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Abstract

Activated carbon was prepared from the seeds of aguaje palm (Mauritia flexuosa L.f.) by a chemical activation with phosphoric acid. This activated carbon was used for adsorbing metal ions: Pb(II), Cd(II), and Cr(III). To understand the mechanism of adsorption of these heavy metals (Cr, Cd, and Pb), the activated carbon surface was oxidized with nitric acid (1 M) increasing the oxygenated surface groups showing an increasing in their adsorption capacities of these metals. The oxidized activated carbon slightly increased the maximum adsorption capacity to 5–7%. The order of adsorption for unoxidized and oxidized activated carbons was Pb> Cd> Cr. This experimental information was corroborated by molecular modeling program Hyperchem 8 based adsorption mainly on two factors: the electron density and orbitals—highest occupied molecular orbital and lowest unoccupied molecular orbital.Activated carbons were characterized by adsorption/desorption of N₂, obtaining an increase of microporous surface area for oxidized activated carbon. An increase of surface acidity and a reduction of isoelectric points were observed in oxidized activated carbon. According to these results, the adsorption of metal ions is favored in contact with an oxidized activated carbon, which has more amount of phenolic and carboxylic functional groups. Similarly, decreasing the isoelectric point indicates that the surface has a higher negative charge. The surface information was corroborated by Hyperchem, which indicates that the surface of the oxidized activated carbon has a higher electron density, indicating a larger amount of electrons on its surface, which means the surface of oxidized activated carbon charges negatively and thereby attracts metal ions.

Keywords

Activated carbon metal ions mechanism of adsorption Hyperchem

Introduction

Water pollution has become one of the most significant environmental problems in recent times. Decrease of water resources by human pollution leads to finding solutions to this problem for removing or reducing pollutants. Heavy metals are one of the most interesting pollutants removal (Ademiluyi and David-West, 2012).

There are several techniques for removal of these heavy metals in aqueous media.

Water treatment techniques include precipitation, reverse osmosis, advanced oxidation reactions, ion exchange, adsorption, among others (Kobya et al., 2005). Adsorption is one of the most used because it is easy to implement, versatility, low cost, and efficiency (Gupta and Ali, 2004). Activated carbon (AC) is one of the most versatile and most industrial application adsorbent materials. It has a large surface area, a good porosity, and a high adsorption capacity due to the surface chemistry (Aygün et al., 2003; Gupta and Ali, 2004; Sheng-Fong et al., 2012).

AC could be from various sources including the lignocellulosic materials such as shells and seeds of fruits stones. At this moment, the most widely used for industrially producing AC raw material is coconut shell (Ioannidou and Zabaniotou, 2007; Kadirvelu et al., 2001). In Peru, one of the emblematic fruits of the Amazon is the aguaje (Mauritia flexuosa L.f.), which is a widely consumed product. Moreover, seeds of aguaje are discarded in large quantities and are an appropriate source of AC.

Some of the interactions between metal ions and AC surface are surface precipitation, ion exchange, and complexing metal ions (Omri and Benzina, 2012). However, according to previous studies, Alfarra et al. (2004) and Parr and Pearson (1983) describe that adsorption of metal ions onto AC corresponds to an acid–base interaction.Such interaction is of great importance on the adsorption phenomenon. For this reason, it is important to note this characteristic of the species because it will impact severely on the adsorption.

In previous works (Alvarez-Puebla et al., 2004; Peles-Lemli et al., 2013; Ras et al., 2013; Shahzad Khan and Shahid Khan, 2011), it was used in computational chemistry to realize a prediction about interactions between a surface and an adsorbate.To do this its proposed do a computational simulation with program Hyperchem, building ACs, and employing semi-empirical methods to model an adsorption process comparing theoretical results with experimental work. The objective of this paper is to identify a mechanism of adsorption of heavy metal ions modifying the surface of carbon by oxidation from the AC of aguaje seeds.

Oxidation of AC does not significantly affect the mesoporous area due to the mild oxidation treatment conditions. Previous works demonstrated that development of a large mesoporous area is the most influence factor in metal adsorption capacity in comparison with total surface area (including microporous). AC with a large mesoporous area demonstrated higher adsorption capacity of cadmium, chromium, and lead adsorption in previous work (Obregón-Valencia and Sun-Kou, 2014; Sun-Kou et al., 2014). It is suggested that first step is the diffusion of metals ions through mesoporous to reach microporous areas.

