Thermal degradation of vegetable-tanned leather fiber (VLF) was investigated by thermogravimetric analysis aiming to know the exact kinetics and degradation mechanism. The thermogravimetric (TG) and differential thermogravimetric (DTG) curves showed that decomposition of the VLF occurs mainly in the range of 150–600 °C, and the latter exhibits asymmetrical peak with a pronounced shoulder. The decomposition process was first analyzed by deconvolution of the experimental DTG curves, followed by reconstruction of the weight loss profiles of two individual processes. Several common isoconversional approaches were applied to calculate the activation energy over a wide range of conversion for the sample, including modified Kissinger-Akahira-Sunose (MKAS), Friedman, and Flynn-Wall-Ozawa. The average activation energy of vegetable-tanned leather fiber was found to be 241.9 kJ mol^− 1 by MKAS method. The activation energy values obtained for the pseudocomponents representing highly-crosslinked and low-crosslinked collagen in VLF were given as 190.6 and 124.8 kJ mol^− 1, respectively. Generalized master plots results suggested that the reaction mechanism for highly-crosslinked collagen follows the random nucleation and growth process at conversion values lower than 0.5. When the conversion is higher than 0.5, the mechanism tends to random scission model. For low-crosslinked collagen, the degradation is mainly governed by random nucleation and nuclei growth. The gaseous products of VLF thermal degradation were analyzed with an online-coupled TG-Fourier transform infrared spectroscopy system.

Green solvents, such as propylene carbonate (PC), can be used in leather processing to improve the efficiency of chrome tanning and reduce wastewater. Here we report a combined small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) study on PC and its efficacy as a carrier medium during chrome tanning. SAXS analysis on the collagen structure of chrome tanned leather using PC, compared to conventionally tanned leather using water, showed an increase in Cr uptake in addition to the more uniform penetration of Cr through the leather cross-section. The increased binding of Cr to the collagen matrix drives the decreased hydration environment of the collagen triple helix. Furthermore, DSC studies show a uniform hydrothermal stability for the PC samples due to the more even distribution of Cr through the collagen matrix. Understanding the mechanisms by which chrome tanning occurs in non-aqueous solvents can guide us towards a more sustainable future for the leather industry.

The removal of chromium (Cr) from wastewater by various adsorbents has been investigated. As compared with the commercial activated carbon, the biosorbents with inexpensive and high adsorption capacities are developed from lignocellulosic wastes. Lignin, existing in lignocellulosic biomass, is the second most abundant resource in nature. Recently, lignin-based bio-sorbents were served as an advanced novel material for the metal ions and dye adsorption from wastewater. It has showed several advantages in the wastewater treatments because of the low-cost, high adsorption capacity, easy recover, and possibility of metal recovery. In this review, the isolation of lignin from lignocellulosic biomass was summarized, and the structural characteristics of lignin were comparably analyzed. The modification of lignin was performed to obtain a large surface area, strong binding-site, and high and quick adsorption properties of lignin-based adsorption materials. The adsorption efficiency of Cr ions was found to be strongly dependent on the pH of the wastewater. To further illustrate the adsorption process, the structural changes and the interactions between the metal ions and the functional groups of the lignin-based biosorbents in the adsorption process should be further investigated. Once the cost-effective and high-efficiency modification techniques are developed, lignin-based adsorbents can be expected to be the most suitable alternatives for Cr ions removal from wastewater in industry.

Fractionational and structural characterization of lignin and its modification as biosorbents for efficient removal of chromium from wastewater.

Collagen structure in biological tissues imparts its intrinsic physical properties by the formation of several covalent crosslinks. For the first time, two major crosslinks in the skin dihydroxylysinonorleucine (HLNL) and histidinohydroxymerodesmosine (HHMD), were isotopically labelled and then analysed by liquid-chromatography high-resolution accurate-mass mass spectrometry (LC-HRMS) and small-angle neutron scattering (SANS). The isotopic labelling followed by LC-HRMS confirmed the presence of one imino group in both HLNL and HHMD, making them more susceptible to degrade at low pH. The structural changes in collagen due to extreme changes in the pH and chrome tanning were highlighted by the SANS contrast variation between isotopic labelled and unlabelled crosslinks. This provided a better understanding of the interaction of natural crosslinks with the chromium sulphate in collagen suggesting that the development of a benign crosslinking method can help retain the intrinsic physical properties of the leather. This analytical method can also be applied to study artificial crosslinking in other collagenous tissues for biomedical applications.

A facile synthesis procedure is proposed to prepare homogeneous Zr and Ti co-doped SBA-15 (Zr-Ti-SBA-15) with high specific surface area of 876.0 m² g^− 1. Based on “masking mechanism” from tanning, lactic acid was used as masking agent to obtain the uniform distribution of Zr and Ti species in the SBA-15 framework. The obtained materials were characterized by powder X-ray diffraction (XRD), nitrogen adsorption-desorption isotherms, scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray photoelectron (XPS). The results reveal that in mesoporous materials, the presence of lactic acid gives rise to the uniform distribution of Zr and Ti species. The adsorption equilibrium and kinetic studies of Zr-Ti-SBA-15 materials show that the adsorption process conforms to the Langmuir isotherm and pseudo-second-order kinetic model, respectively. Regenerational experiments show that the Zr-Ti-SBA-15 displays a significant adsorption ability for methylene blue (MB) (up to 291.6 mg/g), along with good reusability, indicating promising potentials of commercialization. Methodologically, this work provides a wide range of possibilities for further development of SBA-15 based on bimetallic and sewage disposal.

