Fig. 1 The free Pb²⁺ activity estimated from the anodic stripping voltammetry–labile measurements is represented as a function of solution pH and total metal loading (log₁₀) for the different adsorbents. Ferrihydrite represents the synthesized ferrihydrite, SMS-1 represents the most amorphous pedogenic oxide, SMS-2 represents the most crystalline pedogenic oxide, and Leaf represents the leaf compost. The graph surfaces are obtained by distance-weighted least square smoothing, noting that the regions outside the data are extrapolated (Wilkinson, 1992)