Experimental

Sample preparation

The AC is prepared from seeds of aguaje (M. flexuosa L.f.) by chemical activation with phosphoric acid as an activating agent. Initially, the precursor was crushed into granular form and sieved to get a uniform particle size (3–5 mm). Impregnation was carried out at 85°C for 2 h with phosphoric acid solutions at the ratio of impregnation 0,75 H₃PO₄/g raw material.After impregnation, the samples were carbonized at 600°C (heating rate of 8°C/min) using a temperature ramp and under nitrogen flow (250 ml/min). Finally, the sample was washed with distilled water up to pH 5, dried at 100°C, and sieved to 0.25 mm (No. 60-mesh, ASTM) (Obregón-Valencia and Sun-Kou, 2014).

Previously synthesized conditions have demonstrated to be the most optimal in order to improve AC adsorption capacity of metals due to the development of high surface acidity and mesoporosity (Obregón-Valencia and Sun-Kou, 2014; Sun-Kou et al., 2014). In order to evaluate surface acidity influence in metal removal, ACs synthesized were oxidized with nitric acid.

To oxidize the activated carbon was used nitric acid. For this was employed 2g of AC, which was mixed with 160 ml of a 1 M solution of acid for 6 h at 65°C; then the mixture was filtered and the carbon was washed with distilled water in order to remove the remaining acid. Ultimately, the oxidized sample was dried at 100°C for 24 h (Chun et al., 2007). The oxidized activated carbon sample is called AC-OX.

Characterization

The characterization of ACs was carried out using analytical and instrumental techniques. Texture parameters of surface area and pore volume of the AC samples were evaluated by physical adsorption–desorption of N₂ at 77 K in the relative pressure range from 0.005 to 1, on a Gemini VII model 2390; the specific surface area (S_BET) was calculated by the Brunauer–Emmett–Teller (BET) model. Microporous volume and area was determined using the t-plot method. AC surface functional groups were determined by FTIR spectrometry using a Perkin Elmer Spectrum 100 FTIR; the samples were prepared with a KBr pellet and measured in a spectral range of 4000–400 cm⁻¹. To evaluate the acidity of ACs was employed Boehm method, using solutions of three bases: NaOH, Na₂CO₃, and NaHCO₃, the solutions were kept at room temperature with constant stirring for 24 h (Goertzen et al., 2010). The surface charge of the ACs was determined by point of zero (pH_PZC), measured by mass titration with nitric acid solutions at different pH (2–8). The pH_PZC is the point where the curve of pH (final) versus pH (initial) intersects the line pH (initial) = pH(final) (Kosma et al., 2009). The measure of pH was determined by using a pH meter (WTW model 537) and a pH electrode with electrolyte Ag/AgCl₂ (Brand SenTix model 81). The morphological distribution was analyzed by FEI Quanta 600 electron scanning microscope.

Computational modeling

The calculations for modeling the studied compounds were carried out with the program Hyperchem 8. This modeling program is based on quantum mechanics and recognizes molecules as systems with an electron atom, orbital hybridization, and orbitals. The methods used to estimate were the semi-empirical and molecular mechanics.

The geometric optimization of structures of AC and metal ions was performed with AM1 method which is considered within the molecular orbital methods for unrestricted Hartree–Fock state, restricting the spin–spin pairing in different energy levels, not at the same level. Initially structures were modeled with the method of molecular mechanics MM⁺ as well as the interaction of metal ions and the surface of AC was studied using this method.

When the AM1 method was used, it was employed one optimizer steepest-descent followed by a conjugate gradient, Fletcher-Reeves and Polack-Ribiere methods (Shahzad Khan and Shahid Khan, 2011). These were considered to limit convergence of 0.0001 kcal/mol and a gradient limit root mean square of 0.001 kcal/mol-A, allowing to know the level of geometry convergence of geometric optimization.

Results and discussion

Textural analysis

The surface of AC and AC-OX was compared after oxidation treatment. According to Sun-Kou et al. (2014), the acid treatment of AC shows beneficial effects on the adsorption of metal ions; however, this treatment damages the physical aspects of AC, such as BET surface area and volume of pore.

