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Degradation of Caffeine by Heterogeneous Photocatalysis Using ZnO with Fe and Ag

Abstract

The organic compound caffeine when detected in environmental matrices such as surface waters and groundwater is considered as an emerging contaminant, in which its effects are still unknown. Therefore, in the present research, zinc oxide-based catalysts impregnated with iron and silver were prepared for the reaction of caffeine degradation by heterogeneous photocatalysis. The wet impregnation method with excess solvent was applied to the preparation of the materials, later they were characterized by adsorption of N2, X-ray diffraction and photoacoustic spectroscopy. Then, the photodegradation, photolysis and adsorption tests were carried out, in which it was observed that only the presence of the radiation or photocatalysts could not sufficiently degrade the caffeine, however when combined radiation with all the catalysts studied here presented degradation above 70% at the end of 300 minutes of the reaction, and the best catalyst studied was that containing 8% Ag in non-calcined ZnO. Thus, these results point out that the methodology employed in this research, both for the preparation of the catalysts and in the process of the photocatalysis reaction, was efficient in the degradation of the emerging contaminant, caffeine, which could later be used for a mixture of other contaminants.

Keywords:
Caffeine; ZnO; heterogeneous photocatalysis; photodegradation

INTRODUCTION

Resulting mainly from human activity, various organic compounds, called "contaminants or pollutants of emerging concern", are being detected in several environmental matrices, such as in effluent from a wastewater treatment plant [11 Kosma CI, Lambropoulou DA, Albanis TA. Occurrence and removal of PPCPs in municipal and hospital wastewaters in Greece. J Hazard Mater. 2010;179:804-17.], surface waters [22 Kim SD, Cho J, Kim IS, Vanderford BJ, Snyder SA. Occurrence and removal of pharmaceuticals and endocrine disruptors in South Korean surface , drinking , and waste waters. Water Res. 2007;41:1013-21.

3 Meffe R, Bustamante I. Emerging organic contaminants in surface water and groundwater: A first overview of the situation in Italy. Sci Total Environ. 2014;481:280-95.
-44 Pitarch E, Cervera MI, Portolés T, Ibáñez M, Barreda M, Renau-Pruñonosa A, et al. Comprehensive monitoring of organic micro-pollutants in surface and groundwater in the surrounding of a solid-waste treatment plant of Castellón, Spain. Sci Total Environ. 2016;548-549:211-20.], drinking waters [55 Jardim WF, Montagner CC, Pescara IC, Umbuzeiro GA, Di Dea Bergamasco AM, Eldridge ML, et al. An integrated approach to evaluate emerging contaminants in drinking water. Sep Purif Technol. 2012;84:3-8.,66 Rodil R, Quintana JB, Concha-Graña E, López-Mahía P, Muniategui-Lorenzo S, Prada-Rodríguez D. Emerging pollutants in sewage, surface and drinking water in Galicia (NW Spain). Chemosphere. 2012;86:1040-9. A] and groundwaters [44 Pitarch E, Cervera MI, Portolés T, Ibáñez M, Barreda M, Renau-Pruñonosa A, et al. Comprehensive monitoring of organic micro-pollutants in surface and groundwater in the surrounding of a solid-waste treatment plant of Castellón, Spain. Sci Total Environ. 2016;548-549:211-20.,77 Kong L, Kadokami K, Duong HT, Chau HTC. Screening of 1300 organic micro-pollutants in groundwater from Beijing and Tianjin, North China. Chemosphere.2016;165:221-30.,88 Lapworth DJ, Baran N, Stuart ME, Ward RS. Emerging organic contaminants in groundwater?: A review of sources , fate and occurrence. Environ Pollut. 2012;163:287-303.].

The term pollutant or emergent contaminant comprises compounds with diverse characteristics and applications, and may be substances already known and that until then there were no analytical techniques to determine them in the environment, as well as those compounds that were recently discovered and classified as contaminants, the main source of disposal of these compounds into the environment are the sewage directly discharged into hydric bodies [88 Lapworth DJ, Baran N, Stuart ME, Ward RS. Emerging organic contaminants in groundwater?: A review of sources , fate and occurrence. Environ Pollut. 2012;163:287-303.,99 Arfanis MK, Adamou P, Moustakas NG, Triantis TM, Kontos AG, Falaras P. Photocatalytic degradation of salicylic acid and caffeine emerging contaminants using titania nanotubes. Chem Eng J.2017;310:525-36.].

