Quantum chemical and experimental studies on the extraction of acid blue 80 and acid red 1 from their aquatic environment using tetrabutylammonium bromide based deep eutectic solvents

Abstract In this study, the highest occupied molecular orbital (HOMO) energy, the lowest unoccupied molecular orbital (LUMO) energy, HOMO–LUMO energy gap, chemical potential (µ), electronegativity (χ), electrophilicity index (ω), global hardness (η), and global softness (s) of dyes; acid blue 80 (AB80), acid red 1 (AR1) and deep eutectic solvents: tetrabutylammonium bromide with glycerol {[TBAB][Gly]}, tetrabutylammonium bromide with decanoic acid {[TBAB][DA]} and tetrabutylammonium bromide with oleic acid {[TBAB][OA]} were predicted and calculated using quantum chemical calculation. The polarity of all the studied molecules also predicted in terms of screening charge density and plotted sigma profile and sigma potential. Further, the extraction experiment was conducted for the extraction of acid blue 80 (AB80) and acid red 1 (AR1) using tetrabutylammonium bromide with decanoic acid {[TBAB][DA]} and tetrabutylammonium bromide with oleic acid {[TBAB][OA]} as the potential solvent. The process was characterized by means of distribution coefficient and efficiency. Finally, the extraction process feasibility was studied and discussed in terms of and Graphical abstract


Introduction
Dyes are synthetic chemicals that give color to the products on binding with the substance. Since, dyes have a widespread choice of applications in rubber, leather, paper, plastics, textile, cosmetic, printing, pharmaceuticals and food processing industries for the coloration of the desired products, their residues are inevitably found in the effluents discharged from respective industrial wastewater. [1] Synthetic dyes are water-soluble, dispersible and dissolved inorganic solvents that affect the physiochemical properties of fresh water. [2] In addition to the adverse color of industrial wastewater, dyes may degrade to produce toxic products and carcinogens. [3][4][5] Furthermore, colored industrial wastewater consists of complex aromatic hydrocarbons with structures including aryl rings and radical configurations such as carbonyl (¼C ¼ O), azo (-N ¼ N-), nitro (-NO 2 or ¼ NO-OH), carbon-carbon (¼C ¼ C¼), nitroso (-NO or N-OH), carbon-nitrogen (>C ¼ NH or -CH-N), carbon-sulphur (C ¼ S), and ionizing groups like -NH 3 , -COOH, -HSO 3 , and -OH. These radical configurations and ionizing groups present in dye molecules can make them extremely stable, and non-biodegradable. [6][7][8][9][10] Since, most selective dyes are highly toxic, allergenic, mutagenic, and carcinogenic, it leads to affect the aquatic biota, human health, and photosynthetic activities. Moreover, the recent regulations requiring lower dye concentration levels now demand a process for deep decolourization. [11][12][13][14][15][16] Till today, several methods have been adopted for the elimination of such low-concentrated dyes from wastewater to decrease their impact on the ecosystem. [16][17][18][19] Methods such as adsorption, decolourization by photocatalysis, oxidation processes, coagulation, chemical oxidation, microbiological or enzymatic decomposition, solvent extraction process, and photodegradation were repeatedly used to remove few dyes from wastewater because of their low biodegradability and high solubility nature. [18,19] Though, solvent extraction has been recognized as a novel decolourization process for the removal of selective dyes from wastewaters. [20][21][22][23][24][25][26] Moreover, solvent extraction is strongly depending on selection of solvent rather than other parameters such as temperature and pressure. Also, search for new "green" solvents such as ionic liquids and eutectic mixtures, allowing removal of a selective dye molecules from synthetic industrial effluent, shows promise. As a rule, complete removal of a selective dye molecules from industrial effluent by extraction process which requires a multistep extraction process.
Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to their individual components. DESs are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. Most DESs are synthesized using bulk chemicals, often from natural origin, which consequently makes them cheap and biodegradable. DESs cannot be considered as traditional ILs because: (i) DESs are not entirely composed of ionic species, (ii) can also be obtained from nonionic species, (iii) low price, (iv) chemical inertness with water, (v) easy to prepare (since DESs are obtained by simply mixing two components, thus over-passing all problems of purification and waste disposal generally encountered with traditional solvents and ionic liquids), and (vi) most of them are biodegradable, biocompatible and nontoxic, reinforcing the greenness of these media. [27][28][29][30][31][32][33][34] In this work, the highest occupied molecular orbital (HOMO) energy and lowest unoccupied molecular orbital (LUMO) energy of acid blue 80 (AB80), acid red 1 (AR1), and three different nature of DESs such as tetrabutylammonium bromide with glycerol f[TBAB][Gly]g, tetrabutylammonium bromide with decanoic acid f[TBAB][DA]g, and tetrabutylammonium bromide with oleic acid f[TBAB][OA]g were predicted using Gaussion 03 with Quantum chemical principle. From this predicted orbital energy values, HOMO-LUMO energy gap, chemical potential (m), electronegativity (v), electrophilicity index (x), global hardness (g) and global softness (s) were calculated in order to understand the chemical reactivity of all the studied molecules at molecular level. Further, hydrogen bond donor (HBD), hydrogen bond acceptor (HBA), interaction with polar group in both the molecules (dye and DES's molecules), and interaction with non-polar polar group in both the molecules (dye and DES's molecules) were analyzed through sigma profile and sigma potential. Furthermore, the solvent extraction process was conducted to remove AB80 and AR1 from their aqueous solutions using ammoniumbased DESs at room temperature and atmospheric pressure. Finally, the thermodynamic process feasibility was studied through the change in Gibbs free energy (DG 0 T ), change in enthalpy (DH 0 T ), and change in entropy (DS 0 T ).

