Imaging Organophosphate and Pyrophosphate Sequestration on

(12, 13) Phytate has also been detected in plant root exudates. ... Ex situ dissolution experiments (0.1–1.0 mM phytate at pH 2.7–3.7, 1.0 mM ...
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Imaging Organophosphate and Pyrophosphate Sequestration on Brucite by in Situ Atomic Force Microscopy Lijun Wang, Christine V Putnis, Helen E. King, Jörn Hövelmann, Encarnacion Ruiz-Agudo, and Andrew Putnis Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05456 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 9, 2016

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Environmental Science & Technology

Imaging Organophosphate and Pyrophosphate Sequestration on Brucite

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by in Situ Atomic Force Microscopy

3 Lijun Wang,*,† Christine V. Putnis,*,‡,# Helen E. King,§ Jörn Hövelmann,∆ Encarnación

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Ruiz-Agudo,ǁ and Andrew Putnis‡,┴

5 6 †

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College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China ‡

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#

9 §

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Institut für Mineralogie, University of Münster, 48149 Münster, Germany



Department of Chemistry, Curtin University, Perth, WA 6845, Australia

Department of Earth Sciences, Utrecht University, 3584 CD Utrecht, The Netherlands

Interface-Geochemistry, Geoforschungszentrum Potsdam,Telegrafenberg, 14473 Potsdam, Germany ǁ

Department of Mineralogy and Petrology, University of Granada, Fuentenueva s/n 18071, Spain ┴

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The Institute for Geoscience Research (TIGeR), Curtin University, Perth, WA 6102, Australia

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Corresponding authors

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*College of Resources and Environment, Huazhong Agricultural University, 1 Shizishan St.,

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Wuhan 430070, China. Phone/Fax: +86-27-87288382. E-mail: [email protected] (Lijun

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Wang).

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Institut für Mineralogie, University of Münster, 48149 Münster, Germany. Phone

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+49-251-8333454, Fax: +49-251-8338397. E-mail: [email protected] (Christine V.

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Putnis)

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Figures: 7 figures = 7 ×300 = 2100 word eq

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Words: max 7000-2100 = 4900

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Total words: 4577 (the word number of references not included) 1

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ABSTRACT

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In order to evaluate the organic phosphorus (OP) and pyrophosphate (PyroP) cycle and their

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fate in the environment, it is critical to understand the effects of mineral interfaces on the

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reactivity of adsorption and precipitation of OP and PyroP. Here, in situ atomic force

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microscopy (AFM) is used to directly observe the kinetics of coupled dissolution-precipitation

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on cleaved (001) surfaces of brucite [Mg(OH)2] in the presence of phytate,

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glucose-6-phosphate (G6P) and pyrophosphate, respectively. AFM results show that the

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relative order of contribution to mineral surface adsorption and precipitation is phytate >

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pyrophosphate > G6P under the same solution conditions and can be quantified by the

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induction time of OP/PyroP-Mg nucleation in a boundary layer at the brucite-water interface.

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Calculations of solution speciation during brucite dissolution in the presence of phytate or

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pyrophosphate at acidic pH conditions show that the solutions may reach supersaturation with

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respect to Mg5H2Phytate.6H2O as a Mg-phytate phase or Mg2P2O7 as a Mg-pyrophosphate

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phase that becomes thermodynamically stable before equilibrium with brucite is reached. This

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is consistent with AFM dynamic observations for the new phase formations on brucite. Direct

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nanoscale observations of the transformation of adsorption/complexation-surface precipitation,

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combined with spectroscopic characterizations and species simulations may improve the

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mechanistic understanding of organophosphate and pyrophosphate sequestration by mineral

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replacement reactions through a mechanism of coupled dissolution-precipitation occurring at

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mineral-solution interfaces in the environment.

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INTRODUCTION

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Phosphorus (P) is a major growth limiting nutrient of plants and other organisms, and 2

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elevated P inputs into soil solutions and aquatic environments leads to eutrophication,

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therefore increasing the risk of environmental pollution.1 In waters and sediments, dissolved

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or particulate organic phosphorus (OP) is found in similar amounts to inorganic P.2 Moreover,

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OP was recently proposed as a potential pool for bioavailable P, promoting much research

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regarding OP decomposition,3 fractionation,4,5 and characterization.6

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OP forms include phosphonates, myo-inositol hexakisphosphate, and orthophosphate

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mono(di)esters.7 Phosphomonoesters in supra-/macro-molecular structures were found to

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account for the majority of soil OP across diverse soils.8 A considerable proportion of OP in

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soils or wetlands occurs as stereoisomers of inositol hexakisphosphate (IP6).9 The most

