Hydroxamic acid mediated heterogeneous Fenton-like catalysts for the efficient removal of Acid Red 88, textile wastewater and their phytotoxicity studies

Ecotoxicology and Environmental Safety 167 (2019) 385–395

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Hydroxamic acid mediated heterogeneous Fenton-like catalysts for the efficient removal of Acid Red 88, textile wastewater and their phytotoxicity studies

T

Rijuta Ganesh Saratalea, Silojah Sivapathanb, Ganesh Dattatraya Saratalec, J. Rajesh Banud, ⁎ Dong-Su Kimb, a

Research Institute of Biotechnology and Medical Converged Science, Dongguk University-Seoul, Ilsandong-gu, Goyang-si, Gyeonggi-do 10326, Republic of Korea Department of Environmental Science and Engineering, Ewha Womans University, Seoul 120-750, Republic of Korea Department of Food Science & Biotechnology, Dongguk University-Seoul, Dongguk-ro, Ilsandong-gu, Goyang-si, Gyeonggi-do 10326, Republic of Korea d Department of Civil Engineering, Anna University Regional Campus, Tirunelveli, India b c

A R T I C LE I N FO

A B S T R A C T

Keywords: 2-Hydroxypyridine-N-oxide GAC Hydroxamic acid Heterogeneous Fenton-like catalyst 8-Hydroxyquinoline Phytotoxicity

Heterogeneous Fenton-like catalyst and its industrial application are increasingly given importance for its nonselective mineralization of organic pollutants in broad pH range. Current study, utilized an aromatic hydroxamic acid derivative 2-hydroxypyridine-N-oxide (HpO), for the construction of iron-Hpo ligand catalyst supported on granular activated carbon (GAC). 8-Hydroxyquinoline and citric acid as non-hydroxamic aromatic and aliphatic Fenton-like catalysts were used for comparative evaluation of the efficiency with targeted catalyst (iron-HpOGAC). This novel catalyst iron-HpO-GAC exhibits excellent efficiency in Acid Red 88 dye removal in the presence of hydrogen peroxide as oxidant at acidic, basic as well as at neutral conditions. Operational conditions for the catalytic oxidation including temperature, dye concentration, pH and catalyst dosage were systematically investigated and analyzed through kinetic studies. Thermodynamic analysis of the catalytic dye removal revealed that the system could oxidize pollutants faster with less activation energy requirement. Higher level of recyclability and stability of the catalyst with less iron leaching was achieved. Finally, the real time application of the catalyst was investigated through successful repeated treatment for actual industrial wastewater. The phytotoxicity assay (with respect to plant Phaseolus mungo) revealed that the degradation of Acid Red 88 and dye wastewater produced nontoxic metabolites which increases its potential application. This study emphasizes the viability of hydroxamate mediated efficient Fenton-like oxidation as a novel approach in designing economically viable pollutant removal technology.

1. Introduction Advanced oxidation processes remove pollutants using highly reactive intermediates such as hydroxyl radicals through either photochemical or non-photochemical methods. High energy oxidants including ozone, H2O2 or photons are utilized for the production of radicals, which are identified to be non-selective and can decompose huge range of organic pollutants under ambient temperature and atmospheric pressure (Swaminathan et al., 2003). In particular, because of its simplicity in function and high efficiency, Fenton reagent (H2O2/ Fe2+) is considered promising combination for treating various pollutants and also attracted industrial scale application in water treatment (Fayazia et al., 2016; Oliveira et al., 2006). Catalysts in Fenton system

should be cheap, non-toxic, and not require highly complicated apparatus and pressurized systems (Bautista et al., 2007). However, requirement of narrow pH (2.5–3.5) range for optimum production of hydroxyl radical by using large quantity of acid still hindered the vital applicability of the process in wastewater treatment sector (Ramirez et al., 2007; Sun and Pignatello, 1992). A typical Fenton reaction requires the transformation of ferric ion into ferrous ion, which could be inhibited at higher pH by forming ferric oxides or iron sludge causes secondary pollution and increases the cost of the separation processes (Bautista et al., 2007; Pignatello et al., 2006). Heterogeneous catalysts have insignificant metal-leaching rate resulting in negligible sludge generation (Nidheesh, 2015). Porous materials such as silica, alumina, and cation-exchanged resins have been

⁎ Correspondence to: Department of Environmental Science and Engineering, Ewha Womans University, Daehyundong 11-1, Seodaemungu, Seoul 120-750, Republic of Korea. E-mail address: [email protected] (D.-S. Kim).

https://doi.org/10.1016/j.ecoenv.2018.10.042 Received 23 May 2018; Received in revised form 8 October 2018; Accepted 10 October 2018 0147-6513/ © 2018 Elsevier Inc. All rights reserved.

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investigates the immobilization of iron- 2-Hydroxypyridine-N-oxide, iron-citric acid and iron-8-hydroxyquinoline complexes into porous supports using GAC and evaluated its efficiency in producing effective catalyst system. Efficiency of GAC as a support for 2-HydroxypyridineN-oxide ferric is comparatively evaluated with activated carbon fiber. We have explored the utilization of the prepared catalyst for the treatment of individual dyes and actual dye wastewater. Special attention is paid to leaching, pollutant degradation mechanisms and the reusability and recyclability of the catalysts. Further, toxicity of degraded products after treatment was studied to evaluate the ecofriendliness of the process.

