Influence of aging and the presence of dissolved organic matter on caffeine adsorption onto microplastics in aquatic environments

Polyethylene (PE) microplastics and caffeine (CFN) are ubiquitous in water and wastewater systems; however, PE microplastic’s ability to adsorb CFN in organic matter-rich waters and their interaction mechanisms are poorly understood. This study assessed the performance of aged PE microplastics for CFN adsorption in humic acid (HA)-mixed water compared to that of pristine PE microplastics. The effects of solution pH (pH 2-9), contact time (up to 72 h), and initial CFN concentration (1-30 mg/L) on the sorption performances of PE microplastics at different HA concentrations (0.5-2.5 mg/L) were assessed. A PE film was aged in sunshine for three months to obtain aged microplastics. Aged PE microplastics demonstrated the highest CFN adsorption (3.62 mg/g) compared to pristine PE microplastics (2.37 mg/g) at pH 6.0-7.0. The water mixed with the highest concentration of HA (2.5 mg/L) was found to be the most effective medium for sorption, suggesting that CFN adsorption onto PE microplastics is likely favorable in water enriched with HA. The cooperation of nucleophilic attractions, intermolecular hydrogen bonds, and van der Waals and π-π interactions would be prominent in CFN adsorption onto HA-bound PE microplastics. The adsorption results collectively suggested a multifaceted sorption mechanism for CFN adsorption onto PE microplastics through either chemisorption or physisorption. The poor desorption ability on slight changes in the pH further evidenced the existence of chemisorption of CFN and the minimal chance of bioavailability in bodily fluids after exposure. The results of this study indicate that the sorption dynamics within the CFN-microplastic-HA system could potentially impact the transport of CFN in aquatic environments. Further, aging of PE microplastics and HA concentration govern the vector potential of microplastics, which would have the most negative end-to-end impact on human health.