Waste Cotton Fabrics Research Articles

Valorisation of cotton post-industrial textile waste into lactic acid: chemo-mechanical pretreatment, separate hydrolysis and fermentation using engineered yeast

BackgroundThe textile industry has several negative impacts, mainly because it is based on a linear business model that depletes natural resources and produces excessive amounts of waste. Globally, about 75% of textile waste is disposed of in landfills and only 25% is reused or recycled, while less than 1% is recycled back into new garments. In this study, we explored the valorisation of cotton fabric waste from an apparel textile manufacturing company as valuable biomass to produce lactic acid, a versatile chemical building block.ResultsPost-industrial cotton patches were pre-treated with the aim of developing a methodology applicable to the industrial site involved. First, a mechanical shredding machine reduced the fabric into individual fibres of maximum 35 mm in length. Afterwards, an alkaline treatment was performed, using NaOH at different concentrations, including a 16% (w/v) NaOH enriched waste stream from the mercerisation of cotton fabrics. The combination of chemo-mechanical pre-treatment and enzymatic hydrolysis led to the maximum recovery yield of 90.46 ± 3.46%, corresponding to 74.96 ± 2.76 g/L of glucose released, which represents a novel valorisation of two different side products (NaOH enriched wastewater and cotton textile waste) of the textile industry. The Saccharomyces cerevisiae strain CEN.PK m850, engineered for redirecting the natural alcoholic fermentation towards a homolactic fermentation, was then used to valorise the glucose-enriched hydrolysate into lactic acid. Overall, the process produced 53.04 g/L ± 0.34 of l-lactic acid, with a yield of 82.7%, being the first example of second-generation biomass valorised with this yeast strain, to the best of our knowledge. Remarkably, the fermentation performances were comparable with the ones obtained in the control medium.ConclusionThis study validates the exploitation of cotton post–industrial waste as a possible feedstock for the production of commodity chemicals in microbial cell-based biorefineries. The presented strategy demonstrates the possibility of implementing a circular bioeconomy approach in manufacturing textile industries.Graphical

Synthesis of Hydrochars via Hydrothermal Carbonization of Zinc Chloride Activated Cotton Textile Waste

Anthropogenic pollution from fossil fuel combustion due to increasing global populations and emerging industries poses a serious threat to the environment. Therefore, it is vital to explore alternative green energy sources. Hence, this study synthesized renewable solid fuel hydrochars by utilizing biomass of cotton textile waste (CTW) as feedstock. ZnCl2 was selected as a catalyst due to its Lewis acid properties promoting hydrolysis and dehydration of raw materials towards hydrochar formation. The ZnCl2-activated cotton textile waste (ZnCl2-CTW) was prepared via the wet impregnation method at different ZnCl2 loading. The hydrochars were synthesized via hydrothermal carbonization of ZnCl2-CTW in a Parr batch reactor at the reaction temperature of 200 °C for 3 h. The hydrochars and CTW were characterized using a thermogravimetric analyser (TGA) to evaluate the proximate analysis, whereas an elemental analysis and determination of higher heating value (HHV) were conducted employing a CHNS analyser. The proximate analysis indicates the hydrochars volatile matter increases at higher catalyst loading, possibly due to the high content of bio-oil adsorbed on the hydrochars surface. The elemental analysis shows higher carbon and lower oxygen content for hydrochars compared to CTW, mainly due to the dehydration, decarboxylation, and aromatization reactions during HTC. Meanwhile, the HHV of hydrochars increases as catalyst loading increases to 1.5 g, which obtained the highest value of 18.41 MJ/kg, indicating good quality of solid fuel. This demonstrated that the ZnCl2 catalyst was useful in improving the energy density of hydrochars. Overall, ZnCl2 catalyst loading of 1.5 g has exhibited good performance in producing hydrochars via HTC of CTW. Thus, ZnCl2-CTW has the potential to produce hydrochar suitable as solid fuel, contributing to the conversion of waste into energy and the preservation of the environmental.

