A,novel,Fe/HNT,visible,light-driven,heterogeneous,photocatalyst:Development,as,a,semiconductor,and,photocatalytic,application

Gizem Basaran Dindas ,Derya Y.Koseoglu-Imer ,Huseyin Cengiz Yatmaz＊

a Gebze Technical University,Environmental Engineering Department,41400,Gebze,Kocaeli,Turkey

b stanbul Technical University,Environmental Engineering Department,34469,Maslak,Istanbul,Turkey

Keywords:Halloysite nanoclay Fe/HNT nanoparticle Catalyst Semiconductor Photocatalytic degradation Visible light

ABSTRACT Fe/HNT (Iron/Halloysite-nanotube) heterogeneous semiconductor catalysts operating effectively under visible light were developed by using FeCl3,FeSO4 and Fe(OH)3 sludge precipitated after electro-Fenton process and named as Fe/HNT-I,Fe/HNT-II and Fe/HNT-III,respectively.Chemical configuration and particle morphology of the catalysts were characterized with XRD,SEM-EDS and UV–vis DRS.Effect of the developed Fe/HNT photocatalysts was investigated for the degradation of Reactive Orange 16 (RO16) textile dye under visible light.The photocatalytic decolorization of RO16 was 95.6%,99.3% and 96.6%,respectively.It was found that the photocatalytic performance of Fe/HNT-III catalyst under visible light was effective compared to Fe/HNT-I and Fe/HNTII.The iron ratio in the catalyst"s structure(Fe:HNT ratio 0.25,0.5 and 0.75(w/w))and pH value(4,7 and 9)in production phase were also changed to investigate the photocatalytic effect of Fe/HNT-III.An Fe:HNT ratio of 0.25 and a pH of 4 were determined as the optimum conditions for catalyst production.Optimum H2O2 dosage value was also investigated for photocatalytic oxidation process and determined to be 10 mM.Finally,the optimum conditions were further used for the degradation of Terbinafine hydrochloride (TerHCl) active drug and the treatment of wastewater from the textile and pharmaceutical industries.

The wastewater generated by various sources and released into the environment without any treatment spoils the physical,chemical and ecological structure of the receiving environment.Wastes becoming the common problem of the whole world,the increase logarithmically over time and the environmental problems caused by these wastes gain a global dimension [1].In recent years,the problems on water management and waste minimization have also caused the production of concentrated or toxic residues.Therefore,it is inevitable to treat industrial and domestic wastewater and it is extremely important to reduce the organic matter,suspended solids,nutrients and toxic substances in wastewater to the desired level through various treatment methods.In addition,the resistant organic substances (pharmaceutical compounds,personal care products,textile dyes,pesticides,hormones,etc.) are increasingly present in water resources and are not biodegradable[2].In the textile and pharmaceutical industries,the wastewater contains resistant organic components and complex structures (aromatic chemicals,etc.) that also cause toxicity and reduce the activities of microorganisms for the effective biological treatment[3–5].Therefore it is necessary to develop wastewater treatment alternatives and advanced treatment technologies in order to meet the discharge criteria and protect the environment [6,7].The advanced oxidation processes (AOP) (photocatalytic oxidation,Fenton"s chemistry and cavitation) and chemical oxidation processes (use of ozone and hydrogen peroxide) have either complete or partial potential for degradation of the toxic chemicals,biodegradable complexes,pesticides,etc.under ambient conditions[6–10].AOPs are mainly consist of reactions with hydroxyl radicals(⋅OH),the target pollutant is converted into non-toxic and biodegradable oxidation intermediates,and in some cases can be mineralized to oxidation end products(CO2and H2O)[11].⋅OH radicals react with most resistant organic and many inorganic species with high rate constants[12].Photocatalytic oxidation process is an effective way to degrade many organic pollutants and toxic substances in the presence of a light source and a semiconductor catalyst,since complete mineralization occurs under different operating conditions of catalyst and light[13].

Photocatalytic oxidation processes have been successfully performed for the treatment of textile industry effluents [14–16],pharmaceutical industry wastewater [17,18],volatile organic compounds (toluene,benzene,xylene)[19] and resistant organic substances [20] such as aniline [21],tetracycline [22],pentachlorophenol [23].Kryczyk-Poprawa et al.also investigated the degradation of TerHCl using TiO2,ZnO and TiO2/ZnO catalysts under UVA light.The percentage of TerHCl content in solutions with TiO2,ZnO and TiO2/ZnO catalysts were found approximately 30,93,65 after 24 h of exposure to UVA irradiation,respectively[51].

