Upcycling tannery sludge into superior micro/nano protein fibers to reinforce the mechanical and flame retardant properties of cellulose-based film

Upcycling tannery sludge into superior micro/nano protein fibers to reinforce the mechanical and flame retardant properties of cellulose-based film | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Upcycling tannery sludge into superior micro/nano protein fibers to reinforce the mechanical and flame retardant properties of cellulose-based film Jiang Wei, Ting He, Mengke Liu, Fanyu Kong, Weijian Dong, Xin Feng, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5099951/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 05 Feb, 2025 Read the published version in Cellulose → Version 1 posted 15 You are reading this latest preprint version Abstract Micro/nano protein fibers have attracted increasing attention owing to their advantageous properties for applications in advanced materials. Traditional preparation methods often suffer from high costs of raw-materials and energy-intensive manufacturing processes, which hinder large-scale production. Herein, we present an innovative low-carbon approach for converting tannery sludge into micro/nano protein fibers. The alkali-oxygen cooking combined with ultrasonic process shown the potential to reduce carbon emissions. The resulting micro/nano protein fibers reinforced the mechanical properties of the cellulose-based films. The maximum tensile force of the cellulose-based film was increased by 55.40%, and the Young’s modulus was increased by 22.92%. The micro/nano protein fibers also imparted remarkable flame-retardant characteristics, as indicated by an increased peak temperature of heat loss and a 21.60% reduction in the peak heat release rate of cellulose-based films. This low-carbon and ecofriendly process utilizing leather tannery sludge not only provides a sustainable source of raw materials but also contributes to the circular economy by repurposing industrial waste. tannery sludge micro/nano protein fiber flame-retardant alkali-oxygen cooking Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Tannery sludge is a typical hazardous solid waste, which mainly includes animal fur, a large amount of water and chromium could cause serious environmental pollution(Moktadir et al., 2023 ; Moktadir et al., 2023 ; Kilic et al.,2011). The conventional thermal treatment processes, such as incineration and pyrolysis, require dehydration of tannery sludge. Typically only around 60.00% of the moisture could be preliminary eliminated in tannery sludge, necessitating more advanced techniques for further drying, but it is difficult to avoid the impact of high cost(Sunmathi et al., 2022 ; Zhao et al., 2022 ). Moreover, conventional disposal approaches can give rise to secondary pollution concerns. Given that a significant amount of trivalent chromium is utilized as a tanning agent in leather production, incineration of tannery sludge can lead to the release of harmful heavy metals into the environment, while landfilling and composting may result in leachate contamination, causing notable environmental harm(Li et al., 2024 ; Ge et al., 2024 ). Reports have highlighted alternative strategies for tannery sludge treatment to mitigate chromium pollution, these strategies include the production of bricks(Juel et al., 2017 ), cement(Malaiskiene et al., 2019 ), and other useful products from tannery sludge, offering promising avenues for reducing environmental impact(Sunmathi et al., 2022 ). The escalating volume of tannery sludge underscores the urgency of implementing more efficient and eco-friendly disposal solutions to address the long-term environmental repercussions of this waste material. Micro/nano protein fibers have emerged as promising materials owing to their excellent mechanical properties, plasticity, and biocompatibility(Chen et al., 2023 ; Zhang et al., 2023 ; Zhang et al., 2023 ). The production of micro/nano protein fibers typically requires the large-scale collection of raw materials, which increases the associated costs once beyond the collection radius. The variability in raw-materials adversely affect product quality control because of differences in the functional groups of proteins obtained from various sources(Sun et al., 2020 ). In addition, considerable effort has been directed toward the production of regenerated protein fibers using various approaches, such as solvent blowing and electrospinning(Han et al., 2022 ; Dias et al., 2022 ). These methods have several barriers should be overcome, such as complex procedures and the need for relatively large organic solvent quantities which increases the cost of micro/nano protein fibers and the environmental pollution risk(Zhang et al., 2020 ). These challenges highlight the need for more reliable and consistent raw-material supplies and methods to enable the high-quality production of micro/nanoprotein fibers. On one hand, given the development of increasingly strict sewage emissions standards, how to dispose a large amount of tannery sludge reasonably has become a challenging problem(Saira et al., 2023). On the other hand, the development of micro/nano protein fibers lack of reliable and consistent raw-material supplies and green and low-carbon process. One promising two-in-one solution to tackle both problems is using leather tanning sludge as a feedstock for micro/nano protein fiber production. The main component of sludge is animal fur, which is rich in protein fibers, determines that it has the potential for the preparation of protein fibers while achieve the purpose of sludge treatment. Tannery sludge was extracted after tanning and extrusion dehydration process. Only mild reaction conditions are needed to reach the fiber separation point. Alkali-oxygen cooking is commonly used as an effective method for preparing fibers from biomass. This process uses only alkali and oxygen, has the advantages of low energy consumption, low carbon emissions, high efficiency and environmental friendliness. In our previous work, we found that this method could efficiently disperse fibers, making it one of the best candidates for converting tannery sludge into micro/nano protein fibers(Zhang et al., 2022 ; Qian et al., 2023 ; Zhang et al., 2023 ). In this work, tannery sludge was creatively converted into micro/nano protein fibers using an efficient and low-carbon alkali-oxygen cooking process. The physical, chemical, and morphological characteristics of the micro/nano protein fibers were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and laser particle size analysis. The produced micro/nano protein fibers were incorporated into cellulose-based films, and the reinforcement of the mechanical properties was evaluated via tensile tests, while the flame-retardant properties were evaluated via thermogravimetric analysis (TGA) and microscale combustion calorimetry (MCC). Owing to the network structure and dominant nitrogen and sulfur contents, the micro/nano protein fibers reinforced the mechanical properties and flame retardancy of the fiber materials. 2. Experimental section Materials The tannery sludge was initially dehydrated, and the remaining chemicals were used in their original state without further purification. The materials, chemicals, and detailed experimental procedures used in this study are presented in the Supporting Information. Preparation of protein fibers Tannery sludge (3 g) was weighed, and the required amount of NaOH was dissolved in 100 mL of distilled water. All of these were added to the reactor, and the required oxygen was introduced. After the reactor was assembled, the heating rate was set to 20 o C/min, and each experiment was conducted for 60 min. After alkali-oxygen cooking, the slurry was subjected to ultrasonic disintegration and treated at a 540 W output power for 10 min. Preparation of composite films The protein fibers were mixed into nanocellulose, and 50 mL of water was added with stirring to obtain a uniform mixture. To prevent the loss of fibers during the membrane preparation process and ensure the flatness of the fiber film, a nanofiltration membrane (D = 4 cm, 0.45 μm PE) was added to the sintered glass filter, and the cellulose-protein fiber composite membrane was prepared at 140 g/m 2 under 0.08-MPa vacuum suction. 3. Results and discussion Optimization of Reaction Conditions Using Response Surface Methodology Table 1 Experimental conditions of sample preparation and related results. Sample Alkali charge (wt% sludge) Oxygen consumption (MPa) Temperature ( o C) Yield (%) Chromium retention rate (%) 1 5 0.5 140 58.38 89.85 2 5 1 100 4.86 94.78 3 5 0.5 60 1.48 89.3 4 5 0 100 28.39 99.25 5 10 0 60 12.72 91.9 6 10 0.5 100 43.36 90.61 7 10 0.5 100 55.13 89.47 8 10 0.5 100 47.18 92.27 9 10 0.5 100 41.3 90.32 10 10 0 140 54.38 95.52 11 10 1 140 36.85 61.5 12 10 1 60 28.1 98.84 13 10 0.5 100 59.29 91.92 14 15 1 100 53.61 82.32 15 15 0 100 74.54 98.04 16 15 0.5 60 47.55 98.51 17 15 0.5 140 40.83 63.63 To determine the effects of the alkali-oxygen cooking conditions, a single-factor experiment was performed in the early stage of the reaction. The results revealed that the yield of protein fiber was mainly affected by the amount of alkali, while the retention rate of chromium was mainly affected by the reaction temperature(showed in the Supporting Information Fig. S1 -S5). Based on above mentioned results, 17 experimental trials were conducted to optimize the alkali-oxygen cooking parameters, and the results are presented in Table 1 . The three-dimensional (3D) response surface graphs and two-dimensional contour plots of the yield (< 5 µm) and chromium retention rate under different conditions are shown in Figs. 1 and 2 , respectively. The alkali dosage, oxygen consumption and temperature showed the complex interaction effects in the 3D response surfaces, the yield and size of micro/nano protein fibers depends on whether the reaction conditions can reach the fiber separation point(Fig. S8). Figure 1 shows that the yield of protein fiber increases with the increase of alkali dosage, oxygen consumption and temperature. The results shown that the effect of alkali amount is the most obvious which may attribute to the damage of NaOH on the intermolecular force between protein fibers and the fracture of the basic unit of the fiber which similar to the previous works(Gao et al., 2020 ; Zhai et al., 2020 ). The increase of temperature improved the efficiency, and a small consumption of oxygen can reinforce the mass transfer but excessive temperature or oxygen consumption may reduce oxidative cross-linking between protein fibers, resulting in a decrease in yield(Poole et al., 2008 ). The interaction between the amounts