Waste Cotton-Derived Fiber-Based Thermoelectric Aerogel for Wearable and Self-Powered Temperature–Compression Strain Dual-Parameter Sensing

The rapid development of the global economy and population growth are accompanied by the production of numerous waste textiles. This leads to a waste of limited resources and serious environmental pollution problems caused by improper disposal. The rational recycling of wasted textiles and their transformation into high-value-added emerging products, such as smart wearable devices, is fascinating. Here, we propose a novel roadmap for turning waste cotton fabrics into three-dimensional elastic fiber-based thermoelectric aerogels by a one-step lyophilization process with decoupled self-powered temperature–compression strain dual-parameter sensing properties. The thermoelectric aerogel exhibits a fast compression response time of 0.2 s, a relatively high Seebeck coefficient of 43 μV·K−1, and an ultralow thermal conductivity of less than 0.04 W·m−1·K−1. The cross-linking of trimethoxy(methyl)silane (MTMS) and cellulose endowed the aerogel with excellent elasticity, allowing it to be used as a compressive strain sensor for guessing games and facial expression recognition. In addition, based on the thermoelectric effect, the aerogel can perform temperature detection and differentiation in self-powered mode with the output thermal voltage as the stimulus signal. Furthermore, the wearable system, prepared by connecting the aerogel-prepared array device with a wireless transmission module, allows for temperature alerts in a mobile phone application without signal interference due to the compressive strains generated during gripping. Hence, our strategy is significant for reducing global environmental pollution and provides a revelatory path for transforming waste textiles into high-value-added smart wearable devices.

Inspired by Plant Transpiration: Fabrication of a Unique Micro–Nano-Structured Janus Evaporator Using Waste Cotton Fabric for Enhanced Efficiency and Salt Resistance

Inspired by Plant Transpiration: Fabrication of a Unique Micro–Nano-Structured Janus Evaporator Using Waste Cotton Fabric for Enhanced Efficiency and Salt Resistance

Bio‐Based Packaging Materials from Post‐Consumer Cotton Textiles

This study aims to convert cotton‐based post‐consumer textile waste to biodegradable paper, which not only addresses the discarding of waste but also provides a second use of cotton. The post‐consumer garment made with cotton was decolorized by stripping with concentrated NaOH and hydrose. Afterwards, it was chopped, ground, and treated with NaOH solutions. The paper was prepared through a wet‐laid process by mixing carboxymethyl cellulose as a binder with chopped cotton textiles. To reduce water absorbency, the uncoated paper was coated with thermoplastic polyurethane (TPU) using heat pressing technique. The surface morphology and chemical structure of uncoated pristine paper, coated paper, and TPU films were conducted using SEM and FTIR studies. The tensile strength, contact angle, air permeability, and biodegradability tests were investigated according to the standard methods. The tensile properties of the papers were increased after TPU coating, accounting well around 28% compared to the uncoated pristine paper. The elongation at the break of the coated paper was at least 40% greater than the uncoated pristine paper. The coated paper displayed a higher water contact angle of 100°, even after 10 min. The lower air permeability was observed in the coated paper due to TPU film blocking the free spaces of the paper. The TPU‐coated paper exhibited a weight loss of 48.1%–59.8% after 90 days, whereas the uncoated paper was 100% decomposed after 60 days. The burning of post‐consumer cotton textile waste (PCCTW) paper generated ashes resembling those of burning paper, implying clean and environmental friendly biodegradation. The papers can replace the petroleum–plastic materials and serve as food and other packaging applications.

Research on the construction and properties of waste cotton cellulose/carbon dots composite antibacterial hydrogel

The recycling and utilization of waste cotton fabrics is of great significance to implement green pollution reduction and carbon reduction. In order to realize the high value-added utilization of waste cotton cellulose (WCC) by introducing new carbon dots (CDs), WCC/CDs composite hydrogel with good antibacterial effect were prepared for the application in the field of antibacterial dressings. WCC/CDs composite hydrogel formed by doping different contents of carbon dots (CDs) with WCC hydrogel as raw material and epichlorohydrin (EPI) as crosslinking agent. The structure was characterized by X-ray diffraction (XRD) and infrared spectrum analysis, the antibacterial properties and swelling properties of WCC/CDs composite hydrogel were tested. The results show that the addition of CDs promoted the gelation of WCC hydrogel, enhanced its structure stability and endowed it with antibacterial properties. WCC/CDs composite hydrogel with 10wt% CDs was more stable, and its modulus reached 91 KPa. Compared with WCC hydrogel the swelling performance of WCC/CDs composite hydrogel was improved, and the swelling rate was 16.81 g·g-1. By destroying the cellular structure of bacteria and promoting the production of ROS, the inhibition rate of E. coil and S. aureus was more than 99%.