Natural clay minerals modified with various metals/compounds have been widely used as heterogeneous catalysts for the degradation of organic pollutants in wastewater.For instance,iron-exchanged zeolite was applied to degrade RO16[24],Bi2O3–ZnO/Bentonite clay composite catalyst was used for the photocatalytic oxidation of Congo red (CR)[25],halloysite and sepiolite–TiO2nanocomposites was examined in the decomposition of Rhodamine-B dye,tetracycline and paracetamol antibiotics[26],kaolinite loaded TiO2was applied to degrade 4-nitrophenol[27],acid orange 7 [28],halloysite loaded TiO2was also used for degradation of Rodamine B [29],CdS/Halloysite nanoparticles was applied for photocatalytic decomposition of Tetracycline[22].However,photocatalytic processes face common problems such as energy efficiency using natural light sources and the use of additional chemicals to develop the catalysts.The development of innovative and environmentally friendly solutions for photocatalytic oxidation processes are required for the widespread use and treatment of wastewater.

The chemical formula of HNT is Al2Si2O5(OH)4.nH2O and it contains approximately 40% Al2O3in its structure [30].The lengths of HNT mineral vary between 0.2 and 5 μm,while the inner and outer diameters are between 10-30 nm and 40–70 nm,respectively[31].The loads on the inner and outer surfaces of the HNT vary depending on the pH of the aqueous solution in which it is found.Veerabadran et al.(2007)changed the pH value of solution between 3 and 8 and showed that the outer surface of the HNT was positively charged at pH 3.0,while the inner surface was negatively charged.It was also determined that with increasing pH,the positive charge of inner surface decreased,the negative charge of the outer surface increased and at pH 8.0,the charge of the inner surface charge was neutral and the negative charge of the outer surface increased [32].The modification of both surfaces (inner and outer surface)of HNT is a strategy to adjust and develop the properties of nanotube[31].Huo et al.(2014)investigated photocatalytic activities of TiO2/HNTs and Metal/TiO2/HNTs for decomposing of antibiotics(tetracycline,ciprofloxacin,chloromycetin and rifamycin).It was found that Fe3+was the most effective metal tested (Fe3+/TiO2/HNTs) [33].Zheng et al.(2016)examined the photocatalytic effects of TiO2/HNT and TiO2–Fe2O3/HNT nanocomposite heterogeneous catalysts to remove of Methylene Blue (MB)dye under UV-A light.At the end of the 12-h process period,the MB removal efficiency with HNT-TiO2and HNT-TiO2--Fe2O3nanocomposite catalysts were found to be 80% and 100%,respectively [16].G′omez et al.(2014) investigated photocatalytic activities of halloysite/Au nanoparticles in decomposing methyl orange(MO).The results showed that HNTs were suitable and stable as catalyst supports in the decomposition of methyl orange[34].The photocatalytic effect of composite heterogeneous catalysts developed with HNT alone under daylight was not investigated and extra pure chemicals were used for catalyst production and zero waste was not aimed.

In this context,a heterogeneous composite catalyst operating under visible light was developed by combining iron and HNT to investigate their photocatalytic effect with the utilization of recycling raw material from Fe(OH)3sludge waste generated after the Fenton or electro-Fenton process.Fe/HNT composite heterogeneous catalyst operating effectively under visible light was developed using FeCl3,FeSO4and Fe(OH)3sludge precipitated after electro-Fenton process and named as Fe/HNT-I,Fe/HNT-II and Fe/HNT-III,respectively.The photocatalytic effect of the developed Fe/HNT photocatalyst was investigated for the degradation of RO16 textile dye and TerHCl active drug under visible light.Finally,the optimum catalyst production and photocatalytic oxidation conditions were further applied for treatment of textile and pharmaceutical industry wastewater.

