Optimising H₂O₂ digestion and quantifying microplastics in sediment and pacific oyster (Crassostrea gigas) samples

Microplastic pollution continues to threaten marine environments across the world, yet there is inadequate understanding regarding the sources, distribution and impacts of these particles. Marine microplastic pollution is commonly investigated with the use of biomonitors, such as bivalves. However, published methods on chemical tissue digestion lack clarification regarding reagent volumes for small tissue samples <5 g. Therefore, this study aimed to improve H₂O₂ digestion methods and quantify and categorise the microplastics found within sediment and Crassostrea gigas samples collected from Weston Shore, Southampton. Tissue samples of 1 g were digested in varying quantities of 30 % H₂O₂. 20 ml of 30 % H₂O₂ per gram of tissue was sufficient in digesting samples of 2 g or more; 1 g samples require further experimentation. Sediment samples were visually inspected under a light microscope, along with the oyster samples once the microplastics had been extracted using H₂O₂ digestion, followed by density separation using NaCl. For tissue samples ≤5 g, 20 ml H₂O₂ per g of tissue should be used for digestion. For tissue samples >6 g, 6× mass of the sample should be used for digestion. Sediment microplastic concentrations were found to decrease moving south east along the shore, with varying significance, whereas C. gigas microplastic loads did not show any significant spatial differentiation (p = 0.3). Both sediment and C. gigas samples were dominated by fibres (96 % and 97 %, respectively), which is consistent with similar studies worldwide. The new digestion method gives 50 % cost reduction and lessened environmental impacts.