As an example, there are personal care products, pharmaceuticals, food additives, endocrine disruptors, among other compounds such as caffeine. Caffeine is an organic substance belonging to the class of methylxanthines; which have the ability to stimulate the central nervous system [1010 Montagner CC, Vidal C, Sodré FF, Pescara IC, Jardim WF. Cafeína em águas de abastecimento público no Brasil. Capítulo 1- A cafeína no ambiente. 2014. 11-24 p.]. This substance can be found in various food products, such as coffee, teas, chocolates, which are consumed daily by a large part of the world's population [1010 Montagner CC, Vidal C, Sodré FF, Pescara IC, Jardim WF. Cafeína em águas de abastecimento público no Brasil. Capítulo 1- A cafeína no ambiente. 2014. 11-24 p.,1111 Mejia EG, Ramirez-Mares MV. Impact of caffeine and coffee on our health. Trends Endocrinol Metab. 2014;25(10):489-92.].

After the consumption of caffeine, the excretion by humans happens mainly through urine or else by the direct dumping of beverages such as teas and coffees that were not consumed completely, thus reaching the domestic effluent [1010 Montagner CC, Vidal C, Sodré FF, Pescara IC, Jardim WF. Cafeína em águas de abastecimento público no Brasil. Capítulo 1- A cafeína no ambiente. 2014. 11-24 p.]. Then, this effluent or goes to wastewater treatment plants or is dumped directly without treatment in surface water, causing caffeine, among other substances, to be detected in aquatic matrices.

Faced with this problem, researchers have studied the possible elimination of this compound present in the water, so that the technique presents complete mineralization of the pollutant. Among which are, advanced oxidative processes such as the use of heterogeneous photocatalysis with TiO2, photolysis, peroxidation, photoperoxidation, Fenton or Photo-fenton reaction [1212 Centore R, Fusco S, Peluso A, Carotenuto M, Tuzi A. Strong overcrowding in dimethyl 2-(dimethyl-amino)terephthalate. Acta Crystallogr Sect C Cryst Struct Commun. International Union of Crystallography; 2008;64(8):420-2.

13 Fagnani E, Guadagnini RA, Silva GA, Guimarães JR. Degradtion of caffeine and methylene blue by advanced oxidation process (AOPs). Holos Environ. 2013;13:85-97.

14 Chaker H, Chérif-aouali L, Khaoulani S, Bengueddach A, Fourmentin S. Photocatalytic degradation of methyl orange and real wastewater by silver doped mesoporous TiO2 catalysts. J Photochem Photobiol A Chem. 2016;318:142-9.

15 Marques RRN, Sampaio MJ, Carrapiço PM, Silva CG, Morales-Torres S, Dražić G, et al. Photocatalytic degradation of caffeine: Developing solutions for emerging pollutants. Catal Today. 2013;209:108-15.
-1616 Trovó AG, Silva TFS, Gomes Jr O, Machado AEH, Neto WB, Muller Jr PS, et al. Degradation of caffeine by photo-Fenton process: Optimization of treatment conditions using experimental design. Chemosphere. 2013;90(2):170-5.].

Heterogeneous photocatalysis is considered as a clean technology and use irradiation for the excitation of electrons in a given semiconductor, and thus cause electron transition and generating hydroxyl radicals (HO •), which can then react with the organic pollutant by oxidizing them [1717 Herrmann J. Photocatalysis fundamentals revisited to avoid several misconceptions. Appl Catal B Environ.2010;99(3-4):461-8.