2.1.Materials

Raw HNT mineral(with a specific surface area of 125 m2/g and a zeta potential value of -17.3 mV at neutral pH) was obtained from Esan Eczacıbas¸ı,Turkey.FeCl3⋅6H2O (Riedel-de Hain) and FeSO4⋅7H2O(Merck) were used for production Fe/HNT nanocomposites.Fe(OH)3sludge produced after EF (electro-Fenton) process of treating pharmaceutical industry wastewater [17] was also used for production ofFe/HNT nanocomposites.RO16 textile dye used for photocatalytic oxidation was obtained from Sigma Aldrich.TiO2(Degussa P25)was also obtained from Evonik.TerHCl [(E)-N-(6,6-dimethyl-2-hepten-ynyl)-Nmethyl-1-naphthalenementhanamin hydrochloride] drug active substance widely used for treatment of dermatomycoses in human [35,36] and broadly available in pharmaceutical industry wastewater were also supplied from a company in Gebze,TURKEY.General properties of RO16 and TerHCl were given in Table 1.Acetonitrile,Triethylamine(>97%)and H3PO4(85%)were obtained from Sigma Aldrich for analysis of TerHCl by using HPLC(High Performance Liquid Chromatography).

Table 1 Properties of RO16 and TerHCl used in the photocatalytic experiments.

Table 2 Characterization of industrial wastewater samples.

Textile industry wastewater was obtained from the textile dyeing company in Bursa,TURKEY.The wastewater characterizations were performed according to Standard Methods[37]and Table 2 summarizes the results of wastewater samples.BOD5value of textile wastewater sample was not measured due to salinity.However,it was found in the literature that high salinity of wastewater has a significant toxic effect on microorganisms and/or causes operational problems [38,39].BOD5value of pharmaceutical wastewater sample was measured and BOD5/-COD ratio was also calculated as 0.25.Based on obtained values,since the BOD5/COD ratio was below 0.5,the biological degradability of the wastewater was low and biological treatment technique could not be used for pharmaceutical wastewater sample [40,41].H2O2(35% w/w)was used as the oxidizing reagent and pH values were adjusted with 4 N H2SO4and 2 N NaOH in the experiments and reactions.Deionized water was also used throughout the present works and analyses.

Table 3 Zeta potential charges of the HNT and Fe(OH)3 solution at different pH value.

2.2.Preparation of Fe/HNT heterogeneous catalysts

The Fe/HNT nanocomposites were synthesized using the sol-gel method on the basis the article of Szczepanik et al.[42].Iron hydroxide colloid forms were obtained by using FeCl3⋅6H2O and FeSO4⋅7H2O with the method described in the article of Zhou et al.[43].FeCl3⋅6H2O or FeSO4⋅7H2O was weighed and dissolved in purified water so that the Fe:HNT ratio by weight was 0.25.This solution was slowly added into the 180 mL of boiling distilled water and continually stirred at 100°C for 1 h and then 1–2 drops of 2 N NaOH were also dropped to get a dark red solution (iron hydrolysis).The Fe(OH)3or Fe(OH)2colloids containing solution was cooled to room temperature and pH values of solutions were measured as 9.HNT powder (1 g) was added into the iron hydroxide colloid solution and the mixture was continually stirred at 65±5°C for a day.The precipitated nanoparticles were filtered and iron analysis was also performed for the filtrate.The achieved nanocomposite materials were washed several times with deionized water,dried at 100°C and then calcined at 180°C for 2 h [42].Produced heterogeneous catalysts from FeCl3⋅6H2O and FeSO4⋅7H2O with HNT mineral were named as Fe/HNT-I and Fe/HNT-II,respectively.

Within the scope of study,another nanocomposite catalyst was developed by using waste Fe(OH)3sludge generated after the EF process of pharmaceutical industry wastewater treatment [17] and HNT.First,the Fe(OH)3sludge was washed several times with deionized water and then the impurities on the sludge surface were removed with an ultrasonic bath.The purified Fe(OH)3sludge was then centrifuged and dried for 1 h at 100°C.Then,the amount of Fe(OH)3was calculated according to the Fe:HNT ratio by weight(0.25)and slowly added to the 180 mL of distilled water.The pH of 180 mL solution containing red Fe(OH)3particles was measured to be approximately 9.HNT powder was added to the Fe(OH)3colloid solution and the mixture was continuously stirred at 65±5°C for 24 h.The precipitated nanoparticles were filtered and iron analysis was also performed for the filtrate.Based on the analytical results,it was found that all the iron was immobilized on HNT.The obtained nanocomposite materials were washed several times with deionized water,dried at 100°C and then calcined at 180°C for 2 h.Produced heterogeneous catalyst from Fe(OH)3sludge with HNT mineral was named as Fe/HNT-III.

