Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 258−267
pubs.acs.org/journal/ascecg
Photocatalytic Degradation of Rhodamine B Using Zinc Oxide Activated Charcoal Polyaniline Nanocomposite and Its Survival Assessment Using Aquatic Animal Model S. Steplin Paul Selvin,† A. Ganesh Kumar,‡ L. Sarala,§ R. Rajaram,‡ A. Sathiyan,† J. Princy Merlin,† and I. Sharmila Lydia*,† †
PG and Research Department of Chemistry, Bishop Heber College, Tiruchirappalli 620017, Tamil Nadu India DNA Barcoding and Marine Genomics Laboratory, Department of Marine Science, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu India § Strength of Materials Lab, Department of Civil Engineering, Karunya University, Coimbatore 641114, Tamil Nadu India ‡
S Supporting Information *
ABSTRACT: Zinc oxide activated charcoal polyaniline (ZACP) nanocomposite were prepared using a simple precipitation method. The as-synthesized photocatalyst was characterized by XRD, SEM, HRTEM, EDX, UV−vis, PL, XPS, and BET techniques. Results confirmed the successful incorporation of activated charcoal polyaniline (AC/PANI) on ZnO nanoparticles. The photocatalytic activity of the as-synthesized ZACP was assessed by the photodegradation of Rhodamine B (RhB) using visible light irradiation. The improved photocatalytic activity of the sample was ascribed to the synergetic effect between AC/PANI and ZnO which effectively separates the electron hole pair on the interface of PANI and ZnO. Survival assessment was carried out using Artemia salina (AS) to determine the detoxification potential of the degraded products. In survival assessment, treated dye solution exhibited less toxic effect when compared to the untreated dye solution. A mechanism was also proposed for the degradation of RhB dye using ZACP under visible light irradiation. KEYWORDS: Rhodamine B, Nanocomposite, Photodegradation, Artemia salina, Survival assessment
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INTRODUCTION
pollutants that lead to certain problems. As a result of these issues industries are concentrating on innovative technology to meet their demands. Among several treatment approaches heterogeneous photocatalysis in the presence of visible light irradiation have been widely considered for the photocatalytic degradation of textile industry wastewater.6 This technique has advantages over dye degradation without producing any secondary pollution to the environment. However, it is significant to get the degraded products that do not disturb the biological systems.8 TOC and COD measurements are used to find out the quality of water. Nevertheless the effluents not only contain dyes but also have other chemical intermediates with even more toxic nature than the parent compounds. Yet, the chemical evaluation of toxicity of the treated water has restrictions, the ultimate one is being inability to assess the bioavailability of contaminants.9,10 As a result, additional studies are needed to check the quality of the treated water. Among various techniques, survival analysis is one of the main
Industries discharge huge quantities of pollutants into aquatic environments, which may affect both flora and fauna. Especially dye containing wastewater from textile, printing, dyeing, and food industry leads to severe damage to the water surroundings.1,2 Different types of dyes are available based on their chromophore structures, namely, reactive, azo, diazo, anthraquinone, xanthene, disperse, acidic, and basic dyes.3,4 These may also be classified into cationic, anionic, and nonionic dyes. RhB is a type of xanthene dyes, which is extensively used as a colorant in paper, printing, textiles, and food stuffs. It is reported to be toxic and causes irritation to skin, eyes and respiratory tract. Further, it causes carcinogenicity, neurotoxicity, and chronic toxicity to human beings and animals. Similarly dyes containing textile effluents also produce toxicity to the water bodies. Even though it affects the environment the necessity of dyes for industrial application is important to the society.5 Nowadays various treatment methods such as adsorption, biological oxidation, and coagulation/flocculation are used to treat the dye polluted water.6,7 But on the other hand the conventional treatment methods generate some secondary © 2017 American Chemical Society
Received: July 12, 2017 Revised: November 11, 2017 Published: November 23, 2017 258
DOI: 10.1021/acssuschemeng.7b02335 ACS Sustainable Chem. Eng. 2018, 6, 258−267
Research Article
ACS Sustainable Chemistry & Engineering
the photocatalyst. It also supports the catalyst to adsorb more number of dye molecules and helps to degrade the sample more efficiently. On the basis of these facts, herein, we report synthesis of zinc oxide activated charcoal polyaniline (ZACP) nanocomposite by precipitation method. RhB was chosen as a model dye to assess the photocatalytic ability of ZACP under visible light irradiation. Mineralization and survival assessment of the dye sample was studied.