Computational details
The geometry optimization of acid blue 80 (AB80), acid red 1 ( [OA]g were executed using Gaussian03 package. Initially, MOLDEN visualization freeware was used to construct the 3D structure of the studied components. [35] The minimization of the electronic energy was done at Hartree-Fock level theory with the basic set 6-31G Ã for optimization. [36] Here, the optimized .out Gaussian file was used to calculate global scalar properties. After geometry optimization, #P BVP86 density functional theory (DFT); basis set of TZVP (triple zeta valence polarized) and density fitting basis set DGA1 (density gradient approximation) through keyword scrf ¼ COSMORS were used to compute the COSMO file. Since, the chemical 3D structure of deep eutectic solvent [TBAB], [OA] and a selective dye molecule like AB80 and AR1 are not inbuild in the library of COSMOThermX software and therefore we drawn initial structure using MOLDEN visualization package and then geometry optimization was done in Gaussian 03. After geometry optimization, .cosmo file was generated in the Gaussian 03 and then imported to COSMOThermX in order to predict the screening charge density of the studied molecules and then used for the generation sigma profile and sigma potential. The COSMO file contains the COSMO volume, COSMO area, and the ideal screening charges of the molecule. [37][38][39][40] This file was imported in COSMOthermX to generate r profiles and r potential. [41][42][43] 3. Experimental section

Deep eutectic solvents synthesis
Based on the approach studied by Hizaddin et al. [24] and Smith et al. [44] DESs with molar ratio 2:1 of hydrogen bond donor (HBD) with a hydrogen bond acceptor (HBA) was prepared. A stirring speed of 600 rpm at 45 C was maintained until the colorless homogenous solution was formed. Later, the DESs samples were kept undisturbed for 4 hours to attain equilibrium. Wesner weighing balance with an accuracy of ± 0.0001 g was used for weighing all the chemicals. Thereafter, the extract and raffinate phases are separated using a 10 mL separating funnel. The aqueous phase was then analyzed by UV-vis spectrometer (Lark-LI-UV-7000) for determining the water concentration. The AB80 and AR1 concentrations were determined at 620 nm and 532 nm wavelengths, respectively. Using the mass balance equation from the aqueous phase, the extraction efficiency of solvent for removal of AB80 and AR1 was determined.

