Status quo of operation procedures for soil sampling to analyze microplastics

Abstract

Since 2015, the determination of microplastics (MPs; < 5 mm) in soil has gained increasing attention. However, usual analytical protocols still render a comparison of results challenging. This structured review integrates an in-depth screening of scientific articles ( n = 106; from 1980 to 2022) that focused on determining MPs in soils. The different studies were divided into groups regarding (i) MPs input pathways, (ii) study site information, (iii) sampling design, (iv) sample preparation, and (v) analytical methods for MPs detection. Most of the studies (63%) focused on a defined study site influenced by a known point source, whereas 37% measured MPs background contents related to delocalized MPs emission sources. Even though soil was the target compartment, only 26% classified the soil type, mostly using the World Reference Base (WRB) as a classification system. Additional information on soil properties was supplied within only 20% of the studies. However, this information is mandatory for evaluation of MPs recovery tests and analytical results. In nearly all studies, the mass of the single samples equaled the final mass of the composite sample, with a mean of 1.32 kg ± 1.07 kg. However, other procedures that involve a larger sample mass seem promising but are still seldom applied. Our structured review revealed that a standard operation procedure with harmonized methods is urgently needed with a coherent and comprehensive workflow, including field sampling and sample preparation. Such a procedure would ensure the reproducibility and representativeness of analytical results, which are mandatory for evaluating and restricting MPs pollution in soils in the future. Graphical Abstract