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  3. Photodegradation of Reactive Black 5 and raw textile wastewater by heterogeneous photo-Fenton reaction using amino-Fe3O4-functionalized graphene oxide as nanocatalyst

Photodegradation of Reactive Black 5 and raw textile wastewater by heterogeneous photo-Fenton reaction using amino-Fe3O4-functionalized graphene oxide as nanocatalyst

RTW photodegradation experiments exhibited removal efficiencies of 53.25 % for apparent color, and 64.55 % for turbidity. Phytotoxicity assays using cucumber seeds showed low toxicity of the samples after photodegradation, which indicated that toxic compounds were fully mineralized. Environmental Advances 4 (2021) 100064 Contents lists available at ScienceDirect Environmental Advances journal homepage: www.elsevier.com/locate/envadv Photodegradation of Reactive Black 5 and raw textile wastewater

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  1. Environmental Advances 4 (2021) 100064 Contents lists available at ScienceDirect Environmental Advances journal homepage: www.elsevier.com/locate/envadv Photodegradation of Reactive Black 5 and raw textile wastewater by heterogeneous photo-Fenton reaction using amino-Fe3 O4 -functionalized graphene oxide as nanocatalyst Maryne Patrícia da Silva a, Ana Caroline Alves de Souza a, Lettícia Emely de Lima Ferreira a,b, Luiz Martins Pereira Neto c, Bruna Figueiredo Nascimento a, Caroline Maria Bezerra de Araújo a,d, Tiago José Marques Fraga a,∗, Maurício Alves da Motta Sobrinho a, Marcos Gomes Ghislandi a,e a Department of Chemical Engineering, Federal University of Pernambuco (UFPE), 1235 Prof. Moraes Rego Av, Cidade Universitária, 50670-901, Recife/PE, Brazil b Center of Biosciences, Federal University of Pernambuco (UFPE), Engineering Av. Cidade Universitária, 50740-570, Recife/PE, Brazil c Department of Rural Technology, Federal Rural University of Pernambuco (UFRPE), Dom Manuel de Medeiros St. W/N, Dois Irmãos, 521171-900, Recife/PE, Brazil d Faculty of Engineering (FEUP), University of Porto, Dr. Roberto Frias St., w/n, 4200-465, Porto, Portugal e Engineering Campus – UACSA, Federal Rural University of Pernambuco (UFRPE), 300 Cento e sessenta e Três Av., Cabo de Santo Agostinho/PE, Brazil a r t i c l e i n f o a b s t r a c t Keywords: Amino-Fe3 O4 -functionalized graphene oxide (AmGO) was synthesized and had its photocatalytic properties in- 2D nanomaterials vestigated in the degradation of Reactive Black 5 (RB5) dye and raw textile wastewater (RTW). Graphene oxide Functionalization was synthesized via modified Hummers method and functionalized with diethylenetriamine and FeCl3 to obtain Graphene oxide the AmGO. A 23 factorial design was carried out to optimize the best working conditions; the most statistically Heterogeneous photo-Fenton significant effect was the AmGO dosage, followed by the initial pH. Kinetics studies were performed and Chan Textile wastewater & Chu model the most representative of the experimental data with R2 ≥ 0.95. Experiments of adsorption ki- netics were carried out and evidenced that the adsorption of RB5 by AmGO was slower than photodegradation, in which the equilibrium state was reached after 300 min. Moreover, pseudo-second-order model showed the best fit with adsorptive capacity at equilibrium of 53.06 mg∙g−1 . AmGO employment in the photodegradation of RB5 exhibited 75 % removal efficiency in less than 2h for the initial dye concentration of 100 mg∙L−1 . AmGO also showed satisfactory recycling capacity, since it maintained RB5 removal up to 97 % after 6 cycles. RTW photodegradation experiments exhibited removal efficiencies of 53.25 % for apparent color, and 64.55 % for turbidity. Phytotoxicity assays using cucumber seeds showed low toxicity of the samples after photodegradation, which indicated that toxic compounds were fully mineralized. Abbreviations PSO pseudo-second-order (adsorption kinetics model). ADS adsorption (for adsorbed RB5 molecules). RB5 Reactive Black 5. AmGO amino-Fe3O4-functionalized graphene oxide. rGO reduced graphene oxide. AOPs advanced oxidation processes. RGI relative growth index. DETA diethylenetriamine. RTW raw textile wastewater. EDS energy-dispersive spectroscopy. WWTP wastewater treatment plant. EPR electron paramagnetic resonance. GI germination index. 1. Introduction GO graphene oxide. HET heterogeneous electrons transfer. Textile wastewaters usually carry several chemical species present- HPF heterogeneous photo-Fenton. ing various negative effects in the environment and human’s health. IPD intraparticle diffusion (Weber & Morris adsorption kinetics Among these substances, soaps, ions, salts, surfactants, bleaching agents model). and dyes are contained in textiles effluents, being many of these clas- PFO pseudo-first-order (adsorption kinetics model). sified as toxic and carcinogenic species. Besides the high toxicologi- ∗ Corresponding author. E-mail addresses: tiago.fraga2012@gmail.com (T.J.M. Fraga), mauricio.motta@ufpe.br (M.A. da Motta Sobrinho), marcos.ghislandi@ufpe.br (M.G. Ghislandi). https://doi.org/10.1016/j.envadv.2021.100064 Received 5 December 2020; Received in revised form 17 April 2021; Accepted 25 April 2021 2666-7657/© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
  2. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 cal effect, textile dyes are visible to the naked eye, causing the de- that sense, the synthesis of GO was done via modified Hummers method, pletion of dissolved oxygen and poor penetration of sunlight into the and subsequently functionalized in order to obtain AmGO. It was also water bodies, reducing the photosynthetic activity of aquatic plants evidenced that the presence of the amine groups in GO surface favored (Lellis et al., 2019). Usually, textile wastewater treatment is based the RB5 adsorption by the increase of 𝜋–𝜋 interactions between AmGO on physical-chemical processes, such as precipitation-coagulation, fol- and dyes molecules. The effect of AOP working variables in the pho- lowed by biological treatment. However, the large amount of adsorbed tocatalytic process was evaluated by a 23 factorial design - dosage of dye prevents the sludge reuse, generating large amounts of solid waste AmGO, H2 O2 concentration and pH of the reaction medium. Perform- (Sohaimi et al., 2017). In this scenario, investments in new technologies ing the experimental design was important to set up the best working can optimize the treatment processes. conditions to further experiments. Kinetics of the photodegradation re- The employment of advanced oxidation processes (AOPs), and more action was studied, and a brief mechanism was discussed. Assays using specifically the Fenton and photo-Fenton processes in textile wastewa- raw textile wastewater samples and the phytotoxicity for synthetic efflu- ter treatment is a preferred technique for the degradation of contami- ent after degradation were also evaluated and contributed to evidence nants in these types of effluents, since chemical oxidation is one of the the profitability of AmGO. most efficient, low cost, eco-friendly and simple processes, it might be applied. Between the AOPs, photo-Fenton process is the one in which 2. Methodology the oxidation reaction of ferrous to ferric ion oxidation takes place to decompose hydrogen peroxide into hydroxyl free radicals (•OH). This 2.1. Graphene oxide synthesis and functionalization reaction is favored by the presence of ultraviolet (UV) or sunlight ra- diations (Babuponnusami and Muthukumar, 2014). The generation of Graphite powder 99 % A.P. was supplied by Merck Inc., Ger- free radicals causes an attack on the dye molecules. Although this pro- many. Potassium permanganate (KMnO4 , ACS grade), hydrogen perox- cess requires