Catalytic hydrothermal liquefaction of chrome-tanned leather scrap: Bio-oil upgrading, chromium migration and recovery of chromium-containing product

Abstract

Chrome-tanned leather scrap (CLS) is a hazardous solid waste rich in protein and chromium, posing environmental risks while offering significant resource recovery potential. In this study, an integrated catalytic hydrothermal liquefaction (HTL) strategy was developed to couple bio-oil upgrading with chromium retention and recovery from CLS. The effects of reaction parameters and catalytic systems on product distribution were systematically investigated. The optimal reaction conditions were determined to be 340 °C, 60 min, and 20 wt% solid loading, yielding 35.60 wt% bio-oil. A synergistic catalytic system composed of KOH, Fe + Fe 3 O 4 , and an ethylene glycol–water (1:1, v/v) solvent system significantly enhanced the bio-oil yield to 64.62 wt% and improved fuel quality, with carbon contents of 71.32–75.89 wt% and higher heating value (HHV) of 35.53–38.91 MJ·kg −1 . The catalytic system also regulated nitrogen transformation pathways, promoting the conversion of amino acid-derived intermediates into nitrogen-containing heterocycles (i.e., pyridines and pyrazines), while reducing nitrogen accumulation in the aqueous phase. Meanwhile, chromium was predominantly immobilized in the solid residue, while only low total chromium concentrations were detected in the aqueous phase and bio-oil. Based on the chromium-rich solid residue, a regenerated chromium-containing product was prepared through an acid dissolution–masking–alkalization–drying process, showing basic quality indicators comparable to Grade III industrial solid chrome-tanning agents. This study provides a potential pathway for integrating waste-to-energy conversion with chromium recovery, contributing to cleaner production and resource utilization of chromium-containing leather waste.