Mineralogy of lead in a soil developed on a Pb-mineralized sandstone (Largentiere, France)

Lead speciation was determined in a soil developed on a geochemical anomaly arising from a Pb-Zn stratabound deposit in Largentiere (Ardeche, France). This geological setting offers the opportunity to determine the preferred form(s) of Pb following soil formation on this unique anomaly. In the soil profile studied, Pb concentrates in the B-horizon (2055 mg/kg Pb) relative to both the A- (1330 mg/kg Pb) and C- (1874 mg/kg Pb) horizons. Plumbogummi te (PbAl3(PO4)(2)(OH)(5).H2O) is the main host of Pb in the soil profile. P b also appears to be associated with Mn-(hydr)oxides, as shown by micro-ana lyses (EMPA, SEM-EDS, and mu -SXRF),in the form of inner-sphere Pb2+ comple xes, as suggested by Ph L-HI-edge EXAFS spectroscopy. Linear least-squares fitting of background-subtracted, k(3)-weighted Pb L-III-edge EXAFS functio ns derived from balk soil samples was carried out using Ph L-III-EXAFS spec tra of 22 Pb-containing model compounds. Quantitative assessment of Pb spec iation revealed that, whereas plumbogummite is the most abundant Pb phase i n the soil profile, Pb2+-Mn-(hydr)oxide surface complexes are gradually rep laced by Pb2+-surface complexes with other phases, possibly Pb2+-organic co mplexes, upward in the soil profile. The presence of large amounts of Pb-ph osphate in the Largentiere soil suggests that low solubility phosphates may be important long-term hosts of Pb in Pb-contaminated soils that have suff iciently high phosphorous activities to cause formation of these phases.