Effect of Hydraulic Disturbance on Re-release of Heavy Metals in Estuarine Sediments

Sediment re-suspension caused by hydraulic disturbances may result in the release of heavy metals from the sediments, producing potential environmental risks. In order to investigate the effects of first and secondary suspension on releases of Cu, Cd, and Cr in estuarine sediments, a series of experiments were performed by simulated hydraulic disturbances. Based on the incipient motion of sediment transportation, three disturbance intensities with 100, 160, and 240 r·min^-1 were conducted. The results indicate that disturbance could cause the large-size particles to be broken into small-size particles, increasing the clay proportion in sediment. After suspension, the heavy metals in sediment would be released again into the overlying water. The peak values of Cu in the overlying water reached 36 μg·L^-1 under 240 r·min^-1. However, Cd release peaked at low disturbance (100 r·min^-1), which was higher than those under 160 and 240 r·min^-1. Cu in sediment was dominated by acid fraction (49.3%) and residue fraction (30.5%), while Cd was dominated by acid fraction (68.2%) with the shortest time to peak concentration in overlying water compared to other metals. For Cr, residual fraction was the dominant one (82.7%), which was relatively stable, so the release variations kept in a narrow range under three different disturbances intensities. After the first suspension, all heavy metals presented continuous release tendency. However, the peak concentration of Cu after secondary suspension was higher than that in first suspension, while contrary phenomena were observed for Cd and Cr, suggesting Cu release was sensitive to secondary suspension. The result of heavy metal fractions suggests that heavy metal release would be greatly influenced by their geochemical fractions. Overall, acid fraction presented high sensitivity to disturbances, and intensive disturbances enhanced the oxidation of the oxidation fraction heavy metals, increasing the potential risk of heavy metal release.