Surface Engineered Zeolite: An Active Interface for Rapid Adsorption

Apr 19, 2016 - Metal ion adsorption studies under varying conditions of time and concentration indicate that the material follows the Langmuir isother...
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Surface Engineered Zeolite: An Active Interface for Rapid Adsorption and Degradation of Toxic Contaminants in Water Ruchi Shaw,† Richa Sharma,† Sangeeta Tiwari,*,† and Sandeep Kumar Tiwari‡ †

Amity Institute of Applied Sciences, Amity University, Noida, India - 201303 Council of Scientific and Industrial Research, New Delhi, India - 110001

‡

S Supporting Information *

ABSTRACT: Zeolite has been surface modified to form novel multifunctional materials having capability for simultaneous and facile removal of heavy metals [Pb(II)], organic pollutants [methylene blue dye], and microorganisms [E. Coli, S. Aureus, and Pseudomonas] from contaminated water. The unique concept involves formation of core−shell particles with a functional core of zeolite and a porous shell of ZnO nanoflakes which not only imparts photocatalytic and antibacterial properties but also renders the surface negatively charged, thereby facilitating rapid adsorption of Pb(II) and MB. The uniform formation of ZnO nanoflakes (shell) on the zeolite (core) surface has been confirmed by XRD, DRS, FE-SEM, and TEM studies. Metal ion adsorption studies under varying conditions of time and concentration indicate that the material follows the Langmuir isotherm model and pseudo-secondorder kinetics with good correlation to the experimental data. The rapid and high adsorption capacity of the material for both Pb (II) and MB has been established while factors responsible for enhanced adsorption have been discussed. The antibacterial studies against Gram negative bacteria (E. Coli and Pseudomonas) and Gram positive bacteria (S. Aureus) showed good zone inhibition characteristics. The material can be regenerated and reused besides having ease of separation using simple techniques. Being multifunctional, efficient, nontoxic, energy neutral, and recyclable with no effluent generation, the material is an efficient and sustainable alternative for water purification. KEYWORDS: zeolite, zinc oxide, core−shell particles, multifunctional adsorbent, water purification

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INTRODUCTION Water is the most precious resource on the planet earth. However, rapid urbanization and industrialization has contaminated it to such an extent that it can no longer be considered safe for use without prior treatment. Extensive work has been reported on the development of innumerable materials to address the problem. Some of the significant materials developed include membranes,1 porous materials,2 zeolites,3 nano-metal oxides,4 etc. However, they address specific categories of contaminants and lack multifunctional character. The most common technique currently in use is reverse osmosis (RO) membrane filtration,5 which has the capability to simultaneously remove major water contaminants. However, it suffers biofouling6 while generating a significant quantity of reject water.7 Also, the process is costly and energy intensive.8,9 Zeolites are the most important conventionally used materials for heavy metal ion removal in water purification. The efficacy of zeolites as ion exchange materials for removal of heavy metal ions from polluted water has been extensively studied and reported.10−12 On the other hand, photocatalytic metal oxides have gained increased attention in © 2016 American Chemical Society

effective photodegradation of various harmful organic pollutants in water as well as for their antibacterial characteristics.13−15 Use of metal oxides in micro- and nanodimensions for removal of organics from water by photocatalytic degradation is extensively studied and reported. However, none of the aforementioned materials are fully capable of removing all classes of contaminants, which makes water purification all the more difficult and paves the way for further studies to develop materials that can simultaneously remove all major classes of contaminants in a sustainable manner. In an attempt to deliver such material, a new concept is being pursued and discussed in this paper wherein zeolite (CBV500) has been surface engineered by incorporating nano metal oxide (ZnO) on the surface of zeolite particles in the form of nanoflakes (ZnO NFs), not only to bring synergy of the two but also to impart surface characteristics favorable for adsorption. The synergy of the two was envisaged to result in Received: February 10, 2016 Accepted: April 19, 2016 Published: April 19, 2016 12520

