Fig. 4. Correlation between K_d and sorbed metal concentration for all humic acid sorbents. Solid dots are for total sorbed Al plus Fe concentrations (r² = 0.87). Open triangles are for total sorbed Ca plus Mg plus Al and Fe concentrations (r² = 0.39). Note that the high concentration points (105 mmol kg^–1 C) are coincident for both data sets because sorbed Ca and Mg concentrations were negligible for these cases (high Al, high Fe; Table 1).