arsenic 
 is recognized as one of the most serious inorganic contaminants 
 in soil and groundwater worldwide , with significant public health 
 implications .
arsenic often makes its way into soil and water courses 
 by the natural processes of weathering and dissolution of minerals 
 such as arsenian pyrite , fe(as , s)2 , and arsenopyrite , feass .
anthropogenic activities , particularly mineral 
 extraction and processing can also introduce arsenic - rich effluents 
 into the environment if not carefully monitored and controlled .
the effects of arsenic on human health are highly 
 detrimental , with arsenic poisoning being linked to neurological disorders , 
 dermatological and gastrointestinal problems , and it is also a known 
 carcinogen .
arsenic can exist in a range of oxidation 
 states from 3 
 to + 5 , although in aqueous solutions it is most commonly found as 
 as(iii ) or as(v ) oxyacids .
as(iii ) is both more toxic ( 2065 
 times ) and more mobile ( being able to travel five to six times faster ) 
 than as(v ) and is one of the main toxic species in natural waters . analyses of hydrothermal fluids show that as is transported mainly 
 as as(iii ) , and the uptake of as(iii ) 
 from aqueous solutions is reported to occur via neutral molecules , 
 which suggest that arsenous acid ( as(oh)3 ) or related species 
 could be the common form of arsenic in contaminated waters .
an understanding of the geochemistry of as(oh)3 in low 
 temperature anoxic sedimentary environments is therefore crucial to 
 the development of safe drinking water and food supplies in many countries . of the processes influencing arsenite mobility , reactive mineral 
water 
 interfaces exert control on the bioavailability of contaminant arsenic 
 species in natural aqueous systems .
the adsorption of arsenic species 
 onto mineral surfaces strongly affects their concentrations in aqueous 
 environments . in recent years
, 
 iron sulfide mackinawite ( fes ) , has attracted 
 significant interests for environmental remediation due to its natural 
 abundance and high treatment efficiency in anoxic environments .
fes is a layered iron sulfide mineral that crystallizes in the tetragonal 
 structure shown in figure 1 , and it is known to be the first crystalline 
 ferrous sulfide phase to form under sulfate reducing conditions .
fes is a nontoxic mineral and a precursor to other stable iron sulfide 
 minerals , such as greigite and pyrite . like other 
 2d layered materials , for example , mos2
, fes possesses 
 a high specific surface area and reactive surfaces that are ideal 
 for the uptake of aqueous contaminants .
furthermore , fes nanoparticles 
 can be synthesized easily , which makes it a promising candidate for the treatment of groundwater 
 and soil contaminated with arsenic , selenium , and heavy metals , including 
 mercury , and chromium .
layered structure of mackinawite , with 
 the tetragonal unit cell 
 highlighted by dash lines .
owing to its unique structure 
 and surface chemical properties , 
 mackinawite has been reported to be very effective in immobilizing 
 divalent metals such as mg , ca , mn , ni , cd , and hg from aqueous 
 solutions .
fes has also been shown to have a high removal capacity for inorganic 
 oxyanions , including as under anoxic conditions .
it has been reported that mackinawite suspensions and synthetic 
 nanoparticulate mackinawite can effectively remove as(iii ) at a ph 
 range of 510 . a comparative study of the removal 
 capacity of as(iii ) and as(v ) in aqueous solutions by goethite , lepidocrocite , 
 mackinawite , and pyrite , by farquhar et al .
has shown that mackinawite was more efficient than iron - oxide phases 
 or pyrite .
their results suggested that the arsenic uptake by freshly 
 prepared mackinawite was due to outer - sphere complexation , but fundamental aspects of this process , including 
 the registries of the adsorption complexes , adsorption energies , and 
 structural parameters remain unclear .
such information can not be obtained 
 directly from experimental work and the underlying physical driving 
 forces that control the reactivity of arsenic species with the fes 
 surfaces remain poorly understood . the diverse interactions and reactions 
 occurring at the mineral 
however , molecular simulations 
 provide an alternative way to gain fundamental insight into these 
 processes .
