Heterogeneous lead phosphate nucleation at organic-water interfaces

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Heterogeneous lead phosphate nucleation at organicwater interfaces: implications for lead immobilization Chong Dai, Juntao Zhao, Daniel E. Giammar, Jill D. Pasteris, Xiaobing Zuo, and Yandi Hu ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00040 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Earth and Space Chemistry

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Heterogeneous lead phosphate nucleation at organic-

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water interfaces: implications for lead immobilization

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Chong Dai1, Juntao Zhao1, Daniel E. Giammar2, Jill D. Pasteris3, Xiaobing Zuo4, and Yandi Hu1* 1

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Department of Civil & Environmental Engineering, University of Houston, Houston, TX 77004

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Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130 3

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Department of Earth and Planetary Sciences,

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Washington University in St. Louis, St. Louis, MO 63130

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X-ray Science Division, Argonne National Laboratory,

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Argonne, IL 60439

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*To Whom Correspondence Should Be Addressed

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E-mail: yhu11@ uh.edu

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Phone: (713)743-4285

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Fax: (713)743-4260

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http://www.cive.uh.edu/faculty/hu

18 19 20

Revised: July 2018

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ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

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Phosphate is added to Pb-contaminated soils to induce lead immobilization through lead

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phosphate precipitation. Organic coatings on soils, which may affect heterogeneous lead

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phosphate nucleation, can impact the effectiveness of lead immobilization. Here, SiO2 surfaces

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were coated with silanol self-assembled thin films terminated with -COOH and -OH functional

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groups to act as model organic coatings on soil particles. Using grazing incidence small-angle X-

29

ray scattering (GISAXS), heterogeneous lead phosphate nucleation on coatings was measured

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from mixed Pb(NO3)2 and Na2HPO4/NaH2PO4 solutions at pH 7 with varied ionic strengths (IS =

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0.58, 4, and 11 mM). Raman spectroscopy identified the homogeneous precipitates in solution as

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hydroxylpyromorphite (Pb5(PO4)3OH). The smallest lead phosphate nuclei (4.5 ± 0.5 nm) were

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observed on –COOH coatings, which resulted from the highest level of lead and phosphate ion

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adsorption on –COOH coatings. The IS of the solution also affected the sizes of the

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heterogeneous precipitates on –COOH coating, with smaller nuclei (1.3 ± 0.4 nm) forming under

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higher IS (4 and 11 mM). This study provided new findings that can improve our understanding

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of lead immobilization in contaminated soil environments.

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Key Words: heterogeneous nucleation, hydroxylpyromorphite, organic coatings, critical nuclei

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sizes, recrystallization, ion adsorption, local supersaturation near substrates

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1. Introduction

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Lead contamination in environmental systems poses a threat to human health, especially

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that of children and pregnant women.1 Lead can cause damage to the nervous, skeletal,

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circulatory, enzymatic, endocrine, and immune systems.1 Due to health concerns, the United

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States Environmental Protection Agency (USEPA) set a screening level (SL) of 400 mg/kg for

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lead in residential soil.2 However, instances in which lead level exceeds these regulation levels

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are not rare. In natural soil systems, lead contamination can result from geological as well as

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various industrial processes, including mining, ore processing, smelting, sport and military

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shooting, past use of leaded gasoline, and the disposal of lead-bearing wastes.3, 4 Dissolved and

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particulate Pb in various chemical forms (e.g., oxides, carbonates, hydroxides, and sulfates) has

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been found in contaminated soils.5-7 At the original Furnace Creek Site (Missouri), for 510 out of

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2180 collected soil samples, the lead levels exceeded 400 mg/kg with some even greater than

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1200 mg/kg.8 In Eastern Missouri, the lead content in soils collected near two lead smelters and

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two lead mines can be as high as 62,000 ppm.9

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To remediate Pb-contaminated soil, Pb immobilization through phosphate addition is

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currently one of the most cost-effective methods.3, 4, 10 After phosphate addition, lead phosphate

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solids (i.e., Pb5(PO4)3X, with X = OH, Cl, or F) with low solubility and bioavailability can form,

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effectively reducing the dissolved Pb concentrations.3, 4, 10, 11 The precipitation of lead phosphate

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particles starts with nucleation. In fact, lead phosphate nucleation in solution (as homogeneous

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nucleation) and on substrates (as heterogeneous nucleation) can occur simultaneously. The sizes

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of the nuclei can then increase through growth and/or aggregation processes. As illustrated in

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Figure S-1 in Supporting Information, the heterogeneously nucleated lead phosphate precipitates

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can be immobilized on soils; meanwhile, the homogeneously precipitated lead phosphate

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precipitates can suspend in solution and transported with flow before they deposit onto soils. To

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better predict and control the fate of lead after phosphate addition, it is essential to study the

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heterogeneous nucleation of lead phosphate on substrates.

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Previous studies have found that water chemistry, including the solutions’ supersaturation

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level, pH, ionic strength (IS), and the presence of impurity ions (e.g., Ca2+, Mg2+, Cl-, and F-) can

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significantly affect lead phosphate precipitation.5, 12-14 A few studies have reported that dissolved

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organic matter can affect the homogeneous precipitation of lead-containing minerals in solution

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through three possible mechanisms: (1) formation of organic complexes with dissolved Pb,

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thereby increasing its solubility; (2) adsorption onto lead-bearing particles thereby inhibiting

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their growth; and (3) adsorption onto lead-bearing particles thereby increasing their colloidal

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stability and transport.15-18 Organic coatings on soils are common. However, due to the technical

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difficulty to probe the nanoscale interfacial process, heterogeneous nucleation of lead phosphate

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on bare and organic-coated soils has not been investigated yet.

