Effects of ammonium molybdate on phytoremediation by alfalfa plants and (im)mobilization of toxic metals in soils

Ammonium molybdate has been used to remove the toxic metals in waste water by forming precipitates, such as Pb. However, the ammonium molybdate has not been used to remediate the toxic metals contaminated soils. The objectives of this study were to report: (1) the reaction mechanisms of ammonium molybdate with toxic metals; (2) the effects of ammonium molybdate on (im)mobilization of the toxic metals in soils; (3) and the effects of ammonium molybdate on uptake toxic metals from soils by alfalfa (Medicago sativa L.) plants. In this paper, it was found that: the effects of ammonium molybdate on toxic metals are amphoteric-immobilization/mobilization. Cr and Zn can be precipitated with ammonium molybdate, but Cd, Ni, Cu can be formed more soluble fractions with ammonium molybdate. The ammonium molybdate can be named as half-chemical stabilization agent, half-extracting agent, or amphoteric agent. The contents and BCF values of toxic metals in alfalfa plants were increased after the soils were treated with ammonium molybdate. TF values of alfalfa plants (both controls and treatments) were lower than 1. The addition of ammonium molybdate can increase the acid soluble fractions of Cd, Ni, Cu, and residual fractions of Cr, Zn. In other words, the toxicities of Cd, Ni, and Cu to plants in soils have been increasing. Instead, the toxicities of Cr, Zn have been decreasing. The ammonium molybdate has promoted plants to produce more biomass, but the growths of shoot and root in seedling stages have been decreased by the increasing Cd, Ni, and Cu toxicities which caused by ammonium molybdate.