Fig. 4. Diethylenetriaminepentaacetic acid (DTPA)-, acetate-, citrate-, and citrate–ascorbate-extractable Fe and Zn before leaching and after four and eight leaching steps (flow-through experiment): (a) Fe in sludge, (b) Fe in sludge–calcite; (c) Zn in sludge, and (d) Zn in sludge–calcite. Error bars are standard errors (two replicates).