Factors influencing the potential mobility and bioavailability of metals in dried lake sediments

Abstract

Samples taken from polluted creek sediments were dried at temperatures between 20°C and 100°C (either in air or under a nitrogen atmosphere) and selective chemical extraction procedures were then used to examine the effect of drying temperature and oxidation on the bonding mode distribution patterns of Zn, Pb, Cd and Cu. Exposure to air during the drying stage tended to increase the fraction of total metal which was less firmly bound (i.e. ion exchangeable, weakly sorbed). Less metal was present in these categories when higher drying temperatures were used, and temperature changes had a larger effect when the samples were dried under nitrogen. The drying process appears to promote metal migration to phases having a stronger bonding power. In most of the samples studied, heating caused Pb to migrate to the carbonate phase while accumulation in the organic/sulfide phases was the dominant process for Cu and Cd. In the case of Zn, migration to the carbonate phase was favoured in some studies (e.g. using air-dried samples), in other sediments the migration end-point was the organic/sulfide phases. During storage of dried samples at room temperature for prolonged periods (e.g. months) the amount of less firmly bound metal tended to increase, and nullified the distribution changes induced by drying. The effect was most pronounced in the case of Cd. The study highlights that significant errors in metal distribution pattern analyses can arise from sample preparation procedures and it has provided information on the possible mobilisation of metal when dredged sediment is land-dumped, i.e. allowed to dry in air.