Quantum chemical descriptors
In the DFT method, the seven basic parameters give the chemical reactivity and selectivity of the individual molecules. The parameters are; HOMO energy, LUMO energy, HOMO-LUMO energy gap, chemical potential (m), electronegativity (v), electrophilicity index (x), global hardness (g), and global softness (s). These parameters were calculated from the mathematical expression presented in Table S1.

HOMO and LUMO energies
It is evident from Table S2 and Figure S1a that, the HOMO energy of water is À0.496 Hartrees, AB80 possess À0.201 Hartrees and AR1 possess À0.159 Hartrees, whereas the LUMO energy of water is 0.213, AB80 is À0.069 and AR1 is À0.056. This indicates that water is more stable than dye compounds. Similarly, AB80 is more stable and less reactive than AR1. This is because of the lower LUMO energy of the AB80 compound. [ [45][46][47] This implies that AB80 and AR1 molecules occupy the unoccupied state of the DESs. Thus, HBD plays a crucial role in their combinations. The DESs show different HOMO and LUMO energies values, which increases in the sequence as follows ( Figure S1b) [DA] and lower than other HBDs. This confirms that the HBA easily combines with the HBDs to form a good extract. From Figure S1 (b), the HOMO-LUMO gap energy for DESs has seen very smaller values. This implies DESs used in this work possess less stability, more reactivity and have a higher tendency to interact with AB80 and AR1. Also, it is can be said that the interaction of DESs toward the AB80 and AR1 are possessed in the following order: [43,44,47]    and AR1 (0.023 Hartree) are lesser than DESs. [46,47] Therefore, as expected both hydrophobic DESs

Global hardness (g) and softness (s)
From Figure S3a, the individual compounds show more global softness than global hardness. This specifies the compounds are nonresistant toward transferring electrons. Also, the dye compounds AB80 (13.805 Hartrees) and AR1 (22.134 Hartrees) show more global softness than the HBA and HBD, where global softness increases with the decrease of global hardness, also the similar trend is seen in the complex of HBA and HBDs ( Figure S3b). Therefore, it can be said that the DESs that possess the highest softness are more favorable in the removal of dye compounds from the aqueous medium. Based on the highest global softness values, f[TBAB][DA]g (10.404 Hartree) is said to be more favorable followed by f[TBAB][OA]g in the removal of AB80 and AR1 from aqueous medium. [43,46]