The ratio of iron in the Fe/HNT-III composite (the Fe:HNT ratio as 0.25,0.5 and 0.75 by weight)and pH(4,7 and 9)under the production conditions were evaluated to investigate the photocatalytic effect of the heterogeneous composite catalyst structures.

2.3.Characterization of Fe/HNT heterogeneous catalysts

The zeta potentials of HNT and iron solution used in the development of Fe/HNT photocatalyst were measured by using Malvern zeta-nano sizer.The resulting nanocomposite catalysts were characterized by ultraviolet diffuse reflectance spectra (UV–vis DRS,Shimadzu 2101),and the predictable band gap energies of catalysts were determined by using Tauc"s Plot method[44].Fe/HNT photocatalysts were then analyzed by X-ray diffraction (XRD,Rigaku D Max 200) with Cu-Kα radiation for two-theta (2θ) in the range of 20–70°.Morphological and elemental information of the catalysts were also obtained by the Scanning Electron Microscopy (SEM,Philips XL30SFEG) with energy-dispersive X-ray spectrometry(EDS).

2.4.Photocatalytic application of Fe/HNT heterogeneous catalysts

The photocatalytic experiments were carried out in a laboratory-scale column reactor with a capacity of 500 mL.The column reactor was previously used for the degradation of textile dye by using CoPc(-COOH)4–TiO2(Cobalt phthalocyanine-TiO2nanocomposite) catalyst[45].Six hexagonally positioned six lamps (Philips 8W-840T) were placed in the center with a UV cut-off filter (λ >400 nm) to emit only visible light wavelengths.Air was blown into the batch reactor in order to obtain homogeneous mixing and accelerate the reactions.

The photocatalyst(1 g/L)was dispersed in an aqueous solution(500 mL of 20 ppm RO16 or TerHCl solution,textile or pharmaceutical industry wastewater),and exposed to visible light irradiation under continuous stirring.Photo degradation experiments were carried out at a temperature of 25±2°C due to cooled air and at a pH of 3 to effectively ensure the adsorption process according to the zeta potential values of the catalyst and the aqueous solution medium,hence adsorption was the first step of the photocatalytic oxidation process (PcO).Before the photocatalytic degradation,the wastewater was stirred in dark condition for 30 min to confirm the adsorption-desorption equilibrium.At the end of the adsorption process,10 mM H2O2was also added and the lights were turned on for the photocatalytic process.Samples were taken from reactor at regular intervals and centrifuged (Hettich/EBA 20) at 6000 rpm for 5 min to separate the catalysts.The aqueous samples were analyzed using a UV–Vis spectrophotometer(Hach Lange,DR 6000)and a TOC analyzer (Shimadzu TOC-L).The degradation analysis of TerHCl was also followed by HPLC (Shimadzu LC-20AD) equipped with a UV detector at a wavelength of 254 nm.The RP-18 (5 μm) Hibar® RT 150–4.6 HPLC column was selected[46]and its temperature was also set at 45°C.The mobile phase contained acetonitrile: 0.02 M H3PO4+0.012 M triethylamine(52:48)and was used in isocratic flow mode.The injection volume and flow rate were 100 μL and 2.0 mL/min,respectively[36].The calibration was followed by analysis of 4 standard solutions with different concentrations of TerHCl in the range of 2–20 mg/L.The linear regression equation model was determined as y=53918x with acoefficient of determination (R2) of 0.997.

Fig.1.a) UV–vis DRS spectrums and b) Tauc"s Plot analysis of catalysts.

Fig.2.a) Structure, b) SEM image, c) XRD pattern of Fe/HNT-III catalyst and d) photo images of HNT and Fe/HNT-III.

Fig.3.Adsorption effect of HNT on RO16 color removal.

3.1.Characterization of Fe/HNT heterogeneous catalysts

Pure TiO2nanoparticles demonstrated good absorption performance only in the UV region(λ<400 nm)[47],while the absorption of Fe/HNT heterogeneous catalysts extended from the UV region to the visible region(Fig.1-a).The increase in absorption strengths in the visible region for the Fe/HNT catalysts indicates that the catalysts can work under daylight.The predictable band gap energies were calculated and analyzed for pure TiO2,HNT and Fe/HNT catalysts using the Tauc"s Plot method [44].The Tauc"s curves of catalysts are also shown in Fig.1-b.The predictable direct band gap(Eg)for pure TiO2was 3.45 eV,while the Eg values were 4.05,2.12,2.05 and 1.80 eV for HNT,Fe/HNT-I,Fe/HNT-II and Fe/HNT-III catalysts,respectively.According to the results,the catalysts also obtained semiconductor strains that work effectively under daylight and were developed with the modification of HNT mineral.It was found that the spectrum distribution of Fe/HNT-III catalyst prepared under acidic conditions had wider wavelength range and higher absorbance values according to the results of UV–vis DRS analysis(Fig.1-a).