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To fill the important information gaps, the main objectives of this study were 1) to

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monitor heterogeneous lead phosphate nucleation at organic-water interfaces using the

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synchrotron-based grazing-incidence small-angle X-ray scattering (GISAXS); and 2) to explore

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controlling mechanisms through the integration of multiple interfacial characterization

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techniques, including Raman spectroscopy, quartz crystal microbalance with dissipation (QCM-

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D), dynamic light scattering (DLS), and inductively coupled plasma mass spectrometry (ICP-

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MS), to characterize the mineral phase, zeta potential, and compositions of the heterogeneous

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precipitates, as well as to explore the interaction among the aqueous ions, the lead phosphate

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particles, and the substrates.

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2. Experimental Section

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2.1. Substrate Preparation

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One of the main components of soil is SiO2, most entirely in the crystalline form of

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quartz.19 Glass (>72.2 wt.% SiO2) was chosen here as the substrate, as it simulates the SiOH

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groups of quartz but does not suffer from possible crystallographically controlled preferential

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adsorption. Small pieces (10 mm × 10 mm × 1 mm) of glass substrate (> 72.2 wt.% SiO2) were

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cleaned with acetone followed by concentrated sulfuric acid to remove any initial organic

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compounds.20-26 Self-assembled thin films terminated with these different functional groups have

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been widely used to represent model organic-water interfaces.21, 27

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COOH and –OH with quite different pKa values (4.2-4.8 and 17, respectively) are the main

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components of common natural organic matters (i.e., humic and fulvic acids).29-33 Here, silanol

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self-assembled thin films terminated with these common functional groups (i.e., -COOH and -

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OH) were coated on glass (SiO2) as model organic coatings on soils. To coat glass substrates

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with self-assembled organic thin-films terminated with different functional group (-COOH and-

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OH), clean glass substrates were submerged in a mixed solution of 20 wt % (3-

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triethoxysilyl)propylsuccinic anhydride

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polydimethylsiloxane (as “-OH” precursor) and 80 wt % toluene for 24 hours. The organic-

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coated glass substrates were then rinsed with acetone multiple times to remove the excess

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organics, and dried with N2 gas. After coating, the silanol head groups of these precursor

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chemicals were firmly bonded to the glass surfaces, and the functional groups of –COOH and –

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OH were exposed to the aqueous solution. The successful coating was verified by changes in

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water contact angles (Table 2 and Figure S-1 in Supporting Information).

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2.2. Solution Conditions for Lead Phosphate Precipitation

(as

28

Functional groups of –

“-COOH” precursor) or silanol-terminated

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To separately investigate the effects of organic coatings and ionic strength (IS) on

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heterogeneous nucleation, two sets of lead phosphate precipitation experiments were conducted.

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For set I, lead phosphate precipitation on bare and organic-coated (-COOH and -OH) SiO2 was

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conducted under IS = 0.58 mM (Table 1, solution #1). For set II, lead phosphate precipitation on

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–COOH coatings was conducted under IS = 4 and 11 mM (Table 1, solutions #2 and #3).

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Table 1. Experimental Conditions

Solution #1 #2 #3

TPb, µM 3.00 3.20 3.40

TP, µM 300 320 340

NO3-, µM 6.00 3010 10000

Na+, µM 471 3510 10600

ISa, M 0.58 4 11

pHb

SIc

6.94 6.96 7.04

12.7 12.6 12.7

118 119

Note: All solutions were prepared using CO2 equilibrated ultrapure water, so all Geochemist’s

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Workbench (GWB) calculations were set to be in equilibrium with atmospheric CO2. ISa: Ionic

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strength. pHb: GWB calculated pH values, which were consistent with measured pH (7.0 ± 0.1).

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SIc: Saturation indices with respect to hydroxylpyromorphite. SI= Log (Q/Ksp), where Q is the

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actual dissolved composition, and Ksp(hydroxylpyromorphite) = 10-62.79 at 25°C was used for SI

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calculations based on GWB database. Extended Debye-Hückel equation was used to determine

125

the activities of ions.

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To prepare the solutions (Table 1, solutions #1-3) for lead phosphate precipitation, stock

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solutions of 1 mM lead nitrate (Pb(NO3)2), 10 mM disodium hydrogen phosphate (Na2HPO4), 10

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mM monosodium phosphate (NaH2PO4), and 100 mM sodium nitrate (NaNO3) were prepared

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using ultrapure water (resistivity > 18 MΩ-cm) equilibrated with the atmosphere (by purging

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with air overnight). Then stock solutions of Pb(NO3)2, Na2HPO4, NaH2PO4, NaNO3, and

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ultrapure water were mixed to provide the desired concentrations (Table 1) for a given

6


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experiment. For all solutions (#1-#3), the pH values were 7.0, as controlled by the ratios of

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Na2HPO4 and NaH2PO4 added. NaNO3 was added to attain the desired ionic strengths of the

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solutions (IS = 0.58, 4 and 11 mM). The activity coefficients of the aqueous species were

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calculated using the extended Debye Hückel equation, and equations 1 and 2 were used to

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calculate the solution’s saturation index (SI) with respect to hydroxylpyromorphite

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(Pb5(PO4)3OH): =

(1)

= { } { } { } (2) 138

Where, Q is the ion product; Ksp is the solubility, being 10-62.79 at 25 °C; {Pb2+},{PO43-},

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and {OH-} are the activities of Pb2+, PO43-, and H+ ions. The solutions’ pH, ionic strength (IS),

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and saturation index (SI) with respect to hydroxylpyromorphite were calculated using

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Geochemist’s Workbench (GWB, Table 1) with the Minteq database. All solutions had a SI of

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12.7 with respect to hydroxylpyromorphite. Detailed information about the chemical reactions

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and equilibrium constants used in the Minteq database is provided in Table S-1 in Supporting

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Information.