Rodriguez et al. (2014) found a decrease in surface area by treating AC with 5 M nitric acid for 8 h at boiling point (drastic conditions). This effect was also reported by Rios et al. (2003) and Aburub and Wurster (2006) works where concentrated nitric acid is alternated between times of 38 and 24 h, finding the surface area reduction of 33.7 and 6.5%. They suggest that the reduction can be attributed to the destruction of the pore structure within the AC caused by severe oxidation with nitric acid. In this paper, the treatment of AC was avoided in severe conditions in order not to decrease the surface area. Nevertheless, in other works, Moreno et al. (2007) detected that the acid oxidation of ACs produced a decrease in the S_BET surface area before oxidation. The AC used was highly porous (S_BET = 1400 m²/g), after being treated with nitric acid.

In Table 1, the textural characteristics of the AC and the AC-OX are shown. In the case of the ACs worked, the AC-OX increases its surface area compared to AC, especially the microporous area, which demonstrates the formation of oxygenated surface groups. These characteristics are achieved by not exposing the AC to drastic oxidation conditions.

Table 1.

Textural characteristics of AC and AC-OX.

Sample	Area (m²/g)			Micropore volume	Average pore diameter
Sample	Total	Meso	Micro	(cm³/g)	(nm)
AC	1194	111	1083	0.57	2.87
AC-OX	1410	138	1272	0.66	2.81

AC: activated carbon; AC-OX: oxidized activated carbon sample.

In addition, Figure 1 shows the adsorption isotherms of N₂ of AC and AC-OX, which are of type I according to the IUPAC classification (Obregón, 2012) and are corresponding to micropore materials. There is a greater amount of N₂ adsorbed at low pressures by functionalized carbon (AC-OX), which shows a greater surface area, mainly of the microporous type. This might be attributed to the fact that the functionalization with nitric acid eliminates the inorganic material present and opens new pores, thus increasing the surface area. There is not formation of a hysteresis loop, so it is assumed that the pores in both materials are of the cylindrical type.

Figure 1.

Isotherms of N₂ adsorption of samples: AC and AC-OX. AC: activated carbon; AC-OX: oxidized activated carbon sample; STP: Standard Temperature and Presion.

Table 1 shows an increase in the surface area of the functionalized carbon with nitric acid AC-OX (1410 m²/g) in comparison to AC (1194 m²/g). This may be associated with mild oxidation treatment condition, such as a concentration of 1 M nitric acid and 6 h contact, without affecting AC morphology, and favoring the formation of oxygenated groups. As shown in Table 1, there is no significant variation in pore diameter, both of AC (2.87 nm) and AC-OX (2,81 nm) (Rodriguez et al., 2014). Figure 2 shows the pore distribution of the CA and AC-OX samples, which were performed using the technique proposed by Horvath–Kawazoe (Okhovat et al., 2012). Most of the pores lie in the range of 1.5–4 nm in both ACs, specifically between 2.3 and 3.2 nm. This distribution is greater for AC-OX compared to the CA sample.

Figure 2.

Distribution of pores in the samples CA and AC-OX. AC: activated carbon; AC-OX: oxidized activated carbon sample; STP: Standard Temperature and Presion.

Surface acidity

Table 2 shows the results obtained for the determinations of functional groups by Boehm titration and point of zero charge. The starting sample AC, presents a variety of surface groups, with a greater amount of lactone groups. The treatment with nitric acid promoted the formation of surface groups, mainly the formation of carboxylic and phenolic groups.

Table 2.

Characteristics of surface chemistry of AC and AC-OX.

	Carboxilics (mmol H⁺/g)	Lactonics (mmol H⁺/g)	Phenolics (mmol H⁺/g)	Total (mmol H⁺/g)	pH_PCZ
AC	1.12	3.08	0.03	4.22	2.450
AC-OX	4.10	0.05	0.67	4.82	2.014

AC: activated carbon; AC-OX: oxidized activated carbon sample.

