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Structures and properties of As(OH)3 adsorption complexes on hydrated mackinawite (FeS) surfaces: A DFT-D2 study Nelson Y. Dzade, Alberto Roldan, and Nora H. de Leeuw Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00107 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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Environmental Science & Technology

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Structures and properties of As(OH)3 adsorption complexes on hydrated mackinawite (FeS) surfaces: A DFT-D2 study

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Dr. Nelson Y. Dzade1*, Dr. Alberto Roldan2and Prof. Nora H. de Leeuw1, 2

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1

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E-mail: [email protected] (N.Y.D); [email protected] (N.H.dL)

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ABSTRACT

1

Department of Earth Sciences, Utrecht University, Princetonplein 9, 3584 CC, Utrecht, The Netherlands 2

School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 1DF, UK

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Reactive mineral–water interfaces exert control on the bioavailability of contaminant arsenic

11

species in natural aqueous systems. However, the ability to accurately predict As surface

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complexation is limited by the lack of molecular-level understanding of As−water−mineral

13

interactions. In the present study, we report the structures and properties of the adsorption

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complexes of arsenous acid (As(OH)3) on hydrated mackinawite (FeS) surfaces, obtained

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from density functional theory (DFT) calculations. The fundamental aspects of the

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adsorption, including the registries of the adsorption complexes, adsorption energies, and

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structural parameters are presented. The FeS surfaces are shown to be stabilized by hydration,

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as is perhaps to be expected because the adsorbed water molecules stabilize the low-

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coordinated surface atoms. As(OH)3 adsorbs weakly at the water−FeS(001) interface through

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a network of hydrogen-bonded interactions with water molecules on the surface, with the

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lowest-energy structure calculated to be an As−up outer-sphere complex. Compared to the

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water−FeS(001) interface, stronger adsorption was calculated for As(OH)3 on the

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water−FeS(011) and water−FeS(111) interfaces, characterized by strong hybridization

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between the S-p and O-p states of As(OH)3 and the surface Fe-d states. The As(OH)3

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molecule displayed a variety of chemisorption geometries on the water−FeS(011) and

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water−FeS(111) interfaces, where the most stable configuration at the water−FeS(011)

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interface is a bidentate Fe−AsO−Fe complex, but on the water−FeS(111) interface, a

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monodentate Fe−O−Fe complex was found. Detailed information regarding the adsorption

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mechanisms has been obtained via projected density of states (PDOS) and electron density

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difference iso-surface analyses and vibrational frequency assignments of the adsorbed

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As(OH)3 molecule. 1

ACS Paragon Plus Environment


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TOC/ABSTRACT GRAPHICS

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1. INTRODUCTION

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Arsenic is recognized as one of the most serious inorganic contaminants in soil and

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groundwater worldwide, with significant public health implications. Arsenic often makes its

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way into soil and water courses by the natural processes of weathering and dissolution of

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minerals such as arsenian pyrite, Fe(As,S)2, and arsenopyrite, FeAsS.1 Anthropogenic

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activities, particularly mineral extraction and processing can also introduce arsenic-rich

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effluents into the environment if not carefully monitored and controlled.2 The effects of

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arsenic on human health are highly detrimental, with arsenic poisoning being linked to

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neurological disorders, dermatological and gastrointestinal problems, and it is also a known

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carcinogen.3, 4

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Arsenic can exist in a range of oxidation states from –3 to +5, although aqueous

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solutions it is most commonly found as As(III) or As(V) oxyacids. As(III) is both more toxic

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(20–65 times) and more mobile (being able to travel five to six times faster) than As(V) and

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is one of the main toxic species in natural waters.5−7 Analyses of hydrothermal fluids show

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that As is transported mainly as As(III),8 and the uptake of As(III) from aqueous solutions is

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reported to occur via neutral molecules, which suggest that arsenous acid (As(OH)3) or

50

related species could be the common form of arsenic in contaminated waters.9, 2


10

An

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understanding of the geochemistry of As(OH)3 in low temperature anoxic sedimentary

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environments is therefore crucial to the development of safe drinking water and food supplies

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in many countries.11, 12 Of the processes influencing arsenite mobility, reactive mineral–water

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interfaces exert control on the bioavailability of contaminant arsenic species in natural

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aqueous systems. The adsorption of arsenic species onto mineral surfaces strongly affects

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their concentrations in aqueous environments.13

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In recent years, iron sulfide mackinawite (FeS), has attracted significant interests for

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environmental remediation due to its natural abundance and high treatment efficiency in

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anoxic environments.14−26 FeS is a layered iron sulfide mineral that crystallises in the

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tetragonal structure shown in Figure 1, 27, 28 and it is known to be the first crystalline ferrous

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sulfide phase to form under sulfate reducing conditions.29, 30 FeS is a non-toxic mineral and a

