Aluminum and Iron Doped Graphene for Adsorption of

Aluminum and Iron Doped Graphene for Adsorption of Methylated Arsenic Pollutants Diego Cortés-Arriagada* and Alejandro Toro-Labbé Nucleus Millennium Chemical Processes and Catalysis; Laboratorio de Química Teórica Computacional (QTC), Departamento de Química-Física, Facultad de Química, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Macul, Santiago 9900087, Chile. *Corresponding author e-mail: [email protected] Abstract. The ability of Al and Fedoped graphene for the adsorption of trivalent and pentavalent methylated arsenic compounds was studied by quantum chemistry computations. The adsorption of trivalent methylarsenicals is reached with adsorption energies of 1.51.7 eV at neutral conditions; while, adsorption of pentavalent methylarsenicals reaches adsorption energies of 3.34.2 eV and 1.22.4 eV from neutral to low pH conditions, respectively. Moreover, the weakening of the interacting AsO bond in the pollutant structure played an important role in the stability of the adsorbentadsorbate systems, determining the adsorption strength. In addition, the pollutant adsorption appears to be efficient in aqueous environments, with even high stability at ambient temperature; in this regard, it was determined that the trivalent and petavalent forms are mainly adsorbed in their neutral and anionic forms at neutral pH, respectively. Therefore, Al and Fedoped graphene are considered as potential future materials for the removal of methylated arsenic pollutants. Keywords: Al-doped graphene, Fe-doped graphene, arsenic pollution, methylated arsenic, arsenic adsorption 1

1. Introduction Nowadays, the arsenic pollution of waters is a critical problem in many countries on the planet, where the standards for arsenic in drinking waters remain above the recommended values by the World Health Organization (MMAV>iAsV, then, determining the order of desorption. In order to test the accuracy of the Eads values, and due to the anionic character of the pollutants, we perform tests against those obtained with diffuse basis sets. It was used the minimally augmented all-electron def2 basis sets (ma-def2-SVP)[51]; tests were performed for anionic MMAV and DMAV due to the high computational cost of these calculations. As noted in parenthesis in Table 5, the computed Eads values using the diffuse functions change only in up to 0.14 eV with respect to those obtained with the only polarized basis sets, indicating the good accuracy of the Eads values, and discarding that those are some overestimated. Additionally, unlike in the case of MMAIII/DMAIII, the AsO1 bonding remains without significant changes after the interaction with the substrate (dAsO1=1.741.93 Å), so that suggesting the electron density associated to the AsO1 bond is not depleted. Indeed, the Mayer BO of the AsO1 and AO1 (A=Al, Fe) bonds are of BOAsO11.0 and BOAO10.8 (Table 6). Consequently, the high adsorption stability of anionic pentavalent arsenicals is favored because the AsO1 bonding is not weakened to strengthen the dopantoxygen interaction, where the extra electron density is able to balance the charge transfer processes, strengthening both the AsO1 and dopantO1 bonds (even the πAsO2 bond retains a bond order of 2.0). Likewise, the electron density difference indicates that 15

the AsO1 bond loses the extra electron density to increase the electron density of the dopantoxygen bond (among other charge displacements) (see Fig. S5 in the supporting content). On the other hand, the anionic MMAV and DMAV increase the amount of electron transfer towards the acceptor substrate in up to 100% (QPG values in Table 5) with respect to the trivalent forms; the electron transfer using Al and Fedoped graphene is of up to 0.34|e| and 0.53|e|, respectively. Table 6. Mayer bond orders (BO) of the AlO1, FeO1, and AsO1 bonds in iAsV, MMAV and DMAV adsorbed onto Al and Fedoped graphene. (a). This is a bidentate conformation. Pollutant

BOAlO1

BOAsO1

Al-doped graphene

BOFeO1

BOAsO1

Fe-doped graphene

anionic pentavalent arsenicals V

iAs

a b c

0.83 0.87 0.37 (0.56)a

1.01 1.02 0.68 (0.56)a

0.75 1.03 0.79 1.01 0.32 (0.48)a 0.72 (0.60)a

MMAV

a b c

0.91 0.87 0.86

0.99 1.01 1.03

DMAV

a b

0.94 0.88

iAsV

a b c

0.49 0.52 0.66

0.64 0.61 1.19

0.40 0.43 0.59

0.73 0.67 1.26

MMAV

a b c d e

0.53 0.50 0.50 0.66 0.67

0.60 0.61 0.65 1.23 1.23

0.45 0.41 0.41 0.59 0.60

0.66 0.70 0.72 1.29 1.28

DMAV

a b c

0.54 0.66 0.67

0.60 1.24 1.25

0.46 0.46 0.60

0.66 0.66 1.30

0.84 0.79 0.79

0.98 0.88 1.00 0.82 neutral pentavalent arsenicals

0.98 1.02 1.02 0.96 1.00

On the other hand, the metabolic process and the sulfidic environments can favor the formation of thioarsenic species such as dimethylmonothioarsinic acid (DMMTA V)[5255]