used as heterogeneous catalysts, which holds surface for ferrous and ferric ions. Pollutants would be adsorbed on the surface of porous catalysts which increases the collision rate between pollutants and hydroxyl radicals which are produced on the surface by ferrous and ferric ions (He et al., 2002; Gemeay et al., 2003; Ishtchenko et al., 2003; Tachiev et al., 2000). Moreover, ligands such as diethylene triamine pentaacetic acid, ethylene diamine tetraacetic acid, ethylene glycol tetraacetic acid and 3,4-dihydroxybenzoic acid were also experimented with Fe(II) and Fe(III) cations to prepare iron-ligands complex that could act as catalysts (Tachiev et al., 2000; Wang et al., 2017). It was reported that heterogeneous catalysts with clay, zeolite, graphene, supports can avert iron ion leakage and improved pH range to a definite level however due to complex process still limits their large-scale practical applications (Gonzalez-Olmos et al., 2012; Dong et al., 2013; Lo and Boonamnuayvitaya, 2013). Recent studies suggest that the combination of iron-activated carbon fibers could be an effective heterogeneous Fenton catalyst with limited performance at neutral and alkaline conditions (Gemeay et al., 2003; Ishtchenko et al., 2003; Tachiev et al., 2000). Some papers reported that organic ligands complexed with iron proceed through ligand-to-metal charge transfer to extend the pH range from acidic to alkaline environment (Yao et al., 2013; Moon et al., 2004). Common ligands used for this purpose are metallophthalocyanines, and metal coordination complexes with Nbased neutral ligands (Li et al., 2010). In order to improve the Fenton technology, it is crucial to build a heterogeneous catalyst in which Fenton reagents get firmly attached with a solid support and produce optimum amount of hydroxyl radicals even at neutral pH conditions. Hydroxamic acids contain oxime (-N-OH) and carbonyl (C˭O) groups with common structure of RC(=O)NHOH and are weaker than carboxylic acids (McNaught and Wilkinson, 1997). Acidic groups and N-substituted derivatives of hydroxamic acids exhibit chelating characteristics and expresses strong affinities towards cations such as Fe(III) and Cu(II). In addition, also exploited as molecular magnets for tumor tissue imaging in pharmacology (Miller, 1989; Kakkar, 2013). Due to chelating abilities hydroxamic acids are widely utilized in the field of wastewater treatment. One of heterocyclic hydroxamic acids such as, 2Hydroxypyridine-N-oxide is identified to form bidentate stable complexes with trivalent metal ions including Fe(III) (Sun et al., 2014; Yao et al., 2014). 2-Hydroxypyridine-N-oxide ferric (2-HpoFe) as a Fentonlike catalyst supported on granular activated carbon (GAC) can efficiently remove organic pollutants. To enhance the practicability of Fenton system in wastewater treatment, GACs can be a better catalyst support because of its microporous structure, high adsorption capacity of concentrated pollutants which increases the rate of collision of pollutants with catalysts results in higher removal efficiency. GACs also provide catalytic stability by enhancing the stable attachment of catalytic active species through their structure and surface chemistry (Yao et al., 2014). Moreover, 8-Hydroxyquinoline, as an aromatic monoprotic bidentate iron chelating agent with 3 bidentate subunits, has been successfully utilized as Fenton-like catalyst reported earlier (Sun et al., 2014). The possibilities of using ecofriendly food grade substance such as, citric acid could serve as a possible Fenton-like catalysts for comparative studies (Seol and Javandel, 2008). Hydroxamic acid application in Fenton technology can be best evaluated when experimental comparison of non-hydroxamic aliphatic and aromatic Fenton catalysts is conducted. This approach was explored in this research work. Textile dyes are extensively utilized in various industrial sectors including textile, leather tanning, food processing, rubber, cosmetics, and plastic industries (Saratale et al., 2011). Industrial effluent generated constitutes textile dyes which can contaminate water bodies, quality of water and disturbing aquatic ecosystems as well as having serious influence on human health. Thus the existence of colored effluents into the environment has posed one of the major concerns to the environmental problems and need to be decontaminated before their disposal (Ramirez et al., 2007; Saratale et al., 2009, 2016). This study

2. Materials and methods 2.1. Materials and reagents Commercial Granular Activated Carbon (GAC) (500–600 µm) was purchased from Kaya Activated Carbons Inc., Korea. Physical properties of used GAC are listed in Table S1. Ferric chloride hexahydrate, 30% (w/w) hydrogen peroxide, nitric acid, hydrochloric acid, benzene, methanol, isopropanol and calcium chloride were purchased from Samchun Pure Chemical Co. Ltd., Korea as extra pure grade. 2-Hydroxypyridine-N-oxide, 8-Hydroxyquinoline were purchased from SigmaAldrich, USA and, magnesium sulfate, sodium phosphate dibasic, sodium phosphate monobasic, citric acid and sodium carbonate were purchased from Duksan Pure Chemical Co. Ltd., Korea. Commercial dyes such as Acid Red 88 (AR88), Basic Yellow 2 (BY2), Rhodamine B (RB), Methylene Blue (MB), Reactive Black 5 (RB5) and Congo Red (CR) were used without prior purification. Untreated dye wastewater (effluent of reactive and acid dyes production) was collected from Oh Young Industrial Corporation, Ltd., Korea and its initial characteristics are shown in Table S2. 2.2. Catalyst preparation 25 g of GACs was oxidized by 150 mL of 5 M nitric acid, and 12 g of them was modified by 40 mL of 5% thionyl chloride in benzene at 60 °C for 12 h. 2.3. Preparation of HpOFe-GAC, HqFe-GAC and FeCit-GAC complex and their characterization 5 mmol of FeCl3·6H2O, 2-Hydroxypyridine-N-oxide (Hpo) and 15 mmol of 8-Hydroxyquinoline (Hq) were dissolved in methanol separately. Sets of mother liquor were prepared by dropwise addition of FeCl3·6H2O into HpO or Hq solution with continuous stirring. 5 g of modified GACs was immersed separately into mother solution at 25 °C for 24 h to prepare HpOFe-GAC and HqFe-GAC. Sodium citrate solution (5 mmol) was prepared by mixing citric acid and sodium carbonate in 2:3 ratio in distilled water. Prepared sodium citrate solution was slowly added into 5 mmol of FeCl3·6H2O dissolved in distilled water. 5 g of modified GAC was added into the prepared solution and kept at 25 °C for 12 h to prepare FeCit-GAC. The prepared catalyst HpOFe-GAC, HqFe-GAC and HpOFe-GAC were, then, rinsed with distilled water several times to remove residues before drying in vacuum in room temperature. All the prepared catalysts impregnated in GACs were characterized in terms of surface morphology and mineralogical compositions by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) (JEOL-JSM-7500F) at an operating voltage of 20 kV. Ferric content in each heterogeneous catalysts were calculated through atomic absorption spectrometer (AAS). The content of ferric complex in HpOFe-GAC, HqFe-GAC and FeCit-GAC were 416.2, 533 and 416.6 mg/L, respectively. For comparison, Fe-GACs were prepared by direct absorption of Fe3+ into the activated carbon, and the ferric content (5 mmol) was kept pretty much the same as with the catalysts 386

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same amount of dye and H2O2 with same initial concentration were introduced to the catalyst. For understanding the regenerative capacity or catalyst stability the experiments were performed under optimized conditions (mentioned above). To recover the catalyst, after each run of complete dye removal, the final solution was filtered and rinsed carefully with distilled water followed by ethanol. The catalyst was then dried for an hour and then used for the next run of experiment.