treatment methods which could be used to verify both the quality and survival capability of the animal species in the treated industrial effluent. It can be performed under specific conditions on different species, such as daphnias, ceriodaphnias, crustaceans, fish, algae, bacterial cells, and plants. Among different species, Artemia salina (AS) is a small crustacean, which can survive in difficult environment even with low level of oxygen.11,12 Encouraged by all such facts, AS was used as a model species in toxicity tests. A wide range of semiconductor materials are used as a photocatalyst to treat the dye polluted water. Among several photocatalysts ZnO is known to be a better catalyst owing to low cost, nontoxicity, photoactivity, and biological and chemical inertness. However, single-phase semiconductor oxide has some issues relating to photocorrosion, low surface area, wide band gap thus only absorb UV spectrum and further, it leads to high electron hole pair recombination. These drawbacks mainly affect the photocatalytic activity of single phase semiconductor metal oxide nanoparticles. Therefore, suitable modification of these semiconductor nanoparticles has been done to overcome the downsides.13,14,8 Recently, delocalized conjugated polymers, carbon materials have been widely used as composite material to overcome the aforementioned drawbacks. Meanwhile the lowest unoccupied molecular orbital (LUMO) level of conjugated polymer is greater than the conduction band of ZnO, it could transfer the generated electrons under visible light irradiation which is thermodynamically favorable.15,16 The conducting polymers act as photosensitizers and suppress the electron hole pair recombination of metal oxide nanoparticles that could enhance the photocatalytic activity of the composite material under visible light.17 Several conducting polymers, such as polyaniline, polythiophene, and polypyrrole, were used in the modification of semiconductor nanoparticles. Among these conducting polymers, polyaniline with benzenoid and quinonoid parts with delocalized conjugated arrangements have been used as good photosensitizers in the polymeric semiconductor nanocomposite owing to unique optical, electrical, low cost and ease of preparation.18,19 In recent times, PANIbased nanocomposite materials are occasionally reported in photocatalytic degradation of dyes. Especially ZnO/PANI nanocomposite are used as better photocatalysts owing to high adsorption capacity and enhanced ability to suppress the electron hole pair recombination through interfacial charge transfer between ZnO and PANI which improves the photocatalytic activity of the composite material.20,21 Even though polymers have used in many commercial applications the performance of these composites materials is based on its durability, maintenance, and reusability during their outdoor applications. The weakening of these polymer materials rely on the usage and the level of contact with environment.22 Further, photocatalytic reaction, needs catalyst with larger surface area that would allow the catalysts to contact the dyes more easily and thus enhances the catalytic efficiency of the photocatalyst. To overcome these problems polymers should be modified with some carbon materials. Quite a few studies have been devoted to composite materials prepared by mixing different additives with PANI. Among these materials, activated charcoal (AC) have attracted many researchers owing to its low cost, greater surface area and act as a promising supporting material for polymers to improve mechanical, thermal, and physicochemical properties of the composites materials compare to pure polymer.23,24 The three-component system was synthesized to improve the activity and to increase the surface area of
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MATERIALS AND METHOD
All the chemicals were of analytical grade and used without any further purification. All solutions were prepared by deionized water. Preparation of AC/PANI. AC/PANI nanocomposite was synthesized by following steps. Aniline (40 mM) was mixed with 0.5 g of activated charcoal and stirred for 30 min in an ice−water bath under 0 °C. Later, a precooled 50 mM potassium peroxodisulfate (PDS) solution was added dropwise to the resultant solution and stirred for 12 h to initiate the oxidative polymerization. The obtained precipitate was washed quite a few times with water and acetone to eliminate the organic residuals. The purified AC/PANI composite was then dried at 40 °C for 24 h and used for further studies. Preparation of ZACP. ZACP nanocomposite was synthesized using chemical precipitation method. Fifty milliliters of zinc acetate dihydrate (0.1 M) was added with 0.05 g of AC/PANI and magnetically stirred for 30 min at 70 °C. Then 50 mL of NaOH (0.2 M) was added to the above solution and continuously stirred for additional 1 h at 70 °C. The successive product was purified using ethanol and acetone to eliminate the impurities. Then the resultant precipitate was dried at 80 °C for 4 h. Similarly, ZnO was prepared by precipitation method. Fifty milliliters of zinc acetate dihydrate (0.1 M) was added with 50 mL of NaOH (0.2 M) solution and magnetically stirred for 1 h at 70 °C. The obtained precipitate was dried at 80 °C for 4 h. Photocatalytic Activity. The photocatalytic efficiency of the samples were assessed by the degradation of RhB. Photocatalytic reaction was employed in a 100 mL of glass tube container. In a typical experiment, the as-synthesized photocatalyst (50 mg) was suspended in a glass tube containing 50 mL of RhB solution (1 × 10−5 M) which was further stirred in dark for 10 min to confirm the mixing of photocatalyst with dye earlier to the photocatalytic experiment. The compact fluorescent lamp (CFL) (λ > 400 nm 70 WBCB22 220−240 V Philips India) was served as the light source and placed parallel to the 100 mL of glass tube container. Air was bubbled into the reactor for keeping effective mixing of the catalyst and the dye solution. At every 30 min interval, 3 mL of the solution was withdrawn and centrifuged to eliminate the impurities. The adsorption experiment of ZACP nanocomposite was also carried out without light irradiation by the same method as stated above. The concentration of RhB was measured using UV−vis spectrophotometer. The percentage decolorization (%D) of RhB was calculated using the eq 1
%D =
A0 − At × 100 A0
(1)
where A0 is the initial concentration and At is the concentration at time t. Hatching the Shrimp. AS eggs (Brine shrimp) were commercially purchased with the viability around 95% and hatching was carried out with artificial seawater prepared by a commercial salt mixture using sterile double distilled water according to the instruction (3.8% or 35 ppt). Continuous aeration was applied through electronic aerator along with illumination by fluorescent lamp (36 W). After 48 h, the nauplii were hatched and detached from the egg shell. Unhatched eggs and debris were separated by collecting phototropic nauplii with the help of illuminating lamp. Experimental Setup (Survival Assay). The survival experiments were carried out using newly hatched nauplii AS. Around 10 nauplii per tube were added and incubated at room temperature for 48 h 259
DOI: 10.1021/acssuschemeng.7b02335 ACS Sustainable Chem. Eng. 2018, 6, 258−267
Research Article
ACS Sustainable Chemistry & Engineering fortified with different percentage (0, 25, 50, and 100) of both RhB and nanoparticle-modified RhB solution. The numbers of live nauplii in each tube were counted under a stereoscopic microscope after 0, 6, 12, 18, 24 and 48 h intervals. The experiments were conducted in triplicate for each test samples and the negative control were maintained with artificial seawater containing 10 nauplii. Survival rate (%) = no. of live nauplii at each time interval/no. of nauplii at initial time × 100 Statistical Analysis. All the tests were performed in triplicates and the outcomes were represented mean and standard deviation. Significant differences in survival percentage during the exposure condition were analyzed by one way ANOVA. Post hoc Turkey’s methods was applied in order to understand the similarity among the different time of survival (homogeneous subset). Statistical significance was accepted at a level of p < 0.001 (***), p < 0.01 (**), and p < 0.05 (*), while p > 0.05 denotes statistical insignificance. Groups denoted with the same stars are not significantly different, whereas different stars denoted that the statistical significance of the mean values Characterization. The crystalline structure of the products were examined by powder X-ray diffractometer (Rigaku X-ray diffractometers) with Cu Kα radiation (λ= 1.5406 Å). The general morphology of the samples were characterized using scanning electron microscopy (FEI XL30 EFSEM) and high-resolution transmission electron microscope (HRTEM) (JEOL JEM 2100). The optical properties were analyzed by UV−vis absorption spectrophotometer and Photoluminescence (PL) spectra recorded using JASCO. X-ray photoelectron spectroscopy (XPS) analysis was conducted on X-ray Photoelectron Spectroscopy with Auger Electron Spectroscopy (AES) Module: PHI 5000 Versa Prob II. The specific surface area of the sample was determined by Micromeritics ASAP 2020. UV−vis. Absorption studies were carried out to check the dye degradation progress using PerkinElmer Lambda 35 UV−vis spectrophotometer. The degraded end products of RhB were identified using Liquid Chromatography−Mass Spectrometry (LC-MS) (LCMS-2010). CFL (70 W) was used as a visible light source.