a highly acidic conditions (pH < 4), it does not gener- ide (H2 O2 , 99 % A.P.) and iron chloride III hexahydrate (FeCl3 .6H2 O, ate toxic sludge or other waste by-products (Nazari and Salem, 2019; ACS grade) were purchased from Dinâmica Química Ltd., Brazil. Sulfu- Wang et al., 2019). Heterogeneous Fenton oxidation is used when it ric acid (H2 SO4 97 % A.P.) was purchased from Honeywell Co., Ger- is trying to operate at a higher pHs. When iron ions are in crystalline many. Chloride acid (HCl, 37 % A.P.), ethanol (CH3 CH2 OH, 99 % A.P.) or oxidized form and bonded to a support, they do not suffer precip- and ethyleneglycol (HOCH2 CH2 OH, 99 % A.P.) were purchased from itation at high pH values. Various carbonaceous nanomaterials have Química Moderna, Ltd., Brazil. Diethylenetriamine (DETA, C4H13N3, been recently studied as supports for metallic oxides, such as graphene 99 % A.P.) and sodium acetate (H3 CCH2 Na, 99 % A.P.) were purchased oxide (GO), reduced graphene oxide (rGO), covalently functionalized from Sigma-Aldrich, Germany. RB5 powder was supplied by Dystar do graphene and 3D graphene-based hydrogels have been recently stud- Brasil S/A. ied (Gómez-Pastora et al., 2017; Moztahida and Lee, 2020; Wang et al., Graphene oxide synthesis was proceeded following the modified 2019; Zhang et al., 2019). Hummers protocol (Fraga et al., 2019; Hummers and Offeman, 1958). Graphene is defined as a two-dimensional (2D) nanosheet of sp2 hy- 1.0 g of powdered graphite was added into a beaker containing 27 mL of bridized carbon atoms arranged in six-membered rings (Dreyer et al., H2 SO4 under stirring and controlled temperature (0–6°C) in an ice bath 2010; Randviir et al., 2014). Graphene structure gives it interest- until complete homogenization. 3.0 g of KMnO4 was gradually added ing properties such as: transparency, large theoretical specific surface to the flask. Thereafter, the ice bath was removed, and temperature was area (2630 m2 ∙g−1 ), high intrinsic mobility (200,000 cm2 ∙v−1 ∙s−1 ), controlled between 30–40°C under stirring (250–350 rpm) for 6 h. Then, high Young’s modulus (~1 TPa) and thermal conductivity (~5000 distilled water was added along with H2 O2 to finalize the oxidation step. Wm−1 ∙K−1 ). Due to the large surface area, graphene is considered one The sample was washed using HCl 25% and distilled water and the re- of the most promising carbon based adsorbents and an excellent support sulting suspension was mostly graphite oxide. To obtain GO, the freshly for catalysts (Safarpour et al., 2019; Wang et al., 2019; Zhu et al., 2010). prepared solution was submitted to sonication for 4 h in an ultrasound Studies showed that the use of (GO) as a photocatalyst support can bath (Elmasonic, E30H, 37 kHz). improve the efficiency of the process. Arshad and coworkers found Covalent amino functionalization of GO was carried out at the same a 99.24 % removal of methyl orange by heterogeneous photo-Fenton time as the solvothermal method of anchoring Fe3 O4 nanoparticles in (HPF) reaction using a nanocomposite of graphene/Fe3 O4 . On the other the graphene plane. Therefore, 360 mL of graphene oxide was used with hand, when they used Fe3 O4 nanoparticles alone, the removal was of 3.0 g of sodium acetate, 0.5 g of FeCl3 ∙6H2 O and 200 mL of monoethy- 43 % (Arshad et al., 2018). Yu and coworkers concluded that phenol lene glycol. The system was placed under stirring and DETA was added degradation increased from 73.7 to 98.8 % when the GO weight ratio when the temperature reached 170 ºC. The functionalization occurred in was increased from 0 to 15 % in Fe3 O4 -GO nanocomposite (Yu et al., vigorous stirring speed (up to 400 rpm) for 6 h and temperature strictly 2016). When graphene is combined with a semiconductor in a photo- controlled at 180 ºC. After that, the suspension was firstly washed using catalyst, the electrons are photoexcited for the valence band to conduc- 200 mL of ethanol to remove polar organic compounds. For the sub- tive band and are transferred from semiconductor to graphene layers; sequent washes, 1.0 L of distilled water was used and the process was this is known as heterogeneous electron transfer (HET) rate (Chia and repeated totalizing 6 washes. Pumera, 2018). Furthermore, HET eases the conversion of H2 O2 to •OH. Also, the 𝜋–𝜋 interaction between graphene basal planes and 2.2. Characterization of GO and AmGO 𝜋–conjugated electrons of organic pollutants enhance the adsorption and subsequent degradation of the contaminant (Arshad et al., 2018; GO and AmGO were analyzed by Fourier transformed infrared spec- Gogoi et al., 2019; Perreault et al., 2015). Despite several published pa- troscopy (FTIR) in an IR spectrometer with Attenuated Total Reflectance pers which deal with the heterogeneous photocatalytic degradation of (ATR) crystal, Shimadzu model IRAffinity-1S. X-ray diffraction (XRD) dyes, few of them report the investigation of the effect of adsorption in was performed in a diffractometer Rigaku Ultima IV equipped with a the overall efficiency. copper radiation source. XRD patterns of GO, Fe3 O4 and AmGO were This work aims to evaluate the degradation of Reactive Black 5 (RB5) analyzed, and the diffraction angle (2𝜃) ranged from 5 to 80°. Raman dye applying UV assisted Fenton-like process using the Fe3 O4 present on spectra of all samples were acquired by a confocal Raman spectrometer AmGO surface as a photocatalyst and, accordingly, raising the reaction with laser excitation source of 532 nm, WITec, model Alpha 300R. For pH. The objective of anchoring Fe3 O4 nanoparticles over GO structure each sample, 15 individual spectra were measured at random points and is due to GO low band gap energy which favors the electron transfer the average intensity was used to plot the Raman spectra. The thermal to generate •OH under the photoelectric effect (Zhang et al., 2019). In stability of GO and AmGO was characterized using thermogravimetric 2
  3. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 analysis (TGA). All measurements were conducted in a thermogravimet- 2.7. Photodegradation of RTW ric balance Netzsch, model STA 449F3 Jupiter, under nitrogen flux of 50 mL∙min−1 , over a temperature range of 30–900°C and with heat- The textile wastewater samples were collected from a textile finish- ing ramp rate of 3.0 K•min−1 . Scanning electron microscopy (SEM) was ing industry Lavanderia Mamute Ltd., located in the district of Caruaru done to GO and AmGO samples under different magnifications, using (Pernambuco, Brazil), where the daily water consumption is over 60 m3 . a Tescan VEGA3 equipment, using 5 kV and working distance (WD) of Two samples were collected: the first one having passed only through about 5 mm. Powder samples were prepared and gold sputtered previ- a screen for coarse particles separation and a decantation tank in the ous to analysis. Energy-dispersive spectroscopy (EDS) was carried out first stages of the wastewater treatment plant (WWTP); and the second in a EDS spectrometer Oxford model AztecLive. Samples were prepared was collected after coagulation in the WWTP. The samples were stored under similar conditions described above, although without covering at 4 ºC without chemicals addition. HPF experiments were carried out with gold in order to avoid further interferences in the action of elec- with 1.0 mL of AmGO (6.0 mg∙mL−1 ), H2 O2 concentration of 1.0 M, tron beam. Electron paramagnetic resonance (EPR) analysis was carried with 25 mL of RTW, under UV-A radiation during 120 min. Experiments out for AmGO samples in a spectrometer Bruker EMX 10+, with modu- were performed with both wastewater samples. The effectiveness of the lation frequency 100 kHz, amplitude of 4 Gauss and microwave potency UV-assisted HPF was