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2.3.In situ Grazing-Incidence Small-Angle X-ray Scattering (GISAXS)

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The heterogeneous nucleation of lead phosphate nanoparticles at organic-water interfaces

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was measured using GISAXS at Beamline 12 ID-B of the Advanced Photon Source (APS),

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Argonne National Laboratory (Argonne, IL). Before each experiment, a piece of bare or coated

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glass substrate was placed in a custom GISAXS cell. The substrate surface was first aligned to be

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parallel with the incident X-ray beam in the horizontal direction, and the substrate was then

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adjusted vertically to be at the center of the X-ray beam.34 GISAXS image data were collected at

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an X-ray incidence angle of 0.1o for 20 seconds per image. GISAXS data were first collected in 7



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the presence of ultrapure water to provide background scattering data to be subtracted from the

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in situ scattering data. For in situ measurements, after injecting 0.6 mL of mixed solution (Table

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1) into the cell, homogeneous (in solution) and heterogeneous (on substrates) lead phosphate

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nucleation occurred simultaneously, and data collected started immediately. To reduce X-ray

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damage, GISAXS data were collected with 20-s X-ray exposures every 4 minutes. In addition,

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the cell was moved 0.4 mm horizontally after collecting one GISAXS image, so every GISAXS

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image was collected at a fresh spot.

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During a 30-minute measurement, heterogeneous nucleation and deposition of

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homogeneously nucleated precipitates from solution can occur simultaneously on substrates,

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both contributing scattering signals to the collected GISAXS data. To distinguish (a) the

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deposition of homogeneously nucleated lead phosphate precipitates from (b) heterogeneous

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nucleated lead phosphate precipitates on these coatings, GISAXS “deposition only” experiments

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were also performed: solutions (Table 1) were freshly prepared and left to sit for 30 min for lead

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phosphate precipitation to reach equilibrium (more detailed discussion is available in Supporting

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Information). Then 0.6 mL of the equilibrated solution containing homogeneously nucleated lead

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phosphate precipitates was injected into the GISAXS cell, and scattering caused by the particles

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deposited from solution onto bare and organic- (i.e., -COOH and -OH,) coated SiO2 was

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measured. All GISAXS measurements were conducted in duplicate and good reproducibility was

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attained.

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GISAXS data were analyzed using Igor Pro 6.37 and the Irena software package.35, 36 The

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one-dimensional (1-D) scattering curves (i.e., scattering intensity (I) vs. wave vector (q, Å-1))

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were cut from the background-subtracted 2-D images along the Yoneda wing, where the X-ray

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scattering signal is the strongest because of the Vineyard effect.37, 8


38

Particle sizes of lead

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phosphate on substrates were obtained by fitting these 1-D scattering curves with a non-

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interacting spherical particle model using the Irena software package, with average ± standard

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deviation values calculated from the duplicate experiments.35

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Supplementary to the GISAXS measurements, atomic force microscopy (AFM, Veeco

180

Company) was also employed to measure the sizes of particles formed on dried substrates at the

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end of the precipitation experiments.

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2.4.Composition and Phase Characterization of Lead Phosphate Precipitates

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To prepare enough homogeneously nucleated precipitates for compositional analysis and

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phase identification, freshly prepared solutions (Table 1) with a total volume of 1 L were left to

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sit for 30 min to allow for lead phosphate precipitation. After 30 min, the lead phosphate

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particles formed in solution were collected with centrifugal filters (Amicon-15, Millipore) at a

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speed of 5000 rpm, and the dissolved Pb concentration in the filtrate was measured by

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inductively coupled plasma mass spectrometry (ICP-MS, ELAN DRC II). Then ultrapure water

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was used to rinse the slurry collected on the filter multiple times to remove the residual ions.

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For compositional analysis of the lead phosphate particles (homogeneous precipitates),

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the collected slurry with lead phosphate particles (homogeneous precipitates) was digested by 5

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mL 2% HNO3 to complete dissolve the particles. The dissolved Pb and P concentrations were

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measured by ICP-MS, and the atomic ratios of Pb/P (RPb/P) in the homogeneous precipitates were

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calculated. For phase identification of the homogeneously nucleated precipitates, the collected

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lead phosphate slurry was transferred to a clean glass slide for Raman analysis. Raman

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spectroscopy measurements were conducted within 1 week of sample preparation. Details of

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Raman spectroscopic measurements can be found in the Supporting Information. For the

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heterogeneously nucleated precipitates of lead phosphate particles on substrates, their amounts 9



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were too small to be detected by Raman spectroscopy, grazing-incidence wide-angle X-ray

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scattering (GI-WAXS) at Advanced Photon Source (APS), or ICP-MS.

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2.5. Dynamic Light Scattering (DLS) Measurements

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The zeta potential values of homogeneously nucleated precipitates were measured using

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DLS (Zetasizer, Malvern Instrument Ltd.). Zeta potential values of precipitates (ζ, mV) formed

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in solutions were measured in a 1 mL suspension injected into a cuvette (ZEN0040, Malvern

205

Instrument Ltd.) capped by the dip cell (ZEN1002, Malvern Instrument Ltd.). Zeta potential data

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were recorded every 1 min for 20 min. Average zeta potential values (ζ, mV) and standard

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deviations were calculated after the readings became stable.

