Abstract
This study investigated the mechanisms of Humic Acid (HA) precipitation from brown coal waste using hydrochloric, sulfuric, citric, and ascorbic acids. Precipitation was conducted at a pH range of 1.47 to 2.01, and the resulting HAs were characterized by CHN elemental analysis and Fourier Transform Infrared (FTIR) spectroscopy. The highest HA yield was obtained with ascorbic acid (94.81%), followed by sulfuric (93.08%), hydrochloric (91.03%), and citric acid (89.31%). The elemental analysis revealed that citric and ascorbic acids promoted greater preservation of oxygen-containing functional groups (C: 36.4% and 34.8%, respectively), while mineral acids led to more condensed aromatic structures. FTIR spectra confirmed variations in aromaticity, hydroxyl, carboxyl, and carbonyl group intensities, indicating significant structural differences depending on the acid used. These findings suggested that organic acids can offer a milder alternative for HA recovery, preserving functional groups critical for sorption and complexation. The proposed approach contributes to the valorization of coal waste and supports the development of environmentally friendly materials for use in agriculture, water treatment, and soil remediation.