DFT studies on exposure of sulfur impregnated and sulfur functionalized activated carbon to Hg0 vapors

For removal of poisonous vapor emissions such as mercury, it is necessary to select suitable materials based on an understanding of their properties and interactions with the vapor. As mercury has a high affinity towards sulfur, it’s adsorption using sulfur-impregnated activated carbon was explored in this study. The impregnation of sulfur on activated carbon followed by the adsorption of Hg0 vapors was computationally investigated using DFT simulations. Sulfur adsorption was investigated on activated carbon with armchair edge, zigzag edge, and graphene surface. Sulfur adsorption was investigated on activated carbon with edge functional groups such as hydroxyl and carboxylic acid. Activated carbon with edge functional groups such as sulfonic acid, sulfenic acid, and sulfinic acid was further investigated for the adsorption of Hg0 vapors. Among the edge functional groups on the activated carbon, the hydroxyl group was most favored for sulfur adsorption and, subsequently, Hg0 vapors. This was quantified in terms of shortest bond lengths, strongest binding energies, and maximum charge transfer. Among the sulfur-containing functional groups on activated carbon, sulfenic acid was the most favored for the adsorption of Hg0 vapors. Transition state calculations were carried out, and a reaction pathway was proposed for the adsorption of Hg0 on sulfur-impregnated activated carbon. GRAPHICAL ABSTRACT


Introduction
Hg 0 vapors are released through both anthropogenic and industrial sources [1].Hg 0 vapors are released from small-scale gold mines, coal-fired power plants, natural gas producers, cement industries, and many others, as shown in Figure 1.Other sources of Hg 0 vapors include mercury cell processes, battery manufacturing units, catalyst production plants, and industries producing chemicals, pesticides, electric switches, measuring instruments, and fluorescent lamps [2].
Hg 0 vapors are toxic to the central and peripheral nervous systems, respiratory and cardiovascular systems, as well as to the kidneys and liver [3,4].Hg 0 , upon absorption into the bloodstream, oxidizes into Hg + and Hg + 2 and readily binds to the -S-H groups in the cellular matrix, thereby hindering cellular operations [4].Mercury levels above 5.8 ng/mL in the human bloodstream are considered unsafe [4].Combating Hg 0 vapors from the environment is challenging as it is highly volatile and almost insoluble in water [5].
Sorbent injection process is also widely used in combating Hg 0 vapors from flue gases [11].Adsorption is an important preliminary step in heterogeneous catalytic processes [14].The reactant molecules adsorb first on the catalyst surface before initiating chemical reactions.The catalytic efficiency is closely related to the adsorption and activation of reactant molecules [14].One of the strategies to improve catalyst performance is through composite loading [14].
Activated Carbon (AC) is the most widely used adsorbent due to its high surface area, micropore volume, and thermal stability [20].Activated carbon has proven effective in removing Hg 0 vapors [21].Altaf et al. used biochar for combating Hg 0 vapors [22].As observed by Wang et al. [23], the physisorption efficacy of traditional adsorbents may be considerably enhanced by adding substances to the solid surface that lead to chemisorption of the pollutant [23].The nature of the functional groups present on the solid surface may influence the extent of chemisorption [23].Typical literature on Hg 0 removal using different materials is briefly reviewed below.
Li et al. computationally investigated the binding of Hg 0 on CuS (001) surface [16].Results from binding energy suggested that Hg not only binds to S atoms in the structure but also to the Cu atoms.Further, it was found that the edge Cu and S atoms were majorly active in bond formation with Hg and chemisorption of Hg occurred.Li et al. also study the influence of other impurities like oxygen, water vapor and sulfur dioxide on the adsorption of Hg 0 .There was a negligible competitive effect on Hg 0 adsorption on CuS due to the presence of impurities.Lei et al. observed that while precious metal based catalysts are dominant, carbon based systems are attractive alternatives owing to their intrinsic sustainability and practical usability [24].Chompoonut et al. computationally investigated the adsorption and oxidation of Hg on halogenated (Cl, Br and I) activated carbon [25].Binding energy calculations showed that the adsorption and oxidation of Hg was preferred by iodine-modified carbon.These were in line with the experimental results suggested by Chompoonut et al.Padak et al. computationally investigated the binding of mercury on activated carbon with halogens infused into the carbon matrix and containing oxygenated functional groups [26].Their studies based on binding energies revealed that Hg binding was increased in the presence of fluorine-modified carbon.Among the oxygen-containing functional groups, carbonyl and lactone were preferred for binding of Hg.Qu et al. computationally investigated the detailed mechanisms of Hg binding to iodine on the carbon surface [27].The detailed mechanism was proposed based on the results from adsorption energy, atomic bond, and Mulliken charge population.It was found that the Hg-I and Hg-I 2 were probably chemisorbing as a whole or adsorbing in a dissociative fashion.Hsi et al. experimentally investigated the influence of gases like O 2 , SO 2 , NO and HCl on the adsorption of Hg on sulfur-impregnated activated carbon [21].It was found from equilibrium and kinetic studies that adsorption was improved in the presence of NO but was reduced in the presence of O 2 and SO 2 .Zhang et al. experimentally investigated the adsorption of Hg 0 vapors on recycled activated coke [28].It was found that the adsorption was favored on adsorbent rich in functional groups in order of ester > carbonyl > SO 4 −2 .Also, it was found that chemisorption was a dominant mechanism in the adsorption of Hg 0 vapors.Liao et al. investigated the adsorption of Hg 0 vapors on gold electrodeposited activated carbon cloth [29].The authors deduced that the adsorption of Hg 0 on Au deposited adsorbent was due to chemisorption and amalgamation of Hg and Au.However, the mechanism was dominated by physisorption when the Au deposition was absent.
From the above representative review, it is evident that DFT studies involving the adsorption of Hg 0 vapor on sulfur-impregnated activated carbon are scarce.The functionalization was mainly done using halogens, metallic CuS and gold depositions.Exploring new materials development through functionalization using facile synthesis processes is essential.These include using biomass inherently containing functional groups, physical impregnation of sulfur in the carbon matrix, and acid treatment to introduce the sulfur functionality.
To our knowledge, very little work has been done to address computationally the adsorption of Hg 0 vapors on sulfur-impregnated AC and provide essential information with a high level of detail.The problem investigated in the current study is of immense practical relevance, and the work identifies, for the first time through computational studies, suitable ingredients in the development of functional materials for the effective removal of deadly Hg 0 vapors from the air.This theoretical study will provide essential guidelines and facilitate the development of effective materials for addressing a crucial issue.
In the present work, we have demonstrated the following objectives systematically involving the adsorption of Hg 0 vapors on AC: A. Effect of impregnating AC with sulfur B. Influence of hydroxyl and carboxylic acid functional groups inherently present on AC on sulfur impregnation C. Effect of functionalizing sulfenic, sulfonic and sulfinic acids on AC In current study Density Functional Theory (DFT) simulations were carried out to address the above aspects.

