Investigation on Tetracycline degradation and bactericidal properties of binary and ternary ZnO/NiO/g-C₃N₄ composites prepared by a facile co-precipitation method

Highlights

• A facile co-precipitation method was adopted for the synthesis of the composites.

• Study of the effects of Catalyst dosage, Tetracycline concentration and pH using ZnO/NiO/g-C₃N₄ composites.

• Superior and express photocatalytic Tetracycline degradation up to 91.49% within 60 min.

• Fairly consistent photocatalytic performance up to three cycles.

• Exhibit nice antibacterial activity against various bacteria.

Abstract

In this work, a facile co-precipitation method was adopted for the preparation of binary ZnO/g-C₃N₄, NiO/g-C₃N₄ and ZnO/NiO composites and ternary ZnO/NiO/g-C₃N₄ composites. The structural, optical, morphological and compositional analyses were investigated by various characterization techniques. The photocatalytic activity of the prepared composites was analyzed against Tetracycline (TC) under visible light irradiation. The optimized ZnO/NiO/g-C₃N₄ photocatalysts were found to cause superior TC degradation efficiency of 91.49% within 60 min with pseudo-first order kinetics value of 0.05356 min⁻¹. From scavenger’s experiment, it is revealed that •OH and •O₂^- species are core factors for the photocatalytic TC degradation. After three recycles, the ZnO/NiO/g-C₃N₄ composites resulted in 88.76% TC degradation efficiency indicating its advanced photocatalytic ability and high stability. Additionally, bactericidal activity was also analyzed against various bacteria. The ternary mixtures provide moderate bactericidal impact against bacteria. The present work fulfills its primary role on antibiotic degradation and antimicrobial activity with superior performance.

Introduction

In recent decades, the environment has often been affected by the unlimited usage of antibiotics as antibiotics are very essential in the field of pharmaceuticals for new diseases caused by population growth, changing environments and industrial growth. Especially, expired and unused antibiotics are discharged from pharmaceutical industries and mixed directly into water resources, making the environment more affected. Hence, antibiotics have considered as one of the major emerging contaminants that affect directly rivers, lakes and seas [1], [2], [3].

There are various antibiotics like Amoxicillin, Tetracycline, Norfloxacin, Oxytetracycline, Gemifloxacin, Ciprofloxacin, Quinolones, Tylosin, Sulfadimidine and Sulfadiazine used in medical industry for various purposes [4], [5]. Among these, Tetracycline is one of the frequently used antibiotics that have good therapeutic effects. It is generally used in human medicines, veterinary medicines, and agricultural applications. Moreover, it is used to treat many human related issues like bacterial infections of the skin, urinary and respiratory tract, intestines and other body systems. However, excessive usage of tetracycline gives some side effects to humans like teeth and skin discoloration, irritation of mouth and stomach, itching of anus and genital area. Medical wastage (unused and expired) of tetracycline gives more adverse effect to the environmental system. Especially, residues of tetracycline inhibit the microorganisms’ growth in water and soil. In order to decrease the effect of tetracycline in environmental system, it must be decomposed into harmless substance. However, it is very difficult to decompose owing to its chemical stability and antibacterial properties [6], [7], [8].