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Surface zeta potentials of the substrates were also measured. Glass substrates (4 mm × 7

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mm) were coated with organic thin films (i.e., -COOH and -OH) as described earlier. Then, bare

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or organic-coated glass was attached to the bottom of a sample holder (ZEN4060, Malvern

211

Instruments) and placed into a cuvette (ZEN0040, Malvern Instruments) capped by the surface-

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zeta cell (ZEN1020, Malvern Instruments) containing ~ 1 mL tracer solution (Graphene oxide,

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GO, pH = 7.0 ± 0.1). Before each measurement, the substrate surface was aligned to the laser

214

beam. Then zeta potential values of the GO tracer suspensions at distances of 125, 250, 500, 750,

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and 1000 µm from the substrate surface were measured in order to calculate the surface zeta

216

potential values of the substrates. More detailed information about surface zeta potential

217

measurements can be found in our previous work.20, 21, 39, 40

218

2.6. Quartz Crystal Microbalance with Dissipation (QCM-D) Measurements

219

QCM-D measurements were conducted to measure the potential adsorption of lead and

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phosphate ions onto bare and organic-coated substrates. The SiO2 sensors (QSX 303, Q-Sense)

221

were coated with organic thin films (i.e., -COOH and -OH terminated) as described earlier. Then, 10


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the bare or organic-coated SiO2 sensor was placed in a QCM-D flow module, and installed in the

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QCM-D chamber (the temperature was set to 20 ± 0.1 °C, Biolin Scientific). Initially, ultrapure

224

water was pumped through the chamber with a flow rate of 0.2 mL/min until a stable baseline

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was established. The stable baseline indicated that these organic coatings were firmly bonded to

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SiO2 surfaces and could not be washed off by ultrapure water. Then, a test solution (pH = 7.0 ±

227

0.1) of 0.1 mM Pb(NO3)2, 0.1 mM Na2HPO4, or 0.2 mM NaNO3 (control experiment) was

228

pumped into the module at a flow rate of 0.2 mL/min. Changes in vibrational frequency (∆f) and

229

energy dissipation (∆D) of the sensors were recorded, and the Sauerbrey model was used to

230

calculate the mass evolution on the sensors caused by solute adsorption (Q-Tools 3.0, Q-sense).

231

Detailed information about the mass calculations can be found in our previous publications.20, 21,

232

41, 42

233

3. Results and Discussion

234

3.1. Phase and Composition of the Precipitates

235

Based on GWB calculation, at 25 °C, the solutions were all undersaturated with respect

236

to lead carbonates (i.e., cerussite, PbCO3, and hydrocerussite, Pb3(CO3)2(OH)2), lead hydroxide

237

(i.e., Pb(OH)2), and lead oxides (i.e., PbO, litharge and massicot). Meanwhile, the solutions were

238

all supersaturated with respect to Pb3(PO4)2 (SI = 7.3-7.5), PbHPO4 (SI = 1.6-1.7), and

239

hydroxylpyromorphite (Pb5(PO4)3OH, SI = 12.7). Based on the calculated SI values of different

240

lead phosphate phases, hydroxylpyromorphite (Pb5(PO4)3OH) is the most thermodynamically

241

favorable phase. Several previous studies showed that the addition of phosphate to Pb(NO3)2

242

solutions can form hydroxylpyromorphite (Pb5(PO4)3OH) at room temperature, and the

243

formation of PbHPO4 and Pb3(PO4)2 phases required higher temperature (80 °C) or the use of

244

lead acetate.14, 43, 44

11



245

To identify the phases and compositions of homogeneously nucleated lead phosphate

246

particles formed in solutions, Raman spectroscopy and ICP-MS were employed. Based on

247

Raman spectroscopy, the homogeneously nucleated precipitates formed in solutions had spectra

248

with the characteristic peaks of hydroxylpyromorphite (Pb5(PO4)3OH). As shown in Figure 1, the

249

peak at 3565 cm-1 represents the OH stretching band, and the peaks at 924 and 950 cm-1

250

represent P-O stretching bands in hydroxylpyromorphite. Based on ICP-MS measurements of

251

digested solids, the atomic ratios of Pb/P in the lead phosphate particles were 1.67 ± 0.07 under

252

all experimental conditions. These values were identical to the ratios of Pb/P in

253

hydroxylpyromorphite (Pb5(PO4)3OH, Pb/P = 1.67), and they were different from the ratios in

254

PbHPO4 (Pb/P = 1) and Pb3(PO4)2 (Pb/P = 1.5). Therefore, both Raman spectroscopy and ICP-

255

MS measurements indicated the formation of hydroxylpyromorphite, which is one of the most

256

common lead phosphate forms (i.e., Pb5(PO4)3X, with X = OH, Cl, or F) in natural soil

257

environment.14, 43, 44

258 12


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259

Figure 1. Raman spectroscopy of hydroxylpyromorphite standard sample (Figure A), as well as

260

lead phosphate particles formed in solutions (Figures B-D) with IS = 0.58 (B), 4 (C), and

261

11 mM (D).

262 263

3.2. Promoted Heterogeneous Nucleation of Lead Phosphate Nanoparticles on OrganicCoated Substrates

264

The GISAXS scattering profiles caused by particles on substrates during the precipitation

265

experiments (“simultaneous heterogeneous nucleation & deposition of homogeneously nucleated

266

precipitates”) are displayed in Figure 2. For the scattering curves caused by particles formed on

267

bare SiO2, only straight lines (i.e., no knee) were observed over the whole q range (0.005 – 0.12

268

Å-1). This type of scattering curve indicated the formation of large particles beyond the size

269

range (radius of ~1-30 nm) with the current setup. Such observations were generally consistent

270

with AFM measurements: at the end of 30 min precipitation experiments, the radius of dried

271

particles on SiO2 was ~20 nm (Figure S-6 in Supporting Information). The radius measured by

272

ex situ AFM was smaller than that measured by in situ GISAXS (radius > 30 nm), which was

273

probably due to particle dehydration before AFM measurements. On organic-coated substrates,

274

the knees of scattering curves (at q ~ 0.03 Å-1 and ~ 0.05 Å-1 for –OH and–COOH coated

275

substrates) caused by particles on organic-coated substrates were observed, indicating the

276

formation of small particles that can be quantified by GISAXS. By fitting the scattering curves

277

(fitted lines are shown as black curves in Figure 2), the radii (R) of lead phosphate particles on –

278

COOH and -OH coatings remained unchanged (i.e., 4.5 ± 0.5 and 7.0 ± 0.4 nm within 30 min,

279

respectively), indicating no growth of particles on organic coated substrates). The organic

280

coatings increased the surface roughness of the substrates, making it difficult to observe the

281

small nuclei formed on coatings at the end of the precipitation experiments by AFM. For

13



282

scattering curves caused by particles on organic-coated SiO2, straight lines were also observed

283

over low-q range (0.005 – 0.015 Å-1), indicating that large particles (R > 30 nm) also formed.