prepared. 2.4. Experimental design and optimization Central composite design (CCD) was used for optimization of the four independent factors (initial AR 88 concentration, pH, and temperature and catalyst dosage) to attain information about the sole and their interaction. This design is standard second order and usually uses main factors, binary interactions among factors and quadratic terms of factors (Muthukkumaran and Aravamudan, 2017; Dil et al., 2016). For that, 30 sets of batches that was temperature (20, 30, 40, 50 and 60 °C), AR 88 concentration (10, 30, 50, 70 and 90 μM), catalyst dosage (2, 4, 6, 8 and 10 g/L) and pH (3, 5, 7, 9, and 11) were chosen as independent parameters and the rate of dye removal as the desired response. Optimal conditions for the removal of AR 88 were analyzed by Response Surface Methodology (RSM) using Design Expert 10 (Stat-Ease Inc., Minneapolis, MN, USA) (Muthukkumaran and Aravamudan, 2017; Dil et al., 2016; Ghaedi et al., 2015; Khodadoust and Hadjmohammadi, 2011). Ranges and different levels (low, high and center point level) of the variables for the 30 sets of experiment are given in Table S3. These ranges have been selected based on performed preliminary screening experiments. For optimization, the desired goals i.e. to achieve rate of color removal of AR 88 dye of the appropriate experimental responses were chosen (Design-Expert® version 10.0.0, Stat-Ease Inc., Minneapolis, MN, USA). The response models developed during the statistical analysis phase were used to identify factor settings that met the desired AR 88 color removal. The steepest ascent method using multiple starting points was used to find the best suitable condition. According to the best fit suggested by the software, appropriate equation was employed to fit the results. Catalytic oxidation of AR 88 was carried out at a constant temperature (fixed depending on the experimental condition) in a 50 mL glass bottle. A typical reaction mixture had 50 μM of dye along with the 5 g/L catalyst and 40 mM H2O2. All throughout the whole experiment, the amount of hydrogen peroxide introduced in each set of experiment was kept constant (5 mL) and the reaction volume is maintained to be 30 mL. At given time intervals, the samples were taken and analyzed by UV–vis absorption spectra. The maximum absorption of AR 88 was at a wavelength of 503 nm. The experimental parameters which provide optimum results for all three catalysts were chosen for further experiments: temperature 50 °C, catalyst dosage 5 g/L, pH 7 and dye concentration 50 μM.

2.7. Iron leaching condition of HpOFe-GAC The dissolved iron amount during AR 88 (50 μM) degradation at different pH conditions is an important factor which could explain how effectively the system can hold iron ions so that less iron sludge is produced from the reaction. To evaluate this aspect, 5 g/L of each prepared catalyst and 5 mL of 40 mM H2O2 were added into 25 mL of 50 μM AR 88 dye solution with different pH (pH 3, 5, 7, 9 and 11) at 50 °C. After complete degradation of the dye, a particular quantity of the solution was taken and the iron amount was measured using atomic absorption spectroscopy (AAS). 2.8. Investigation of active species in catalytic system The presence of hydroxyl radicals in a heterogeneous Fenton-like system would prove that hydroxyl radicals are the predominant active species in this system. Isopropanol is a hydroxyl radical scavenger. When adding isopropanol in a catalytic system, if the reaction rate is highly reduced, it can be an indication of hydroxyl radical being the active species. Four different batches of 25 mL of 50 μM AR 88 were taken (pH 7 and 50 °C). One set was introduced only with 5 g/L HpOFeGAC while 5 g/L HpOFe-GAC and 300 mM of isopropanol were introduced in another set. In other two batches, HpOFe-GAC and H2O2 were introduced, while only one among them received 300 mM isopropanol. The results were evaluated to determine whether hydroxyl radicals play a vital role in HpOFe-GAC catalytic oxidation. 2.9. Kinetic study The decolorization kinetics of AR88 was studied for different catalyst dosages HpOFe-GAC (2.5, 5, 7.5 and 10 g/L), dye concentration (30, 50, 70 and 90 μM), pH (3, 5, 7, 9 and 11) and temperature (30, 40, 50 and 60 °C). Influence of above mentioned parameters on effective removal of dye was evaluated through oxidation mechanism from pseudo-first order and pseudo-second order kinetic models. Pseudo-first order kinetic equation (He et al., 2014)

2.5. Control experiments

ln(Ct − C0) = −k1 t

To clarify that the prepared catalysts have significant impacts on removal rate, control experiments were conducted. 12 sets of 25 mL of AR 88 (50 μM) were taken. Initial pH of the dye solution was maintained to be 7 in all sets, temperature was kept as 50 °C and catalyst dosage as 5 g/L. One set was introduced with only H2O2 to understand the oxidizing nature of H2O2 alone. Only GAC was added in one system to clarify its contribution in adsorption related dye removal. HpOFeGAC, HqFe-GAC and FeCit-GAC were introduced into separate dye sets without and with H2O2 to analyze the impact of adsorption and oxidation of dye by the prepared catalysts. H2O2 was added with GAC and Fe-GAC in separate sets to understand how iron addition as heterogeneous catalyst impact on the removal rate.

(1)

Pseudo-second order kinetic equation (He et al., 2014)

1 1 − = k2 t Ct C0

(2)

where C0 and Ct are dye concentration (μM) at initial and time t, respectively, and k1 (min−1) and k2 (M−1.min−1) are the rate constants of the pseudo-first order and pseudo-second order kinetics, respectively. 2.10. Decolorization of textile effluent using constructed HpOFe-GAC catalyst The performance of HpOFe-GAC catalysts was studied for color removal from the textile effluent by withdrawing samples at definite time intermissions. Samples were centrifuged at 3000×g for 20 min and the resulted supernatant was utilized to evaluate the decolorization. The decolorization of textile effluent was determined by measuring the true color level was measured using the American Dye Manufacturers’ Institute (ADMI 3WL) tristimulus filter method. Decolorization of textile wastewater was by measuring ADMI removal percent (%) from the aqueous solutions by following the methods that have been reported

2.6. Sustaining catalytic stability and regenerative capacity To evaluate how stable the catalysts for repetitive use in removing dye, all three catalysts (HpOFe-GAC, HqFe-GAC and FeCit-GAC) were put in separate batch of experiment: each having 5 g/L catalyst, 25 mL of 50 μM AR 88, 5 mL of 40 mM H2O2 at pH 7.0 and 50 °C. Catalytic stability of the prepared catalyst was evaluated by repetitive oxidation experiments where after each cycle of complete removal of the dye, 387

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Fig. 1. In comparison with GAC (Fig. 1a), the prepared FeCit-GAC, HqFe-GAC and HpOFe-GAC (Fig. 1b, c and d) contain predominant microporous structures and possible attachment of ferric ions. The SEM images also proves the modification of GAC where much of the macroporous structures are removed from initial condition and are more exposed in the outer surface layer. It is possible indication of improvement in stable attachment/ adherence of iron ligand complexes in microporous sites after modification. Further, SEM photographs (Fig. 1a to d) were taken for detailed EDS microanalysis, and the elements detected from each sample are shown in Fig. S1. EDS reveals a new peak of iron (Fe) in all prepared catalysts (Figs. S1b, 2c and d), which is absent in raw GAC (Fig. S1a). Also, iron content was observed to be higher in FeCit-GAC than HqFe-GAC and HpOFe-GAC.

(Saratale et al., 2015). In addition, catalytic stability of HpOFe-GAC for textile effluent was also evaluated through repetitive experiments of dye wastewater removal using the same catalyst. 2.11. Phytotoxicity studies of AR 88, textile effluent and their degraded metabolites after treatment The phytotoxicity of AR 88, actual textile effluent, and their degraded metabolites after catalytic degradation was executed to find out their toxic effects on usual agricultural crop (Phaseolus mungo) using the procedure reported earlier (Saratale et al., 2016). The phytotoxicity experiments were carried out at ambient room temperature (~28 °C). Phytotoxicity effect was measured by quantifying the germination rate, plumule and radicle length of seedling growth of the plants after 7 days of incubation. All the experiments were conducted in triplicates. The Tukey-Kramer multiple comparison test (one-way analysis of variance) was used to statistically analyze the data using the Graph Pad Prism version 5.0 software for Windows (USA) (Saratale et al., 2016). The values obtained after taking triplicates with mean ± SD were only considered significant when P was ≤ 0.05.