DOI: 10.1021/acsami.6b01754 ACS Appl. Mater. Interfaces 2016, 8, 12520−12527

Research Article

ACS Applied Materials & Interfaces

4. Antibacterial zone inhibition test. 0.65 g of nutrient broth and 0.75 g of agar−agar were mixed together with 100 mL of DI water and autoclaved for 15 min to synthesize the nutrient medium. Twenty milliliters of this nutrient medium was put in three sterilized petri dishes; 100 μL of the bacterial solution of E. coli, S. Aureus, and Pseudomonas were added on these petri dishes. After 15 min, 100 mg of the Ze/ZnO CSPs were added to these petri dishes and incubated overnight at 37 °C. The zone of inhibition measurements were carried out thereafter. The same procedure was adopted for zeolite. 5. Characterization. The FE-SEM micrographs and elemental analysis were carried out on a Nova Nano SEM 450/EDAX. The phase compositions of the synthesized Ze/ZnO CSPs were determined using a Philips X’Pert PRO diffractometer with nickelfiltered with Cu Kα. HR-TEM was performed using IFEI, Tecnai F30, FEG system. UV−vis analysis of the water samples was carried out on a Shimadzu UV-1800 double beam spectrophotometer. The diffuse reflectance spectra (DRS) of the samples were recorded with a UV− vis-NIR spectrophotometer (PerkinElmer Lambda-950). For photodegradation studies, a solar simulator (AM 1.5 100 mW/cm2 Newport Sideways Solar simulator Class AAA) was used. The zeta potentials were recorded on a Zetasizer, version 7.11 (Malvern), while BET analysis was carried out using a Quantachrome Autosorb Automated Gas Sorption System, version 3.01.

a multifunctional material wherein the zeolitic core contributes to the high adsorption of heavy metals [Pb(II)] and organic contaminants [MB dye], while the nano-ZnO NF shell facilitates photodecomposition of organic matter, provides antibacterial characteristics, and imparts negative surface charge to enhance adsorption. The optimized synthesis facilitates uniform formation of a ZnO nanoflakes shell, which provides high surface area, porosity, and negative charge to the surface, resulting in enhanced adsorption of Pb(II) and MB. The synergy between the core and the shell not only results in rapid and effective removal of all categories of pollutants but also provides additional benefits of ease in separation and recyclability. The material is envisaged to have multifunctional characteristics, efficient adsorption and photodegradation capability, ease of separation, and to be recyclable and energy neutral with no effluent generation.

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MATERIALS AND METHODS

Materials. Zeolite Y (CBV 500; Zeolyst International), zinc acetate dihydrate (Qualigens), potassium hydroxide (CDH), hexamine (CDH), zinc nitrate (CDH), polyethylenimine (Sigma-Aldrich), lead acetate trihydrate (Fisher Scientific), EDTA (Fisher Scientific), methylene blue (CDH), zinc oxide (Sigma-Aldrich). The chemicals used were of analytical-reagent grade of the highest available purity. Methods. 1. Synthesis of Zeolite@ZnO Core−Shell Particles (Ze/ ZnO CSPs). Formation of a porous shell of ZnO on the zeolite core was an important aspect to ensure the synergy of the two materials. The conditions for growth of ZnO NFs on zeolite were optimized. The optimized conditions involved use of zinc oxide nanoparticle seed solution. The seed solution was prepared using zinc acetate dihydrate as the precursor; methanolic solution of KOH was added dropwise at 65 °C over a period of 15 min. The reaction mixture was then stirred for 2.5 h at 65 °C. After cooling to room temperature, the precipitate was washed twice with methanol. N-Butanol (70 mL), methanol (5 mL), and chloroform (5 mL) were added to disperse the precipitate. The seed solution was mixed with the zeolite powder and stirred overnight. Seeded/nucleated zeolite powder was obtained with seeds of zinc oxide on its surface. This seeded zeolite powder was used to make the Ze/ZnO CSPs. For growing ZnO nanoflakes on the seeded zeolite, three solutions were prepared: solution of zinc nitrate in DI water; solution of HMTA (hexamethylenetetramine) in DI water, and solution of PEI (polyethylenimine) in DI water. The solution of zinc nitrate was mixed with the solution of PEI, and the resulting solution was mixed with the solution of HMTA. Thereafter, ammonia was added. The seeded zeolite was added to the final solution and kept at 95 °C with stirring for 4 h. The resulting product was filtered, washed, dried, and annealed at 450 °C for 30 min to get the final product. 2. Heavy metal ion removal. Time and concentration variation studies were conducted by mixing 200 mg of Ze/ZnO CSPs in 50 mL of the Pb(II) (model heavy metal) ion solutions of known concentrations. After 60 min of stirring, the resulting solutions were centrifuged and supernatant was collected for determination of ion concentration using UV−vis spectroscopy. The complex of Pb(II) was obtained using EDTA and estimated by UV vis analysis at 242 nm.16 For comparison purposes, ion adsorption using zeolite was also studied. 3. Organic methylene blue (MB) dye adsorption and photodegradation. 200 mg of Ze/ZnO CSPs were added to 50 mL solution of different concentrations of methylene blue dye (model organic molecule). The resulting solutions were centrifuged. Supernatant and powder was collected separately and subjected to UV and diffuse reflectance spectroscopy (DRS) analysis to study the dye adsorption characteristics. In another experiment, the dye solution with Ze/ZnO CSPs was exposed to a solar simulator to study the rate of photodegradation. For comparison purposes, the same studies were conducted on zeolite and ZnO NF particles.