assessed by the parameters of chemical oxygen of 0.632 mW. demand (COD), biochemical oxygen demand (BOD), turbidity and ap- parent color, evaluated before and after the degradation by UV-assisted HPF. 2.3. HPF experimental procedure 3. Theory and calculation HPF experiments were carried out in Petri dishes with 25 mL of RB5 solution 100 mg∙L−1 (and also with 50 mg∙L−1 in HPF kinetics assays) 3.1. HPF kinetics and mathematical modelling and under room temperature (28 ± 3 ºC). The AmGO dosage, H2 O2 con- centration and pH of RB5 solution varied according to the experiments Kinetics studies of HPF were conducted according to the best condi- (as described in factorial design 23 and kinetics assays). The dishes were tions found in the factorial design, evaluating the change in dye concen- placed in a UV light reactor with ultraviolet A lamps (UV-A, 40 W, 254 tration over time in system with initial RB5 concentrations of 50 mg∙L−1 nm, Taschibra). The samples were manually mixed each 30 min to keep and 100 mg∙L−1 . Thereafter, the samples were placed in the reactor and homogenization in the reaction medium, thus the experiments were per- the contact time ranged from 1 to 480 min. The final concentrations of formed without continuous agitation. RB5 were measured on a UV-Visible spectrophotometer (Aquamat Gen- Regarding the adsorption experiments, they were carried out follow- esis 10S, ThermoFisher). Experiments were carried out at room temper- ing the same procedure described for HPF experiments, however with- ature (28 ± 3 ºC), and UV-A light. 18.0 mg of AmGO was added to 25 mL out adding the H2 O2 and in darkness. of RB5 solution, at pH 8.0. The dosage of hydrogen peroxide solution was 1.0 mL [H2 O2 = 1.0 mol∙L−1 ]. Experimental data were modeled by both linear regression and non-linear fitting of the Chan & Chu model 2.4. Comparative study of catalytic activity Eq. (1): A study with different catalyst were realized to compare the effi- 𝐶 𝑡 =1− (1) ciency of AmGO in RB5 degradation. The photocatalytic activity of GO, 𝐶0 (ρ + σt ) Fe3 O4 and Fe3 O4 -GO (5% wt of GO) were evaluated under the same In which, C is the concentration of RB5 (mg∙L−1 ) remaining in the conditions of AmGO. In addition, the homogeneous process was eval- system after the reaction time t (min), C0 is the initial concentration uated by photoperoxidation. For this, 25 mL of RB5 (100 mg.L−1 ), 1.0 of RB5 (mg∙L−1 ), 𝜌 (min) and 𝜎 (dimensionless) are two characteristic mL of H2 O2 (1.0 mol.L−1 ) and 18 mg of catalyst was putting in a Petri constants regarding the reaction kinetics and oxidation capacities. The dish under UV-A light. After 120 minutes, the final concentration was constants 1/𝜌 (min−1 ) and 1/𝜎 represent the initial removal rate of RB5 measured using a UV-Vis spectrophotometer model Genius 20S (Ther- and the maximum oxidation capacity in the process, respectively. Ki- moFisher). netics first-order Eq. (2) and second-order Eq. (3) kinetic models were also applied to determine the rate constants, being k1 (min−1 ) and k2 (L∙mg−1 ∙min−1 ) the first- and second-order rate constants. 2.5. Optimizing HPF conditions: Factorial design 23 𝐶𝑡 = 𝑒−𝑘1 𝑡 (2) The experimental design was performed to set up the best working 𝐶0 conditions for the removal of RB5 molecules using AmGO. Therefore, the 𝐶 1 23 factorial design was proceeded for the variables: pH of the reaction = ( ) (3) 𝐶0 1 + 𝐶0 𝑘2 𝑡 medium, H2 O2 concentration and AmGO dosage (in mL of AmGO sus- pension with 6.0 mg∙mL−1 ). The contact time was stablished in 90 min The linearized forms of the first-order and second-order models are for all experiments and initial concentration of RB5 was 100 mg∙L−1 . given by Eq. (4) and Eq. (5). HPF assays were performed with all possible combinations in replicate. ( ) 𝐶0 Factorial design factors and levels are presented in Table S1 in the Sup- ln = 𝑘1 𝑡 (4) 𝐶𝑡 plementary Material. 1 1 − = 𝑘2 𝑡 (5) 𝐶𝑡 𝐶0 2.6. AmGO regeneration 3.2. Adsorption kinetics and modeling AmGO regeneration assays were carried out by HPF experiments with 100 mL of RB5 solutions (50 mg∙L−1 ), 4.0 mL of H2 O2 (1.0 mol∙L−1 ) For adsorption kinetics, the same experiments were carried out in the and 5.0 mg∙mL−1 of AmGO under UV-A light. After HPF, AmGO was sep- absence of H2 O2 and UV radiation in order to evaluate the influence of arated from RB5 solutions by centrifugation. AmGO was regenerated by RB5 adsorption onto AmGO in the overall removal process. Initial con- washing with 50 mL of distilled water prior to be reutilized in further centration of RB5 was 100 mg∙L−1 . Moreover, it was analyzed how the cycles of HPF. pseudo-first-order (PFO), pseudo-second-order (PSO) and Weber-Morris 3
  4. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 Fig. 1. a) FTIR of AmGO, AmGO after the ad- sorption of RB5 (ADS) and AmGO after the pho- todegradation of RB5 (HPF); b) XRD patterns of AmGO and GO samples; c) TGA and DTG pat- terns of GO and AmGO; d) Raman spectra of GO and AmGO, showing ID /IG ratio for both spectra. intraparticle diffusion (IPD) (Weber and Morris, 1963) kinetic models. possible to observe the characteristic bands at 1554.6 cm−1 , attributed These models are respectively given in their non-linear forms by the to the GO carbonyl (–C=O) stretching of –COOH groups, and at 1527.6 Eqs. 6, 7 and 8. cm−1 , due to O–H bending vibration, epoxide groups and the skele- ( ) tal ring vibration of unoxidized graphite (Ţucureanu et al., 2016). The 𝑞𝑡 = 𝑞𝑒 1 − 𝑒−𝑘𝐹 𝑡 (6) band at 1003 cm−1 is due to the C–O present in several oxygen con- taining groups (Li et al., 2013). The peak at 1658.8 cm−1 is attributed 𝑘𝑆 𝑞𝑒2 𝑡 to stretching vibrations of the bond –C=O of the adjacent carbons of 𝑞𝑡 = ( ) (7) 1 + 𝑘𝑆 𝑞𝑒 𝑡 amide and also carboxyl groups. The peak at 1122.5 cm−1 for ADS might be attributed to –S=O– groups from RB5 molecules (Bilal et al., 2018). 𝑞𝑡 = 𝑘𝐼𝐷 𝑡1∕2 + 𝑘0 (8) This peak disappears in the spectrum after HPF indicating that the RB5 Where qt is the adsorption capacity in the time t (mg∙g−1 ); qe is the molecules adsorbed onto AmGO were degraded. The peak at 1076.3 adsorption capacity at the equilibrium state (mg∙g−1 ); kF is the the PFO cm−1 characterizes the sulfoxide nature of the RB5 (Bilal et al., 2018). rate constant (min−1 ); kS is the PSO rate constant (in g∙mg−1 ∙min−1 ); kID Finally, a broad band, located between 2900 and 3500 cm−1 is related is the intraparticle diffusion rate coefficient (mg.g−1 .min−1/2 ); while k0 to the stretching vibration of H–O– bonds of hydroxyl groups, present in is a constant related to the resistance to the diffusion (in mg∙g−1 ). GO samples, and also in AmGO, since hydroxyl groups remain attached to the graphenic plane even after the nucleophilic attack of the amines. 