Enhanced Immobilization of Pb by Phosphorus Amendment in Rhizosphere Soils Investigated by Thermodynamic Modeling and XAFS Spectroscopy.

In order to formulate successful phytostabilization strategies in contaminated soils, understanding how metals are immobilized at the molecular level is critical at root-metal interfaces. The present study assessed mineralogical transformations of Pb-containing phases induced by different plant species in a shooting range soil treated with a phosphorus amendment. Rhizosphere and bulk (non-rhizosphere) soil samples from a pot experiment were analyzed using synchrotron X-ray absorption spectroscopy (XAS), thermodynamic modeling and chemical extraction. In the bulk and rhizosphere soils without amendment application, the dominant species were Pb-carbonates (cerussite and hydrocerussite, 42-47%) and Pb-sorbed with organic substances (21-38%). In the amended soil without plant growth, the proportion of these species were reduced and replaced by chloropyromorphite with 38% of total Pb. Application of amendment in conjunction with plant growth enhanced Pb immobilization in the rhizosphere soil, which was indicated by the decrease of chemically-extractable Pb concentration and increase of chloropyromorphite proportion to 52%. Regardless of plant growth, Pb in an aqueous phase of the amended soil was near saturation to supersaturation with respect to chloropyromorphite. Our study suggests that the combined application of phosphorus amendment and plant have the potential to induce transformation of original Pb into more geochemically stable species