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abundant myo-IP6 occurs as a phosphorus storage compound in seeds.10,11 When IP6 chelates

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metals such as Ca, Mg, or Fe it forms an insoluble phytate, for example the common cereal

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component phytin (Ca−Mg−K phytate).12,13 Phytate has also been detected in plant root

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exudates.14,15 Glucose-6-phosphate (G6P), a representative sugar monoester, has also been

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detected in soils.16

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In addition to OPs, increased amounts of pyrophosphate form after pyrolysis of raw

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sewage sludge,17 and pyrophosphate is present in aquatic ecosystems including marine

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particulates18

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of pyrophosphate in terrestrial and aquatic P cycles is unclear, though it has been shown to be

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bioavailable even in the presence of high ambient soluble reactive P.19

and estuary sediments,19 and also is common in soil samples.20 The function

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Most inorganic phosphorus (IP) and OP in soils, eutrophic water bodies or sewage

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sludges are present as surface adsorbed species on minerals.21,22 The concentrations of both

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dissolved (DP) and particulate phosphorus (PP) are correlated with the organic component

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concentration23 in sediment containing different mineral components.24 Various OP and IP

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compounds including PyroP in these natural environments exhibit significant differences in

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their chemical behavior with mineral surfaces, reactivity and bioavailability.25 3

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OP and PyroP constitute a major component of soil phosphorus and play a key role in the

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P cycle.26 Therefore, the dynamics of OP and PyroP in the environment,27 particularly the

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kinetics associated with OP/PyroP-mineral surface reactions, including adsorption,

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complexation and precipitation, will play a significant role in the biogeochemistry of these

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molecules and their fate in different ecosystems.27 Although the roles of different mineral

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surfaces in OP and PyroP adsorption and immobilization have been widely studied, little is

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known regarding the kinetics of dissolution and precipitation at the brucite mineral-solution

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interface. As a model mineral, brucite was chosen because 1) it exhibits a simple composition

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and structure and could be an analogue for brucite-like minerals such as hydrotalcite-like

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layered double hydroxides (LDHs that include clay minerals) with a high chemical affinity for

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both OP and PyroP; 2) dissolution and precipitation reactions at the brucite-solution interface

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occur easily within the AFM experimental time frame. Therefore, the objective of this study

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was to observe the kinetic processes of brucite dissolution in the presence of phytate, G6P,

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and pyrophosphate and coupled precipitation. In addition, we aimed to determine how the

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number of phosphate groups in molecules regulates complexation of OP/PyroP-Mg and

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subsequent precipitation, providing insights into the fundamental mineral interfacial

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phenomenon in controlling OP and PyroP sequestration in the environment.

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EXPERIMENTAL SECTION

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Brucite Dissolution. In situ dissolution experiments were performed using a Digital

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Instruments (Bruker) Nanoscope IIIa AFM (Multimode) operating in contact mode. A natural

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optically clear brucite crystal (Norberg, Sweden) was cleaved in order to expose a fresh

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cleavage (001) surface. The brucite was characterized as single phase by X-ray diffraction.

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The solutions of phytate, G6P, and pyrophosphate (0.01 to 5.0 mM) at different pH values 4

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(2.7-9.6), adjusted by the addition of 0.01 M NaOH or HCl, were passed over the cleaved

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brucite crystals in the AFM fluid cell. NaCl (1.0-100.0 mM at pH 7.0) was added to the

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phytate or pyrophosphate solutions (1.0 mM) to observe the effects of ionic strength on

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brucite dissolution. In situ experiments were conducted at a constant flow rate of 1 mL/min to

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ensure surface-controlled reaction rather than diffusion (transport) control. Different locations

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of three different crystals per solution condition were imaged to ensure reproducibility of the

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results. In the present study, the formation of precipitates followed brucite dissolution; thus

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we used the time before the first particles (about 2.0-6.0 nm in height) were observed on the

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brucite surface to characterize the kinetics of nucleation.

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Ex situ dissolution experiments (0.1-1.0 mM phytate at pH 2.7-3.7, 1.0 mM

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pyrophosphate at pH 4.3, and 5.0 mM G6P at pH 3.5) were performed following in situ AFM

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experiments. The reacted brucite samples were removed from the AFM fluid cell and placed

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in a beaker filled with about 10 mL of the different solutions at room temperature for 16-24 h

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in order to observe further precipitation reactions. After 16 h or 24 h, the samples were

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removed from the solution and quickly dried using absorbent paper and then immediately

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imaged in the AFM.

