Trends in Sourcing for Filter Media Applications
In recent years, natural materials, such as silk, wood, and cotton, have held great attraction from the filtration industry due to their superior sustainability and low carbon footprint features – they reduce our reliance on oil and petrochemicals and put less pressure on the environment, by the global trend of sustainability and low carbon footprint. In addition, their unique structures and properties also make them ideal choices for certain filtration application scenarios and fields – in some cases, they could outperform their petrochemical counterparts with higher efficiency, less energy consumption, or other benefits due to their advantages in mechanical properties, stability, and compatibility, among many others.
In this article, we will examine recent trends in filter media materials developed by taking advantage of natural materials’ unique features. These materials include plant and animal materials such as silk and wood, as well as regenerated materials such as chitosan and cellulose, which are, of course, made from plant and animal materials such as cotton and shellfish.
Mechanical Strength, Compatibility, Functional Groups
Recently, the University of Hong Kong (HKU) announced that their research team, led by Prof. Tang Chuyang, had developed a nanofiltration membrane incorporating a silk nanomaterial interlayer, sandwiched by a porous substrate and a selective layer, e.g., a polyamide nanofilm acting as a rejection layer. This membrane filters water nearly 10 times faster than most commercially available nanofiltration membranes, while offering close to 80% reduction in energy consumption and greenhouse gas emissions. HKU says this technology “could revolutionize the water purification or treatment process.”
University of Hong Kong has developed a nanofiltration membrane incorporating a silk nanomaterial interlayer. This membrane filters water nearly 10 times faster than most commercially available nanofiltration membranes, while offering close to 80% reduction in energy consumption.
In fact, this membrane’s performance is partially built on silk materials’ distinguished advantages:
The silk nanomaterials are strong enough to facilitate the formation and improve the stability of the polyamide rejection layer and the whole membrane. Silk is one of the strongest natural fiber and consists of two main proteins, fibroin and sericin. Fibroin is the silk’s structural center, while sericin is the glue-like sticky material surrounding fibroin. Sericin is removed from raw silk to form degummed silk, which is then hydrolyzed to be broken into silk nanomaterials, which inherit and even improve the good mechanical properties of natural silk.
In HKU’s invention, the pore sizes of the membrane’s microfiltration substrate are much larger than those of a conventional substrate. This requires a strong material to support the polyamide rejection layer over a large distance, and silk nanomaterials do a good job of meeting this requirement.
At the same time, silk nanomaterials are also compatible with polyamide, the polymer used to make the rejection layer. Silk nanomaterials and polyamide both have amide functional groups, which provide good compatibility between the two layers and maintain good stability and selectivity.
In addition, silk nanomaterials have abundant surface charges, which can form strong electrostatic forces with charged polymers or other particles.
All the above advantages form a silk nanomaterial layer that allows for fast water transport and a nanofiltration membrane with improved water permeance.
In other applications, silk materials are also used in filter media for their biocompatibility. Sericin could result in allergy and immune rejection responses, but degummed silk materials that completely remove sericin have excellent cytocompatibility and biocompatibility with other commonly used biomaterials. Therefore, degummed silk materials are often used for medical filtration purposes, such as making a rejection layer or incorporating a rejection layer in a dialysis filter media.
Other research focuses on improving silk nanofibers’ properties to expand their usage. For example, although the existence of sericin do negatively affect the forming process of silk nanofibers, keeping a small amount of sericin under a certain level could significantly increase the nanofibers’ mechanical properties and stability in water filtration applications while still maintaining acceptable fiber-forming results, according to recent research from China’s Anhui Polytechnic University.
The above research has led to some good results in filtration applications. For example, HKU’s nanofiltration membrane has shown improved water permeance and a high rejection rate against target ions, such as divalent ions and multivalent ions, in gravity- and vacuum-driven water filtration systems that incorporate single or multiple nanofiltration membranes for treating seawater, surface water, groundwater, and wastewater. Unlike most commercially available nanofiltration membranes, which operate under a high pressure of up to 10 bar, HKU’s new product achieves water purification at much lower pressure, for example, at smaller than one bar by using a partial vacuum.
Structure
The above developments take advantage of natural materials’ mechanical and chemical features. On the other hand, some other researchers focus on natural materials’ intrinsic structure. Some noticeable examples are from wood membranes.
Wood consists of four major layers: outer bark, phloem, cambium, and xylem (sapwood and heartwood). Xylem is often used to make filtration membranes. In hardwood, the xylem consists of a vessel, tracheid, wood fiber, xylem ray, and xylem parenchyma cell, and the vessel is used to deliver water and inorganic salt; but in softwood, there is no vessel, and the tracheid is used for the same purpose.