3.2. Treatment efficiency of prepared catalysts In this study, an attempt was made to investigate the catalytic activity of all prepared catalysts, by choosing Acid Red 88 (AR 88) as the target dye for catalytic oxidation and its removal. AR 88 which is an anionic dye (CAS Number 1658-56-6; Molecular formula: C20H13NaN2O4S; molecular weight; 400.38). It is mainly composed of an azo (N = N) linkage, with two naphthalene rings. Ten treatment processes (GAC, H2O2, GAC+H2O2, HpOFe-GAC, HqFe-GAC, FeCitGAC, Fe-GAC+H2O2, HpOFe-GAC+H2O2, HqFe-GAC+H2O2 and FeCit-GAC+H2O2) were conducted through comparative control experiments to assess their color removal efficacy. The removal

3. Results and discussion 3.1. Characterization of constructed catalysts SEM micrographs of GAC and prepared catalysts were presented in

Fig. 1. FESEM images of samples: (a) GAC (b) FeCit-GAC (c) HqFe-GAC; (d) HpOFe-GAC at 3000 x magnification. 388

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3.3. Screening and optimization of various parameters for three prepared catalysts Thirty sets of experimental results showed more than 40% of AR88 dye removal in all experimental conditions. HpOFe-GAC showed highest dye removal rate (99.8%) in pH 7, at 50 °C, 5 g/L catalyst, 50 μM dye concentration and 40 mM H2O2. Maximum dye removal rate for HqFe-GAC was observed as 88.1% in pH7, at 50 °C, 5 g/L catalyst, 50 μM dye concentration and 40 mM H2O2. Whereas FeCit-GAC showed the significant color removal rate (92.1%) in pH 7, 50 °C, 5 g/L catalyst, 50 μM dye concentration and 40 mM H2O2. The overall trend observed in the study that with the increase of temperature and catalyst dosage, removal efficiency increases whereas increase in dye concentration reduces the efficiency. Regarding pH, neutral pH conditions showed good removal rate in all catalysts. In current study, fluctuations in removal rate was observed with a pH change for all three catalysts, but the efficiency did not reduce drastically as compared with the removal rate at lower pH values. It confers that these three constructed catalysts would yield significant removal efficiency at neutral pH. Several other studies also suggest that heterogeneous catalysts do not give the highest removal efficiency at higher pH values (pH 9–11), but the efficiency was higher than that in conventional Fenton reaction (Yao et al., 2013, 2014; Sun et al., 2014). Based on the CCD design study, AR 88 removal by HpOFe-GAC is expressed through a second order polynomial equation. The empirical relationship between the variables and experimentally found response is calculated. In contrast to HpOFe-GAC, linear equation was found to be best fit for both HqFe-GAC and FeCit-GAC. The best fit equations for each catalyst are given below. (i) Removal rate of HpOFe-GAC = +64.46520 – 5.06041X1 – 1.04154X2 + 2.97063X3 – 0.688204X4 – 4.21250E-003X1X2 + 0.045875X1X3 – 3.65312E-003X1X4 + 0.089419X2X3 + 3.80500E-003X2X4 −5.56250E-003X3X4 + 0.28628X12 2 2 + 8.18375E-003X2 −0.2381 X3 + 5.52344E-003X42 (ii) Removal rate of HqFe-GAC = +52.96980 – 3.90529X1 + 0.20312X2 + 3.78475X3 + 0.10750X4 (iii) Removal rate of FeCit-GAC = +58.44589 – 1.35114X1 + 0.12791X2 + 1.82091X3 – 0.10689X4

Fig. 2. Concentration changes of AR 88 under various conditions keeping constant [catalysts] = 5 g/L, [AR 88] = 50 μM; [H2O2] = 40 mM; [pH] = 7; [Temperature = 50 °C conditions.

efficiencies of the GAC, HpOFe-GAC, HqFe-GAC and FeCit-GAC processes were approximately 50.2%, 62.1%, 55.1% and 58.2%, respectively (Fig. 2a). The obtained dye removal rates found considerably greater than activated carbon-FeOOH heterogenous Fenton-like catalyst which showed only 21% color removal of Reactive Brilliant Orange without using H2O2 in 2 h of incubation (Wu et al., 2013). The color removal rate of the H2O2, GAC+H2O2, FeGAC+H2O2, HpOFeGAC+H2O2, HqFe-GAC+H2O2, and FeCit-GAC+H2O2 processes were approximately 33.5%, 60.2%, 71.0%, 99.8%, 88.1% and 92.1%, respectively (Fig. 2b). Higher color removal in the presence of H2O2 indicates a positive catalytic role of GAC and Fe-GAC. H2O2 alone is not an effective process for the treatment of AR88 however Fe-GAC+H2O2 increased the AR 88 removal efficacy. The maximum removal efficiency of HpOFe-GAC+H2O2 (99.88%) was comparatively higher than FeGAC+H2O2 (71.0%) and H2O2 (33.5%) processes in 60 min was observed (Fig. 2b). Whereas, HqFe-GAC+H2O2, and FeCit-GAC+H2O2 showed lower color removal efficiency (88.1% and 92.1%) compared to HpOFe-GAC+H2O2. The highest removal efficiency of HpOFeGAC+H2O2 process was possibly due to the catalytic reactions that arise between coated iron oxides and H2O2 by which HpOFe-GAC could improve the oxidative ability of H2O2 for the removal of AR 88. This higher rate of removal confers that iron complex being attached on the surface of activated carbon retards or constrains the ferrous or ferric hydroxide precipitation (Xu et al., 2009). These results also suggest that ferric ion ligands with GAC were catalytically efficient in H2O2 activation to remove AR 88 and comparatively higher synergistic effect in terms of catalytic activity between HpOFe and GAC than that of other two ligands was noticed.