284

Therefore, we hypothesized that the large particles (R > 30 nm) on both bare and organic-coated

285

glasses were mainly deposits of homogeneously nucleated particles from solution, whereas the

286

small particles (R = ~ 4.5 and ~ 7.0 nm) on organic-coated glasses were formed on the surface by

287

heterogeneous nucleation.

A. SiO2 2 min 10 min 14 min 18 min 22 min 26 min

2

1000 4 2

100 4 2

10 5 6

2

0.01

288

B. -OH

3

q_xy (Å-1)

2

1000 4 2

100 4 2

10

7.0 ± 0.4 nm

5 6 7

4 5 6

0.1

C. -COOH

4

Intensity (counts)

4

Intensity (counts)

Intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

2

3

2

1000 4 2

100 4 2

10

4.5 ± 0.5 nm

5 67

4 5 6 7

0.01

4

0.1

q_xy (Å-1)

2

3

4 5 67

0.01

0.1

q_xy (Å-1)

289

Figure 2. GISAXS (A-C) scattering intensities generated by lead phosphate particles formed on

290

glass (A), -OH (B), and –COOH (C) coatings under IS = 0.58 mM and pH = 7.0. The

291

knee positions of scattering curves (indicated by red boxes) did not change over time,

292

indicating no growth within 30 min. The colored dots are measured data, while the

293

black lines are the fitted curves. To minimize X-ray damage, the scattering curves

294

acquired at different times were collected at fresh spots. Due to the slight

295

heterogeneity among spots, intensity fluctuation among scattering curves is observed

296

for some samples.

297

To test this hypothesis, GISAXS scattering curves generated by particles on different

298

substrates during the precipitation (“simultaneous heterogeneous nucleation & deposition of

299

homogeneously nucleated precipitates”) and the “deposition only” experiments (Figure S-2 in 14


Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

Supporting Information) were compared. On both bare and organic coated SiO2 (Figure S-2), the

301

GISAXS scattering intensities generated by the deposited particles on all substrates during

302

“deposition only” experiments were very similar: all showed only straight lines over our q range,

303

indicating large deposits (R > 30 nm) on all substrates; while no knees of scattering curves over

304

the whole q range were observed, indicating no presence of small nanoparticle (R < 30 nm) on

305

any of the substrate. On bare SiO2 substrate, the GISAXS scattering curves generated during

306

precipitation experiments were similar as “deposition only” experiments, indicating that the lead

307

phosphate particles on bare SiO2 substrates were mainly deposited from solution. More

308

discussion about the limited heterogeneous nucleation of lead phosphate nanoparticles on bare

309

SiO2 is available in the Supporting Information. In contrast, on organic-coated substrates, the

310

determined sizes of the small particles (Figure 2, 4.5 ± 0.5 and 7.0 ± 0.4 nm on –COOH and -OH

311

coatings, respectively) did not change during the 30-min precipitation experiments. This

312

behavior is indicative of small nuclei formation through heterogeneous nucleation without

313

further growth.

314

3.3. Local Supersaturation near Organic-Coated Substrates Affected Sizes of Heterogeneous

315

Nuclei

316

As discussed earlier, the sizes of heterogeneous nuclei on -COOH (4.5 ± 0.5 nm) coatings

317

were much smaller than those on -OH (7.0 ± 0.4 nm) coatings. According to classical nucleation

318

theory (CNT), the sizes of heterogeneous nuclei that form on substrates can be calculated using

319

eqn. (3)45:

320

=

2 !"#"$% & (3) '( ×

15



!"#"$% ,

Page 16 of 31

mJ/m2 is the effective energy barrier for the heterogeneous nucleation of

321

Where,

322

hydroxylpyromorphite

323

hydroxylpyromorphite (Pb5(PO4)3OH), m3/mol; R is the gas constant, J/(mol·K); T is the

324

temperature, K; and SI is the local saturation index with respect to hydroxylpyromorphite near

325

the substrates. Based on eqn. (3), the size of the heterogeneous nuclei could be affected by both

326

the effective energy barrier for heterogeneous nucleation (

327

(SI). For the effective energy barrier (δhetero) for heterogeneous nucleation, it could be affected by

328

both the energy barrier for crystal growth (∆Gr = -2.303 × SI, controlled by solution

329

supersaturation) and the interfacial energy for heterogeneous nucleation (∆Ginterface) (details in

330

Supporting Information). Due to the lack of the interfacial energies for the substrate-precipitate-

331

solution system, ∆Ginterface could not be calculated here (more detailed discussion is available in

332

Supporting Information). Therefore, we considered the potential differences energy barrier for

333

crystal growth (∆Gr), which was controlled by the supersaturation (SI) of local solutions near

334

substrate surfaces with respect to hydroxylpyromorphite. The heterogeneous precipitation

335

experiments on different substrates were conducted with the same solution (solution #1, Table 1).

336

We therefore considered the potential attraction of lead and phosphate ions onto the different

337

substrates, which could affect the level of supersaturation locally in solution volumes near the

338

substrates.