Theory
The sulfur impregnation may be represented as Eq. ( 1).AC Surface or edge + S cluster → AC Surface or edge − S cluster (1) The impregnated sulfur on AC reacts with Hg 0 vapors in the atmosphere to form mercury sulphide (HgS) [3], as shown by Eq. ( 2).This aspect is discussed in objective (A).
In objective (B), the activated carbon with edge functionalization has been used for sulfur impregnation, followed by Hg 0 adsorption.This is of interest, as the AC is obtained frequently from biomass, and this medium possesses considerable amounts of edge functional groups on the carbon.These functional groups are responsible for surface interactions leading to stronger adsorption.In the present study, hydroxyl and carboxylic acid functional groups were studied.
Also, it would be interesting to study the adsorption of Hg 0 vapors directly on AC edge with sulfur-containing functional groups.This is explored in objective (C).These functional groups may be located on the edge of AC.The adsorption may be represented by Eq.(3).Sulfur functionalized AC could be easily realized in practice by acid treatment to carbon [30][31][32] or acid treatment to biomass [33].In the present study, AC with three different functional groups were selected for analysis: sulfenic acid, sulfonic acid, and sulfinic acid.

Computational details
Gauss View 5 software was used for visualizing the molecule and creating input files.Gaussian 16 software was used for performing DFT calculations [34].B3LYP was the functional used, the 6-31G basis set was used for O, H, C, and S atoms, and the LANL2DZ basis set was used for Hg.The calculations were performed in a vapor state as the system is for the adsorption of Hg 0 vapor.The transition state calculations were done using the Berny optimization algorithm in Gaussian 16 software [34].The use of alternate basis sets is discussed in supplementary section S1.