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precursor to other stable iron sulfide minerals, such as greigite and pyrite.29, 30 Like other 2D

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layered materials, for example, MoS2, FeS possesses a high specific surface area and reactive

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surfaces that are ideal for the uptake of aqueous contaminants. Furthermore, FeS

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nanoparticles can be synthesized easily,31−35 which makes it a promising candidate for the

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treatment of groundwater and soil contaminated with arsenic,14−18 selenium,19, 20 and heavy

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metals, including mercury,21−23 and chromium.24−26

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Owing to its unique structure and surface chemical properties, mackinawite has been

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reported to be very effective in immobilizing divalent metals such as Mg2+, Ca2+, Mn2+, Ni2+,

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Cd2+, and Hg2+ from aqueous solutions.36−39 FeS has also been shown to have a high removal

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capacity for inorganic oxyanions, including As under anoxic conditions.14−20 It has been

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reported that mackinawite suspensions and synthetic nanoparticulate mackinawite can

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effectively remove As(III) at a pH range of 5−10.14, 40 A comparative study of the removal

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capacity of As(III) and As(V) in aqueous solutions by goethite, lepidocrocite, mackinawite,

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and pyrite, by Farquhar et al.17 has shown that mackinawite was more efficient than iron3



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oxide phases or pyrite. Their results suggested that the arsenic uptake by freshly prepared

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mackinawite was due to outer-sphere complexation,17 but fundamental aspects of this

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process, including the registries of the adsorption complexes, adsorption energies, and

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structural parameters remain unclear. Such information cannot be obtained directly from

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experimental work and the underlying physical driving forces that control the reactivity of

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arsenic species with the FeS surfaces remain poorly understood. The diverse interactions and

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reactions occurring at the mineral–water interfaces often create complex situations that are

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difficult to interpret. However, molecular simulations provide an alternative way to gain

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fundamental insight into these processes.41−44 Calculations based on the density functional

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theory (DFT) have become indispensable in unravelling the interactions of organic and

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inorganic molecules with solid surfaces as they are capable of accurately predicting lowest-

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energy adsorption geometries and identifying charge transfer and other electronic effects.45−47

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For example, DFT-based studies have been instrumental in elucidating the complex

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adsorption processes of arsenic and arsenate on iron oxide mineral surfaces.42, 43 Goffinet and

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Mason employed spin-polarized DFT calculations to study the inner-sphere As(III)

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complexes on hydrated α-Fe2O3(0001) surface models.42 Blanchard and co-workers have

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modelled arsenate adsorption on the hydrated (1−12) hematite surface, investigating charged

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inner- and outer-sphere complexes using DFT calculations.43 To date, no systematic

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theoretical study has been conducted to investigate the detailed adsorption mechanism of

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arsenous acid at the water−FeS interface, which makes this investigation timely.

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In this study, the structures and properties of the adsorption complexes of As(OH)3 on

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hydrated mackinawite (FeS) surfaces was studied using dispersion-corrected density

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functional theory calculations (DFT-D2). The energetically preferred As(OH)3 surface

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complexes on the hydrated (001), (011), and (111) surfaces of mackinawite have been

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identified. Detailed structural analysis of the adsorption complexes and insight into the nature 4


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of adsorption on the different surfaces was determined via analysis of projected density of

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states and differential charge density iso-surfaces. Vibrational frequency assignment of the

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different identified adsorption complexes of As(OH)3 was carried out, which will be useful

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for comparison with any future experimental studies.

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2. COMPUTATIONAL DETAILS

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The calculations were carried out using the VASP code,48, 49 which employs a basis

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set of plane-waves to solve the Kohn-Sham (KS) equations of the density functional theory

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(DFT) in a periodic system. Long-range dispersion forces were accounted for in our

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calculations using the Grimme DFT-D2 method,50 which is essential for the accurate

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description of the FeS interlayer interactions,51−54 as well as the interactions between the

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As(OH)3 molecule and the water−FeS surfaces. The D2 method was used in this study to

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remain consistent with previous work and to ensure that direct comparison could be made

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with our earlier studies. However, we have carried out a number of test calculations using the

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DFT-D3 method, as mentioned in the text where relevant, but no significant differences

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between the two methods were observed.

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The generalized gradient approximation (GGA), with the PW91 functional55 was used

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to calculate the total free energies. The interactions between the valence electrons and the

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cores were described with the projected augmented wave (PAW) method56 in the

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implementation of Kresse and Joubert.57 The on-site potential, GGA+U, was not employed

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for these calculations as previous studies on FeS using VASP have shown that the extra

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localization of the d-electrons through the inclusion of +U correction term provides

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inadequate structural optimizations.54 An energy cut-off of 400 eV for the plane-wave basis

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set was tested to be sufficient to converge the total energy of mackinawite to within 0.0001

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eV and the Brillouin zone was sampled using 11 x 11 x 7 and 5 x 5 x 1 Monkhorst-Pack58 K5



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points mesh for bulk and surface calculations, respectively, which ensures electronic and

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ionic convergence. Geometry optimizations were performed using the conjugate gradient

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minimization algorithm until the magnitude of the residual Hellman−Feynman force on each

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relaxed atom reached 0.001 eV/Å.