where, X1, X2, X3 and X4 are pH, temperature, catalyst dosage and initial dye concentration, respectively. The determination coefficient and residuals of the ANOVA was applied as criterion to check the statistical adequacy of the model (Shi et al., 2014). The results are given in Table S4, S5(a) and (b) for different catalysts. According to judgement based on the F test which explains the relationship between the mean square of the model and the residual error (Ghaedi et al., 2016). The F value and p value for the quadratic model of HpOFe-GAC are 27.83 and < 0.0001 respectively, suggesting the model is significant (Table S4). Similarly, linear model of HqFe-GAC provide 5.50 F value and p value about 0.0026 (Table S5(a)) and for FeCit-GAC it has 4.3 F value and 0.0088 p value (Table S5(b)). The above results indicates that these models are also significant. Many values in all three models are found to be insignificant at the 95% confidence level, therefore, only significant values were included in the ANOVA table of HpOFe-GAC (Table S4). By analyzing the monomical coefficient values in each model, pH, temperature and catalyst dosage were identified to be significant for HpOFe-GAC. At the same time, pH and catalyst dosage seem to be significant factors for both HqFe-GAC and FeCit-GAC. The combined effect of any parameters were not significant for any models indicating that each parameter independantly influence on dye removal rate. Therefore, changes in pH, catalyst dosage would highly change the dye removal rate for three catalyst systems while temperature also influences the dye removal rate in the HpOFe-GAC system. Neutral pH condition was required for the purpose of this study. Therefore, pH 7, AR 88 concentration, 50 μM; catalyst 389

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Fig. 3. Sustaining catalytic stability of catalysts ((a) HpOFe-GAC; (b) HqFe-GAC and (c) FeCit-GAC) for the decomposition of AR 88. Conditions: [catalyst] = 5 g/L), [H2O2] = 40 mM (addition of AR 88 = 50 μM/run, H2O2 = 50 mM/ run, pH = 7.0, Temperature = 50 °C), and (d) Regenerative stability of different catalysts for the decomposition of AR 88. Conditions: [Catalyst] = 5 g/L, [AR 88] = 50 μM; [H2O2] = 40 mM; [pH] = 7; [Temperature] = 50 °C.

Hydroxypyridine-N-oxide and 8-Hydroxyquinoline could have played a major role in producing high removal rate in comparison to ferric citrate, because π-π conjugate interaction of benzene rings exchanges free electron between pollutants (Nagashima and Takatsuka, 2012). Both HpO and Hq have ability to form tautomeric equilibrium that leads towards possible co-ligand complexes could also have improved the catalytic nature (Hendrickson and Pierpont, 2004). These findings indicate that the catalyst utilizing hydroxamic iron ligand remains more efficient, stabilized and sustained performance for the catalytic oxidation of AR 88 in repetitive dye removal with negligible reduction in removal rate. Regeneration of the catalyst is an important issue for the practical application of catalyst at industrial level. Heterogenous Fenton catalyst (Fe-GAC+H2O2) and heterogeneous Fenton-like catalysts (HpOFeGAC+H2O2, HqFe-GAC+H2O2 and FeCit-GAC+H2O2) were used as the catalyst in the reaction as per the conditions mentioned and then it was drawn out, washed thoroughly with deionized water to make the catalyst free from the residual H2O2 and then further dried in vacuum at room temperature. This process was conducted at least four times. The results were graphically explained in Fig. 3(d). In first run, all the four catalyst systems showed better color removal efficiency at pH 7. This explains that all four catalysts break the pH limitation (pH 2.5–3.5) in Fenton technology. Wu, (2013) also confirmed in their study using FeOOH-activated carbon catalyst that heterogeneous system operates efficiently at neutral pH conditions. As expressed in Fig. 3(d), both the HpOFe-GAC and HqFe-GAC managed to maintain the removal rate more than 95% and 90%, respectively. (HpOFe-GAC from 99.8% to 98.2% and HqFe-GAC 88.2–84.5%). This explains that aromatic iron ligands form firm bonding with functional groups from the surface layer

dosage, 5 g/L; temperature at 50 °C and 40 mM H2O2 were selected as optimized conditions for the further experiments (as seen in Fig. 2a and b).

3.4. Sustaining catalytic ability and regenerative capacity Although heterogeneous catalyst is more advantageous compared to the conventional Fenton process, but possess certain limitations in practical application of wastewater treatment such as moderate activity but low stability of the catalysts used in the process (Yang et al., 2009). To investigate the sustaining catalytic abilities of three iron ligands catalysts system were investigated through several continuous oxidation processes (up to tenth cycle) to catalyze AR 88 by adding the same amount of H2O2 and AR 88 after each cycle of dye removal. Their performances are presented in Fig. 3a, b and c. In this experiment, in case of HpOFe-GAC+H2O2, it was found that 99.8% of the dye was removed in first cycle whereas 99.2% removal rate could be perceived when dye added for tenth repeated consecutive cycles however the time requisite for the color removal was increased from 60 min to 650 min (Fig. 3a). Similar results were observed in case of Aminopyridine-ferric@ACF catalyst system which showed Acid Red 1 dye removal about 98.3% up to fifteenth cycle (Yao et al., 2014). However HqFeGAC+H2O2 removed 88.1% and 72.0% of dyes in first and 10th runs (Fig. 3b), respectively and in case of FeCit-GAC+H2O2 processes, the dye had removal rate of 92.1% in first run that reduced to 55.6% in the tenth run (Fig. 3c). A significant reduction in the removal rate of FeCitGAC+H2O2 might be because citric acid tends to compete more for hydroxyl radicals and get degraded thus redox cycling by ferric citrate complex reduces (Adam et al., 2015). Aromatic nature of 2390

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reactions to form ferrous ions that are dissolved out of the constructed system into aqueous media (Gallard et al., 1999). With the increase of pH, peroxo-complex amount reduces resulting in less iron dissolution. HpOFe-GAC+H2O2, HqFe-GAC+H2O2 yield less than 2 ppm of iron content in the range of 0.006–1.0 mg/dm3, (0.13–2.4%) in aqueous medium at all pH values (2 ppm is the maximum allowed amount of iron concentration by European Standard) (Demir, 2013). In contrast, the concentration of iron leached from FeCit-GAC catalyst is more than 30 ppm in all pH conditions: proving the inefficiency of the catalyst. These results was in accordance with the results obtained in catalytic stability and regenerative capacity experiments. Higher level of iron contents in aqueous medium proves the tendency of citric acid to compete more for hydroxyl radicals than other ligands. Also, the dissolved iron amount is fluctuating in Fe-citrate system; increasing up to pH 7 followed by a reduction at pH 9 which then showed an increase at pH 11. Citric acid forms various types of ligand complexations with ferric ion in different pH values that could have dissolved in different rates with increasing pH (Silva et al., 2009). Therefore, it can be concluded that FeCit-GAC+H2O2 should not be considered for vast scale application for Fenton oxidation. However, HpOFe-GAC+H2O2 and HqFe-GAC+H2O2 form very less amount of iron sludge in the aqueous medium credited by high affinity between activated carbon medium and iron complexes coupled with free electron flowing due to aromatic nature (Li et al., 2010; Nagashima and Takatsuka, 2012). A possible mechanism for this occurrence could be explained through interactions between oxygen containing species on activated carbon surface and iron species (Yao et al., 2013). The stability of the catalyst was also verified by determining the iron leaching to the solution. The small amount of iron leaching by the catalyst of HpOFe-GAC + H2O2 proves that it is comparatively stable for dye decomposition, which is constant with the above results and thus makes the HpOFe-GAC+H2O2 system more favorable to practical applications. Fig. 4. (a). The dissolved iron during AR 88 (50 μM) degradation at different pH conditions. ([Catalyst] =5 g/L; [H2O2] =40 mM; [Temperature] = 50 °C), and (b) Concentration changes of AR 88 (50 μM) in the presence of HpOFe-GAC (5 g/L) with isopropanol (300 mM) or without isopropanol and with H2O2 (40 mM) or without H2O2.