To tackle this aquatic based issue, the scientific research community has been making various attempts like conventional processes (membrane bioreactors, activated sludge processes, moving bed biofilm reactors, reverse osmosis, chemical precipitation, coagulation, sedimentation-flocculation etc.), biological treatment, advanced oxidation processes (AOPs) (photocatalysis, UV/H₂O₂, UV-ozonation, sonochemical, photo-Fenton, etc.). Among these, conventional and biological treatments have some limitations based on cost, high energy expenditure, and complete destruction of pollutants [9], [10], [11], [12]. In contrast, AOPs are promising technologies that offer more advantages like high treatment efficiency, creation of stable end products, robustness and ease of automation. The main advantage of AOPs is the formation of reactive oxygen species (ROS) during the process. AOPs involve in the process of in-situ production of ROS like •OH, •O₂^-, and H₂O₂ that stimulate oxidation process which leads to efficient degradation of pollutants. Among the various AOPs, photocatalysis has concerned more consideration in the degradation process of antibiotics owing to its high proficient and photonic sustainability [13], [14]. Over the last few years, many compounds based on unary, binary, ternary, quaternary like TiO₂, ZnO, WO₃, ZnS, NiO, Fe₂O₃ [15], [16], [17], TiO₂/g-C₃N₄, ZnO/BiOBr, BaTiO₃/Bi₂O₃ [18], [19], [20], Fe₃O₄/BiOCl/BiOI, Fe₃O₄/BiVO₄/CdS, g-C₃N₄/carbon nanotubes/Bi₂₅FeO₄₀ [21], [22], [23], g-C₃N₄/NiO/ZnO/Fe₃O₄, Fe₃O₄/ZnO/Ag₃VO₄/AgI, BiOCl/g-C₃N₄/Cu₂O/Fe₃O₄ [24], [25], [26], have been used for the degradation of antibiotic based effluents. Nevertheless, the research on cost effective and non-toxic materials is still challenge. For that, the metal oxide and carbonaceous material based photocatalysts with cost effective and non-toxic nature have been developed for antibiotic degradation.

Metal oxides like NiO, TiO₂, ZnO, and WO₃ have often been explored as a photocatalyst in antibiotic degradation. As promising metal oxides, ZnO and NiO have paid more attention in antibiotic degradation owing to their prestigious properties. ZnO has been paid immense consideration owing to its superior properties like non-toxicity, n-type conductivity, wide band gap (3.37 eV), number of electrons, high mechanical and thermal stability, and UV light blocking capability. Moreover, ZnO is considered as one of the GRAS materials pointed out by U.S. FDA [27], [28]. NiO has also been explored owing to its prominent properties like p-type, wide band gap (3.5 eV), electric and magnetic properties and high chemical stability [29], [30]. Researchers have recently concentrated on one of the carbonaceous semiconductors called graphitic carbon nitride (g-C₃N₄) owing to the following properties: non-metallic nature, high visible light response, high heat and chemical resistance, band gap energy of 2.7 eV, non-toxic nature, cost-effective and simple synthesis. Due to the abovementioned reasons, g-C₃N₄ is used in all kinds of applications like effluent degradation, water splitting, CO₂ reduction and energy storage [31], [32]. Moreover, it is one of the reliable materials used in antibiotic degradation. As a visible light activist, g-C₃N₄ gives more favourable effect to the system.

Even ZnO and NiO have lots of advantages, they absorbs photons in UV region due to their wide band gap and suffers from fast charge recombination. Hence, in order to improve their wide range photon responsibility/absorption, they coupled with g-C₃N₄ which leads electron-hole recombination. Especially, owing to sheet like carbonaceous support of g-C₃N₄, it acts as a conductive ground for ZnO and NiO which leads high charge carriers mobility and restricts the photo corrosion of metal oxides. It bestows mass diffusability and enhances specific surface area and reusable capability through long-term photo stability. ZnO/K-CN [33], P & S codoped g-C₃N₄ [34], Ag-Bi₅FeTi₃O₁₅/g-C₃N₄ [35], g-C₃N₄/C/Fe₃O₄ [36], NiO/g-C₃N₄ [37], MnO₂/NiO [38], NiO/NiC/g-C₃N₄ [39], Cu₂O/ZnO [40], Ag₂O-ZnO [41], N-ZnO/MoS₂ [42], are some composites developed using g-C₃N₄(CN), NiO and ZnO for tetracycline degradation.

In this work, we prepared ZnO/g-C₃N₄, NiO/g-C₃N₄, ZnO/NiO, ZnO/NiO/g-C₃N₄, composites using co-precipitation method to explore the photocatalytic degradation of tetracycline in detail. Moreover, the bactericidal performance of the prepared composites is also explored against various bacteria.