, which has been determined to show an increased cytotoxicity in comparison with 16

DMAV, and similar to DMAIII. Because the latter, we perform calculations with anionic DMMTAV (dimethylmonothioarsenate) as adsorbate to get insights into the removal of toxic thiolated methylarsenicals; their results are summarized in Table 5. In this regard, the same efficiency observed for MMAV/DMAV was reached for the anionic DMMTAV adsorption, with adsorption energies of up to 4.02 and 3.95 eV using Al and Fedoped graphene, respectively. Onto both adsorbents, the covalent dopantoxygen interaction was the preferred conformation (Fig. 56), with a high electron transfer in the DMMTAVadsorbent direction of up to 0.63|e|. Therefore, Al and Fedoped graphene are expected to be good adsorbents of arsenic pollutants, including thiolated arsenicals. As explained above, the adsorption of pentavalent arsenicals at neutral conditions takes place mainly in their anionic forms; however, we perform calculations with neutral iAsV, MMAV and DMAV in order to get insights about the removal in a broad range of pH. The results for these systems are summarized in Table 5. Two main interaction modes were found (See Fig. 7a); in the first one, the dopant atom interacts with the protonated O 1 atom of the pollutant, which belongs to the AsO1 bond, causing the decrease of the pollutant stability as discussed above; these conformations are the ab, ac, and a for iAsV, MMAV and DMAV, respectively, and their Eads values are similar as in the trivalent cases, with Eads=1.181.56 eV (see some conformations in Fig. 7b). The second (and preferred) interaction mode appears in conformations c, de, and bc for iAsV, MMAV and DMAV, respectively, displaying the interaction of the dopant with the non-protonated O2 atom in the pollutant, which belongs to the πAsO2 bonding (Fig. 7a). In these cases, the bond lengths range from dAlO2=1.861.87 Å and dFeO2=1.911.92 Å, and improved adsorption energies on the range of 2.142.36 eV and 2.042.21 eV for the pollutant adsorption onto Al and Fedoped graphene, respectively (see some conformations in Fig. 7b). The difference between the two interaction modes (AO1 and AO2 ; A=Al, Fe) is explained because in the case of the AO2 mode, the AsO1 bond in the pollutant is partially broken without significantly destabilization in the pollutant structure unlike in the case of the depletion of the AsO1 bond (this can be noted from the Mayer BO of the AsO1, which decrease from 2.0 to 1.2, Table 6); therefore, the weakening of the AsO1 bond plays an important role in the stability of the adsorbentadsorbate interactions. Furthermore, the 17

proposed adsorbents would insure for efficient removal of pentavalent arsenicals compounds in a broad range of pH, specifically at neutral and oxidizing conditions. Other interesting result is that the adsorption strength also can be sorted as: (anionic pentavalent arsenicals) > (neutral pentavalent arsenicals) > (neutral trivalent arsenicals).

Figure 7. a) Interaction modes (AO1 and AO2) found in interaction of neutral pentavalent arsenicals onto A doped graphene (A=Al or Fe); R1 and R2 groups are changing between the different arsenic compounds; green lines stand for double bonds. b) Some molecular structures of neutral iAsV, MMAV and DMAV adsorbed onto Fedoped graphene taken as representatives; all of them including Al-doped graphene are included as supporting content in Fig. S1S2. Distances are in angstroms Å; color code by atom: white (H); grey (C); red (O); green (Al); orange (Fe); purple (As). In this point, it is important to note that the high Eads values could indicate a low recovery of the adsorbent materials after the pollutant removal; however, it is known that the adsorption of anionic pollutants is competitive in alkaline conditions with the adsorption of OH- particles, avoiding the pollutant uptake by weakening the electrostatic interaction between the negatively charged adsorbent surface and the anionic arsenicals[11]. Subsequently, the recovery of the adsorbent material by high pH solutions could be done. Furthermore, in comparison with other adsorbent materials, these adsorption energies appear improved; for instance, the arsenate binding onto gibbsite is estimated in ~2.6 eV[42], and onto mineral iron oxides (such as goethite, lepidocrocite and hematite) was computed to be 2.4 eV[56]; also, the adsorption of iAsV and DMAV onto 18

