The rapid industrialization and population growth is a cause for environmental pollution and become a major global concern [[1], [2], [3], [4]]. In every year, the levels of pollutants are increasing due to the discharging of industrial effluents to the environment [5]. Particularly, the industrialization process largely contributed to severe deterioration of freshwater resources [6,7]. Moreover, large amount of untreated wastewater from the production process of carpet, paper, leather, printing, pharmaceuticals, and paint and textile industries have been discharged in to the environment and become serious global warning issues. Most of the contaminants are also soluble in water and easily transported which can be the cause for direct and long-term toxic threat [[8], [9], [10]]. Specifically, pollutants such as persistent organic pollutants (POPs) are highly toxic and can't be degraded in the natural environment [11]. Among the toxic pollutants, organic and inorganic pollutants [12] including volatile organics [13], dioxins [14], dibenzofurans [15], pesticides [16], nitro-aromatic compounds [17], polychlorinated biphenyls [18], chlorophenols [19], and heavy metals and radionuclides [20,21], etc. are well reported. Since water is very important for human and animal survival, wastewater containing toxic chemicals should be treated before it is discharged to the environment [22,23].

Recently, different kinds of wastewater treatment methods have been employed [[24], [25], [26], [27], [28], [29]]. However, mainly, wastewater treatment techniques should be low cost, non-toxic, and efficient [30,31] Among the methods used, advanced oxidation processes (AOPs) consider as green and an effective method, have been utilized for the decontamination of toxic pollutants [32]. Since an extensive range of hazardous substances are being released in to the surrounding continuously, the complete mineralization or changing them in to less harmful intermediates is a vital process [33,34]. For this purpose, the removal of harmful contaminants with photocatalytic degradation processes has been reported by different researchers [35,36]. Moreover, semiconductors nanomaterials are not only useful for the removal of organic and inorganic pollutants but also applicable in the antimicrobial, and other activities [37,38]. It is also suggested that metal oxide nanoparticles such as ZnO exhibited a very strong antibacterial agent. In the antimicrobial activity, an electrostatic interaction of ZnO NPs and cell walls is responsible for the distraction of the bacterial cell wall [39]. However, searching and designing of cost-effective, environmentally friend, and efficient photocatalyst materials is still the main challenge [40,41].

In recent years, metal oxide-based materials are considered as effective photocatalysts and attract a great attention owing to their high chemical stability, nontoxic nature, and low cost [[42], [43], [44]]. Moreover, the catalysts with appropriate band alignment and multifunctional activities also become promising materials [45]. Among materials, ZnO has been extensively studied owing to its environmentally friendly, unique properties, stability, catalytic activity, and low cost [46,47]. However, the photocatalytic activities of ZnO is limited for practical application as a result of its wider band gap energy, higher recombination rate of the photogenerated charge carriers, and poor adsorption abilities to the visible light [48]. Owing to this reason, modification of ZnO with different techniques is needed to enhance its photocatalytic performance [[49], [50], [51], [52]]. For example, doping of the transition metals or making composites with ZnO and other semiconductors are effective methods to enhance the photocatalytic activities with the benefit of lowering band gap energy, enhancing charge separation, and boosting catalytic activity through electron trapping mechanism [[53], [54], [55], [56]].

In addition to performance enhancement, the fabrication method is also vital and it should be cheaper, greener, and facile method [57,58]. For example, the conventional physicochemical synthesis process requires hazardous chemicals, high energy, and expensive equipment. Due to this reason, the implementation of green and environmentally friendly, cheaper, and biogenic mediated catalyst fabrication method is needed [59,60]. Although there are numerous reports on green synthesis of ZnO-based catalysts with plant extract for environmental remediation, recent and well-organized review papers are limited. Most of the review papers written were mainly focused on the synthesis of either metal nanoparticles or metal oxide only [[61], [62], [63]]. On the other hand, the green synthesized ZnO-based such as ZnO, transition metal doped ZnO, or ZnO coupled with metal and other semiconductors for environmental remediation have rarely been reviewed to the best of our knowledge. To fill this gap, we aim to provide a timely and comprehensive review of the green synthesized ZnO-based catalyst for environmental remediation.

This review summarizes the basic principles of heterogeneous photocatalysis, strategies for photocatalytic performance enhancements, and the current trend on green synthesized ZnO-based nanocatalyst for environmental remediation. In detail, the plant extract methods, the catalyst synthesis mechanism and characterizations, and the optical, structural and morphological properties of the resulting synthesized materials are included. The green-synthesized ZnO-based nanocomposite and their potential applications for environmental remediation are also highlighted. Moreover, the opportunities, challenges, and future outlooks in the development of a new ecofriendly treatment method with green and sustainable strategic approaches are included.