Effects of centrifugation speed on measurements of thallium in sediment pore water

Pore water recovery increases directly with the speed of centrifugation. After centrifugation at 1,000 rpm, a range of 36 to 17% of total pore water was recovered from the surface and at 20 cm sediment depth. After centrifugation at 20,000 rpm, about 30% of pore water remained unrecovered due to sediment compaction and pore size reduction. The results of this study indicate that sediment pore water can be divided in three fractions, free-, trapped-, and bound-pore water. The concentration of thallium (Tl) is significantly different in each pore water fraction and increases with increasing centrifugation speed. Centrifugation of the sediments at low speed (<4,000 rpm) and in situ dialysis yield similar concentrations of Tl in sediment pore water, corresponding to the free-fraction. Pore water data obtained by different centrifugation speeds may be significantly different with serious environmental consequences when studying fluxes and transport of trace elements.