Comparing methods for optimising microplastic extraction in sediment through density separation

Oceans constitute a significant sink of microplastics, mostly accumulating in sediment. Extracting MPs from sediment remains challenging, highlighting the need for a standardised methodology to allow comparison across studies. We compared the efficiency of commonly used solutions for density separation and two transferring methods. A CaCl₂-saturated solution with decantation was the most efficient, recovering 90 ± 1 % of MPs regardless of polymer, size or shape, and proved safer, cheaper, and faster than the second-best alternative, ZnCl₂ with decantation. High-density polymers were harder to recover, especially with low-density solutions (e.g. NaCl) or the overflow method. Although size had no effect in the case of CaCl₂ and decantation, larger MPs generally had better extraction when using other solutions or transferring methods. Shape affected PET fibre recovery, which was particularly low with NaCl and moderately lower with ZnCl₂ and CaCl₂. Adding H₂O₂ or NaCl + H₂O₂ enhanced fibre extraction. Our study indicates that salts yielding intermediate densities at saturation may outperform denser alternatives due to lower viscosity and less particle adhesion to container surfaces. We recommend CaCl₂ with decantation as a standard protocol to maximise MP recovery and ensure comparability. If using H₂O₂, it should be included in the first extraction run. This approach may also apply to similar matrices like soil.