3.6. Evaluation of active species in HpOFe-GAC+H2O2 catalytic system The formation of hydroxyl radicals (.OH) are the main reactive oxygen species that oxidize organics in homogenous Fenton reaction. (Yao et al., 2013). To check the generation of hydroxyl radicals in the created heterogeneous Fenton-like catalyst system, we have studied the influence of isopropanol (IPA), a known hydroxyl radical scavenger on the removal of AR 88 using ferric iron supported by granular activated carbon (HpOFe-GAC) (Mrowetz and Selli, 2005). It is assumed that this hydroxyl radical scavenger (IPA) would consume the radicals, thereby, reduce or inhibit the oxidative dye removal. As shown in Fig. 4(b), without H2O2, about 19.3% of AR 88 was adsorbed by the HpOFe-GAC and the adsorption rate was almost similar and did not show any significant variation in the presence of isopropanol (20.3%) for HpOFeGAC with IPA in 10 min. However, in the presence of H2O2 (HpOFeGAC+H2O2 without IPA), the dye removal rate increased to about 99.8%. Moreover, the removal rate of AR 88 HpOFe-GAC/H2O2 system was suppressed or significantly reduced up to 74.2% on adding of IPA, which specifies that .OH plays an important role in the catalytic reaction. Similar results was recorded for the removal of Reactive Red M3BE by Fe@ACF with H2O2 when IPA was added (Yao et al., 2013). Also, similar effects were observed while oxidizing the same dye using 8-Hydroxyquinoline-ferric catalyst which reduced its oxidation drastically in the presences of IPA (Sun et al., 2014). Nevertheless, as per the results, IPA did not completely inhibit the oxidation. It can be because other oxidative species such as Fe (IV) species could have been produced in the system (Chanderia et al., 2013). In the heterogeneous catalyst system superoxide radical having lower oxidation stability relative to ∙OH, which was not generated and is the contrasting feature as that of the homogeneous Fenton system. This property makes the heterogeneous catalytic system to utilize H2O2 very efficiently, thereby generation of ∙OH radicals gets increased and

of GAC and maintain the stable attachment more than aliphatic iron ligands. FeCit-GAC also showed considerable reduction in removal rate (from 92.1% to 78.2%) possibly caused by higher dissolution of ferric ion complexes into aqueous system. In case of HpOFe-GAC+H2O2 after repeatedly performing the experiment four times, the sustained and higher color removal efficacy which was about 98.2% suggesting that the HpOFe-GAC+H2O2 exhibited superior catalytic activity and regenerative capacity over HqFe-GAC, FeCit-GAC and Fe-GAC after repeatedly removing AR88. These results indicated that HpOFeGAC+H2O2 not only can be used for sustainable catalytic oxidation, but also amended ability to regenerate on-site which increases its commercial applicability in practical wastewater treatment.

3.5. Iron leaching condition of the catalyst Iron leaching ability of catalysts under different pH (3, 5, 7, 9, 11) conditions was investigated in this study. The leaching iron concentration was measured by AAS. As seen in Fig. 4(a), HpOFeGAC+H2O2, HqFe-GAC+H2O2 and FeCit-GAC+H2O2 had higher iron leaching at lower pH value (pH 3) and continued to reduce towards pH 11; a similar trend was identified while removing reactive Brilliant Red X-3B dye by 8-Hydroxyquinoline-ferric catalyst (Sun et al., 2014). This trend could be explained through Fe(III) speciation (Salgodo et al., 2013). With the addition of hydrogen peroxide in to ferric catalyst system, [Fe(OH)]2+ and [Fe(OOH)]2+ peroxo-complexes are produced in highest amount at pH 3. These species go through intermediary 391

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Table 1 Pseudo first-order and second-order kinetic studies of AR88 oxidation by HpOFe-GAC at different operational parameters. Sr. No.

pH

Temperature (°C)

[AR88]0 (μM)

Catalyst (g/L)

Pseudo-first-order k1 (min

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

7 7 7 7 7 7 7 7 7 7 7 7 3 5 7 9 11

50 50 50 50 50 50 50 50 30 40 50 60 50 50 50 50 50

50 50 50 50 30 50 70 90 50 50 50 50 50 50 50 50 50

2.5 5 7.5 10 5 5 5 5 5 5 5 5 5 5 5 5 5

−1

)

0.2341 0.2735 0.2964 0.3197 0.2582 0.2735 0.2208 0.2069 0.2331 0.2498 0.2735 0.294 0.256 0.2651 0.2735 0.2541 0.2351

Pseudo-second-order R

2

0.7221 0.5378 0.4471 0.3732 0.3172 0.5378 0.6289 0.627 0.5999 0.5782 0.5378 0.49 0.6114 0.5781 0.5378 0.5648 0.5684

k2 (M-1 min-1)

R2

0.1176 0.205 0.2869 0.3993 0.3521 0.205 0.0901 0.0571 0.1131 0.145 0.205 0.2754 0.1603 0.1827 0.205 0.154 0.1159

0.9838 0.9909 0.9836 0.9869 0.9952 0.9909 0.9921 0.9931 0.9939 0.9923 0.9909 0.9907 0.9904 0.9902 0.9909 0.9925 0.9952

R2 is the coefficients of determination for the fit of adsorption and oxidation curves to kinetic models. k1 (min-1) and K2 (M-1 min-1) are the rate constants of the pseudo-first order and pseudo-second order kinetics, respectively.

(Table 1(5–8)). p-Nitroaniline removal by Fenton oxidation also yield similar results (Sun et al., 2007). It could be probably due to the constant supply of hydrogen peroxide which could only produce certain amount of reactive oxygen species. With the increase of dye concentration, higher level of intermediates could have formed which need more hydroxyl radicals for complete removal. Oxidative removal of dye at different pH values (3–11) were also evaluated through kinetic studies (Table 1 (13–17)). From pH 3–7, an increasing trend was observed in terms of reaction rate while pH 7 being the optimum point. After pH 7, the removal rate depicted by rate constants continued to reduce up to pH 11. 8-Hydroxyquinoline-ferric catalyst in a previous study also showed similar results deriving almost 100% of reactive red X-3B removal at pH 3, 5 and 7 which then reduced to 92% at pH 9 (Sun et al., 2014). Below the optimum pH value, H+ might have reacted with hydrogen peroxide to form electrophilic oxonium ions (H3O2+) resulting in less interactions between iron ions and hydrogen peroxide (Kwon et al., 1999). Above the optimal point, less availability of H+ ions leads towards the increase in H2O2 decomposition creating shortage of OOH- in the system to carry out Fe2+ regeneration (Kwon et al., 1999). Additionally, higher level of OH- ions in the system at higher pH values could have changed the functional groups and surface structural properties of GAC. All reasons together leads towards fluctuation in removal efficiency with pH. pH 7 being the optimal condition in dye oxidization implies that organic pollutants in wastewater could be mineralized by HpOFe-GAC without any additional pH adjustments. Interestingly, apart from the fluctuations in removal efficiency, almost a complete dye removal was observed in all pH conditions within one hour time period. The same trend was observed in Fe@ACF catalyst system removing remarkable amount of RR M-3BE dye in neutral or basic conditions (Yao et al., 2013). The broader pH scale could be a result of interactions between oxygen functional groups of GAC and ferric ions which hinder the formation of iron hydroxide complexes. The overall results confer the possibility of utilizing HpOFeGAC for catalytically oxidizing organic pollutants at neutral pH.

