Biofilm heterogeneity affects the mobility of nanoplastics during riverbank filtration

The presence of NPs in drinking water has raised wide concerns due to their potential impacts on human health. Riverbank filtration (RBF), a natural water treatment process that involves the passage of water through soil, is employed by some Dutch drinking water companies that source water from rivers. Given the emergence of NPs as pollutants, it is essential to understand their transport and removal behavior during riverbank filtration to ensure the safety and quality of drinking water.

The deposition of NPs in porous media is strongly influenced by the physicochemical properties of aquifer grain surfaces. Natural biofilms, consisting of complex communities of bacteria and other microorganisms, typically form on these surfaces and alter their properties. During RBF, biofilm-associated microbial activity leads to rapid oxygen consumption due to the degradation of organic matter, resulting in the formation of localized anoxic and anaerobic zones. However, the impact of this spatial heterogeneity of biofilms on the removal of NPs remains unclear. In this study, we aim to investigate how biofilm spatial heterogeneity influences NP deposition during RBF.

We constructed several long columns (90 cm in length), each composed of ten short columns (9 cm in length, 2 cm in diameter), packed with technical sand (particle size: 0.4–0.6 mm). River water sourced from an RBF site was pumped through the columns at a flow rate of 0.1 mL/min (0.054 m/h) to facilitate biofilm growth over periods of 1, 3, and 6 months. After biofilm formation, columns were segmented into ten short columns to assess NP transport behavior by analyzing breakthrough and retention curves at different biofilm depths. Europium-doped polystyrene NPs (30 mg/L) suspended in synthetic river water with a similar ionic composition to natural river water were used as tracers to evaluate NP transport and retention at an increased flow rate of 0.75 mL/min.

So far, we have obtained breakthrough curves for NP transport in columns with 1- and 3-month biofilms. Preliminary results indicate that site blocking contributed to the concentration- and time-dependent deposition of NPs. The inherent surface roughness of the technical sand created heterogeneous sites that contributed to multi-site NP deposition. Compared to the original sand grains, biofilms exhibited a less negative surface charge, facilitating stronger interaction with NPs. It suggest that biofilms created more favorable heterogeneous sites, enhancing both irreversible and reversible deposition of NPs. The maximum retention capacity of sand grains decreased with depth, with the shallow biofilm layers, which had the highest biomass, greatly enhancing NP retention. Additionally, biofilms grown for 3 months demonstrated a stronger capacity to retain NPs compared to those grown for 1 month.

The remaining experiments are expected to be completed by May next year. This study will enhance the understanding of NP deposition mechanisms during RBF, with a particular focus on the critical role of the spatial heterogeneity of natural biofilms. The findings will provide valuable insights into the potential removal efficiency of NPs in RBF systems, as well as the associated risks of NP exposure in downstream environments and drinking water sources.