4. Results and discussion XRD patterns of GO (Fig. 1b) shows a strong peak at 2𝜃 ~ 14° - (002) diffraction plane - evidencing an increase in interlayer space, inferring 4.1. Characteristics of Reactive Black 5 dye that graphite was effectively oxidated via the modified Hummers pro- tocol. XRD patterns of Fe3 O4 , by its turn, exhibits characteristic peaks RB5, commercially known as Remazol Black B, is a reactive diazo at 36º, 43º and 57º, that corresponds to the diffraction of (311), (400), anionic dye, with dark-blue color in concentrated aqueous solution - see (422) and crystal planes of pure Fe3 O4 nanoparticles with spinal struc- photograph of RB5 solution in Fig. S1(a), in Supplementary Material. It ture, as reported in previous works (Habibi, 2014). Some of these peaks has in its molecular structure the chromophore group azo (N=N), in appear in small intensity in AmGO sample (43º and 63º), which evi- which its binding stretching vibration are represented by the FTIR band dences the presence of Fe3 O4 anchored over its surface. The (002) peak at 1450 cm−1 ; while its auxhochrome group is the sulfonate (–SO3− ), in at 25° is present in XRD pattern of AmGO, this is characteristic of func- which S=O stretching vibration is confirmed by the band at 1050 cm−1 tionalized or reduced graphene samples (Fang et al., 2009). Therefore, (Fig. S1-b) (Bilal et al., 2018). RB5 molecular weight is 991.82 g∙mol−1 , according to XRD diffractograms, both samples GO and AmGO are com- and its maximum absorbance wavelength in the UV-Visible spectrum is posed by multilayer GO or even graphite oxide, which could be con- at 595 nm (Fig. S1-a). firmed by further in-depth analyses of the nanoflakes thicknesses, using atomic force microscopy (AFM), for example. The AFM results exhibit- 4.2. Characterization ing the produced GO (average thickness 5.79 nm), as well as the amino- functionalized sample of GO (nGO-(NH)R, ~ 8.43 nm) can be found in FTIR spectra for AmGO, AmGO after adsorption of RB5 (ADS) and a previous work by Fraga et al. (2019). Although the nGO-(NH)R is not the spectrum of AmGO after HPF process are depicted in Fig. 1a. It is exactly the same material as AmGO, the samples were produced with the 4
  5. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 same precursors and under similar conditions (amino-functionalization Table 1 with DETA using ethyleneglycol as solvent). This allows us to infer that EDS data for pure AmGO sample and AmGO after 6 cycles of HPF- both GO and AmGO present multilayer nature. reutilization Fig. 1c depicts the Raman spectra for GO and AmGO samples, in Before HPF After 6 cycles of reutilization which the presence of the characteristic bands for carbonaceous mate- Element Element Weight (%) 𝜎 (%) Element Weight (%) 𝜎 (%) rials can be detected - the D-band (1364 cm−1 ) and the G-band (1580 C 57.52 1.86 86.48 0.7 cm−1 ). The G-band presents the stretching movement of the sp2 bonds O 25.50 1.40 12.50 0.8 between carbon atoms of the basal 2D plane (Ferrari et al., 2006). The Fe 13.56 2.20 N/E (1) - D-band is reflects disordered structures such as defects, crystal bound- Si 3.20 0.38 N/E (1) - Na 0.50 0.10 N/E (1) - aries and symmetry breaking (Araujo et al., 2012). The intensity ratio Al 0.20 0.10 N/E (1) - of D to G band (ID /IG ) is usually used to evaluate the lattice disorder of S N/Ea - 1.02 0.2 graphite derivatives (Peng et al., 2016). Thus, comparing the ratio ID /IG a in both samples (see Fig. 1c), there was an increased from 1.02 to 1.17 N/E = not evidenced. after NH2 functionalization and Fe3 O4 anchoring on GO nanosheets, in- dicating that the AmGO presents a higher degree of disorder, which can as the pH increases (– for +). From previous works, the ideal pH for be attributed to the functionalization process. The high degree of defects the photo-Fenton reaction is usually around 3.0 (Babuponnusami and and disorder at the pristine sp2 structure increases HET rate of AmGO Muthukumar, 2014; Clarizia et al., 2017; Xavier et al., 2015). Even (Chia and Pumera, 2018). Moreover, band gap energy levels in defect though, the factorial design showed that the pH positively influenced states lie between conduction and valence bands in a disordered basal the photodegradation. These results can be justified by the fact that the plane and contribute to enhance its catalytic potential to generate •OH reaction studied here is heterogeneous, as the iron source is impregnated radicals from H2 O2 molecules under UV radiation. in the catalyst. Therefore, the iron oxide did not coagulate or precipi- TGA analysis shows that the weight loss was 10 % for GO and 5 % for tate, neither formed complexes at high pH values. This hypothesis can be AmGO between 0 and 100 ºC. These mass decreases are relative to mois- confirmed by FTIR spectrum of the AmGO sample after the HPF process. ture content, which is eliminated in this temperature range. It was evi- Through which the characteristic band attributed to Fe–O stretching vi- denced that graphite powder was sufficiently oxidized in modified Hum- bration (500–600 cm−1 ) remained in the spectrum after the adsorption mers method, once substantial decrease in GO weight is observed be- and HPF degradation of RB5 (Fig. 3b). These results are advantageous tween 200–425°C (Fig. 1d), reaching 2.02 %∙min−1 DTG. This mass loss from the economical point of view, since the proposed employment for is due to the uptake of oxygenated groups (–OOH, –OH) from graphene AmGO catalyst is the HPF reaction for wastewater treatment from tex- basal plane (You et al., 2020). Such loss occurs in small intensity when tile industries. This type of effluent usually presents high pH, so it would it comes to AmGO, indicating that most oxygenated groups were re- not be necessary to proceed with pH adjustments during the treatment duced during amino-functionalization process. Moreover, the high ther- process. mal stability of amines groups from AmGO is evidenced by a nearly Analysis of the Pareto diagram (Fig. S2, Supplementary Material) constant mass of AmGO between 150 and 300°C. This is explained by evidenced that all effects were statistically significant with 95 % of con- covalent bonds formed between amine and the carbon atoms from oxy- fidence level. Furthermore, the effect of AmGO dosage over HPF effi- genated groups and graphene basal plane, which need great amount of ciency stood out compared to the other variables, followed by the ini- energy to break these bonds (Zhang et al., 2014). At temperature range tial pH of the reaction. Furthermore, the effects of all variables and their 50–120°C, reactants with primary amines in their structures (DETA, combined interactions were statistically significant. ethylenediamine, dodecylamine, etc.) can break the epoxide rings dis- AmGO dosage exhibited the most significant effect for the 23 facto- tributed over GO surface; however, at higher temperatures (150–200°C), rial design. This can be explained because of larger amounts of AmGO primary amines attack the hydroxyl groups of GO (Zhang et al., 2014, represent a larger availability of catalytic sites in the reaction medium 2013). Finally, the residual 60 % in AmGO mass (temperature > 800 and, consequently, a greater formation of •OH radicals. Therefore, the ºC) is attributed to carbon, metallic iron and oxides, as well as other value in the Pareto chart is positive (Fig. S2), indicating an increase impurities derived from the combustion. in removal efficiency. From the Pareto diagram, it is also possible to SEM analyses of GO (Fig. 2a, b and c) exhibited a clean and nearly observe that the concentration of hydrogen peroxide contributed pos- smooth morphology for the GO prepared samples. The multilayer sheets itively, as higher concentrations of H2 O2 lead to a more efficient HPF tend to stack up and fold over each other, with some wrinkles (see (Kapoor, 2017; Starling et al., 2017). However, it is already expected as Fig. 2b). For the AmGO samples the sheets have a higher roughness as the degradation efficiency increases with increasing of peroxide concen- compared to GO and the surfaces are, in general, much more wrinkled tration until it reaches a maximum value. From that, any H2 O2 concen- and corrugated (see Fig. 2d, e and f). This is probably because of the tration above this maximum value would contribute to the recombina- covalent amino functionalization, which promoted bonds between the tion of the hydroxyl radicals into hydroperoxyl (•OOH) (Trovó et al., sheets and the anchored chemical structures. 