thus improved the catalytic activity in the HpOFe-GAC system. The result can be inferred that hydroxyl radicals are the predominant reactive oxygen species in the constructed heterogeneous Fenton-like system. During strong oxidation process ∙OH radicals those get formed can effectively and rapidly remove the dyes even at higher concentration.

3.7. Oxidation kinetics studies pH, temperature, AR 88 dye concentration and HpOFe-GAC catalyst dosage parameters and their effects on oxidative removal of dye were experimentally evaluated and pseudo-first-order and pseudo-secondorder kinetic models were utilized for the analysis. The rate constants (k1 and k2) and regression coefficient (R2) for the kinetic studies are listed in Table 1. It was observed that a better fit to the pseudo–secondorder model confers the adsorption rate of dye is majorly relies on the accessibility of the adsorption sites of the adsorbent instead of the amount of dye in the solution. Similar performance was observed in case of biosorption of Acid Yellow 17 dye onto rambutan (Nephelium lappaceum) peel (Njoku et al., 2014). In 60 min time, all the studies yield rate constant (k2) for pseudo-second-order kinetics with more than 0.98 regression coefficient values, which are very much higher than that of pseudo-first-order kinetics (Table 1). It indicates that pseudo-second-order kinetic model could describe the catalytic oxidation of AR88 more than pseudo-first-order kinetic model. In this studies ln (Ct/Co) versus time graph did not show a linear relationship however, the plots between (1/Ct–1/C0) and time are essentially linear; demonstrating Fenton-like catalytic oxidation of AR88 follows pseudo-secondorder kinetics (data not shown). Similarly, catalytic oxidations of catechol or 4-chlorocatechol by nano-Fe3O4 also expressed pseudo-second order kinetics (He et al., 2014). With the increase of catalyst dosage, the reaction rate has also been increased (Table 1(1–4)), expressing similar relationship with previous study which oxidized RR M-3BE dye using 8-Hydroxyquinoline-ferric catalyst (Sun et al., 2014). Similar to the catalyst dosage, increasing trend of reaction rate was observed with the increase of temperature (Table 1(9–12)). Temperature improves collision rate of molecules that increase the proximity of hydrogen peroxide and catalyst resulting in more hydroxyl radical formation (Sun et al., 2007). Also, increased temperature could have provided required activation energy to the system to proceed through oxidation (Xu et al., 2009). However, the rate constants significantly reduced with increasing dye concentration

3.8. Thermodynamic analysis Activation energy for the removal of AR88 using HpOFe-GAC was calculated from Arrhenius equation.

ln K2 = −

Ea 1 + ln A R T

(4)

where Ea, R, T and A refer to activation energy, universal gas constant, 392

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GAC+H2O2 exhibits higher efficiency in terms of pH tolerance and nonselectivity in dye removal which is novel and increases its practical applicability.

absolute temperature and pre-exponential factor, respectively. The activation enthalpy and entropy are calculated through the linear form of Erying equation.

ln

ΔS ≠ ΔH ≠ 1 K2 K = ln B + − T h R R T

3.10. Industrial wastewater treatment and catalytic stability of the prepared catalysts

(5)

where KB , h, ΔS ≠ and ΔH ≠ are Boltzmann constant, Planck constant, activation entropy and activation enthalpy, respectively. Linear regressions for equations were drawn and utilized to calculate Ea, ΔH‡ and ΔS‡, and the values were 25.29 kJ mol-1, 22.64 kJ mol-1 and −188.69 Jmol-1 K-1, respectively. Positive value of ∆H ≠ indicates an endothermic nature of the system (Hashemian and Mirshamsi, 2012). Activation energy value in this current study is comparatively lower than the previous studies of Fenton oxidation. For example, hydroxyl-iron pillered bentonite catalyst needed 31 kJ mol-1 of activation energy to degrade azo dye through UV-Fenton oxidation (Chen and Zhu, 2007). Also, 32.8 kJ mol-1 of activation energy was needed for granular size goethite (R-FeOOH) particles to carry out Fenton reaction (Lin and Gurol, 1998). Therefore, HpOFe-GAC catalyst could be easily activated with less energy requirement. This finding indicates that the constructed catalyst could be utilized in practical application where organic pollutants are efficiently and cost-effectively removed. Many previous studies that tried to expand the pH range of Fenton system had used additional energy applications or longer time requirement for the complete mineralization of dye (Yang et al., 2009; Zhao et al., 2012; Cheng et al., 2006; Flores et al., 2008; Hadjltaief et al., 2013).

Dye containing industrial wastewater was collected for the evaluation of the applicability of constructed catalyst to be used in real time wastewater treatment. The initial color of used dye wastewater in terms of ADMI value is 1024 ± 3.5. The evaluation was conducted as comparative study between HpOFe-GAC, HqFe-GAC and FeCit-GAC under the optimized conditions. HpOFe-GAC showed about 95.4% color removal of dye wastewater (in terms of ADMI value) whereas moderate color removal by HqFe-GAC was 80.8% and in case of FeCit-GAC it was only 49.6% at 50 °C. Thus, both the HpOFe-GAC and HqFe-GAC showed more than 80% of dye wastewater color removal, and HpOFe-GAC has comparatively higher removal efficiency. Thus, prepared hydroxamic acid mediated catalyst displayed significant performance as compared to both non-hydroxamic iron ligands. In a previous study, photo Fenton-like oxidation of azo dye production wastewater removed 100% of the color in 30 min time (Arslan-Alaton et al., 2009). However, with no additional energy requirement, HpOFe-GAC managed to remove more than 95% of color showing its superiority over other catalytic systems that required additional energy. Soares et al. (2015) tried to utilize ferric-oxalate, ferric-citric and ferric-EDDS ligands as solar photo Fenton catalysts to treat synthetic acrylic textile dye wastewater. Ferricoxalate system yield 83.3% of dissolved organic carbon (DOC) removal at pH 2.8 and 30 °C while citric acid and EDDS showed 75% and 66.67% of DOC removal rate, respectively. Comparing these results with HpOFe-GAC, the constructed catalyst showed higher color removal rate which is noteworthy. In addition, sustaining catalytic stability of HpOFe-GAC in treating industrial wastewater was investigated through 5 cycles of continuous oxidation by adding same amount of H2O2 into the system. The catalyst could be repeatedly used and the system had removal rate efficiency of 95.4% in first run that reduced to only 78.2% after fifth cycle. A similar catalytic stability was observed in a study that used Iron Zeolite Socony Mobil-5 catalyst to completely remove a mixture of methyl orange and Reactive Black 5 dyes in three repetitive cyclic Fenton oxidations (Ahmad et al., 2015). This revealed the higher level of efficiency of HpOFe-GAC catalyst that can be industrially used in vast scale with considerable cost-effectiveness.

