How Doping Regulates As(III) Adsorption at TiO2 Surfaces: A DFT + U Study

The efficient adsorption and removal of As(III), which is highly toxic, remains difficult. TiO2 shows promise in this field, though the process needs improvement. Herein, how doping regulates As(OH)3 adsorption over TiO2 surfaces is comprehensively investigated by means of the DFT + D3 approach. Doping creates the bidentate mononuclear (Ce doping at the Ti5c site), tridentate (N, S doping at the O2c site), and other new adsorption structures. The extent of structural perturbation correlates with the atomic radius when doping the Ti site (Ce >> Fe, Mn, V >> B), while it correlates with the likelihood of forming more bonds when doping the O site (N > S > F). Doping the O2c, O3c rather than the Ti5c site is more effective in enhancing As(OH)3 adsorption and also causes more structural perturbation and diversity. Similar to the scenario of pristine surfaces, the bidentate binuclear complexes with two Ti-OAs bonds are often the most preferred, except for B doping at the Ti5c site, S doping at the O2c site, and B doping at the O3c site of rutile (110) and Ce, B doping at the Ti5c site, N, S doping at the O2c site, and N, S, B doping at the O3c site of anatase (101). Doping significantly regulates the As(OH)3 adsorption efficacy, and the adsorption energies reach −4.17, −4.13, and −4.67 eV for Mn doping at the Ti5c site and N doping at the O2c and O3c sites of rutile (110) and −1.99, −2.29, and −2.24 eV for Ce doping at the Ti5c site and N doping at the O2c and O3c sites of anatase (101), respectively. As(OH)3 adsorption and removal are crystal-dependent and become apparently more efficient for rutile vs. anatase, whether doped at the Ti5c, O2c, or O3c site. The auto-oxidation of As(III) occurs when the As centers interact directly with the TiO2 surface, and this occurs more frequently for rutile rather than anatase. The multidentate adsorption of As(OH)3 causes electron back-donation and As(V) re-reduction to As(IV). The regulatory effects of doping during As(III) adsorption and the critical roles played by crystal control are further unraveled at the molecular level. Significant insights are provided for As(III) pollution management via the adsorption and rational design of efficient scavengers.


Introduction
Arsenic (As) is a ubiquitously distributed toxic metalloid.Its pollution has seriously endangered the ecological environment and human health and aroused global concern [1,2].In order to remediate As-polluted sites, a number of techniques have been developed, such as adsorption, precipitation, flocculation, membrane separation, and reverse osmosis [3][4][5].Among them, adsorption by minerals pronouncedly regulates arsenic migration, bioavailability, and fate and ranks to be the most widely used technique for pollution management [6][7][8].Inorganic arsenic in natural environments predominates in the As(III) and As(V) forms.Although much more toxic, As(III) is highly mobile and more difficult for adsorption and removal because it exists predominantly as the uncharged As(OH) 3 moiety [9][10][11].It thus represents an imperative task to explore the usage efficient As(III) scavengers.