of alkali and oxygen has the least significant impact on the chromium retention rate in the three-way interaction of alkali, oxygen amount, and temperature(shown in Fig. 2 ). The chromium retention rate is particularly sensitive to temperature changes, and chromium can be easily released from protein fibers at high temperatures-especially when the alkali or oxygen dosage is higher than 100 o C. At low temperatures or alkali dosages, the chromium retention rate exhibited a parabolic trend with an increase in the temperature. This may be because the chromium release rate exceeded the capture capacity of the protein fiber network at high temperatures(Kar et al., 2004). To maximize both the yield and chromium retention rate, Design-Expert v12 software was used to comprehensively optimize the analysis of the two objectives from the perspective of cost. The reaction conditions for realizing the optimal theoretical yield of 63.32% and chromium retention rate of 97.78% were determined to be an alkali dosage of 15 wt%, oxygen consumption of 0.79 MPa, and a temperature of 69.57 o C. The predicted results were verified three times, and under these conditions, the micro/nano protein fiber yield reached 62.14%, and the chromium retention rate reached 97.61%. The experimental values agreed well with the predicted values as showed in Figs. S6, S7, Tables S1, S2. Morphology of micro/nano protein fibers As shown in Fig. 3 , large protein fiber bundles resembling hemp ropes (several to tens of micrometers in diameter) are observed in the tannery sludge, which were converted into micro/nanofiber networks after alkali-oxygen cooking and ultrasonic treatment. Alkali-oxygen cooking can cleave the intermolecular covalent bonds (disulphide crosslinks between cysteine residues) between protein fibers and destroy the covalent bonds of the primary protein chain(Poole et al., 2008 ). When the fiber bundles were treated with alkali-oxygen cooking, the fiber bundles were initially dispersed or broken into micro/nano scale, whereas the protein fibers were not totally dispersed. After the ultrasonic treatment process, the protein fibers exhibited an obvious regular network nano structure under TEM rather than being entangled or crosslinked into bundles.The micro/nano network structure was also observed by AFM and SEM after freeze-drying(Figs. S9, S10). This is attributed to the ultrasound having a cavitation effect that reinforced the mass transfer, accelerating the dispersion of fibers(Biswas et al., 2022). Additionally, the ultrasound modified the protein fiber surface by increasing the surface area and embedding hydroxyl group in it. These changes in the morphology and surface chemical properties also helped to capture chromium in the solution(Kar et al., 2004). Lifecycle assessment To assess the environmental impact of this process, we conducted a carbon lifecycle analysis using SimaPro, with the calculation boundary extending from the raw materials to the final products. The ecoinvent database served as the foundation for our analysis, and this method was compared with two alternative approaches: preparation of protein fibers via solution blowing and electrospinning(the data were obtained from published works(Sinha-Ray et al., 2011 ; Goyal et al., 2022 ). Our findings revealed that the proposed method is more ecofriendly and more conducive to environment and health. For example, it is 73.26% lower than the solution blowing method and 90.91% lower than the electrospinning method in reducing carbon dioxide emissions. As shown in Fig. S12, in order to judge the relative contribution of different preparation processes of protein fibers, the related factors of various preparation methods are divided into materials and chemicals, electricity consumption. The impact of solution blowing on the environment is mainly reflected in the amount of electricity consumed in the pretreatment of raw materials. Because the development of electrospinning technology is relatively perfect, low energy consumption leads to less impact on the environment. But in the raw material preparation stage, the treatment usually requires organic solvents and other chemicals, which will have a considerable impact on the environment. In contrast, the alkaline oxygen cooking method used in this work has the characteristics of using environmentally friendly chemicals and low energy consumption. X-ray and FTIR analysis of protein fibers at different reaction stages Tannery sludge is rich in inorganic salts because the tanning process, which can form micelles with the original fibers in the sludge, potentially affecting the XRD results. As shown in Fig. 5 a, the protein fibers in tannery sludge have a typical β-sheet structure, with a diffraction peak at 2θ = 20.90 o (Ha et al., 2005 ). Moreover, the diffraction peaks of inorganic salts such as CaSO 4 and Na 2 SO 4 appeared at 11.80 o and 29.20 o , while the diffraction peak of chromium oxide appeared at 37.2 o . After alkali-oxygen cooking, the micellar structure in the sludge was destroyed, and the internal crystal morphology was exposed. Thus, Fig. 5 a shows sharp peaks at 29.30 o , 31.70 o , and 33.90 o for the treated sludge sample, corresponding to inorganic salts such as CaSO 4 and Na 2 SO 4, which is consistent with previous results. However, the intensity of the diffraction peak of the protein fibers after cooking and ultrasonic treatment at 22.56 o was significantly increased, indicating that the structure of the protein fibers was changed after cooking, it was similar to that of silk I(Asakura et al., 2001 ). The abundant amino acids in the protein fibers were instrumental in crosslinking with cellulose as well as sulfur-containing groups, enhancing the flame-retardant properties. Fourier transform infrared spectroscopy was used to identify some of the functional groups that may play a role (Fig. 5 b). In the wavenumber range of 1700 − 1600 cm − 1 , the amide I band was observed, which is typically due to C-O stretching vibrations(Chen et al., 2020 ). The band at 1654 cm − 1 is often assigned to the α-helix structure of the protein fiber(Ling et al., 2011 ). According to the above XRD, FTIR and the earlier studies, this structure of the protein fiber similar to silk I at the 1653-cm − 1 band could still be assigned to the β-sheet structure(Jafari et al., 2023 ). The characteristic peak at 1442 cm − 1 corresponds to the vibration of CH 2 in the protein fibers, and the peak at 1137 cm − 1 in the range of 1000–1200 cm − 1 may be due to cysteine-S-sulfonated residues or crosslinked residues(Ramirez et al., 2021 ; Ebrahimgol et al., 2014 ). The bands at 624 cm − 1 are assigned to C-S stretching vibrations, and the intense peak at 873 cm − 1 corresponds to dehydroalanine groups, which are mainly formed by the cleavage of the C-S bond(Pourjavaheri et al., 2019 ). Most of the absorption peaks remained unchanged, but the peak intensity increased, indicating that the cooking process exposed more functional groups of the protein fibers. Reinforcement of mechanical properties of film The reinforcements in the mechanical properties of the composite films with different protein fiber contents (0, 5, 10, 15, and 20 wt%) are presented in Fig. 6 . Generally, the strength of fiber materials depends on the dispersion of the fibers and interactions between the fibers. As indicated by the stress-strain curves of the composite films (Fig. 6 b), with an increase in the protein fiber content, the break strength of the films first increased and then decreased. At a protein fiber content of 10 wt%, the tensile strength, Young’s modulus, and toughness of the composite film were significantly improved and reach the maximum value. After ultrasonic treatment, the protein fibers formed a network structure, interweaving with cellulose to make stress diffusion more uniform(Wan et al., 2022 ). In addition, new hydrogen bonds may formed between the protein fibers and cellulose, and the amide bonds may induce an orderly arrangement of fibers, improving their mechanical properties(Zhao et al., 2022 ). At a higher protein fiber content, the possibility of forming an interface layer between the protein fibers and cellulose increased, which blocked the continuity of the cellulose matrix and further disrupted the formation of hydrogen bonds(Wang et al., 2014). In addition, an appropriate protein fiber content can increase the transmittance, as shown in Fig. 6 e. In particular, for the 10 wt% sample, the transmittance was increased by 16.67% compared with that of the 0 wt% sample, indicating its potential for application in materials with high transparency requirements. Flame-retardant potential of films Protein fibers are among the most commonly used flame retardants in fiber materials because they form a carbon layer and a noncombustible gas, which hinder flame combustion(Alongi et al., 2014 ). The burning time of films with different protein fibers content was preliminarily evaluated(Fig. S11). Figure 7 shows the results of thermogravimetric experiment and microcalorimetry experiment, indicating that the protein fibers reinforce the flame retardancy of the cellulose-based film. The thermal stability of the films doped with protein fibers under nitrogen and air was evaluated to evaluate their degradation behavior. The protein fiber-added samples and blank samples exhibited a main weight-loss stage at 300–400 o C, which was due to the depolymerization of cellulose and the formation of aliphatic char. In the air atmosphere, the pyrolysis-rate peak at 500–600 o C corresponded to the conversion of aliphatic carbons into an aromatic form, which produced CO 2 (Rosace et al., 2018 ). In both environments, compared with the original sample, the initial decomposition temperature of the protein fiber-added samples was significantly increased by about 16 o C, which indicates that the incorporation of protein fibers can significantly improve the thermal stability of the film. As shown in Fig. 7 d and Fig. 7 e, the residual carbon content of the films doped with protein fibers was generally increased, which played a protective role and limits the contact between oxygen and combustibles(Liu et al., 2020 ). Amino acids, as the main component of protein fibers, were also found to be positively correlated with flame-retardant effect(Leong et al., 2021 ). More specifically, amino acids such as cysteine in protein fibers have been evaluated as effective flame-retardant systems for cellulose substrates because these components can affect the formation of carbon from cellulose pyrolysis(Alongi et al., 2014 ). Interestingly, we found that the film in the combustion process exhibited obvious stratification in addition to carbonization as shown in Fig. 7 a(ⅰ). The combustion of protein fibers can generate a certain amounts of noncombustible gases (including H 2 O, NO, and NO 2 ), which play the role of reducing the concentration of combustible gases in the layered gap and on the film surface(Sun et al., 2021 ). The fire-retardant properties of composite films with different protein fiber contents were investigated using MCC, as shown in Fig. 7 f and Fig. 7 g. The peak heat release rate (PHRR) and total heat release (THR) for the blank sample were 138.87 w/g and 7.10 kJ/g, respectively. The PHRR value and THR value of cellulose-based films decreased by 21.60% and 28.17% with the incorporation of protein fibers, respectively. This is mainly attributed to the formation of an effective carbon layer on the film surface, the mechanism is described in Fig. 7 a(ⅲ). The formation of a carbon layer inhibited the diffusion of volatile gases and heat transfer during combustion. 