The structure and SEM image of Fe/HNT-III catalyst are present in Fig.2-a and Fig.2-b,respectively.The Fe/HNT catalyst consisted of the plat-like tubes organized in various dimensions and directions.SEM image of the developed Fe/HNT catalyst confirmed the successful development of Fe(OH)3nanoparticles attached to the surface of HNT.These particles were comparatively rather small,and distributed along the HNT surface.According to the results of EDS analysis,this catalyst consisted of Fe,Al,Si and O elements.XRD patterns of the samples:HNT and Fe/HNT-III are shown in Fig.2-c.An increase in the intensity of iron peaks was detected in the diffraction pattern of Fe/HNT catalyst.

3.2.Adsorption effect of HNT

Adsorption experiments were carried out in erlenmeyer flask containing 0.025 g,0.05 g or 0.1 g of adsorbent(HNT)and 100 mL of 20 ppm RO16 dye solution.The flask was shaken in a shaker at 150 rpm for 30 min at room temperature to confirm adsorption equilibrium.Then,the samples were taken from the flask at regular intervals and centrifuged at 6000 rpm for 10 min to separate HNT.The residual dye concentration in the supernatant was analyzed using a UV–Vis spectrophotometer.According to the results,it was observed that increasing the amount of adsorbent increased the color removal efficiency and the adsorptiondesorption process reached equilibrium within 30 min (Fig.3).The amount of RO16 adsorbed at equilibrium(qe,mg/g)was also calculated using Eq.(1):

Fig.4.Photocatalytic effect of Fe/HNT heterogeneous catalysts under visible light (PcO conditions: 20 mg/L RO16 dye solution,1 g/L catalyst,pH～3,10 mM H2O2).

Fig.5.Photocatalytic effect of Fe/HNT-III and H2O2 under visible light (PcO conditions: 20 mg/L RO16 dye solution,1 g/L catalyst,pH～3,10 mM H2O2).

where: C0and Ce(mg/L) -initial and equilibrium concentrations of RO dye solution,V(L)–the volume of RO16 dye solution,m(g)–the dosage of HNT sample.Adsorption capacity of HNT was calculated as approximately 11 mg/g.

3.3.Photocatalytic effect of Fe/HNT heterogeneous catalysts on RO16 color removal

The time course of photocatalytic decolorization of RO16 (dye concentration 20 mg/L)in the presence of Fe/HNT-I,Fe/HNT-II,Fe/HNT-III and TiO2(P25)catalysts(Fig.4)was 95.6%(Fe/HNT-I),99.3%(Fe/HNTII),96.6% (Fe/HNT-III) and 19.5% (TiO2),respectively.The photocatalytic performance of Fe/HNT-III catalyst developed using waste Fe(OH)3was also found to be effective under visible light compared to Fe/HNT-I and Fe/HNT-II.The Fe/HNT-III catalyst was then selected for the rest of the experiments.

Fig.6.Effect of the H2O2 concentration on color removal by using Fe/HNT-III catalyst under visible light (PcO conditions: 20 mg/L RO16 dye solution,1 g/L catalyst,pH～3).

Fig.7.Production of Fe/HNT-III with different pH and effect on color removal under visible light(PcO conditions: 20 mg/L RO16 dye solution,1 g/L catalyst,pH～3,10 mM H2O2).