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Raman Spectroscopy. Raman spectra were collected using a WITec Alpha 300R confocal

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Raman spectrometer operating with the 785 nm line of a diode laser. A 50x long working

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distance lens (NA 0.5) was used to obtain spatially constrained information from surface

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precipitates produced during the AFM and batch experiments. As the precipitates were

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sensitive to the laser beam, the laser intensity was lowered until no change was observed

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optically or in the spectrum. Depth scans of the surfaces were taken using with an integration

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time of 3 s. The brucite spectrum was generated from an average of 4 spectra taken from 5

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µm below the surface. After positions were identified with additional peaks not related to

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brucite at the surface, longer collection times were used to help identify the precipitates. 5

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Single spectra were taken for 3 s and integrated 30 times to obtain the best signal to noise

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ratio. All spectra were obtained using a grating of 300 grooves/mm and a pinhole of 20 µm.

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Spectral background removal and peak fitting was conducted using the WITec Project Plus

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software (version 4.0).

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Scanning electron microscopy (SEM). Imaging of the phases precipitated at the brucite

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surfaces was conducted using a FEI Helios Nanolab G3 SEM. Images of the surface

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precipitates were taken at 10 kV acceleration voltage and 0.8 nA probe current.

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PHREEQC Simulations. The PHREEQC program28 was used to simulate the reactions

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between brucite and OP/PyroP solutions using the database wateq.dat. First, we calculated the

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solution speciation using thermodynamic constants (Table S1-S7) during brucite dissolution

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in the presence of 1.0 mM phytate at pH 2.5 and 7.0 by PHREEQC. This provided

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simulations of brucite dissolving until equilibrium with the solution was reached. For the

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pyrophosphate system (1.0 mM) at more acidic pH (4.3) conditions the solution reaches

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saturation with respect to Mg2P2O7 as a Mg-pyrophosphate phase. Moreover, we made further

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simulations for the same solutions of 1.0 mM phytate and pyrophosphate at a fixed pH of 4.3

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or 7.0, while allowing the pH to vary as brucite dissolved.

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RESULTS AND DISCUSSION

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Coupled Dissolution-Precipitation at the Brucite-Solution Interface in the Presence of

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Phytate. Following 329 s of injection of 1 mM phytate (pH 9.0), slow dissolution occurred on

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the exposed (001) surfaces with the formation of shallow, equilateral triangular etch pits with

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a depth of 0.5-1.0 nm (Figure 1A, B). Pit depth increased to about 1.5 nm and 2.5-3.0 nm after

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7 min and 43 min, respectively (Figure 1C, D). The measured depth of shallow etch pits

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(about 0.5 nm) closely resembles the thickness of one unit-cell layer (0. 47 nm) (Figure 1E). 6

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Moreover, according to the brucite sheet structure with edge-sharing Mg(OH)6 octahedra

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(Figure 1E), the equilateral triangular morphology of the etch pits results from the 3-fold

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rotation axis normal to the (001) cleavage surface.29

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When the brucite surface was exposed to 1 mM phytate solutions with 10 mM NaCl at

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pH 7.0, the pit depth gradually increased to about 6.0 nm after 28 min of reaction (Figure 2A,

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B). At the same time, spherical particles (2.0-16.0 nm in height) (Figure 2A, B) and 2-D

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plates (14.0-16.0 nm in thickness) (Figure 2A, C) formed. As the pH of the 1 mM phytate

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solutions was further decreased to 3.0, simultaneous occurrence of shallow and deep (>10 nm)

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dissolution pits was observed following 279 s of dissolution reactions (Figure 2D). Rows of

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etch pits associated with linear defects were evident on areas of the surface that remained free

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of precipitate (Arrows in Figure 2D, E). The brucite surface became heavily pitted after 360 s

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(Figure 2E), and finally, the surface was completely covered by nanoparticles (about 40-60

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nm in height) after 497 s of dissolution (Figure 2F, F’). Dissolution and precipitation was

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occurring simultaneously in a coupled process at the brucite-solution interface.30,31 The

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dissolution of brucite releases Mg2+ ions to the interfacial solution in contact with the brucite

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surface, so that this solution boundary layer becomes supersaturated with respect to a new

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Mg-OP phase.

