Evaluation of phosphate fertilizers and red mud in reducing plant availability of Cd, Pb, and Zn in mine tailings

Mono-potassium phosphate (MKP), natural phosphate fertilizer, and red mud (RM) were tested individually and as mixed agents for stabilizing Cd, Pb, and Zn in mine tailings. The mixed stabilizer containing MKP/RM was the most effective among the tested stabilizers. This is based on the efficiency of conversion from the plant-available metal fractions to the non-plant-available metal fractions and on the performances of toxicity characteristic leaching procedure tests. MKP/RM converted Cd and Zn mainly to carbonate and Fe/Mn oxide fractions, and Pb mainly to the residual fraction. When the mine tailings were treated with MKP/RM, the pH value of the stabilized soil increased from 3.4 to 6.4 after 28 days, which is advantageous for plant growth. The X-ray diffraction, scanning electron microscopy/energy dispersive spectroscopy, and the sequential extraction results imply that the major stabilization mechanism for Cd and Zn by MKP and RM involves the formation of surface complexes with Fe (hydr)oxides or Fe phosphates. The major stabilization mechanism of Pb by MKP is believed to involve the formation of Pb-phosphate precipitates, and that by RM is proposed to involve the formation of inner sphere complexes with Fe/Al (hydr)oxides or incorporation within the grids of Fe/Al (hydr)oxides or silicate minerals.