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RESULTS AND DISCUSSION The chemical composition of Ze/ZnO CSPs determined using ICP showed 4.5% of zinc oxide in the composite core−shell material. The corresponding XRD pattern and DRS spectra of zeolite and the Ze/ZnO CSPs are shown in Figure 1.

Figure 1. (a) XRD spectra of the zeolite and Ze/ZnO CSPs. (b) DRS spectra of the zeolite and the Ze/ZnO CSPs.

The characteristic diffraction peaks of ZnO NFs match with the standard ZnO data (JCPDS Card: 65-3411), indicating formation of a hexagonal wurtzite structure. Sharp peaks indicate high crystallinity of the material. Overlapping of the diffraction peaks of ZnO with the peaks of the zeolite could be an indication of formation of a core−shell structure. DRS spectra also indicate formation of ZnO particles on zeolite [Figure1(b)]. The Ze/ZnO CSPs exhibit absorbance in the UV−visible region with a sharp inflection at 380 nm showing typical band edge adsorption of ZnO.17 The observed band edge shift could be attributed to nitrogen doping of ZnO NFs, possibly due to the use of ammonia during synthesis. Raman spectral analysis confirms formation of N-doped ZnO [S 1]. Morphological studies by SEM (Figure 2) indicate the formation of the core−shell structure with a uniform distribution of ZnO NFs over the zeolite surface. The morphology of ZnO NFs on the zeolite surface shows that the presence of a ZnO NF shell would not significantly obstruct the interaction of contaminants with the zeolite surface. SEM studies therefore indicate that the core would carry its 12521