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It is apparent that phytate-mediated brucite dissolution occurs via a different process

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depending on the pH range. Dissolution at point defects dominated at higher pH (>7.0), and at

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linear defects (including screw defects, line dislocations, and dislocation loops) at lower pH

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( indicates a surface

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site).33,35 Increasing H+ activity results in enhanced protonation of >MgOH0 surface sites and

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the formation of >MgOH2+, thus promoting brucite dissolution.33 In addition, etch pit

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spreading is also promoted by the presence of background electrolytes such as NaHCO3 at a

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constant pH.33

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Step velocity in the presence of phytate perceptibly increased as pits deepened (depth >

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10 nm in Figure 2D), although the relationship between step retreat velocity and pit depth and

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dependence on pH could not be determined in the present AFM experiments. The increase of

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step velocity in phytate solutions would have contributions from the shallower, slow-retreat

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pits at point defects promoted by water and the deeper, fast-retreat pits along linear defects

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promoted by organic ligands such as phytate. These observations are similar to those reported

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for the case of calcite dissolution in the presence of ethylenediamine tetraacetate (EDTA).32

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Linear defects (such as dislocations) penetrate deeply into the mineral structure, resulting in

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deeper pits.32,36,37

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Coupled Dissolution-Precipitation at the Brucite-Solution Interface in the Presence of

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Pyrophosphate. In the presence of 1 mM pyrophosphate at pH 9.6 for 23 h, scarce isolated

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nanoparticles (about 2.0-6.0 nm) were formed along step edges on dissolving brucite surfaces,

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where dissolution occurred through the formation of typical equilateral triangular etch pits

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(Figures 3A, S1A’). When the pH was lowered to 7.0 and the pyrophosphate concentration

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was raised to 5 mM, some particles of size 10-20 nm were present after 43 min of brucite

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dissolution (Figures 3B, S1B’). When the pH was further decreased to 4.3 in 1 mM

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pyrophosphate, the immediate formation of pits associated with linear defects (depths in 8

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2.0-4.0 nm) was observed after 103 s of dissolution (Figures 3C, D, and S1D’). Spherical

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particles (about 2.0-10.0 nm in height) (Figures 3E, S1E’) and conical precipitates (about

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40.0-120.0 nm in height) (Figures 3F, S1F’) were formed after 192 s and 37 min, respectively.

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Coupled Dissolution-Precipitation at the Brucite-Solution Interface in the Presence of

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G6P. In the presence of 1 mM G6P at pH 5.9, dissolution occurred with the formation of

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equilateral triangular etch pits on the brucite (001) surfaces, and their depth gradually

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increased from about 4.0 nm at 13 min, to about 8.0 nm at 41 min, and 8.0-10.0 nm at 81 min

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(Figure 4). At the same time, their sizes (width) also increased (Figure 4A’-C’). However, no

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precipitates were observed even after 17 h of ex situ reactions (Figure S2). As the G6P

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concentration was increased to 5 mM at pH 5.8, precipitates were formed after 18 h in ex situ

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reactions (Figure S3). When the pH of 5 mM G6P solutions was decreased to 3.5, etch pits

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rapidly formed (Figure 5); the brucite surfaces were completely covered by particles (Figure 5)

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with a height of 40-50 nm (Figure S4A, B) after 16 h of dissolution reactions.

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Dynamics of Nucleation and Growth of Mg-OP/PyroP on Brucite. The dissolving brucite

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surfaces in the presence of OPs and pyrophosphate provided a source of Mg2+ ions, resulting

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in supersaturation of the solution in the brucite-solution boundary layers with respect to an

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OP/PyroP-Mg phase.38 This will drive surface-induced nucleation and growth on the brucite

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surface. According to classical nucleation theory (CNT), the steady-state nucleation rate

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(number of nucleation events per square meter per second), J, can be calculated as:39,40

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exp



(1)

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where A is a kinetic constant, depending on physical parameters such as diffusional

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barriers,39,40 ΔG* is the total free-energy cost to form a spherical crystallite, kB is Boltzmann’s

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constant, and T is temperature.39,40 The technical difficulties involved in directly evaluating

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crystal nucleation rates41 have led to an approach to measure the induction period (t) prior to

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nucleation based on the equation J =1/(tV),41 where V is the volume of the entire system (i.e. 9

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fluid + crystal, about 50 μL). In the present study, we used the time required to observe the

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occurrence of the first smallest particles (about 2.0-6.0 nm in height) on brucite to

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characterize the kinetics of nucleation. Moreover, we only compared the induction times for

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phytate and pyrophosphate solutions on brucite surfaces because a relatively longer time was

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required to form nucleated particles for solutions containing G6P (Figure S2). The most likely

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explanation for delayed nucleation in GP6 solutions is that G6P contains the lowest number of

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phosphate groups 1) compared to the other two molecules tested, 2) and hence indicates a

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lower chelation ability. The induction time significantly increased with the increase in pH for

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both phytate and pyrophosphate solutions at 1 mM, and at the same alkaline pH, the induction

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time for pyrophosphate (1 mM) is much longer than that of phytate (1 mM)

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Shorter induction times were observed with increasing phytate or pyrophosphate solution

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concentration (Figure 6B) or NaCl concentration (in 1 mM phytate or pyrophosphate) (Figure

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6C) when the solution pH was kept constant (7.0).

(Figure 6A).

