Adherence of Polystyrene Microspheres on Cave Sediment: Implications for Organic Contaminants and Microplastics in Karst Systems

Interactions of karst aquifer sediments with organic contaminants or microplastics (MPs) have received relatively little attention even though the susceptibility of karst aquifers to contamination and their ability to store and transport sediment is well documented. Studies using polystyrene microspheres as surrogate tracers for bacteria transport in karst systems have generally observed low recovery of microspheres and attributed this to microsphere adsorption onto aquifer sediments. In addition to being used as surrogate tracer for bacteria, microspheres have the potential to be used as surrogate material for organic contaminants and MPs. Using cave sediments as a proxy for karst aquifer sediments, the adherence of two types of microspheres (carboxylated and nonfunctionalized) was measured in three different types of solutions: deionized water, a calcium carbonate solution, and a karst spring water. Both types of microspheres adhered to the sediments; the most influential factor in adherence was solution type not microsphere type. Average adherence ranged from 51%-94% with average adsorption coefficients (KD) ranging from 11.8-442. Average estimated organic-carbon water partition coefficients (KOC) and retardation factors (RF) ranged from 1.64×103 - 6.13×104 and 6.20×101 - 2.29 × 103, respectively. KD, KOC, and RF were an order of magnitude higher in the karst water than in DI or CaCO3 solution. The results illustrate the importance of sediment interactions with potential organic or MP contaminants in karst systems.