DOI: 10.1021/acsami.6b01754 ACS Appl. Mater. Interfaces 2016, 8, 12520−12527

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Experiments were carried out for selected model pollutants from each category, i.e., Pb(II) (heavy metals), MB dye (Organic), and E. coli, Pseudomonas, and S. Aureus bacterial strains (microorganisms). Adsorption kinetics and isotherm for heavy metal ion [Pb(II)] removal. The application of various kinetics and adsorption isotherm models has revealed that pseudo-secondorder and Langmuir isotherms are the best suited models to describe the mechanism of adsorption. The intraparticle diffusion model was used to study the diffusivity in zeolite and Ze/ZnO CSPs, which indicates that the removal of analytes follow a diffusion as well as an adsorption mechanism (Supporting Information section S7). Figure 4(a) shows the effect of time on the adsorption of Pb(II) on the Ze/ZnO CSPs. The initial concentration of Pb(II) was 300 mg/L while 200 mg of Ze/ZnO CSPs was used. For an initial concentration of 300 mg/L of Pb(II) solution, Ze/ZnO CSPs reach the adsorption equilibrium in ∼60 min, showing the equilibrium adsorption capacity (qe) of the 74.661 mg/g. The observed qe of Ze/ZnO CSPs is much higher than that of zeolite, which has a qe of 45.96 mg/g and reaches equilibrium in ∼70 min. The enhanced rate of adsorption observed in Ze/ZnO CSPs can be attributed to the change in the surface characteristics, which attain a negative charge (zeta potential: −7) after formation of the ZnO NF layer (from positive in the case of zeolite; zeta potential: 1.05). Being positively charged, the Pb(II) ions are rapidly adsorbed on Ze/ZnO CSPs. The study of adsorption kinetics establishes that the formation of a shell shifts the adsorption mechanism from chemisorption to physiochemisorption, which helps to significantly increase the equilibrium adsorption capacity. The pseudo-second-order kinetic model19 was applied to study the mechanism of heavy metal ion adsorption on Ze/ ZnO CSPs. Pseudo-second-order kinetic model implies:

Figure 2. (a) SEM image of the zeolite. (b−d) SEM images of the Ze/ ZnO CSPs at different magnifications.

functional characteristics even after the formation of the ZnO NF shell. Table S1 describes the elemental compositions of zeolite and Ze/ZnO CSPs found using EDAX and ICP analysis. TEM images of the Ze/ZnO CSPs shown in Figure 3(a,b) indicate the flaky nature of the ZnO on the zeolite core. The

t/q t = 1/kq e 2 + t/q e Figure 3. TEM images of Ze/ZnO CSPs showing nanoflakes (a), Nanoflakes at the edges (b), Indexing of ZnO NF showing lattice constant of 0.266 nm (c).

where qe and qt are the adsorption capacity of the adsorbent in mg/g at equilibrium and at time t respectively. k is the rate constant of the second-order adsorption (g mg−1 min−1). The adsorption capacity “qe” of the Ze/ZnO CSPs was determined to be 74.661 mg g−1. From the regression graph, the values of the rate constant k and R2 are found to be 0.0081 g mg−1 min−1 and 0.9541, respectively, indicating that the theoretical calculation is in agreement with the experimental data. Adsorption isotherms indicate the interaction at the solid− liquid interfaces. The Langmuir adsorption isotherm20 was used to analyze the equilibrium adsorption data, which is as follows:

NFs are seen to be uniformly distributed on the surface and the edges of zeolite particles. The lattice constant value of 0.266 nm matches with the (0002) plane of ZnO18 [Figure 3(c)]. Formation of ZnO NFs on the surface of zeolite results in an increase in surface area and pore volume (Table 1). BET isotherm studies of the zeolite and the Ze/ZnO CSPs (Figure S2) indicate the porous nature of the shell. Table 1. Difference in the Pore Radius, Pore Volume, and BET Surface Area of the Zeolite and the Ze/ZnO CSPs Zeolite Ze/ZnO CSPs

Pore radius 15.299 Ȧ 17.055 Ȧ

Pore Volume 0.031 cc/g 0.039 cc/g

(1)

Ce /q e = 1/q maxKL + Ce /q max

(2)

where Ce is the equilibrium concentration of Pb(II) in solution, qe is the equilibrium amount of the ion adsorbed, qmax is the maximum adsorption capacity, and KL is the Langmuir adsorption constant. The Langmuir adsorption isotherm is based on the monolayer adsorption formula while neglecting the lateral interactions between the adsorbent and the adsorbate.21 Figure 5(a) shows the equilibrium adsorption isotherm for the adsorption of Pb(II) by 200 mg of the Ze/ZnO CSPs. It indicated that the adsorption capacity increased gradually with the increase in concentration of the Pb(II) in solution. The values of KL (energy of the reaction) and qm (maximum adsorption capacity) were obtained by the intercept and slope