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Mineral dissolution kinetics is complicated in the presence of inorganic/organic ligands42

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and background electrolytes.34,43 Background electrolytes can enhance the mineral dissolution

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rate due to the ability of salt ions to modify water structure as well as solute and mineral

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surface hydration.43,44 The amount of released Mg2+ ions by dissolution is increased in the

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presence of NaCl, thereby resulting in rapid nucleation of a Mg-OP/PyroP phase with shorter

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induction times because a higher degree of supersaturation is reached within the boundary

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layers. This phenomenon has also been observed in the calcite-G6P system.44

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Identification of Precipitates on Brucite (001) Cleavage Surfaces. Using Raman

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spectroscopy, the cleaved brucite surface shows two sharp, intense peaks at 281 and 446 cm-1

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and two broad peaks with centres at 725 and 809 cm-1 (Figure 7A). This is consistent with

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previous analysis of brucite using spectroscopic methods.45 In contrast, precipitates on the

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brucite surface show additional peaks depending on the organophosphate present in the 10

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experiment solution. In experiments with G6P, the precipitates were highly reactive to the

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laser beam and even at the lowest laser intensity were burnt during analysis. However, the

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burnt material produced broad peaks at 1332 and 1575 cm-1 (Figure 7B) that are characteristic

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for amorphous carbon material46 implying the original material was carbon-bearing,

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consistent with the precipitation of an organic-C phase during the experiments. Precipitates on

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the surface of brucite produced during the phytate experiments were stable in the Raman laser

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and produced clear peaks in the Raman spectra (Figure 7C). Additional peaks were observed

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as a broad peak at 544 cm-1, three sharp and one broad peak at 954, 993, 1034 and 1100 cm-1

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and two broad bands at 1282 and 1372 cm-1. These peaks are consistent with known Raman

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peaks for solid samples of phytate47,48 and particularly those between 950 and 1100 cm-1 are

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related to the symmetrical stretch of the phosphate moiety within the phytate molecule.

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However, the peaks are consistently shifted to higher wavenumbers in comparison to previous

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studies by as much as 10 cm-1 compared to the initial sodium salt.49 Such a shift could imply a

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difference in chemistry of the precipitates through the incorporation of Mg released from the

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dissolving brucite, as Mg-acetate has been found to show a shift to higher wavenumbers in

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comparison to the Na-acetate salt.50 No additional bands to those belonging to brucite were

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observed on the pyrophosphate sample surface. This can be explained from the SEM

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observations because the precipitate forms very thin, spread out crystals on the surface

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(Figure S5). In this case the laser beam would not sample enough of the precipitate during

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Raman analysis to produce bands in the spectra, precluding any possible identification of

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other vibrational peaks from any nanometer thin precipitate layer.

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PHREEQC Simulations. Finally, we calculated the solution speciation using thermodynamic

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constants (Table S1-S7) during brucite dissolution in the presence of 1.0 mM phytate at pH

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2.5 and 7.0 by PHREEQC.28 This allowed simulations of brucite dissolving until equilibrium.

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The simulations show that for both initial pH values, the solution (volume of solution is 11

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equivalent to that of the AFM fluid cell, about 35-50 μL) would reach supersaturation with

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respect to Mg5H2Phytate.6H2O as a Mg-phytate phase (Supporting Data 1 and 2). This is

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consistent with the AFM results (Figure 2). For the pyrophosphate system (1.0 mM) at more

279

acidic pH (4.3) the solution reaches saturation with respect to Mg2P2O7 as a

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Mg-pyrophosphate phase, that may become stable before equilibrium with brucite is attained

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(Supporting Data 3). At pH ≥7.0, the solution is saturated with respect to brucite before

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saturation with respect to Mg2P2O7 is attained (Supporting Data 4 and 5). This means for the

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formation of Mg2P2O7 at higher pH more brucite would need to dissolve (to release enough

284

Mg2+) through the addition of higher pyrophosphate concentrations and/or longer reaction

285

times. These simulation results are also consistent with the AFM observations (Figure 3).

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Moreover, we made further simulations for the same solutions of 1.0 mM phytate and

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pyrophosphate at a fixed pH of 4.3 or 7.0, while allowing the pH to vary as brucite dissolved.

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Results showed that both Mg5H2Phytate.6H2O and Mg2P2O7 become thermodynamically

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stable due to the solutions reaching supersaturation with respect to these two phases

290

(Supporting Data 6-8). Because it is impossible to include all possible species in the database

291

and appropriate equilibrium constants are mostly lacking, the simulations are incomplete,

292

especially for the formation of other solid phases, for example hydrated Mg2P2O7.

