Adsorption of Common Analgesics On Secondary Microplastics in Treated Wastewater

Introduction
Microplastics and pharmaceuticals are both important classes of contaminants of emerging concerns (CEC) in water and wastewater 1-3. The primary route of pharmaceuticals entry into municipal wastewater is through human excretion4,5. Analgesics such as acetaminophen (ACE), ibuprofen (IBU), diclofenac (DCF), are some of the most frequently detected and associated with high concentrations in municipal wastewater due to their common use 6,7. While conventional water resource reclamation facilities (WRRFs) are capable of removing the majority of these pharmaceuticals, high residuals could still be found in wastewater effluent1. Upon their release to the aquatic environment via treated wastewater, they can impose adverse effects on living organisms and aquatic species such as impaired ion regulation, cardiac and reproductive abnormalities, and mortalities 8,9.
Microplastics, commonly defined as plastics pieces between 1 µm to 5 mm in at least three dimensions 10,11, and are generally categorized as primary microplastics and secondary microplastics based on sources 2. Primary MPs are intentionally manufactured in small sizes for particular applications; secondary MPs result from the fragmentation of larger plastic particles. Due to microplastics' large surface area and hydrophobicity, microplastics may serve as vectors other contaminants such as heavy metal and pharmaceuticals 12,13. Concentrations of pharmaceuticals, which had been detected in parts per trillion in some effluent samples, could be adsorbed and enriched on microplastics' surfaces 13,14. Regardless of the fact that the majority of microplastics can be retained in sludge 15, abundant microplastics residues can be found in treated wastewater, and potentially transport remaining pharmaceuticals to reclaimed water systems or receiving water bodies. Therefore, it is critical to investigate the interaction between pharmaceutical and microplastics residuals in the final stages of WRRFs. The objective of this study is to understand the adsorption kinetics of frequently detected analgesics on secondary microplastics, simulating pharmaceutical concentrations in typical treated wastewater while using a commonly detected secondary microplastics in wastewater.
Materials and methods
Polypropylene (PP) was chosen in this study as it was found to be abundant in wastewater in previous studies 16-18. Commercial grade PP pellets (2-3 mm) (Poly-fit ® Polypellets, Fairfield Inc.) were grinded into secondary microplastics in Milli-Q water using a kitchen grinder to simulate the fragmentation of microplastics in sink garbage disposal, and were sieved with stainless steel meshes with openings of 900, 150, and 25 µm. Produced secondary PP were examined under a Scanning Electron Microscope (SEM) and Raman Spectroscopy for surface morphology and any chemical composition change.
For the purpose of adsorption test, ACE, IBU and DCF solutions were prepared with Milli-Q water (Table 1). Three sets experiments were designed to in order to understand pharmaceutical adsorption on microplastics of different sizes and the adsorption kinetics of each pharmaceutical as well as in a mixture (Table 1). The concentration of solutions was determined based on Adeleye et al. 1.
Following the adsorption, solids were then retrieved via vacuum filtration, and sonicated for 15 minutes for the sequential solid-phase extraction. To more accurately assess the adsorption, both remaining solution as well the resulting solution from solids sonication underwent solid-phase extraction which was modified from Guart et al.19. Samples were then analyzed by a gas chromatography mass spectrometer (GC/MS).
Preliminary results
The Raman Spectra were obtained for secondary PP of all three size ranges (Fig.1). The differences in intensity between the original PP pellets and secondary PP was caused by the difference in material thickness and the chemical nature of the secondary PP was unaltered. According to the SEM images of secondary PPs (Fig.2), the morphology and size were random, thus the production of secondary PPs can be replicated. Figure 3 shows the sorption of the three analgesics on small, medium and large PP particles. In general, the uptake of pharmaceuticals was larger on smaller microplastics which had higher surface area . The cracks and irregularities on the larger PP particles (Fig.2) could result in a high specific surface area 12 regardless of their larger particle size, thus this might be causing large variation in the adsorption of pharmaceuticals on large PPs. The results also revealed that ibuprofen exhibited a higher sorption affinity to PP compared to diclofenac and acetaminophen within 24 hours. The primary purpose of the study is to show that secondary microplastics can act as carriers for pharmaceutical residuals, potentially transporting them into drinking water system via water reclamation and into aquatic environment via discharge. The first stage of the study demonstrates the sorption affinity of the three analgesics and effects of microplastics' size. The study shall continue as listed in Table 1, and be expanded to other types of abundant secondary microplastics such as polyester fibers.
Acknowledgement
The research project is funded by Santa Margarita Water District and Brown & Caldwell.