BET Surface Area 2

319.941 m /g 382.882 m2/g

After establishing the formation of morphologically engineered Ze/ZnO CSPs having a layer of ZnO on zeolite particles, studies on the multifunctional behavior of Ze/ZnO CSPs and their efficacy to remove contaminants from water belonging to different categories were performed. Comparative studies between zeolite, ZnO NF, and the Ze/ZnO CSPs were carried out under different conditions to establish the efficiency and multifunctional characteristic of the Ze/ZnO CSPs. 12522

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Figure 4. (a) Variation in adsorption capacity with adsorption time for the adsorption of Pb(II) ions on the Ze/ZnO CSPs (Adsorbent dose = 200 mg, Initial concentration = 300 mg/L). (b) Pseudo-second-order kinetics for the adsorption of Pb(II) on Ze/ZnO CSPs (Adsorbent dose = 200 mg, Initial concentration = 300 mg/L).

Figure 5. (a) Adsorption isotherm for Pb(II) on Ze/ZnO CSPs (adsorbent dose = 200 mg, Pb(II) concentration = 10−500 mg/L). (b) Langmuir plot for Pb(II) on Ze/ZnO CSPs.

of the linear regression curve of Cf/qe against “Ce”, where Cf is the final concentration. The maximum adsorption capacity of the Ze/ZnO CSPs using the Langmuir adsorption isotherm was found to be 78.85 mg/g whereas KL and R2 were found to be 0.0006 L mg−1 and 0.999, respectively. This indicates that the Pb(II) adsorption is well represented by the monolayer adsorption model. The maximum adsorption capacity of the Ze/ZnO CSPs is 78.85 mg/g, which is observed to be much higher than that of pristine zeolite (CBV 500; 47.93 mg/g). It may be noted that the pH of the Pb(II) solution remains more or less unchanged on addition of zeolite and Ze/ZnO CSPs (Table S3). The studies conducted by varying the pH revealed that the maximum adsorption of Pb(II) ions takes place at pH of 6.5−7.0 (Figure S7). The ion exchange capacity (IEC) determined as per standard procedure was observed to be 2.4 and 3.8 mequiv/g (pH: 6−7) for zeolite and Ze/ZnO CSPs, respectively, indicating the adsorption synergy of the core−shell in the enhancement of the IEC. Adsorption kinetics and isotherm for methylene blue (MB) dye removal. Figure 6(a) illustrates the time dependent adsorption kinetics of MB molecules on the Ze/ZnO CSPs at an initial concentration of 300 mg/L of MB dye and 200 mg of the Ze/ZnO CSPs. From Figure 6(a), it is observed that Ze/

ZnO reaches an adsorption equilibrium in 60 min with an equilibrium adsorption capacity (qe) of 74.532 mg/g while zeolite reaches qe of 55.27 mg/g in ∼65 min and ZnO nanoflakes show equilibrium at around 70 min, having qe of 35.018 mg/g. The enhanced adsorption in Ze/ZnO CSPs is facilitated by favorable surface characteristics imparted by formation of a shell on the zeolite. It is emphasized that adsorption plays a significant role in the photodecomposition process, as the organic molecule is first adsorbed followed by its photodecomposition.22 The low adsorption in the case of ZnO NF is due to the absence of the organophilic zeolite surface, which significantly contributes to adsorption. The rapid adsorption of dye on the material surface is expected to enhance photodegradation as well. Analysis of adsorption by various models reveals that the adsorption of MB dye by the zeolite best follows the Elovich model, which suggests chemisorption whereas Ze/ZnO CSPs follow pseudo-secondorder kinetics in addition to the Elovich model, suggesting that the mechanism for MB dye adsorption by the CSPs is governed by physiochemisorption. The pseudo-second-order kinetic model, as described in the previous section (eq 1), was also applied to study the mechanism of methylene blue dye adsorption. The linear plot 12523

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Figure 6. (a) Variation in adsorption capacity with adsorption time for the adsorption of methylene blue molecules on the Ze/ZnO CSPs (Adsorbent dose = 200 mg, Initial concentration = 300 mg/L). (b) Pseudo-second-order kinetics for the adsorption of methylene blue molecules on the Ze/ZnO CSPs (Adsorbent dose = 200 mg, Initial concentration = 300 mg/L).