2013). The excess of •OOH (oxidation potential 1.42 V) in reaction EDS analysis evidenced that most of Fe3 O4 was leached from the medium decreases the RB5 oxidation rate, since these radicals are much nanocatalyst sites (Fig. 2g and Fig. 2h and Table 1) after the 6 cycles of less reactive than •OH (oxidation potential 2.80 V). It was also observed HPF-reutilization of AmGO. This might be explained by the incidence in the response surface (Fig. 3b) that the optimum value of peroxide of weak interactive forces between the ligands (Fe) and the graphenic concentration was near to the central point, [H2 O2 ] = 0.55 mol∙L−1 . plane and its functional groups (mostly amines, amides, hydroxyl and Moreover, from Pareto diagram (Fig. S2), it is seen that the single effect carboxyl). The presence of sulfur in trace amount (1.02 %) in the sample of [H2 O2 ] is very small in comparison to the most significant variables, of AmGO after 6 cycles of HPF is related to the occurrence of chemical and near the statistical insignificance (limit at p 0.05). bonds between the aforementioned groups of AmGO and the sulfur of sulfonate groups from degraded RB5 molecules. 4.4. Kinetics data and modeling 4.3. Optimizing HPF conditions: Factorial design Experimental data of UV-assisted Fenton-like degradation kinetics of RB5 are depicted in Fig. 4a along with the adsorption process – results From surface response methodology, it is possible to observe the in- are given in terms of removal efficiency. The color behavior during ki- crease in percentage of degradation (% Rem) with pH, since the response netic assessment is depicted in Fig. 4b, and it is possible to verify a surface (Fig. 3a) shows an increase in intensity (red area in the surface) diminishing in the color intensity with time. 5
  6. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 Fig. 2. SEM comparison of the GO (a, b, c) and AmGO samples (d, e, f) under three different magnifications; EDS spectra for AmGO: before HPF (g); after six cycles of HPF-reutilization (h). Fig. 3. a) response surface of RB5 removal (%Rem) by HPF – AmGO dosage [OGf] ver- sus pH; b) response surface of RB5 removal (%Rem) by HPF – AmGO dosage [OGf] versus [H2 O2 ]; Experimental conditions: C0 (RB5) ~ 100 mg∙L−1 , 25 mL of solution, 28 ± 3°C. 6
  7. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 Table 2 Parameters of kinetics for HPF and adsorption after mathematical modeling HPF RB5 initial concentration (mg∙L−1 ) Chan & Chu nonlinear fit 1st order linear fit 2nd order linear fit 1/𝜌 (min−1 ) 1/𝜎 R2 k1 (min−1 ) R2 k2 (L∙mg−1 ∙min−1 ) R2 50.00 17.5 0.8 0.99 7.30∙10−3 0.97 8.00∙10−4 0.98 100.00 2.3 0.7 0.95 7.10∙10−3 0.95 2.00∙10−4 0.97 Adsorption PFO PSO IPD (Weber and Morris, 1963) Exp qe (mg∙g−1 ) qe (mg∙g−1 ) kF (min−1 ) R2 qe (mg∙g−1 ) kS (g∙mg−1 ∙min−1 ) R2 k0 (mg∙g−1 ) kID (L∙mol−1 ) R2 −3 53.06 44.6 0.21 0.66 46.24 9.10∙10 0.88 21.36 1.72 0.73 Similarly, Baptisttella and coworkers investigated the magnetic Fe3 O4 - GO nanocomposite in the photodegradation of RB5 and other textile dyes (Baptisttella et al., 2020). Although their work presented advan- tages regarding the magnetic separation and easy regeneration of the catalyst, kinetic tests showed a removal efficiency of 54 % for RB5, with the initial concentration of the dye 20.0 mg∙L−1 (Baptisttella et al., 2020). Since the amount of GO in Fe3 O4 -GO nanocomposite was much lower compared to the amount of magnetite (5 % of GO), the effect of adsorption was quite small, different from what is reported in the present work. The contribution of RB5 adsorption onto AmGO in the overall re- moval process is depicted in Fig. 5d. From the graph in Fig. 4a, is noted that adsorption played a very important role in the process, being re- sponsible for the removal of almost 30 % of RB5 (Fraga et al., 2019). When adsorption and photodegradation were combined (in HPF reac- tion), the removal efficiency was 85 % for the system with initial RB5 concentration of 100 mg∙L−1 . From Fig. 5d it is observed that the adsorp- tion of RB5 reached the equilibrium within 300 min, despite the HPF process were quicker (60 min, as depicted in Fig. 4a). It might be ex- plained by the catalytic generation of •OH, which attacks RB5 molecules quicker than the RB5 reach the active sites of AmGO. Moreover, ac- cording to data reported in Table 2, it is concluded that the PSO model showed the best fit for RB5 adsorption by AmGO (R2 0.88), which evi- dences that the resistances of mass transfer in bulk phase governed the Fig. 4. a) kinetics of RB5 removal efficiency for systems with initial dye concen- adsorption mechanisms. tration of ~ 100 mg∙L−1 ; b) RB5 solutions after the HPF process using AmGO. Although the object of this investigation was photo-Fenton-like pro- cess, it was necessary to analyze the entire mechanism of heterogeneous photo-Fenton reaction. For that, it is important to understand the influ- ence of RB5 adsorption by AmGO, since it is a core step in the process From Fig. 4a, it is possible to observe that the reaction occurred (Fig. 6). In this sense, it must be remarked the amine and amides role quickly in the first 60 min and the process stabilized after 180 min. In on RB5 attractive forces, since these nitrogenized groups contributed order to estimate the rate constants, the linear first-order and second- to increase the distribution of delocalized 𝜋–electrons throughout the order models were fit for experimental data RB5 initial concentrations AmGO plane (Caliman et al., 2018). This behavior is reported as be- of 50 and 100 mg∙L−1 (Fig. 5). Quick photodegradation of RB5 is due to ing responsible for increase 𝜋–𝜋 stacking interactions between AmGO hydrogen peroxide is consumed instantly for the formation of •OH. As and the dyes molecules (Fraga et al., 2019; Wang et al., 2018). Addi- the H2 O2 concentration decreases, the formation of the hydroxyl radi- tionally, the amines also contribute to form strong dipole and early co- cals is impaired, initiating the slow reaction step (Starling et al., 2017). valent bonds with certain functional groups of RB5 molecules, mostly In addition, observing the data in Table 2, it can be stated that first –SO3− , –OH and –NH (Fraga et al., 2019). Moreover, RB5 molecules may and second order models fitted the experimental data by linear regres- also engage in interface interactions with the surface of the AmGO by sion with great determination coefficients ≥ 0.95 (Fig. 5a and Fig. 5b). van der Waals forces, H-bonds, electron donor-receptor interactions, and The mathematical modeling of first and second order models did not other weak interactive forces (Tavassoli Larijani et al., 2015; Wang et al., converge by non-linear fitting. Finally, by analyzing the R2 values, it is 2018). concluded that non-linearized Chan & Chu model showed the best fit On the other hand, amino-graphene oxide (RNH-GO) is reported as a for HPF. From Table 2, the great values obtained for 1/𝜌 indicate that material of high adsorptive capacity towards cationic and reactive dyes the system with lower concentration of RB5 showed a higher initial rate (Fraga et al., 2019; Wang et al., 2018). Therefore, the photo-Fenton- of degradation and greater oxidation capacity. This can be explained like occurred in overall by the adsorption of RB5 on AmGO surface, in by the quantity of AmGO and H2 O2 , compared to the dye concentra- parallel with the generation of •OH radicals by the photoelectric effect. tion. For homogeneous Fenton-reactions, Chan & Chu found that the This phenomenon is catalyzed by high HET rate at the edges and basal oxidation rate increases with the initial concentration of Fe2+ and H2 O2 planes from defective sp2 structure of GO, associated to lower band gap in the system (Chan and Chu, 2003). Similarly, it was observed that energy and the increase of electron conductivity from Fe3 O4 anchor- the higher the dosage of AmGO, the higher the oxidation capacity, due age (Chia and Pumera, 2018). Finally, the subsequent degradation of to a larger availability of active sites of the catalyst in the reactions the remaining RB5 molecules in liquid phase, and of those adsorbed in medium, which increases the HET and the generation of •OH radicals. 7
