See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/347555479 Carbon-mediated visible-light clay-Fe 2 O 3 -graphene oxide catalytic nanocomposite for the removal of steroid estrogens from water Article  in  Journal of Water Process Engineering · December 2020 DOI: 10.1016/j.jwpe.2020.101865 CITATION 1 READS 154 12 authors, including: Some of the authors of this publication are also working on these related projects: New photonic materials based on Cu(I) complexes, upconverting nanoparticles and quantum dots View project PhD Thesis View project Ajibola Bayode Redeemer's University 6 PUBLICATIONS   36 CITATIONS    SEE PROFILE Dayana Moscardi Santos University of São Paulo 28 PUBLICATIONS   406 CITATIONS    SEE PROFILE Dr Martins O. 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xxx-xxx Contents lists available at ScienceDirect Journal of Water Process Engineering journal homepage: http://ees.elsevier.com Carbon-mediated visible-light clay-Fe2O3–graphene oxide catalytic nanocomposite for the removal of steroid estrogens from water Ajibola A.Bayode a,b,c, Dayana M.dos Santos c, Martins O.Omorogie a,b, Olumide D.Olukanni b,d, RoshilaMoodley e, OlusolaBodede e, Foluso O.Agunbiade f, AndreasTaubert g, Andrea S.S. de Camargo h, HellmutEckert h, Eny MariaVieira c, Emmanuel I.Unuabonah a,b,⁎ a Department of Chemical Sciences, Faculty of Natural Sciences, Redeemer’s University, PMB 230, Ede, Osun State, Nigeria b African Centre of Excellence for Water and Environmental Research (ACEWATER), Redeemer’s University, PMB 230, Ede, 232101, Osun State, Nigeria c Laboratório de Química Analítica Ambiental e Ecotoxicologia (LaQuAAE), Departamento de Química e Física Molecular, Instituto de Química de Sao Carlos, Universidade de Sao Paulo, Sao Carlos, Brazil d Department of Biochemistry, Faculty of Basic Medical Sciences, Redeemer’s University, PMB 230, Ede, Osun State, Nigeria e School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban, 4000, South Africa f Department of Chemistry, University of Lagos, Lagos, Nigeria g Institute of Chemistry, University of Potsdam, D-14476, Potsdam, Germany h Sao Carlos Institute of Physics, University of Saõ Paulo, Avenida Trabalhador Saõcarlense 400, Saõ Carlos, SP, 13566-590, Brazil A R T I C L E I N F O Keywords Visible-light Photocatalyst Steroid estrogens Wastewater treatment Carbon Graphene oxide Hematite A B S T R A C T This study reports the development of an efficient photosensitive nanocomposites made from Clay, Fe2O3, and Graphene oxide (GO). These nanocomposites were used for the removal of steroid estrogens (E1, E2, E3 and EE2) from water under visible-light. The use of these photocatalytic nanocomposites lead to complete oxidation of the steroid estrogens at >80 % even under simultaneous presence of all estrogens in water. Complete mineralization was obtained for these estrogens with the range of 58–73 %. The presence of Fe-oxide in the nanocomposites increased the photocatalytic efficiency but addition of GO further improved the photocatalytic efficiency. This improved efficiency was further doubled when the nanocomposites were prepared with a carbon source (carica papaya seeds). The presence of carbon in the nanocomposite matrix was confirmed using X-ray Photoelectron Spectroscopy and Elemental Analysis. The main contributors to photocatalytic efficiency of the nanocomposites are superoxide radicals (· ) and holes (h+). Under competitive conditions, the catalysts are still active although the extent of estrogen oxidation is somewhat lower. Changes in the ionic strength did not significantly influence the efficiency of the photocatalyst. This signifies that adsorption only plays a minor role in estrogen removal from water. Toxicity tests show that the treated water is safe for human consumption and the most efficient nanocom- posite can be recycled three times without any significant loss of performance. Overall, the nanocomposite shows high potential for the effective removal of a cocktail of estrogens in raw wastewater, tap and rain water, attaining contamination levels that are within WHO safe limits. 1. Introduction Over the past decades, industrialization and urbanization have led to an increased amount of endocrine-disrupting chemicals (EDCs) released into water bodies and the environment. This poses serious environmen- tal and health challenges due to high toxicity, persistence, and estro- genicity of EDCs [1,2]. The most important groups of EDCs are the natural and synthetic hormones, pesticides, pharmaceuticals, personal care products, and some heavy metals [3,4]. Estrogens are primary female sex hormones that are responsible for the development and regulation of the female reproductive system and secondary sex character [3,5]. Of these estro- gens, estrone (E1), 17β-estradiol (E2), estriol (E3), and 17-α ethynyl estradiol (EE2) (Fig. 1) are major contributors to estrogenicity and eco- logical risk [6,7] and are therefore the major focus of this study. E1, E2, and E3 are endogenous (naturally occurring) estrogens pre- sent in women while EE2 is the synthetic derivative of estradiol (E2). It is used as an active ingredient in post-menopausal hormonal appurte- nances and oral contraceptive pills [6]. ⁎ Corresponding author at: Department of Chemical Sciences, Faculty of Natural Sciences, Redeemer’s University, PMB 230, Ede, Osun State, Nigeria. E-mail address: unuabonahe@run.edu.ng (E.I. Unuabonah) https://doi.org/10.1016/j.jwpe.2020.101865 Received 29 September 2020; Received in revised form 10 November 2020; Accepted 3 December 2020 Available online xxx 2214-7144/© 2020. UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx Fig. 1. Chemical Structures of the Steroid Hormones E1, E2, E3 and EE2. These estrogens occur in the natural environment, such as seawater [8], surface water [9], or soil, [10] and reports have linked them to breast cancer, ovarian cancer, feminization in fishes, low sperm count in adult males, endometriosis, obesity, cardiovascular endocrinology, and fibroid even at trace concentrations [11–13]. Approximately 30,000 kg of natural steroidal estrogens (E1, E2, and E3) and an additional 700 kg of synthetic estrogens (EE2) solely from birth control pills are discharged into water bodies yearly [11]. Humans and animals largely excrete these estrogens via urine and faeces as active free forms or inactive glucuronide and sulfate conjugates with animals excreting more than humans [11]. Other sources of these estrogens in the environment include effluents from the wastewater treatment plant and animal waste [14,15]. In the developing world, especially in most countries in Africa, mu- nicipal water treatment is non-existent. The population depends gener- ally on streams (surface water), groundwater, point-of-use water treat- ment systems, and packaged water as sources of drinking water. Most of these sources still provide drinking water with some levels of estrogens either because the water is polluted via transport to the underground drinking water sources or because they are not adequately treated before they are