Low-Density All-Cellulose Composites Made from Cotton Textile Waste with Promising Thermal Insulation and Acoustic Absorption Properties

Low-Density All-Cellulose Composites Made from Cotton Textile Waste with Promising Thermal Insulation and Acoustic Absorption Properties

Weldable and calligraphy programmable humidity-actuated regenerated cellulose film from waste cotton fabric

The textile industry has caused significant pollution and resource consumption. To address the issue of textile waste, it is crucial to effectively recycle most common natural fiber - cotton. However, current methods face limitations in terms of cost and efficiency. In this study, we developed a weldable and programmable humidity-actuated regenerated cellulose (RC) film based on discoloration, dissolution, regeneration, and vacuum filtration of cotton textiles waste. Therein, the dissolution of cotton waste by phosphoric acid (PA) offers advantages of high efficiency, low cost, and mild dissolution conditions. Especially, we proposed a water-assisted welding process to connect independent films, allowing for the design of a smart cuff with adjustable flaps showing the potential for improved thermal-moisture comfort. Furthermore, inspired by Chinese calligraphy, a hydrophobicity programming strategy is introduced, facilitating diverse actuation and the creation of an information encryption device. Overall, this work shows promise for efficient, cost-effective recycling and intelligent application of cotton textiles waste. The proposed water-assisted welding and calligraphy-inspired programming would pave the way to advance multiscale, adaptive, and intelligent actuation of cellulose materials.

Assessment of Properties and Microstructure of Concrete with Cotton Textile Waste and Crushed Bricks.

Cotton textile waste (CW) and crushed bricks (CB) are wastes generated by the textile and construction industries that cause adverse effects on the environment. This paper explores the effect of adding 1, 2, 5, and 10 wt.% of CW and CB, instead of natural sand under 1 mm (50 to 100 vol.%), on the properties of concrete. The study included the analysis of workability, density, water absorption, thermal conductivity, mechanical strengths, and electron microscopy. The results show that the presence of CW and CB increased the water required to obtain the same slump value as reference, R. Concretes with CW provided better performance in terms of density, water absorption (for 1 wt.%), and splitting strength (for 1 to 2 wt.%). The 28 days of compressive strength decreased with increasing CW (33.3 MPa for R and 26.9 MPa for 2 wt.% of CW). The partial substitution of sand decreased the workability and density and increased the mechanical strength of concrete. The presence of both CW and CB decreased workability, density, and mechanical strengths. Regarding the ability of concrete to transfer heat, the addition of CW and CB decreased the thermal conductivity value (e.g., 0.32 W/(m·K) for 1 wt.% of CW compared to 0.37 W/(m·K) for reference).

Waste cotton fabric-derived multimodal heating textile for comfortable and reliable personal thermal management

Wearable textiles with multiple desirable thermal functions still face significant challenges due to the sophisticated manufacturing techniques and limited sources. This work aimed to innovatively propose a convenient means to up-recycle waste cotton fabrics into an all-in-one heating textile integrating electric heating, outdoor solar heating, and indoor radiative heating for efficacious personal thermal management. The multimodal heating textile is realized by the in-situ growth of vertically-aligned polyaniline nanofibers similar to moth compound eye structure on the surface of cotton fibers and assembling silver nanowires on the backside. Methods for instrument analysis and simulating human skin were employed to evaluate the heating performance of the prepared heating fabric. The heating cotton fabric exhibits superb visible light absorptivity (∼98.5%), allowing the fabric 36.7 °C higher than that of the normal cotton under one solar irradiance. Meanwhile, the high mid-infrared reflectivity (∼61.0%) endows the fabric with exceptional radiative heating performance for human body. The high conductivity ensures the terrific electric heating ability (∼74 °C at 0.7 V) of the fabric. Fortunately, the fabric also possesses excellent electromagnetic interference shielding (∼60 dB), breathability, flexibility, self-cleaning performance, and antibacterial capacity. The findings in this work open a promising direction on how to convert waste fabrics into high value-added products and develop efficacious advanced heating textile, which is benefit for realizing the ideal personal thermal management and mitigating global climate change.