(Pb5(PO4)3OH)

on

substrates;24

v

is

!"#"$% )

the

molar

volume

of

and the local supersaturation

339

The adsorption of lead and phosphate ions onto different substrates under our

340

experimental conditions (pH ~ 7) was measured by QCM-D. As shown in Figure 3, decreases in

341

vibrational frequencies of the organic-coated sensors were observed in 0.1 mM Pb(NO3)2 or

342

NaH2PO4 solutions, indicating mass increases on these sensors caused by ion adsorption. For the

343

control experiment with just 0.2 mM NaNO3 solution, no frequency changes were observed

16


Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


344

(Figure S-3), indicating no Na+ or NO3- adsorption. Thus, the mass increases on sensors in 0.1

345

mM Pb(NO3)2 or NaH2PO4 solutions were caused only by the adsorption of lead or phosphate. As

346

shown in Table 2, QCM-D results showed more lead adsorption on -COOH (91.0 ± 4.8 ng/cm2)

347

than on -OH (22.7 ± 1.7 ng/cm2), and phosphate also adsorbed more to -COOH (22.8 ± 1.4

348

ng/cm2) coated than -OH (11.6 ± 1.1 ng/cm2) coated substrates. Lead ion adsorption on –COOH

349

and –OH coated substrates has been reported before.29-33 A previous study also reported that

350

phosphate ions can be largely adsorbed by the carboxyl (-COOH) functional groups on citric acid

351

coated magnetite nanoparticles.46 Phosphate was also found to adsorb on hydroxylated minerals

352

(e.g., Al(OH)3 and Fe(OH)3) through ligand exchange, or formation of P-O bonds on hydroxyl (-

353

OH) coated silica wafers during calcium phosphate nucleation and growth processes.47, 48 The

354

adsorbed lead and phosphate ions on organic coated substrates might generate preferential sites

355

for heterogeneous nucleation. The –COOH coatings can adsorb higher amounts of both lead and

356

phosphate ions, which might result in more nucleation sites on -COOH coatings as well as higher

357

supersaturation in local solution near the –COOH coatings than near the –OH coatings.

358

According to eqn. (3), the higher local SI in solution near the –COOH coatings resulted in the

359

formation of smaller nuclei on the –COOH coatings. A systematic investigation and better

360

understanding of the lead and phosphate ion adsorption onto the functional groups are important

361

and interesting research direction under our current investigation, but they cannot be thoroughly

362

discussed here and are not the focus of the current paper.

17



A. -OH

B. -COOH 8

4

0.1 mM Pb(NO3)2

∆f (Hz) f (Hz)

2 0 -2

22.7 ± 1.7

∆f (Hz)

6

ng/cm2

-6

0

5

10 15 t t(min) (min)

0

91.0 ± 4.8 ng/cm2

10

20

30

t (min) 2

0.1 mM NaH2PO4 ∆f (Hz)

1

f (Hz)

0.1 mM Pb(NO3)2

-8 0

20

2

0 -1 -2

4

-4

-4

∆f (Hz)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

11.6 ± 1.1 ng/cm2 0

5

10

15

20

0.1 mM NaH2PO4

1 0 -1 -2

22.8 ± 1.4 ng/cm2 0

5

363

10

15

20

t (min) t (min)

t (min)

364

Figure 3. QCM-D measurements of lead and phosphate ion adsorption onto different organic

365

coatings. To establish a stable baseline, measurements were conducted by flowing

366

ultrapure water over sensors coated with organics terminated with different functional

367

groups (–OH and –COOH). Then the inlet solution was switched to 0.1 mM Pb(NO3)2

368

or NaH2PO4 solution (the arrows indicate the switching point). The decreases in

369

frequency (∆f) indicate the adsorption of lead and phosphate ions onto coatings on

370

sensors (ng/cm2). The amounts of lead ions adsorbed onto –OH and –COOH coatings

371

were measured to be 22.7 ± 1.7 and 91.0 ± 4.8 ng/cm2, respectively. The amounts of

372

phosphate ion adsorbed on –OH and –COOH were measured to be 11.6 ± 1.1 and 22.8

373

± 1.4 ng/cm2, respectively.

374 375

The

local

supersaturation

could

potentially

be

affected

not

only

by

ion

adsorption/attraction to substrates, but also by the attraction of pre-nucleation clusters (PNCs) or

18


Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


376

Posner clusters to substrates.49-51 The nucleation pathways of lead phosphate have not been

377

studied. For calcium phosphate, pre-nucleation clusters (PNCs) or Posner clusters were reported

378

as the precursors of nuclei formation.49 The potential formation of pre-nucleation clusters (PNCs)

379

or Posner clusters of lead phosphate was considered here, and the attraction of PNCs or Posner

380

clusters to substrates could be affected by the electrostatic forces between the PNCs or Posner

381

clusters and the substrates.

382

According to Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, the electrostatic

383

forces between particles and substrates can be largely affected by the zeta potentials of the

384

particles and substrates, as well as the Debye length of solution.52 Assuming the electrostatic

385

interactions between PNCs and substrates would be similar to that between the particles and the

386

substrates, we adopted the DLVO theory to explain the potential electrostatic interactions

387

between the lead phosphate PNCs and the substrates. As the precipitation experiments on

388

different substrates were conducted with the same solution (solution #1 in Table 1), the Debye

389

length of the solution was the same. Since the zeta potential of lead phosphate PNCs or Posner

390

clusters cannot be measured by DLS, the zeta potential of lead phosphate nanoparticles (-30.3 ±

391

2.9 mV) was used to represent the zeta potential of PNCs or Posner clusters. The zeta potentials

392

of –OH and -COOH coated glasses were measured to be -71.9 ± 5.4 and -49.6 ± 0.6 mV,

393

respectively. Compared with –OH (-71.9 ± 5.4 mV) coating, –COOH coatings were less

394

negatively charged (-49.6 ± 0.6 mV). Thus, the electrostatic repulsion between the –COOH

395

coated substrate and the negatively charged PNCs or Posner clusters (-30.3 ± 2.9 mV) would be

396

weaker. The local PNC or Posner cluster concentrations near the –COOH coatings therefore

397

could be higher than those near the –OH coatings, leading to a higher level of local

398

supersaturation near the –COOH coatings and thus smaller nuclei on –COOH coatings than on –

19



399

OH coatings. The formation of PNC or Posner clusters of lead phosphate is speculative here, but

400

it suggests a future direction well worth exploring. Nevertheless, consideration of the attraction

401

of lead/phosphate ions or PNCs to the substrates will result in the same trends in the size of

402

heterogeneous nuclei on coatings, consistent with our observations. Another possibility is that

403

lead and phosphate ion adsorption onto coated substrates might change the interfacial energy

404

between solution and substrate (δls), thus affecting lead phosphate heterogeneous nucleation.

