ABSTRACT KOH has traditionally been used to extract lignite or weathered coal to produce water-soluble potassium humate (KHA) fertilizer, which is essential in agriculture. However, the KHA yield from this method is constrained by the free humic acid (HA) content in the raw material, and the resulting product shows limited resistance to hard water. In this study, lignite, KOH, and copper-based catalysts, including commercial copper nitrate (CCN), hydrothermal copper oxide (HCO), and commercial copper oxide (CCO) were used in hydrothermal treatment at 150°C for 2 h. The findings indicate that CCN, HCO, and CCO increased KHA yields by 9.94%, 4.37%, and 3.76%, respectively, with a maximum yield of 66.94% achieved using CCN. Copper-based catalysts reduced the oxygen-containing functional groups in HA and promoted the release of calcium and magnesium ions from HA, thereby improving KHA’s resistance to hard water. Among the catalysts, KHA prepared with CCO exhibited the highest resistance to hard water. The time of flocculation in hard water at 20 °dH and 21 °dH was delayed by 10 h and 2.4 h respectively. Additionally, the hydrothermal residue demonstrated effective methylene blue (MB) removal, highlighting the potential for lignite’s non-fuel applications and the resource-efficient utilization of lignite waste.
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