Investigation of Sequential Chemical Extraction of Arsenic from Sediments: Variations in Sample Treatment and Extractant

Abstract

The sequential chemical extraction procedure (SCEP) is a very useful technique as an indirect measurement for arsenic (As) fractionation in soils and sediments. However, SCEP can produce variations in results due to sample conditions, types of samples, and extracting reagents used. Therefore, this study has evaluated the SCEP for determination of As in subsurface sediments using different sample treatments and extractants. When the same amounts of dry and wet sediment samples were treated with 6N HCl, the dry sediment samples released more As than wet sediments, reflecting that the drying process causes the increase of leachable As pool and dispersion of As from one phase to another (e.g., sulphide, silicate, and organic phase to oxide phase). This is presumed due to the moderation (i.e., change) of the oxide, organic, sulphide, silicate, and residual phases of As in sediments. Furthermore, the dry sediment (without HCl treatment) released more As than the wet samples (without HCl treatment) from the oxide phase; however, the dry samples released less As than the wet samples from the organic and silicate-sulphide phases. The results suggest that the SCEP conducted using dry and wet sediment samples doesn't reproduce the same results due to changes in redox conditions, organic matter re-mineralization, and net desorption reactions resulting in phase dispersion during drying of sediments. The sequential leaching of As using wet and dried sediments showed that 6N HCl can leach not only reactive phase of As from (1) oxide phase of Fe and Mn, but also from (2) organic phase and (3) silicate-sulphide phase. There was a significant amount of As in the silicate-sulphide phase.