Green Salt-Free High-Exhaustion Chrome Tanning Strategy: Biomass-Derived Aldehyde Acid–Chrome Tanning

Conventional chrome (Cr) tanning systems generate substantial wastewater containing Cl^– and Cr³⁺, which presents huge environmental challenges. This study proposes a sustainable alternative, biomass-derived aldehyde acid–chrome (AA–Cr) tanning, to mitigate Cl^– and Cr³⁺ pollution. Pretanning with 3 wt % AA effectively prevented acid-induced swelling of pelts in the absence of salts at a pickling pH of 1 to 4. The dialdehyde and carboxyl groups in AA facilitated the formation of a robust AA–Cr cross-linking network. Subsequent tanning with 3 wt % chrome tanning agent led to an exceptional Cr uptake rate that exceeded 98.5%, with the AA–Cr system exhibiting superior tanning performance compared to glyoxylic acid–Cr (GA–Cr). Compared to the Cr system, the AA–Cr system achieved 89.4% and 96.3% reductions in Cl^– and Cr³⁺ loads, respectively, while exhibiting enhanced biodegradability. Life cycle assessment revealed that the AA–Cr system demonstrates remarkably lower carbon emissions, reduced resource consumption, and diminished human toxicity. This study presents a feasible strategy for salt-free, high-exhaustion chrome tanning and provides novel insights into the molecular design of biomass-derived AA.