Binding Strength and Transport Kinetics of Organic\nDyes into Different Live Diatoms Using Second Harmonic Scattering\nSpectroscopy

Dye-contaminated\nwastewater poses serious environmental risks to\necosystems and human health. Diatoms, algae with nanoporous frustules\n(cell walls), offer promising potential for wastewater remediation\ndue to their high surface area and adsorption properties. While dead\ndiatom biomass is well-studied for biosorption, research on living\ndiatoms’ bioaccumulation and biotransformation potential is\nlimited, with gaps in kinetic and equilibrium modeling of dye adsorption.\nHere, we analyzed the adsorption of crystal violet (CV) dye onto living <i>Phaeodactylum tricornutum</i> (P-cell) and <i>Navicula\ncryptocephala var. veneta</i> (N-cell) diatoms by characterizing\nthe physiochemical properties of the species’ outer surfaces\nand monitoring the adsorption of CV using surface-specific second\nharmonic scattering (SHS) spectroscopy. Direct monitoring of dye adsorption,\nrather than its removal from the solution, enables a more accurate\ninvestigation of adsorption kinetics and thermodynamics, revealing\nstrong correlations with the cell surface structure and composition.\nWe found that the N-cell has a greater adsorption capacity for CV\nthan the P-cell, though with slightly less favorable adsorption free\nenergy. Ionic strength could impact uptake capacities, likely due\nto competition between metal cations and the dye cation as well as\nsurface screening. SHS experiments revealed a simple adsorption process\nfor N-cells, while P-cells exhibited a multistep process involving\nCV transport through thinner, nonporous cell walls to the plasmic\nmembrane, contributing to favorable adsorption free energy. The thicker,\nporous walls of N-cells provided more surface sites, increasing the\ncapacity, while P-cells facilitated deeper uptake. Ionic strength\nhad only a significant effect on adsorption capacity, not adsorption\nfree energy, reflecting the intricacies that govern adsorption and\nuptake by living organisms. The comprehensive analysis presented herein\ndemonstrates great potential for diatoms to be used as biosorbents\nin dye remediation and provides systematic relationships between the\nstructure and function of diatom cell walls, which will inform the\nuse of tailored species for more efficient remediation.