A sustainable approach to manufacture mélange yarn from waste jute fiber and pre-consumer cotton fabric waste using I-optimal mixture design

Today, the exploitation of recycled fibers emanating from textile waste has substantially increased owing to the growing environmental consciousness, legal sustainability mandates, and escalating raw material expenses. Herein, for the very first time, an innovative and sustainable approach was employed to utilize the dyed waste jute fibers accumulated beneath the jute weaving machines to create mélange yarns. Recycled cotton was used as well to reduce the amount of virgin cotton in mélange yarn. First-time, a precise prediction of mixing proportions in mélange yarn was yielded by using an I-optimal mixture design, a part of response surface methodology. The design recommended performing 16 experimental runs with 3 mixing components: dyed waste jute fiber (5–30%), recycled cotton (5–15%) and virgin cotton (55–80%), resulting in the production of 16 distinct yarn samples. Surface response models such as special quartic, special cubic, linear, and quadratic were developed for predicting the yarn unevenness, imperfection, hairiness, strength, and elongation. All the models were found to be statistically significant with p-values below 0.0500. Using desirability functions based on the response models, the optimized mixture components in the yarn were derived to be 15.8% dyed waste jute, 7.4% recycled cotton, and 76.8% virgin cotton fibers. This formulation was used to produce an 18 Ne mélange yarn whose quality parameters closely matched the predicted values, substantiating the effectiveness of the response model in navigating and predicting the attributes of mélange yarn.

Sustainable Textile Raw Materials: Review on Bioprocessing of Textile Waste via Electrospinning

The fashion and textile industry in its current fast-rising business model has generated a huge amount of textile waste during and after the production process. The environmental impact of this waste is well documented as it poses serious threats to lives on earth. To confront the menace of this huge pollution problem, a number of research works were carried out to examine the possible re-utilization of these waste materials without further damaging the environment; for instance, reusing, generating valuable products, or regenerating fibrous materials to form a closed loop in the cotton textile waste lifecycle. This review covers different methodologies to transform cellulosic textile materials into various products with added value, such as cellulosic glucose, cellulase, etc., and finally, to regenerate the fibrous materials for re-application in textiles and fashion. This article presents an overall picture to researchers outlining the possible value addition of textile waste materials. Furthermore, the regeneration of cellulosic fibrous materials from textile waste will be brought into the limelight.

Effect of microcrystalline cellulose on rheology, hydration kinetics and early-age properties of Portland cement-based and alkali-activated slag-fly ash blend

The research examines the influence of microcrystalline cellulose (MCC) on the evolution of rheological behaviour, micro-structural build-up, hydration kinetics and early-age strength (up to seven days) of OPC and AAM matrices subject to dry and sealed curing. Rheological measurements show that the addition of MCC improves recoverability of viscosity and thixotropy of OPC-based matrices. While viscosities of MCC-OPC matrices increase with relatively low change in yield stress, MCC- AAM shows drastic increase in yield stress, up to 2.2 folds within 15 min, due to an increase in MCC content from 0.25% to 1%. This is attributed to gelation and swelling of MCC in NaOH. Hydration kinetics of MCC-OPC in the first 24 h is severely retarded with increase in MCC dosage (about 5.3 h for 1% MCC), while that of AAM is less sensitive to MCC dosage, attributed to rupture of hydrogen bonds in MCC after exposure to NaOH. The findings also show that addition of MCC reduces the heat evolution of AAM after seven days of hydration, due to its suppressing effect on fly ash dissolution. Early-age compressive strength of MCC-OPC reduces with increase in MCC content, registering a maximum reduction of 21% at 1% MCC dosage. Compressive strength of MCC-based AAM is less sensitive to curing conditions and the dosage of MCC than OPC-based matrices. The findings suggest that MCC can be a potential admixture for modifying rheology and hardening of OPC and AAM matrices, while providing an avenue for upcycling cotton textile and biomass wastes.

Production and characterization of activated carbon from pyrolysis biochar of cellulosic cotton-based textile wastes

The increased in the production of textile wastes has been associated to the to the excessive formation of solid wastes resulting to detrimental threat to both human and the environment. Therefore, the upcycling of these wastes materials is in utmost importance. In this study, cotton textile wastes (CTW) undergone slow pyrolysis in a static bed reactor at different temperatures (350 °C − 550 °C), to produce biochar as a precursor for the production of activated carbon (AC). The biochar were then chemically activated using H3PO4 at different impregnation ratios and then heat-treated to obtain the (AC). The produced biochar and AC were characterized using scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Results showed that highest char yield (27.64 wt%) was obtained at 450 °C, with a microporosity structure increased as the temperature increases. On the other hand, the micropore volume of AC increases as the impregnation ratio increases. Finally, the AC exhibit an acidic surface with an abundance of oxygen surface groups, making it promising candidates for different applications.

