Amelioration Of Heavy –metal- Contaminated Sediment By Composting And Its Reutilization As Resources

Nowadays, river pollution has been largely concerned by the world. Ecological dredging is a common and efficient method for river pollution control. However, the following safe treatment and utilization of dredging sediment have been hot issues in the field of environmental protection in recent years. In this paper, sediments taken from two river segments of Nanfeihe, named Phoenix Bridge and the Ma Jia Du, were analyzed in terms of physical and chemical characteristics, and the determination of heavy metals to study the sediment properties and pollution status. Then the sediments were ameliorated by composting, of which the best composting groups were utilized for the pot experiments to test the amelioration effect and the impact of plants on the fractions of heavy metals in different cultivation conditions. Conclusions could be drawn as follows:There were similar proportions of Cr and Cd in river segments of Phoenix Bridge and the Ma Jia Du. The content of Pb was higher in Majiadu than that in Phoenix Bridge, but with opposite situation of Cu. In addition, Majiadu showed bigger silt concentration and higher bulk density.Composting could improve physical and chemical characteristics of sediments, and enrich the nutrient elements that would offer more suitable cultivation environment for the growth of plants. Withered straw and dead leaves were fermented at a mass ration of 1 to 1, which were used as composting admixture mixing with sediments in accordance with a proportion of 2%. The pot experiments indicated that the yield of water spinach for Phoenix Bridge group was larger than the blank group, while Majiadu showed lower yield that the blank. In this way, the sediment from Majiadu segment had better value of resourceful utilization.Acid-extractable Pb, Cu, Cr and Cd in sediments from Phoenix Bridge were decreased by 53.1%, 33.5%, 53.1% and 57.8%, respectively, after composting. Reducible Pb, Cu, Cr were reduced by 22%, 51.9% and 50%, respectively, but reducible Cr were increased by 47.2%. Acid-extractable and reducible fractions of heavy metals in sediments from Majiadu were decreased as well, which would be in favor of the mobility and bioavailability of heavy metals.The result showed that pH of rhizosphere soil was decreased during the early growth stage of water spinach, which caused the rise of acid-extractable fraction. However, acid-extractable fraction still was found to be declined both in rhizosphere and non-rhizosphere soils because of the enrichment of roots on heavy metals. The content of reducible Pb and Cu in the rhizosphere soil had been increased obviously,and acid-extractable and reducible Cr were increased slightly, while oxidizable Cr was significantly reduced. Acid extractable and oxidizable Cd were decreased with the reducible and residual fractions content increased in Phoenix Bridge group, while only reducible Cd increased and other three fractions of Cd decreased in Majiadu group.Furthermore, all kinds of heavy metals were enriched because the roots of water spinach adsorbed heavy metals, part of which would transfer up to the leaves and stems but a few transformed to reducible metals. There was no positive correlation for heavy metals between roots and non-roots including stems and leaves. After 30 days planting, only the content of Cr in stems and leaves exceeded the national food safety standards, but all of other three heavy metals(Pb, Cr and Cd) exceeded the standard after 90 days planting. Phoenix Bridge Group Water spinach in Phoenix Bridge group enriched heavy metals faster than Majiadu group.During the process of growing plants, it can be found that the roots of plants were propitious to the adsorption of heavy metals on surface sediments. Some non-decomposed straws, celluloses and other substances in composting sediments would adsorb heavy metals in later decomposable period, so as to reduce the mobility of pollutants in sediments.