sold to consumers. Both situations are primarily caused by the high-cost of purchase and maintenance of state-of-the-art water treat- ment technology such as membrane filtration, ultrafiltration, nanofiltra- tion and reverse osmosis [16–18] which are best suited for the removal of these estrogens from water. Elimination of these highly toxic molecules, even at low concen- tration, from wastewater is challenging, especially because of their low bioavailability, persistence, and complicated structures [19]. Tradi- tional water treatment plants are not designed to remove or reduce these hormones in drinking water effectively [20]. This prompted the devel- opment of additional, more specific and more efficient techniques for their removal from water. Some of these techniques include ozonation [21], the use of activated carbon for adsorption [5], biological degra- dation [14], advanced oxidation processes (AOPs) including free radi- cal oxidation [22], electrochemical oxidation [23], sonolysis [24], and membrane technology [25]. Of these techniques, electrochemical oxi- dation is effective but slow and requires frequent replacement of elec- trodes. The membrane approach is the most efficient as well as the most expensive one. Moreover, it also requires frequent membrane re- placements due to fouling. The adsorption technique is useful but lim- ited by the molecular size of the estrogens and their very low concen- tration in environmental samples. The AOPs including photocatalysis technique, can generate reactive oxygen species that mineralize organic contaminants and their intermediates to CO2 and H2O, have been used mostly for removal of estrogen from water under ultraviolet radiation [22,26,27]. The present study describes new visible-light active photocatalytic nanocomposites generated from Kaolinite, crushed Carica papaya seeds, 3-aminopropyltriethoxysilane (APTES), hematite (Fe2O3) and graphene oxide (GO), for the effective oxidation and subsequent mineralization of the steroid hormones, estrone, 17β-estradiol, estriol, and ethinyl estra- diol. The use of Kaolinite and crushed Carica papaya seeds is moti- vated by the demand for low-cost, efficient, visible-light-driven, and sustainable photocatalysts, which function without the additional ex- penses for supplemental UV lamps and power supplies. The photocatalytic system designed in the present study contains the following components; (1) kaolinite serving as a low-cost matrix sup- port, (2) graphene oxide known to be very effective as a photocatalyst in visible light [28], (3) Fe2O3, with its ability to absorb a large portion of visible solar spectrum, good chemical stability in aqueous medium, low cost, abundance and non-toxic nature, (4) carica papaya seeds, which serve as a carbon source that assists in the creation of mesoporosity in the composite and reduce the rate of recombination of photogener- ated e-/h + pair generated by Fe2O3-Graphene oxide composite under visible light [29], and (5) APTES which serves as a source of nitrogen atoms, which broaden the composites’ absorption spectrum in the visi- ble region considering that the present study is to be conducted under laboratory fluorescent lamps as the excitation source in the photocat- alytic process. It is the aim of this study to show the synergy between these components of prepared photocatalytic nanocomposite material and to what extent they enhance photoactivity. The prepared photocatalyst has advantages such as the use of en- vironmentally friendly chemical for synthesis and synthesis ease, thus making it cost effective and easily scalable. To the authors best knowl- edge this is one of the first reports on the use of carbon-mediated clay- Fe2O3-GO visible-light catalysts for the mineralization of estro- gens in water. The photocatalytic materials developed in this study hold promise for small scale treatment of drinking water treatment, mostly needed in developing countries. 2. Materials and methods 2.1. Materials and precursors Materials and precursors used for the preparation of composite ma- terials in this study are shown in section S.1 (SI document). 2.2. Material synthesis Kaolinite clay and the biomass (crushed Carica papaya seeds) were pretreated and purified as previously reported [30]. The nomenclature, composition and ratio of components for the various materials prepared in this study are shown in Table 1. However, the details of preparation for the various composites are presented in the supporting information (S 2.1–2.5). The graphene ox- ide was synthesized in the laboratory from graphite using Hummer’s method [31]. These materials were used for preliminary study to ascer- tain the influence of each of the components and their various combi- nations on the removal efficiency of the nanocomposites for steroid es- trogens in water. Similar method described in section 2.5 was used but samples were only withdrawn and analyzed for the presence of steroid estrogen after 720 min. 2.3. Characterization of the nanocomposite photocatalysts Electron microscopy was done on a Zeiss FEG-SEM Ultra plus field emission gun scanning electron microscope (SEM) operated at 5 kV and 2 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx Table 1 Information on the Nomenclature, composition and ratio of components of various mate- rials prepared in the study. Nanocomposite nomenclature Composition aRatio of components PSK Crushed carica papaya seeds, kaolinite, ZnCl2.6H2O, 20 mL 0.1 M NaOH 1:1:2 KGO Kaolinite, Graphene oxide 2:1 respectively +50 mL methanol N-K Kaolinite in 2:1 of Toluene: Water 2 g +3 mL APTES N-KGO kaolinite, Graphene oxide 2:1 respectively +3 mL APTES Fe@KGO FeCl2.4H2O, FeCl3.6H2O, kaolinite, Graphene oxide 1.6:4.4:2:1 respectively Fe@K FeCl2.4H2O, FeCl3.6H2O, kaolinite 1.6:4.4:2 respectively N-Fe@K FeCl2.4H2O, FeCl3.6H2O, kaolinite 1.6:4.4:2 respectively +3 mL APTES N-Fe@KGO FeCl2.4H2O, FeCl3.6H2O, kaolinite, Graphene oxide 1.6:4.4:2:1 respectively +3 mL APTES Fe@PSK FeCl2.4H2O, FeCl3.6H2O, PSK 1.6:4.4:2 respectively Fe@PSK@GO FeCl2.4H2O, FeCl3.6H2O, PSK, Graphene oxide 1.6:4.4:2:1 respectively N-Fe@PSK FeCl2.4H2O, FeCl3.6H2O, PSK 1.6:4.4:2 respectively +3 mL APTES N-Fe@PSK@GO FeCl2.4H2O, FeCl3.6H2O, PSK, Graphene oxide 1.6:4.4:2:1 respectively +3 mL APTES a Details of how each of these materials were prepared are shown in section S.2 in the supporting information document. equipped with an energy-dispersive X-ray (EDX) spectrometer (Aztec Analysis Software, England) as well as on a HR-TEM, JEOL 2100 trans- mission electron microscope (TEM, field emission gun, 200 kV) equipped with diffraction apertures for selected area electron diffraction (SAED). Infrared spectra were collected on a Perkin Elmer (USA) Spec- trum 100 Fourier transform infrared (FTIR) spectrometer with universal attenuated total reflectance (ATR) sampling accessory. Raman spectra were measured on a WITec Alpha 300 RAS Microscope with a 785 nm laser (WITec, Ulm, Germany). Thermogravimetric analysis (TGA) and differential thermal analysis (DTG) was done on a Netzsch TG 209F1 220−10-055-K from 30 to 1000 °C with a heating rate of 10 °C/ min in air. Raman spectroscopy was performed on a WITec Alpha 300 RAS Microscope (WITec, Ulm, Germany). Nitrogen sorption was done on a Quantachrome NOVA1000e BET system. Electron paramagnetic reso- nance (EPR) spectroscopy was performed at 77 and 298 K using a TE102 rectangular cavity with a microwave power of 2 mW and modulation amplitude of 1 G (EMX plus, Bruker BioSpin, Rheinstetten, Germany). X-ray photoelectron spectroscopy (XPS) was done on a Thermo Scien- tific K-Alpha spectrometer with a monochromatic Al Kα (1486.6 eV) ra- diation source. 