Manufacturing of window shutters from cotton waste fabrics for sustainable textiles

Purpose This study aims to manufacture alternative window shutters using waste cotton fabrics by stiffening using polyvinyl acetate (PVA) with vinyl acrylic binder solutions. Design/methodology/approach The manufactured fabrics were evaluated for their tensile strength, drapeability, bending length by weight and color fastness to light. And finally, an analysis of variance was done for each parameter. Findings As the percent of PVA with a vinyl acrylic solution and the number of layers increased, the tensile strength, drape coefficient (percent), bending length (cm), and color fastness to light increased in both directions. The percent of PVA with a vinyl acrylic solution and the number of layers are statistically significant for each response such as tensile strength, drape coefficient (percent), bending length (cm), color fastness to light and water repellency at a 95% confidence interval. Tensile strength, drape coefficient (%) and bending length (cm) are always greater in the warp direction than in the weft direction. The tensile strength, drape coefficient (percent), bending length (cm) and color fastness to light of treated fabrics samples are greater than those of the untreated fabrics. Originality/value The factory waste fabrics can be recycled into window shutters which will provide the cheaper raw material for window shutter manufacturers.

Environmentally friendly oxidation pretreatments to produce sugar-based building blocks from dyed textile wastes via enzymatic hydrolysis

Given the increasing concern over textile waste management and the proliferation of textile landfills, enzymatic hydrolysis of cotton represents a potential pathway to upcycle textile waste into valuable chemical building blocks. However, this pathway is challenged by the presence of persistent dyes, hindering enzyme performance. To overcome this issue, environmentally friendly and total chlorine free oxidation methods such as ozone and alkaline hydrogen peroxide were used in combination with mechanical refining pretreatment. The results showed that the enzymatic conversion of black-dyed cotton, without oxidation, resulted in a glucose yield of only 60% as compared to 95% for undyed cotton fibers. On the other hand, the inclusion of oxidation processes in the pretreatment stage resulted in a glucose yield of 90% via enzymatic hydrolysis at expense of using low oxidation chemicals and low enzyme charges. This work highlights the potential of oxidation methods, enzymatic hydrolysis, and mechanical refining as an ecofriendly pathway for generating value-added chemicals from cotton textile waste while promoting economic circularity.

Cationic Waste Cotton Fabric for the Adsorptive Colorimetric Detection of Chromium Ion Traces

Hexavalent chromium (Cr(VI)) exists in water as carcinogenic anionic species. Here, waste cotton fabric (WCF) was recycled and given a second life as a support for Cr(VI) colorimetric detection. WCF was prepared by grafting 2,3-epoxypropyltrimethylammonium chloride on its surface to make it cationic (cWCF) and suitable for Cr anion adsorption via electrostatic interactions. Two strategies were assessed for Cr(VI) detection in water. In the first method, cWCF was functionalized with color-developing 1,5-diphenylcarbazide (DPC), and the resulting strip was evaluated for Cr(VI) quantification. This sensor exhibited a short response time of 30 s with a limit of detection (LOD) of 0.029 mg/L. In the second method, Cr(VI)-containing samples were initially preconcentrated by adsorption on the cWCF substrate, and then, DPC was added for color development. With this system, the LOD was excellent at 0.0013 mg/L, but the response time was longer at 5 min. Another advantage of the robust cWCF support is that it could be washed and reused multiple times for Cr(VI) detection. The two approaches described here are inexpensive and can be selected for different purposes depending on the sensitivity required as well as the volume of samples and time available to complete the Cr ion assay.