Metal (hydr)oxides [12,13], zero-valent iron [14,15], zeolites [7,16], clay minerals [17,18], and activated carbon [19,20] have been used for As(III) adsorption.However, they generally suffer from limited adsorption capacity and strength, and hence chemical modification is necessitated [21][22][23].Mn doping of γ-Al 2 O 3 enables the transfer of more electrons into a stable status, which further promotes As(III) adsorption [24].As indicated by periodic density functional theory (DFT) calculations, doping of gibbsite results in strong M-3d and O As -2p orbital interactions (M = Fe(III), Mn(III), Mn(IV)) that facilitate As(III) adsorption [25].Owing to its superior stability, non-toxicity, and high activity, titanium dioxide (TiO 2 ) has been applied extensively for environment-associated adsorption and catalysis [26][27][28][29][30]. Pena et al. [27] found that 0.5 mmol/g of As(III) is adsorbed by nanocrystalline TiO 2 , and the adsorption thermodynamics and kinetics can be well described by the Freundlich isotherm and pseudo-second-order model.Wu et al. [28] showed that due to adsorption by TiO 2 nanoparticles, the accumulation of As in plants reduces by 40~90%.The controllable configuration of TiO 2 nanoparticles was realized using 3D-printing technology, and after being used more than 10 times, the TiO 2 nanoparticles remained effective for the treatment of raw-arsenic-polluted groundwater samples [29].X-ray absorption spectra (XAS) revealed that As(III) at the TiO 2 surface tends to adopt the bidentate binuclear configuration, and the Ti-As distance is approximately 3.32 Å [30], which is in good agreement with DFT calculated results [31][32][33].A density of states (DOS) analysis [34] demonstrated that As(III) at the TiO 2 surface generates anti-bonding orbitals above the Fermi level, and the Ti-O As bonds are attributed mainly to the electron sharing between the O-2p and Ti-3d orbitals.Doping is widely documented to be capable of regulating the adsorption and catalytic performances of TiO 2 , e.g., Fe [35,36], Ce [37], B [38], V [39], and Mn [39] at the Ti site and N [40,41], F [42], and S [43] at the O site.Owing to the high efficiency of H 2 O 2 utilization (99.1%), almost all As(III) adsorbed at the Ce-doped TiO 2 surface is catalytically oxidized to As(V), and the activity of Ce-doped TiO 2 remains after five cycles [37].The incorporation of Fe into the TiO 2 lattice improves the adsorption capacity of As by arresting the grain growth, which results in a higher affinity [44].N doping into the TiO 2 lattice effectively promotes the decolorization of double azo reactive black 5 (RB5) dye and shows significant bactericidal activity against Escherichia coli, with an inhibition rate of up to 92.47% [45].F doping of TiO 2 significantly enhances the adsorption capacity of Pb(II) [42], while S-doped TiO 2 causes a redshift of the optical absorption edge, and hepatotoxin microcystin-LR is efficiently degraded under visible-light irradiation [43].Despite their wide application, how these dopants within TiO 2 affect As(III) adsorption and the associated structure-activity relationship remain enigmatic.DFT approaches have been testified to be well suited for structural engineering, mechanistic unraveling, and catalyst design due to their atomicscale spatial resolution and absolute accuracy (<4.0 kJ/mol) [46,47].In this study, DFT calculations were carried out, considering (1) two dominant polymorphs of TiO 2 (rutile and anatase) to probe crystal dependence during As(III) adsorption; (2) all types of doping (at the Ti 5c , O 2c , and O 3c sites, see Figure 1), to explore the most effective type of doping and site specificity for As(III) adsorption; and (3) a number of dopants, including those aforementioned, to establish the structure-activity relationship during As(III) adsorption.Then, the regulatory mechanisms of doping and crystal control during As(III) adsorption were unraveled at the molecular level.The results provide valuable insight into As(III) adsorption and removal by TiO 2 -based materials and feed back for the design of efficient scavengers for As(III) and other pollutants.

As(OH) 3 Adsorption by Rutile (110) with the Ti Site Being Doped
Adsorption configurations of As(OH) 3 at the rutile (110) surface with the Ti 5c site being doped (D Ti = Fe, Mn, V, Ce, B) are depicted in Figure 2 and S2.They exhibit a marked difference from those of a pristine surface (Figure S3), wherein R1Pr, R2Pr, and R3Pr correspond to the physisorbed, monodentate, and bidentate binuclear complexes [31].
