Recovery of Heavy Metals from Acid Digestion of an Estuarine Sediment and Concentration Profiles of Sediment Cores from Hebe Haven and NGAU MEI HOI, Hong Kong

Abstract

The recovery efficiencies of Cd, Cr, Cu, Fe, Ni, Pb and Zn during acid digestion by different acid mixtures and by block heater and microwave heating were determined for an estuarine sediment sample. For block heater heating, the aqua regia mixture was found the most powerful digestion agent, giving a high yield of 112% for Fe and a poorer yield of 83% for Pb. Compared to block heater heating, HNO₃ digestion by a domestic microwave oven gave a lower yield for Cr, Cu, Fe and Ni. The digestion time of 3–6 min was possibly not sufficient to cause a complete release of the heavy metals. The recoveries for a heating time of 3.8 min and under ‘High Heat’ were determined as follows: Cd, 99%; Cr, 55%; Cu, 95%; Fe, 87%; Ni, 76%; Pb, 75%; Zn, 88%. Microwave digestion using HNO₃ was used to determine the concentration of the above metals in two sediment cores from Hebe Haven and Ngau Mei Hoi, Hong Kong. Hebe Haven, which is close to a suburban area with some dyeing factories, has its surficial sediment enriched in Cd, Cr, Cu, Pb and Zn, with mean surficial concentrations of 0.259, 65.0, 30.3, 52.4 and 115 mg kg⁻¹, respectively. The corresponding enrichment factors (EFs) were calculated as 3.6, 2.0, 3.0, 1.4 and 1.6. Ngau Mei Hoi is further away from the urbanized coast and only Cd, Cu and Pb are slightly enriched in the surficial sediment, with EFs of 1.1, 1.3 and 1.4, respectively.