An innovative microplastic extraction technique: The switchable calcium chloride density separation column tested for biodegradable polymers, polyethylene, and polyamide

Extracting microplastics from complex matrices poses challenges due to the potential impact of harsh chemical treatments on microplastic properties. For fate and hazard assessment reliable techniques are needed to not only quantify the particle number but also to assess the physicochemical properties of environmental microplastics with minimum changes induced by extraction. Here we present the method development for an innovative and non-destructive extraction protocol based on a switchable calcium chloride density separation column. In contrast to commonly reported extraction protocols, the presented technique is suitable for targeted microplastic property analysis (e.g., surface chemistry and texture) by keeping chemical treatments (such as oxidation and enzymatic digestion) to a minimum. By adjusting the temperature we can control the aggregate state of the highly concentrated salt solution, allowing to separate the microplastics from matrix by cutting of purified, solidified samples. Harsh chemical treatments are avoided, as well as obstruction of microplastic extraction by adsorption to matrix components when passing the tap at the bottom of traditional density separation funnels.The use of microplastics that were prelabeled with a fluorescence dye helped to solve difficulties observed during method development by visual inspection before measurement of extraction efficiency: We spiked a blank compost with low-density polyethylene (LDPE) and polyamide (PA). Additionally, UV aged LDPE was used to demonstrate applicability to more hydrophilic, more environmentally relevant microplastics. The obtained initial results show high recovery of both unaged and aged LDPE over 97 wt.-% and an efficient compost removal but a lower and less robust recovery (between 68 and 18 wt.-%) for PA particles that are more challenging to extract due to an unfortunate synergistic combination of smaller particle size and higher density. Method adaptation to other microplastic types may still be necessary. In short: • A low-cost and simple approach without oxidation to extract (pre-aged) microplastics from compost • Method development by visual observation using fluorescent labelled microplastics and method validation by spike-recovery tests