Model details
A graphene sheet with 16 C atoms was used to model the AC surface.Three different types of surfaces were used for the AC, namely armchair edge, zigzag edge, and graphene surface containing 16 carbon atoms, as seen in Figure 2(a-c), respectively.The reason for the small size of the carbon matrix is that this study is of screening nature where the selection of the type of carbon edge or functional group was important.Further detailed analysis with more carbon atoms may be performed based on the screening results.This will save the computational time and effort.The armchair edge is formed when deprotonation occurs in the same carbon ring, whereas for zigzag edge is the one where deprotonation occurs in the adjacent carbons in the chain [35].AC carbon prepared from biomass contains edge functional groups, typically hydroxyl and carboxylic acid [36].AC surface containing 16 carbon atoms and hydroxyl and carboxylic acid were also studied, as seen in Figure 2(d, e).Elemental sulfur is used for impregnation onto the activated carbon.Sulfur exists in various allotropes, namely S 2 , S 5 , S 6 , S 7, and S 8 , where S 8 is the most stable form at room  temperature and the most abundant [37].The impregnation of sulfur is carried out at high temperatures of 600 to 700°C [37].At these temperatures, S 2 and S 6 allotropes are dominantly present [37].Hence only S 2 and S 6 allotropes will be impregnated on the carbon and considered in the discussions ahead.(Figure 3) The sulfur-functionalized AC edges may be modelled, as seen in Figure 4.For this study, three functional groups, namely sulfenic acid (-SOH, Figure 4(a)), sulfinic acid (-SO 2 H, Figure 4(b)), and sulfonic acid (-SO 3 H, Figure 4(c)), have been considered.The cartesian coordinates for all the optimized structures are given in supplementary section S2.
The affinity of sulfur to different AC surfaces was evaluated based on binding energy ( E) [38], which is defined as the difference between the energy of the products and the reactants as per Eq. ( 4).'E' in turn, represents the minimum electronic energy of the optimized molecular complex with zero-point energy correction.
The lower the value of the 'E,' the more stable is the chemical species.E represents the energy transfer that occurs during the formation of the products [38].This suggests that the more negative the value of E, the higher the feasibility of the formation of products [38].The interaction and affinity among molecules has been investigated through charge transfer analysis [39].Mulliken atomic charge was used to evaluate the charge transfer during adsorption [38].Mulliken atomic charge is the distribution of charge of overlapping electron clouds for a molecule [40].A charge transfer on (OH) oxygen atom was studied during adsorption for AC with hydroxyl and carboxylic acid edges, as the Mulliken atomic charge is relative.The charge transfer is defined as shown in Eq. ( 5).

= |Mulliken charge on O atom on AC|
− |Mulliken charge on O atom on AC after S 2 cluster adsorption| (5)

Analysis of sulfur clusters
Among the two sulfur clusters (S 2 and S 6 ) stable at an impregnation temperature of 600-700°C [37], it was found that the S 2 cluster was the most reactive.This conclusion is based on the higher electronic energy (with zero-point correction) of S atoms in the S 2 cluster compared to that of the S 6 cluster.Figure 5 reveals that the energy of S atoms in S 2 (on a per sulfur atom basis) is much higher than that in the S 6 cluster, implying that the former is more reactive than the latter.This is due to the less coordinated sulfur atoms in the S 2 cluster.Owing to the higher reactivity, this cluster will be considered further.

Analysis of S 2 cluster impregnation on AC surfaces
From Figure 6(a), the electronic energy of AC with the zigzag edge is the highest and hence the most reactive among the three cases considered.This finding is in agreement with the literature [35].This reactive nature is manifested in the greater affinity for adsorption of the S 2 cluster.The binding energy of the S 2 cluster for the zigzag edge is the lowest (more negative) and hence is the most favorable for the adsorption of S 2 .The trend in binding energy is zigzag edge < armchair edge < graphene surface.This is due to the reverse trend in electronic energy graphene surface < armchair edge < zigzag edge.This implies that the most reactive zigzag edge results in the strongest adsorption of the S 2 cluster on the activated carbon.The optimized structure of the adsorbed S 2 cluster on the armchair and zigzag edge is seen in Figure 7(a, b), respectively.From Figure 7, the atomic distance of the S 2 cluster from the AC surface is 1.6 Å for the  armchair edge and 2.4 Å for the zigzag edge.This implies that although the affinity of the zigzag edge is higher than that of the armchair edge for S 2 cluster adsorption, the S 2 cluster adsorbs more strongly on the armchair edge than on the zigzag edge.The E for the zigzag edge is less when compared to the armchair edge, and hence it is more favorable and readily available edge for adsorption [35].