  8. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 Fig. 5. Linear fit of experimental kinetic data according to first order (a) and second order (b) models applied for HPF reaction under initial concentration of 50 mg∙L−1 and 100 mg∙L−1 ; non-linear fit of HPF kinetic data by Chan & Chu model for systems with initial concentra- tion of RB5 ~100 mg∙L−1 (c); adsorption kinet- ics with modeling according to the models PFO, PSO and IPD (d). Experimental conditions: 28 ± 3 ºC, 18.0 mg of AmGO, 25 mL of RB5 solution (C0 = 100 mg∙L−1 ), pH 8.0, UV-A radiation, 1.0 mL of H2 O2 1.0 mol∙L−1 ; adsorption exper- iments were carried out without H2 O2 and in darkness. Fig. 6. a) Comparative histogram graph for the percentage of degradation of RB5 (100 mg.L−1 ) after 120 minutes for each catalyst tested; b) regeneration cycles for AmGO in RB5 pho- todegradation by HPF and adsorption tests; c) UV-Vis spectra of RB5 solutions after each re- generation cycle. Experiments carried out with 5.0 mg∙mL−1 of AmGO, 100 mL of RB5 solution (50 mg∙L−1 ) at pH 8.0 and UV-A light; d) EPR spectrum of AmGO. AmGO, is provoked by •OH radicals. This mechanism is better explained 4.5. Comparative study of catalytic activity and illustrated in topic 4.5 (Fig. 7). That can be confirmed by the dis- appearance of the characteristic band of –S=O– stretching vibration at A comparative histogram (Fig. 6a) shows the percentage of degra- 1122.5 cm−1 from the FTIR spectra, that is attributed to SO3− groups of dation for each catalyst tested where it is possible seen that the AmGO RB5 (Fig. S1-b and Fig. 1a). present the best performance. When GO is introduced, the catalyst also worked as an adsorbent. In addition, the GO reduce the recombination of 8
  9. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 Fig. 7. Schematic representation of UV- assisted heterogeneous photo-Fenton reaction for the degradation of RB5 in aqueous medium under the catalytic effects of AmGO. electrons and holes (e− and h+ ) photo-generated from Fe3 O4 nanoparti- strong interactions, ΔHº ~ 93.0 kJ•mol−1 (Fraga et al., 2019). More- cles, resulting in an improved photodegradation performance (da Silva over, from the investigations of HPF-reusability and the absence of char- et al., 2020; Wang et al., 2019). The different in degradation perfor- acteristic bands of in FTIR spectra of RB5 sample post-HPF (Fig. 1a), it mance between Fe3 O4 -GO and AmGO system can be attributed to the can be verified that the RB5 molecules attached to AmGO were also amount of graphene present in the catalyst. In Fe3 O4 -GO only 5% by photodegraded by •OH radicals, as described by Perreault et al. (2015). weight is due to the GO and, therefore, the adsorption process does not The growing presence of non-degraded RB5 molecules in HPF-treated influence in the final result. The presence of amine groups in AmGO can solutions can be verified by the arising of the absorbance band between also contributed to the improvement of the RB5 degradation. The results 555–595 nm (Fig. 6c), which is attributed to the wavelength of max- also shows that the AmGO has much better catalytic activity than Fe3 O4 imum absorbance for RB5. This result points to great perspectives to nanoparticles where it is possible conclude that the Fe3 O4 nanoparticles further use of AmGO in the scale-up treatment of textile wastewaters, can agglomerated and difficult the exposure of active Fe sites (Xu et al., which recycling capacity would allow the reduction of operational costs. 2018). The photoperoxidation was also tested due to the H2 O2 gener- ates hydroxyl radicals by direct photodecomposition under UV light. 4.7. Photodegradation mechanisms The H2 O2 +UVA system shows a low percentage of degradation (4.8% after 120 minutes). Despite the great affinity towards amino-graphene Analyzing the experimental data, the mechanisms of HPF reaction frameworks (Fraga et al., 2019; Wang et al., 2018), the structure of was proposed in terms of mass transport phenomena for RB5 from bulk the RB5, with four aromatic rings, can difficult the photodegradation phase to the surface of the catalyst. From the disappearance of iron sig- by the UV/oxidant systems in the absent of a highly selective catalyst nal in EDS analysis after 6 cycles of HPF degradation/regeneration, it is (Baptisttella et al., 2020). evidenced that Fe3 O4 nanoparticles were leached from AmGO in form The results obtained in this work, compared to those reported in pre- of Fe2+ /Fe3+ to the aqueous medium. AmGO maintained the removal vious works that also investigated the degradation of RB5 are exhibited efficiency (97 %) after six cycles, which is regarded the significant role in Table 3. From the table, it is observed that in overall, AmGO presented played by Fe3+ /Fe2+ in the photodegradation of RB5, which confirm the higher degradation % of RB5, when compared to the magnetic Fe3 O4 - existence of two types of photodegradation reaction. Firstly there was GO nanocomposite (Baptisttella et al., 2020), NGO-Mn3 O4 and Mn3 O4 the heterogeneous photo-Fenton reaction, under the catalytic activity of (Saroyan et al., 2019). It was also indicated that UV-C radiation alone is Fe3 O4 plus the GO adsorptive capacity and the effect of low band gap not sufficient to degrade the dye. On the other hand, the photo-Fenton energy; then, the leaching of iron in the form of Fe3+ /Fe2+ actuated in process applied showed high degradation efficiency of RB5, however, the generation of •OH/•OOH by homogeneous photo-Fenton Eq. 9 and some of the disadvantages of using this method include sludge forma- (10). As reported in previous works, the leaching of Fe3 O4 from the cata- tion, and limitations regarding the need for extremely low pH (Bali et al., lyst into Fe3+ /Fe2+ ionic form can be accounted for the enhancement of 2004). the photodegradation of pollutants (Saleh and Taufik, 2019; Tolba et al., Other catalysts showed higher degradation efficiencies compared to 2019). AmGO, such as the Feo and Feo -UV; although, the catalyst dosage in It is well known from the literature that homogeneous photo-Fenton this case was more than double that of AmGO (Rahmani et al., 2010). reaction occurs following a chain of a series of radical chemical reac- In addition, as in many other catalysts exhibited in Table 3, the pH indi- tions, so that the free radicals •O2 and •OH are generated from H2 O2 cated for the best process efficiency is 3, which represents a limitation in by the photoelectric effect Perreault et al., 2015). These free radicals terms of practical application, since pH values of real textile wastewater allow the degradation of dye molecules (Araujo et al., 2011). The pho- are usually ranging from 6 to 9. toactivation mechanism for photo-Fenton reaction is demonstrated by Eqs. (9) to ((13). Finally, OH radicals engaged RB5 molecules, promot- 4.6. Regeneration of AmGO ing the cleavage of aromatic rings and aliphatic chains and generating CO2 and water Eq. (13). AmGO was successfully reutilized in 6 cycles of HPF-reusability with 𝐹 𝑒3+ + 𝐻2 𝑂2 → 𝐹 𝑒2+ + 𝐻 𝑂2∙ + 𝐻 + (9) 97 % of RB5 removal efficiency at the end of sixth cycle (Fig. 6b). More- over, the decrease of nearly 3 % in removal efficiency can be explained by the saturation of active sites of AmGO even with the leaching of 𝐹 𝑒2+ + 𝐻2 𝑂2 → 𝐹 𝑒3+ + 𝑂𝐻 − + 𝑂𝐻 ∙ (10) Fe3 O4 nanoparticles into Fe2+ /Fe3+ throughout the cycles. Adsorption efficiency of RB5 over 6 cycles of adsorption-reusability decreased 9 %, 𝐻 𝑂2∙ ↔ 𝐻 + + 𝑂2∙− (11) which is explained by the saturation of AmGO active sites with RB5 molecules. This is in agreement with previous work, which reported the chemical nature of RB5 adsorption by amino-GO, characterized by 𝐹 𝑒3+ + 𝐻 𝑂2 → 𝐹 𝑒2+ + 𝐻 + + 𝑂2 (12) 9
