end{eqnarray*}$$

$$\begin{eqnarray*}&&\displaystyle \frac{{\rm{t}}}{{{\rm{q}}}_{{\rm{t}}}}=\displaystyle \frac{1}{{{\rm{k}}}_{2}{{\rm{q}}}_{{\rm{e}}}^{2}}+\displaystyle \frac{{\rm{t}}}{{{\rm{q}}}_{{\rm{e}}}}\left(\mathrm{pseudo} \mbox{-} \mathrm{second} \mbox{-} \mathrm{order}\right)\end{eqnarray*}$$

$$\begin{eqnarray*}&&{{\rm{q}}}_{{\rm{t}}}={{\rm{k}}}_{{\rm{i}}}{{\rm{t}}}^{1/2}+{\rm{C}}\,\left({\rm{intraparticle}}\,{\rm{diffusion}}\right)\end{eqnarray*}$$

where q_e (mg/g) and q_t (mg/g) are the amounts of RhB adsorbed on the PVA-20%SiO₂ hybrid microspheres at equilibrium and a given time t (min), respectively; k₁ (min⁻¹) and k₂ (g·mg⁻¹ · min⁻¹) are the rate constants of the pseudo-first-order and pseudo-second-order models, respectively; the intercept C (mg/g) is related to the boundary layer thickness and k_i is the intraparticle diffusion rate constant (mg·g⁻¹·min^−1/2).

The linear fitness plots of three adsorption models of RhB on the PVA-20%SiO₂ hybrid microspheres are shown in figure 6. It can be seen that good linear relationships were obtained for both pseudo-first-order and pseudo-second-order models with the R² values of 0.9943 and 0.9975, respectively. However, the calculated q_e,cal value from the pseudo-second-order model (1.39 mg g⁻¹) exhibited a much better consistency with the experimental result (q_e,exp, 1.33 mg g⁻¹) than that from the pseudo-first-order model (0.70 mg g⁻¹). In contrast, the intraparticle diffusion kinetic model presented a poor linear relationship with the R² value of 0.9177, as shown in figure 6(c). Moreover, the plot of the intraparticle diffusion model did not pass through the origin (C=0), indicating that it was not the predominant process. Therefore, it was concluded that the adsorption process of RhB on the PVA-20%SiO₂ hybrid microspheres could be well described by the pseudo-second-order kinetic model, which revealed that the overall adsorption rate of RhB during the entire sorption process was controlled by chemisorption.

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The dependence of the removal efficiency of RhB on the SiO₂ content of the hybrid microspheres was represented in figure 7. It was revealed that even the pure PVA microspheres exhibited a good adsorption capacity of the RhB dye, with a removal percentage of about 75.3%. The adsorption capacity of the PVA microspheres for extracting the water-soluble RhB dye was mainly related to the formation of H-bonding and electrostatic attraction between the abundant –OH groups on the PVA chains and the RhB molecules. With an increase of the SiO₂ content, the percentage of the adsorbed RhB increased rapidly and reached a maximum at 20 wt% SiO₂. This may be ascribed to the availability of more adsorption sites by involving the SiO₂ phase as well as the improved uniformity in morphology of the hybrid microspheres. Afterwards, the absorption ability of the hybrid microspheres was gradually decreased as the SiO₂ content increased, which was probably attributed that the total surface area of the hybrid microspheres to adsorb the RhB molecules decreased with a decrease in the uniformity of the particle size. Thus, the PVA-20%SiO₂ hybrid microspheres with a good morphology and a high dispersity were desirable in the industrial applications.

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In the present study, the sol-gel combined emulsification technique was applied to fabricate the PVA/SiO₂ hybrid microspheres with different SiO₂ contents. The SEM observation presented that the hybrid microspheres had the most desirable morphology and an average particle size of about 13.3 μm when the SiO₂ content was 20 wt%. The formation of amphorous SiO₂ xerogel in the hybrid microspheres was confirmed by the FT-IR and XRD results, and its actual content was determined by the TG analysis. Furthermore, the PVA/SiO₂ hybrid microspheres, especially those containing 20 wt% SiO₂, exhibited an improved adsorption capacity to remove the RhB dye from aqueous solution as compared to the pure PVA microspheres, thus potentially serving as an efficient adsorbent for water treatment.

This work was financially supported by the Fund for the Frontier Research of the Discipline (No. 2015XKQY03).