In Figure 3, the FTIR spectrum of AC-OX and AC is presented. It shows an increase in the intensity of the peak at 3425 cm⁻¹, which is due to the stretching of the O–H of the carboxylic groups. This corroborated the increase of carboxylic groups with respect to the original sample. In addition, groups derived from phosphorus (990–1220 cm⁻¹) increased, likely for fixation of nitros groups (Moreno et al., 1995).

Figure 3.

Espectrum FTIR of CA and AC-OX with nitric acid (6 h, [HNO₃] =1 M and 65°C functionalization conditions). AC: activated carbon; AC-OX: oxidized activated carbon sample.

In the AC-OX sample, the appearance of a very intense band attributed to the N–O link can be observed at 782 cm−1 (Silverstein et al., 1981). These signals are attributed to nitrogen groups as a result of nitric acid treatment (Moreno et al., 1995). An increase in the intensity of the band can be seen at 671 cm−1, which corresponds to the out-of-plane annular CC flexion in phenols (Silverstein et al., 1981), which is consistent with the increase in the phenolic groups shown in Table 2.

In similar studies, nitric acid oxidation increase in these groups was also reported (Figueiredo et al., 1999; Jaramillo et al., 2010a, 2010b). The high porosity and the surface groups enhance the capacity of adsorption.

The isoelectric point (pH) 2.45 PZC in AC would indicate that in aqueous solutions at lower pH, AC is positively charged and could create electrostatic repulsions with the Cd²⁺ and Cr ions. Moreover, low pH presented a competitive effect between H⁺ ions and metal ions, which contributes to a reduction of the adsorption capacity of metals. On contrary to pHs higher than the isoelectric point, the surface of the CA would be charged in a negative way with PZC. It would favor the electrostatic attractions with the metal ions Cd²⁺ and Cr_Total.

The functionalization of AC-OX increased acidic groups on its surface. The total surface acidity increased from 4.22 to 4.82 mmol H⁺/g, and the zero charge point decreased from 2.45 to 2.01 (Table 2). It could be observed in FTIR spectra, for example in AC-OX signals related to oxygenated groups increased. Carboxylic groups contributed significantly in this increase (from 1.12 to 4.10 mmol H⁺/g).

The formation of carboxylic groups occurs on the aliphatic side of the molecules, especially in the side chains where there is a greater amount of carbon atoms (Figure 4). The reaction is carried out by dividing the C–C bond of the ortho position of the benzyl carbon atom, which ends up forming dicarboxylic groups.

Figure 4.

Formation of carboxylic groups in the AC surface.

Moreno et al. (1995) studied ACs with three different oxidizing agents (oxygen peroxide, nitric acid, and ammonium peroxydisulfate) and found that nitric acid introduced the largest amount of oxygenated groups on the surface of AC with more drastic changes in the pores of the material.

It was obtained zero charge points less than 7 value, which shows that these carbons were acidic. This is explained by the use of the impregnating agent, phosphoric acid, in the activation of AC and is enhanced by the acid functionalization process (AC-OX).

Scanning electron microscopy (SEM)

Figure 5 shows the SEM micrographs for both porous materials unoxidized AC and AC-OX. Meso and micropores are evident in both micrographs. It may be noted that carbon unoxidized presents a uniform surface, with a pore network more homogeneous than AC-OX. In addition, the AC-OX micrograph corroborated that the material maintained its porosity after oxidation treatment.

Figure 5.

Micrographs of (a) AC and (b) AC-OX.

Experimental adsorption

Kinetics test was conducted at room temperature (20°C) with 15 mg of AC with 20 ml aqueous solution (10 ppm of Pb, Cd, Cr) kept under agitation at pH 5. The contact time varied between 5 min and 4 h. The results, which can be seen in Table 3, showed the following adsorptive order for the AC—0.75–600 and for oxidized, AC-OX—0.75–600: Pb> Cd> Cr, finding that the adsorption capacities below: AC—0.75–600: Pb (12.73 mg/g)> Cd (12.32 mg/g)> Cr (9.38 mg/g), AC-OX—0.75–600: Pb (12.73 mg/g)> Cd (12.97 mg/g)> Cr (10 mg/g).

Table 3.

Results of kinetic adsorption onto activated carbon.

