Coupled Dissolution and Precipitation at the Cerussite-Phosphate

Nov 15, 2013 - In situ atomic force microscopy (AFM) has been used to study the interaction of phosphate-bearing solutions with cerussite, PbCO3, (010...
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Coupled Dissolution and Precipitation at the Cerussite-Phosphate Solution Interface: Implications for Immobilization of Lead in Soils Lijun Wang,*,† Christine V. Putnis,*,‡ Encarnación Ruiz-Agudo,§ Helen E. King,‡,∥ and Andrew Putnis‡ †

College of Resources and Environment, Huazhong Agricultural University, Wuhan, Hubei 430070, China Institut für Mineralogie, University of Münster, 48149 Münster, North Rhine-Westphalia, Germany § Department of Mineralogy and Petrology, University of Granada, Granada, Granada 18071, Spain ∥ Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, United States ‡

S Supporting Information *

ABSTRACT: In situ atomic force microscopy (AFM) has been used to study the interaction of phosphate-bearing solutions with cerussite, PbCO3, (010) surfaces. During the dissolution of cerussite we observed simultaneous growth of needle-shaped or spherical pyromorphite phases. This occurred at two different pH values and ionic strengths relevant to soil solution conditions. The initial dissolution processes occurring at the cerussite solid-phosphate solution interface were clearly distinguished, and heterogeneous nucleation and growth rates of pyromorphites at phosphate concentrations ranging from 0.1 μM to 10 mM were quantitatively defined. Enhanced cerussite dissolution in the presence of high salt (NaCl or NaF) concentrations leads to an increase in pyromorphite nucleation and growth rates. The newly formed pyromorphites were found to be stable upon contact with water or citrate-bearing solutions under acidic or alkaline conditions in the pH range 4−8. These in situ observations may improve the mechanistic understanding of processes resulting in lead immobilization in diverse soil systems as well as to enhance the effectiveness of phosphate-based treatments for remediation of lead-polluted soils.

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INTRODUCTION The greatly increased circulation of toxic metals through soils, sediments, and aquatic environments and their inevitable transfer to the human food chain remains an important environmental issue which entails some unknown health risks.1 Given that the toxicity of a contaminant is mainly related to its bioavailability rather than to its total concentration in a certain environment, remediation approaches focus on the reduction of contaminant bioavailability without their removal from the contaminated environments.2,3 Coupled dissolution and precipitation reactions that lead to the formation of sparingly soluble crystalline compounds containing these toxic metals appears to be a promising strategy to reduce their chemical mobility and bioavailability.4,5 Thus, the solubility of contaminant-containing minerals in these environments can directly influence the chemical reactivity, toxicity, and mobility of their constituent ions. In many situations, the solution concentrations of heavy metals such as lead (Pb (II)) in soils and sediments are frequently controlled by the dissolution and precipitation of discrete mineral phases.2 Pb-contaminated soils contain the minerals cerussite (PbCO3), anglesite (PbSO4), crocoite (PbCrO4), and various lead oxides including litharge (PbO) as well as Pb2+ adsorbed to calcite in calcareous soils or Fe/Mn oxides in noncalcareous soils.2 The lead oxides, sulfates, and carbonates are all soluble at acidic and neutral conditions, and © 2013 American Chemical Society

these dissolved Pb ions which are released to soil solutions can pose a significant environmental risk. In contrast, the pyromorphite [PY, Pb5(PO4)3X, where X is OH−, Cl−, or F−] group of lead phosphate (Pb−P) minerals including chloropyromorphite Pb5(PO4)3Cl (CPY), hydroxypyromorphite Pb 5 (PO 4 ) 3 OH (HPY), and fluoropyromorphite Pb5(PO4)3F (FPY) are much less soluble and geochemically stable over a wide pH range. The solubility decreases FPY > HPY > CPY, with log Ksp of −72, −77, and −84,6 respectively. Nriagu provided the first indication that the extremely insoluble Pb pyromorphite minerals would be formed in Pb contaminated soils.7,8 The possibility of using phosphate compounds to stabilize and immobilize Pb in remediation of contaminated soils has been extensively investigated at the field-scale.9,10 In light of their intrinsically low solubilities and the natural occurrence of pyromorphites in some contaminated soils,11,12 effort has been given to promoting the dissolution of Pbcontaining minerals, and the subsequent in situ formation of pyromorphites in Pb-contaminated soils through the addition of phosphate sources. Pyromorphite formation from a range of different initial lead-containing solids including cerussite has Received: Revised: Accepted: Published: 13502