Photocatalytic reactions take place in the presence of main reactants i.e.light source,catalyst,and oxygen.Whereas H2O2enhances the oxidation potential.The experiments were also performed to determine the H2O2effect in the presence of catalyst with the experimental conditions:20 mg/L RO16 initial dye concentration,pH 3,1.0 g/L Fe/HNT-III and 10 mM H2O2.While the effect of H2O2presence as alone on the removal efficiency was negligible,the Fe/HNT-III catalyst alone was found to indicate a semiconductor effect and showed competitive color removal efficiency(Fig.5).On the other hand,the combined use of Fe/HNT-III catalyst with H2O2together greatly increased the removal efficiency in the treatment process.With the combination of Fe/HNT-III and H2O2,both PcO and heterogeneous Fenton-like reactions occurred simultaneously in the reaction.While the catalyst surface acted as a semiconductor and produced radicals that confirmed the degradation of the resistant organic materials,Fe3+on the Fe/HNT-III surface was also reduced to Fe2+under visible light.Then the formed Fe2+accelerated the breakdown of H2O2in solution,generating highly oxidative additional OH·radicals.These OH·radicals also attacked resistant organic matter,and eventually the intermediate products were mineralized to CO2and H2O.Similar processes were confirmed for the decolorization of Remazol Brilliant Orange 3R with Fe(III)loaded on zeolit and Acid Violet 7 with Fe(III) loaded on Al2O3[24,48].

3.4.Photocatalytic effect of H2O2 dosage on RO16 color removal with Fe/HNT catalyst

The significance of H2O2dosage on RO16 color removal was investigated under the following PcO conditions:20 mg/L dye solution,pH 3 and 1.0 g/L Fe/HNT-III catalyst.The effect of H2O2concentration on removal efficiency increased up to 50 mM(Fig.6).The improvement incolor removal efficiency by addition of H2O2was due to the increase in⋅OH radicals.However,the higher H2O2concentration after 10 mM started to negatively affect the removal process.This is because higher H2O2dosage acts as a ⋅OH radical quencher and thus decreases the ⋅OH radical concentration[24,49].The results showed that the optimal H2O2concentration was 10 mM and thereon selected as optimum amount.

Fig.8.EDS analysis results of Fe/HNT-III catalyst developed under a) pH:4, b)pH:7 and c) pH:9 conditions.

3.5.Production of Fe/HNT-III with different pH and photocatalytic effect on RO16 color removal

The pH values(4,7 and 9)in the Fe/HNT-III production were used to investigate the photocatalytic effect of the composite heterogeneous catalysts using Fe(OH)3sludge precipitated after the electro-Fenton process.The importance of pH in catalyst production was also investigated for RO16 removal using the following PcO conditions:20 mg/L dye solution,pH 3,10 mM H2O2and 1.0 g/L Fe/HNT-III catalyst(Fig.7).

Fig.9.Effect of iron ratio in Fe/HNT-III structure on color removal under visible light(PcO conditions:20 mg/L RO16 dye solution,1 g/L catalyst,pH～3,10 mM H2O2).

The color removal results supported the UV–vis DRS analysis results(inset Fig.7),and it was observed that the optimum pH value for catalyst production was 4.The zeta potential charges of the HNT and Fe(OH)3solution were measured as negative under basic pH conditions,and the results are given in Table 3.

EDS analysis results of Fe/HNT-III catalyst developed under pH 4,7 and 9 conditions are shown in Fig.8,and the weight percentage iron content was measured as 24.6,22.1,18.8,respectively.According to the pC-pH diagram for iron ions,the ideal conditions occur at pH 4 that the Fe3+ions present from HNT/Fe3+complexes and Fe(OH)3solid particles fill the cavities of HNT mineral in the reaction solution[50].Moreover,as the pH value of the solution increases,the positive charge of the HNT inner surface decreases and the negative charge of HNT outer surface increases[32].The charge of both the inner and outer surfaces of HNT is negative at pH 9.

3.6.Amount of Fe on catalyst structure and photocatalytic effect on RO16 color removal

Fig.11.Effect of Fe/HNT-III catalyst on TOC removal for industrial wastewater under visible light (500 mL,1 g/L catalyst,pH～3,10 mM H2O2).

The iron ratio in the structure of catalyst (the Fe:HNT weight ratio:0.25,0.5 and 0.75)was changed in the production of Fe/HNT-III,which is used to study the photocatalytic effect of the composite heterogeneous catalyst under the following conditions:20 mg/L dye solution,pH 3 and 1.0 g/L Fe/HNT-III catalyst.The effect of iron ratio in Fe/HNT-III structure on RO16 removal is shown in Fig.9.The increasing amount of Fe ratio on catalyst structure increased the adsorption effect on RO16 removal rates(in 30 min).However,increasing the amount of Fe had no significant effect on the removal rates,and Fe content of 25%gave better results.Hence similar removal efficiencies of the catalysts were obtained at the end of 150 min.It was also found that the similar spectral distributions of Fe/HNT-III catalysts were obtained based on the results of UV–vis DRS analysis (insert Fig.9).