calculations based on the density functional theory ( dft ) have become 
 indispensable in unravelling the interactions of organic and inorganic 
 molecules with solid surfaces as they are capable of accurately predicting 
 lowest - energy adsorption geometries and identifying charge transfer 
 and other electronic effects .
for example , dft - based studies 
 have been instrumental in elucidating the complex adsorption processes 
 of arsenic and arsenate on iron oxide mineral surfaces .
goffinet and mason employed spin - polarized dft calculations to study 
 the inner - sphere as(iii ) complexes on hydrated -fe2o3(0001 ) surface models .
blanchard 
 and co - workers have modeled arsenate adsorption on the hydrated ( 1012 ) 
 hematite surface , investigating charged inner- and outer - sphere complexes 
 using dft calculations . to date , no systematic 
 theoretical study has been conducted to investigate the detailed adsorption 
 mechanism of arsenous acid at the water 
, the structures 
 and properties of the adsorption 
 complexes of as(oh)3 on hydrated mackinawite ( fes ) surfaces 
 were studied using dispersion - corrected density functional theory 
 calculations ( dft - d2 ) .
the energetically preferred as(oh)3 surface complexes on the hydrated ( 001 ) , ( 011 ) , and ( 111 ) surfaces 
 of mackinawite have been identified .
detailed structural analysis 
 of the adsorption complexes and insight into the nature of adsorption 
 on the different surfaces was determined via analysis of projected 
 density of states and differential charge density iso - surfaces .
vibrational 
 frequency assignment of the different identified adsorption complexes 
 of as(oh)3 was carried out , which will be useful for comparison 
 with any future experimental studies .
the calculations were carried out using the vasp 
 code , which employs a basis set of plane - waves 
 to solve the kohn 
sham 
 ( ks ) equations of the density functional theory ( dft ) in a periodic 
 system .
long - range dispersion forces were accounted for in our calculations 
 using the grimme dft - d2 method , which 
 is essential for the accurate description of the fes interlayer interactions , as well as the interactions between the as(oh)3 molecule 
 and the water 
the d2 method was used in this 
 study to remain consistent with previous work and to ensure that direct 
 comparison could be made with our earlier studies .
however , we have 
 carried out a number of test calculations using the dft - d3 method , 
 as mentioned in the text where relevant , but no significant differences 
 between the two methods were observed .
the generalized gradient 
 approximation ( gga ) , with the pw91 functional was used to calculate the total free energies . 

the interactions between the valence electrons and the cores were 
 described with the projected augmented wave ( paw ) method in the implementation of kresse and joubert .
the on - site potential , gga+u , was not employed 
 for these calculations as previous studies on fes using vasp have 
 shown that the extra localization of the d - electrons 
 through the inclusion of a + u correction term provides inadequate 
 structural optimizations .
an energy cutoff 
 of 400 ev for the plane - wave basis set was tested to be sufficient 
 to converge the total energy of mackinawite to within 0.0001 ev and 
 the brillouin zone was sampled using 11  11  7 and 5  
 5  1 monkhorst 
pack k - points 
 mesh for bulk and surface calculations , respectively , which ensures 
 electronic and ionic convergence .
geometry optimizations were performed 
 using the conjugate gradient minimization algorithm until the magnitude 
 of the residual hellman 
the bulk fes was modeled in the tetragonal 
 structure ( figure 1 ) . from a full geometry 
 optimization
, the equilibrium lattice parameters were predicted to 
 be a = 3.587  , c = 4.908  , 
 and c / a = 1.368 , which agree well with those measured 
 experimentally ( a = 3.674  , c = 5.033  , and c / a = 1.370 ) .
similar results 
 were obtained within the dft - d3 scheme , which predicted the lattice 
 parameters to be a = 3.590  , c = 4.992  , and c / a = 1.390 . from the fully relaxed bulk structure
, 
 we created the ( 001 ) , ( 011 ) , and ( 111 ) surfaces of fes , which are 
 the commonly observed facets in mackinawite nanoparticles .
