A versatile nanocomposite made of Cd/Cu, chlorophyll and PVA matrix utilized for photocatalytic degradation of the hazardous chemicals and pathogens for wastewater treatment

Highlights

• A novel CdS/Cu(OH)₂/CuO@PVA-Chl nanocomposite has been preaped and characterized.

• The prepared nanocomposite is applied as photocatalyst for degradation of the hazardous dyes.

• Great degradation efficiency for the MB dye has been achieved.

• The prepared nanocomposite shows high reusability investigations after five times recycles.

• The inhibition zones evaluated for S. aureus and E. coli bacterial cells, respectively.

Abstract

In this work, a novel composition of cadmium sulfide (CdS), copper(II) hydroxide (Cu(OH)₂), copper(II) oxide nanoparticles, and chlorophyll (Chl) is prepared based on polyvinyl alcohol (PVA) matrix. The characterization of the CdS/Cu(OH)₂/CuO@PVA-Chl nanocomposite has been carried out by the various analytical methods. Then, the prepared CdS/Cu(OH)₂/CuO@PVA-Chl is then applied as a very efficient photocatalyst for degradation of the hazardous dyes such as methylene blue (MB), Congo red (CR), and 4-chlorophenol (4-CP) in the aqueous samples. For the preparation of the intended nanocomposite, the Chl has been extracted from fresh spinach via a convenient mechanical adsorption/desorption method. In the obtained optimum conditions, a great degradation efficiency (ca. 97.6%) for the MB dye achieved by simultaneous application of the prepared CdS/Cu(OH)₂/CuO@PVA-Chl photocatalyst and the visible-light (LED 70 W, λ = 425 nm), over a 60 min contact time. Moreover, the degradation efficiency of the CR and 4-CP dyes were ca. 92% and 88%, respectively. The reusability investigations have revealed that no significant loss in the catalytic performance is occurred over seven times recycles. As well, degradation of the pathogens by the CdS/Cu(OH)₂/CuO@PVA-Chl nanocomposite has been screened under the obtained optimal conditions. Briefly, the diameter of the inhibition zones by the prepared composite film has been evaluated to be ca. 11.0 (± 0.1) and 9.0 (± 0.1) mm for S. aureus and E. coli bacterial cells, respectively.

Introduction

As a major concern in the environmental sciences, the industrial pollutants like the wastewater residual dyes cause serious dangers for the human health, so it is of high importance to remove/degrade these substances via efficient and convenient strategies [1]. Briefly, the small-molecule hazardous chemicals can diffuse into the soil and promote the toxicity in the plants, and also spread into the underground water resources [2]. Among various species of the hazardous chemical compounds, dyes are one of the most known compounds which include acidic, basic, vat, and azo dyes [3]. The azo dyes like methylene blue (MB) can be converted to the carcinogen aromatic amines that increase the risk of breast cancer [4]. This is why the researchers have always made endeavors to quickly detect, degrade and remove the hazardous dyes from the food/water cycles [5], [6], [7], [8], [9]. Besides, the microbial pathogens are an important issue in the food industry, causing the bacterial infection diseases that bring serious health problems and in some cases include severe complications [10,11]. Hence, it would be very valuable to design the novel strategies to simultaneously remove/degrade the hazardous chemical dyes and the pathogens from the aqueous and food sources. In this way, Singh et al. have reported a magnetic composite made of iron and zinc oxide nanoparticle and utilized that for the removal of the hazardous ions, dyes, and the bacterial pathogens [12]. The properties of the nanomaterials deeply depend on their size and morphology, therefore design of the various nanomaterials in different size and shape is considered as a hot topic in the field of materials engineering [13], [14], [15], [16], [17]. Among diverse species of the nanomaterials, cadmium sulfide (CdS) is one of the most attractive nanostructures including many effective features such as large surface area, various morphologies (nano-rod, nano-sheet, and microsphere), significant sensitivity to the visible light through having an appropriate band gap (2.5 eV), and good crystallinity leading to the extensive use as a semiconductor photocatalytic system [18]. So far, many studies have been performed about the photocatalytic ability of the CdS nanostructures, in which various composite structures including CdS and other active ingredients such as Au, Ag, Pt, Bi, TiO₂, MO₆, WO₆, and VO₄ have been designed and used for different purposes. The CdS-containing composites are used in various fields such as photo-detectors, bio-sensors, gas sensors, nano-fluids, field emissions, removal of inorganic pollutants, photo-catalysis [19,20]. The copper(II) hydroxide particles has shown great antifouling  effects, and also have been used in the photocatalytic processes due to having high structural stability and sensitivity to the light photons [21]. Since, it has a band gap in a range of 2.73−4.5 eV, the coupling of these crystals with CdS nanoparticles seemed to be constructive for the photocatalytic aims [22], [23], [24], [25], [26], [27]. Therefore, we decided to design a composition of CdS and Cu(OH)₂ particles integrated by a polymeric matrix, and investigate its photocatalytic activity for degradation of the chemical dyes and the bacterial pathogens.