2.4. Optical properties The optical properties of the prepared nanocomposites were deter- mined using a Shimadzu UV/Vis spectrophotometer equipped with a solid sample reflectance kit, using BaSO4 as the reference standard and the apparent band gap of the nanocomposites were estimated using the Tauc equation [32]. (1) where α is the absorption coefficient, h is the Planck’s constant, ν is the, light frequency, Eg is the apparent band gap energy, and A is a con- stant [33]. 2.5. Photo-oxidation of steroid estrogens A kinetic study was performed to determine the efficiencies of the nanocomposites using 90 mL of a solution containing the steroid es- trogens (concentration 5 mg/L) with 0.03 g/L of the photocatalytic nanocomposite. The pH of the mixture was adjusted to pH 7.0 using 0.01 M HCl and NaOH respectively. The mixture, in a transparent glass beaker, was agitated for 720 min with 35 W white fluorescent lamps as light source. Previous study of the fluorescent lamp source indicate its absorption in the visible region at wavelengths of 400, 450, 550 and 580 nm [34]. Solution aliquots of 1 mL were withdrawn at spe- cific time intervals. The composites were separated by filtration through 0.45 µm PTFE syringe filters. To ascertain that the filter does not ad- sorb the estrogens, untreated samples of the steroid estrogens were fil- tered through the PTFE filter as well. The concentration of the remnant steroid estrogens was determined using high-performance liquid chro- matography (HPLC, Agilent 1200 series HPLC-DAD detector at wave- length of 285 nm). Gradient elution was conducted with a mixture of acetonitrile and water spiked with 0.1 % formic acid, (3 m in. 30/70 %; 4 m in. 40/60 %; 10 m in. 45/65 %; 13 m in. 30/70 %) with a flow rate of 1 mL/min, and an injection volume of 20 µL for 12 min. A Zorbax SB-CN (5 µm) reverse phase (4.6 × 250 mm) column was used for sep- aration. The degree of steroid estrogen oxidation was calculated via: (2) where Co and Ce are the initial and final concentrations of the steroid estrogen (mg/L). The contribution of adsorption to the estrogen removal process was determined using a similar setup but in the absence of light. Finally, to evaluate the direct influence of laboratory lighting on the removal of these estrogens from water, the same experiment was carried out in the absence of composite material present in the solution. All samples were withdrawn and analyzed after 720 min. The effect of some variables (composite dose, pH, and initial concen- tration of steroid hormones, ionic strength and anions) with respect to the photocatalytic process and toxicity test using treated and untreated water effluents were also studied using the nanocomposite with the best efficiency: Fe@PSK@GO. The details of these studies are described in the supporting information document (S 2.0) and all analysis of samples were carried out in triplicate. 2.6. Degree of oxidation The degree of complete oxidation of estrogens after photocatalysis was determined by chemical oxygen demand, COD (determination of the oxygen equivalent of the organic matter of the degraded steroid es- trogen effluent) using a Nanocolor® Vario 4 spectrophotometer. The reagents for COD analysis (Solution A K2Cr2O7 + H2SO4 and Solution B concentrated H2SO4 and Ag2SO4) and a 2.5 mL treated water sample were mixed in a glass cell and digested in a Nanocolor® Vario 4 diges- tion reactor for 2 h at 150 °C. After digestion, the mixture was cooled to room temperature, and the COD was measured using the Nanocolor® spectrophotometer. The COD was measured for blank (milli-Q water) used in the preparation of the steroid estrogen solution and for cen- trifuged treated water samples. The fraction of estrogen completely oxi- dized was calculated via (3) Where, CODo is the initial COD before degradation and CODe is the fi 3 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx nal COD after degradation. Results of % complete oxidation were sub- tracted from results obtained from kinetic studies after 720 min. This provided the amount of estrogen actually mineralized to CO2 and H2O. All analysis of samples were carried out in triplicate. 2.7. Toxicity test To test the estrogenicity and toxicity of the treated water, Ceriodaph- nia silvestrii, a freshwater cladoceran species was used. Further details of the toxicity test are shown in section S 3.0 in the supporting informa- tion document. 2.8. Detection of active species To identify reactive species generated by the Fe@PSK@GO nanocomposite (the nanocomposite with the best efficiency), a radical trapping study was performed. Reactive species of focus were hydroxyl radical (·OH), superoxide radical (· ), and holes (h+). To solutions with 5 mg/L of steroid estrogen, 10 mL of an aqueous 0.05 mM iso- propanol (IPA) solution, 10 mL of an aqueous 0.05 mM benzoquinone (BQ) and 10 mL of an aqueous 0.05 mM ammonium oxalate (AO) was added and mixed with 0.1 g/L of the nanocomposite. IPA, BQ and AO are used to detect the presence of ·OH, · and h+, respectively, in aqueous systems [32]. All analysis of samples were carried out in tripli- cate. 2.9. Treatment of natural, treated and environmental waste water The performance of the most efficient photocatalytic nanocompos- ite, Fe@PSK@GO, for the photo-oxidation of steroid estrogens (E1, E2, E3 and EE2) in tap water collected from a point-of-use water system in the laboratory, rainwater, and wastewater collected from around the wastewater sewer in the University of São Paulo, São Carlos, Brazil (22.0027 °S, 47.8986 °W), was studied. All samples were collected in amber bottles with Teflon-liner caps. Physicochemical properties (con- ductivity, temperature, pH and total dissolved solids) were measured initially in the collected samples. Thereafter, the samples were fixed with sulfuric acid (1%, V/V) to prevent degradation since the samples were not analyzed immediately. Samples were subsequently stored in the refrigerator at 4 °C until analysis. All samples were filtered through a PTFE membrane with a pore size of 0.45 µm to remove suspended solids. Estrogens from these samples were then pre-concentrated using solid phase extraction (SPE) as described in section S 4.0 in the sup- porting document. 