Micro- and nanoplastics retention in porous media exhibits different dependence on grain surface roughness and clay coating with particle size

Authors: LIANG, YAN - Guangxi University; LUO, YONGLU - Guangxi University; SHEN, CHONGYANG - China Agricultural University; Bradford, Scott

Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2022
Publication Date: 6/7/2022
Citation: Liang, Y., Luo, Y., Shen, C., Bradford, S.A. 2022. Micro- and nanoplastics retention in porous media exhibits different dependence on grain surface roughness and clay coating with particle size. Water Research. 221. Article 118717. https://doi.org/10.1016/j.watres.2022.118717.
DOI: https://doi.org/10.1016/j.watres.2022.118717

Interpretive Summary: Most studies examining the transport and retention of colloids in soils and aquifers have been conducted in the absence of clay coatings, but natural systems will always contain clay. This work examines the influence of different fractions of clay coatings on the migration of colloids. The clay coating was found to have a strong and complex influence of the retention of colloids that changed with the sand roughness, colloid size, and the solution composition. The presence of clay coatings enhanced or reduced the sand roughness and this altered the strength of chemical and hydrodynamic forces that controlled colloid retention. This information will be of interest to scientists, engineers, and government regulators that are concerned with assessing and managing the risk of colloidal contaminants (e.g., pathogens, nanoparticles, and plastics) on groundwater resources.

Technical Abstract: The presence and/or coating of natural colloids (e.g., clays and metal oxides or hydroxides) on collector surfaces has frequently been demonstrated to enhance the retention of engineered colloids that are negatively charged due to favorable electrostatic interactions. However, this work demonstrates that the presence of natural clay coating can lead to a reduced or a nonmonotonic retention of micro- and nanosized plastics (MNPs). Column experiments were carried out to systematically investigate the transport of MNPs with different sizes in relatively smooth and rough sands that had various clay coating fractions. These coating fractions on the collector were found to significantly influence MNP retention in a complex manner that changed with the colloid size and the roughness properties of the sand. This observation was attributed to the impact of clay coatings on the roughness and morphology properties of collector surfaces that were dominant over surface charge. Scanning electron microscopy and interaction energy calculations on surfaces with pillars or valleys indicate that mechanisms that contributed to MNP retention changed with the colloid size. Particularly, retention of nanosized plastics was mainly controlled by interactions on convex/concave locations that change with the solution chemistry, whereas microsized plastics were also strongly influenced by the applied hydrodynamic torque and straining processes. Additionally, the significant sensitivity of MNP retention under a low-level ionic strength also reflects the importance of physicochemical heterogeneities. These observations are important for investigating the mechanisms of colloid transport in natural systems that ubiquitously exhibit clay coating on their surfaces.