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Due to the limitation of appropriate thermodynamic data, all PHREEQC simulations were

294

an approximation to the real scenario. However the simulations predicted the fast formation of

295

a Mg-phytate phase at the pH values tested. The Mg-PyroP phase did not form at pH ≥ 7 and

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only became significant at lower pH values when dissolution of brucite was enhanced. This is

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in accordance with the AFM observations and Raman analyses, especially, as Raman analyses

298

for ex situ formation of precipitates on the surface of brucite verify the utility of speciation

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modeling. The lack of equilibrium constant data did not allow for simulations with G6P.

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In summary, from our observations a higher number of phosphate groups in a phosphate 12

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molecule seems to enhance the chelating ability of the molecule to sequester Mg2+. The

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reaction rates of dissolution and subsequent precipitation followed the order: phytate (with 6

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phosphate groups per molecule) > pyrophosphate (2 phosphate groups) > G6P (1 phosphate

304

group) at all pH values tested.

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Implications. A fundamental challenge in the study of soil organic and inorganic phosphate

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dynamics is to assess the relative importance of adsorption and precipitation processes on

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mineral surfaces in the environment as this plays an important role in the global P cycle.44 In

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this study, direct AFM observations and kinetic analyses have been applied to observe and

309

predict the behavior of OPs and PyroP on brucite mineral surfaces. Our results may provide

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dynamic insights into how OP and PyroP can be sequestered through mineral interfacial

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dissolution-precipitation reactions.51 The approach used in this study may readily extend to

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further investigate the adsorption and precipitation dynamics of guest anions/cations on the

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surfaces and in the interlayer spaces in brucite-like minerals such as hydrotalcite-like layered

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double hydroxides (LDHs).52 These minerals are of considerable environmental relevance

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because of their anion-exchange capacity that can affect the mobility of chemical species and

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removal of toxic anions.52 Moreover, these materials can accommodate a wide range of

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different cations in the interlayer spaces for use in environmental remediation of heavy

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metals.52,53

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ASSOCIATED CONTENT

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Supporting Information. AFM images and depth profiles of brucite dissolution and

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precipitation in the presence of pyrophosphate (Figure S1); ex situ reaction of 1 mM G6P with

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brucite (Figure S2); ex situ reaction of 5 mM G6P with brucite (Figure S3); depth profile of

325

brucite substrates covered by larger nanoparticles in 5 mM G6P at pH 3.5 (Figure S4); SEM 13

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images of brucite with OPs under different reaction conditions (Figure S5); Protonation

327

constants for phytate (Table S1); Association constants for Mg-phytate complexes (Table S2).

328

Solubility constants of possible solid phases (Mg-phytate) (Table S3). Protonation constants

329

for pyrophosphate (PyroP) (Table S4). Association constants for Na-pyrophosphate

330

complexes (Table S5). Association constants for Mg-pyrophosphate complexes (Table S6).

331

Solubility constants of possible solid phases (Mg-PyroP) (Table S7), and PHREEQC

332

calculations (Supporting Data 1-8). This material is available free of charge via the Internet at

333

http://pubs.acs.org.

334 335

AUTHOR INFORMATION

336 337

Corresponding authors

338

*E-mail: [email protected] (Lijun Wang); [email protected] (Christine V. Putnis)

339 340

ACKNOWLEDGMENT

341

This work was supported by the National Natural Science Foundation of China (41471245

342

and 41071208), a Specialized Research Fund for the Doctoral Program of Higher Education

343

(20130146110030), and the Fundamental Research Funds for the Central Universities

344

(2662015PY206, 2662015PY116). C.V. Putnis and A. Putnis acknowledge support through

345

EU Marie Curie initial training networks (FlowTrans and CO2 React). E.R.-A. acknowledges

346

a Ramón y Cajal grant and support from the Research Group RNM-179 of the Junta de

347

Andalucía.

348 349

REFERENCES 14

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dynamics and Kinetics of Water–Rock Interaction. Reviews in Mineralogy & Geoche

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rhombohedral carbonate minerals. Am. Mineral. 2004, 89, 554−563.

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nanoscale dissolution of dicalcium phosphate dihydrate by an organic ligand:

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electrolytes on the kinetics and mechanism of calcite dissolution. Geochim.

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atomic-force microscopy. Environ. Sci. Technol. 2016, 50, 259−268.

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(45) Dawson, P.; Hadfield, C. D.; Wilkinson, G. R. The polarized infra-red and Raman

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spectra of Mg(OH)2 and Ca(OH)2. J. Phys. Chem. Solids 1973, 34, 1217–1225.

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amorphous carbon. Phys. Rev. B 2000, 61, 14095–14107.

