Enhancement in Formation of Insoluble Arsenic Phases without Re-Leaching by Particle Size of Excavated Sediment with Iron Sulfate Heptahydrate

Massive quantities of naturally arsenic-containing sediments are excavated for construction. Chemical immobilization is applied to such excavated materials for reuse. Appropriate physicochemical properties should be understood to enhance the formation of the insoluble arsenic phases without re-leaching. The up-flow column retention test flowing 10 mg L−1 of arsenic solution was conducted to understand how the retention of arsenic and the phases of arsenic immobilized by iron sulfate heptahydrate depend on the particle size of the excavated sediment. The amount of arsenic retained on the immobilization material (particle size of 0.5 mm) during the retention test was lower for the larger particle size of the excavated sediment. However, arsenic was more stably retained on the immobilization material for the large particle sizes (1.0–2.0 and 2.0–5.0 mm) and the amount of arsenic re-leached was 12% of that in the small one (<0.5 mm). The percentages of arsenic in fraction 3 of sequential extraction increased from 45% to 68% with increasing the particle size. These results indicate that the excavated sediment with a larger particle size than that of immobilization material retains less arsenic on the immobilization material, but the arsenic phases immobilized are more stable, resulting in less arsenic re-leaching. This study suggests that the use of excavated sediment with a particle size larger than that of the immobilization material is desirable for the application of immobilization technique without re-leaching.