(102), (110), (103), (200), (112), and (201) planes of hexagonal wurtzite structure of ZnO. These pattern are fully matched with JCPDS card 36-1451. Additionally, no impurity or AC/PANI related peaks are detected which shows the phase purity of as-synthesized photocatalyst when doped with AC/ PANI in the ZACP nanocomposite. The corresponding peaks of ZnO were also observed, which suggests that the crystal structure was not affected by the addition of AC/PANI.25 Surface morphology of ZnO and ZACP is shown in Figure S1. It shows that the morphology is irregular in shape with some agglomeration were found for both ZnO and ZACP nanocomposite. Further, the SEM image of ZACP nanocomposite clearly exhibit the uniform distribution of ZnO nanoparticles on the surface of AC/PANI composites. To get further insight in to the nanostructure, the as-prepared ZACP nanocomposite was observed by HRTEM analysis. The HRTEM image of ZACP nanocomposite is shown with different magnification in Figure 2a and 2b. It clearly shows that ZnO is evenly coated on the AC/PANI matrix. The HRTEM image (Figure 2c) of the sample further demonstrates the fringes of ZACP nanocomposite. It can be clearly observed that three components are present in the nanocomposite which is represented by arrow marks in Figure 2c. Similarly, Li Qiang et al.26 obtained three distinct layers for ternary composites in HRTEM analysis. The selective area electron diffraction (SAED) analysis shows the polycrystalline nature of ZACP nanocomposite (Figure 2d). Figure S2 shows the EDX spectrum of ZACP nanocomposite. The strong peaks observed in the spectrum related to zinc, carbon, and oxygen. The prepared ZACP nanocomposite have atomic percentage at 54.81 of zinc, 41.54 of carbon, and 3.65 of oxygen. The prepared ZACP nanocomposite have weight percentage at 18.53 of zinc, 76.43 of carbon, and 5.05 of oxygen. This confirmed the formation of ZACP nanocomposite in the process. The Cu Kα peak at 8 keV obtained from the TEM Cu grid on which the sample was mounted.
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RESULTS AND DISCUSSIONS Characterization of ZACP Nanocomposite. The XRD patterns of bare ZnO and ZACP nanocomposite are shown in Figure 1. The observed XRD pattern indicates strong and sharp peaks, with 2θ values of 31.80, 34.63, 36.63, 47.69, 56.72, 62.55, 66.57, 67.97, and 69.58 are assigned to (100), (002), (101),
Figure 2. HRTEM with different scale of magnification of ZACP (a, b, c) and SAED pattern of ZACP (d) nanocomposite
Figure 1. XRD patterns of pure ZnO and ZACP nanocomposite 260
DOI: 10.1021/acssuschemeng.7b02335 ACS Sustainable Chem. Eng. 2018, 6, 258−267
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ACS Sustainable Chemistry & Engineering The UV−vis spectra of the ZnO and ZACP nanocomposite were depicted in Figure 3. The as-synthesized photocatalysts were subjected to UV−vis analysis to ascertain the visible light activity of the photocatalyst. The results indicates that the ZnO strongly absorb UV light with a sharp peak at 360 nm and ZACP nanocomposite absorbs both UV and visible light region of the spectrum with a sharp peak at 360 and a very strong absorption in the visible region which may be due to strong absorption capability in the entire UV and visible region, resulting from the interactions between PANI and ZnO nps and π−π* transition of the PANI molecules.27 Therefore, PANI can act as a photosensitizer, which is probable to improve the photocatalytic activity of ZACP nanocomposite under visible light irradiation. The photoluminescence (PL) spectroscopy has been widely used to investigate the optoelectronic nature of doped semiconductor nanoparticles. For instance, it is used to observe the efficacy of charge carrier trapping and to understand the electron−hole pair recombination of semiconductors nanoparticles. Semiconductor material like ZnO normally shows PL signals owing to surface oxygen vacancies, recombination of electron−hole pair and defects of semiconductor particles.28 As shown in Figure 4, the as synthesized ZnO nanoparticles have high intensity band peak. The PL spectra shows UV emission band at 365 nm for bare ZnO and ZACP