The effect of pH on the lability of lead and cadmium sorbed on humic acid particles

Abstract

Sorption of lead(II) or cadmium(II) ions on humic acid particles (at pH 3.5) yields surface complexes which are sparingly soluble in the pH 3.55 region. Interaction of these species with acetic acid or dilute nitric acid released ASV labile metal species (mainly hydrated metal ion). When the pH was adjusted between 4 and 7, increasing amounts of the metal humate species (and humic acid substrate) dissolved and low levels of ASV labile species were detected. Overnight interaction with an excess of acetate ions (0.01 M) led to higher lability values (attributed to acetate/humate ligand exchange). The percentage of labile metal ion detected in acetate solutions varied with the amount of metal ion initially sorbed (range 100 to 500 mmol g−1). With the lower loadings, the ASV labile levels peaked in the pH 6–7 region (at around 12% of total sorbed cadmium, and 4% of total lead). In alkaline solutions, the percentage of labile Cd fell to about half of the peak value, but with Pb, lability values increased at pH > 8, due possibly to the formation of hydroxy complex species. The ASV lability of the Cd and Pb humates, and their 24 hour lability values (determined using a transfer to cation exchanger technique) has been compared with the behaviour of Cu and Zn humates (using similar conditions).