Colloids are ubiquitous within the aquatic environment. Examples include both natural and anthropogenic substances such as nanomaterials, mineral oxides, particulate natural organic matter, and microplastics. They can dictate fate and transport of emerging contaminants via molecular adsorption and surface-mediated photochemistry. More importantly, these colloidal substances are heterogeneous in size. Thus, our objective has been to elucidate the size effect, in the range of sub-100 nm to few microns, that a polystyrene-based (PS) particle has on the adsorption of organic molecules from an aqueous solution. Using an interfacial selective tool, second harmonic generation (SHG) spectroscopy, we have carried out <i>in-situ</i> measurements of adsorption kinetics and isotherms of a SHG probe molecule, a styryl dye, as a function of particle diameter. Furthermore, dynamic light scattering has been employed to investigate the effect of molecular adsorption on the surface charge and aggregation properties of the particles. Our results reveal that while there is no aggregation upon adsorption of the dye, the binding kinetic profile does depend on the particle size. As the particle diameter increases, multi-step kinetics become prominent. This unique feature is also a function of the initial concentration of the dye. A reduction in zeta-potential upon binding has also been observed. Using this <i>in-situ</i> methodology, we have further explored competitive binding of pharmaceuticals, personal care products (PPCPs), and endocrine disrupting compounds (EDCs) onto PS particles. The findings suggest that size-dependent surface morphology play a role on the adsorption kinetics. These fundamental results and their implication on the fate and transport of PPCPs and EDCs within the aquatic environment will be discussed.

Effect of particle size on the affinity and adsorption kinetics of organic molecules from aqueous solution: An interfacial selective study

ACS Fall 2021

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