Removal of Water-Soluble Inorganic Arsenicals with Phosphorene Oxide Nanoadsorbents: A First-Principles Study

A complete picture of the phosphorene oxide (PhosO) sorption properties for the simultaneous removal of inorganic As(III) and As(V) pollutants from water has been developed using first-principles calculations. Calculated adsorption energies, competitive adsorption with co-existing species, energy decomposition analyses (ALMO-EDA), implicitly/explicitly solvated geometries, and adsorption free energies provide deep insights into the adsorption mechanism as well as the origin of the strong selectivity sorption ability. The PhosO nanoadsorbents establish inner-sphere surface complexes with arsenicals even under competition with water molecules. These proposed structures also show a strong affinity with the highly mobile As(III), where energy saving is achieved by avoiding the pre oxidation process to convert As(III) into As(V) as requested in related materials. Results show that electrostatic driving forces govern the adsorption of neutral arsenicals, while the interplay between electrostatic and polarization phenomena drives the uptake of anionic arsenicals. By computing the adsorption strength as a function of the oxidation degree, the optimum adsorption efficiency is reached with a 25% in the content of oxidizing groups. In this oxidation degree, the strong repulsive surface charge at high pH turns the PhosO nanoadsorbents convenient to recycle via simple treatment with alkaline eluents. Finally, the adsorption ability remains thermodynamically allowed in a wide range of ambient temperatures (enthalpically governed reaction). Conceptually understanding the sorption properties of phosphorene-oxide-based materials towards arsenic pollutants provides a useful framework for future water treatment technologies.


Introduction
correction avoid basis set superposition errors [38]. Moreover, adsorption energies were 99 further decomposed into physical contributions by the energy decomposition analysis based 100 on absolutely localized molecular orbitals (ALMO-EDA) in the Q-Chem program [39,40]. 101 Accordingly, the adsorption energy for one AB complex is expressed as: 102 −Eads =∆ECT+∆EPOL+∆EDISP+∆EELEC+∆EPREP+∆EPAULI (2)     the Eads values up to 160% with respect to intrinsic phosphorene. As an illustration, As(III), 192 As(V) − , and As(V) 2− reach Eads values of 1.1, 1.7, and 1.8 eV onto PhosO(1:4), respectively.   It is also important to highlight that because of the weak affinity towards As(III) of 203 several adsorbents, the arsenic treatment technologies require the pre-oxidation of As(III) to 204 As(V) to allow the efficient uptake. Pre-oxidation is used in technologies employing iron 205 coagulants, nanofiltration by thin-films, and membrane-integrated hybrid systems[58-60].

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These processes turn costly and time-consuming due to the operational complexity, make use 207 of strong oxidants (H2O2 and KMnO4) or photocatalysts (TiO2) in the pre-oxidation process.

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Therefore, medium-oxidized PhosO structures could be implemented as excellent 209 nanoadsorbents for simultaneously and directly removal of As(III) and As(V), where energy 210 saving is achieved by avoiding the pre-oxidation process to convert As(III) into As(V).   219 We also analyzed the adsorption stability of water molecules (H2O) and hydroxide 220 anions (OH − ) onto phosphorene oxide (Fig. 2). First, the adsorption energy of the   256 We attempt to provide a quantitative and readily physical interpretation of the 257 adsorption mechanism by analyzing the specific role of physically intuitive meaningful terms 258 with the ALMO-EDA method. EDA terms were organized as stabilizing (EELEC, EDISP, 259 ECT, EPOL), and destabilizing terms (EPAULI, EPREP). Table 2 shows the EDA terms, and   (Fig. 3), denoting that adsorption on both materials is governed by similar driving forces. In  EDA terms were also obtained for the As(V)−PhosO complexes to rationalize the 82% of the total stabilizing energy (Fig. 3). Therefore, the inner-sphere surface complexation  Finally, destabilizing effects emerge at least 80% due to Pauli repulsion (EPAULI) 306 in all the cases because the geometrical structure of As(III)/As(V) is weakly influenced upon 307 adsorption. A high EPAULI term appears in the As(V)−PhosO complexes due to the strong 308 negative charge excesses in both the adsorbent and adsorbate (up to EPAULI14 eV, Table   309 2). However, the magnitude of steric repulsion is lower than the main stabilizing forces adsorption performance for arsenic removal than medium-oxidized phosphorene.

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An overview of the whole adsorption process along the dissociation path of the 319 As(III)−PhosO and As(V) − −PhosO complexes as representative cases is presented (Fig. 4).

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The dynamic adsorption process is determined in the first stages by the mass transport  Fig. 4b). Therefore, the time to reach the adsorption equilibrium is 338 expected to be lesser for pentavalent arsenicals than trivalent ones. After the diffusion step,   8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4. 344 The adsorption stability in a water environment was examined to ensure low pollutant due to the adsorption mechanism via inner-sphere surface complexation. 403 We explored the adsorption process spontaneity in a temperature range of 300−1000 404 K (Fig. 6). At room temperature (300 K), the adsorption free energy (ΔGads) of the As−PhosO 405 and As−Phos complexes is negative, denoting a spontaneous adsorption process. The   429 We have theoretically elucidated the sorption properties of phosphorene oxide 430 nanoadsorbents for the simultaneous removal of inorganic As(III) and As(V) pollutants from 431 water. We found that phosphorene oxide forms stable inner-sphere surface complexes with 432 arsenicals even under aqueous environments, and it shows a strong affinity with highly 433 mobile As(III). Electrostatic driving forces govern the adsorption of neutral arsenicals, while 434 the interplay between electrostatic and polarization phenomena drives the uptake of anionic 435 arsenicals. Furthermore, the optimum adsorption efficiency is reached with a 25% in the 436 content of oxidizing groups, which also turns the phosphorene oxide nanoadsorbents 437 convenient to be recycled via simple alkaline treatment. Moreover, adsorption-free energies 438 show that the adsorption process is allowed in a wide range of ambient temperatures. nanocomposite adsorbents for arsenic removal, Colloids Surfaces A Physicochem.