Lignin nanoparticles and nanocelluloses for the removal of pharmaceutical residues from water

Active pharmaceutical ingredients (APIs), either as synthetic or natural chemicals are used in various types of medical and veterinary drugs. APIs are emerging pollutants that have caused global concern because they have been widely found in various types of water systems. Even though APIs have typically passed stringent tests for their safety at ‘normal’ dose thresholds, their accumulation in aquatic systems raises concerns about their potential to be hazardous to aquatic life and humans.

The main route of entry of APIs in to aquatic environments is through municipal wastewater as current wastewater treatment methods are not designed or optimized to remove them. Removal of APIs prior to their discharge in to rivers or other water bodies is therefore needed in order to mitigate their accumulation.

With the ultimate goal of developing bio-based adsorbents targeting pharmaceutical pollutants in water, this study screened the adsorption potential of several lignin- and cellulose-based nanomaterials upon various types of APIs. These materials include lignin nanoparticles (LNPs), cationic LNPs, cellulose nanofibrils (CNF), and tempo-oxidized CNF (tempo-CNF). Adsorption experiments involved mixing a known mass of the nanomaterial to a specific volume of artificial wastewater prepared from standard APIs. A portion of the mixture was withdrawn after one hour and centrifuged. A supernatant was collected and analyzed for the concentration of the remaining APIs using ultra-high performance liquid chromatography.

Adsorption results suggested that electrostatic attraction primarily drove the removal of APIs towards charged nanomaterials. Cationic LNPs showed the highest removal for diclofenac and ibuprofen, both of which have deprotonated carboxyl groups at the unadjusted pH (5-6) of the API solutions. Tempo-CNF showed strong adsorption for tramadol and metoprolol, which have both hydroxyl and amine groups and pKa>9. These groups are possibly protonated at the unadjusted pH of the API solutions enabling their attraction to the negative carboxylate groups of the tempo-CNF. The weak adsorption of cationic LNPs towards positively charged APIs, and of unmodified LNPs towards negatively charged APIs, suggested a different mode of action, possibly by pi-pi interactions of aromatic rings. Further adsorption tests revealed the effect of variation in initial pH, initial concentration of the APIs, and contact time towards the adsorption capacity of selected nanomaterials.