2.10. Reuse efficiency The reusability and durability of the most efficient nanocomposite photocatalyst, Fe@PSK@GO, was evaluated under the following exper- imental conditions. Photocatalyst dose: 0.02 g/L; steroid estrogen con- centration: 5 mg/L; reaction time: 720 min; steroid estrogen solution: 30 mL. The experiment was carried out three times under the same con- ditions by using the same nanocomposite photocatalyst used in the pre- vious experiment. After each experiment, the nanocomposite was recov- ered, rinsed with Millipore water, and dried in the oven at 60 °C for 5 h before reuse. All analysis of samples were carried out in triplicate. 3. Results and discussion 3.1. Photocatalyst characterization Fig. 2A shows Raman spectra of Fe@PSK, GO, Fe@PSK@GO, N-Fe@PSK, and N-Fe@PSK@GO nanocomposites. The spectra show two bands near 1350 and 1600 cm−1, known in the literature as d- and G-bands, respectively. The D band indicates disorder in GO originat- ing from defects associated with vacancies, grain boundaries and amor- phous carbon species, while the G band is attributed to the E2g phonon of sp2 C atoms in a 2-dimensional hexagonal lattice [35]. The D bands appear at 1350, 1352, 1347 and 1345 cm−1 for Fe@PSK, GO, Fe@PSK@GO, N-Fe@PSK and N-Fe@PSK@GO nanocomposites, respec- tively. This variability is consistent with chemical interactions between Fe@PSK, GO and APTES in the composites. In contrast, the G band, which is generally stronger in the current spectra, always appears at 1598 cm−1. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) analysis (Fig. 2B) of Fe@PSK, GO, Fe@PSK@GO, N-Fe@PSK, and N-Fe@PSK@GO nanocomposites was done to confirm the chemical functionalities present in these composites. The prepared GO material shows bands at 1724, 1622, 1400, 1219, 1383 and 1055 cm−1 assigned to CO and O = CO vibrations of carboxyl groups, C OC vibrations of epoxy groups along with C O, CC stretch vibrations and the C O band, respectively [36,37]. Moreover, all spectra show a broad peak centered around 3269 cm−1 that denotes the stretching vibration of −OH. This band is broad and intense in GO and all GO-modified nanocomposites. The peak at 1624 cm−1 in all the samples except GO is indicative of the presence of C O stretching from amide groups [38]. However, this peak in GO sample is due to the stretching vibrations of C C or unoxidized graphitic domains in GO [39] with a shoulder peak attributed to C O group [39]. The peak at 1020 cm−1 in all samples except GO Fig. 2. (A) Raman spectra (B) Attenuated total reflectance-FTIR (C) X-ray diffraction pattern of prepared visible-light photocatalytic nanocomposites Fe@PSK, GO, Fe@PSK@GO, N-Fe@PSK and N-FeO@PSK @GO, respectively. 4 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx suggests the presence of Si-O-Si [40] in the nanocomposites. Further- more, all spectra show bands at 1611 (−OH), 1031 (Si-O), 910 (Al−OH), 783 (diagnostic band for quartz), 664 (Al-Fe−OH), and 525 (Zn-O) cm−1 [41–43]. The X-ray diffraction (XRD, Fig. 2C) shows the characteristic graphene oxide peak at 2θ = 10.1° [44], the presence of kaolinite at 2θ = 20.3 (110) and 24.9° (002) and the characteristic quartz peak at 26.4° [45]. The reflections at 2θ = 23.8 (012), 32.8 (104), 40.3° (113) arise from hematite (ICSD 201,096). Further confirmation for the pres- ence of Fe in the nanocomposites can be found from X-ray fluorescence spectroscopy (Table S1, SI). Judging from XRD patterns, no new crys- tal phase was formed in the process of developing the nanocomposites. This suggests that modification of the nanocomposites were mere sur- face modification. Electron paramagnetic resonance (EPR) spectra of the nanocom- posites at 77 K (see Fig. S1, SI document) show a sharp signals at g = 2.01, a broader signal component with g near 2.63, and a weak sig- nal at g = 4.34 for nanocomposites without GO. While the sharp sig- nal most likely arises from organic free radicals left behind after the py- rolysis of the biomass, the other signals show the typical signature of high-spin Fe3+ species in hematite and various siliceous inorganic ma- trices. Spectra of this kind and their suggestions for their interpretation assignments have been extensively discussed in the literature [46–48], and are beyond the scope of this study. Fig. S1 indicates that the spec- troscopic variations for the present Fe-containing samples are insignif- icant, confirming that the nature of the iron species are unchanged by both the addition of GO and the surface modification with APTES. Table 2 Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size of the prepared photocatalytic composite. Composite Surface Area (m 2/g) Pore Volume (cm 3/g) Pore Size (Å) Fe@PSK 189.8 0.208 22.1 N-Fe@PSK 59.8 0.074 24.8 Fe@PSK@GO 233.8 0.281 24.1 N-Fe@PSK@GO 125.8 0.171 27.2 The four composite materials show type IV nitrogen sorption isotherms [1] typical of porous materials (Fig. S2, SI document), and indeed, the pore sizes calculated from that data are in the microporous range (Table 2). Moreover, the adsorption-desorption loop is a H3 loop; this may be related to the presence of slit-shaped pores [1,49]. The pres- ence of APTES in the synthesis procedure significantly decrease the sur- face area of the prepared nanocomposites (Table 2). The field emission scanning electron microscopy (FE-SEM) images of the nanocomposites show agglomerated materials with rough sur- faces. The typical scale-like structure of GO is visible in the GO-modified nanocomposites (Fig. 3A–E). This further confirms the successful incor- poration of the GO into the composites, consistent with IR and Raman spectroscopy (Fig. 2A-B). Complementary high-resolution transmission electron microscopy (HR-TEM) images of Fe@PSK nanocomposite (Fig. S3A, SI document) shows rhombohedral Fe2O3 particles distributed on the surface of Fe@PSK nanocomposite. In contrast all other materials show much smaller particles distributed on a light gray but crumpled and folded background (the GO) (Fig. S3B-D, SI document). All composites are dif- ferent from kaolinite which show typical plate-like structures (Fig. S3E, SI document). The selected area electron diffraction (SAED), pattern for the nanocomposites show that only the Fe@PSK nanocomposite show some level of crystallinity, similar to kaolinite (Fig. S4, SI document). The other nanocomposites are more amorphous than crystalline. How- ever, the presence of APTES is seen to decrease the crystallinity of the nanocomposites and with the addition of GO, the nanocomposites lose more of their crystallinity (Fig. S4, SI document). X-ray photoelectron spectroscopy (XPS) analysis of Fe@PSK@GO nanocomposite (Fig. 4A) confirmed that it contains Fe, O, C, N, Si and Zn which agrees well with the elemental mapping result (Fig. S5). The binding energies peak at 284.2 and 286.2 eV confirm the presence of sp3C of C C and CO in Fe@PSK@GO nanocomposite (Fig. 4B) and the peak at 531.03 eV corresponds to O 1s chemical state (Fig. 4B-C) [50]. The auger electron for oxygen is seen at 974 eV [51]. These are all due to the presence of the organic component from the biomass and graphene oxide [52,53]. The complex multiple splitting peak and de-convoluted peaks at 710.7, and 724.7 eV (Fig. 4D) confirms the pres- ence of Fe 2p3/2 and Fe 2p1/2 lines with their associated satellite peaks at 718 and 732.6 eV (Fig. 4D) respectively due to charge transference Fig. 3. Field Emission-Scanning Electron Microscopy images of (A) Fe@PSK (B) GO (C) Fe@PSK@GO (D) N-Fe@PSK (E) N-Fe@PSK@GO nanocomposites (F) kaolinite (All scale bar A-E =200 nm, F = 2 µm). 