Lead phosphate minerals: solubility and dissolution by model and natural ligands.
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.
Martínez
Carmen Enid
CE
Department of Crop and Soil Sciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. cem17@psu.edu
Jacobson
Astrid R
AR
Mcbride
Murray B
MB
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
D006812:Humic Substances; D008024:Ligands; D008903:Minerals; D009930:Organic Chemicals; D012989:Soil Pollutants; D013438:Sulfhydryl Compounds; D007854:Lead; C036682:lead phosphate; D002244:Carbon