Raman microspectroscopic identification of microplastic particles in freshwater bivalves (Unio pictorum) exposed to sewage treatment plant effluents under different exposure scenarios

We investigated the uptake of microplastic (MP, <5 mm) particles by using freshwater bivalves (Unio pictorum) as biological samplers in the environment. They were exposed either directly to the biologically purified sewage of a North Bavarian sewage treatment plant (STP) or placed in a small river up- and downstream of the wastewater discharge for 28 days and 6 months, respectively. A control group was maintained in a pond. After acid digestion, the soft tissue was analyzed for MP particles by means of Raman microspectroscopy (RM, over 3000 particles individually measured), which allows for identification and quantification of particles down to 1 μm. Only in the bivalve collective exposed to STP effluents MP was found, however a very small amount (maximum of nine MP particles in the bivalve sample exposed for 6 months). In the bivalves up- and downstream of the wastewater discharge and in control organisms from a pond, no microplastic was identified. The amount of microplastic particles was small in absolute terms and small in relative terms (ca. 1:100 (6 months) and below 1:1000 (28 days)) as hundreds of particles per sample were analyzed which turned out to be non-plastic. Including the results for the river, this indicates a rather low MP contamination level for organisms in close vicinity to a sewage treatment plant.