Fe(oxyhydr)oxides reaches adsorption energies of up to ~1.6 eV[57]; while, adsorption energies of up to 3.0 eV were obtained for the arsenate adsorption onto TiO2 based surfaces[58, 59] . 3.3 Electronic properties Density of states plots of some representative adsorbent-adsorbate systems are depicted in Figs. 8 and 9. At first glance, DOS plots indicate that the electronic structure of the substrates are weakly affected by the pollutant adsorption, and only differences can be seen near to the frontier molecular orbitals. We observe that the adsorption of trivalent and pentavalent methylarsenicals onto Aldoped graphene (in their neutral forms) has a weak effect on the electronic structure of the substrate; the HOMO-LUMO gap (HLg) of the isolated Al-G (1.19 eV) is decreased in about 0.15 eV in all the cases (with mono or dimethylated molecules). The latter suggest an slight increase in the reactivity of the substrate after the adsorption. In the case of the Fe-doped substrate, its HLg (0.54 eV) is increased in about 0.42 eV, indicating a higher effect of the pollutant adsorption onto the frontier molecular orbitals compared with Al-G; the latter is due to the high hybridization of the unoccupied 3d z2 orbital of Fe (with high contribution to the LUMO of Fe-G) with the lone pair orbital of the oxygen atom in the pollutant molecule, causing an antibonding orbital with high energy. The latter also indicates that the reactivity of the Fe-G adsorbent is decreased by the pollutant adsorption, and the adsorbent-pollutant system is highly stable. On the other hand, as expected, the adsorption of anionic pollutant increase the energy of the HOMO level due to the excesses of negative charge; for Fe-G as adsorbent, the increase in the HLg is lower in comparison with the neutral compounds (with an increase of 0.16 eV ); conversely, the adsorption of anionic pentavalent methylarsenicals decreases the HL g of Al-G in 0.86 eV. Therefore, the effects on the frontier molecular orbitals are only dependent of the pollutant charge, and not of the oxidation state, or of the mono or dimethylated forms.

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Figure 8. Total density of states (DOS) plots for some representative conformations of the adsorption of MMAIII and DMAIII onto Al and Fedoped graphene; most stable conformations were selected in all the cases. Vertical dotted line indicate the position of the HOMO level.

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Figure 9. Total density of states (DOS) plots for some representative conformations of the adsorption of anionic MMAV and neutral MMAV onto Al and Fedoped graphene; most stable conformations were selected in all the cases. Vertical dotted line indicate the position of the HOMO level. 3.4 Stability at aqueous and dynamic conditions As noted above, the arsenic pollutants show a high mobility from adsorbent surfaces in aqueous environments, which turns inefficient their removal, mainly for the trivalent forms[49]. Hence, it is important to determine the adsorbentadsorbate stability in water in order to insure for the applicability of the proposed adsorbents in an aqueous medium. In this regard, we computed the H2O interaction onto Al and Fedoped graphene with adsorption energies of 1.11 and 0.68 eV, respectively (Table 5), and bond lengths of dAlO=1.99 Å and dFeO=2.04 Å, respectively, which agree with DFT results for the H2O adsorption onto doped carbon nanotubes[22]. Given that the arsenic adsorption onto the doped adsorbents appears to be stronger than those for H2O molecules from comparing 21

their adsorption energies, the proposed doped graphene surfaces can serve as superior adsorbent materials for the removal arsenic pollutants. With regard to this last point, explicit solvent calculations were done to get insights into the structural effects of water molecules onto the adsorbentadsorbate systems; an explicit/implicit methodology was adopted by surrounding the adsorbate with explicit 15 H2O molecules and reoptimizing the whole system in presence of the implicit COSMO solvent model[39] to create the “water environment”; 36 adsorbentadsorbate systems were taken for statistics, including as pollutants to MMAIII, DMAIII, iAsV, MMAV, DMAV and DMMTAV. From the explicit solvent calculations, it was observed that the H2O molecules tend to form clusters surrounding the adsorbate, which stabilizes the adsorption, and avoid the diffusion of the pollutants from the adsorbent surface (see Fig. 10). Moreover, the geometrical parameters of the adsorbentadsorbate conformations are slightly affected with respect to the gas phase conformations (see Table S1 in the supporting content). In the case of MMAIII/DMAIII, the dopantO1 bond lengths appear in the range of dAlO1=1.822.00 Å and dFeO1=1.841.92 Å; while, the AsO1 bond appears stable with bond lengths in the range of 1.812.20 Å. In the case of pentavalent pollutants, the dopantO1 bonds are found in the range of dAlO1=1.801.96 Å and dFeO1=1.842.12 Å; while, the AsO1 bond is retained in the range of 1.691.79 Å, which is in agreement with the high stability of the As-O1 bond in the anionic forms as noted above. Furthermore, the explicit solvent calculations also show the trivalent methylarsenicals remain protonated in water environments as suggested from experimental results for related adsorbent materials 3, 35-36; the latter was determined by analyzing the bond length of the interacting OH groups with average values of 1.05 Å. Conversely, the H atoms turns labile from the interacting O1H group when the pentavalent pollutants are computed in the explicit solvent environment (with OH bond above ~1.4 Å, and forming H3O+ molecules), indicating that the pollutants are hydrolyzed in water, and then, mainly adsorbed in their anionic forms.