nanocomposite displays UV emission at 355 nm and other band peak shows in the violet region at 411 and 406 nm for ZnO and ZACP respectively.29,30 The intensity of this peak is reduced markedly when a calculated amount of AC/PANI was added to ZnO. A possible reason for the decrease in the intensity may be due to the suppression of recombination of electron−hole pairs occurred by loading AC/PANI with ZnO. Generally, lower intensity of PL band peak confirms the lower electron hole pair recombination which enhances the photocatalytic activity of the as synthesized photocatalyst.31 During the photocatalytic reaction, the excited electrons are transferred to AC/PANI from ZnO and thereby reducing the electron hole recombination rate in a significant manner which further improves the photocatalytic activity. The chemical composition of the surface of the ZACP nanocomposite was investigated by the XPS measurement. As shown in Figure 5 the XPS survey spectrum reveals that the ZACP nanocomposite is comprised of Zn, O and C. Figure 5 show the high-resolution scans of Zn 2p and O 1s. The peaks located at 1021.6 and 1044.4 eV corresponded to Zn 2p3/2 and Zn 2p1/2, respectively. The peak located at 531 eV should be
Figure 4. PL spectra of pure ZnO and ZACP nanocomposite
Figure 5. XPS spectra of ZACP nanocomposite
corresponded to the electronic state of O 1s and the peak at 284 eV are relates to the electronic state of C 1s, respectively. The N2 adsorption−desorption isotherms and Barrett− Joyner−Halenda (BJH) pore size distribution curve were studied so as to examine the porous nature of the bare ZnO and ZACP nanocomposite. The ZnO curve in Figure 6 belongs to type II, which shows the ZnO have macro porous structure. The isotherms of ZACP are type IV N2 adsorption−desorption isotherm with an H1 hysteresis loop at relative pressure (P/P0) between 0.4 and 1.0, revealing of their mesoporous structure.32,33 Further, BJH pore size distribution of bare ZnO and ZACP exposes the macroporous and mesoporous nature of the samples in the inset of Figure 6 whose mean pore diameters are 54.34 and 31.79 nm, respectively. The specific BET surface areas of bare ZnO and ZACP nanocomposite were measured to be 11.455 and 23.504 m2/g, respectively. The smaller particle size of ZACP nanocomposite, increases the surface area of the catalyst to support good adsorption capability of dye molecules. Because of the high surface area of the catalyst, it could adsorb more number of dye molecules on the surface of the photocatalyst which leads to effective photodegradation of RhB.34 Photowork. The photodegradation efficacies of bare AC, PANI, AC/PANI, ZnO, and ZACP photocatalysts were studied with RhB as a model dye under CFL light irradiations and are
Figure 3. UV−visible spectra of pure ZnO and ZACP nanocomposite. 261
DOI: 10.1021/acssuschemeng.7b02335 ACS Sustainable Chem. Eng. 2018, 6, 258−267
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Figure 6. N2 adsorption−desorption isotherm (BET) and BJH pore size distribution (inset) of the as prepared pure ZnO and ZACP nanocomposite.
shown in Figure 7. The concentration of RhB was nearly less than 10% degradation under visible light irradiation after 120 min, showing that the degradation of target organic dye could not occur properly under visible-light irradiation. This confirms that a smaller extent of degradation had occurred when the reaction was carried out in the presence of CFL light without any photocatalyst. Further, the reaction carried out in the presence of catalyst under dark conditions shows around 17% degradation over a time of 120 min. The increased percentage degradation of RhB under dark conditions may be responsible due to the presence of AC in the ZACP composites which increase the surface area that attracts more number of dye molecules on the surface of the photocatalyst. Further, PANI, AC/PANI, AC particles show around 4%, 14%, and 62% degradation under visible light irradiation, respectively. The reduction in the degradation percentage in AC/PANI composites is due to the surface of AC are covered by PANI. The photocatalytic activity of bare ZnO has resulted in 52% degradation of RhB. These observations show that both CFL light and photocatalyst are desirable for effective degradation of RhB. In addition, the ZACP had higher photocatalytic activity around 95% over RhB dye under CFL light irradiation (Figure 7). The enhanced photocatalytic activity of the sample could be attributed to the accumulation of dye molecules on the surface
