Montmorillonite and phosphorus enrichment in sediments as the causative factor for mud bank formation along the southwest coast of India

Abstract

Mud banks of Kerala (southwest coast of India) are renowned for immense fish harvesting during the southwest monsoon season. Although inclement weather and turbulent waves during monsoon prevent vessels sailing into the sea, the tranquil mud banks provide a natural platform for artisanal fishing. Despite several attempts over the past 6 decades, the formation and sustenance of these mud banks remain scientifically unexplained. In this study, we analyzed the geochemical properties of sediments and fluid mud collected from Alappuzha mud banks. The sediments were mostly silty clay (relative abundance of kaolinite [35%–67%] and montmorillonite [25%–45%]), whereas the fluid mud was purely fine clay (montmorillonite [47%–67%] and kaolinite [29%–45%]). Phosphorus content was exceptionally high in the fluid mud (2240–2634 μg/g) and sediments (1559–3013 μg/g). Laboratory experiments showed an increase in volume and a decrease in the settling rate of sediments spiked with varying proportions of montmorillonite and phosphorus. Natural conditions such as winds, waves, currents, or terrestrial factors dislodged the sediments into the water column. Natural sedimentation of clay particles results in the rapid flocculation of large particles (kaolinite and illite), whereas light particles (montmorillonite) remain suspended in the water column for a longer time. During this period, phosphate anions (or any oxyanions) geochemically interact with montmorillonite as well as with its interlayer cations in the following ways: (i) the ligand exchange reaction between the edge hydroxyl groups of mineral with phosphate anions increase the size and negative charge of the clay particles and (ii) phosphate anions effectively adsorb to the interlayer cations. The resultant strong interparticle repulsive force stabilizes the clay suspension, and thereafter, (iii) the intercalation of organic moieties stably converts the hydrophilic clay suspension into a hydrophobic suspension. Results of bulk geochemical parameters along the southwest coast of India show that montmorillonite and phosphorus-rich sediments are the unique properties of Alappuzha mud banks, which lead to their recurrent formation.