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mode and equipped with a fluid cell. A natural cerussite crystal (Otavi, Namibia) was cleaved in order to expose a fresh cleavage (010) surface for each experiment. Prior to the input of reaction solutions, the cerussite (010) cleavage face was exposed to deionized water (pH 5.8−6.0). Solutions of diammonium phosphate, (NH4)2HPO4, in concentrations ranging from 0.1 μM to 10 mM were passed through the fluid cell at a rate of 2 mL per 1.5 min. The chosen flow rate was to ensure surface-controlled reaction rather than diffusion control.24 AFM images were collected using Si3N4 tips (Bruker, tip model NP-S20) with spring constants of 0.12 N/m and 0.58 N/m and were analyzed using the NanoScope software (Version 5.31r1) and WSxM v5.0 Develop 6.4.25 Additionally, NaCl or NaF at concentrations ranging from 1−250 mM was added to the phosphate solutions to study the effects of ionic strength. Also ammonium dihydrogen phosphate (NH4H2PO4) and sodium dihydrogen phosphate (NaH2PO4) solutions were used to study the effects of different pHs and phosphate speciation. The pH values of NaH2PO4 solutions were adjusted to pH 7.6 using 0.01 M NaOH or pH 5.3 using 0.01 M HCl. Following the formation of lead phosphate precipitates on cerussite (010) surfaces, their stability was investigated by in situ observation of possible dissolution of the precipitates in H2O (pH 5.8) or monosodium citrate (0.1 mM, pH 7.9). Nucleation rate measurements were determined from data collected shortly after the onset of precipitate formation, where the increase in Pb−P particle density exhibited a linear dependence on time. Data used in our analyses were typically obtained within the first 10−30 min of each experiment. Measurements were made on three different crystals per solution composition to ensure reproducibility of the results, and each was imaged in different locations on the crystal. All data with their mean values ± standard deviation (SD) of three independent sets of experiments are presented. Ex situ AFM dissolution experiments were performed following a few AFM scans of a cerussite surface in air and deionized water. The sample was then removed from the AFM fluid cell and placed in a beaker filled with ca. 10 mL of different reaction solutions for 7 days at room temperature. The reacted crystal surface was observed in a scanning electron microscope (SEM, JEOL JSM 6460 LV) equipped with an energy dispersive X-ray (EDX) detector (Oxford Instruments) for elemental analyses of precipitates formed on the cerussite surfaces. To identify the precipitated phase spatially resolved Raman spectra were collected using the 532 nm Nd:YAG laser of a Jobin Yvon Xplora Raman microscope calibrated using the 520.7 cm−1 band of silicon. A 300 μm hole, 50 μm entrance slit, and 1800 grooves/mm grating were used to obtain the spectra. Spectra from an unreacted cerrusite surface and samples after reaction with the different phosphate-bearing solutions were collected for at least 60 s and integrated multiple times to enhance the signal-to-noise ratio. The Raman bands were fitted using the Fityk program by first removing the background using a linear function followed by fitting each peak using a Voigt function. The spectral resolution, 3.1 cm−1 in the region associated with phosphate stretching (900−1000 cm−1), was measured using a Neon lamp. The PHREEQC program26 was used to calculate surface and solution speciation with increasing cerussite dissolution in a volume of 38 μL of phosphate-bearing solution in the AFM fluid cell in our experiments. For the surface speciation, we used the surface complexation model (SCM) for calcite and other