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The bulk FeS was modelled in the tetragonal structure (Figure 1). From a full geometry

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optimization, the equilibrium lattice parameters were predicted to be a = 3.587 Å, c = 4.908

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Å, and c/a = 1.368,44, 51−54 which agree well with those measured experimentally (a = 3.674

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Å, c = 5.033 Å, and c/a = 1.370).27,

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scheme, which predicted the lattice parameters to be a = 3.590 Å, c = 4.992 Å, and c/a =

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1.390. From the fully relaxed bulk structure, we created the (001), (011), and (111) surfaces

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of FeS, which are the commonly observed facets in mackinawite nanoparticles.44,

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surfaces were created using the METADISE code,60 which generates non-polar supercells,

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avoiding dipole moments perpendicular to the surface plane, as is required for reliable and

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realistic surface calculations.61

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Similar results were obtained within the DFT-D3

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The

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For each surface, a minimum slab thickness of 10 Å was used in each simulation cell,

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and a vacuum region of 15 Å was tested to be sufficient to avoid interactions between

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periodic slabs. The converged slab thickness used to model the (001), (011), and (111)

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surfaces were constructed of 6, 9, and 12 atomic layers, respectively. Because the processes

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take place in an aqueous environment, the FeS surfaces were hydrated through associative

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adsorption of a monolayer of water, to provide a realistic picture of the As(OH)3

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complexation in natural aqueous systems at the mackinawite–water interface. In an earlier

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study, we showed that the dissociative water adsorption did not occur spontaneously at FeS

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surfaces.62 We considered that a monolayer of water was obtained when all surface

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cations/anions had been terminated by water. The hydrated (001), (011), and (111) surfaces

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are modelled by large slabs constructed as (3 x 3)−9water, (4 x 2)−8water, and 6


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(3 x 2)−6water supercells, respectively. These simulation supercells are large enough to

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minimize lateral interaction between the As(OH)3 molecules in neighbouring image cells.

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Different binding modes of the As(OH)3 molecule were considered, for example,

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monodentate or bidentate adsorption configurations, in order to obtain the lowest-energy

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adsorption complexes. The adsorption energy (Eads) of the As(OH)3 on the hydrated FeS

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surfaces was calculated as follows:

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Eads = Ewater −surf + As (OH )3 − ( Ewater −surf + E As (OH )3 )

(1)

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where E water − surf + As (OH )3 represents the total energy of the adsorbate-substrate system, E water − surf

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represents the total energy of the relevant hydrated FeS substrate, and E As (OH )3 is the energy

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of the free As(OH)3 molecule. Differences in the adsorption energies reflect trends in surface

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reactivity, thus Eads is useful for characterizing activity trends and relative energetics. A

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Bader population analysis was carried out for all the As(OH)3−water−FeS complexes, using

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the code developed by Henkelman and co-workers63 in order to quantify any charge transfer

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between the substrate surfaces and the adsorbate molecule. Vibrational frequency assignment

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of the As−O and O−H bond stretching modes were performed within the framework of the

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self-consistent density functional perturbation theory.64

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3. RESULTS AND DISCUSSIONS

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3.1 Hydrated FeS (001), (011), and (111) surface models

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Prior to studying the adsorption and surface reactions of As(OH)3, we have

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characterised the interaction of water with the (001), (011), and (111) surfaces of FeS and

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how hydration affects their relative stabilities. Shown in Figure 2 are the optimized structures

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of the hydrated (001) and (011), and (111) surfaces. The relaxed surface energies ( γ r ) of the

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pure symmetric stoichiometric slabs were calculated using the equation: 7



γr =

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relaxed E unrelaxed − nEbulk Eslab − nEbulk − γ u ; γ u = slab A 2A

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(2)

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relaxed unrelaxed where E slab and Eslab are the energies of the relaxed and unrelaxed slabs, respectively,

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nEbulk is the energy of an equal number (n) of bulk FeS units, and A is the area of one side of

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the slab. Considering that the adsorption of water on the FeS surfaces may affect their

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stability, we have also calculated the surface energies of the surfaces after water adsorption

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using Equations 3.

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γ water =

relaxed E slab + n ( water ) − nE water − nEbulk

A

−γu

(3)

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relaxed where E slab + water is the energy of the surface with adsorbed water molecules and nEwater is the

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energy of an equivalent number of free water molecules.

