Recent advances in applications for air pollutants purification and perspectives of electrospun nanofibers

Abstract

At present, the air is polluted to by sources like particulate matters and industrial waste gases, etc. Air pollution not only causes serious harm to human body, but also is the reason for the climate changes. Electrospun nanofibers are one of the most effective air purification materials to rapidly capture fine particles and gaseous wastes due to their high porosity and special skeleton structures. Thus, the present paper reviews the recent progress in the design of electrospun nanofibers and their practical applications in air purification. Firstly, the basic equipment and principles of electrospinning are introduced for a better understanding of the technology. Secondly, the latest development trends of electrospinning technology are summarized, including the raw materials, methods, structures, types and simulations of electrospinning nanofibers, and the structure optimization strategies for electrospinning nanofibers are outlined in detail. Third, recent reports on the use of electrospun nanofibers in gas purification and particulate filtration are extensively reviewed, focusing on the most prominent examples. Finally, perspectives are provided on growing opportunities, challenges and emerging trends. Electrospun nanofibers and their derived composites will be key to meeting future demands for high-performance nanofiber purification materials.

Introduction

Given the presence of poisonous pollutants in the air, the ongoing problem of air pollution across the globe greatly harms public health (Wang et al., 2021). A World Health Organization (WHO) report states that more than 90% of the world's population is still exposed to air quality levels that exceed WHO limit in the 2018 (Lv et al., 2018). The sources of air pollutants are quite wide, among which, the emissions from the consumption of fossil fuels and biomass are the main sources of various air pollutants, including particulate matters (PMs), volatile organic compounds (VOCs), CO, H₂S, NO_x and so on (Fig. 1) (Feng et al., 2020b; Khan et al., 2019). Besides that, industrial processes (Asghar et al., 2021), solvent use (Zhang et al., 2020c), volcanic activities (Saxena and Sonwani, 2019), spontaneous fires (Chen et al., 2021), plant and microbial releases (Liao et al., 2021) etc., can also produce large amounts of air pollutants.

An efficient air filter is a suitable choice to capture different air pollutants. Ideal air filters can not only capture aerosol particles and gaseous pollutants, but also allow air to pass through them easily at the same time (Sun et al., 2022b). Various materials, including zeolites, metal-organic-frameworks (MOFs), carbon nanotubes, nano- and micro-fibers have been explored for efficient air filtration (Nicosia et al., 2015). Among them, nanofibrous membranes with small diameter, high specific surface area, porosity and filtration efficiency are excellent materials for the purification of gaseous wastes or isolation of small size particulates (Naragund and Panda, 2022).

Compared with other preparation methods of nanofibers, such as phase separation, self-assembly, drawing, flash spinning, melt blown method and centrifugal spinning, etc., electrospinning technology has obvious advantages in process controllability, continuity and repeatability (Jain et al., 2020). Electrospun nanofibers have the advantages of smaller diameter, larger specific surface area, higher porosity and better homogeneity (Liao et al., 2018). Reports on electrospun air filtration materials can be dated back to the late 20th century, when Rangarajan et al. found that the direct deposition of electrospun nanofibers on nonwovens could significantly improve their filtration performance (Rangarajan et al., 1999). With the widespread application of computer technology in scientific experiments, the understanding of the dynamic process of air filtration and its underlying mechanism have been continuously strengthened (Li et al., 2014). Under the pursuit of higher standards of environmental protection, a large number of research results on advanced naturally degradable bio-based electrospinning nanofiber filtration membranes have emerged (Deng et al., 2021). To date, the development of electrospun air filtration materials has shown a vigorous trend, which has been considered as one of the most promising materials to control air pollution (Li et al., 2019b).

In conceiving this review, the comprehensive overview of electrospinning is reviewed, including the principle, methods and perspectives, and more importantly, its applications of electrospun materials in air purification, namely the removal of gaseous contaminants and PMs. Firstly, the principle and equipment of electrospinning are described. Then, routes to flexible modification of pristine electrospinning to obtain nanofibers with special structures are summarized. Afterwards, design strategies based on the compositions, structures, and properties of electrospun nanofibers to purify various air pollutants are discussed. At the end, views on the challenges and opportunities of electrospinning development trend are proposed.