also been previously observed.13 The use of soluble forms of orthophosphate for remediation seems to be highly effective; however, excess P in soil may increase the risk of P contamination through the migration and eutrophication of surface and underground waters and of phosphate inducing the mobilization of arsenic, chromium(VI), and other anionic contaminants.14 An alternative approach is to use a lower solubility source of P such as apatite.15−17 Efficient Pb immobilization using P compounds requires an increase in the solubility of the phosphate phase under acidic conditions.18 In situ soil treatments using various other phosphates including single/triple superphosphate, phosphoric acid, rock phosphate, mono/diammonium phosphate, and bone have been extensively explored to reduce the phyto- and bioavailability of soil Pb.19−21 Various Pb species and their corresponding reaction behavior during the reaction with different P sources in soils vary widely and depend upon the sources of Pb contamination and environmental conditions. In a dynamic soil system, the kinetics of a reaction must be taken into account in order to determine the possibility and the extent of a reaction occurring for given conditions and time scale. Most studies on the formation of pyromorphites were derived largely from macroscopic investigations; therefore, the kinetic pathways and mechanisms of pyromorphite formations on mineral surfaces at microscopic levels remain insufficiently understood for predicting Pb immobilization from various soluble and insoluble lead species. In addition, according to a long-term field study, Pb in contaminated soils before P treatment was primarily associated with the carbonate fraction (up to 44.5%), followed by Fe−Mn oxides > water-soluble and exchangeable > organic,9,22 demonstrating the importance of the form of Pb contamination and soil pH in determining the effectiveness of Pb immobilization in soils. Therefore, we have chosen cerussite (PbCO3, log Ksp of −13)23 as a mineral substrate and investigated, by direct microscopic observations, the pyromorphite nucleation and subsequent growth on cerussite surfaces using in situ atomic force microscopy (AFM) coupled with a fluid reaction cell through which solutions flowed with varying phosphate concentrations under acidic to alkaline pH conditions. We then compare the effects of different P sources on the pyromorphite formation through the interactions between soluble Pb(NO3)2 and apatite surfaces. To our knowledge, there has been no experimental effort to directly measure the thermodynamic and kinetic contributions to the surface nucleation and growth rates of pyromorphite phases on related mineral surfaces even under very low phosphate concentrations where significant P migration and eutrophication of waters would not occur. These direct nanoscale observations may provide a new recognition for Pb immobilization, allowing for a reconsideration of a remediation strategy that would be more effective in natural environments. In addition, the current microscopic work and numerous macroscopic studies have largely established that the rate of pyromorphite precipitation is considerably faster than the rates of dissolution of lead-containing minerals. The remediation applications should therefore focus on the ability to deliver phosphate to soils and the rates of lead and phosphate mass transfer. This, however, deserves a separate and further study.

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EXPERIMENTAL SECTION In situ dissolution experiments were performed using a Digital Instruments (Bruker) Nanoscope IIIa AFM working in contact 13503

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Figure 1. A sequence of AFM deflection (A-C) and the corresponding height images (D-F) illustrating dissolution and precipitation on the surface of cerussite when exposed to the 0.5 μM (NH4)2HPO4 solution at pH 7.7 at (A) t = 48, (B) 55, and (C) 67 min, respectively. (G-I) show the crosssectional analyses of the height (depth) of etch pits along a dotted line in each height image. An arrow shows an etch pit on the dissolving cerussite substrate in (F). Following 60 min of injecting reaction solutions, precipitation became increasingly evident on the cerussite (010) surface. AFM images, 5 × 5 μm.

retreat and isolated deep dissolution pits (Figure 1). The pits (the dotted ellipse in Figure 1D) coalesce into larger irregular pits. However, some pits have relatively regular shape, close to a rectangular form with 96 ± 4° and 86 ± 4° (n = 10) (Figure 1F). These rectangular shaped etch pits are consistent with the orthorhombic symmetry of cerussite, and their depth gradually increases from 3.7 to 9.5 nm during this dissolution process (Figure 1G-I). At the same time, irregular shallow pits are filled by surface precipitates (marked by arrows in Figure 1E). Following 60 min of injecting reaction solutions, precipitation is increasingly evident on the cerussite (010) surface (Figure 1F). Lead Phosphate Nucleation and Growth on Cerussite (010) Cleavage Surfaces. Cerussite dissolution in the presence of all the experimental phosphate solutions used in this study resulted in the nucleation of Pb-phosphate crystals on the cerussite (010) surfaces as shown in Figure 2A. The average height of the initially formed needle-like crystals was about 20 nm (Figure 2D). The height of the growing crystals remained constant and just elongated during the reaction time period (Figure 2E). After 10 min, no further growth

divalent metal carbonates (see the Supporting Information). This thermodynamic model for the carbonate/solution interface postulates the formation of the two primary hydration sites, >MeOH0 and >CO3H0, having a 1:1 stoichiometry. Note that the notation > is used here to mean potential bonding/ hydration surface sites. According to this model, the following complexes are formed from these primary species exposed to the aqueous solution: >MeOH2+, >MeO−, >MeHCO30, >MeCO3−, >CO3Me+, and >CO3−. Each cation surface site can be divided into two different sites: weak and strong (s). The equilibria between these surface groups and aqueous species are described using thermodynamic stability constants (Tables S1−S2).