Figure 7. (a) Adsorption isotherm for MB dye on Ze/ZnO CSPs (adsorbent dose = 200 mg, MB concentration = 10−500 mg/L). (b) Langmuir plot for MB adsorption on Ze/ZnO CSPs.

of “t/qt” vs ‘t’ is shown in Figure 6(b), which shows a linear pseudo-second-order kinetic pattern having an R2 value of 0.9695, indicating that the theoretical calculation is in agreement with the experimental data. The value of the rate constant ‘k’ is found to be 0.0073g mg−1 min−1. The Langmuir isotherm model (eq 2) was used to describe the adsorption isotherm. Figure 7(a) shows the equilibrium adsorption isotherm for the adsorption of MB dye on the Ze/ ZnO CSPs. Figure 7(b) shows the linear regression plot of Cf/ qe against Ce. The maximum adsorption capacity and energy of the reaction KL were found to be 79.83 mg/g and 0.0005 L/mg, respectively. The Langmuir adsorption isotherm for Ze/ZnO CSPs is in accordance with the experimental data, with the R2 value of 0.999. The data indicates that the MB adsorption is well represented by the monolayer adsorption model, and the maximum adsorption capacity (79.83 mg/g) has significantly increased in comparison to that of zeolite (63.14 mg/g) and ZnO NF (24.11 mg/g). The MB molecules [C16H18N3SCl] would dissociated into [C16H18N3S+] and Cl− ions in aqueous medium.23 [C16H18N3S+] ions are expected to have strong affinity toward the negatively charged Ze/ZnO CSPs. The adsorption would be further aided by the organophilic nature of the zeolite surface. Thus, the organic molecules are expected to rapidly migrate from the solution to the interface of the

photocatalytic core−shell material. As seen in Figure 7(a), ZnO NF attains adsorption equilibrium at 200−300 mg/L of MB dye and a sharp decrease in adsorption is seen thereafter. This dip may be attributed to the increased presence of the large MB molecules at higher concentrations, resulting in saturation of the adsorbent surface. Photocatalytic activity of the Ze/ZnO CSPs. 200 mg of the Ze/ZnO CSPs was added to MB dye solutions of varied concentrations (5−75 ppm) and exposed to a solar simulator (SS). It was observed that, with the increase in concentration, the time for photodegradation also increases. Five ppm of dye solution was completely degraded in 75 min whereas 50 ppm solution took 2.5 h to degrade completely. Adsorption of MB dye on Ze/ZnO CSPs is confirmed using the DRS technique, and the concentration of MB in the resultant solution was studied using UV spectroscopy. The DRS of MB adsorbed Ze/ZnO CSPs (Figure 8) shows the characteristic peak of MB, signifying the adsorption of MB by the particles. On exposure to a solar simulator, complete decomposition of MB molecules is indicated by the disappearance of the MB peak and appearance of the Ze/ ZnO CSPs adsorption peak. Complete photodegradation is marked by the conversion of the blue color of the MB solution into a milky white solution [Figure S3]. The UV−visible 12524

DOI: 10.1021/acsami.6b01754 ACS Appl. Mater. Interfaces 2016, 8, 12520−12527

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Figure 8. DRS of the core−shell particles; Ze/ZnO CSPs before and after the exposure to SS.

Figure 11. Drop in activity with recycling of Ze/ZnO CSPs.

Figure 12. Zone of inhibition of the core−shell particles against E. coli, S. Aureus, and Pseudomonas. Number 1 indicates the zeolite, and Number 2 indicates the Ze/ZnO CSPs.

solutions. Ze/ZnO CSPs showed complete photodegradation of MB within 75 min. The commercial ZnO showed 92.2% degradation, which in the case of ZnO NFs was only 85.3%, even after 90 min illumination (Figure 9). The photodecomposition kinetics of MB for ZnO NF and Ze/ZnO CSPs was studied by using various kinetic models, and it was found that the adsorption mechanism best followed the first-order kinetics model. The following equation describes the linear form of the first-order reaction.

