title: Lead phosphate minerals: solubility and dissolution by model and natural ligands.
abstract: Due to their relatively low solubility, lead-phosphate minerals may control Pb solution levels at a low value in natural environments. We reportthe solubility of Pb from two lead-orthophosphate mineral suspensions (beta-Pb9(PO4)6 and PbHPO4) after aging for 3 years. Lead (Pb2+) activity in the aged suspensions was compared to the activity calculated using the Ksp values of various Pb-PO4 minerals reported in the literature. We also determine the time-dependent dissolution of the aged lead-phosphate minerals by organic and inorganic ligands containing S-functional groups (cysteine, methionine, and thiosulfate) and by a soil extracted humic acid. We find the activity of Pb2+ in the aged lead-phosphate suspensions to be 1-2 orders of magnitude higher than predicted by the Ksp values reported in the literature. Disagreement between measured and Ksp-calculated activities has been reported in other investigations of Pb-PO4 minerals; we compiled some of the data and present them together with our results. Furthermore, the time-dependent dissolution experiments indicate that, in most cases, lead phosphates are partly dissolved in the presence of soluble ligands, i.e., model sulfides and humic acid. The soil-extracted humic acid enhanced the dissolution of Pb from the high pH (7.2) lead-phosphate (beta-Pb9(PO4)6) mineral while suppressing Pb dissolution from the low pH (3.8) lead-phosphate (PbHPO4) mineral. While the low molecularweight sulfur-containing ligands enhanced Pb dissolution, their effect was less pronounced. We conclude that (i) nonequilibrium conditions prevail in the mineral suspensions even after 3 years of aging; and (ii) soluble ligands present in soils, sediments, and natural waters can potentially dissolve Pb from lead-phosphate minerals; such ligands, then, may enhance the biological availability and mobility of Pb in the environment.
authors.affiliation: Department of Crop and Soil Sciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. cem17@psu.edu
authors.forename: Carmen Enid
authors.initials: CE
authors.lastname: Martínez
authors.forename: Astrid R
authors.initials: AR
authors.lastname: Jacobson
authors.forename: Murray B
authors.initials: MB
authors.lastname: Mcbride
mesh_terms: D001682:Biological Availability; D002244:Carbon / Q000737:chemistry; D006801:Humans; D006812:Humic Substances / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D007854:Lead / Q000737:chemistry* / Q000378:metabolism / Q000633:toxicity; D008024:Ligands; D008903:Minerals / Q000737:chemistry*; D008954:Models, Biological; D009930:Organic Chemicals / Q000737:chemistry; D012989:Soil Pollutants / Q000032:analysis*; D012995:Solubility; D013438:Sulfhydryl Compounds / Q000737:chemistry; D013997:Time Factors; D014961:X-Ray Diffraction
chemical_list: D006812:Humic Substances; D008024:Ligands; D008903:Minerals; D009930:Organic Chemicals; D012989:Soil Pollutants; D013438:Sulfhydryl Compounds; D007854:Lead; C036682:lead phosphate; D002244:Carbon