Three digestion methods to determine concentrations of Cu, Zn, Cd, Ni, Pb, Cr, Mn, and Fe in mangrove sediments from Sai Keng, Chek Keng, and Sha Tau Kok, Hong Kong.

Heavy metals in sediments have received increasing attention in recent years, partly because of the growing scientific and public awareness of environmental issues, and partly because of the improvement in the analytical techniques for accurately measuring their concentrations. Trace metal determinations in soils and sediments commonly involve some type of mineral acid extraction (wet oxidation) followed by atomic absorption analysis. Wet oxidation is normally carried out by digestion of the sample in a mixture of strong acids such as sulphuric, hydrochloric, hydrofluoric, nitric and perchloric acids (APHA 1995). However, sulphuric acid is not recommended for simultaneous extraction of different metals, especially in sediments rich in alkaline earth metals, because of the formation of insoluble sulphate salts (Berrow and Stein 1983). Recently, hydrofluoric and perchloric acids are not recommended to use especially for routine metal analysis in laboratories (De Groot et al. 1982; Berrow and Stein 1983; Abreu et al. 1996). Hydrofluoric acid requires special attention to handle as it is dangerous and highly corrosive, while perchloric acid always poses a danger of explosion especially when the perchlorate vapor is trapped in the wooden or plasticware of the fume exhausted system (Berrow and Stein 1983; Alloway 1995). Nitric acid has been used either on its own or in combination with hydrochloric acids and proved to be effective (Berrow and Stein 1983). Agemian and Chau (1976) reported that sediment samples digested with either aqua-regia, hot concentrated nitric acid or hot concentrated nitric-perchloric acid mixture gave comparable results. Therefore, the use of either concentrated nitric acid alone or nitric-hydrochloric acid mixture is a common digestion practice for metal determination in most accredited routine laboratories in Hong Kong including Government and private testing laboratories. Most of the digestion methods for heavy metal determination were developed for bottom sediments in marine environment or soils in terrestrial ecosystems. When these acid extraction methods are applied to coastal sediments, their completeness, accuracy, precision and reproducibility might be different from those of the marine sediments as their matrices were different and need further examinations.