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Figure 10. Representative conformations of the adsorption of MMAIII, DMAIII, MMAV and DMAV onto Aldoped graphene in a explicit solvent environment; H2O molecules are depicted in white with pointed hydrogen bonds. All the systems are included in the supplementary content; color code by atom. Taking into account the above explicit solvent calculations, and to get insights about the stability of the adsorbateadsorbent interactions at dynamic conditions, we carriedout semiempirical molecular dynamics trajectories via the Verlet velocity algorithm[60] at 300 K, and using the DFT optimized structures as starting points; the Berendsen thermostat was used for the temperature control [61]. Solvent effects were included with the COSMO solvent model[39]. The potential was determined “on-the-fly” using the semiempirical PM6 Hamiltonian implemented in the MOPAC2012 program[62]; we have successfully used this methodology to analyze the stability onto graphene of 4chlorophenol[63], bisphenolA[64] and inorganic trivalent arsenic (iAsIII)[20]. The time-step for all the simulations was of 0.5 fs, and 5.0 ps were used in the production step; data were collected for statistics after 1.0 ps of both heating and equilibrium steps. Note that the from these calculations are obtained reasonable differences in the bond lengths, which are changed in up to ±0.2 Å with respect to those obtained from the DFT calculations.

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Figure 11. Radial pair distribution [gab (r)] of the AsO1, AlO1 and FeO1 distances in selected adsorbentadsorbate systems; conformation a was selected as representative. 10000 conformations per system were used for statistics, during a time t=5.0 ps at 300 K. a) Neutral trivalent methylarsenicals; b) anionic pentavalent methylarsenicals. All the conformations are included in the supporting content. We focused the analysis in the propagation of the AlO1, FeO1 and AsO1 bond length by means of the radial pair distribution function [gab(r)], which allows determining the distribution of distances between two atoms in the overall trajectory (10000 conformations by system were used for statistics). In this regard, the gab (r) function for the trajectory of MMAIII and DMAIII adsorbed onto the doped adsorbent shows that the AsO1 and dopantO1 bonds remain strong even in dynamic conditions with values on the range of 1.72.2 Å (Fig. 11a). On the other hand, the anionic forms of the pentavalent methylarsenicals were determined to be the main species at neutral pH; therefore, these conformations were selected for the molecular dynamics calculations; in these cases, the AlO1 and AsO1 bond lengths are found in the range of 1.62.0 Å (Fig. 11b), which also 24

agree with stronger interaction of anionic pentavalent arsenicals compared to the neutral forms. Therefore, these results indicate the high stability for the adsorption of methylarsenicals onto the proposed doped adsorbent even in aqueous conditions at ambient temperature. 4. Conclusions In summary, the removal of all the pollutants appears to be efficient onto Al and Fedoped graphene both in gas phase and water conditions, where both substrates performs well without significant differences. The adsorption of trivalent methylarsenicals graphene was determined with adsorption energies of up to 1.7 eV; while, adsorption of pentavalent methylarsenicals was determined with adsorption energies of up to 4.2 eV and 2.4 eV from neutral to low pH conditions, respectively. Moreover, the weakening of the AsO1 bond in the pollutant structure was determined to play an important role in the stability of the adsorbentadsorbate systems, determining the adsorption strength. , where the trivalent and pentavalent compounds are expected to be adsorbed in their neutral and anionic forms at neutral conditions; the removal in gas phase was determined to serve even for detection of methylarsenicals in particulate matter. With regard to the differences in the adsorption strength, which turns important for experimental purposes, the adsorption strength of all the arsenic pollutants can be sorted as: (anionic pentavalent arsenicals) > (neutral pentavalent arsenicals) > (neutral trivalent arsenicals). While trivalent arsenicals do not show differences between them with respect their adsorption strengths, the adsorption strength of pentavalent arsenicals can be sorted as the methyl groups in the pollutant increases, this is DMA V>MMAV>iAsV, then, determining the order of desorption. Therefore, Al and Fedoped graphene are proposed as a potential future class of high efficient adsorbent materials for the removal of hazardous methylarsenicals, including their trivalent and pentavalent forms, in addition to thiolated arsenicals.

