The contents of the tubes were centrifuged at 10,050 × g for 10 min.
An aliquot of 100 ml of the supernatant was diluted to 1 ml with Milli-Q water.
The sodium concentration was measured using graphite furnace atomic absorption spectroscopy (VarianSpectrAA 880 spectrophotometer; Varian, Palo Alto, California, USA).