Chromium­(III) sulfate is extensively used in leather processing to stabilize the collagen molecules in hides and skins. Although its excess usage causes severe environmental pollution and health concerns, the role of chromium in stabilizing collagen still remains poorly understood. For the first time, by integrating a number of techniques, including real-time small-angle X-ray scattering, differential scanning calorimetry and natural cross-link analysis, we reveal crucial molecular-level indicators of collagen stability. The results indicate that collagen molecules achieve maximum molecular stability at concentrations as low as 1.8 wt % even if excess chromium (>3.7%) is introduced into the collagen matrix. At low concentrations (1.8% to 3.7%), the active amino acid residues are saturated via covalent bonding with chromium. Any excess chromium interacts purely non-covalently with the collagen molecule and, we propose, can be substituted by environment-friendly alternatives. Further, important natural cross-links, which are crucial in imparting mechanical strength, were observed to decrease with increasing chromium concentration, highlighting the adverse impact of chromium­(III) sulfate on collagen matrix and the importance of identifying alternative cross-linking agents. Our findings provide tools which will enable the evaluation of greener tanning agents to facilitate a more sustainable future for the leather industry.

The effect of sodium silicates on collagen structure during leather processing was investigated. Small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) reveal that the molecular structure and thermal stabilities of the sodium silicate treated leathers (So-Si and So-Si + BCS) were different to the conventionally processed chromium treated leathers (BCS). The collagen fibrils were observed to be coated by aggregates of silica, which did not affect the axial periodicity (D-period) of the collagen molecules. However, an increase in collagen fibril diameter was observed during the main tanning step when sodium silicates were used. This could be due to the swelling of collagen fibers from the high alkaline conditions of sodium silicates. From DSC studies, it was also found that sodium silicate treated samples impart no effect on collagen stabilization in the absence of chromium(III). However, a pseudostabilization effect is observed in the So-Si + BCS samples, possibly due to the inability of the collagen molecules to undergo conformational changes due to the silica coating on the collagen fibrils.The tanning of leather involves chemically intense processes leading to environmental pollution, resulting in a demand for cleaner but effective collagen stabilization mechanisms for the leather industry.1,2 Basic chromium(III) sulphate is the most common mineral tanning agent and, is preferred industrially because of the high hydrothermal stability. It has excellent properties in addition to relative short times required to produce nished leathers.3 However, poor uptake of chromium salts leads to high chemical and biological oxygen demand, and toxicity concerns relating to hexavalent chromium(VI) exposure, have led researchers to seek more environmentally friendly alternatives. 4-6Mineral tanning agents such as chromium, zirconium, aluminium, titanium and iron can all stabilize collagen and impart varying degrees of hydrothermal stability.3,7 Synthetic tanning agents (syntans) and vegetable tanning agents from plant polyphenols along with aldehydic cross-linkers are also commonly used, but typically in conjunction with mineral tannages.8 Combination tannages however, with or without mineral tanning agents, can overcome the issues that single tanning systems have with hydrothermal stability.9,10 For example, Vitolo and co-workers observed an increase in chromium uptake when sodium silicate was used in combination with basic chromium sulphate.11 Whilst pre-tans can improve the penetration and even distribution of the main tanning agents in the collagen matrix, a co-stabilizing agent can increase the efficiency of chrome uptake by reducing the amount of chrome required.8 With this idea of combining a weak and strong tanning agent, further studies have been carried out by investigating combinations of a number of tanning options and their effect on collagen stabilization. 4,6,9,12 Soluble silicates such as sodium silicate belong to a group of compounds that contain varying compositions of an alkali met...

The collagen structure in skins is significantly influenced by the cross-linking chemistry adopted during leather processing. We have developed an in situ technique to measure real-time collagen structure changes using synchrotron-based small-angle X-ray scattering (SAXS). Three common mineral tanning systems, basic chromium sulfate (BCS), zirconium sulfate (ZIR) and an aluminosilicate-based reagent (ALS) were used to stabilize collagen in ovine skin. Studying the molecular changes by in situ SAXS revealed a range of tanning mechanisms: a complex combination of covalent cross-linking, electrostatic interactions and hydrogen bonding by BCS, hydrogen bonding interactions by ZIR, and the formation of colloidal aggregates by ALS. These results unravel the mechanisms of producing leathers with different properties, explaining why ZIR produces denser leathers while ALS produces softer leathers compared to conventional BCS leathers. ZIR and ALS are environment-friendly alternatives to BCS, and understanding their mechanisms is important for a more sustainable future for the leather industry.

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