Fabric Close- Loop Recycling: An Overview
To achieve the closed-loop recycling of Natural and Artificial Fabrics into their corresponding monomers, we have developed a multi-module, multi-functional, and high-biosafety fabric degradation bioreactor for the project Fabrevivo. The construct includes a fabric pre-treatment module, two detachable, interconvertible fermentation modules, and one autoclave module. This model is designed for the complete bio-degradation and closed-loop recycling of natural cotton fibres, artificial polyester fibres, and cotton-polyester blends.
figure1 An overview of our multi-module Cellulose-PET degradation Bioreactor. The whole design includes a pre-treatment model (top-left), two fermentation modules (bottom-left), and a separate autoclave module (right). The untreated fabrics enter the machinery through the large, cylindrical blades of the pre-treatment module.
figure2 The longitudinal section of our whole model. The autoclave(left) functions for the final sterilization of our product of degradation. The pre-treatment module(top-right) consists of two large cylindrical blades, which reduce the surface area of the substrate, and a pre-treatment tank for chemical pre-treatment. The two fermentation modules(bottom-right), one loaded with scaffoldin and Cellulase-expressing K.marxianus strains, serves for the degradation of cellulose to glucose, whilst the second, loaded with scaffoldin and PETase-expressing K.marxianus strains, serves for the degradation of Polyethylene terephthalic acid (PET).
The Chemical Pre-treatment Module
The first module - the fabric pre-treatment module - consists of two large cylindrical blades and a chemical pre-treatment tank with built-in electric-heating apparatus. The untreated, useless, and contaminated commercial fabrics are initially sliced into small-sized threads to increase the surface area for chemical treatment and cellulosome-based enzymatic degradation. The treatment tank, with a constant core temperature of 75℃, is filled with hydrogen peroxide or other dye-removing agents and then treated with a dilute sulfuric acid solution to thoroughly remove the acetyl groups on the cellulose acetate fibres. This treatment removes most of the dyes and impurities from the fabric from our desired degradation target.
figure3 The 3D structure of the pre-treatment module. The two intersecting cylindrical blades (opaque black) slice the target of degradation into minuscule threads, facilitating the subsequent chemical pre-treatment(the first transparent tank) and enzymatic degradation(the two transparent fermentation modules). A core temperature of 75℃ is maintained in the pre-treatment tank, with dilute sulfuric acid, H2O2 or other dye-removing agents filled within the space.
Detailed Arichitecture of The Fermentation Modules
The two fermentation modules - the cellulose degradation module and PET degradation module, are comprised of a cylindrical fermentation chamber with a cone-shaped bottom surface equipped with an electro-magnetic ring, an electric-heating apparatus which maintains the core temperature of the modules to 50℃, and a six-bladed shutter which controls the movement of substances in and out of the module. The first module (cellulose fermentation module) is designed for completely degrading natural fibres and hosts the scaffoldin-expressing K.marxianus strains and cellulase-expressing K.marxianus strains. The second module (PET fermentation module) is designed for completely degrading synthetic fibres (PET) and hosts the scaffoldin-expressing K.marxianus strains and PETase-expressing K.marxianus strains. The high temperature within the container will attenuate the glucose-consuming activity of the K.marxianus strains. Thus the end-product of cellulose digestion can be mostly protected from microbial consumption.
figure4 An outer view of the two fermentation modules(lustrous-silver). The upper fermentation module contains the scaffoldin K.marxianus strains and cellulase K.marxianus strains. Together they express the protein components of a fully functional membrane-anchored cellulose-degrading cellulosome complex, whilst the second module contains K.marxianus strains that express the protein components of a functional PET-degrading cellulosome-based complex. Both modules contain a six-bladed shutter-like opening(bottom-middle) on their base, which controls the flux of degradation products and degradation intermediates through the different modules.
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To avoid the loss of K.marxianus in each cycle of degradation, the peripheral circular electromagnet powers on at the end of each recycling period and attracts the ferritin-expressing magnetic sensitive K.marxianus to the periphery of the cylindrical fermentation container, further away from the shutter exit in the middle. After most of the K.marxianus yeast is immobilized on the gradient of the cone-shaped bottom of the container via electro-magnetism, the six-bladed shutter automatically opens and delivers the reacted products to the next chamber. The two fermentation modules are interconvertible, so their combination or position could be switched based on the type of untreated fabric for recycling. When the target of degradation is purely constituted of natural fibres, a single cellulose-degradation module will be sufficient; when polyester is the only substance present in the untreated mass, a PET degradation module should be deployed. For the complete degradation of Cotton-polyester blends or other combinations of natural and synthetic fibres, two modules should be simultaneously applied.
figure6 The transverse section of the degradation consortium on the plane of a fermentation module. When a degradation cycle is completed, and all the insoluble substrates are converted into their target soluble products, the electromagnet chunks on the periphery(lustrous silver) will be powered on and attract magnetic-sensitive K.marxianus yeasts along the inclined bottom plane. After most of the yeasts have been localized to the periphery, away from the central exit, the six-bladed mechanical shutter will automatically open, delivering the degradation product to the next module of this degradation consortium.
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Biosafety First - The Autoclave Module
The last module of this construct - the autoclave module, with a core temperature of 121℃, is applied in the end to sterilize the degradation product. Although K.marxianus only has a Biosafety Level of BSL-1, strict commercial or industrial usage protocols must be applied. The end-product of degradation: glucose, ethylene glycol, and terephthalic acid, could be further separated via HPLC. The former could be used to make new bacterial celluloses (BC), while the latter could synthesize new artificial fibres via chemical polymerization.
figure9 The 3D longitudinal section of the autoclave module. After the end-product of degradation is settled in the perforation basket(middle), which is hovered from the pressurized container via a circular steel scaffold, the core temperature of this module will be increased up to 121℃, the sterilizing temperature. The pressure of this apparatus is controlled by two air valves(top).
