Formation of formaldehyde in dialdehyde polysaccharides tanning agent: Effect of polysaccharide structural characteristic.

Dialdehyde polysaccharides (DAPs) were prepared as tanning agents via periodate oxidation, intentionally omitting ethanol precipitation to maintain molecular weight polydispersity, thus optimizing tanning performance. However, the presence of formaldehyde in these DAPs compromised their environmental sustainability. This study systematically explored the impact of polysaccharide structures on formaldehyde formation in DAPs. Five dialdehyde derivatives were successfully synthesized: dialdehyde maltodextrin (DMD), dialdehyde chitosan (DCTS), dialdehyde sodium hyaluronate (DSH), dialdehyde starch (DST), and dialdehyde sodium alginate (DSA). Formaldehyde content analysis revealed the following concentration hierarchy: DMD (4.59 mg/g) > DCTS (3.33 mg/g) > DSH (1.14 mg/g) > DST (0.99 mg/g), with no detectable formaldehyde in DSA. The formaldehyde formation mechanism was elucidated by identifying intermediate structures. It was demonstrated that adjacent hydroxymethyl and hydroxyl/aldehyde groups, generated during periodate oxidation, underwent overoxidation to produce formaldehyde. Polysaccharides with more labile glycosidic bonds were more prone to forming these reactive intermediates, resulting in higher formaldehyde levels. This mechanism was further confirmed by periodate oxidation of monosaccharides. In practical tanning applications, DAP-tanned leathers thoroughly evaluated. DSA-tanned leather, notably formaldehyde-free, exhibited an outstanding shrinkage temperature of 80.8 °C, highlighting its potential as an environmentally friendly tanning agent. These findings indicate that selecting polysaccharides with stable glycosidic bonds and avoiding hydroxymethyl precursors (e.g., sodium alginate) can eliminate formaldehyde in both DAPs and the tanned leather products. This research offers both a theoretical framework for minimizing formaldehyde in DAP synthesis and provides practical guidance for advancing sustainable tanning technologies.