the surfaces were created using the metadise code , which generates nonpolar supercells , avoiding dipole moments 
 perpendicular to the surface plane , as is required for reliable and 
 realistic surface calculations . for each surface ,
a minimum slab thicknesses of 10  was used 
 in each simulation cell , and a vacuum region of 15  was tested 
 to be sufficient to avoid interactions between periodic slabs . the 
 converged slab thickness used to model the ( 001 ) , ( 011 ) , and ( 111 ) 
 surfaces
because the processes take place in an aqueous environment , the fes 
 surfaces were hydrated through associative adsorption of a monolayer 
 of water , to provide a realistic picture of the as(oh)3 complexation in natural aqueous systems at the mackinawite  water 
 interface . in an earlier study
, we showed that dissociative water 
 adsorption did not occur spontaneously at fes surfaces .
we considered that a monolayer of water was 
 obtained when all surface cations / anions had been terminated by water . 

the hydrated ( 001 ) , ( 011 ) , and ( 111 ) surfaces are modeled by large 
 slabs constructed as ( 3  3)9water , ( 4  2)8water , 
 and ( 3  2)6water supercells , respectively .
these simulation 
 supercells are large enough to minimize lateral interaction between 
 the as(oh)3 molecules in neighboring image cells .
different binding modes of the as(oh)3 molecule were 
 considered , for example , monodentate or bidentate adsorption configurations , 
 in order to obtain the lowest - energy adsorption complexes .
the adsorption 
 energy ( eads ) of the as(oh)3 on the hydrated fes surfaces was calculated as follows:1where ewater 
surf represents 
 the total energy of the relevant hydrated fes substrate , and eas(oh)3 is the energy of the free 
 as(oh)3 molecule .
differences in the adsorption energies 
 reflect trends in surface reactivity , thus eads is useful for characterizing activity trends and relative 
 energetics .
a bader population analysis was carried out for all the 
 as(oh)3water  fes complexes , using the code 
 developed by henkelman and co - workers in order to quantify any charge transfer between the substrate surfaces 
 and the adsorbate molecule .
vibrational frequency assignment of the 
 as  o and o  h bond stretching modes was performed within 
 the framework of the self - consistent density functional perturbation 
 theory .
prior to studying the adsorption and surface reactions 
 of as(oh)3 , we have characterized the interaction of water 
 with the ( 001 ) , ( 011 ) , and ( 111 ) surfaces of fes and how hydration 
 affects their relative stabilities .
shown in figure 2 are the optimized structures of the hydrated 
 ( 001 ) , ( 011 ) , and ( 111 ) surfaces .
the relaxed surface energies ( r ) of the pure symmetric stoichiometric slabs 
 were calculated using the equation:2where eslabrelaxed and eslabunrelaxed are the energies of the relaxed and unrelaxed slabs , respectively , nebulk is the energy of an equal number ( n ) of bulk fes units , and a is the area 
 of one side of the slab .
considering that the adsorption of water 
 on the fes surfaces may affect their stability , we have also calculated 
 the surface energies of the surfaces after water adsorption using eqs 3.3where eslab  +  waterrelaxed is 
 the energy of the surface with adsorbed water molecules and newater is the energy of an equivalent number 
 of free water molecules .
side view of the geometry - optimized structures 
 of hydrated fes 
 ( a ) ( 001 ) , ( b ) ( 011 ) , and ( 111 ) surfaces .