405

However, such a possibility could not be tested due to the difficulty in quantifying the changes of

406

solution-substrate interfacial energies due to ion adsorption.

407

3.4. Ionic Strength (IS) Affected Nuclei Sizes on –COOH Substrates

408

The effects of IS on heterogeneous nucleation of lead phosphate (pH = 7) were

409

investigated on –COOH coated substrates using GISAXS. Under higher IS (4 and 11 mM, Figure

410

4), the initial lead phosphate nuclei were much smaller (1.3 ± 0.4 nm) than the nuclei (4.5 ± 0.5

411

nm) formed under low IS (0.58 mM). According to eqn. 3, the nuclei sizes can be affected by

412

changes in the effective energy barrier (δhetero) for heterogeneous nucleation and/or the local

413

supersaturation (SI). For the effective energy barrier (δhetero), it could be affected by both the

414

energy barrier for crystal growth (∆Gr = -2.303 × SI, decreases with the increase of the local

415

supersaturation) and the interfacial energy for heterogeneous nucleation (∆Ginterface). As all these

416

experiments were conducted on the same substrate (-COOH), the same interfacial energy for

417

heterogeneous nucleation (∆Ginterface) was expected in all experiments. Therefore, the decrease in

418

nuclei sizes under high IS conditions should be caused only by the increase in local

419

supersaturation (SI).

420

20


Page 20 of 31

Page 21 of 31

A. 4 mM

B. 11 mM

5

5

10

Intensity (counts)

10

Intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.3 ± 0.4 nm

4

10

3

10

2

10

1

10

5 6

2

0.01

421

3

1.3 ± 0.4 nm

4

10

3

10

2

10

1

4 5 6

q_xy (Å-1-1)

2 min 10 min 14 min 18 min 22 min 26 min

10

0.1

5 6

2

0.01

3

4 5 6

q_xy (Å-1)

0.1

422

Figure 4. GISAXS (A-C) scattering intensities generated by lead phosphate particles formed on

423

–COOH under IS = 4 mM (A) and 11 mM (B) at pH = 7.0. The colored dots are

424

measured data, while the black lines are the fitted curves. Bimodal size distribution

425

was observed, representing large particles (R > 30 nm, out of our measurable range

426

with current set up) and small nuclei (1.3 ± 0.4 nm, represented by red boxes). To

427

minimize X-ray damage of the sample, the scattering curves acquired at different

428

times were collected at fresh spots. Due to the slight heterogeneity among spots,

429

intensity fluctuation among scattering curves is observed.

430

Based on DLVO theory, the ionic strength (IS) of solution can affect the attraction of

431

particles towards substrates.53,

54

432

nanoparticles (representing the charges of PNCs or Posner clusters, Table 2) were similar, being

433

-30.3 ± 2.9, -30.9 ± 1.6, and -29.9 ± 1.6 mV in solutions with IS = 0.58, 4, and 11 mM,

434

respectively. The zeta potentials of the substrates under varied IS (0.58, 4 and 11 mM) also did

435

not show significant differences.

The absolute zeta potential values of lead phosphate

436

Here we assume that the electrostatic interaction between PNCs and substrates is similar

437

to that between particles and substrates, and that the interaction can be explained by DLVO

21



Page 22 of 31

438

theory. The absolute zeta potential values of lead phosphate nanoparticles (representing the

439

charges of PNCs or Posner clusters, Table 2) decreased slightly with the increase of IS, being -

440

30.3 ± 2.9, -30.9 ± 1.6, and -29.9 ± 1.6 mV in solutions with IS = 0.58, 4, and 11 mM,

441

respectively. Under high IS (4 and 11 mM), the electric double layers were also compressed,

442

resulting in a decreased Debye length. Under high IS, decreases in both the absolute zeta

443

potentials of the particles and the Debye length could result in the decreased electrostatic

444

repulsive forces between the negatively charged PNCs or Posner clusters and –COOH coatings.

445

Accordingly, more lead phosphate PNCs or Posner clusters can be attracted to –COOH surfaces,

446

which would increase the local supersaturation level (SI) of lead phosphate near the –COOH

447

substrates. According to eqn. 3, smaller nuclei should form under high IS (4 and 11 mM), as

448

observed.

449

Table 2. Properties of substrates Sample Name Contact angle

ζa, mV

Mleadb, ng/cm2 M phosphate b, ng/cm2

SiO2

16.0 ± 4.3 °

-48.8 ± 1.4

25.3 ± 3.9

BDLc

-OH

43.7 ± 4.1 °

-71.9 ± 5.4

22.7 ± 1.7

11.6 ± 1.1

-COOH

22.1± 0.5 °

-49.6 ± 0.6

91.0 ± 4.8

18.9 ± 6.9

450 451

Note: ζa : surface zeta potential of substrates, measured by DLS; Mleadb and M phosphate b: masses

452

of lead and phosphate ions adsorbed on substrates measured by QCM-D; BDLc: below detection

453

limit.

454

3.5. The Effects of Ionic Strength (IS) on Recrystallization

455

Under high IS (4 and 11 mM), with ongoing reaction, the GISAXS scattering peaks

456

caused by the small nuclei (1.3 ± 0.4 nm, q ~ 0.7 Å-1) disappeared (Figure 4). The disappearance

457

of the small heterogeneous nuclei has been observed in the ferrihydrite system before, as one 22


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


458

type of recrystallization (i.e., Ostwald ripening) process, i.e., the dissolution of small particles

459

coincident with the growth of large particles.23-25 Recrystallization was attributed to the presence

460

of particles with different sizes and the large solubility difference in nanoparticles of different

461

sizes. The size-dependent solubility of nanoparticles can be calculated according to eqn. (4).

