Pb Mineral Precipitation in Solutions of Sulfate, Carbonate and Phosphate: Measured and Modeled Pb Solubility and Pb2+ Activity

Xinxin Li,Murray B Mcbride,Behrooz Azimzadeh,Carmen Enid Martinez

doi:10.3390/min11060620

Abstract

Lead (Pb) solubility is commonly limited by dissolution–precipitation reactions of secondary mineral phases in contaminated soils and water. In the research described here, Pb solubility and free Pb2+ ion activities were measured following the precipitation of Pb minerals from aqueous solutions containing sulfate or carbonate in a 1:5 mole ratio in the absence and presence of phosphate over the pH range 4.0–9.0. Using X-ray diffraction and Fourier-transform infrared spectroscopic analysis, we identified anglesite formed in sulfate-containing solutions at low pH. At higher pH, Pb carbonate and carbonate-sulfate minerals, hydrocerussite and leadhillite, were formed in preference to anglesite. Precipitates formed in the Pb-carbonate systems over the pH range of 6 to 9 were composed of cerussite and hydrocerussite, with the latter favored only at the highest pH investigated. The addition of phosphate into the Pb-sulfate and Pb-carbonate systems resulted in the precipitation of Pb3(PO4)2 and structurally related pyromorphite minerals and prevented Pb sulfate and carbonate mineral formation. Phosphate increased the efficiency of Pb removal from solution and decreased free Pb2+ ion activity, causing over 99.9% of Pb to be precipitated. Free Pb2+ ion activities measured using the ion-selective electrode revealed lower values than predicted from thermodynamic constants, indicating that the precipitated minerals may have lower KSP values than generally reported in thermodynamic databases. Conversely, dissolved Pb was frequently greater than predicted based on a speciation model using accepted thermodynamic constants for Pb ion-pair formation in solution. The tendency of the thermodynamic models to underestimate Pb solubility while overestimating free Pb2+ activity in these systems, at least in the higher pH range, indicates that soluble Pb ion-pair formation constants and KSP values need correction in the models.

Highlights

The chemical speciation and mineral forms of Pb in soil and water are critically important in determining the solubility, bioavailability and potential toxicity of this metal [1,2,3]
Precipitation reactions of Pb were conducted in 50-milliliter tubes containing aqueous solutions of 1 mM Pb(NO3)2 and 5 mM of either Na2SO4, Na2CO3, Na2HPO4, Na2SO4Na2HPO4 or Na2CO3-Na2HPO4, with pH adjusted over a wide range
Pb solubilities were similar in the SO4-PO4, CO3-PO4 and PO4 systems when compared over the pH range of 6 to 8, indicating that the formation of insoluble Pb phosphate minerals limited solubility whether PO4 was present alone or in combination with CO3 or SO4

Summary

Introduction

The chemical speciation and mineral forms of Pb in soil and water are critically important in determining the solubility, bioavailability and potential toxicity of this metal [1,2,3]. Whether Pb carbonates (cerussite, hydrocerussite), Pb oxides, Pb sulfates (anglesite, lanarkite) or Pb phosphates (such as pyromorphite) are the weathering products from the Pb ore mineral, galena, is dependent on the initial ore rock composition. In the alkaline environment of carbonate-rich rocks, cerussite [PbCO3], hydrocerussite [Pb3(CO3)2(OH)2] and occasionally shannonite [Pb2OCO3] have been identified [8,9] In addition to these minerals, lanarkite [Pb2OSO4], leadhillite [Pb4SO4(CO3)2(OH)2] and plumbojarosite [PbFe6(SO4)4(OH)2] have been identified in soils contaminated by Pb mine wastes [6,10]. Since the oxidative weathering of PbS to plumbojarosite or plumbogummite requires the availability of soluble Fe or Al, these latter minerals are most likely to be formed in acidic environments

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