For Fe 5c doping, R2Fe 5c with the Ti1-O2 bond remains structurally similar as it appears at a pristine surface (R2Pr), while the other adsorption structures may be distinct: R1Fe 5c with the Fe-As interaction vanishes, R4Fe 5c with the Fe-O2 bond becomes monodentate, and R3Fe 5c (Ti2-O1 + Fe-O2) and R5Fe 5c (Fe-O2 + As-O4) are bidentate binuclear.R2Fe 5c and R4Fe 5c are further stabilized by the Fe-As (3.160 Å) and Ti2-As (3.047 Å) interactions, and R5Fe 5c is featured by the As-O S bond that is absent at a pristine surface (O S refers to the surface O atom).Mn 5c and V 5c doping has As(OH) 3 adsorption that resembles Fe 5c doping, while Ce 5c and B 5c doping differs substantially, which originates from the divergence of their atomic radii: Ce (1.82 Å) >> Ti and V, Mn, Fe (1.17~1.32Å) >> B (0.82 Å).Owing to the large atomic radius, Ce protrudes outside the plane of its bonded O S atoms (Figure S4) [48], and the interaction with As(OH) 3 is promoted so that R6Ce 5c of the bidentate mononuclear motif is generated.However, R6Ce 5c is disfavored sterically by the hepta-coordination for Ce, and this can be reflected by the longer Ce-O As bonds and a less negative E ad than in R3Ce 5c of the bidentate binuclear motif (2.616, 2.571 vs. 2.428 Å, and −2.12 vs. −3.43eV).B has the smallest atomic radius and forms only three B-O S bonds [49], which affects the coordination environment and reactivity of the adjacent surface atoms (Figure S5).B is not directly involved during adsorption, and hence R3B 5c and R4B 5c vanish while the B-As interaction lacks in R2B 5c .R7B 5c arises due to O4 transformation from O 3c to O 2c , and albeit with similar bonding interactions as R5B 5c , it is less preferred due to a four-membered ring (Ti1O2AsO4): E ad = −3.20 vs. −5.15eV.The stability of different As(OH) 3 adsorption configurations generally declines as bidentate (R3, R5, R6, R7) > monodentate (R2, R4) > physisorbed (R1).R3 of the bidentate binuclear motif is the most favorable for all dopants at a pristine surface (Figures 2 and S2), which is consistent with EXAFS and ATR-FTIR observations [30,50,51].The E ad amounts to −2.43, −2.73, −3.43, −3.79, and −4.17 eV for R3V 5c , R3Fe 5c , R3Ce 5c , R3Pr, and R3Mn 5c , respectively [50].This agrees with the exergonic nature of As(III) adsorption onto Fe-doped TiO 2 (B) [52].As(OH) 3 adsorption in R3Mn 5c is further enhanced by the auto-transfer of H1 and H2 to the O S atoms [31].Hence, the adsorption efficacy of As(OH) 3 is regulated pronouncedly by doping the Ti 5c site and varies within a wide range.B 5c doping is an exception, and R5B 5c formation is strongly benefited: the reactivity of the Ti1 site is enhanced due to the elongation of Ti1-O S bonds (Figure S5) and the superior stability of the AsO 4 tetrahedron.The O2AsO5 angle in R5B 5c is closer to 109.5 • for the regular tetrahedron than in the other adsorption configurations: 103.7 • vs. 93.8• , 94.4 • , 93.7 • , 79.7 • for R5B 5c vs. R5Fe 5c , R5Mn 5c , R5V 5c , and R5Ce 5c .The deviation degree of the O2AsO5 angle also interprets the lowest relative stability of R5 vs. R3 for Ce 5c doping (∆E ad = 1.49eV).

As(OH) 3 Adsorption by Rutile (110) with Doping the O Site
As(OH) 3 adsorption over a rutile (110) surface with the O 2c , O 3c site being doped (D O = N, F, S, B), as shown in Figures 3 and S6, is more perturbed than with the Ti 5c site being doped.N doping creates the additional adsorption sites [40,41], and As(OH) 3 bonding to N 2c and N 3c generates three and two new adsorption structures, respectively.R1, R2, and R3 remain similar to how they appear at a pristine surface, albeit with some differences: the Ti-As interaction is promoted in R1N 2c (2.673 vs. 2.917 Å in R1Pr) and R2N 3c (2.795 vs. 3.078 Å in R2Pr), while dual proton transfers occur in R3N 2c and R3N 3c instead of the single-proton transfer in R3Pr, and H3 in R3N 2c bonds directly to N 2c .The As-N bond is created in R4N 2c and R4N 3c , and a second bonding (Ti-O2) leads to bidentate R6N 2c and R6N 3c , while R6N 3c is less favorable due to the formation of a four-membered ring (Ti1NAsO2) (see the E ad in Figure 3).It is surprising to find that R5N 2c with only an As-O4 bond exists stably (Figure S6).