3.9. Oxidative removal of other organic dyes Efficiency of oxidative removal of AR88 by hydroxamic iron ligand was investigated in terms of its stability and regenerative ability at neutral pH. For the vast scale of industrial application in real time organic pollutant removal, hydroxamic iron ligand catalyst should be able to remove all types of dyes, regardless of their acidic, basic and reactive nature. Therefore, this section investigated the removal rate of Rhodamine B (RB), Reactive Black 5 (RB5), Basic Yellow 2 (BY2), Congo Red (CR) and Methylene Blue (MB) in the presence of HpOFeGAC+H2O2 system at neutral pH. Table S6 shows the percentage ratio of final and initial dye concentration (C/C0%) and the removal rate (%). When HpOFe-GAC and H2O2 added together at 50 °C, highly efficient removal of all dyes were observed. Interestingly, more than 90% of all dyes were removed in 60 min. It is an indication of non-selective catalytic oxidation process of the designed HpOFe-GAC+H2O2 catalyst which is more aval of AR88 using HpOFe-GAC was cdvantageous to its real applications for the removal of different types of dyes such as; acid dyes, reactive dyes, basic dyes and so on. Selected literature studies using different heterogeneous Fenton catalyst based on various supports for the removal of dyes and its comparative performance with our studies are summarized in Table 2. In comparison with those studies, HpOFe-

3.11. Phytotoxicity studies There are some reports where it was reported that the toxicity of degraded products found highly toxic in nature compared to the parent dye compound (Saratale et al., 2011). The phytotoxicity study of the AR88, dye wastewater, and their degraded metabolites after treatment

Table 2 Comparison of decolorization of dyes by heterogeneous Fenton or Fenton-like oxidation that utilized various supports. Dye

Catalyst

Conditions

Results

Reference

Rhodamine B (80 μM) Rhodamine B (20 μM) Reactive Black 5 (61 μM) Congo Red (230 mg/ L) Methylene Blue (100 mg/L) Acid Red 88 (50 μM)

Iron-modified rectorite

[Cat.] = 0.4 g/L, [H2O2] = 6 mM, pH 3, Temperature = 25 °C under visible light or sunlight [Cat.] = 0.1 g/L, [H2O2] = 2 mM, visible light irradiation at room temperature and 2 h [Cat.] = 0.1 g/L, [H2O2] = 4.56 mM pH 2.8

Removal under visible light was 90% and sunlight was 99% in 1 h Dye removal rates at pH 3, 7 and 11 were 90%, 92% and 35% COD removal rate was 80% in 2 h time period Complete mineralization in one hour time Removal rate constant (kobs) was 0.00215 min−1 Dye removal was 99.8%

Zhao et al. (2012)

Iron(II) bipyridine complex-clay Iron (III)-fly ash

Titanomagnetite

[Cat.] = 0.3 g/L, [H2O2] = 5.88 mM pH 2.5 – 3 UV/light irradiation [Cat.] = 1 g/L, [H2O2] = 300 mM pH 7; 24 h

Iron(III)- 2-Hydroxypyridine-N-oxide on GAC support (HpOFe-GAC)

[Cat.] = 5 g/L, [H2O2] = 40 mM pH 7; Temperature = 50 oC, 60 min

Iron-pillered Tunisian clay

393

Cheng et al. (2006) Flores et al. (2008) Hadjltaief et al. (2013) Yang et al. (2009) This study

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References

Table 3 Phytotoxicity studies of AR 88, actual dye wastewater, and its degraded metabolites formed after catalytic degradation on Phaseolus mungo. Parameter

Germination (%) Plumule (cm) Radical (cm)

Control

AR 88

D/W 100 11.81 ± 0.88 3.15 ± 0.25

AR 88a 70* 6.45 ± 0.34* 1.12 ± 0.31 *

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Dye wastewater EMa 100 10.25 ± 0.65 2.85 ± 0.32 *

Effluenta 50* 5.12 ± 0.32* 1.25 ± 0.26*

EMa 90 9.65 ± 0.88 2.65 ± 0.41

AR 88: Acid Red 88; D/W: distilled water; EM: extracted products of dye and dye wastewater after catalytic degradation. Values are mean of three replicates, followed by standard errors. Asterisks indicated significant differences from the control (seeds germinated in distilled water) at *P < 0.05 (ANOVA followed by Tukey's HSD test). a 400 ppm concentration.

are depicted in Table 3. Phytotoxicity carried out using P. mungo demonstrated the degradation products of AR 88 and textile effluent showed no inhibition in the seed germination, plumule length and radicle length which is almost similar with the seeds treated with distilled water (Table 3). However, the germination rate, plumule and radicle length of P. mungo were drastically affected when treated with AR 88 and textile wastewater relative to its degraded products. The foregoing results suggest that Fenton oxidations using HpOFe-GAC catalyst for degradation of dye containing effluent is safe, ecofriendly and inexpensive increasing the importance of this technology in real applications.

4. Conclusions The current study investigated a novel heterogeneous Fenton-like catalyst (HpOFe-GAC) of iron-hydroxamate ligand on to granular activated carbons to amend the pH limitation and other disadvantages of conventional Fenton technology. In order to experimentally analyze the stability of hydroxamic acid-iron ligand, two non-hydroxamic iron ligands (aliphatic and aromatic) were also constructed using 8Hydroxyquinoline and citric acid. Interestingly, in comparison to HqFeGAC and FeCit-GAC, HpOFe-GAC exhibits higher level of catalytic stability, regenerative capacity with excellent pH tolerance and insignificant iron leaching. The results from the kinetic studies of Iron-2Hydroxypyridine-N-oxide showed that the oxidation follows pseudosecond-order kinetics. Thermodynamic studies proved the spontaneous nature of the oxidation in which the oxidation process could be carried out easily with less activation energy. We have also investigated the applicability of HpOFe-GAC and its stability for the oxidation of industrial dye wastewater. The catalyst was found to be more excellent in removing mixture of textile dyes relative to other two prepared catalysts. Lastly, we have found that after treatment dye and textile effluent degraded products are less phytotoxic in nature which proves that the developed method is ecofriendly in nature and increases its practical application for dye removal from textile wastewater.

Acknowledgements Author GDS and RGS are thankful and acknowledge Dongguk University-Seoul, South Korea under research fund 2018–2020.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.ecoenv.2018.10.042.

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