Improved Method of Background Value Determination for Sb and Cd in Freshwater Sediment-Insights from Controlling Factors on Spatial Variability.

Variability in the distribution of natural total Sb and Cd in freshwater sediments leads to difficulties in background value (BV) determination. This study aimed to establish a method to determine BV more accurately by investigating the vertical distribution of Sb and Cd in sediment cores collected from a typical river in alluvial plain in China and revealed the factors that control the variation of Sb and Cd BV, which has not been studied in alluvial freshwater sediment. The results suggested that uncontaminated samples for BV calculation should be determined by statistical analysis as human and natural disturbance led to high variation in contamination depth, from <5 cm to >55 cm. The sequential chemical extraction method showed a considerable amount of non-residual fractions of Sb and Cd, which accounted for 48% and 43% of the total, respectively. Acid extractable Cd (16%) was associated to the limestone geology in the area. Fine particles which governed by sedimentary environment contained more natural Sb and Cd, as strong positive correlation was found between clay content and Sb concentration (r = 0.89, p < 0.01), as well as Cd concentration (r = 0.54, p < 0.01). Based on these findings, a method combined with standard deviation and geochemical method was established to calculate the BV of Sb and Cd, and counter maps were made to cover the variation of BV in the Taipu river sediment. The pollution level has been evaluated by the geoaccumulation index more accurately.

Effect of Humus on the Solidification and Stabilization of Heavy Metal Contaminated River Sediment.

To better reutilize heavy metal contaminated river sediment containing organic matter, the sediments in a river located in Chongming District, Shanghai were collected and Portland cement was used as a curing agent along with commercial organic matter to conduct the solidification/stabilization experiment. The unconfined compressive strength and heavy metal leaching concentrations of solidified blocks with different water content, organic matter content, and cement content were tested and analyzed to determine the optimal ratio. The effects of fulvic acid (FA), humic acid (HA), and an HA/FA ratio on the solidification and stabilization, as well as the speciation of heavy metals in sediment before and after solidification and stabilization, were studied. The results showed that when the organic content of the sediment is 6.16%, the water content is 65% and the cement content is greater than 38%, so the curing effect proves to be satisfactory. Fulvic acid has a stronger inhibiting effect on cement hydration than humic acid, and its consumption in the curing process is more significant. The addition of humic acid contributes to the stabilization of heavy metals, while the increase in fulvic acid greatly weakens the stability of heavy metals. The exchangeable state of heavy metals in the sediment has been reduced to varying degrees after solidification and stabilization. The research results can provide a basis for the reclamation and utilization of heavy metal contaminated river sediment with organic matter.