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Supplementary Information: Optimized molecular structures, Mayer bond orders, fragmental electron density analysis, and explicit solvent calculations are included as supplemental information. Acknowledgments This work was supported by the projects FONDECYT/Postdoctorado no. 3140314, FONDECYT no. 1130072 and ICM grant no.120082. References [1] W.H. Organization, Guidelines for drinking-water quality: recommendations, World Health Organization, 2004. [2] K.T. Kitchin, Recent advances in arsenic carcinogenesis: modes of action, animal model systems, and methylated arsenic metabolites, Toxicol. Appl. Pharmacol., 172 (2001) 249-261. [3] E. Dopp, L. Hartmann, U. Von Recklinghausen, A. Florea, S. Rabieh, U. Zimmermann, B. Shokouhi, S. Yadav, A. Hirner, A. Rettenmeier, Forced uptake of trivalent and pentavalent methylated and inorganic arsenic and its cyto-/genotoxicity in fibroblasts and hepatoma cells, Toxicol. Sci., 87 (2005) 46-56. [4] V.K. Sharma, M. Sohn, Aquatic arsenic: toxicity, speciation, transformations, and remediation, Environ. Int., 35 (2009) 743-759. [5] J.E. Laine, K.A. Bailey, M. Rubio-Andrade, A.F. Olshan, L. Smeester, Z. Drobná, A.H. Herring, M. Stýblo, G.G. García-Vargas, R.C. Fry, Maternal Arsenic Exposure, Arsenic Methylation Efficiency, and Birth Outcomes in the Biomarkers of Exposure to ARsenic (BEAR) Pregnancy Cohort in Mexico, Environ. Health Perspect., 123 (2015) 186. [6] L. Yang, Y. Chai, J. Yu, B. Wei, Y. Xia, K. Wu, J. Gao, Z. Guo, N. Cui, Associations of arsenic metabolites, methylation capacity, and skin lesions caused by chronic exposure to high arsenic in tube well water, Environ. Toxicol., (2015) in press: DOI: 10.1002/tox.22209. [7] D. Melak, C. Ferreccio, D. Kalman, R. Parra, J. Acevedo, L. Pérez, S. Cortés, A.H. Smith, Y. Yuan, J. Liaw, C. Steinmaus, Arsenic methylation and lung and bladder cancer in a case-control study in northern Chile, Toxicol. Appl. Pharmacol., 274 (2014) 225-231. [8] B.A. Peters, M.N. Hall, X. Liu, V. Slavkovich, V. Ilievski, S. Alam, A.B. Siddique, T. Islam, J.H. Graziano, M.V. Gamble, Renal function is associated with indicators of arsenic methylation capacity in Bangladeshi adults, Environ. Res., 143, Part A (2015) 123-130. [9] E. Dopp, L. Hartmann, A.-M. Florea, U. Von Recklinghausen, R. Pieper, B. Shokouhi, A. Rettenmeier, A. Hirner, G. Obe, Uptake of inorganic and organic derivatives of arsenic associated with induced cytotoxic and genotoxic effects in Chinese hamster ovary (CHO) cells, Toxicol. Appl. Pharmacol., 201 (2004) 156-165. [10] E.M. Kenyon, M.F. Hughes, A concise review of the toxicity and carcinogenicity of dimethylarsinic acid, Toxicology, 160 (2001) 227-236. [11] Y. Cao, X. Li, Adsorption of graphene for the removal of inorganic pollutants in water purification: a review, Adsorption, 20 (2014) 713-727. [12] S. Wang, H. Sun, H.-M. Ang, M. Tadé, Adsorptive remediation of environmental pollutants using novel graphene-based nanomaterials, Chem. Eng. J., 226 (2013) 336-347. [13] G.Z. Kyzas, E.A. Deliyanni, K.A. Matis, Graphene oxide and its application as an adsorbent for wastewater treatment, J. Chem. Technol. Biotechnol., 89 (2014) 196-205. [14] X.-L. Wu, L. Wang, C.-L. Chen, A.-W. Xu, X.-K. Wang, Water-dispersible magnetite-graphene-LDH composites for efficient arsenate removal, J. Mater. Chem., 21 (2011) 17353-17359. [15] K. Zhang, V. Dwivedi, C. Chi, J. Wu, Graphene oxide/ferric hydroxide composites for efficient arsenate removal from drinking water, J. Hazard. Mater., 182 (2010) 162-168.

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