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The calculated surface energies of different surfaces (pristine and hydrated) as listed

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in Table 1, show that the order of increasing surface energies, and therefore decreasing

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stability, before and after hydration is (001) < (011) < (111). All the FeS surfaces were

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stabilized through hydration, as is perhaps to be expected because the adsorbed water

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molecules stabilize the low-coordinated surface atoms. At the FeS(001) surface, we found

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that the water molecules were only physisorbed with the hydrogen atoms pointing towards

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the terminating surface sulfur ions (Figure 2a), similar to results obtained from previous

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DFT,53, 62 and molecular dynamics (MD) simulations65 of the structure and dynamics of water

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at the FeS(001) surface. The shortest H−S distance is calculated at 2.319 Å, which is larger

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than the typical hydrogen-bond length in water of 1.97 Å,66 and therefore suggests that

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dispersion forces may play an important role in stabilizing the water molecule on the

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FeS(001) surface. In a previous study, we showed that the dispersion interactions contribute

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approximately 87% of the total adsorption energy of water on the FeS(001).62 The average 8


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hydrogen to oxygen (H---O) interatomic distance between the water molecules on the (001)

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surface is calculated at 1.824 Å.

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Compared to the (001) surface, the water molecules on the (011) surface are oriented

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in such a way that now the O atoms are closest to the surface Fe sites (average Fe−O =2.253

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Å) as shown in Figure 2b. The hydrogen atoms are oriented towards the sulfur ions in the

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next FeS layer at an average distance of 2.703 Å, which is larger than the average Fe−O bond

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length of 2.253 Å and therefore suggests that the major interactions between the adsorbing

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water molecules and the (011) surface is through the interaction of their oxygen ions with

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surface Fe ions. In the case of the water−FeS(111) complex (Figure 2c), the water molecules

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are located above the bridge sites between adjacent Fe ions (average Fe−O = 2.205 Å). The

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hydrogen atoms are oriented towards the sulfur ions in the next FeS layer at an average

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distance of 2.043 Å, compared to 2.703 Å at the FeS(011) surface, which indicates stronger

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hydrogen-bonding at the FeS(111) surface than at the FeS(011). Generally, the FeS surfaces

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were found to undergo modest relaxations relative to the bulk interlayer spacings upon

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hydration, where the topmost three percentage relaxations of the interlayer spacings are

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calculated to be +6.5 %, +3.3 %, and −3.4 % for the (001), −24.1 %, +10.9 %, and −2.3 % for

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the (011), and +29.3 %, +12.1 %, and −6.6 % for the (111). The multilayer relaxations for the

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hydrated surfaces were calculated as the percentage difference in the surface interlayer

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spacing, dij-hydrated, from the layer spacing of the same orientation in the geometry of the

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unrelaxed surface structure, dij-unrelaxed, created from the equilibrium bulk material. In these

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simulations, since the models are constructed from the optimized bulk structure, the required

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surface layer spacing is given by the spacing of the unrelaxed bulk-terminated slab structure.

∆d ij =

(d

ij − hydrated

− d ij −unrelaxed ) d ij −unrelaxed

218

× 100

9


(1)


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Within this definition, negative values correspond to inward relaxation (contraction) and

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positive values denote outward relaxation (dilation) of the interlayer spacings.

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3.2 As(OH)3 structural conformations

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Arsenous acid (As(OH)3) exists in two conformations in the gas phase with either C1 or C3

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symmetry. The optimized geometries of the C1 and C3 conformations are shown in Figure 3

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and the calculated interatomic bond distances and bond angles along with earlier theoretical

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results41, 67 are listed in Table 2. From our geometry optimization calculations, we found that

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the C1 symmetry is 0.03 eV more stable than the C3 symmetry, in agreement with earlier

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theoretical results of Ramírez-Solís et al.67 and Tossell et al.68 We show from climbing-image

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nudged elastic band (cNEB) calculations that the C1 conformation has to overcome an

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activation barrier of 0.34 eV to transform to the higher-energy C3 conformation. The three

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As−O bond distances of the C1 and the C3 conformers do not differ significantly, calculated

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to be 1.798, 1.801, and 1.811 Å for the C1 symmetry and 1.810, 1.811 and 1.813 Å for the C3

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symmetry. Our calculated bond distances (As‒O and O‒H) and angles (O‒As‒O and

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As−O−H) show good agreement with earlier theoretical results41,

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obtained from X-ray absorption and EXAFS analysis.69, 70 In our study, we have explored

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several possible adsorption structures including monodentate or bidentate binding geometries

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on the different hydrated FeS surfaces.

