Arsenate and Arsenite Reaction Kinetics with Ferric Hydroxides Using Quantum Chemical Calculations 144 D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 03 .0 3 Cijin J. George, Sougata Santra, Zyryanov G. V., Kousik Giri Chimica Techno Acta. 2018. Vol. 5, No. 3. P. 144–149. ISSN 2409–5613 Cijin J. George1, Sougata Santra2, G. V. Zyryanov2,3, Kousik Giri1* 1Department of Computational Sciences, Central University of Punjab, Bathinda, Punjab, India 2Department of Organic and Biomolecular Chemistry, Chemical Engineering Institute, Ural Federal University, 19 Mira St., Ekaterinburg, 620002, Russian Federation 3I. Ya. Postovskiy Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, 22 S. Kovalevskoy St., Ekaterinburg, 620219, Russian Federation *E-mail: kousikgiri@gmail.com Arsenate and Arsenite Reaction Kinetics with Ferric Hydroxides Using Quantum Chemical Calculations The knowledge of the mechanism involved in the process of adsorption and desorption of arsenate and arsenite with ferric hydroxides is important to address the water toxicity problems and to tackle the adverse effect of these substances in nature. An essential outcome of previous studies on the kinetics of the arsenate adsorption on aluminum and iron oxide was that the adsorption is a two-phase (bi-phase) process. Quantum mechanical calculations using density functional theory were used to determine the thermodynamic variables governing the adsorption process to get an insight into the stability of the complexes formed. The previous investigation showed that the positively charged ferric hydroxide cluster had better stability at neutral pH. The chemisorbed charged monoden- tate complexes had Gibbs free energy of reaction –55.97 kcal/mol where the bidentate complex formation had Gibbs free energy of reaction –62.55 kcal/mol. The bidentate complex having a negative charge had more Gibbs free energy of reaction compared to uncharged one. The results of the study indicate that Gibbs free energy for the reaction has a significant role in controlling the kinetics of the adsorption and sorption process of arsenate on ferric hydroxide clusters. Keywords: Received: 01.10.2018. Accepted: 25.10.2018. Published: 31.10.2018. © Cijin J. George, Sougata Santra, Zyryanov G. V., Kousik Giri, 2018 Introduction Arsenic is a significant contami- nant present in the groundwater due to natural processes like weathering of rocks, discharge of effluents and waste disposal from industries, arsenical herbicides and pesticides used in agricultural activities and many other sources [1]. The various oxidation states arsenic exhibits are –3, 0, +3, and +5, where the inorganic forms +3 (arsenite) and +5 (arsenate) states of As are 145 predominant depending on the reducing and oxidizing conditions, respectively. The relatively strong adsorption affinity of As shown towards ferric hydroxides has a vi- tal role in the detoxification of As, thereby controlling the arsenic water pollution and employed in techniques for purification of drinking water. Several works were done on the struc- tural determination of arsenate-ferric hydroxide complex. The studies of arse- nic (III) stability on goethite using X-ray absorption spectroscopy and batch tech- niques indicated that there is inner sphere bidentate complexation through ligand exchange [2]. Monodentate and bidentate complexes are observed in the adsorption of arsenate on goethite [3–5]. The models produced by molecular modeling and the X-ray absorption fine structure spectros- copy (EXAFS) results are compared, and the nature of the chemisorption complex is determined [6, 7]. An essential outcome of previous studies on the kinetics of the arsenate adsorption on aluminum and iron oxide was the two-phase adsorption pro- cess constituting a fast phase of the time order of a minute or less and slow phase which attains an equilibrium within a time scale greater than 162 hours [5, 8–10]. The factors responsible for this phenomenon are considered to be slow diffusion mass transport [9, 11, 12], availability of hetero- geneous sites, monodentate to bidentate complex conversions, the surface precipi- tate formation and the rearrangement of surface complexes. Further, the studies on the arsenate desorption by use of different extractants like phosphate or high pH so- lutions indicate a slow release of arsenate from the adsorbed complexes. The conclu- sion from these studies implies that only a small portion of arsenate is released by the use of extractants like phosphate or hydroxide ions. This leads to the interesting fact that a part of arsenate-ferric hydroxide complex may be irreversible in nature. Despite the studies done on arsenate- ferric hydroxide complex formation, a little is known about the mechanism of this re- action. In this regard, the motive of this theoretical study is to get an insight into the mechanism governing the arsenate- ferric hydroxide complex formation. The thermodynamic parameters and reaction rates governing the mechanism are deter- mined by quantum chemical calculations. Materials and Methods All calculations were carried out using the Gaussian 09 software [13]. Full geo- metry optimizations