( color scheme : fe = gray , 
 s = yellow , o = red , and h = white ) .
the calculated surface energies of different surfaces ( pristine 
 and hydrated ) as listed in table 1 , show that the order of increasing surface energies , 
 and therefore decreasing stability , before and after hydration is 
 ( 001 ) < ( 011 ) < ( 111 ) .
all the fes surfaces were stabilized 
 through hydration , as is perhaps to be expected because the adsorbed 
 water molecules stabilize the low - coordinated surface atoms . at the 
 fes(001 ) surface , we found that the water molecules were only physisorbed 
 with the hydrogen atoms pointing toward the terminating surface sulfur 
 ions ( figure 2a ) , similar 
 to results obtained from previous dft , and molecular 
 dynamics ( md ) simulations of the structure 
 and dynamics of water at the fes(001 ) surface .
the shortest h  s 
 distance is calculated at 2.319  , which is larger than the typical 
 hydrogen - bond length in water of 1.97  , and therefore suggests that dispersion forces may play an important 
 role in stabilizing the water molecule on the fes(001 ) surface . in 
 a previous study
, we showed that the dispersion interactions contribute 
 approximately 87% of the total adsorption energy of water on the fes(001 ) .
the average hydrogen to oxygen ( h --- o ) interatomic 
 distance between the water molecules on the ( 001 ) surface is calculated 
 at 1.824  .
compared to the ( 001 ) surface , the 
 water molecules on the ( 011 ) 
 surface are oriented in such a way that now the o atoms are closest 
 to the surface fe sites ( average fe  o = 2.253  ) as shown 
 in figure 2b .
the hydrogen 
 atoms are oriented toward the sulfur ions in the next fes layer at 
 an average distance of 2.703  , which is larger than the average 
 fe  o bond length of 2.253  and therefore suggests that 
 the major interactions between the adsorbing water molecules and the 
 ( 011 ) surface is through the interaction of their oxygen ions with 
 surface fe ions . in the case of the water 
fes(111 ) complex 
 ( figure 2c ) , the water 
 molecules are located above the bridge sites between adjacent fe ions 
 ( average fe  o = 2.205  ) .
the hydrogen atoms are oriented 
 toward the sulfur ions in the next fes layer at an average distance 
 of 2.043  , compared to 2.703  at the fes(011 ) surface , 
 which indicates stronger hydrogen - bonding at the fes(111 ) surface 
 than at the fes(011 ) .
generally , the fes surfaces were found to undergo 
 modest relaxations relative to the bulk interlayer spacings upon hydration , 
 where the topmost three percentage relaxations of the interlayer spacings 
 are calculated to be + 6.5% , + 3.3% , and 3.4% for the ( 001 ) , 
  24.1% , + 10.9% , and 2.3% for the ( 011 ) , and + 29.3% , 
 + 12.1% , and 6.6% for the ( 111 ) .
the multilayer relaxations 
 for the hydrated surfaces were calculated as the percentage difference 
 in the surface interlayer spacing , dij - hydrated , from the layer spacing of the same orientation in the geometry 
 of the unrelaxed surface structure , dij - unrelaxed , created from the equilibrium bulk material . in these simulations , 
 since the models are constructed from the optimized bulk structure , 
 the required surface layer spacing is given by the spacing of the 
 unrelaxed bulk - terminated slab structure.4 within this definition , negative values 
 correspond to inward relaxation 
 ( contraction ) and positive values denote outward relaxation ( dilation ) 
 of the interlayer spacings .
arsenous acid ( as(oh)3 ) exists in 
 two conformations 
 in the gas phase with either c1 or c3 symmetry .
the optimized geometries of the c1 and c3 conformations are shown in figure 3 and the calculated interatomic bond distances 
 and bond angles along with earlier theoretical results are listed in table 2 . from our geometry optimization calculations , we found that the 
 c1 symmetry is 0.03 ev more stable than the c3 symmetry , in agreement with earlier theoretical 
 results of ramrez - sols et al . and tossell et al .
we show from climbing - image 
 nudged elastic band ( cneb ) calculations that the c1 conformation has 
 to overcome an activation barrier of 0.34 ev to transform to the higher - energy 
 c3 conformation .
the three as  o bond distances of the c1 and the c3 conformers do not differ significantly , 
 calculated to be 1.798 , 1.801 , and 1.811  for the c1 symmetry and 1.810 , 1.811 , and 1.813  for the c3 symmetry .
our calculated bond distances ( as  o 
 and o  h ) and angles ( o  as  o and as  o  h ) 
 show good agreement with earlier theoretical results and with those obtained from 
 x - ray absorption and exafs analysis . in our study
, 
 we have explored several possible adsorption structures including 
 monodentate or bidentate binding geometries on the different hydrated 
 fes surfaces .