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3.3 As(OH)3 adsorption complexes at water-FeS(001) interface

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Several possible modes of adsorption sites and configurations were studied for As(OH)3

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adsorption at the water-FeS(001) interfaces but only the lowest-energy structure (denoted

240

As–up–outer) is shown in Figure 4a (the remaining conformations and calculated binding

241

energies are given in the Supporting Information (Figure S1 and Table S1, respectively)). In

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the lowest-energy As–up–outer complex, the As(OH)3 is adsorbed outside the water layer 10


67, 68

and with those

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with the As atom pointing upwards, while the hydroxyl groups form hydrogen-bonded

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interactions with the surface-bound water molecules. The adsorption energy of this structure

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is −1.14 eV, which is 0.2 eV more favourable than the As–up inner-sphere complex (Figure

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S1b), in which the As(OH)3 molecule is adsorbed within the water layer by displacing some

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of the water molecules during the adsorption process. In the case of As–down configurations,

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the inner-sphere complex (Figure S1c) is found to be energetically more favourable than the

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outer-sphere complex (Figure S1d) by 0.23 eV. In all adsorption geometries, we observe

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only small elongations in the As−O and O−H bonds (Table 3 and Table S1) compared to the

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structural data of the free As(OH)3 molecule (Table 2), which may be attributed to the

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hydrogen-bonded interactions with the surface water molecules. In the lowest-energy outer-

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sphere As–up complex, the three hydrogen atoms of the As(OH)3 molecule interact with three

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different surface water molecules at Hmol‒Owat distances of 1.702, 1.747, and 1.960 Å. We

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also observe hydrogen-bonded interactions between hydrogen atoms of two water molecules

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and O atoms of As(OH)3 at Hwat‒Omol distances of 1.639 and 1.783 Å. The Hmol‒Owat and

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Hwat‒Omol bond lengths calculated in the present study compare closely with the typical

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hydrogen-bond length in water of 1.97 Å,66 which therefore suggests that hydrogen-bonded

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interactions contribute significantly to the stabilization of As(OH)3 at the water−FeS(001)

260

interface.

261


262

As with the water−FeS(001) surface, we have considered different possible adsorption

263

structures for As(OH)3 on the water−FeS(011) surface. During the adsorption, some of the

264

water molecules were displaced from the surface by the As(OH)3, enabling direct stronger

265

interactions with the surface cations sites. Shown in Figure 4b is the lowest-energy

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adsorption configuration identified (the remaining conformations are given in the Supporting 11



267

Information (Figure S2), whereas the calculated adsorption energies and optimized structural

268

parameters are reported in Table 3 and Table S2. The lowest-energy adsorption structure of

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As(OH)3 at the water−FeS(011) interface is calculated to be a bidentate Fe−AsO−Fe complex

270

(Figure 4b), wherein the As(OH)3 molecule interacts with the surface Fe atoms via the As

271

and one O atom. The adsorption energy of this structure is calculated at −1.82 eV, compared

272

to the adsorption energy of −1.43 eV for the monodentate Fe−O complex (Figure S2b),

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wherein the As(OH)3 molecule interacts with the surface Fe atoms via only one O atom,

274

−1.06 eV for the monodentate Fe−As complex (Figure S2c), wherein the As(OH)3 molecule

275

interacts with the surface Fe atoms via the As atom, and −0.89 eV for the As−bridge complex

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(Figure S2d), wherein the As(OH)3 is adsorbed in a bridging position between the FeS layers

277

and stabilized through hydrogen-bonded interactions with the surface water molecules. The

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As−S interatomic distances are calculated in the range of 2.960−4.147 Å, whereas the As−Fe

279

are calculated in the range of 2.269−3.787 Å (Table 3 and Table S2). Similar interatomic

280

distances were reported from spectroscopic and extended X-ray absorption fine structure

281

(EXAFS) data fitting of As(III) sorbed on mackinawite (As−S =3.1 Å and As−Fe =3.4−3.5

282

Å) in aqueous solution.17

283


284

Again, we have explored several possible sites and modes of adsorption of As(OH)3 on the

285

water−FeS(111) surface. Similar to the water−FeS(011) surface, some of the water molecules

286

were displaced by As(OH)3 during the adsorption process, which allows for the formation of

287

direct interactions with the surface cation sites. Displayed in Figure 4c is the lowest-energy

288

adsorption complex identified (the remaining conformations are given in the Supporting

289

Information (Figure S3).The lowest-energy As(OH)3 adsorption configuration at the

290

water−FeS(111) interface was calculated to be the Fe−O−Fe complex (Figure 4c), wherein 12


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the As(OH)3 molecule adsorbs at the bridge site between adjacent surface Fe atoms via one O

292

atom. The adsorption energy of this structure (Fe−O−Fe complex) is calculated at −1.76 eV,

293

whereas the energies of the other stable adsorption configurations are calculated at −1.57 eV

294

for the Fe−AsO−Fe complex (Figure S3b), −1.17 eV for the Fe−As complex (Figure S3c),

295

and −0.86 eV for the Hwat−OH−Ssurf complex (Figure S3d). In the lowest-energy Fe−O−Fe

296

complex, the bridging O−Fe distances were calculated at 2.159 Å and 2.138 Å, and the

297

average values are reported in Table 3. The As−S and As−Fe interatomic distances

298

are converged at 3.675 Å and 3.365 Å, respectively. Similar interatomic distances

299

were calculated for the As atom interacting with the surface S and Fe ions in the

300

other adsorption configurations (Table S3). At all three water−FeS interfaces, we

301

have observed elongations in the As−O bonds in all adsorption complexes (1.765−1.946 Å),

302

especially in the complexes in which the O atom interacts directly with the surface Fe ions.