5 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx Fig. 4. (A) X-ray photoelectron spectroscopy (XPS) spectrum, (B) C 1s spectrum, (C) O 1s spectrum, (D) Fe 2p spectrum, (E) Si 2p spectrum and (F) N 1s spectrum of Fe@PSK@GO nanocomposite. processes [54,55]. This corroborates our earlier results from XRD and IR analysis suggesting the presence of Fe2O3 in the nanocomposite. A low intensity peak at 400.1 eV (Fig. 4F) confirms N 1s in Si chemical environment [56] suggesting the presence of amine groups from carica papaya seeds which is known to contain nitrogenous compounds [57]. The peak at 101.8 eV indicate oxide phase of Si in an environment of C with an Si 2p 3/2 peak with very low intensity (Fig. 4E). In addi- tion, there are Zn 2p1/2 and Zn 2p3/2 chemical states of Zn in ZnO in the Fe@PSK@GO nanocomposite which are found at 1022 and 1044.7 eV [58]. The thermal stability of the nanocomposites was characterized via thermogravimetric analysis (TGA) and the derivative loss (DTG) as shown in (Fig. 5). The Fe@PSK and N-Fe@PSK nanocomposites showed weight loss in three regions between 31 and 594 °C representing the elimination of physiosorbed or adsorbed water in the interlayer spaces at the edge and external surface of the composites (31−110 °C), volatilization 110−430 °C), and dehydroxylation of kaolinite (>430 °C) in the nanocomposites respectively [59,60]. However, for N-Fe@PSK Fig. 5. Thermal gravimetric analysis (TGA) and derivative thermogravimetry (DTG) plots of (A) Fe@PSK, (B) Fe@PSK@GO, (C) N-Fe@PSK and (D) N-Fe@PSK@GO nanocomposites. 6 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx nanocomposite, there is a second loss of adsorbed water from the pres- ence of APTES in the composite at 286 °C. Every weight loss is accom- panied by the corresponding percentage derivative weight loss (Fig. 5A-B). The GO-modified nanocomposites exhibit four regions of weight loss with their corresponding percentage derivative weight loss, which are linked to loss of absorbed water (31−110 °C), pyrolysis of GO (ca. 200 °C), loss of aliphatic groups (212−430 °C) and dehydroxylation of Kaolinite (>430 °C) [59–61]. The corresponding differential thermo- gram that supports these weight losses are seen at ca. 75, 210, 403 and 586 °C (Fig. 5C-D). It should be noted that while the dehydroxylation of Kaolinite is pronounced in Fe@PSK and Fe@PSK@GO nanocompos- ites, it appears very weak in nanocomposites containing APTES. Further- more, the N-Fe@PSK@GO sample show residual weight significantly lower (40 %) than in the other three samples. This supported by the Energy Dispersive X-ray data (Table S2, SI document) that suggest a higher carbon content in this sample than for others. The optical absorbance properties of the nanocomposites were probed with UV-diffuse reflectance spectroscopy (UV-DRS) and photo- luminescence spectroscopy (PL) (Fig. 6). Fig. 6A shows absorbance plots for the nanocomposites with peaks at 288 nm. The indirect band gap fitting mode of the Tauc plot was used to determine the approx- imate apparent band gaps of Fe@PSK, N-Fe@PSK, Fe@PSK@GO and N-Fe@PSK@GO nanocomposites as 1.75, 1.86, 1.56 and 1.69, respec- tively (Fig. 6B). It is suggested that the presence of carbon (from the biomass) and GO reduce band gap in materials [62]. The photoluminescence spectra of the same nanocomposites are as shown in Fig. 6C. At excitation wavelength of 288 nm, emission wave- lengths for all nanocomposites are observed in the visible region (Fig. 6C) suggesting that these materials could be active as catalysts in the visible portion of the electromagnetic spectrum. 3.2. Photocatalytic-oxidation of steroid estrogens To understand the influence of each component of the nanocompos- ites on the removal of steroid estrogens from water, a series of control experiments were carried out using experimental method described in section 2.5 except that samples were only withdrawn and analyzed for steroid estrogen after 720 min. Fig. 7 shows that the material without Fe or GO (N-K) exhibit removal efficiency of less than 5% while those with the GO, Fe and GO/Fe components showed expected higher effi- ciency between 20 and 56 %. Infact, most of these materials show less efficiency for the removal of these steroid estrogens from water than GO itself (Fig. 7A–D). The more considerable influence on photocatalytic efficiency is seen with the presence of carbon as seen in Fe@PSK@GO nanocomposite for all steroid estrogens (Fig. 7). This result suggests that carbonaceous species, arising from biomass oxidation, may be re- sponsible for this effect, perhaps by decreasing electron-hole recombina- tion rates [63,64] It is important to state that kaolinite serves as a support base, and the hydrolysis of ZnCl2, followed by calcination in the presence of bio- mass for Fe@PSK@GO nanocomposite creates a nanoporous and nanos- tructured material (as seen from its specific surface area Table 2 and it TEM images in Fig. S3, SI document), which is imparted with photocat- alytic activity via precipitating it together with either graphene oxide, and/or nanocrystalline iron oxide. The role of the APTES modification is principally to introduce a heteroatom (nitrogen) which is exoected to in- crease photoactivity of the nanocomposite. Indeed, APTES modification reduced the effective surface area and pore volume, diminishing to some extent the photocatalytic performance of the material (Table 2 and Fig. 7). Since photocatalytic processes consist of three concomitant steps (adsorption unto the photocatalyst surface, photolysis, and degrada- tion), it is important to evaluate how these steps influence the photo Fig. 6. (A) UV–vis diffuse reflectance spectroscopy (UV-DRS), (B) Tauc plot and (C) photoluminescence spectra at room temperature for Fe@PSK, Fe@PSK@GO, N-Fe@PSK, N-Fe@PSK@GO nanocomposites. 