  10. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 𝑅𝐵5 + 𝑂𝐻 ∙ + 𝐻𝑂2∙ + 𝑂2∙− → 𝑖𝑛𝑡𝑒𝑟𝑚𝑒𝑑𝑖𝑎𝑡𝑒𝑠 → 𝐻2 𝑂 + 𝐶 𝑂2 (13) Additionally, according to reports by Perreault and coworkers Baptisttella et al. (2020) (Perreault et al., 2015), the heterogeneous photo-Fenton reaction, by which dyes molecules are degraded, takes place in three steps, as de- picted in Fig. 7: firstly, the adsorption of the pollutant in the active (Rahmani et al., (Saroyan et al., sites of catalyst (Wang et al., 2019). Secondly, photoactivation through (Bali et al., light (UV, LED, solar, etc.), which provokes the photoelectric effect and This work Reference 2019) consequently the in situ generation of free radicals •OH (Moztahida and 2010) 2004) Lee, 2020). The peak intensity of AmGO (G = 1.9974) in electron param- agnetic resonance spectra (EPR) exhibited a shorter value than Fe3 O4 (G ~ 2.20, according to literature reports), which indicates the decrease in Degradation efficiency (%) the single electron density around Fe. Moreover, EPR spectra indicates that the recombination of Fe3 O4 and the electronic sites of –O=C–NH from AmGO leads to an increase in electrons density around Fe3+ in Not significant Fe–O=CNH–[GO plane]. This allows the formation of a rich region in Fe3 O4 favoring the generation of •OH radicals. Studies carried out by ~100.0 ~100.0 ~55.0 ~99.0 ~40.0 Zuo et al. (2021) support these conclusions. Furthermore, it has been 54.0 89.0 99.0 98.0 85.0 well established in the literature that tunable metal oxide-graphenes exhibit elevated HET rate and high band gap energy in comparison to Time (min) pristine graphene which deliver electrons to enhance photoelectric ef- < 180 fect. Finally, the attack of free radicals, promoting breaks in the dye 180 120 120 120 120 60 60 60 molecules. Regarding the first step, as it is exhibited in the figure, the 5 - NH2 groups from amino functionalization are important as they con- UV light type tribute to the adsorption of the anionic dye RB5 in the active sites. UV-A UV-A UV-A UV-A UV-C UV-C UV-C 4.8. Photodegradation of RTW - - - - Table 4 exhibits characteristics of the RTW samples collected in the H2 O2 (mM) textile mill before and after HPF-assisted treatment using AmGO. Char- acterization of this type of effluent can be found in previous works 38 25 25 38 (de Araújo et al., 2020; Hayat et al., 2015; R Ananthashankar, 2013). - - - - - - - The untreated samples presented a dark blue color to the naked eye. RB5 initial concentration (mg.L−1 ) The COD/BOD ratio of the RTW was equal to 2.68. COD/BOD ratios > 2 indicate the high recalcitrance level of the effluent. In this case, the most appropriate treatment would be using physical-chemical methods (de Araújo et al., 2020). Degradation efficiencies of 12 % for COD, 53.25 % for apparent color, and 64.55 % for turbidity were observed in the RTW as a con- sequence of the photodegradation. According to Table 3, it is observed Different materials and processes used for the degradation of RB5 dye. an increase in pH values after the HPF process, being slightly above the 100 100 100 100 100 100 100 100 limit established by the Brazilian Environmental Legislation; and also 20 50 50 that the removal efficiency of COD was < 60 %, which is below the limit established by CONAMA 430 (Brasil., 2005; Brasil, 2011). Although the 5.5 pH 3 3 3 3 3 3 3 7 7 8 efficiency of the process for removing COD was below what is indicated, the system proposed in this work could be a good alternative, for exam- Dosage (g.L−1 ) ple, for a tertiary treatment of industrial textile effluents, contributing to remove part of the remaining organic matter before disposal. The 0.005 0.72 0.40 0.50 0.50 0.50 0.50 2.00 2.00 apparent color and turbidity levels of both the RTW and coagulated - - RTW wastewater decreased after de photo-Fenton process, indicating the degradation of the dyes and possibly other organic substances. magnetic Fe3 O4 -GO nanocomposite Hayat and coworkers (Hayat et al., 2015) evaluated and compared Photocatalyst/degradation process two processes for the removal of dye pollutants by using an anaerobic IC Photo-Fenton (UV/H2 O2 /Fe2+ ) reactor and Fenton’s oxidation. As a result, in their work the maximum color removal efficiency was 19 % for the anaerobic reactor and over 88 % using Fenton process without pH adjustment (pH ~ 7). Turbidity removal efficiency of the anaerobic reactor was around 36 %, and 94 % for Fenton process; and COD removal for the IC reactor and Fenton UV radiation NGO-Mn3 O4 NGO-MnO2 process (also without pH adjustment) were 87 % and 26 %, respectively. UV/H2 O2 Feo –UV Mn3 O4 AmGO Table 3 In that case, for Fenton oxidation, turbidity removal value was also the MnO2 Feo highest, followed by color and COD removal. Table 5 exhibits the results obtained using AmGO to treat RTW, com- pared to other materials and processes proposed in previous works for wastewater treatment. It is noted that AmGO presented quite similar val- ues of removal efficiency for apparent color, when compared to those 10
  11. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 Table 4 Parameters of RTW before and after photodegradation treatmenta Before photo-Fenton-like After photo-Fenton-like Parameters RTW Coagulated RTW Reference RTW Coagulated RTW Legal parametersb Turbidity (NTU) 91.4 54.2 (Hayat et al., 2015) 32.4 24.9 100.0 pH 7.86 7.51 (Hayat et al., 2015; R Ananthashankar, 2013) 9.18 9.01 6.00–9.00 Apparent Color (Hazen) 385 266 (de Araújo et al., 2020; R Ananthashankar, 2013) 180 164 75 COD (mg O2 ∙L−1 ) 131.99 31.53 (R Ananthashankar, 2013) 115.73 75.58 N/Ac BOD (mg O2 ∙L−1 ) 55 47 (R Ananthashankar, 2013) N/Ac N/Ac minimum 60% removal a Experimental conditions: AmGO dosage 6.0 mg∙mL−1; 28 ± 3°C; 25 mL of wastewater; UV-A radiation and without pH adjustment; b Parameters for wastewater discharge in water bodies, stablished at Brazilian legislation CONAMA 357 and CONAMA 430 (Brasil., 2005; Brasil, 2011); c No available data. from the magnetic Fe3 O4 -functionalized mGO (da Silva et al., 2020), PSO model showed the best fit in comparison to PFO and IPD, which and nGO-(NH)R (Fraga et al., 2019). The % removal of turbidity, using contributed to understand the governing effect of bulk mass transfer re- the AmGO, was higher than that obtained by the anaerobic IC reactor sistance to adsorption. In this sense, the amines and amides distributed (Hayat et al., 2015), although that was smaller compared to the other over AmGO surface and edges played a significant role on RB5 adsorp- treatment options exhibited in Table 5. tion by establishing covalent bonds with sulfonate and hydroxyl groups Although the efficiency obtained for COD removal was the lowest present in RB5 molecules. Regeneration experiments showed that AmGO presented, the material developed and proposed here can still be con- maintained its satisfactory removal efficiency of RB5 after 6 cycles, with sidered as promising for the degradation of organic compounds in textile 97 % of RB5 removal. This result was achieved even with almost all affluents, since it can be used in small quantities to treat the RTW, and Fe3 O4 being leached after the 6 cycles of HPF-reutilization, as observed there is also no need to make major corrections to pH value. Moreover, by EDS technique. Such data evidence the great contribution of amino- the treated effluent did not show potential phytotoxicological effects. GO in the photodegradation of RB5 by its great affinity towards RB5 Phytotoxicity analyses were performed and indicated that the treated