An ultralow base weight of nanocellulose boosting filtration performance of hierarchical composite air filter inspired by native spider web

Highlights

• A biomimetic composite air filter with ultralow base weight has been developed.

• Cellulose nanofibrils were self-assembled into robust nano-networks.

• A high efficiency of 94.5% and low pressure drop of 296.3 Pa were achieved.

• A filtration efficiency of 75% for PM0.3 could be achieved at 80% RH.

• All the materials we used are biodegradable.

Abstract

The beauty of nature is inexhaustible to human beings, and the process of human understanding and learning nature promotes the progress of science and technology. Inspired by the natural hierarchical spider-web structure, a biomimetic composite air filter with high filtration performance and good wet stability is prepared for the first time. Cellulose nanofibrils (CNF) generated from regenerable jute plants are implanted into the pores of commercialized biodegradable porous corrugated papers and self-assembled into robust nano-networks as filter building blocks to prepare a biodegradable composite air filter. When the base weight of CNF goes up to only 0.2500 g/m², the CNF nano-networks completely fill the pores of corrugated paper, which greatly reduces the consumption of raw materials. More importantly, the filtration efficiency for PM0.3 removal of resultant corrugated paper is increased from 6% to 94.5%, while maintaining a relatively low pressure drop of 296.3 Pa. After a silane modification of CNF, the filtration efficiency of 75% can be achieved at high humidity. Overall, the successful synthesis of biomimetic composite air filters provides inspiration for the design of separation materials, satisfying the growing demand for renewability, sustainability, and biodegradability.

Introduction

Nature is an inexhaustible treasure from which humans can draw inspiration in all aspects. Many creatures have unique abilities: bats use ultrasound to achieve accurate positioning [1]; octopus suckers have strong adsorption capacity [2]; plants dissipate heat by water transport and evaporation through the vascular tissue [3,4]. Nature endows them unique abilities that humans can use to their own advantages through study and imitation, leading to the development of numerous biomimetic structures [[5], [6], [7], [8]].

Spiders can catch food more efficiently by weaving webs among tree branches, which is similar to particulate filtering. The branches can enhance the mechanical properties of the spider webs and significantly reduce the base weight of spider silk without sacrificing prey capture. Inspired by the hierarchical spider web structure, an air filtration material with a biomimetic structure has been conceived: easily available porous corrugated paper is used as a frame, and smaller networks are constructed in the pores. It is envisaged that this biomimetic structure can not only provide stronger mechanical properties for smaller networks, but also greatly reduce the consumption of raw materials without compromising filtration performance. In addition, conventional air filters are basically derived from petroleum-derived polymers such as polypropylene (PP) and polyethylene (PE) [[9], [10], [11]], which cannot meet the needs of human beings, the green and biodegradable materials are what we are hunting for [12,13].

Cellulose is a promising candidate as the most abundant renewable biopolymer on Earth [[14], [15], [16], [17], [18]]. Cellulose nanofibrils (CNF), which are the basic elements of cellulose, have many advantages such as high specific surface area (SSA), easy functionalization, outstanding mechanical properties, and biodegradability [[19], [20], [21], [22], [23], [24]]. Hence, CNF is considered a potential green filtration material. For example, Fan et al. [25] used cellulose microfibers and bacterial nanocellulose coated by protein nanoparticles as frame and functional fillers, respectively. The protein-functionalized CNF could help expose the functional groups of protein for trapping pollutants, a high filtration efficiency of above 99.5% for PM1-2.5 and an extremely low normalized pressure drop of 0.194 kPa/g were achieved. Liu et al. [26] prepared CNF aerogels with a coexisting structure of larges fibers and films using a freeze-drying technique. And the CNF aerogels exhibited super-hydrophobicity and high-efficiency filtration. Excellent filtration performance of CNF-based air filters was achieved, however, a very large demand for raw materials of CNF was also indispensable.

In this work, a biomimetic composite air filter based on the natural hierarchical spider-web structure with high filtration performance and wet stability is prepared. Initially, the CNF suspension was obtained from the jute plant by nanofibrillation, as can be seen in Fig. 1a. Then the CNF suspension was modified with MTMS and sprayed on the corrugated paper by a nebulizer. Subsequently, the sample was frozen by being immersed in liquid nitrogen and then immediately sublimated in a freeze-dryer. Eventually, CNF was self-assembled into robust nano-networks among corrugated paper fibers. CNF nano-networks almost completely filled the pores of corrugated paper at an extremely low base weight of CNF, which greatly reduced the consumption of raw materials. By silanization hydrophobic modification of CNF, nano-networks were stable at high relative humidity (RH). CNF nano-networks would be able to intercept and adsorb PM, resulting in a significant improvement in the PM filtration capacity, as can be seen in Fig. 1b. This work shows great potential for the application in air filtration of CNF and provides inspiration for the design of separation materials.