7 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx Fig. 7. Control experiments for the efficiency of each component of photocatalytic nanocomposite materials prepared. catalytic degradation of the steroid hormones (E1, E2, E3, and EE2). To this end, a control experiment involving photolysis of the estrogens in the presence of visible-light in the absence of nanocomposite was used. The results clearly show that simple irradiation already leads to some degradation in all cases (via photolysis), but that the fraction of de- graded estrogens is rather low: E1 (28 %), E2 (22 %), E3 (22 %), and EE2 (24 %) after 720 min of irradiation (Fig. 8 A–D). To understand the influence of adsorption on estrogen removal, the nanocomposites were agitated in a solution of the estrogens in the dark for 720 min (Fig. 8 E–H). The results show an increased estrogen re- moval from water when compared with results from photolysis as a re- sult of adsorption, suggesting that adsorption also contributes to estro- gen removal from solution. However, in the presence of visible light, the efficiencies of the nanocomposites in estrogen removal are further enhanced (Fig. 8 E–H). GO-modified nanocomposites consistently give the best results between ca. 89 and ca. 94 % of estrogen removal. This suggests that the nanocomposites are efficient adsorbents for the removal of estrogens but that the true benefit in the materials lies in the combination of ad- sorption and photocatalytic degradation of steroid estrogens. A simple estimation shows that, for example, E1 is degraded to about 28 % by photolysis only (Fig. 8A) and removed the estrogen from solution to about 35 % by adsorption on Fe@PSK@GO nanocomposite and photoly- sis as well (Fig. 8E). Irradiation of estrogen solutions in the presence of Fe@PSK@GO nanocomposite, however, leads to a significantly higher degree of estrogen removal of about 81 % (Fig. 8E), a >2-fold increase in the efficiency of the photocatalyst. The degree of complete oxidation to carbon dioxide and water was investigated for the most efficient nanocomposite, Fe@PSK@GO. Fig. 8I shows that complete oxidation (conversion of the organic fraction to CO2) is rather high and reaches its highest value for EE2 with slightly over 73 %. The fact that the overall levels of complete oxidation is somewhat lower that the total percentage of estrogen removal supports the above statement that a certain fraction of the estrogens is indeed removed by adsorption and others could be transformation products. To ascertain toxicity levels of treated and untreated water, based on the possible presence of these chemical intermediates in the water, an acute toxicity test was carried out following the standards by the Brazil- ian Association of Technical Standards [65]. Fig. S6 A–D (SI document) shows a clear dose-response relationship between C. daphnia spp and the steroid estrogens, since increasing steroid estrogen concentrations increased mortality. The ranking of relative toxicity (LC50 in mg/L) is E1 (7.69) ≈ E3 (7.70) < EE2 (6.71) < E2 (6.56). The lethal effects of these estrogens become significant above 6.50 mg/L (acute level). The results of the toxicity test for treated water effluents show no lethal influence on C. daphnia spp suggesting that the possible presence of intermediates in the treated water does not pose any health risk to humans. Our kinetic studies (Fig. S7, SI document) indicate that the rate of photocatalytic oxidation follows a first-order kinetic model and that the reaction occurs at the solid-liquid interface. The results of this analy- sis are summarized in Table 3. Again, the Fe@PSK@GO nanocompos- ite shows the best photocatalytic performance, indicating that the intro 8 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx Fig. 8. Photolysis, adsorption, and photodegradation of (A, E) E1, (B, F) E2, (C, G) E3, and (D, H) EE2 steroid estrogens. (I) Fraction (%) of estrogens removed by Fe@PSK@GO nanocom- posite. Black data points in A-D show degradation from pure photolysis (no composite present) vs. time. Colored datasets in A-D show effect of different composites on photodegradation. Figures E-H provides information on the influence of the various components of the nanocomposites and their various combination on the removal of steroid estrogens in water. The term “adsorption” in E-H refers to percentage of steroids removed by adsorption on the composites in the dark and “photodegradation” refers to the combined effect of adsorption and irradiation in the presence of composite. duction of GO into the nanocomposite increases the rate of photocat- alytic oxidation of steroid estrogens. The rates of photocatalytic oxida- tion of EE2 and E2 estrogens are greater than those for E3 and E1 es- trogens. This trend was also seen by Coleman [66] using TiO2 as photo- catalyst. It is possible that the rapid photocatalytic degradation of EE2 is due to its low stability arising from the presence of the ethynyl group which absorbs light and is easily oxidized [9]. On the other hand, the addition of −OH groups may make the phenolic rings more stable, re- sulting in a considerable drop in the removal rate of E2 and a significant decrease for E3 [67]. In any case, the EE2 and E2 estrogens are the most potent among the four hormones discussed here while E3 has the lowest estrogenicity [67]. 3.3. Influence of process variables Certain process variables are required to be optimized to fully un- derstand the operational limits for optimal performance of the photo- catalyst. These variables include solution pH, photocatalyst dose, ionic strength, presence of anions, and initial concentration of steroid estro gens. The influence of these parameters was studied with the photocat- alyst with the best efficiency for the estrogen removal, Fe@PSK@GO nanocomposite. Fig. 9A shows that the efficiency of the photocatalytic oxidation in- creases as more catalyst is present in the mixture. Consistent with the data shown above (Fig. 7I), EE2 is degraded most effectively. Likely, this increased efficiency vs. increasing weight is related to an increasing number of the available active catalytic sites [32,68]. It must be noted however, that the photocatalytic oxidation efficiency decreases again at catalyst loadings of 0.17–1.67 g/L. This is because catalyst loadings above lead to turbid dispersions and agglomeration reducing both the number of available catalytic cycles and the light intensity on the cata- lyst [69]. Fig. 9B shows that the initial concentration of the estrogens corre- lates with photodegradation efficiency. Likely, the decline in efficiency with increasing concentration is associated with the fact that at higher estrogen concentrations the number of catalytically active sites available is not sufficient to degrade all molecules. Moreover, some of the sites may be inactivated by adsorption of some estrogen molecules. 9 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx Table 3 Kinetic parameters for the photocatalytic oxidation of steroid estrogens with Fe@PSK, Fe@PSK@GO, N-Fe@PSK, and N-Fe@PSK@GO photocatalytic nanocomposites. E1 PHOTOCATALYST kapp (min −1) t1/2 (min) ;r2 Fe@PSK 0.0017 239.6 0.9742 N-Fe@PSK 0.0039 178.2 0.9261 Fe@PSK@GO 0.0060 112.1 0.9902 N-Fe@PSK @GO 0.0043 158.5 0.9896 E2 PHOTOCATALYST kapp (min −1) t1/2 (min) ;r2 Fe@PSK 0.0042 172.4 0.9622 N-Fe@PSK 0.0049 146.8 0.8924 Fe@PSK@GO 0.0063 120.0 0.9012 N-Fe@PSK @GO 0.0050 136.4 0.9428 E3 PHOTOCATALYST kapp (min −1) t1/2 (min) ;r2 Fe@PSK 0.0023 145.2 0.9742 N-Fe@PSK 0.0045 153.9 0.9228 Fe@PSK@GO 0.0053 196.8 0.9821 N-Fe@PSK@GO 0.0042 170.4 0.8999 EE2 PHOTOCATALYST kapp (min −1) t1/2 (min) ;r2 Fe@PSK 0.0048 152.2 0.9712 N-Fe@PSK 0.0057 124.4 0.9962 Fe@PSK@GO 0.0068 105.2 0.9982 N-Fe@PSK@GO 0.0059 130.8 0.8241 Fig. 9C shows that the ionic strength of the estrogen solution us- ing different concentrations of NaCl (a common electrolyte in drinking water) does not significantly influence the photodegradation efficiency of Fe@PSK@GO nanocomposite. The efficiency remains at >80 % irre spective of the electrolyte