of photocatalyst, high adsorption capacity of the catalyst and suppression of electron hole pair recombination. The time-based progress of the spectral changes taking place during the course of photocatalysis under CFL light irradiation at different time intervals was shown in Figure 8. RhB shows a major absorption band at 554 nm. Addition of ZACP nanocomposite in RhB solution under visible light illumination leads to an apparent reduction in absorption with a slight wavelength shift. The intensity at 554 nm decreases progressively in the course of the photodegradation of RhB via aromatic ring opening. Moreover, it is exposed that the band at 554 nm decreased noticeably with hypochromic shift35 and nearly disappeared after 120 min of irradiation. Langmuir−Hinshelwood model 36,37 was used in the following simplified forms (eq 2) to describe the relationship between photocatalytic degradation of RhB as a function of irradiation time. ⎛C ⎞ ln⎜ o ⎟=kappt ⎝ Ct ⎠
(2)
Where kapp is apparent pseudo-first-order rate constant (min−1), C0 is initial concentration of RhB, and Ct is concentration of RhB after degradation. By plotting (ln C0/Ct) vs irradiation time, we obtained a straight line, and the linear relationship is shown in Figure S3. This specifies that the photocatalytic degradation of RhB follows the pseudo-first order kinetics. The rate constants are calculated to be 0.0073 and 0.0310 min−1 for ZnO, ZACP nanocomposite, respectively. Analysis of the reaction intermediates and final products may expose certain details of the degraded solution. The photodegradation end products were identified and detected by LC/ MS analysis. The results are indicated in Figure 9. The photodegradation degraded products of RhB were identified as 4-(methoxycarbonyl) benzoic acid (m/z − 1 = 180), 2(methoxycarbonyl) benzoic acid (m/z − 1 = 180), phthalic acid (m/z − 1 = 166), isophthalic acid (m/z − 1 = 166), terephthalic acid (m/z − 1 = 166), phthalic anhydride (m/z − 1 = 148), 2-hydroxypentanedioic acid (m/z − 1 = 148), and maleic acid (m/z − 1 = 116) by LC/MS and these are well matched with the previous reports.38−40 The plausible photocatalytic degradation path for RhB is proposed based on the identified products is as shown in Figure 10. According to Xu, Hui et al.41 two possible familiar mechanisms may be accountable for the degradation of RhB (i) cleavage of
Figure 7. Photocatalytic efficiency of as-synthesized photocatalysts in the degradation of RhB (1 × 10−5 M) solution under visible light irradiation. 262
DOI: 10.1021/acssuschemeng.7b02335 ACS Sustainable Chem. Eng. 2018, 6, 258−267
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Figure 8. Changes in UV−vis spectrum of aqueous RhB solution (1 × 10−5 M) under visible light assisted degradation using pure ZnO and ZACP nanocomposite.
Figure 9. LC/MS spectra of (a) RhB and (b) end products of the photocatalytic degradation of RhB.
chromophore aromatic ring with hypochromic shift (ii) and succeeding de-ethylation from the aromatic ring with hypsochromic shift). It can be seen from the LC/MS results that the chromophore cleavage and ring-opening would be the key photodegradation pathway of RhB. Further, RhB was transformed to smaller organic products, such as CO2 and
water. These identified end-products may be responsible for the slight toxic nature of degraded solution which is confirmed by survival assessment studies. The EPR technique with DMPO and TEMPO as a spintrapping agent were used to find out the production of OH and O2 radical in the RhB/ZACP solution under visible light 263
DOI: 10.1021/acssuschemeng.7b02335 ACS Sustainable Chem. Eng. 2018, 6, 258−267
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Figure 10. Photocatalytic degradation pathway of RhB in the presence of ZACP nanocomposite under visible light irradiation.