r-profile and r-potential analysis
In Figure 1, the r-profile of AB80 and AR1 are mainly distributed in the region À0.028 e/Å 2 to þ0.020 e/Å 2 . It is observed that the two peaks in the region À0.008 e/Å 2 < r < þ0.008 e/Å 2 indicate the presence of benzene ring present in AB80 and AR1 which is weak for hydrogen bond interaction. The negative peaks of r-profile are due to the H and N atoms present in AB80 and AR1, while the positive peak are due to the C and S atoms present in AB80 and AR1. [48] Also, the small peaks in r-profile of AB80 and AR1 distributed beyond r < À0.008 e/Å 2 indicates, weak HBD ability due to (N-H) in the molecule, whereas the small peaks beyond r > þ0.008 e/Å 2 is due to O atom indicating to weak HBA. Also, the symmetrical shape of r-profile indicate a favorable interaction between the benzene and itself, explaining its high boiling point and surface tension of AB80 and AR1. For water, the r-profile is distributed broadly over À0.03 e/Å 2 < r < þ0.03 e/Å 2 region. It indicates the hydrogen atom act as HBD and the O atom as HBA. Also, the peaks distributed in a wide range in the non-polar region are below the peaks of the polar region thereby hinting that the complete surface of the water molecule is polarizable. Therefore, the r-profile of water assigns the polar nature and hydrogen bonding formation with other molecules. In such a way, water dissolves AB80 and AR1 easily. [49,50] Deep eutectic solvents are typically synthesized by combining HBA and HBD. [DA]g in the non-polar region remove the broadly distributed AR1 in the non-polar region. [48][49][50] In Figure 2, the r-potential plot showing higher positive values in the H-bonding threshold (r < þ0.008 e/Å 2 ) signifies an increase of repulsive behavior and negative values (À0.008 e/Å 2 > r) signifies higher affinity between the molecules. From Figure 2, the AB80 and AR1 shows negative values at r < À0.008 e/Å 2 and positive at r > þ0.008 e/Å 2 . This specifies that they have a higher affinity toward both H-bond donors and H-bond acceptors. In the case of water, the r-potential shows a broad plot at r ¼ 0. It signifies the lipophilic nature of water and the broad peaks of all compounds showing r-potential away from r ¼ 0 indicates the hydrophobicity nature. The DESs compounds show negative r-potential values beyond r < À0.008 e/Å 2 and positive peaks beyond r < þ0.008 e/Å 2 . It signifies that they have an affinity toward both donor and acceptors compounds. Therefore, the acceptor compounds of DESs have an affinity toward the donor compounds of AB80 and AR1 and vice-versa by strong H-bonding formation.

Different volume ratio of DES (f[TBAB][DA]g and/or f[TBAB]
[OA]g) rich phase to aqueous dye phase (AB80/AR1) were tested at different ppm of aqueous dye solutions (from 25 to 100 ppm). Here, the extraction efficiency (% g) were determined using the following equation as follows, [51,52] % g ¼ Here, C 1 denotes the initial aq.AB80 (or) aq.AR1 concentrations and C 2 denotes the final AB80 and AR1 concentration in their raffinate phase (aqueous phase) after extraction with the DESs.
The results presented in Table S3 and the extraction efficiency of AB80 and AR1 from their aqueous solution versus phase volume ratio of 7:3; 8:2; and 9:1 were shown in Figure  3a, b, Figure 4a, (20) and applied for evaluation of used DES's. [53][54][55] Table S3. It was noted that the distribution coefficient increased with increasing the phase volume ratio of aqueous AB80 or AR1/DES's at 7:3; 8:2; and 9:1, respectively. Hence, the distribution coefficient of AB80 and AR1 in DES's is strongly depending on the molecular configurations of dyes as well as DES's which include azo (-N ¼ N-); sulfonyl (-SO 3 ); carbonyl (¼C ¼ O); hydroxyl (-OH); carbon-nitrogen (>C ¼ NH or -CH ¼ N-) and alkyl groups. Moreover, the distribution coefficients of AB80 The transfer enthalpies for AB80 and AR1 extraction was determined by using the slopes ÀDH 0 T =2:3R À Á (Figures S4a,  b and S5a, b). Likewise, the entropy DS 0 T À Á was calculated from the following equation [55] TDS 0 showing positive values indicate that the process is endothermic and the affinity of AB80 and AR1 toward the DESs are higher at the liquid-liquid interface. Therefore, the temperature ranges (298.15-328.15 K) can be considered as more favorable for an effective separation process. The findings are consistent with the result reported by Pei et al. for the removal of selective dyes using ionic liquids. These thermodynamic data characterize the interactions as the driving force for extraction of AB80 and AR1 from their aqueous solution driven by entropy terms. [56][57][58][59]

Conclusion
The [OA]g gave higher efficiency (82.02-97.74%) for AR1. Finally the process feasibility was studied at temperatures ranging from T ¼ 298.15 to 328.15 K and at 1 atm. Finally, it was concluded that the hydrophobic based DES's has the potential to remove such a low concentration of dye molecules from an industrial dye effluent at atmospheric conditions. Since three is no a prior publication available with respect to DES's based extraction process for the removal of dye molecules in the literature.