and corresponding harmonic vibrational frequency compu- tations, to confirm their minima on the potential energy surface, were carried out using the Austin-Frisch-Petersson func- tional with dispersion (APFD) [14] and M06 hybrid functional of Truhlar and Zhao [15] suite of density functional theories (DFT) as implemented in Gaussian 09. AP- FD functional has been used as a primary method in Gaussian 09 for its best trade-off between accuracy and computational cost for the largest range of molecular systems and chemical problems. M06 functional perform better for a model system with dispersion and ionic hydrogen-bonding interactions. For both structural optimiza- tions and frequency calculations the ba- lanced basis set of triple zeta valence from Ahlrichs and coworkers (DEF2TZVP) [16] was employed. For all calculations, solvent effects of water were introduced using the polarizable continuum model (PCM) us- ing the integral equation formalism variant 146 (IEFPCM) with APFD functional and the SMD variation of IEFPCM of Truhlar and workers with M06 functional. Quantum-chemical calculations involv- ing DFT were used to calculate the heat of formation of reactants and products and their corresponding Gibbs free energies. The ferric hydroxides clusters were mo- deled following the previous studies. The ferric hydroxides clusters consisted of two iron atoms, ten oxygen atoms which were octahedrally coordinated to the iron atoms. The clusters can be represented by the ge- neral formula Fe2O3 (H2O)7. The numbers of the protons were varied to calculate the effect of binding of arsenate with the cation and anion cluster’s binding sites. To minimize the effect of gross dis- tortions of the di-octahedral geometry observed in previous studies [17], the co- ordinates of six peripheral oxygen atoms which were not part of the binding reac- tion was fixed to simulate the binding sites of the iron hydroxide clusters also includ- ing bound arsenic species. The enthalpies of the reaction are determined by calculat- ing the heats of formation. The equation used is as follows: ∆r H0(298K) = = Σ(E0 + Hcorr)products – (E0 + Hcorr)reactants Similarly, Gibbs free energy change of the reaction is computed by the key equa- tion: ∆r G0(298K) = = Σ(E0 + Gcorr)products − (E0 + Gcorr)reactants Results and Discussion The heat of formation of the clusters at 0K was determined and is given in Table 1. Table 1 Calculated heat of formation for three different possible species Cluster Charge Heat of Formation (kcal/mol) Fe2O10H13 – –1 –1261.64 Fe2O10H14 0 –1186.33 Fe2O10H15 + +1 –1087.94 The results indicate that the heat of formation of the positive charged fer- ric hydroxide has the least value at 0K, whereas the other clusters are more stable than Fe2O10H15 +. The previous investigation showed that the positively charged ferric hydroxide cluster had better stability at neutral pH [17]. This may be possible due to the better stabilization of the Fe2O10H15 + at neutral pH and hence increase in the heat of formation at the neutral pH. The optimized geometry of the Fe2O10H15 + is represented by Fig. 1. The reaction of Fe2O10H15 + with HAsO4 2– resulted in the formation of charged (–1 charge) monodentate (Fig. 2) and bidentate complex (Fig. 3). Meanwhile, the reaction of Fe2O10H15 + with H2AsO4 - resulted in the formation of uncharged monodentate (Fig.  4) and bidentate complex (Fig. 5). The corresponding Gibbs free energy for the reaction was determined for the monodentate and bidentate complexes formed. The values are in given in table two and three, respectively. The mono- and bidentate complex bear- ing a negative charge showed higher stabi- lity than the uncharged complexes formed. This indicates that the charge on the species improved the stability of the complex. The chemisorbed charged monodentate com- plexes had Gibbs free energy of reaction –55.97 kcal/mol, whereas the bidentate complex formation had Gibbs free ener- gy of reaction –62.55 kcal/mol. Thus, the desorption process of arsenate with com- 147 petitive ligands would depend on Gibbs free energy of the reaction. The process is feasible only when the higher Gibbs free energy values are attained. The results of the study indicate that Gibbs free energy for the reaction has a significant role in controlling the kinetics of the adsorption and sorption process of arsenate on ferric hydroxide clusters. Fig. 1. Structure of Fe2O10H15 + complex carrying a net positive charge Fig. 3. Structure of negatively charged bidentate complex Fig. 2. Structure of negatively charged monodentate complex Fig. 4. Structure of uncharged monodentate complex Fig. 5. Structure of uncharged bidentate complex 148 Acknowledgments K. Giri acknowledges financial support from UGC, Govt. of India for Start-up Pro- ject Funding. S.  Santra and G. V.  Zyryanov thank Russian Science Foundation (Ref # 18-73-00301) for financial help. References 1. Bhumbla DK, Keefer RF. Arsenic mobilization and bioavailability in soils. Arsenic in the Environment. Part I: Cycling and Characterization. Nriagu JO, Editor. John Wiley & Sons: New York, 1994. pp 51−82. 2. 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