concentration or estrogen type. This is differ- ent from a previous report on the adsorption of E2 onto GO suggesting an increase in the adsorption efficiency of GO [26] with an increase in ionic strength. Overall, however, there is a dearth of data on the influ- ence of ionic strength on the photocatalytic removal of estrogens from water under visible-light conditions so at the moment a final conclusion on these effects is difficult. Fig. 9D shows the effects of pH. The highest removal efficiencies are observed at pH 6.0, with EE2 being removed the best. Above pH 6.0, the efficiency of the photocatalyst is lower and keeps decreasing with increasing pH. The pH of solution is known to influence the surface charge of material as well as the dissociation properties of organic con- taminants. Even though we expect the surface charge of Fe@PSK@GO photocatalytic nanocomposite to be negatively charged above its pHpzc of 4.2 (Fig. S8, SI document) in estrogen solution of pH 7.3, yet the electrostatic theory will not be able to explain the trend of results shown in Fig. 9D especially because the pKa of these estrogens is ca. 10.4 [67]. Hydrophobic interaction, hydrogen bonding and dissociation properties is postulated to explain the trend. When the initial pH of aqueous so- lution is less than 9.0, the dissociation of the estrogen molecules (pKa ≈10.4) is partially suppressed, causing them to exist in their non-disso- ciated form. Therefore, even though the electrical charge of the photo- catalyst gradually changes from positive to negative (when pH exceeded 4.2), electrostatic repulsion would not be the major force of interaction. In this case, it will be hydrogen bonding between the hydroxyl of the estrogens and nitrogen and/or oxygen atoms on the surface of the pho- tocatalyst [70]. Another important factor that could possibly affect the adsorption of the estrogen is hydrophobic interaction wherein the hy- drophobic phase in the interlayer region of the photocatalyst is expected to interact with the estrogens via π-π interaction. As pH of the estrogen solution exceeds from 8.0, the increased hydroxyl ion could result in re- pulsive electrostatic interactions leading to a further decline in the effi- ciency of the photocatalyst to remove these estrogens from water [70]. Fig. 9. Effect of process parameters on estrogen photodegradation. (A) Photodegradation vs. catalyst dose (0.17, 0.33, 0.67, 1.0, 1.33, 1.67 g/L) for initial estrogen concentration of 5 mg/L. (B) Photodegradation vs. initial steroid estrogen concentration using a photocatalyst dose of 0.67 g/L. (C) Photodegradation vs. ionic strength with photocatalyst concentration of 0.67 g/L and steroid concentration of 5 mg/L. (D) Photodegradation vs. initial solution pH with photocatalyst concentration of 0.67 g/L and steroid concentration of 5 mg/L. Photocatalyst is Fe@PSK@GO nanocomposite in all experiments. 10 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx In any case, these results support our earlier conclusion that adsorp- tion is one of the mechanisms for the removal of these estrogen mole- cules from water using the Fe@PSK@GO photocatalytic nanocomposite. However, our results differ from those obtained with TiO2 suspensions for the photocatalytic removal of E2 where a continuous decline in effi- ciency from pH 2.0 till pH 9.0 was noted [71]. Fig. 10 shows the effects of inorganic anions on the performance of the composites. Inorganic anions are important because they are com- mon in drinking water. A small decrease in the photocatalytic efficiency was observed in the presence of sulphate while bicarbonate and phos- phate leads to more drastic reductions. One possible explanation for this difference between sulfate and the other ions could be that bicarbon- ate and phosphate ions are efficient ·OH radical scavengers [72]. Some other report suggests that their presence increase the rate of h+ oxida- tion which reduces the overall efficiency of the photocatalyst. This hole (h+) oxidation is shown to be more in the presence of phosphate than in carbonate [72]. Given the result in Fig. 10, it is plausible to attribute the significant decrease in the efficiency of Fe@PSK@GO photocatalytic nanocomposite observed here, to hole (h+) oxidation by these anions (phosphate and carbonate). This is an indication that the photocatalytic function of the nanocomposite is based on hole (h+) generation. The re- sults shown in Fig. 10 indicate a need for the initial removal of these anions via an anion exchange resin or other suitable techniques. To ascertain the kind of reactive species produced by Fe@PSK@GO nanocomposite, the formation of hydroxyl radicals (·OH), superoxide radicals (· ), and holes (h+) were investigated by introducing propanol (IPA), benzoquinone (BQ), and ammonium oxalate (AO) scav- engers, respectively, into the solutions containing a mixture of steroid estrogens and Fe@PSK@GO nanocomposite (see section 2.8). Fig. 11 shows that the addition of scavengers does reduce the efficiency of Fe@PSK@GO nanocomposite to photo-oxidize. The strong effect of BQ and AO radical scavengers on the photocatalytic oxidation efficiency of Fe@PSK@GO nanocomposite is more profound than that of IPA. More over, the strongest effect is seen when E1 is present. This presumes that the composite produces more superoxide radicals ( ) and hole (h+) reactive species than the hydroxyl radicals (·OH) in solution when ex- posed to visible-light and in water. Fig. 11 therefore, suggests that the most reactive species h+, followed by (· ) and finally ·OH. The strong negative influence of BQ and AO radical scavengers on the photocatalytic oxidation efficiency of Fe@PSK@GO nanocomposite is more pronounced than that of IPA, and even so, this influence is more with E1 estrogen (Fig. 11) than with the other estrogens. This presumes that the composite produces more superoxide radicals (· ) and hole (h+) reactive species than the hydroxyl radicals (HO·) in solution when exposed to visible-light and in water. With respect to the results ob- tained in Fig. 11, the order of significance of reactive radicals produced by the composite is h+ > · > HO·. Scheme 1 shows the tentative reaction mechanism of Fe@PSK@GO photocatalytic nanocomposite leading to the release of reactive oxygen species responsible for degradation of steroid estrogens. The theoretical band edge for both Fe2O3 have been calculated to be 2.4 eV and 0.3 eV [73] while that for graphene oxide (GO) is 3.4 eV and -0.51 eV [74] The reactive species (superoxide radicals and holes) can react with H2O to form hydroxyl radicals (HO·) which then interacts with estrogens to form CO2, H2O and transformed products (scheme 1). (4) Where E denotes the estrogen and E*product is the photo-oxidized prod- uct of estrogen. In addition, the GO component and carica papaya seeds into the composite does decrease the high recombination rate of e_/h+ in Fe2O3 [75,76], increasing the photocatalytic oxidation efficiency of the nanocomposite material. To confirm this, there is increase in photocat- alytic efficiency from 28 to 33% with Fe@K material (Fig. 7) to 39–57 % when carbon was introduced to the Fe@K (Fe@PSK) as shown in Fig. 8E–H. This efficiency was further extended to 80–90% when GO included in Fe@PSK [Fe@PSK@GO] (Fig. 8E–H). Fig. 10. Effect of Anions on Photo-oxidation of Steroids Estrogens using Fe@PSK@GO nanocomposite. 