EFFECTS OF DRUG ADSORPTION ON THE SETTLING OF VERY FINE PARTICLES IN AQUEOUS SUSPENSIONS

Pharmaceutical & personal care products (PPCPs) such as antibiotics and antivirals have been identified in wastewater effluents in a multitude of studies, and their impact on the environment is of growing concern. In light of the recent SARS-CoV-2 pandemic, understanding the fate and transport (FT) of antiviral drugs, in particular, has gained attention. Drugs in wastewater, groundwater, and other natural aqueous systems often adsorb to fine suspended particles (FSPs). Understanding the FT of FSPs, which act as substrates for dissolved PPCPs, is fundamental to predicting the spread of drug contaminants. In this study, interactions between two PPCPs and a substrate of FSPs were investigated by a combination of batch mixing and timed settling experiments. The question investigated was if PPCP adsorption to sub-micron size particles ( ~ 1000-10 nm) could affect the rates of settling and subsequent size distribution of FSPs over time. NIST-SRM 1978 ZrO₂ was used as a substrate because its particle size distribution is a reference standard. Experimental sample solutions consisting of a 1:1 volume ratio of SRM 1978 and 1, 5, 10, and 30 ppm ofloxacin (antibiotic) or amantadine (antiviral) solution were mixed for 24 hours in the dark at room temperature. After mixing, attendant solutions were analyzed with LC/MS/MS techniques to determine the amount of drug adsorbed. Duplicate samples of substrate-drug solutions were homogenized by sonication, followed by measurement of the particle size distribution by dynamic light scattering (DLS) techniques every 15 minutes over a 13 hour settling time. Preliminary results indicate a difference in grain size distribution (d₅₀ and d₁₀) between the drug-attached particles and drug-free particles at some settling times. At times 15 and 32 minutes, the d₅₀ and d₁₀, respectively, of the drug-free sample and the amantadine drug-attached sample match the certified value within error. On the other hand, the ofloxacin drug-attached sample is significantly larger than the SRM 1978 at comparable settling times. However, it is uncertain whether differences are significant over the duration of the settling experiment. Ongoing investigations include the quantification of the amount of drug attached and experiments on various PPCP-nanoparticle substrate combinations.