4. Conclusion In this work, micro/nano protein fibers were successfully prepared from tannery sludge through a low-carbon alkali-oxygen cooking combined with ultrasonic crushing. The yield of micro/nano protein fibers reached 62.15% while the retention rate of chromium reached 97.61%. This process was evaluated by LCA that indicts the carbon emissions was reduced by 73.26% compared with the conventional solution blowing method and 90.91% compared with electrospinning. Micro/nano fibers can significantly reinforce the mechanical properties and flame-retardancy of cellulose-based films. The tensile strength of the cellulose-based film was increased by 55.40%, and the Young 's modulus and toughness were also significantly improved. The peak heat release rate was reduced by 21.60%, and the total heat release rate was also reduced by 28.17%. The results showed that these functions were derived from the network structure of the protein fibers and the substances such as amino acids. This work broadens the source of raw materials for protein fiber preparation and provides an effective strategy for the resource treatment of tannery sludge. Declarations Declaration of interests The authors have no relevant financial or non-financial interests to disclose. Human Ethics and Consent to Participate declarations Not applicable Funding This work was financially supported by National Natural Science Foundation of China (No. 21908127, No. 91934302, No. 21978128) and the Project Supported by the Foundation (GZKF202103) of State Key Laboratory of Biobased Material and Green Papermaking. Author Contribution J.W., and H. T., performed the experiments, collected and analyzed the data. M. L., F. K.,X. F., and X. L., helped with data analysis and discussions. L.Z. and J. Z. conceived the experiments, planned synthesis, analyzed results, and wrote the paper. All authors reviewed the manuscript. Acknowledgement This work was financially supported by National Natural Science Foundation of China (No. 21908127, No. 91934302, No. 21978128) and the Project Supported by the Foundation (GZKF202103) of State Key Laboratory of Biobased Material and Green Papermaking. References Alongi, J., Bosco, F., Carosio, F., Blasio, A.D., Malucelli, G., 2014. A new era for flame retardant materials?. Materials Today. 17, 152–153. Asakura, T., Yamane, T., Nakazawa, Y., Kameda, T., 2001. K. Ando, Structure ofBombyx mori silk fibroin before spinning in solid state studied with wide angle x-ray scattering and13C cross-polarization/magic angle spinning NMR. Biopolymers., 58. 521–525. Biswas, S., Rashid, T.U., 2022. Effect of ultrasound on the physical properties and processing of major biopolymers-a review. Soft Matter. 18, 8367–8383. Chen, C., Feng, J., Li, J., Guo, Y., Shi, X., Peng, H., 2023. Functional Fiber Materials to Smart Fiber Devices. Chem Rev. 123, 613–662. Chen, D., Narayanan, N., Federici, E., Yang, Z., Zuo, X., Gao, J., Fang, F., Deng, M., Campanella, O.H., Jones, O.G., 2020. Electrospinning Induced Orientation of Protein Fibrils. Biomacromolecules. 21, 2772–2785. Dias, A., Cena, C., Lutz-Bueno, V., Mezzenga, R., Marques, A., Ferreira, I., Roque, A.C.A., 2022. Solvent modulation in peptide sub-microfibers obtained by solution blow spinning. Front Chem. 10, 1054347. Ebrahimgol, F., Tavanai, H., Alihosseini, F., Khayamian, T., 2014. Electrosprayed recovered wool keratin nanoparticles. Polym Advan Technol. 25, 1001–1007. Gao, J., Wang, Y., Yan, Y., Li, Z., Chen, M., 2020. Protein extraction from excess sludge by alkali-thermal hydrolysis. Environ Sci Pollut Res Int. 27, 8628–8637. Ge, R., E,T., Cheng, Y., Wang, Y., Yu, J., Li, Y., Yang, S., 2024. NaH(2)PO(4) synergizes with organic matter to stabilize chromium in tannery sludge. J Environ Manage. 351, 119843. Goyal, S., Dotter, M., Diestelhorst, E., Storck, J.L., Ehrman, A.n, Mahltig, B., 2022. Extraction of keratin from wool and its use as biopolymer in film formation and in electrospinning for composite material processing. J Eng Fiber Fabr. 17, 1–11. Ha, S.W., Tonelli, A.E., Hudson, S.M., 2005. Structural Studies of Bombyx mori Silk Fibroin during Regeneration from Solutions and Wet Fiber Spinning. Biomacromolecules. 6. 3, 1722–1731. Han, W., Wang, L., Li, Q., Ma, B., He, C., Guo, X., Nie, J., Ma, G., 2022. A Review: Current Status and Emerging Developments on Natural Polymer-Based Electrospun Fibers. Macromol Rapid Commun. 43, e2200456. Jafari, M.J., Bäcklund, F.G., Arndt, T., Schmuck, B., Greco, G., Rising, A., Barth, A., Ederth, T., 2023. Force-Induced Structural Changes in Spider Silk Fibers Introduced by ATR-FTIR Spectroscopy. ACS Appl. Polym. Mater. 5, 9433–9444. Juel, M.A.I., Mizan, A., Ahmed, T., 2017. Sustainable use of tannery sludge in brick manufacturing in Bangladesh. Waste Manag. 60, 259–269. Kar, P., Misra, M., 2004. Use of keratin fiber for separation of heavy metals from water. J Chem Technol Biot. 79, 1313–1319. Kilic, E, Font, J., Puig, R., Colak, S., Celik, D., 2011. Chromium recovery from tannery sludge with saponin and oxidative remediation. J Hazard Mater. 185, 456–462. Leong, W.I., Lo, O.L.I., Cheng, F.T., Cheong, W.M., Seak, L.C.U., 2021. Using recombinant adhesive proteins as durable and green flame-retardant coatings. Syn Syst Biotechno. 6, 369–376. Li, Z., Yu, D., Wang, X., Liu, X., Xu, Z., Wang, Y., 2024. A novel strategy of tannery sludge disposal - converting into biochar and reusing for Cr(VI) removal from tannery wastewater. J Environ Sci (China). 138, 637–649. Ling, S., Qi, Z., Knight, D.P., Shao, Z., Chen, X., 2011. Synchrotron FTIR microspectroscopy of single natural silk fibers. Biomacromolecules. 12, 3344–3349. Liu, Q., Yang, S., Ren, J., Ling, S., 2020. Flame-Retardant and Sustainable Silk Ionotronic Skin for Fire Alarm Systems. ACS Materials Lett. 2, 712–720. Malaiskiene, J., Kizinievic, O., Kizinievic, V., 2019. A Study on Tannery Sludge as a Raw Material for Cement Mortar. Materials (Basel). 12(9), 1652. Moktadir, M.A., Ren, J., Zhou, J., 2023. A systematic review on tannery sludge to energy route: Current practices, impacts, strategies, and future directions. Sci Total Environ. 901, 166244. Moktadir, M.A., Ren,J., 2023. Promoting sustainable management of hazardous waste-to-wealth practices: An innovative integrated DPSIR and decision-making framework. J Environ Manage. 344, 118470. Poole, A.J., Church, J.S., Huson, M.G., 2008. Environmentally Sustainable Fibers from Regenerated Protein. Biomacromolecules. 10.1, 1–8. Pourjavaheri, F., Pour, S.O., Jones, O.A.H., Smooker, P.M., Brkljača, R., Sherkat, F., Blanch, E.W., Gupta, A., Shanks, R.A., 2019. Extraction of keratin from waste chicken feathers using sodium sulfide and l-cysteine. Process Biochem. 82, 205–214. Qian, Y., Ping, T., Huang, H., Xing, H., Liu, M., Feng, X., Lu, X., Zhang, L., Zhu, J., 2023. Highly efficient and green micro/nano cellulose preparation with micro-nano bubbles from mild alkali pretreated bagasse, Cellulose. 30, 9379–9390. Ramirez, D.O.S., Cruz-Maya, I., Vineis, C., Guarino, V., Tonetti, C., Varesano, A., 2021. Wool Keratin-Based Nanofibres-In Vitro Validation. Bioengineering (Basel). 8, 224. Rosace, G., Castellano, A., Trovato, V., Iacono, G., Malucelli, G., 2018. Thermal and flame retardant behaviour of cotton fabrics treated with a novel nitrogen-containing carboxyl-functionalized organophosphorus system. Carbohydr Polym. 196, 348–358. Saira, G.C., Shanthakumar, S., 2023. Zero waste discharge in tannery industries - An achievable reality? A recent review. J Environ Manage. 335, 117508. Sinha-Ray, S., Zhang, Y., Yarin, A.L., Davis, S.C., Pourdeyhimi, B., 2011. Solution Blowing of Soy Protein Fibers. Biomacromolecules. 12, 2357–2363. Sun, J., Su, J., Ma, C., Gostl, R., Herrmann, A., Liu, K., Zhang, H., 2020. Fabrication and Mechanical Properties of Engineered Protein-Based Adhesives and Fibers. Adv Mater. 32, e1906360. Sun, L., Wang, H., Li, W., Zhang, J., Zhang, Z., Lu, Z., Zhu, P., Dong, C., 2021. Preparation, characterization and testing of flame retardant cotton cellulose material: flame retardancy, thermal stability and flame-retardant mechanism. Cellulose. 28, 3789–3805. Sunmathi, N., Padmapriya, R., Sudarsan, J.S., Nithiyanantham, S., 2022. Optimum utilization and resource recovery of tannery sludge: a review. Int J Environ Sci Te. 20, 10405–10414. Wan, S., Cheng, W., Li, J., Wang, F., Xing, X., Sun, J., Zhang, H., Liu, K., 2022. Biological composite fibers with extraordinary mechanical strength and toughness mediated by multiple intermolecular interacting networks. Nano Research. 15, 9192–9198. Wang, Y., Chen, L., 2014. Cellulose nanowhiskers and fiber alignment greatly improve mechanical properties of electrospun prolamin protein fibers. ACS Appl Mater Interfaces. 6, 1709–1718. Zhai, S., Xiong, Y., Li, M., Wang, D., Fu, S., 2020. Roles of hydrothermal-alkaline treatment in tannery sludge reduction: rheological properties and sludge reduction mechanism analysis. RSC Adv. 10, 14291–14298. Zhang, J., Liu, Y., Sun, J., Gu, R., Ma, C., Liu, K., 2020. Biological fibers based on naturally sourced proteins: mechanical investigation and applications. Mater Today Adv. 8, 100095. Zhang, L., Liu, M., Sun, J., Huang, H., Sheng, L., Chen, K., 2023. Low-temperature alkaline soaking coupled with circulating alkali-oxygen cooking: A novel method for whole utilization of bagasse. Ind Crop and Prod, 204.117251. Zhang, L., Ping, T., Xu, H., Yao, S., Wu, Y., 2022. Simultaneous cellulose nanocrystals extraction and organic halides removal from paper mill excess sludge via alkali-oxygen cooking and ultrasonication. J. Clean. Prod. 379, 134512. Zhang, P., Sun, J., Li, J., Zhang, H., Liu, K., 2023. Biosynthesis, Assembly, and Biomedical Applications of High-Performance Engineered Proteins. ACS Chem Biol. 18, 1460–1472. Zhang, X., Li, J., Ma, C., Zhang, H., Liu, K., 2023. Biomimetic Structural Proteins: Modular Assembly and High Mechanical Performance. Acc Chem Res. 56, 2664–2675. Zhao, L., Li, J., Zhang, L., Gu, X., Wei, W., Sun, J., Wang, F., Chen, C., Zhao, Y., Zhang, H., Liu, K., 2022. Biosynthetic protein and nanocellulose composite fibers with extraordinary mechanical performance. Nano Today. 