3.7.Photocatalytic effect of Fe/HNT-III heterogeneous catalyst on TerHCl removal

The photocatalytic degradation of TerHCl in the presence of Fe/HNTIII catalysts was further investigated under optimum catalyst production(pH:4,Fe:HNT=0.25) and PcO process conditions (20 mg/L drug additive solution,1 g/L catalyst,pH～3,10 mM H2O2).According to HPLC results,TerHCl was decomposed and converted into other organic structures in 150 min of process time.In both HPLC and UV spectrum analysis,it was also observed that TerHCl was degraded into by-products with time (Fig.10).TOC removal efficiency for TerHCl mineralization was also attained as 38%in the photocatalytic process at 150 min reaction time.Kryczyk-Poprawa et al.determined the identification of photocatalytic degradation products of TerHCl based on HPLC/MS/MS analyses.1-methylaminomethylnaphtalene or 1-naphthalenemethanol were also the possible products as a result of oxidative deamination of TerHCl[51].Therefore,determining the mineralization efficiency of the TerHCl pollutant is essential for environmental side.Kryczyk-Poprawa et al.also investigated the degradation of TerHCl using TiO2,ZnO and TiO2/ZnO catalysts under UVA light.The percentage of TerHCl content in solutions with TiO2,ZnO and TiO2/ZnO catalysts were found approximately 30,93,65 after 24 h of exposure to UVA irradiation,respectively[51].

Fig.10.Photocatalytic degradation of TerHCl a) HPLC chromatograms and TerHCI removal b) UV spectrums by time.

3.8.The treatment of textile and pharmaceutical industry wastewater by using Fe/HNT-III heterogeneous catalyst

The optimum values of catalyst production (pH:4,Fe:HNT=0.25)and PcO process conditions (20 mg/L drug additive solution,1 g/L catalyst,pH～3,10 mM H2O2) were further applied to the treatment of the wastewater samples from textile and pharmaceutical industries(Fig.11).The percentage of TOC removal of these samples in the presence of Fe/HNT-III catalysts was examined under visible light,and was found as 30.2%and 33.8%,respectively.

Heterogeneous semiconductor catalysts have been developed by modifying the Halloysite mineral with Fe solutions as Fe/HNT.In order to investigate the photocatalytic effect of the developed Fe/HNT catalysts by using FeCl3,FeSO4and Fe(OH)3sludge precipitated after electro-Fenton process,the degradation of RO16 textile dye has been initially studied under visible light.The photocatalytic decolorization of RO16 under visible light has been found to be 95.6%(Fe/HNT-I),99.3%(Fe/HNT-II),96.6% (Fe/HNT-III) and 19.5% (TiO2),respectively.The photocatalytic performance of Fe/HNT-III catalyst produced by recycling the EF process sludge has been found to work effectively under visible light compared to Fe/HNT-I and Fe/HNT-II.It has been also observed that Fe/HNT-III catalyst alone has a semiconductor effect (1.8 eV) and shows color removal efficiency.On the other hand,the combined use of Fe/HNT-III catalyst with H2O2together greatly increases the removal efficiency in the treatment process.However,the higher H2O2concentration after 10 mM started to negatively affect the removal process.The optimum catalyst production conditions have been also studied for photocatalysis,and found to be Fe:HNT weight ratio 0.25 and pH 4 that the Fe3+ions present from HNT/Fe3+complexes and Fe(OH)3solid particles fill the cavities of HNT mineral in the reaction solution.The optimum catalyst production(25%Fe/HNT-III,pH 4)and photocatalytic oxidation conditions (1 g/L catalyst,10 mM H2O2) have been further applied for the decomposition of TerHCl and the treatment of textile and pharmaceutical industry wastewater.The decomposition of TerHCl into byproducts over time has been observed in both HPLC and UV spectrum analysis.The TOC removal efficiency of Fe/HNT-III under visible light for mineralization of TerHCl and treatment of textile and pharmaceutical industry wastewater samples have been also carried out and were 38,30.2 and 33.8%,respectively.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors would like to thank ESAN ECZACIBAS¸I for providing HNT clay mineral to develop innovative and environmentally friendly catalysts.