18 Ziolli RL, Jardim WF. Mechanism reactions of photodegradation of organic compounds catalyzed by TiO2. Quim Nova. 1998;21:319-25.
-1919 Samran B, Krongkitsiri P, Chaiwichian S. Effect of Copper Dopants on Visible-Light-Driven Photocatalytic Activity of BiFeO3 Photocatalysts. Mod Environ Sci Eng. 2018;4(3):234-43.]. Thus, among other variables, the photocatalyst or semiconductor is of fundamental importance in heterogeneous photocatalysis. To select the to be used material is a decisive step for the good performance of the reaction. Among the most used semiconductors as photocatalysts are TiO2, which is a material with good photostability, is non-toxic and is widely found in nature [2020 Bellardita M, Paola AD, Megna B, Palmisano L. Determination of the crystallinity of TiO2 photocatalysts. J. Photochem. Photobiol., A.2018;367: 312-320.]. Another material that has also been investigated as a photocatalyst is ZnO, which, like TiO2 is non-toxic, and has a strong oxidation capacity, good photocatalytic property [2121 Lee KM, Lai CW, Ngai KS, Juan JC. Recent developments of zinc oxide based photocatalyst in water treatment technology: A review. Water Res. 2016; 88: 428-448.] and a lower cost when compared to TiO2-P25. Moreover, ZnO has a band gap energy close to that of TiO2 (approximately 3.3 eV) and is capable of absorbing a wide range of the UV spectrum [2121 Lee KM, Lai CW, Ngai KS, Juan JC. Recent developments of zinc oxide based photocatalyst in water treatment technology: A review. Water Res. 2016; 88: 428-448.,2222 Mirzaei A, Chen Z, Haghighat F, Yerushalmi L. Removal of pharmaceuticals and endocrine disrupting compounds from water by zinc oxide-based photocatalytic degradation: A review. Sustain Cities and Soc. 2016; 27: 407-418.] which makes it very attractive for photocatalysis studies.

In addition, an improvement of the semiconductor can be made, and such enhancement may be affected, for example, by the addition of metals in semiconductors (for example, TiO2 and ZnO) for the purpose to benefit from the photocatalytic performance of the material [1919 Samran B, Krongkitsiri P, Chaiwichian S. Effect of Copper Dopants on Visible-Light-Driven Photocatalytic Activity of BiFeO3 Photocatalysts. Mod Environ Sci Eng. 2018;4(3):234-43.].

Iron is one the most abundant metals on Earth and the use of this metal in science is very diversified, being that in catalysis and adsorption when impregnated in supports can improve the behavior and performance of the catalyst, and this happens due to its redox and textural characteristics [2323 Jo WK, Lee JY. Iron - impregnated titania composites for the decomposition of low - concentration aromatic organic pollutants under UV and visible light irradiation. Chinese J Catal. 2013;34(12):2209-16.,2424 Oliveira LCA, Fabris JD, Pereira M. Iron Oxides and Their Applications in catalytic process: A review. Quim Nova. 2013;36(1):123-30.].

Still in this context, in a given study [2525 Mahy JG, Lambert SD, Léonard GL, Zubiaur A, Olu P, Mahmoud A, et al. Towards a large scale aqueous sol-gel synthesis of doped TiO2?: Study of various metallic dopings for the photocatalytic degradation of p - nitrophenol. J Photochem Photobiol A Chem .2016;329:189-202.], the researches verified that among the metallic ions that were added to the TiO2, the iron with the percentage of 0.5% showed an improvement in the photoefficiency in relation to the pure TiO2 P-25 Degussa, in this way the evaluation of the impregnation of iron in substrates like ZnO becomes interesting because the cost of iron is relatively low in relation to other, more noble, such as gold.

Silver, on the other hand, has the lowest resistivity of metals, high resistance to oxidation and is non-toxic. Using silver in ZnO, for example, can lead to an improvement in surface charge distribution, optical properties and can prevent recombination of the electron-hole pair [2626 Nghia N Van, Trung TN, Ngoc N, Truong K, Thuy DM. Preparation and Characterization of Silver Doped ZnO Nanostructures. Open J Synth Theory Appl. 2012;1:18-22.].

Some studies have reported improvement in the degradation of some contaminants such as Rhodamine B, water disinfection with E. coli, when using Ag in TiO2 in the photocatalysis reaction [2727 Xiong Z, Ma J, Ng WJ, Waite TD, Zhao XS. Silver-modified mesoporous TiO2photocatalyst for water purification. Water Res.2011;45(5):2095-103.], as well as the reduction of mercury (II) when was utilized Ag/TiO2 catalyst [2828 Lenzi GG, Fávero CVB, Colpini LMS, Bernabe H, Baesso ML, Specchia S, et al. Photocatalytic reduction of Hg (II) on TiO2 and Ag/TiO2 prepared by the sol - gel and impregnation methods. Desalination. 2011;270:241-7.].

