No abstract

The role of calcium (Ca) in the cycle of phosphorus in water environment depends on the speciation of Ca in sediments, and reliable Ca sequential extraction method is useful for obtaining information about Ca transport processes from sediment to water. Experiments have been carried out to develop a simple but robust Ca sequential extraction method for identification of Ca portioning in sediments. The sequential extraction protocol was modified for extractants, extracting time, concentration and pH of the reagents in the first three steps of Tessier sequential procedure. With optimum conditions, there were 1:25 sediment to solution ratio and 3 h of extracting time shaken with NH 4 AC (1 mol L -1 , pH 7.0) (exchangeable Ca fraction); 1:25 sediment to solution ratio and extracted with NH 4 AC (1 mol L -1 , pH 5.0) for 5 h (carbonatebound Ca fraction); and 1:30 sediment to solution ratio and extracted with NH 2 OHÁHCl (0.5 mol L -1 , pH 1.5) for 6 h (Fe and Mn oxides bound Ca fraction). The recovery for Tessier procedure ranged form 35.6 to 71.1 %, while the recovery for the modified Tessier procedure ranged form 46.1 to 86.9 %. These results revealed that Tessier procedure underestimated the proportion of Ca in exchangeable fraction. In sediments, Ca was mainly exchangeable and carbonate bound, which was considered as the most liable fractions.