Comparison of alternative remediation technologies for recycled gravel contaminated with heavy metals.

To evaluate the effects of different remediation methods on heavy metals contaminated recycled gravel, three immobilization agents (monopotassium phosphate, lime, nano-iron) and two mobilization agents (glyphosate, humic acid (HA)) were studied and compared. Results indicated that nano-iron powder was found to be more effective to immobilize Zn, Cu, Pb and Cd. Meanwhile, glyphosate presents a higher mobilization effect than HA with removal rates of about 66.7% for Cd, more than 80% for Cr, Cu and Zn, and the highest removal percentage of 85.9% for Cr. After the mobilization by glyphosate, the leaching rates of Zn, Cu and Cr were about 0.8%, and below 0.2% for Pb and Cd. The leaching rates after nano-iron powder treatment were 1.18% for Zn, 0.96% for Cr, 0.61% for Cu, 0.45% for Pb and Cd not detected. The formation and disappearance of metal (Zn/Cu/Cr/Pb/Cd) compounds were firmly confirmed through X-ray diffraction and scanning electron microscopy analyses on crystalline phases and morphological surface structures.