( color scheme : 
 as = 
 green , o = red and h = white ) .
several possible modes of 
 adsorption sites and configurations were studied for as(oh)3 adsorption at the water - fes(001 ) interfaces but only the lowest - energy 
 structure ( denoted as  up 
outer ) is shown in figure 4a ( the remaining 
 conformations and calculated binding energies are given in the supporting information ( si ) figure s1 and table s1 , respectively ) . in the lowest - energy as  up 
outer complex , 
 the as(oh)3 is adsorbed outside the water layer with the 
 as atom pointing upward , while the hydroxyl groups form hydrogen - bonded 
 interactions with the surface - bound water molecules .
the adsorption 
 energy of this structure is 1.14 ev , which is 0.2 ev more 
 favorable than the as  up inner - sphere complex ( si figure s1b ) , in which the as(oh)3 molecule is adsorbed within the water layer by displacing some of 
 the water molecules during the adsorption process . in the case of 
 as  down configurations , the inner - sphere complex ( si figure s1c ) is found to be energetically more 
 favorable than the outer - sphere complex ( si figure s1d ) by 0.23 ev . in all adsorption geometries , we observe only 
 small elongations in the as  o and o 
h bonds ( table 3 and si table s1 ) compared to the structural data of the free as(oh)3 molecule ( table 2 ) , which may be attributed to the hydrogen - bonded interactions 
 with the surface water molecules . in the lowest - energy outer - sphere 
 as  up complex , the three hydrogen atoms of the as(oh)3 molecule interact with three different surface water molecules at 
 hmol  owat distances of 1.702 , 1.747 , 
 and 1.960  .
we also observe hydrogen - bonded interactions between 
 hydrogen atoms of two water molecules and o atoms of as(oh)3 at hwat  omol distances of 1.639 and 
 1.783  .
the hmol  owat and hwat  omol bond lengths calculated in the present 
 study compare closely with the typical hydrogen - bond length in water 
 of 1.97  , which therefore suggests 
 that hydrogen - bonded interactions contribute significantly to the 
 stabilization of as(oh)3 at the water 
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 .
( color scheme : fe = grey , s = yellow , 
 as = green , owater = red , oas(iii ) = pink , and 
 h = white ) . as with the water 
fes(001 ) 
 surface , we have considered different possible adsorption structures 
 for as(oh)3 on the water 
some of the water molecules were displaced from the 
 surface by the as(oh)3 , enabling direct stronger interactions 
 with the surface cations sites .
shown in figure 4b is the lowest - energy adsorption configuration 
 identified ( the remaining conformations are given in the si figure s2 , whereas the calculated adsorption 
 energies and optimized structural parameters are reported in table 3 and si table s2 .
fes(011 ) interface is calculated to be 
 a bidentate fe  aso  fe complex ( figure 4b ) , wherein the as(oh)3 molecule 
 interacts with the surface fe atoms via the as and one o atom .
the 
 adsorption energy of this structure is calculated at 1.82 
 ev , compared to the adsorption energy of 1.43 ev for the monodentate 
 fe  o complex ( si figure s2b ) , wherein 
 the as(oh)3 molecule interacts with the surface fe atoms 
 via only one o atom ,  1.06 ev for the monodentate fe  as 
 complex ( si figure s2c ) , wherein the as(oh)3 molecule interacts with the surface fe atoms via
the as atom , 
 and 0.89 ev for the as  bridge complex ( si figure s2d ) , wherein the as(oh)3 is adsorbed in a bridging position between the fes layers and stabilized 
 through hydrogen - bonded interactions with the surface water molecules . 