Chlorophyll (Chl), as the most important ingredient in the photosynthesis process in the green plants, has been suggested to be used in the structure of the photocatalytic systems [28]. Briefly, the Chl acts as the starting agent for the absorption of the light photons, and decreases the band-gap energy in a composite structure [29]. In the photoelectric reactions, Chl acts as a sensitizer and also an electron donor to the acceptors [30]. In the photocatalytic degradation processes, Chl begins the absorption of the light photons (light harvesting) that stimulate the electrons located in the lower energy orbitals (ground state) and lead them to move to the orbitals with higher energy. Then, the excited electrons are transferred to the conduction band of the neighbor ingredient with a close band gap [31]. Chl as a green inexpensive pigment has also many other properties such as nontoxicity, durability, slow charge recombination kinetics, fast electron injection speed, and high absorption coefficient [32]. In chemical structure of the Chl, there are four pyrrole rings and side chains in each ring (such as phytol), and also a magnesium which are not stable, this is why the researchers typically use various types of the Chl-containing composites for the photo-absorption purposes [33]. Polyvinyl alcohol (PVA) has been widely used as a suitable substrate for immobilization of the nanoparticles due to including an integrated structure and also numerous hydroxyl groups [34]. Its noticeable features such as permanence, chemical stability, biocompatibility, high hydrophilicity, and nontoxicity led researchers to apply that in different scopes of science [35].

Moreover, its jelly nature helps to make an integrated composition of the ingredients that will be poured on a laminate to form a filmed structure. Based on the above explained features, herein, a novel photocatalytic system has been designed including an efficient composition of CdS, Cu(OH)₂/CuO, Chl, and PVA. Firstly, simultaneous use of the Chl oil (extracted from plant) and the PVA network provides a highly flexible matrix suitable for incorporation of the other components. In fact, Chl as the starting agent for light harvesting is well mixed with the PVA strands, hence it can be expressed that the PVA strands are activated for the photocatalytic process. Furthermore, composition of the prepared PVA-Chl with the active ingredients creates significant synergies in the photocatalytic process through improvement of the photoelectrochemical properties. For this purpose, CdS and Cu(OH)₂/CuO nanostructures with the band gap values of 2.5 and 4.5 eV, respectively, have been used. Also, the rod-shaped structure of the Cu(OH)₂/CuO helps the stability and strength of the composite's structure. The employed characterization methods (particularly electron microscopy) have confirmed successful formation of the designed composite with the exclusive naming of CdS/Cu(OH)₂/CuO@PVA-Chl. Then, the prepared nanocomposite has been used for the photocatalytic degradation of the methylene blue (MB), Congo red (CR), and 4-chlorophenol (4-CP) hazardous dyes, under irradiation of the visible light through applying a LED source with 70 W power and a specific wavelength of 425 nm. Also the reusability of the CdS/Cu(OH)₂/CuO@PVA-Chl nanocomposite has been studied through the consecutive runs. The cleaning ability of the designed composite has also been investigated on the bacterial pathogens including both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacterial cells. Concisely, it has been found out that the presence of the visible light is so effective in the photocatalytic degradation of the mentioned hazardous dyes (degradation efficiency = 97.6%) and also the bacterial pathogens.