Binding Strength and Transport Kinetics of Organic Dyes into Different Live Diatoms Using Second Harmonic Scattering Spectroscopy

Dye-contaminated wastewater poses serious environmental risks to ecosystems and human health. Diatoms, algae with nanoporous frustules (cell walls), offer promising potential for wastewater remediation due to their high surface area and adsorption properties. While dead diatom biomass is well-studied for biosorption, research on living diatoms' bioaccumulation and biotransformation potential is limited, with gaps in kinetic and equilibrium modeling of dye adsorption. Here, we analyzed the adsorption of crystal violet (CV) dye onto living Phaeodactylum tricornutum (P-cell) and Navicula cryptocephala var. veneta (N-cell) diatoms by characterizing the physiochemical properties of the species' outer surfaces and monitoring the adsorption of CV using surface-specific second harmonic scattering (SHS) spectroscopy. Direct monitoring of dye adsorption, rather than its removal from the solution, enables a more accurate investigation of adsorption kinetics and thermodynamics, revealing strong correlations with the cell surface structure and composition. We found that the N-cell has a greater adsorption capacity for CV than the P-cell, though with slightly less favorable adsorption free energy. Ionic strength could impact uptake capacities, likely due to competition between metal cations and the dye cation as well as surface screening. SHS experiments revealed a simple adsorption process for N-cells, while P-cells exhibited a multistep process involving CV transport through thinner, nonporous cell walls to the plasmic membrane, contributing to favorable adsorption free energy. The thicker, porous walls of N-cells provided more surface sites, increasing the capacity, while P-cells facilitated deeper uptake. Ionic strength had only a significant effect on adsorption capacity, not adsorption free energy, reflecting the intricacies that govern adsorption and uptake by living organisms. The comprehensive analysis presented herein demonstrates great potential for diatoms to be used as biosorbents in dye remediation and provides systematic relationships between the structure and function of diatom cell walls, which will inform the use of tailored species for more efficient remediation.