44, 101485. Zhao, R., Guo, Y., Huang, X., Qian, J., Wu, Y., Li, Z., Lu, S., 2022. Synergistic immobilization of chromium in tannery sludge by ZnO and TiO(2) and the oxidation mechanism of Cr(III) under alkaline in high temperature. J Hazard Mater. 424, 127290. Additional Declarations No competing interests reported. Supplementary Files floatimage1.jpeg Graphical Abstract SupportingInformation.docx Cite Share Download PDF Status: Published Journal Publication published 05 Feb, 2025 Read the published version in Cellulose → Version 1 posted Editorial decision: Revision requested 30 Oct, 2024 Reviews received at journal 28 Oct, 2024 Reviews received at journal 28 Oct, 2024 Reviews received at journal 22 Oct, 2024 Reviews received at journal 20 Oct, 2024 Reviews received at journal 17 Oct, 2024 Reviewers agreed at journal 14 Oct, 2024 Reviewers agreed at journal 14 Oct, 2024 Reviewers agreed at journal 14 Oct, 2024 Reviewers agreed at journal 12 Oct, 2024 Reviewers agreed at journal 12 Oct, 2024 Reviewers invited by journal 12 Oct, 2024 Editor assigned by journal 11 Oct, 2024 Submission checks completed at journal 24 Sep, 2024 First submitted to journal 16 Sep, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5099951","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":372147581,"identity":"af9f6f48-7627-4d89-8ce3-7135e248ac69","order_by":0,"name":"Jiang Wei","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Jiang","middleName":"","lastName":"Wei","suffix":""},{"id":372147583,"identity":"d001f776-0085-4566-b79f-05066609bc3b","order_by":1,"name":"Ting He","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Ting","middleName":"","lastName":"He","suffix":""},{"id":372147586,"identity":"abd075af-7442-4177-b7c3-261e704d6bee","order_by":2,"name":"Mengke Liu","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Mengke","middleName":"","lastName":"Liu","suffix":""},{"id":372147589,"identity":"4fc5b4ea-84ed-4515-be4b-dc8e56fdfe34","order_by":3,"name":"Fanyu Kong","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Fanyu","middleName":"","lastName":"Kong","suffix":""},{"id":372147591,"identity":"32960c38-b3d8-421c-80be-5ae9300c8530","order_by":4,"name":"Weijian Dong","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Weijian","middleName":"","lastName":"Dong","suffix":""},{"id":372147592,"identity":"394c04de-4be9-44df-8592-ddc78f02527a","order_by":5,"name":"Xin Feng","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Xin","middleName":"","lastName":"Feng","suffix":""},{"id":372147594,"identity":"519efe8f-a96e-48e4-ae0c-eb2630c9f7f6","order_by":6,"name":"Xiaohua Lu","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Xiaohua","middleName":"","lastName":"Lu","suffix":""},{"id":372147596,"identity":"f9f7bac0-28f5-4815-b2b2-076176765566","order_by":7,"name":"Lilong Zhang","email":"data:image/png;base64,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","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":true,"prefix":"","firstName":"Lilong","middleName":"","lastName":"Zhang","suffix":""},{"id":372147597,"identity":"df348f35-4b34-49f0-8e58-35dd27793e00","order_by":8,"name":"Jiahua Zhu","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Jiahua","middleName":"","lastName":"Zhu","suffix":""}],"badges":[],"createdAt":"2024-09-17 00:41:54","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5099951/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5099951/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10570-024-06358-x","type":"published","date":"2025-02-05T15:57:55+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":69358023,"identity":"022f36e4-7eb2-46f1-9828-fc47fc1c88ca","added_by":"auto","created_at":"2024-11-19 13:55:27","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":167261,"visible":true,"origin":"","legend":"\u003cp\u003eThree-dimensional surface and contour plots of yield showing the effects of alkali dosage, oxygen consumption, and temperature.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5099951/v1/9272e097b50ca2b52d002b80.jpeg"},{"id":69358030,"identity":"bbbe166b-d702-4523-aef1-c25f14df24e9","added_by":"auto","created_at":"2024-11-19 13:55:28","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":167219,"visible":true,"origin":"","legend":"\u003cp\u003eThree-dimensional surface and contour plots of retention rate of Cr showing the effects of alkali dosage, oxygen consumption, and temperature.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5099951/v1/4099f9e7265f3c18f83059be.jpeg"},{"id":69358029,"identity":"84d4b447-59e2-467f-8119-76ada0f21edb","added_by":"auto","created_at":"2024-11-19 13:55:27","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1151885,"visible":true,"origin":"","legend":"\u003cp\u003eMorphological changes of protein fibers in each stage: (a) schematic diagram of the morphological changes of the protein fibers; (b)scanning electron microscope (SEM) images of tannery sludge; (c)transmission electron microscope (TEM) images of protein fibers after alkali-oxygen cooking; (d) protein fiber network after ultrasonic.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5099951/v1/ad69a1a14f6286cf313b7bb8.jpeg"},{"id":69358022,"identity":"9906a06e-e9f7-48fa-a999-9420d5cf9887","added_by":"auto","created_at":"2024-11-19 13:55:27","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":103886,"visible":true,"origin":"","legend":"\u003cp\u003eLifecycle assessment of protein fiber production compared with conventional solution blowing and electrospinning. Assessment of environmental and health impacts, chlorofluorocarbons(CFC) and paradichlorobenzene(1,4-DCB) are common toxic substances and are used as reference substances to evaluate the impact of the process on the environment or health.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5099951/v1/2ba7192805adf8d3b8ab2701.jpeg"},{"id":69358024,"identity":"b8762413-2a8c-4861-8a16-2a5204b230d2","added_by":"auto","created_at":"2024-11-19 13:55:27","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":147252,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XRD patterns of tannery sludge, protein fibers after alkali-oxygen cooking, and the protein fiber network after ultrasound, (b) the FTIR spectra of tannery sludge(×10), protein fibers after alkali-oxygen cooking, and the protein fiber network after ultrasound.\u003c/p\u003e","description":"","filename":"floatimage612.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5099951/v1/ea337362d293e6fa255812c3.jpeg"},{"id":69358025,"identity":"b4f11683-5f8e-4bc1-b110-e2046fa02edd","added_by":"auto","created_at":"2024-11-19 13:55:27","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":814267,"visible":true,"origin":"","legend":"\u003cp\u003eReinforcement of the mechanical properties of the film: (a) preparation process of films; (b) stress-strain curves; (c) Young’s modulus; (d) toughness; (e) visible spectrum.\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5099951/v1/c0a7a035a34cbc5de1d8c7cb.jpeg"},{"id":69358028,"identity":"8af6bf7b-cda3-4a7c-83b6-bdad5d040ea5","added_by":"auto","created_at":"2024-11-19 13:55:27","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1016442,"visible":true,"origin":"","legend":"\u003cp\u003eFlame-retardant potential of the protein fibers: (a) flame-retardant mechanism; (b, c) derivative thermogravimetric curves of the films in nitrogen and air; (d, e) thermogravimetric curves of the films in nitrogen and air; (f) peak heat release rate(PHRR) of the films; (g) total heat release(THR) of the films.\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5099951/v1/a5f0f4b28009fe5f345640b3.jpeg"},{"id":75931194,"identity":"2a0a4c9c-39ea-450b-a3f8-7bf58c4945ed","added_by":"auto","created_at":"2025-02-10 16:14:01","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4322877,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5099951/v1/be8c1004-7478-4200-ae84-dfa171ab7640.pdf"},{"id":69358363,"identity":"83ce1c5e-ffb4-44e5-8eee-2608b1b636f6","added_by":"auto","created_at":"2024-11-19 14:03:27","extension":"jpeg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":480425,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical Abstract\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-5099951/v1/68d2dfa02c3643bd1c428f32.jpeg"},{"id":69358027,"identity":"524e0bcc-df26-4256-bef3-c605833251f1","added_by":"auto","created_at":"2024-11-19 13:55:27","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":2460114,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5099951/v1/d4196b471466a6dde597e01d.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Upcycling tannery sludge into superior micro/nano protein fibers to reinforce the mechanical and flame retardant properties of cellulose-based film","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eTannery sludge is a typical hazardous solid waste, which mainly includes animal fur, a large amount of water and chromium could cause serious environmental pollution(Moktadir et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Moktadir et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Kilic et al.,2011). The conventional thermal treatment processes, such as incineration and pyrolysis, require dehydration of tannery sludge. Typically only around 60.00% of the moisture could be preliminary eliminated in tannery sludge, necessitating more advanced techniques for further drying, but it is difficult to avoid the impact of high cost(Sunmathi et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhao et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Moreover, conventional disposal approaches can give rise to secondary pollution concerns. Given that a significant amount of trivalent chromium is utilized as a tanning agent in leather production, incineration of tannery sludge can lead to the release of harmful heavy metals into the environment, while landfilling and composting may result in leachate contamination, causing notable environmental harm(Li et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Ge et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Reports have highlighted alternative strategies for tannery sludge treatment to mitigate chromium pollution, these strategies include the production of bricks(Juel et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), cement(Malaiskiene et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), and other useful products from tannery sludge, offering promising avenues for reducing environmental impact(Sunmathi et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The escalating volume of tannery sludge underscores the urgency of implementing more efficient and eco-friendly disposal solutions to address the long-term environmental repercussions of this waste material.