There are pits on the cell walls of vessel and tracheid. These pits have many nanometer pores, which allows water and small impurities to pass through, while holding back large particles such as bacteria. Therefore, in softwood, the water and inorganic salt are in fact transported through the nanopores in the cell walls of tracheid, which block bacteria and other large impurities and deliver cleaner water, and thus form an excellent natural filtration membrane – research shows that natural softwood membranes could remove 99.9% of the bacteria from an inactivated Escherichia coli (E. coli) solution.
Currently, research and development on wood membranes mostly focus on modifying them to meet the needs of diversified filtration applications. Examples include delignification, metal modification, etherification, esterification, carbonization, and incorporating activated carbon.
Delignification is a process that removes lignin and sometimes partial hemicellulose from the cell walls of vessels and trachea. These cell walls consist of three main materials: cellulose, hemicellulose, and lignin. Removing the lignin or partial hemicellulose could increase the number of nanopores, thus improving the filtration efficiency.
Metal modification is a process of modifying wood membranes with metal nanoparticles or metal-organic framework, which can utilize metals’ antibacterial activities and other advantages in water filtration.
Research in Africa in 2019 showed that a gravity-driven water filtration system using wood membranes made from four indigenous species, with activated carbon incorporated, could remove 99.9% of E.coli from water. This research encouraged several subsequent studies and showed the potential of making simple and low-cost water filtration systems for poor populations in Africa and other regions.
China’s Qingdao University developed a chitosan fiber-based composite for medical filtration applications, made by electrospinning a skinny layer of chitosan and polyethylene oxide (PEO) composite nanofiber membrane on the surface of a 100% spun lace chitosan nonwoven fabric.
Antibacterial Activity
Natural materials are also used in filter media for their antibacterial activity. For example, China’s Qingdao University developed a chitosan fiber-based composite for medical filtration applications. The material is made by electrospinning a skinny layer of chitosan and polyethylene oxide (PEO) composite nanofiber membrane on the surface of a 100% spun lace chitosan nonwoven fabric.
This composite exhibits good air filtration performance (99.56% filtration efficiency against 300-nanometer sodium chloride aerosol particles) and excellent antibacterial ability (99.97% blocking rate against E. coli and 99.88% against S. aureus), while maintaining a good strength similar to the original 100% chitosan nonwoven. All these benefits allow it to be used as a filtration media in surgical masks, medical air filters, and other medical applications.
Chitosan is produced from chitin, the second largest natural polymer in the world after celluloses. The outer skeleton of shellfish, including crab, lobster, and shrimp, produces over 10 billion metric tons of chitin every year.
Biodegradability
The above efforts often incorporate petroleum-based materials or use chemicals that pollute our environment, thus reducing natural materials’ advantages in regard to sustainability. Therefore, some other efforts focus on making filter media solely from natural materials or their composites through a green production process.
An example is from silk nanomaterials again. China’s Wuhan Textile University (WTU) has developed a high-efficiency low-resistance microporous aerogel with silk microfibers and nanofibers. The manufacturing process includes several rounds of freezing, dissolving, and drying, with the help of alcohol, ether, or ketone regulators and other chemicals that are volatile and easy to recover. The result is a fully biodegradable aerogel made through a process that has little environmental pollution.
The researchers used silk-based aerogel to make filter cotton, which has a density ranging from 1 to 50 mg/cm3, for air purification. This cotton shows advantages of high filtration efficiency, low filtration resistance, and high dust holding capacity. The researchers ascribed these advantages to the aerogel’s high porosity, dense and uniform pore sizes, big specific surface area, and excellent mechanical properties and structural integrity.
In addition to using a single source of natural material, researchers also mix different types of plant and animal materials to form new composites with better performance or unique advantages to meet the needs of diversified filtration applications. Examples include cellulose and chitosan composites for making aerogels and cellulose and protein composites for air purification.
Recycling Waste Natural Materials
The chitosan filter material mentioned above is made from shellfish waste. In addition, researchers are using waste of cotton, wood, bamboo, silk, among many others, for making filter media materials. For example, waste cotton, wood, and bamboo can be turned into cellulose fibers for making cellulosic aerogels for air filtration. Another example is that waste silk can be used for making aerogel that shows excellent filtration and absorption ability in removing oil pollution – the oil can be recovered and the aerogel can be reused, according to a recent research by Dalian Polytechnic University.
Even the natural materials removed from the previously discussed process, such as lignin and sericin, have their fans. For example, research by China’s Changzhou University found that adding lignin to a polyamide selective layer of a nanofiltration membrane would significantly increase the permeation flux and the retention rate of the membrane in filtering sodium sulfate and dyes.
