Keywords: 
• grainsize measurement /
• peaty sediments /
• pretreatment method /
• measuring time

Abstract: One or two very low subpeaks appear in the coarse end of the grain size differential curves of the peaty sediments from the Yangtze Estuary with the conventional pretreatment method, which is difficult to interpret. The experimental results indicate that different pretreatment procedures and measuring time has obvious influence on the grainsize distribution. In order to obtain accurate measurement, three different methods are used in the present study. In Method 1, sample is immersed in a beaker successively by the distilled water and boiled with 10 ml 10% hydrogen peroxide (H2O2). The beaker is filled up with distilled water and supernate is then removed after settled for 24 hours. Repeat the settlement and removal to eliminate the surplus H2O2. The remained sample is then dispersed by (NaPO3)6 and ready for measurement. In Method 2, which is also called conventional method, samples are immersed by 10ml 10% hydrochloric acid (HCl) in addition to the H2O2. Other steps are the same as in Method 1. In Method 3, the difference from Method 2 is that concentrations of both H2O2 and HCl are increased from 10% to 30%, and other steps are the same as in Method 1. The results show that the Method 1 is preferred and grainsize differential distribution measured in the third minute since they are put in the lasergrain equipment should be selected. Compared with both in the first and the second minute, the grain size measured in the third minute is ideal, which possibly indicate that the flocculated particles need three minutes to disperse. The coarse composition demonstrated in the differential curves might be resulted from the insufficient pretreatment. Our experimental result of mineral grainsize measurement of peaty sediments suggest that clayey silt and silty clay, with mean grain size ranging between 9 μm and 25 μm, prevail and reflect the relative stable environment of still water.