Analysis of Hg 0 vapor adsorption on S 2 cluster impregnated AC surfaces
From Figure 8(a), the electronicenergy of the S 2 cluster adsorbed on AC is highest for the zigzag edge and lowest for the graphene surface.Hence, the S 2 adsorbed on the zigzag edge will be the most reactive; from Figure 8(b), the binding energy is the lowest for the zigzag edge.The trend in binding energy is zigzag edge < armchair edge < graphene surface.This trend is due to the reverse trend in electronic energy: graphene surface < armchair edge < zigzag edge.Figure 8(c) compares the binding energy of Hg 0 vapors adsorption on AC with and without S 2 cluster for armchair and zigzag edge.It may be seen from binding energy data that the adsorption of Hg 0 vapors is enhanced by an S 2 cluster for both armchair and zigzag edge AC.Also, the trend in binding energy is the same among armchair and zigzag edges, irrespective of the presence or absence of the S 2 cluster.The optimized structure of Hg 0 vapors adsorbed on S 2 -impregnated AC surfaces is seen in Figure 9.In all cases, the Hg 0 is bonding to only one of the sulfur atoms, which is farthest from the carbon surface and adsorbing on it.Also, the bond lengths for Hg-S are the same for armchair and zigzag edge cases and equal to 2.5 Å, which is in agreement with the value reported by Manceau et al. [41].The atomic distance of Hg-S for the graphene surface is 2.9 Å.This is due to lower affinity, as seen in the binding energy results.

Transition state (TS) analysis of Hg 0 vapor adsorption on AC impregnated with S 2 cluster
TS calculations were performed for the adsorption of Hg 0 vapors at the S 2 adsorbed AC on the armchair and zigzag edges.From Figure 10, it may be seen that the energy barrier is  less for the zigzag edge than for the armchair edge.The zigzag edge is more suitable for the adsorption of Hg 0 vapors.This is in line with the results from binding energy calculations as discussed in sections 3.2 and 3.3.The following mechanism may be proposed based on the TS calculations and the atomic distance data given in Table 1.
Step 1 (Weakening of S-S bond): The S-S bond weakens, and the S atom near to AC surface starts approaching the AC surface.
Step 2 (Bond formation between Hg and S atoms): The weakened S-S bond causes the sulfur atom away from the surface to interact with Hg atoms leading to bond formation.
Step 3 (Formation of Hg-S bond): The HgS molecule is finally formed and stabilized.

Analysis of S 2 cluster adsorption on AC surfaces with different functional groups
From Figure 11(a), the electronic energy of AC with a hydroxyl edge is higher when compared to the carboxylic acid edge.This reactive nature is manifested in the adsorption of the S 2 cluster.As seen from Figure 11(b), the binding energy of the S 2 cluster for the hydroxyl edge is less than the carboxylic acid edge and is more favorable for the adsorption of S 2 .
The optimized structure of the adsorbed S 2 cluster on hydroxyl and carboxylic acid edge AC is seen in Figure 12(a, b), respectively.From the structure, the atomic distance of the S 2 cluster from the AC surface is 2.01 Å for the hydroxyl edge and 3.23 Å for a carboxylic acid edge.This implies the affinity and strength of S 2 cluster adsorption are relatively higher for the hydroxyl edge than for the carboxylic acid edge.

Analysis of Hg 0 vapor adsorption on S 2 cluster impregnated AC surfaces possessing different functional groups
From Figure 13(a) the electronic energy of the S 2 cluster adsorbed on AC is higher for the hydroxyl edge than for the carboxylic acid edge.So, the S 2 adsorbed on the hydroxyl edge  will be more reactive.Figure 13(b) shows that the binding energy of adsorption of Hg 0 vapors is lower for the hydroxyl edge when compared to the carboxylic acid edge due to the higher reactive nature of the former.The optimized structure of Hg 0 vapors adsorbed on S 2 -impregnated AC surfaces is seen in Figure 14.In both cases, the Hg 0 is bonding to only one of the sulfur atoms, away from the carbon surface.
Also, the bond lengths for Hg-S are 2.4 Å for the hydroxyl edge and 2.6 Å for the carboxylic acid edge.This implies that Hg 0 is more strongly bonded to sulfur atoms to form an HgS molecule at hydroxyl edge AC.Mulliken charge transfer analysis shows that the charge transfer on the oxygen atom for the hydroxyl group is 0.087 and for carboxylic acid is 0.009.This indicates that the AC with hydroxyl edge bonds more strongly with S 2 cluster than with the carboxylic acid edge.This is the possible reason for the AC with hydroxyl edge having lower binding energy and higher affinity.

