Optimisation of the Extraction Process of Toluene and Humic Acid Extract from Brown Coal

Lignite (brown coal) is a promising source of humic acids (HAs) and toluene-soluble extracts (bitumen "A"), which have applications as soil conditioners/biostimulants (HAs) and hydrophobic coatings or polymer additives (toluene extract). This study optimized their sequential extraction from Ukrainian lignite, evaluating yield trade-offs and structural properties. Four extraction variants were tested. Conventional toluene-first extraction (Variant 0) yielded the highest toluene extract (14.86 wt. %), but lower HAs (41.0 wt. %), while reversing the sequence (Variants 2–3) increased HA yields (47.39–51.70 wt. %) at the expense of toluene extract (1.79–5.28 wt.%). Structural analysis revealed toluene extracts had higher aromaticity (60.1–61.9% aromatic carbon) and molecular association (1.9–2.2) than HAs (48.5–60.3%; 1.6–2.0), reflecting their distinct chemistries. Alkaline pretreatment enhanced HA aromaticity but reduced toluene extract recovery. The findings enable tailored protocols for agricultural (HAs) or industrial (toluene extract) applications, supporting sustainable lignite valorization.