F doping alters As(OH) 3 adsorption less than N doping, and a majority of adsorption structures (R1F 2c , R2F 2c , R3F 2c , R2F 3c , and R3F 3c ) remain similar to how they appear at a pristine surface.The As-O S bond is created in R5F 2c and R7F 3c , and the additional Ti1-O2 bond greatly stabilizes R7F 3c (E ad = −2.38 vs. −0.25 eV, Figures 3 and S6).Owing to the close chemo-properties of S and O, R1S 2c , R2S 2c , and R2S 3c are structurally similar and have a comparable E ad as those at a pristine surface (Figures 3, S3 and S6).In addition to R5S 2c and R5S 3c with the As-O S bond, R4S 2c with the As-S bond is produced and exists stably.The large atomic radius causes S to protrude outwards and facilitates the interaction with As(OH) 3 , which further leads to a tridentate complex: R8S 2c (Ti1-O2 + Ti2-O1 + As-S).B 2c doping is not considered due to its markedly lower stability compared to B 3c doping (5.17 eV), which is distinct from other dopants (e.g., 0.15 eV for N 2c vs. N 3c doping).B 3c doping causes the same surface rearrangement as B 5c doping (Figure S7).In addition to R2B 3c and R3B 3c , which are structurally similar to how they appear at a pristine surface, R9B 3c and R10B 3c appear and the As-O4 bond forms at expense of the As-O2 bond rupture.R10B 3c is further stabilized by the Ti3-O1 bonding.
The adsorption configurations of As(OH) 3 at the rutile (110) surface have the following stability trend: tridentate (R8) > bidentate (R3, R6, R7) > monodentate (R2, R4, R5) > physisorbed (R1) (see the E ad in Figures 3 and S6).R3 is generally the most preferred: −3.12, −2.93, −3.18, −3.00, −4.13, and −4.67 eV for R3F 2c , R3F 3c , R8S 2c (transformed barrierlessly from R3S 2c ), R3S 3c , R3N 2c , and R3N 3c , respectively.This also suggests that doping the O 2c , O 3c site may be more effective in enhancing As(OH) 3 adsorption than doping the Ti 5c site (Figure 2).For B 3c doping, the preference of R10B 3c over R3B 3c (−4.72 vs. −2.59eV) is ascribed to the structural reconstruction.This is confirmed by the superior stability of R9B 3c (−4.65 eV) that is monodentate while it undergoes a similar reconstruction.Rutile (110) with doping of the O site is also efficient for As(OH) 3 adsorption and removal, and the efficacy can be regulated within a wide range through the choice of dopants.The preferred adsorption with the O 3c vs. O 2c site being doped ranks as N > S > F, consistent with the likelihood to form more bonds: As(OH) 3 forms a direct bond with N 3c in R4N 3c and R6N 3c , creating the tetra-coordinated N site, and S 2c in R4S 2c and R8S 2c , creating the tri-coordinated S site, whereas it forms no bond with F 2c and F 3c .R3N 3c and R3F 2c are the most favorable for N and F doping, while due to the tridentate motif, R8S 2c is slightly preferred over R3S 3c .This is corroborated by the stability trend of adsorption structures with the As-O/As-D O bond (N > S > F): for As-O4 bonding, R5N 2c (−2.All new adsorption configurations are featured by the direct bonding of As(OH) 3 with the doped sites, except B, while those with only the As-O S bond vanish.This confirms less alteration at the anatase (101) than at the rutile (110) surface: (1) As(OH) 3 adsorption is similar for Fe 5c , Mn 5c , and V 5c doping.The bidentate A5 (D Ti -O2 + As-O4) is destabilized by the formation of a four-membered ring (D Ti O2AsO4), while in A4, the ring strain is alleviated where the As center forms the interaction with O4 and O5 instead of direct bonding.(2) Ce 5c and B 5c doping behaves distinctly due to a structural disruption (Figures S10 and S11), as discussed for rutile (110).However, A6Ce 5c with two Ce-O S bonds becomes preferred over A3Ce 5c (Figure 4), probably due to the inferior reactivity of the Ti 5c sites at the anatase (101) surface [53,54].Ce 5c doping enhances the reactivity, and the formation of more Ce-O As bonds promotes adsorption (|E ad |: A6Ce 5c > A3Ce 5c > A3Pr), consistent with shorter Ce-O As bonds in A6Ce 5c vs. A3Ce 5c (2.573, 2.594 vs. 2.640 Å).Although all of these have the As-O 2c bond, A5B 5c , A7B 5c , and A8B 5c have distinct chemical environments for O 2c : Ti-O 2c -Ti, O 2c transformed from O 3c , and B-O 2c -Ti, respectively.(3) N 2c doping leads to monodentate A4N 2c (As-N) and tridentate A5N 2c (As-N + Ti1-O1 + Ti2-O3), while N 3c doping produces bidentate A6N 3c (As-N + Ti1-O2).( 4) S 2c doping results in tridentate A5S 2c (Ti1-O1 + Ti2-O3 + As-S), in addition to bidentate A6S 2c (Ti1-O2 + As-S) that is disfavored by the large Ti-S distances (ca.2.665 vs. 2.349 Å in A1S 2c ) (Figures 5 and S9).