\u003c/p\u003e \u003cp\u003eMicro/nano protein fibers have emerged as promising materials owing to their excellent mechanical properties, plasticity, and biocompatibility(Chen et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The production of micro/nano protein fibers typically requires the large-scale collection of raw materials, which increases the associated costs once beyond the collection radius. The variability in raw-materials adversely affect product quality control because of differences in the functional groups of proteins obtained from various sources(Sun et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In addition, considerable effort has been directed toward the production of regenerated protein fibers using various approaches, such as solvent blowing and electrospinning(Han et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Dias et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). These methods have several barriers should be overcome, such as complex procedures and the need for relatively large organic solvent quantities which increases the cost of micro/nano protein fibers and the environmental pollution risk(Zhang et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). These challenges highlight the need for more reliable and consistent raw-material supplies and methods to enable the high-quality production of micro/nanoprotein fibers.\u003c/p\u003e \u003cp\u003eOn one hand, given the development of increasingly strict sewage emissions standards, how to dispose a large amount of tannery sludge reasonably has become a challenging problem(Saira et al., 2023). On the other hand, the development of micro/nano protein fibers lack of reliable and consistent raw-material supplies and green and low-carbon process. One promising two-in-one solution to tackle both problems is using leather tanning sludge as a feedstock for micro/nano protein fiber production. The main component of sludge is animal fur, which is rich in protein fibers, determines that it has the potential for the preparation of protein fibers while achieve the purpose of sludge treatment. Tannery sludge was extracted after tanning and extrusion dehydration process. Only mild reaction conditions are needed to reach the fiber separation point. Alkali-oxygen cooking is commonly used as an effective method for preparing fibers from biomass. This process uses only alkali and oxygen, has the advantages of low energy consumption, low carbon emissions, high efficiency and environmental friendliness. In our previous work, we found that this method could efficiently disperse fibers, making it one of the best candidates for converting tannery sludge into micro/nano protein fibers(Zhang et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Qian et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn this work, tannery sludge was creatively converted into micro/nano protein fibers using an efficient and low-carbon alkali-oxygen cooking process. The physical, chemical, and morphological characteristics of the micro/nano protein fibers were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and laser particle size analysis. The produced micro/nano protein fibers were incorporated into cellulose-based films, and the reinforcement of the mechanical properties was evaluated via tensile tests, while the flame-retardant properties were evaluated via thermogravimetric analysis (TGA) and microscale combustion calorimetry (MCC). Owing to the network structure and dominant nitrogen and sulfur contents, the micro/nano protein fibers reinforced the mechanical properties and flame retardancy of the fiber materials.\u003c/p\u003e"},{"header":"2. Experimental section","content":"\u003cp\u003e\u003cstrong\u003eMaterials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe tannery sludge was initially dehydrated, and the remaining chemicals were used in their\u0026nbsp;original state without further\u0026nbsp;purification. The materials, chemicals, and detailed experimental procedures used in this study are presented in the Supporting Information.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparation of protein fibers\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTannery sludge (3 g) was weighed, and the required\u0026nbsp;amount of NaOH was dissolved in 100 mL of distilled water. All of these\u0026nbsp;were added to the reactor,\u0026nbsp;and the required oxygen was introduced. After the reactor was assembled, the heating rate was set to 20 \u003csup\u003eo\u003c/sup\u003eC/min, and each experiment was conducted for 60 min. After alkali-oxygen cooking, the slurry\u0026nbsp;was subjected to\u0026nbsp;ultrasonic disintegration and treated at a 540 W output power for 10 min.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparation of composite films\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe protein fibers were mixed into nanocellulose, and 50 mL of water was added with stirring to obtain a uniform mixture. To prevent the loss of fibers during the membrane preparation process and ensure the flatness of the fiber film, a nanofiltration membrane (D = 4 cm, 0.45 \u0026mu;m PE) was added to the sintered glass filter, and the cellulose-protein fiber composite membrane was prepared at 140 g/m\u003csup\u003e2\u003c/sup\u003e under 0.08-MPa vacuum suction.\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003e \u003cb\u003eOptimization of Reaction Conditions Using Response Surface Methodology\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eExperimental conditions of sample preparation and related results.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAlkali charge (wt% sludge)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOxygen consumption (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTemperature (\u003csup\u003eo\u003c/sup\u003eC)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eYield (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eChromium retention rate (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e140\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e58.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e89.85\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e4.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e94.78\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e89.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e28.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e99.25\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e12.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e91.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e43.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e90.61\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e55.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e89.47\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e47.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e92.27\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e41.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e90.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e140\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e54.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e95.52\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e140\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e36.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e61.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e28.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e98.84\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e59.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e91.92\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e53.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e82.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e74.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e98.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e47.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e98.51\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e140\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e40.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e63.63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTo determine the effects of the alkali-oxygen cooking conditions, a single-factor experiment was performed in the early stage of the reaction. The results revealed that the yield of protein fiber was mainly affected by the amount of alkali, while the retention rate of chromium was mainly affected by the reaction temperature(showed in the Supporting Information Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e-S5). Based on above mentioned results, 17 experimental trials were conducted to optimize the alkali-oxygen cooking parameters, and the results are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The three-dimensional (3D) response surface graphs and two-dimensional contour plots of the yield (\u0026lt;\u0026thinsp;5 \u0026micro;m) and chromium retention rate under different conditions are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe alkali dosage, oxygen consumption and temperature showed the complex interaction effects in the 3D response surfaces, the yield and size of micro/nano protein fibers depends on whether the reaction conditions can reach the fiber separation point(Fig. S8). Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows that the yield of protein fiber increases with the increase of alkali dosage, oxygen consumption and temperature. The results shown that the effect of alkali amount is the most obvious which may attribute to the damage of NaOH on the intermolecular force between protein fibers and the fracture of the basic unit of the fiber which similar to the previous works(Gao et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Zhai et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The increase of temperature improved the efficiency, and a small consumption of oxygen can reinforce the mass transfer but excessive temperature or oxygen consumption may reduce oxidative cross-linking between protein fibers, resulting in a decrease in yield(Poole et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe interaction between the amounts of alkali and oxygen has the least significant impact on the chromium retention rate in the three-way interaction of alkali, oxygen amount, and temperature(shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The chromium retention rate is particularly sensitive to temperature changes, and chromium can be easily released from protein fibers at high temperatures-especially when the alkali or oxygen dosage is higher than 100\u003csup\u003eo\u003c/sup\u003eC. At low temperatures or alkali dosages, the chromium retention rate exhibited a parabolic trend with an increase in the temperature. This may be because the chromium release rate exceeded the capture capacity of the protein fiber network at high temperatures(Kar et al., 2004).