Tuning ZnO/GO p-n heterostructure with carbon interlayer supported on clay for visible-light catalysis: Removal of steroid estrogens from water

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Highlights

  • Synthesis of new ZnO/GO p-n heterostructured visible-light catalyst on kaolinite.

  • Catalyst photodegradation efficiency of the estrogens, >90%.

  • New photocatalyst efficient for the treatment of real wastewater.

  • Mild toxicity from treated water which is below the acute level of estrogens.

  • Efficiency of photocatalyst stable over three cycles.

Abstract

This study demonstrates the efficiency of a new visible-light p-n ZnO/GO heterostructured composite catalyst supported on clay with carbon interlayer. This photocatalyst was prepared via microwave assisted technique for the removal of four steroid estrogens in water: Estrone (E1), 17-β-estradiol (E2), Estriol (E3) and the synthetic estrogen 17-α ethinylestradiol (EE2). The prepared catalyst was characterized by different techniques: FE-SEM, EDX, RAMAN, ATR-FTIR, XPS, BET, UV–VIS, and PL. Studies confirmed that the presence of carbon interlayer (from carica papaya seeds) and graphene oxide (GO) were important for the visible-light efficiency of the photocatalyst. In single solute systems, estrogen removal was >89% and as high as 98% and this was not significantly different in a competitive system. In real wastewater samples, efficiency was 63–78% estrogen removal. A reuse study suggested that the photocatalyst efficiency was slightly >80% after 3 reuse cycles. The presence of humic acid reduced the efficiency to ≥70% for all estrogens while the addition of 1% H2O2 raised photodegradation of estrogens to 100% in 10 min. However, using the chemical oxidation demand test, the actual oxidation level of steroid estrogens after photodegradation was 51–77% for the various steroid estrogens. Important reactive oxygen species responsible for photodegradation was hydroxyl radical (HOradical dot) via superoxide radical (radical dotO2) and hole (h+) formation from the photocatalytic composite. Test with ceriodaphnia silvestrii suggests very mild toxicity from treated water which is below the acute level of these estrogens (LC50 = ca. 0.89 mg/L).

Introduction

Recently, the discharge of liquid waste into water bodies has drastically increased. These wastewater streams are composed of a wide range of natural and synthetic substances such as medicines, inorganic and organic chemicals, and human waste. One of the major chemical pollutants discharged into the environment and water bodies that has attracted a lot of attention recently, is the endocrine-disrupting chemicals (ECDs) which is being classified as contaminants of emerging concern [1], [2], [3].

Endocrine-disrupting chemicals are chemicals that interfere with the function of the endocrine system of humans and wildlife by blocking or mimicking the normal way hormones control metabolism, growth, and body function [4], [5]. Chemical compounds exhibiting endocrine-disrupting properties include a wide range of industrial chemicals, pesticides, natural and synthetic hormones with their derivatives among which are the steroid estrogens [6].

Estrogens are steroid hormones/chemicals that are important for the human brain, reproduction, and bone development, and are produced by the endocrine system. This group of chemicals and particularly the natural hormones: Estrone (E1), 17-β-estradiol (E2), Estriol (E3) and the synthetic estrogen 17-α ethinylestradiol (EE2) have shown much stronger pollutant effects than other EDCs [1], [7], [8].

The presence of these hormones in water and the environment has become an important issue [2]. The presence of E1, E2, E3 and EE2 in the environment have been associated with increase in endometriosis, fibroid, testicular and breast cancer, in humans, abnormal reproductive processes, infertility, behavioral and reproductive organ modification and feminization in fishes [7], [9], [10], [11]. Humans and wildlife are exposed to these hormones via different pathways including drinking water [12]. Water is one important medium for the distribution of these hormones in the environment. The occurrence and activity of endocrine-disrupting chemicals which include these hormones have been investigated in several studies. These studies show that a range of hormones can be present in drinking water sources, particularly surface water (river), as well as in wastewater effluent [9], [13], [14].

Most water treatment systems still use conventional water treatment techniques which were not originally designed to cater for the removal or reduction of these hormones in drinking water. Among several new methods such as electrochemical oxidation, sonophotocatalytic and ultrasound induced oxidation [15], [16] and photocatalytic decomposition [17] routes developed to remove organic pollutants from water, the latter has received a lot of attention as one technique useful at the tertiary stage of water treatment because of their “green nature” such as the use of solar light, environmental friendliness, and reaction at room temperature [18], [19], [20].