462

55-57

463

,

"+ = "+ exp

464

23,

2 0 &

(4) '(

, Where, "+ is the size-dependent solubility of lead phosphate nanoparticles; "+ is the

= 10-62.79 at 25°C);

0 is

465

solubility for large hydroxylpyromorphite crystals (Ksp,

466

interfacial energy for hydroxylpyromorphite in water, mJ/m2; v is the molar volume of lead

467

phosphate particles, m3/mol; r is the radius of lead phosphate nanoparticles, nm; R is the gas

468

constant, J/mol·K; and T is temperature, K.

Pb5(PO4)3OH

the

469

Bimodal size distributions of lead phosphate particles on –COOH coatings under high IS

470

(4 and 11 mM) were observed. As shown in Figure 4, under high IS (4 and 11 mM), besides the

471

scattering peaks of the ~ 1 nm small nuclei, straight lines with high intensities at low q range

472

were also observed, indicating the existence of large deposits (R > 30 nm, out of GISAXS

473

measurable range) on substrates as well. The large particles together with the small ~ 1 nm

474

heterogeneous nuclei resulted in a bimodal size distribution on –COOH coatings. Based on

475

Equation (4), the small nuclei (R ~ 1 nm) had a much higher solubility than the large deposits

476

(>30 nm), resulting in higher local lead and phosphate ion concentrations in solution near the

477

small nuclei. The concentration gradient in local solute concentrations can cause the diffusion of

478

ions from the small nuclei to the large deposits, leading to dissolution of the small nuclei, as

23



479

observed by GISAXS. Under high IS (4 and 11 mM), the presence of the small nuclei (1.3 ± 0.4

480

nm) and the large deposits resulted in the fast recrystallization process.

481

4. Conclusions

482

In this study, heterogeneous lead phosphate nucleation on bare and organic-coated SiO2

483

substrates terminated with common functional groups (i.e., -COOH and –OH) was investigated.

484

The particles formed in all solutions were identified as the thermodynamically most stable phase

485

of lead phosphate, i.e., hydroxylpyromorphite (Pb5(PO4)3OH). Compared with bare SiO2, organic

486

coatings promoted heterogeneous nucleation, and the sizes of lead phosphate heterogeneous

487

nuclei formed on organic coatings were quite different, being 4.5 ± 0.5 nm on -COOH and 7.0 ±

488

0.4 nm on –OH coatings. The functional groups of the organic coatings (-COOH and –OH) could

489

attract lead and phosphate ions, possibly the lead phosphate PNCs, from the bulk solution

490

towards substrates. Thus the organic coatings significantly altered the supersaturation levels of

491

lead phosphate in local solutions near substrates, affecting heterogeneous lead phosphate

492

nucleation on substrates.

493

Furthermore, heterogeneous lead phosphate nucleation on –COOH coatings was explored

494

under three different IS conditions (i.e., 0.58, 4, and 11 mM). On –COOH coatings, the nuclei

495

formed under higher IS (IS = 4 and 11 mM) had sizes of ~ 1 nm, much smaller than those (~ 4

496

nm) formed under lower IS (IS = 0.58 mM). This may be due to the reduced electrostatic

497

repulsive forces between PNCs or Posner clusters and a –COOH coating under higher IS, which

498

can result in higher supersaturation near the –COOH coated substrates, leading to the smaller

499

nuclei at higher IS.

500

This study provided new fundamental knowledge of the nanoscale interfacial processes

501

controlling heterogeneous lead phosphate nucleation. For phosphate addition to remediate lead

24


Page 24 of 31

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


502

contaminated soils, the lead phosphate precipitates could form in solution (by homogeneous

503

nucleation) and potentially be transported with the flow of water through the soil, or the

504

precipitates could be immobilized on soils at the onset of their formation through heterogeneous

505

nucleation and deposition. Our investigation showed promotion of heterogeneous nucleation on

506

organic coatings over bare sand, indicating that the organic coatings on soils could potentially

507

improve the effectiveness of lead immobilization by phosphate addition. A higher ionic strength

508

of the pore solution could also promote both heterogeneous nucleation and deposition of lead

509

phosphate particles, thus improving lead immobilization by phosphate addition.

510

Acknowledgments

511

This work was supported by the National Science Foundation (NSF, Awards # 1604042

512

and #1603717). We thank Dr. Soenke Seifert for helping with preliminary GISAXS experiments

513

at beamline 12-ID-C, Advanced Photon Source (APS), Argonne National Laboratory. Use of the

514

facilities at beamlines Sector 12-ID-B and 12 ID-C at APS was supported by the US Department

515

of Energy, Office of Science, Office of Basic Energy Science, under Contract No. DE-AC02-

516

06CH11357.

517

Supporting Information

518

Raman spectroscopy, design of “deposition only” experiments, discussion of the limited

519

heterogeneous nucleation on SiO2, the chemical reactions and equilibrium constants in GWB

520

Minteq database (Table S-1), diagram of “homogeneous nucleation” and “heterogeneous

521

nucleation” (Figure S-1), contact angle measurements (Figure S-2), GISAXS scattering curves of

522

“deposition only” experiments on different substrates (Figure S-3), QCM-D control experiments

523

with 0.2 mM NaNO3 (Figure S-4), QCM-D measurements of lead and phosphate ion adsorption

25



524

on SiO2 (Figure S-5), and AFM measurements (Figure S-6). This material is available free of

525

charge via the Internet at http://pubs.acs.org.

526

26


Page 26 of 31

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for TOC only

Electrostatic interactions Lead or phosphate ion adsorption Local supersaturation near substrates Nuclei ~ 7 nm

Nuclei ~ 4 nm

-OH

-COOH SiO2 substrate

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Heterogeneous lead phosphate nucleation at organic-water interfaces

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