Spectroscopic characterization of humic substances isolated from tropical mangrove sediments

Mangroves are tropical wetlands that are among the most productive ecosystems on Earth which cover up to 75% of the coastlines. Humic substances are the ever-present natural organic compounds, being a major component of organic carbon in the global carbon cycle. The present study focuses on the organic carbon dynamics as well as spectroscopic characterization (UV, FT- IR, and NMR) of humic substances—humic acids (HAs) and fulvic acids (FAs)—isolated from three tropical mangrove sediments representing different environmental settings belonging to Kerala, in the southwest coast of India during a premonsoon sampling conducted in April 2018. The study revealed a higher concentration of HAs than FAs in all the stations which was also complemented by a higher concentration of tannin and lignin as well as TOC. The E4/E6 ratios of FAs were also higher than that of HAs confirming its low molecular weight and less polymerized nature. The FTIR spectrum of both HAs and FAs showed peaks corresponding to the existence of carboxyl, phenol, carbonyl, and amide group. The proton NMR spectrum showed the presence of aliphatic regions slightly more controlled with long chains and/or alicyclic moieties rather than methyl groups which are also supported by the higher hydrogen content in HAs than FAs, and these peaks were absent in FAs. It also showed the presence of more or less common aromatic core involved in unsaturated structures as well as other aromatic groups like phenols and ionized carboxylic groups slightly higher in FAs than in HAs.