Thus, this study evaluated the photocatalytic performance of ZnO impregnated with iron and silver, on the degradation of caffeine in aqueous medium by heterogeneous photocatalysis.

MATERIAL AND METHODS

Synthesis of Catalysts

Pure commercial ZnO (DINAMICA Ltda) and Fe and Ag impregnated were tested as catalysts. The impregnation of Fe and Ag in ZnO was carried out by means of the wet impregnation technique with excess solvent. The methodology proceeded with the following steps: the oxide used (ZnO) and the aqueous solution containing the metal precursor (AgNO3 or Fe (NO3).9H2O) were mixed and kept under constant stirring for 24 hours at room temperature. The metal solution was made so that the Fe or Ag nominal load was of 8% of the ZnO total weight by previously described in the literature [2929 Coelho ALS, Almeida Neto AF, Ivashita FF, Jorge LMM, Santos OAA. TiO2 and modified TiO2/Fe on photocatalytic reduction og Hg2+ using artificial and solar radiation. Desalin Water Treat. 2017;97: 304-314.]. After obtaining the mixture, the excess solvent (water) was removed by vacuum evaporation in the rotavaporation system with heating, at 90 °C and then the catalyst was dried at 100 ºC for 20 hours. Subsequently, half of the already dried catalysts were calcined at 400 °C in a muffle.

Characterization of the Catalysts

Texture properties: The porous properties of the catalysts, such as specific surface area (B.E.T method), pore volume and mean pore diameter were determined using QUANTACHROME - Nova-1200 with adsorption of N2 at 77 K. The samples were previously subjected to a thermal treatment at 573 K under vacuum of 2 hours to eliminate possible water within the pores of the solids.

X-Ray Diffraction (XRD): Was used a D8 Advance diffractometer (Bruker), with initial theta of 5 and final of 80. The obtained patterns were then compared with the diffraction data cards of the Joint Committee of Powder Drifraction Standards [3030 JCPDS- International Centre for Diffraction Data. PCPDFWIN. 1997. p. V. 130.].

Photoacoustic spectroscopy (PAS): Technique that allows to determine the minimum energy necessary for excitation of the electron, that is, its band gap energy. The conditions used were: 20Hz modulation frequency and recorded between 220 and 270 nm. The spectra were standardized with charcoal sample. The band gap energy was then calculated by Equation 1:

λ = h c E g a p = 1240 E g a p (1)

Experimental tests of photocatalysis

Caffeine solutions (20 mg L-1) were prepared using caffeine (Vetec, 99% purity) with ultra-pure water. The reactions were carried out in a 2000 mL cylindrical reactor with 1000 mL of caffeine solution and 0.3 g of photocatalyst. The reactor was jacketed to keep the water at constant temperature ~ 23 °C, the system still contained oxygen flow (0.5 L min-1) and magnetic stirring and to obtain a homogeneous mixture. The lamp used for these experiments was installed approximately 20 cm above the aqueous solution (mercury vapor lamp with medium pressure of 125 W) and the tests were run with a time of 300 minutes. The radiations emitted by the lamp are UVA and UVB (280 to 400 nm) and the radiation incidence was measured at 5.32 mW cm-2, information collected by means of a Uv light meter radiometer (Sentry ST-500).

Figure 1 shows the scheme of the experimental system. At established times, samples were collected, filtered (0.22 μm syringe filter, 13 mm diameter and nylon membrane), followed by determination of the caffeine concentration by means of a UV-Vis spectrophotometer (Femto-800XI), where at first a calibration curve was made and the wavelength of 275 nm was defined.

Figure 1
System for Photocatalysis Reactions.

The photolysis and adsorption test were performed with the same characteristics of the photocatalysis test, however, without light presence and catalysts, respectively.

RESULTS AND DISCUSSION

Synthesis of catalysts

In relation to the textural properties, the results obtained by N2 adsorption analysis are shown in Table 1 and the adsorption isotherms are showed in Figure 2, in which all the catalysts are calcined (cal) and not calcined.