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RESULTS AND DISCUSSION Dissolution Features on Cerussite (010) Cleavage Surfaces. On contact with deionized water, dissolution immediately began with the formation of irregular shallow or deep etch pits on the exposed surfaces (Figure S1). In the presence of low concentration (NH4)2HPO4 (0.5 μM, pH 7.7), the AFM images show widespread surface etching with step 13504

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Figure 2. AFM time sequence (deflection images) of the growth evolution of precipitates on a dissovling cerussite surface in 1 mM (NH4)2HPO4 + 1 mM NaF (pH 7.7) at (A) t = 228 s, (B) 326 s, and (C) 10 min, respectively. The scan area was almost fully covered by the needle-shaped crystals after 10 min. (D, E) Height profile for the needle-like crystals along section 1 to 1′ or 2 to 2′ (dashed lines in (A) and (B)). The average height of single needle-shaped crystals is about 20.0 nm, which was almost unchanged in a short period of reaction time. (F) Dependence of the growth (elongation) rate of needle-like crystals on time. AFM images, 5 × 5 μm.

Figure 3. Growth (elongation) rate of needle-like crystals on cerussite (010) surfaces under different experimental conditions. (A) A range of (NH4)2HPO4 concentrations at pH 7.7, and (B) NaF concentrations with constant concentration of 1 mM (NH4)2HPO4 at pH 7.7. Data are means ± SD (n > 10, crystals) for three independent experiments.

solution at pH 7.7. A similar trend can also be found in 1 mM NaH2PO4 solutions at the same pH 5.1 (Figure S2). In the presence of high concentration NaF (0.1 or 0.25 M) with a constant concentration of 1 mM (NH4)2HPO4 (pH 7.7), the nucleated crystals grew as rounded forms, with sizes ranging from 130 to 240 nm (Figure S3). Similarly, in the presence of NaCl (1, 10, 100, or 250 mM) with 1 mM (NH4)2HPO4 (pH 7.7) or 1 mM NH4H2PO4 (pH 5.1), spherical crystals formed on cerussite (010) surfaces as seen in Figure 5. These precipitates were loosely attached at the initial stage and can be removed by scanning the AFM tip at higher contact force. No preferential locations were observed for these precipitates; they appeared both at edge steps and terraces on the surface. The number of Pb−phosphate nuclei increased linearly with time for a given NaCl concentration, and faster nucleation rates were observed with increasing NaCl concentration when the solution pH was kept constant (pH 7.7) (Figure 5D), whereas the nucleation rate significantly increased at lower pH (5.1)

(elongation and/or widening) of the needle-like crystals was observed. This could be related to the fact that the cerussite surface was rapidly covered by a layer of precipitates (Figure 2C), effectively passivating the surface so that no Pb2+ ions could be released to the interfacial fluid layer. The length of needle-like crystals increased linearly with time for a given phosphate concentration in the absence and presence of NaF as a background electrolyte (Figure 2F). Faster growth (elongation) rates were observed with increasing the (NH4)2HPO4 solution concentration (Figure 3A) or NaF concentration (1−10 mM) for a constant concentration of 1 mM (NH4)2HPO4 (Figure 3B) when the solution pH was kept constant (pH 7.7). Moreover, the cerussite (010) surface displayed a greater depth of elongated etch pits (up to 30 nm) (Figure 4E) and a greater height of needle-shaped Pbphosphate crystals (up to 150 nm) (Figure 4F) after injecting 1 mM NH4H2PO4 solution at pH 5.1 compared with the 13505