(5) B 3c doping causes the structural rearrangement producing three Ti-B bonds (Figure S12).A7B 3c has a B-O As bond, as further stabilized by the H2 transfer to O5, while A4B 3c has an As-B bond and A8B 3c has an As-O S bond, and a second bonding (Ti-O As ) produces A6B 3c and A9B 3c .
The stability of different adsorption configurations generally declines as tridentate (A5), bidentate (A3, A6, A9) > monodentate (A2, A4, A8) > physisorbed (A1) (see Figures 4, 5, S8 and S9).The most favorable adsorption configurations may not be A3, and have E ad values of −1.24 eV for A3Fe 5c , −1.50 eV for A3Mn 5c , −1.45 eV for A3V 5c , −1.99 eV for A6Ce 5c , −2.05 eV for A7B 5c , −2.29 eV for A4N 2c , −2.24 eV for A6N 3c , −1.54 eV for A3F 2c , −1.83 eV for A3F 3c , −1.45 eV for A5S 2c , −1.10 eV for A2S 3c , and −4.30 eV for A9B 3c .These values agree with the literature reports available: the Ce-Ti hybrid oxide shows enhanced adsorption for As(III) compared to pure TiO 2 [55], and Pb 2+ adsorption by anatase increases due to F doping [42].The choice of different dopants causes the As(OH) 3 adsorption efficiency to vary within a wide range, and doping the O 2c , O 3c rather than the Ti 5c site may exhibit larger promoting effects for As(OH) 3 adsorption, which is in line with the results of the rutile (110) surface.However, the doped anatase (101) surface is apparently less efficient for As(OH) 3 adsorption and removal than the doped rutile (110) surface, implying the critical role of crystal control played therein.Albeit both being bidentate, A9B 3c is preferred over A6B 3c due to the B-O2H bond formation, as verified by the superior stability of A8B 3c that has B-O2H bonding, although it is monodentate.In order to form the tridentate motif, A5N 2c has the stretched As-O1 and As-O3 bonds (ca.1.880 Å), which lead to the lower stability compared to A4N 2c .A7B 5c has As-O 2c bonding similar to A5B 5c and A8B 5c , while it is preferred due to the high reactivity of its O 2c site that evolves from O 3c .Albeit being the bidentate motif, A3S 3c is destabilized by a serious structural distortion: the strong electrostatic repulsion with O1 and O3 causes the S atom to be pushed below the top surface, and the Ti1-Ti2 distances are significantly enlarged (4.387 vs. 3.815 Å in A3Pr).
Doping alters the electronic structures of TiO 2 and further As(OH) 3 adsorption [61] (see the charge density difference and spin density isosurfaces for Fe 5c , N 2c , and N 3c doping in Figures S17 and S18).Fe doping is used to illustrate the change in charged states during doping of the TiO 2 surfaces (see Figure S19).Charge transfer occurs from Fe to the adjacent O atoms, and the Bader charge of Fe shows some increase.Note that Fe(OH) 3 is used as a reference for Fe in the +III state.The alteration of charged states due to Fe doping is a localized behavior, and one or several adjacent Fe-bonded O atoms fall between the O 2− and O •− states, while the rest of the atoms of the TiO 2 surfaces remain nearly intact.Similar changes can be found for other dopants.Accordingly, the doped sites and proximal-O atoms undertake the change in charged states due to doping [35][36][37][38][39][40][41][42][43].Electron transfer occurs considerably between the As(OH) 3 and TiO 2 surface and is promoted by the direct interaction of the As center with the doped site that further leads to As(III) oxidation.In addition to the doped site, the adjacent Ti and O atoms participate closely during As(OH) 3 oxidation (Figures S13-S16), e.g., O4 in R5Fe 5c in O •− form and subsurface Ti3 in R5N 2c in Ti(III) • form, consistent with the XPS spectra indicating that doping renders the Ti atoms to be more electronegative [61].Electron redistribution and dispersion greatly stabilize the As species, especially As(IV), where more atoms may be involved, e.g., at least six O atoms in A4Fe 5c have clear spin densities and fall between O 2− and O •− .Electron back-donation from TiO 2 to As(OH) 3 is promoted due to the formation of the multidentate motif, and As(V) is likely to be re-reduced to As(IV), e.g., Ti1 in R4N 3c is in Ti(III) • form, while it turns to Ti(IV) in R6N 3c due to the Ti1-O2 bonding that causes As(V) re-reduction.

