PARTITIONING OF TRACE METALS IN SUSPENDED SEDIMENTS FROM HUANGHE AND CHANGJIANG RIVERS IN EASTERN CHINA

Assessing metal contamination of sediments requires knowledge of the geochemical partitioning of trace metals at the sediment-water interface. Under controlled laboratory conditions, sequential extraction was conducted to determine the associations of metals (Cd, Cr, and Zn) and radiotracers (¹⁰⁹Cd, ⁵¹Cr, and ⁶⁵Zn with various geochemical phases and the different partitioning and mobility of metals for two types of surface sediments collected from the Huanghe and Changjiang Rivers in Eastern China. The residual phase was the major phase for stable metal binding, indicating that these sediments had little subjection to recent anthropogenic influences. Fe-Mn oxides were the next important binding phases for metals. The partitioning of metals in various geochemical phases as a function of the duration of the radiolabeling was also examined. Trace metals transferred among the different geochemical phases over the 30 days radiolabeling period, particularly between the carbonate and Fe-Mn oxides phases. The freshwater-sediment distribution coefficients (K_d ) of three metals were investigated in batch experiments using the radiotracer technique. The decreasing K_d  with increasing metal concentration (from 0.5 to 200 μg L^-1) may be explained by competitive adsorption. The metal K_d  in sediments from the Changjiang River was greater than those from the Huanghe River, presumably because of the higher Fe/Mn and organic carbon contents in Changjiang River sediment. The K_d  decreased with increasing total suspended solid load from 3 to 500 mg L^-1, and was Cr > Zn > Cd. For Cd and Zn, increasing the pH from 5 to 8 resulted in an increase in K_d  due to the reduced H⁺ competition and increasing sorption potential. However, the K_d  for Cr in the sediments from both rivers showed no relationship with pH, presumably because of the complexity of the Cr species and environmental behavior.