Comparison of molecular characteristics of Type A humic acids derived from fish waste and sugarcane bagasse co-compost influenced by various alkaline extraction protocols

An analytical investigation on different alkaline extraction methods to extract “Type A” humic acids (HAs) from fish waste and sugarcane bagasse co-compost using 0.1 M concentration of Na4P2O7, NaOH and KOH was experimentally studied. The influence of each alkaline extraction method in producing heterogeneous characteristics of the humic acids (HAs) was critically compared to identify the variations in yield, elemental composition; spectroscopic properties and morphological features. Contrasting variations in the yield; elemental Carbon (C) and Oxygen (O) composition was identified in HAs extracted with 0.1 M Na4P2O7 and NaOH. UV–vis and fluorescence spectroscopy exhibited slight differences in the absorption maxima (λmax) and emission spectra (λem) values. FTIR analysis showed contrasting variations in the peak intensities of the functional moieties. Especially, high resolved IR spectra was observed in HAs extracted with Na4P2O7 for CC of amide I (1631 cm−1) and CO of polysaccharides (1085 cm−1) whereas NaOH and KOH derived HAs exhibited prominent shoulder for alkyl CH groups (2935 cm−1). ICP-MS results showed drastic variations in the concentration of micro elements and heavy metals in HAs obtained from each method. 1H NMR spectra showed intense signals in 3–4 ppm region for HAs extracted with NaOH. Stereo microscopic and photon correlation spectroscopic measurements revealed variations in the particle sizes of HAs. Tiny diffused amorphous structures of HAs extracted using Na4P2O7 and large rock-like macromolecular structures of HAs isolated with KOH and NaOH was apparent from SEM analysis. The comparative findings of the study provide an insight towards understanding the sensitive variations observed in the molecular characteristics of humic acids influenced by the alkaline extraction protocols.