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(47) Kolozsvari, B.; Firth, S.; Saiardi, A. Raman spectroscopy detection of phytic acid in

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Electrochemical and surface enhanced Raman scattering spectroelectrochemical study

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of phytic acid on the silver electrode. J. Phys. Chem. B 2004, 108, 17412–17417.

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inositol hexaphosphate monolayers at the copper surface from a Na-salt of phytic acid

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solution studied by in situ surface enhanced Raman scattering spectroscopy, Raman

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mapping and polarization measurements. Anal. Chim. Acta 2005, 458, 150–165.

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potassium and magnesium at liquid nitrogen temperature. J. Mol. Struct. 2000, 526,

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the environment. Elements 2013, 9, 177–182.

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cations on layered double hydroxides. Colloids and Surfaces A: Physicochem. Eng.

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Aspects 2013, 433, 122– 131.

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Figure Legends

498 499

Figure 1. Time sequence AFM (deflection mode) images of natual brucite dissolution. (A-D)

500

Time-lapse images show equilateral triangular etch pit formation and evolution on the (001)

501

cleavage plane (the white circles in B-D) after injection of 1 mM phytate (pH 9.0) into a

502

flow-through cell. Images A-C, 3×3 μm; D, 5×5 μm. (A’-D’) Depth profile of the etch pit at

503

t =245 s, 329 s, 7 min, or 43 min, respectively, measured along the white dashed lines in

504

(A-D). (E) The brucite atomic structure projected along the b-axis (in right panel of E; 0.47

505

nm of the thickness of one Mg(OH)2 layer), and the c-axis (in left panel of E; the morphology

506

and crystallographic orientation of the edges of the triangular etch pits is indicated).

507 508

Figure 2. Time sequence AFM (deflection mode) images of brucite dissolution and

509

precipitation in the presence of phytate. (A) In situ AFM images show both etch pit formation

510

and precipitation on a brucite (001) surface in 1 mM phytate + 10 mM NaCl (pH 7.0) at t =28

511

min. (B and C) Height profiles of nucleated nanoparticles (about 2-16 nm) and 2-D nanoplates

512

(about 16 nm in thickness) along the dashed lines 1→1’ and 2→2’, respectively. (D and E) 21

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Page 22 of 31

513

Time-resolved AFM images show the coexistence of point and line defects formed in 1 mM

514

phytate (pH 3.0). Pits associated with observable line defects are apparent (Arrows). (D’ and

515

E’) Depth profiles of etch pits and line defects along the dashed lines 3→3’ and 4→4’,

516

respectively. (F) After 497 s, brucite substrates were covered by larger nanoparticles (about

517

50 nm in F’). Image A, 3×3 μm; D-F, 5×5 μm.

518 519

Figure 3. Time sequence AFM (deflection mode) images of brucite dissolution and

520

precipitation in the presence of pyrophosphate. In situ AFM images show coupled dissolution

521

(by etch pit formation) and precipitation on a brucite (001) surface in (A) 1 mM

522

pyrophosphate (pH 9.6) at t =23 h and (B) 5 mM pyrophosphate (pH 7.0) at t =43 min.

523

Time-resolved AFM images show (C and D) the formation of pits associated with line defects,

524

and (E and F) spherical particles and conical precipitates in 1 mM pyrophosphate (pH 4.3).

525

Images A and C-F, 3×3 μm; B, 5×5 μm.

526 527

Figure 4. Time sequence AFM (deflection mode) images of brucite dissolution in the

528

presence of G6P. (A-C) In situ AFM images of a brucite (001) surface dissolving in 1 mM

529

G6P (pH 5.9). (A’-C’) Depth profiles of the etch pits evaluated along the white dashed lines

530

in A-C. Images A-C, 3×3 μm.

531 532

Figure 5. Time-resolved AFM (deflection mode) images of brucite dissolution and

533

precipitation in the presence of G6P. In situ AFM images show both (A-C) etch pit formation

534

and evolution, and (D) precipitation on a brucite (001) surface in 5 mM G6P (pH 3.5). After

535

16 h, brucite substrates were covered by larger nanoparticles. Images A-D, 3×3 μm.

536 537

Figure 6. Induction times for precipitate formation on the brucite (001) surfaces under 22

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538

various experimental conditions in the presence of phytate or pyrophosphate. (A) A range of

539

pH values, (B) Phytate or pyrophosphate concentrations at pH 7.0, and (C) NaCl

540

concentrations with 1 mM phytate or pyrophosphate at pH 7.0. Induction times are reported as

541

mean ± SD.

542 543

Figure 7. Raman spectra taken from (A) brucite, and surface precipitates from an AFM

544

experiment run with (B) G6P or (C) phytate. Asterisks indicate peaks unrelated to the

545

underlying brucite generated by the surface precipitates in the different organic phosphate

546

experiments.