Models
The optimized lattice parameters of TiO 2 polymorphs amount to a = b = 4.608 Å, c = 2.973 Å for rutile and a = b = 3.793 Å, c = 9.594 Å for anatase, and show good agreement with the literature reports [62,63]; (110) and (101) stand for the most frequently exposed and extensively studied facets for rutile and anatase [53,64], and their models are stoichiometric with the molecular formulas of Ti 60 O 120 and Ti 48 O 96 , respectively (Figure 1).All models consist of six atomic layers [65][66][67], and the slabs were separated by 20.000 Å to avoid image interactions.The effects of water at TiO 2 surfaces on As(OH) 3 adsorption [68] were further investigated and found to be slight (see more details in Supplementary Materials S1: Effects of Adsorbed Water).
The adsorption energies (E ad ) of As(OH) 3 over the pristine and doped TiO 2 surfaces were calculated similarly: where E As(OH)3 , E TiO2 , and E D-TiO2 stand for the electronic energies of As(OH) 3 and pristine and doped TiO 2 surfaces, while E As(OH)3/TiO2 and E As(OH)3/D-TiO2 refer to the electronic energies of As(OH) 3 adsorption structures corresponding to the pristine and doped TiO 2 surfaces, respectively.
To further the understanding of interactions between As(OH) 3 and TiO 2 surfaces and effects of doping and facet control onto As(OH) 3 adsorption, Bader charge and spin density calculations were conducted [81], and the oxidation states of the As centers in adsorption configurations were then determined.The isosurfaces of spin densities were visualized by means of the VESTA 3 software [82].In addition, the PDOS was calculated to gain insight into the bonding mechanisms between As(OH) 3 and TiO 2 surfaces [83].

Conclusions
This study presents a comprehensive understanding of how doping regulates As(III) adsorption and removal by TiO 2 and the critical roles of crystal control played therein.As(OH) 3 adsorption is more altered at the rutile (110) rather than anatase (101) surface.
(1) All dopants except B 5c , F 2c , and F 3c form direct bonding with As(OH) 3 that can be detected in a majority of new adsorption structures.(2) The atomic radius is critical for adsorption when doping the Ti 5c site (Ce >> Ti, Fe, Mn, V >> B).Ce 5c and B 5c doping may have distinct As(OH) 3 adsorption from Fe 5c , Mn 5c , and V 5c doping: Ce 5c doping leads to the bidentate mononuclear R6Ce 5c and A6Ce 5c , while B 5c doping transforms O 3c to O 2c that further forms a direct bond with As(OH) 3 in R7B 5c and A7B 5c .(3) For doping the O 2c , O 3c site, the perturbation extent of As(OH) 3 adsorption is N > S > F and accords with the likelihood of forming more bonds.N doping stabilizes R5N 2c with only the As-O S bonding and has the largest potential to form the As-N bond, with production of monodentate (R4N 2c , R4N 3c , and A4N 2c ), bidentate (R6N 2c , R6N 3c , and A6N 3c ) and tridentate (A5N 2c ) motifs.S doping facilitates the interaction with As(OH) 3 and leads to tridentate R8S 2c and A5S 2c .(4) Doping the O 2c , O 3c rather than the Ti 5c site causes more structural perturbation and diversity to As(OH) 3 adsorption.
