Effect of overlying water pH, dissolved oxygen, salinity and sediment disturbances on metal release and sequestration from metal contaminated marine sediments

Abstract

Experiments were undertaken to examine the key variables affecting metal release and sequestration processes in marine sediments with metal concentrations in sediments reaching up to 86, 240, 700, and 3000 mg kg(-1) (dry weight) for Cd, Cu, Pb and Zn, respectively. The metal release and sequestration rates were affected to a much greater extent by changes in overlying water pH (5.5-8.0) and sediment disturbance (by physical mixing) than by changes in dissolved oxygen concentration (3-8 mg l(-1)) or salinity (15-45 practical salinity units). The physical disturbance of sediments was also found to release metals more rapidly than biological disturbance (bioturbation). The rate of oxidative precipitation of released iron and manganese increased as pH decreased and appeared to greatly influence the sequestration rate of released lead and zinc. Released metals were sequestered less rapidly in waters with lower dissolved oxygen concentrations. Sediments bioturbated by the benthic bivalve Tellina deltoidalis caused metal release from the pore waters and higher concentrations of iron and manganese in overlying waters than non-bioturbated sediments. During 21-day sediment exposures, T. deltoidalis accumulated significantly higher tissue concentrations of cadmium, lead and zinc from the metal contaminated sediments compared to controls. This study suggests that despite the fact that lead and zinc were most likely bound as sulfide phases in deeper sediments, the metals maintain their bioavailability because of the continued cycling between pore waters and surface sediments due to physical mixing and bioturbation.

Effect of overlying water pH, dissolved oxygen and temperature on heavy metal release from river sediments under laboratory conditions

“…The exception was Cu, which exhibited a contrary trend of having a lower average dissolved concentration of 51.14 μg L -1 at low DO concentration and a higher dissolved concentration of 61.49 μg L -1 at high DO concentration. These results were consistent with several other studies (Atkinson et al 2007, Ho et al 2012. Atkinson et al 2007 explained the reason to be related with the slower oxidative precipitation rate and scavenging of the dissolved heavy metal ions by iron and manganese hydroxide formed from Fe and Mn cations simultaneously released from pore waters under low DO (Atkinson et al 2007, Santana-Casiano et al 2004.…”

“…These results were consistent with several other studies (Atkinson et al 2007, Ho et al 2012. Atkinson et al 2007 explained the reason to be related with the slower oxidative precipitation rate and scavenging of the dissolved heavy metal ions by iron and manganese hydroxide formed from Fe and Mn cations simultaneously released from pore waters under low DO (Atkinson et al 2007, Santana-Casiano et al 2004. At high DO, oxygen penetration was stronger, Fe and Mn oxidative precipitation was more rapid, so the generated hydroxide or oxide would adsorb more dissolved heavy metals, thus leading to reduced metals release.…”

“…However, these researches mainly focused on metal release from sediments under more or less extreme environments. For example, Atkinson (Atkinson et al 2007) studied the release of metals from marine sediments with very high metal concentrations (86, 240, 700, and 3000 mg kg -1 dry weight for Cd, Cu, Pb and Zn). Equeenuddin (Equeenuddin et al 2013) studied the metal behavior in sediment of an acid mine drainage stream, and the sediments had very low pH of 2.5 and very high electrical conductivity of 1816 μS/cm.…”

“…The fl ux and speciation of metals released from sediments are affected by many factors, such as pH, salinity, dissolved oxygen (DO), suspended solids, sediment particle size, etc. (Butler 2009, Atkinson et al 2007). …”

“…Moreover, the fate and toxicity of heavy metals are greatly dependent on their partitioning between the sediment particles and the pore waters, dissolved heavy metals in the waters are more bio available and toxic than particulate metals (Atkinson et al 2007). …”