11 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx Fig. 11. The effect of scavengers (propanol - IPA, benzoquinone – BQ, and ammonium oxalate – AO) on the efficiency of Fe@PSK@GO nanocomposite for estrogen photodegradation. Scheme 1. Generation of Reactive Oxygen Species by Fe@PSK@GO nanocomposite (CB and VB). 3.3.1. Treatment of raw wastewater and drinking water To determine the efficiency of Fe@PSK@GO nanocomposite for es- trogen removal from raw wastewater, tap water, and rainwater, these matrices were spiked with 10 mg L−1 of each of the estrogen. Milli- pore water is used as the control and was spiked with the same con- centration of the estrogens. Table S3 (SI document) shows some ba- sic physicochemical properties of the various water matrices. Table 4 shows that only the wastewater matrix contains a measurable amount of estrogens prior to spiking. This is because contraceptives (birth con- trol pills) are massively used globally; they contain EE2 and are of- ten excreted and transported into the central municipal wastewater sys- tems. Table 4 shows that the levels of estrogen removal follows the se Table 4 Levels of removal of steroid estrogens from Millipore water, Rainwater, Tap water and Wastewater samples using Fe@PSK@GO nanocomposite. Removal of E1 % Removal of E2 % Removal of E3 % Removal of EE2 % Millipore water 81 (ND) 89 (ND) 84 (ND) 93 (ND) Rainwater 77 (ND) 84 (ND) 77 (ND) 91 (ND) Tap water 75 (ND) 79 (ND) 79 (ND) 89 (ND) Wastewater 66 (0.42)* 67 (0.38)* 63 (0.40)* 78 (0.59)* ND = Not detected. * Values in bracket are the initial concentrations of the estrogens (mg/L) before spiking. 12 UN CO RR EC TE D PR OO F A.A. Bayode et al. Journal of Water Process Engineering xxx (xxxx) xxx-xxx quence: Millipore water (control) > rain water > tap water > waste- water. Clearly however, Table 4 also shows that there is a significant reduc- tion in the efficiency of Fe@PSK@GO in “real” systems with the most challenging matrix being the wastewater matrix. Likely, some other solutes (other than the estrogens) compete for active sites on the pho- tocatalytic nanocomposite and reactive species present in the reaction medium. In any case, the Fe@PSK@GO nanocomposite holds significant potential for the treatment of real wastewater systems containing these (and possibly related) steroid estrogens. 3.3.2. Competitive degradation of the steroid estrogens To evaluate the efficiency of the photocatalyst in the presence of multiple estrogens at different concentrations, a test system with all es- trogens (E1 = 100 µg/L, E2 = 100 µg/L, E3 = 100 µg/L, EE2 = 200 µg/L) was studied. The estrogen concentrations are such that they repre- sent their relative concentration in environmental matrices, i.e., the con- centration of EE2 is usually higher than the other three steroids (even though a previous study cites E1 as more abundant in the environment [77]). Fig. 12A shows that the steroids are degraded effectively also when present as a mixture. The order of degradation is EE2 > E2 > E1 > E3 with the percentage of removal being 93, 89, 84, and 82 %, respectively. A similar order has been observed in Frontistis et al. [77]. Aside from that article, the current study is one of the first reports on the use of heterogeneous photocatalyst for visible-light oxidation of estrogens in a competitive system simultaneously in water. Other reports have re- ported on the use of harsher methods such as UV-light [22], potassium permanganate [78], UV/H2O2 [27], or potassium permanganate com- bined with ultrasound [79]. 3.3.3. Reuse efficiency Finally, the reuse efficiency and stability of Fe@PSK@GO nanocom- posite was evaluated. Fig. 12B shows that the catalyst is stable at least for three cycles and does not significantly lose efficiency over the three-cycle test period. Clearly, there is a need for longer test phases but these initial data are quite promising. 4. Conclusion The synthesis of low cost-high performance water treatment mate- rials is a tremendous challenge, in particular for endocrine disruptors, which are only present in low concentrations in aqueous media. Here we report a group of new and highly promising candidates for removal of steroid estrogens from water. Nanocomposites containing graphene ox- ide (GO) are superior in their photocatalytic activities over those with- out GO. While the mechanism of the photocatalyst is not entirely clear, the most promising material, Fe@PSK@GO nanocomposite, clearly holds potential for the treatment of wastewater or other waters in real systems. The key findings of the study are that (1) the materials also work with mixtures of steroids and keep their efficiency over at least three cycles, (2) the treated water samples show no toxicity towards Ce- rio daphnia silvestrii spp, and (3) there is no need to use UV light for suc- cessful application of the catalyst. Overall, the materials and processes introduced here are highly potent materials with high application poten- tial in wastewater treatment, in particular for steroid removal. Declaration of Competing Interest The authors declare that they have no known competing financial in- terests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement Bayode Ajibola Abiodun expresses her thanks to the African-German Network of Excellence in Science (AGNES) for granting a Mobility Grant in 2018. The grant is generously sponsored by German Federal Ministry of Education and Research and supported by the Alexander von Hum- boldt Foundation. This research is also supported by The World Acad- emy of Sciences- Brazilian National Council for Scientific and Techno- logical Development (TWAS-CNPq) award number 315710/2018-7 and the fund from FAPESP: 2018/16244-0. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jwpe.2020.101865. Fig. 12. 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Public Health 12, 15434–15448. 15 View publication statsView publication stats https://www.researchgate.net/publication/347555479 Carbon-mediated visible-light clay-Fe2O3–graphene oxide catalytic nanocomposite for the removal of steroid estrogens from water Keywords Abstract Introduction Materials and methods Materials and precursors Material synthesis Characterization of the nanocomposite photocatalysts Optical properties Photo-oxidation of steroid estrogens Degree of oxidation Toxicity test Detection of active species Treatment of natural, treated and environmental waste water Reuse efficiency Results and discussion Photocatalyst characterization Photocatalytic-oxidation of steroid estrogens Influence of process variables Treatment of raw wastewater and drinking water Competitive degradation of the steroid estrogens Reuse efficiency Conclusion Declaration of Competing Interest Acknowledgement Supplementary data References