Abstract
The control of airborne contaminants is of great interest in improving air quality, which has deteriorated more and more in recent years due to strong industrial growth. In the last decades, cellulose has been largely proposed as suitable feedstock to build up eco-friendly materials for a wide range of applications. Herein, the issue regarding the use of cellulose to develop air-filtering systems is addressed. The review covers different cellulose-based solutions, ranging from aerogels and foams to membranes and films, and to composites, considering either particulate filtration (PM10, PM2.5, and PM0.3) or gas and water permeation. The proposed solutions were evaluated on the bases of their quality factor (QF), whose high value (at least of 0.01 Pa−1 referred to commercial HEPA (high-efficiency particulate air) filters) guarantees the best compromise between high filtration efficiency (>99%) and low pressure drop (<1 kPa/g). To face this aspect, we first analyzed the different morphological aspects which can improve the final filtration performance, outlining the importance on using nanofibers not only to increase surface area and to modulate porosity in final solutions, but also as reinforcement of filters made of different materials. Besides the description of technological approaches to improve the mechanical filtration, selected examples show the importance of the chemical interaction, promoted by the introduction of active functional groups on cellulose (nano)fibers backbone, to improve filtration efficiency without reducing filter porosity.
Highlights
Air pollution has increased globally as a result of rapid industrial growth [1]
Limiting our overview to the results reported in the literature mainly in the last decade, in the following, after a brief introduction to the main filtration properties which should be sought, we critically analyze several cellulose-based and cellulose-reinforced systems with characteristics suitable for ultrafiltration, mainly focusing on the difference in chemical and mechanical barrier mechanisms
The presence of an additional layer of cellulose nanofibers, which have excellent transparency, flexibility, and tensile strength properties, led to a drastic reduction in the oxygen permeability of the filters from a value of 746 to a value of 1 mL m−2 day−1. Another example of cellulose-reinforced filters is the work of Nemoto et al [13], previously reported for the synthesis of TEMPO-oxidized cellulose nanofibers (TOCNF) spider-like aerogels, where commercially available HEPA filters were combined with TOCNF/water/t-butyl alcohol (TBA) dispersions
Summary
Several aspects affect the distribution of contaminants in the air and soil, namely natural, chemical, and socio-economic factors [2]. Air pollution aerosol particles are typically composed by microscopic solid or liquid substances generally classified as a particulate matter under 10 (PM10) or 2.5 μm (PM2.5) [5,6] (Figure 2). For particles smaller than 0.3 μm, proper filtering systems are still under development. Cellulose-based filters and composites have found ample application in the field of CO2 capture and storage. For this topic, which would require an ad hoc discussion and for this reason is not object of this contribution, we refer the reader to recent and specific review literature [10]
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