Figure 2 shows that for all catalysts the adsorbed amount tends to infinity when p/po →1, which, according to the literature [3030 JCPDS- International Centre for Diffraction Data. PCPDFWIN. 1997. p. V. 130.], corresponds to the overlap of multilayer adsorption characterizing macroporous solids, non-porous or mesopore. Moreover, verified that the Fe or Ag addition in the ZnO surface did not change the isotherm shape. Flores and coauthors [3232 Flores N M, Pal U, Galeazzi R, Sanfoval A. Effects of morphology, surface area, and defect content on the photocatalytic dye degradation performance of ZnO nanostructures. RSC Adv. 2014; 4:41099-41110.] analyzing ZnO nanomaterials synthesized by ultrasound-assisted hydrolysis identified adsorption isotherms similar to those found in this paper.

Figure 2
Adsorption isotherms.

Table 1
Specific surface area (S0), pore volume (Vp) and mean pore diameter (dm).

In Table 1, it can be verified that the values obtained for the specific areas are of the same order of magnitude as those obtained for ZnO, indicating that the method of preparation of the catalysts by impregnation maintains the textural characteristics of the material used as support. Also regarding specific area, with regard to the calcination process, it is observed that this thermal treatment exerts little or no influence. However, the small increase observed in the value of the specific area with the calcination of the ZnO catalyst may be related to the release of adsorbed residues on the surface of the catalyst during the manipulation of the samples, or even that they were not totally eliminated during the drying process.

For the 8% Fe -ZnO catalyst, the thermal treatment of calcination caused a decrease in the value obtained for the specific area, indicating that for this catalyst the calcination at 400 ºC could have caused the agglomeration of the particles, for sintering, and loss of surface hydroxyl groups [2828 Lenzi GG, Fávero CVB, Colpini LMS, Bernabe H, Baesso ML, Specchia S, et al. Photocatalytic reduction of Hg (II) on TiO2 and Ag/TiO2 prepared by the sol - gel and impregnation methods. Desalination. 2011;270:241-7.,3030 JCPDS- International Centre for Diffraction Data. PCPDFWIN. 1997. p. V. 130.,3333 Mital GS, Manoj T. A review of TiO2 nanoparticles. Chinese Sci Bull. 2011;56(16):1639-57.]. The results of the XRD are shown in Figure 3.

Figure 3
XRD for catalysts with ZnO calcined and non-calcined.

As shown in Figure 3, it can be seen that for pure, non-calcined and calcined zinc oxides, the characteristics of crystallinity in wurtzite hexagonal form are the most common for this material. Characteristic peaks for this material are located at approximately 2Ө= 32, 34, 36, 47, 57, 62, 66, 68 and 69º (JCPDS:19-0206). So, the thermal treatment did not alter the crystalline structure of the material. Similar results were found in literature [3434 Pan L, Muhammad T, Ma L, Huang Z, Wang S, Wang L, et al. MOF-derived C-doped ZnO prepared via a two-step calcination for efficient photocatalysis. Appl Catal B Environ. 2016;189:181-91.] that, varying the calcination temperature of the ZnO between 350 and 450 °C did not identify change in the typical peaks of the hexagonal wurtzite of ZnO.

About the catalysts containing 8% Ag-ZnO non-calcined, besides the characteristic peaks of zinc oxide with wurtzite hexagonal crystallinity, silver nitrate (AgNO3) peaks were also identified in orthorhombic crystalline form 2Ө = 19, 22, and 36º (JCPDS:43-0649). On the other hand, silver-related peaks (Ag) with cubic crystalline system 2Ө=38, 44 and 64º (JCPDS: 87-0597) were identified for the calcined catalyst. Similar results were found by [3535 Ravishankar TN, Manjunatha K, Ramakrishnappa T, Nagaraju G, Kumar D, Sarakar S, et al. Comparison of the photocatalytic degradation of trypan blue by undoped and silver-doped zinc oxide nanoparticles. Mater Sci Semicond Process.2014;26:7-17.], when studying silver-doped ZnO nanocatalysts. The researchers identified Ag in cubic phase, with characteristic peaks located in the same position as those found in this work as well as zinc oxide in hexagonal wurtzite format.