Electrooxidation Performance of a Cotton-Cloth-Derived, Ni-Based, Hollow Microtubular Weave Catalytic Electrode for Methanol and Urea

Increasing consumption produces a large amount of cotton textile waste, the conversion of which into porous metals used for energy purposes is of practical value. In this paper, a porous, Ni-based, hollow microtubular weave (Ni-HTW) is obtained from cotton weave by high-temperature carbonization and hydrothermal synthesis with high-temperature reduction. The Ni-based, hollow microtubules in this weave have a diameter of 5–10 μm and a wall thickness of about 1 μm, and every 15 microtubules form a loose bundle with a diameter of 150–200 μm. For improved performance, Ni(OH)2 nanosheets are further electrodeposited on the fibers’ surface of the Ni-HTW to form a nano-Ni(OH)2/Ni-HTW composite electrode with a core–shell heterostructure where Ni is the core and Ni(OH)2 the shell. The combination of hollow microtubule weave morphology and nanosheet structure results in a large specific surface area and abundant active sites, and the composite electrode shows excellent electrocatalytic performance and long-term stability for methanol oxidation (MOR) and urea oxidation (UOR). The current densities can reach 303.1 mA/cm2 and 342.5 mA/cm2 at 0.8 V, and 92.29% (MOR) and 84.41% (UOR) of the pre-cycle current densities can be maintained after 2000 consecutive cycles.

Fabrication and Characterization of Functional Biobased Membranes from Postconsumer Cotton Fabrics and Palm Waste for the Removal of Dyes

Textile industries currently face vast challenges for the active removal of colored wastewater. Indeed, sustainable, recyclable, and green approaches are still lacking to achieve this aim. Thus, the present study explored the utilization of highly functional, green, recyclable, fully bio-based, and cost-effective composite membranes from post-consumer cotton fabrics and palm waste for wastewater treatment purposes. Highly functional cellulose nanofibers (CNF) were produced from waste cotton fabrics and filter paper using an acid hydrolysis technique. The yield of nanofibers extracted from waste cotton fabrics and filter paper was 76.74 and 54.50%, respectively. The physical, chemical, and structural properties of nanofibers were studied using various advanced analytical techniques. The properties of isolated nanofibers were almost similar and comparable to those of commercial nanofibers. The surface charge densities were -94.0, -80.7, and -90.6 mV for the nanofibers of palm waste, cotton fibers, and filter paper, respectively. After membrane fabrication using vacuum and hot-pressing techniques, the characteristics of the membrane were analyzed. The results showed that the average pore size of the palm-waste membrane was 1.185 nm, while it was 1.875 nm for membrane from waste cotton fibers and filter paper. Congo red and methylene blue dyes were used as model solutions to understand the behavior of available functional groups and the surface ζ-potential of the membrane frameworks' interaction. The membrane made from palm waste had the highest dye removal efficiency, and it was 23% for Congo red and 44% for methylene blue. This study provides insights into the challenges associated with the use of postconsumer textile and agricultural waste, which can be potentially used in high-performance liquid filtration devices for a more sustainable society.

Effects of ZnCl2 Catalyst Loading in Hydrothermal Carbonization of Cotton Textile Waste to Produce Hydrochars

Hydrothermal carbonization (HTC) is a thermochemical process that utilizes biomass as feedstocks to produce hydrochars as solid fuel. Cotton textile wastes (CTW) are abundant; most are landfilled or incinerated, causing environmental pollution. CTW is rich in cellulose, thus suitable as biomass feedstock. HTC at a moderate temperature below 230 °C yields low-value hydrochars with low carbon content; however, an acid catalyst enhances the reaction. In this study, zinc-activated cotton textile waste (Zn-CTW) was synthesized via incipient wetness impregnation and used as a catalyst in hydrothermal carbonization to produce hydrochars. The reactions were conducted in a batch reactor at a temperature of 200 °C for 3 h. The effects of ZnCl2 catalyst loading on CTW were studied. The characteristics of hydrochars in terms of surface morphologies, hydrogen/carbon (H/C) and oxygen/carbon (O/C) ratios and surface functional groups were affected by the ZnCl2 loading on CTW. The results show that the hydrothermal carbonization of Zn-CTW-1.5 obtained hydrochars with the lowest H/C and O/C ratio values of 1.286 and 0.614, respectively. The FTIR analysis indicates stretching vibration of the C-O bond, which are the carboxylic acids and esters formed in the hydrochars. The hydrochars' surface morphologies show irregular and rough surfaces. It is concluded that Zn-CTW has the potential as a heterogeneous catalyst to produce hydrochars via hydrothermal carbonization