Characterizing the availability of metals in contaminated soils. II

. Methods that can be used to determine the distribution of metal species in soil solution are critically reviewed and assessed. They are divided into two groups: those that can provide free ion activity, and those that measure labile species in solution. Ion selective electrodes have long been regarded as a promising technique, but there are practical problems in performing accurate measurements and only the Cu electrode has been used routinely. The Donnan membrane technique is capable of measuring the free ion activity of many metals, but adequate sensitivity can be a problem. Although resin competition methods are versatile, care must be exercised to avoid perturbing the solution excessively. Anodic stripping voltammetry (ASV) measures labile species, so the approximation involved in interpretation as simple inorganic species, from which free ion activities can be derived, should be recognized. Diffusive gradients in thin‐films also measures labile species, but it is applicable to a much wider range of metals than ASV. It requires larger volumes of solution, but it can be used directly on the whole soil where it also measures the metal that can be rapidly supplied to solution. Other techniques such as permeable liquid membranes have yet to be used for measurements on soil solution. All of these methods have strength and weaknesses, and measure different aspects of speciation. Knowledge of the availability of the metals to biota is likely to be best advanced by the critical use of one or more of these speciation methods with a thorough understanding of exactly what is being measured.