As for the catalysts containing 8% iron (8% Fe-ZnO), the presence of characteristic peaks relative to hexagonal ZnO was observed, however, with lower intensity. For the catalysts, calcined and not calcined, iron oxide (Fe3O4) was found with crystalline orthorhombic system 2θ = 9, 12, 18 and 43º (JCPDS: 76-0958), more was identified in 2θ = 25º a peak related to Pigment Brown 38 (JCPDS: 47-2148).

Recent research [3636 Kumar K, Chitkara M, Singh I, Mehta D, Kumar S. Photocatalytic , optical and magnetic properties of Fe-doped ZnO nanoparticles prepared by chemical route. J Alloys Compd.2014;588:681-9.] -iwhich studied ZnO based catalysts doped with different amounts of iron, with the XRD analysis it was possible to verify that the intensity of the ZnO peaks was also smaller/less due to the addition of the metal. In the same way, they observed a decrease of the intensity of the peaks with the increase of the amount of the metal in the sample, besides, there was even small displacement of these same peaks, which was not observed in this work. For the results of the PAS, Table 2 shows the values obtained.

Table 2
Band gap and absorption threshold (λ) of the catalysts calcined and non-calcined

The band gap obtained for the non-calcined ZnO was approximately 3.12 eV and for the calcined ZnO was 3.11 eV, this small difference indicates that the calcination process at 400 ºC exerted little or no influence on the ZnO band gap energy. On the other hand, in general, both iron and silver addition led to an increase in ZnO band gap energy. According to Rodnyi and Khodyuk [3737 Rodnyi PA, Khodyuk I V. Optical and Luminescence Properties of Zinc Oxide (Review). Opt Spectrosc. 2011;111(5):776-85.] the addition of some elements, such as Ga, In, Mg, can cause a shift of the optical adsorption edge of the crystals causing the band gap value to increase, which may have happened in this work. With respect to the wavelength (λ) calculated from Equation (1) it can be verified that all the photocatalysts are in the ultraviolet region λ <400 nm.

Experimental tests of photocatalysis

The concentration curves of caffeine versus reaction time are shown in Figure 4.

Figure 4
Photolysis and photocatalytic degradation of caffeine, ph= 6.5; T=23 ºC; Concentration of catalysts = 0.3 g L-1 and 125 W lamp.

In general, all the catalysts studied showed good caffeine degradation, that is, above 70% at the end of the 300 minutes of reaction.

In order to evaluate the kinetics of caffeine degradation not only visually the first order model expressed by ln(C0/C)=kt. In the tests, 83.3% of the cases the correlation coefficient (R²) was above 0.99. The k velocity constant values are presented according to the sequence in order of decreasing performance: 8%Ag-ZnO non-cal (k=-0.039) > ZnO cal (k=-0.034) > 8%Ag-ZnO cal (k=-0.013) > ZnO non-cal (k=-0.012) > 8%Fe-ZnO cal (k=-0.003) > 8%Fe-ZnO non-cal (k= 6.4 x10-6). The catalyst with 8%Ag-ZnO non-cal, in 120 minutes already showed a degradation of the caffeine of 100%.

For the catalysts of calcined and non-calcined ZnO, it was verified that both the band gap energy and the crystalline structure of ZnO were practically unchanged with the calcination, an increase in the specific area (8 to 10 m² g-1) which may have favored the performance of the catalyst. According to Pan and coauthors [3434 Pan L, Muhammad T, Ma L, Huang Z, Wang S, Wang L, et al. MOF-derived C-doped ZnO prepared via a two-step calcination for efficient photocatalysis. Appl Catal B Environ. 2016;189:181-91.] larger specific area and larger porous structure favor the adsorption of the reagents on the surface of the ZnO that can facilitate the reaction at the interface, in addition the porous structure improves the optical absorption through the dispersion of light.

Still in analysis of Figure 3, we note that the addition of iron did not favor the performance of the ZnO catalyst, since 8% Fe in ZnO non-calcined and 8% Fe in ZnO calcined presented at the end of 300 minutes a degradation of 73.5% and 76.8%, respectively.