Figure 9. Time variation studies with respect to % photodegradation of MB dye.

spectra of the solutions after adsorption of MB dye on the Ze/ ZnO CSPs show reduction of color due to significant removal/ adsorption of dye on CSPs (Figure S4). The photodegradation of MB dye was studied by analyzing the change in color of the MB dye solution under illumination (solar simulator) at regular time intervals. The DRS of adsorbents indicates that initially the adsorption of MB takes place on adsorbents. In the case of Ze/ZnO CSPs, the dye molecules are completely adsorbed in 15−20 min. The photodecomposition of dye is expected to start after its adsorption on adsorbents. For comparison of decomposition efficiencies, studies were carried out by using 5 ppm MB

ln[A] = ln[A 0] − kt where [A0] is the initial concentration of MB dye solution at t = 0 and k is the rate constant. Figure 10(a) shows the photodecomposition of MB dye as a function of time on exposure of a solution containing ZnO NF and Ze/ZnO CSPs to a solar simulator. The initial

Figure 10. Effect of time t vs qt on the photodegradation of MB dye by the CSPs and ZnO NF. (b) First-order kinetic plot for the MB degradation. 12525

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Adsorption isotherms indicate that the analytes are adsorbed by a synergy between the core and the shell involving a surface and multilayer heterogeneous adsorption process. The excellent adsorption and photodegradation behavior was attributed to the porous structure, large surface area, and negative surface charge on the Ze/ZnO CSPs. The multifunctional characteristics of the material were established involving adsorption, photodegradation, and antibacterial activities while reflecting the significance and novelty of designing Ze/ZnO CSPs for water treatment. The material is multifunctional, nontoxic, and recyclable; therefore, it qualifies as a sustainable alternative to the existing water treatment methodologies and holds significant promise for comprehensive water treatment.

concentration of MB dye used was 50 ppm with 200 mg of the adsorbents. For Ze/ZnO CSPs, the complete photodegradation of 50 ppm dye solution takes place within 150 min while in the case of ZnO NF it was observed that the photodegradation process reached saturation after 75% of photodegradation of MB dye. The correlation data obtained showed an R2 value of 0.95 for CSPs and 0.93 for ZnO NF (Figure 10(b)). Under the experimental conditions used, the rate constant k was found to be 18.5 × 10−3 s−1 for Ze/ZnO CSPs and 1.8 × 10−3 s−1 for ZnO NF. Regeneration and recycle. The effect of regeneration of the Ze/ZnO CSPs was studied up to 5 cycles. The adsorption of Pb(II), organic pollutant (MB), and photodegradation efficiencies were determined after each cycle. The experimental procedures for each cycle were kept the same. After ion adsorption, the Ze/ZnO CSPs were regenerated using 1 M NaOH and HCl solutions (2 h; 40 °C). As the organic molecules (MB dye) degrade on exposure to light, the Ze/ZnO CSPs were regenerated by 2 h exposure to light. After 5 cycles, the adsorption capacity for the Pb(II) ion was observed to progressively decrease to 86%. With regard to the adsorption of MB, the percent activity remains high at 94% even after five cycles. A significant drop to 72% was observed in the photocatalytic activity for MB dye decomposition. This decrease may be attributed to the formation of lecuo-methylene blue (LMB) following the adsorption of the MB on the Ze/ ZnO CSPs. The LMB would block further electron transfer from the Ze/ZnO CSPs to the methylene blue molecules.24 The results indicate the robustness of the synthesized material. This implies that the Ze/ZnO CSPs can be used several times for the heavy metal ion removal and degradation of organic pollutants from water, thereby indicating they are a costeffective alternative in water purification (Figure 11). Antibacterial studies. The bactericidal experiments carried out on Escherichia coli and Pseudomonas (Gram positive bacteria)25 and S. Aureus (Gram negative bacteria)26 show a clear zone surrounding the Ze/ZnO CSPs in the presence of the bacterial solutions. The experiments confirm significant zone inhibition characteristics for Ze/ZnO CSPs (Figure 12). The length of the zone of inhibition of Ze/ZnO CSPs is shown in Table S2. Predictably, the zeolite does not show any zone. The Ze/ZnO CSPs therefore show the potential of an efficient antibacterial material, which adds to its efficient adsorbent characteristics.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01754. Raman data, composition analysis of the zeolite and the Ze/ZnO CSPs using EDAX and ICP studies, BET adsorption isotherms, experimental picture of the photodegradation process, UV visible spectra of solutions after adsorption of MB dye, DRS spectra of the zeolite before and after exposure to solar simulator and length of zone of inhibition of the Ze/ZnO CSPs, proposed mechanism of photodegradation, mechanism of antibacterial activity, error analysis and pH study data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. Onkar Game, Senior Research Scholar at CSIR-NCL, Pune, for his valuable input in conducting a part of the experiments.