molecules and low band-gap energy. RTW had no toxic effect over the germination of cucumber seeds (Cu- These satisfactory results evidenced that AmGO plays an important cumis sativus L.). Relative growth indexes (RGIs) were between 0.8 and role in textile wastewater photodegradation, exhibiting removal effi- 1.1, which means that it was not observed a significant effect of toxicity ciencies over 53 % for apparent color and 64 % for turbidity. Addition- for HPF-treated RTW. Phytotoxicity assessment is entirely reported in ally, these values are in agreement with Brazilian legislation standards. Supplementary Material. Finally, the low phytotoxicity levels of the textile effluent after HPF was Regarding the degradation mechanism of RTW, according to da Silva another evidence of the mineralization of toxic compounds. Finally, the et al. (2020), the photodegradation of RTW takes place in the same way main findings and results of this work bring excellent prospects for fu- as exhibited in Fig. 7 for RB5. At first the pollutant is adsorbed on the ture employments of AmGO in the treatment of industrial wastewaters active sites of the catalyst. Next, the hydroxyl radicals are generated by by photodegradation techniques. the photoelectric effect promoted by UV-A radiation. Then, •OH pro- motes the cleavage of aromatic rings and aliphatic chain of the organic pollutants. Declaration of competing interest It is worth mentioning that besides the mixture of dyes in RTW samples, there are also various organic and inorganic substances, such The authors declare that they have no known competing financial as metallic ions, soaps, salts, surfactants, and lubricants agents, which interests or personal relationships that could have appeared to influence turns more difficult to stablish a mechanism to accurately predict the work reported in this paper. the degradation mechanism of pollutants (de Araújo et al., 2020; Fraga et al., 2019). CRediT authorship contribution statement 5. Conclusions Maryne Patrícia da Silva: Conceptualization, Writing – orig- inal draft. Ana Caroline Alves de Souza: Investigation, Data In this work, the amino functionalized graphene oxide was employed curation. Lettícia Emely de Lima Ferreira: Investigation, Data as support of Fe3 O4 nanoparticles and evaluated as catalyst of RB5 dye curation. Luiz Martins Pereira Neto: Investigation, Data cura- and real textile wastewater samples. Characterization analysis demon- tion. Bruna Figueiredo Nascimento: Investigation, Data curation. strated that GO synthesis, its amino-functionalization and anchoring of Caroline Maria Bezerra de Araújo: Visualization, Writing – review & Fe3 O4 nanoparticles in the graphene plane were successful, as confirmed editing. Tiago José Marques Fraga: Supervision, Validation, Writing by FTIR, XRD and TGA results. From the 23 factorial design results, it – review & editing. Maurício Alves da Motta Sobrinho: Conceptual- was concluded that statistically, the most significant effects on the RB5 ization, Resources, Project administration. Marcos Gomes Ghislandi: removal process were the volume of AmGO and the initial pH. This is jus- Supervision, Methodology. tified by the fact that a higher volume of AmGO means a larger availabil- ity of GO/Fe3 O4 catalytic sites in the system to act on the degradation of hydrogen peroxide and formation of the hydroxyl radicals. Moreover, Acknowledgments AmGO showed the greatest RB5 degradation efficiency when compared with single H2 O2 /UV-assisted photodegradation (without catalyst), and The authors acknowledge the technical support provided by the De- the H2 O2 /UV-assisted photodegradation with pristine GO, Fe3 O4 and partment of Nuclear Energy (DEN) of UFPE, the Center of Northeast GO-Fe3 O4 as catalyst. It was also observed that the reaction did not Strategic Technologies (CETENE), the Pernambuco Institute of Technol- need to occur at acidic pH to exhibit a higher efficiency. ogy (ITEP), the Center for Advanced Research in Graphene, Nanoma- HPF kinetic study indicated that Chan & Chu model fitted the experi- terials and Nanotechnologies (MackGraphe) of Mackenzie Presbyterian mental data with non-linear regression coefficient over 0.95. Adsorption University, São Paulo/SP and the Fundação de Amparo à Ciência e Tec- kinetics demonstrated the slow adsorption of RB5 onto AmGO and the nologia do Estado de Pernambuco (FACEPE). 11
  12. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Table 5 Different materials and processes applied for textile wastewater treatment. Treatment Material process Type of effluent Parameters Dosage (g.L−1 ) pH Time (min) Efficiency (%) Reference GO batch RTW Turbidity, 0.46 6.0 ~60 89.5% turbidity, (de Araújo adsorption apparent color, 76.6% apparent et al., 2020) COD color, 60.9% COD, all for raw RTW nGO-(NH)R batch RTW Apparent color, 0.228 7.25 - 7.67 120 53.6% apparent (Fraga et al., adsorption COD color (raw 2019) RTW), 55% COD (after co- agulation + ad- sorption) gas diffusion electro-Fenton RTW Turbidity, COD, 0.10 (Fe3 O4 / 3 180 98.5% turbidity, electrode using total organic GO15%) 81.7% COD, (Geraldino et al., improved with heterogeneous carbon (TOC) 70.3% TOC, all 2020) rGO intensified Fenton-like for in natura with Fe3 O4 /GO catalyst RTW 12 magnetic heterogeneous RTW Turbidity, 0.12 8.08 180 77.52% (da Silva et al., Fe3 O4 - photo-Fenton apparent color, turbidity, 2020) functionalized reaction COD 55.57% mGO apparent color, 76.35% COD, all for raw RTW FeSO4 and Fenton RTW Turbidity, 1 part FeSO4 ~7 30 94.3% turbidity, (Hayat et al., H2 O2 oxidation apparent color, and 25 parts 88.8% color, 2015) process COD H2 O2 26% COD, all for raw RTW sludge taken anaerobic IC RTW Turbidity, - ~7 30 days 36.4% turbidity, from anaerobic reactor apparent color, 19% color, 87% digester COD COD, all for raw RTW AmGO heterogeneous RTW Turbidity, 6.00 7.86 120 64.55% This work photo-Fenton apparent color, turbidity, reaction COD 53.25% Environmental Advances 4 (2021) 100064 apparent color, 12% COD, all for raw RTW
  13. M.P. da Silva, A.C.A. de Souza, L.E. de Lima Ferreira et al. Environmental Advances 4 (2021) 100064 Funding Gogoi, J., Choudhury, A.D., Chowdhury, D., 2019. Graphene oxide clay nanocomposite as an efficient photo-catalyst for degradation of cationic dye. Mater. Chem. Phys. 232, 438–445. doi:10.1016/j.matchemphys.2019.05.010. This work was supported by the Conselho Nacional de Desenvolvi- Gómez-Pastora, J., Dominguez, S., Bringas, E., Rivero, M.J., Ortiz, I., Dionysiou, D.D., mento Científico e Tecnológico (CNPq) [Grant numbers 311133/2015-0 2017. Review and perspectives on the use of magnetic nanophotocatalysts (MNPCs) and 150734/2020-4]; and the Fundação de Amparo à Ciência e Tecnolo- in water treatment. Chem. Eng. J. 310, 407–427. doi:10.1016/j.cej.2016.04.140. Habibi, N., 2014. Preparation of biocompatible magnetite-carboxymethyl cellu- gia do Estado de Pernambuco (FACEPE) [Grant numbers IBPG-1008- lose nanocomposite: Characterization of nanocomposite by FTIR, XRD, FESEM 3.06/19, IBPG-1917-3.06/16 and APQ-1086-3.06/15]. and TEM. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 131, 55–58. doi:10.1016/j.saa.2014.04.039. Hayat, H., Mahmood, Q., Pervez, A., Bhatti, Z.A., Baig, S.A., 2015. Comparative decol- Supplementary materials orization of dyes in textile wastewater using biological and chemical treatment. Sep. Purif. Technol. 154, 149–153. doi:10.1016/j.seppur.2015.09.025. Hummers, W.S., Offeman, R.E., 1958. Preparation of Graphitic Oxide. J. Am. Chem. Soc. Supplementary material associated with this article can be found, in 80, 1339. doi:10.1021/ja01539a017. the online version, at doi:10.1016/j.envadv.2021.100064. Kapoor, S.S.S., 2017. 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