s interatomic distances are calculated in the range 
 of 2.9604.147  , whereas the as  fe are calculated 
 in the range of 2.2693.787  ( table 3 and si table s2 ) .
similar interatomic distances were reported from spectroscopic 
 and extended x - ray absorption fine structure ( exafs ) data fitting 
 of as(iii ) sorbed on mackinawite ( as  s = 3.1  and as  fe 
 = 3.43.5  ) in aqueous solution .
again , we have explored several 
 possible sites and modes of adsorption of as(oh)3 on the 
 water 
fes(011 ) 
 surface , some of the water molecules were displaced by as(oh)3 during the adsorption process , which allows for the formation 
 of direct interactions with the surface cation sites . displayed in figure 4c is the lowest - energy 
 adsorption complex identified ( the remaining conformations are given 
 in the si figure s3).the lowest - energy 
 as(oh)3 adsorption configuration at the water 
fes(111 ) 
 interface was calculated to be the fe  o  fe complex ( figure 4c ) , wherein the as(oh)3 molecule adsorbs at the bridge site between adjacent surface 
 fe atoms via one o atom .
the adsorption energy of this structure ( fe  o  fe 
 complex ) is calculated at 1.76 ev , whereas the energies of 
 the other stable adsorption configurations are calculated at 1.57 
 ev for the fe  aso  fe complex ( si figure s3b ) ,  1.17 ev for the fe  as complex 
 ( si figure s3c ) , and 0.86 ev for 
 the hwat  oh  ssurf complex ( si figure s3d ) . in the lowest - energy fe  o  fe 
 complex , the bridging o 
fe distances were calculated at 2.159 
 and 2.138  , and the average values are reported in table 3 . the as 
s 
 and as  fe interatomic distances are converged at 3.675 and 
 3.365  , respectively .
similar interatomic distances were calculated 
 for the as atom interacting with the surface s and fe ions in the 
 other adsorption configurations ( si table s3 ) .
at all three water  fes interfaces , we have observed elongations 
 in the as 
o bonds in all adsorption complexes ( 1.7651.946 
  ) , especially in the complexes in which the o atom interacts 
 directly with the surface fe ions .
o  h bond elongations were 
 also observed ( 0.9761.046  ) , which can be attributed 
 to the presence of hydrogen - bonded interactions between the hydrogen 
 atom of as(oh)3 and the o atom of the surface water molecules 
 as reported in table 3 and si tables s1s3 . to gain insight 
 into the nature of the interactions between the as(oh)3 molecule and the different hydrated fes surfaces , we have carried 
 out an atom - by - atom projected density of states ( pdos ) analysis of 
 the free molecule and compared it to those of the adsorbed states . 

the pdos for the free as(oh)3 molecule is shown in figure 5a1 , whereas those 
 for the lowest - energy adsorption configurations at the water 
fes 
 ( 001 ) , ( 011 ) , and ( 111 ) interfaces are shown in figures 5b1 , c1 , and d1 , respectively . in the free 
 as(oh)3 pdos , we note that the states around the fermi 
 level are dominated by p - states of as and o , which 
 are associated with the lone pair electron density of the as and o 
 atoms as shown in the highest occupied molecular orbital ( homo ) in figure 5a2 .
these orbitals 
 are therefore expected to interact strongly with the orbitals of the 
 surface species during sorption processes at the mineral surfaces . 

fes ( 011 ) and ( 111 ) interfaces 
 where the as(oh)3 interacts directly with the surface fe 
 ions , we observe disappearance or reduction of the as - p and o - p states of as(oh)3 around the 
 fermi level , due to their strong hybridization with the interacting 
 surface fe - d states ( figures 5c1 and d1 ) . at the water 
fes(001 ) 
 interface , however , we only observe a shift toward lower energy levels 
 ( figure 5b1 ) , which 
 signifies stabilization of the as(oh)3 via physisorption . 