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indicated these crystallites are composed of Pb, P, O, and/or F(Cl) (Figure 7B and Figures S4E, F). These precipitates formed on cerussite (010) surfaces were identified using Raman spectroscopy (Figure 7C). All spectra show bands related to the cerussite substrate, detectable due to υ1 and υ2 carbonate stretching bands at 1055 and 838 cm−1, respectively.28 However, spectra from the reacted samples show two additional peaks in the phosphate stretching region between 900 and 1000 cm−1. The symmetric υ1 and asymmetric υ3 P−O stretch bands were in almost identical positions for precipitates from the pure phosphate solution (924 and 950 cm−1) and Cl-bearing solution (922 and 946 cm−1).29 In contrast, the bands associated with the precipitated phase from F-bearing solutions are significantly shifted to 934 and 969 cm−1, as observed in other Raman studies.30 Only the precipitates from the pure phosphate solution showed a small but distinct peak above the background in the water region of the Raman spectra (Figure S5). At 3565 cm−1 this peak is typical for the OH stretch in HPY,31 thus the phases precipitated can be characterized as HPY from pure phosphate solutions, CPY from the Cl-bearing phosphate solution and FPY from the F-bearing phosphate solution. Although the formation of these pyromorphites on cerussite surfaces is kinetically rapid, they are stable in contact with deionized water or citrate solutions (Figure 6). Batch dissolution studies32 also suggest that the dissolution rate of a 1-day experiment sample was not significantly different than a 1-year old specimen. Other works on CPY stability through dissolution experiments showed a reaction order of 0.65 and an intrinsic rate constant of 0.0039 mol m−2 h−1 at pH 3−7,33 whereas fluorapatite had a reaction order of 0.81 and a much higher rate constant of 0.021 mol m−2 h−1 for pH < 6.34 Moreover, concentrations of organic acids such as citrate in the bulk soil solution typically range from 0.1 μM to 0.1 mM and are estimated to be PbCO3− and >CO3−).47 This disagreement could be related to the fact that our model did not include electrostatic terms in the mass-action equations for surface species and no charge-balance equations for the surfaces are used. Consequently, the ionic strength primarily influences the activity coefficients for aqueous phosphate species in addition to its influence to increase the cerussite dissolution rate (Figure S6). In addition to the influence of the ionic strength, the effect of changing the concentrations of chloride and fluoride ions, which are key reactants to producing CPY and FPY, may be much more significant than that of ionic strength. The spherical morphology of precipitates with high NaCl concentrations may be due to a much higher degree of supersaturation with respect to CPY than HPY, causing homogeneous nucleation to dominate over heterogeneous nucleation in ways that could affect particle morphology. A long-term field study and model (the use of Visual MINTEQ) on the potential Pb species distribution in a soil profile 480 days after phosphate application showed that cerussite may control Pb solubility in soils.9 Pb activity in the Ptreated soils decreased with time, and the soil solution was supersaturated with respect to CPY in all P-treated soils.9 However, organic matter in soils may exert an inhibitory influence on the formation of PY.48 A very recent result further showed that reduced PY formation in the presence of oxalic acid may be associated with the increase in organic-bound Pb and decrease in Pb-carbonate phases.49 Our results show that citric acid seemed to have a less adverse effect in PY formation than oxalic acid. Finally, we used AFM to explore the interactions of dissolved Pb (in the form of Pb(NO3)2) with fluor/chlorapatite surfaces (Figure S7). These solid apatitic surfaces seem to have less capacity to remove Pb from solution in a short period of time due to their intrinsically low dissolution rates compared to treatments using the more soluble cerussite surfaces exposed to phosphate solutions. Further evidence that HPY did not require an HAP surface for nucleation came from ex situ SEM images showing needles of HPY nucleated in solution and simply fell onto the cantilever/tip of the AFM.50 In addition, under the actual conditions of soil environments, adsorption of aqueous Pb to other mineral surfaces (such as calcite in alkaline soils or Fe/Mn oxides in acidic soils) as well as its precipitation as mineral phases (i.e., lead carbonates) may occur. Using a calcite surface, we tested various Pb(NO3)2 concentrations and a rapid sorption of Pb2+ ions on calcite was observed, based on morphology changes of the etch pits captured immediately following the addition of Pb(NO3)2 solution into the AFM fluid cell (Figure S8). Subsequently, cerussite or hydrocerussite quickly formed on calcite with an increasing concentration of Pb(NO3)2 solution at varying pH values (4.0−8.0). It has been shown that the majority of Pb(II) in aqueous solutions precipitates on the surface of calcite even at concentrations as low as 0.1 μM51 via the initial formation of Pb mononuclear inner-sphere complexes at the calcite-water interface.52 The same phenomenon has been observed on the surface of aragonite at short interaction times, hydrocerussite is formed, and it is later replaced or transformed into cerussite.53 These collective results suggest that Pb ions can be readily removed from the aqueous environment by soil minerals (e.g., calcite)

