Evaluation of chloropyromorphite stability in the rhizosphere of and in a sand culture.

Chloropyromorphite (Pb(PO)Cl), CP, is the most stable lead (Pb) mineral under normal environmental conditions and precipitates in Pb-contaminated soils by addition of phosphorous (P). A sand culture experiment was conducted to evaluate the efficiency of and roots to dissolve CP in the presence and in the absence of P source. The results showed that the rhizosphere of the plants had lower soluble P and Pb compared with the bulk, which can be attributed to a higher pH in the rhizosphere. Mineralogical transformations of CP in the root surface of the plants including lanarkite (PbSO.PbO) has been confirmed by X-ray diffraction and scanning electron microscopy techniques. Decrease in soluble P in the rhizosphere as a consequence of P uptake by the plant roots may be a reason for CP dissolution. This study indicates that the dissolution of CP can be promoted by rhizosphere processes.