547 548 549 550 551

23

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A’

A

Page 24 of 31

3.0

Depth (nm)

2.5 2.0 1.5 1.0 0.5

~0.5 nm

0.0 0.7

B’

0.8

0.9 1.0 1.1 1.2 Distance (μm)

1.3

1.4

1.7

1.8

3.5 3.0

1 mM phytate, pH 9.0 t = 245 s

Depth (nm)

2.5

B

2.0 1.5 1.0

~1.0 nm

0.5 0.0 1.1

C’

1.2

1.3

1.4 1.5 1.6 Distance (μm)

5

Depth (nm)

4

329 s

3 2 1

~1.5 nm

0 1.0

C

1.2 1.4 Distance (μm)

1.6

D’ 7 6

Depth (nm)

5

~3 nm

4 3 2

~2.5 nm

1 0 1.4

1.6

1.8

2.0 2.2 2.4 Distance (μm)

2.6

2.8

E

7 min

0.47 nm

D

b

552 553

43 min

c

c a

b

a

Figure 1

24

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A B

2

C

25

15

~2.0 nm

10 5

2’

1

~16.0 nm

~6.0 nm 0.5

1.0 1.5 2.0 Distance (μm)

1 mM phytate + 10 mM NaCl pH 7.0 t = 28 min

2.5

3’

4

15 10 5

555

E’

1

2 3 Distance (μm)

4

5

2.5

3.0

497 s

4’

4

30 20 10

7000 6000 5000 4000 3000 2000 1000 0

0

1.0 1.5 2.0 Distance (μm)

F’ 8000

50

0

0

0.5

4’

40

20

2 0.0

360 s

Depth (nm)

Depth (nm)

25

3’

3

2’

F

1 mM phytate, pH 3.0 t = 279 s

D’ 30

18 16 14 12 10 8 6 4 2 0

3.0

E

3

554

1’

0 0.0

D

Height (nm)

20

Number of events

1’

Height (nm)

1

0

1

2 3 4 Distance (μm)

5

0

10 20 30 40 50 60 70 80 Height (nm)

Figure 2

556 557 558 559 560 561 562 563 564 565 25

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A

1 mM pyrophosphate, pH 9.6 t = 23 h

B

C

H2O, pH 4.3

D

E

192 s

F

Page 26 of 31

5 mM pyrophosphate, pH 7.0 t = 43 min

1 mM pyrophosphate, pH 4.3 t = 103 s

37 min

566 567

Figure 3

568 569 570 571 572 573 574 575 576 26

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A

1 mM G6P, pH 5.9 t = 13 min

10 Depth (nm)

B’ 12

4 Depth (nm)

578

41 min

A’ 5 3 2 1

0.0

1.0 1.5 2.0 Distance (μm)

2.5

3.0

10

8 6 4

0.0

81 min

12 8 6 4 2

0

0.5

C

C’ 14

2

0

577

B

Depth (nm)

Page 27 of 31

0 0.5

1.0 1.5 2.0 Distance (μm)

2.5

3.0

0.0

0.5

1.0 1.5 2.0 Distance (μm)

2.5

3.0

Figure 4

579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 27

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Page 28 of 31

A

H2O, pH 6.0

B

5 mM G6P, pH 3.5 t = 109 s

C

43 min

D

16 h

594 595

Figure 5

596 597 598 599 600 601 602 603 604 605 606 607 608 28

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A 1600 1400 1200 1000 800 600 400 200 0

Induction time (min)

Pyrophosphate 1 mM Phytate  or Pyrophosphate

Phytate

3

4

5

6

7

8

9

10

pH

Induction time (min)

B 250 Phytate pH 7.0

200 150 100 50 0

0.01

0.1 1 Concentration (mM)

Induction time (min)

C 180 160 140 120 100 80 60 40 20 0

610

10

1 mM Phytate  or Pyrophosphate pH 7.0

Pyrophosphate Phytate

0

609

Pyrophosphate pH 7.0

20

40 60 NaCl (mM)

80

100

Figure 6

611 612 613 614 615 616 617 618 619 29

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Intensity (a. u.)

Environmental Science & Technology

*

* * ** * * * *

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Phytate on brucite

C G6P on brucite

*

B brucite

A

0 620

500 1000 1500 -1 Wavenumbers (cm )

2000

621 622

Figure 7

623 624 625 626 627 628 629 630 631 632 633 634 635 636 30

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1 mM phytate, pH 3.0

5 mM pyrophosphate, pH 7.0

1 mM pyrophosphate, pH 4.3

or 5 mM G6P, pH 3.5

637 638

TOC Graphical abstract

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