Figure 7. (A) SEM image showing the ex situ formation of needleshaped crystals on cerussite after 7 days of interfacial reaction in 1 mM (NH4)2HPO4 solutions in the presence of 0.1 M NaF (pH 7.7). (B) EDX spectrum taken from an individual needle- shaped precipitate (inset in (B)), indicating a F-containing lead phosphate phase. (C) Raman spectra for unreacted cerussite (green), a sample reacted in 1 mM (NH4)2HPO4 solution (black), 1 mM (NH4)2HPO4 + 0.1 M NaCl solution (red), and 1 mM (NH4)2HPO4 + 0.1 M NaF solution (blue). Inset in (C) shows detailed spectra of the region between 800 and 1000 cm−1.

determination, solution composition (ionic strength) may influence the precipitate morphologies. We assume that a dissolution reaction with increased ionic strength (Figures 5 and S3) is too fast to form needle-shaped crystals. The distribution of surface-active ions determines the cerussite dissolution rates at two different solution pHs (Figures 4 and 5): H+ at pH 2−5 and HCO3− at pH 5−8,45 which also will influence surface speciation of cerussite (Table S2) and phosphate primary speciation in the reaction solution.46 For the formation of a Pb-phosphate phase on cerussite under our experimental conditions, the thermodynamically most stable phase is CPY. Calculated total concentration of lead carbonate (assuming a crystal size of ca. 4 × 3 × 1 mm) dissolved in 38 μL of 1 mM (NH4)2HPO4 solution (free volume in the AFM fluid cell) at pH 7.7 in equilibrium is 8.83 μM. When 0.01 or 0.1 M NaF/NaCl was added into 1 mM (NH 4 ) 2 HPO 4 solutions, the total concentration of cerussite dissolved in each of the phosphate solutions increased to 16.4 and 15.74 μM, respectively. Saturation indices (SI) of possible solid phases including hydrocerussite (Pb3(CO3)2(OH)2), HC), CPY, HPY, and FPY calculated using MINTEQ database at the last step of dissolution reaction are shown in Table S3. Our results suggest that the presence of high concentrations of NaCl is more favorable for HPY and CPY to form on cerussite than at low NaCl concentrations (Figure 5), and adsorbed phosphate increases with the ionic strength. However, this is not in 13508

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and indicate the importance of precipitation processes (mainly surface precipitation leading to the overgrowth of cerussite and/or hydrocerussite crystals) taking place in parallel with mineral surface dissolution processes. Ultimately no free Pb ions would be available in soil solutions for the reaction with apatite, and, in turn, the effectiveness for removing aqueous Pb using sparingly soluble Ca−P solids such as apatite and bone meal would be very low. Similarly, AFM observations have shown that the interaction of calcite (1014̅ ) surfaces with Cdbearing aqueous solutions involves the dissolution of the calcite substrate and the simultaneous epitaxial growth of multilayer three-dimensional islands of Cd-rich members of the CdxCa1‑xCO3 solid solution. The formation of such passivation layers armors the calcite substrate from further dissolution and determines the end of the process at a ‘partial’ pseudoequilibrium end point.54 Collectively, the conditions in previous macroscopic studies were such (high dissolved Pb (II) and phosphate concentrations in combination with high pH > 6) that the formation of pyromorphite-type mineral phases were favorable.55 However, natural soil conditions may be completely different, for example very low dissolved Pb and low pH values. Cao et al.9 demonstrated that for more efficient Pb immobilization, pH reduction using H 3 PO 4 became necessary to dissolve carbonate-bound Pb and make Pb readily available for geochemically stable Pb phosphate formation. Our experiments show that during the interaction of phosphate-bearing solutions with cerussite, PbCO3, the dissolution of this mineral is tightly coupled at the mineralfluid interface with the precipitation of a new more stable phase (pyromorphite). However the dissolution and precipitation at the cerussite-phosphate solution interface is dependent on the solution composition at the mineral surface, including the activity of phosphate species, pH and ionic strength (NaCl/ NaF), thereby influencing immobilization and bioavailability of Pb. Based on the present study, we predict a significant increase in Pb immobilization due to increased phosphate adsorption and the corresponding precipitation on mineral surfaces at elevated phosphate concentrations. The bioavailability of Pb will be reduced even further at high salt concentrations. In addition, the stability of the newly formed Pb-phosphate phases on cerussite in water or citrate solutions (Figure 6) was confirmed. This may enhance the immobilization of Pb in the rhizosphere. Although such an approach does not remove the heavy metal element from the biosphere, it can significantly reduce its bioavailability. This approach would not only be limited to Pb. Similar treatments can stabilize other toxic ions such as Cd and Zn in contaminated soils and sediments. Our results provide a mechanistic understanding of mineral surfaceinduced Pb immobilization, converting labile forms of toxic elements into less reactive solids more consistent with longterm capture at geochemical equilibrium. We conclude that the use of soluble phosphates could be highly effective in immobilizing Pb in contaminated soils and other applications such as the inhibition of lead corrosion in drinking water distribution systems.