\u003c/p\u003e \u003cp\u003eTo maximize both the yield and chromium retention rate, Design-Expert v12 software was used to comprehensively optimize the analysis of the two objectives from the perspective of cost. The reaction conditions for realizing the optimal theoretical yield of 63.32% and chromium retention rate of 97.78% were determined to be an alkali dosage of 15 wt%, oxygen consumption of 0.79 MPa, and a temperature of 69.57 \u003csup\u003eo\u003c/sup\u003eC. The predicted results were verified three times, and under these conditions, the micro/nano protein fiber yield reached 62.14%, and the chromium retention rate reached 97.61%. The experimental values agreed well with the predicted values as showed in Figs. S6, S7, Tables S1, S2.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMorphology of micro/nano protein fibers\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, large protein fiber bundles resembling hemp ropes (several to tens of micrometers in diameter) are observed in the tannery sludge, which were converted into micro/nanofiber networks after alkali-oxygen cooking and ultrasonic treatment. Alkali-oxygen cooking can cleave the intermolecular covalent bonds (disulphide crosslinks between cysteine residues) between protein fibers and destroy the covalent bonds of the primary protein chain(Poole et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). When the fiber bundles were treated with alkali-oxygen cooking, the fiber bundles were initially dispersed or broken into micro/nano scale, whereas the protein fibers were not totally dispersed. After the ultrasonic treatment process, the protein fibers exhibited an obvious regular network nano structure under TEM rather than being entangled or crosslinked into bundles.The micro/nano network structure was also observed by AFM and SEM after freeze-drying(Figs. S9, S10). This is attributed to the ultrasound having a cavitation effect that reinforced the mass transfer, accelerating the dispersion of fibers(Biswas et al., 2022). Additionally, the ultrasound modified the protein fiber surface by increasing the surface area and embedding hydroxyl group in it. These changes in the morphology and surface chemical properties also helped to capture chromium in the solution(Kar et al., 2004).\u003c/p\u003e \u003cp\u003e \u003cb\u003eLifecycle assessment\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo assess the environmental impact of this process, we conducted a carbon lifecycle analysis using SimaPro, with the calculation boundary extending from the raw materials to the final products. The ecoinvent database served as the foundation for our analysis, and this method was compared with two alternative approaches: preparation of protein fibers via solution blowing and electrospinning(the data were obtained from published works(Sinha-Ray et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Goyal et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Our findings revealed that the proposed method is more ecofriendly and more conducive to environment and health. For example, it is 73.26% lower than the solution blowing method and 90.91% lower than the electrospinning method in reducing carbon dioxide emissions.\u003c/p\u003e \u003cp\u003eAs shown in Fig. S12, in order to judge the relative contribution of different preparation processes of protein fibers, the related factors of various preparation methods are divided into materials and chemicals, electricity consumption. The impact of solution blowing on the environment is mainly reflected in the amount of electricity consumed in the pretreatment of raw materials. Because the development of electrospinning technology is relatively perfect, low energy consumption leads to less impact on the environment. But in the raw material preparation stage, the treatment usually requires organic solvents and other chemicals, which will have a considerable impact on the environment. In contrast, the alkaline oxygen cooking method used in this work has the characteristics of using environmentally friendly chemicals and low energy consumption.\u003c/p\u003e \u003cp\u003e \u003cb\u003eX-ray and FTIR analysis of protein fibers at different reaction stages\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTannery sludge is rich in inorganic salts because the tanning process, which can form micelles with the original fibers in the sludge, potentially affecting the XRD results. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, the protein fibers in tannery sludge have a typical β-sheet structure, with a diffraction peak at 2θ\u0026thinsp;=\u0026thinsp;20.90\u003csup\u003eo\u003c/sup\u003e(Ha et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Moreover, the diffraction peaks of inorganic salts such as CaSO\u003csub\u003e4\u003c/sub\u003e and Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e appeared at 11.80\u003csup\u003eo\u003c/sup\u003e and 29.20\u003csup\u003eo\u003c/sup\u003e, while the diffraction peak of chromium oxide appeared at 37.2\u003csup\u003eo\u003c/sup\u003e. After alkali-oxygen cooking, the micellar structure in the sludge was destroyed, and the internal crystal morphology was exposed. Thus, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea shows sharp peaks at 29.30\u003csup\u003eo\u003c/sup\u003e, 31.70\u003csup\u003eo\u003c/sup\u003e, and 33.90\u003csup\u003eo\u003c/sup\u003e for the treated sludge sample, corresponding to inorganic salts such as CaSO\u003csub\u003e4\u003c/sub\u003e and Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4,\u003c/sub\u003e which is consistent with previous results. However, the intensity of the diffraction peak of the protein fibers after cooking and ultrasonic treatment at 22.56\u003csup\u003eo\u003c/sup\u003e was significantly increased, indicating that the structure of the protein fibers was changed after cooking, it was similar to that of silk I(Asakura et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2001\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe abundant amino acids in the protein fibers were instrumental in crosslinking with cellulose as well as sulfur-containing groups, enhancing the flame-retardant properties. Fourier transform infrared spectroscopy was used to identify some of the functional groups that may play a role (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). In the wavenumber range of 1700\u0026thinsp;\u0026minus;\u0026thinsp;1600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the amide I band was observed, which is typically due to C-O stretching vibrations(Chen et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The band at 1654 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is often assigned to the α-helix structure of the protein fiber(Ling et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). According to the above XRD, FTIR and the earlier studies, this structure of the protein fiber similar to silk I at the 1653-cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e band could still be assigned to the β-sheet structure(Jafari et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The characteristic peak at 1442 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the vibration of CH\u003csub\u003e2\u003c/sub\u003e in the protein fibers, and the peak at 1137 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the range of 1000\u0026ndash;1200 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e may be due to cysteine-S-sulfonated residues or crosslinked residues(Ramirez et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Ebrahimgol et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The bands at 624 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are assigned to C-S stretching vibrations, and the intense peak at 873 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to dehydroalanine groups, which are mainly formed by the cleavage of the C-S bond(Pourjavaheri et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Most of the absorption peaks remained unchanged, but the peak intensity increased, indicating that the cooking process exposed more functional groups of the protein fibers.\u003c/p\u003e \u003cp\u003e \u003cb\u003eReinforcement of mechanical properties of film\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe reinforcements in the mechanical properties of the composite films with different protein fiber contents (0, 5, 10, 15, and 20 wt%) are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Generally, the strength of fiber materials depends on the dispersion of the fibers and interactions between the fibers. As indicated by the stress-strain curves of the composite films (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb), with an increase in the protein fiber content, the break strength of the films first increased and then decreased. At a protein fiber content of 10 wt%, the tensile strength, Young\u0026rsquo;s modulus, and toughness of the composite film were significantly improved and reach the maximum value. After ultrasonic treatment, the protein fibers formed a network structure, interweaving with cellulose to make stress diffusion more uniform(Wan et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In addition, new hydrogen bonds may formed between the protein fibers and cellulose, and the amide bonds may induce an orderly arrangement of fibers, improving their mechanical properties(Zhao et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). At a higher protein fiber content, the possibility of forming an interface layer between the protein fibers and cellulose increased, which blocked the continuity of the cellulose matrix and further disrupted the formation of hydrogen bonds(Wang et al., 2014). In addition, an appropriate protein fiber content can increase the transmittance, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee. In particular, for the 10 wt% sample, the transmittance was increased by 16.67% compared with that of the 0 wt% sample, indicating its potential for application in materials with high transparency requirements.