2.5. pH measurements.
To measure pH, 0.5-1 mL samples were taken by a pipette from the water column 10-15 cm above the sediment, from the water-sediment interface (including 1-2 mm of water above the sediment and the uppermost sediment layer), and from the middle of the sediment. For the montmorillonite variant, samples were taken 5 cm under the water surface, 1 cm above the red spot (see below), in the middle of the red spot, 1 cm under the red spot, and 1 cm above the bottom of the tube.
All pH measurements were performed within 2 days after opening the tubes; during this time the samples were stored at +4°C. Liquid samples were tested according to the standard protocol using a combination glass pH electrode, calibrated in a standard buffered solution. Portions of the sediment were stirred using a vortex type stirrer without preliminary filtration until a uniform suspension was formed, and then the pH was measured with the same pH-meter 3-5 times for each sample (the results represent an average of these measurements).