Homogenous Zr-Ti-SBA-15 with high specific surface area was successfully by direct synthesis method. The as-prepared Zr-Ti-SBA-15 possessed excellent adsorption capacity and reusability.

The dissolution of collagen in ionic liquids (ILs) was highly dependent on the polarity of ILs, which was influenced by their sorts and concentrations. Herein, the solubility and dispersion degree of collagen in two sorts of ILs, namely 1-ethyl-methylimidazolium tetrafluoroborate ([EMIM][BF₄]) with low polarity and 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) with high polarity in a concentration range from 10% to 70% at 10 °C were investigated. When 150 mg of collagen was added to 30 mg of ILs, the minimum soluble collagen concentration was 0.02 mg/mL in 70% [EMIM][BF₄] with lowest polarity and the maximum was 3.57 mg/mL in 70% [EMIM][Ac] with highest polarity, which indicates that soluble collagen and insoluble collagen fibers were both present. For insoluble collagens, differential scanning calorimetry showed that the thermal-stability was weakened when increasing the ILs concentration and polarity, and the fiber arrangement was looser with a more uniform lyophilized structure, observed by atomic force microscopy and scanning electron microscopy. For soluble collagens, electrophoresis patterns and Fourier transform infrared spectroscopy showed that no polypeptide chain degradation occurred during dissolution, but the thermal denaturation temperature decreased by 0.26 °C~ 7.63 °C with the increase of ILs concentrations, measured by ultra-sensitive differential scanning calorimetry. Moreover, the aggregation of collagen molecules was reduced when ILs polarity was increased as determined by fluorescence measurements and dynamic light scattering, which resulted in an increased loose fiber arrangement observed by atomic force microscopy. If the structural integrity of collagen needs to be retained, then the ILs sorts and concentrations should be considered.

Pressure sensor can be applied in a wide range of fields, such as voice recognition, human motions detection and artificial electronic skin, the sensing of which is greatly influenced by the flexibility and stretchability of substrate materials. Here, based on the piezoresistive effect, new kinds of flexible pressure sensors have been realized from a pair of flexible and biocompatible collagen films: one is coated by silver nanowires (Ag NWs) and the other by interdigital electrode, respectively. The collagen films are regenerated from leather waste and could bring economic benefits to society. The prepared pressure sensors are applied for voice recognition and human motion detection.

A flexible pressure sensor based on two layers of regenerated collagen films coated silver nanowires (Ag NWs) and interdigital electrodes respectively are obtained. The flexible collagen films are prepared by dissolving leather waste into ionic liquid, [BMIM]Cl. The fabricated pressure sensor displays potential applications in voice recognition and human physical motion detection, such as finger and wrist bending-releasing, which are expected to access the training data of athletes, monitor some physical activities of patients synchronously and act as a voice recognition device.

Leather making is the process of converting raw hides into leather. Amounts of solid waste containing hazardous and high value components are generated during this process. Therefore, the elimination of the potential pollution and resource utilization of leather solid waste are the primary research hotspots. Herein, we comprehensively review the recent advances in the resource utilization of leather solid waste generated from chrome tanning including the utilization of tannery hair waste, untanned solid waste, chrome-containing leather solid waste, tannery sludge and finished leather waste, emphasizing on the general and novel utilization approaches. Finally, the remaining challenges and perspectives were summarized, in order to accelerate the development of resource utilization of leather solid waste.

Nowadays, the increasing demands from consumer challenges the traditional leather products. Traditional polymer leather finishes gradually lose their dominant position in the market. To address this issue, recent research effort has been devoted to developing polymer-based organic-inorganic nanocomposite leather finishes due to their various functional properties including antibacterial, self-cleaning and water-resistant property. In this review, we provide a comprehensive overview of synthesis of polymer-based organic-inorganic nanocomposites and their application as functional leather finishes. With the perspective of their properties and current challenges, an outlook in the future development about crafting functional and high-quality leather finishes are further proposed.

Bating pelt with protease is an important process, which removes unwanted non-collagenous proteins from the pelt and moderately disperse hide collagen network. However, the grain surface, may be excessively hydrolyzed during bating due to the longer retention time of protease in the grain than in the middle layer caused by the low mass transfer rate of protease in pelt. Here, the effects of protease dosage, common auxiliaries and molecular weight of protease on protease transfer during bating were investigated so that we can find the key points to avoid excessive collagen damage, particularly in the grain. Observably, increasing protease dosage led to faster protease transfer and softer leather, but along with more considerable grain damage. Using penetrating agent JFC (fatty alcohol-polyoxyethylene ether) and ammonium sulfate enhanced protease transfer and simultaneously alleviated collagen damage due to the decrease in interfacial tension and electrostatic attraction between protease and pelt, respectively. Additionally, proteases with lower molecular weight transferred faster in pelt, which suggests that a potential strategy to solve the conflict between the mass transfer and the reaction of protease in pelt might be to produce/employ smaller bating proteases.