FIELD-BASED PARTITION-COEFFICIENTS FOR TRACE-ELEMENTS IN SOIL SOLUTIONS

A total of 48 elements was detected in the soil solutions centrifuged from two acid sandy (humus-iron podzol) profiles from southern England. Concentrations ranged from mM for the major ions to nM for trace metals such as U and the rare earth elements. Field-based solid/solution partition coefficients, K-d, were determined by calculating the ratio of the amount of an element extracted by 0.43 M HNO3 or a neutral salt (0.01 M CaCl2 or 0.1 M Ba(NO3)2) to the concentration in the soil solution. These partition coefficients did not show the expected trend in selectivity. For example, Cd consistently had one of the highest K-d values, higher even than Cu. This was thought to be due in part to the nature of the K-d which reflects a balance between binding to the soil solids and to the dissolved organic carbon (DOC), which is present at relatively high concentrations (1-20 mM) in the soil solutions. Because of the underlying functional similarity between metal binding by the solid and dissolved organic matter, the partition coefficient (and hence element mobility) will be relatively insensitive to changes in pH and metal-ion activity in the soil solution.