303

O−H bond elongations were also observed (0.976−1.046 Å), which can be attributed to the

304

presence of hydrogen-bonded interactions between the hydrogen atom of As(OH)3 and the O

305

atom of the surface water molecules as reported in Table 3 and supporting information

306

Tables S1−S3.

307

3.6. Electronic structures

308

To gain insight into the nature of the interactions between the As(OH)3 molecule and the

309

different hydrated FeS surfaces, we have carried out an atom-by-atom projected density of

310

states (PDOS) analysis of the free molecule and compared it to those of the adsorbed states.

311

The PDOS for the free As(OH)3 molecule is shown in Figure 5a1, whereas those for the

312

lowest-energy adsorption configurations at the water−FeS (001), (011), and (111) interfaces

313

are shown in Figures 5b1, 5c1, and 5d1, respectively. In the free As(OH)3 PDOS, we note

314

that the states around the Fermi level are dominated by p-states of As and O, which are 13



315

associated with the lone pair electron density of the As and O atoms as shown in the highest

316

occupied molecular orbital (HOMO) in Figure 5a2. These orbitals are therefore expected to

317

interact strongly with the orbitals of the surface species during sorption processes at the

318

mineral surfaces. Indeed, we found that at the water−FeS (011) and (111) interfaces where

319

the As(OH)3 interacts directly with the surface Fe ions, we observe disappearance or

320

reduction of the As-p and O-p states of As(OH)3 around the Fermi level, due to their strong

321

hybridization with the interacting surface Fe-d states (Figures 5c1 and 5d1). At the

322

water−FeS(001) interface, however, we only observe a shift towards lower energy levels

323

(Figure 5b1), which signifies stabilization of the As(OH)3 via physisorption. The PDOS for

324

the interacting surface Fe d-states before and after the adsorption of As(OH)3 at the

325

water−FeS(111) and water−FeS(011) interface, and for the interacting surface S p-states at

326

the water−FeS(001) interface are shown in Figure 6. We found that the electronic properties

327

of the surfaces were essentially preserved after the adsorption of As(OH)3, with only small

328

shifts in the peak positions and heights, which indicates adsorption induced changes due to

329

the interactions between the As(OH)3 specie and the water-FeS interfaces. The electron

330

density redistributions within the adsorbate-substrate systems were determined through

331

analyses of the iso-surface plots of the differential charge density, which is obtained by

332

subtracting from the charge density of the total adsorbate-substrate complex, the sum of the

333

charge densities of the As(OH)3 molecule and the hydrated FeS surface. The atomic positions

334

of the water−FeS surface and of the As(OH)3 molecule are kept the same as those of the total

335

adsorbate-substrate system. In this way, the presentation highlights local electron density

336

rearrangement and bond formation in the As(OH)3−water−FeS complexes. Shown in Figures

337

5b2, 5c2, and 5d2 are the isosurfaces of the electron density differences due to As(OH)3

338

adsorption at the water−FeS (001), (011), and (111) interfaces, respectively. An inspection of

339

the iso-surfaces reveals electron density accumulation within the bonding regions between 14


Page 14 of 33

Page 15 of 33


340

As(OH)3 and the water−FeS (011) and (111) interacting surface Fe ions, which is consistent

341

with the formation of new bonds. In the case of the As(OH)3−water−FeS (001) complex, we

342

see electron density accumulation between the hydrogen and O atoms indicative of hydrogen-

343

bonded interactions. Despite the strong electron density redistribution within the

344

As(OH)3−water−FeS complexes, only little charge transfer occurs from the interacting

345

surface species to the adsorbed As(OH)3 molecule, as revealed from our Bader charge

346

population analyses (Tables 3 and Tables S1−S3). The charge gained by the As(OH)3 in the

347

different adsorption complexes is calculated to be in the range of 0.01−0.04 e− at the

348

water−FeS(001) surface, 0.08−0.30 e− at the water−FeS(011) surface, and 0.01−0.28 e− at the

349

water−FeS(111) surface (Tables 3 and Tables S1−S3).