Time (min)	q_t
Time (min)	Pb	Cd	Cr
5	7.97	1.71	0.74
15	9.21	–	0.86
30	10.24	5.36	2.16
50	10.48	5.50	2.59
60	10.65	5.83	3.69
120	10.72	6.05	4.00

One can see that the adsorption improves slightly with activated oxidation, especially for the cadmium and chromium. In percentage terms, the increase is 5–7%.

All kinetics curves reached the equilibrium state within the first hour except for chromium kinetic curve with AC-OX. The experimental adsorption allowed to determine the order of adsorption capacities for the three metals studied in both adsorbent materials at the same conditions.

These results are corroborated with previous studies. Madhava Rao et al. (2009) found that the adsorption capacity of lead was superior to that of cadmium (Cd(II) < Cu(II) < Zn(II) < Pb(II)). The same behavior is also observed in the work done by Kermit Wilson et al. (2006). The adsorption capacity of lead is much greater than that of cadmium (Kermit Wilson et al., 2006).

According to Abdel-Nasser A. El-Hendawy (2009), lead is removed more efficiently compared to cadmium due to its low solubility and high degree of complexing, which is related to the interaction between the metal and the functional groups on the surface of the carbon. Moreover, the deposition of lead, due to its low solubility, promotes its removal from the solution (El-Hendawy, 2009). As shown in Table 4, the solubility of lead is much lower compared to cadmium and chromium.

Table 4.

Solubility expressed as the number of grams of the substance that when dissolved in 100 g of water produces a saturated solution at a temperature of 20°C (Dean, 1999).

Compound	Solubility (g/100 g _H2O)
Pb(NO₃)₂	54.3
Cd(NO₃)₂	150
Cr(NO₃)₃	130

It is known that AC has on its surface a series of functional groups (hydroxyl, carboxyl, carbonyl, among others); these functional groups are some of an acidic nature and others are basic, and give the carbon its amphoteric nature. In the process of adsorption with metals, it is possible that electrostatic interactions of different magnitude (of the van der Waals type and/or chemical bond) could occur between the metal and the surface functional groups of the carbon. In a study conducted by Pandey et al. (2002), it was established that coordination compounds between humic acid and metals can be formed. The stability presented in the metal–humic acid complexes can help to understand this complexing effect. The stability constant (log K) found indicates the following increasing order of stability for metal–humic acid complexes (Pandey et al., 2002)

Cu> Fe> Pb> Ni> Co> Ca> Cd> Zn> Mn> Mg

It can be seen that the Pb complexes are much more stable than those of cadmium.

The kinetic adsorption is shown in Figure 6. The adsorption order is according to the theory explained before.

Figure 6.

Kinetic adsorption of metals onto AC (m = 0.015 g of activated carbon; [C]_o = 10 ppm de Cd, Cr y Pb; Vol =20 ml of solution).

Apparently, the adsorption process is more influenced by the greater acidity of the surface, the solubility, pH of the solution, and the presence of mesopores that facilitate the diffusion of cations to the active centers.

Previous work (Sun-Kou et al., 2014) has shown that kinetic curves were adjusted to the pseudo-second-order model indicating an interaction preferably of the chemical type (chemisorption).

In addition, the adsorption isotherms were estimated and data fit with Redlich–Peterson. Therefore, the parameter G was used to determine the influence of each of these models in the adsorption process and a behavior of the Langmuir type was preferably obtained. This model assumes that adsorption occurs in a finite, defined number of identical and energetically equivalent localized sites that form a monolayer. This occurs without any lateral interaction or steric hindrance between the adsorbed molecules.

This information was compared with the modeling program Hyperchem.

Possible mechanism

Surface of AC has an amphoteric nature (Chun et al., 2007). Chemical adsorption (chemisorption) is due to the formation of bonds between the adsorbed species and the acidic surface functional groups present in ACs. Metals are incorporated or retained on AC surface by the formation of complexes due to the chelating, attribute possessed by functional groups as shown in equation (1)

M^{n +} + n (- COOH) ⇄ {(- COO)}_{n} M + {nH}^{+}

(1)where:

Mⁿ⁺: Cation in the solution

n(–COOH): Acid group onto AC surface

(–COO)_n M: Complexes formed

nH⁺: Protons

The previous reaction occurs due to the cation exchange mechanism, in which the metal cation exchanges site with the hydrogen ion. This has motivated the research selected with the modification of the surface of AC with the aim of increasing surface acid groups and thus increasing the ability to incorporate or retain metals. Other researchers have studied the mechanism of adsorption finding similar results (Biswas et al., 2019, 2020; Chen and Wu, 2004; Yalçin and Sevinç, 2000)

Hyperchem simulation

Electron densities

According to the results of surface acidity, models of AC consistent with these findings were made. Therefore, the percentage of functional groups contained in each carbon was considered and the corresponding AC was modeled with Hyperchem program.For example, AC analyzed by Boehm treatment presented more lactone groups and AC-OX showed more phenolic and carboxilic groups.