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AUTHOR INFORMATION

Corresponding Authors

*Phone/Fax: +86-27-87288095. E-mail: [email protected]. cn (L.W.). *Phone: +49-251-8333454. Fax: +49-251-8338397. E-mail: [email protected] (C.V.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Natural Science Foundation of China (Grant No. 41071208), the Fundamental Research Funds for the Central Universities (2011PY150), and a startup grant from the Huazhong Agricultural University (2010BQ063) (L.J.W.). E.R-.A. acknowledges the receipt of a Ramón y Cajal grant from Spanish Ministry of Economy and Competitiveness, as well as additional funding from the Spanish government (grants MAT2012-37584) and the Junta de Andaluciá (research group RNM-179 and project P11-RNM-7550). The authors thank V. Rapelius for help with ICP-OES analyses.

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REFERENCES

(1) Nriagu, J. O.; Pacyna, J. M. Quantitative assessment of worldwide contamination of air, water and soils by trace metals. Nature 1988, 333, 134−139. (2) Traina, S. J.; Laperche, V. Contaminant bioavailability in soils, sediments, and aquatic environments. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3365−3371. (3) Berti, W. R.; Cunningham, S. C. In-place inactivation of Pb in Pbcontaminated soils. Environ. Sci. Technol. 1997, 31, 1359−1364. (4) Valsami-Jones, E.; Ragnarsdottir, K. V.; Putnis, A.; Bosbach, D.; Kemp, A. J.; Gressey, G. The dissolution of apatite in the presence of aquetous metal cations at pH 2−7. Chem. Geol. 1998, 151, 215−233. (5) Perez-Garrido, C.; Fernandez-Diaz, L.; Pina, C. M.; Prieto, M. In situ AFM observations of the interaction between calcite surfaces and Cd-bearing aqueous solutions. Surf. Sci. 2007, 601, 5499−5509. (6) Flis, J.; Manecki, M.; Bajda, T. Solubility of promorphitemimetite solid solution series. Geochim. Cosmochim. Acta 2011, 75, 1858−1868. (7) Nriagu, J. O. Lead orthophosphates. I: Solubility and hydrolysis of secondary lead orthophosphate. Inorg. Chem. 1972, 11, 2499−2503. (8) Nriagu, J. O. Lead orthophosphates-IV. Formation and stability in the environment. Geochim. Cosmochim. Acta 1974, 38, 887−898. (9) Cao, X.; Ma, L. Q.; Chen, M.; Singh, S. P.; Harris, W. G. Impacts of phosphate amendments on lead biogeochemistry at a contaminated site. Environ. Sci. Technol. 2002, 36, 5296−5304. (10) Ma, L. Q.; Santos, J.; Cao, X.; Saha, U.; Harris, W. Field application of phosphate rock for remediation of metal-contaminated soils, Publication No. 01-148-226; Stewart, K. J., Ed.; Florida State Institute of Phosphate Research: Florida, USA, 2008. (11) Ruby, M. V.; Davis, A.; Nicholson, A. In situ formation of lead phosphates in soils as a method to immobilize lead. Environ. Sci. Technol. 1994, 28, 646−654. (12) Cotter-Howells, J. Lead phosphate formation in soils. Environ. Pollut. 1996, 1, 9−16. (13) Zhang, P. C.; Ryan, J. A. Transformation of Pb(II) from cerrusite to chloropyromorphite in the presence of hydroxyapatite under varying conditions of pH. Environ. Sci. Technol. 1999, 33, 625− 630. (14) Kaplan, D. I.; Knox, A. S. Enhanced contaminant desorption induced by phosphate mineral additions to sediment. Environ. Sci. Technol. 2004, 38, 3153−3160.

ASSOCIATED CONTENT

* Supporting Information S

Surface speciation of cerussite, saturation indices (SI) with respect to different carbonate and phosphate phases (Tables S1−S3); AFM images of cerussite, calcite, and apatite surfaces under various reaction conditions (Figures S1−S8). This 13509

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dx.doi.org/10.1021/es4041946 | Environ. Sci. Technol. 2013, 47, 13502−13510