350

3.7. Vibrational properties

351

In order to propose an assignment for the As–O and O–H stretching vibrational modes

352

of the adsorbed As(OH)3, which can serve as a guide for future experimental identification of

353

the different adsorption complexes of As(OH)3 at the water−FeS interfaces, we have

354

computed the wavenumbers of the normal modes of all the stable adsorption complexes at the

355

different water−FeS interfaces (Table 4 and Table S4). Our calculated As–O and O–H

356

stretching vibrational modes for the free As(OH)3 molecule compare closely with

357

experimental data,71 as shown in Table 4, which ensures the reliability and accuracy of our

358

approximate assignments. The three As−O stretching vibrational modes for the free As(OH)3

359

molecule were calculated at 700.8, 639.1, and 638.3, which compares with the experimental

360

values of 710.0, 655.0, and 655.0 cm-1.71 The O–H stretching vibrational modes are

361

calculated at 3743.5, 3715.3 and 3674.6 cm-1 which are similar to the O–H stretching modes

362

of water.72 Compared to the free As(OH)3 molecule, we observe a reduction in the stretching

363

vibrational modes of the As−O bonds upon As(OH)3 adsorption, indicative of weakening of 15



364

these bonds, in agreement with the elongated As−O bonds calculated for the As(OH)3

365

adsorption complexes at the different water−FeS surfaces (Table 4). For example, the three

366

stretching As−O bands of As(OH)3 adsorbed in the lowest-energy configuration at the

367

water−FeS(011) and water−FeS(111) surfaces can be assigned at 580.2, 501.5, 488.9 cm-1

368

and 673.5, 616.5, 456.1 cm-1, respectively, which are lower than the gas phase stretching

369

As−O band assigned at 700.8, 639.1, and 632.3 cm-1. We have also observed reductions in

370

the stretching O–H bands of the adsorbed As(OH)3 compared to the free unbound state

371

(Table 4), which can be attributed to the formations of hydrogen-bonded interactions with the

372

oxygen ions of the surface water molecules.

373

The unique information provided by our atomic-level investigations provide

374

fundamental insights into the structure–property relationships of FeS–water−As(OH)3

375

interfaces. Our simulations show that As(OH)3 adsorbs weakly onto the water−FeS(001)

376

interface through a network of hydrogen-bond interactions with water molecules at the

377

surface. Stronger interaction is however, calculated for As(OH)3 adsorption on the

378

water−FeS(011) and water−FeS (111) interfaces, which is characterized by hybridization

379

between the S-p and O-p states of As(OH)3 and the surface Fe-d states. Our calculated As‒Fe

380

and As‒S interatomic distances in the lowest-energy adsorption complexes at the various

381

water-mackinawite interfaces (As‒Fe = 2.269−3.369 Å and As‒S = 3.382−3.675 Å) show

382

good agreement with those obtained from K-edge EXAFS and XANES spectroscopic data

383

(As‒Fe = 3.4‒3.5 Å and As‒S = 3.1 Å).17 The long distances obtained from experiment

384

clearly suggest As interactions via outer sphere complexes at the FeS surface. However, from

385

our simulation results, the short As‒Fe distances (2.217−2.530 Å) calculated for the

386

Fe−AsO−Fe and Fe−As adsorption complexes at the water-FeS (011) and (111) interfaces

387

indicate that, depending on the surface structure and composition, inner-sphere complexation

388

with respect to the As atom is also possible at the water-mackinawite interface. Future 16


Page 16 of 33

Page 17 of 33


389

investigations will expand the work presented here to include classical MD simulations which

390

will provide a complete description of the dynamic processes occurring at the As(OH)3–

391

water−FeS interfaces. The calculated interatomic distances and adsorption energies from this

392

work will be useful in the derivation of forcefields to be employed in the classical MD

393

simulations to simulate more complex systems, including single and multiple As(OH)3

394

species adsorption from an explicit 3-dimensional aqueous environment.

395

396

ASSOCIATED CONTENT

397 398

*S Supporting Information

399

Figures of all other As(OH)3 adsorption conformations, and Tables of adsorption energies,

400

structural parameters and vibrational frequencies of As(OH)3 adsorbed on water-FeS (001),

401

(011), and (111) interfaces. It contains three Figures and four Tables.

402

403

AUTHOR INFORMATION

404

Corresponding Author

405

*Telephone: +31-6-8523-9288 (N.Y.D); +44-29-2087-0658 (N.H.dL)

406

Fax: +31-30-253-5096 (N.Y.D); +44-29-2087-4030 (N.H.dL)

407

E-mail: [email protected] (N.Y.D); [email protected] (N.H.dL)

408

409

ACKNOWLEDGMENTS

410

We acknowledge the Netherlands Foundation for Fundamental Research on Matter (FOM)

411

for funding (Grant No. 13CO26-2). This work made use of the facilities of ARCHER

17



412

(http://www.archer.ac.uk), the UK’s national supercomputing service via our membership of

413

the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202).

414

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597 598 599 600 601 602 603 604 605 606 23



Page 24 of 33

607 608

LIST OF TABLES

609 610

Table 1: Calculated surface energies of pristine (γr) and hydrated (γhydrated) FeS. The corresponding percentage relaxation after hydration is denoted as % Relaxation.