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REFERENCES

(1) Zularisam, A. W.; Ismail, A. F.; Salim, R. Desalination 2006, 194 (1−3), 211−231. (2) Shukla, A.; Zhang, Y.-H.; Dubey, P.; Margrave, J. L.; Shukla, S. S. The Role of Sawdust in the Removal of Unwanted Materials from Water. J. Hazard. Mater. 2002, 95 (1−2), 137−152. (3) Á lvarez-Ayuso, E.; García-Sánchez, A.; Querol, X. Purification of Metal Electroplating Waste Waters using Zeolites. Water Res. 2003, 37 (20), 4855−4862. (4) Zhong, L.-S.; Hu, J.-S.; Liang, H.-P.; Cao, A.-M.; Song, W.-G; Wan, L.-J. Self-Assembled 3D Flowerlike Iron Oxide Nanostructures and Their Application in Water Treatment. Adv. Mater. 2006, 18 (18), 2426−2431. (5) Greenlee, L. F.; Lawler, D. F.; Freeman, B. D.; Marrot, B.; Moulin, P. Reverse Osmosis Desalination: Water Sources, Technology, and Today’s Challenges. Water Res. 2009, 43 (9), 2317−2348. (6) Matin, A.; Khan, Z.; Zaidi, S. M. J.; Boyce, M. C. Biofouling in Reverse Osmosis Membranes for Seawater Desalination. Desalination 2011, 281, 1−16. (7) Gedam, V. V.; Patil, J. L.; Kagne, S.; Sirsam, R. S.; Labhasetwar, P. Performance Evaluation of Polyamide Reverse Osmosis Membrane for Removal of Contaminants in Ground Water Collected from Chandrapur District. J. Membr. Sci. Technol. 2012, 2 (3), 10.4172/ 2155-9589.1000117.

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CONCLUSION In summary, porous Ze/ZnO CSPs were successfully synthesized via a simple one pot homogeneous precipitation method. The resulting Ze/ZnO CSPs showed enhanced adsorption capability to remove Pb(II) (model heavy metal ion) and MB dye (model organic molecule). The particles exhibited rapid photodegradation of MB dye. Ze/ZnO CSPs also possess antibacterial properties against Gram positive and Gram negative bacteria. The adsorption isotherms of Pb(II) and MB fitting to the Langmuir adsorption model indicate a monolayer adsorption on Ze/ZnO CSPs. The adsorption kinetics followed the pseudo-second-order kinetic model. A significant increase in the equilibrium adsorption capacities and maximum adsorption capacities was observed for Ze/ZnO CSPs in comparison to other adsorbents, viz., zeolite and ZnO NF. A shift in adsorption mechanism from chemisorption to physiochemisorption suggests that the ZnO nanoflake shell facilitates rapid adsorption of the analytes from the solution. 12526

DOI: 10.1021/acsami.6b01754 ACS Appl. Mater. Interfaces 2016, 8, 12520−12527

Research Article

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DOI: 10.1021/acsami.6b01754 ACS Appl. Mater. Interfaces 2016, 8, 12520−12527