\u003c/p\u003e \u003cp\u003e \u003cb\u003eFlame-retardant potential of films\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eProtein fibers are among the most commonly used flame retardants in fiber materials because they form a carbon layer and a noncombustible gas, which hinder flame combustion(Alongi et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The burning time of films with different protein fibers content was preliminarily evaluated(Fig. S11). Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the results of thermogravimetric experiment and microcalorimetry experiment, indicating that the protein fibers reinforce the flame retardancy of the cellulose-based film.\u003c/p\u003e \u003cp\u003eThe thermal stability of the films doped with protein fibers under nitrogen and air was evaluated to evaluate their degradation behavior. The protein fiber-added samples and blank samples exhibited a main weight-loss stage at 300\u0026ndash;400 \u003csup\u003eo\u003c/sup\u003eC, which was due to the depolymerization of cellulose and the formation of aliphatic char. In the air atmosphere, the pyrolysis-rate peak at 500\u0026ndash;600 \u003csup\u003eo\u003c/sup\u003eC corresponded to the conversion of aliphatic carbons into an aromatic form, which produced CO\u003csub\u003e2\u003c/sub\u003e(Rosace et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). In both environments, compared with the original sample, the initial decomposition temperature of the protein fiber-added samples was significantly increased by about 16\u003csup\u003eo\u003c/sup\u003eC, which indicates that the incorporation of protein fibers can significantly improve the thermal stability of the film.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed and Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ee, the residual carbon content of the films doped with protein fibers was generally increased, which played a protective role and limits the contact between oxygen and combustibles(Liu et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Amino acids, as the main component of protein fibers, were also found to be positively correlated with flame-retardant effect(Leong et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). More specifically, amino acids such as cysteine in protein fibers have been evaluated as effective flame-retardant systems for cellulose substrates because these components can affect the formation of carbon from cellulose pyrolysis(Alongi et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Interestingly, we found that the film in the combustion process exhibited obvious stratification in addition to carbonization as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea(ⅰ). The combustion of protein fibers can generate a certain amounts of noncombustible gases (including H\u003csub\u003e2\u003c/sub\u003eO, NO, and NO\u003csub\u003e2\u003c/sub\u003e), which play the role of reducing the concentration of combustible gases in the layered gap and on the film surface(Sun et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe fire-retardant properties of composite films with different protein fiber contents were investigated using MCC, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ef and Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eg. The peak heat release rate (PHRR) and total heat release (THR) for the blank sample were 138.87 w/g and 7.10 kJ/g, respectively. The PHRR value and THR value of cellulose-based films decreased by 21.60% and 28.17% with the incorporation of protein fibers, respectively. This is mainly attributed to the formation of an effective carbon layer on the film surface, the mechanism is described in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea(ⅲ). The formation of a carbon layer inhibited the diffusion of volatile gases and heat transfer during combustion.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this work, micro/nano protein fibers were successfully prepared from tannery sludge through a low-carbon alkali-oxygen cooking combined with ultrasonic crushing. The yield of micro/nano protein fibers reached 62.15% while the retention rate of chromium reached 97.61%. This process was evaluated by LCA that indicts the carbon emissions was reduced by 73.26% compared with the conventional solution blowing method and 90.91% compared with electrospinning. Micro/nano fibers can significantly reinforce the mechanical properties and flame-retardancy of cellulose-based films. The tensile strength of the cellulose-based film was increased by 55.40%, and the Young 's modulus and toughness were also significantly improved. The peak heat release rate was reduced by 21.60%, and the total heat release rate was also reduced by 28.17%. The results showed that these functions were derived from the network structure of the protein fibers and the substances such as amino acids. This work broadens the source of raw materials for protein fiber preparation and provides an effective strategy for the resource treatment of tannery sludge.\u003c/p\u003e "},{"header":"Declarations","content":" \u003ch2\u003eDeclaration of interests\u003c/h2\u003e \u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e \u003ch2\u003eHuman Ethics and Consent to Participate declarations\u003c/h2\u003e \u003cp\u003eNot applicable\u003c/p\u003e \u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work was financially supported by National Natural Science Foundation of China (No. 21908127, No. 91934302, No. 21978128) and the Project Supported by the Foundation (GZKF202103) of State Key Laboratory of Biobased Material and Green Papermaking.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eJ.W., and H. T., performed the experiments, collected and analyzed the data. M. L., F. K.,X. F., and X. L., helped with data analysis and discussions. L.Z. and J. Z. conceived the experiments, planned synthesis, analyzed results, and wrote the paper. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work was financially supported by National Natural Science Foundation of China (No. 21908127, No. 91934302, No. 21978128) and the Project Supported by the Foundation (GZKF202103) of State Key Laboratory of Biobased Material and Green Papermaking.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAlongi, J., Bosco, F., Carosio, F., Blasio, A.D., Malucelli, G., 2014. A new era for flame retardant materials?. Materials Today. 17, 152\u0026ndash;153.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAsakura, T., Yamane, T., Nakazawa, Y., Kameda, T., 2001. K. Ando, Structure ofBombyx mori silk fibroin before spinning in solid state studied with wide angle x-ray scattering and13C cross-polarization/magic angle spinning NMR. Biopolymers., 58. 521\u0026ndash;525.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBiswas, S., Rashid, T.U., 2022. Effect of ultrasound on the physical properties and processing of major biopolymers-a review. Soft Matter. 18, 8367\u0026ndash;8383.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChen, C., Feng, J., Li, J., Guo, Y., Shi, X., Peng, H., 2023. Functional Fiber Materials to Smart Fiber Devices. Chem Rev. 123, 613\u0026ndash;662.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChen, D., Narayanan, N., Federici, E., Yang, Z., Zuo, X., Gao, J., Fang, F., Deng, M., Campanella, O.H., Jones, O.G., 2020. Electrospinning Induced Orientation of Protein Fibrils. Biomacromolecules. 21, 2772\u0026ndash;2785.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDias, A., Cena, C., Lutz-Bueno, V., Mezzenga, R., Marques, A., Ferreira, I., Roque, A.C.A., 2022. Solvent modulation in peptide sub-microfibers obtained by solution blow spinning. Front Chem. 10, 1054347.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eEbrahimgol, F., Tavanai, H., Alihosseini, F., Khayamian, T., 2014. Electrosprayed recovered wool keratin nanoparticles. Polym Advan Technol. 25, 1001\u0026ndash;1007.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGao, J., Wang, Y., Yan, Y., Li, Z., Chen, M., 2020. Protein extraction from excess sludge by alkali-thermal hydrolysis. Environ Sci Pollut Res Int. 27, 8628\u0026ndash;8637.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGe, R., E,T., Cheng, Y., Wang, Y., Yu, J., Li, Y., Yang, S., 2024. NaH(2)PO(4) synergizes with organic matter to stabilize chromium in tannery sludge. J Environ Manage. 351, 119843.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGoyal, S., Dotter, M., Diestelhorst, E., Storck, J.L., Ehrman, A.n, Mahltig, B., 2022. Extraction of keratin from wool and its use as biopolymer in film formation and in electrospinning for composite material processing. J Eng Fiber Fabr. 17, 1\u0026ndash;11.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHa, S.W., Tonelli, A.E., Hudson, S.M., 2005. Structural Studies of Bombyx mori Silk Fibroin during Regeneration from Solutions and Wet Fiber Spinning. Biomacromolecules. 6. 3, 1722\u0026ndash;1731.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHan, W., Wang, L., Li, Q., Ma, B., He, C., Guo, X., Nie, J., Ma, G., 2022. A Review: Current Status and Emerging Developments on Natural Polymer-Based Electrospun Fibers. Macromol Rapid Commun. 43, e2200456.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJafari, M.J., B\u0026auml;cklund, F.G., Arndt, T., Schmuck, B., Greco, G., Rising, A., Barth, A., Ederth, T., 2023. Force-Induced Structural Changes in Spider Silk Fibers Introduced by ATR-FTIR Spectroscopy. ACS Appl. Polym. Mater. 5, 9433\u0026ndash;9444.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJuel, M.A.I., Mizan, A., Ahmed, T., 2017. Sustainable use of tannery sludge in brick manufacturing in Bangladesh. Waste Manag. 60, 259\u0026ndash;269.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKar, P., Misra, M., 2004. Use of keratin fiber for separation of heavy metals from water. J Chem Technol Biot. 79, 1313\u0026ndash;1319.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKilic, E, Font, J., Puig, R., Colak, S., Celik, D., 2011. Chromium recovery from tannery sludge with saponin and oxidative remediation. J Hazard Mater. 185, 456\u0026ndash;462.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLeong, W.I., Lo, O.L.I., Cheng, F.T., Cheong, W.M., Seak, L.C.U., 2021. Using recombinant adhesive proteins as durable and green flame-retardant coatings. Syn Syst Biotechno. 6, 369\u0026ndash;376.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi, Z., Yu, D., Wang, X., Liu, X., Xu, Z., Wang, Y., 2024. A novel strategy of tannery sludge disposal - converting into biochar and reusing for Cr(VI) removal from tannery wastewater. J Environ Sci (China). 138, 637\u0026ndash;649.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLing, S., Qi, Z., Knight, D.P., Shao, Z., Chen, X., 2011. Synchrotron FTIR microspectroscopy of single natural silk fibers. Biomacromolecules. 