Fig. 2 The calculated pH-dependent free Pb²⁺ activity [shown as negative M log₁₀(Pb²⁺)] is represented for three arbitrary metal loadings (20, 200, and 2000 mg Pb kg^-1). The lines are obtained from the regressions reported in Table 2. They do not represent actual measurements; they simply reflect the shape of the regressions. The triangles () represents ferrihydrite; the circles (•) represents the most amorphous pedogenic oxide (SMS-1); the asterisks (*) represents the most crystalline pedogenic oxide (SMS-2); the squares () represents the leaf compost; and the lozenges () represent data obtained from an independent study of field-collected Pb-contaminated soils (Sauvé et al., 1997a). The inserts use the same x–y scales to compare the pH-dependent free Pb²⁺ relationship calculated using a model similar to Eq. [1] and Table 2 but based on different arbitrary surface coverage levels of 0.5, 5, and 50 µg Pb m^-2 (regression data not presented)