the pdos for the interacting surface fe - d - states 
 before and after the adsorption of as(oh)3 at the water 
fes(011 ) interface , and for the interacting surface 
 s - p - states at the water 
we found that the electronic properties of the surfaces were essentially 
 preserved after the adsorption of as(oh)3 , with only small 
 shifts in the peak positions and heights , which indicates adsorption 
 induced changes due to the interactions between the as(oh)3 species and the water 
substrate systems were 
 determined through analyses of the iso - surface plots of the differential 
 charge density , which is obtained by subtracting from the charge density 
 of the total adsorbate 
substrate complex , the sum of the charge 
 densities of the as(oh)3 molecule and the hydrated fes 
 surface . the atomic positions of the water  fes surface and 
 of the as(oh)3 molecule are kept the same as those of the 
 total adsorbate  substrate system . in this way , the presentation 
 highlights local electron density rearrangement and bond formation 
 in the as(oh)3water  fes complexes .
shown 
 in figures 5b2 , c2 , 
 and d2 are the isosurfaces of the electron density differences due 
 to as(oh)3 adsorption at the water  fes ( 001 ) , ( 011 ) , 
 and ( 111 ) interfaces , respectively .
an inspection of the iso - surfaces 
 reveals electron density accumulation within the bonding regions between 
 as(oh)3 and the water 
fes ( 011 ) and ( 111 ) interacting 
 surface fe ions , which is consistent with the formation of new bonds . 
 in the case of the as(oh)3water  fes ( 001 ) 
 complex ,
we see electron density accumulation between the hydrogen 
 and o atoms indicative of hydrogen - bonded interactions . despite the 
 strong electron density redistribution within the as(oh)3water  fes complexes ,
only little charge transfer occurs 
 from the interacting surface species to the adsorbed as(oh)3 molecule , as revealed from our bader charge population analyses 
 ( tables 3 and si tables s1s3 ) .
the charge gained by 
 the as(oh)3 in the different adsorption complexes is calculated 
 to be in the range of 0.010.04 e at the 
 water 
( right ) pdos for as(oh)3 in the ( a ) 
 free state and adsorbed 
 in the lowest - energy geometry at the water 
( 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 /  , respectively .
pdos for the interacting surface fe d - states before 
 and after the adsorption of as(oh)3 at the ( a ) water 
fes(011 ) interface , and ( c ) for the interacting 
 surface s - p - states at the water 
fes(001 ) interface . in order to 
 propose an assignment for the as  o and o 
h stretching 
 vibrational modes of the adsorbed as(oh)3 , which can serve 
 as a guide for future experimental identification of the different 
 adsorption complexes of as(oh)3 at the water 
fes 
 interfaces , we have computed the wavenumbers of the normal modes of 
 all the stable adsorption complexes at the different water  fes 
 interfaces ( table 4 and si table s4 ) .
h stretching vibrational modes for the free as(oh)3 molecule compare closely with experimental data , as shown in table 4 , which ensures the reliability and accuracy 
 of our approximate assignments .
the three as  o stretching vibrational 
 modes for the free as(oh)3 molecule were calculated at 
 700.8 , 639.1 , and 638.3 , which compares with the experimental values 
 of 710.0 , 655.0 , and 655.0 cm .
h stretching vibrational modes are calculated 
 at 3743.5 , 3715.3 , and 3674.6 cm which are similar 
 to the o 
compared to the free as(oh)3 molecule , we observe a reduction 
 in the stretching vibrational modes of the as  o bonds upon 
 as(oh)3 adsorption , indicative of weakening of these bonds , 
 in agreement with the elongated as  o bonds calculated for the 
 as(oh)3 adsorption complexes at the different water 
for example , the three stretching as  o bands of as(oh)3 adsorbed in the lowest - energy configuration at the water 
fes(111 ) surfaces can be assigned at 580.2 , 501.5 , 
 488.9 cm and 673.5 , 616.5 , 456.1 cm , respectively , which are lower than the gas phase stretching as 
h bands of 
 the adsorbed as(oh)3 compared to the free unbound state 
 ( table 4 ) , which can 
 be attributed to the formations of hydrogen - bonded interactions with 
 the oxygen ions of the surface water molecules .