611

Surface

γr (J m−2)

γhydrated (J m−2)

% Relaxation

(001)

0.19

0.14

26.31

(011)

0.95

0.71

25.26

(111)

1.51

1.21

19.87

612 613 614 615

Table 2: Structural data (interatomic bond distance and angles) of As(OH)3. The experimental As‒O bond length is 1.77−1.82 Å.69, 70 C1 Symmetry

C3 Symmetry

Parameter

This Work

PBE41

B3LYP67

This work

PBE41

B3LYP67

d(As‒O) /Å

1.798

1.811

1.796

1.810

1.829

1.813

1.801

1.818

1.800

1.811

1.829

1.813

1.811

1.841

1.826

1.813

1.829

1.813

0.975

0.977

0.967

0.975

0.982

0.970

0.978

0.980

0.969

0.978

0.982

0.970

0.978

0.983

0.970

0.978

0.982

0.970

90.17

88.79

90.86

97.25

96.92

97.34

99.84

99.75

99.61

97.48

96.94

97.36

100.94

103.22

100.89

97.57

96.99

97.37

108.6

105.33

110.16

108.6

104.87

109.93

110.9

109.83

112.68

110.8

104.98

109.94

111.2

111.77

112.78

111.0

105.02

109.93

d(O‒H) /Å

α(O−As−O) /°

α(As−O−H) /°

616

24


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Table 3: Adsorption energies, variation of the total Bader charge, representative geometrical parameters, and interatomic distances of the lowestenergy As(OH)3 adsorption complexes at water−FeS (001), (011), and (111) interfaces. The DFT-D3 Eads are shown in parenthesis. Surface

FeS(001)

FeS(011)

FeS(111)

Configuration

As−up−outer

Fe−AsO−Fe

Fe−O−Fe

Eads /eV

‒1.14 (‒1.06)

‒1.82 (‒1.73)

‒1.76 (‒1.68)

∑q /e−

0.04

0.30

0.28

d(As‒O) /Å

1.834

1.889

1.946

1.835

1.838

1.810

1.781

1.877

1.765

0.988

1.024

1.018

1.003

0.977

1.004

1.005

0.980

0.977

d(Hmol‒Owat) /Å

1.702, 1.747, 1.960

1.645

1.817

d(Hwat‒Omol) /Å

1.639, 1.783

1.803

3.240

d(Hwat‒S) /Å

2.301

−−−

2.034

d(As‒S) /Å

−−−

3.382

3.675

d(As‒Fe) /Å

−−−

2.269

3.365

d(O‒Fe) /Å

−−−

2.133

2.149

d(O‒H) /Å

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Table 4: Molecular vibrational frequencies (in cm-1) of adsorbed As(OH)3 at water−FeS interfaces.

ν(As−O) Surface

Configuration Free As(OH)3

ν(O−H)

As−O1 700.8 (710)71

As−O2 639.1 (655)71

As−O3 638.3 (655)71

O1−H

O2−H

O2−H

3738.1

3711.5

3674.7

FeS(001)

As−up−outer

695.1

620.7

585.8

3465.9

3182.9

3140.5

FeS(011)

Fe−AsO−Fe

580.2

501.5

488.9

3715.1

3670.9

2829.1

FeS(111)

Fe−O−Fe

673.5

616.5

456.1

3731.2

3204.2

2884.3

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LIST OF FIGURES Figure 1: The layered structure of mackinawite, with the tetragonal unit cell highlighted by dash lines. (Colour scheme: Fe = grey, S = yellow).

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Figure 2: Side view of the geometry-optimized structures of hydrated FeS (a) (001), (b) (011), and (111) surfaces. (Colour scheme: Fe = grey, S = yellow, O = red, and H = white).

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Figure 3: Optimized structures and energetics of C1 and C3 conformations of As(OH)3. (Colour scheme: As = green, O = red and H = white).

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Figure 4: Lowest-energy adsorption complexes of As(OH)3 at the (a) (001), (b) (011), and (c) (111) water−FeS interfaces, in side (top) and top (bottom) views. (Colour scheme: Fe = grey, S = yellow, As = pink, O = red and H = white).

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Figure 5: (Right) PDOS for As(OH)3 in the (a) free state and adsorbed in the lowest-energy geometry at the water−FeS interfaces (b−d). (Left) the corresponding isosurfaces of the differential charge density, where the purple and orange contours indicate electron density increase and decrease by 0.02 e/Å3, respectively.

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Figure 6: PDOS for the interacting surface Fe d-states before and after the adsorption of As(OH)3 at the (a) water−FeS(111) and (b) water−FeS(011) interface, and (c) for the interacting surface S p-states at the water−FeS(001) interface.

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Adsorption complex of As(OH)3 at water-FeS(001) interface. 79x39mm (300 x 300 DPI)


FeS - ACS Publications - American Chemical Society

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