Zinc oxide is one of the most widely used photocatalysts that have been developed within different architectures for water treatment. Besides being highly efficient, ZnO as a photocatalyst for water treatment has several advantages. Lee et al [21] published a thorough review of ZnO-based photocatalysts for water treatment showing its excellent chemical and photochemical stability, non-toxic nature [22], [23], low-cost and large exciton binding energy [21]. Still, the low-charge separation efficiency of ZnO, its inability to absorb energy in the visible region and its dissolution under UV-irradiation [24], [25], [26] which increases the difficulty in recovering it from solution after photocatalysis, are its drawbacks. This has necessitated its modification. To this end, several approaches have been employed including doping with noble metal via deposition [27], modulation of vacancies to lower bandgap energy and reduction of hole-electron pair recombination [28], [29] and the use of carbon nanotubes and quantum dots [30], [31]. While all of these approaches have truly improved the photocatalytic efficiency of ZnO for removal of pollutant from water, they also increased the cost of preparing the photocatalyst since some of the modifying agents and some of the processes involved in the modification are quite expensive. Furthermore, some of these dopants are not environmentally friendly, raising concerns over their impact on human health if they leach into the treated water meant for drinking.

Even though there have been attempts to prepare Graphene and carbon-based ZnO [32], [33], [34], [35] as a way to enhance the photoactivity of ZnO, there is no report on the use of agro-waste (as carbon source) for the development of visible-light active graphene oxide (GO)-Carbon-ZnO photocatalyst supported on clay architecture. Therefore, this study focuses on the microwave synthesis of a new, efficient and sustainable visible-light active p-n heterojunction composite with carbon interlayer from ZnO, GO and carica papaya seeds as carbon source for the removal of steroid estrogens: Estrone (E1), Estradiol (E2), Estriol (E3) and 17α-ethynylestradiol (EE2) from water. It further considers the influence of certain process variables on the efficiency of the photocatalytic composite in the removal of these estrogens from water including the presence of natural organic matter.

The choice of clay (kaolinite) and Carica papaya seeds (as a carbon source) stems from their abundance in the environment and as agro-waste respectively. The clay is expected to act as a support that will reduce photocorrosion and allow for easy recovery of the photocatalyst from solution after use. On the other hand, the carbon content (from the Carica papaya seeds and GO) should act in a synergistic manner to extend the absorption of ZnO to the visible-light region by reducing its band gap and provide excellent electron transfer leading to better charge separation efficiency that ultimately improves photocatalytic efficiency of estrogen removal from water. Graphene oxide in nanocomposites are known to act as electron acceptor/transporter, cocatalyst, photocatalyst and photosensitizer [36].

Section snippets

Precursor materials

The precursor materials and consumables used for this work were: Polytetrafluoroethylene (PTFE) 0.45 µm membrane filters (Analitca Brasil), filter paper, cellulose nitrate filter (0.45 µm Sartorius Stedim Germany), Nylon filter (47 mm, 0.45 µm, Unifil), Strata-X Solid Phase cartridge (Phenomex, polymeric SPE), graphite (lab synth Brazil), E1 (98% Sigma Aldrich, Germany), E2 (99% Sigma Aldrich, Germany), E3 (98% Sigma Aldrich, Germany), EE2 (99% Sigma Aldrich, Germany), H2SO4, HCL, KMnO4 (Lab

Materials characterization

The attenuated total reflectioance-fourier transformed infra-red (ATR-FTIR) Spectroscopy was used to confirm the presence of chemical functionalities present in the composite materials. The spectrum of K-ZnO/C composite material show a broad band centered at 3311 cm−1 (Fig. 1A) which is intense and shifted to 3292 cm−1 with the introduction of GO to the composite (K-ZnO/C/GO). This indicate the presence of –O–H/N–H overlap stretching vibrations. The peak at 1605 cm−1 represents C = C stretching

Conclusion

A new visible-light p-n heterojunction photocatalyst with carbon interlayer K-ZnO/C/GO composite (kaolinite-ZnO/Carbon/Graphene oxide) was successfully developed via a microwave assisted mode. Its composition was confirmed with different techniques: FE-SEM, EDX, RAMAN, ATR-FTIR, XPS, BET, UV–VIS, HR-TEM and PL. This composite showed enhanced photocatalytic activity under visible light when compared with its components for the degradation of steroid estrogens E1, E2, E3, and EE2. Its enhanced

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

Bayode Ajibola Abiodun expresses her thanks to the African-German Network of Excellence in Science (AGNES) for granting mobility in 2018. This grant is generously sponsored by the German Federal Ministry of Education and Research and supported by Alexander Von Humboldt Foundation. This research is also supported by The World Academy of Science-Brazilian National Council for Scientific and Technological Development (TWAS-CNpq) award number 315710/2018-7. Professor Eny Maria Vieira expresses her

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