On the other hand, the addition of silver increased the catalytic activity in relation to the pure ZnO, however the calcination negatively influenced the performance in the first hour of reaction, since the caffeine degradation was approximately 96% and 60% respectively for the samples non-calcined and calcined. Although the non-calcined and calcined 8% Ag-ZnO catalysts reach 100% caffeine degradation at the end of the 300 min reaction. Still on the catalysts with Ag, when we observe the XRD spectra, it is noted that there is difference in composition of the catalyst structure (8%Ag-ZnO) calcined and non-calcined, that is, AgNO3 for the catalyst non-calcined and just Ag for the catalyst calcined. Possibly the presence of AgNO3 favored the catalytic activity of ZnO.

Elhalil and coauthors [3838 Elhalil A, Elmoubarki R, Farnane M, Machrouhi A, Mahjoubi F Z, Sadiq M, Qouzal S, Abdennouri M, Barka N. Novel Ag-ZnO-La2O2CO3 photocatalysts derived from the Layered Double Hydroxide structure with excellent photocatalytic performance for the degradation of pharmaceutical compounds. J. Sci.:Adv Mater Devices. 2019; 4:34-46.] studied ZnO-La2O2CO3 nanomaterials as catalysts and found that catalysts when doped with Ag showed better experimental results compared to pure ZnO and P-25, the best result was obtained with 5% Ag with degradation 99.4% after 40 minutes of reaction.

Chuang and coauthors [3939 Chuang L, Luo C, Huang S, Wu Y, Huang Y. Photocatalytic Degradation Mechanism and Kinetics of Caffeine Aqueous Suspension of Nano-TiO2. Adv. Mater. Res. 2011; 214: 97-102.] investigated the degradation of caffeine by TiO2 nano-powders through a modified HPPLT and found that there was a total pollutant degradation within 360 min of reaction. Sacco and coauthors [4040 Sacco O, Sannino D, Matarangolo M, Vaiano V. Room Temperature Synthesis of V-doped TiO2 and Its Photocatalytic Activity in the Removal Of Caffeine under UV Irradiation. Materials. 2019;12:1-10.] used a vanadium-doped TiO2 catalyst synthesized by the sol-gel method in caffeine degradation and found that the amount of 1 mg of Vanadium in TiO2 significantly increased catalyst performance showing 96% degradation in 360 min. The catalysts studied in this study, besides being synthesized by a simple methodology, showed good results of caffeine degradation when compared to those already presented in the literature.

The result obtained with the photolysis test showed that the caffeine degradation only in the presence of the irradiation of the lamp of 125 W, reached a percentage of the degradation, at the end 300 minutes of reaction of only 7% , and in the first 120 minutes the degradation remained below 2.5%, which demonstrates the need to the introduce a photocatalyst.

The adsorption test is shown in Figure 5. Thus, as the photolysis test, the adsorption test was not efficient for caffeine degradation complete. This leads to the need for the presence of radiation and catalysts for the complete degradation of pollutant.

Figure 5
Adsorption tests.

CONCLUSION

In the present work, in general, the catalysis based on ZnO promoted with iron and silver, showed a good degradation of caffeine, the difference in the performance of each catalyst can be better understood with the help the characterization techniques that presented important results with respect to its textural and structural characteristics, showing, for example, that the presence of the silver nitrate favored the degradation caffeine reaction.

In relation the experimental photolysis test it was possible to conclude that only in the presence of the irradiation was not efficient to satisfactorily remove the caffeine present in aqueous solution. Soon, the 8%Ag-ZnO, which presented itself as the best catalyst, could be applied in degradation studies of a mixture of emerging contaminants and on a larger scale tested here.

Acknowledgments:

The authors wish to thank the Brazilian Agencies CNPq and CAPES for the financial support of this study, UTFPR and COMCAP - UEM for the analyzes carried out.

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    Fagnani E, Guadagnini RA, Silva GA, Guimarães JR. Degradtion of caffeine and methylene blue by advanced oxidation process (AOPs). Holos Environ. 2013;13:85-97.
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HIGHLIGHTS

  • 1
    Fe and Ag were added to the ZnO surface using an excess solvent technique.
  • 2
    ZnO doped with Fe and Ag is efficient in the Caffeine photoreduction.
  • 3
    Presence of silver nitrate in ZnO favored the catalytic performance.
  • 4
    Photolysis and Adsorption did not show significant results.

Publication Dates

  • Publication in this collection
    24 July 2020
  • Date of issue
    2020

History

  • Received
    02 Nov 2018
  • Accepted
    17 Mar 2020
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