12, 3344\u0026ndash;3349.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLiu, Q., Yang, S., Ren, J., Ling, S., 2020. Flame-Retardant and Sustainable Silk Ionotronic Skin for Fire Alarm Systems. ACS Materials Lett. 2, 712\u0026ndash;720.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMalaiskiene, J., Kizinievic, O., Kizinievic, V., 2019. A Study on Tannery Sludge as a Raw Material for Cement Mortar. Materials (Basel). 12(9), 1652.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMoktadir, M.A., Ren, J., Zhou, J., 2023. A systematic review on tannery sludge to energy route: Current practices, impacts, strategies, and future directions. Sci Total Environ. 901, 166244.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMoktadir, M.A., Ren,J., 2023. Promoting sustainable management of hazardous waste-to-wealth practices: An innovative integrated DPSIR and decision-making framework. J Environ Manage. 344, 118470.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePoole, A.J., Church, J.S., Huson, M.G., 2008. Environmentally Sustainable Fibers from Regenerated Protein. Biomacromolecules. 10.1, 1\u0026ndash;8.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePourjavaheri, F., Pour, S.O., Jones, O.A.H., Smooker, P.M., Brkljača, R., Sherkat, F., Blanch, E.W., Gupta, A., Shanks, R.A., 2019. Extraction of keratin from waste chicken feathers using sodium sulfide and l-cysteine. Process Biochem. 82, 205\u0026ndash;214.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eQian, Y., Ping, T., Huang, H., Xing, H., Liu, M., Feng, X., Lu, X., Zhang, L., Zhu, J., 2023. Highly efficient and green micro/nano cellulose preparation with micro-nano bubbles from mild alkali pretreated bagasse, Cellulose. 30, 9379\u0026ndash;9390.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRamirez, D.O.S., Cruz-Maya, I., Vineis, C., Guarino, V., Tonetti, C., Varesano, A., 2021. Wool Keratin-Based Nanofibres-In Vitro Validation. Bioengineering (Basel). 8, 224.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRosace, G., Castellano, A., Trovato, V., Iacono, G., Malucelli, G., 2018. Thermal and flame retardant behaviour of cotton fabrics treated with a novel nitrogen-containing carboxyl-functionalized organophosphorus system. Carbohydr Polym. 196, 348\u0026ndash;358.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSaira, G.C., Shanthakumar, S., 2023. Zero waste discharge in tannery industries - An achievable reality? A recent review. J Environ Manage. 335, 117508.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSinha-Ray, S., Zhang, Y., Yarin, A.L., Davis, S.C., Pourdeyhimi, B., 2011. Solution Blowing of Soy Protein Fibers. Biomacromolecules. 12, 2357\u0026ndash;2363.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSun, J., Su, J., Ma, C., Gostl, R., Herrmann, A., Liu, K., Zhang, H., 2020. Fabrication and Mechanical Properties of Engineered Protein-Based Adhesives and Fibers. Adv Mater. 32, e1906360.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSun, L., Wang, H., Li, W., Zhang, J., Zhang, Z., Lu, Z., Zhu, P., Dong, C., 2021. Preparation, characterization and testing of flame retardant cotton cellulose material: flame retardancy, thermal stability and flame-retardant mechanism. Cellulose. 28, 3789\u0026ndash;3805.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSunmathi, N., Padmapriya, R., Sudarsan, J.S., Nithiyanantham, S., 2022. Optimum utilization and resource recovery of tannery sludge: a review. Int J Environ Sci Te. 20, 10405\u0026ndash;10414.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWan, S., Cheng, W., Li, J., Wang, F., Xing, X., Sun, J., Zhang, H., Liu, K., 2022. Biological composite fibers with extraordinary mechanical strength and toughness mediated by multiple intermolecular interacting networks. Nano Research. 15, 9192\u0026ndash;9198.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang, Y., Chen, L., 2014. Cellulose nanowhiskers and fiber alignment greatly improve mechanical properties of electrospun prolamin protein fibers. ACS Appl Mater Interfaces. 6, 1709\u0026ndash;1718.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhai, S., Xiong, Y., Li, M., Wang, D., Fu, S., 2020. Roles of hydrothermal-alkaline treatment in tannery sludge reduction: rheological properties and sludge reduction mechanism analysis. RSC Adv. 10, 14291\u0026ndash;14298.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang, J., Liu, Y., Sun, J., Gu, R., Ma, C., Liu, K., 2020. Biological fibers based on naturally sourced proteins: mechanical investigation and applications. Mater Today Adv. 8, 100095.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang, L., Liu, M., Sun, J., Huang, H., Sheng, L., Chen, K., 2023. Low-temperature alkaline soaking coupled with circulating alkali-oxygen cooking: A novel method for whole utilization of bagasse. Ind Crop and Prod, 204.117251.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang, L., Ping, T., Xu, H., Yao, S., Wu, Y., 2022. Simultaneous cellulose nanocrystals extraction and organic halides removal from paper mill excess sludge via alkali-oxygen cooking and ultrasonication. J. Clean. Prod. 379, 134512.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang, P., Sun, J., Li, J., Zhang, H., Liu, K., 2023. Biosynthesis, Assembly, and Biomedical Applications of High-Performance Engineered Proteins. ACS Chem Biol. 18, 1460\u0026ndash;1472.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang, X., Li, J., Ma, C., Zhang, H., Liu, K., 2023. Biomimetic Structural Proteins: Modular Assembly and High Mechanical Performance. Acc Chem Res. 56, 2664\u0026ndash;2675.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhao, L., Li, J., Zhang, L., Gu, X., Wei, W., Sun, J., Wang, F., Chen, C., Zhao, Y., Zhang, H., Liu, K., 2022. Biosynthetic protein and nanocellulose composite fibers with extraordinary mechanical performance. Nano Today. 44, 101485.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhao, R., Guo, Y., Huang, X., Qian, J., Wu, Y., Li, Z., Lu, S., 2022. Synergistic immobilization of chromium in tannery sludge by ZnO and TiO(2) and the oxidation mechanism of Cr(III) under alkaline in high temperature. J Hazard Mater. 424, 127290.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"cellulose","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cels","sideBox":"Learn more about [Cellulose](https://www.springer.com/journal/10570)","snPcode":"10570","submissionUrl":"https://submission.nature.com/new-submission/10570/3","title":"Cellulose","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"tannery sludge, micro/nano protein fiber, flame-retardant, alkali-oxygen cooking","lastPublishedDoi":"10.21203/rs.3.rs-5099951/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5099951/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMicro/nano protein fibers have attracted increasing attention owing to their advantageous properties for applications in advanced materials. Traditional preparation methods often suffer from high costs of raw-materials and energy-intensive manufacturing processes, which hinder large-scale production. Herein, we present an innovative low-carbon approach for converting tannery sludge into micro/nano protein fibers. The alkali-oxygen cooking combined with ultrasonic process shown the potential to reduce carbon emissions. The resulting micro/nano protein fibers reinforced the mechanical properties of the cellulose-based films. The maximum tensile force of the cellulose-based film was increased by 55.40%, and the Young\u0026rsquo;s modulus was increased by 22.92%. The micro/nano protein fibers also imparted remarkable flame-retardant characteristics, as indicated by an increased peak temperature of heat loss and a 21.60% reduction in the peak heat release rate of cellulose-based films. This low-carbon and ecofriendly process utilizing leather tannery sludge not only provides a sustainable source of raw materials but also contributes to the circular economy by repurposing industrial waste.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e","manuscriptTitle":"Upcycling tannery sludge into superior micro/nano protein fibers to reinforce the mechanical and flame retardant properties of cellulose-based film","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-19 13:55:22","doi":"10.21203/rs.3.rs-5099951/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-10-30T09:23:26+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-28T22:15:50+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-28T08:20:00+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-22T13:56:13+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-20T13:59:51+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-17T15:59:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"84476863663419490172698786351242383182","date":"2024-10-14T19:57:25+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"33577047024078530212497427584716883204","date":"2024-10-14T12:31:43+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"254735506542833186367060741161571419206","date":"2024-10-14T05:06:51+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"139557591990068576673127715205877789291","date":"2024-10-12T22:04:38+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"281462296137125914608919448336187521108","date":"2024-10-12T18:54:17+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-10-12T17:47:13+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-10-11T19:35:05+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-09-24T10:28:12+00:00","index":"","fulltext":""},{"type":"submitted","content":"Cellulose","date":"2024-09-17T00:40:29+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"cellulose","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cels","sideBox":"Learn more about [Cellulose](https://www.springer.com/journal/10570)","snPcode":"10570","submissionUrl":"https://submission.nature.com/new-submission/10570/3","title":"Cellulose","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"c222e7a3-a6ef-47d3-a2fa-cc70cb940dd7","owner":[],"postedDate":"November 19th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-02-10T16:08:49+00:00","versionOfRecord":{"articleIdentity":"rs-5099951","link":"https://doi.org/10.1007/s10570-024-06358-x","journal":{"identity":"cellulose","isVorOnly":false,"title":"Cellulose"},"publishedOn":"2025-02-05 15:57:55","publishedOnDateReadable":"February 5th, 2025"},"versionCreatedAt":"2024-11-19 13:55:22","video":"","vorDoi":"10.1007/s10570-024-06358-x","vorDoiUrl":"https://doi.org/10.1007/s10570-024-06358-x","workflowStages":[]},"version":"v1","identity":"rs-5099951","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5099951","identity":"rs-5099951","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}