The Important Role of Natural Organic Ligands in the Distribution of Dissolved Fe(III) in Tropical Costal Water

Abstract

Abstract The role of natural organic ligands on Fe(III) ion distribution was determined in the Mersing coastal water. It was studied by determining the concentration of natural organic ligand ([LT]) and its constant stability (Log KFeL) in the seawater collected during two different monsoon seasons, which were pre-monsoon in August 2020 and post-monsoon in April 2021. Both seasons have different physical and chemical conditions due to the influence of the Northeast monsoon. The seawater samples from both sampling periods were analysed using a competitive ligand equilibration-adsorptive cathodic stripping voltammetry (CLE-AdCSV) method. The [LT] in August 2020 ranged from 13.9±0.1 nM to 21.4±0.8 nM, which was higher compared to its concentration in April 2021. In April 2021, its concentration ranged from 9.1±0.1 nM to 19.3±1.2 nM. For the binding strength (Log KFeL) of natural organic Fe(III) binding ligand in August 2020 and April 2021, it ranged between 11.1–11.8 and 10.6–11.9, respectively. This indicated the presence of a similar strong organic Fe (III) binding ligand class (L1) in our study area. Our present results suggested that the [LT] and log K were similar in Mersing coastal water for both August 2020 and April 2021. Despite the changes in in-situ parameters such as temperature, salinity, and DO caused by northeast monsoon phenomena, they had no significant impact on the distribution of dissolved Fe(III) speciation due to the presence of excess natural organic ligands in this coastal area. Hence, these strong organic ligands are important as they bind to the dissolved Fe(III) ions and create a Fe(III)-binding ligand complex that remains in the dissolved phase and is bioavailable to organisms such as phytoplankton.