Separation and quantification of microplastics from beach and sediment samples using the Bauta microplastic-sediment separator

Microplastic pollution is a topic of scientific interest globally. The environmental impacts caused by microplastics have spurred research focusing on separation and quantification of microplastics from soil and sediments in both fresh and marine waters. Different approaches are used for studies and continuous improvements are made, thus developing a reliable, standardized separation and quantification method has been a challenge. The Bauta microplastic-sediment separator, based on the concept of Munich plasticsediment separator, was constructed at NGI, Oslo. An optimized separation protocol was developed using different dense solutions, and a range of laboratory microplastics (lab MP): LDPE pellets, PE fibers, HDPE pellets and PET powder. Optimized method includes the use of Zinc chloride and Calcium chloride solution (d ~1.6 g/mL) as separation solution and a steel mesh (45µm) as filter. Laboratory sand (0.2-0.7 mm, d =2.6 g/mL) and beach sand (> 200 µm, d > 1.6 g/mL) were spiked with the lab MP to obtain recovery rates. Organic matter separated with the MP were dissolved following a digestion protocol which uses sodium hydroxide: urea: thiourea solution for dissolution, followed by 30% hydrogen peroxide and 10M sodium hydroxide for oxidization of organic matter. Microplastics (< 8mm and > 45m) from environmental samples: beach plastic debris from Bygdøy sjøbad and effluent sediment from Bekkelaget WWTP were also separated. Spiking resulted in lower recovery rates of PE fibers from lab sand and beach sand (77 ± 0.05 (s.d) % and 82 ± 0.10% respectively). The beach sand from Bygdøy sjøbad had a significant difference (p < 0.05) for the concentration of MP and hard-to-digest organic matter; the difference was found between the high-water mark (HWM) region and HWM -6m region, also between HWM and HWM +6m region. Bekkelaget WWTP samples had an average MP concentration of 27.92 ± 37.37 mg/kg. This novel technique is a reliable approach to separate microplastics from soil and sediments. Microplastics were present in Bekkelaget WWTP sediment and Bygdøy sjøbad debris samples. The concentration of MP was highest at the high-water zone. Advanced quantification of the separated microplastics based on their polymers should be performed using available identification techniques. This study will aid as an information tool for further optimization and development of more accurate separation, filtration, quantification and identification methods for microplastics from soil and sediments.