Figure 11. Plausible photodegradation mechanism of RhB using ZACP under visible light irradiation
irradiation (Figure S4). The characteristics peaks of DMPO− OH spin adduct shows quartet lines with intensity 1:2:2:1 and TEMPO-O2 adduct shows three peaks which confirm the production of O2 radical in the solution. From the above results, the photocatalytic mechanism of RhB degradation was proposed. The possible reaction mechanism of photocatalytic degradation of RhB in the presence of ZACP under visible light irradiation is shown in Figure 11. The LUMO and HOMO
levels of PANI are higher when compared to conduction band and valence band positions of ZnO. Upon visible light irradiation PANI transfers the excited electrons from π to π* orbital. These excited electrons are moved from LUMO of PANI to the CB of ZnO and consequently react with water and oxygen to produce hydroxyl and superoxide radicals. These OH and superoxide radicals react with the dye and subsequently transfer the organic pollutants into less toxic materials. Hence, a 264
DOI: 10.1021/acssuschemeng.7b02335 ACS Sustainable Chem. Eng. 2018, 6, 258−267
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ACS Sustainable Chemistry & Engineering
Figure 12. Survival assessment of untreated (a) and treated (b) Rhodamine B solution after different time of exposure on Artemia salina animal model
The mortality caused by RhB exposed to AS was analyzed between different time of intervals with respect to various percentage of solutions. The experimental results were shown in Figure 12. During the starting period 100% survival was observed in all the samples and it was rapidly reduced when compared with control until 18 h. The survival rate of all the (100%, 50%, and 25%) samples were significantly varied up to 22% at 0.001 level. No significant reduction in survival rate was observed after 18−48 h. The effect of degraded RhB solution was exposed to AS from 0 to 48 h with differently diluted dye solutions and the results were shown in Figure 12. No significant different was found during the initiation of experiment and after 6, 12, and 48 h of 25% solution and 48 h of 50% solution (Figure 12). The survival rate was gradually reduced with respect to concentration of dye and the exposure time and the significant variation was observed between 6 to 48 h of exposure in all the dilution at 0.001 level (p < 0.001). Hence, the survival assessment results, indicated that the toxicity level was decreased in relation to the initial dye concentration, which suggests that the obtained products during photodegradation are less toxic than the untreated dye solution.
rapid photogenerated electrons are separated effectively by the addition of PANI, which encouragingly increase the photocatalytic activity of ZACP nanocomposite. The plausible photocatalytic reactions accountable for the photodegradation of RhB under visible light irradiation are summarized according to some literature.42,43 PANI/ZnO + visible light → PANI(h+) + ZnO(e−)
(3)
PANI(h+) + H 2O → OH• + H+
(4)
e− + O2 → O2•−
(5)
O2•− + H 2O → OOH• + OH−
(6)
OOH• + H 2O → H 2O2 + OH•
(7)
H 2O2 → OH• + OH•
(8)
OH• + dye → CO2 + H 2O
(9)
Toxicity Assays with Artemia SalinaEcotoxicity. The supplement toxicity studies were used to find out the chemical and biological responses of the living organisms from the photocatalytically treated and untreated RhB dye solution using ZACP nanocomposite. During the pollutant degradation some of the compounds obtained from breaking the dye molecules in to smaller products may be even more toxic than the parent compound. Therefore, it is necessity to examine the survival capacity of the living organism after degradation of dyes.10 This work focuses on the survival capacity of the species AS in the treated and untreated RhB solution. The acute ecotoxicity study showed that AS was more sensitive to high concentration of untreated RhB. The toxic effects of RhB was increased with increasing concentration as well as increasing exposure time. Many researchers have used AS as a pollutant indicator of toxicity solution owing to its ease of handling and its great analysis responses. Dilutions (100%, 50%, and 25%) of untreated and treated RhB solution were used in these trials to validate the impact of dilution on the mortality of AS. The AS assays exhibited that the RhB dye solutions were toxic in nature and after treatment, the toxicity decreased considerably to a safer boundary.
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CONCLUSION Visible-light-driven ZACP nanocomposite was synthesized by a simple precipitation method. The as-synthesized ZACP nanocomposite exhibits excellent photocatalytic activity than pure ZnO on the degradation of RhB under visible light irradiation. This can be attributed to photosensitization and separation of electron−hole pair effect of PANI in the composites. Also, the composite materials have good stability and can be used three times with minimum loss of activity. The chemical and toxicity assessment confirmed the mineralization RhB. Thus, ZACP nanocomposite are proved to be effective photocatalytic materials for degrading RhB under visible light irradiation.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02335. 265
DOI: 10.1021/acssuschemeng.7b02335 ACS Sustainable Chem. Eng. 2018, 6, 258−267
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SEM image of pure ZnO and ZACP nanocomposite, energy dispersive x-ray diffraction spectroscopy of ZACP nanocomposite, first-order kinetic plot of RhB degradation for pure ZnO, ZACP and their rate constant values, and EPR spectra obtained by spin trapping (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
I. Sharmila Lydia: 0000-0002-9066-1835 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Principal, Management of Bishop Heber College, Tiruchirappalli, for providing necessary research facilities and also the authors gratefully acknowledge Bharathidasan University, Tiruchirappalli, to accomplish the work successfully.
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