It was found that AC unoxidized had a lower electron density than the AC oxidized (Figure 7). Therefore, AC-OX showed a negative charge to attract better the metal ions.

Figure 7.

Electron densities of (a) AC unoxidized (electronic density: −1.38) and (b) AC oxidized (electronic density: −2.81).

Frontier orbitals

Two orbitals are extremely important in the study of interactions and chemical reactions. These are the frontier orbitals: highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Between these orbitals occur a charge donation and a charge acceptation, respectively. When the LUMO from metal ion is closer to the E_f of the AC, there is an increase in the amount of net charge on the adsorbate (metal ion) contributed by AC. The net charge strongly suggests that empty orbitals on metal ions overlap with filled orbitals localized on AC, and the charge transfer from AC to the metal ions increases the strength of adsorption (Paredes-Doig et al., 2014; Rochefort and Wuest, 2009).

To evaluate which ions are more readily adsorbed, it is necessary to compare the HOMO of the AC and the LUMO of the metal ion. The metal ions are considered Lewis acids and the AC as a Lewis base, by classifying Parr and Pearson (1983) and subsequently studied by Alfarra et al. (2004). Both authors consider the acid–base theory, which means that hard acids interact with hard bases and soft acids with soft bases.

Table 5 shows the energy differences of the frontier orbitals values have an order. The values are higher for Cr and Cd, and much smaller for Pb. This indicates that the lead was adsorbed in greater quantity and predictable order of adsorption is as follows: Pb(II)> Cd(II)> Cr(III).

Table 5.

Frontier orbital values obtained using Hyperchem.

	HOMO	LUMO	ΔE
Cr(III)	−49.78	−27.81	18.49
Cd(II)	−38.92	−14.71	5.39
Pb(II)	−37.52	−13.55	4.23
AC unoxidized	−9.32	−2.24
Cr(III)	−49.78	−27.81	18.7
Cd(II)	−38.92	−14.71	5.6
Pb(II)	−37.52	−13.55	4.44
AC-OX	−9.11	−1.82

AC: activated carbon; AC-OX: oxidized activated carbon sample; HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital.

This table also presents that the adsorption capacity for AC-OX and AC is similar. Therefore, orbitals frontier is not enough or the unique factor to understand the adsorption process. Consequently, the electronic densities should be considered.

Conclusions

Oxidation of AC increased the negative charge present on the surface of AC and increased the adsorption of metal ions. Therefore, it is proposed that the initial stage of adsorption of metal ions occurs through an electrostatic interaction.

Hyperchem program results show a dependency between the energy difference of frontier orbital (between the adsorbate and the adsorbent) and the adsorption capacity of metals.

In the case of Cr(III), because it is a hard acid, adsorption on the carbon surface may be more influenced by the presence of oxygenated functional groups.

In both CA, oxidized and unoxidized, the order of adsorption had the following relation: Pb(II)> Cd(II)> Cr(III).

Footnotes

Acknowledgements

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors acknowledge the support provided by Pontificia Universidad Católica del Perú (PUCP) and Peruvian Council of Science (CONCYTEC).

ORCID iD

AL Paredes-Doig

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The interaction of metallic ions onto activated carbon surface using computational chemistry software

Abstract

Keywords

Introduction

Experimental

Sample preparation

Characterization

Computational modeling

Results and discussion

Textural analysis

Surface acidity

Scanning electron microscopy (SEM)

Experimental adsorption

Possible mechanism

Hyperchem simulation

Electron densities

Frontier orbitals

Conclusions

Footnotes

Acknowledgements

Declaration of Conflicting Interests

Funding

ORCID iD

References