the unique information provided by 
 our atomic - level investigations 
 provide fundamental insights into the structure  property relationships 
 of fes  water 
fes(001 ) 
 interface through a network of hydrogen - bond interactions with water 
 molecules at the surface .
fes(011 ) and 
 water  fes ( 111 ) interfaces , which is characterized by hybridization 
 between the s - p and o - p states of 
 as(oh)3 and the surface fe - d states .
our 
 calculated as  fe and as  s interatomic distances in the 
 lowest - energy adsorption complexes at the various water - mackinawite 
 interfaces ( as  fe = 2.2693.369  and as  s 
 = 3.3823.675
 ) show good agreement with those obtained 
 from k - edge exafs and xanes spectroscopic data ( as  fe = 3.43.5 
  and as  s = 3.1  ) .
the long distances obtained from experiment clearly suggest as interactions 
 via outer sphere complexes at the fes surface . however , from our simulation 
 results , the short as  fe distances ( 2.2172.530  ) 
 calculated for the fe  aso  fe and fe  as adsorption 
 complexes at the water - fes ( 011 ) and ( 111 ) interfaces indicate that , 
 depending on the surface structure and composition , inner - sphere complexation 
 with respect to the as atom is also possible at the water - mackinawite 
 interface .
future investigations will expand the work presented here 
 to include classical md simulations which will provide a complete 
 description of the dynamic processes occurring at the as(oh)3water  fes interfaces .
the calculated interatomic distances 
 and adsorption energies from this work will be useful in the derivation 
 of force fields to be employed in the classical md simulations to 
 simulate more complex systems , including single and multiple as(oh)3 species adsorption from an explicit 3-dimensional aqueous 
 environment .
 reactive 
 mineral  water interfaces exert control on the bioavailability 
 of contaminant arsenic species in natural aqueous systems . however , 
 the ability to accurately predict as surface complexation is limited 
 by the lack of molecular - level understanding of as  water  mineral 
 interactions . in the present study , we report the structures and properties 
 of the adsorption complexes of arsenous acid ( as(oh)3 ) 
 on hydrated mackinawite ( fes ) surfaces , obtained from density functional 
 theory ( dft ) calculations . 
 the fundamental aspects of the adsorption , 
 including the registries of the adsorption complexes , adsorption energies , 
 and structural parameters are presented . 
 the fes surfaces are shown 
 to be stabilized by hydration , as is perhaps to be expected because 
 the adsorbed water molecules stabilize the low - coordinated surface 
 atoms . 
 as(oh)3 
 adsorbs weakly at the water  
 fes(001 ) 
 interface through a network of hydrogen - bonded interactions with water 
 molecules on the surface , with the lowest - energy structure calculated 
 to be an as  up outer - sphere complex . compared to the water  
 fes(001 ) 
 interface , stronger adsorption was calculated for as(oh)3 on the water  
 fes(011 ) and water  
 fes(111 ) interfaces , 
 characterized by strong hybridization between the s - p and o - p states of as(oh)3 and the surface 
 fe - d states . 
 the as(oh)3 molecule displayed 
 a variety of chemisorption geometries on the water  
 fes(011 ) 
 and water  
 fes(111 ) interfaces , where the most stable configuration 
 at the water  
 fes(011 ) interface is a bidentate fe  aso  fe 
 complex , but on the water  
 fes(111 ) interface , a monodentate 
 fe  o  fe complex was found . 
 detailed information regarding 
 the adsorption mechanisms has been obtained via projected density 
 of states ( pdos ) and electron density difference iso - surface analyses 
 and vibrational frequency assignments of the adsorbed as(oh)3 molecule . 
Introduction
Computational
Details
Results
and Discussion