Optimization of extraction method and characterization of humic acid derived from coals and composts (Pengoptimuman kaedah pengekstrakan dan pencirian asid humik daripada arang batu dan kompos)

Humic acid (HA), known as the black gold of agriculture, is increasingly becoming popular for use in agriculture. HA from sub-bituminous coals from Mukah, Sarawak, commercial HA product (leonardite), humified peat and various types of compost were extracted and their chemical characteristics compared. HA yield from various coals ranged from 1.5–11.1% while composts yielded HA from a low of 4.2% to 16.5%. Humified peat and commercial leonardite contained 31.6% and 58.5 % HA respectively. Functional group analyses of extracted HA from Mukah coals showed a narrow range of carboxylic group from 3.2–3.5 meq g–1 while phenol-OH group was between 6.8–7.3 meq g–1. Functional groups in HA from composts were, however, lower than those obtained from coals. The total acidity of HA from composts ranged from 5.6–9.5 meq g–1, carboxylic group at 1.9–3.5 meq g–1 and phenol-OH group, 3.5–6.2 meq g–1. Fourier Transformed Infrared (FTIR) spectra of extracted HA from coals, leonardite and chrysanthemum compost had distinct clear absorption bands indicating the presence of major HA structural elemental groups such as H bonded OH (3400 cm–1 peak), C=O of carbonyl (1710 cm–1 peak) functional groups, aliphatic components CH2 and CH3 (2930, 1420 and 1370 cm–1 peaks) and C=C of aromatic ring (1620 cm–1). Keywords: humic acid; coals and composts; extraction techniques; chemical characteristics