Effect of Reducing Soil pH and Cultivation of Thlaspi caerulescens on Distribution of Cd and Zn in Metal-contaminated Soils.

           This study used two soils differing in levels of Cd and Zn and was adjusted to 5 or 6 different pH levels. Metals were extracted into 5 sequential fractions and the pH effect on the mobilization of metals from each fraction and T. caerulescens phytoextraction was assessed. Reducing pH redistributed Cd and Zn among the five fractions. The most soluble metal form (F1) was greatly increased with decreasing pH. Sequentially more recalcitrant fractions F2, F3, F4, and F5 all had different degrees of mobilization at low pH. Most of the “new” mobile Cd was from F2 and for Zn it was mainly from F2 and F3. Plant uptake of metal also modified the rhizosphere soil metal environment. Thlaspi caerulescens was able to reduce Cd concentration in all 5 fractions, although F1-F3 were most significantly reduced. For Zn, T. caerulescens significantly reduced metal concentration in F1 and F2 pools, while no significant changes in F3-F5 pools were observed.