ABSTRACT

Chemical and physical properties of humic acids (HAs) isolated from Mudbanks (locally known as "Chakara"—a unique phenomenon occurring along the south west coast of India during the southwest monsoon season) sediments were characterized by using CHNS, UV-Vis, Fluorescence, FTIR, ¹H NMR, solid state CP/MAS ¹³C NMR, MALDI-TOF-MS and HRTEM. Elemental analysis by CHNS revealed the presence of excess organic sulfur content which indicates the prevailing of sulphate-reducing conditions during the active phase of mudbanks. The HAs origin under the influence of terrestrial environments which contributes more aliphatic fragments is substantially supported by lower N/C ratio and higher E4/E6 ratio (i.e., a rapid decrease in absorptivity with an increase in wavelength). Intense short wavelength maxima in fluorescence emission spectra characterises the presence of terrestrially derived lignin-like moieties. FTIR indicates the presence of hydroxyl, methyl, methylene, carbonyl, carboxyl, phenol, and alcohol and amide groups, probably sulfur groups also. The ¹H NMR spectra shows that in mudbank HAs, the relative content of aliphatic, lignin-like and methoxyl protons is typically greater than the content of aromatic protons. Alkyl carbon components (such as methyl, methylene and methine carbons) in ¹³C NMR spectra constitute the highest structural moiety, and the percentage of carbon signaling carboxyl, amide, ester, ketone, and aldehyde groups is significantly higher than the O-substituted alkyl or substituted aromatic carbon entities. The resonance of unsubstituted aromatic structures shows a higher proportion of the structural components. Mass fragmentation pattern in MALDI-TOF-MS revealed the presence of lignin moieties with β-O-4 in the HAs. These spectral informations further correlated to the quasi-spherical assembling of HAs revealed from HRTEM images. Structural features of HAs isolated from the sediments of mudbanks are significantly marked with lignin and/or lignin-like moieties which are characteristic of the terrestrially derived organic debris, and thus it endorses various postulates for the formation of mudbanks along the Kerala coast during southwest monsoon season, based on the flow of water and mud from the inland rivers and backwaters through the "subterranean channels" into the coastal sea.