As(OH) 3 adsorption structures generally have the following stability trend: tridentate, bidentate > monodentate > physisorbed.Similar to the scenario of pristine TiO 2 surfaces, R3 and A3 with two Ti-O As bonds often remain to be the most preferred adsorption configurations, while this is not the case for B 5c (Ti-O As + As-O Ti ), S 2c (Ti-O As + Ti-O As + As-S) and B 3c (As-O Ti ) doping of rutile (110), and Ce 5c (Ce-O As + Ce-O As ), B 5c (As-O Ti ), N 2c (As-N), S 2c (Ti-O As + Ti-O As + As-S), N 3c (Ti-O As + As-N), S 3c (Ti-O As ), and B 3c (As-O Ti ) doping of anatase (101).Doping is crystal-dependent, and the doped rutile (110) rather than anatase (101) surface is apparently more efficient for As(III) adsorption.In addition, doping the O 2c , O 3c rather than the Ti 5c site may lead to larger promoting effects for adsorption, and the E ad reaches −4.17, −4.13, and −4.67 eV for Mn 5c , N 2c , and N 3c doping of rutile (110) and −1.99, −2.29, and −2.24 eV, for Ce 5c , N 2c , and N 3c doping of anatase (101), respectively.
Distinct from pristine TiO 2 surfaces, where the As centers are always in +III form, doping is prone to cause As(III) auto-oxidation when the As centers interact directly with the TiO 2 surfaces.Rutile rather than anatase is more ready to trigger the auto-oxidation of As(III) to As(V).All adsorption structures with As(V) have superior stability, and doping also stabilizes the As(IV) species: R8S 2c and A7B 5c become the most preferred for S 2c and B 5c doping.The multidentate adsorption of As(OH) 3 causes electron back-donation and may cause As(V) re-reduction to As(IV).The mechanisms regarding how doping regulates As(III) adsorption by TiO 2 and the critical roles of crystal control played therein are further rationalized at the molecular level.
Author Contributions: Data curation, methodology, software, writing-original draft, X.H.; validation, writing-review and editing, M.W. and R.H.; supervision, conceptualization, writing-review and editing, G.Y.All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Innovative Research Groups of CQ, China, grant number CXQT19006, the Fundamental Research Funds for the Central Universities, grant number XDJK2019B038, and the Chongqing Scientific Innovation Project for Postgraduates, grant number CYB22122.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Figure 3 .
Figure 3. Adsorption configurations of As(OH) 3 over rutile (110) surface with O 2c , O 3c site being doped (D O = N, F, S, B), together with adsorption energies (E ad ) and oxidation states for the As centers (in parentheses).Color scheme: Ti (blue), O (red), As (green), H (white), N (gray), F (orange), S (yellow), and B (pink).Selected H-bonds are indicated by dashed gray lines.Distances are given in Å.

Figures 4 , 5 ,
Figures 4, 5, S8 and S9 depict the adsorption structures of As(OH) 3 over the anatase (101) surface with the Ti 5c , O 2c , O 3c site being doped (D Ti = Fe, Mn, V, Ce, B; D O = N, F, S, B).Doping causes less alteration for As(OH) 3 adsorption over anatase (101) vs. the rutile (110) surface, which can be deduced partially from the number of the disruption-prone physisorbed complexes (A1 and R1): nine vs. three remain after doping.Particularly, F 2c , F 3c , and S 3c doping has a total of three adsorption structures (A1, A2, and A3) that are exactly identical to those at a pristine surface.

Figure 5 .
Figure 5. Adsorption configurations of As(OH) 3 over the anatase (101) surface with the O 2c , O 3c site being doped (D O = N, F, S, B), together with adsorption energies (E ad ) and oxidation states for the As centers (in parentheses).Color scheme: Ti (blue), O (red), As (green), H (white), N (gray), F (orange), S (yellow), and B (pink).Selected H-bonds are indicated by dashed gray lines.Distances are given in Å.

Figure 6 .
Figure 6.Projected density of states (PDOS) for As(OH) 3 adsorption at rutile (110) and anatase (101) surfaces in pristine and doped forms (D Ti = Fe; D O = N).The Fermi level (E F ) is set to zero energy and highlighted by gray dotted line.