Anionic nanoparticle and microplastic non-exponential distributions from source scale with grain size in environmental granular media

• Published in: Water research (Oxford) 2020-09, Vol.182, p.116012-116012, Article 116012
• Main Authors: Johnson, William P., Rasmuson, Anna, Ron, Cesar, Erickson, Brock, VanNess, Kurt, Bolster, Diogo, Peters, Brett
• Format: Article
• Language: English
• Subjects: Colloids; Groundwater; Microplastics; Nanoparticles; Retention; Transport
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2020.116012

Nanoparticle and microplastic (colloid) transport behaviors impact strategies for groundwater protection and remediation. Complex colloid transport behaviors of anionic nano- and micro-sized colloids have been previously elucidated via independent experiments in chemically-cleaned and amended granular media with grain sizes in the range of fine to coarse sand (e.g., 200–1000 μm). Such experiments show that under conditions where a repulsive barrier was present in colloid-collector interactions (unfavorable conditions), the distribution of retained colloids down-gradient from their source deviates from the exponential decrease expected from compounded loss across a series of collectors (grains). Previous experiments have not examined the impact of colloid size or granular media grain size on colloid distribution down-gradient from their source, particularly in streambed-equilibrated granular media. To address this gap, a field transport experiment in constructed wetland stream beds to distances up to 20 m were conducted for colloids ranging in size from micro to nano (60 nm–7 μm) in streambed-equilibrated pea gravel and sand (4200 and 420 μm mean grain sizes, respectively). All colloid sizes showed non-exponential (hyper-exponential) distributions from source, over meter scales in pea gravel versus cm scales reported for fine sand. Colloids in the ca. 1 μm size range were most mobile, as expected from mass transfer to surfaces and interaction with nanoscale heterogeneity. The distance over which non-exponential colloid distribution occurred increased with media grain size, which carries implications for the potential mechanism driving non-exponential colloid distribution from source, and for strategies to predict transport. [Display omitted] •Distributions from source were hyper-exponential regardless of colloid size (n-μ).•Sizes in the n-μ transition range (0.2–2.0 μm diameter) were most mobile.•Distances across which non-exponential distribution occurred scaled with grain size.•Transport reflected complexities observed in chemically-cleaned unfavorable media.