Analysis of Hg 0 vapor adsorption on AC surfaces functionalized with S-containing groups
From Figure 15(a), the electronic energy AC with the sulfenic acid edge is the highest and sulfonic acid is the least.AC with the sulfenic acid edge will be the most reactive to interact  with Hg 0 vapors.Figure 15(b) shows that the binding energy of Hg 0 vapors is the least for AC with the sulfenic acid edge due to its higher reactive nature.The trend in binding energy is sulfenic acid edge < sulfinic acid edge < sulfonic acid edge.This trend is due to the reverse trend in electronic energy sulfenic acid edge > sulfinic acid edge > sulfonic acid edge.The optimized structure of Hg 0 adsorbed on AC with different sulfur-containing functional groups may be seen in Figure 16.It may be seen from Table 2 the atomic distances between the Hg and the sulfur atom at the AC edge are similar for the sulfinic acid and sulfenic acid edge.Hg atoms strongly bond with O atoms in the sulfonic acid edge AC.This shows that Hg 0 vapors interact with oxygen atoms more strongly than sulfur atoms in all the cases considered.Oxygen atom being more electronegative, draws electrons towards itself in the functional groups [26].This causes higher affinity of oxygen atoms towards Hg atoms; hence, instead of sulfur atoms, Hg atoms bind to oxygen.
The sulfur impregnated materials are concluded to be better than sulfenic, sulfinic, and sulfonic acids for adsorption of mercury based on their respective binding energies.Sulfur, when impregnated, is present on the AC surface and its activity is not shielded by the presence of other atoms.The freely available sulfur cluster in sulfur impregnated AC may readily interact with the Hg 0 .
In the cases involving functionalization with the three acids, the sulfur is not directly bound to the AC surface or edge.Moreover, the reactivity of sulfur towards Hg 0 in the alkyl acidic functional groups may be affected by the presence of surrounding atoms such as oxygen and hydrogen.The Mulliken charge transfer to Hg 0 is more from oxygen rather than sulfur.Due to these reasons, the ability of S to interact with Hg 0 reduces.

Conclusions
DFT calculations enabled us to understand that sulfur impregnation increases the affinity of Hg 0 towards AC.The DFT calculations established that zigzag edge AC was preferred for both S 2 adsorption and Hg 0 adsorption.A mechanism for Hg 0 adsorption was evaluated for AC with an armchair and zigzag edge.Among the functionalized AC, hydroxyl functionalization was preferred for S 2 impregnation and Hg 0 adsorption.Among the activated carbons with sulfur-functionalized edges, sulfenic acid was preferred for Hg 0 adsorption.The AC with higher amounts of hydroxyl groups should be used for the most practical applications, as tailoring a zigzag edge AC may not be economical.In terms of binding energies, sulfur impregnation may be a better alternative for Hg 0 adsorption rather than sulfur functionalization.The detailed mechanism for the adsorption of Hg 0 vapor on sulfur-impregnated hydroxyl group edge AC is recommended for further insights.These fundamental studies will allow researchers to provide a reliable framework based on DFT theory to experimentalists to build new functional materials for specific applications.Experimental studies on Hg 0 adsorption using sulfenic, sulfinic, and sulfonic acids functionalized AC are recommended.For mercury removal, multifunctional catalysts exploiting the synergy between different types of catalyst materials may be analyzed using DFT simulations.One of the catalysts may be activated carbon, and the other could be a metal catalyst, as suggested by Zhang et al. [42].

Figure 5 .
Figure 5. Electronic energy per S atom in a cluster, with S representing the number of sulfur atoms in the cluster (2 for S 2 and 6 for S 6 ).

Figure 6 .
Figure 6.(a) Electronic energies of optimized structures of AC surfaces (b) Binding energy of S 2 cluster adsorption on AC edges/surfaces.

Figure 8 .
Figure 8.(a) Electronic energy of optimized structure of S 2 adsorbed on AC surfaces (b) Binding energy of Hg 0 vapors on S 2 adsorbed AC surfaces (c) Comparison of the binding energy of Hg 0 vapors adsorption on S 2 adsorbed AC and AC edges.

Figure 10 .
Figure 10.Energy barrier diagram for adsorption of Hg 0 vapors on S 2 impregnated AC for armchair and zigzag edge.

Figure 11 .
Figure 11.(a) Electronic energy of optimized structure of AC surfaces with edge functional groups (b) Binding energy of S 2 cluster adsorption on AC surfaces with edge functional groups.

Figure 13 .
Figure 13.(a) Electronic energy of optimized structure of S 2 adsorbed on functionalized AC edges (b) Binding energy of Hg 0 vapors on S 2 adsorbed on functionalized AC edges.

Figure 15 .
Figure 15.(a) Electronic energy of optimized structure of sulfur functionalized AC edges (b) Binding energy of Hg 0 vapors on sulfur functionalized AC edges.

Table 1 .
Atomic distances of reactants, products, and transition sates structures.

Table 2 .
Atomic Distance of Hg on various sulfur-functionalized AC edges.