Process optimization and method validation for efficient valorization of low- grade coal into humic substances

The exploration of low-grade coal resources and progress in the extraction method of humic substances are the core aims of this study. Ultrasonic and microwave assisted alkali extractions were compared with conventional shake flask alkali extraction to trace the efficiency of the extraction of humic substances from a wide range of coal samples. Among the seven coal samples collected from coal mines in Chakwal district, Pakistan, the observed humification index values ranged from 1.22 to 1.35 for humic acids, and from 5.0 to 6.9 for fulvic acids. This indicates high ratio of polycondensed aromatic moieties. The coal sample with the highest humic content was used to assess extraction methods targeted to humic substances. The extractions supported with microwave and ultrasound, when compared to shake flask extraction, increased the yield in 6.3 % and 3.2 % for humic acids, and just under 1 % for fulvic acids along with 4 times reduction in extraction time. The highest recoveries of humic acids (39.2 ± 0.9 %) and fulvic acids (9.1 ± 0.3 %) were obtained with a microwave assisted alkali extraction using KOH (0.1 M)-urea (0.5 M) (4:1, v/v) solution, and microwave irradiation time 180 s. In general, the optimised method improved the humic and fulvic acids extraction yield up to 10.8 % and 4.4 %, respectively. Furthermore, the Yates model was well suited for predicting the efficiency of the study extractions.